Perhaps the most critical aspect of understanding the geologic, climatological and biologic history of Mars is establishing the absolute timing of events. Presently, only a relative chronology has been established using the frequency of impact craters on different surfaces (higher frequencies indicate older ages). Attempts to extrapolate the lunar cratering chronology (established by dating Apollo and Luna samples from known locations with well-defined crater frequencies) to a martian crater rate have large uncertainties. For example, the youngest major volcanic surface (Cerberus Plains) has model ages ranging from a few million years to hundreds of millions of years. To calibrate the absolute cratering chronology for Mars, an absolute radiometric age must be determined for an igneous rock from a known location from a surface with a well-defined impact crater frequency. A promising approach to establishing the chronology is to determine the radio metric ages using an in situ experiment. While in situ techniques are less precise than terrestrial analyses, they have precisions of the order 10-20% which are sufficient for these objectives. Various techniques have been proposed and are under development in cluding K/Ar and Rb/Sr. A mission with the objective of determining a radiometric age would be relatively simple and need survive only a short time.