A study reveals that both Neptune and Uranus are greenish-blue in color, not the deep blue and pale cyan previously believed. Modern telescope data have been used to correct these historical color misinterpretations. Credit: Patrick Irwin, edited
Recent research led by Professor Patrick Irwin shows that Neptune And Uranus Both have similar green-blue hues, challenging previous perceptions of their colors. The study used modern telescope data to correct historical color errors and explain small color changes in Uranus' orbit.
Neptune likes to be a rich blue and Uranus a green – but a new study has revealed that the two ice giants are much closer in color than commonly thought.
The exact shadows of the planets have been confirmed with the help of research led by Professor Patrick Irvine. University of OxfordIt was published today in the Monthly Notices of the Royal Astronomical Society.
Although it is commonly believed that Neptune is a deep blue and Uranus has a pale cyan appearance, he and his team found that the two worlds are actually a similar shade of greenish-blue.
Voyager 2/ISS images of Uranus and Neptune, released after the Voyager 2 flybys in 1986 and 1989, respectively, compared to reprocessing the separate filter images in this study, determined a better estimate of the true colors of these planets. Credit: Patrick Irwin
Misconceptions of planetary colors
Astronomers have long known that most modern images of the two planets do not accurately reflect their true colors.
During the 20th century the misconception arose as images of the two planets were captured – including NASAThe Voyager 2 mission, the only spacecraft to fly past these worlds – recorded images in discrete colors.
The single-color images were then recombined to create mixed-color images, which were not always precisely balanced to achieve a “true” color image, and – especially in the case of Neptune – were often made “too blue”.
Uranus as seen by HST/WFC3 from 2015-2022. During this sequence, the light green North Pole swings down toward the Sun and Earth. In these images, the equator and latitude lines are marked at 35N and 35S. Credit: Patrick Irwin
In addition, Voyager 2's early Neptune images were strongly contrast-enhanced to better reveal the clouds, bands, and winds that shape our modern view of Neptune.
Professor Irwin said: “While the familiar Voyager 2 images of Uranus were released in a form close to 'true' colour, the images of Neptune were actually stretched and enhanced and therefore artificially much bluer.”
“Although the artificially saturated color was known among planetary scientists at the time — images were published with captions explaining it — the distinction was lost over time.”
“By applying our model to the original data, we were able to reproduce a very accurate representation of the color of both Neptune and Uranus.”
Clarifying true colors through modern research
In the new study, researchers used the data Hubble Space TelescopeSpace Telescope Imaging Spectrograph (STIS) and Multi-Unit Spectroscopic Explorer (Muse) on the European Southern Observatory A very large telescope. In both instruments, each pixel is a continuous spectrum.
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 and the Hubble Space Telescope's Wide Field Camera 3 (WFC3).
This revealed that Uranus and Neptune are actually similar shades of greenish-blue. The main difference is that Neptune has a slight hint of blue, which the model reveals is due to the planet's thin haze layer.
An animation of the seasonal changes on Uranus over two Uranus years (one Uranus year is 84.02 Earth years), runs from 1900 to 2068 and begins just before the southern summer solstice, when Uranus' south pole points almost directly toward the Sun.
The left-hand disc shows the appearance of Uranus to the naked eye, while the right-hand disc has been enhanced to make the atmospheric features clearer. In this animation, Uranus' spin has been scaled down more than 3000 times so that the planet's rotation can be seen, and individual storm clouds can be seen moving across the planet's disk.
As the planet moves toward its solstices, the pale polar 'hood' of cloud opacity and reduced methane abundance can be seen filling in more of the planet's disk, leading to seasonal changes in the planet's overall color.
The change in size of Uranus' disk is caused by Uranus' distance from the Sun changing during its orbit.
Credit: Patrick Irwin, University of Oxford
Explaining the color variations of Uranus
The study also answers the long-standing mystery of why Uranus' color changes so little during its 84-year orbit around the Sun.
The authors reached their conclusions after first comparing images of the ice giant with measurements of its brightness recorded by the Lowell Observatory in Arizona from 1950 – 2016 in blue and green wavelengths.
These measurements show that Uranus appears slightly greener at its solstices (i.e., summer and winter), when one of the planet's poles points toward our star. But during its equinox – when the sun is above the equator – it has a somewhat bluer hue.
This was known to be due to the fact that Uranus has a very unusual cycle.
It effectively rotates on its side 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 because any changes in the reflectivity of the polar regions can have a large effect on the overall brightness of Uranus as seen from our planet.
Astronomers are unclear about how or why this reflection varies.
This led the researchers to develop a model that compared the spectra of Uranus' polar regions with those of its equatorial regions.
It found that the polar regions have higher reflectance in green and red wavelengths than in blue wavelengths because red-absorbing methane is half as abundant near the poles as near 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 'hood' of gradually thickening fog, previously seen in the summer, when the sun's rays were the polar planet. Moving from the equinox to the solstice.
Astronomers think it is made of methane ice particles.
When simulated in the model, the ice particles further increased reflectance at the poles in green and red wavelengths, providing an explanation for why Uranus is green at the solstice.
Professor Irvine said: “This is the first study to match a quantitative model with imaging data to explain why the color of Uranus changes during its orbit.”
“In this way, we have demonstrated that Uranus is greener at the solstice due to the reduced abundance of methane in the polar regions and the thickening of bright-scattering methane ice particles.”
Dr Heidi Hammel of the Association of Universities for Astronomical Research (AURA), who has studied Neptune and Uranus for decades, but was not involved in the study, said: “Misunderstandings of the color of Neptune and the unusual color changes of Uranus have fooled us for decades. This comprehensive study finally puts both issues to rest.” want
Future study and follow-up research
The ice giants Uranus and Neptune are an exciting place for future robotic explorers to build on Voyager's legacy in the 1980s.
Professor Lee Fletcher, a planetary scientist at the University of Leicester and co-author of the new study, said: “A mission to explore the Uranian system – from its peculiar seasonal atmosphere, to its diverse rings and moons – is a high priority for space agencies in the coming decades.”
However, even long-lived planetary explorers in orbit around Uranus will capture only a small snapshot of the Uranian year.
“Earth-based studies like these, which show how the appearance and color of Uranus have changed over the decades in response to the different seasons in the Solar System, will be essential in putting the findings of this future mission into their wider context,” added Professor Fletcher.
Reference: “Modeling the seasonal cycle of color and size of Uranus and comparing it with Neptune” Patrick GJ Irwin, Jack Dobinson, Arjuna James, Nicholas A Deanby, Amy A Simon, Lee N Fletcher, Michael T Roman, Glenn S Orton, Michael H Wong, Daniel Toledo , by Santiago Perez-Hoyos and Julie Peck, 12 September 2023, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/stad3761
