Jeans parameter and thermal escape
At an exobase radius r_ex and temperature T_ex, λ=GMm/(r_ex k_BT_ex) compares a species' binding energy with thermal energy.
Key knowledge
At an exobase radius r_ex and temperature T_ex, λ=GMm/(r_ex k_BT_ex) compares a species' binding energy with thermal energy. In the ideal isotropic Maxwellian Jeans model, the upward escaping number flux is n_ex v_th(1+λ)e^(−λ)/(2√π), with v_th=√(2k_BT_ex/m). A larger λ suppresses this high-speed tail.
Values and equations
λ=GMm/(r_ex k_BT_ex); v_th=√(2k_BT_ex/m); Φ_J=n_ex v_th(1+λ)e^(−λ)/(2√π)
- λ
- Jeans binding/thermal parameter 1
- G
- gravitational constant m³ kg⁻¹ s⁻²
- M,m
- planet and escaping-particle masses kg
- r_ex
- exobase radius from planet center m
- T_ex
- exobase temperature K
- k_B
- Boltzmann constant J K⁻¹
- n_ex
- species number density at the exobase m⁻³
- v_th
- most-probable Maxwell speed m s⁻¹
- Φ_J
- escaping particle-number flux m⁻² s⁻¹
Conditions and scope
- Use the specified species mass m and exobase number density, a collisionless outward escape boundary and a near-Maxwellian distribution without a large bulk outflow.
- Hydrodynamic flow, nonthermal loss, heating above the exobase, stellar tides and collisions can invalidate this estimate. A temperature from planetary radiative balance is not automatically T_ex, and no universal atmospheric lifetime or sharp transition threshold follows.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
- Thermal escape: scale height, Jeans speed and parameter equations2–4Frank X. Timmes — Arizona State University, cococubedReference guide
- Thermally-driven atmospheric escape,2010 original research abstract: model-dependent kinetic/hydrodynamic transitionAlexey N. Volkov, Robert E. Johnson, Orenthal J. Tucker and Justin T. ErwinReference guide
Evidence and review
Read the actual Jeans-speed/parameter expressions on the original code reference and original paper abstract for its restricted simulation boundary/model. Flux=n_ex times the stated Jeans speed is explicitly derived here; the abstract's model-specific numeric transition thresholds are not universalized.
2026-10-10 · Independent AI editorial review: All44 exact root-authored compact records, equations/variable units/conditions and140 origin edges accepted after independently reading final6/18 with bounded primary bodies and matching prior38/122 scientific fingerprints unchanged. Exact claim/edge review is distinct from combined inventory scope and separate CLE26 approval; no entire cited books or old-corpus experimental revalidation is asserted. Receipt: cosmos-completion-independent-content-and-inventory-review.1.
Entry details
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- uni-cosmos-jeans-atmospheric-escape
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- core.1
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- University core expansion