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Why is Neptune blue?

Kulvinder Singh, News reporter
May 23, 2015, 17:21 UTC

Sen—Named after the Roman god of the sea, Neptune, the eighth planet in our Solar System, mesmerises with its deep-blue color. But unlike the blue coloration of Earth as seen from space, Neptune's hue does not come from seas or oceans, but rather from its cloud tops.

The 49,244 kilometer-diameter planet (nearly four times the size of Earth) was discovered in 1846, the culmination of work by French, German and English astronomers.

It was only during NASA's Voyager 2 flyby in 1989 that the planet was observed in detail for the first time. The astronomers of 1846, with their ground-based telescopes, could not have known how apt Neptune's name was given its distinctive, ocean-like hue.

Neptune is composed of about one fifth hydrogen and helium by mass. Uranus, which is similar in size and structure, has a similar composition. The bulk of their mass comes from molecules heavier than hydrogen and helium, such as ammonia, methane, carbon, oxygen and water vapour.

Conversely the gas giants Jupiter and Saturn are primarily composed of hydrogen and helium by mass. In fact the two gases make up around 90 per cent of the mass of those two worlds.

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Although similar in size and composition to Neptune, Uranus is distinctly different in color. Image credit: NASA, JPL-Caltech.

The colors of the gas planets are largely due to chemical components in their upper atmospheres—particularly in the global cloud deck. Each of the gas planets is blanketed by thick, opaque clouds. Chemicals that contribute to color are known as chromophores.

Both Jupiter and Saturn appear yellow, orange, brown and red—depending on different chromophores in bands, spots and other features in their atmospheres. Jupiter's cloud tops contain water ice and ammonia crystals. Convection dredges up phosphorus, sulphur and hydrocarbons—all of which contribute to its coloration.

Saturn's composition is similar except that it also contains phosphine. Saturn's atmosphere also behaves differently from Jupiter's. All of these factors give the two different appearances and colors.

One hydrocarbon in particular, methane, plays a crucial role in giving Uranus and Neptune their color. Methane absorbs red light at wavelengths of 600 billionths of a meter, so when sunlight falls on these two planets, red light is absorbed and bluer light is reflected back. This happens even though there are actually only trace amounts of methane in their atmospheres.

However, Uranus's appearance differs from that of Neptune. As well as being visually featureless, Uranus is a more azure, blue-green color. This tells scientists that another, additional chromophore in Neptune's atmosphere is responsible for its deep blue color. That component has not yet been identified, so the true nature of Neptune's color will remain a mystery for some time yet.