Fragmentation of ice by low velocity impact

Low velocity impact experiments (0.14 to l km/ s) carried out in polycrystalline water ice targets at 257 and 81 K resulted in interactions which can be assigned to four fragmentation classes, cratering, erosion, disruption, and total fragmentation. Specific kinetic energies for the transitions between these classes range from l x 105 to 7 x 105 ergs/g for 81 K ice and from 3 x 105 to 2 x 106 ergs/g for 257 K ice. These values are about one to two orders of magnitude below those for silicate rocks. The mass vs. cumulative number distribution of fragments in our experiments can be described by a simple power law, similar to that observed in fragmented rocks in both the laboratory and in nature. The logarithmic slopes of cumulative number vs. fragment weight vary between 0.9 and 1.8 decreasing with increasing projectile energy and are approximately independent of target temperature. The shapes of fragments resulting from erosion and disruption of ice targets are significantly less spherical for 257 K targets than for 81 K targets. Fragment sphericity increases with increasing projectile energy at 257 K, but no similar trend is observed for 81 K ice. Our results support the hypothesis that the specific projectile energy is a measure for target comminution for a relatively wide range of projectile energies and target masses. We apply our results to the collisional interaction of icy planetary bodies and find that the complete destruction of a target body with radii between 50 m and 100 km· range from 1017 to 1027 ergs. Energies corresponding to basaltic bodies of the same size range from 1018 to 1028 ergs. Our experiments suggest that regolith components on icy planets resemble those on rocky planetary bodies in size and shape. We predict that the initial shapes of icy particles in the Saturnian ring system were roughly spherical. The initial mass distribution of ring particles should follow a power law with a slope of 1.5.