In the late 1980s, the Voyager 2 spacecraft snapped the “canonical” up-close images of Uranus and Neptune. In those views, Uranus was a pretty greenish-blue and Neptune appeared a deep azure color. It turns out that both planets are pretty close in color: a greenish-blue more akin to Uranus’s appearance.
No, Uranus and Neptune haven’t swapped color values. It turns out that those images aren’t precise recordings of their actual colors. Planetary scientists re-examined the Voyager 2 images and compared them to more recent observations made with both space-based and ground-based observatories. Then they created a model of what the colors should be. After that, they re-processed the images to come up with a more “true color” view of each planet. The result is a more realistic view of both worlds.
Having a better sense of each planet’s true color allows scientists to better understand actual changes in their atmospheres due to internal activity and seasonal shifts in position and temperature. In particular, the observations and color redefinitions help reveal something about the mysterious color changes that Uranus undergoes during its 84-year orbit.
What Voyager 2 Saw at the Ice Giants
The Voyager 2 spacecraft whizzed past Uranus and Neptune in 1986 and 1989, respectively. It was on a lightning-fast visit to each planet. Both close approaches lasted less than a day apiece. That gave the spacecraft a finite amount of time to gather as much information as possible. To get good pictures of the planets, Voyager’s cameras took images through different filters. The imaging team was under tremendous time constraints, with press conferences to prepare for nearly every day. They combined the single-color images and processed them to make the “press release” views we all know and love.
Think of it like taking a landscape image with your smartphone in black-and-white mode. Then you take the same one with different-colored filters. After that, you’d drag them into an image-processing software package and combine them. Depending on how they look, you might tweak them a bit in the contrast. Or you could enhance some colors to bring out specific features.
That’s what the Voyager imaging team did. For Neptune, they cranked up the image contrast to bring out specific clouds, cloud bands, and storms. That meant applying a bit more blue to increase the contrast. The resulting image was pretty and certainly did showcase those odd features—which caught everyone’s attention during the flybys. But, did the images reflect reality?
The same question gets asked about Uranus’s color, especially since its appearance has changed slightly from the canonical Voyager 2 image in the intervening decades. The Voyager 2 mission took spectra of Uranus as it flew by and confirmed that the planet’s atmosphere is mainly hydrogen and helium, with a small amount of methane. That composition was well-known from ground-based observations and spectra since the early part of the 20th century.
Determining True Planetary Colors of Uranus and Neptune
Professor Patrick Irwin and a team of scientists at Oxford University in the UK set out to answer questions about the actual colors of Uranus and Neptune. They analyzed the Voyager 2 images, plus studies made by the Hubble Space Telescope, ESO’s Very Large Telescope, and others, to come up with a model of the planets’ actual colors.
“Although the familiar 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,” said Irwin. “Even though the artificially saturated color was known at the time amongst planetary scientists – and the images were released with captions explaining it – that distinction had become lost over time. Applying our model to the original data, we have been able to reconstitute the most accurate representation yet of the color of both Neptune and Uranus.”
Essentially, the team “rebalanced” the colors of both planets. The result is that both are similar shades of greenish-blue, although Neptune still has a bit more blue to it than Uranus does. However, Uranus sometimes develops a bit more green over its poles.
These colors seem to better match long-stand observations of both planets made at Lowell Observatory between 1950 and 2016. According to Heidi Hammel, a scientist who has studied the two planets for years, rebalancing the colors in Uranus and Neptune imagery is a good thing. “The misperception of Neptune’s color, as well as the unusual color changes of Uranus, have bedeviled us for decades,” she said. “This comprehensive study should finally put both issues to rest.”
What Causes Color Changes on Uranus?
Scientists may be happy with the colors of these ice giants now, although they do still note some slight seasonal color shifts in both planets throughout their orbits. In particular, the changing appearance of Uranus over time remains something of a mystery to be solved. For example, the Lowell observations show something intriguing: Uranus appears a little greener at its winter and summer solstices. That’s the point in its orbit when one of the planet’s poles is pointed towards our star. Things change during the equinoxes when the Sun is over the equator. Then, it has a somewhat bluer tinge.
Part of that change is due to Uranus’s unusual spin. It rolls around the Sun on its side, pointing one or the other of its poles at the Sun during the solstices. (Recently, JWST took advantage of that positioning to grab a look at its northern polar region.) The “tipped” position of Uranus likely forces some changes in its reflectivity at those times, making it look brighter to us here on Earth. Now the big question is, do those changes indicate something else happening in the atmosphere?
In the paper they published about this work, Irwin and the team suggest that the changes that Lowell Observatory saw could be caused by Uranus’s distance from the Sun. That affects the production of a dark haze—a sort of “polar hood”. It settles over the upper atmosphere at the poles. Production would be more intense when Uranus is closest to the Sun. That would explain a change in reflectivity and brightness. The team modeled a hood that produced a steadily thickening haze, probably consisting mostly of methane ice. The model simulation showed that the ice particles increased reflection at green and red wavelengths at the poles. That could explain the greenish tint astronomers see at solstice.
Tracking Changes on Uranus
The modeling and re-jiggering of imagery of Uranus, in particular, go a long way toward explaining Uranus’s color changes. “This is the first study to match a quantitative model to imaging data to explain why the color of Uranus changes during its orbit,” said Irwin.
At the moment, Uranus is heading into its high summer season. That should cause its northern polar hood to thicken and grow. It may end up looking more like the hood seen in the Voyager 2 images and the team’s model. Irwin suggests that the Hubble Space Telescope and the Very Large Telescope should focus on spectroscopic studies of the planet to watch it change. Future Hubble observations should also use filtering methods that correlate with the Voyager imaging system filters to make better comparisons. It’s likely that future studies should be able to replicate the work Irwin and his team have done, and do more to explain the changes that Uranus’s atmosphere appears to experience as it moves through its seasons.