3D modelling of the early martian climate under a denser CO ₂ atmosphere: Temperatures and CO ₂ ice clouds

On the basis of geological evidence, it is often stated that the early martian climate was warm enough for liquid water to flow on the surface thanks to the greenhouse effect of a thick atmosphere. We present 3D global climate simulations of the early martian climate performed assuming a faint young Sun and a CO ₂ atmosphere with surface pressure between 0.1 and 7bars. The model includes a detailed radiative transfer model using revised CO ₂ gas collision induced absorption properties, and a parameterisation of the CO ₂ ice cloud microphysical and radiative properties. A wide range of possible climates is explored using various values of obliquities, orbital parameters, cloud microphysic parameters, atmospheric dust loading, and surface properties.Unlike on present day Mars, for pressures higher than a fraction of a bar, surface temperatures vary with altitude because of the adiabatic cooling and warming of the atmosphere when it moves vertically. In most simulations, CO ₂ ice clouds cover a major part of the planet. Previous studies had suggested that they could have warmed the planet thanks to their scattering greenhouse effect. However, even assuming parameters that maximize this effect, it does not exceed +15K. Combined with the revised CO ₂ spectroscopy and the impact of surface CO ₂ ice on the planetary albedo, we find that a CO ₂ atmosphere could not have raised the annual mean temperature above 0°C anywhere on the planet. The collapse of the atmosphere into permanent CO ₂ ice caps is predicted for pressures higher than 3bar, or conversely at pressure lower than 1bar if the obliquity is low enough. Summertime diurnal mean surface temperatures above 0°C (a condition which could have allowed rivers and lakes to form) are predicted for obliquity larger than 40° at high latitudes but not in locations where most valley networks or layered sedimentary units are observed. In the absence of other warming mechanisms, our climate model results are thus consistent with a cold early Mars scenario in which nonclimatic mechanisms must occur to explain the evidence for liquid water. In a companion paper by Wordsworth et al. we simulate the hydrological cycle on such a planet and discuss how this could have happened in more detail.