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The formation of planetary atmospheres less efficient than anticipated

Researchers from the IPGP - Université Paris Cité show that the degassing of volatiles at the surface of magmatic oceans is far from instantaneous.

The formation of planetary atmospheres less efficient than anticipated

Artist's view of a planetary magma ocean the outgassing its volatiles at the surface. © Adobe Stock - IPGP

Publication date: 06/12/2022

Press, Research

The early evolutionary stages of terrestrial planets such as Mercury, Venus, the Earth or Mars are characterized by the presence of gigantic magma oceans. Until now, it was commonly accepted that volatile species dissolved in these magma oceans were instantaneously outgassed, thereby contributing to the genesis of planetary atmospheres. Indeed, it was thought that the very rapid convective motions due to the low-viscosity magma would lead the bulk of the magma ocean to reach almost immediately the low pressures close to the surface, where outgassing occurs, and would thus loose its volatiles on this occasion. However, this hypothesis had never been tested.

Result of high-resolution numerical simulations of a fluid in vigorous convection, analogous to a silicate magma ocean. Hot ascending (red/yellow) and cold descending (blue) thermal plumes transport fluid to the surface and to the bottom of the magma ocean, respectively. © IPGP

For the first time, researchers from the IPGP – Université Paris Cité performed high-resolution numerical simulations of a fluid in vigorous convection. Contrary to the common assumption, these numerical experiments show that magma ocean outgassing is far from being instantaneous, and that despite elevated convective velocities, dissolved volatile species approach the surface without necessarily reaching the depths at which the exsolution and outgassing occurs. Therefore, a considerable fraction of volatiles can remain dissolved in the magma ocean and are trapped in the planetary interiors at the end of this evolutionary phase.

View of a convective system analogous to a magma ocean after 15 convective transits (the time required for a parcel of fluid to travel 15 times the distance between the surface and the base). Even after several dozen convective cycles, a large fraction of the magma ocean has retained its dissolved volatiles (blue), the rest has reached the exsolution depth, dc, and outgassed its volatiles (yellow). Contrary to what was commonly accepted until now, magma ocean outgassing can be much slower and even limited to shallow depths between the surface and dc. © IPGP

These results suggest that the formation of planetary atmospheres is much less efficient than anticipated, thus revealing a different distribution of water between the different planetary reservoirs, and affecting the appearance of the first water oceans where life can develop, but also the overall subsequent evolution of rocky planets. In several cases, magma ocean convective dynamics restricts water outgassing to the most superficial areas, leading to the rapid cooling of a water-rich mantle bounded by a dry atmosphere, where the formation of a water ocean is then compromised.
 
The new scenarios reshape our understanding of the conditions necessary for the emergence of life at the surface of rocky planets, and impact the search and the characterization of the environments favorable to its development in extra-solar planetary systems.

Ref : A. Salvador, H. Samuel, Convective outgassing efficiency in planetary magma oceans: insights from computational fluid dynamics, Icarus, 390, doi:10.1016/j.icarus.2022.115265, 2023

 

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