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Elevated atmospheric escape of atomic hydrogen from Mars induced by high-altitude water

Nature Geoscience volume 10, pages 174–178 (2017)Cite this article

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Abstract

Atmospheric loss has controlled the history of Martian habitability, removing most of the planet’s initial water through atomic hydrogen and oxygen escape from the upper atmosphere to space. In standard models, H and O escape in a stoichiometric 2:1 ratio because H reaches the upper atmosphere via long-lived molecular hydrogen, whose abundance is regulated by a photochemical feedback sensitive to atmospheric oxygen content. The relatively constant escape rates these models predict are inconsistent with known H escape variations of more than an order of magnitude on seasonal timescales, variation that requires escaping H to have a source other than H2. The best candidate source is high-altitude water, detected by the Mars Express spacecraft in seasonally variable concentrations. Here we use a one-dimensional time-dependent photochemical model to show that the introduction of high-altitude water can produce a large increase in the H escape rate on a timescale of weeks, quantitatively linking these observations. This H escape pathway produces prompt H loss that is not immediately balanced by O escape, influencing the oxidation state of the atmosphere for millions of years. Martian atmospheric water loss may be dominated by escape via this pathway, which may therefore potentially control the planet’s atmospheric chemistry. Our findings highlight the influence that seasonal atmospheric variability can have on planetary evolution.

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Figure 1: Equilibrium photochemical model output.
Figure 2: Time response of the atmosphere to water parcel introduction and subsequent removal.
Figure 3: Sensitivity of atmospheric response to altitude and magnitude of the water parcel injected.

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Acknowledgements

The photochemical model was developed with support from NASA NESSF NNX11AP49H, NASA MDAP NNX14AM20G, and a NASA Astrobiology Institute Early Career Collaboration Award.

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Authors and Affiliations

  1. Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA

    M. S. Chaffin, J. Deighan, N. M. Schneider & A. I. F. Stewart

Authors
  1. M. S. Chaffin
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  2. J. Deighan
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  3. N. M. Schneider
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  4. A. I. F. Stewart
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Contributions

M.S.C. developed the model and performed the analysis. J.D. oversaw development of and contributed to the model. All authors contributed to interpretation of the results and their presentation.

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Correspondence to M. S. Chaffin.

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The authors declare no competing financial interests.

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Chaffin, M., Deighan, J., Schneider, N. et al. Elevated atmospheric escape of atomic hydrogen from Mars induced by high-altitude water. Nature Geosci 10, 174–178 (2017). https://doi.org/10.1038/ngeo2887

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