Galatea (moon)
Fourth-closest inner moon of Neptune, shepherd of the Adams ring.
Galatea (Neptune VI) is Neptune's fourth-closest inner moon and its fifth-largest. Its name comes from a Nereid in Greek myth who was the object of the Cyclops Polyphemus's unrequited love.
The moon was discovered in late July 1989 from Voyager 2 images, announced on 2 August 1989 (IAUC 4824) under the temporary designation S/1989 N 4. The announcement noted ten frames taken over five days, suggesting a discovery date before July 28. It received its permanent name on 16 September 1991.
Galatea has an irregular shape with no signs of geological activity. It is probably a rubble pile formed from fragments of Neptune's original satellites, shattered by gravitational disturbances from Triton shortly after that moon was captured into a highly eccentric orbit. Its composition resembles other small inner Neptunian moons, with a strong 3.0-micron feature indicating water ice or hydrated silicate minerals. The surface shows hydrated materials—a deep 3-μm OH band and phyllosilicates—but no clear water ice signatures, suggesting a composition like carbonaceous chondrites or main-belt asteroids such as Ceres. The phyllosilicates imply the material was altered by prolonged contact with water at temperatures between 300 and 400 K, and never heated above 700 K, which would have dehydrated it. Its surface albedo is 0.08 at 1.4 and 2.0 microns, drops to 0.04 at 3.0 microns, and rises to 0.12 at 4.6 microns.
Galatea's orbit lies inside Neptune's synchronous orbit radius, so tidal forces are slowly pulling it inward. It may eventually crash into Neptune or break apart into a new ring system after crossing the Roche limit. The moon acts as a shepherd for the Adams ring, located about 1,000 kilometers outside its orbit. A 42:43 orbital resonance with Galatea is thought to confine the ring's unique arcs. Its mass is estimated at (2.12±0.08)×10¹⁸ kg, based on the radial disturbances it causes in the ring.
- Designation
- Neptune VI
- Discovery date
- late July 1989
- Discovery probe
- Voyager 2
- Temporary designation
- S/1989 N 4
- Announcement date
- 2 August 1989
- Name given date
- 16 September 1991
- Mass
- (2.12±0.08)×10^18 kg
- Surface albedo at 1.4 and 2.0 microns
- 0.08
- Surface albedo at 3.0 microns
- 0.04
- Surface albedo at 4.6 microns
- 0.12
Lore & Background
Galatea was discovered in late July 1989 from images taken by the Voyager 2 probe, with the temporary designation S/1989 N 4. The discovery was announced on 2 August 1989, and the name was given on 16 September 1991. The moon is irregularly shaped and shows no sign of any geological modification, likely a rubble pile re-accreted from fragments of Neptune's original satellites smashed up by perturbations from Triton soon after that moon's capture into a very eccentric initial orbit. Compositionally, Galatea appears similar to other small inner Neptunian satellites, with a deep 3.0 micron feature attributed to water ice or hydrated silicate minerals. Its surface contains hydrated materials, as evidenced by a deep 3-μm OH band and phyllosilicates, and lacks clear signatures of water ice, suggesting its composition may resemble carbonaceous chondrites and main belt asteroids such as Ceres. The presence of phyllosilicates indicates that Galatea accreted materials altered by prolonged exposure to water at temperatures between less than 300 to 400 K, and the bulk of this material has never been exposed to temperatures greater than 700 K. Its surface albedo is 0.08 at 1.4 and 2.0 microns, dropping to 0.04 at 3.0 microns, and increasing to 0.12 at 4.6 microns.
Reader's Guide
Galatea's significance lies in its role as a shepherd moon for the Adams ring, located 1,000 kilometers outside its orbit. Resonances with Galatea in the ratio 42:43 are considered the most likely mechanism for confining the unique ring arcs that exist in this ring. Its mass has been estimated at (2.12±0.08)×10^18 kg based on the radial perturbations it induces on the ring. Additionally, 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. The moon's composition, resembling carbonaceous chondrites and Ceres, provides insight into the history of Neptune's satellite system, particularly the disruption caused by Triton's capture. Its irregular shape and lack of geological modification suggest it is a primordial rubble pile, offering clues about the violent early dynamics of the Neptunian system.
Did You Know?
- It is named after Galatea, one of the fifty Nereids of Greek legend, with whom Cyclops Polyphemus was vainly in love.
- Its surface albedo varies from 0.08 at 1.4 and 2.0 microns to 0.04 at 3.0 microns and 0.12 at 4.6 microns.
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