In-situ measurement of high velocoty impact fragments of icy planets

冰系行星间的高速碰撞是外太阳系行星吸积过程中一个重要的物理现象。然后，利用安装在冷室（-10℃）的微型两级光气枪对水冰进行了撞击实验，阐明了撞击破碎的基本过程，研究了撞击碎片的再聚集条件。结果表明，无论10^-3和10^-5之间的质量比如何，冰靶中的冲击压力都按照P = a (Lp/d)^2的关系衰减，其中Lp为弹丸尺寸，d为传播距离。在10 ~ 200gpa的冲击压力下，冲击波以4.4 km/s的速度传播；在200兆帕时达到2.6公里/秒。在200 MPa以下，剪切破裂区域以1.7 km/s的速度扩展，直到冲击压力低于60 MPa。对冰态行星在高速碰撞中的再聚集条件进行了估计，结果表明，在撞击速度为500 ~ 5 km，质量比为10^-2 ~ 10^ -5的任何条件下，半径大于500 km的母天体破碎碎片都可以再聚集。

High velocity impact among icy planets is an important physical phenomenon related to the planetary accretion process in the outer solar system. Then, an impact experiment on water ice was made by using a micro two-stage light gas gun installed in a cold room (-10ﾟC) to clarify elementary processes of the collisional disruption and to study reaccumulation condition of the impact fragments. As a result, we clarified that the shock pressure attenuated in the ice target according to the relation of P = a (Lp/d)^2 irrespective of the mass ratio between 10^-3 to 10^-5, where Lp is a projectile size and d is a propagation distance. We also observed that the shock wave propagated with the velocity of 4.4 km/s at the shock pressure from 10 to 200 GPa ; 2.6 km/s at 200 MPa. Below 200 MPa, the shear fracture region expanded with the velocity of 1.7 km/s until the shock pressure decreased lower than 60 MPa. The reaccumulation condition of icy planets in the high velocity collisions was estimated to show that fragments disrupted from parent bodies with the radius larger than 500 km could reaccumulate at any conditions of the impact velocity from 500 m/s to 5 km/s and the mass ratio from 10^-2 to l0^-5.