Carbon Reservoir History of Mars Constrained by Atmospheric Isotope Signatures

Abstract:

The evolution of the atmosphere on Mars is one of the most intriguing problems in the exploration of the Solar System, and the climate of Mars may have evolved from a warmer, wetter early state to the cold, dry current state. Because CO₂ is the major constituent of Mars’s atmosphere, its isotopic signatures offer a unique window to trace the evolution of climate on Mars. Here we use a box model to trace the evolution of the carbon reservoir and its isotopic signature on Mars, with carbonate deposition and atmospheric escape as the two sinks and magmatic activity as the sole source. We derive new quantitative constraints on the amount of carbonate deposition and the atmospheric pressure of Mars through time, extending into the Noachian, ~3.8 Gyr before present. This determination is based on recent Mars Science Laboratory (MSL) isotopic measurements of Mars’s atmosphere, recent orbiter, lander, and rover measurements of Mars’s surface, and a newly identified mechanism (photodissociation of CO) that efficiently enriches the heavy carbon isotope. In particular, we find that escape via CO photodissociation on Mars has a fractionation factor of 0.6 and hence, photochemical escape processes can effectively enrich ¹³C in the Mars’s atmosphere during the Amazonian. As a result, modest carbonate deposition must have occurred early in Mars’s history to compensate the enrichment effects of photochemical processes and also sputtering, even when volcanic outgassing up to 200 mbar occurred during the Hesperian. For a photochemical escape flux that scales as the square of the solar EUV flux or more, at least 0.1 bar of CO₂ must have been deposited as carbonates in the Noachian and Hesperian. More carbonate deposition would be required if carbonate deposition only occurred in the Noachian or with low fractionation factors.