14 January: Matt Roche

Speaker: Matt Roche (University of Bristol)

Date: Wednesday 14 January 2026

Time: 15:00

Location: Physics 3.21 Berry

Atmosphere–ocean loss during giant impacts and the volatile evolution of terrestrial planets

The primary epoch of planetary accretion concludes with giant impacts – highly energetic collisions between protoplanets that can play a key role in shaping a planet’s inventory of volatile elements. Previous work has shown that single giant impacts have the potential to eject a significant amount of a planet’s atmosphere and ocean but that the efficiency of atmospheric loss depends strongly on the impact parameters, atmospheric properties, and pre-impact surface conditions. Fully quantifying the role of giant impacts in planetary volatile evolution requires a more complete understanding of the mechanisms driving loss during impacts. Here, we use suites of high-resolution 3D smoothed particle hydrodynamics simulations to determine the mechanisms by which atmosphere and ocean are lost during a giant impact. Considering these mechanisms operating separately, we derive new scaling laws that can precisely approximate loss from a wide range of planet masses with thin, terrestrial-like atmospheres for any combination of impactor mass, impact velocity, and angle. We then apply our scaling laws to the results of existing N-body simulations for solar system formation to gain insight into the role that giant impacts play in the volatile evolution of the terrestrial planets. Finally, we show that sequences of giant impacts onto an ocean-bearing proto-Earth are responsible for Earth’s characteristic depletion in carbon, nitrogen, and hydrogen relative to its chondritic building blocks. We suggest that late giant impacts could be necessary to produce Earth-like habitable worlds and, as a stochastic process, could explain the divergent histories of otherwise similar planets, such as Earth and Venus.