In situ, measurements of 12C/13C of atmospheric CO2 on Mars made by Viking are, within error, similar to values normally encountered on Earth. However, high precision isotopic measurements made on SNC meteorites show there to be a 40% fractionation between carbon associated with silicate minerals (Δ13C≈-25%, 12C/13C≈91.3) and trapped CO2 gas or carbonate minerals (Δ13C≈+15%, 12C/13C≈87.7). To a first approximation, it can be considered that the silicate-sited carbon in SNC meteorites carriers the isotopic signature of the element present at depth in the Martian crust; herein it is considered that this magmatic carbon is representative of bulk Mars. On the other hand, carbon in the form of carbonate most probably results from secondary processes in operation at the surface of the planet. Trapped CO2 is thought to be a sample of Martian atmospheric gas. The significant difference in isotropic composition between the different components in SNC meteorites is difficult to interpret in terms of closed-system equilibrium or kinetic isotopic fractionation effects. Rather, it seems that the relative enrichment of 13C in surficial carbon is due to preferential removal of the lighter isotope during an atmospheric loss process. Between 30 and 70 mbar of CO2 may have been lost from the atmosphere to outer space. ¿American Geophysical Union 1990