Impact crater formation on comet surface and its implications for thermal history of comet nucleus

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We carried out impact experiments on snow to simulate the impact crater found on the surface of comet nuclei. Especially, in order to clarify the relationship between the thermal history of the comet surface and the crater morphology, the impact experiments were conducted using sintered snow samples. The size of the impact crater was found to increase with the increase of the impact velocity up to 150m/s when the snow projectile was impacted on the snow with the porosity of 40% sintered at-10℃ during 15 min. These results show that the crater volume has a power law relationship to the projectile kinetic energy (Ek) with the power law index of 0.5 for the Ek. The crater size was found to decrease with the increase of the sintering time from 3 min. to 60 hrs. at -10℃. The crater volume has a power law relationship to the sintering time. A high velocity impact of a nylon projectile with the velocity of 2.7km was carried out on the snow with the porosity of 50%, and then we found that the outer edge of the crater was spalled out to form a ring like structure of the crater rim and the crater had a very spherical shape with the refrozen thin ice layer on the inner wall. This results show that the impact of the nylon projectile higher than 2.7km/s (impact pressure could be 3.9GPa) can induce impact melting of snow to spread it out on the crater wall. In order to estimate the temperature increase of the high velocity impact quantitatively, ice-on-ice impact experiments were conducted to measure the post shock temperature on the crater by using an infrared video camera. As a result, we found that the post shock temperature was measured to be from 2K to 9K when the ice projectile was impacted up to 300m/s. Thus, it was found that the projectile kinetic energy was partitioned into the crater heated at the post shock temperature with the partition coefficient of 1% to 10%.