SOLAR SYSTEM/Neptune, Pluto and Uranus 293 liquid nitrogen If any of this migrates upward, the pressure is relaxed and the nitrogen explodes in a shower of ice and vapour, travelling quickly up the nozzle of the geyser-like vent – fast enough to make it rise to several kilometres before falling back; the outrush sweeps dark de´ bris along it, blown by winds in the tenuous atmosphere The edge of the cap is well-defined, and north of it there is a darker, redder region Monad Regio is part smooth, part hummocky, with rimless pits (paterae), mushroom-like features (guttae) and low-walled plains; Bubembe Regio is characterized by the so-called cantaloupe terrain – a nickname given to it because of its superficial resemblance to a melon-skin! Fissures cross it, meeting at elevated X or Y junctions; this is probably the oldest part of Triton’s surface Of course, we have no information about the hemisphere which was in darkness at the time of the Voyager encounter Nereid, the other satellite known before the Voyager mission, was discovered by G Kuiper in 1949 It is small, only 340 km in diameter, and has a very eccentric orbit, so that its distance from Neptune ranges between 1.35 million km and 9.62 million km The orbital period is just over 360 days, but it is unlikely that its rotation period is synchronous; as with Hyperion in Saturn’s system, the rotation period may be chaotic It has direct motion, and although not well imaged from Voyager 2, it seems to be fairly regular in shape; there were vague indications of a few large craters Of the other satellites, Proteus is actually larger than Nereid, but its closeness to Neptune means that it is not observable with Earthbased telescopes The rotation is synchronous, and the albedo is low; it has been said that Proteus is ‘as dark as soot’ The area imaged by Voyager is dominated by a circular depression, Pharos, 225 km in diameter and up to 15 km deep The remaining inner satellites are small, icy, and presumably cratered; Voyager sent back one image of Larissa The orbit of the 158 km satellite Galatea is very close to the Adams Ring Three outer asteroidal satellites were discovered in 2001 Pluto Pluto, discovered by Clyde Tombaugh in 1930, is an enigma; it is smaller than the Moon or even Triton, and has an orbit which is both eccentric and inclined It has one satellite, Charon Data for Pluto and Charon are given in Table The calculations leading to the discovery of Pluto were made from 1905 by Percival Lowell, and were based upon perturbations of Neptune and (particularly) Uranus Searches carried out with the Lowell refractor at Flagstaff in Arizona were unsuccessful, Table Pluto and Charon Pluto Distance from Sun, km Orbital period, days Rotation periods Mean orbital velocity Axial inclination, degrees Orbital inclination, degrees Orbital eccentricity Diameter, km Density, water Volume, Earth Mass, Earth Max surface temperature, C Escape velocity, km s Surface gravity, Earth Albedo Max 381 200 000 Mean 906 400 000 Min 445 800 000 90 465 (247.7 years) d h 17 m 4.75 km s 122.46 Charon d h 17 m (round Pluto) d h 17 m 0.23 17.14 (to Pluto) c 0.2488 324 2.05 0.006 0.0022 about 233 0.0076 1270 1.18 0.06 0.55 0.58 0.21 0.36 but the planet was finally identified at Flagstaff by Tombaugh, not too far from the position predicted by Lowell After some discussion it was named Pluto, after the God of the Underworld When near perihelion, it is closer-in than Neptune, as was the case between 1979 and 1999, but there is no danger of collision, because Pluto’s orbit is inclined at an angle of 17 After the discovery it was found that Pluto had been photographed at Flagstaff in 1915 and Mount Wilson in 1919, but had been missed because it was so much fainter than had been expected A telescope of fair size is needed to show it at all; Voyager went nowhere near it, but some surface features have been recorded by the Hubble Space Telescope (Figure 2) There are indications of a dark equatorial band and brighter polar areas The axial inclination is 122 , so that, as with Uranus, the rotation is technically retrograde; the rotation period is 6.34 days Pluto’s density, twice that of water, indicates that the globe contains more rock than in the icy satellites of the giant planets, but our knowledge of the internal structure is decidedly meagre Pluto may or may not be differentiated, but the gravitational pressure may not be adequate to increase the rock density deep inside the globe to a marked degree The surface is coated with methane ice at least in some areas There is a tenuous but surprisingly extensive atmosphere, made up chiefly of nitrogen together with methane As Pluto moves out toward aphelion, due in the year 2114, the temperature will drop and the atmosphere may freeze out, so that for part of its long ‘year’ there may be no atmosphere at all