An Ocean May Have Been Necessary to Produce the Surface Geology on Early Mars

Abstract:

The debate over the early Martian climate is among the most exciting in planetary science. Although the geologic evidence generally supports a warmer and wetter climate, climate models have had difficulty simulating such a scenario, leading to suggestions that the observed surface geology (e.g., valley networks and modified craters) may have formed in a cold climate instead ^1,2. However, such models have assumed nearly pure CO₂ atmospheres with no other major greenhouse gases. As we have originally shown using a single‐column radiative‐convective climate model ³, warming from CO₂‐H₂ collision‐induced absorption on a volcanically active early Mars could have raised mean surface temperatures above the freezing point. Later calculations showed that this is achievable with hydrogen concentrations as low as ~1%. Here we use an advanced energy balance model to test these predictions. Our analysis includes a northern lowlands ocean and shows that mean surface temperatures near the freezing point of water were necessary to carve the valley networks (see Figure). We find that a relatively large ocean, as previously suggested ⁴, was necessary to explain the surface geology. Valley networks would have been distributed globally prior to their partial removal. At lower mean surface temperatures and smaller ocean sizes, precipitation and surface erosion become insufficient to erode effectively. The warm period may have been approximately <10⁷ years, perhaps suggesting that episodic warming mechanisms were not needed. Atmospheric collapse and permanently glaciated conditions occur once surface ice coverage exceeds a threshold, depending on collision‐induced absorption assumptions. Our results support an early warm and semiarid climate consistent with many geologic observations ⁵. We also find that rain may have been the dominant precipitation on this warm early Mars.

REFERENCES

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