Why is Phobos so dark? Phobos , the largest and innermost of two Martian moons, is the darkest moon in the entire Solar System . Its unusual orbit and color indicate that it may be a captured asteroid composed of a mixture of ice and dark rock. The above picture was captured recently by the robot spacecraft Mars Global Surveyor (MGS) orbiting Mars. Phobos is a heavily cratered and barren moon, with its largest crater located on the far side. From MGS images like this, Phobos has been determined to be covered by perhaps a meter of loose dust . Phobos orbits so close to Mars that from some places it would appear to rise and set twice a day, but from other places it would not be visible at all. Phobos ' orbit around Mars is continually decaying -- it will likely break up with pieces crashing to the Martian surface in about 50 million years.
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Credit: Malin Space Science Systems , MGS , JPL , NASA