Preamble

The elements of the Solar System are products of several nucleo-synthetic events but were almost completely mixed in the solar nebula before planetary accretion began. Fine grains in carbonaceous chondrites have preserved a record of early events; the final one was a supernova that preceded planetesimal accretion by no more than a million years. Elements heavier than iron and all or most of the radioactive species were supernova products. The non-volatile elements accreted in the inner Solar System, forming the terrestrial planets and the meteorite parent bodies. The mixture is dominated by elements with atomic masses that are multiples of 4, a nuclear structure favoured by the strong binding of 4He nuclei (Table 2.1). This selection of available nuclides places a restriction on hypotheses concerning planetary composition. It adds confidence to our understanding that the same major elements formed all of the terrestrial planets as well as the meteorites, and that the meteorites give us a broad picture of planetary chemistry.

As in the meteorites, the most abundant elements in the Earth are oxygen, iron, magnesium and silicon (masses 16, 56, 24, 32). For the compositional model summarized by Table 2.2, the proportions by mass are 31.5%, 30.3%, 15.4% and 14.2%, with all the other elements together making up the remaining 8.6%. The uncertainties in these numbers are indicated by the differences between proportions of major refractory elements in the meteorite and Earth columns of Table 2.1.