Phobos over Mars
Why is Phobos so dark? Phobos , the largest and innermost of the 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 featured assigned-color picture of Phobos near the edge of Mars was captured in late 2021 by ESA 's robot spacecraft Mars Express , currently orbiting Mars. Phobos is a heavily cratered and barren moon, with its largest crater located on the far side. From 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, while 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. Your Sky Surprise: What picture did APOD feature on your birthday? (post 1995)