海卫六1989

概述

由航海家2号于1989年7月底所观测到，并于同年的8月2日公开成果；并给予S/1989 N 4的暂时编号，但公开内容仅有5天内所拍摄到的10张照片。

表面特征

海卫六是从海王星内侧算来第四颗卫星，表面十分的凹凸不平；但是根据观察结果显示，似乎并不是因为地质运动所造成的。可能的造成原因是因为海王星捕获海卫一时，海卫一初期轨道的干扰；使卫星的碎片再度结合在一起，而形成海卫六的。

由于海卫六十分靠近海王星，受潮汐力的影响；持续靠近海王星，未来有可能因为小于洛希极限而坠毁于海王星大气或是形成一个环。

Galatea, also known as Neptune VI, is the fourth-closest inner moon of Neptune, and fifth-largest moon of Neptune. It is named after Galatea, one of the fifty Nereids of Greek legend, with whom Cyclops Polyphemus was vainly in love.

Discovery

Galatea was discovered in late July 1989 from the images taken by the Voyager 2 probe. It was given the temporary designation S/1989 N 4. The discovery was announced (IAUC 4824) on 2 August 1989, and mentions "10 frames taken over 5 days", implying a discovery date of sometime before July 28. The name was given on 16 September 1991.

Physical properties

Galatea is irregularly shaped and shows no sign of any geological modification. It is likely that it is a rubble pile re-accreted from fragments of Neptune's original satellites, which were smashed up by perturbations from Triton soon after that moon's capture into a very eccentric initial orbit.

Compositionally, Galatea appears to be similar to other small inner Neptunian satellites, with a deep 3.0 micron feature attributed to water ice or hydrated silicate minerals.

Galatea's surface contains hydrated materials, as evidenced by a deep 3-μm OH band, phyllosilicates, and lacks clear signatures of water ice, suggesting that its composition may resemble carbonaceous chondrites and main belt asteroids such as Ceres. The presence of phyllosilicates indicate that Galatea accreted materials which was altered by prolonged exposure to water at temperatures between less than 300 to 400 K, and the bulk of this material have never been exposed to temperatures greater than 700 K, which would have dehydrated it. Its surface albedo is 0.08 albedo at 1.4 and 2.0 microns, dropping to 0.04 at 3.0 microns, and increasing to 0.12 at 4.6 microns.

Orbit

Galatea's orbit lies below Neptune's synchronous orbit radius, so it is slowly spiralling inward due to tidal deceleration and may eventually impact the planet or break up into a new planetary ring system upon passing its Roche limit due to tidal stretching.

Galatea appears to be a shepherd moon for the Adams ring that is 1,000 kilometres (620 mi) outside its orbit. Resonances with Galatea in the ratio 42:43 are also considered the most likely mechanism for confining the unique ring arcs that exist in this ring. Galatea's mass has been estimated at (2.12±0.08)×1018 kg based on the radial perturbations it induces on the ring.