To my knowledge, no free oxygen has ever been detected anywhere except on the Earth, because it is such a reactive chemical. The atmospheric composition is hydrogen = 74%, helium=25% and methane=1%, and the interior is believed to consist of a rocky core with an overlying mantle of water ice. This ice may be slushy because Neptune produces about 10% more energy than it receives from the Sun. This apparently allows currents to flow in the interior ice mantle which generates Neptune's magnetic field.
Here is a good summary of Neptune from the online class notes from Astronomy 161
It is slightly smaller than Uranus, but its density is 1.6 g/cc (compared with 1.2 g/cc for Uranus), which makes it the 3rd most massive planet. The relatively low density indicates large concentrations of hydrogen and helium, but Uranus and Neptune both have much larger concentrations of heavier elements than Jupiter and Saturn. As for all the gas giant planets, models suggest rocky cores of maybe 15 Solar masses, but there is no direct confirmation of this. The bluish color of the adjacent image is, as for Uranus, because of methane in the atmosphere, which absorbs red light, leaving the light scattered from Neptune preferentially enhanced at blue wavelengths. The period of rotation is about 16 hours, comparable to that of Uranus and much slower than for Jupiter and Saturn. The temperatures at the cloud tops are about -216 degrees Celsius, slightly warmer than for Uranus. Neptune, like Jupiter and Saturn but unlike Uranus, has an internal heat source and produces 2.7 times more heat than it absorbs.
Copyright 1997 Dr. Sten Odenwald
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