Speaker: Anders Johansen (Copenhagen U.)
Title: Rapid accretion of terrestrial planets and the outgassing of their first atmospheres
Abstract: 
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 10-100 million years. However, comparison of the isotopic composition of meteorites to the composition of the Earth have revealed that our planet incorporated a large fraction of material originating in the outer regions of the protoplanetary disc. This could be the first evidence that Earth formed mainly by accretion of pebbles that drifted through the protoplanetary disc, on a time-scale as short as 3-5 million years. I present here a theoretical model for the growth and migration of terrestrial planets within the protoplanetary disc. I discuss how the immense 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.