Preservation of pristine materials under impact craters formed on comet nuclei

Comet nuclei are the most primitive small bodies in the solar system, and would contain highly volatiles and amorphous water ice. However, comet nuclei may lose their primordial properties after high-velocity impacts with other small bodies. Such high-velocity impacts might also cause the characteristic circular structures and pinnacles on the comet-nucleus surface. However, the true origin of these geological structure remains uncertain. We here sought to clarify the shape of impact craters and the effects of post-shock heating on the comet surfaces. We conducted high-velocity impact experiments at velocities of 3-5.8 km s(-1) on porous-ice targets with 40%, 50% and 60% porosity, and measured the post-shock temperature around the impact crater. The hard, dense refrozen layer formed on the crater wall resembled the pinnacles and sharp pit rims observed on comet nuclei, suggesting that such structures on comets may originate from impacts. In addition, the crater size scaling law for porous ice targets was successfully obtained in the strength-dominated regime. From the temperature profiles obtained, the energy partition coefficient of the projectile kinetic energy to the post-shock heat, gamma, was estimated as follows: gamma=0.08, 0.27 and 0.63 for 40%, 50% and 60% porosity at 4.2 km s(-1) impact velocity, and gamma=0.58 and 0.22 for 50% porosity at 3 km s(-1) and 5.8 km s(-1) impact velocities. These results, together with our crater size scaling law for porous ice, were used to estimate the temperature distribution induced by post-shock heat on the comet-nucleus surface. Volatile depletion and crystallization of amorphous water ice can occur at distances less than twice the crater radius from the crater center-that is, the post-shock heat effects are confined to the area around the crater. This might suggest that the comet nucleus surface remains relatively primitive after impacts.