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 (fO2) 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 log10 units below- and above the iron-wüstite 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 ∼IW, ceding to O2(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, fO2 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 pSiO 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 fO2 of the atmosphere. Depending on the emission spectroscopy metric of the target, deriving the fO2 of known nearby LOPs is possible with a few secondary occultations observed by JWST.
Spectra of all 140 simulations of this study. Each column contains all spectra of equal irradiation temperature, and each row constant composition. In each box, all spectra of constant irradiation temperature are plotted (grey), but only the ones corresponding to the composition indicated in the row are coloured. The colour-coding is made according to their oxygen fugacity (yellow most reducing, brown intermediate, light blue most oxidising). Species responsible for important spectral features are indicated by the pointers, important wavebands by coloured patches. — astro-ph.EP
Effect of composition on the spectrum of a mineral atmosphere at constant fO2 relative to the IW buffer (∆IW-2) and irradiation temperature (2500 K) for an Earth-sized planet. Shown are the stellar spectrum (top), the spectrum of planetary emission (center, coloured) and the secondary occultation depth Fp/F⋆, the ratio between the two (bottom, colours). Flux ratios for outgassed atmospheres may overlap, as evident for the CORL and ARS compositions. Only the ultra-SiO2-poor composition XTREM ist slightly distinct by showing less emission in the TiO band. Both iron-free compositions (HERM and XTREM) do not express iron lines (red field). — astro-ph.EP
Fabian L. Seidler, Paolo A. Sossi, Simon L. Grimm
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2408.16548 [astro-ph.EP] (or arXiv:2408.16548v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2408.16548
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Submission history
From: Fabian Seidler
[v1] Thu, 29 Aug 2024 14:10:43 UTC (38,541 KB)
https://arxiv.org/abs/2408.16548
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