In planetary science, volatiles are the group of chemical elements and chemical compounds with low boiling points that are associated with a planet’s or moon’s crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, methane and sulfur dioxide. In astrogeology, these compounds, in their solid state, often comprise large proportions of the crusts of moons and dwarf planets. In contrast with volatiles, elements and compounds with high boiling points are known as refractory substances. Planetary scientists often classifly 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”, and Uranus and Neptune are referred to as “ice giants”, even though the vast majority of the “gas” and “ice” in their interiors is a hot, highly dense fluid that gets denser as the center of the planet is approached. 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.

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