PHYSICS!
Neptune has much more methane in its atmosphere than Jupiter and Saturn. Methane is what gives Neptune its blue color. Below Neptune's atmosphere is a liquid ocean of water, ammonia and methane this region is also called "ices," The heat left over from planetary formation is why a planet about 2.8 billion miles from the sun has an ocean that is so hot. There were theroys that the neptune was creating dimonds. Untill the Italians' discovered a more likely posibility that what happens is that methane is converted into the colorless, odorless, flammable hydrocarbon gas called ethane, which bubbles up from the ocean and into Neptune's atmosphere. Although Neptune still may be forming diamonds according to the scenario earlier described. But if it's happening, it's happening at much deeper layers with in the planet.
