The average chemical and isotopic composition of the solar system is appropriately referred to as cosmic, since this elemental abundance distribution is found to be nearly the same for interstellar gas and for young stars associated with gas and dust in the spiral arms of galaxies. The Sun makes up more than 99.9% of the mass of the solar system, so the bulk chemical composition of the solar system is essentially the same as that of the Sun. The cosmic abundances of the nonvolatile elements are determined from chemical analyses of a type of meteorite known as CI chondrites, whereas the relative abundances of the volatile elements are determined from quantitative measurements of the intensities of elemental emission lines from the Sun's photosphere. In most silicate-rich meteorites and the Earth, Moon, Venus, and Mars, the most abundant elements are oxygen, magnesium, silicon, iron, aluminum, and calcium. Average solar-system composition consists of 70.7 wt % hydrogen, 27.4 wt % helium, and only 1.9 wt % of all remaining elements, lithium to uranium. Cosmic abundances are now widely referred to as standard abundances in the astrophysical literature. See also Astronomical spectroscopy; Element (chemistry); Elements, geochemical distribution of.
Cosmic abundances of elements have several important uses. First, by comparing cosmic abundances to chemical analyses of various types of meteorites, inferences can be made about chemical fractionation processes that occurred in the primitive solar nebula, such as condensation and vaporization. Also, by comparing them to rock compositions, inferences can be made about processes that occurred early in the history of rocky planets, such as separation of a metallic core and differentiation of silicates into mantle and crust. Second, cosmic abundances serve as a standard of comparison for spectroscopic measurements of elemental abundances of the photospheres of other stars and for measurements of elemental and isotopic abundances in cosmic rays. Finally, nucleosynthesis occurs in many different stellar environments. Explanations of nucleosynthesis must account for how elements and isotopes from various astrophysical sources are made and then mixed to form the solar system's average chemical and isotopic composition. See also Nucleosynthesis; Solar system.
