Mars has two moons, Phobos and Deimos, which were discovered in 1877. Both are thought to be C-type asteroids (which resemble the class of "carbonaceous chondrite" asteroids) which were "captured" by Mars. Phobos is in an orbit 9,380 km from Mars and has a period of less than 8 hours (making the Martian month shorter than the Martian day!). It is about 26 km long. Deimos is in an orbit 23,460 km from Mars and has a period of about 30 hours. It is about 10 km long. Our first close-up look of these moons came with the Mariner 9 probe.

Mariner 9 image of Phobos taken from 5760 km. Phobos is 26 km long (in its longest dimension). Craters as small as 300 m are visible here.

Viking 1 orbiter view of Phobos. The 10 km crater Stickney is visible. Notice the grooves and crater chains around Stickney.

Viking 2 orbiter image of Deimos from a distance of 500 km. Deimos is about 10 km across (in its longest dimension). The surface appears smooth, but many craters are partially hidden by regolith (soil and broken-up material).

Goto Section 8 - Life on Mars: The Myths.