Mantle Fraction Enhancement in Targets of Giant Impacts

Collisions between similar-sized bodies are prevalent throughout the planet-building stages in the formation of the Solar System. These energetic events can produce a single body, as is the case in merging collisions, attributed to low impact velocity and low impact angles. At moderate impact angles and velocities the collisions enter the hit-and-run regime, the most common impact phenomenon to occur in a self-stirred disc of planetesimals (vimp 1-3 vesc). Here the impactor exchanges material with the target and deflects downrange. At higher velocities and non-grazing angles, half the target mass can be lost as debris and the event is called catastrophic disruption. In any collision, debris are dynamically bound to the system or escape. At low to moderate impact velocities, debris is preferentially sourced from mantle material in the impactor. Previous studies have shown that for differentiated bodies, the core mass fraction of the largest remnant increases with impact velocity; this is widely used to assert that giant impacts cannot enhance the relative abundance of mantle material. But by analyzing a large database of giant impact simulations using Smoothed Particle Hydrodynamics code, at a relatively high granularity of parameter space, we find a regime of hit-and-run impacts where mantle mass fraction increases from pre-impact conditions, depositing mantle onto the target. This result arises both in cases of rock mantle over iron core (`chondritic') and water mantle over rock core (`outer solar system'). This suggests a more nuanced picture of hit-and-run impacts, that includes a transition over the range of impact velocities vimp 1.1-1.8vesc between accretionary hit-and-run, where mantle is dumped onto the target by the impactor, and erosive hit-and-run. We examine and discuss the implications for debris production and planetary evolution in giant impacts.