Phobos, Mars' Large and Incredibly Close Moon

Phobos, the larger of Mars' two moons, is a fascinating place. The moon, discovered back in 1877, is small -- only about 17 miles long -- and orbits incredibly close to the surface of its host planet. Phobos, in fact, is so close to Mars that it moves around the planet faster than the planet itself rotates. If you were standing on the surface of Mars, you would see Phobos rise in the west and move across the sky in only a little more than four hours before disappearing to the east. And it's speeding up. As the orbital radius decreases, Phobos moves closer to the planet's surface and will one day either smash into Mars or break up into a planetary ring.
"[Its] origins are still something of a mystery, and the surface featured on Phobos are not totally understood either," Phil Plait wrote at Discover last week after these new photographs were taken. "Specifically, all those parallel grooves are pretty weird! The current thinking is that they were actually caused by impacts on Mars! It works like this: some giant rock hits Mars and blasts vast quantities of material up and out, some of which reaches up into space. Phobos plows into this material, and the direct impacts with big chunks can form craters."
These new images were taken by the High Resolution Stereo Camera (HRSC) aboard the European Space Agency's Mars Express probe after it passed within 66 miles of Mars' surface.