Diagram illustrating secondary and hybrid atmospheres. Secondary atmospheres derive entirely from geochemical outgassing, while hybrid atmospheres derive from outgassing in a primordial hydrogen-helium envelope left over from the process of formation and evolution. — astro-ph.EP

The atmospheres of small exoplanets likely derive from a combination of geochemical outgassing and primordial gases left behind by formation.

Secondary atmospheres, such as those of Earth, Mars, and Venus, arise from outgassing. Persistent outgassing in long-lived primordial hydrogen-helium envelopes produces hybrid atmospheres of which there are no examples in the solar system.

We construct a unified theoretical framework for calculating the outgassing chemistry of secondary and hybrid atmospheres, where the input parameters are surface pressure, mantle oxidation and sulphidation states, and primordial atmospheric hydrogen content, helium and nitrogen.

Non-ideal gases (quantified by the fugacity coefficient) and non-ideal mixtures of gaseous components (quantified by the activity coefficient) are taken into account. Secondary and hybrid atmospheres exhibit a rich diversity of chemistries, including hydrogen-dominated atmospheres.

The abundance ratio of carbon dioxide to carbon monoxide serves as a powerful diagnostic for mantle oxygen fugacity, which could possibly be constrained by spectra from the James Webb Space Telescope in the near future.

Methane-dominated atmospheres are difficult to produce and require specific conditions: surface atmospheric pressures greater than ∼10 bar, reduced mantle (little oxidized) and decreased magma temperatures (compared to modern Earth).

Future work should include photochemistry in these calculations and clarify the general role of atmospheric escape. Exoplanet science should quantify the relationship between oxygen mass and fugacity for a sample of super-Earths and sub-Neptunes; such an empirical relation already exists for the bodies of the solar system.

Meng Tian, ​​Kevin Heng

Comments: 24 pages, 23 figures, 1 table
Subjects: Terrestrial and planetary astrophysics (astro-ph.EP); Atmospheric and Oceanic Physics (physics.ao-ph); Geophysics (physics.geo-ph)
Cite as: arXiv:2301.10217 [astro-ph.EP] (or arXiv:2301.10217v1 [astro-ph.EP] for this release)
Submission History
By: Kevin Heng
[v1] Tue Jan 24 2023 6:45:55 PM UTC (5198 KB)
Astrobiology, Astrochemistry

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