292 SOLAR SYSTEM/Neptune, Pluto and Uranus hydrogen sulphide, above which are layers of hydrocarbons, with a methane layer and an upper methane haze Above the hydrogen sulphide layer there are discrete clouds with diameters up to 100 km, casting shadows on the cloud deck 50 to 75 km below These clouds may be described as ‘methane cirrus’ Temperature measurements from Voyager show that there is a cold mid-latitude region, with a warmer equator and pole (we know little about the north pole, which was in darkness during the Voyager encounter) There are strong winds; most of them blow in a westerly direction (that is to say, opposite to the planet’s rotation), and are distinctively zonal At the equator they blow westward at up to 450 m s Further south they slacken, and beyond latitude À50 they become eastward (prograde) up to 300 m s 1, decreasing once more near the south pole There is, in fact, a broad equatorial retrograde jet between latitudes ỵ45 and 50 , with a relatively narrow prograde jet at around latitude À70 At the time of the Voyager encounter the most conspicuous feature on the disk was the Great Dark Spot, a huge oval with a longer axis of 10 000 km, drifting westward relative to the adjacent clouds, it was a high-pressure area, rotating counter-clockwise and showing all the signs of an atmospheric vortex Hanging above it were methane cirrus clouds, and Table Rings of Neptune Galle Le Verrier Lassell Arago Adams Distance from centre of Neptune, km Width, km 41 900 53 200 53 200 57 200 61 950 62 933 2000 110 4000 100 (indistinct) 50 between these and the main cloud deck there was a clear region 50 km deep Other, smaller spots were seen at different latitudes, and the whole disk was extremely active Later images obtained with the Hubble Space Telescope show that the Great Dark Spot has disappeared, so that the surface shows marked changes over relatively short periods Neptune has an obscure ring system (Table 4) The outer ring, named after Adams, is ‘clumpy’, with three brighter arcs, while the Lassell ring is a diffuse band of material containing a high percentage of very small particles There may be ‘dust’ extending from the inner Galle ring almost down to the cloud-tops Eleven satellites are known (Table 5), but of these only two, Triton and Nereid, were discovered before the Voyager fly-by Triton is one of the most remarkable bodies in the entire Solar System It was found by the English astronomer W Lassell a few weeks after the discovery of Neptune itself, and is brighter than any of the satellites of Uranus; it is also more reflective, with an albedo in places of 0.8, and it is the coldest world ever encountered by a space-craft – the temperature is À235 C, a mere 18 above absolute zero The globe seems to be made up of a mixture of rock and ice There is an extensive though very tenuous atmosphere, made up almost entirely of nitrogen with a trace of methane Triton has retrograde motion, and there seems no doubt that it is a captured body rather than a bona fide satellite The surface is very varied There is a general coating of ice, presumably water ice overlain by nitrogen ice; there is little surface relief, and there are few craters The area surveyed by Voyager was divided into three parts: polar (Uhlanga Regio), eastern equatorial (Monad) and western equatorial (Bubembe Regio) The polar area is covered with a pink cap of nitrogen snow and ice, and there are geysers, completely unexpected before the Voyager mission Apparently there is a sub-surface layer of Table Satellites of Neptune Naiad Thalassa Despina Galatea Larissa Proteus Triton Nereid S/2002 N2 S/2002 N3 S/2002 N1 Mean distance from Neptune, km Orbital period, d Orbital eccentricity Orbital inclination Diameter, km Magnitude 48 227 50 075 52 526 61 953 71 548 17 647 33 76 5513 20 200 21 390 21 990 0.29 0.31 0.33 0.43 0.55 4.12 5.87 360.14 2525 2751 2868 0.0003 0.0002 0.0001 0.0001 0.0014 0.0004 0.0000016 0.7512 0.17 0.47 0.43 4.74 0.21 0.07 0.05 0.20 0.04 157.34 7.23 57 43 121 58 80 148 158 208 436 2705 340 40 40 40 25 24 23 23 21 20 13.6 18.7 25 25 25