The Unresolved Problem with Deriving Lunar Thermal Profiles When Including Heat Producing Elements

Despite more than four decades of research, first-order knowledge about lunar evolution and structure remains unresolved, including: (a) the dynamic development of the interior through lunar history starting from crystallization of an early magma ocean; and (b) the presence of a low-rigidity basal mantle layer, a potential remnant of an overturned Fe-Ti- rich layer that formed below the crust and sank to the core-mantle boundary. Understanding the thermal state of the present-day lunar interior is a primary challenge for improving estimates of internal structure. The existing estimates of thermal profiles (selenotherms) derived from inversions of seismic, gravity, and electromagnetic data differ by ~800 °C; too broad to discriminate between proposed petrologic stratigraphies. Constraining the heat-producing element (HPE) concentrations and distribution in the various reservoirs of the Moon would directly inform the thermal state of the interior. Estimates of bulk lunar mantle HPE concentrations can range from that of an ordinary chondrite (U = 0.0068; Th = 0.025; K = 17 ppm) to higher estimates (U = 0.039; Th = 0.15; K = 212 ppm) based on measurements of Apollo pyroclastic glasses that might represent the least fractionated, near-primary lunar mantle melts. We show preliminary results of selenotherms and their corresponding mantle properties from lunar interior models. The selenotherms were calculated by incorporating the HPE estimates into a 1D thermal conduction equation. The total mass and moment of inertia of each interior model were calculated through the Birch-Murnaghan equation of state and compared to observations. Here we illustrate the difficulties of producing an HPE-based selenotherm that falls within geophysically based estimates, as well as highlight future effort to address these problems. Our preliminary search has found selenotherms on the hot edge of or hotter than this range. At the extreme, the higher HPE concentration estimates yield an impossibly hot mantle with temperatures in excess of 4,000 K, melting large portions of the mantle. This study emphasizes the need for future in-situ observations and sample analysis to better inform modeling of the Moons interior.