Molecular-Kinetic Simulations of Escape from the Ex-planet and Exoplanets: Criterion for Transonic Flow

TL;DR

This paper uses molecular-kinetic simulations to analyze atmospheric escape from planets, providing a criterion for when the outflow transitions to transonic flow, which impacts escape rate estimates and atmospheric modeling.

Contribution

It introduces a new criterion for predicting transonic flow in planetary atmospheres using molecular-kinetic simulations, improving upon traditional energy-limited escape models.

Findings

Energy-limited escape approximation is valid during adiabatic cooling.

In the sonic regime, escape rates can be significantly underestimated.

Large escape rates can produce subsonic, highly extended atmospheres.

Abstract

The equations of gas dynamics are extensively used to describe atmospheric loss from solar system bodies and exoplanets even though the boundary conditions at infinity are not uniquely defined. Using molecular-kinetic simulations that correctly treat the transition from the continuum to the rarefied region, we confirm that the energy-limited escape approximation is valid when adiabatic expansion is the dominant cooling process. However, this does not imply that the outflow goes sonic. In fact in the sonic regime, the energy limited approximation can significantly under estimate the escape rate. Rather large escape rates and concomitant adiabatic cooling can produce atmospheres with subsonic flow that are highly extended. Since this affects the heating rate of the upper atmosphere and the interaction with external fields and plasmas, we give a criterion for estimating when the outflow goes transonic in the continuum region. This is applied to early terrestrial atmospheres, exoplanet atmospheres, and the atmosphere of the ex-planet, Pluto, all of which have large escape rates. The paper and its erratum, combined here, are published: ApJL 768, L4 (2013); ApJ, 779, L30 (2013).