Could Cirrus Clouds Warm Early Mars?
Could cirrus clouds warm early Mars?
The presence of the ancient valley networks on Mars indicates that the climate
at 3.8 Ga was warm enough to allow substantial liquid water to flow on the
martian surface for extended periods of time. However, the mechanism for
producing this warming continues to be debated. One hypothesis is that Mars
could have been kept warm by global cirrus cloud decks in a CO2-H2O atmosphere containing at
least 0.25 bar of CO2 (Urata and Toon, 2013). Initial warming from some other
process, e.g., impacts, would be required to make this model work. Those
results were generated using the CAM 3-D global climate model. Here, we use a
single-column radiative convective climate model to further investigate the
cirrus cloud warming hypothesis. Our calculations indicate that cirrus cloud
decks could have produced global mean surface temperatures above freezing, but
only if cirrus cloud cover approaches ~100% and if other cloud properties
(e.g., height, optical depth, particle size) are chosen favorably. However, at
more realistic cirrus cloud fractions, or if cloud parameters are not optimal,
cirrus clouds do not provide the necessary warming.