COMPOSITION

Composition

The Sun, which comprises nearly all the matter in the Solar System, is composed of roughly 98% hydrogen and helium.^[10] Jupiter and Saturn, which comprise nearly all the remaining matter, possess atmospheres composed of roughly 99% of those same elements.^[11][12] A composition gradient exists in the Solar System, created by heat and light pressure from the Sun; those objects closer to the Sun, which are more affected by heat and light pressure, are composed of elements with high melting points. Objects farther from the Sun are composed largely of materials with lower melting points.^[13] The boundary in the Solar System beyond which those volatile substances could condense is known as the frost line, and it lies at roughly 4 AU from the Sun.^[14]

The objects of the inner Solar System are composed mostly of rock,^[15] the collective name for compounds with high melting points, such as silicates, iron or nickel, that remained solid under almost all conditions in the protoplanetary nebula.^[16] Jupiter and Saturn are composed mainly of gases, the astronomical term for materials with extremely low melting points and high vapor pressure such asmolecular hydrogen, helium, and neon, which were always in the gaseous phase in the nebula.^[16] Ices, like water, methane, ammonia, hydrogen sulfide and carbon dioxide,^[15] have melting points up to a few hundred kelvins, while their phase depends on the ambient pressure and temperature.^[16] They can be found as ices, liquids, or gases in various places in the Solar System, while in the nebula they were either in the solid or gaseous phase.^[16] Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (the so-called "ice giants") and the numerous small objects that lie beyond Neptune's orbit.^[15][17] Together, gases and ices are referred to as volatiles.^[18]