The 24-km-diameter Tertiary Haughton impact structure formed in rocks that contained preexisting liquid hydrocarbons. Biomarker ratios in the hydrocarbons show a consistent pattern of variation in degree of heating across the structure. The heating reached a maximum at the crater center and is attributed to hydrothermal activity following impact. Kinetic modeling suggests a time scale of ∼5 k.y. for the heating, at a maximum temperature of 210 °C. The short time scale suggests that in moderate-sized craters, which are abundant on Mars, heating is not so extensive that fossil or extant organic matter would be obliterated.