Jupiter
Composition

By mass, Jupiter's atmosphere is approximately 76% hydrogen and 24% helium, though, because helium atoms are more massive than hydrogen molecules, Jupiter's upper atmosphere is about 90% hydrogen and 10% helium by volume. The atmosphere also contains trace amounts of methane, water vapour, ammonia, and silicon -based compounds, as well as fractional amounts of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, and sulfur. The outermost layer of the atmosphere contains crystals of frozen ammonia. Through infrared and ultraviolet measurements, trace amounts of benzene and other hydrocarbons have also been found. The interior of Jupiter contains denser materials—by mass it is roughly 71% hydrogen, 24% helium, and 5% other elements.

The atmospheric proportions of hydrogen and helium are close to the theoretical composition of the primordial solar nebula. Neon in the upper atmosphere only consists of 20 parts per million by mass, which is about a tenth as abundant as in the Sun. Jupiter's helium abundance is about 80% that of the Sun due to precipitation of these elements as helium-rich droplets, a process that happens deep in the planet's interior.

Based on spectroscopy, Saturn is thought to be similar in composition to Jupiter, but the other giant planets Uranus and Neptune have relatively less hydrogen and helium and relatively more of the next most common elements, including oxygen, carbon, nitrogen, and sulfur. These planets are known as ice giants because during their formation these elements are thought to have been incorporated into them as ices; however, they probably contain little ice today.
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