Is the Moon Hot or Cold?

Baldwin (1) has summarized some of the evidence for the early and present internal thermal states of the moon. He argued that the interior of the moon, below about 200 to 300 kilometers, is presently "hot," that is, has interior temperatures close to or exceeding the lunar solidus curve. The evidence for a rapid differentiation of the moon about 4.6 X 10^9 on years ago, the extensive igneous episode resulting in mare formation 3.7 X 10^9 to 2.8 X 10^9 years ago, the depletion of the moon in volatiles and its enrichment in calcium, aluminum, and the trace refractory elements all argue for a hot origin and high initial temperatures. A straightforward consequence of the lunar thermal inertia is that if the interior of the moon were ever hot it would remain hot to the present. The high surface concentrations of uranium, thorium, and potassium, and the Apollo 15 heat flow value of 33 ergs per square centimeter per second (2), indicate high present-day temperatures in the lunar interior (3).