Neptune is not as blue as we concept it was once

Neptune’s true colour is a pale greenish-blue similar to that of Uranus, contrary to popular images that show it to be a much deeper shade of blue.

NASA’s Voyager 2 spacecraft flew past the outer planets in the 1980s and sent back photos showing that Uranus and Neptune were markedly different colours.

This is puzzling, given their similar size, mass and chemical make-up. Models of the planets’ atmospheres can explain some of the variation – such as a “haze layer” that is thicker on Uranus and reflects more white light, making the planet appear lighter – but these don’t fully explain why the planets should have such different hues.

Now, Patrick Irwin at the University of Oxford and his colleagues have processed the Voyager 2 images to show how the human eye might see the planets.

The original photos of Neptune taken by Voyager 2 had an enhanced contrast ratio to highlight hard-to-see atmospheric features. Along with the way that the colours were balanced to make a final composite image, this made the planet appear bluer.

Scientists at the time knew this and included these changes in picture captions, but over time the captions were separated from the images and Neptune’s deep blue shade became enshrined as fact in the public consciousness, says Irwin.

He and his team developed a model to convert the raw image data to a true-colour image using shots taken by the Hubble Space Telescope, which contain more complete information about the light. This produced similar shades for both planets. “The true-colour image is much more boring and bland because of the way the eye works,” says Irwin.

The researchers also used the Hubble images, along with images from Lowell Observatory in Arizona, to build a model that predicts how Uranus’s colour changes during its long, 84-year orbit around the sun. Because of the planet’s spin, we see more of the equator during the equinoxes and more of the poles during the solstices. At the equator, there is more methane, which absorbs red light. The planet also has a hood of reflective, brightening ice particles that forms at the sun-facing pole during the equinoxes, increasing the reflectivity of red and green wavelengths.

This helps explain the long-standing mystery of why Uranus appears slightly greener in its solstices. “We knew there was a hood, and we knew there’s less methane at the poles, but no one had put it all together to explain what’s actually happening seasonally,” says Irwin.