Phobos (pronounced FOH-buhs) is one of two moons of the planet Mars. Phobos is the larger innermost moon, and is thought to be a rubble pile held together by a dark thin crust. In 30 to 50 million years, it will either collide with Mars or break apart and become a ring system. Both Phobos and Deimos may be asteroids captured by Mars in the distant past.
Pan, zoom, or go full-screen with this megapixel image of Phobos.
There are a number of notable surface features on Phobos, including the infamous Phobos monolith.