Telescopes reveal why Neptune is bluer than Uranus


New observations from space and ground telescopes have revealed what lies behind this difference in tone.

The farthest planets from the sun in our solar system, Neptune and Uranus have similar sizes, masses and atmospheric conditions. Looking 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 paler shade of cyan blue.

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

Scientists have determined that excess haze is building up in Uranus’ atmosphere, giving it a lighter appearance. This haze is thicker on Uranus than a similar atmospheric layer on Neptune, so it whitens Uranus’ appearance from our perspective.

Without this haze in either planetary atmosphere, astronomers believe the two planets would be almost identically blue. A study detailing the findings published Tuesday in the Journal of Geophysical Research: Planets.

Previous attempts to understand this difference have focused on planetary atmospheres above specific wavelengths of light.

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

The model also probed deeper atmospheric layers that include haze particles, in addition to methane and hydrogen sulfide ice clouds.

New observations from the Gemini North Telescope, located near the summit of Mauna Kea in Hawaii, have been combined with other archival data from the telescope. The team analyzed three layers of aerosols at different heights on Uranus and Neptune. The middle layer of haze particles is what has the most impact 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 the sluggish slow-moving one of Uranus, so methane particles and snow showers keep haze from building up on Neptune.

Scientists believe this pattern could also help explain why dark spots appear on Neptune, but are less common on Uranus. This is likely due to the innermost atmospheric layer darkening, which would be more visible on Neptune.

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

We could learn more about these mysterious worlds, which were never visited by Voyager 2 except during rapid flybys.

The 10-year planetary study, released in April, recommended the first dedicated Uranus orbiter and probe as NASA’s next big mission. The authors of the report consider the orbiter and the Uranus probe as a means of revolutionizing the knowledge of astronomers about the ice giants.

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