Distribution of Radioactive Heat Sources and Thermal History of the Moon

Abstract

AbstractThe initial distribution of heat sources in crustal and mantle reservoirs plays a major role in the thermal evolution of the Moon. We use new constraints on the thickness of the crust, the size of a nearside low in crustal magnetization, surface composition data from orbit, Apollo samples, and mass balance considerations to generate a set of plausible post magma ocean initial conditions. We then test those initial conditions using the 3‐D thermochemical mantle convection code Gaia and compare with observables. Models that use Lunar Prospector gamma‐ray spectrometer values of thorium throughout the highland crust cannot sustain long lasting volcanic activity, as low abundances of heat‐producing elements are left in the mantle to keep an Earth‐like bulk silicate composition. The low magnetic field intensities of the innermost Procellarum KREEP Terrane are consistent with a higher heat production than in the outermost portion and delayed cooling below the Curie temperature of iron metal until after 3.56 Ga when the dynamo field strength is known to have decreased by an order of magnitude. The distribution of crustal heat sources also influences the depth evolution of isotopic closure isotherms for a range of isotopic systems relevant to radiometric dating, which may be important for sample age estimation. Core crystallization can sustain a continuous dynamo for about 1 billion years, after which dynamo activity is potentially more episodic.