A comparative study of the early terrestrial atmospheres with interactive cloud formation

Abstract

Due to their formation at about the same time in the same region of the early solar nebula, it is reasonable to assume that the primitive atmospheres of Earth, Mars and Venus were similar and that present-day differences have arisen as a result of their differing masses and incident solar fluxes. Using a radiative-convective model, we determine maximum and minimum carbon dioxide levels for the early atmospheres which are consistent with this assumption and with climatic conditions thought to have existed on the three terrestrial planets 4.0 billion years ago. Rather than employ the cloud-free atmosphere approach of earlier studies, we include an interactive water vapor transport and cloud formation scheme in the model. Due to uncertainties about the direction of cloud cover feedback, we treat cloud cover as fixed. For most cases examined, we set the cloud cover at 50%, but the effect of varying cloud cover is also explored.