Neptune & Uranus are of similar shade of greenish blue, study confirms

Washington: While Neptune is fondly known for being a rich blue and Uranus green, a new study revealed that the two ice giants are actually far closer in colour than typically thought.

Despite the commonly-held belief that Neptune is a deep azure and Uranus has a pale cyan appearance, the study, published in the Monthly Notices of the Royal Astronomical Society, showed that both worlds are in fact a similar shade of greenish blue.

Astronomers have long known that most modern images of the two planets do not accurately reflect their true colours.

The misconception arose because images captured of both planets during the 20th century -- including by NASA’s Voyager 2 mission, the only spacecraft to fly past these worlds – recorded images in separate colours.

The single-colour images were later recombined to create composite colour images, which were not always accurately balanced to achieve a "true" colour image, and – particularly in the case of Neptune – were often made "too blue".

In addition, the early Neptune images from Voyager 2 were strongly contrast enhanced to better reveal the clouds, bands, and winds that shape our modern perspective of Neptune.

"Although the familiar Voyager 2 images of Uranus were published in a form closer to ‘true’ colour, those of Neptune were, in fact, stretched and enhanced, and therefore made artificially too blue," said Professor Patrick Irwin from the University of Oxford.

"Even though the artificially-saturated colour 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 colour of both Neptune and Uranus," he said.

In the new study, the researchers used data from Hubble Space Telescope’s Space Telescope Imaging Spectrograph (STIS) and the Multi Unit Spectroscopic Explorer (MUSE) on the European Southern Observatory’s Very Large Telescope. In both instruments, each pixel is a continuous spectrum of colours.

This means that STIS and MUSE observations can be unambiguously processed to determine the true apparent colour of Uranus and Neptune. The researchers used these data to re-balance the composite colour images recorded by the Voyager 2 camera, and also by the Hubble Space Telescope’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.