DPS 34th Meeting, October 2002
Session 23. Other (than Io and Europa) Planetary Satellites
Oral, Chair(s): C.A. Hibbitts and W.M. Grundy, Wednesday, October 9, 2002, 9:20-10:30 and 11:00-11:20am, Room M

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[23.09] Sizes and Shapes of Neptune's Inner Satellites

E. Karkoschka (Univ. Arizona)

I measured resolved images of the inner Neptunian satellites by Voyager 2. The best-fitting tri-axial radii are 48x30x26 km for Naiad, 54x50x26 km for Thalassa, 90x74x64 km for Despina, 102x92x72 km for Galatea, and 108x102x84 km for Larissa. These sizes are within uncertainty limits by Thomas and Veverka (1991) who provided a shape for Larissa (104x89 km), radii with assumed spherical shapes for Despina (74 km) and Galatea (79 km) and estimated radii based on assumed albedos for Naiad (29 km) and Thalassa (40 km). The uncertainties of the new radii are smaller. Estimates of volumes and masses of Naiad and Galatea need to be increased by some 50 percent, which is interesting since Galatea's gravity is considered to cause the unique arcs of Neptune's Adams ring.

The moderately elongated shapes of the medium-sized satellites Despina and Galatea, and the strongly elongated shapes of the small satellites Naiad and Thalassa are typical for bodies of their size, although the shape of Thalassa is almost oblate (like a lens) while the shapes of other, strongly elongated satellites such as Naiad are closer to prolate (like a cigar).

While previous uncertainties allowed the same reflectivity for the inner six Neptunian satellites, this is no longer true. There is a trend of albedos increasing with distance from Neptune, similar to the trend observed for the Uranian satellites. By estimating phase curves, I predict brighter albedos for inner six Neptunian satellites (0.07-0.10) than for the inner 10 Uranian satellites (0.05-0.07), opposite to previous estimates, which could be tested using recent images by the Hubble Space Telescope and ground-based observatories.

The measured shapes of the inner Neptunian satellites cause orbital light curves, even if their surfaces lack spatial albedo variations. Indeed, photometry by Thomas and Veverka (1991) reveals amplitudes and phases of the light curves consistent with those inferred from the measured shapes, although most data are quite noisy.

This work was supported by NASA grant NAG5-12014.

Reference: Thomas, P. and J. Veverka 1991. Neptune's small inner satellites. J. Geophys. Res. 96, suppl., 19,261-19,268.

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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.