Neptune is composed of 80 percent hydrogen, 19 percent helium and small amounts of methane, hydrogen deuteride and ethane. As a gas giant, the planet has no solid surface. The surface is defined as the area where the pressure is equal to the pressure of the Earth at sea level.
Scientists believe that methane is partially responsible for giving Neptune its blue hue. The methane, ethane and ethyne are at higher concentrations around the equator than at the poles. Nearer to the planet's core, the gases take on a liquid form and may crystallize at lower depths. The core is composed of metals.