Origin, evolution and present thermal state of the moon.

The relative absence of lunar volcanism in the last 3,000,000,000 years and the Apollo 15 heat flow measurement suggest that present-day temperatures in the moon are approximately steady-state to depths of about 100 km. An exponential distribution of heat sources with depth may then be scaled by equating the surface heat flow to the integrated heat production of this exterior shell. Presumed present-day interior temperatures, as well as the present-day surface heat flow of about 30 erg/sq cm-sec, may be obtained with an initial temperature roughly corresponding to the Apollo 11 basalt solidus and exponential scaling of heat sources. The concentrations of U for an originally homogeneous moon are estimated to be about 0.009 micrograms per gram, close to that measured for eucrites and inferred for primitive inclusions of the Allende meteorite. The estimated homogeneous concentrations of U, the chemistry of the lunar surface material and inferences to modest depth, and the short accretion time of the moon necessary to provide large-scale differentiation at 4.6 AE suggest that the moon had its origin in the rapid accretion of compounds first condensing from the protoplanetary nebula.