Why Twin-Like Ice Giants Uranus and Neptune Are Different Shades of Blue

A whiteish layer of haze forms where methane reacts with sunlight

A side-by-side comparison of planet neptune and planet uranus
The ice giants appear blue because of the methane in their atmospheres. Methane reflects blue light and absorbs red light. Pictured: Uranus on the left and Neptune on the right. NASA/JPL-Caltech

Uranus and Neptune have plenty of similarities, and like siblings, the two are often compared to each other.

The pair of ice giants are the furthest two major planets in our solar system, and they are about the same size, roughly 15 times the size of Earth. Both planets spin much faster than Earth, completing their daily rotations in less than 17 hours. Near their superheated cores, it may even rain diamonds. But don't call them twins just yet. 

Puzzlingly, Neptune is a vibrant shade of cobalt blue, whereas Uranus has a pale green tint—and now, scientists may know why, reports Science Alert's Michelle Starr.

A new study suggests variance in their hues may due to a hazy layer where methane reacts with sunlight found in both planets' atmospheres. The milky layer is twice as opaque on Uranus than it is on Neptune, which would make Uranus appear more pale, according to a paper uploaded to the preprint server arXiv last month. 

The only spacecraft to study the furthest planets in our solar system, NASA's Voyager 2, was launched in 1977 and reached Uranus in 1986 and Neptune in 1989. The spacecraft gave astronomers an up-close look at the ice giants for the first time. The thick, freezing, and gassy atmospheres of Uranus and Neptune contain a mixture of hydrogen, helium, and methane. In the planets' lower atmospheres, methane absorbs red light and reflects blue light, giving both icy worlds their cerulean undertones, explains study author Leigh Fletcher, a planetary scientist at the University of Leicester in the United Kingdom, to New Scientist 's Will Gater.

However, patches of chemicals create haziness throughout the atmospheres of both faraway worlds. These cloudy-looking layers occur where the sun's ultraviolet radiation breaks down aerosols. This photochemical process occurs on Venus, Earth, Jupiter, and Saturn as well, per Science Alert. For example, a brownish-haze called photochemical smog forms in Los Angeles when nitrogen oxide from car exhaust reacts with sunlight in Earth's lower atmosphere. 

Using data collected by Voyager 2, the Hubble Space Telescope, and other ground-based observatories on Earth, the research team created new models of the both planets' atmospheres, per New Scientist. They identified a blanket of haze where methane breaks down and called it the Aerosol-2 layer. On the visible light spectrum, this layer would look milky white, per New Scientist. Like a sheet of wax paper over blue cardstock, Uranus' double-thick Aerosol-2 layer would make the planet look a few shades paler than Neptune's bold cobalt. 

"We conclude that photochemical haze produced in the upper atmospheres of both planets is steadily mixed down to lower layers, where it concentrates in a vertically-thin statically stable layer near the methane condensation level," the study authors state in the paper.

The researchers also suggest the Aerosol-2 may be critical for producing both planets' signature methane ice and snow storms. "We suggest that methane condenses so rapidly upon these haze particles that it efficiently 'snows' out at the base of this layer, falling to lower, warmer levels, where the methane evaporates, releasing the core haze particles to 'seed' [hydrogen sulfide] cloud formation," per the study.

Dark spots on both planets are suspected to originate in the layer beneath Aerosol-2 called Aerosol-1. Because Neptune's Aerosol-2 layer is less thick, these darker regions would be easier to see, which explains why fewer have been spotted on Uranus.

"Seeing both planets successfully compared directly in the analysis is rare," says Naomi Rowe-Gurney, an ice giant expert at NASA's Goddard Space Flight Center who was not involved with the study, to New Scientist. "The authors state that future observations will help to answer remaining questions, and I am sure that the James Webb Space Telescope will help with this during the planned observations of both planets within the first year of operations." 

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