Ivan D.S. Grey

Laboratory investigations of hypervelocity impact cratering in ice

Abstract Hypervelocity impact experiments on water ice targets have been performed using a two stage light gas gun. The resulting craters were measured to obtain the crater depth and diameter. From the data set for 23 impact craters, damage equations have been obtained which give the crater depth (diameter) in terms of the dependence on impact velocity, projectile diameter and projectile density. The resulting damage equations are compared to those for another brittle material, glass. Scaling of the excavated crater volume with energy is shown to obey a simple power law over 10 orders of magnitude in energy (10 −7 to 10 3 J).

Oblique Hypervelocity Impacts on Thick Glass Targets

Abstract Data are reported for hypervelocity impact cratering in glass arising from impacts of 1 mm diameter aluminium spheres impacting thick glass targets at 5.11±0.13 km s−1. The angle of incidence of the projectiles was varied from 0° (normal incidence) to 80° (glancing incidence). The crater resulting from each impact was measured and it was found that the crater size does not change significantly unless impacts are at >45° from the normal. Crater shape (as given by ratios of crater measurements, e.g. depth/diameter) is insensitive to impact angle up to extreme angles of incidence.

Laboratory investigations of the temperature dependence of hypervelocity impact cratering in ice

Abstract Laboratory investigations by hypervelocity impact cratering in water ices are usually carried out at temperatures of approximate 250–265 K. However, icy surfaces in the Solar System are typically at lower temperatures. Accordingly a study of the temperature dependence of cratering in water ice has been carried out using a two-stage light gas gun firing mm-sized projectile at 5 – 6 km s −1 . The temperature of the water ice targets has been varied in the range 152–253 K. The variation in depth, diameter and volume of the resulting craters is presented as a function of temperature.

Survivability of Bacteria in Hypervelocity Impacts on Ice

As part of the arrival stage of the Panspermia process, organisms must endure a hypervelocity impact either into the atmosphere or onto the surface of the destination planet. The impacts associated with this arrival stage are studied in this paper. To this end, the two-stage light gas gun at the University of Kent has been used to fire bacteria-laden projectiles, at velocities of approximately 5 km s−1, onto semi-solid nutrient medium, and solid and porous ice targets, representing planetary oceans and icy surfaces. The targets were then analysed to investigate whether the bacteria survived the impacts. It was found that bacteria can survive hypervelocity impacts at 5 km s−1, with a survival rate of 1 per 3.5 million using targets of nutrient gel. With ice targets no survival has been found yet with a limit on survival of less than 1 per 0.4 million.

Estimating Crater Size for Hypervelocity Impacts on Small Icy Bodies (e.g. Comet Nucleus)

The morphology and size expected for impact craters on small icy bodies are presented. Such bodies are for example minor satellites of the outer planets (some of which are ice covered) or comet nuclei. The differences between the impact craters that result on such bodies, compared to those on more traditional ice targets (effectively large, well consolidated, semi-infinite ice surfaces) is discussed with particular reference to the impact on a comet nucleus expected in the Deep Impact space mission. Finally extrapolation of laboratory scale experiments is carried out to try and quantify crater size and shape for impacts on small, porous bodies. It is found that given our present knowledge, simple scaling with impact energy produces a result compatible to scaling via more sophisticated methods. The handling of the influence of the detailed composition of the ice target (porosity, volatile content, silicate content etc.) is less certain.