Calculations of the Moon's Thermal History at Different Concentrations of Radioactive Elements, Taking Into Account Differentiation on Melting

Calculations of the thermal history of the moon were carried out by solving the thermal conductivity equation for the case in which the heat sources are the long-lived radioactive elements Th, U, and K40. The concentrations of these elements were adjusted to give four variations of the heat flow: 1.35 x 10-8 and 0.91 x 10-6 cal cm-2 s-1 (I and I**), 0.61 x 10-6 (variant II, the terrestrial mixture of Lyubimova and Starkova), and 0.236 x 10-6 (variant III, the chondrite model of Urey and MacDonald). In the same calculations, we considered layering of the differentiated material with transport to the surface of the radioactive elements after the temperature of the layer rose to 200 K above the melting temperature, which is considered in five variants that differ in the amount of transported radioactive elements: 100 percent (n = 1), 80 percent n = 0.8), 40 percent, 20 percent (n = 0.6, 0.4, 0.2). During fusion the heat capacity and heat conductivity were changed.

We considered two variants of an initially cold (273 K) and hot (900 K) Moon. Calculations show that the interior of the Moon was heated to melting during the first 0.7 to 2.3 x 109 years. The maximum fusion involved practically the entire Moon to a distance from 15 to 45 km beneath the surface, and started 3.5 to 4.0 x 109 years ago (I, I**), or 2.5 to 3.0 x 109 years ago (II, III) and continued for 1 to 2 x 109 years. Today the Moon is cooling. The current thickness of the solid crust is from 150 to 200 km and the heat flow exceeds the stationary value 1.5 fold. Apparently the most realistic variant is II (terrestrial mixture) for an initially hot Moon, and gives, regardless of the moderate concentration of radioactive elements, a heat flow of 0.9 to 0.95 x 10-6 cal cm-2 s-1, which agrees with the radioastronomical measurements of Troitsky and Krotikov and with the Apollo 15 data.