Many of us are familiar with Neptune’s intense blue color and Uranus’ green hue. However, a new study has found that these two ice giants are more similar in color than previously believed. Led by Professor Patrick Irwin of the University of Oxford, researchers discovered that both planets actually have a similar bluish-green tone, despite the widespread idea that Neptune is a vivid blue and Uranus has a pale cyan appearance.

Astronomers have long known that most modern images of the planets do not accurately reflect their true colors. This is because images taken of them during the 20th century – including those from NASA’s Voyager 2 mission, the only spacecraft to fly by the planets – recorded separate color images.

Those single-color images were later combined to create composite color pictures, but this process did not always precisely balance the colors to achieve a “real” color image. Especially for Neptune, the images were often made artificially “too blue”.

Additionally, early images of Neptune from Voyager 2 underwent strong contrast enhancement to better reveal clouds, bands, and winds in our modern view of Neptune. According to Professor Irwin, While the known Uranus images from Voyager 2 were published in a form closer to the ‘true’ color, Neptune’s were stretched and enhanced, making them artificially too blue.

For the new study, researchers used data from Hubble Space Telescope’s Imaging Spectrograph and the Multi Unit Spectroscopic Explorer on the Very Large Telescope in Chile. This allowed the planets’ colors to be determined unambiguously.

They rebalanced Voyager 2 and Hubble camera images using these new spectroscopic data. This revealed that Uranus and Neptune actually have a fairly similar bluish-green hue. The main difference is Neptune has a slight additional blue tint believed to come from a thinner haze layer.

The study also offers an explanation for why Uranus’ color changes slightly during its 84-year orbit. It compares images to measurements of brightness from 1950-2016. Uranus appears a bit greener at solstices when a pole points toward the Sun, but looks somewhat bluer at equinoxes.

Researchers developed a model comparing polar and equatorial spectra, discovering polar regions reflect more green and red light due to less abundant methane. However, this did not fully explain the color shift, so researchers added a “hood” of icy haze thickening over the sunlit summer pole. This increased green and red reflection, providing an explanation for Uranus’ greener solstice color.

In summary, this new study has provided the first quantitative model matching data to explain why Uranus’ color shifts during its orbit. The research also settles longstanding debates about the true hues of Neptune and unusual color changes on Uranus.

Understanding these icy worlds remains an important goal as scientists plan future missions to explore the outer planets and unravel their mysteries.


Sources

University of Oxford | Patrick Irwin et al., Modelling the seasonal cycle of Uranus’s colour and magnitude, and comparison with Neptune, Monthly Notices of the Royal Astronomical Society (2023). DOI:10.1093/mnras/stad3761


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