Investigating the past atmospheric escape rate from Mars using a semi-empirical model

Water was abundant on early Mars but disappeared, likely escaping into interplanetary space.

Large-scale planetary magnetic fields were long thought to shield planetary atmospheres and limit atmospheric escape, suggesting that Mars lost most of its water after its intrinsic magnetic field vanished. However, observations of atmospheric escape from Mars, Venus and Earth as well as recent numerical models question the protective effect of planetary magnetic fields on atmospheric erosion.

We use a semi-empirical model of atmospheric escape to investigate the past oxygen and hydrogen escape rate from Mars. This model uses physical considerations and a magnetic field model to extrapolate present-day observations to past solar and planetary conditions. It accounts for the variation of the planetary magnetic field and of the solar wind dynamic pressure and EUV/UV flux. Our modelling results show that for a more active Sun, atmospheric escape peaks for a weak planetary magnetization level as both unmagnetized escape processes like ion pick-up and sputtering can occur at the same time as magnetized escape processes in the polar regions. This study suggests that the water loss rate from the Martian atmosphere may have peaked when Mars was (still) magnetized rather than when it was unmagnetized.