Carbon isotopic gradients in the Martian crust: implications for past or present life on Mars

Recent missions to Mars raise the possibility of surface sedimentary sequences that may contain the organic remains of past or present Martian biota. Irrespective of the mechanism of any biological processes on Mars, it seems reasonable to presume that they will involve the transfer and reaction of carbon-bearing molecules. In this case, following the example of terrestrial life forms such as plants and bacteria, it is almost certain that these processes will be accompanied by changes in 12C/13C ratios (which are themselves the result of kinetic isotope effects imparted during the embedded chemical/physical processes). Thus, just as carbon in biological organic matter on Earth is enriched in the lighter carbon isotope relative to mantle (juvenile) carbon, the logical consequence of Martian life is a stable carbon isotopic gradient from the top of the mantle to the surface sedimentary rocks. Stepped combustion-isotope ratio mass spectrometry is a proven technique for measuring the isotopic composition of ambient carbon trapped in crystals during magma solidification. Data from SNC meteorites extracted from different depths on Mars are not inconsistent with a biologically-produced carbon isotope gradient in the Martian crust and provide directions for future research and exploration.