Rapid accretion of terrestrial planets and the outgassing of their first atmospheres

by Anders Johansen (University of Copenhagen, Lund University)

Wednesday 5 Jan 2022, 13:30 → 14:30 Europe/Stockholm

Albano 3: 6228 - Mega (22 seats) (Albano Building 3)

Hybrid talk: Mega (6228, Hus 3, Albano) + https://stockholmuniversity.zoom.us/j/530682073

Terrestrial planets have traditionally been thought to form by collisions between protoplanets taking place mostly after the dissipation of the protoplanetary disc, on time-scales of 30-100 million years. I present here a new theoretical model where terrestrial planets grow instead by accreting small pebbles in the protoplanetary disc within 3-5 million years. I discuss how the immense pebble accretion heat leads to extensive melting of the growing planets and to the emergence of deep magma oceans. Volatiles such as water, carbon and nitrogen are accreted with the pebbles and partitioned between atmosphere, magma ocean and core. The end of the accretion phase leads to rapid crystallisation of the magma ocean and outgassing of the first planetary atmospheres. The degree of oxidation of the magma ocean is key to determining the atmospheric composition. Earth and Venus outgas thick atmospheres of CO2 and water, while the more reduced martian magma ocean outgasses primarily H2 and CO. The martian atmosphere undergoes hydrodynamical escape of hydrogen atoms that drag along most of the carbon and water budget of the planet. This way the key differences between Venus, Earth and Mars can be understood within a single framework of rapid terrestrial planet formation by pebble accretion with just a single giant impact needed to form the Earth’s moon.

Johansen_2022-Jan-05.mp4