Proteus (moon)

Proteus ( PROH-tee-əs), also known as Neptune VIII, is the second-largest moon of Neptune and the planet's largest inner satellite. Discovered by Voyager 2 in 1989, it is named after Proteus, the shape-shifting sea god in Greek mythology. Proteus is about 400 km (250 mi) in diameter and orbits Neptune in a nearly equatorial orbit at a distance of about 4.75 Neptune radii from the planet's center. Proteus has a highly irregular shape that differs significantly from an ellipsoid. It resembles an irregular polyhedron with several slightly concave facets and surface relief of up to 20 km (12 mi). Its surface is dark, nearly neutral in color, and heavily cratered. The largest crater, Pharos, is more than 230 km (140 mi) across. The surface also contains numerous cliffs, grooves, and valleys associated with large impact craters. Proteus is unlikely to have formed alongside Neptune. Instead, it probably accreted from debris produced after the capture of Neptune's largest moon, Triton.