Influence of carbon dioxide clouds on early martian climate

Recent studies have shown that clouds made of carbon dioxide ice may have w armed the surface of early Mars by reflecting not only incoming solar radia tion but upwelling IR radiation as well. However, these studies have not tr eated scattering self-consistently in the thermal TR. Our own calculations, which treat IR scattering properly, confirm these earlier calculations but show that CO2 clouds can also cool the surface, especially if they are low and optically thick. Estimating the actual effect of CO2 clouds on early m artian climate will require three-dimensional models in which cloud locatio n, height, and optical depth, as well as surface temperature and pressure, are determined self-consistently. Our calculations further confirm that CO2 clouds should extend the outer boundary of the habitable zone around a sta r but that there is still a finite limit beyond which above-freezing surfac e temperatures cannot be maintained by a CO2-H2O atmosphere. For our own So lar System, the absolute outer edge of the habitable zone is at similar to 2.4 AU, (C) 2000 Academic Press.