Do Giant Impacts Kickstart Rocky Planet Habitability?

IAP weekly specialised seminars / 21 June 2024
Ramon Brasser (Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Budapest, Hungary & Centre for Planetary Habitability, University of Oslo, Norway)
Earth is the only inhabited planet that we know of. Life may have begun about 4.36 billion years ago in the aftermath of a colossal impact with a lunar-sized object some 4.48 billion years ago that produced a temporary hydrogen atmosphere due to the reaction of the impactor’s metal core with terrestrial surface water. This atmosphere provided the strongly reducing environment that created the pre-biotic molecules that may have been suitable to begin life.
Since a rocky planet’s mantle oxidation state is somewhat dependent on its mass, the question arises whether colossal impacts facilitate the kickstarting of origins of life on planets whose masses are in excess of 0.5 Earth mass, on what timescale and with what frequency these impacts take place.
To test this, I ran numerical N-body simulations of exoplanet formation around low-mass stars (M8 to K5) to establish the frequency and timing of large impacts in such systems. The frequency of planet-planet collisions decreases monotonically until about 15 Myr of simulation time, after which most systems have settled into a quasi-stable state. This is much earlier than in the terrestrial planet region of the solar system. Since crust formation on Mars did not start until about 20 Myr in the history of the solar system, it is plausible that most large collisions in systems of close-in rocky exoplanets in the habitable zone of low-mass stars are over before they have begun to form a crust. In that case rocky planets in the habitable zone of more massive stars could be preferred to kickstart origins of life than those around low-mass stars.