Neptune is a fascinating planet. For example, its most memorable characteristic is its glorious blue hue. But what causes this unique coloring? To answer that, we have to look at what Neptune’s atmosphere is made of. To break things down a bit, Neptune’s atmosphere is made of about 70-80% hydrogen, 15-20% helium, and about 1.5% methane. It also has trace amounts of other gases and ices, such as ammonia and water. The methane is what we need to look at in order to answer why Neptune is so blue.
Unlike the other gases, methane stays near the top of the atmosphere in clouds. Notably, methane gas absorbs a large chuck of the visible wavelengths, most of the red side of the spectrum, leaving the blue light reflected back to the observer. Since Neptune doesn't produce its own light, it has to reflect the Sun’s light. All the red light is absorbed by the methane rich clouds, and the Sun’s blue light waves are reflected back to us here on Earth. Hence, the sky looks blue.
Want to get more interesting facts about Neptune? Check out the video below: