New images reveal what Neptune and Uranus really look like

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The true colors of Neptune and Uranus

A study reveals that Neptune and Uranus are both blue-green, not the deep blue and pale cyan previously believed. Data from modern telescopes were used to correct these historical color misrepresentations. Credit: Patrick Irwin, edited

Recent research led by Professor Patrick Irwin shows that Neptune and Uranus Both are a similar shade of blue-green, challenging previous perceptions of their colors. The study used modern telescopic data to correct historical color inaccuracies and explained minor color changes in Uranus throughout its orbit.

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

The correct hues of the planets have been confirmed with the help of research led by Professor Patrick Irwin of the Oxford Universitywhich has been published today in Monthly Notices of the Royal Astronomical Society.

He and his team discovered that both worlds are, in fact, a similar shade of blue-green, despite the common belief that Neptune is a deep blue and Uranus has a pale cyan appearance.

Colors of Neptune and Uranus

Voyager 2/ISS images of Uranus and Neptune released shortly after the Voyager 2 flybys in 1986 and 1989, respectively, compared to a reprocessing of the individual filter images in this study to determine the best estimate of the true colors of these planets. Credit: Patrick Irwin

The misconception of planetary colors

Astronomers have long known that most modern images of the two planets do not accurately reflect their true colors.

The misconception arose because images captured of both planets during the 20th century, even by POTThe Voyager 2 mission, the only spacecraft to pass by these worlds, recorded images in separate colors.

The single-color images were later recombined to create composite color images, which were not always precisely balanced to achieve a “true” color image and, particularly in the case of Neptune, were often made “too blue.”

Uranus color variations

Uranus as seen by HST/WFC3 between 2015 and 2022. During this sequence, the north pole, which is a paler green color, descends toward the Sun and Earth. In these images, the equator and latitude lines are marked at 35N and 35S. Credit: Patrick Irwin

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

Professor Irwin said: “Although the well-known images of Uranus taken by Voyager 2 were published in a form closer to ‘true’ colour, those of Neptune were, in fact, stretched and enhanced, and were therefore artificially made too blues”.

“Although artificially saturated color was known to planetary scientists at the time (and the images were published with captions explaining it), that distinction had been lost over time.”

“By applying our model to the original data, we have been able to reconstitute the most accurate representation yet of the color of Neptune and Uranus.”

Clarifying true colors through modern research

In the new study, the researchers used data from Hubble space telescopeThe Space Telescope Imaging Spectrograph (STIS) and Multi-Unit Spectroscopic Explorer (MUSE) at the European Southern Observatory Very large telescope. In both instruments, each pixel is 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 color composite images recorded by the Voyager 2 camera and also by the Hubble Space Telescope’s Wide Field Camera 3 (WFC3).

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

Animation of the seasonal color changes in Uranus during two Uranus years (one Uranus year is equal to 84.02 Earth years), ranging from 1900 to 2068 and beginning just before the southern summer solstice, when the south pole of Uranus points almost directly towards the Sun.
The disk on the left shows what Uranus looks like to the naked eye, while the disk on the right has been enlarged and color enhanced to clarify atmospheric features. In this animation, Uranus’ spin has been slowed more than 3,000 times so that planetary rotation can be seen, with discrete storm clouds passing across the planet’s disk.
As the planet moves toward its solstices, a pale polar “hood” of increasing cloud opacity and reducing methane abundance can be seen filling more of the planet’s disk, causing seasonal changes in color. general of the planet.
The change in size of Uranus’ disk is due to Uranus’s distance from the Sun changing during its orbit.
Credit: Patrick Irwin, University of Oxford

Explaining the color variations of Uranus

The study also provides an answer to the long-standing mystery of why Uranus’s color changes slightly during its 84-year orbit around the Sun.

The authors reached this conclusion after first comparing images of the ice giant with measurements of its brightness, which were recorded by the Lowell Observatory in Arizona between 1950 and 2016 at blue and green wavelengths.

These measurements showed that Uranus appears slightly greener on its solstices (i.e. summer and winter), when one of the planet’s poles points toward our star. But during its equinoxes – when the Sun is above the equator – it has a somewhat bluer tint.

Part of the reason for this was known to be that Uranus has a very unusual spin.

Indeed it rotates almost sideways during its orbit, meaning that during the planet’s solstices its north or south pole points almost directly towards the Sun and Earth.

This is important, the authors said, because any change in the reflectivity of the polar regions would have a large impact on the overall brightness of Uranus when viewed from our planet.

What astronomers were not so clear about is how or why this reflectivity differs.

This led researchers to develop a model that compared the spectra of Uranus’s polar regions with its equatorial regions.

He found that polar regions are 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 freezing fog that had previously been observed during the summer, at the sunlit pole of the planet. passes from the equinox to the solstice.

Astronomers believe it is likely made up of methane ice particles.

When simulated in the model, the ice particles further increased reflection in green and red wavelengths at the poles, offering an explanation for why Uranus is greener on the solstice.

Professor Irwin said: “This is the first study to link a quantitative model with imaging data to explain why Uranus’s color changes during its orbit.”

“In this way, we have shown that Uranus is greener at the solstice because the polar regions have a lower abundance of methane but also a greater thickness of brightly scattering methane ice particles.”

Dr. Heidi Hammel of the Association of Universities for Research in Astronomy (AURA), who has spent decades studying Neptune and Uranus but was not involved in the study, said: “The misperception of Neptune’s color, as well as the “Uranus’s unusual color changes have plagued us for decades. This comprehensive study should finally put both issues to rest.”

Future exploration and continued research

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 at the University of Leicester and co-author of the new study, said: “A mission to explore the Uranus system, from its strange seasonal atmosphere to its diverse collection of rings and moons, is a great opportunity.” priority for space agencies in the coming decades.”

However, even a long-lived planetary explorer, orbiting Uranus, would only capture a brief snapshot of a Uranian year.

“Ground-based studies like this one, which show how Uranus’s appearance and color have changed over the decades in response to the Solar System’s strangest seasons, will be vital to placing the discoveries of this future mission in their broader context.” “said Professor Fletcher. aggregate.

Reference: “Modeling the seasonal cycle of color and magnitude of Uranus, and comparison with Neptune” by Patrick GJ Irwin, Jack Dobinson, Arjuna James, Nicholas A Teanby, Amy A Simon, Leigh N Fletcher, Michael T Roman, Glenn S Orton, Michael H Wong, Daniel Toledo, Santiago Pérez-Hoyos and Julie Beck, September 12, 2023, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/stad3761

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