Abstract
It is still unknown how much water has escaped from Mars during its history. Hydrogen escape from Mars’s atmosphere probably played a major role in drying the planet, but present-day H loss rates (~3 × 1026 atoms per second on average) cannot explain the geological evidence for the large volumes of liquid water on ancient Mars. Here we used the three-dimensional Mars-Planetary Climate Model to show that H loss rates could have increased by more than one order of magnitude (6 × 1027 atoms per second) during higher spin axis obliquity periods, notably in the last few million years when Mars’s obliquity was about 35° on average. The resulting accumulated H escape over Mars’s history translates into an ~80 m global equivalent layer, which is close to the lower limit of geological estimates, assessing the major role of atmospheric escape in drying Mars.
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Data availability
The outputs from Mars-PCM used in this paper are available in NetCDF format via Zenodo at https://doi.org/10.5281/zenodo.15041471 (ref. 55).
Code availability
Mars-PCM is freely available from https://svn.lmd.jussieu.fr/Planeto/trunk. For the high-obliquity simulations, the starting files for obliquity 30° and obliquity 35° (in NetCDF format) are available via Zenodo at https://doi.org/10.5281/zenodo.15041471 (ref. 55).
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Acknowledgements
The IAA team (F.G.-G., G.G., M.Á.L.-V. and A.B.) were funded by the Spanish Ministerio de Ciencia, Innovación y Universidades, the Agencia Estatal de Investigación and EC FEDER funds (Projects RTI2018-100920-J-I00, PGC2018-101836-B-100 and PID2022-137579NB-I00), and they acknowledge financial support from a Severo Ochoa grant (No. CEX2021-001131-S), funded by MCIN/AEI/10.13039/501100011033. G.G. acknowledges financial support from the Junta de Andalucía through the programme EMERGIA 2021 (EMC21 00249). J.-Y.C. was partially funded by the Programme National de Planetologie of CNRS-INSU co-funded by CNES and the Programme National Soleil Terre of CNRS-INSU co-funded by CNES and CEA. This project has received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement No. 835275).
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G.G. performed the simulations for high obliquity, analysed the results, discussed their implications and wrote the first version of the paper. F.G.-G. conceived the study, contributed to the development and extension of the photochemical model and performed the simulations for the current obliquity. J.-Y.C. contributed to the extension of the photochemical model and implemented the calculation of escape rates in Mars-PCM. E.M. and F.F. are the main developers of Mars-PCM and provided support for running the simulations. F.M., J.N., M.V. and L.R. contributed to improving the water cycle in the model. M.Á.L.-V. and A.B. contributed to validating the water cycle in the model. F.L. and Y.L. contributed to the development of the photochemical model and performed the spin-up for the high-obliquity simulations. All authors participated in discussions on the results and contributed to the preparation of the paper.
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Extended data
Extended Data Fig. 1 Effect of the different processes over the simulated H escape.
Panel a: H escape simulated by the Mars-PCM for solar average conditions and the climatological dust scenario, for a reference simulation using the same physics as in19 (black line), a simulation including the H2O microphysical model but not the H2O-ion photochemistry (blue line line), and a simulation including the both the H2O microphysical model and the extended H2O ionospheric reactions (blue line). Panel b: Ratio of the simulated H escape rate with respect to the reference simulation.
Extended Data Fig. 2 Effect of the inclusion of H2O microphysical scheme at high obliquity.
Simulations with (red) and without (purple) microphysics after 1 Mars year: the H-loss rate is up to two orders of magnitude larger when the H2O microphysics is included, and the annually integrated H-loss is a factor 28.6 larger. This suggests that radiative active clouds also played a key role in changing the water content and H loss in the Mars past conditions.
Extended Data Fig. 3 Effects of global dust storms on the current escape rate.
Globally integrated H escape rate for MY25 (panel a), MY28 (panel b) and MY34 (panel c) simulated by the Mars-PCM forced with the UV solar flux for each year, and the observed dust load for each year (black lines) or the climatological dust scenario (red lines). The vertical dotted lines mark the approximate starting point of each storm (Ls=185 for MY25 and MY34, Ls=265 for MY2858. Including the dust storms further increases the H escape rate in between 16% and 33%.
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Gilli, G., González-Galindo, F., Chaufray, JY. et al. Increased hydrogen escape from Mars atmosphere during periods of high obliquity. Nat Astron (2025). https://doi.org/10.1038/s41550-025-02561-3
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DOI: https://doi.org/10.1038/s41550-025-02561-3
