Atmospheric Erosion by Giant Impacts onto Terrestrial Planets: A Scaling Law for any Speed, Angle, Mass, and Density

Abstract:

Giant impacts dominate many planets' late accretion and evolution, including the Earth's, and can build, erode, or completely destroy a young atmosphere. We examine the mechanisms by which atmosphere can be eroded by giant impacts, based on 3D smoothed particle hydrodynamics simulations. We present a new scaling law to predict the loss of atmosphere from planetary collisions for any speed, angle, impactor mass, target mass, and body compositions, in the regime of broadly terrestrial planets with relatively thin atmospheres.

Different collision scenarios lead to extremely different behaviours and consequences for the planets. In spite of this complexity, the fraction of lost atmosphere is fitted well by a power law, and is independent of the total system mass for a constant impactor:total mass ratio. We find no evident departure from the trend at the extremes of the parameters explored. The scaling law can readily be incorporated into models of planet formation.

Slow impactors can also deliver a significant mass of atmosphere, but always accompanied by larger proportions of their mantle and core. In the Moon-forming collision, only around 10% of the atmosphere would have been lost from the immediate effects of a "canonical" impact, up to about 60% in more violent scenarios.

Left figure: An early cross-section snapshot from a high-speed, grazing impact simulation using ∼10⁸ SPH particles, coloured by their material. The colour luminosity varies with the internal energy.

Right figure: The lost mass fraction of atmosphere from the primary set of 259 simulations, as a function of the speed, masses, densities, and impact angle. The black line shows our scaling law. The colours and shapes indicate the impactor:target mass ratio and the target mass, respectively.