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

冰系行星间的高速碰撞是与太阳系外行星吸积过程相关的重要物理现象。然后，利用微型二级轻气炮在-10 ° C的冷室中对水冰进行了碰撞实验，以阐明碰撞破裂的基本过程，并研究碰撞碎片的再积聚条件。结果表明，无论质量比在10^-3到10^-5之间如何，冰靶中的冲击压力都按照P = a（Lp/d）^2的关系衰减，其中Lp是射弹尺寸，d是传播距离。在10 ~ 200 GPa的冲击压力范围内，冲击波的传播速度为4.4km/s，在200 MPa时为2.6km/s。在200 MPa以下，剪切破裂区以1.7 km/s的速度扩展，直至冲击压力降至60 MPa以下。对高速碰撞中冰质行星的再聚集条件进行了估算，结果表明，在碰撞速度为500 ~ 5 km/s、质量比为10^-2 ~ 10 ^-5的任何条件下，从半径大于500 km的母体分裂出来的碎片都可以再聚集。

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.