Neptune is known for its deep blue colour and Uranus for its pale green hue, but a new study has shown that these planets are much closer in colour than previously thought. The correct hues of the planets were confirmed by a study led by Professor Patrick Irwin of the University of Oxford. He and his team discovered that the two planets have a similar shade of greenish-blue.
Astronomers have long known that most modern images of these two planets do not accurately reflect their true colours. The misconception arose because images of both planets taken during the 20th century, including by NASA’s Voyager 2 mission, did not reflect their true colours.
Voyager 2 recorded them in different colours, and the single-colour images were later recombined to create composite colour images that were not accurate. This was especially true of Neptune, who was often “too blue.” In addition, Voyager 2’s early images of Neptune were highly contrasted to better show the clouds, streaks, and winds.
“Although the known images of Uranus obtained by Voyager 2 were published in a form close to the “true” colour, the images of Neptune were stretched and enlarged, and therefore artificially made too blue, – note the scientists. “By applying our model to the original data, we were able to reproduce the most accurate representation of the colour of Neptune and Uranus to date.”
In the new study, scientists used data obtained with the Hubble Space Telescope Spectrograph (STIS) and the MUSE spectroscope on the European Southern Observatory’s Very Large Telescope. The STIS and MUSE observations can be unambiguously processed to determine the true apparent colour of Uranus and Neptune. Using this data, scientists balanced the images from the Voyager 2 camera and the Hubble telescope.
Observations have shown that Uranus and Neptune have a similar shade of greenish-blue. The main difference is that Neptune has a slight tint of extra blue, which is due to the thinner layer of fog on this planet. The research also provides an answer to the long-standing mystery of why Uranus changes colour during its 84-year orbit around the Sun.
Measurements have shown that Uranus appears slightly greener during the solstices (that is, summer and winter), when one of the planet’s poles points toward our star. But during the equinox, it has a bluer hue. This is partly because Uranus rotates almost on its side, which means that during the solstices, either the planet’s north or south pole points almost directly at the Sun and Earth. According to the authors, changes in the reflectivity of the polar regions will have a large effect on the overall brightness of Uranus.
However, this was not enough, so the researchers added a new variable to the model in the form of a “cap” of gradually thickening ice haze, previously observed over the sunlit pole during the summer, when the planet moves from the equinox to the solstice. Astronomers believe that it probably consists of particles of methane ice. In the simulations, ice particles increased the reflection of green and red waves at the poles, explaining why Uranus is greener during the solstice.
“This is the first study to correlate a quantitative model with imaging data to explain why Uranus’ colour changes as it orbits,” the scientists said. – Thus, we demonstrated that during the solstice, Uranus becomes greener since the content of methane decreases in the polar regions and the thickness of brightly scattering particles of methane ice increases.”
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