Modeling Tenuous Atmospheres with ROCKE-3D GCM: Application to the Paleo-Moon Volcanically-Induced Atmosphere

The currently released version of the ROCKE-3D planetary General Circulation Model (GCM) has a low surface pressure limit of ~5 millibar (Way et al. 2017). This proves to be too restrictive if one wants to simulate climate on such planets as Pluto (~10 microbar surface pressure) or investigate transient atmospheres generated by volcanic outgassing or cometary impacts on otherwise airless bodies. Here we present a version of ROCKE-3D adapted to handle atmospheric pressures down to ~1 microbar at the planetary surface and investigate its behaviour on an example of a paleo-Moon volcanically-induced atmosphere at ~3.5 Ga. The possibility that the ancient Moon could have a tenuous transient atmosphere due to volcanic outgassing has been discussed for decades (Stern 1999), but its estimated thickness is a subject of ongoing research. While instantaneous release of all lunar volcanic volatiles could have produced an atmosphere ~10 millibar thick (Needham & Kring 2017), it is more likely that long intervals between the eruptions would not allow the atmosphere to accumulate to this level (Head et al. 2020) , and at its peak it would not exceed the surface pressure of a few microbars. Here we study such a hypothetical atmosphere with the ROCKE-3D GCM for a range of surface pressures from 1 microbar to 10 millibar. We investigate its circulation patterns, temperature distribution and the ability to transport volatiles. In particular, we conducted a single major volcanic outgassing experiment (Wilson & Head 2018) and investigated the fate of the outgassed water. In all our simulations the outgassed water was quickly delivered to the polar regions, and the efficiency of its transport increased with the decreasing thickness of the atmosphere, likely due to increased relative humidity. This water could have been preserved indefinitely in the lunar polar cold traps and can constitute a significant part of currently observed lunar polar water. The isotopic analysis of lunar polar deposits in future missions will help to validate this hypothesis. The presented research is a work in progress. While most of ROCKE-3D algorithms were adapted to treat thin atmospheres properly, some modules require further work. In particular, the radiation needs to account for non-local thermal equilibrium processes. This will be addressed in our future research.