Evolution of Mercury’s Earliest Atmosphere

Abstract MESSENGER observations suggest a magma ocean formed on proto-Mercury, during which evaporation of metals and outgassing C- H-bearing volatiles produced an early atmosphere. Atmospheric escape subsequently occurred by plasma heating, photoevaporation, Jeans escape, photoionization. To quantify atmospheric loss, we combine constraints the lifetime surficial melt, melt composition, composition. Consideration two initial Mercury sizes four compositions determines speciation at given surface temperature. A coupled interior–atmosphere model cooling rate therefore melt. Combining flux calculations provides estimates for total mass loss from Mercury. Loss rates are negligible. Plasma heating photoionization limited homopause diffusion ∼10 6 kg s −1 . photoevaporation depends timing formation assumed efficiency ranges 6.6 to 9.6 The material is sourced below energy rather than limited. timescale efficient chemical exchange less 10,000 yr. Therefore, processes only account equivalent 2.3 km crust (0.3% Mercury’s mass). Accordingly, ≤0.02% H 2 O Na lost. cumulative cannot significantly modify bulk mantle composition stage. high core:mantle ratio volatile-rich may instead reflect variations in its building blocks resulting solar-proximal accretion environment.