Atmospheres & Climate

Novel Physics of Escaping Secondary Atmospheres May Shape the Cosmic Shoreline

By Keith Cowing

Status Report

astro-ph.EP

December 9, 2024

Filed under astro-ph.EP, atmosphere, Earth, exoplanet, faint young sun, imaging, JWST, LHS-1140 b, LHS-1140 c, Mars, TRAPPIST-1, TRAPPIST-1 b

Novel Physics of Escaping Secondary Atmospheres May Shape the Cosmic Shoreline — Regime diagram for high molecular weight escape in terms of the polytropic index 0.65 ≤ γ ≤ 0.95 and the hydrodynamic escape parameter 10 ≤ λ0 ≤ 400. The heat map indicates the ratio of sonic to base temperatures 10 − 100. Contours of uniform ratio of sonic to base radius (red-dashed) and uniform sonic to base density in log units (green dashed). The Young Sun Earth (c.f. Figures 4 & 8B) and Young Sun Mars (see Figures 5 & 8A) calculations are represented by images of each planet. This figure can be compared to the solution space of polytropic solutions (Fig 3). For large enough λ0, the polytropic model of steep XUV inversions breaks down because line cooling will begin to dominate over advection. The overlaying pink patch is to illustrate that the shift is broad and dependent on boundary conditions and dimensional parameters. Rightward of the transition, the collisional radiative thermostat will control the resulting global ion outflow (see Section 5). — astro-ph.EP

Recent James Webb Space Telescope observations of cool, rocky exoplanets reveal a probable lack of thick atmospheres, suggesting prevalent escape of the secondary atmospheres formed after losing primordial hydrogen.

Yet, simulations indicate that hydrodynamic escape of secondary atmospheres, composed of nitrogen and carbon dioxide, requires intense fluxes of ionizing radiation (XUV) to overcome the effects of high molecular weight and efficient line cooling. This transonic outflow of hot, ionized metals (not hydrogen) presents a novel astrophysical regime ripe for exploration. We introduce an analytic framework to determine which planets retain or lose their atmospheres, positioning them on either side of the cosmic shoreline.

We model the radial structure of escaping atmospheres as polytropic expansions – power-law relationships between density and temperature driven by local XUV heating. Our approach diagnoses line cooling with a three-level atom model and incorporates how ion-electron interactions reduce mean molecular weight. Crucially, hydrodynamic escape onsets for a threshold XUV flux dependent upon the atmosphere’s gravitational binding. Ensuing escape rates either scale linearly with XUV flux when weakly ionized (energy-limited) or are controlled by a collisional-radiative thermostat when strongly ionized.

Thus, airlessness is determined by whether the XUV flux surpasses the critical threshold during the star’s active periods, accounting for expendable primordial hydrogen and revival by volcanism.

We explore atmospheric escape from Young-Sun Mars and Earth, LHS-1140 b and c, and TRAPPIST-1 b. Our modeling characterizes the bottleneck of atmospheric loss on the occurrence of observable Earth-like habitats and offers analytic tools for future studies.

Richard D. Chatterjee, Raymond T. Pierrehumbert

Comments: Submitted to ApJ; comments welcome
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2412.05188 [astro-ph.EP] (or arXiv:2412.05188v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2412.05188
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Submission history
From: Richard Chatterjee
[v1] Fri, 6 Dec 2024 17:09:33 UTC (5,235 KB)
https://arxiv.org/abs/2412.05188
Astrobiology, Astrogeology,

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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