Mineral reaction buffering of Venus’ atmosphere: A thermochemical constraint and implications for Venus-like planets

The equilibrium suggested as a buffer for CO2 in the Venus atmosphere, CaCO3 + SiO2 = CaSiO3 + CO2, cannot act as a buffer at the Venus surface/troposphere – the pressure–temperature slope of the equilibrium and that of the atmosphere (dry adiabat with significant greenhouse heating) do not provide buffering capacity (if indeed CaCO3 were present). Instead, perturbations to T or P(CO2) can produce catastrophic expansion or collapse of the atmosphere. This instability can be generalized to all devolatilization reactions that produce a radiatively active gas in a planetary atmosphere dominated by such gases, and gives a simple thermochemical criterion for whether a reaction could buffer such an atmosphere. Simple decarbonation reactions fail this criterion, suggesting that the abundance of CO2 in a CO2-dominated atmosphere cannot be buffered by chemical reactions with the surface; a similar conclusion holds for the abundance of H2O in an H2O-dominated (steam) atmosphere. Buffering of minor gases is more likely; a mineral buffer equilibrium for SO2 proposed for Venus, FeS2 + CO2 = Fe3O4 + SO2 + CO, passes the thermochemical criterion, as does a reaction involving Ca sulfate. These inferences can be generalized to atmospheres in ‘moist’ adiabatic equilibria, and to extrasolar Venus-like planets, and will help in interpreting the compositions of their atmospheres.