Telescopes reveal why Neptune is bluer than Uranus

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New space and ground observations have revealed what’s behind this difference in tone.

The planets furthest from the sun in our solar system, Neptune and Uranus, have similar sizes, masses and atmospheric conditions. If we look at the two planets side by side, which was made possible after NASA’s Voyager 2 spacecraft flew past them in the 1980s, Neptune has a bright blue appearance. Uranus is a lighter shade of cyan blue.

Astronomers used the Gemini North Telescope and NASA Infrared Telescope Facility, both in Hawaii, and the Hubble Space Telescope to create a model that could rival observations of Neptune and Uranus.

The scientists found that excess haze builds up in Uranus’ atmosphere, making it appear lighter. This haze is thicker on Uranus than a comparable atmospheric layer on Neptune, so it whitens Uranus’s appearance from our perspective.

Without this haze in both planetary atmospheres, astronomers think both planets would be almost identically blue. A study detailing the findings was published Tuesday in the Journal of Geophysical Research: Planets.

Previous efforts to understand this difference have focused on the atmospheres of the upper planets at specific wavelengths of light.

“This is the first model to simultaneously fit observations of reflected sunlight from ultraviolet to near-infrared wavelengths,” study lead author Patrick Irwin, a professor of planetary physics at the University of Oxford, said in a statement. “It is also the first to explain the difference in visible color between Uranus and Neptune.”

The model also explored deeper atmospheric layers containing nebula particles, in addition to clouds of methane and hydrogen sulfide ice.

New observations from the Gemini North telescope, located near the summit of Mauna Kea in Hawaii, were combined with other archival data from the telescope. The team analyzed three layers of aerosols at different elevations on Uranus and Neptune. The middle layer of haze particles has the most influence on the color.

On both planets, the middle layer is where methane ice turns into methane snow showers. Neptune has a turbulent atmosphere that is more active than Uranus’ slow, sluggish atmosphere, so the methane particles and snow showers prevent a haze from forming on Neptune.

Scientists think this model may also help explain why dark spots appear on Neptune, but are less common on Uranus. It’s probably due to the deepest atmospheric layer getting darker, which would be more visible on Neptune.

“We hoped developing this model would help us understand clouds and mists in the ice giant atmospheres,” study co-author Mike Wong, an astronomer at the University of California, Berkeley, said in a statement. “Explaining the color difference between Uranus and Neptune was an unexpected bonus!”

We could learn more about these mysterious worlds, visited only by Voyager 2 on high-speed scenic flights.

The planetary decade study, published in April, recommended the first dedicated Uranus Orbiter and Probe as NASA’s next major mission. The report’s authors see the Uranus Orbiter and Probe as a way to revolutionize astronomers’ knowledge of ice giants.



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