The geological and climatological case for a warmer and wetter early Mars


The climate of early Mars remains a topic of intense debate. Ancient terrains preserve landscapes consistent with stream channels, lake basins and possibly even oceans, and thus the presence of liquid water flowing on the Martian surface 4 billion years ago. However, despite the geological evidence, determining how long climatic conditions supporting liquid water lasted remains uncertain. Climate models have struggled to generate sufficiently warm surface conditions given the faint young Sun—even assuming a denser early atmosphere. A warm climate could have potentially been sustained by supplementing atmospheric CO2 and H2O warming with either secondary greenhouse gases or clouds. Alternatively, the Martian climate could have been predominantly cold and icy, with transient warming episodes triggered by meteoritic impacts, volcanic eruptions, methane bursts or limit cycles. Here, we argue that a warm and semi-arid climate capable of producing rain is most consistent with the geological and climatological evidence.

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Fig. 1: Geomorphic evidence for water on ancient Mars compared to Earth.
Fig. 2: Martian valley networks.
Fig. 3: A schematic of the geologic evolution of Mars with time.
Fig. 4: Illustration of simplified energy balance for Mars.


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R.M.R. wishes to thank J. F. Kasting for lively discussions about limit cycles and transient warming episodes. R.M.R. acknowledges support from the Simons Foundation (SCOL # 290357, Kaltenegger), Carl Sagan Institute, Cornell Center for Astrophysics and Planetary Science, and the Earth-Life Science Institute. R.A.C. acknowledges support from NASA grant 80NSSC17K0454 and a grant from the Smithsonian’s Universe Consortium.

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R.M.R conceived idea, wrote and edited much of the main text and Supplementary Information and created the figures. R.A.C. defined the initial conditions of the geologic environment before valley network formation (early–mid Noachian) and co-wrote and co-edited the main text and Supplementary Information. Both authors discussed and analysed the results and implications.

Correspondence to Ramses M. Ramirez.

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Ramirez, R.M., Craddock, R.A. The geological and climatological case for a warmer and wetter early Mars. Nature Geosci 11, 230–237 (2018).

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