Neptune is undoubtedly one of the most beautiful planets in the solar system. It is the only planet in the solar system that was predicted mathematically before it was directly observed. Much of what we know about this bright blue planet comes from the data gathered by NASA’s Voyager 2 spacecraft during its Neptune flyby in 1989.

Both Uranus and Neptune belong to the category of ice giants. Ice giants are different than gas giants (Jupiter and Saturn). As opposed to gas giants (that contain more than 90% of hydrogen and helium), ice giants consist of only 20% of hydrogen and helium in mass. Ice giants are mainly composed of heavier elements like oxygen, carbon, nitrogen and sulfur.

The upper atmosphere of Neptune is mainly composed of hydrogen and helium, with trace concentrations of methane. As you go deeper into the planet, the concentration of compounds like ammonia, methane, carbon (diamond) and other hydrocarbons increases.

At high altitudes, the composition of Uranus and Neptune is very similar. The blue color of both the ice giants can be contributed to the presence of methane in their upper atmospheres. Methane absorbs most of the red light incident on the planets which makes them appear blue in color. But there is a difference in the color of these planets: Uranus has a cyan blue color (blue-green) whereas Neptune has a vivid azure blue color (bright blue).

Hence, there must be another unknown compound in Neptune’s atmosphere that is yet to be identified. The reason for the deep blue hue of Neptune is still a mystery: the very reason why we need to put an orbiter around Neptune soon.