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Often appearing in images to have starkly different hues, the true colors of Neptune and Uranus may be more similar than previously thought, new research has found.
The first detailed glimpses of the two ice giants on the outer edge of our solar system were made possible by NASA’s Voyager 2 mission, which conducted flybys of Uranus in 1986 and Neptune in 1989. Voyager 2 remains the only spacecraft to fly by both worlds.
Uranus appeared to be a pale cyan color, while Neptune was depicted as a striking deep blue. Voyager 2 captured images of each planet in separate colors, and the single-color images were combined to create composites. The images of Neptune were enhanced to show the white clouds and winds of the planet’s atmosphere.
“Although the familiar Voyager 2 images of Uranus were published in a form closer to ‘true’ colour, those of Neptune were, in fact, stretched and enhanced, and therefore made artificially too blue,” said Patrick Irwin, a professor of planetary physics at the University of Oxford and author of a new study about the images, in a statement.
“Even though the artificially-saturated colour was known at the time amongst planetary scientists — and the images were released with captions explaining it — that distinction had become lost over time.”
Hubble Space Telescope’s Imaging Spectrograph and the Very Large Telescope’s Multi Unit Spectroscopic Explorer instrument both capture a continuous spectrum of colors, resulting in better color accuracy. Irwin and his team applied data collected using those instruments to the original Voyager 2 images.
The corrected images show that Neptune and Uranus have a similar greenish-blue hue. Both planets have atmospheric haze, but Neptune appears slightly bluer because it has a thinner haze layer.
“Applying our model to the original data, we have been able to reconstitute the most accurate representation yet of the colour of both Neptune and Uranus,” Irwin said.
The team’s results and the new image, published Thursday in the Monthly Notices of the Royal Astronomical Society, not only address a long-standing question about the ice giants, they pave the way for a better understanding of the enigmatic planets, researchers say.
Uranus’ shifting colors
While solving one planetary mystery, the research team seized the opportunity to answer another: why Uranus seems to change colors as it orbits the sun, as seen in the video loop below.
A year on Uranus lasts 84 Earth years. While the planet seems greener during its summer and winter solstices, it has a bluer hue during the equinoxes.
The unusual world spins on its side, so one of the planet’s poles points toward Earth and the sun during solstices.
During their comparison of images of Uranus for the study, the researchers looked at measurements of the planet’s brightness recorded by the Lowell Observatory in Arizona from 1950 to 2016.
The team developed a model comparing light data from the polar regions versus the equatorial regions and determined that the polar regions are more reflective in green and red wavelengths of light. This model involved adding a “hood” of a gradually thickening haze made of methane ice, which has been observed when the planet moves from equinox to solstice.
“In this way, we have demonstrated that Uranus is greener at the solstice due to the polar regions having reduced methane abundance but also an increased thickness of brightly scattering methane ice particles,” Irwin said.
Dr. Heidi Hammel, vice president for science at the Association of Universities for Research in Astronomy, has spent decades studying the ice giants.
“The misperception of Neptune’s colour, as well as the unusual colour changes of Uranus, have bedevilled us for decades,” said Hammel, who was not involved in the study, in a statement. “This comprehensive study should finally put both issues to rest.”
Investigating ice giants
Many mysteries remain about the ice giants. The James Webb Space Telescope recently revealed a new portrait of Uranus showcasing its often invisible rings and hidden features of its atmosphere.
In recent years, researchers have detected X-rays coming from Uranus. Scientists also found a weird “blip” in Voyager 2 data indicating the spacecraft flew through a plasmoid, a giant magnetic bubble that likely pinched off part of the planet’s atmosphere and floated out into space.
“A mission to explore the Uranian system — from its bizarre seasonal atmosphere, to its diverse collection of rings and moons — is a high priority for the space agencies in the decades to come,” said study coauthor Leigh Fletcher, a planetary scientist at the University of Leicester, in a statement.
“Earth-based studies like this, showing how Uranus’ appearance and colour has changed over the decades in response to the weirdest seasons in the Solar System, will be vital in placing the discoveries of this future mission into their broader context.”
