Heat Flow Measurement and Drilling from the Moon
Measurement of heat flow that originates in the deep interior of the moon and is released from the surface regolith has long been considered important in understanding the moon’s origin, because mapping its geographic variation will help researchers in constraining the internal thermal structure and distribution of heat-producing elements (U, Th, and K). An international group of space agencies including NASA hopes to deploy a network of geophysical instruments on the moon via a series of robotic missions in the next decade. Heat flow measurement capability will likely be included in the instrument packages. Heat flow is obtained as a product of the thermal gradient in, and the thermal conductivity of, the subsurface. The measurement requires drilling into lunar regolith and installing sensors in the borehole. The thermal gradient is determined from temperature measurements at different depths down the borehole. The thermal conductivity is obtained by applying heat to the well bore formation and measuring how quickly it dissipates. The sensors must be deployed below ~5-m depth in order to avoid the influence of the diurnal, annual, and other types of time fluctuation of the insolation, which dominates the shallow subsurface thermal regime. The two heat flow measurements made on the Apollo 15 and 17 missions did not reach such depth. Drilling deep holes may be one of the biggest technological challenges for the payload-constrained robotic missions. The instrumentation we propose for future missions utilizes a new, light-weight (~6 kg), percussive penetrometer in inserting a heat flow sensor assembly to the desired depth. Preliminary tests of several penetrometer designs were recently conducted in manual operation modes. Each design consisted of three parts: the rotary percussive actuator for creating the hammering penetrating force, the detachable cone that was hammered into lunar regolith simulant, and the connecting rod that held the detachable cone to the rotary percussive hammer. For the simulant, we used finely crushed diabase powders with physical texture much-like that of lunar regolith. The simulant was held in a 10-meter tall, 0.5-m wide ‘chimney’. The penetrating rod was extended by a 1-m section at a time. All the penetrators tested reached near the bottom of the chimney within 3-7 minutes depending on the percussive energy of the actuator. The test results give optimism for deep-penetrating heat flow sensor deployment on future missions.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009