Why is Neptune so blue in color?

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.

Neptune flyby of Voyager 2 in 1989 (Credit: NASA)

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.

Neptune and its ring system (Image: NASA/GSFC)

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. You can know more about the interior of Neptune by watching the video below:

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.

Several missions are being planned to explore Neptune. A mission like Cassini (that spent 13 years orbiting Saturn) would not only help us explore the secrets of Neptune but also help us to learn about the formation and properties of ice giants. According to a study, distant exoplanets around other stars are more likely to be ice giants like Neptune. 

Artist concept of Neptune Orbiter (NASA)

Neptune Orbiter is a proposed mission by NASA that would explore Neptune and its moons just like Cassini. Although there is no official date for its launch, it would take about 8 to 10 years to reach Neptune after it is launched. 

Do you think it is important for us to launch an orbiter to Neptune soon? Let us know your thoughts in the comments.

Cover Image Credit: ESA/NASA


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