Saturn's interior. Computer artwork showing the interior structure of the planet Saturn. Usually, Saturn and Jupiter are known as gas giants, but they are not made entirely of gas. Saturn is composed of 96% hydrogen, 3% helium and 1% other elements, but these are just gaseous in the atmosphere (beige), which is about 1000 km thick. Below the atmosphere is a layer of liquid hydrogen and helium about 30,000 km thick (light blue), located above another layer of metallic hydrogen (dark blue) about 15,000 km thick. There is thought to be a rocky core (sphere, centre), with a radius of about 12,000 km, at the centre of the planet.