Origin, evolution and present thermal state of the Moon

Abstract

The relative absence of lunar volcanism in the last 3 × 10 9 y and the Apollo 15 heat flow measurement suggest that present-day temperatures in the Moon are approximately steady-state to depths of ≈ 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 ≈ erg/cm 2 · s, may be obtained with an intial temperature roughly corresponding to the Apollo 11 basalt solidus, the exponential scaling of heat sources, and a parameter Q/ U 0 K = 1.6 × 10 −4 K/ppm( U 0) · cm ( U 0) is the surface concentration of U in ppm, K is the average thermal conductivity in erg/cm · K · s and Q is the present-day surface heat flow in erg/cm 2 · s); the nonuniqueness is constrained by the observations of U 0 and Q and inferences concerning K. The “best” models require strong concentration of heat sources in the upper 100–200 km, within the √( kτ) depth of ≈ 300 km for which the buried heat sources may be felt at the Moon's surface. The concentrations of U for an originally homogeneous Moon are estimated to be ≈ 9 × 10 −8 g/g, close to that measured for eucrites and infrared for primative inclusions of the Allende meteorite. The estimated 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. Accretion of the Earth and Moon may well have kept pace with condensation in the originally hot nebula. In the later stages of accretion-condensation, the Moon competed unsuccessfully with the more favorably disposed Earth. The present thermal state of the Moon may well involve at least some partial melting through all of the lunar interior deeper than 200 km. This would eliminate the large density changes which would otherwise occur for CaAl rich compositions at depth. Such a present-day thermal configuration in neither inconsistent with temperatures inferred from electrical conductivity studies now with the nonhydrostatic shape of Moon. In the first place, the lunar interiors is probably more deficient in total. Fe than had previously been suspected, and in the second place the Moon is closer to being in hydrostatic equilibrium than is the aseismicity of the Moon need only reflect the absence of plate motions, as they are known on the Earth, rather than a “cold” interior. For otherwise “hot” interiors, plate motions seem less likely on the Moon than on Earth for simple geometrical reasons.