The color of Uranus. Fine, and Neptune, too

January 15, 2024   Issue #669

Pic o’ the Letter

A cool or lovely or mind-bending astronomical image/video with a description so you can grok it

Colors can tell you a lot about an astronomical object. Turns out, they can tell us about ourselves, too.

For example, Uranus is well known to be a greenish tint, caused by methane in its upper atmosphere. That gas absorbs red light from the Sun, reflecting away green and blue. There is also a haze layer in the giant planet’s atmosphere, muting the colors and giving Uranus an overall muted appearance.

That color changes over time as Uranus orbits the Sun. The haze layer changes with season, for example, so the color changes with the seasons as well. New research [link to paper] shows this to be true, with color changes caused by seasonal changes in the amount methane in the atmosphere, together with the aerosol particles making up the haze changing in their reflectivity as well.

That will inform researchers on how the planet’s atmosphere behaves over time, which is something we don’t understand too well. But there’s more…

They used the color data (taken using a various telescopes and cameras, including STIS, which I’m always delighted to mention for reasons) to determine the true color of Uranus and Neptune (they had observations of the latter to use as a comparison to Uranus), and found something funny.

The image everyone uses of Neptune in science communication (and pretty much everywhere) was taken by the Voyager 2 spacecraft which passed the distant planet in 1989. Those images revealed Neptune to be a deep blue, even reminiscent of Earth. Unlike Uranus, Neptune lacks the haze layer, so the deep red absorption by methane is more obvious, making the planet more cerulean.

What the astronomers found, though, is that this isn’t the case. Neptune isn’t quite that deep a blue! This has been known for a while, in fact — over at The Planetary Society’s website, space image processor Björn Jónsson showed that the Voyager image is misleading, showing it a much deeper blue than it really is.

But even that isn’t the whole picture (so to speak). Getting the true color is actually difficult, because it depends on the filters used (and how well they match our eyes) together with the detailed actual colors Neptune reflects. A filter typically lets through a broad range of wavelengths, covering, say, what we’d call green. But Neptune may emit different amounts of light across those wavelengths, and if you don’t account for that the colors you get for your image may be off.

Complicating things is the fact that the planet’s colors will look different if you look at the very center of its disk versus the edges, since colors get absorbed differently by different layers in the atmosphere — near the edge you’re looking through more atmosphere, so that changes things. It also depends on how you set the brightness contrast in the image, and more.

The result is the image above, showing Uranus to be that pale, featureless seawater green, while Neptune is only a slightly deeper version of the same color. IN reality they’re pretty similar. The astronomers note that the Voyager image had its colors enhanced when it was released, and while that was noted at he time and known among astronomers, that information has been gradually lost over time in the public perception.

And mine! I’ll admit I think of Neptune as being deeper blue than it really is. I even mention it in Crash Course Astronomy. There was a discussion on Blue Sky (a twitter alt) about this after the news came out, with Catherine Heymans (the Astronomer Royal of Scotland) and Mike Brown (astronomer best known for killing Pluto); Heymans noted the spectra of Uranus and Neptune are very similar, and I responded that they are but that Uranus reflects less blue and more red, so Neptune is in fact bluer, if only by a little.

And I’ve seen both planets through my own telescope, and I remember Neptune looking bluer than Uranus! But it’s entirely possible my memory has been affected by seeing the Voyager image a zillion times over the years. It’s been a while since I’ve looked at both planets through my ‘scope, and it turns out they’re both up and easy(ish) to spot right now. If I get a chance I’ll take another look and see just how deep my bias runs.

Astronomy teaches us deeper truths about the Universe, but it also is a reflection of ourselves, so it can also teach us about how we see the Universe. If we also know what the Universe really looks like, that can reveal our own internal fallacies. I think these revelations are equally important.

News Roundup

Who can keep up with everything these days?

• Venus is well known to be ridiculously hot on its surface, enough to melt lead. Life as we know is not possible there. But the planet is covered in clouds, and at high altitudes the temperatures are much lower. Could there be life there? Interestingly, there appears to be something suspended in the clouds that absorbs ultraviolet radiation at a couple of different broad wavelength ranges, and one possible explanation is bacteria. I know, but hang on: New work in a lab on Earth shows that two different iron-bearing minerals reproduce the same absorption features seen link to research paper]. This isn’t conclusive, but it’s a whole lot more likely than wee beasties floating around in the Cytherean air.

• How can you tell if an exoplanet has liquid water on its surface? That’s a really hard observation to make, but scientists have come up with a new method: Compare how much carbon dioxide is in its atmosphere with other planets in the same system [link to research paper]. Water absorbs CO2 (this is what happened on Earth, which has much less CO2 in its air compared to Venus and Mars) so a depletion of the gas may indicate liquid water. These kinds of measurements can be made for multiple planets in one system, like TRAPPIST-1, using ‘scopes like JWST, too. I don’t think this would be a definitive test but it can be used as a way to see what planets we should investigate more deeply.

Et alia

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