Increased Hydrogen Escape From Mars Atmosphere During Periods Of High Obliquity

IMAGE Caption: Sketch showing processes leading to larger H-loss in the past 20 million years (Myr). The loss rate is not expected to have been constant with time, and may vary significantly during Martian history.

Panel A: In current obliquity conditions (25.2^◦) the water ice in the polar caps sublimes in Summer, and then it is recycled back in Winter. Large dust load in the lower atmosphere facilitates the transport of water to the upper atmosphere, where it is chemically converted into atomic H that can easily escape to space.

Panel B: In the last 20 million year, when the obliquity of Mars was higher than today, the climate of Mars was different. Larger north pole insolation induced a more intense water cycle: the amount of sublimated water vapour in the atmosphere of Mars was much larger than today, and localised surface water ice reservoirs were created after precipitation in tropics and midlatitudes [44].

In addition, the formation of thick clouds warmed the middle atmosphere (up to 50 K at 45 km) by absorbing both solar radiation and IR radiation emitted by the surface, inducing positive feedback. All this favoured water penetration into the mesosphere (e.g. with up to 5 order of magnitude increased water abundances at about 45 km, near the aphelion), resulting in larger H escape rate. Other processes not accounted for in our study could also contribute to further changes in the H escape rate. Buried deposits of CO₂ ice within the south polar layer could have been released in the atmosphere at the time of high obliquity, producing an atmosphere with double its current pressure [45]. With higher pressure and warmer temperature conditions, is uncertain if the seasonal dust activity was more or less intense than today, due to higher water content and changes in the circulation patterns. (Mars image credit: NASA/JPL/USGS).

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 (about 3×10^26 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 (6×10^27 atoms per second) during higher spin axis obliquity periods, notably in the last few million years when Mars’s obliquity was about 35 deg on average. The resulting accumulated H escape over Mars’s history translates into an approx. 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.

Gabriella Gilli, Francisco González-Galindo, Jean-Yves Chaufray, Ehouarn Millour, François Forget, Franck Montmessin, Franck Lefèvre, Joseph Naar, Yangcheng Luo, Margaux Vals, Loïc Rossi, Miguel Ángel López-Valverde, Adrián Brines

Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Atmospheric and Oceanic Physics (physics.ao-ph)
Cite as: arXiv:2505.16692 [astro-ph.EP] (or arXiv:2505.16692v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2505.16692
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Journal reference: Nat Astron (2025), published on-line on 21 May 2025
Related DOI:
https://doi.org/10.1038/s41550-025-02561-3
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Submission history
From: Gabriella Gilli Gg
[v1] Thu, 22 May 2025 13:57:09 UTC (1,547 KB)
https://arxiv.org/abs/2505.16692
Astrobiology,