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Mars' Moon Phobos
The High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter took two images of the larger of Mars' two moons, Phobos, within 10 minutes of each other on March 23, 2008.
Phobos is the larger of the two heavily-cratered Martian moons and is dominated by three large craters. The largest of Phobos' craters, Stickney, was named after the wife of Asaph Hall, the astronomer who discovered the moons of Mars. Stickney crater is 10 km in diameter, which is almost half of the average diameter of Phobos! The crater is so large relative to the size of Phobos that the satellite probably came close to breaking up. Radiating away from Stickney are sets of parallel grooves or striations. These fractures undoubtably formed as a result of the impact that produced Stickney.
Another interesting feature about Phobos is the duration of its orbit. Phobos revolves around Mars at an astounding rate. In fact, it revolves around Mars 3 times during one Martian day! As a result, Phobos appears to rise in the west, and set in the east!