Habitable Zones & Global Climate

Tracing the Inner Edge of the Habitable Zone with Sulfur Chemistry

By Keith Cowing
Status Report
astro-ph.EP
January 31, 2025
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Tracing the Inner Edge of the Habitable Zone with Sulfur Chemistry
Cartoon depiction of the sulfur-cycle in different planetary regimes. In the Earthlike case (A), atmospheric SO2 is scrubbed from the atmosphere by wet deposition (19). In the Venus-like case with high-UV irradiation (B), for example irradiation by the Sun, SO2 can be abundant in the deep atmosphere, however in the upper atmosphere SO2 is efficiently catalysed into H2SO4 by UV photons and subsequent reaction with H2O (80). In the Venus-like case with low-UV irradiation (C), such as irradiation by a low-UV M-dwarf, SO2 can survive in the upper atmosphere (20), potentially enabling SO2 to be identified in spectroscopic observation and thereby revealing the lack of a surface water ocean. — astro-ph.EP

The circumstellar liquid-water habitable zone guides our search for potentially inhabited exoplanets, but remains observationally untested. We show that the inner edge of the habitable zone can now be mapped among exoplanets using their lack of surface water, which, unlike the presence of water, can be unambiguously revealed by atmospheric sulfur species.

Using coupled climate-chemistry modelling we find that the observability of sulfur-gases on exoplanets depends critically on the ultraviolet (UV) flux of their host star, a property with wide variation: most M-dwarfs have a low UV flux and thereby allow the detection of sulfur-gases as a tracer of dry planetary surfaces; however, the UV flux of Trappist-1 may be too high for sulfur to disambiguate uninhabitable from habitable surfaces on any of its planets.

We generalise this result to show how a population-level search for sulfur-chemistry on M-dwarf planets can be used to empirically define the Habitable Zone in the near-future.

IMAGE

Transit features and cloud albedo for different atmospheric sulfur abundances. Polar plots showing surface temperature, cloud mass, Bond albedo, and the transmission feature heights of the sulfur-gases SO2, OCS, and H2S, in a Venus-like atmosphere irradiated by a low-UV M-dwarf and a high-UV M-dwarf. From the top, going clockwise, segments of the polar plots show: surface temperature, Tsurf (K); cloud mass loading, mcloud (mg cm−2 ); Bond albedo, ABond; H2S transmission feature height (ppm); OCS transmission feature height (ppm); SO2 transmission feature height (ppm). The radial coordinate corresponds to decreasing instellation flux (or increasing distance from host star) in units S⊕. The azimuthal coordinate, going anti-clockwise around the edge of each segment, corresponds to increasing mixing ratio of the input sulfur-species, from 10−6−2 (1%). The Tsurf, mcloud, and ABond segments are plotted for the case where the azimuthal coordinate is tracking an increase of the input SO2 abundance anti-clockwise. For the SO2, OCS, and H2S feature height segments, the azimuthal coordinate corresponds to the input abundance of SO2, OCS, and H2S, respectively, with the other sulfur-gases not being input to the atmosphere. — astro-ph.EP

Sean Jordan, Oliver Shorttle, Paul B. Rimmer

Comments: Published in Science Advances
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2501.17948 [astro-ph.EP] (or arXiv:2501.17948v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2501.17948
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Journal reference: Science Advances 29 Jan 2025: Vol. 11, Issue 5, eadp8105
Related DOI:
https://doi.org/10.1126/sciadv.adp81055
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Submission history
From: Sean Jordan
[v1] Wed, 29 Jan 2025 19:17:58 UTC (1,329 KB)
https://arxiv.org/abs/2501.17948
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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