No. 20: Mar-Apr 1982
Crater-density studies of the small, icy Saturn satellites Rhea, Dione, Mimas, and Tethys reveal important non-uniformities in crater distribution and age. The anomalies are so large that astronomers have concluded that these objects must have undergone considerable evolution after they were formed by accretion (the currently accepted mode of formation). Unfortunately these four satellites are so small that they could not have accommodated any reasonable energy source capable of causing the observed crustal evolution. The authors suggest strong local concentrations of radioactive heat generators rather than uniformly distributed radiogenic substances, such as those that helped mould the earth's surface.
(Plescia, J.B., and Boyce, J.M.; "Crater Densities and Geological Histories of Rhea, Dione, Mimas and Tethys," Nature, 295:285, 1982.)
Comment. Interestingly enough, local concentrations of radioactivity have been discovered on the moon.