First real colour images of Uranus and Neptune

A new study has revealed what Uranus and Neptune really look like – and they are far closer in colour than typically thought.

Neptune is usually depicted as being rich blue and Uranus a pale turquoise.

The correct hues of the gas giants have been confirmed in research published in the Monthly Notices of the Royal Astronomical Society. The study shows that both planets have a similar greenish-blue tinge.

But how did we get the colours wrong in the first place?

Images of uranus and neptune true colour compare with voyager 2
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 colours of these planets. Credit: Patrick Irwin / University of Oxford.

The misconceptions arose out of images captured of the planets in the 20th century, including by NASA’s Voyager 2 spacecraft.

Those images were produced by combining single-colour images. The final composite images were not always accurately balanced to achieve the “true” colour. Voyager 2 images in particular were also strongly contrast enhanced to better reveal cloud bands and other atmospheric phenomena that are now well known on Neptune’s surface.

“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,” says lead researcher Professor Patrick Irwin from the UK’s University of Oxford.

The team used data from the 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.

They went to work re-balancing the composite colour images recorded by Voyager 2’s camera and by Hubble.

Why Uranus appears to change colour during its orbit was also answered in the study.

It has long been a mystery why Uranus appears a little greener during solstice, and a little bluer during equinox.

Because of Uranus’s highly unusual horizontal spin, changes in reflectivity of its polar regions – which point almost directly to the Earth and Sun – will have a big impact on how the planet is viewed from Earth. But why would Uranus’ polar reflectivity change?

Animation of seasonal changes in colour on Uranus during two Uranus years (one Uranus year is 84.02 Earth years), running from 1900 to 2068 and starting just before southern summer solstice, when Uranus’s south pole points almost directly towards the Sun. The left-hand disc shows the appearance of Uranus to the naked eye, while the right-hand disc has been colour stretched and enhanced to make atmospheric features clearer. Credit: Patrick Irwin / University of Oxford.

Irwin’s team simulated in their model one theory which is that Uranus has a ‘hood’ of gradually thickening, icy haze at the polar regions. These hazes reflect green at red wavelengths as opposed to red-absorbing methane which is more abundant away from the poles.

“This is the first study to match a quantitative model to imaging data to explain why the colour of Uranus changes during its orbit,” Irwin explains.

“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.”

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