CO2 Snow on Mars and Early Earth: Experimental Constraints

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2002-11, Vol.160 (1), p.66-72
• Main Author: Glandorf, D
• Format: Article
• Language: English
• ISSN: 0019-1035
• DOI: 10.1006/icar.2002.6953

Greenhouse warming due to carbon dioxide atmospheres may be responsible for maintaining the early Earth's surface temperature above freezing and may even have allowed for liquid water on early Mars. However, the high levels of CO2 required for such warming should have also resulted in the formation of CO2 clouds. These clouds, depending on their particle size, could lead to either warming or cooling. The particle size in turn is determined by the nucleation and growth conditions. Here, we present laboratory studies of the nucleation and growth of carbon dioxide on water ice under Martian atmospheric conditions. We find that a critical saturation, S = 1.34, is required for nucleation, corresponding to a contact parameter between solid water and solid carbon dioxide of m = 0.95. We also find that after nucleation occurs, growth of CO2 is very rapid, and we report the growth rates at a number of supersaturations. Because growth would be expected to continue until the CO2 pressure is lowered to its vapor pressure, we expect particles larger than those being currently suggested for the present and past Martian atmospheres. Using this information in a microphysical model described in a companion paper, we find that CO2 clouds are best described as "snow", having a relatively small number of large particles. (Author)