Outgassing of the Hadean magma ocean: a computational perspective

The solubility of various volatiles in magma oceans plays a significant role in the formation and evolution of planetary atmospheres. Using ab initio molecular dynamics simulations, we investigate the dissolution of various volatiles in a magma ocean with bulk silicate Earth composition under conditions relevant to both early Earth and exoplanetary systems.

We find that hydrogen is highly soluble in silicate magma oceans, and its solubility increases dramatically with pressure and temperature. In particular for exoplanets, like sub-Neptunes, this solubility influences the structure and functioning of the entire planet. It significantly alters the redox state of the system and causes a massive outflux of oxygen. The results are large-scale formation of water vapor and the release of other complex chemical species. This process profoundly impacts the thermal and chemical evolution of exoplanets, particularly sub-Neptunes, whose atmospheres may show observable spectral signatures linked to magma ocean interactions. At conditions characteristic to the beginning of the Hadean, the Earth’s magma ocean could have easily dissolved large amounts of hydrogen. As a result, the amount of water present in the early atmosphere was determined by a fine balance between water degassing and hydrogen solubility. Changes in the redox state of the magma at shallow conditions would further influence this balance.

With regard to noble gases and CO/CO2, our simulations show that they are profoundly incompatible in silicate melts. They easily degas under lower pressure conditions, particularly when they are present jointly in the melt. The partial pressures of either of these gases need to reach at least a couple GPa to prevent degassing. These results suggest that the magma ocean contributed to the CO2-reach atmosphere of the Hadean, by both limited ingassing in the aftermath of the giant impact, and by massive outgassing, once the magma ocean was put in place.