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

我们在雪上进行了撞击实验，以模拟在彗星核表面发现的撞击坑。特别是，为了弄清彗星表面的热历史与陨石坑形态之间的关系，利用烧结的雪样进行了撞击实验。当弹丸撞击孔隙率为40%、烧结温度为-10℃、时间为15min的雪地时，弹坑尺寸随撞击速度的增加而增大，最高为150m/S。结果表明，弹坑体积与弹丸动能呈幂函数关系，其幂指数为0.5。从3min开始，随着烧结时间的延长，凹坑尺寸逐渐减小。到60小时。在-10℃。凹坑体积与烧结时间呈幂函数关系。用速度为2.7公里的尼龙弹高速撞击孔隙率为50%的雪面，发现弹坑的外缘被剥落，形成环形的环形结构，弹坑的形状非常球形，内壁有一层重新冻结的薄冰层。结果表明，尼龙弹的撞击速度高于2.7公里/S(撞击压力可达3.9 Gpa)时，会导致雪的撞击融化，使雪在弹坑壁上铺展开来。为了定量估计高速撞击的温升，利用红外摄像机进行了冰-冰撞击实验，测量了撞击后陨石坑的温度。结果表明，当冰弹撞击速度达到300m/S时，冲击后温度为2K~9K，从而发现在冲击后温度下，弹丸动能被分配到被加热的弹坑中，分配系数为1%~10%。

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%.