In-Situ Heat Flow Measurements In The Maria Outside The Procellarum Kreep Terrane
During the Apollo program, heat flow measurements were considered high priority and planned on 4 of the landing missions (Apollo 13, 15, 16, and 17). Measurements of the heat flow originating from the interior of the Moon help us better understand its thermal evolution and the differentiation history of the lunar crust and mantle. The successful measurements obtained at the Apollo 15 and 17 sites (21 mW/m2 and 14 mW/m2, respectively), along with the data from the gamma-ray spectrometer onboard Lunar Prospector, lead to the hypothesis that the Procellarum KREEP terrane (PKT) being hotter than the surrounding areas because of the relative abundance of heat-producing elements, K, Th, and U in its crust. Many researchers agree that radiogenic heat-producing elements are more highly concentrated within the crust than in the mantle. While their abundance on the lunar surface has already been mapped globally, their vertical distribution within the crust is unknown. It is highly likely that their abundance decreases with depth. The crust’s contribution to the Moon’s total heat budget is still relatively unknown, and that has been a major hindrance in the effort for further constraining the thermal structure of the deeper interior. For future robotic lunar-landing missions, we propose that heat flow measurements be made in areas where lunar crust is known to be very thin and also outside the PKT. Crustal heat production in such areas should be minimal, and thus heat flow measurements should yield values close to the flow originating from the mantle. One might argue that the measurements reported at the Apollo 17 site serve such a purpose, but that area is likely underlain by Th-rich ejecta from the PKT basins and craters. Further, the Apollo 17 site was located in a valley in a transition zone between highland and maria. The heat flow through the lunar surface there may have been affected by the local topography and the sharp variation in the regolith and the underlying crust. For future measurements, we suggest sites further away from the PKT on the flat central floor of mare basins, at least 100 km from the basin rim. Although data from multiple locations on both sides of the Moon are ultimately needed to fully understand the Moon’s thermal evolution, this data can be obtained one station at a time and does not require a network mission. We believe that Mare Crisium and Mare Nectaris are good candidates. The GRAIL mission revealed that lunar crust is in these two basins is less than 10 km. Data from the Lunar Orbiter Laser Altimetry (LOLA) and images from the Lunar Reconnaissance Orbiter Camera (LROC) will be useful for landing site choice and characterization. In recent years, our group has been developing a compact, modular heat flow instrumentation specifically for robotic lunar-landing missions. Our current prototype is at Technical Readiness Level 5. The entire system weighs less than 2 kg. It does not require roving capability. The probe, mounted on a lander’s platform, is designed to penetrate 3 m into regolith, well below the thermal skin depth, and measure thermal gradient and thermal conductivity in situ. Heat flow is obtained as the product of the two measurements. The entire sequence of probe deployment and thermal measurements can be completed within 6 to 10 hours. Finally, both Crisium and Nectaris are located on the near side and at low latitudes. The heat flow measurement does not require returned samples. These traits are suitable for maiden voyage missions by private companies.