Neptune's small inner satellites

Abstract

Voyager 2 images revealed six small satellites orbiting within five planetary radii of Neptune. The largest, Proteus (1989N1), has a mean radius of 208 ± 8 km; the smallest, Naiad (1989N6), is 29 ± 6 km in radius. The four satellites for which reasonable radius measurements can be made all have geometric albedos (not accounting for opposition effects) of 0.06 at 0.48 pm. The albedos and phase coefficients of these satellites are very similar to those measured for the inner satellites of Uranus but are quite distinct from the values for Nereid (p= 0.16‐0.20, β = 0.028 magnitude/degree) as determined by Thomas et al. (this issue). Color measurements (0.4 to 0.6 μm) of Proteus show a neutral color. The two largest satellites are resolved as irregular objects and have limb roughnesses comparable to those of similar‐sized satellites of other planets. One image of Proteus shows a 150‐km crater with some overlapping smaller craters; this feature allows measurement of a rotation rate consistent with a synchronous period. The irregular shape and heavily cratered, photometrically bland surface of Proteus are consistent with characteristics of a primitive, undifferentiated object. Evidently, Proteus has escaped the tidal heating believed to have been responsible for the remarkable geologic processes on the similar‐sized bodies Miranda and Enceladus and perhaps Mimas.