Exoplanetology: Exoplanets & Exomoons

Phase Equilibria of Sub-Neptunes and Super-Earths

By Keith Cowing
Status Report
astro-ph.EP
August 22, 2024
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Phase Equilibria of Sub-Neptunes and Super-Earths
Number densities of condensate droplets and gas molecules, and their respective molecular weights, as a function of height R/Rp for the atmosphere shown in Figure 8 with Kzz = 1×104 . Left: Numbers of dots at each height in the atmosphere represent the relative number densities of droplets, and the symbol sizes are proportional to droplet sizes. Right: Numbers of dots represent the relative number densities of molecules. In both plots, colors indicate molecular weights at each height (note different color scales in the two panels). Curves for absolute number densities of droplets (left) and gas molecules (right) are overlain for comparison. — astro-ph.EP

We investigate the consequences of non-ideal chemical interaction between silicate and overlying hydrogen-rich envelopes for rocky planets using basic tenets of phase equilibria. Based on our current understanding of the temperature and pressure conditions for complete miscibility of silicate and hydrogen, we find that the silicate-hydrogen binary solvus will dictate the nature of atmospheres and internal layering in rocky planets that garnered H2-rich primary atmospheres.

The temperatures at the surfaces of supercritical magma oceans will correspond to the silicate-hydrogen solvus. As a result, the radial positions of supercritical magma ocean-atmosphere interfaces, rather than their temperatures and pressures, should reflect the thermal states of these planets.

The conditions prescribed by the solvus influence the structure of the atmosphere, and thus the transit radii of sub-Neptunes. Separation of iron-rich metal to form metal cores in sub-Neptunes and super-Earths is not assured due to prospects for neutral buoyancy of metal in silicate melt induced by dissolution of H, Si, and O in the metal at high temperatures.

Edward D. Young, Lars Stixrude, James G. Rogers, Hilke E. Schlichting, Sarah P. Marcum

Comments: 13 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2408.11321 [astro-ph.EP] (or arXiv:2408.11321v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2408.11321
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Submission history
From: Edward Young
[v1] Wed, 21 Aug 2024 04:02:29 UTC (14,477 KB)
https://arxiv.org/abs/2408.11321

Astrobiology

Keith Cowing

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) 🖖🏻

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