Both Earth and Mars show geologic evidence of flowing liquid water in the distant past, nearly four billion years ago. The presence of liquid water on the surface of these planets is difficult to reconcile with the reduced luminosity of the sun at the time, so scientists have continued to search for possible explanations for the warm climates of early Earth and early Mars. A team of researchers recently suggested that early Earth may have had larger oceans and fewer clouds than today, which would have reflected away less incoming sunlight and might have allowed the planet to remain warm in spite of a fainter sun. Whether or not this solution will pan out for early Earth, it at least suggests the possibility of a similar mechanism on early Mars.
In a recent paper published in Astronomy & Astrophysics, on which I am co-author, we examine the possibility that reduced reflectivity could have kept early Mars above freezing. We use a computer climate model to calculate the global average temperature at various values of ocean fraction and cloud coverage. We find that our model does indeed produce warm conditions for early Earth, but it fails to do the same for early Mars. In fact, our model can only produce warm conditions if early Mars were nearly entirely covered by oceans and also free of clouds, a result which is unlikely as well as inconsistent with geologic evidence. We conclude that some combination of climate and geochemical mechanisms, as yet unknown, may provide clues for understanding the stability of liquid water on early Mars.
If early Mars did harbor oceans, then the possibility remains that life could have developed. Examining the climates of both Earth and Mars in the past may therefore help in the quest to understand the origin of life. Future Mars exploration missions, as well as continued research on Earth, will slowly shed light on this mystery.