Proteus is one of Neptune's moonss. It is one of the darkest objects in the solar system, as dark as soot; like Saturn's moon Phoebe, it reflects only 6 percent of the sunlight that strikes it. Proteus is about 400 kilometers in diameter, larger than Nereid, but it wasn't discovered from Earth because it is so close to Neptune that it is lost in the glare of reflected sunlight. Proteus is irregularly shaped and shows no sign of any geological modification; scientists believe Proteus is about as large as a moon can be without being pulled into a spherical shape by its own gravity. It is heavily cratered.