Martian Moon Phobos from Mars Express . ... Mars Express , DLR , ESA ; Acknowledgement: Peter Masek Explanation: 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. ...