What element makes up about 90% of Jupiter's atmosphere?

Hydrogen makes up about 90% of Jupiter’s atmosphere by volume. The planet’s upper atmosphere is dominated by molecular hydrogen, with helium accounting for most of the remainder. This composition resembles the gases present in the early solar nebula from which Jupiter and the other planets formed.

The exact percentage depends on how it is measured. By volume, Jupiter’s upper atmosphere is roughly 90% hydrogen and 10% helium. By mass, hydrogen accounts for about 76% and helium about 24%, because helium atoms are heavier than hydrogen molecules. These different percentages describe the same atmosphere rather than a contradiction.

Jupiter also contains tiny quantities of methane, ammonia, water vapour, hydrogen sulfide, phosphine, and other compounds. Although they are present in much smaller amounts, these trace chemicals help create the planet’s colourful cloud bands and contribute to its weather. Jupiter has no solid surface separating atmosphere from planet; pressure and temperature increase downward until the hydrogen gradually becomes a dense liquid and, deeper still, metallic hydrogen.