Abstract
Evidence from Arrokoth and comets strongly suggests a very low density
for this and similar small Kuiper belt objects. Plausible compositions
imply very high porosities, in excess of 70%, and low compaction crush
strengths. If so, craters on Arrokoth (especially Sky, its largest)
formed largely by compaction of pore space and material displacement.
This is consistent with geological evidence from New Horizons
imaging. High porosity reduces cratering efficiency in the gravity
regime whereas compaction moves it towards a crush strength scaling.
Compaction also guarantees that most impactor kinetic energy is taken up
as waste heat near the impact point, with momentum transferred to the
rest of the body by elastic waves only. Monte Carlo simulations of
Sky-forming conditions indicate that the momentum imparted likely
separated Arrokoth’s two lobes, but displacement was limited by
dissipation at the neck. Unusual strength properties are not required to
preserve Arrokoth’s bilobate configuration.