When you think of the solar system’s outermost planets, Uranus and Neptune, you might think of two distinct hues: pale turquoise and cobalt blue. But astronomers say the true colors of these distant ice giants are more similar than what is commonly depicted.
Neptune is slightly bluer than Uranus, but the difference in hue is not as large as seen in typical images, according to the study published Friday in the Monthly Notices of the Royal Astronomical Society.
The results help “set the record straight,” said Lee Fletcher, professor of planetary science at the University of Leicester in the UK and author of the study. “There are subtle differences in the blue hues of Uranus and Neptune, but subtle is the operative word.”
The deep blue color attributed to Neptune dates back to artificial enhancement in the 1980s, when NASA’s Voyager 2 became the first (and still only) spacecraft to visit the two planets.
Scientists at the time intensified the blue in images of Neptune taken by Voyager’s cameras to highlight the planet’s many interesting points, such as its polar waves and dark spots. But as many skywatchers have known for decades, both Neptune and Uranus appear pale greenish-blue to the human eye.
“The Uranus that Voyager saw was pretty bland, so they tried to get it as close to its real color as possible,” said Patrick Irwin, professor of planetary physics at the University of Oxford and author of the study. “But Neptune has all kinds of strange things,” he said, noting that with proper color correction “it gets a little washed out.”
Enhanced images of Neptune often include captions featuring artificial colors, but the deep blue planet’s vision survives.
Dr. Irwin and his colleagues used advanced instruments from the Hubble Space Telescope and the Very Large Telescope in Chile to resolve the planet’s colors as accurately as possible.
They also examined extensive observation records of both planets captured by the Lowell Observatory in Arizona between 1950 and 2016.
The results confirmed that Uranus is only slightly thinner than Neptune. This is due to the thick layer of aerosol haze that brightens its color.
Lowell’s dataset also sheds new light on the mysterious color changes that Uranus experiences during its extreme seasons.
Astronomers have long puzzled as to why Uranus turns green at the summer solstice, but glows bluer at the equinox. This pattern has to do with Uranus’ odd position: tilted almost completely sideways. During its 84-year orbit around the Sun, Uranus’ poles experience decades of eternal light and darkness in summer and winter, while its equatorial regions face the Sun near the equinoxes.
Uranus’ color changes can be partially explained by methane in its atmosphere. Methane absorbs red and green light, so the equator reflects more blue light. In contrast, the poles with half the amount of methane are colored slightly green. New research confirms this dynamic, showing that “hoods” of ice particles coalesce above Uranus’s summer sunlit poles, driving the greening effect.
Rabbit Heled, professor of theoretical astrophysics at the University of Zurich who was not involved in the study, said the study “opens the door to many future studies aimed at understanding Uranus’ atmosphere and its seasons. It is a thing,” he said. The study could “improve our understanding of the planet’s internal structure and thermal evolution,” she added.
For astronomer Heidi Hammel, who worked on Voyager’s imaging team in 1989, the new study is the latest chapter in a years-long quest to uncover the planet’s true colors.
“For the general public, we hope this paper will help undo decades of misinformation about Neptune’s color,” said Dr. Hummel, currently vice president for science at the Association of Universities for Research in Astronomy. Ta. “When discussing Neptune, please erase the word ‘azure’ from your vocabulary!”
The gap between public perception and Neptune’s reality illustrates one of the many ways data can be manipulated to emphasize certain features or make astronomical visualizations more appealing. For example, the stunning images released by the James Webb Space Telescope are synthetic pseudocolor versions of the original infrared observations.
“I never tried to deceive anyone,” Dr. Fletcher said. Perhaps it makes more sense than the fuzzy gray amorphous blob in the distance. ”