Neptune isn't as blue as we thought it was

The renowned image of Neptune, captured by Voyager 2, has been reexamined, revealing that Neptune's actual colour is a subdued blue-green, contrasting popular representations that present the planet in a strong, deep blue hue. Voyager 2 had sent earlier images in the 1980s that portrayed Neptune and Uranus in strikingly distinct colours, which is perplexing given their shared physical and chemical characteristics. Various models have been used to explain such discrepancies, often citing factors like a denser "haze layer" on Uranus that reflects increased white light, causing the planet to appear lighter. However, these models fail to provide a comprehensive explanation for the largely varying colours.

Researchers from the University of Oxford recently reprocessed Voyager 2's images, presenting what could potentially be the human eye's perception of the planets. The initial photos of Neptune taken by Voyager 2 were manipulated to emphasize less visible atmospheric aspects and balanced colouring, causing the planet to seem bluer. Despite these amendments being specified in the captions, as time passed, the captions and images were detached and Neptune's intense blue shade came to be regarded as a public fact, according to researcher Patrick Irwin.

The Oxford team designed a model to transform the raw image data into a true colour image utilizing photos from the Hubble Space Telescope, which offers more comprehensive light information. This process generated similar shades for both Neptune and Uranus. "The true colour image doesn't capture the eye's attention because of its ordinary and dull nature," Irwin adds.

Adding to this, the scientists used the Hubble images together with images from the Lowell Observatory in Arizona to develop a model predicting Uranus's colour alteration during its extensive 84-year orbit around the sun. Due to Uranus's rotation, we observe more of its equator at equinoxes and its poles at solstices. A higher concentration of methane at the equator absorbs red light, while a cover of light-enhancing ice particles forms at the pole facing the sun during equinox, increasing the reflection of red and green wavelengths. This model helps solve the enigma of why Uranus looks marginally greener during its solstices. "We were aware of the ice cover and the reduced methane at the poles, but not of how they all interact to determine the seasonal changes," concludes Irwin.