Warming Early Mars with Scattering Clouds

F. Forget (LMD, Paris, France), R.T. Pierrehumbert (Univ. of Chicago)

In spite of a solar luminosity significantly lower than it is today, geological evidence shows that early in its history, Mars was warm enough to support flowing water. To explain this fact, it had been speculated that high concentration of certain gases, most notably carbon dioxide, could create a greenhouse effect sufficient to counteract the faint young sun. However, Kasting [1] suggested that such a CO tex2html_wrap_inline19 greenhouse effect may be strongly reduced because, in such atmospheres, CO tex2html_wrap_inline19 would form highly reflective CO tex2html_wrap_inline19 clouds, reducing the lapse rate and severely limiting the surface warming. They estimated that, because CO tex2html_wrap_inline19 ice (unlike water ice) has very low infrared absorbance, CO tex2html_wrap_inline19 ice clouds should cool the planet through reflection of solar radiation uncompensated by infrared trapping. We have studied this effect using a CO tex2html_wrap_inline19 ice cloud radiative model (previously used to study the radiative properties of CO tex2html_wrap_inline19 ice clouds in present Mars [2]) in combination with the Kasting's energy-balance climate model. We show that such high altitude CO tex2html_wrap_inline19 clouds are in fact able to compensate their high reflectivity at solar wavelength by scattering the infrared radiation back to the ground. Overall, this scattering greenhouse effect could have even been sufficient to strongly warm the planet at that time.
- [1] Kasting, Icarus 94, 1-13 (1991)
- [2] Forget et al., J.G.R. 100, 21,119-21,234 (1995)