Neptune has no identifiable surface layer, and the icy, gaseous atmosphere eventually transitions to a slushy water and liquid ammonia ocean with a rocky, iron core at the center. As the depth increases, the intense pressure may cause methane to crystallize into diamonds.
Neptune's dense atmosphere is composed of 80 percent hydrogen, 19 percent helium and up to 1.5 percent methane, with trace amounts of hydrogen deuteride, ethane, ammonia ice, water ice and ammonia hydrosulfide. The cloud composition is arranged in layers, changing with altitude and pressure. The highest level of clouds are frozen methane, followed by clouds of ammonia and hydrogen sulfide, and it is possible that clouds of water ice could be found below these layers.