Why Is Neptune Blue?

Our First Close-Up View of Neptune

Though Neptune is so far away from the Earth that it only looks like a tiny faraway star when observed with binoculars, we have discovered that this planet is actually a startlingly brilliant blue celestial object upon closer inspection. When Voyager 2 flew by Neptune in 1989 (twelve whole years after it was originally launched from Earth in 1977), we got our first glimpse the planet, revealing it to be a brilliant blue. So, what gives Neptune its blue color? We must first clarify what we are actually seeing and we also must know exactly what to analyze to discover the answer.

What We Are Actually Seeing

When we see the planets in the night sky, we are actually viewing the light from the sun that is reflecting off of them. Therefore, we can ascertain that we are seeing reflected sunlight off of Neptune. But, why does this reflected light appear blue and not yellow like the sun's light normally looks?

Where Do We Look?

The answer to why Neptune is blue lies within Neptune's atmosphere, which merges into its liquid mantle. More specifically, we must examine the components of its outer atmosphere to find out the solution our question. What do the high cloud tops of Neptune's atmosphere reveal?

Unveiling Neptune's Atmosphere

There are three major gases that make up Neptune's atmosphere: hydrogen, helium, and methane (along with trace amounts of water and ice particles). While hydrogen and helium make up about 99% of its atmosphere, it is the remaining roughly 1% of methane that is important. The presence of methane is why Neptune appears blue.

What Does This Methane Do?

These icy methane particles in the clouds take the red and orange light waves and absorb them, leaving the blue light waves to escape and reflect outward to our eyes. This is how the sunlight is reflected off of Neptune's atmosphere and appears blue.

Conclusion

Why is Neptune blue? We've seen that after the sun's light reaches Neptune, the methane in the atmosphere of Neptune absorbs the red end of the spectrum's light waves, allowing only the blue light to be reflected back towards Earth.
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