Giant impacts preferentially remove low-mass atmospheres: a generalised scaling law for impact-driven atmospheric loss

Impact-driven atmospheric loss is a key process shaping the diversity of planetary atmospheres, but existing prescriptions for calculating loss from a given impact neglect the role of atmospheric mass and therefore often severely miscalculate how much atmosphere is removed.

Using 3D smoothed particle hydrodynamics simulations, the authors quantify atmospheric loss during giant impacts onto planets with 1–20% mass-fraction atmospheres. They find that less massive, lower-pressure atmospheres are significantly easier to remove. From this, they derive a new scaling law for atmospheric loss that includes atmospheric mass and is applicable to collisions ranging from volatile-poor planetary embryos to super-Earths and sub-Neptunes.

The authors apply the scaling law to N-body simulations of solar system formation and show that including the coupling between atmospheric mass and loss efficiency results in significantly more atmosphere removal. Substantial atmospheric loss events are common for smaller planets (≲0.6 M⊕), while even roughly Earth-mass bodies lose 30–100% of their atmospheres through multiple impacts. They also show that atmospheric loss during the Moon-forming impact strongly depends on both the pre-impact atmospheric mass and the impact style: for a representative ~100 bar pre-impact atmosphere, Earth could have lost between 20% and 80% of its atmosphere.

In any system that undergoes a phase of giant impacts, the atmospheres of its host planets will be unavoidably shaped by the stochasticity of those impacts. The new scaling law can be readily incorporated into models of planet formation and system-wide evolution, offering new insights into the origins of planetary diversity.