Deimos ("DEE mos") is the smaller and outermost of Mars' two moons. It is the smallest known moon in the solar system. orbit : 23,459 km from Mars diameter : 12.6 km (15 x 12.2 x 11) mass : 1.8e15 kg . ... Deimos and Phobos are composed of carbon-rich rock like C-type asteroids and ice. ... Deimos and Phobos are probably asteroids perturbed by Jupiter into orbits that allowed them to be captured by Mars. more images . ... Mars .. Phobos .. ... Jupiter .. ...