The interiors of the Jovian planets, also known as gas giants, are different because of their sizes. Jupiter and Saturn, the largest of the four Jovian planets, are massive enough to compress materials into a rocky core. Uranus and Neptune exert less pressure, which causes their cores to be in an icy, liquid state.
The size of each gas giant affects more than just the state of its core—it also affects the planet’s chemical composition. All the Jovian planets are rich in hydrogen, but because Jupiter and Saturn exert such a massive amount of gravity, the elements found in their cores are very heavy. A liquid layer of metallic hydrogen is located around each rocky core, which includes oxygen and certain metals. It is supposed that the smaller Jovian planets contain only liquid water, as well as other compounds like methane and ammonia, in their cores. The presence of methane lends the two smaller Jovian planets a pale blue tint.
The greater Jovian planets are so massive that they actually generate more energy in the form of heat than they receive from the sun. In contrast, Neptune only creates a very small amount of energy, while Uranus is not known to emit any energy.