Non-thermal escape of the Martian CO$_2$ atmosphere over time: constrained by Ar isotopes

TL;DR

This study models the non-thermal escape of Mars' CO2 atmosphere over 4.1 billion years, using Ar isotopes to constrain escape rates and assess historical atmospheric pressures and isotope fractionation.

Contribution

It provides the first comprehensive simulation of Martian atmospheric escape over geological time, integrating ion escape, sputtering, and isotope fractionation with surface interactions.

Findings

Non-thermal escape could have removed up to 1.8 bar of CO2 by 4.1 Ga.

Ion escape was dominant before 2.6 Ga under high solar EUV flux.

Isotope fractionation patterns match observed meteorite data for certain atmospheric pressures.

Abstract

The ion escape of Mars' CO$_2$ atmosphere caused by its dissociation products C and O atoms is {simulated} from present time to $\sim 4.1$ billion years ago (Ga) by {numerical models of the upper atmosphere and its interaction with the solar wind}. The planetward-scattered pick-up ions are used for sputtering estimates of exospheric particles including $^{36}$Ar and $^{38}$Ar isotopes. Total ion escape, sputtering and photochemical escape rates are compared. For solar EUV fluxes $\geq$\,3 times that of today's Sun (earlier than $\sim 2.6$ Ga) ion escape becomes the dominant atmospheric non-thermal loss process until thermal escape takes over during the pre-Noachian eon (earlier than $\sim 4.0\,-\,4.1$ Ga). If we extrapolate the total escape of CO$_2$-related dissociation products back in time until $\sim$ 4.1 Ga we obtain a {maximum} theoretical equivalent to CO$_2$ partial pressure of more than {$\sim 0.4$ bar through non-thermal escape during quiet solar wind conditions}. {However, surface-atmosphere interaction and/or extreme solar events such as frequent CMEs could have increased this value even further. By including the surface as a sink, up to 0.9\,bar, or even up to 1.8\,bar in case of hidden carbonate reservoirs, could have been present at 4.1\,Ga} The fractionation of $^{36}$Ar/$^{38}$Ar isotopes through sputtering and volcanic outgassing from its initial chondritic value of 5.3, as measured in the 4.1 billion years old Mars meteorite ALH 84001, until the present day, however, can be reproduced for assumed CO$_2$ partial pressures of {$\sim0.01 -- 0.4$\,bar without, and $\sim0.4 -- 1.8$\,bar including surface sinks, and} depending on the cessation time of the Martian dynamo (assumed between 3.6\,--\,4.0 Ga) - if atmospheric sputtering of Ar started afterwards.