Volatiles are the group of chemical elements and chemical compounds that can be readily vaporized. In contrast with volatiles, elements and compounds that are not readily vaporized are known as refractory substances. On planet Earth, the term 'volatiles' often refers to the volatile components of magma. In astrogeology volatiles are investigated in the crust or atmosphere of a planet or moon. Volatiles include nitrogen, carbon dioxide, ammonia, hydrogen, methane, sulfur dioxide, water and others. Planetary science Planetary scientists often classify volatiles with exceptionally low melting points, such as hydrogen and helium, as gases (as in gas giant), whereas those volatiles with melting points above about 100 K (–173 °C, –280 °F) 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 gas giants, and Uranus and Neptune are 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. Inside of Jupiter's orbit, cometary activity is driven by the sublimation of water ice. Supervolatiles such as CO and CO2 have generated cometary activity as far out as . Igneous petrology In igneous petrology the term more specifically refers to the volatile components of magma (mostly water vapor and carbon dioxide) that affect the appearance and explosivity of volcanoes. Volatiles in a magma with a high viscosity, generally felsic with a higher silica (SiO2) content, tend to produce 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. Volatiles in magma Some volcanic eruptions are explosive because of the mixing between water and magma reaching the surface, which releases energy suddenly. However, in some cases, the eruption is caused by volatiles dissolved in the magma itself. Approaching the surface, pressure decreases and the volatiles come out of solution, creating bubbles that circulate in the liquid. The bubbles become connected together, forming a network. This promotes the fragmentation into small drops or spray or coagulate clots in gas. Generally, 95-99% of magma is liquid rock. However, the small percentage of gas present represents a very large volume when it expands on reaching atmospheric pressure. Gas is thus important in a volcano system