The original Voyager 2 photo of Neptune (left) and the reprocessed image from the new study (right)<\/p>\n
Patrick Irwin<\/p>\n<\/div>\n<\/figcaption><\/figure>\n<\/p>\n
Neptune\u2019s 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.<\/p>\n
NASA\u2019s Voyager 2 spacecraft flew past the outer planets in the 1980s and sent back photos showing that Uranus and Neptune were markedly different colours.<\/p>\n
This is puzzling, given their similar size, mass and chemical make-up. Models of the planets\u2019 atmospheres can explain some of the variation \u2013 such as a \u201chaze layer\u201d that is thicker on Uranus and reflects more white light, making the planet appear lighter \u2013 but these don\u2019t fully explain why the planets should have such different hues.<\/p>\n
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.<\/p>\n
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.<\/p>\n
<\/p>\n 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\u2019s deep blue shade became enshrined as fact in the public consciousness, says Irwin.<\/p>\n Images of Uranus (left) and Neptune (right) produced previously and in the new study<\/p>\n Patrick Irwin<\/p>\n<\/div>\n<\/figcaption><\/figure>\n<\/p>\n 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. \u201cThe true-colour image is much more boring and bland because of the way the eye works,\u201d says Irwin.<\/p>\n The researchers also used the Hubble images, along with images from Lowell Observatory in Arizona, to build a model that predicts how Uranus\u2019s colour changes during its long, 84-year orbit around the sun. Because of the planet\u2019s 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.<\/p>\n This helps explain the long-standing mystery of why Uranus appears slightly greener in its solstices. \u201cWe knew there was a hood, and we knew there\u2019s less methane at the poles, but no one had put it all together to explain what\u2019s actually happening seasonally,\u201d says Irwin.<\/p>\n Topics:<\/p>\n<\/section><\/div>\n![\"\"](\"https:\/\/images.newscientist.com\/wp-content\/uploads\/2024\/01\/04155454\/SEI_185947728.jpg?width=1200\")
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