![]() Nereid was discovered in 1949 by Gerard P. Only Triton and the outer satellite Nereid were known before the reconnaissance of Neptune by the Voyager 2 spacecraft in 1989. Triton has an appreciable atmosphere, seasons, and currently active geologic processing. Five more moons were discovered in 2003, including one with a period of 26.3 years, which corresponds to a distance of 47 million km from Neptune. At the time of the Voyager 2 encounter, Nereid, which moves in an eccentric prograde orbit bringing it from 57 to 385 planetary radii from Neptune, was the only known satellite in the latter category. The small satellites can be divided into two categories: the 6 inner bodies, which move in highly regular, circular orbits close to Neptune (<5 planetary radii), and the irregular outer satellites. Neptune has 13 known satellites: one is the large moon Triton and the remaining 12 are small, irregularly shaped bodies (see Table 1). , in Encyclopedia of the Solar System (Second Edition), 2007 4.5.1 INTRODUCTION The North Pole was not observed by Voyager, so it is unknown whether that hemisphere displays aurora.īonnie J. Voyager 2 UVS measurements tentatively identified weak auroral emissions at Neptune's South Pole, interpreted as H 2 emissions. HST measurements also detected weak carbon monoxide lines at 19 Å, suggesting a mixing ratio of ∼3 ± 2 × 10 −6 in the upper troposphere. The HST images showed that the Great Dark Spot seen by Voyager no longer existed, but a new large dark feature of comparable size had appeared in the northern latitudes. The goal of these observations was to study the cloud structure on Neptune and compare the measurements with the observations made by Voyager 2. In 1994, HST imaged Neptune in six broadband filters, one of which was in the ultraviolet. CH 4 and C 2H 6 abundances inferred from the Voyager UVS solar occultation experiment are between 0.0006 and 0.005 mole fraction for CH 4 in the lower stratosphere (with a mixing ratio of 5–100 × 10 −5) and a density of C 2H 6 estimated to reach 3 × 10 9 cm −3. Most of the important data for Neptune at ultraviolet wavelengths have come from the UVS onboard the Voyager 2 spacecraft and from HST. Below 1500 Å, Neptune's albedo is reduced by the higher hydrocarbon abundance carried into its stratosphere by its more vigorous vertical transport. The geometric albedo of Neptune measured by IUE is 0.5, which, like that of Uranus, is twice that of Jupiter and Saturn. Neptune is so distant that only broadband ultraviolet measurements are possible from Earth orbit. Domingue, in Encyclopedia of the Solar System (Second Edition), 2007 3.7. ![]()
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