Megaregolith thickness, heat flow, and the bulk composition of the Moon

Abstract

The Moon's bulk composition is a major constraint on its origin and evolution. Bulk density alone shows that the Moon is depleted in metallic FeNi relative to the Earth or to chondrites with earthlike MgO/FeO ratios (such as H-group chondrites). Depletions of minor-trace volatile elements are also obvious from geochemical data. The simplest assumption would be that the Moon is not much different in terms of nonsiderophile, nonvolatile elements from the silicate portions of the Earth or H chondrites. Lunar heat flow data¹ have been interpreted to imply that the Moon's uranium content is about 46 ng g⁻¹, about twice that of the Earth's mantle, and three times that of H-chondrite silicates. As it is difficult to envisage any process preceding the formation of planets that would fractionate refractory lithophiles such as uranium from major lithophiles such as Si, disparity in the U/Si ratio implies disparity in provenance or origin. Based on a new model that takes into account the considerable, but variable, thickness of porous, low-conductivity megaregolith, the thickness of the lunar lithosphere, and the nonrepresentative composition of the crust at one of the two sites where heat flow was measured, we estimate here that the Moon's uranium content is roughly 19 ng g⁻¹. The Moon's bulk composition appears far less exotic than generally assumed.