The Martian Carbon Cycle and Implications for the C value of the Atmosphere
D.M. Kass, Y.L. Yung (Caltech)
As a result of laboratory measurements on the SNC meteorites, and those of the Martian atmosphere, we have data on the carbon isotopic system for Mars. The question then becomes, what do these measurements mean? In order to interpret the results, it is necessary to consider the carbon cycle on Mars.
Unlike on the Earth, carbon does not cycle through the Martian environment. Instead, primordial carbon is out-gassed from the mantle into the linked atmosphere-cap-regolith reservoir. It is removed from this reservoir either by sputtering and other exospheric processes or by carbonate formation. On the Earth, carbonate is subducted back into the mantle and the carbon is re-released during volcanic out-gassing, closing the cycle. On Mars, there appears to be little recycling of carbonate, which becomes a permanent sink for carbon. Thus on Mars, primordial carbon out-gasses from the mantle into the atmosphere and is then either lost to space or permanently sequestered in carbonate.
While the carbon system on Mars is, in many ways, much simpler than on the Earth, it is also significantly different. On the Earth, since the ocean acts as a large carbon reservoir, the carbonate is recycled, and escape processes are extremely slow, the atmospheric C value does not vary much with time. On Mars, where these effects do not occur, the C value of the atmospheric reservoir can change with time. Using the SNC meteorites, we can make reasonable assumptions for the C of the out-gassed source carbon. Then by examining the loss processes (which can enrich or deplete), we created a model of the C behavior of the atmosphere over the history of Mars. Using constraints from the current atmosphere and the various loss processes, we find that the atmosphere of Mars started out quite light and has been enriched by at least 50 per mil as it evolved.
Our modeling implies that when measuring C values for Mars, it is
critical to know, not only where the carbon is, but also when it was
stored. This is especially true for mineral phases that acquired
their carbon from the atmosphere-cap-regolith system.