Our previous understanding of Neptune has been shown incorrect by a recent examination of some historical photos.
Scientists have been perplexed as to why Neptune and Uranus, which are remarkably similar in most respects, have such distinct hues for decades, ever since Voyager 2 passed past them and took close-up pictures along the route. According to the Voyager 2 images, Uranus looked like a lovely duck egg in aqua. Conversely, Neptune appeared to be the deeper shade of lapis lazuli. The discrepancy was striking because the two worlds’ atmospheric compositions are nearly the same. and perplexing.
Scientists have since reanalyzed the data, though, and found that our original interpretation was wildly inaccurate. The hues of Uranus and Neptune are actually rather similar. Although a difference exists, it is far less than originally believed. The misperception stems from the fact that Voyager 2 captured photographs of the planets in two distinct color bands; as a result, Neptune’s image was processed to enhance contrast and deepen its actual hue, giving the impression that it is bluer than it actually is.
Planetary physicist Patrick Irwin of the University of Oxford says, “Although the well-known Voyager 2 images of Uranus were published in a form closer to ‘true’ color, those of Neptune were, in fact, stretched and enhanced, and therefore made artificially too blue.”
“That distinction had faded over time, even though planetary scientists were aware of the artificially-saturated color at the time and the photos were published with captions clarifying it. We have recreated the most accurate depiction of Neptune and Uranus’ colors to date by using our model with the original data.”
Although it was long known that the Voyager 2 photos did not fully capture the hues of Uranus and Neptune, scientists were unsure of the precise nature of these colors. Irwin and colleagues used two potent instruments, the Multi Unit Spectroscopic Explorer of the Very Large Telescope and Hubble’s Space Telescope Imaging Spectrograph (STIS), to find out.
They independently ascertained the true color of Neptune and Uranus using the data from each telescope. Based on this information, they reprocessed not only the Voyager 2 images but also the Wide Field Camera 3 images from Hubble, which had also been previously processed, yielding a darker-hued Neptune. The recently reconstructed photos reveal that Neptune is considerably lighter than previously believed and has a color more akin to Uranus. The primary distinction is that Neptune seems somewhat bluer, most likely due to a lower coating of atmospheric haze.
The reason behind Uranus’s minor hue changes throughout its 84 Earth-year year was also clarified by the new data. During the solstice, the planet appears noticeably greener when one of its poles faces the Sun. Uranus is somewhat bluer at the equinox when the equator faces the Sun. This is related to the peculiar orientation of Uranus. Its rotational axis is skewed sideways, making it perpendicular to the Solar System’s orbital plane. Because methane absorbs red wavelengths, it is significantly less common in the planet’s poles than it is in the equator, which affects how Uranus reflects sunlight there.
Further simulation showed that there is a methane ice haze that thickens and increases reflectivity as the planet goes from equinox to solstice. This explains why the planet’s color changes so fascinatingly—like an odd mood ring.
“We have been plagued by the misconception about Neptune’s hue and the peculiar color shifts of Uranus for many years,” says astronomer Heidi Hammel of the Association of Universities for Research in Astronomy, who was not involved in the study.
“This comprehensive study should finally put both issues to rest.”
The Royal Astronomical Society’s Monthly Notices has published the team’s results.
By Nikola 
