Thalassa (moon)

Thalassa

Naiad or Thalassa as seen by Voyager 2 (smearing has caused excessive elongation)
Discovery
Discovered by	Richard J. Terrile[1] and Voyager Imaging Team
Discovery date	September 1989
Orbital characteristics[2]
Epoch 18 August 1989
Semi-major axis	50 075 ± 1 km
Eccentricity	0.0002 ± 0.0002
Orbital period	0.31148444 ± 0.00000006 d
Inclination	0.21 ± 0.02° (to Neptune equator) 0.21° (to local Laplace plane)
Satellite of	Neptune
Proper orbital elements
Physical characteristics
Dimensions	108×100×52 km[3][4]
Mean radius	41 ± 3 km[5]
Volume	~2.9×105km³
Mass	~3.5×1017 kg (based on assumed density)
Mean density	~1.2 g/cm³ (estimate)[5]
Rotation period	synchronous
Axial tilt	zero
Albedo	0.09[3][5]
Temperature	~51 K mean (estimate)
Apparent magnitude	23.3[5]

A simulated view of Thalassa orbiting Neptune.

Thalassa ( /θəˈlæsə/ thə-lass-ə; Greek: Θάλασσα), also known as Neptune IV, is the second innermost satellite of Neptune. Thalassa was named after sea goddesss Thalassa, a daughter of Aether and Hemera from Greek mythology. "Thalassa" is also the Greek word for "sea".

A Voyager 2 image of Thalassa (1989 N5), Naiad (1989 N6) and Despina (1989 N3)

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.^[6] 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. The name was given on 16 September 1991.^[7]

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.^[8] 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 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.

References

1. ^ Planet Neptune Data http://www.princeton.edu/~willman/planetary_systems/Sol/Neptune/
2. ^ Jacobson, R. A.; Owen, W. M., Jr. (2004). "The orbits of the inner Neptunian satellites from Voyager, Earthbased, and Hubble Space Telescope observations". Astronomical Journal 128 (3): 1412–1417. Bibcode 2004AJ....128.1412J. doi:10.1086/423037.  edit
3. ^ ^{a b} Karkoschka, Erich (2003). "Sizes, shapes, and albedos of the inner satellites of Neptune". Icarus 162 (2): 400–407. Bibcode 2003Icar..162..400K. doi:10.1016/S0019-1035(03)00002-2.  edit
4. ^ Williams, Dr. David R. (2008-01-22). "Neptunian Satellite Fact Sheet". NASA (National Space Science Data Center). http://nssdc.gsfc.nasa.gov/planetary/factsheet/neptuniansatfact.html. Retrieved 2008-12-13. 
5. ^ ^{a b c d} "Planetary Satellite Physical Parameters". JPL (Solar System Dynamics). 2010-10-18. http://ssd.jpl.nasa.gov/?sat_phys_par. Retrieved 2011-10-11. 
6. ^ Green, Daniel W. E. (September 29, 1989). "Neptune". IAU Circular 4867. http://www.cbat.eps.harvard.edu/iauc/04800/04867.html. Retrieved 2011-10-26. 
7. ^ Marsden, Brian G. (September 16, 1991). "Satellites of Saturn and Neptune". IAU Circular 5347. http://www.cbat.eps.harvard.edu/iauc/05300/05347.html. Retrieved 2011-10-26. 
8. ^ Banfield, Don; Murray, Norm (October 1992). "A dynamical history of the inner Neptunian satellites". Icarus 99 (2): 390–401. Bibcode 1992Icar...99..390B. doi:10.1016/0019-1035(92)90155-Z.  edit

External links

• Thalassa Profile by NASA's Solar System Exploration
• Neptune's Known Satellites (by Scott S. Sheppard)