CO2 ice clouds induced warming of early Martian climate and its stability

Abstract

The scattering greenhouse effect of CO₂ ice clouds is proposed as a mechanism to make the Martian climate warm enough to support flowing water under a faint young Sun. Previous studies have shown that such warm climate is possibly achieved if a cloud layer with suitable ranges of particle size and optical depth is given. However, it has not been examined whether or not such a suitable cloud layer could exist stably in the early Martian atmosphere. In this study, we construct a 1-D radiative model for the CO₂-H₂O atmosphere and analyze cloud stability on the basis of the numerical estimation of the ice condensation or evaporation rate in a cloud layer. We assume that the latent heat release of CO₂ condensation is balanced with the radiative cooling in cloud layer. Our numerical analysis suggests that a CO₂ ice cloud layer is stabilized and the global mean surface temperature rises above melting point of H₂O when the atmospheric pressure is larger than 1 bar and the number column density of cloud condensation nuclei is fixed at nearby 10¹⁰ m^-2.