Measurements of ejection velocities in collisional disruption of ice spheres

Impact experiments are performed on ice spheres to measure the velocity field of ejected ice fragments and the conditions under which the fragments would reaccumulate during accretion in the outer solar system are considered. A single-stage light gas gun set in a cold room at −18°C and an image-converter camera running at 2 × 105-1 × 104 frames per second with a xenon flash lamp are used for observing the collisional phenomena. Spherical projectiles of ice (mp = 1.5 g) collide head-on with spherical targets (Mt = 1.5, 12, 172 g) at 150–690 m s−1. The ejection velocity is observed to vary with the initial position and ranges from 3 to 110 of the impact velocity (Vi). The ejection velocity of fragments at the rear side of the target (Ve) varies with distance from the impact point according to a power law relation, Ve = Va(1D)−n, where Va is the antipodal velocity, l and D are the distance and the target diameter, and n = 1.5–2.0. Va depends on the specific energy (Q) at a constant mass ratio (mpMt = 0.13) and the empirical dependence is written as Va = 0.35 × Q0.52. The ejection velocity of fine fragments formed by the jetting process near the impact point is determined to be 1.7–2.9 times as large as the impact velocity irrespective of the target size and the impact velocity.