Carbon reservoirs on Mars: Constraints from Martian meteorites

Introduction: The search for life on Mars is a subject of enormous scientific and public interest; it is anticipated that remote missions to Mars over the coming few years will garner the requisite data that will allow the presence, or otherwise, of martian biological carbon to be determined. The only previous attempt to detect the presence of life in martian soil, made by instruments on the Viking landers of 1976, yielded ambiguous results [1]. In the three decades since Viking, analysis of carbon in martian meteorites has given insight to the complexities of carbon in martian rocks: it is clear that carbon from several reservoirs (mantle, crust, atmosphere) is present. Unless we have a full understanding of these reservoirs, their relative abundances and isotopic compositions, then interpretation of data acquired directly at Mars' surface will be difficult to interpret. Carbon in martian rocks: There are several contributions to the carbon budget in martian meteorites (not including potential martian biological material): primary indigenous carbon dissolved in minerals (sili-cates, phosphates); secondary carbonates produced by surficial aqueous activity; atmospheric CO 2 trapped during ejection from the martian surface; spallogenic carbon produced during irradiation in space and organic contamination following arrival on Earth. It is assumed that the first two of these components will be present in martian rocks analysed in situ; it remains to be seen whether there is an additional reservoir emanating from a martian biosphere. Many measurements have been made of the abundance and stable isotopic compositions of secondary carbonates in martian meteorites, particularly in ALH 84001 and Nakhla, the two specimens in which they can most readily be identified. Broadly speaking, several tens to hundreds of ppm