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Scientists learn why Neptune is a darker hue than Uranus


Neptune and Uranus are similar in lots of respects: Each are ice giants between 15 (Uranus) and 17 (Neptune) times the mass of Earth, possess atmospheres largely product of hydrogen and helium, and take a long time to make their long, extremely cold orbits across the Sun.

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

Uranus as seen by the Voyager 2 spacecraft in 1986


In a paper published within 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, results in the planet’s paler color.

An analogous haze layers exists on Neptune, and will explain the dark spots that appear in its atmosphere, but that planet’s more turbulent atmosphere disrupts the haze layer greater than on Uranus.

Generally known as ice giants — versus gas giants Jupiter and Saturn — each 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, together with the Hubble Space Telescope, to construct a model of each planets’ atmospheres.

“That is the primary model to concurrently fit observations of reflected sunlight from ultraviolet to near-infrared wavelengths,” Dr Irwin said in an announcement. “It’s also the primary to clarify the difference in visible color between Uranus and Neptune.”

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

The invention is, nevertheless, somewhat incidental, as a greater understanding of the colour of Neptune and Uranus’s atmospheres was not the important thrust of the research program when it began, in line with team member and UC Berkeley astronomer Mike Wong.

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

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