The basic thermal evolution model, described by Weizman et al. <1996>, was modified to include the effect of phase transitions and the dependence of melting temperatures on pressure. The total amount of water release through Martian history was estimated from this model using the wet solidus of peridotite. Assuming a water content of 0.1% in the basalt, we find that the total amount of water released would be sufficient to cover the planet with a layer at least 3--300 m thick.¿ 1997 American Geophysical Union