The great mixture of ingredients that were in the original disk and from which the Sun and planets were built, can be reduced to three basic components: gas, ice and rock (Fig. 5.1). The gas was mostly hydrogen and helium that made up 98% of the mass of the initial solar nebula. The remaining 2% of material in the nebula consisted of various ices and rock composed of the heavier elements. The ices can be divided into three groups[1]. Water ice is the dominant component and displays many polymorphs. Other ices are more volatile and include NH3, CO2 and methanol (CH3OH). A “super-volatile” group includes methane (CH4), CO and N2 that, in contrast to the former group, do not form phases with water.

Volatility is the most important chemical parameter under nebular conditions, so the elements present as “rock” are classified as “very volatile” (e.g., Bi, Tl); “volatile” (e.g., K, Rb, Cs); “moderately volatile” (e.g., Mn, Ba, Sr); “moderately refractory” (e.g., Si, Fe, Mg, Cr, V, Eu); “refractory” (e.g., Ca, Al, U, La), and “super-refractory” (e.g., Zr) (Table 5.1). Elements are also classified as lithophile, chalcophile or siderophile according to their preference for entering silicate, sulfide or metal (Fe) phases.

The term “solar-system abundances” refers to the composition of the solar system, including the Sun.