Thermally-driven atmospheric escape: Transition from hydrodynamic to Jeans escape

TL;DR

This paper investigates the transition from hydrodynamic to Jeans atmospheric escape using simulations, identifying key Jeans parameter thresholds and their implications for planetary atmospheres.

Contribution

It provides a detailed simulation-based analysis of the transition from hydrodynamic to Jeans escape, clarifying the role of the Jeans parameter and scaling laws for planetary atmospheres.

Findings

Transition occurs over a narrow Jeans parameter range (~2-3).

Escape rate aligns with Jeans rate for high Jeans parameters (>6).

Results applicable to planetary atmospheres like Pluto and early Earth.

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, R0, and well below the exobase, this transition is shown to occur over a surprisingly narrow range of Jeans parameters evaluated at R0: {\lambda}0 ~ 2-3. The Jeans parameter {\lambda}0 ~ 2.1 roughly corresponds to the upper limit for isentropic, supersonic outflow and for {\lambda}0 >3 escape occurs on a molecule by molecule basis. For {\lambda}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.