The average densities of the giant planets should indicate how much dense material they contain, but in practice, this determination is not so simple. Although the actual numbers are very uncertain, the dense components of the cores of Jupiter and Saturn have masses of about 5–10 Earth masses (M¢). As Table 10.1 shows, Jupiter and Saturn have total masses of 318 and 95 M¢, respectively. The dense materials in their cores contribute little to their average chemical composition. This means you can think of both Jupiter and Saturn as having approximately the same composition as the Sun and the rest of the universe: about 98 percent hydrogen and helium, and only 2 percent everything else. Why, with nearly identical composition, should Jupiter’s density be almost twice as great as Saturn’s? The internal pressure created by Jupiter’s much greater mass compresses its hydrogen and helium and its core to a higher average density than in the core of Saturn. What does all this imply about Uranus and Neptune?