A model of the interior of Jupiter is compared with that of Earth, to scale. Jupiter is mostly hydrogen, with some helium and a dusting of heavier elements. The gas giant's outer envelope is in the form of molecular hydrogen and, beneath that, the hydrogen transitions to metallic hydrogen. Most models include a layer of metallic hydrogen stabilized by exsolution of helium (aka "helium rain") at the top of the metallic hydrogen region.

More information about Juno is at https://www.nasa.gov/juno and https://missionjuno.swri.edu.