TRUE-COLOUR IMAGES REVEAL THAT URANUS AND NEPTUNE ARE SIMILAR BLUES

Although the familiar Voyager 2 images of Uranus were published in a form closer to true colour, those of Neptune were stretched and enhanced, making them too blue

In the summer of 1989, from a remote expanse of our Solar System where sunlight is merely a tepid glow, NASA’s Voyager 2 spacecraft radioed to Earth humankind’s very first images of Neptune. The pictures revealed the Sun’s outermost planet was a stunning deep-blue orb. In contrast, Uranus – Neptune’s planetary neighbour and the first to be discovered with a telescope – appeared noticeably paler. Both seemingly twin worlds have a lot in common. They’re roughly the same size, almost equally massive and are both enveloped with deep atmospheres made of similar materials. So why were the two orbs different shades of blue? This is a question that has puzzled scientists for decades. Now a fresh analysis of Voyager 2’s images shows both ice giants are in fact a similar shade of greenish blue, which is the most accurate representation yet of the planets’ colours.

The images Voyager 2 recorded of Uranus and Neptune were in single colours that were combined to create composite images that showed the planets to be cyan and azure respectively. While Uranus’ published pictures were processed close to its true colour, early Neptune images had been “stretched and enhanced” to display its clouds, bands and winds, “and therefore made artificially too blue,” study lead Patrick Irwin, a planetary physicist at Oxford University said. “Even though the artificially saturated colour was known at the time among planetary scientists – and the images were released with captions explaining it – that distinction had become lost over time.”

To resolve the misconception, Irwin and his colleagues used new images from NASA’s Hubble Space Telescope and the European Space Agency’s Very Large Telescope, whose instruments capture a rich spectrum of colours in each pixel. Processing them determined the true apparent colours of Uranus and Neptune. Next the team revisited Voyager 2’s images and rebalanced them in line with the new data, showing both planets are actually similar shades of blue. The colour comes from a layer of methane in the planets’ atmospheres, which absorbs red from the Sun’s light.

Uranus is slightly whiter, the new study finds, possibly because its somewhat “stagnant, sluggish” atmosphere permits the methane haze to accumulate, which reflects red portions of sunlight to a greater extent than Neptune does. The presence of amassed methane ice particles may also explain why Uranus changes its colour slightly during its

84-year orbit around the Sun. Images recorded between 1950 and 2016 by the Lowell Observatory in Arizona show the planet appears greener during its solstices – when one of its poles points towards the Sun – and bluer during equinoxes, when the Sun shines directly above its equator. By comparing the brightness of Uranus’ poles to its equatorial regions in these images, Irwin and his team concluded methane is likely half as abundant near the poles than at the equator, which accounts for the changing colours. “The misperception of Neptune’s colour, as well as the unusual colour changes of Uranus, have bedevilled us for decades,” said Heidi Hammel of the Association of Universities for Research in Astronomy, who is not affiliated with the new study. “This comprehensive study should finally put both issues to rest.”

Images of Neptune were stretched and enhanced, made to look bluer like this artist’s impression