A crater nine kilometers across dominates weird little Phobos, the larger of the two Martian moons. Brilliant streaks on the crater walls suggest that even the feeble gravity of this moon is strong enough to erode loose material; bluish regions near the crater rim may be younger exposed surfaces. This enhanced-color photo was taken by the Mars Reconnaissance Orbiter in 2008.

Some proponents of a manned mission to Mars suggest that a trip to Phobos could serve as a trial run to the Martian neighborhood. Because Phobos has no atmosphere and only weak gravity, they say, it would be easier for a spacecraft to land there than on Mars.