Reevalution of Lunar Heat Flow Experiment: (Construction of a LUnar Thermal Model using Lunar Prospector Data)

由于月球热流为我们提供了一种估算月球中难熔元素总体丰度的独特手段，因此根据近期月球任务获得的新的全球数据，重新评估阿波罗15号和17号任务获得的热流数据非常重要。本文结合探勘者号和克莱门汀号探测到的地形、重力场和钍丰度数据，对阿波罗号的热流数据进行了分析。首先，利用有限元方法研究了地形变化和巨风化层厚度变化对测量热流的影响。这种影响被认为对阿波罗热流值非常重要，因为在阿波罗15号和17号地点的测量都是在月球海和高地的边界附近进行的，那里的地形和大风化层厚度变化很大。数值实验结果表明，阿波罗15号和阿波罗17号站点的热流观测值比这些站点的代表性值大10 ~ 20%。通过对地形和重力场资料的分析，我们建立了两个地壳结构模型，一个模型（厚壳模型）的平均地壳厚度为61 kin，另一个模型（薄壳模型）的平均地壳厚度为48 km。将这些地壳模型与探矿者的钍丰度图相结合，我们计算了地壳模型所期望的地表热流。理论热流值与重新评估的阿波罗热流值的比较表明，月球的体积U丰度为20 ~ 30 ppb，与地球上地幔的丰度或多或少相似。这表明，月球上的难熔元素并不像以前许多研究返回月球样品的化学工作者所提出的那样富集。这一结论对我们对月球起源的认识有重要的影响。

Since lunar heat flow provides us with a unique means to estimate the bulk abundance of refractory elements in the moon, it is very important to re-assess the heat flow data obtained in the Apollo 15 and 17 missions, in terms of new global data obtained by recent lunar missions. In this paper, we analyze the Apollo heat flow data combining the topography, gravity field, and Thorium abundance data by Prospector mission and Clementine mission.First we investigated the effect of topography variation and mega-regolith thickness variation on the measured heat flow, using a finite element method. This effect is believed to be very significant on Apollo heat flow values, because the measurements at the both Apollo 15 and 17 sites are made near the boundary of lunar mare and highland where the topography and mega-regolith thickness changes drastically. The result of the numerical experiments show that the observed heat flow values at Apollo 15 and 17 sites are 10 to 20 % larger than the representative values of those sites.We construct two crustal-structure models by analyzing the topography and gravity field data one model (thick crust model) has a mean crustal thickness of 61 kin, and the other model (thin crust model) has a mean crustal thickness of 48 km. Combing these crustal models with Prospector's Thorium abundance map, we calculated the surface heat flow expected from the crustal models. Comparison of the theoretical heat flow values with the reevaluated Apollo heat flow values indicates that the bulk U abundance in the Moon is 20 to 30 ppb which is more or less similar to that of the upper mantle of the Earth. This suggests that refractory elements in the Moon are not so much enriched as those proposed by previous many workers on chemistry of returned lunar samples. The conclusion has significant influence of our view of the lunar origin.