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Uranus and Neptune, the so-called ice giants, are the most distant giant planets in our Solar System. Our knowledge of these worlds was revolutionized by the flybys of NASA’s Voyager 2 spacecraft on January 24, 1986 and August 25, 1989, respectively. Since these Voyager encounters, our perception of the visible appearance of these worlds comes primarily from images reconstructed from observations from Voyager 2’s Imaging Science System (ISS), which recorded images in several separate filters, running from ultraviolet to orange. In those images, Uranus appears pale green, while Neptune appears darker blue, and this perception of the relative colors of these planets has become commonly accepted. But a new study has revealed that the two ice giants are actually far closer in color.
Voyager 2/ISS images of Uranus and Neptune released shortly after the Voyager 2 flybys in 1986 and 1989, respectively, compared with a reprocessing of the individual filter images in this study to determine the best estimate of the true colors of these planets. Image credit: Irwin et al., doi: 10.1093/mnras/stad3761.
“Although the familiar Voyager 2 images of Uranus were published in a form closer to ‘true’ color, those of Neptune were, in fact, stretched and enhanced, and therefore made artificially too blue,” said University of Oxford’s Professor Patrick Irwin.
“Even though the artificially-saturated color was known at the time amongst planetary scientists — and the images were released with captions explaining it — that distinction had become lost over time.”
“Applying our model to the original data, we have been able to reconstitute the most accurate representation yet of the color of both Neptune and Uranus.”
In the study, Professor Irwin and his colleagues used data from the Space Telescope Imaging Spectrograph (STIS) on board the NASA/ESA Hubble Space Telescope and the Multi Unit Spectroscopic Explorer (MUSE) on ESO’s Very Large Telescope.
This means that STIS and MUSE observations can be unambiguously processed to determine the true apparent color of Uranus and Neptune.
The astronomers used these data to re-balance the composite color images recorded by the Voyager 2 camera, and also by Hubble’s Wide Field Camera 3 (WFC3).
This revealed that Uranus and Neptune are actually a rather similar shade of greenish blue.
The main difference is that Neptune has a slight hint of additional blue, which the model reveals to be due to a thinner haze layer 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 of the Sun.
The authors came to their conclusion after first comparing images of the ice giant to measurements of its brightness, which were 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 over the equator — it has a somewhat bluer tinge.
Part of the reason for this was known to be because Uranus has a highly unusual spin.
It effectively spins almost on its side during its orbit, meaning that during the planet’s solstices either its north or south pole points almost directly towards the Sun and Earth.
This is important because any changes to the reflectivity of the polar regions would therefore have a big impact on Uranus’ overall brightness when viewed from our planet.
What astronomers were less clear about is how or why this reflectivity differs.
This led the researchers to develop a model which compared the spectra of Uranus’ polar regions to its equatorial regions.
It found that the polar regions are more reflective at green and red wavelengths than at blue wavelengths, partly because methane, which is red absorbing, is about half as abundant near the poles than the equator.
However, 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 ‘hood’ of gradually thickening icy haze which has previously been observed over the summer, sunlit pole as the planet moves from equinox to solstice.
Astronomers think this is likely to be made up of methane ice particles.
When simulated in the model, the ice particles further increased the reflection at green and red wavelengths at the poles, offering an explanation as to why Uranus is greener at the solstice.
“This is the first study to match a quantitative model to imaging data to explain why the color of Uranus changes during its orbit,” Professor Irwin said.
“In this way, we have demonstrated that Uranus is greener at the solstice due to the polar regions having reduced methane abundance but also an increased thickness of brightly scattering methane ice particles.”
“The misperception of Neptune’s color, as well as the unusual color changes of Uranus, have bedevilled us for decades. This comprehensive study should finally put both issues to rest,” said Dr. Heidi Hammel, a researcher at the Association of Universities for Research in Astronomy (AURA).
The results appear in the Monthly Notices of the Royal Astronomical Society.
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Patrick G.J. Irwin et al. 2024. Modelling the seasonal cycle of Uranus’s colour and magnitude, and comparison with Neptune. MNRAS 527 (4): 11521-11538; doi: 10.1093/mnras/stad3761
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