Mars has two small moons, the larger of which is Phobos. Like most small bodies in the solar system, Phobos is quite irregular in shape, with a long dimension of 27 kilometers and a short dimension of 19 kilometers. The large crater in this image is Stickney, with a diameter of 10 kilometers. There is a moonwide system of grooves on Phobos, some of which can be seen on this slide, that was evidently produced by the impact that produced Stickney. Phobos and Deimos may be captured asteroids. Their very low surface reflectivity, low density, and spectral reflectance properties all are consistent with a composition similar to a class of meteorites known as carbonaceous chondrites.
Mosaic of Viking Orbiter images 854A81, 854A82, and 854A83. Image processing by Brian Fessler, Lunar and Planetary Institute