Planet Science

Skylab

2008-09-15T18:51:00.000-07:00

Skylab

A view of Skylab from the departing Skylab 4 mission

Station statistics
Call sign:	Skylab
Crew:	3
Launch:	1973-05-14 17:30:00 UTC
Launch pad:	LC-39A, Kennedy Space Center
Reentry:	1979-07-11 16:37:00 UTC near Perth, Australia
Mass:	77,088 kg
Living volume:	10,000 ft³
Perigee:	269.7 miles (434 km)
Apogee:	274.6 miles (442 km)
Orbit inclination:	50 degrees
Orbital period:	93.4 minutes
Orbits per day:	15.4
Days in orbit:	2,249 days
Days occupied:	171 days
Number of orbits:	34,981
Distance travelled:	~890,000,000 miles (~1,400,000,000 km)
Statistics as of deorbit on 1979-07-11.
Configuration

Skylab configuration with docked Apollo Command/Service Module
Skylab

Skylab was the first space station the United States launched into orbit, and the second space station ever visited by a human crew. The 100 ton space station was in Earth's orbit from 1973 to 1979, and it was visited by crews three times in 1973 and 1974. It included a laboratory for studying the effects of microgravity and the Apollo Telescope Mount solar observatory.

Background

The origin of the project is difficult to pinpoint because a number of different but related proposals were floated by various NASA centers before Skylab itself was launched.

Early studies

A key event took place in 1959, when Wernher von Braun submitted his final Project Horizon plans to the US Army. The overall goal of Horizon was to place man on the moon, a mission that would soon be taken over by the rapidly-forming NASA. Although concentrating on the moon missions, von Braun also detailed an orbiting laboratory built out of a Horizon upper stage. This basic concept of re-using existing boosters would lead directly to a number of follow-on designs, and eventually the Skylab that actually flew.

In 1963, the US Air Force started development of the Manned Orbital Laboratory (MOL), a small space station primarily intended for photo reconnaissance using large telescopes directed by a two-man crew. It also had the designation KH-10 - Dorian. The station was the same diameter as a Titan II upper stage. The stations were to be launched with the crew riding atop in a modified Gemini capsule with a hatch cut into the heat shield on the bottom of the capsule.^[1]^[2]

A number of NASA centers saw the MOL as something of a threat, and started back-room studies on various space station designs of their own.^{[citation needed]} Most of these were simply "back of a napkin" type designs with no official backing. Studies generally looked at platforms launched by the Saturn V, followed up by crews launched on Saturn IB using an Apollo Command and Service Module (CSM), or alternately Gemini capsule on a Titan II-C, the latter being much less expensive in the case where cargo was not needed.

But at the same time NASA was also looking for proposals for a major post-Apollo follow-on mission, including studies of a very large 24-man station with an operating lifetime of about five years. Lockheed Martin was involved in this project, and proposed a station that they felt would be a natural follow-on to the moon missions. One requirement for a permanent station would be periodic resupply, and for this role Lockheed suggested both Apollo-derived cargo vehicles and a new lifting body craft. After a lengthy and circuitous history, the new supply vehicle would emerge as the Space Shuttle, and their space station proposal as Space Station Freedom.

The Apollo Applications Program

In June 1964, NASA headquarters in Washington set up the Apollo Logistic Support System Office, originally intended to study various ways to modify the Apollo hardware for scientific missions. The office initially proposed a number of projects for direct scientific study, including an extended-stay lunar mission which required two Saturn V launchers, a "lunar truck" based on the Lunar Module (LM), a large manned solar telescope using an LM as its crew quarters, and small space stations using a variety of LM or CSM-based hardware. Although it didn't look at the space station specifically, over the next two years the office would become increasingly dedicated to this role. In 1965 the office was renamed, becoming the Apollo Applications Program (AAP).

As part of their general work, in August 1964 MSC presented studies on an expendable lab known as Apollo "X", short for Apollo Extension System. Apollo X replaced the LM carried on the top of the S-IVB stage with a small space station just larger than the CSM's service area, containing supplies and experiments for missions between 15 and 45 days' duration. Using this study as a baseline, a number of different mission profiles were looked at over the next six months.

Wernher von Braun proposed a more ambitious plan to build a much larger station. His design replaced the S-IVB stage of a complete Saturn V with an aeroshell, primarily as an adaptor for the CSM on top. Inside the shell was a cylindrical equipment section slightly smaller in diameter than the CSM. On reaching orbit, the S-II booster would be vented to remove any remaining hydrogen fuel, then the equipment section would be slid into it via a large inspection hatch. The station filled the entire interior of the S-II stage's hydrogen tank, with the equipment section forming a "spine" and living quarters between it and the walls of the booster. This would have resulted in a very large 33 x 45 foot living area. Power was to be provided by solar cells lining the outside of the S-II stage.

One problem with this proposal was that it required a dedicated Saturn V launch to fly the station. Engineers could not "piggyback" the station's launch on a lunar mission, which required a working S-IVB stage. At the time the design was being proposed, all of the then-contracted Saturn V's were already earmarked for moon launches. Further work led to the idea of launching a smaller station based on the S-IVB instead, launching it on a surplus Saturn IB. Several planned Earth-orbit test missions for the LM and CSM had been canceled, leaving a number of Saturn IB's free for use.

An early "wet workshop" version of Skylab.

Since the Saturn I had a much lower throw weight capability, the S-IV stage could not be left empty; its thrust would be needed for the mission. This limitation led to the development of the wet workshop concept, which led naturally out of von Braun's idea of using an existing stage after its fuel had burned off. However, in this case the station was to be built out of the S-IVB stage itself, as opposed to the S-II below it. A number of S-IVB-based stations were studied at MSC, but even the earliest, from mid-1965, had much in common with the Skylab design that actually flew. An airlock was placed in the equipment area immediately below where the LM sat on a moon mission, and a minimum amount of equipment was installed in the tank itself in order to avoid taking up too much fuel volume. After launch, a follow-up mission launched by a Saturn IB would carry up additional equipment in place of its LM, including solar panels, an equipment section and docking adaptor, and various experiments. Douglas Aircraft, builder of the S-IVB stage, was asked to prepare proposals along these lines.

On 1 April 1966, MSC sent out contracts to Douglas, Grumman, and McDonnell for conversion of a S-IVB spent stage under the name Saturn S-IVB spent-stage experiment support module (SSESM). In May the Apollo astronauts voiced concern over purging the stage's hydrogen tank in space. Nevertheless, in late July it was announced that the Orbital Workshop would be launched as a part of Apollo mission AS-209, originally one of the Earth-orbit CSM test launches, followed by two Saturn I/CSM crew launches, AAP-1 and AAP-2.

Design work continued over the next two years, in an era of shrinking budgets. In August 1967 NASA announced that the lunar mapping and base construction missions examined by the AAP were being canceled. Only the Earth-orbiting missions remained, namely the Orbital Workshop and Apollo Telescope Mount solar observatory. Later several Moon missions were canceled as well, originally to be Apollo missions 18 through 20. The cancellation of these missions freed up three Saturn V boosters for the AAP program. Although this would have allowed them to develop von Braun's original S-II based mission, by this time so much work had been done on the S-IV based design that work continued on this baseline. With the extra power available, the wet workshop was no longer needed; the S-IC and S-II lower stages could launch a "dry workshop" directly into orbit.

Vehicle

On 8 August 1969, McDonnell Douglas received a contract for the conversion of two existing S-IVB stages to the Orbital Workshop configuration. One of the S-IV test stages was shipped to McDonnell for the construction of a mockup in January 1970. The Orbital Workshop was renamed Skylab as a result of a NASA contest. The actual stage that flew was the upper stage of the AS-212 vehicle). The mission computer used aboard Skylab was the IBM System/4Pi TC-1, a relative of the AP-101 Space Shuttle computers.

Mission

Launch of the last Saturn V rocket (Actually a Saturn INT-21) carrying the Skylab space station

Skylab was launched 14 May 1973 by a Saturn INT-21 (a two-stage version of the Saturn V launch vehicle) into a 235 nautical mile (435 km) orbit. The launch is sometimes referred to as Skylab 1, or SL-1. Severe damage was sustained during launch, including the loss of the station's micrometeoroid shield/sun shade and one of its main solar panels. Debris from the lost micrometeoroid shield further complicated matters by pinning the remaining solar panel to the side of the station, preventing its deployment and thus leaving the station with a huge power deficit. The station underwent extensive repair during a spacewalk by the first crew, which launched on 25 May 1973 (the SL-2 mission) atop a Saturn IB. If the crew had failed to repair Skylab in time, the plastic insulation inside the station would have melted, releasing poisonous gas and making Skylab completely uninhabitable. They stayed in orbit with Skylab for 28 days. Two additional missions followed on 28 July 1973 (SL-3) and 16 November 1973 (SL-4) with stay times of 59 and 84 days, respectively. The last Skylab crew returned to Earth on 8 February 1974.

View of Skylab space station cluster in Earth orbit from the departing Skylab 4 command module.

Operations in orbit

Skylab orbited Earth 2,476 times during the 171 days and 13 hours of its occupation during the three manned Skylab missions. Astronauts performed ten spacewalks totaling 42 hours 16 minutes. Skylab logged about 2,000 hours of scientific and medical experiments, including eight solar experiments. The Sun's coronal holes were discovered thanks to these efforts. Many of the experiments conducted investigated the astronauts' adaptation to extended periods of microgravity. Each Skylab mission set a record for the amount of time astronauts spent in space.^{[citation needed]}

End of Skylab

Planned Fourth Mission (SL-5)

This would have been a short 20 day mission to conduct scientific experiments and boost Skylab into a higher orbit. Astronauts Vance Brand (commander), Don Lind (science pilot), and William B. Lenoir (command module pilot) would have been the crew for this mission, with Brand and Lind being the prime crew for the never-flown Skylab Rescue flights.

Skylab B

NASA considered use of the existing Apollo/Saturn material for launching a second Skylab B station in May 1973, but decided to donate all the hardware to museums.

Planned shuttle missions

Following the last mission, Skylab was left in a parking orbit expected to last at least eight years. The Space Shuttle was planned to dock with and elevate Skylab to a higher safe altitude in 1979; however, the shuttles were not able to launch until 1981. A planned unmanned satellite called the Teleoperator was to be launched to save Skylab, but funding never materialized.

Skylab was in need of a major overhaul, including new gyroscopes, and was low on fuel. Some systems were not designed for maintenance in space; however this type of problem had been overcome before such as when the primary coolant loop was repaired.

A four phase plan to use the Skylab with the Space Shuttle was as follows:

A small Skylab Reboost Module would be docked to Skylab on the second Shuttle flight (STS-2A) and boost it to a higher storage orbit.

In two shuttle flights, Skylab would be refurbished. In January 1982, the first mission would attach a docking adapter and conduct repairs. In August 1983, a second crew would replace several system components.

Beginning in March 1984, shuttle crews would attach a solar-powered Power Expansion Module, refurbish scientific equipment, and conduct 30 to 90 day missions using the Apollo Telescope Mount and the earth resources experiments.

From 1985 to 1989 Skylab would be expanded for up to eight astronauts, with a new large docking/interface module, additional logistics modules, Spacelab modules and pallets, and an orbital vehicle space dock using the shuttle's external tank.

Reentry

Vanguard (T-AGM-19) seen here as a NASA Skylab tracking ship. Note the SatCom tracking radar and telemetry antennas.

Increased solar activity, heating the outer layers of the Earth's atmosphere and thereby increasing drag on Skylab, led to an early reentry at approximately 16:37 UTC 11 July 1979. In the weeks leading up to the reentry, ground controllers had re-established contact with the six year old vehicle, and were able to adjust its attitude for optimal reentry dynamics. Earth reentry footprint was a narrow band (approx. 4° wide) beginning at about 48° S 87° E and ending at about 12° S 144° E, an area covering portions of the Indian Ocean and Western Australia. Debris was found between Esperance and Rawlinna, 31–34°S, 122–126°E. The Shire of Esperance fined the United States $400 for littering, a fine which, to this day, remains unpaid.^[3]

The largest fragment of Skylab recovered after its reentry through Earth's atmosphere. It is on display at the United States Space & Rocket Center.

Skylab's demise was an international media event, with merchandising, wagering on time and place of re-entry and nightly news reports. The San Francisco Examiner offered a $10,000 prize for the first piece of Skylab to be delivered to their offices. 17-year-old Stan Thornton scooped a few pieces of Skylab off the roof of his home in Esperance, Western Australia and caught the first flight to San Francisco, where he collected his prize. In a coincidence for the organizers, the annual Miss Universe pageant was scheduled to be held a few days later, on 20 July 1979 in nearby Perth, Western Australia. A large piece of Skylab debris was displayed on the stage.^[4]

Two flight-quality Skylabs were built. The first was de-orbited and crashed in Western Australia in 1979; the second, a backup, is on display at the National Air and Space Museum in Washington, DC. A full scale training mockup once used for astronaut training is located at the Lyndon B. Johnson Space Center visitor's center in Houston, Texas. Another full scale training mockup is now kept at Huntsville, Alabama, made from spare parts. It is currently being restored.^[5]

Skylab missions

The number identification of the manned Skylab missions is the cause of much confusion. Originally, the unmanned launch of Skylab and 3 manned missions were numbered SL-1 through SL-4. During the preparations for the manned missions, some documentation was created with a different scheme--SLM-1 through SLM-3--for those missions only. William Pogue credits Pete Conrad with asking the Skylab program director which scheme should be used for the mission patches and the astronauts were told to use 1-2-3, not 2-3-4. By the time NASA administrators tried to reverse this decision, it was too late, as all the in-flight clothing had already been manufactured and shipped with the 1-2-3 mission patches.^[6]

Mission	Commander	Pilot	Science Pilot	Launch date	Landing date	Duration (days)
Skylab 1 SL-1	unmanned launch			1973-05-14 17:30:00 UTC	1979-07-11 16:37:00 UTC	2248.96
Skylab 2 SL-2 (SLM-1)	Pete Conrad	Paul Weitz	Joseph Kerwin	1973-05-25 13:00:00 UTC	1973-06-22 13:49:48 UTC	28.03
Skylab 3 SL-3 (SLM-2)	Alan Bean	Jack Lousma	Owen Garriott	1973-07-28 11:10:50 UTC	1973-09-25 22:19:51 UTC	59.46
Skylab 4 SL-4 (SLM-3)	Gerald Carr	William Pogue	Edward Gibson	1973-11-16 14:01:23 UTC	1974-02-08 15:16:53 UTC	84.04

The waste management facilities of Skylab, as seen in the mockup at the National Air and Space Museum.

An astronaut dines aboard Skylab, as depicted by the mockup at the Smithsonian NASM.

Astronomical symbols

2008-09-14T21:44:00.000-07:00

Chinese Celestial symbols on an antique bronze mirror

Astronomical symbols are symbols used to represent various celestial objects, theoretical constructs and observational events in astronomy. The symbols listed here are commonly used^{[weasel words]} by professional and amateur astronomers.^{[citation needed]} Many of the symbols are shared with western astrology, which uses multiple variant forms.

Planets
Name	Symbol	Symbol Represents
Mercury		Mercury's winged helmet and caduceus
Venus		Venus' hand mirror
Earth		Globe with equator and a meridian
Earth		globus cruciger
Mars		Mars' shield and spear
Jupiter		Jupiter's thunderbolt, eagle or "Z" for Zeus, Jupiter's Greek name.^[1] Or a "4" for the fourth day of the week (Thursday) in some cultures. It also strongly represents the letter bha in the Devangari script.
Saturn		Saturn's sickle or scythe
Uranus		One of the two symbols for platinum or a combination of the symbols for Mars and the Sun
Uranus		"H" from the discoverer's last name (Herschel)
Neptune		Neptune's trident
Dwarf Planets
Name	Symbol	Symbol Represents
Ceres		Handle-down sickle; cf. the handle-up sickle symbol of Saturn
Pluto		PL monogram for Pluto and Percival Lowell
Makemake	No symbol
Eris	No symbol	Unlikely to gain an official symbol (although there have been a number of proposals, such as the Hand of Eris )
Asteroids
Name	Symbol	Symbol Represents
2 Pallas		Modified symbol for female/Shield of Athena?
3 Juno		Peacock (totem of Juno).
4 Vesta		Hearth or fire-altar.
5 Astraea		Anchor (inverted), or possibly scales of justice.
6 Hebe		Cup
7 Iris		Rainbow with star under it (asteroid means star-like)
8 Flora		Stylised flower
9 Metis		Eye, with star above it (asteroid means star-like)
10 Hygeia		Rod of Asclepius
11 Parthenope
12 Victoria
14 Irene	Never drawn	Described as "A dove carrying an olive-branch, with a star on its head"
15 Eunomia
28 Bellona
35 Leukothea
37 Fides
Moons
Name	Symbol	Symbol Represents
Moon (crescent)		A crescent moon
Moon (decrescent)		A decrescent moon
Other Celestial Bodies
Name	Symbol	Symbol Represents
Sun		Solar symbol
Other Symbols
Name	Symbol
comet	☄
ascending node	☊
descending node	☋
conjunction	☌
opposition	☍

Neptune

2008-09-14T21:36:00.000-07:00

This article is about the planet. For other uses, see Neptune (disambiguation).

Neptune

Neptune from Voyager 2

Discovery

Discovered by

Urbain Le Verrier
John Couch Adams
Johann Galle

Discovery date

September 23, 1846^[1]

Designations

Adjective

Neptunian

Orbital characteristics^[2]^[3]

Epoch J2000

Aphelion

4,553,946,490 km
30.44125206 AU

Perihelion

4,452,940,833 km
29.76607095 AU

Semi-major axis

4,503,443,661 km
30.10366151 AU

Eccentricity

0.011214269

Orbital period

60,190^[4] days
164.79 years

Synodic period

367.49 day^[5]

Average orbital speed

5.43 km/s^[5]

Mean anomaly

267.767281°

Inclination

1.767975°
6.43° to Sun's equator

Longitude of ascending node

131.794310°

Argument of perihelion

265.646853°

Satellites

Physical characteristics

Equatorial radius

24,764 ± 15 km^[6]^[7]
3.883 Earths

Polar radius

24,341 ± 30 km^[6]^[7]
3.829 Earths

Flattening

0.0171 ± 0.0013

Surface area

7.6408×10⁹ km²^[4]^[7]
14.98 Earths

Volume

6.254×10¹³ km³^[5]^[7]
57.74 Earths

Mass

1.0243×10²⁶ kg^[5]
17.147 Earths

Mean density

1.638 g/cm³^[5]^[7]

Equatorial surface gravity

11.15 m/s²^[5]^[7]
1.14 g

Escape velocity

23.5 km/s^[5]^[7]

Sidereal rotation
period

0.6713 day^[5]
16 h 6 min 36 s

Equatorial rotation velocity

2.68 km/s
9,660 km/h

Axial tilt

28.32°^[5]

North pole right ascension

19^h 57^m 20^s^[6]

North pole declination

42.950°^[6]

Albedo

0.290 (bond)
0.41 (geom.)^[5]

Surface temp.
1 bar level
0.1 bar

min	mean	max
	72 K^[5]
	55 K^[5]

Apparent magnitude

8.0 to 7.78^[5]^[8]

Angular diameter

2.2″—2.4″^[5]^[8]

Atmosphere^[5]

Scale height

19.7 ± 0.6 km

Composition

80±3.2%	Hydrogen (H₂)
19±3.2%	Helium
1.5±0.5%	Methane
~0.019%	Hydrogen deuteride (HD)
~0.00015%	Ethane
Ices:
	Ammonia
	Water
	Ammonium hydrosulfide(NH₄SH)
	Methane (?)

Neptune (pronounced /ˈnɛptjuːn/^[9] [AmE: [ˈnɛptun] (help·info)]) is the eighth and farthest planet from the Sun in the Solar System. It is the fourth largest planet by diameter, and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 Earth masses and less dense.^[10] The planet is named after the Roman god of the sea. Its astronomical symbol is , a stylized version of the god Neptune's trident.

Discovered on September 23, 1846,^[1] Neptune was the first planet found by mathematical prediction rather than regular observation. Unexpected changes in the orbit of Uranus led astronomers to deduce the gravitational perturbation of an unknown planet. Neptune was found within a degree of the predicted position. The moon Triton was found shortly thereafter, but none of the planet's other 12 moons were discovered before the 20th century. Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989.

Neptune is similar in composition to Uranus, and both have different compositions from those of the larger gas giants Jupiter and Saturn. As such, astronomers sometimes place them in a separate category, the "ice giants". Neptune's atmosphere, while similar to Jupiter's and Saturn's in being composed primarily of hydrogen and helium, contains a higher proportion of "ices" such as water, ammonia and methane, along with the usual traces of hydrocarbons and possibly nitrogen.^[11] In contrast the interior of Neptune is mainly composed of ices and rocks like that of Uranus.^[12] Traces of methane in the outermost regions, in part, account for the planet's blue appearance.^[13]

Neptune has the strongest winds of any planet in the solar system, measured as high as 2100 km/h.^[14] At the time of the 1989 Voyager 2 flyby, its southern hemisphere possessed a Great Dark Spot comparable to the Great Red Spot on Jupiter. Neptune's temperature at its cloud tops is usually close to −218 °C (55.1 K), one of the coldest in the solar system, due to its great distance from the Sun. The temperature in Neptune's centre is about 7,000 °C (7,270 K), which is comparable to the Sun's surface and similar to most other known planets. Neptune has a faint and fragmented ring system, which may have been detected during the 1960s but was only indisputably confirmed by Voyager 2.^[15]

History

Discovery

Main article: Discovery of Neptune

Galileo's drawings show that he first observed Neptune on December 28, 1612, and again on January 27, 1613; on both occasions, Galileo mistook Neptune for a fixed star when it appeared very close—in conjunction—to Jupiter in the night sky.^[16] Hence he is not credited with Neptune's discovery. During the period of his first observation in December 1612, it was stationary in the sky because it had just turned retrograde that very day. This apparent backward motion is created when the orbit of the Earth takes it past an outer planet. Since Neptune was only beginning its yearly retrograde cycle, the motion of the planet was far too slight to be detected with Galileo's small telescope.^[17]

In 1821, Alexis Bouvard published astronomical tables of the orbit of Uranus.^[18] Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesize that an unknown body was perturbing the orbit through gravitational interaction. In 1843, John Couch Adams calculated the orbit of a hypothesized eighth planet that would account for Uranus' motion. He sent his calculations to Sir George Airy, the Astronomer Royal, who asked Adams for a clarification. Adams began to draft a reply but never sent it and did not aggressively pursue work on the Uranus problem.^[19]^[20]

Urbain Le Verrier, the mathematician who codiscovered Neptune.

In 1845–46, Urbain Le Verrier, independently of Adams, rapidly developed his own calculations but also experienced difficulties in encouraging any enthusiasm in his compatriots. In June, however, upon seeing Le Verrier's first published estimate of the planet's longitude and its similarity to Adams's estimate, Airy persuaded Cambridge Observatory director James Challis to search for the planet. Challis vainly scoured the sky throughout August and September.^[21]^[22]

Meantime, Le Verrier by letter urged Berlin Observatory astronomer Johann Gottfried Galle to search with the observatory's refractor. Heinrich d'Arrest, a student at the observatory, suggested to Galle that they could compare recently drawn chart of the sky in the region of Le Verrier's predicted location with the current sky to seek the displacement characteristic of a planet, as opposed to a fixed star. The very evening of the day of receipt of Le Verrier's letter, Neptune was discovered, September 23, 1846, within 1° of where Le Verrier had predicted it to be, and about 12° from Adams' prediction. Challis later realized that he had observed the planet twice in August, failing to identify it owing to his casual approach to the work.^[21]^[23]

In the wake of the discovery, there was much nationalistic rivalry between the French and the British over who had priority and deserved credit for the discovery. Eventually an international consensus emerged that both Le Verrier and Adams jointly deserved credit. However, the issue is now being re-evaluated by historians with the rediscovery in 1998 of the "Neptune papers" (historical documents from the Royal Observatory, Greenwich), which had apparently been misappropriated by astronomer Olin J. Eggen for nearly three decades and were only rediscovered (in his possession) immediately after his death.^[24] After reviewing the documents, some historians now suggest that Adams does not deserve equal credit with Le Verrier. Since 1966 Dennis Rawlins has questioned the credibility of Adams's claim to co-discovery. In a 1992 article in his journal Dio he deemed the British claim "theft".^[25] "Adams had done some calculations but he was rather unsure about quite where he was saying Neptune was", said Nicholas Kollerstrom of University College London in 2003.^[26]^[27]

Naming

Shortly after its discovery, Neptune was referred to simply as "the planet exterior to Uranus" or as "Le Verrier's planet". The first suggestion for a name came from Galle, who proposed the name Janus. In England, Challis put forward the name Oceanus.^[28]

Claiming the right to name his discovery, Le Verrier quickly proposed the name Neptune for this new planet, while falsely stating that this had been officially approved by the French Bureau des Longitudes.^[29] In October, he sought to name the planet Le Verrier, after himself, and he was patriotically supported in this by the observatory director, François Arago. However, this suggestion met with stiff resistance outside France.^[30] French almanacs quickly reintroduced the name Herschel for Uranus, after that planet's discoverer Sir William Herschel, and Leverrier for the new planet.^[31]

Struve came out in favour of the name Neptune on December 29, 1846, to the Saint Petersburg Academy of Sciences.^[32] Soon Neptune became the internationally accepted name. In Roman mythology, Neptune was the god of the sea, identified with the Greek Poseidon. The demand for a mythological name seemed to be in keeping with the nomenclature of the other planets, all of which, except for Uranus and Earth, were named for Roman gods.^[33]

Status

From its discovery until 1930, Neptune was the farthest known planet. Upon the discovery of Pluto in 1930, Neptune became the penultimate planet, save for a 20-year period between 1979 and 1999 when Pluto fell within its orbit.^[34] However, the discovery of the Kuiper belt in 1992 led many astronomers to debate whether or not Pluto should be considered a planet in its own right or as part of the belt's larger structure.^[35]^[36] In 2006, the International Astronomical Union defined the word "planet" for the first time, reclassifying Pluto as a "dwarf planet" and making Neptune once again the last planet in the Solar System.^[37]

Composition and structure

A size comparison of Neptune and Earth.

With a mass of 1.0243×10²⁶ kg,^[5] Neptune is an intermediate body between Earth and the larger gas giants: its mass is seventeen times that of the Earth but just 1/19th that of Jupiter.^[10] Neptune's equatorial radius of 24,764 km^[6] is nearly four times that of the Earth. Neptune and Uranus are often considered a sub-class of gas giant termed "ice giants", due to their smaller size and higher concentrations of volatiles relative to Jupiter and Saturn.^[38] In the search for extrasolar planets Neptune has been used as a metonym: discovered bodies of similar mass are often referred to as "Neptunes",^[39] just as astronomers refer to various extra-solar "Jupiters."

Internal structure

Neptune's internal structure resembles that of Uranus. Its atmosphere forms about 5–10 percent of its mass and extends perhaps 10–20 percent of the way towards the core, where it reaches pressures of about 10 GPa. Increasing concentrations of methane, ammonia, and water are found in the lower regions of the atmosphere.^[40]

The internal structure of Neptune.

Gradually this darker and hotter region condenses into a superheated liquid mantle, where temperatures reach 2–5,000 K. The mantle is equivalent to 10–15 Earth masses, and is rich in water, ammonia, methane, and other compounds.^[1] As is customary in planetary science, this mixture is referred to as icy even though it is a hot, highly dense fluid. This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean.^[41] At a depth of 7,000 km, the conditions may be such that methane decomposes into diamond crystals that then precipitate toward the core.^[42]

The core of Neptune is composed of iron, nickel and silicates, with an interior model giving a mass about 1.2 times that of the Earth.^[43] The pressure at the centre is 7 Mbar—millions of times more than that on the surface of the Earth, and the temperature may be 5,400 K.^[40]^[44]

Atmosphere

At high altitudes, Neptune's atmosphere is 80% hydrogen and 19% helium.^[40] A trace amount of methane is also present. Prominent absorption bands of methane occur at wavelengths above 600 nm, in the red and infrared portion of the spectrum. As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue,^[45] although Neptune's vivid azure differs from Uranus's milder aquamarine. Since Neptune's atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune's colour.^[13]

Neptune's atmosphere is sub-divided into two main regions; the lower troposphere, where temperature decreases with altitude, and the stratosphere, where temperature increases with altitude. The boundary between the two, the tropopause, occurs at a pressure of 0.1 bars.^[11] The stratosphere then gives way to the thermosphere at a pressure lower than 10⁻⁴–10⁻⁵ microbars.^[11] The thermosphere gradually transitions to the exosphere.

A band of high altitude clouds is shown casting shadows on Neptune's lower cloud deck.

Models suggest that Neptune's troposphere is banded by clouds of varying compositions depending on altitude. The upper level clouds occur at pressures below one bar, where the temperature is suitable for methane to condense. For pressures between one and five bars, clouds of ammonia and hydrogen sulfide are believed to form. Above a pressure of five bars, the clouds may consist of ammonia, ammonium sulfide, hydrogen sulfide and water. Deeper clouds of water ice should be found at pressures of about 50 bars, where the temperature reaches 0 C. Underneath, clouds of ammonia and hydrogen sulfide may be found.^[46]

High altitude clouds on Neptune have been observed casting shadows on the opaque cloud deck below. There are also high altitude cloud bands that wrap around the planet at constant latitude. These circumferential bands have widths of 50–150 km, and lie about 50–110 km above the cloud deck.^[47]

Neptune's spectra suggest that its lower stratosphere is hazy due to condensation of products of ultraviolet photolysis of methane, such as ethane and acetylene.^[40]^[11] The stratosphere is also home to trace amounts of carbon monoxide and hydrogen cyanide.^[11]^[48] The stratosphere of Neptune is warmer than that of Uranus due to elevated concentration of hydrocarbons.^[11]

For reasons that remain obscure, the planet's thermosphere is at an anomalously high temperature of about 750 K.^[49]^[50] The planet is too far from the Sun for this heat to be generated by ultraviolet radiation. One candidate for a heating mechanism is atmospheric interaction with ions in the planet's magnetic field. Other candidates are gravity waves from the interior that dissipate in the atmosphere. The thermosphere contains traces of carbon dioxide and water, which may have been deposited from external sources such as meteorites and dust.^[46]^[48]

Magnetosphere

Neptune also resembles Uranus in its magnetosphere, with a magnetic field strongly tilted relative to its rotational axis at 47° and offset at least 0.55 radii (about 13,500 kilometres) from the planet's physical centre. Before Voyager 2's arrival at Neptune, it was hypothesised that Uranus's tilted magnetosphere was the result of its sideways rotation. However, in comparing the magnetic fields of the two planets, scientists now think the extreme orientation may be characteristic of flows in the planets' interiors. This field may be generated by convective fluid motions in a thin spherical shell of electrically conducting liquids (probably a combination of ammonia, methane and water)^[46] resulting in a dynamo action.^[51]

The magnetic field at the equatorial surface of Neptune is estimated at 1.42 μT, for a magnetic moment of 2.16×10¹⁷ Tm³. Neptune's magnetic field has a complex geometry that includes relatively large contributions from non-dipolar components, including a strong quadrupole moment that may exceed the dipole moment in strength. By contrast, Earth, Jupiter and Saturn only have relatively small quadrupole moments and their fields are less tilted from the polar axis. The large quadrupole moment of Neptune may be the result of offset from the planet's center and geometrical constraints of the field's dynamo generator.^[52]^[53]

Neptune's bow shock, where the magnetosphere begins to slow the solar wind, occurs at a distance of 34.9 times the radius of the planet. The magnetopause, where the pressure of the magnetosphere counterbalances the solar wind, lies at a distance of 23–26.5 times the radius of Neptune. The tail of the magnetosphere extends out to at least 72 times the radius of Neptune, and very likely much further.^[52]

Planetary rings

Main article: Rings of Neptune

Neptune's rings, taken by Voyager 2.

Neptune has a planetary ring system, though one much less substantial than that of Saturn. The rings may consist of ice particles coated with silicates or carbon-based material, which most likely gives them a reddish hue.^[54] In addition to the narrow Adams Ring, 63,000 km from the centre of Neptune, the Leverrier Ring is at 53,000 km and the broader, fainter Galle Ring is at 42,000 km. A faint outward extension to the Leverrier Ring has been named Lassell; it is bounded at its outer edge by the Arago Ring at 57,000 km.^[55]

The first of these planetary rings was discovered in 1968 by a team led by Edward Guinan,^[15]^[56] but it was later thought that this ring might be incomplete.^[57] Evidence that the rings might have gaps first arose during a stellar occultation in 1984 when the rings obscured a star on immersion but not on emersion.^[58] Images by Voyager 2 in 1989 settled the issue by showing several faint rings. These rings have a clumpy structure,^[59] the cause of which is not currently understood but which may be due to the gravitational interaction with small moons in orbit near them.^[60]

The outermost ring, Adams, contains five prominent arcs now named Courage, Liberté, Egalité 1, Egalité 2, and Fraternité (Liberty, Equality, and Fraternity).^[61] The existence of arcs was difficult to explain because the laws of motion would predict that arcs would spread out into a uniform ring over very short timescales. Astronomers now believe that the arcs are corralled into their current form by the gravitational effects of Galatea, a moon just inward from the ring.^[62]^[63]

Earth-based observations announced in 2005 appeared to show that Neptune's rings are much more unstable than previously thought. Images taken from the W. M. Keck Observatory in 2002 and 2003 show considerable decay in the rings when compared to images by Voyager 2. In particular, it seems that the Liberté arc might disappear in as little as one century.^[64]

Climate

One difference between Neptune and Uranus is the typical level of meteorological activity. When the Voyager 2 spacecraft flew by Uranus in 1986, that planet was visually quite bland. In contrast Neptune exhibited notable weather phenomena during the 1989 Voyager 2 fly-by.^[65]

The Great Dark Spot (top), Scooter (middle white cloud),^[66] and the Small Dark Spot (bottom).

Neptune's weather is characterized by extremely dynamic storm systems, with winds reaching near-supersonic speeds of nearly 600 m/s.^[67] More typically, by tracking the motion of persistent clouds, wind speeds have been shown to vary from 20 m/s in the easterly direction to 325 m/s westward.^[68] At the cloud tops, the prevailing winds range in speed from 400 m/s along the equator to 250 m/s at the poles.^[46] Most of the winds on Neptune move in a direction opposite the planet's rotation.^[69] The general pattern of winds showed prograde rotation at high latitudes vs. retrograde rotation at lower latitudes. The difference in flow direction is believed to be a "skin effect" and not due to any deeper atmospheric processes.^[11] At 70° S latitude, a high speed jet travels at a speed of 300 m s⁻¹.^[11]

The abundance of methane, ethane and acetylene at Neptune's equator is 10–100 times greater than at the poles. This is interpreted as evidence for upwelling at the equator and subsidence near the poles.^[11]

In 2007 it was discovered that the upper troposphere of Neptune's south pole was about 10°C (10 K) warmer than the rest of Neptune, which averages approximately −200 °C (73.1 K).^[70] The warmth differential is enough to let methane gas, which elsewhere lies frozen in Neptune's upper atmosphere, leak out through the south pole and into space. The relative 'hot spot' is due to Neptune's axial tilt, which has exposed the south pole to the Sun for the last quarter of Neptune's year, or roughly 40 Earth years. As Neptune slowly moves towards the opposite side of the Sun, the south pole will be darkened and the north pole illuminated, causing the methane release to shift to the north pole.^[71]

Because of seasonal changes, the cloud bands in the southern hemisphere of Neptune have been observed to increase in size and albedo. This trend was first seen in 1980 and is expected to last until about 2020. The long orbital period of Neptune results in seasons lasting forty years.^[72]

Storms

The Great Dark Spot, as seen from Voyager 2.

In 1989, the Great Dark Spot, an anti-cyclonic storm system spanning 13,000 × 6,600 km,^[65] was discovered by NASA's Voyager 2 spacecraft. The storm resembled the Great Red Spot of Jupiter. However, on November 2, 1994, the Hubble Space Telescope did not see the Great Dark Spot on the planet. Instead, a new storm similar to the Great Dark Spot was found in the planet's northern hemisphere.^[73]

The Scooter is another storm, a white cloud group further south than the Great Dark Spot. Its nickname is due to the fact that when first detected in the months before the 1989 Voyager 2 encounter it moved faster than the Great Dark Spot.^[69] Subsequent images revealed even faster clouds. The Small Dark Spot is a southern cyclonic storm, the second most intensive storm observed during the 1989 encounter. It initially was completely dark, but as Voyager 2 approached the planet, a bright core developed and can be seen in most of the highest resolution images.^[74]

Neptune's dark spots are thought to occur in the troposphere at lower altitudes than the brighter cloud features,^[75] so they appear as holes in the upper cloud decks. As they are stable features that can persist for several months, they are thought to be vortex structures.^[47] Often associated with dark spots are brighter, persistent methane clouds that form around the tropopause layer.^[76] The persistence of companion clouds shows that some former dark spots may continue to exist as a cyclone even though they are no longer visible as a dark feature. Dark spots may also dissipate either when they migrate too close to the equator or possibly through some other unknown mechanism.^[77]

Internal heat

Neptune's more varied weather when compared to Uranus is believed to be due in part to its higher internal heat.^[78] Although Neptune lies half again as far from the Sun as Uranus, and receives only 40% its amount of sunlight,^[11] the two planets' surface temperatures are roughly equal.^[78] The upper regions of Neptune's troposphere reach a low temperature of −221.4 °C (51.7 K). At a depth where the atmospheric pressure equals 1 bar, the temperature is −201.15 °C (72.0 K).^[79] Deeper inside the layers of gas, however, the temperature rises steadily. As with Uranus, the source of this heating is unknown, but the discrepancy is larger: Uranus only radiates 1.1 times as much energy as it receives from the Sun;^[80] Neptune radiates about 2.61 times as much, which means the internal heat source generates 161% of the solar input.^[81] Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System. Several possible explanations have been suggested, including radiogenic heating from the planet's core,^[82] dissociation of methane into hydrocarbon chains under atmospheric pressure,^[83]^[82] and convection in the lower atmosphere that causes gravity waves to break above the tropopause.^[84]^[85]

Orbit and rotation

The average distance between Neptune and the Sun is 4.55 billion km (about 30.1 times the average distance from the Earth to the Sun, or 30.1 AU) and it completes an orbit every 164.79 years. On July 12, 2011, Neptune will have completed the first full orbit since its discovery in 1846,^[86]^[4] although it will not appear at its exact discovery position in our sky due to the Earth being in a different location in its 365.25 day orbit.

The elliptical orbit of Neptune is inclined 1.77° compared to the Earth. Because of an eccentricity of 0.011, the distance from Neptune and the Sun varies by 101 million km between perihelion and aphelion, or the nearest and most distant points of the planet along the orbital path respectively.^[2]

The axial tilt of Neptune is 28.32°,^[87] which is similar to the tilt of Earth and Mars. As a result this planet experiences similar seasonal changes. However, the long orbital period of Neptune means that the seasons last for forty Earth years.^[72] Its sidereal rotation period (day) is roughly 16.11 hours long.^[4] Since its axial tilt is comparable to the Earth's (23°), the variation in the length of its days over the course of its long year is not any more extreme.

Because Neptune is not a solid body, its atmosphere undergoes differential rotation. The wide equatorial zone rotates with a period of about 18 hours, which is slower than the 16.1 hour rotation of the planet's magnetic field. By contrast, the reverse is true for the polar regions where the rotation period is 12 hours. This differential rotation is the most pronounced of any planet in the Solar System,^[88] and it results in strong latitudinal wind shear.^[47]

Orbital resonances

Main article: Kuiper belt

A diagram showing the orbital resonances in the Kuiper belt caused by Neptune: the highlighted regions are the 2/3 resonance (Plutinos), the "classical belt", with orbits unaffected by Neptune, and the 1/2 resonance (twotinos).

Neptune's orbit has a profound impact on the region directly beyond it, known as the Kuiper belt. The Kuiper belt is a ring of small icy worlds, similar to the asteroid belt but far larger, extending from Neptune's orbit at 30 AU out to about 55 AU from the Sun.^[89] Much in the same way that Jupiter's gravity dominates the asteroid belt, shaping its structure, so Neptune's gravity completely dominates the Kuiper belt. Over the age of the Solar System, certain regions of the Kuiper belt become destabilized by Neptune's gravity, creating gaps in the Kuiper belt's structure. The region between 40 and 42 AU is an example.^[90]

There do, however, exist orbits within these empty regions where objects can survive for the age of the Solar System. These resonances occur when an object's orbit around the Sun is a precise fraction of Neptune's, such as 1/2, or 3/4. If, say, an object orbits the Sun once for every two Neptune orbits, it will only complete half an orbit every time Neptune returns to its original position, and so will always be on the other side of the Sun. The most heavily populated resonant orbit in the Kuiper belt, with over 200 known objects,^[91] is the 2/3 resonance. Objects in this orbit complete 1 orbit for every 1½ of Neptune's, and are known as Plutinos because the largest of the Kuiper belt objects, Pluto, lies among them.^[92] Although Pluto crosses Neptune's orbit regularly, the 2/3 resonance means they can never collide.^[93] Other, less populated resonances exist at 3/4, 3/5, 4/7 and 2/5.^[94]

Neptune possesses a number of trojan objects, which occupy its L₄ and L₅ points; gravitationally stable regions leading and trailing it in its orbit. Neptune trojans are often described as being in a 1/1 resonance with Neptune. Neptune trojans are remarkably stable in their orbits and are unlikely to have been captured by Neptune, but rather to have formed alongside it.^[95]

Formation and migration

Main article: Formation and evolution of the Solar System

A simulation showing Outer Planets and Kuiper Belt: a)Before Jupiter/Saturn 2:1 resonance b)Scattering of Kuiper Belt objects into the solar system after the orbital shift of Neptune c)After ejection of Kuiper Belt bodies by Jupiter

The formation of the ice giants, Neptune and Uranus, has proven difficult to model precisely. Current models suggest that the matter density in the outer regions of the Solar System was too low to account for the formation of such large bodies from the traditionally accepted method of core accretion, and various hypotheses have been advanced to explain their evolution. One is that the ice giants were not created by core accretion but from instabilities within the original protoplanetary disc, and later had their atmospheres blasted away by radiation from a nearby massive OB star.^[96] An alternative concept is that they formed closer to the Sun, where the matter density was higher, and then subsequently migrated to their current orbits.^[97]

The migration hypothesis is favoured for its ability to explain current orbital resonances in the Kuiper belt, particularly the 2/5 resonance. As Neptune migrated outward, it collided with the objects in the proto-Kuiper belt, creating new resonances and sending other orbits into chaos. The objects in the scattered disc are believed to have been placed in their current positions by interactions with the resonances created by Neptune's migration.^[98] A 2004 computer model by Alessandro Morbidelli of the Observatoire de la Côte d'Azur in Nice, suggested that the migration of Neptune into the Kuiper belt may have been triggered by the formation of a 1/2 resonance in the orbits of Jupiter and Saturn, which created a gravitational push that propelled both Uranus and Neptune into higher orbits and caused them to switch places. The resultant expulsion of objects from the proto-Kuiper belt could also explain the Late Heavy Bombardment 600 million years after the Solar System's formation and the appearance of Jupiter's Trojan asteroids.^[99]

Moons

Neptune (top) and Triton (bottom).

Main article: Moons of Neptune

For a timeline of discovery dates, see Timeline of discovery of Solar System planets and their moons

Neptune has 13 known moons.^[5] The largest by far, comprising more than 99.5 percent of the mass in orbit around Neptune^[100] and the only one massive enough to be spheroidal, is Triton, discovered by William Lassell just 17 days after the discovery of Neptune itself. Unlike all other large planetary moons in the Solar System, Triton has a retrograde orbit, indicating that it was captured rather than forming in place, and probably was once a dwarf planet in the Kuiper belt.^[101] It is close enough to Neptune to be locked into a synchronous rotation, and is slowly spiraling inward because of tidal acceleration and eventually will be torn apart when it reaches the Roche limit.^[102] In 1989, Triton was the coldest object that had yet been measured in the solar system,^[103] with estimated temperatures of −235 °C (38 K).^[104]

Neptune's second known satellite (by order of discovery), the irregular moon Nereid, has one of the most eccentric orbits of any satellite in the solar system. The eccentricity of 0.7512 gives it an apoapsis that is seven times its periapsis distance from Neptune.^[105]

Neptune's moon Proteus.

From July to September 1989, Voyager 2 discovered six new Neptunian moons.^[52] Of these, the irregularly shaped Proteus is notable for being as large as a body of its density can be without being pulled into a spherical shape by its own gravity.^[106] Although the second most massive Neptunian moon, it is only one quarter of one percent of the mass of Triton. Neptune's innermost four moons, Naiad, Thalassa, Despina, and Galatea, orbit close enough to be within Neptune's rings. The next farthest out, Larissa was originally discovered in 1981 when it had occulted a star. This had been attributed to ring arcs, but when Voyager 2 observed Neptune in 1989, it was found to have been caused by the moon. Five new irregular moons discovered between 2002 and 2003 were announced in 2004.^[107]^[108] As Neptune was the Roman god of the sea, the planet's moons have been named after lesser sea gods.^[33]

Observation

Neptune is never visible to the naked eye, having a brightness between magnitudes +7.7 and +8.0,^[8]^[5] which can be outshone by Jupiter's Galilean moons, the dwarf planet Ceres and the asteroids 4 Vesta, 2 Pallas, 7 Iris, 3 Juno and 6 Hebe.^[109] A telescope or strong binoculars will resolve Neptune as a small blue disk, similar in appearance to Uranus.^[110]

Because of the distance of Neptune from the Earth, the angular diameter of the planet only ranges from 2.2–2.4 arcseconds;^[5]^[8] the smallest of the Solar System planets. Its small apparent size has made it challenging to study visually; most telescopic data was fairly limited until the advent of Hubble Space Telescope and large ground-based telescopes with adaptive optics.^[111]^[112]

From the Earth, Neptune goes through apparent retrograde motion every 367 days, resulting in a looping motion against the background stars during each opposition. These loops will carry it close to the 1846 discovery coordinates in April and July 2010 and in October and November 2011.^[86]

Observation of Neptune in the radio frequency band shows that the planet is a source of both continuous emission and irregular bursts. Both sources are believed to originate from the planet's rotating magnetic field.^[46] In the infrared part of the spectrum, Neptune's storms appear bright against the cooler background, allowing the size and shape of these features to be readily tracked.^[113]

Exploration

Main article: Exploration of Neptune

Voyager 2's closest approach to Neptune occurred on August 25, 1989. Since this was the last major planet the spacecraft could visit, it was decided to make a close flyby of the moon Triton, regardless of the consequences to the trajectory, similarly to what was done for Voyager 1's encounter with Saturn and its moon Titan. The images relayed back to Earth from Voyager 2 became the basis of a 1989 PBS all-night program called Neptune All Night.^[114]

A Voyager 2 image of Triton

During the encounter, signals from the spacecraft required 246 minutes to reach the Earth. Hence, for the most part, the Voyager 2 mission relied on pre-loaded commands for the Neptune encounter. The spacecraft performed a near-encounter with the moon Nereid before it came within 4,400 km of Neptune's atmosphere on August 25, then passed close to the planet's largest moon Triton later the same day.^[115]

The spacecraft verified the existence of a magnetic field about the planet, and discovered that the field was offset from the centre and tilted in a manner similar to the field around Uranus. The question of the planet's rotation period was settled using measurements of radio emissions. Voyager 2 also showed the Neptune had a surprisingly active weather system. Six new moons were discovered, and the planet was shown to have more than one ring.^[115]^[52]

In 2003, there was a proposal to NASA's "Vision Missions Studies" to implement a "Neptune Orbiter with Probes" mission that does Cassini-level science without fission-based electric power or propulsion. The work is being done in conjunction with JPL and the California Institute of Technology.^[116]

Uranus

2008-09-14T21:21:00.000-07:00

This article is about the planet. For other uses, see Uranus (disambiguation).

Uranus

Uranus, as seen by Voyager 2

Discovery

Discovered by

William Herschel

Discovery date

March 13, 1781

Designations

Adjective

Uranian

Orbital characteristics^[1]^[a]

Epoch J2000

Aphelion

3,004,419,704 km
20.08330526 AU

Perihelion

2,748,938,461 km
18.37551863 AU

Semi-major axis

2,876,679,082 km
19.22941195 AU

Eccentricity

0.044405586

Orbital period

30,799.095 days
84.323326 yr

Synodic period

369.66 days^[2]

Average orbital speed

6.81 km/s^[2]

Mean anomaly

142.955717°

Inclination

0.772556°
6.48° to Sun's equator

Longitude of ascending node

73.989821°

Argument of perihelion

96.541318°

Satellites

Physical characteristics

Equatorial radius

25,559 ± 4 km
4.007 Earths^[3]^[c]

Polar radius

24,973 ± 20 km
3.929 Earths^[3]^[c]

Flattening

0.0229 ± 0.0008^[b]

Surface area

8.1156×10⁹ km²^[4]^[c]
15.91 Earths

Volume

6.833×10¹³ km³^[2]^[c]
63.086 Earths

Mass

8.6810 * 13×10²⁵ kg
14.536 Earths^[5]
GM=5,793,939 ± 13 km³/s²

Mean density

1.27 g/cm³^[2]^[c]

Equatorial surface gravity

8.69 m/s²^[2]^[c]
0.886 g

Escape velocity

21.3 km/s^[2]^[c]

Sidereal rotation
period

−0.71833 day
17 h 14 min 24 s^[3]

Equatorial rotation velocity

2.59 km/s
9,320 km/h

Axial tilt

97.77°^[3]

North pole right ascension

17 h 9 min 15 s
257.311°^[3]

North pole declination

−15.175°^[3]

Albedo

0.300 (bond)
0.51 (geom.)^[2]

Surface temp.
1 bar level^[7]
0.1 bar
(tropopause)^[8]

min	mean	max
	76 K
49 K	53 K	57 K

Apparent magnitude

5.9^[6] to 5.32^[2]

Angular diameter

3.3"–4.1"^[2]

Atmosphere^[8]^[9]^[10]^[d]

Scale height

27.7 km^[2]

Composition

(Below 1.3 bar)

83 ± 3%	Hydrogen (H₂)
15 ± 3%	Helium
2.3%	Methane
0.009% (0.007–0.015%)	Hydrogen deuteride (HD)^[11]
Ices:
	Ammonia
	water
	ammonium hydrosulfide (NH₄SH)
	methane (CH₄)

Uranus (/ˈjʊərənəs/ (help·info) or /jʊˈreɪnəs/ (help·info)^[12]) is the seventh planet from the Sun and the third-largest and fourth-most massive planet in the solar system. It is named after the ancient Greek deity of the sky (Uranus, Οὐρανός), the father of Kronos (Saturn) and grandfather of Zeus (Jupiter). Though it is visible to the naked eye like the five classical planets, it was never recognized as a planet by ancient observers due to its dimness and slow orbit.^[13] Sir William Herschel announced its discovery on March 13, 1781, expanding the known boundaries of the solar system for the first time in modern history. This was also the first discovery of a planet made using a telescope.

Uranus is similar in composition to Neptune, and both have different compositions from those of the larger gas giants Jupiter and Saturn. As such, astronomers sometimes place them in a separate category, the "ice giants". Uranus' atmosphere, while similar to Jupiter and Saturn in being composed primarily of hydrogen and helium, contains a higher proportion of "ices" such as water, ammonia and methane, along with the usual traces of hydrocarbons.^[8] It is the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (−224 °C). It has a complex, layered cloud structure, with water thought to make up the lowest clouds, and methane thought to make up the uppermost layer of clouds.^[8] In contrast the interior of Uranus is mainly composed of ices and rocks.^[7]

Like the other giant planets, Uranus has a ring system, a magnetosphere, and numerous moons. The Uranian system has a unique configuration among the planets because its axis of rotation is tilted sideways, nearly into the plane of its revolution about the Sun; its north and south poles lie where most other planets have their equators.^[14] Seen from Earth, Uranus' rings can sometimes appear to circle the planet like an archery target and its moons revolve around it like the hands of a clock, though in 2007 and 2008 the rings appear edge-on. In 1986, images from Voyager 2 showed Uranus as a virtually featureless planet in visible light without the cloud bands or storms associated with the other giants.^[14] However, terrestrial observers have seen signs of seasonal change and increased weather activity in recent years as Uranus approached its equinox. The wind speeds on Uranus can reach 250 meters per second.^[15]

Discovery

Uranus had been observed on many occasions prior to its discovery as a planet, but it was generally mistaken for a star. The earliest recorded sighting was in 1690 when John Flamsteed observed the planet at least six times, cataloging it as 34 Tauri. The French astronomer, Pierre Lemonnier, observed Uranus at least twelve times between 1750 and 1769,^[16] including on four consecutive nights.

Sir William Herschel observed the planet on 13 March 1781 while in the garden of his house at 19 New King Street in the town of Bath, Somerset (now the Herschel Museum of Astronomy),^[17] but initially reported it (on 26 April 1781) as a "comet".^[18] Herschel "engaged in a series of observations on the parallax of the fixed stars",^[19] using a telescope of his own design.

He recorded in his journal "In the quartile near ζ Tauri … either [a] Nebulous star or perhaps a comet".^[20] On March 17, he noted, "I looked for the Comet or Nebulous Star and found that it is a Comet, for it has changed its place".^[21] When he presented his discovery to the Royal Society, he continued to assert that he had found a comet while also implicitly comparing it to a planet:^[22]

“

The power I had on when I first saw the comet was 227. From experience I know that the diameters of the fixed stars are not proportionally magnified with higher powers, as planets are; therefore I now put the powers at 460 and 932, and found that the diameter of the comet increased in proportion to the power, as it ought to be, on the supposition of its not being a fixed star, while the diameters of the stars to which I compared it were not increased in the same ratio. Moreover, the comet being magnified much beyond what its light would admit of, appeared hazy and ill-defined with these great powers, while the stars preserved that lustre and distinctness which from many thousand observations I knew they would retain. The sequel has shown that my surmises were well-founded, this proving to be the Comet we have lately observed.

”

Herschel notified the Astronomer Royal, Nevil Maskelyne, of his discovery and received this flummoxed reply from him on April 23: "I don't know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it".^[23]

While Herschel continued to cautiously describe his new object as a comet, other astronomers had already begun to suspect otherwise. Russian astronomer Anders Johan Lexell estimated its distance as 18 times the distance of the Sun from the Earth, and no comet had yet been observed with a perihelion of even four times the Earth–Sun distance.^[24] Berlin astronomer Johann Elert Bode described Herschel's discovery as "a moving star that can be deemed a hitherto unknown planet-like object circulating beyond the orbit of Saturn".^[25] Bode concluded that its near-circular orbit was more like a planet than a comet.^[26]

The object was soon universally accepted as a new planet. By 1783, Herschel himself acknowledged this fact to Royal Society president Joseph Banks: "By the observation of the most eminent Astronomers in Europe it appears that the new star, which I had the honour of pointing out to them in March 1781, is a Primary Planet of our Solar System."^[27] In recognition of his achievement, King George III gave Herschel an annual stipend of £200 on the condition that he move to Windsor so the Royal Family could have a chance to look through his telescopes.^[28]

Naming

Maskelyne asked Herschel to "do the astronomical world the faver [sic] to give a name to your planet, which is entirely your own, & which we are so much obliged to you for the discovery of."^[29] In response to Maskelyne's request, Herschel decided to name the object Georgium Sidus (George's Star), or the "Georgian Planet" in honour of his new patron, King George III.^[30] He explained this decision in a letter to Joseph Banks:^[27]

William Herschel, discoverer of Uranus

“

In the fabulous ages of ancient times the appellations of Mercury, Venus, Mars, Jupiter and Saturn were given to the Planets, as being the names of their principal heroes and divinities. In the present more philosophical era it would hardly be allowable to have recourse to the same method and call it Juno, Pallas, Apollo or Minerva, for a name to our new heavenly body. The first consideration of any particular event, or remarkable incident, seems to be its chronology: if in any future age it should be asked, when this last-found Planet was discovered? It would be a very satisfactory answer to say, 'In the reign of King George the Third.

”

Herschel's proposed name was not popular outside of Britain, and alternatives were soon proposed. Astronomer Jérôme Lalande proposed the planet be named Herschel in honour of its discoverer.^[31] Bode, however, opted for Uranus, the Latinized version of the Greek god of the sky, Ouranos. Bode argued that just as Saturn was the father of Jupiter, the new planet should be named after the father of Saturn.^[28]^[32]^[33] Bode's suggestion was the most widely used, and became universal in 1850 when HM Nautical Almanac Office, the final holdout, switched from using Georgium Sidus to Uranus.^[32]

Nomenclature

The preferred pronunciation of the name Uranus among astronomers is [ˈjʊərənəs], with the first syllable stressed and a short a (ūrănŭs);^[34] this is more classically correct than the alternate [jʊˈɹeɪ.nəs], with stress on the second syllable and a "long a" (ūrānŭs), which is often used in the English-speaking world.

Uranus is the only planet whose name is derived from a figure from Greek mythology rather than Roman mythology. (The Roman equivalent would have been Caelus.) The adjective of Uranus is "Uranian". The element uranium, discovered in 1789, was named in its honour by its discoverer, Martin Klaproth.^[35]

Its astronomical symbol is . It is a hybrid of the symbols for Mars and the Sun because Uranus was the Sky in Greek mythology, which was thought to be dominated by the combined powers of the Sun and Mars.^[36] Its astrological symbol is , suggested by Lalande in 1784. In a letter to Herschel, Lalande described it as "un globe surmonté par la première lettre de votre nom" ("a globe surmounted by the first letter of your name").^[31] In the Chinese, Japanese, Korean, and Vietnamese languages, the planet's name is literally translated as the sky king star (天王星).^[37]^[38]

Orbit and rotation

HST image of Uranus showing cloud bands, rings, and moons

Uranus revolves around the Sun once every 84 Earth years. Its average distance from the Sun is roughly 3 billion km (about 20 AU). The intensity of sunlight on Uranus is about 1/400 that of Earth.^[39] Its orbital elements were first calculated in 1783 by Pierre-Simon Laplace.^[24] With time, discrepancies began to appear between the predicted and observed orbits, and in 1841, John Couch Adams first proposed that the differences might be due to the gravitational tug of an unseen planet. In 1845, Urbain Le Verrier began his own independent research into Uranus' orbit. On September 23, 1846, Johann Gottfried Galle located a new planet, later named Neptune, at nearly the position predicted by Le Verrier.^[40]

The rotational period of the interior of Uranus is 17 hours, 14 minutes. However, as on all giant planets, its upper atmosphere experiences very strong winds in the direction of rotation. In effect, at some latitudes, such as about two-thirds of the way from the equator to the south pole, visible features of the atmosphere move much faster, making a full rotation in as little as 14 hours.^[41]

Axial tilt

Uranus' axis of rotation lies on its side with respect to the plane of the solar system, with an axial tilt of 97.77 degrees. This makes its exchange of seasons completely unlike those of the other major planets. Other planets can be visualized to rotate like tilted spinning tops relative to the plane of the solar system, while Uranus rotates more like a tilted rolling ball. Near the time of Uranian solstices, one pole faces the Sun continually while the other pole faces away. Only a narrow strip around the equator experiences a rapid day-night cycle, but with the Sun very low over the horizon as in the Earth's polar regions. At the other side of Uranus' orbit the orientation of the poles towards the Sun is reversed. Each pole gets around 42 years of continuous sunlight, followed by 42 years of darkness.^[42] Near the time of the equinoxes, the Sun faces the equator of Uranus giving a period of day-night cycles similar to those seen on most of the other planets. Uranus reached its most recent equinox on 7 December 2007.^[43]^[44]

Northern hemisphere	Year	Southern hemisphere
Winter solstice	1902, 1986	Summer solstice
Vernal equinox	1923, 2007	Autumnal equinox
Summer solstice	1944, 2028	Winter solstice
Autumnal equinox	1965, 2049	Vernal equinox

One result of this axis orientation is that, on average during the year, the polar regions of Uranus receive a greater energy input from the Sun than its equatorial regions. Nevertheless, Uranus is hotter at its equator than at its poles. The underlying mechanism which causes this is unknown. The reason for Uranus' unusual axial tilt is also not known with certainty, but the usual speculation is that during the formation of the Solar System, an Earth sized protoplanet collided with Uranus, causing the skewed orientation.^[45] Uranus' south pole was pointed almost directly at the Sun at the time of Voyager 2's flyby in 1986. The labeling of this pole as "south" uses the definition currently endorsed by the International Astronomical Union, namely that the north pole of a planet or satellite shall be the pole which points above the invariable plane of the solar system, regardless of the direction the planet is spinning.^[46]^[47] However, a different convention is sometimes used, where a body's north and south poles are defined according to the right-hand rule in relation to the direction of rotation.^[48] In terms of this latter coordinate system it was Uranus' north pole which was in sunlight in 1986. Astronomer Patrick Moore, commenting on the issue, summed it up by saying "Take your pick!"^[49]

Visibility

From 1995 to 2006, Uranus' apparent magnitude fluctuated between +5.6 and +5.9, placing it just within the limit of naked eye visibility at +6.5.^[6] Its angular diameter is between 3.4 and 3.7 arcseconds, compared with 16 to 20 arcseconds for Saturn and 32 to 45 arcseconds for Jupiter.^[6] At opposition, Uranus is visible to the naked eye in dark, un-light polluted skies, and becomes an easy target even in urban conditions with binoculars.^[4] In larger amateur telescopes with an objective diameter of between 15 and 23 cm, the planet appears as a pale cyan disk with distinct limb darkening. With a large telescope of 25 cm or wider, cloud patterns, as well as some of the larger satellites, such as Titania and Oberon, may be visible.^[50]

Physical characteristics

Internal structure

Size comparison of Earth and Uranus

Uranus' mass is roughly 14.5 times that of the Earth, making it the least massive of the giant planets, while its density of 1.27 g/cm³ makes it the second least dense planet, after Saturn.^[5] Though having a diameter slightly larger than Neptune (roughly four times Earth's), it is less massive.^[3] These values indicate that it is made primarily of various ices, such as water, ammonia, and methane.^[7] The total mass of ice in Uranus' interior is not precisely known, as different figures emerge depending on the model chosen; however, it must be between 9.3 and 13.5 Earth masses.^[7]^[51] Hydrogen and helium constitute only a small part of the total, with between 0.5 and 1.5 Earth masses.^[7] The remainder of the mass (0.5 to 3.7 Earth masses) is accounted for by rocky material.^[7]

The standard model of Uranus' structure is that it consists of three layers: a rocky core in the center, an icy mantle in the middle and an outer gaseous hydrogen/helium envelope.^[7]^[52] The core is relatively small, with a mass of only 0.55 Earth masses and a radius less than 20 percent Uranus'; the mantle comprises the bulk of the planet, with around 13.4 Earth masses, while the upper atmosphere is relatively insubstantial, weighing about 0.5 Earth masses and extending for the last 20 percent of Uranus' radius.^[7]^[52] Uranus' core density is around 9 g/cm³, with a pressure in the center of 8 million bars (800 GPa) and a temperature of about 5000 K.^[51]^[52] The ice mantle is not in fact composed of ice in the conventional sense, but of a hot and dense fluid consisting of water, ammonia and other volatiles.^[7]^[52] This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean.^[53] The bulk compositions of Uranus and Neptune are very different from those of Jupiter and Saturn, with ice dominating over gases, hence justifying their separate classification as ice giants.

While the model considered above is more or less standard, it is not unique; other models also satisfy observations. For instance, if substantial amounts of hydrogen and rocky material are mixed in the ice mantle, the total mass of ices in the interior will be lower, and, correspondingly, the total mass of rocks and hydrogen will be higher. Presently available data does not allow us to determine which model is correct.^[51] The fluid interior structure of Uranus means that it has no solid surface. The gaseous atmosphere gradually transitions into the internal liquid layers.^[7] However for the sake of convenience an oblate spheroid of revolution, where pressure equals 1 bar (100 kPa), is designated conditionally as a ‘surface’. It has equatorial and polar radii of 25,559 ± 4 and 24,973 ± 20 km, respectively.^[3] This surface will be used throughout this article as a zero point for altitudes.

Internal heat

Uranus' internal heat appears markedly lower than that of the other giant planets; in astronomical terms, it has a low thermal flux.^[54]^[15] Why Uranus' internal temperature is so low is still not understood. Neptune, which is Uranus' near twin in size and composition, radiates 2.61 times as much energy into space as it receives from the Sun.^[15] Uranus, by contrast, radiates hardly any excess heat at all. The total power radiated by Uranus in the far infrared (i.e. heat) part of the spectrum is 1.06 ± 0.08 times the solar energy absorbed in its atmosphere.^[55]^[8] In fact, Uranus' heat flux is only 0.042 ± 0.047 W/m², which is lower than the internal heat flux of Earth of about 0.075 W/m².^[55] The lowest temperature recorded in Uranus' tropopause is 49 K (−224 °C), making Uranus the coldest planet in the Solar System.^[55]^[8]

Hypotheses for this discrepancy include that when Uranus was "knocked over" by the supermassive impactor which caused its extreme axial tilt, the event also caused it to expel most of its primordial heat, leaving it with a depleted core temperature.^[56] Another hypothesis is that some form of barrier exists in Uranus' upper layers which prevents the core's heat from reaching the surface.^[7] For example, convection may take place in a set of compositionally different layers, which may inhibit the upward heat transport.^[8]^[55]

Atmosphere

Main article: Atmosphere of Uranus

Although there is no well-defined solid surface within Uranus' interior, the outermost part of Uranus' gaseous envelope that is accessible to remote sensing is called its atmosphere.^[8] Remote sensing capability extends down to roughly 300 km below the 1 bar (100 kPa) level, with a corresponding pressure around 100 bar (10 MPa) and temperature of 320 K.^[57] The tenuous corona of the atmosphere extends remarkably over two planetary radii from the nominal surface at 1 bar pressure.^[58] The Uranian atmosphere can be divided into three layers: the troposphere, between altitudes of −300 and 50 km and pressures from 100 to 0.1 bar; (10 MPa to 10 kPa) the stratosphere, spanning altitudes between 50 and 4000 km and pressures of between 0.1 and 10^–10 bar (10 kPa to 10 µPa), and the thermosphere/corona extending from 4,000 km to as high as 50,000 km from the surface.^[8] There is no mesosphere.

Composition

The composition of the Uranian atmosphere is different from the composition of Uranus as a whole, consisting as it does mainly of molecular hydrogen and helium.^[8] The helium molar fraction, i.e. the number of helium atoms per molecule of gas, is 0.15 ± 0.03^[10] in the upper troposphere, which corresponds to a mass fraction 0.26 ± 0.05.^[8]^[55] This value is very close to the protosolar helium mass fraction of 0.275 ± 0.01,^[59] indicating that helium has not settled in the center of the planet as it has in the gas giants.^[8] The third most abundant constituent of the Uranian atmosphere is methane (CH₄).^[8] Methane possesses prominent absorption bands in the visible and near-infrared (IR) making Uranus aquamarine or cyan in color.^[8] Methane molecules account for 2.3% of the atmosphere by molar fraction below the methane cloud deck at the pressure level of 1.3 bar (130 kPa); this represents about 20 to 30 times the carbon abundance found in the Sun.^[8]^[9]^[60] The mixing ratio^[e] is much lower in the upper atmosphere due to its extremely low temperature, which lowers the saturation level and causes excess methane to freeze out.^[61] The abundances of less volatile compounds such as ammonia, water and hydrogen sulfide in the deep atmosphere are poorly known. However they are probably also higher than solar values.^[8]^[62] In addition to methane, trace amounts of various hydrocarbons are found in the stratosphere of Uranus, which are thought to be produced from methane by photolysis induced by the solar ultraviolet (UV) radiation.^[63] They include ethane (C₂H₆), acetylene (C₂H₂), methylacetylene (CH₃C₂H), diacetylene (C₂HC₂H).^[61]^[64]^[65] Spectroscopy has also uncovered traces of water vapor, carbon monoxide and carbon dioxide in the upper atmosphere, which can only originate from an external source such as infalling dust and comets.^[65]^[64]^[66]

Troposphere

Temperature profile of the Uranian troposphere and lower stratosphere. Cloud and haze layers are also indicated.

The troposphere is the lowest and densest part of the atmosphere and is characterized by a decrease in temperature with altitude.^[8] The temperature falls from about 320 K at the base of the nominal troposphere at −300 km to 53 K at 50 km.^[57]^[60] The temperatures in the coldest upper region of the troposphere (the tropopause) actually vary in the range between 49 and 57 K depending on planetary latitude.^[8]^[54] The tropopause region is responsible for the vast majority of the planet’s thermal far infrared emissions, thus determining its effective temperature of 59.1 ± 0.3 K.^[54]^[55]

The troposphere is believed to possess a highly complex cloud structure; water clouds are hypothesised to lie in the pressure range of 50 to 100 bar (5 to 10 MPa), ammonium hydrosulfide clouds in the range of 20 to 40 bar (2 to 4 MPa), ammonia or hydrogen sulfide clouds at between 3 and 10 bar (0.3 to 1 MPa) and finally directly detected thin methane clouds at 1 to 2 bar (0.1 to 0.2 MPa).^[8]^[57]^[67]^[9] The troposphere is a very dynamic part of the atmosphere, exhibiting strong winds, bright clouds and seasonal changes, which will be discussed below.^[15]

Upper atmosphere

The middle layer of the Uranian atmosphere is the stratosphere, where temperature generally increases with altitude from 53 K in the tropopause to between 800 and 850 K at the base of the thermosphere.^[58] The heating of the stratosphere is caused by absorption of solar UV and IR radiation by methane and other hydrocarbons,^[68] which form in this part of the atmosphere as a result of methane photolysis.^[63] Heat is also conducted from the hot thermosphere.^[68] The hydrocarbons occupy a relatively narrow layer at altitudes of between 100 and 280 km corresponding to a pressure range of 10 to 0.1 mbar (1000 to 10 kPa) and temperatures of between 75 and 170 K.^[61]^[64] The most abundant hydrocarbons are methane, acetylene and ethane with mixing ratios of around 10⁻⁷ relative to hydrogen. The mixing ratio of carbon monoxide is similar at these altitudes.^[61]^[64]^[66] Heavier hydrocarbons and carbon dioxide have mixing ratios three orders of magnitude lower.^[64] The abundance ratio of water is around 7×10⁻⁹.^[65] Ethane and acetylene tend to condense in the colder lower part of stratosphere and tropopause (below 10 mBar level) forming haze layers,^[63] which may be partly responsible for the bland appearance of Uranus. However, the concentration of hydrocarbons in the Uranian stratosphere above the haze is significantly lower than in the stratospheres of the other giant planets.^[61]^[69]

The outermost layer of the Uranian atmosphere is the thermosphere and corona, which has a uniform temperature around 800 to 850 K.^[8]^[69] The heat sources necessary to sustain such a high value are not understood, since neither solar far UV and extreme UV radiation nor auroral activity can provide the necessary energy. The weak cooling efficiency due to the lack of hydrocarbons in the stratosphere above 0.1 mBar pressure level may contribute too.^[58]^[69] In addition to molecular hydrogen, the thermosphere-corona contains a large proportion of free hydrogen atoms. Their small mass together with the high temperatures explain why the corona extends as far as 50,000 km or two Uranian radii from the planet.^[58]^[69] This extended corona is a unique feature of Uranus.^[69] Its effects include a drag on small particles orbiting Uranus, causing a general depletion of dust in the Uranian rings.^[58] The Uranian thermosphere, together with the upper part of the stratosphere, corresponds to the ionosphere of Uranus.^[60] Observations show that the ionosphere occupies altitudes from 2,000 to 10,000 km.^[60] The Uranian ionosphere is denser than that of either Saturn or Neptune, which may arise from the low concentration of hydrocarbons in the stratosphere.^[69]^[70] The ionosphere is mainly sustained by solar UV radiation and its density depends on the solar activity.^[71] Auroral activity is insignificant as compared to Jupiter and Saturn.^[69]^[72]

Planetary rings

Main article: Rings of Uranus

Uranus' inner rings. The bright outer ring is the ε ring, eight other rings are present.

Uranian ring system

Uranus has a complicated planetary ring system, which was the second such system to be discovered in the Solar System after Saturn's.^[73] The rings composed of extremely dark particles, which vary in size from micrometers to a fraction of meter.^[14] Thirteen distinct rings are presently known, the brightest being the ε ring. All rings of Uranus (except two) are extremely narrow—they are usually a few km wide. The rings are probably quite young; the dynamics considerations indicate that they did not form with Uranus. The matter in the rings may once have been part of a moon (or moons) which was shattered by high-speed impacts. From numerous debris that formed as result of those impacts only few particles survived in a limited number of stable zones corresponding to present rings.^[73]^[74]

William Herschel claimed to have seen rings at Uranus in 1789, however this is doubtful as in the two following centuries no rings were noted by other observers.^[75] Still, it has been claimed by some that Herschel actually gave accurate descriptions of the ring's size relative to Uranus, its changes as Uranus travelled around the Sun, and its colour.^[76] The ring system was definitively discovered on March 10, 1977 by James L. Elliot, Edward W. Dunham, and Douglas J. Mink using the Kuiper Airborne Observatory. The discovery was serendipitous; they planned to use the occultation of the star SAO 158687 by Uranus to study the planet's atmosphere. However, when their observations were analyzed, they found that the star had disappeared briefly from view five times both before and after it disappeared behind the planet. They concluded that there must be a ring system around the planet.^[77] Later they detected four additional rings.^[77] The rings were directly imaged when Voyager 2 passed Uranus in 1986.^[14] Voyager 2 also discovered two additional faint rings bringing the total number to eleven.^[14]

In December 2005, the Hubble Space Telescope detected a pair of previously unknown rings. The largest is located at twice the distance from the planet of the previously known rings. These new rings are so far from the planet that they are being called the "outer" ring system. Hubble also spotted two small satellites, one of which, Mab, shares its orbit with the outermost newly discovered ring. The new rings bring the total number of Uranian rings to 13.^[78] In April 2006, images of the new rings with the Keck Observatory yielded the colours of the outer rings: the outermost is blue and the other red.^[79]^[80] One hypothesis concerning the outer ring's blue colour is that it is composed of minute particles of water ice from the surface of Mab that are small enough to scatter blue light.^[79]^[81] In contrast, the planet's inner rings appear grey.^[79]

Magnetic field

The magnetic field of Uranus as seen by Voyager 2 in 1986. S and N are magnetic south and north poles.

Prior to the arrival of Voyager 2, no measurements of the Uranian magnetosphere had been taken, so its nature remained a mystery. Before 1986, astronomers had expected the magnetic field of Uranus to be in line with the solar wind, since it would then align with the planet's poles that lie in the ecliptic.^[82]

Voyager's observations revealed that the magnetic field is peculiar, both because it does not originate from the planet's geometric center, and because it is tilted at 59° from the axis of rotation.^[82]^[83] In fact the magnetic dipole is shifted from the center of the planet towards the south rotational pole by as much as one third of the planetary radius.^[82] This unusual geometry results in a highly asymmetric magnetosphere, where the magnetic field strength on the surface in the southern hemisphere can be as low as 0.1 gauss (10 µT), whereas in the northern hemisphere it can be as high 1.1 gauss (110 µT).^[82] The average field at the surface is 0.23 gauss (23 µT).^[82] In comparison, the magnetic field of Earth is roughly as strong at either pole, and its "magnetic equator" is roughly parallel with its physical equator.^[83] The dipole moment of Uranus is 50 times that of Earth.^[82]^[83] Neptune has a similarly displaced and tilted magnetic field, suggesting that this may be a common feature of ice giants.^[83] One hypothesis is that, unlike the magnetic fields of the terrestrial and gas giant planets, which are generated within their cores, the ice giants' magnetic fields are generated by motion at relatively shallow depths, for instance, in the water–ammonia ocean.^[53]^[84]

Despite its curious alignment, in other respects the Uranian magnetosphere is like those of other planets: it has a bow shock located at about 23 Uranian radii ahead of it, a magnetopause at 18 Uranian radii, a fully developed magnetotail and radiation belts.^[82]^[83]^[85] Overall, the structure of the magnetosphere of Uranus is different from that of Jupiter's and more similar to that of Saturn's.^[82]^[83] Uranus' magnetotail trails behind the planet into space for millions of kilometers and is twisted by the planet's sideways rotation into a long corkscrew.^[82]^[86]

Uranus' magnetosphere contains charged particles: protons and electrons with small amount of H₂⁺ ions.^[83]^[85] No heavier ions have been detected. Many of these particles probably derive from the hot atmospheric corona.^[85] The ion and electron energies can be as high as 4 and 1.2 megaelectronvolts, respectively.^[85] The density of low energy (below 1 kiloelectronvolt) ions in the inner magnetosphere is about 2 cm⁻³.^[87] The particle population is strongly affected by the Uranian moons that sweep through the magnetosphere leaving noticeable gaps.^[85] The particle flux is high enough to cause darkening or space weathering of the moon’s surfaces on an astronomically rapid timescale of 100,000 years.^[85] This may be the cause of the uniformly dark colouration of the moons and rings.^[74] Uranus has relatively well developed aurorae, which are seen as bright arcs around both magnetic poles.^[69] However, unlike Jupiter's, Uranus' aurorae seem to be insignificant for the energy balance of the planetary thermosphere.^[72]

Climate

Main article: Climate of Uranus

Uranus' southern hemisphere in approximate natural colour (left) and in higher wavelengths (right), showing its faint cloud bands and atmospheric "hood" as seen by Voyager 2

At ultraviolet and visible wavelengths, Uranus' atmosphere is remarkably bland in comparison to the other gas giants, even to Neptune, which it otherwise closely resembles.^[15] When Voyager 2 flew by Uranus in 1986, it observed a total of ten cloud features across the entire planet.^[14]^[88] One proposed explanation for this dearth of features is that Uranus' internal heat appears markedly lower than that of the other giant planets. The lowest temperature recorded in Uranus' tropopause is 49 K, making Uranus the coldest planet in the Solar System, colder than Neptune.^[55]^[8]

Banded structure, winds and clouds

Zonal wind speeds on Uranus. Shaded areas show the southern collar and its future northern counterpart. The red curve is a symmetrical fit to the data.

In 1986 Voyager 2 found that the visible southern hemisphere of Uranus can be subdivided into two regions: a bright polar cap and dark equatorial bands (see figure on the right).^[14] Their boundary is located at about −45 degrees of latitude. A narrow band straddling the latitudinal range from −45 to −50 degrees is the brightest large feature on the visible surface of the planet.^[14]^[89] It is called a southern "collar". The cap and collar are thought to be a dense region of methane clouds located within the pressure range of 1.3 to 2 bar (see above).^[90] Unfortunately Voyager 2 arrived during the height of the planet's southern summer and could not observe the northern hemisphere. However, at the beginning of the twenty-first century, when the northern polar region came into view, Hubble Space Telescope (HST) and Keck telescope observed neither a collar nor a polar cap in the northern hemisphere.^[89] So Uranus appears to be asymmetric: bright near the south pole and uniformly dark in the region north of the southern collar.^[89] In addition to large-scale banded structure, Voyager 2 observed ten small bright clouds, most lying several degrees to the north from the collar.^[14] In all other respects Uranus looked like a dynamically dead planet in 1986.

However in the 1990s, the number of the observed bright cloud features grew considerably partly because new high resolution imaging techniques became available.^[15] The majority of them were found in the northern hemisphere as it started to become visible.^[15] An early explanation—that bright clouds are easier to identify in the dark part of the planet, whereas in the southern hemisphere the bright collar masks them—was shown to be incorrect: the actual number of features has indeed increased considerably.^[91]^[92] Nevertheless there are differences between the clouds of each hemisphere. The northern clouds are smaller, sharper and brighter.^[92] They appear to lie at a higher altitude.^[92] The lifetime of clouds spans several orders of magnitude. Some small clouds live for hours, while at least one southern cloud may have persisted since Voyager flyby.^[15]^[88] Recent observation also discovered that cloud features on Uranus have a lot in common with those on Neptune.^[15] For example, the dark spots common on Neptune had never been observed on Uranus before 2006, when the first such feature was imaged.^[93] The speculation is that Uranus is becoming more Neptune-like during its equinoctial season.^[94]

The first dark spot observed on Uranus. Image obtained by ACS on HST in 2006.

The tracking of numerous cloud features allowed determination of zonal winds blowing in the upper troposphere of Uranus.^[15] At the equator winds are retrograde, which means that they blow in the reverse direction to the planetary rotation. Their speeds are from −100 to −50 m/s.^[15]^[89] Wind speeds increase with the distance from the equator, reaching zero values near ±20° latitude, where the troposphere's temperature minimum is located.^[54]^[15] Closer to the poles, the winds shift to a prograde direction, flowing with the planet's rotation. Windspeeds continue to increase reaching maxima at ±60° latitude before falling to zero at the poles.^[15] Windspeeds at −40° latitude range from 150 to 200 m/s. Since the collar obscures all clouds below that parallel, speeds between it and the southern pole are impossible to measure.^[15] In contrast, in the northern hemisphere maximum speeds as high as 240 m/s are observed near +50 degrees of latitude.^[15]^[89]^[95]

Seasonal variation

Uranus in 2005. Rings, southern collar and a bright cloud in the northern hemisphere are visible.

For a short period from March to May 2004, a number of large clouds appeared in the Uranian atmosphere, giving it a Neptune-like appearance.^[92]^[96] Observations included record-breaking wind speeds of 229 m/s (824 km/h) and a persistent thunderstorm referred to as "Fourth of July fireworks".^[88] On August 23, 2006, researchers at the Space Science Institute (Boulder, CO) and the University of Wisconsin observed a dark spot on Uranus' surface, giving astronomers more insight into the planet's atmospheric activity.^[93] Why this sudden upsurge in activity should be occurring is not fully known, but it appears that Uranus' extreme axial tilt results in extreme seasonal variations in its weather.^[44]^[94] Determining the nature of this seasonal variation is difficult because good data on Uranus' atmosphere has existed for less than 84 years, or one full Uranian year. A number of discoveries have however been made. Photometry over the course of half a Uranian year (beginning in the 1950s) has shown regular variation in the brightness in two spectral bands, with maxima occurring at the solstices and minima occurring at the equinoxes.^[97] A similar periodic variation, with maxima at the solstices, has been noted in microwave measurements of the deep troposphere begun in the 1960s.^[98] Stratospheric temperature measurements beginning in 1970s also showed maximum values near 1986 solstice.^[68] The majority of this variability is believed to occur due to changes in the viewing geometry.^[91]

However there are some reasons to believe that physical seasonal changes are happening in Uranus. While the planet is known to have a bright south polar region, the north pole is fairly dim, which is incompatible with the model of the seasonal change outlined above.^[94] During its previous northern solstice in 1944, Uranus displayed elevated levels of brightness, which suggests that the north pole was not always so dim.^[97] This information implies that the visible pole brightens some time before the solstice and darkens after the equinox.^[94] Detailed analysis of the visible and microwave data revealed that the periodical changes of brightness are not completely symmetrical around the solstices, which also indicates a change in the meridional albedo patterns.^[94] Finally in the 1990s, as Uranus moved away from its solstice, Hubble and ground based telescopes revealed that the south polar cap darkened noticeably (except the southern collar, which remained bright),^[90] while the northern hemisphere demonstrates increasing activity,^[88] such as cloud formations and stronger winds, bolstering expectations that it should brighten soon.^[92]

The mechanism of physical changes is still not clear.^[94] Near the summer and winter solstices, Uranus' hemispheres lie alternately either in full glare of the Sun's rays or facing deep space. The brightening of the sunlit hemisphere is thought to result from the local thickening of the methane clouds and haze layers located in the troposphere.^[90] The bright collar at −45° latitude is also connected with methane clouds.^[90] Other changes in the southern polar region can be explained by changes in the lower cloud layers.^[90] The variation of the microwave emission from the planet is probably caused by a changes in the deep tropospheric circulation, because thick polar clouds and haze may inhibit convection.^[99] Now that the spring and autumn equinoxes are arriving on Uranus, the dynamics are changing and convection can occur again.^[88]^[99]

Formation

See also: Nebular hypothesis

Many argue that the differences between the ice giants and the gas giants extend to their formation.^[100]^[101] The Solar System is believed to have formed from a giant rotating ball of gas and dust known as the presolar nebula. As it condensed, it formed into a disc with a slowly collapsing Sun in the middle.^[100]^[101] Much of the nebula's gas, primarily hydrogen and helium, formed the Sun, while the dust grains collected together to form the first protoplanets. As the planets grew, some of them eventually accreted enough matter for their gravity to hold onto the nebula's leftover gas.^[100]^[101] The more gas they held onto, the larger they became; the larger they became, the more gas they held onto until a critical point was reached, and their size began to increase exponentially. The ice giants, with only a few Earth masses of nebular gas, never reached that critical point.^[101]^[102]^[100] Current theories of solar system formation have difficulty accounting for the presence of Uranus and Neptune so far out from Jupiter and Saturn. They are too large to have formed from the amount of material expected at that distance. Rather, some scientists expect that both formed closer to the Sun but were scattered outward by Jupiter.^[100] However, more recent simulations, which take into account planetary migration, seem to be able to form Uranus and Neptune near their present locations.^[101]

Moons

Main article: Moons of Uranus

See also: Timeline of discovery of Solar System planets and their natural satellites

Major moons of Uranus compared, at their proper relative sizes (montage of Voyager 2 photographs)

Uranus has 27 known natural satellites.^[102] The names for these satellites are chosen from characters from the works of Shakespeare and Alexander Pope.^[52]^[103] The five main satellites are Miranda, Ariel, Umbriel, Titania and Oberon.^[52] The Uranian satellite system is the least massive among the gas giants; indeed, the combined mass of the five major satellites would be less than half that of Triton alone.^[5] The largest of the satellites, Titania, has a radius of only 788.9 km, or less than half that of the Moon, but slightly more than Rhea, the second largest moon of Saturn, making Titania the eighth largest moon in the Solar System. The moons have relatively low albedos; ranging from 0.20 for Umbriel to 0.35 for Ariel (in green light).^[14] The moons are ice-rock conglomerates composed of roughly fifty percent ice and fifty percent rock. The ice may include ammonia and carbon dioxide.^[104]^[74]

Among the satellites, Ariel appears to have the youngest surface with the fewest impact craters, while Umbriel's appears oldest.^[14]^[74] Miranda possesses fault canyons 20 kilometers deep, terraced layers, and a chaotic variation in surface ages and features.^[14] Miranda's past geologic activity is believed to have been driven by tidal heating at a time when its orbit was more eccentric than currently, probably as a result of a formerly present 3:1 orbital resonance with Umbriel.^[105] Extensional processes associated with upwelling diapirs are likely the origin of the moon's 'racetrack'-like coronae.^[106]^[107] Similarly, Ariel is believed to have once been held in a 4:1 resonance with Titania.^[108]

A picture of Uranus taken by Voyager 2 as it headed to Neptune

Exploration

Main article: Exploration of Uranus

In 1986, NASA's Voyager 2 visited Uranus. This visit is the only attempt to investigate the planet from a short distance and no other visits are currently planned. Launched in 1977, Voyager 2 made its closest approach to Uranus on January 24, 1986, coming within 81,500 kilometers of the planet's cloud tops, before continuing its journey to Neptune. Voyager 2 studied structure and chemical composition of the atmosphere,^[60] discovered 10 new moons and studied the planet's unique weather, caused by its axial tilt of 97.77°; and examined its ring system.^[109]^[14] It also studied the magnetic field, its irregular structure, its tilt and its unique corkscrew magnetotail brought on by Uranus' sideways orientation.^[82] It made the first detailed investigations of its five largest moons, and studied all nine of the system's known rings, discovering two new ones.^[74]^[14]

Saturn

2008-09-14T21:20:00.000-07:00

This article is about the planet. For the Roman God, see Saturn (mythology). For other uses, see Saturn (disambiguation).

Saturn

Saturn, as seen by Cassini

Designations

Adjective

Saturnian

Orbital characteristics^[1]^[2]

Epoch J2000

Aphelion

1,513,325,783 km
10.11595804 AU

Perihelion

1,353,572,956 km
9.04807635 AU

Semi-major axis

1,433,449,370 km
9.58201720 AU

Eccentricity

0.055723219

Orbital period

10,832.327 days
29.657296 yr

Synodic period

378.09 days^[3]

Average orbital speed

9.69 km/s^[3]

Mean anomaly

320.346750°

Inclination

2.485240°
5.51° to Sun's equator

Longitude of ascending node

113.642811°

Argument of perihelion

336.013862°

Satellites

60 confirmed
(up to 63 seen)

Physical characteristics

Equatorial radius

60,268 ± 4 km^[4]^[5]
9.4492 Earths

Polar radius

54,364 ± 10 km^[4]^[5]
8.5521 Earths

Flattening

0.09796 ± 0.00018

Surface area

4.27×10¹⁰ km²^[6]^[5]
83.703 Earths

Volume

8.2713×10¹⁴ km³^[3]^[5]
763.59 Earths

Mass

5.6846×10²⁶ kg^[3]
95.152 Earths

Mean density

0.687 g/cm³^[3]^[5]
(less than water)

Equatorial surface gravity

8.96 m/s²^[3]^[5]
0.914 g

Escape velocity

35.5 km/s^[3]^[5]

Sidereal rotation
period

0.439 – 0.449 day^[7]
(10 h 32 – 47 min)

Equatorial rotation velocity

9.87 km/s^[5]
35,500 km/h

Axial tilt

26.73°^[3]

North pole right ascension

2 h 42 min 21 s
40.589°^[4]

North pole declination

83.537°^[4]

Albedo

0.342 (bond)
0.47 (geom.)^[3]

Surface temp.
1 bar level
0.1 bar

min	mean	max
	134 K^[3]
	84 K^[3]

Apparent magnitude

+1.2 to -0.24^[8]

Angular diameter

14.5" — 20.1"^[3]
(excludes rings)

Atmosphere^[3]

Scale height

59.5 km

Composition

~96%	Hydrogen (H₂)
~3%	Helium
~0.4%	Methane
~0.01%	Ammonia
~0.01%	Hydrogen deuteride (HD)
0.0007%	Ethane
Ices:
	Ammonia
	water
	ammonium hydrosulfide(NH₄SH)

Saturn (/ˈsætɚn/ (help·info)^[9]) is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant. Together, these four planets are sometimes referred to as the Jovian planets, where Jovian is the adjectival form of Jupiter.

Saturn is named after the Roman god Saturnus, equated to the Greek Kronos (the Titan father of Zeus) and the Babylonian Ninurta. Saturn's symbol represents the god's sickle (Unicode: ♄).

The planet Saturn is composed of hydrogen, with small proportions of helium and trace elements.^[10] The interior consists of a small core of rock and ice, surrounded by a thick layer of metallic hydrogen and a gaseous outer layer. The outer atmosphere is generally bland in appearance, although long-lived features can appear. Wind speeds on Saturn can reach 1,800 km/h, significantly faster than those on Jupiter. Saturn has a planetary magnetic field intermediate in strength between that of Earth and the more powerful field around Jupiter.

Saturn has a prominent system of rings, consisting mostly of ice particles with a smaller amount of rocky debris and dust. Sixty known moons orbit the planet. Titan, Saturn's largest and the Solar System's second largest moon (after Jupiter's Ganymede), is larger than the planet Mercury and is the only moon in the Solar System to possess a significant atmosphere.^[11]

Physical characteristics

A rough comparison of the sizes of Saturn and Earth.

Due to a combination of its lower density, rapid rotation, and fluid state, Saturn is an oblate spheroid; that is, it is flattened at the poles and bulges at the equator. Its equatorial and polar radii differ by almost 10% – 60268 km vs. 54364 km.^[3] The other gas planets are also oblate, but to a lesser extent. Saturn is the only planet of the Solar System that is less dense than water. Although Saturn's core is considerably denser than water, the average specific density of the planet is 0.69 g/cm³ due to the gaseous atmosphere. Saturn is only 95 Earth masses,^[3] compared to Jupiter, which is 318 times the mass of the Earth^[12] but only about 20% larger than Saturn.^[13]

Composition

The outer atmosphere of Saturn consists of about 93.2% molecular hydrogen and 6.7% helium. Trace amounts of ammonia, acetylene, ethane, phosphine, and methane have also been detected.^[14] The upper clouds on Saturn are composed of ammonia crystals, while the lower level clouds appear to be composed of either ammonium hydrosulfide (NH₄SH) or water.^[15] The atmosphere of Saturn is significantly deficient in helium relative to the abundance of the elements in the Sun.

The quantity of elements heavier than helium are not known precisely, but the proportions are assumed to match the primordial abundances from the formation of the Solar System. The total mass of these elements is estimated to be 19–31 times the mass of the Earth, with a significant fraction located in Saturn's core region.^[16]

Internal structure

Saturn's temperature emissions: the prominent hot spot at the bottom of the image is at Saturn's south pole.

Though there is little direct information about Saturn's internal structure, it is thought that its interior is similar to that of Jupiter, having a small rocky core surrounded mostly by hydrogen and helium. The rocky core is similar in composition to the Earth, but denser. Above this, there is a thicker liquid metallic hydrogen layer, followed by a layer of liquid hydrogen and helium, and in the outermost 1,000 km a gaseous atmosphere.^[17] Traces of various ices are also present. The core region is estimated to be about 9–22 times the mass of the Earth.^[18] Saturn has a very hot interior, reaching 11,700 °C at the core, and it radiates 2.5 times more energy into space than it receives from the Sun. Most of the extra energy is generated by the Kelvin-Helmholtz mechanism (slow gravitational compression), but this alone may not be sufficient to explain Saturn's heat production. An additional proposed mechanism by which Saturn may generate some of its heat is the "raining out" of droplets of helium deep in Saturn's interior, the droplets of helium releasing heat by friction as they fall down through the lighter hydrogen.^[19]

Cloud layers

Saturn's celestial body atmosphere exhibits a banded pattern similar to Jupiter's (the nomenclature is the same), but Saturn's bands are much fainter and are also much wider near the equator. At the bottom, extending for 10 km and with a temperature of -23 °C, is a layer made up of water ice. After that comes a layer of ammonium hydrosulfide ice, which extends for another 50 km and is approximately at -93 °C. Eighty kilometers above that are ammonia ice clouds, where the temperatures are about -153 °C. Near the top, extending for some 200 km to 270 km above the clouds, come layers of visible cloud tops and a hydrogen and helium atmosphere.^[20] Saturn's winds are among the Solar System's fastest. Voyager data indicate peak easterly winds of 500 m/s (1,800 km/h).^[10] Saturn's finer cloud patterns were not observed until the Voyager flybys. Since then, however, Earth-based telescopy has improved to the point where regular observations can be made.

Saturn's usually bland atmosphere occasionally exhibits long-lived ovals and other features common on Jupiter. In 1990, the Hubble Space Telescope observed an enormous white cloud near Saturn's equator which was not present during the Voyager encounters, and, in 1994, another smaller storm was observed. The 1990 storm was an example of a Great White Spot, a unique but short-lived phenomenon which occurs once every Saturnian year, or roughly every 30 Earth years, around the time of the northern hemisphere's summer solstice.^[21] Previous Great White Spots were observed in 1876, 1903, 1933, and 1960, with the 1933 storm being the most famous. If the periodicity is maintained, another storm will occur in about 2020.^[22]

In recent images from the Cassini spacecraft, Saturn's northern hemisphere appears a bright blue, similar to Uranus, as can be seen in the image below. This blue color cannot currently be observed from Earth, because Saturn's rings are currently blocking its northern hemisphere. The color is most likely caused by Rayleigh scattering.

Saturn's northern hemisphere, as seen by Cassini. Note the planet's blue appearance through the ring.

North polar hexagonal cloud feature, discovered by Voyager 1 and confirmed in 2006 by Cassini.^[23]

Astronomers using infrared imaging have shown that Saturn has a warm polar vortex and that it is the only such feature known in the solar system. This, they say, is the warmest spot on Saturn. Whereas temperatures on Saturn are normally -185 °C, temperatures on the vortex often reach as high as -122 °C.^[24]

A persisting hexagonal wave pattern around the north polar vortex in the atmosphere at about 78°N was first noted in the Voyager images.^[25]^[26] Unlike the north pole, HST imaging of the south polar region indicates the presence of a jet stream, but no strong polar vortex nor any hexagonal standing wave.^[27] However, NASA reported in November 2006 that the Cassini spacecraft observed a 'hurricane-like' storm locked to the south pole that had a clearly defined eyewall.^[28] This observation is particularly notable because eyewall clouds had not previously been seen on any planet other than Earth (including a failure to observe an eyewall in the Great Red Spot of Jupiter by the Galileo spacecraft).^[29]

The straight sides of the northern polar hexagon are each about 13,800 km long. The entire structure rotates with a period of 10h 39 m 24s, the same period as that of the planet's radio emissions, which is assumed to be equal to the period of rotation of Saturn's interior. The hexagonal feature does not shift in longitude like the other clouds in the visible atmosphere.

The pattern's origin is a matter of much speculation. Most astronomers seem to think some sort of standing-wave pattern in the atmosphere; but the hexagon might be a novel sort of aurora. Polygon shapes have been replicated in spinning buckets of fluid in a laboratory.^[30]

Magnetosphere

Saturn has an intrinsic magnetic field that has a simple, symmetric shape—a magnetic dipole. Its strength at the equator—0.2 gauss (20 µT)—is approximately one twentieth than that of the field around Jupiter and slightly weaker than Earth's magnetic field.^[31] As a result the cronian magnetosphere is much smaller than the jovian and extends slightly beyond the orbit of Titan.^[32] Most probably, the magnetic field is generated similarly to that of Jupiter—by currents in the metallic-hydrogen layer, which is called a metallic-hydrogen dynamo.^[32] Similarly to those of other planets, this magnetosphere is efficient at deflecting the solar wind particles from the Sun. The moon Titan orbits within the outer part of Saturn's magnetosphere and contributes plasma from the ionized particles in Titan's outer atmosphere.^[31]

Orbit and rotation

Animation of hexagonal cloud feature.

The average distance between Saturn and the Sun is over 1,400,000,000 km (9 AU). With an average orbital speed of 9.69 km/s,^[3] it takes Saturn 10,759 Earth days (or about 29½ years), to finish one revolution around the Sun.^[3] The elliptical orbit of Saturn is inclined 2.48° relative to the orbital plane of the Earth.^[3] Because of an eccentricity of 0.056, the distance between Saturn and the Sun varies by approximately 155,000,000 km between perihelion and aphelion,^[3] which are the nearest and most distant points of the planet along its orbital path, respectively.

The visible features on Saturn rotate at different rates depending on latitude, and multiple rotation periods have been assigned to various regions (as in Jupiter's case): System I has a period of 10 h 14 min 00 s (844.3°/d) and encompasses the Equatorial Zone, which extends from the northern edge of the South Equatorial Belt to the southern edge of the North Equatorial Belt. All other Saturnian latitudes have been assigned a rotation period of 10 h 39 min 24 s (810.76°/d), which is System II. System III, based on radio emissions from the planet in the period of the Voyager flybys, has a period of 10 h 39 min 22.4 s (810.8°/d); because it is very close to System II, it has largely superseded it.

However, a precise value for the rotation period of the interior remains elusive. While approaching Saturn in 2004, the Cassini spacecraft found that the radio rotation period of Saturn had increased appreciably, to approximately 10 h 45 m 45 s (± 36 s).^[33] The cause of the change is unknown—it was thought to be due to a movement of the radio source to a different latitude inside Saturn, with a different rotational period, rather than because of a change in Saturn's rotation.

Later, in March 2007, it was found that the rotation of the radio emissions did not trace the rotation of the planet, but rather is produced by convection of the plasma disc, which is dependent also on other factors besides the planet's rotation. It was reported that the variance in measured rotation periods may be caused by geyser activity on Saturn's moon Enceladus. The water vapor emitted into Saturn's orbit by this activity becomes charged and "weighs down" Saturn's magnetic field, slowing its rotation slightly relative to the rotation of the planet itself. At the time it was stated that there is no currently known method of determining the rotation rate of Saturn's core.^[34]^[35]^[36]

The latest estimate of Saturn's rotation based on a compilation of various measurements from the Cassini, Voyager and Pioneer probes was reported in September 2007 is 10 hours, 32 minutes, 35 seconds.^[37]

Planetary rings

The rings of Saturn (as imaged here by Cassini in 2007) are the most conspicuous in the Solar System.^[17]

Main article: Rings of Saturn

Saturn is probably best known for its system of planetary rings, which makes it the most visually remarkable object in the solar system.^[17]

History

The rings were first observed by Galileo Galilei in 1610 with his telescope, but he was unable to identify them as such. He wrote to the Duke of Tuscany that "The planet Saturn is not alone, but is composed of three, which almost touch one another and never move nor change with respect to one another. They are arranged in a line parallel to the zodiac, and the middle one (Saturn itself) is about three times the size of the lateral ones [the edges of the rings]." He also described Saturn as having "ears." In 1612 the plane of the rings was oriented directly at the Earth and the rings appeared to vanish. Mystified, Galileo wondered, "Has Saturn swallowed his children?", referring to the myth of the god Saturn eating his own children to prevent them from overthrowing him.^[38] Then, in 1613, they reappeared again, further confusing Galileo.^[39]

In 1655, Christiaan Huygens became the first person to suggest that Saturn was surrounded by a ring. Using a telescope that was far superior to those available to Galileo, Huygens observed Saturn and wrote that "It [Saturn] is surrounded by a thin, flat, ring, nowhere touching, inclined to the ecliptic."^[39]

In 1675, Giovanni Domenico Cassini determined that Saturn's ring was composed of multiple smaller rings with gaps between them; the largest of these gaps was later named the Cassini Division. This division in itself is a 4,800 km wide region between the A Ring and B Ring.^[40]

In 1859, James Clerk Maxwell demonstrated that the rings could not be solid or they would become unstable and break apart. He proposed that the rings must be composed of numerous small particles, all independently orbiting Saturn.^[41] Maxwell's theory was proven correct in 1895 through spectroscopic studies of the rings carried out by James Keeler of Lick Observatory.

Physical characteristics

Saturn's rings cut across an eerie scene that is ruled by Titan's luminous crescent and globe-encircling haze, broken by the small moon Enceladus, whose cryovolcanos are dimly visible at its south pole. North is up. Imaged by Cassini in 2006.

The rings can be viewed using a quite modest modern telescope or with good binoculars. They extend from 6,630 km to 120,700 km above Saturn's equator, average approximately 20 meters in thickness, and are composed of 93 percent water ice with a smattering of tholin impurities, and 7 percent amorphous carbon.^[42] They range in size from specks of dust to the size of a small automobile.^[43] There are two main theories regarding the origin of Saturn's rings. One theory, originally proposed by Édouard Roche in the 19th century, is that the rings were once a moon of Saturn whose orbit decayed until it came close enough to be ripped apart by tidal forces (see Roche limit). A variation of this theory is that the moon disintegrated after being struck by a large comet or asteroid. The second theory is that the rings were never part of a moon, but are instead left over from the original nebular material from which Saturn formed.

While the largest gaps in the rings, such as the Cassini Division and Encke Gap, can be seen from Earth, both Voyager spacecraft discovered that the rings have an intricate structure of thousands of thin gaps and ringlets. This structure is thought to arise, in several different ways, from the gravitational pull of Saturn's many moons. Some gaps are cleared out by the passage of tiny moonlets such as Pan, many more of which may yet be discovered, and some ringlets seem to be maintained by the gravitational effects of small shepherd satellites such as Prometheus and Pandora. Other gaps arise from resonances between the orbital period of particles in the gap and that of a more massive moon further out; Mimas maintains the Cassini division in this manner. Still more structure in the rings consists of spiral waves raised by the moons' periodic gravitational perturbations.

Data from the Cassini space probe indicate that the rings of Saturn possess their own atmosphere, independent of that of the planet itself. The atmosphere is composed of molecular oxygen gas (O₂) produced when ultraviolet light from the Sun interacts with water ice in the rings. Chemical reactions between water molecule fragments and further ultraviolet stimulation create and eject, among other things O₂. According to models of this atmosphere, H₂ is also present. The O₂ and H₂ atmospheres are so sparse that if the entire atmosphere were somehow condensed onto the rings, it would be on the order of one atom thick.^[44] The rings also have a similarly sparse OH (hydroxide) atmosphere. Like the O₂, this atmosphere is produced by the disintegration of water molecules, though in this case the disintegration is done by energetic ions that bombard water molecules ejected by Saturn's moon Enceladus. This atmosphere, despite being extremely sparse, was detected from Earth by the Hubble Space Telescope.^[45]

Saturn shows complex patterns in its brightness.^[8] Most of the variability is due to the changing aspect of the rings,^[46]^[47] and this goes through two cycles every orbit. However, superimposed on this is variability due to the eccentricity of the planet's orbit that causes the planet to display brighter oppositions in the northern hemisphere than it does in the southern.^[48]

In 1980, Voyager I made a fly-by of Saturn that showed the F-ring to be composed of three narrow rings that appeared to be braided in a complex structure; it is now known that the outer two rings consist of knobs, kinks and lumps that give the illusion of braiding, with the less bright third ring lying inside them.

Spokes of the rings

Spokes in the B ring, imaged by Voyager 2 in 1981

Until 1980, the structure of the rings of Saturn was explained exclusively as the action of gravitational forces. The Voyager spacecraft found radial features in the B ring, called spokes, which could not be explained in this manner, as their persistence and rotation around the rings were not consistent with orbital mechanics.^[49] The spokes appear dark in backscattered light, and bright in forward-scattered light. It is assumed that they are microscopic dust particles that have levitated away from the ring plane and that they are connected to electromagnetic interactions, as they rotate almost synchronously with the magnetosphere of Saturn. However, the precise mechanism generating the spokes is still unknown.^[50]

These are three images of the spokes imaged by Cassini in 2005.

Twenty-five years later, the spokes were observed again, this time by Cassini. They appear to be a seasonal phenomenon, disappearing in the Saturnian midwinter/midsummer and reappearing as Saturn comes closer to equinox. The spokes were not visible when Cassini arrived at Saturn in early 2004. Some scientists speculated that the spokes would not be visible again until 2007, based on models attempting to describe spoke formation. Nevertheless, the Cassini imaging team kept looking for spokes in images of the rings, and the spokes reappeared in images taken on September 5, 2005.^[51]

Natural satellites

Main article: Moons of Saturn

Four of Saturn's moons: Dione, Titan, Prometheus (edge of rings), Telesto (top center)

Saturn has a large number of moons. The precise figure is indeterminate, as the orbiting chunks of ice in Saturn's rings are all technically moons, and it is difficult to draw a distinction between a large ring particle and a tiny moon. As of 2007, 60 moons had been identified, plus 3 unconfirmed moons that could be large dust clumps in the rings. Of those, 52 had been given proper names. Many of the moons are very small: 34 are less than 10 km in diameter, and another 13 less than 50 km.^[52] Only seven are massive enough to have collapsed into hydrostatic equilibrium under their own gravitation. These are compared with Earth's moon in the table below.

Titan, Saturn's largest moon, is the only moon in the Solar System to have a dense atmosphere. While most of the moons in the Saturnian system are small in size, Titan is, relatively speaking, gigantic. After the Sun, the eight planets and Jupiter's moon Ganymede, Titan is the most massive object in the Solar System.^[11] Titan comprises more than 90 percent of the mass in orbit around Saturn, including the rings, and the other moons range from one hundredth to one hundred millionth its mass.^[53]

Saturn's second largest moon Rhea may have a tenuous ring system of its own.^[54]

Traditionally, most of Saturn's moons have been named after Titans of Greek mythology. This started because John Herschel—son of William Herschel, discoverer of Mimas and Enceladus—suggested doing so in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope,^[55] because they were the sisters and brothers of Cronos (the Greek Saturn).

Saturn's major satellites, compared with Earth's Moon.
Name (Pronunciation key)		Diameter (km)	Mass (kg)	Orbital radius (km)	Orbital period (days)
Mimas	ˈmaɪməs	400 (10% Moon)	0.4×10²⁰ (0.05% Moon)	185,000 (50% Moon)	0.9 (3% Moon)
Enceladus	ɛnˈsɛlədəs	500 (15% Moon)	1.1×10²⁰ (0.2% Moon)	238,000 (60% Moon)	1.4 (5% Moon)
Tethys	ˈtiːθɨs	1,060 (30% Moon)	6.2×10²⁰ (0.8% Moon)	295,000 (80% Moon)	1.9 (7% Moon)
Dione	daɪˈoʊni	1,120 (30% Moon)	11×10²⁰ (1.5% Moon)	377,000 (100% Moon)	2.7 (10% Moon)
Rhea	ˈriːə	1,530 (45% Moon)	23×10²⁰ (3% Moon)	527,000 (140% Moon)	4.5 (20% Moon)
Titan	ˈtaɪtən	5,150 (150% Moon)	1350×10²⁰ (180% Moon)	1,222,000 (320% Moon)	16 (60% Moon)
Iapetus	aɪˈæpɨtəs	1,440 (40% Moon)	20×10²⁰ (3% Moon)	3,560,000 (930% Moon)	79 (290% Moon)

For a timeline of discovery dates, see Timeline of discovery of Solar System planets and their natural satellites.

History and exploration

Main article: Exploration of Saturn

A Hubble Space Telescope image, captured in October 1996, shows Saturn's rings from just past edge-on. Credit: NASA/ESA.

Ancient times and observation

See also: Planet#Etymology

Saturn has been known since prehistoric times.^[56] In ancient times, it was the most distant of the five known planets in the solar system (excluding Earth) and thus a major character in various mythologies. In ancient Roman mythology, the god Saturnus, from which the planet takes its name, was the god of the agricultural and harvest sector.^[57] The Romans considered Saturnus the equivalent of the Greek god Kronos.^[57] The Greeks had made the outermost planet sacred to Kronos,^[58] and the Romans followed suit.

In Hindu astrology, there are nine astrological objects, known as Navagrahas. Saturn, one of them, is known as "Sani" or "Shani," the Judge among all the planets, and by everyone accordingly to their own performed deeds bad or good.^[57] Ancient Chinese and Japanese culture designated the planet Saturn as the earth star (土星). This was based on Five Elements which were traditionally used to classify natural elements. In ancient Hebrew, Saturn is called 'Shabbathai'. Its angel is Cassiel. Its intelligence, or beneficial spirit, is Agiel (layga), and its spirit (darker aspect) is Zazel (lzaz). In Ottoman Turkish, Urdu and Malay, its name is 'Zuhal', derived from Arabic زحل.

Saturn's rings require at least a 15 mm diameter telescope^[59] to resolve and thus were not known to exist until Galileo first saw them in 1610.^[60] He, though, thought of them as two moons on Saturn's sides. It was not until Christian Huygens used greater telescopic magnification that the rings were assumed to be rings. Huygens also discovered Saturn's moon Titan. Some time later, Jean-Dominique Cassini discovered four other moons: Iapetus, Rhea, Tethys, and Dione. In 1675, Cassini also discovered the gap now known as the Cassini Division.^[61]

No further discoveries of significance were made until 1789 when William Herschel discovered two further moons, Mimas and Enceladus. The irregularly shaped satellite Hyperion, which has a resonance with Titan, was discovered in 1848 by a British team.

In 1899 William Henry Pickering discovered Phoebe, a highly irregular satellite that does not rotate synchronously with Saturn as the larger moons do. Phoebe was the first such satellite found, and it takes more than a year to orbit Saturn in a retrograde orbit. During the early twentieth century, research on Titan led to the confirmation in 1944 that it had a thick atmosphere - a feature unique among the solar system's moons.

Pioneer 11 flyby

Saturn was first visited by Pioneer 11 in September 1979. It flew within 20,000 km of the planet's cloud tops. Low resolution images were acquired of the planet and a few of its moons; the resolution of the images was not good enough to discern surface features. The spacecraft also studied the rings; among the discoveries were the thin F-ring and the fact that dark gaps in the rings are bright when viewed towards the Sun, or in other words, they are not empty of material. Pioneer 11 also measured the temperature of Titan.^[62]

Voyager flybys

In November 1980, the Voyager 1 probe visited the Saturn system. It sent back the first high-resolution images of the planet, rings, and satellites. Surface features of various moons were seen for the first time. Voyager 1 performed a close flyby of Titan, greatly increasing our knowledge of the atmosphere of the moon. However, it also proved that Titan's atmosphere is impenetrable in visible wavelengths; so, no surface details were seen. The flyby also changed the spacecraft's trajectory out from the plane of the solar system.^[63]

Almost a year later, in August 1981, Voyager 2 continued the study of the Saturn system. More close-up images of Saturn's moons were acquired, as well as evidence of changes in the atmosphere and the rings. Unfortunately, during the flyby, the probe's turnable camera platform stuck for a couple of days, and some planned imaging was lost. Saturn's gravity was used to direct the spacecraft's trajectory towards Uranus.^[63]

The probes discovered and confirmed several new satellites orbiting near or within the planet's rings. They also discovered the small Maxwell gap (a gap within the C Ring) and Keeler gap (a 42 km wide gap in the A Ring).

Cassini orbiter

Saturn eclipses the Sun, as seen from Cassini.

On July 1, 2004, the Cassini–Huygens spacecraft performed the SOI (Saturn Orbit Insertion) maneuver and entered into orbit around Saturn. Before the SOI, Cassini had already studied the system extensively. In June 2004, it had conducted a close flyby of Phoebe, sending back high-resolution images and data.

Cassini's flyby of Saturn's largest moon, Titan, has captured radar images of large lakes and their coastlines with numerous islands and mountains. The orbiter completed two Titan flybys before releasing the Huygens probe on December 25, 2004. Huygens descended onto the surface of Titan on January 14, 2005, sending a flood of data during the atmospheric descent and after the landing. During 2005, Cassini conducted multiple flybys of Titan and icy satellites. Cassini's last Titan flyby commenced on March 23, 2008.

Since early 2005, scientists have been tracking lightning on Saturn, primarily found by Cassini. The power of the lightning is said to be approximately 1000 times than that of the lightning on Earth. In addition, scientists believe that this storm is the strongest of its kind ever seen.^[64]

On March 10, 2006, NASA reported that, through images, the Cassini probe found evidence of liquid water reservoirs that erupt in geysers on Saturn's moon Enceladus. Images had also shown particles of water in its liquid state being emitted by icy jets and towering plumes. According to Dr. Andrew Ingersoll, California Institute of Technology, "Other moons in the solar system have liquid-water oceans covered by kilometers of icy crust. What's different here is that pockets of liquid water may be no more than tens of meters below the surface."^[65]

On September 20, 2006, a Cassini probe photograph revealed a previously undiscovered planetary ring, outside the brighter main rings of Saturn and inside the G and E rings. Apparently, the source of this ring is the result of the crashing of a meteoroid off two of the moons of Saturn.^[66]

In July 2006, Cassini saw the first proof of hydrocarbon lakes near Titan's north pole, which was confirmed in January 2007. In March 2007, additional images near Titan's north pole discovered hydrocarbon "seas", the largest of which is almost the size of the Caspian Sea.^[67]

In October 2006, the probe detected a 5,000 km diameter hurricane with an eyewall at Saturn's South Pole.^[68]

As of 2006, the probe has discovered and confirmed 4 new satellites. Its primary mission will end in 2008 when the spacecraft will be expected to have completed 74 orbits around the planet. The probe, however, is expected to have at least one mission extension.

Best viewing

Saturn Oppositions: 2001–2029

Saturn is the most distant of the five planets easily visible to the naked eye, the other four being Mercury, Venus, Mars, and Jupiter (Uranus and occasionally 4 Vesta are visible to the naked eye in very dark skies), and was the last planet known to early astronomers until Uranus was discovered in 1781. Saturn appears to the naked eye in the night sky as a bright, yellowish point of light whose magnitude is usually between +1 and 0 and takes approximately 29½ years to make a complete circuit of the ecliptic against the background constellations of the zodiac. Most people will require optical aid (large binoculars or a telescope) magnifying at least 20X to clearly resolve Saturn's rings.^[17]

While it is a rewarding target for observation for most of the time it is visible in the sky, Saturn and its rings are best seen when the planet is at or near opposition (the configuration of a planet when it is at an elongation of 180° and thus appears opposite the Sun in the sky). During the opposition of December 17, 2002, Saturn appeared at its brightest due to a favorable orientation of its rings relative to the Earth.^[47]

Jupiter

2008-09-14T21:18:00.000-07:00

Jupiter

This processed color image of Jupiter was produced in 1990 by the U.S. Geological Survey from a Voyager image captured in 1979. The colors have been enhanced to bring out detail.

Designations

Adjective

Jovian

Orbital characteristics^[1]^[2]

Epoch J2000

Aphelion

816,520,800 km (5.458104 AU)

Perihelion

740,573,600 km (4.950429 AU)

Semi-major axis

778,547,200 km (5.204267 AU)

Eccentricity

0.048775

Orbital period

4331.572 days
11.85920 yr

Synodic period

398.88 days^[3]

Average orbital speed

13.07 km/s^[3]

Mean anomaly

18.818°

Inclination

1.305°
6.09° to Sun's equator

Longitude of ascending node

100.492°

Argument of perihelion

275.066°

Satellites

Physical characteristics

Equatorial radius

71,492 ± 4 km^[4]^[5]
11.209 Earths

Polar radius

66,854 ± 10 km^[4]^[5]
10.517 Earths

Flattening

0.06487 ± 0.00015

Surface area

6.21796×10¹⁰ km²^[6]^[5]
121.9 Earths

Volume

1.43128×10¹⁵ km³^[3]^[5]
1321.3 Earths

Mass

1.8986×10²⁷ kg^[3]
317.8 Earths

Mean density

1.326 g/cm³^[3]^[5]

Equatorial surface gravity

24.79 m/s²^[3]^[5]
2.528 g

Escape velocity

59.5 km/s^[3]^[5]

Sidereal rotation
period

9.925 h^[7]

Equatorial rotation velocity

12.6 km/s
45,300 km/h

Axial tilt

3.13°^[3]

North pole right ascension

268.057°
17 h 52 min 14 s^[4]

North pole declination

64.496°^[4]

Albedo

0.343 (bond)
0.52 (geom.)^[3]

Surface temp.
1 bar level
0.1 bar

min	mean	max
	165 K^[3]
	112 K^[3]

Apparent magnitude

-1.6 to -2.94^[3]

Angular diameter

29.8" — 50.1"^[3]

Atmosphere^[3]

Surface pressure

20–200 kPa^[8] (cloud layer)

Scale height

27 km

Composition

89.8±2.0%	Hydrogen (H₂)
10.2±2.0%	Helium
~0.3%	Methane
~0.026%	Ammonia
~0.003%	Hydrogen deuteride (HD)
0.0006%	Ethane
0.0004%	water
Ices:
	Ammonia
	water
	ammonium hydrosulfide(NH₄SH)

Jupiter (pronounced /ˈdʒuːpɨtɚ/ (help·info)^[9]) is the fifth planet from the Sun and the largest planet within the Solar System. It is two and a half times as massive as all of the other planets in our Solar System combined. Jupiter is classified as a gas giant, along with Saturn, Uranus and Neptune. Together, these four planets are sometimes referred to as the Jovian planets, where Jovian is the adjectival form of Jupiter.

The planet was known by astronomers of ancient times and was associated with the mythology and religious beliefs of many cultures. The Romans named the planet after the Roman god Jupiter.^[10] When viewed from Earth, Jupiter can reach an apparent magnitude of −2.8, making it the third brightest object in the night sky after the Moon and Venus. (However, at certain points in its orbit, Mars can briefly exceed Jupiter's brightness.)

The planet Jupiter is primarily composed of hydrogen with a small proportion of helium; it may also have a rocky core of heavier elements under high pressure. Because of its rapid rotation, Jupiter's shape is that of an oblate spheroid (it possesses a slight but noticeable bulge around the equator). The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence and storms along their interacting boundaries. A prominent result is the Great Red Spot, a giant storm that is known to have existed since at least the 17th century. Surrounding the planet is a faint planetary ring system and a powerful magnetosphere. There are also at least 63 moons, including the four large moons called the Galilean moons that were first discovered by Galileo Galilei in 1610. Ganymede, the largest of these moons, has a diameter greater than that of the planet Mercury.

Jupiter has been explored on several occasions by robotic spacecraft, most notably during the early Pioneer and Voyager flyby missions and later by the Galileo orbiter. The latest probe to visit Jupiter was the Pluto-bound New Horizons spacecraft in late February 2007. The probe used the gravity from Jupiter to increase its speed and adjust its trajectory toward Pluto, thereby saving years of travel. Future targets for exploration include the possible ice-covered liquid ocean on the Jovian moon Europa.

Structure

Jupiter is one of the four gas giants; that is, it is not primarily composed of solid matter. It is the largest planet in the Solar System, having a diameter of 142,984 km at its equator. Jupiter's density, 1.326 g/cm³, is the second highest of the gas giant planets, but lower than any of the four terrestrial planets.

Composition

Jupiter's upper atmosphere is composed of about 88-92% hydrogen and 8-12% helium by percent volume or fraction of gas molecules (see table to the right). Since a helium atom has about four times as much mass as a hydrogen atom, the composition changes when described in terms of the proportion of mass contributed by different atoms. Thus the atmosphere is approximately 75% hydrogen and 24% helium by mass, with the remaining one percent of the mass consisting of other elements. The interior contains denser materials such that the distribution is roughly 71% hydrogen, 24% helium and five percent other elements by mass. The atmosphere contains trace amounts of methane, water vapor, ammonia, and silicon-based compounds. There are also traces of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, and sulfur. The outermost layer of the atmosphere contains crystals of frozen ammonia.^[11]^[12] Through infrared and ultraviolet measurements, trace amounts of benzene and other hydrocarbons have also been found.^[13]

The atmospheric proportions of hydrogen and helium are very close to the theoretical composition of the primordial solar nebula. However, neon in the upper atmosphere only consists of 20 parts per million by mass, which is about a tenth as abundant as in the Sun.^[14] Helium is also depleted, although to a lesser degree. This depletion may be a result of precipitation of these elements into the interior of the planet.^[15] Abundances of heavier inert gases in Jupiter's atmosphere are about two to three times that of the sun.

Based on spectroscopy, Saturn is thought to be similar in composition to Jupiter, but the other gas giants Uranus and Neptune have relatively much less hydrogen and helium.^[16] However, because of the lack of atmospheric entry probes, high quality abundance numbers of the heavier elements are lacking for the outer planets beyond Jupiter.

Mass

Approximate size comparison of Earth and Jupiter, including the Great Red Spot

Jupiter is 2.5 times more massive than all the other planets in our Solar System combined — this is so massive that its barycenter with the Sun actually lies above the Sun's surface (1.068 solar radii from the Sun's center). Although this planet dwarfs the Earth (with a diameter 11 times as great) it is considerably less dense. Jupiter's volume is equal to 1,317 Earths, yet is only 318 times as massive.^[17]^[18]

Theoretical models indicate that if Jupiter had much more mass than it does at present, the planet would shrink. For small changes in mass, the radius would not change appreciably, and above about four Jupiter masses the interior would become so much more compressed under the increased gravitation force that the planet's volume would actually decrease despite the increasing amount of matter. As a result, Jupiter is thought to have about as large a diameter as a planet of its composition and evolutionary history can achieve. The process of further shrinkage with increasing mass would continue until appreciable stellar ignition is achieved as in high-mass brown dwarfs around 50 Jupiter masses.^[19] This has led some astronomers to term it a "failed star", although it is unclear whether or not the processes involved in the formation of planets like Jupiter are similar to the processes involved in the formation of multiple star systems.

Although Jupiter would need to be about 75 times as massive to fuse hydrogen and become a star, the smallest red dwarf is only about 30 percent larger in radius than Jupiter.^[20]^[21] In spite of this, Jupiter still radiates more heat than it receives from the Sun. The amount of heat produced inside the planet is nearly equal to the total solar radiation it receives.^[22] This additional heat radiation is generated by the Kelvin-Helmholtz mechanism through adiabatic contraction. This process results in the planet shrinking by about 2 cm each year.^[23] When it was first formed, Jupiter was much hotter and was about twice its current diameter.^[24]

Internal structure

This cut-away illustrates a model of the interior of Jupiter, with a rocky core overlaid by a deep layer of metallic hydrogen. NASA background image

Jupiter is thought to consist of a dense core with a mixture of elements, a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen.^[23] Beyond this basic outline, there is still considerable uncertainty. The core is often described as rocky, but its detailed composition is unknown, as are the properties of materials at the temperatures and pressures of those depths (see below). In 1997, the existence of the core was suggested by gravitational measurements.^[23] indicating a mass of from 12 to 45 times the Earth's mass or roughly 3%-15% of the total mass of Jupiter.^[25]^[22] The presence of a core during at least part of Jupiter's history is suggested by models of planetary formation involving initial formation of a rocky or icy core that is massive enough to collect its bulk of hydrogen and helium from the protosolar nebula. Assuming it did exist, it may have shrunk as convection currents of hot liquid metallic hydrogen mixed with the molten core and carried its contents to higher levels in the planetary interior. A core may now be entirely absent, as gravitational measurements aren't yet precise enough to rule that possibility out entirely.^[23]^[26]

The uncertainty of the models is tied to the error margin in hitherto measured parameters: one of the rotational coefficients (J₆) used to describe the planet's gravitational moment, Jupiter's equatorial radius, and its temperature at 1 bar pressure. The JUNO mission, scheduled for launch in 2011, is expected to narrow down the value of these parameters, and thereby make progress on the problem of the core.^[27]

The core region is surrounded by dense metallic hydrogen, which extends outward to about 78 percent of the radius of the planet.^[22] Rain-like droplets of helium and neon precipitate downward through this layer, depleting the abundance of these elements in the upper atmosphere.^[15]^[28]

Above the layer of metallic hydrogen lies a transparent interior atmosphere of liquid hydrogen and gaseous hydrogen, with the gaseous portion extending downward from the cloud layer to a depth of about 1,000 km.^[22] Instead of a clear boundary or surface between these different phases of hydrogen, there is probably a smooth gradation from gas to liquid as one descends.^[29]^[30] This smooth transition happens whenever the temperature is above the critical temperature, which for hydrogen is only 33 K (see hydrogen).

The temperature and pressure inside Jupiter increase steadily toward the core. At the phase transition region where liquid hydrogen (heated beyond its critical point) becomes metallic, it is believed the temperature is 10,000 K and the pressure is 200 GPa. The temperature at the core boundary is estimated to be 36,000 K and the interior pressure is roughly 3,000–4,500 GPa.^[22]

Cloud layers

Main article: Atmosphere of Jupiter

This looping animation shows the movement of Jupiter's counter-rotating cloud bands. In this image, the planet's exterior is mapped onto a cylindrical projection

Jupiter is perpetually covered with clouds composed of ammonia crystals and possibly ammonium hydrosulfide. The clouds are located in the tropopause and are arranged into bands of different latitudes, known as tropical regions. These are sub-divided into lighter-hued zones and darker belts. The interactions of these conflicting circulation patterns cause storms and turbulence. Wind speeds of 100 m/s (360 km/h) are common in zonal jets.^[31] The zones have been observed to vary in width, color and intensity from year to year, but they have remained sufficiently stable for astronomers to give them identifying designations.^[18]

The cloud layer is only about 50 km deep, and consists of at least two decks of clouds: a thick lower deck and a thin clearer region. There may also be a thin layer of water clouds underlying the ammonia layer, as evidenced by flashes of lightning detected in the atmosphere of Jupiter. (Water is a polar molecule that can carry a charge, so it is capable of creating the charge separation needed to produce lightning.)^[22] These electrical discharges can be up to a thousand times as powerful as lightning on the Earth.^[32] The water clouds can form thunderstorms driven by the heat rising from the interior.^[33]

The orange and brown coloration in the clouds of Jupiter are caused by upwelling compounds that change color when they are exposed to ultraviolet light from the Sun. The exact makeup remains uncertain, but the substances are believed to be phosphorus, sulfur or possibly hydrocarbons.^[34]^[22] These colorful compounds, known as chromophores, mix with the warmer, lower deck of clouds. The zones are formed when rising convection cells form crystallizing ammonia that masks out these lower clouds from view.^[17]

Jupiter's low axial tilt means that the poles constantly receive less solar radiation than at the planet's equatorial region. Convection within the interior of the planet transports more energy to the poles, however, balancing out the temperatures at the cloud layer.^[18]

Great Red Spot and other storms

Main article: Great Red Spot

This dramatic view of Jupiter's Great Red Spot and its surroundings was obtained by Voyager 1 on February 25, 1979, when the spacecraft was 9.2 million km (5.7 million mi) from Jupiter. Cloud details as small as 160 km (100 mi) across can be seen here. The colorful, wavy cloud pattern to the left of the Red Spot is a region of extraordinarily complex and variable wave motion. To give a sense of Jupiter's scale, the white oval storm directly below the Great Red Spot is approximately the same diameter as Earth.

The best known feature of Jupiter is the Great Red Spot, a persistent anticyclonic storm located 22° south of the equator that is larger than Earth. It is known to have been in existence since at least 1831,^[35] and possibly since 1665.^[36] Mathematical models suggest that the storm is stable and may be a permanent feature of the planet.^[37] The storm is large enough to be visible through Earth-based telescopes.

The oval object rotates counterclockwise, with a period of about six days.^[38] The Great Red Spot's dimensions are 24–40,000 km × 12–14,000 km. It is large enough to contain two or three planets of Earth's diameter.^[39] The maximum altitude of this storm is about 8 km above the surrounding cloudtops.^[40]

Storms such as this are common within the turbulent atmospheres of gas giants. Jupiter also has white ovals and brown ovals, which are lesser unnamed storms. White ovals tend to consist of relatively cool clouds within the upper atmosphere. Brown ovals are warmer and located within the "normal cloud layer". Such storms can last as little as a few hours or stretch on for centuries.

Time-lapse sequence from the approach of Voyager I to Jupiter, showing the motion of atmospheric bands, and circulation of the great red spot. NASA image.

Even before Voyager proved that the feature was a storm, there was strong evidence that the spot could not be associated with any deeper feature on the planet's surface, as the Spot rotates differentially with respect to the rest of the atmosphere, sometimes faster and sometimes more slowly. During its recorded history it has traveled several times around the planet relative to any possible fixed rotational marker below it.

In 2000, an atmospheric feature formed in the southern hemisphere that is similar in appearance to the Great Red Spot, but smaller in size. This was created when several smaller, white oval-shaped storms merged to form a single feature—these three smaller white ovals were first observed in 1938. The merged feature was named Oval BA, and has been nicknamed Red Spot Junior. It has since increased in intensity and changed color from white to red.^[41]^[42]^[43]

Planetary rings

Main article: Rings of Jupiter

The rings of Jupiter.

Jupiter has a faint planetary ring system composed of three main segments: an inner torus of particles known as the halo, a relatively bright main ring, and an outer "gossamer" ring.^[44] These rings appear to be made of dust, rather than ice as is the case for Saturn's rings.^[22] The main ring is probably made of material ejected from the satellites Adrastea and Metis. Material that would normally fall back to the moon is pulled into Jupiter because of its strong gravitational pull. The orbit of the material veers towards Jupiter and new material is added by additional impacts.^[45] In a similar way, the moons Thebe and Amalthea probably produce the two distinct components of the gossamer ring.^[45]

Magnetosphere

Jupiter's broad magnetic field is 14 times as strong as the Earth's, ranging from 4.2 gauss (0.42 mT) at the equator to 10–14 gauss (1.0–1.4 mT) at the poles, making it the strongest in the Solar System (with the exception of sunspots).^[17] This field is believed to be generated by eddy currents — swirling movements of conducting materials—within the metallic hydrogen core. The field traps a sheet of ionized particles from the solar wind, generating a highly-energetic magnetic field outside the planet — the magnetosphere. Electrons from this plasma sheet ionize the torus-shaped cloud of sulfur dioxide generated by the tectonic activity on the moon Io. Hydrogen particles from Jupiter's atmosphere are also trapped in the magnetosphere. Electrons within the magnetosphere generate a strong radio signature that produces bursts in the range of 0.6–30 MHz.^[46]

At about 75 Jupiter radii from the planet, the interaction of the magnetosphere with the solar wind generates a bow shock. Surrounding Jupiter's magnetosphere is a magnetopause, located at the inner edge of a magnetosheath, where the planet's magnetic field becomes weak and disorganized. The solar wind interacts with these regions, elongating the magnetosphere on Jupiter's lee side and extending it outward until it nearly reaches the orbit of Saturn. The four largest moons of Jupiter all orbit within the magnetosphere, which protects them from the solar wind.^[22]

Aurora borealis on Jupiter. Three bright dots are created by magnetic flux tubes that connect to the Jovian moons Io (on the left), Ganymede (on the bottom) and Europa (also on the bottom). In addition, the very bright almost circular region, called the main oval, and the fainter polar aurora can be seen.

The magnetosphere of Jupiter is responsible for intense episodes of radio emission from the planet's polar regions. Volcanic activity on the Jovian moon Io (see below) injects gas into Jupiter's magnetosphere, producing a torus of particles about the planet. As Io moves through this torus, the interaction generates Alfven waves that carry ionized matter into the polar regions of Jupiter. As a result, radio waves are generated through a cyclotron maser mechanism, and the energy is transmitted out along a cone-shaped surface. When the Earth intersects this cone, the radio emissions from Jupiter can exceed the solar radio output.^[47]

Orbit and rotation

The average distance between Jupiter and the Sun is 778 million km (about 5.2 times the average distance from the Earth to the Sun, or 5.2 AU) and it completes an orbit every 11.86 years. The elliptical orbit of Jupiter is inclined 1.31° compared to the Earth. Because of an eccentricity of 0.048, the distance from Jupiter and the Sun varies by 75 million km between perihelion and aphelion, or the nearest and most distant points of the planet along the orbital path respectively.

The axial tilt of Jupiter is relatively small: only 3.13°. As a result this planet does not experience significant seasonal changes, in contrast to Earth and Mars for example.^[48]

Jupiter's rotation is the fastest of all the Solar System's planets, completing a rotation on its axis in slightly less than ten hours; this creates an equatorial bulge easily seen through an Earth-based amateur telescope. This rotation requires a centripetal acceleration at the equator of about 1.67 m/s², compared to the equatorial surface gravity of 24.79 m/s²; thus the net acceleration felt at the equatorial surface is only about 23.12 m/s². The planet is shaped as an oblate spheroid, meaning that the diameter across its equator is longer than the diameter measured between its poles. On Jupiter, the equatorial diameter is 9275 km longer than the diameter measured through the poles.^[30]

Because Jupiter is not a solid body, its upper atmosphere undergoes differential rotation. The rotation of Jupiter's polar atmosphere is about 5 minutes longer than that of the equatorial atmosphere; three "systems" are used as frames of reference, particularly when graphing the motion of atmospheric features. System I applies from the latitudes 10° N to 10° S; its period is the planet's shortest, at 9h 50m 30.0s. System II applies at all latitudes north and south of these; its period is 9h 55m 40.6s. System III was first defined by radio astronomers, and corresponds to the rotation of the planet's magnetosphere; its period is Jupiter's "official" rotation.^[49]

Observation

Jupiter is usually the fourth brightest object in the sky (after the Sun, the Moon and Venus);^[17] however at times Mars appears brighter than Jupiter. Depending on Jupiter's position with respect to the Earth, it can vary in visual magnitude from as bright as −2.8 at opposition down to −1.6 during conjunction with the Sun. The angular diameter of Jupiter likewise varies from 50.1 to 29.8 arc seconds.^[3] Favorable oppositions occur when Jupiter is passing through perihelion, an event that occurs once per orbit. As Jupiter approaches perihelion in March 2011, there will be a favorable opposition in September 2010.^[50]

The retrograde motion of an outer planet is caused by its relative location with respect to the Earth.

Earth overtakes Jupiter every 398.9 days as it orbits the Sun, a duration called the synodic period. As it does so, Jupiter appears to undergo retrograde motion with respect to the background stars. That is, for a period of time Jupiter seems to move backward in the night sky, performing a looping motion.

Jupiter's 12-year orbital period corresponds to the dozen constellations in the zodiac.^[18] As a result, each time Jupiter reaches opposition it has advanced eastward by about the width of a zodiac constellation. The orbital period of Jupiter is also about two-fifths the orbital period of Saturn, forming a 5:2 orbital resonance between the two largest planets in the Solar System.

Because the orbit of Jupiter is outside the Earth's, the phase angle of Jupiter as viewed from the Earth never exceeds 11.5°, and is almost always close to zero. That is, the planet always appears nearly fully illuminated when viewed through Earth-based telescopes. It was only during spacecraft missions to Jupiter that crescent views of the planet were obtained.^[51]

Research and exploration

Ground-based telescope research

In 1610, Galileo Galilei discovered the four largest moons of Jupiter, Io, Europa, Ganymede and Callisto (now known as the Galilean moons) using a telescope; thought to be the first observation of moons other than Earth's.

Note, however, that Chinese historian of astronomy, Xi Zezong, has claimed that Gan De, a Chinese astronomer, made this discovery of one of Jupiter's moons in 362 BC with the unaided eye, nearly two millennia before any Europeans.^[52]^[53] Galileo's was also the first discovery of a celestial motion not apparently centered on the Earth. It was a major point in favor of Copernicus' heliocentric theory of the motions of the planets; Galileo's outspoken support of the Copernican theory placed him under the threat of the Inquisition.^[54]

During 1660s, Cassini used a new telescope to discover spots and colorful bands on Jupiter and observed that the planet appeared oblate; that is, flattened at the poles. He was also able to estimate the rotation period of the planet.^[12] In 1690 Cassini noticed that the atmosphere undergoes differential rotation.^[22]

False-color detail of Jupiter's atmosphere, imaged by Voyager 1, showing the Great Red Spot and a passing white oval.

The Great Red Spot, a prominent oval-shaped feature in the southern hemisphere of Jupiter, may have been observed as early as 1664 by Robert Hooke and in 1665 by Giovanni Cassini, although this is disputed. The pharmacist Heinrich Schwabe produced the earliest known drawing to show details of the Great Red Spot in 1831.^[55]

The Red Spot was reportedly lost from sight on several occasions between 1665 and 1708 before becoming quite conspicuous in 1878. It was recorded as fading again in 1883 and at the start of the twentieth century.^[56]

Both Giovanni Borelli and Cassini made careful tables of the motions of the Jovian moons, allowing predictions of the times when the moons would pass before or behind the planet. By the 1670s, however, it was observed that when Jupiter was on the opposite side of the Sun from the Earth, these events would occur about 17 minutes later than expected. Ole Rømer deduced that sight is not instantaneous (a finding that Cassini had earlier rejected^[12]), and this timing discrepancy was used to estimate the speed of light.^[57]

In 1892, E. E. Barnard observed a fifth satellite of Jupiter with the 36-inch (910 mm) refractor at Lick Observatory in California. The discovery of this relatively small object, a testament to his keen eyesight, quickly made him famous. The moon was later named Amalthea.^[58] It was the last planetary moon to be discovered directly by visual observation.^[59] An additional eight satellites were subsequently discovered prior to the flyby of the Voyager 1 probe in 1979.

In 1932, Rupert Wildt identified absorption bands of ammonia and methane in the spectra of Jupiter.^[60]

Three long-lived anticyclonic features termed white ovals were observed in 1938. For several decades they remained as separate features in the atmosphere, sometimes approaching each other but never merging. Finally, two of the ovals merged in 1998, then absorbed the third in 2000, becoming Oval BA.^[61]

In 1955, Bernard Burke and Kenneth Franklin detected bursts of radio signals coming from Jupiter at 22.2 MHz.^[22] The period of these bursts matched the rotation of the planet, and they were also able to use this information to refine the rotation rate. Radio bursts from Jupiter were found to come in two forms: long bursts (or L-bursts) lasting up to several seconds, and short bursts (or S-bursts) that had a duration of less than a hundredth of a second.^[62]

Scientists discovered that there were three forms of radio signals being transmitted from Jupiter.

Decametric radio bursts (with a wavelength of tens of meters) vary with the rotation of Jupiter, and are influenced by interaction of Io with Jupiter's magnetic field.^[63]
Decimetric radio emission (with wavelengths measured in centimeters) was first observed by Frank Drake and Hein Hvatum in 1959.^[22] The origin of this signal was from a torus-shaped belt around Jupiter's equator. This signal is caused by cyclotron radiation from electrons that are accelerated in Jupiter's magnetic field.^[64]
Thermal radiation is produced by heat in the atmosphere of Jupiter.^[22]

During the period July 16, 1994 to July 22, 1994, over 20 fragments from the comet Shoemaker-Levy 9 hit Jupiter's southern hemisphere, providing the first direct observation of a collision between two Solar System objects. This impact provided useful data on the composition of Jupiter's atmosphere.^[65]^[66]

Exploration with space probes

Main article: Exploration of Jupiter

Since 1973 a number of automated spacecraft have visited Jupiter. Flights to other planets within the Solar System are accomplished at a cost in energy, which is described by the net change in velocity of the spacecraft, or delta-v. Reaching Jupiter from Earth requires a delta-v of 9.2 km/s,^[67] which is comparable to the 9.7 km/s delta-v needed to reach low Earth orbit.^[68] Fortunately, gravity assists through planetary flybys can be used to reduce the energy required to reach Jupiter, albeit at the cost of a significantly longer flight duration.^[67]

Flyby missions

Flyby missions
Spacecraft	Closest approach	Distance
Pioneer 10	December 3, 1973	130,000 km
Pioneer 11	December 4, 1974	34,000 km
Voyager 1	March 5, 1979	349,000 km
Voyager 2	July 9, 1979	570,000 km
Ulysses	February 1992	409,000 km
Ulysses	February 2004	240,000,000 km
Cassini	December 30, 2000	10,000,000 km
New Horizons	February 28, 2007	2,304,535 km

Voyager 1 took this photo of the planet Jupiter on January 24, 1979 while still more than 25 million mi (40 million km) away.

Beginning in 1973, several spacecraft have performed planetary flyby maneuvers that brought them within observation range of Jupiter. The Pioneer missions obtained the first close-up images of Jupiter's atmosphere and several of its moons. They discovered that the radiation fields in the vicinity of the planet were much stronger than expected, but both spacecraft managed to survive in that environment. The trajectories of these spacecraft were used to refine the mass estimates of the Jovian system. Occultations of the radio signals by the planet resulted in better measurements of Jupiter's diameter and the amount of polar flattening.^[18]^[69]

Six years later, the Voyager missions vastly improved the understanding of the Galilean moons and discovered Jupiter's rings. They also confirmed that the Great Red Spot was anticyclonic. Comparison of images showed that the Red Spot had changed hue since the Pioneer missions, turning from orange to dark brown. A torus of ionized atoms was discovered along Io's orbital path, and volcanoes were found on the moon's surface, some in the process of erupting. As the spacecraft passed behind the planet, it observed flashes of lightning in the night side atmosphere.^[18]^[11]

The next mission to encounter Jupiter, the Ulysses solar probe, performed a flyby maneuver in order to attain a polar orbit around the Sun. During this pass the spacecraft conducted studies on Jupiter's magnetosphere. However, since Ulysses has no cameras, no images were taken. A second flyby six years later was at a much greater distance.^[70]

In 2000, the Cassini probe, en route to Saturn, flew by Jupiter and provided some of the highest-resolution images ever made of the planet. On December 19, 2000, the spacecraft captured an image of the moon Himalia, but the resolution was too low to show surface details.^[71]

The New Horizons probe, en route to Pluto, flew by Jupiter for gravity assist. Closest approach was on February 28, 2007.^[72] The probe's cameras measured plasma output from volcanoes on Io and studied all four Galilean moons in detail, as well as making long-distance observations of the outer moons Himalia and Elara.^[73] Imaging of the Jovian system began September 4, 2006.^[74]^[75]

Galileo mission

Jupiter as seen by the space probe Cassini. This is the most detailed global color portrait of Jupiter ever assembled.

So far the only spacecraft to orbit Jupiter is the Galileo orbiter, which went into orbit around Jupiter on December 7, 1995. It orbited the planet for over seven years, conducting multiple flybys of all of the Galilean moons and Amalthea. The spacecraft also witnessed the impact of Comet Shoemaker-Levy 9 as it approached Jupiter in 1994, giving a unique vantage point for the event. However, while the information gained about the Jovian system from Galileo was extensive, its originally-designed capacity was limited by the failed deployment of its high-gain radio transmitting antenna.^[76]

An atmospheric probe was released from the spacecraft in July 1995, entering the planet's atmosphere on December 7. It parachuted through 150 km of the atmosphere, collecting data for 57.6 minutes, before being crushed by the pressure to which it was subjected by that time (about 22 times Earth normal, at a temperature of 153 °C).^[77] It would have melted thereafter, and possibly vaporized. The Galileo orbiter itself experienced a more rapid version of the same fate when it was deliberately steered into the planet on September 21, 2003 at a speed of over 50 km/s, in order to avoid any possibility of it crashing into and possibly contaminating Europa—a moon which has been hypothesized to have the possibility of harboring life.^[76]

Future probes

NASA is planning a mission to study Jupiter in detail from a polar orbit. Named Juno, the spacecraft is planned to launch by 2011.^[78]

Because of the possibility of a liquid ocean on Jupiter's moon Europa, there has been great interest in studying the icy moons in detail. A mission proposed by NASA was dedicated to doing so. The JIMO (Jupiter Icy Moons Orbiter) was expected to be launched sometime after 2012. However, the mission was deemed too ambitious and its funding was canceled.^[79] A European Jovian Europa Orbiter mission is being studied, but its launch is unscheduled.^[80]

Moons

Main article: Moons of Jupiter

See also: Timeline of discovery of Solar System planets and their moons

Jupiter has 63 named natural satellites. Of these, 47 are less than 10 kilometres in diameter and have only been discovered since 1975. The four largest moons, known as the "Galilean moons", are Io, Europa, Ganymede and Callisto.

Jupiter's 4 Galilean moons, in a composite image comparing their sizes and the size of Jupiter (Great Red Spot visible). From the top they are: Callisto, Ganymede, Europa and Io.

Galilean moons

Main article: Galilean moons

The orbits of Io, Europa , and Ganymede , some of the largest satellites in the Solar System, form a pattern known as a Laplace resonance; for every four orbits that Io makes around Jupiter, Europa makes exactly two orbits and Ganymede makes exactly one. This resonance causes the gravitational effects of the three large moons to distort their orbits into elliptical shapes, since each moon receives an extra tug from its neighbors at the same point in every orbit it makes. The tidal force from Jupiter, on the other hand, works to circularize their orbits.^[81]

The eccentricity of their orbits causes regular flexing of the three moons' shapes, with Jupiter's gravity stretching them out as they approach it and allowing them to spring back to more spherical shapes as they swing away. This tidal flexing heats the moons' interiors via friction. This is seen most dramatically in the extraordinary volcanic activity of innermost Io (which is subject to the strongest tidal forces), and to a lesser degree in the geological youth of Europa's surface (indicating recent resurfacing of the moon's exterior).

The Galilean moons, compared to Earth's Moon
Name (Pronunciation key)		Diameter		Mass		Orbital radius		Orbital period
Name (Pronunciation key)		km	%	kg	%	km	%	days	%
Io	eye'-oe ˈaɪəʊ	3643	105	8.9×10²²	120	421,700	110	1.77	7
Europa	ew-roe'-pə jʊˈrəʊpə	3122	90	4.8×10²²	65	671,034	175	3.55	13
Ganymede	gan'-ə-meed ˈgænəmid	5262	150	14.8×10²²	200	1,070,412	280	7.15	26
Callisto	kə-lis'-toe kəˈlɪstəʊ	4821	140	10.8×10²²	150	1,882,709	490	16.69	61

Callisto, Ganymede, Jupiter and Europa

Classification of moons

Europa, one of Jupiter's many moons.

Before the discoveries of the Voyager missions, Jupiter's moons were arranged neatly into four groups of four, based on commonality of their orbital elements. Since then, the large number of new small outer moons has complicated this picture. There are now thought to be six main groups, although some are more distinct than others.

A basic sub-division is a grouping of the eight inner regular moons, which have nearly circular orbits near the plane of Jupiter's equator and are believed to have formed with Jupiter. The remainder of the moons consist of an unknown number of small irregular moons with elliptical and inclined orbits, which are believed to be captured asteroids or fragments of captured asteroids. Irregular moons that belong to a group share similar orbital elements and thus may have a common origin, perhaps as a larger moon or captured body that broke up.^[82]^[83]

Regular moons	Inner group	The inner group of four small moons all have diameters of less than 200 km, orbit at radii less than 200,000 km, and have orbital inclinations of less than half a degree.
Regular moons	Galilean moons^[84]	These four moons, discovered by Galileo Galilei and by Simon Marius in parallel, orbit between 400,000 and 2,000,000 km, and include some of the largest moons in the Solar System.
Irregular moons	Themisto	This is a single moon belonging to a group of its own, orbiting halfway between the Galilean moons and the Himalia group.
	Himalia group	A tightly clustered group of moons with orbits around 11,000,000–12,000,000 km from Jupiter.
	Carpo	Another isolated case; at the inner edge of the Ananke group, it revolves in the direct sense.
	Ananke group	This group has rather indistinct borders, averaging 21,276,000 km from Jupiter with an average inclination of 149 degrees.
	Carme group	A fairly distinct group that averages 23,404,000 km from Jupiter with an average inclination of 165 degrees.
	Pasiphaë group	A dispersed and only vaguely distinct group that covers all the outermost moons.

Interaction with the Solar System

Along with the Sun, the gravitational influence of Jupiter has helped shape the Solar System. The orbits of most of the system's planets lie closer to Jupiter's orbital plane than the Sun's equatorial plane (Mercury is the only planet that is closer to the Sun's equator in orbital tilt), the Kirkwood gaps in the asteroid belt are mostly due to Jupiter, and the planet may have been responsible for the Late Heavy Bombardment of the inner Solar System's history.^[85]

This diagram shows the Trojan Asteroids in Jupiter's orbit, as well as the main asteroid belt.

In addition to its moons, Jupiter's gravitational field controls numerous asteroids that have settled into the regions of the Lagrangian points preceding and following Jupiter in its orbit around the sun. These are known as the Trojan asteroids, and are divided into Greek and Trojan "camps" to commemorate the Iliad. The first of these, 588 Achilles, was discovered by Max Wolf in 1906; since then more than two thousand have been discovered.^[86] The largest is 624 Hektor.

Jupiter has been called the Solar System's vacuum cleaner,^[87] because of its immense gravity well and location near the inner Solar System. It receives the most frequent comet impacts of the Solar System's planets.^[88] In 1994 comet Shoemaker-Levy 9 (SL9, formally designated D/1993 F2) collided with Jupiter and gave information about the structure of Jupiter. It was thought that the planet served to partially shield the inner system from cometary bombardment. However, recent computer simulations suggest that Jupiter doesn't cause a net decrease in the number of comets that pass through the inner Solar System, as its gravity perturbs their orbits inward in roughly the same numbers that it accretes or ejects them.^[89]

The majority of short-period comets belong to the Jupiter family—defined as comets with semi-major axes smaller than Jupiter's. Jupiter family comets are believed to form in the Kuiper belt outside the orbit of Neptune. During close encounters with Jupiter their orbits are perturbed into a smaller period and then circularized by regular gravitational interaction with the Sun and Jupiter.^[90]

Possibility of life

In 1953, the Miller-Urey experiment demonstrated that a combination of lightning and the chemical compounds that existed in the atmosphere of a primordial Earth could form organic compounds (including amino acids) that could serve as the building blocks of life. The simulated atmosphere included water, methane, ammonia and molecular hydrogen; all molecules still found in the atmosphere of Jupiter. However, the atmosphere of Jupiter has a strong vertical air circulation, which would carry these compounds down into the lower regions. The higher temperatures within the interior of the atmosphere breaks down these chemicals, which would hinder the formation of Earth-like life.^[91]

It is considered highly unlikely that there is any Earth-like life on Jupiter, as there is only a small amount of water in the atmosphere and any possible solid surface deep within Jupiter would be under extraordinary pressures. However, in 1976, before the Voyager missions, it was hypothesized^[92]^[93] that ammonia- or water-based life, such as the so-called atmospheric beasts, could evolve in Jupiter's upper atmosphere. This hypothesis is based on the ecology of terrestrial seas which have simple photosynthetic plankton at the top level, fish at lower levels feeding on these creatures, and marine predators which hunt the fish.

Human culture

The planet Jupiter has been known since ancient times. It is visible to the naked eye in the night sky and can occasionally be seen in the daytime when the sun is low.^[94] To the Babylonians, this object represented their god Marduk. They used the roughly 12-year orbit of this planet along the ecliptic to define the constellations of their zodiac.^[18]^[95]

The Romans named it after Jupiter (Latin: Iuppiter, Iūpiter) (also called Jove), the principal god of Roman mythology, whose name comes from the Proto-Indo-European vocative form *dyeu ph₂ter, meaning "god-father."^[10] The astronomical symbol for the planet, , is a stylized representation of the god's lightning bolt. The Greek equivalent Zeus supplies the root zeno-, used to form some Jupiter-related words, such as zenographic.^[96]

Jovian is the adjectival form of Jupiter. The older adjectival form jovial, employed by astrologers in the Middle Ages, has come to mean "happy" or "merry," moods ascribed to Jupiter's astrological influence.^[97]

The Chinese, Korean, Japanese, and Vietnamese referred to the planet as the wood star, 木星,^[98] based on the Chinese Five Elements. The Greeks called it Φαέθων, Phaethon, "blazing". In Vedic Astrology, Hindu astrologers named the planet after Brihaspati, the religious teacher of the gods, and often called it "Guru," which literally means the "Heavy One".^[99] In the English language Thursday is rendered as Thor's day, with Thor being associated with the planet Jupiter in Germanic mythology.^[100]