The puzzling moons of Mars

Viking Orbiter observations of Phobos and Deimos are presented with attention to physical dimension and surface features. Both satellites are approximately 1.4 times as long as they are wide; Phobos having a length of 27 km, while Deimos is half that size. In addition, both satellites are tidally locked, as is earth's moon, and are held together by cohesive forces as well as by gravity. Phobos and Deimos are both heavily cratered, and Phobos has deep (30 m) grooves in the vicinity of its largest crater, Stickney; indicating surface fracturing under meteorite impact. The craters on Deimos are largely filled with a fine-grained substance, probably crater ejecta.