Quantifying Thermal and Non-Thermal H and D Escape at Mars Using a Fully-Coupled Ion-Neutral Photochemical Model

Atmospheric models of Mars produce low water loss estimates compared to geological studies. Because water is the main reservoir of hydrogen (H) and deuterium (D) on Mars, H and D escape is the primary vector for water loss. Although H escape from Mars has rightly received a great deal of attention, the same cannot be said of D escape, and especially of non-thermal D escape. As non-thermal escape is expected to be much more important for D than thermal escape, this is a clear hole in the literature.

To plug this hole, we set out to quantify non-thermal D escape, which depends heavily on both ion processes and the neutral water isotope HDO. We had three scientific aims and one technical aim: (1) to model the structure of the deuterated ionosphere of Mars; (2) to understand the processes controlling non-thermal D escape, (3) to quantify thermal and non-thermal escape of H and D; and (4) to do all of the above in a surface-to-space photochemical model without a fixed background atmosphere or the assumption of photochemical equilibrium. Such a model is better suited to understanding the feedbacks between ion and neutral chemistry, as well as lower-upper atmospheric coupling, which is important to the water cycle.

We have substantially upgraded our existing surface-to-space photochemical model accordingly, and also added a self-consistent ionosphere, D-bearing ions, and the associated chemistry and 1D dynamics. We then use the model to simulate three model atmospheres for solar minimum, mean, and maximum conditions with different insolation and neutral temperature profiles. The model atmospheres are evolved to long-term equilibrium in order to obtain reasonable bounds on the possible escape regimes.

We find that non-thermal escape dominates D escape under all solar conditions (from 65-99%), which agrees with the literature for minimum and mean but differs for solar maximum. Our findings for H escape also agree with existing studies. We will present a detailed analysis of the escape, processes, and structure of the deuterated ionosphere, a comparison of results as a function of modeling assumptions with previous work, and also discuss the modeling approaches used and how they can benefit the larger Mars modeling community.