A diagram of Jupiter and Earth, with a section cut out of each of them, showing the interiors. For Jupiter, the outer layer is labelled gaseous hydrogen, the second layer is labelled liquid hydrogen, and the third layer is labelled metallic hydrogen. The centre is labelled, with a question mark, core of rock, metals, and hydrogen compounds. For Earth, the outer layer is labelled atmosphere, the second layer is labelled crust, and the third layer is labelled mantle. The centre is labelled core of iron.