In planetary science, volatiles are that group of chemical elements and chemical 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, as well as sulfur dioxide. In astrogeology, these compounds, in their solid state, often comprise large proportions of the crusts of moons and dwarf planets. 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 in a magma with a high viscosity, generally felsic with a higher silica (SiO2) content, tend to eruptions that are explosive. Volatiles in a magma with a low viscosity, generally mafic with a lower silica content, tend to vent and can give rise to a lava fountain.
Planetary scientists often class volatiles with exceptionally low melting points, such as hydrogen and helium, as gases (as in gas giant), while those volatiles with melting points above about 100 K are referred to as ices. The terms "gas" and "ice" in this context can apply to compounds that may be solids, liquids or gases. Thus, Jupiter and Saturn are referred to as "gas giants" even though the vast majority of the "gas" in their interiors is liquid, and Uranus and Neptune are called "ice giants" 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 e.g. silicates), but negligible amounts of hydrogen, nitrogen or carbon.
In contrast, those elements and compounds with high boiling points are known as refractory substances.[1]
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