The early history of the terrestrial planets was punctuated by a period of heavy bombardment. Frequent large impacts condensed in a short, 0.02–0.5-Ga-long time period resulted in heavily cratered planetary surfaces. In volatile-bearing planetary crusts, each of these large impacts created a volume of hot rock and melt that drove vast subsurface hydrothermal systems. In Mars' basaltic crust, these systems produced a variety of alteration phases, the nature of which we explored with thermochemical modeling. Using the computer code CHILLER, we found a variety of oxides, hydroxides, and hydrous and water-free silicates resulting from the hydrothermal alteration. The main hydrous silicates are serpentine, chlorite, nontronite, and other clay minerals. Some of the resulting assemblages contain up to 15 wt% bound water. Our results constrain the temperature and water rock ratio at the time of alteration and compare well to the mineral assemblages found on Mars by OMEGA (Observatoire pour la Minéralogie, l'Eau, les Glaces, et l'Activité) and CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) and, therefore, provide an interpretative framework for analyzing Martian mineralogy from orbital data. Our results also provide a geological template needed to select landing locations for future missions that search for water and energy sources associated with potential habitats on early Mars.