Lunar Heat Flow Measurements: Previous Findings and Future Plans

Seiichi Nagihara¹, Patrick Taylor³, Bruce Milam³, Yosio Nakamura², and Paul Lowman^3
1Department of Geosciences, Texas Tech University, Lubbock, TX
²Institute for Geophysics, University of Texas at Austin, Austin, TX
³NASA Goddard Space Flight Center, Greenbelt, MD

Knowledge of heat flow through lunar crust and its geographic variation is important in further understanding of the moon’s compositional variation and its origin. Heat flow is obtained as a product of the thermal gradient and the thermal conductivity of the geologic interval of interest. On earth, thermal gradient is typically determined from temperature measurements made at different depths down a borehole. Thermal conductivity is measured on rock samples recovered from the hole. The hole must be deep enough (~100 m below surface) to avoid the thermal noise associated with the diurnal and seasonal temperature fluctuations at the surface. In measuring heat flow through lunar regolith, similar principles apply. Because thermal conductivity of lunar regolith is about 1/100 of that of typical igneous and sedimentary rocks on earth, surface temperature changes do not penetrate as deep as they do on earth. Therefore, the borehole does not need to be as deep. During the Apollo missions, heat flow was measured at two sites (Apollo 15 and 17). At these sites, the measurement holes reached only 1.6-m to 2.3-m below surface, but diurnal noise was almost negligible below 1-m depth. However, recent studies indicate that precession of the lunar orbit with an 18.6-year period yields large enough thermal noise to affect the entire depths of the Apollo temperature measurements. If corrected for the long-term noise, the heat flow values from the two sites (21 mW/m² at the Apollo 15 site and 16 mW/m² at the other) may be revised downward by a factor of 3 to 4. In preparing for heat flow experiments to be conducted in the next series of human lunar missions, we must identify all possible long-term thermal noise and determine the depth and magnitude of its influence. In addition, we must develop technologies for drilling holes much deeper than those used for the Apollo measurements.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas