Preamble

The notion that the Earth and Sun had a common origin has a long history, predating by many years modern ideas about their ages. It underlay the paradox that paralyzed geological thinking in the late 1800s: there was no known source of the Sun's energy that could warm the Earth for the apparent duration of the sedimentary record. The discovery of radioactivity by H. Becquerel in 1896 was deemed to release geological thinking from the conceptual difficulty of a very limited age for the Earth, although the release was not logically satisfying until thermonuclear fusion was recognized in the 1930s. Following the discovery of radioactivity, its two principal roles in studies of the Earth were promptly recognized. Measurements of radiogenic heat in igneous rocks, especially by Strutt (1906), and early ideas about dating, initiated by Rutherford, confirmed its significance. This chapter considers the global and Solar System questions that are illuminated by studies of isotopes; evidence for the evolution of the Earth is considered in the following chapter and radiogenic heat in Chapter 21.

Meteorites are especially important to our understanding of the early Solar System. Unlike the planets, they have suffered little modification since their common origin, 4.57 × 109 years ago. Isotopic studies on meteorites date the Solar System (Section 4.3); a precise independent age for the Earth cannot be obtained from terrestrial rocks, which have evolved in many ways from the original nebular mix.