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Hydrogen escape on Mars dominated by water vapour photolysis above the hygropause

Nature Astronomy volume 8, pages 827–837 (2024)Cite this article

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Abstract

The loss of water on Mars to space largely occurs through the decomposition of water vapour after transport to high altitudes with the subsequent escape of atomic hydrogen. However, there are still open questions about the relative importance of water photolysis and ion chemistry as decomposition mechanisms. In addition, the relevance of seasonally recurring compared with impulsive vertical water transport driven by dust storms is not fully understood. Using photochemical modelling based on a synergistic dataset from three Mars orbiters, we show that water photolysis above the main region of cloud formation (above the hygropause) is the dominant source of hydrogen available for escape, significantly exceeding hydrogen production through ion chemistry. We also show that seasonally recurring transport dominates hydrogen escape over impulsive transport, suggesting that dust storms play only a minor role in atmospheric water loss. Modelled hydrogen escape rates show good agreement with available measurements, which demonstrates the importance of the mechanisms investigated here for improving quantitative estimates of long-term water loss on Mars.

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Fig. 1: Climatological water vapour mixing ratios and corresponding modelled hydrogen mixing ratios in the Martian atmosphere.
Fig. 2: Globally averaged hydrogen formation rates and rates that lead to the reformation of water.
Fig. 3: Globally averaged hydrogen escape flux calculated by the photochemical model (lines) in comparison to hydrogen escape rates derived from MAVEN measurements (symbols).

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Data availability

MCS data are publicly available on the Atmospheres Node of NASA’s Planetary Data System (https://atmos.nmsu.edu/data_and_services/atmospheres_data/MARS/mcs.html). ACS and NOMAD water vapour data are available as supplementary datasets to ref. 14 and ref. 35, respectively. IUVS ECH data on hydrogen escape are available as a supplementary dataset to ref. 22. Other data are available as Supplementary Information.

Code availability

An executable version of the KINETICS code with adjustable input parameters, together with all the input files required to reproduce all scenarios in this paper, is available upon request.

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Acknowledgements

We thank S. Suzuki for preparing the supplementary video. This work was funded by the NASA Mars Data Analysis Program (80NM0018F0719). L.P. appreciates the support of the Ecole Superieure des Techniques Aeronautiques et de Construction Automobile, which allowed his participation in the Visiting Student Research Program of JPL. J.S.H. acknowledges the MAVEN mission for supporting the analysis of SWIA data. Work at the Jet Propulsion Laboratory, California Institute of Technology, is performed under contract with the National Aeronautics and Space Administration.

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Authors and Affiliations

  1. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

    Armin Kleinböhl, Karen Willacy & Marek J. Slipski

  2. Ecole Superieure des Techniques Aeronautiques et de Construction Automobile, Paris, France

    Loïc Poncin

  3. Department of Physics and Astronomy, University of Iowa, Iowa City, IA, USA

    Jasper S. Halekas

  4. Boston University, Boston, MA, USA

    Majd Mayyasi

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  1. Armin Kleinböhl
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Contributions

A.K. conceived the study, developed the climatologies, and analysed and interpreted the data. K.W. performed and analysed the simulations made with the photochemical model. M.J.S. prepared the NGIMS data and contributed information on the upper atmosphere. L.P. prepared the NOMAD data and contributed to the model simulations. J.S.H. and M.M. provided the MAVEN data on hydrogen escape and contributed to the interpretation. A.K. prepared the original draft of the paper. All authors contributed to the reviewing and editing of the paper.

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Correspondence to Armin Kleinböhl.

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Nature Astronomy thanks Juan Alday and Shohei Aoki for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Upper atmospheric temperature data during seasons covered by this study.

Profiles were derived from Ar measurements by NGIMS during MAVEN deep dips32, while crosses correspond to temperatures at altitudes around 170 km derived from IUVS measurements43. The black profile was chosen for the study for being representative for conditions experienced in the dusty season at intermediate solar zenith angles.

Extended Data Fig. 2 Zonally averaged water ice cloud extinction vs. latitude and pressure observed by the Mars Climate Sounder.

Data is shown at a reference frequency of 843 cm−1 around 3 AM local time (left) and around 3 PM local time (right) for four different ranges of Ls corresponding to northern fall equinox (top row), the peak of the MY34 GDS (2nd row), northern winter solstice (3rd row) and the peak of the LRDS of MY34 (bottom row). Note that the altitude scale is only approximate and varies depending on the average temperature of the atmosphere, which is particularly noticeable in the 2nd row when the atmosphere expands due to heating driven by the effects of the GDS.

Extended Data Fig. 3 Chemical reactions involving water as well as molecular and atomic hydrogen as implemented in the photochemical model KINETICS.

Bi- and termolecular reactions are marked in blue while photolysis reactions are marked in red and ion reactions are marked in green.

Extended Data Fig. 4 CO2+ concentration profiles as calculated by the KINETICS photochemical model.

Calculations are global averages given for several ranges of Ls and compared with data based on the Viking Landers (ref. 58, and references therein) as well as the NGIMS instrument on MAVEN during the ‘Deep Dip 2’ and ‘Deep Dip 4’ measurement campaigns31.

Extended Data Fig. 5 J-values for water photolysis.

Calculations are based on the idealized atmosphere by Chaffin et al.10 (black), the same atmosphere used in the KINETICS model (blue), and the atmosphere based on the climatology from this work at 5°-10°N, for Ls = 180°-185° (yellow) and Ls = 200°-205° (green).

Supplementary information

Supplementary Information

Supplementary Tables 1–5.

Supplementary Video

Climatological water vapour mixing ratios in the Martian atmosphere (left) and corresponding modelled hydrogen mixing ratios (right) for all Ls ranges of MY34 covered in this study. Black lines indicate the hygropause.

Supplementary Data 1

SWIA measurements of the H escape flux for an assumed exobase temperature of 200 K.

Supplementary Data 2

Climatologies of temperature, pressure, density, dust opacity and water vapour mixing ratio as well as corresponding modelled hydrogen mixing ratios for all Ls ranges of MY34 covered in this study.

Supplementary Data 3

Zonally and meridionally averaged climatology of temperature, pressure, density, dust opacity and water vapour mixing ratio used for the spin-up of the KINETICS photochemical model.

Supplementary Data 4

Hygropause altitude based on the top of saturated water derived from MCS water-ice clouds.

Supplementary Data 5

Globally averaged hydrogen formation rates and rates that lead to the reformation of water versus altitude for relevant reactions as calculated by the KINETICS model for four Ls periods as given in Fig. 2.

Supplementary Data 6

Hydrogen escape flux as calculated by the photochemical model KINETICS using the KIDA database for the four water scenarios given in Fig. 3.

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Kleinböhl, A., Willacy, K., Slipski, M.J. et al. Hydrogen escape on Mars dominated by water vapour photolysis above the hygropause. Nat Astron 8, 827–837 (2024). https://doi.org/10.1038/s41550-024-02268-x

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