[PPS08-P06] High-velocity impact cratering experiments on snow simulating porous icy bodies: Effect of impact melting on crater size scaling law
Keywords:porous icy bodies, impact crater, impact melting
The snow targets were prepared by compaction of ice particles smaller than several tens of mm. Ice particles were poured into the container and compressed to control the porosity. The porosity was 50 or 60%. The sintering duration changed from 2 to 6 days. Two types of projectile, an aluminum sphere with the diameter of 2 mm and a polycarbonate sphere with the diameter of 4.7 mm, were launched from two-stage light gas gun installed in Kobe University. The impact velocity was 0.9 km/s for polycarbonate projectile, and it changed from 2 to 4 km/s for aluminum projectile. The target was set in the vacuum chamber at -15 ºC evacuated below about 150 Pa. The collisional disruption was observed using a high-speed digital video camera with the frame rate of 105 fps and the shutter speed of 380 ns. After the shot, the target was cut using the band-saw to measure the crater diameter and depth and to observe the impact melting layer.
Just after the impact, the snow particles were ejected from the impact point to make the cone-shaped envelope. As the time passed, the shape of the neck of ejecta envelope changed from cone to pillar. The pillar-shaped ejecta envelope has not been observed on snow so far [e.g., Arakawa & Yasui, 2011]. After the pillar-shaped ejecta envelope was observed, some large spall fragments were ejected. The number of spall fragments for 50%-porosity target was larger.
Next, we observed the shape of impact crater. On the target surface, the shallow spalling area was observed. This spalling area had the irregular shape such as circle, ellipse, and rectangle. The circular pit entrance was observed at the center of the bottom of spalling area. From the cross section of impact crater, it was found that the width of the pit entrance was narrow and the pit bulged from the entrance. On the bottom of pit, the long and narrow hole formed by the projectile penetration was observed. The intact polycarbonate projectile was recovered from the bottom of penetration hole while the aluminum projectile was destructed and a part of the projectile was recovered. The impact melting layer was observed on the wall of pit and around the projectile at the impact energy larger than 40 J. Furthermore, in the case of 50%-porosity target, the compaction layer with the thickness of 1-2 mm was observed outside the impact melting layer.
Finally, we examined the relationship between the impact energy and the maximum spalling diameter, Ds, or the pit diameter, Dp. The Ds for 50%-porosity target was larger than that for 60%-porosity target: the Ds for 50% was 48 mm, 1.4 times larger than that for 60% at 100 J. On the other hand, the Dp for 60% was larger than that for 50%: the Dp for 60% was 20 mm, 1.6 times larger than that for 50% at 100 J.
[References] Zahnle et al. (2003), Icarus 163, 263; Arakawa & Yasui (2011), Icarus 216, 1.