Coring Planetary Ices; Their Thermomechanical Behaviour

Spacecraft missions are underway that will mechanically probe the material found at the surface of cometary nuclei. Little is known about the physical properties of these bodies and how their surface material will respond to a probe's sampling mechanisms. Tools such as rotating drills or hammering bits can cause the otherwise pristine material to be damaged and heated. This work addresses the phenomenon of tool-induced heating and examines the properties of ices in three planetary environments in which such coring processes may occur. A unique drilling system has been built to provide data on the thermal response of ices under space-like conditions. Cold (-150 K) samples of water ice and carbon dioxide ice have been formed from the vapour phase and cored with an instrumented cutting head under low pressures. The effort used to core these ices has been measured as a function of temperature, coring speed, and depth rate. Three broad conclusions can be drawn from these experiments. 1) Carbon dioxide ice grown from its vapour at 150 K requires approximately one third of the mechanical effort needed to core or drill the same volume of water ice at a given rate at the same temperature. 2) The power needed to drive a coring tool through water ice at a fixed rotation speed and a given vertical rate trebles as the ice's temperature falls from 240 to 140 K. Specific cutting energies greater than 55 J m-3 have been recorded for ice at 140 K, and dense carbon dioxide at the same temperature has been shown to have one half of that strength displayed by water ice. 3) Mechanical effort is mostly converted into heat in a coring process. Temperature rises of at least 10 K have been measured in dense samples of cryogenic (-140 K) carbon dioxide ice around a coring tool that operated with power levels of no more than 2 W. For the same applied power, water ices are expected to display one third of this temperature rise seen when coring void-free carbon dioxide ice at the same temperature. The magnitude of this heating effect is assessed for a comet sampling tool on the Rosetta comet lander. It is expected that temperature rises of a few degrees will occur in water-ice rich material retrieved by the tool if the comet, at depths of several tens of centimetres, shows little porosity. It is also suggested that higher temperatures will be developed by extracting material from greater depths, where carbon dioxide may be present as a void-free ice