In planetary science, volatiles, commonly called ices in the extraterrestrial context, are that group of compounds with low boiling points (see volatile) that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen and methane, all compounds of C, H, O and/or N. In terrestrial geology, the term more specifically refers to components of magma (mostly water vapor and carbon dioxide) that affect the appearance and strength of volcanoes. Volatiles affect the viscosity of the magma, and the tendency to explosive eruptions. The term "ice" in this context can apply to compounds that may be solids, liquids or gases. Thus, Uranus and Neptune are called "ice giants", because they contain large amounts of methane and ammonia, despite the fact that neither planet contains more than trace amounts of solid ice. The Earth's Moon is considered very low in volatiles: its crust contains oxygen chemically bound into the rocks (as eg silicates), but negligible amounts of hydrogen, nitrogen or carbon.
