Proteus is one of the darkest objects in the solar system. Like Saturn's moon Phoebe, it reflects only 6 percent of the sunlight that hits it. Proteus is very cratered, showing no sign of any geological changes. It is also not a sphere; scientists believe Proteus is about as big as a body of its density can be without being pulled into a spherical shape by its own gravity. Saturn's moon Mimas has much more spherical shape despite being less massive than Proteus.