Diagram showing the theoretical interior of the gas giant planet Saturn At the centre there is probably a rocky and icy core several times the mass of the Earth. This is surrounded by an extensive inner mantle, about half of the planet's total radius, of liquid metallic hydrogen under great pressure. On top of this is a shell of helium 'rain', topped with a thick outer mantle of helium-saturated liquid molecular hydrogen. This gives way to a thick shell of hydrogen gas, and eventually, to an extensive hydrogen-helium atmosphere.