Uranus and Neptune are similar in many ways: the solar system’s outermost planets are both made of gas and ice, and they’re roughly the same size and mass. Yet, curiously, they’re not the same color. Uranus, the seventh planet from the sun, is a light, pale blue, while Neptune, the eighth planet, is a deeper, richer blue. But why?
Now, scientists think they have an answer. Both planets have methane in their atmospheres, which is what makes them appear blue in the first place (the gas absorbs red from the sun’s light, leaving blue behind), but one of Uranus’ methane layers is twice as thick as the layer on Neptune. The whitish haze of Uranus’ extra-thick methane layer is responsible for the planet’s overall lighter blue color.
The findings, published last week in the Journal of Geophysical Research: Planets, help solve a longstanding astronomical mystery.
“Something has to explain the difference in colors,” Imke de Pater, a planetary scientist at the University of California, Berkeley, who was not involved in the study, tells the New York Times’ Jonathan O’Callaghan.
Astronomers got their first look at Uranus and Neptune in the late 1980s thanks to Voyager 2, a spacecraft launched in 1977 that traveled to the edge of the solar system. To probe the color difference of the so-called “Ice Giants,” researchers used observations and data from the Gemini North telescope, the NASA Infrared Telescope Facility and the Hubble Space Telescope to piece together an atmospheric model of both planets.
They found that the middle layer of the atmosphere—a layer made up of frozen hydrogen sulfide mixed with gaseous haze particles—is thicker on Uranus. Neptune’s haze layer is thinner, they suspect, because the planet has a more active atmosphere, which mixes up methane ice-laden particles and turns them into literal methane snow. This methane snow gets pulled deeper into the atmosphere, which thins out some of the haze in Neptune’s middle layer, thus causing its blue color to appear stronger.
These atmospheric variations could simply be the result of physical differences between the two planets. Or, as some scientists believe, Uranus may have collided with another object, which knocked the planet on its side and created a more stagnant planet, per the New York Times.
“We hoped that developing this model would help us understand clouds and hazes in the ice giant atmospheres,” Mike Wong, an astronomer at the University of California, Berkeley, and one of the study’s authors, says in a statement. “Explaining the difference in color between Uranus and Neptune was an unexpected bonus.”
The model may also help explain the dark spots researchers occasionally see on Neptune and, less frequently, on Uranus. The spots are the result of a darkening or clearing of the deepest atmospheric layer, they suspect, which may stem from evaporating hydrogen sulfide ice.
Still, there are a lot of unanswered questions about the far reaches of the solar system, like how outer planets formed and attained their sometimes eccentric orbits, or why Uranus is tilted a full 90 degrees on its side relative to its orbit. To that end, NASA wants to launch a Uranus orbiter and atmospheric probe, ideally by sometime in the 2030s, which could help shine even more light on one of the icy blue planets to start.
"A flagship mission to the Uranian system will provide an incredible opportunity to explore how ice giant systems, which are common in the galaxy, formed and evolved," Chloe Beddingfield, an astronomer at NASA’s Ames Research Center, told Space.com’s Jamie Carter earlier this year.