Thermally Driven Atmospheric Escape: Transition from Diffusion-limited to Drag-off Escape
We examine the transition from diffusion-limited to drag-off escape using a direct simulation Monte Carlo (DSMC) model developed to simulate planetary atmospheres, referred to as Harrah. A 1D spherically symmetric DSMC model is used to simulate two-component atmospheres to focus on the impact of rar...
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| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
IOP Publishing
2025-01-01
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| Series: | The Planetary Science Journal |
| Subjects: | |
| Online Access: | https://doi.org/10.3847/PSJ/ada369 |
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| Summary: | We examine the transition from diffusion-limited to drag-off escape using a direct simulation Monte Carlo (DSMC) model developed to simulate planetary atmospheres, referred to as Harrah. A 1D spherically symmetric DSMC model is used to simulate two-component atmospheres to focus on the impact of rarefaction on thermal diffusion. The results for the escape rates and thermal structure are characterized using a mass-averaged Jeans parameter, λ _avg,0 , and Knudsen number, Kn _a,0 , defined at a reference radial distance r _0 . As a test of the model, we simulated the escape of N _2 and H _2 self-consistently for conditions of Pluto’s upper atmosphere observed during the New Horizons mission. As expected, the DSMC result for H _2 escape is consistent with the diffusion-limited approximation, and H _2 escape has little effect on the background N _2 atmosphere. Then we examined the transition from diffusion-limited to drag-off escape for two-component atmospheres characterized by mass-averaged Knudsen numbers and Jeans parameters of Kn _a,0 ≈ 0.01–0.1 and λ _avg,0 ≈ 0.5–13, respectively. We found that escape transitioned from hydrodynamic escape to an enhanced Jeans-like escape at λ _avg,0 ≈ 3.0–3.6. For values of λ _avg,0 < 3.0–3.6, escape was hydrodynamic and in the blow-off regime. For values of λ _avg,0 >≈ 3.6, the DSMC results indicated that using the diffusion-limited and drag-off approximations can lead to inaccurate estimates of the escape rate. |
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| ISSN: | 2632-3338 |