A Study of Convection Caused by Sample Acquisition in Solar System Geologic Formations

摘要

In order to accurately model thermal distributions created during coring or drilling into rock or soil formations on solar system bodies, the roles of heat transfer via convection need to be investigated. An understanding of thermal distributions is necessary in order to protect the scientific content of samples, as well as to prevent sublimation of any ice present in the formation. Sublimated water may deposit ice onto cooler areas of the bit, causing the bit to become permanently stuck in its borehole. Three primary sources of convection are discussed herein, namely, increased heat transfer caused by drill-generated heat convecting from the surface of the formation, the "propeller" action of a bit's auger flutes drawing air into a permeable rock and out through the borehole, and pressure waves caused by any recoil of a percussive bit while sampling a permeable rock. An experiment was conducted to test the role of the rotating auger flutes in generating air currents. Surface convection was calculated to remove about 0.71% of heat generated by sample acquisition processes from the surface of a rock in fast wind gusts on Mars. The auger flutes were shown experimentally to be ineffective at driving convective currents because air flow in the borehole was found to be purely rotational with no axial component. At the borehole, even large bit recoils of 1.0 mm were calculated to drive flows of only 4.5 mm/s on Mars in permeable formations with large permeability of κ = 10~(10) m~2.