On the thermal history of the Moon

Abstract

The thermal history of the moon is investigated on the basis of theoretical thermal models, and the results are compared with astrophysical and geological evidence. Previous calculations are extended to include the effects of melting and fluid convection. A numerical procedure is developed for studying the consequences of differentiation by concentration of radioactive isotopes toward the surface with time. A ‘chondritic’ and a ‘terrestrial’ model for the radioactive abundances together with several limiting distributions of the initial temperature in a solid moon are considered for these calculations. The results indicate that melting and, consequently, differentiation have occurred in the moon. It is demonstrated that, for a wide range of assumed conditions, the redistribution of radioactive elements is achieved by partial melting without the development of a significant amount of complete melting. The extent of the partially molten regions through time suggests that volcanic activity, possibly on a large scale, has taken place, the maximum activity having occurred between 1 and 3 b.y. ago. Cooling and resolidification within the outer layers commence after the period of intensive heating which occupies a time span of the order of 1 b.y. Although partially molten matter may exist in the interior, the present study indicates that the bulk of the moon is solid at present. Such a largely solid body would be consistent with an explanation of the inequalities in the moon's figure on the basis of internal strength.