Hypervelocity impact craters in ammonia rich ice

Research on the impact cratering process on icy bodies has been largely based on the most abundant ice, water. However little is known about the influence of other relatively abundant ices such as ammonia. Accordingly, data are presented studying the influence on cratering in ammonia rich ice using spherical 1 mm diameter stainless steel projectiles at velocities of 4.8±0.5 km s⁻¹. The ice target composition ranged from pure water ice, to solutions containing 50% ammonia and 50% water by weight. Results for crater depth, diameter, volume and depth/diameter ratio are given. The results showed that the presence of ammonia in the ice had a very strong influence on crater diameter and morphology. It was found that with only a 10% concentration of ammonia, crater diameter significantly decreased, and then at greater concentrations became independent of ammonia content. Crater depth was independent of the presence of ammonia in the ice, and the crater volume appeared to decrease as ammonia concentration increased. Between ammonia concentrations of 10 and 20% crater morphology visibly changed from wide shallow craters with a deeper central pit to craters with a smoothly increasing depth from the crater rim to centre. Thus, a small amount of ammonia within a water ice surface may have a major effect on crater morphology.