Extreme hydrodynamic atmospheric loss near the critical thermal escape regime

TL;DR

This study models hydrodynamic atmospheric escape near the critical thermal escape regime for Mars-like planets, revealing a sharp increase in escape rates when the Jeans escape parameter drops below a critical value of about 2.5.

Contribution

It introduces a 1-D hydrodynamic model to analyze atmospheric escape near the critical thermal escape regime, highlighting the transition point and its impact on escape rates.

Findings

Steady hydrodynamic solutions exist for $eta > 2.5$

Escape rates increase with lower boundary temperature

No stationary solutions for $eta \,\leq\, 2.5$, leading to rapid atmospheric loss

Abstract

By considering martian-like planetary embryos inside the habitable zone of solar-like stars we study the behavior of the hydrodynamic atmospheric escape of hydrogen for small values of the Jeans escape parameter $\beta < 3$, near the base of the thermosphere, that is defined as a ratio of the gravitational and thermal energy. Our study is based on a 1-D hydrodynamic upper atmosphere model that calculates the volume heating rate in a hydrogen dominated thermosphere due to the absorption of the stellar soft X-ray and extreme ultraviolet (XUV) flux. We find that when the $\beta$ value near the mesopause/homopause level exceeds a critical value of $\sim$2.5, there exists a steady hydrodynamic solution with a smooth transition from subsonic to supersonic flow. For a fixed XUV flux, the escape rate of the upper atmosphere is an increasing function of the temperature at the lower boundary. Our model results indicate a crucial enhancement of the atmospheric escape rate, when the Jeans escape parameter $\beta$ decreases to this critical value. When $\beta$ becomes $\leq$2.5, there is no stationary hydrodynamic transition from subsonic to supersonic flow. This is the case of a fast non-stationary atmospheric expansion that results in extreme thermal atmospheric escape rates.