Atmospheres & Climate

Efficient And Inefficient Hydrodynamic Escape of Exo-satellite Atmospheres Driven By Irradiation From Their Young Giant Planets

By Keith Cowing

Status Report

astro-ph.EP

March 26, 2025

Filed under astro-ph.EP, atmosphere, Exomoon, exoplanet, gas giant

Efficient And Inefficient Hydrodynamic Escape of Exo-satellite Atmospheres Driven By Irradiation From Their Young Giant Planets — A comparison between the values of the various mean opacities defined by Eqns. 14-16, the wavelength dependence of the different radiation fields and actual cross-sections. The left panel sketches the conditions under which 𝛾 > 1 occurs when averaging opacity functions 𝜅 (𝜆) (solid black line). When irradiated by a stellar black-body, the opacity ratio of incoming to outgoing mean opacities is 𝛾 < 1 (the constant red value is below the constant green value). When the irradiating black-body cools, it moves up to the high opacity range in the infrared, but the outgoing radiation remains in the far-infrared, resulting in 𝛾 > 1 (the constant blue line is above the constant green one) and vigorous hydrodynamic escape occurs. The right panel shows that 𝜅_out readily remains low for realistic and cold, molecules such as NH₃, as the natural spacing between the molecular band peaks outpaces the width of a black-body. — astro-ph.EP

The bolometric radiation from a central body is potentially a powerful driver of atmospheric escape from planets or satellites.

When heated above their equilibrium temperatures those satellites, due to their low surface gravity, are be prone to significant atmospheric erosion. Such high temperatures can be reached through a known mechanism: a large ratio of the irradiation to re-radiation opacities of the atmospheric species.

We investigate this mechanism for irradiating black-bodies of sub-stellar temperatures and find that specific molecules exist, such as NH₃ and CH₄, which develop temperature inversions under the irradiation of young post-formation giant planets. These non-isothermal temperature profiles lead to escape rates that can significantly exceed isothermal Parker-model escape rates evaluated at the satellite’s equilibrium temperature. Our results indicate that exo-satellites can lose most of their atmospheric mass through this mechanism if the cooling of the exo-satellite’s interior is not too rapid.

In all scenarios, we find a hierarchical ordering of escape rates of atmospheric species due to thermal decoupling in the upper atmosphere. This thermal decoupling leads to a natural depletion of CH₄ and retention of NH₃ in our models.

We find that giant planets with masses above 2mJ_up, for cold starts and above 1m_Jup in hot start scenarios are able to remove the majority of a Titan analogue’s atmosphere. Hence, finding and characterizing exomoon atmospheres in hypothetical future surveys can constrain the post-formation cooling behaviour of giant planets.

Matthäus Schulik, James E. Owen, Richard A. Booth, Shun Fai Ling, Shun Ping Wong

Comments: 22 pdf pages. Accepted for publication in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2503.18049 [astro-ph.EP] (or arXiv:2503.18049v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2503.18049
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Submission history
From: Matthäus Schulik
[v1] Sun, 23 Mar 2025 12:34:36 UTC (1,318 KB)
https://arxiv.org/abs/2503.18049
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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