CO-oxidation on surface hematite in hot atmospheres of rocky planets

Abstract

Hematite surface minerals can play a key role for the stability in hot, CO2 exoplanetary atmospheres. In a previous work we applied a heterogeneous mechanism for the oxidation of atmospheric CO(g) into CO2(g) occurring on the surface of hematite to planetary atmospheres. In that work we calculated CO2(g) production rates via this “hematite mechanism“ for specific planetary atmospheric scenarios both in and out of the Solar System. We perform a general parameter study of the hematite mechanism in which we change key initial variables (CO and O2 gas-phase abundances) and temperature, pressure covering the diverse range of conditions for terrestrial planetary atmospheres; we investigate the response of the CO(g) oxidation rate and hence discuss the implications for the atmospheric CO2(g) budget. We apply a numerical integration scheme based on the Gear method to a system of seven chemical equations to investigate the rate of CO(g) oxidation via the hematite mechanism. Results suggest the mechanism has a potentially important influence on the evolution of hot atmospheres of terrestrial-type planets, especially for temperatures above about 550K. The abundance of CO(g) was found to be not important for the rate of CO oxidation, whereas the abundance of O2(g) begins to play a role above about 10-5 volume mixing ratio. Above about 550K, the efficiency of CO(g) oxidation increases because the rate determining step involving CO2 desorption is faster. Subsequently switching off the rather uncertain rate of diffusion of O atoms from the crystal bulk to the surface led to a strong lowering in reaction rates and a stronger dependency of the CO(g) oxidation rate upon O2(g). For example, on increasing the volume mixing ratio of O2(g) from 10−5 to 10−4 for a scenario without diffusion (with Venus-like surface conditions) the percentage conversion of initial CO(g) into CO2(g) increased from ~30% up to ~60%.