Phobos: Doomed Moon of Mars . ... Credit Copyright: HiRISE , MRO , LPL (U. Arizona) , NASA Explanation: This moon is doomed. ... But Phobos orbits so close to Mars - about 5,800 kilometers above the surface compared to 400,000 kilometers for our Moon - that gravitational tidal forces are dragging it down. A recent analysis of the long grooves indicates that they may result from global stretching caused by tides -- the differing force of Mars' gravity on different sides of Phobos . ...