Heating, Excitation, Dissociation, and Ionization of Molecules by High-Energy Photons in Planetary Atmospheres

Photoionization by high-energy photons creates nonthermal electrons with a broad range of energies that heat and chemically transform the atmospheres of planets.

The specifics of the interactions are notably different when the gas is atomic or molecular. Motivated by the idea that molecules survive to high altitudes in some exoplanets, we built a model for the energy transfer from nonthermal electrons to the H₂O, H₂, and O₂ molecules.

Our calculations show that the primary electrons of energy above about a hundred eV, a likely outcome from X-ray photoionization at moderately high atmospheric densities, expend most of their energy in ionization, dissociation, and electronic excitation collisions.

In contrast, the primary electrons of less than about ten eV, such as those produced by extreme-ultraviolet photons at low densities, expend most of their energy in momentum transfer (heating), rotational, and vibrational excitation collisions.

The partitioning between channels with weak thresholds is particularly sensitive to local fractional ionization. The transition between these two situations introduces a parallel transition in the way that the stellar energy is deposited in the atmosphere.

Our calculations show that the nonthermal electrons enhance the ionization rate by a factor of a few or more with respect to photoionization alone but may not greatly contribute to the direct dissociation of molecules unless the local flux of far-ultraviolet photons is relatively weak.

These findings highlight the importance of tracking the energy from the incident photons to the nonthermal electrons and onto the gas for problems concerned with the remote sensing and energy balance of exoplanet atmospheres.

Antonio García Muñoz, Ewan Bataille

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2506.12956 [astro-ph.EP] (or arXiv:2506.12956v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2506.12956
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Journal reference: ACS Earth and Space Chemistry, vol. 8, issue 12, pp. 2652-2663 Pub Date: December 2024 DOI: ACS Earth and Space Chemistry, vol. 8, issue 12, pp. 2652-2663
Related DOI:
https://doi.org/10.1021/acsearthspacechem.4c00273
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Submission history
From: Antonio García Muñoz
[v1] Sun, 15 Jun 2025 20:38:29 UTC (626 KB)
https://arxiv.org/abs/2506.12956

Astrobiology, Astrochemistry,