Direct observations of chemicalcomposition can be made for Earth, theMoon, and meteorites, although there are some problems of interpretation. The chemical composition ofEarth’s crust, oceans, andatmosphere can be studied, but this is only a minute fraction of the mass of Earth, and there are many composition differences even within this small sample. Some information about the chemical properties of Earth’s unobserved interior can be obtained by the study of themotion of earthquake waves and byEarth’s magnetic field, which originates in the interior (see belowGeochemical distribution of the elements).

Until recently, more was known about element abundances in distant stars than in Earth’s nearest neighbour, the Moon. The lunar landings have provided samples that have been intensively analyzed in many laboratories throughout the world. The data for theApollo 11 material, collected in the Sea of Tranquility (Mare Tranquillitatis), are given in theTable. Analyses of Apollo 12 collections are similar for most of the elements. Comparison of theanalytical data with those for carbonaceous chondrites (a type of meteorite that provides a good average sample of nonvolatile solar system material) shows that the lunar material has undergone marked geochemical fractionation (segregation of elements).Meteorites suffer from heating in Earth’s atmosphere, so that what is found on Earth is not necessarily the original chemical composition of the meteorites, especially for the volatiles,light gases that are easily lost. When allowance is made for the loss of volatile light gases and for effects of chemical separation, it seems quite possible that the overall chemical composition of Earth, the Moon, the Sun, and the meteorites is essentially the same and that they have a common origin.

If elemental abundances are the same in Earth and stars,isotopic abundances are likely to be the same. Theories predict the relative production of the different isotopes, and it is desirable to be able to compare these with observation. The study of terrestrial abundances of radioactive elements yields information about the age of the solar system, which is discussed below.

Summary of observations

The chemical composition of all objects in theuniverse is not quite the same, and not all elements can be observed in any one object, even if they are present. Nevertheless, thecompositions of many objects are sufficiently similar to make it worthwhile to try to construct a typical table of abundances. Suchcompilations have been made by several authors and the best known is thework of the American physicists Hans Suess and Harold Urey. Although it dates from 1956, and later compilations differ in some details, its general character is not in dispute.

The main properties shown in the abundance table are quite clear.Hydrogen andhelium are much more common than all of the other elements. There is a gradual decline toward higheratomic number with a great underabundance oflithium,beryllium, andboron. There is a significant peak in the region ofiron, the element with the highest fractional bindingenergy, and the decline continues to higher atomic number with some subsidiary peaks. These peaks are associated with nuclei containing 50, 82, or 126 neutrons; the theory of nuclear structure predicts that these nuclei should be particularly stable, and these numbers are known as“magic” numbers.

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