A magma ocean origin for Mercury's earliest exosphere

MESSENGER observations used to constrain surface composition suggest a global magma ocean formed on early Mercury [1, 2]. Our study models the coupled evolution of the Hermean magma ocean and atmosphere, and determines the extent of exospheric loss from an atmosphere formed by evaporation of a magma ocean. Using our framework to couple the interior and exterior chemical reservoirs, we evaluate a range of possible atmospheric evolution scenarios for early Mercury. These include the possibility that Mercury was fully-accreted before a mantle stripping event caused by a giant impact [3] led to degassing, or alternatively that the building blocks of Mercury were already relatively volatile-free.

Assuming an initial surface temperature of 2500 K and an oxygen fugacity fixed at 1 log unit below the Iron-Wüstite buffer, we find that the Hermean magma ocean cooled to 1500 K (the rheological transition) in around 400 to 9000 yrs, depending on the efficiency of radiative heat transfer in the atmosphere. We investigate the behaviour of two endmember cases: (1) a present-day sized Mercury that is volatile free and thus cools in a manner similar to that of a blackbody, and (2) a larger Mercury that is sufficiently volatile-rich to provide a greenhouse atmosphere that delays cooling. During the magma ocean stage, evaporation and sublimation of oxide components from the molten silicate magma contribute to the growth of the atmosphere, in addition to volatile outgassing. For the endmember case with initial volatile abundances based on enstatite chondrite-like precursors, the atmosphere is dominated by CO, CO₂, H₂, and H₂O, with minor abundances of SiO, Na, K, Mg, and Fe gas species. For the endmember case without major volatiles (C, H), the atmospheric composition is dominated by metal- and metal oxide gas species only.

We apply a Monte Carlo (MC) atmospheric loss model [4] to calculate exospheric losses for all pathways and find that photoionization is the dominant loss mechanism for early Mercury. Cases both with and without major volatiles reveal that the atmosphere lasts between 100 and 250 years before the near-surface of Mercury becomes solid. Preliminary MC results show that photo-dissociation considerably alters the atmospheric composition, efficiently breaking SiO₂ into SiO⁺ and O. Solar wind and magnetospheric plasma leads to rapid evacuation of the ionized species from the neutral exosphere whereas oxygen is lost efficiently by Jeans escape.

Using our coupled framework, future modelling efforts will aim to understand if and how the evaporation of the magma ocean of early Mercury has modified its surface composition, with a view to interpreting BepiColombo observations. Hence our work provides an important step to connect Mercury’s formation, earliest evolution, and upcoming observations.

References: [1] Vander Kaaden, K. E. and McCubbin, F. M. (2016). Cosmochim. Ac., 173, 246–263. [2] Berthet, S., Malavergne, V., and Righter, K. (2009). Geochim. Cosmochim. Ac., 73(20), 6402–6420. [3] Benz, W., et al. (2008). Mercury, pp. 7–20. Springer. [4] Wurz, P., and Lammer, H. (2003). Icarus, 164(1), 1–13