A mineralogical reason why all exoplanets cannot be equally oxidizing

Abstract

ABSTRACT From core to atmosphere, the oxidation states of elements in a planet shape its character. Oxygen fugacity ($f_{\rm O_2}$) 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 $f_{\rm O_2}$ at the top of their silicate mantles. This $f_{\rm O_2}$ 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 oxidizing a mantle is – a factor modulating $f_{\rm O_2}$ 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 $f_{\rm O_2}$. 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 $f_{\rm O_2}$ 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 oxidizing an exoplanet’s mantle is.