Scientists learn why Neptune is a darker hue than Uranus

New observations and research has shed unexpected light on the hues of our Solar System’s two ice giant planets

The deep azure atmosphere of Neptune is apparent in this image taken by Voyager 2 in 1989 (Nasa)

Neptune and Uranus are similar in many respects: Both are ice giants between 15 (Uranus) and 17 (Neptune) times the mass of Earth, possess atmospheres largely made of hydrogen and helium, and take decades to make their long, extremely cold orbits around the Sun.

But while Neptune appears a dark, rich hue of blue to the human eye, with swirling storms and clouds. Uranus is a nearly featureless pale greenish-blue orb. Scientists now think they know why.

Uranus as seen by the Voyager 2 spacecraft in 1986 (Nasa)

In a paper published in the Journal of Geophysical Research: Planets, a team of researchers led by Oxford University professor of planetary physics Patrick Irwin conclude that a layer of aerosolised particle haze, combined with Uranus’s comparatively more quiescent atmosphere, leads to the planet’s paler colour.

A similar haze layers exists on Neptune, and could explain the dark spots that appear in its atmosphere, but that planet’s more turbulent atmosphere disrupts the haze layer more than on Uranus.

Known as ice giants — as opposed to gas giants Jupiter and Saturn — both planets possess voluminous gas atmospheres wrapped around a mantle of water, ammonia and methane ices, with a small core of rock and ice. Neither have been visited by a spacecraft since Nasa’s Voyager 2 mission flyby of Uranus in 1986, and the spacecraft’s flyby of Neptune in 1989.

The researchers used observations in visible, ultraviolet and infrared light from the Gemini North and Nasa Infrared Telescope Facility in Hawaii, along with the Hubble Space Telescope, to build a model of both planets’ atmospheres.

“This is the first model to simultaneously fit observations of reflected sunlight from ultraviolet to near-infrared wavelengths,” Dr Irwin said in a statement. “It’s also the first to explain the difference in visible color between Uranus and Neptune.”

The researchers believe that methane condenses on the aerosol haze particles in both planets’ atmospheres, eventually falling downward as methane snow. Neptune, possessing the fastest winds in the Solar System — gusts have been clocked at 1,300 miles per hour — is simply better at mixing methane in with the haze and clearing it out. A thinner haze layer on Neptune allows the planet’s true colors to show through, while the thicker haze layer on Uranus washes out the deeper hues.

The discovery is, however, somewhat incidental, as a better understanding of the color of Neptune and Uranus’s atmospheres was not the main thrust of the research program when it started, according to team member and UC Berkeley astronomer Mike Wong.

“We hoped that developing this model would help us understand clouds and hazes in the ice giant atmospheres,” he said in a statement. “Explaining the difference in color between Uranus and Neptune was an unexpected bonus!”