New images reveal what Neptune and Uranus really look like

Neptune is affectionately known for being rich blue and Uranus green, but a new study has revealed that the two ice giants are actually much closer in color than commonly thought.

The correct shades of the planets have been confirmed using research led by Professor Patrick Irwin of the University of Oxford, published today in the journal Monthly Notices of the Royal Astronomical Society.

He and his team discovered that the two worlds actually have a similar shade of greenish blue, despite the widespread belief that Neptune is a deep azure and Uranus has a pale cyan appearance.

Astronomers have long known that most modern images of the two planets do not accurately reflect their true colors. This misconception arose because images captured of the two planets during the 20th century, including by NASA’s Voyager 2 mission, the only spacecraft to fly over these worlds, recorded images in distinct colors.

The monochrome images were then recombined to create composite color images, which were not always precisely balanced to obtain a “true” color image and, particularly in the case of Neptune, were often rendered “too blue”.

Additionally, Voyager 2’s first images of Neptune featured greatly enhanced contrast to better reveal the clouds, bands, and winds that shape our modern perspective of Neptune.

Professor Irwin said: “Although the familiar images of Uranus taken by Voyager 2 were released in a form closer to ‘real’ colors, those of Neptune were actually stretched and enhanced, and therefore made artificially too blue . Although artificially saturated images Color was known at the time among planetary scientists (and the images were published with captions explaining it) and this distinction had been lost over time. By applying our model to the original data, we were able to reconstruct the most accurate representation of color to date. of Neptune and Uranus.

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

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

This revealed that Uranus and Neptune are actually a fairly similar shade of greenish blue. The main difference is that Neptune has a slight hint of extra blue, which the model reveals is due to a thinner layer of haze on that planet.

The study also provides an answer to the long-standing mystery of why Uranus’ color changes slightly during its 84-year orbit around the sun. The authors reached their conclusion after first comparing images of the ice giant to measurements of its brightness, recorded by the Lowell Observatory in Arizona from 1950 to 2016 at blue and green wavelengths.

These measurements showed that Uranus appears a little greener at its solstices (i.e. summer and winter), when one of the planet’s poles is pointed towards our star. But during its equinoxes, when the sun is above the equator, its hue is a little bluer.

We know that this is partly explained by the fact that Uranus has a very unusual rotation. In fact, it turns almost sideways during its orbit, meaning that during the planet’s solstices its north or south pole points almost directly toward the sun and Earth. This is important, according to the authors, because any change in the reflectivity of the polar regions would therefore have a significant impact on the overall brightness of Uranus as seen from our planet.

What astronomers were less clear about was how and why this reflectivity differs. This led researchers to develop a model comparing the spectra of Uranus’ polar regions to those of its equatorial regions. Polar regions have been found to be more reflective at green and red wavelengths than at blue wavelengths, in part because methane, which absorbs red, is about half as abundant near the poles as ‘at the equator.

However, this was not enough to fully explain the color change, so the researchers added a new variable to the model in the form of a gradually thickening “hood” of icy mist that has already been observed above the sunny summer pole like the planet. passes from the equinox to the solstice.

Astronomers think they are likely methane ice particles. When simulated in the model, the ice particles further increased reflection at green and red wavelengths at the poles, offering an explanation as to why Uranus is greener at the solstice.

Professor Irwin said: “This is the first study that combines a quantitative model with imaging data to explain why the color of Uranus changes during its orbit. In this way, we demonstrated that Uranus is greener at the solstice due to the polar regions having a reduced abundance of methane but also an increased thickness of brightly dispersed methane ice particles.

Dr Heidi Hammel, of the Association of Universities for Astronomical Research (AURA), who spent decades studying Neptune and Uranus but was not involved in the study, said: “The misperception of color of Neptune, as well as the unusual color changes of Uranus, have tormented us for decades. This in-depth study should finally put an end to these two problems.

The ice giants Uranus and Neptune remain a tantalizing destination for future robotic explorers, building on the legacy of Voyager in the 1980s.

Professor Leigh Fletcher, planetary scientist from the University of Leicester and co-author of the new study, said: “A mission to explore the Uranian system, from its strange seasonal atmosphere to its diverse collection of rings and moons, is a major challenge. priority of space agencies in the decades to come. »

However, even a long-time planetary explorer orbiting Uranus would only be able to capture a brief snapshot of a Uranian year.

“Earth-based studies like this, showing how the appearance and color of Uranus have changed over the decades in response to the solar system’s strangest seasons, will be essential for putting the findings of this future mission into context. wider,” added Professor Fletcher. .