Thalassa (moon)

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Thalassa

Naiad or Thalassa as seen by Voyager 2 (smearing has caused excessive elongation)
Discovery
Discovered by	Voyager Imaging Team
Discovered in	September 1989
Orbital characteristics
Semi-major axis	50 075 ± 1 km
Eccentricity	0.0002 ± 0.0002
Orbital period	0.31148444 ± 0.00000006 d
Inclination	0.72 ± 0.02° (to Neptune equator) 0.21° (to local Laplace plane)
Is a satellite of	Neptune
Physical characteristics
Dimensions
Mass	(based on assumed density)
Mean density	~1.2 g/cm3 (estimate)
Rotation period	assumed synchronous
Axial tilt	~zero presumably
Albedo (geometric)	0.09
Surface temp.	~51 K mean (estimate)
Atmosphere	none

A simulated view of Thalassa orbiting Neptune.

Thalassa (thə-las'-ə, IPA /θəˈlæsə/, Greek Θάλασσα), or Neptune IV, is the second moon of Neptune. Thalassa was named after a daughter of Aether and Hemera from Greek mythology. "Thalassa" is also the Greek word for "sea".

Thalassa was discovered sometime before mid-September, 1989 from the images taken by the Voyager 2 probe. It was given the temporary designation S/1989 N 5 [IAUC 4867] discovery IAUC circular. The discovery was announced (IAUC 4867) on September 29, 1989, but the text only talks of "25 frames taken over 11 days", giving a discovery date of sometime before September 18.

Thalassa 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 . Unusually for irregular bodies, it appears to be roughly disk-shaped.

Since the Thalassian orbit is below Neptune's synchronous orbit radius, it is slowly spiralling inward due to tidal decceleration and may eventually impact Neptune's atmosphere, or break up into a planetary ring upon passing its Roche limit due to tidal stretching. Relatively soon after, the spreading debris may impinge upon Despina's orbit.

References