Neptune is not the deep azure color it was believed to be for decades – it’s actually a greenish blue like Uranus, according to new research from Oxford University.
New images reveal what Uranus and Neptune, the seventh and eighth planets from the Sun, really look like. They show that the two ice giants are much more similar in appearance than previously thought.
The findings regarding Neptune’s color are particularly noteworthy, as the planet has traditionally been depicted as being deep blue. Meanwhile, the actual greenish blue of Uranus is closer to the pale cyan it was thought to be.
The misconception arose because images captured of Neptune and Uranus during the 20th century were recorded in separate colors and recombined later. Much of the imagery comes from NASA’s Voyager 2 mission, the only spacecraft to fly past these planets.
These recombined single-color images were not always accurately balanced, while Neptune’s early pictures were also strongly contrast-enhanced to better reveal its clouds, bands, and winds.
“Although the familiar Voyager 2 images of Uranus were published in a form closer to ‘true’ color, those of Neptune were, in fact, stretched and enhanced, and therefore made artificially too blue,” said lead researcher Professor Patrick Irwin, from Oxford’s Department of Physics.
Planetary scientists knew from the beginning that the color was artificially saturated, but while the images were released with captions explaining it, the distinction “had become lost over time,” according to Irwin.
To achieve a more accurate representation of the planets’ colors, the researchers utilized data from the Hubble Space Telescope’s Space Telescope Imaging Spectrograph (STIS) and the Multi Unit Spectroscopic Explorer (MUSE) on the European Southern Observatory’s Very Large Telescope.
These instruments provided a continuous spectrum of colors for each pixel, the university said, which could be “unambiguously” processed to determine the true colors of the planets.
The research, published by the Monthly Notices of the Royal Astronomical Society, also addressed the mystery of why the color of Uranus slightly changes during its 84-year orbit around the Sun.
The findings indicated that Uranus appears to be greener during its solstices – when one of the planet’s poles points towards the Sun – and bluer during its equinoxes. The variation was also partially a result of the planet’s unique spin, researchers said.
“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, of the Association of Universities for Research in Astronomy, who was not involved in the study, said that “the misperception of Neptune’s color, as well as the unusual color changes of Uranus, have bedeviled us for decades.”
Hammel added: “This comprehensive study should finally put both issues to rest.”
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