Phobos. Mars Global Surveyor (MGS) image of the surface of the Martian moon Phobos. The large crater Stickney is seen at upper left. It is 10 kilometres wide. The grooves extending from the rim of the crater towards centre are thought to be fractures in the moon caused by the impact which made Stickney. Phobos is the larger of the two Martian moons. It is irregular in shape, measuring 19x21x27 kilometres. It orbits Mars in just over 7.6 hours at an altitude of 9400 kilometres. Image taken by the Mars Orbiter Camera on the MGS on 19th August 1998.