254 Part III Learning from Other Worlds summary of key concepts 8.1 A Different Kind of Planet • What are jovian planets made of? Jupiter and Saturn are made almost entirely of hydrogen and helium, while Uranus and Neptune are made mostly of hydro-gen compounds mixed with metal and rock. These differences arose because all four planets started from ice-rich planetesi-mals of about the same size but captured different amounts of hydrogen and helium gas from the solar nebula. • What are jovian planets like on the inside? The jovian planets have layered interi-ors with very high internal tempera-tures and pressures. All have a core about 10 times as massive as Earth, consisting of hydrogen compounds, metals, and rock. They differ mainly in their surrounding layers of hydrogen and helium, which can take on unusual forms under the extreme internal con-ditions of the planets. • What is the weather like on jovian planets? The jovian planets all have multiple
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