--- The interior of Mars appears to be as wet as the interior of Earth.

everal lines of evidence indicate that the surface of Mars was sculptured by flowing water and glaciation, but how much water was needed for this planetary art work is not easy to determine. In particular, how much is in the mantle that underlies the basaltic crust of Mars? How much was inside the planet when it formed? Lacking pieces of the mantle, the best way to address this question is to calculate how much water was in magmas produced by melting portions of the mantle. Francis McCubbin and colleagues at the University of New Mexico and the Carnegie Institution of Washington measured the amount of water (actually the hydroxyl molecule, OH) in the mineral apatite in two shergottites, martian meteorites formed by partial melting of the interior. They measured several tenths of a percent by weight H2O in apatite in two shergottites. By making reasonable assumptions about crystallization history and the amount of melting required to produce the shergottite magmas, they estimate that the interior contains between 73 and 290 parts per million H2O, which overlaps the lower range of water estimates for the Earth's mantle. This water was likely incorporated into both bodies as they formed by accretion of smaller bodies. Reference