A thermal model for analysis and control of drilling in icy formations on mars

Geologic samples acquired from Mars or solar system bodies via dry coring or drilling operations undergo substantial heating. A finite element thermal model has been created to provide knowledge of the complete thermal environment throughout the bit and rock formation during a drilling process, using only properties of the rock formation and the sampling system. A set of experimental drilling operations was performed upon a block of 45MPa Indiana limestone outfitted with thermal sensors to validate the accuracy of the model during penetration as well as during periods of rest designed to allow the system to cool. Tests were performed by a prototype Mars Sample Return (MSR) drill operating in a 7Torr atmosphere. The maximum error between predicted and observed temperatures was found to be 1.26°C for a test predicted to generate in excess of 100° of temperature rise within the rock. Analysis of this error, in conjunction with penetration rate data, suggest that this difference may be caused by the drill bit's depth-dependent ability to clear hot, cut material from the borehole. The thermal model will be an essential tool in preventing potentially catastrophic freeze-in conditions caused by melting and subsequent re-freezing of contained ice, as well as helping to minimize and record the thermal alternation of the acquired samples prior to in situ analysis or Earth return.