Out of all the planets in the solar system, Neptune is the one that looks most peaceful. When seen through a telescope, the eighth and most distant planet from the sun appears sky-blue or as a uniform, peaceful ocean world that would have made the Roman god of the sea proud.
In reality, Neptune is anything but peaceful and its atmosphere is actually mainly made of three gases: hydrogen (80%), helium (19%), and methane (1%).
It’s actually clouds of methane gas that are responsible for the distant planet’s blue marble appearance. Despite the fact it makes up a relatively tiny proportion of Neptune’s atmosphere, methane absorbs red wavelengths of light and reflects blue light outward.
A distant blue gem
Neptune is the only planet in the solar system that isn’t visible to the naked eye. As such, it was among the last to be discovered in 1846 by Johann Galle, based on mathematical predictions made by Urbain Le Verrier.
But during these initial observations, astronomers had no idea what Neptune looked like.
The planet, which is about four times larger than Earth, was first visited by a spacecraft in 1989 when NASA’s Voyager 2 made a flyby. Voyager beamed back images showing Neptune’s ocean-like hue for the first time. It was really a stroke of luck that Neptune was so aptly named when astronomers could not have known that the planet is all blue.
Similarly to Uranus, Neptune is one-fifth hydrogen and helium by mass. The bulk of the planet’s mass, however, is owed to heavier molecules such as ammonia, methane, carbon, oxygen, and water.
Despite their similarities in size and composition, Neptune and Uranus are distinctly colored. This is explained by different chemical components in each of the planets’ upper atmospheres, particularly in the global cloud deck.
Neptune’s clouds are known to vary with altitude, just like on Earth. Methane clouds condense in the highest layers of the planet’s atmosphere due to frigidly cold temperatures. Further down, there may be clouds of hydrogen sulfide, ammonium sulfide, ammonia, and water. The blue-toned methane isn’t evenly distributed; ten to a hundred times more methane, ethane, and ethyne can be found at Neptune’s equator than at its poles.
Being present in the outermost layer of the atmosphere, the most important compound that influences the color of both planets’ atmospheres is methane. The greenhouse gas absorbs red light at wavelengths of 600 billionths of a meter, reflecting back bluer light. Uranus, however, is more azure, blue-green in appearance due to an additional chromophore that Neptune seems to lack. This particular chromophore hasn’t been identified yet so the true nature of Neptune’s color is still a mystery.