Non-thermal escape of molecular hydrogen from Mars

TL;DR

This paper presents a detailed quantum-mechanical analysis of non-thermal molecular hydrogen escape from Mars, highlighting the role of hot atomic oxygen collisions and its implications for atmospheric evolution.

Contribution

It provides the first comprehensive quantum calculations of H$_2$ collision cross sections and estimates non-thermal escape fluxes, including isotopic variations, from Mars.

Findings

Non-thermal H$_2$ escape flux is about 1.9×10^5 cm$^{-2}$s$^{-1}$.

Escape rates of HD and D$_2$ exceed Jeans rates significantly.

Escaping H$_2$ molecules have higher rotational states than thermal distribution.

Abstract

We present a detailed theoretical analysis of a non-thermal escape of molecular hydrogen from Mars induced by collisions with hot atomic oxygen from martian corona. To accurately describe the energy transfer in O + H$_2(v,j)$ collisions, we performed extensive quantum-mechanical calculations of state-to-state elastic, inelastic, and reactive cross sections. The escape flux of H$_2$ molecules was evaluated using a simplified 1D column model of the martian atmosphere with realistic densities of atmospheric gases and hot oxygen production rates for the low solar activity conditions. An average density of the non-thermal escape flux of H$_2$ of $1.9\times10^5$ cm$^{-2}$s$^{-1}$ was obtained considering energetic O atoms produced in dissociative recombinations of O$_{2}^{+}$ ions. Predicted rovibrational distribution of the escaping H$_2$ was found to contain a significant fraction of higher rotational states. While the non-thermal escape rate was found to be lower than Jeans flux for H$_2$ molecules, the non-thermal escape rates of HD and D$_2$ are significantly higher than their respective Jeans rates. The accurate values of non-thermal escape fluxes of different molecular isotopes of H$_2$ may be important in analyses of evolution of the martian atmosphere. The described molecular ejection mechanism is general and expected to contribute to atmospheric escape of H$_2$ and other light molecules from planets, satellites, and exoplanetary bodies.