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

冰行星间的高速碰撞是与太阳系外行星吸积过程有关的一种重要物理现象。然后，利用安装在冷室(-10ﾟC)中的微型二级轻气炮对水冰进行了撞击实验，以阐明碰撞破裂的基本过程，并研究了撞击碎片的再积累条件。结果表明，无论质量比在10^-3到10^-5之间，冲击波压力在冰靶中的衰减关系为P=a(Lp/d)^2，其中Lp是弹丸尺寸，d是传播距离。我们还观察到激波在10~200 GPa时以4.4公里/S的速度传播，在200 Mpa时以2.6公里/S的速度传播。在200 Mpa以下，剪切断裂区以1.7 Km/S的速度扩展，直至冲击压力降至60 Mpa以下。冰行星在高速碰撞中的再聚集条件估计表明，在撞击速度从500m/S到5公里/S，质量比从10^-2到10^-5的任何条件下，半径大于500公里的母体碎块都可以重新积累。

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.