Martian Moon Phobos from Mars Express

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 of Phobos near the limb of Mars was captured last month by the robot spacecraft Mars Express currently orbiting Mars.