Isotopic constraints on planetary evolution

Abstract

Direct observations are considered that have been made on terrestrial and lunar materials in the search for answers to questions such as: when did the major structural elements (core, mantle, and crust) come into being and evolved. Measurements of isotope ratios of Pb, Sr, Nd, Hf, Ar, and other elements provide information about the internal evolution of the earth and moon since their formation 4.5 Gyr ago. The existing evidence indicates the core formed more than 4.4 Gyr ago, possibly while the earth was still accreting. Though the moon crust is similarly very old, the moon ceased to differentiate after about 1 Gyr whereas earth differentiation has continued to the present. Cataclysmic episodes have occurred, the largest 2.8 Gyr ago, when about half of the present continental mass formed.