Volcanic outgassing of CO 2 and H 2O on Mars

Abstract

Volcanic outgassing is one of the main sources of volatiles for the martian atmosphere and degassing of the martian interior potentially influenced the early martian climate. Using a parameterized thermo-chemical evolution model and considering two end-member melting models, we self-consistently calculate the amount of CO 2 and H 2O outgassed during the martian evolution. Outgassing rates are found to depend primarily on a factor describing the outgassing efficiency, the bulk mantle water content, the mantle oxygen fugacity, and the local melt fraction in the magma source regions. We find that significant outgassing ceased around 3.5–2 Gyr ago, depending on the adopted melting model. A total of 0.9–1 bar CO 2 is outgassed during this time period if a mantle oxygen fugacity corresponding to one log 10 unit above the iron–wustite buffer is assumed. Additionally, a total of 17–61 m of water is delivered to the surface. Outgassing is most efficient in the pre-Noachian (up to 4.1 Gyr), but still significant during the Noachian, and 5–15 m of water and ∼250 mbar of CO 2 are outgassed between 4.1 and 3.7 Gyr. Although this amount is probably insufficient for an appreciable greenhouse effect, pressures are found to be sufficient to stabilize transient liquid water on the surface well into the Hesperian period. Therefore, our results support the hypothesis that rather than being warm-and-wet, the martian climate was probably cold-and-wet. Outgassing is found to strongly decline during the Hesperian, and is insignificant during the Amazonian period. A simple parameterization for the outgassing of CO 2 and H 2O as a function of time is presented.