Mass loss of Mercury's earliest atmosphere during its magma ocean state
Abstract
The origin of Mercury remains unknown. Its large iron core and surface composition inferred from MESSENGER supports the presence of a magma ocean early in its history. Therefore, we modeled the cooling, degassing and atmospheric loss of proto-Mercury. Our models consider two planetary radii for Mercury (assuming either its present-day size or a larger mantle), three magma compositions, and three scenarios for the radiative-transfer in the outgassed atmosphere. Our sequential modeling workflow ultimately provides the input to model atmospheric loss using a Monte Carlo code. Key outputs of our workflow including the timescale of magma ocean cooling and volatile degassing, the vapor pressure of oxides in the atmosphere, and atmospheric photochemistry. For an initial surface temperature of 2500 K, the magma ocean cools to 1500 K within 400 to 9000 years, depending on the efficiency of radiative transfer in the atmosphere. A cooling time of 400 years corresponds to a volatile-free proto-Mercury radiating as a blackbody with its present-day size. In contrast, a cooling time of 9000 years results from a volatile rich proto-Mercury having a greenhouse atmosphere, with a larger mantle, before a mantle stripping removed most of the mantle due to a giant impact or similar. Based on chondritic abundances, we found that outgassed atmospheres at 2000 K for both small and large proto-Mercury are massive and dominated by CO$ _{2}$ and H$ _{2}$O with minor contributions of species containing Si, Na, K, Mg, and Fe. For either of the magma compositions that we consider, ignoring interior outgassing of volatiles produces a thin atmosphere of about 10$ ^{-3}$ bar, which is dominated by Na, K, NaOH and KOH. The simulations of atmospheric loss through ionization showed that under the extreme UV of a potent early Sun, 100 % of the particles reaching Mercury's exosphere are ionized and are lost to the magnetospheric plasma because of ion pick-up processes. Our results revealed a mass loss flux of about 10$ ^{6}$ kg/s during the first few 100 years after Mercury was accreted.
- Publication:
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43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E.450J