Evolution of Mercury's Earliest Atmosphere

MESSENGER observations suggest a magma ocean formed on proto-Mercury, during which evaporation of metals and outgassing of C- and H-bearing volatiles produced an early atmosphere. Atmospheric escape subsequently occurred by plasma heating, photoevaporation, Jeans escape, and photoionization. To quantify atmospheric loss, we combine constraints on the lifetime of surficial melt, melt composition, and atmospheric composition. Consideration of two initial Mercury sizes and four magma ocean compositions determines the atmospheric speciation at a given surface temperature. A coupled interior-atmosphere model determines the cooling rate and therefore the lifetime of surficial melt. Combining the melt lifetime and escape flux calculations provides estimates for the total mass loss from early Mercury. Loss rates by Jeans escape are negligible. Plasma heating and photoionization are limited by homopause diffusion rates of similar to 10(6) kg s(-1). Loss by photoevaporation depends on the timing of Mercury formation and assumed heating efficiency and ranges from similar to 10(6.6) to similar to 10(9.6) kg s(-1). The material for photoevaporation is sourced from below the homopause and is therefore energy limited rather than diffusion limited. The timescale for efficient interior-atmosphere chemical exchange is less than 10,000 yr. Therefore, escape processes only account for an equivalent loss of less than 2.3 km of crust (0.3% of Mercury's mass). Accordingly, <= 0.02% of the total mass of H2O and Na is lost. Therefore, cumulative loss cannot significantly modify Mercury's bulk mantle composition during the magma ocean stage. Mercury's high core:mantle ratio and volatile-rich surface may instead reflect chemical variations in its building blocks resulting from its solar-proximal accretion environment.

Automated summary

Messenger observations indicate that a magma ocean may have formed on proto-Mercury, leading to the early atmosphere through evaporation of metals and outgassing of volatiles. Atmospheric escape processes include plasma heating, photoevaporation, Jeans escape, and photoionization, which result in relatively small mass loss rates but do not significantly alter the mantle composition of Mercury.