HOW HOT IS THE MOON? A PERSPECTIVE FROM HEAT-PRODUCING ELEMENTS ON THE PRESENT-DAY SELENOTHERM THROUGH PARTITIONING EXPERIMENTS AND CONDUCTIVE THERMAL MODELING

The lunar thermal profile, or selenotherm, is crucial for understanding the Moon's mantle dynamics, thermoelastic properties, and volcanic history. Techniques like analyzing seismic waves, electrical conductivity, and heat flow have helped define the selenotherm but still leave uncertainties of up to 800°C, which is too broad for petrogenetic or geodynamic models. The present-day selenotherm is mainly influenced by radiogenic heat from within the Moon, with heat primarily being transported by conduction.

We propose a new approach to study how the selenotherm is a function of the distribution and concentration of heat-producing elements (HPEs) and mantle overturn. As the Moon’s magma ocean cooled, minerals crystallized and created the initial framework of the lunar mantle and plagioclase floatation crust. The partition coefficient (K_D) measures an element’s concentration in a mineral versus the melt from which it crystallized. We use a range of K_D values for HPEs like uranium, thorium, and potassium from piston-cylinder experiments to determine their distribution in the Moon's layers. All three HPEs are incompatible (K_D < 1), meaning they prefer to stay in the magma rather than in minerals. Fractional crystallization models show that an upper mantle layer, called ilmenite-bearing cumulates (IBCs), contains the most HPEs which concentrate in the last dregs of the magma ocean. The shallow IBCs are denser than the underlying mantle, which may cause them to sink towards the core-mantle boundary in a process known as mantle overturn and potentially transport HPEs to the Moon’s core-mantle boundary (CMB).

The heat-producing element (HPE) content and distribution of the IBCs significantly affect the selenotherm. Key findings from our thermal modeling are: (i) The initial HPE concentration in the magma ocean strongly affects the final HPE content in the IBC layer, (ii) From the perspective of the selenotherm, enstatite chondrites are more likely building blocks of the Earth-Moon system compared to carbonaceous chondrites, as the later results in large swathes of the lunar interior being molten, and (iii) Mantle overturn was an inefficient process with only ~25% of the IBC mass being potentially transported to the CMB.