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CATASTROPHIC DISRUPTION OF MIXED ICE:SAND BODIES

Introduction: Impact driven catastrophic disruption of Solar System bodies is an important evolutionary driver for the size distributions of bodies found today. The recent discoveries of more and more Pluto sized bodies (not just small Edgeworth Kuiper belt objects but bodies comparable in size to Pluto itself, e.g. Sedna, Quaoar etc.) indicates that as a class the smaller icy planetary bodies are the most frequent class of planetary body in this Solar System (assuming one accepts them as planets). Although there is a large volume of space in the outer solar system, the large number of small bodies predicted out there means that these bodies will be subject to collisions from smaller bodies [1] at speeds of a few km s [2]. Indeed it is interesting to note the recent report that Pluto has 3 observable satellites and not just one (Charon), indicating that there are still many interesting bodies to be discovered in that part of the Solar System. Experimental studies of the energy needed to disrupt targets have long been carried out. Since laboratory scales are much smaller than those of real bodies in space, the results have to be scaled to larger sizes and combined with the energy need to disperse a shattered body against gravitational re-assembly. Nevertheless a knowledge of the critical energy for disruption at one scale (the laboratory) is a fixed point that can be obtained. Although an increasing number of studies of disruption have looked at ice targets, few have considered mixed ice: sand targets. Since the bulk density of Pluto is estimated to be 2000 kg m, with 60-70% rocky materials and the remainder ices (mostly water ice), it is not immediately obvious that data from disruption of pure ice targets is relevant. Thus in this work we have carried out a set of impact experiments on ice: sand targets with a bulk density of 1800 kg m and approximately 80% sand to 20% water ice. In our experiments, for simplicity, we have a uniform mix of sand and ice. It is of course suspected that Pluto will be a differentiated body (i.e. rocky core, icy surface layer) although this has not yet been demonstrated. Experimental Method: The two stage light gas gun of the University of Kent was used in this work [3]. The ice targets were made by cold water and sand and rapidly freezing. This provided targets with minimum porosity and uniform mixture of sand and ice throughout. The targets were then left in a freezer at 255 K for 4 days before use. We have also experimented with making targets with lower concentrations of sand, but in these the sand settles out nonuniformly in the frozen target. Accordingly we have used the method of mixing crushed ice, sand and water to make 50:50 sand: ice mixture targets. But, whilst uniform in their distribution of sand, these targets are slightly porous and so results from them are not included here (as porosity also influences disruption energies [4, 5, 6]). The targets were not spheres (due to practical issues with moulds) but were instead squat cylinders (mass typically 740 g) with a diameter equal to their height. During each shot a flat face of the cylinder was used as the target face. Impacts were at normal incidence on the centre of this face. The projectiles were copper spheres, either 1 or 2 mm diameter. The projectile diameter was varied from shot to shot along with the impact speed to provide a range of impact energies. The projectile speed was measured in each shot to better than 1%. The impact energy was divided by the target mass to give the impact energy density Q (J kg) for each impact. If the result of an impact were a crater, the crater depth, diameter and volume were measured. If the result was a disrupted target, the masses of all largest fragments were obtained along with their largest linear dimensions.