Thermally Driven Atmospheric Escape: Transition From Hydrodynamic to Jeans Escape

Abstract

Thermally-driven atmospheric escape evolves from an organized outflow (hydrodynamic escape) to escape on a molecule by molecules basis (Jeans escape) with increasing Jeans parameter, the ratio of the gravitational to thermal energy of molecules in a planet's atmosphere.This transition is described here using the direct simulation Monte Carlo method for a single component spherically symmetric atmosphere. When the heating is predominantly below the lower boundary of the simulation region, R 0 , and well below the exobase, this transition is shown to occur over a surprisingly narrow range of Jeans parameters evaluated at R 0 : λ 0 ~ 2-3. The Jeans parameter λ 0 ~ 2.1 roughly corresponds to the upper limit for isentropic, supersonic outflow and for λ 0 >3 escape occurs on a molecule by molecule basis. For λ 0 > ~6, it is shown that the escape rate does not deviate significantly from the familiar Jeans rate evaluated at the nominal exobase, contrary to what has been suggested. Scaling by the Jeans parameter and the Knudsen number, escape calculations for Pluto and an early Earth's atmosphere are evaluated, and the results presented here can be applied to thermally-induced escape from a number of solar and extrasolar planetary bodies.

Extreme hydrodynamic atmospheric loss near the critical thermal escape regime

“…It is also worth to mention that our results agree with the kinetic simulations of Volkov et al (2011) whose simulations yield a very sharp growth of the escape rate, when the β parameter decreases from 2.7 to 2. For β values 2 this kinetic simulations predict only supersonic flow.…”

“…Generally, the main necessary condition for hydrodynamic escape modeling is that the sonic point c 2014 RAS should be within the collision dominated atmosphere where the Knudsen number Kn is sufficiently small. Volkov et al (2011) suggested a criterion where hydrodynamic models should be valid without problems as long as the Kn 0.1. In our present study we investigate the escape criteria and efficiency of atomic hydrogen from a Mars-like planetary embryo at 1 AU that experiences XUV flux values between 45-100 times higher than that of today's solar value.…”

XUV-Exposed, Non-Hydrostatic Hydrogen-Rich Upper Atmospheres of Terrestrial Planets. Part I: Atmospheric Expansion and Thermal Escape

“…For instance, in the case of the well-studied hydrogen-rich hot gas giant HD 209458b with a skin temperature T 0 * 1350 K and a XUV flux *453 times higher compared to that of today's solar value, Eq. 29 leads to a thermal mass loss rate in the order of a few *10 10 g s -1 (e.g., Erkaev et al, 2007;Lammer et al, 2009), which is comparable to models solving similar hydrodynamic equations of mass, momentum, and energy conservation as in the present study (e.g., Yelle, 2004;Tian et al, 2005a;García Muñ oz, 2007;Penz et al, 2008;Volkov et al, 2011;Koskinen et al, 2013).…”

Thermal escape from extrasolar giant planets

“…These observations indicate that close-in EGPs such as HD209458b are surrounded by a hot thermosphere composed of atomic hydrogen that extends to several planetary radii and provide the required line of [20,21] and their application to close-in EGPs [8,9]. We then proceed to generalize the results to a sample of known EGPs to make specific predictions about the nature of their upper atmospheres.…”