Icarus 168 (2004) 467–474 www.elsevier.com/locate/icarus Hypervelocity impact craters in ammonia rich ice I.D.S. Grey and M.J. Burchell ∗ Centre for Astrophysics and Planetary Science, School of Physical Sciences, University of Kent, Kent CT2 7NR, UK Received 20 February 2003; revised 3 October 2003 Abstract 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.  2003 Elsevier Inc. All rights reserved. Keywords: Ammonia ice; Cratering; Impact processes 1. Introduction Data from ground based observations and spacecraft ex- plorations have confirmed the presence of ammonia ice on a number of icy bodies in the Solar System (e.g., Yarger et al., 1993, and Brown and Calvin, 2000). Ammonia ice (NH 3 ) is one of three principle ices in the Solar System. Oxygen (O) is however much more abundant than nitrogen (N) (Hunten et al., 1984) which explains why H 2 O ice is the predomi- nant form of ice. For this reason it is often suggested (e.g., see Johnson and Nicol, 1987) that rather than existing in a pure form, ammonia ice is more likely to be stable as an am- monia dihydrate (NH 3 –2H 2 O) or monohydrate (NH 3 –H 2 O), i.e., with water mixed in. Of these, ammonia monohydrate is the phase that condenses from a vapour and is stable at low pressures and is thus initially more likely. However, once in the interior of a planetary body, ammonia dihydrate should be a major constituent due to the internal pressure of the body (Johnson and Nicol, 1987). There is evidence that ammonia is present in various icy Solar System bodies. In comets it has been observed * Corresponding author. E-mail address: m.j.burchell@kent.ac.uk (M.J. Burchell). at the level of 1 to 2% (relative to water) as measured in the gas content of the comet tail (e.g., Allen et al., 1987; Meier et al., 1994; Altenhoff et al., 1983; Bird et al., 1997). The presence of ammonia on Pluto’s satellite Charon has been reported (Brown and Calvin (2000) and see Brown (2002) for an extended discussion). It has also been sug- gested that the presence of ammonia is the cause of flow like features that have been observed on the surfaces of many icy bodies (Brown and Calvin, 2000) although the presence of ammonia ices is usually inferred and not confirmed by ob- servations. In general only the surfaces of icy bodies (the satellites of outer planets, comets), etc. have been observed and ammonia ice is known to be destroyed by irradiation (Lanzerotti et al., 1984) making observation difficult. For icy bodies with atmospheres, e.g., Titan, (part of the saturnian system where the planetary nebula condensation tempera- ture was low enough to encourage the formation of ammonia ices), the presence of ammonia in the surface is difficult to determine spectroscopically because of the dense methane rich atmosphere of the satellite. Thus determining the true amount of ammonia ice present on such bodies is difficult. One possible method to confirm the presence of ammonia ices could be to see how ammonia could affect the appear- ance of impact craters on icy bodies. 0019-1035/$ – see front matter  2003 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2003.11.006