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Hydrogen and Helium
Hydrogen and helium are the most common elements in the solar system, so it's not surprising that these are the elements that make up the majority of the outer planets. The hydrogen and helium form visible clouds that glide along the surface of the planets, which can be seen through a telescope from Earth. These elements also form liquids and solid ice beneath the clouds because of the low temperatures and extreme pressure.
Hydrogen Compounds
The composition of the outer planets also includes hydrogen compounds, such as methane, ammonia, ammonium hydrosulfide, benzene and water. These compounds can also form visible clouds or pressurized liquid formations that are so dense they are essentially solids. There is no real distinction between land and atmosphere on the outer planets because the elements gradually change from gas, to liquid, to ice and back again as they move around the planet, powered by natural heating and cooling cycles.
A Dense Core
Though mostly made up of fluid compounds, the outer planets do have very dense cores made up of a small amounts of rock and metal, such as olivine and pyroxene. Astronomers also find metallic hydrogen in the planets' cores. The pressure is so high that the individual hydrogen molecules begin to overlap one another. When this happens the electrons no longer orbit individual atoms but rather flow freely from one atom to another. This process makes hydrogen act like a metallic conductor, such as copper.
Other Trace Elements and Compounds
The outer planets also contain other trace elements, such as carbon, silicon, neon, oxygen and sulfur. Some of these elements combine with silicon to make silicon-based compounds. These elements make up a very small percentage of the overall composition of the outer planets by mass, however.