For decades, images taken of Neptune have looked like the planet has a deep blue hue, while Uranus seemed more green. However, these two ice giants may actually look more similar to eachother than astronomers previously believed. According to a study published January 5 in Monthly Notices of the Royal Astronomical Society, our solar system’s furthest planets’ true colors could both be similar pale shades of greenish blue.
Images versus reality
NASA’s Voyager 2 mission remains the only flyby of both ice giants conducted by a spacecraft. It gave us the first detailed images of these far-flung planets. Voyager 2 conducted a flyby of Uranus in 1986, and the images revealed a planet with a more pale cyan or blue color. The vessel flew by Neptune in 1989 and the imagery showed a planet with a rich blue color.
However, astronomers have long understood that most modern images of both planets don’t accurately reflect their true colors. Voyager 2 captured images of each planet in separate colors and these single-color images were then put together to make composites. These composite images were not always accurately balanced, particularly for the planet Neptune which was believed to appear too blue. The contrast on the early Voyager images of Neptune were also strongly enhanced to better reveal the clouds and winds of the planet.
“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,” study co-author and University of Oxford astronomer Patrick Irwin said in a statement. “Even though the artificially-saturated color was known at the time amongst planetary scientists–and the images were released with captions explaining it–that distinction had become lost over time.”
Creating a more accurate view
In the new study, the team applied data taken 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.
With both the STIS and MUSE, each pixel is a continuous spectrum of colors, so their observations can be processed more clearly to determine the more accurate color of the planets, instead of what is being seen with a filter.
The team used the data to rebalance the composite color images that were recorded by Voyager 2’s onboard camera and by the Hubble Space Telescope’s Wide Field Camera 3. The rebalancing revealed that both Uranus and Neptune are actually a similar pale shade of greenish blue. Neptune has a slight hint of more blue, which the model showed to be a thin layer of haze on the planet.
The changing colors of Uranus
This research also provides a likely answer to why Uranus changes color slightly during its 84 year-long orbit around the sun. The team first compared images of Uranus to measurements of its brightness that were taken at blue and green wavelengths by the Lowell Observatory in Arizona from 1950 to 2016. These measurements showed that Uranus looks a little greener during its summer and winter solstices, when its poles are pointed towards the sun. However, during the equinoxes–when the sun is over the planet’s equator–it appears to have a more blue tinge.
One already established reason for the change is due to Uranus’ a highly unusual spin. The planet spins almost on its side during orbit, so its north and south poles point almost directly towards the sun and Earth during its solstices. Any changes to the reflectivity of Uranus’ poles would have a major impact on the planet’s overall brightness when viewed from the Earth, according to the authors. What was less clear to astronomers was how and why this reflectivity differs. The team developed a model to compare the bands of colors of Uranus’s polar regions to its equatorial regions.
They found that polar regions are more reflective at green and red wavelengths than at blue wavelengths. Uranus is more reflective at these wavelengths partially because gas methane absorbs the color red and methane is about half as abundant near Uranus’ poles than the equator.
However, this wasn’t enough to fully explain the color change so the researchers added a new variable to the model in the form of a ‘hood’ of gradually thickening icy haze which has previously been observed when Uranus moves from equinox to summer solstice. They believe that this haze is likely made up of methane ice particles.
After simulating this pole shift in the model, the ice particles further increased the reflection at green and red wavelengths at the planet’s poles, which explained that Uranus looks greener at the solstice due to less methane at the poles and increased thickness of the methane ice particles.
“The misperception of Neptune’s color, as well as the unusual color changes of Uranus, have bedeviled us for decades,” Heidi Hammel, of the Association of Universities for Research in Astronomy said in a statement. “This comprehensive study should finally put both issues to rest.” Hammel is not an author of the new study.
Filling in this gap between the public perception of Neptune and its reality shows how data can be manipulated to show off certain features of a planet or enhance visualizations.
“There’s never been an attempt to deceive,” study co-author and University of Leicester planetary scientist Leigh Fletcher told The New York Times. “But there has been an attempt to tell a story with these images by making them aesthetically pleasing to the eye so that people can enjoy these beautiful scenes in a way that is, maybe, more meaningful than a fuzzy, gray, amorphous blob in the distance.”