A Mineralogical Reason Why All Exoplanets Cannot Be Equally Oxidising

By Keith Cowing
Status Report
August 21, 2023
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A Mineralogical Reason Why All Exoplanets Cannot Be Equally Oxidising
Cross plots of log(fO2) expressed as ∆FMQ, resulting from pMELTS calculations, shown at 1 GPa (top) and 4 GPa (bottom) and 1373.15 K. From left to right, columns show the dependence of ∆FMQ on bulk mantle Mg/Si, Al/Si, stellar metallicity [Fe/H]⋆, and the total refractory oxygen present in the mantle, Orock (i.e., a sum over oxygen in all metal oxides). Each point (N = 1198) represents a bulk mantle composition (Ca-Na-Fe-Mg-Al-Si-O-Ti) inferred for a planet-hosting star in the Hypatia Catalog, assuming Fe3+/ΣFe = 3% and Femantle/(Fecore + Femantle) = 12%. Points are coloured by the Fe2O3 wt.% composition of orthopyroxene (opx). Histograms of the ∆FMQ distribution are projected on the y-axis. — astro-ph.EP

From core to atmosphere, the oxidation states of elements in a planet shape its character. Oxygen fugacity (fO2) is one parameter indicating these likely oxidation states.

The ongoing search for atmospheres on rocky exoplanets benefits from understanding the plausible variety of their compositions, which depends strongly on their oxidation states — and if derived from interior outgassing, on the fO2 at the top of their silicate mantles. This fO2 must vary across compositionally-diverse exoplanets, but for a given planet its value is unconstrained insofar as it depends on how iron (the dominant multivalent element) is partitioned between its 2+ and 3+ oxidation states.

Here we focus on another factor influencing how oxidising a mantle is — a factor modulating fO2 even at fixed Fe3+/Fe2+ — the planet’s mineralogy. Only certain minerals (e.g., pyroxenes) incorporate Fe3+. Having such minerals in smaller mantle proportions concentrates Fe3+, increasing fO2. Mineral proportions change within planets according to pressure, and between planets according to bulk composition. Constrained by observed host star refractory abundances, we calculate a minimum fO2 variability across exoplanet mantles, of at least two orders of magnitude, due to mineralogy alone. This variability is enough to alter by a hundredfold the mixing ratio of SO2 directly outgassed from these mantles.

We further predict that planets orbiting high-Mg/Si stars are more likely to outgas detectable amounts of SO2 and H2O; and for low-Mg/Si stars, detectable CH4, all else equal. Even absent predictions of Fe3+ budgets, general insights can be obtained into how oxidising an exoplanet’s mantle is.

Claire Marie Guimond, Oliver Shorttle, Sean Jordan, John F. Rudge

Comments: 16 pages, 7 figures, accepted for publication in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2308.09505 [astro-ph.EP] (or arXiv:2308.09505v1 [astro-ph.EP] for this version)
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Submission history
From: Claire Guimond
[v1] Fri, 18 Aug 2023 12:30:03 UTC (2,195 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) 🖖🏻