Heating,\nExcitation, Dissociation, and Ionization\nof Molecules by High-Energy Photons in Planetary Atmospheres

Photoionization by high-energy photons creates nonthermal\nelectrons\nwith a broad range of energies that heat and chemically transform\nthe atmospheres of planets. The specifics of the interactions are\nnotably different when the gas is atomic or molecular. Motivated by\nthe idea that molecules survive to high altitudes in some exoplanets,\nwe built a model for the energy transfer from nonthermal electrons\nto the H<sub>2</sub>O, H<sub>2</sub>, and O<sub>2</sub> molecules.\nOur calculations show that the primary electrons of energy above about\na hundred eV, a likely outcome from X-ray photoionization at moderately\nhigh atmospheric densities, expend most of their energy in ionization,\ndissociation, and electronic excitation collisions. In contrast, the\nprimary electrons of less than about ten eV, such as those produced\nby extreme-ultraviolet photons at low densities, expend most of their\nenergy in momentum transfer (heating), rotational, and vibrational\nexcitation collisions. The partitioning between channels with weak\nthresholds is particularly sensitive to local fractional ionization.\nThe transition between these two situations introduces a parallel\ntransition in the way that the stellar energy is deposited in the\natmosphere. Our calculations show that the nonthermal electrons enhance\nthe ionization rate by a factor of a few or more with respect to photoionization\nalone but may not greatly contribute to the direct dissociation of\nmolecules unless the local flux of far-ultraviolet photons is relatively\nweak. These findings highlight the importance of tracking the energy\nfrom the incident photons to the nonthermal electrons and onto the\ngas for problems concerned with the remote sensing and energy balance\nof exoplanet atmospheres.