Could Cirrus Clouds Have Warmed Early Mars?

TL;DR

This study uses a climate model to evaluate if cirrus clouds alone could have warmed early Mars enough for liquid water, finding that only near-complete cloud cover could achieve this, implying other greenhouse effects were likely necessary.

Contribution

It provides a detailed analysis of cirrus cloud warming potential on early Mars using a single-column climate model, refining previous global model results.

Findings

Cirrus clouds could warm Mars if coverage is 75-100%.

Realistic cloud coverage likely insufficient for warming.

Additional greenhouse effects probably contributed to early Mars warmth.

Abstract

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 ~75 - 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, suggesting that other greenhouse mechanisms are needed.