Abstract
The lunar heat flow experiments were made at Apollo 15 and 17 landing sites, both of which are located near the margin of the circular basins. In such locations, the megaregolith thickness is expected to have a large lateral variation. Warren and Rasmussen (1987) pointed out that the Apollo heat flow measurements are significantly affected by lateral variation of megaregolith thickness, because a very low thermal conductivity of megaregolith tends to focus the heat flux to thin regolith area from thick regolith area.
But the lunar surface heat flow is affected not only by regolith thickness variation but also topographic variation and distribution of heat-generating elements. The present study aims to study these effects on the measured heat flow values at Apollo 15 and 17 sites, using a numerical simulation of the thermal state of the Moon. The models used here are much more quantitative and much finer in spatial resolution than the previous studies.
In the present models, we assume the abundance distribution of the heat-generating elements, such as U, and Th, in the lunar crust is represented by the following equation:
C(h) = Co exp (-h/D)
where h is depth, Co is the surface abundance, and D is the skin depth which indicates the degree of upper-crust concentration of the heat-generating elements.
The results indicate that the effects of topography and regolith thickness variation on the surface heat flow amount to about 10 to 50% of the observed values and suggest that a significant correction must be made in order to obtain a regionally representative value from Apollo heat flow experiments. Comparison of the corrected heat-flow values at Apollo 15 and 17 sites suggests that the skin depths, D's at Apollo 15 and 17 sites are 20 to 40 km, which is much larger than terrestrial skin depth, D=10 km (Lachenbruch, 1970). This large D value suggests that the incompatible elements is more homogeneously distributed in the Moon than in the Earth and that the lunar crust was generated in large quantities in a short time.
