Astronomy Picture of the Day . ... 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 . ... The above picture of Phobos near the limb of Mars was captured last month by the robot spacecraft Mars Express currently orbiting Mars. Phobos is a heavily cratered and barren moon, with its largest crater located on the far side. ... Intrepid Crater on Mars . ...