Voyager 2 image of Proteus.

PROTEUS - MOON OF NEPTUNE

Proteus was a god of the sea who could change his shape at will. Proteus was discovered in 1989 in Voyager 2 images.

Orbit

Proteus orbits 117,600 kilometres from Neptune. Its orbit has a very low inclination with respect to Neptune's equatorial plane, only 0.55o. The orbit is circular (zero eccentricity). Proteus takes 1.122 days to complete one orbit about Neptune, the same time it takes to spin on its own axis.

Physical properties

Proteus, with an average diameter of 418 kilometres, is Neptune's second largest satellite. Though relatively large, its dark surface (albedo 0.06) and proximity to Neptune make Proteus a difficult object to see.

Its size is at the limit at which a rocky body can be, without being pulled into a sphere by its own gravity. It is oddly shaped, measuring 401 x 416 x 436 kilometres. Proteus is in fact the largest irregular satellite in the Solar System. Proteus' mass and density are unknown.

Interior

The interior and composition of Proteus is unknown.

Magnetic field

No magnetic field has been detected.

Atmosphere

No atmosphere has been detected.

Surface

The Voyager images of Proteus reveal a very rough looking surface. It appears cratered and pitted, and has what appears to be an impact basin 250 kilometres wide.The impact basin also appears to have an inner ring, and ridges can be made out on the rest of the surface. The low albedo of Proteus, and the appearance of its surface, suggest it is similar to a C-type asteroid. This does not mean though that it is a captured asteroid. One theory suggests that the capture of Triton so perturbed the orbits of Neptune's moon system, that the moons collided and fragmented. Proteus may have re-combined from those fragments - that is, Proteus could be a second generation moon.