Impacts into Ice–Silicate Mixtures: Crater Morphologies, Volumes, Depth-to-Diameter Ratios, and Yield

Abstract

In this and a companion paper, we analyze the strength-dominated impact process into ice–silicate mixtures via experimental techniques, which is important in understanding the evolution of icy satellite surfaces as well as the dynamics of planetary rings. We used the plasmadynamic accelerator (PDA) and the electrothermal accelerator (ET) of the Fachbereich Raumfahrt of the Technische Universität München to perform impact experiments. The projectile velocities were varied between 0.9 and 11.8 km/s and projectile masses were between 1.7·10 −11 kg and 7.2·10 −10 kg at the plasma accelerator (projectile material was glass); at the electrothermal accelerator, projectile velocities were between 1.1 to 3.3 km/s and projectile masses between 6·10 −6 kg and 3·10 −5 kg (projectile material: Nylon). The kinetic energies ranged over 6 orders of magnitude, from 8.8·10 −5 J to 70 J. Silicate contents of the target were varied from 5% to 20% (mass), the rest was water ice. The target temperature was 250K±5K. Typical crater diameters were in the order of 0.1 to 1 mm at the PDA and several cm at the ET. In general, the crater morphology can be described by a conical spallation zone, in which a central crater, at the electrothermal accelerator with raised rim, was observed. In the center, the craters were much brighter than the undisturbed target surface. At the ET experiments, we measured the silicate content in different regions of the crater. It does not change significantly; actually, there is a slight tendency for the silicate content to be higher in the center of the crater as compared to the undisturbed surface. Together with data from Frisch (1990, Dissertation, TU München; and 1992, Proceedings of the Workshop on Hypervelocity Impacts in Space) for pure ice and Moore et al. (1963, Trans. Min. Eng. 229, 258–262) for basalt, an empirical relationship for the crater volume depending on the target and projectile parameters is found. Typically, crater volumes in pure ice are about two orders of magnitude larger than craters in pure silicates, for identical projectile mass and velocity. The crater yield, defined as the ratio of ejected mass to projectile mass, was found to be in the order of 1 to 10 3. We derived an empirical relationship for the yield as a function of the target and projectile properties. This relationship was compared to the necessary yields found by Durisen et al. (1992, Icarus 100, 364–393) for the description of mass transfer in Saturn's A- and B-ring. We find that their required yields are about one to two orders of magnitude larger than our experiments showed and conclude that more knowledge of the flux of incoming meteoroids and the ring's viscosity are necessary.