Neptune

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, 1846Hamilton, Calvin J. (August 4, 2001). Neptune. Views of the Solar System. Retrieved on 2007-08-13.
Orbital characteristicsYeomans, Donald K. (July 13, 2006). HORIZONS System. NASA JPL. Retrieved on 2007-08-08.—At the site, go to the "web interface" then select "Ephemeris Type: ELEMENTS", "Target Body: Neptune Barycenter" and "Center: Sun".Orbital elements refer to the barycentre of the Neptune system, and are the instantaneous osculating values at the precise J2000 epoch. Barycentre quantities are given because, in contrast to the planetary centre, they do not experience appreciable changes on a day-to-day basis from to the motion of the moons.
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 days 164.79 years
Synodic period	367.49 dayWilliams, David R. (September 1, 2004). Neptune Fact Sheet. NASA. Retrieved on 2007-08-14.
Average orbital speed	5.43 km/s
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	13
Physical characteristics
Equatorial radius	24,764 ± 15 kmP. Kenneth, Seidelmann; Archinal, B. A.; A’hearn, M. F. et al (2007). "Report of the IAU/IAGWorking Group on cartographic coordinates and rotational elements". Celestial Mechanics and Dynamical Astronomy 90: 155–180. Springer Netherlands. doi:10.1007/s10569-007-9072-y. ISSN (Print) 0923-2958 (Print). Retrieved on 2008-03-07. Refers to the level of 1 bar atmospheric pressure 3.883 Earths
Polar radius	24,341 ± 30 km 3.829 Earths
Flattening	0.0171 ± 0.0013
Surface area	7.6408×109 km² 14.98 Earths
Volume	6.254×1013 km³ 57.74 Earths
Mass	1.0243×1026 kg 17.147 Earths
Mean density	1.638 g/cm³
Equatorial surface gravity	11.15 m/s² 1.14 g
Escape velocity	23.5 km/s
Sidereal rotation period	0.6713 day 16 h 6 min 36 s
Equatorial rotation velocity	2.68 km/s 9,660 km/h
Axial tilt	28.32°
North pole right ascension	19h 57m 20s
North pole declination	42.950°
Albedo	0.290 (bond) 0.41 (geom.)
Surface temp.    1 bar level    0.1 bar	min mean max 72 K 55 K
Apparent magnitude	8.0 to 7.78
Angular diameter	2.2″—2.4″
Adjectives	Neptunian
Atmosphere
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/Walter, Elizabeth (April 21, 2003). Cambridge Advanced Learner\'s Dictionary, Second Edition, Cambridge University Press. ISBN 0521531063. ) 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. 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, 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\'s atmosphere is primarily composed of hydrogen and helium along with traces of methane. The methane in the atmosphere, in part, accounts for the planet\'s blue appearance.Munsell, Kirk; Smith, Harman; Harvey, Samantha (November 13, 2007). Neptune overview. Solar System Exploration. NASA. Retrieved on 2008-02-20. Neptune also has the strongest winds of any planet in the solar system, measured as high as 2,100 kilometres per hour (1,300 mph).Suomi, V. E.; Limaye, S. S.; Johnson, D. R. (1991). "High Winds of Neptune: A possible mechanism". Science 251 (4996): 929–932. AAAS (USA). doi:10.1126/science.251.4996.929.  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 (55.2 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.

Contents 1 History 1.1 Discovery 1.2 Naming 1.3 Status 2 Composition and structure 2.1 Internal structure 2.2 Atmosphere 2.3 Magnetosphere 2.4 Planetary rings 3 Climate 3.1 Storms 3.2 Internal heat 4 Observation 5 Exploration 6 See also 7 References 7.1 Notes 7.2 Book references 8 External links

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.Hirschfeld, Alan (2001). Parallax:The Race to Measure the Cosmos. New York, New York: Henry Holt. ISBN 0-8050-7133-4.  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.Littmann, Mark; Standish, E. M. (2004). Planets Beyond: Discovering the Outer Solar System. Courier Dover Publications. ISBN 0-4864-3602-0. 

In 1821, Alexis Bouvard published astronomical tables of the orbit of Uranus.Bouvard, A. (1821). Tables astronomiques publiées par le Bureau des Longitudes de France. Paris: Bachelier.  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.O\'Connor, John J.; Robertson, Edmund F. (March 2006). John Couch Adams\' account of the discovery of Neptune. University of St Andrews. Retrieved on 2008-02-18. Adams, J. C. (November 13, 1846). "Explanation of the observed irregularities in the motion of Uranus, on the hypothesis of disturbance by a more distant planet". Monthly Notices of the Royal Astronomical Society 7: 149. Blackwell Publishing. Retrieved on 2008-02-18. 

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.Airy, G. B. (November 13, 1846). "Account of some circumstances historically connected with the discovery of the planet exterior to Uranus". Monthly Notices of the Royal Astronomical Society 7: 121–144. Blackwell Publishing. Retrieved on 2008-02-18.  Challis, Rev. J. (November 13, 1846). "Account of observations at the Cambridge observatory for detecting the planet exterior to Uranus". Monthly Notices of the Royal Astronomical Society 7: 145–149. Blackwell Publishing. Retrieved on 2008-02-18. 

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.Galle, J. G. (November 13, 1846). "Account of the discovery of the planet of Le Verrier at Berlin". Monthly Notices of the Royal Astronomical Society 7: 153. Blackwell Publishing. Retrieved on 2008-02-18. 

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. Kollerstrom, Nick (2001). Neptune\'s Discovery. The British Case for Co-Prediction.. University College London. Archived from the original on 2005-11-11. Retrieved on 2007-03-19. 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 the journal Dio he deemed the British claim "theft".Rawlins, Dennis (1992). The Neptune Conspiracy: British Astronomy\'s Post­Discovery Discovery. Dio. Retrieved on 2008-03-10. "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.McGourty, Christine (2003). Lost letters\' Neptune revelations. BBC News. Retrieved on 2008-03-10.Summations following the Neptune documents\' 1998 recovery appeared in DIO 9.1 (1999) and William Sheehan, Nicholas Kollerstrom, Craig B. Waff (December 2004), The Case of the Pilfered Planet - Did the British steal Neptune? Scientific American.

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.Moore (2000):206

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.Littmann (2004):50 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.Baum & Sheehan (2003):109–110 French almanacs quickly reintroduced the name Herschel for Uranus, after that planet\'s discoverer Sir William Herschel, and Leverrier for the new planet.Gingerich, Owen (1958). "The Naming of Uranus and Neptune". Astronomical Society of the Pacific Leaflets 8: 9–15. Retrieved on 2008-02-19. 

Struve came out in favor of the name Neptune on December 29, 1846, to the Saint Petersburg Academy of Sciences.Hind, J. R. (1847). "Second report of proceedings in the Cambridge Observatory relating to the new Planet (Neptune)". Astronomische Nachrichten 25: 309. Retrieved on 2008-02-18.  Smithsonian/NASA Astrophysics Data System (ADS). 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.Blue, Jennifer (December 17, 2008). Planet and Satellite Names and Discoverers. USGS. Retrieved on 2008-02-18.

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.Tony Long (2008). Jan. 21, 1979: Neptune Moves Outside Pluto\'s Wacky Orbit. wired.com. Retrieved on 2008-03-13. 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.Weissman, Paul R.. The Kuiper Belt. Annual Review of Astronomy and Astrophysics. Retrieved on 2006-10-04.The Status of Pluto:A clarification. International Astronomical Union, Press release (1999). Retrieved on 2006-05-25. 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."IAU 2006 General Assembly: Resolutions 5 and 6", IAU, 24 August 2006. 

Composition and structure

A size comparison of Neptune and Earth.

With a mass of 1.0243×10²⁶ kg, 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.The mass of the Earth is 5.9736×10²⁴ kg, giving a mass ratio of:

$\backslash begin\{smallmatrix\}\backslash frac\{M\_\{Neptune\}\}\{M\_\{Earth\}\}$

\ =\ \frac{1.02 \times 10^{26}}{5.97 \times 10^{24}} \ =\ 17.09\end{smallmatrix} The mass of Uranus is 8.6810×10²⁵ kg, giving a mass ratio of:

$\backslash begin\{smallmatrix\}\backslash frac\{M\_\{Uranus\}\}\{M\_\{Earth\}\}$

\ =\ \frac{8.68 \times 10^{25}}{5.97 \times 10^{24}} \ =\ 14.54\end{smallmatrix} The mass of Jupiter is 1.8986×10²⁷ kg, giving a mass ratio of:

$\backslash begin\{smallmatrix\}\backslash frac\{M\_\{Jupiter\}\}\{M\_\{Neptune\}\}$

\ =\ \frac{1.90 \times 10^{27}}{1.02 \times 10^{26}} \ =\ 18.63\end{smallmatrix} See: Williams, David R. (November 29, 2007). Planetary Fact Sheet - Metric. NASA. Retrieved on 2008-03-13. Neptune\'s equatorial radius of 24,764 km 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.See for example: Boss, Alan P. (2002). "Formation of gas and ice giant planets". Earth and Planetary Science Letters 202 (3–4): 513–523. doi:10.1016/S0012-821X(02)00808-7.  In the search for extrasolar planets Neptune has been used as a metonym: discovered bodies of similar mass are often referred to as "Neptunes",Lovis, C.; Mayor, M.; Alibert Y.; Benz W.. "Trio of Neptunes and their Belt", ESO, May 18, 2006. Retrieved on 2008-02-25.  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% of its mass and extends perhaps 10–20% 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.

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. 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.Atreya, S.; Egeler, P.; Baines, K. (2006). "Water-ammonia ionic ocean on Uranus and Neptune?" (pdf). Geophysical Research Abstracts 8: 05179.  At a depth of 7,000 km, the conditions may be such that methane decomposes into diamond crystals that then precipitate toward the core.Kerr, Richard A. (1999). "Neptune May Crush Methane Into Diamonds". Science 286 (5437): 25. Retrieved on 2007-02-26. 

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.Podolak, M.; Weizman, A.; Marley, M. (1995). "Comparative models of Uranus and Neptune". Planetary and Space Science 43 (12): 1517–1522. doi:10.1016/0032-0633(95)00061-5.  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.Nettelmann, N.; French, M.; Holst, B.; Redmer, R.. Interior Models of Jupiter, Saturn and Neptune (PDF). University of Rostock. Retrieved on 2008-02-25.

Atmosphere

At high altitudes, Neptune\'s atmosphere is 80% hydrogen and 19% helium.Hubbard, W. B. (1997). "Neptune\'s Deep Chemistry". Science 275 (5304): 1279–1280. Retrieved on 2008-02-19.  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,Crisp, D.; Hammel, H. B. (June 14, 1995). Hubble Space Telescope Observations of Neptune. Hubble News Center. Retrieved on 2007-04-22. 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.

Neptune\'s atmosphere is 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.Lunine, Jonathan I. (1993). The Atmospheres of Uranus and Neptune (PDF). Lunar and Planetary Observatory, University of Arazona. Retrieved on 2008-03-10. The stratosphere then gives way to the thermosphere at a pressure lower than 10⁻⁴–10⁻⁵ microbars. The thermosphere gradually trasitions 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.

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.

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. The stratosphere is also home to trace amounts of carbon monoxide and hydrogen cyanide.Encrenaz, Therese (2003). "ISO observations of the giant planets and Titan: what have we learnt?". Planet. Space Sci. 51: 89–103. doi:10.1016/S0032-0633(02)00145-9.  The stratosphere of Neptune is warmer than that of Uranus due to elevated concentration of hydrocarbons.

For reasons that remain obscure, the planet\'s thermosphere is at an anomalously high temperature of about 750 K.Broadfoot, A.L.; Atreya, S.K.; Bertaux, J.L. et.al. (1999). "Ultraviolet Spectrometer Observations of Neptune and Triton" (pdf). Science 246: 1459–1456. Herbert, Floyd; Sandel, Bill R. (1999). "Ultraviolet Observations of Uranus and Neptune". Planet.Space Sci. 47: 1119–1139.  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.

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)Elkins-Tanton (2006):79–83. resulting in a dynamo action.Stanley, Sabine; Bloxham, Jeremy (March 11, 2004). "Convective-region geometry as the cause of Uranus\' and Neptune\'s unusual magnetic fields". Nature 428: 151–153. doi:10.1038/nature02376. 

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.Ness, N. F.; MAcuña, M. H.; Burlaga, L. F.; Connerney, J. E. P.; Lepping, R. P.; Neubauer, F. M. (1989). "Magnetic Fields at Neptune". Science 246 (4936): 1473–1478. Retrieved on 2008-02-25.  Russell, C. T.; Luhmann, J. G. (1997). Neptune: Magnetic Field and Magnetosphere. University of California, Los Angeles. Retrieved on 2006-08-10.

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.

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.Cruikshank (1996):703–804 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.Blue, Jennifer (December 8, 2004). Nomenclature Ring and Ring Gap Nomenclature. Gazetteer of Planetary. USGS. Retrieved on 2008-02-28.

The first of these planetary rings was discovered in 1968 by a team led by Edward Guinan,Wilford, John N.. "Data Shows 2 Rings Circling Neptune", The New York Times, June 10, 1982. Retrieved on 2008-02-29. Guinan, E. F.; Harris, C. C.; Maloney, F. P. (1982). "Evidence for a Ring System of Neptune". Bulletin of the American Astronomical Society 14: 658. Retrieved on 2008-02-28.  but it was later thought that this ring might be incomplete.Goldreich, P.; Tremaine, S.; Borderies, N. E. F. (1986). "Towards a theory for Neptune\'s arc rings". Astronomical Journal 92: 490–494. Retrieved on 2008-02-28.  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.Nicholson, P. D. et al (1990). "Five Stellar Occultations by Neptune: Further Observations of Ring Arcs". Icarus 87: 1. Retrieved on 2007-12-16.  Images by Voyager 2 in 1989 settled the issue by showing several faint rings. These rings have a clumpy structure,Missions to Neptune. The Planetary Society (2007). Retrieved on 2007-10-11. the cause of which is not currently understood but which may be due to the gravitational interaction with small moons in orbit near them.Wilford, John Noble. "Scientists Puzzled by Unusual Neptune Rings", Hubble News Desk, December 15, 1989. Retrieved on 2008-02-29. 

The outermost ring, Adams, contains five prominent arcs now named Courage, Liberté, Egalité 1, Egalité 2, and Fraternité (Liberty, Equality, and Fraternity).Cox, Arthur N. (2001). Allen\'s Astrophysical Quantities. Springer. ISBN 0387987460.  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.Munsell, Kirk; Smith, Harman; Harvey, Samantha (November 13, 2007). Planets: Neptune: Rings. Solar System Exploration. NASA. Retrieved on 2008-02-29.Salo, Heikki; Hänninen, Jyrki (1998). "Neptune\'s Partial Rings: Action of Galatea on Self-Gravitating Arc Particles". Science 282 (5391): 1102–1104. Retrieved on 2008-02-29. 

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.Staff (March 26, 2005). Neptune\'s rings are fading away. New Scientist. Retrieved on 2007-08-06.

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.

The Great Dark Spot (top), Scooter (middle white cloud),Lavoie, Sue (January 8, 1998). PIA01142: Neptune Scooter. NASA. Retrieved on 2006-03-26. and the Small Dark Spot (bottom).

Neptune\'s weather is characterized by extremely dynamic storm systems, with winds reaching supersonic speeds of up to around 600 m/s.Suomi, V. E.; Limaye, S. S.; Johnson, D. R. (1991). "High Winds of Neptune: A Possible Mechanism". Science 251 (4996): 929–932. doi:10.1126/science.251.4996.929. Retrieved on 2008-02-25.  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.Hammel, H. B.; Beebe, R. F.; De Jong, E. M.; Hansen, C. J.; Howell, C. D.; Ingersoll, A. P.; Johnson, T. V.; Limaye, S. S.; Magalhaes, J. A.; Pollack, J. B.; Sromovsky, L. A.; Suomi, V. E.; Swift, C. E. (1989). "Neptune\'s wind speeds obtained by tracking clouds in Voyager 2 images". Science 245: 1367–1369. Retrieved on 2008-02-27.  At the cloud tops, the prevailing winds range in speed from 400 m/s along the equator to 250 m/s at the poles. Most of the winds on Neptune move in a direction opposite the planet\'s rotation.Burgess (1991):64–70. 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. At 70° S latitude, a high speed jet travels at a speed of 300 m s⁻¹.

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.

In 2007 it was discovered that the upper troposphere of Neptune\'s south pole was about 10 °C (283 K) warmer than the rest of Neptune, which averages approximately −200 °C (73.2 K).Orton, G. S., Encrenaz T., Leyrat C., Puetter, R. and Friedson, A. J. (2007). Evidence for methane escape and strong seasonal and dynamical perturbations of Neptune\'s atmospheric temperatures. Astronomy and Astrophysics. Retrieved on 2008-03-10. 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 40 Neptunian years, a Neptunian year being 165 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.Orton, Glenn; Encrenaz, Thérèse. "A Warm South Pole? Yes, On Neptune!", ESO, September 18, 2007. Retrieved on 2007-09-20. 

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.Villard, Ray; Devitt, Terry. "Brighter Neptune Suggests A Planetary Change Of Seasons", Hubble News Center, May 15, 2003. Retrieved on 2008-02-26. 

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,Lavoie, Sue (February 16, 2000). PIA02245: Neptune\'s blue-green atmosphere. NASA JPL. Retrieved on 2008-02-28. 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.Hammel, H. B.; Lockwood, G. W.; Mills, J. R.; Barnet, C. D. (1995). "Hubble Space Telescope Imaging of Neptune\'s Cloud Structure in 1994". Science 268 (5218): 1740–1742. doi:10.1126/science.268.5218.1740. Retrieved on 2008-02-25. 

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. 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.Lavoie, Sue (January 29, 1996). PIA00064: Neptune\'s Dark Spot (D2) at High Resolution. NASA JPL. Retrieved on 2008-02-28.

Neptune\'s dark spots are thought to occur in the troposphere at lower altitudes than the brighter cloud features,S. G., Gibbard; de Pater, I.; Roe, H. G.; Martin, S.; Macintosh, B. A.; Max, C. E. (2003). "The altitude of Neptune cloud features from high-spatial-resolution near-infrared spectra" (PDF). Icarus 166 (2): 359–374. doi:10.1016/j.icarus.2003.07.006. Retrieved on 2008-02-26.  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. Often associated with dark spots are brighter, persistent methane clouds that form around the tropopause layer.Stratman, P. W.; Showman, A. P.; Dowling, T. E.; Sromovsky, L. A. (2001). "EPIC Simulations of Bright Companions to Neptune\'s Great Dark Spots" (PDF). Icarus 151 (2): 275–285. doi:10.1006/icar.1998.5918. Retrieved on 2008-02-26.  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.Sromovsky, L. A.; Fry, P. M.; Dowling, T. E.; Baines, K. H. (2000). "The unusual dynamics of new dark spots on Neptune". Bulletin of the American Astronomical Society 32: 1005. Retrieved on 2008-02-29. 

Internal heat

Neptune\'s more varied weather when compared to Uranus is believed to be due in part to its higher internal heat.Williams, Sam (2004). Heat Sources within the Giant Planets. University of California, Berkeley. Retrieved on 2008-03-10. Although Neptune lies half again as far from the Sun as Uranus, and receives only 40% its amount of sunlight, the two planets\' surface temperatures are roughly equal. The upper regions of Neptune\'s troposphere reach a low temperature of −221.4 °C (51.8 K). At a depth where the atmospheric pressure equals 1 bar, the temperature is −201.15 °C (72.0 K).Lindal, Gunnar F. (1992). "The atmosphere of Neptune - an analysis of radio occultation data acquired with Voyager 2". Astronomical Journal 103: 967–982. Retrieved on 2008-02-25.  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;Class 12 - Giant Planets - Heat and Formation. Laboratory of Atmospheric and Space Physics, Colorado University, Boulder. Retrieved on 2008-03-13.{r_{per}} = \frac{9.655 \times 10^6 \text{km}}{1.372 \times 10^6 \text{km}} = 7.037.\end{smallmatrix}

Neptune\'s moon Proteus.

From July to September 1989, Voyager 2 discovered six new Neptunian moons.Stone, E. C.; Miner, E. D. (1989). "The Voyager 2 Encounter with the Neptunian System". Science 246 (4936): 1417–1421. AAAS (USA). Retrieved on 2008-02-29.  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.Brown, Michael E.. The Dwarf Planets. California Institute of Technology, Department of Geological Sciences. Retrieved on