Radioactive heat sources in the lunar interior

Abstract

Published models for the Moon's thermal history typically imply present day central temperatures far too high to be consistent with the recently proposed lunar temperature profile of Sonett and others, inferred from apparent electrical conductivity of the lunar interior. Furthermore, chemical data on Apollo samples show that the Moon is depleted relative to chondrites in volatile elements, including potassium, and possibly enriched relative to chondrites in refractory elements, including uranium and thorium. Additional thermal models have therefore been investigated in order to set upper limits on lunar radioactivity consistent with the proposed temperature distribution. For an initially cold, uniform Moon, devoid of potassium, a maximum uranium content of 23 parts per billion is inferred. This is about twice the chondritic value and an order of magnitude less than the least radioactive lunar rock samples. Accretional heating and more realistic K/U ratios imply still lower uranium contents. Surface concentration of radioactive heat sources is probable only if deep melting has occurred. This, too, implies present day central temperatures greater than 1400 °C. Either the Moon has a most unusual geochemistry or the Sonett et al. temperature distribution is grossly in error.