Helium is the second most abundant gas in Jupiter’s atmosphere, after molecular hydrogen. Together, hydrogen and helium account for nearly all of the planet’s atmosphere, with helium present in roughly solar-system proportions but somewhat depleted compared with the Sun.
Jupiter’s atmosphere formed from the same broad material as the early Solar System, which is why its composition is dominated by the two lightest elements. Deeper inside the planet, extreme pressure turns hydrogen into a liquid metallic state. Scientists think some helium separates from that hydrogen and falls inward as “helium rain,” helping explain Jupiter’s lower-than-expected helium abundance.
Small quantities of methane, ammonia, water vapour, hydrogen sulfide, and other compounds produce Jupiter’s familiar clouds and colours. These trace gases are visually important even though they make up only a tiny fraction of the atmosphere.
A common mix-up is choosing nitrogen or oxygen because they are prominent in Earth’s air. Jupiter’s atmosphere is fundamentally different: oxygen, nitrogen, and carbon dioxide are minor constituents rather than the dominant gases.