Neptune and Uranus are a similar shade of blue-green, despite the common belief that Neptune has a deep blue shade and Uranus a pale cyan appearance.
The exact colors of the planets have been confirmed with the help of research led by Professor Patrick Irwin of Oxford University. It is published in the Monthly Notices of the Royal Astronomical Society.
Astronomers have long known that most modern images of the two planets do not accurately reflect their true colors. The misconception arose as images of the two planets were captured (including by NASA's Voyager 2 mission) during the 20th century. (the only spacecraft to pass through these worlds) recorded images in discrete colors.
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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.”
Additionally, Voyager 2's first images of Neptune had strong contrast to better reveal the clouds. Bands and winds that shape our modern view of Neptune.
Professor Irwin said in a statement: “While the well-known images of Uranus taken by Voyager 2 were released in a form close to 'true' color, the images of Neptune were actually stretched and enhanced, so they were artificially produced. They are very blue.
Although artificially color-saturated images were known among planetary scientists at the time (and images were published with captions explaining this), 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.”
In the new study, researchers used data from the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope and the Multi-Unit Spectroscopic Explorer (MUSE) on the European Southern Observatory's Largest 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 reconstruct color composite 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 blue-green. 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.
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 came to this conclusion after comparing images of the ice giant recorded in blue and green wavelengths by the Lowell Observatory in Arizona between 1950 and 2016 with measurements of its brightness.
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.
One reason for this was known to be that Uranus has a very unusual cycle. It rotates nearly 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 because any change 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. He found that polar regions have higher reflectance in green and red wavelengths than in blue wavelengths because methane, which absorbs red, is half as abundant near the poles as it is at 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 gradually thickening fog “hood” previously seen over the sunlit pole. Summer like the planet. Goes from Uttarayana to Solstice.
Astronomers believe it is made of methane ice particles. When simulated in the model, the ice particles further increased reflectance in green and red wavelengths at the poles, providing an explanation for why Uranus is greener at the solstice.
Professor Irvine said: “This is the first study to compare a quantitative model with imaging data to explain why Uranus' color changes during its orbit. “In this way, we have shown that Uranus is greener at the solstice, because the polar regions contain less methane, but there is a greater thickness of brightly dispersed methane ice particles.”
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