Thalassa (moon)

Thalassa /θəˈlæsə/, also known as Neptune IV, is the second-innermost satellite of Neptune. Thalassa was named after sea goddess Thalassa, a daughter of Aether and Hemera from Greek mythology. "Thalassa" is also the Greek word for "sea".

Thalassa

Thalassa (1989 N5) seen by Voyager 2 on 23 August 1989
Discovery
Discovered by	Richard J. Terrile[1] and Voyager Imaging Team
Discovery date	September 1989
Designations
Designation	Neptune IV
Pronunciation	/θəˈlæsə/[2]
Named after	Θάλασσα Thalassa
Adjectives	Thalassian /θəˈlæsiən/[3]
Orbital characteristics[4][5]
Epoch 18 August 1989
Semi-major axis	50074.44 km
Eccentricity	0.00176±0.00054
Orbital period (sidereal)	0.31148444±0.00000006 d
Inclination	0.21 ± 0.02° (to Neptune equator) 0.21° (to local Laplace plane)
Satellite of	Neptune
Physical characteristics
Dimensions	(108±6) × (100±12) × (52±6) km[6]
Mean radius	41±3 km[6]
Mass	~3.54×1017 kg (calculated)
Mean density	1.23±0.43 g/cm3[7]
Synodic rotation period	synchronous
Axial tilt	zero
Albedo	0.091[6][8]
Temperature	~51 K mean (estimate)
Apparent magnitude	23.32[8]

Discovery

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.^[9] The discovery was announced (IAUC 4867) on 29 September 1989, and mentions "25 frames taken over 11 days", implying a discovery date of sometime before 18 September. The name was given on 16 September 1991.^[10]

Physical properties

Thalassa is irregularly shaped. 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.^[11] Unusually for irregular bodies, it appears to be roughly disk-shaped.

Orbit

Depiction of Naiad's orbital motion (red) in a view that co-rotates with Thalassa (central yellow dot)

Since the Thalassian orbit is below Neptune's synchronous orbit radius, it is slowly spiralling inward due to tidal deceleration 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.

Thalassa is currently in a 69:73 orbital resonance with the innermost moon, Naiad, in a "dance of avoidance". As it orbits Neptune, the more inclined Naiad successively passes Thalassa twice from above and then twice from below, in a cycle that repeats every ~21.5 Earth days. The two moons are about 3540 km apart when they pass each other. Although their orbital radii differ by only 1850 km, Naiad swings ~2800 km above or below Thalassa's orbital plane at closest approach. Thus this resonance, like many such orbital correlations, stabilizes the orbits by maximizing separation at conjunction. However, the role of Naiad's nearly 5° orbital inclination in this avoidance in a situation where eccentricities are minimal is unusual.^[12][7]