Neptune is known for its dark blue and Uranus for its green, but a new study has shown that the colors of the two ice giants are much closer than previously thought. The correct hues of the planets have been confirmed in research published today led by Professor Patrick Irwin of the University of Oxford. Monthly Notices of the Royal Astronomical Society.

He and his team found that, despite the popular belief that Neptune is dark blue and Uranus is pale blue, the two worlds actually have a similar shade of greenish-blue. Astronomers have long known that most modern images of the two planets do not accurately reflect their true colors. This misconception arose because images of both planets taken in the 20th century, including by NASA’s Voyager 2 mission, the only spacecraft to fly past these worlds, produced images of different colors.

Monochrome images were then recombined to create composite color images, which were not always fully balanced to create a “true” color image and were often shown as “too blue”, especially in the case of Neptune.

In addition, Voyager 2’s first images of Neptune were greatly enhanced with contrast to better reveal the clouds, streaks, and winds that shape our modern view of Neptune.

Professor Irwin said: “While the familiar Voyager 2 images of Uranus were broadcast in a format closer to ‘true’ colour, images of Neptune were actually stretched and enhanced and therefore artificially too blue. Although the artificially saturated color was popular among planetary scientists at the time “Although it is known (and the images are published with captions explaining it) this distinction has been lost over time. By applying our model to the raw data, we were able to obtain the most accurate representation of the color of both Neptune and Uranus.”

In the new study, researchers used data from the Hubble Space Telescope Imaging Spectrograph (STIS) and the Multi-Unit Spectroscopic Explorer (MUSE) on the European Southern Observatory’s Very Large Telescope. In both tools, each pixel is a continuous spectrum of colors.

This means that STIS and MUSE observations can be explicitly processed to determine the apparent true color of Uranus and Neptune. The researchers used this data to rebalance composite color images taken by the Voyager 2 camera and the Hubble Space Telescope’s Wide Field Camera 3 (WFC3).

This showed that Uranus and Neptune actually have a very similar shade of greenish-blue. The main difference is that Neptune has a slight extra blue tint; the model shows that this is due to the planet’s thinner haze layer.

The research also offers an answer to the long-standing mystery of why Uranus changes color slightly during its 84-year orbit around the Sun. The authors first reached this conclusion after comparing images of the ice giant with brightness measurements recorded in the blue and green bands between 1950 and 2016 by the Lowell Observatory in Arizona.

These measurements showed that during Uranus’ solstices (i.e., summer and winter), the planet appears slightly greener when one of its poles points toward our star. However, at the equinox, when the sun is above the equator, it takes on a slightly bluer hue.

This is known to be partly due to the unusual rotation of Uranus. In fact, it rotates almost on its side during its orbit, which means that the planet’s north or south pole points almost directly toward the Sun and Earth during the solstices. This is important, the authors say, because any change in the reflectance of the polar regions would have a major impact on the overall brightness of Uranus as seen from our planet.

What astronomers are less clear on is how and why this reflection is different. This led researchers to develop a model that compared the spectra of Uranus’ polar regions with its equatorial regions. Polar regions were found to have greater reflectivity at green and red wavelengths than at blue wavelengths; This is because red-absorbing methane is about half as abundant near the poles as it is at the equator.

But 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 “cap” of increasingly thick icy haze previously observed above the summer solar pole as a planet. It moves from the equinox to the solstice.

Astronomers believe it is probably composed of methane ice particles. When simulated in the model, the ice particles further strengthened reflections in green and red wavelengths at the poles, offering an explanation for why Uranus is greener during the solstice.

Professor Irwin said: “This is the first study to match a quantitative model with imaging data to explain why Uranus changes color as it orbits. Thus, during Uranus’ solstice, the abundance of methane in the polar regions decreases and at the same time the thickness of the brightly scattered methane ice particles increases.” We showed that it is greener because of

Dr. from the Association of Universities for Research in Astronomy (AURA), who has been working on Neptune and Uranus for decades but was not involved in the research. Heidi Hummel said: “Misperceptions about the color of Neptune, as well as unusual color changes on Uranus, have plagued us for decades. “This comprehensive study will eventually solve both problems.”

Ice giants Uranus and Neptune remain an attractive target for future robot explorers, building on Voyager’s legacy in the 1980s.

Professor Leigh Fletcher, planetary scientist from the University of Leicester and co-author of the new study, said: “The mission to explore the Uranian system, from its strange seasonal atmosphere to its diverse collection of rings and moons, is a high priority for space agencies in the coming years.”

However, even a planetary explorer with a long-lived Uranus orbiter would only be able to take a one-year snapshot of Uranus.

“Such ground-based studies that show how the appearance and color of Uranus change over decades in response to the strangest seasons in the Solar System will be vital to place the findings of this future mission in a wider context,” Professor Fletcher added. .