Atmospheres & Climate

Atmospheres As A Window To Rocky Exoplanet Surfaces

By Keith Cowing
Status Report
December 19, 2023
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Atmospheres As A Window To Rocky Exoplanet Surfaces
The timescales of surface–atmosphere interaction (red line) and atmospheric equilibration (blue dashed line) across different rocky planet climaticgeodynamic regimes. A reference timescale is shown for pure gas-phase chemical equilibrium to be reached, which is a simple monotonic function of temperature (Arrhenius in nature). Timescales of surface-atmosphere interaction on rocky planets deviate from this as a function of temperature as the climatic/geodynamic phases mediating the reactions change: schematics of these various planetary climatic-geodynamic regimes are shown at top, above their corresponding region of the reaction timescale graph. Cold planets feature slow solid-solid interactions between rock and ice layers (red line) and similarly cold atmospheres (dashed blue line). Where the temperature allows liquid surface water, weathering processes efficiently couple the atmospheric, oceanic, and surface chemistry, albeit in a disequilibrium state. Reaction rates in this regime increase with temperature as water temperature and increased rainfall drive more reaction (Walker et al. 1981; Maher & Chamberlain 2014). In the absence of liquid water, the rate of interaction between the atmosphere and surface may sharply decrease. However, at higher surface temperatures warm rocky planets such as Venus feature increasingly strong surface–atmosphere coupling. In this regime thermochemical equilibrium is achievable potentially both in the lower atmosphere (Liggins et al. 2023) and between the atmosphere and surface. Very hot terrestrial worlds have molten surfaces, and there is likely efficient mass transfer (and therefore equilibration) between the surface and atmosphere. — astro-ph.EP

As the characterization of exoplanet atmospheres proceeds, providing insights into atmospheric chemistry and composition, a key question is how much deeper into the planet we might be able to see from its atmospheric properties alone.

For small planets with modest atmospheres and equilibrium temperatures, the first layer below the atmosphere will be their rocky surface. For such warm rocky planets, broadly Venus-like planets, the high temperatures and moderate pressures at the base of their atmospheres may enable thermochemical equilibrium between rock and gas.

This links the composition of the surface to that of the observable atmosphere. Using an equilibrium chemistry code, we find a boundary in surface pressure-temperature space which simultaneously separates distinct mineralogical regimes and atmospheric regimes, potentially enabling inference of surface mineralogy from spectroscopic observations of the atmosphere.

Weak constraints on the surface pressure and temperature also emerge. This regime boundary corresponds to conditions under which SO2 is oxidized and absorbed by calcium-bearing minerals in the crust, thus the two regimes reflect the sulphidation of the crust.

The existence of these atmospheric regimes for Venus-like planets is robust to plausible changes in the elemental composition. Our results pave the way to the prospect of characterizing exoplanetary surfaces as new data for short period rocky planet atmospheres emerge.

Xander Byrne, Oliver Shorttle, Sean Jordan, Paul B. Rimmer

Comments: 12 pages, 9 figures, 2 tables. Accepted 17 Dec 2023 for publication in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2312.11133 [astro-ph.EP] (or arXiv:2312.11133v1 [astro-ph.EP] for this version)
Submission history
From: Xander Byrne
[v1] Mon, 18 Dec 2023 12:07:01 UTC (2,790 KB)

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) 🖖🏻