CO_2 Greenhouse in the Early Martian Atmosphere: SO_2 Inhibits Condensation

Abstract

Many investigators of the early martian climate have suggested that a dense carbon dioxide atmosphere was present and warmed the surface above the melting point of water (J. B. Pollack, J. F. Kasting, S. M. Richardson, and K. Poliakoff 1987.Icarus71,203–224). However, J. F. Kasting (1991.Icarus94,1–13) pointed out that previous thermal models of the primitive martian atmosphere had not considered the condensation of CO_2. When this effect was incorporated, Kasting found that CO_2 by itself is inadequate to warm the surface. SO_2 absorbs strongly in the near UV region of the solar spectrum. While a small amount of SO_2 may have a negligible effect by itself on the surface temperature, it may have significantly warmed the middle atmosphere of early Mars, much as ozone warms the terrestrial stratosphere today. If this region is kept warm enough to inhibit the condensation of CO_2, then CO_2 remains a viable greenhouse gas. Our preliminary radiative modeling shows that the addition of 0.1 ppmv of SO_2 in a 2 bar CO_2 atmosphere raises the temperature of the middle atmosphere by approximately 10 degrees, so that the upper atmosphere in a 1D model remains above the condensation temperature of CO_2. In addition, this amount of SO_2 in the atmosphere provides an effective UV shield for a hypothetical biosphere on the martian surface.