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Warm early Mars surface enabled by high-altitude water ice clouds

E. Kite,L. Steele,M. Mischna,M. Richardson

Published 2021 in Proceedings of the National Academy of Sciences of the United States of America

ABSTRACT

Significance Mars is cold today but once had lakes. We evaluated the water ice cloud greenhouse hypothesis for warming early Mars. Our results reconcile previous discrepant results by showing that the cloud greenhouse provides strong warming if the surface has patchy surface H2O but not if there is very extensive surface H2O. In our model, arid, warm, stable climates emerge with surface H2O (and low clouds) only at locations much colder than average surface temperature. At locations horizontally distant from the surface cold traps, clouds are found only at high altitudes, which maximizes cloud warming. As this scenario is consistent with geologic data that suggest a warm, arid early Mars climate, our results support the cloud greenhouse hypothesis for warming early Mars. Despite receiving just 30% of the Earth’s present-day insolation, Mars had water lakes and rivers early in the planet’s history, due to an unknown warming mechanism. A possible explanation for the >102-y-long lake-forming climates is warming by water ice clouds. However, this suggested cloud greenhouse explanation has proved difficult to replicate and has been argued to require unrealistically optically thick clouds at high altitudes. Here, we use a global climate model (GCM) to show that a cloud greenhouse can warm a Mars-like planet to global average annual-mean temperature (T¯) ∼265 K, which is warm enough for low-latitude lakes, and stay warm for centuries or longer, but only if the planet has spatially patchy surface water sources. Warm, stable climates involve surface ice (and low clouds) only at locations much colder than the average surface temperature. At locations horizontally distant from these surface cold traps, clouds are found only at high altitudes, which maximizes warming. Radiatively significant clouds persist because ice particles sublimate as they fall, moistening the subcloud layer so that modest updrafts can sustain relatively large amounts of cloud. The resulting climates are arid (area-averaged surface relative humidity ∼25%). In a warm, arid climate, lakes could be fed by groundwater upwelling, or by melting of ice following a cold-to-warm transition. Our results are consistent with the warm and arid climate favored by interpretation of geologic data, and support the cloud greenhouse hypothesis.

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