Impact of oxygen fugacity on atmospheric structure and emission spectra of ultra hot rocky exoplanets

Atmospheres above lava-ocean planets (LOPs) hold clues as to the properties of their interiors, owing to the expectation that the two reservoirs are in chemical equilibrium.

Here we consider `mineral' atmospheres produced in equilibrium with silicate liquids.

We treat oxygen fugacity ($f$O$_2$) as an independent variable, together with temperature ($T$) and composition ($X$), to compute equilibrium partial pressures ($p$) of stable gas species at the liquid-gas interface.

Above this boundary, the atmospheric speciation and the pressure-temperature structure are computed self-consistently to yield emission spectra.

We explore a wide array of plausible compositions, oxygen fugacities (between 6 log$_{10}$ units below- and above the iron-w\"ustite buffer, IW) and irradiation temperatures (2000, 2500, 3000 and 3500 K) relevant to LOPs.

We find that SiO(g), Fe(g) and Mg(g) are the major species below $\sim$IW, ceding to O$_2$(g) and O(g) in more oxidised atmospheres.

The transition between the two regimes demarcates a minimum in total pressure ($P$).

Because $p$ scales linearly with $X$, emission spectra are only modest functions of composition.

By contrast, $f$O$_2$ can vary over orders of magnitude, thus causing commensurate changes in $p$.

Reducing atmospheres show intense SiO emission, creating a temperature inversion in the upper atmosphere.

Conversely, oxidised atmospheres have lower $p$SiO and lack thermal inversions, with resulting emission spectra that mimic that of a black body.

Consequently, the intensity of SiO emission relative to the background, generated by MgO(g), can be used to quantify the $f$O$_2$ of the atmosphere.

Depending on the emission spectroscopy metric of the target, deriving the $f$O$_2$ of known nearby LOPs is possible with a few secondary occultations observed by JWST.