Saturn is a gas planet and is not solid like Earth. It is predominantly composed of hydrogen and helium, which are the two basic gases of the universe. The planet also contains small amounts of methane and ammonia. Unlike the rocky terrestrial planets, gas giants such as Saturn lack the layered crust-mantle-core structure because they formed differently from their rocky siblings. It is thought that there might be a molten, rocky core about the size of Earth deep within Saturn. The core of Saturn is probably made out of iron and rock, which is possibly surrounded by ammonia, methane, and water. The hydrogen gas that makes up most of the atmosphere slowly changes to liquid with depth as the pressure increases. Below the liquid hydrogen rests the heavier liquid helium. Deep in the depths of the body of Saturn, the hydrogen is then under tremendous pressure and is transformed to liquid metallic hydrogen. It is believed that a rocky core about ten times the mass of the Earth exists at the center.