The Seasonal Cycle of the Hadley Cell on Early Mars

Radiative-convective models of the mean surface temperature of Early Mars indicate unambiguously that without cloud effects the surface would be too cold to support extensive surface flow of liquid water. Rather more ambiguously, they indicate that above freezing surface temperatures might be possible under certain circumstances if CO2 cloud effects are taken into account; the result is highly dependent on cloud microphysics however. A proper consideration of the problem with or without clouds requires consideration of the geographic and seasonal variation of the surface temperature of the planet. In this presentation, I will discuss the problem in the context of axisymmetric models of the Early Mars climate, using both simulations and idealized Hadley cell theory. The emphasis will be on the asymmetric Hadley cells that occur during the Solstice conditions, with a particular eye to determining the extent of the planet that may be above freezing with and without cloud effects. The seasonal cycle is estimated to be weaker than that of Present Mars, but stronger than that of Earth, owing to the lack of a planetary ocean but presence of a thick CO2 atmospere. This allows hot spots to develop in high latitude summer regions, but also allows a very cold polar night. The possible role of clouds in limiting the polar night chill is discussed.