The formation, cooling, and alteration of the impact ejecta deposits at the Ries crater are an analog for impact processes and the formation of regoliths and soils on Mars. We have studied the suevite deposit, an impact-melt-bearing breccia found outside of the crater rim. This suevite contains large numbers of chimney-like degassing pipes, apparently formed by high gas pressures in the hot (>500¿ C) fluidized suevite immediately after deposition of the suevite. The likely sources of the gas are volatiles from the melted and shocked basement inclusions. The suevite outside of the crater rim contains about 15 wt % clay, indicating a large amount of alteration of the suevite. The clay is an Fe-rich montmorillonite with less than 5% interstratified illite, indicating that the temperature at which most of the alteration occurred was less than 100¿--130¿ C. The duration of this low-temperature period may have been extended by additional heating from the exothermal alteration reactions. The surfaces of the degassing pipes are not altered to clays, however, but have a thin Fe-rich oxide or hydroxide coating. The absence of alteration in the pipes suggests that the main alteration event outside the crater occurred under unsaturated conditions, such that the walls of the pipes were dry. Suevite deposits are likely to exist on Mars, because of the presence of volatile-rich target rock. Martian suevites may have also been fluidized and hydrothermally altered, contributing significantly to the formation of the clay-like phases inferred to exist in the Martian soil.