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
sufficient to counteract the faint young sun. However, Kasting 
suggested that such a CO greenhouse effect
may be strongly reduced because, in such atmospheres, CO would
form highly reflective CO clouds, reducing the lapse rate and
severely limiting the surface warming.
They estimated that, because CO ice
(unlike water ice) has very low infrared absorbance, CO ice clouds
should cool the planet through reflection of solar radiation
uncompensated by infrared trapping.
We have studied this effect using a CO ice cloud radiative
model (previously used to study the radiative properties of CO ice
clouds in present Mars ) in combination with the Kasting's
energy-balance climate model.
We show that such high altitude
CO 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.
-  Kasting, Icarus 94, 1-13 (1991)
-  Forget et al., J.G.R. 100, 21,119-21,234 (1995)