In light of new spacecraft data and advances in computer modeling tools, fundamental questions about the Apollo Heat Flow Experiments (HFE) and their implications for the lunar interior can now be addressed in unprecedented detail. Recent crustal thickness models from the GRAIL and Selene missions, laser altimetry data from the LRO and its predecessors, and crustal radiogenic composition from several Gamma ray instruments can all be combined with thermal models to produce major advances in our ability to predict local heat flow. The Apollo HFE will serve as a calibration for global models. These models will also help separate the importance of several competing theories for why the two Apollo HFE sites (Apollo 15 and 17) differ and what implications that has on mantle heat production.
The two successful Apollo Heat Flow Experiments (HFE), differ from each other dramatically, with the Apollo 15 measured heat flux of 21± 3 mWm-2 and the Apollo 17 values of 14-16 ± 2 mWm-2. Many explanations have been put forward to explain these differences, but no single coherent model has looked at the relative impact of combining them. Previous models to explain the differences between the Apollo HFE measurements can be summarized into 4 classes: 1) Crustal thickness variations , 2) Crustal thermal conductivity variations , 3) Near Surface radiogenic (KREEP) enrichment, and 4) Deep radiogenic (KREEP) enrichment. Large temperature changes at depth (~50 K) over short distances (~10 km) will also affect heat flow, but are only an important factor in the polar regions of the Moon.
These models show the importance of regional context on localized surface heat flow measurements. We find measured heat flow values can be greatly altered by deep sub-surface radiogenic content and crustal density. However, total crustal thickness and the presence of a near surface radiogenic-rich ejecta provide less leverage, representing only minor (<1.5mWm-2) perturbations on values measured at the lunar surface. Using the model of crustal thickness and density implied by GRAIL results, we found a roughly 13-16 mWm-2 mantle heat flux as a best fit to produce the heat flux observed at both Apollo sites. These high heat flow values could imply that the lunar interior is similar to, or even slightly more radiogenic than, the Earth’s mantle; perhaps implying a greater fraction of terrestrial crust was incorporated at the time of formation from a presumed giant impact. These results may also imply that heat flux at the crust-mantle boundary beneath the Procellarum KREEP Terrain (PKT) is anomalously elevated compared to the rest of the Moon. Such models will greatly enhance future landed measurements in areas distant from the radiogenic-rich PKT that can provide a more detailed understanding of the lunar interior.