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Have you ever seen an aurora?
I haven’t. Well, not really; twice in my life I’ve seen a dull glow on the northern horizon, but seeing a full-blown shimmering curtain of light sweeping down from the heavens is something that’s — hopefully — still in my future.
Part of the problem is my location. And no, I don’t mean Colorado, though we’re just far enough south that it’s really rare to see one here.
What I mean is Earth. Yeah, we get our skies lighting up near the poles every now and again, but if you really want to see an aurora, you need to move a bit farther away from Colorado than that. Like, say, Saturn.
Or, you could save yourself a billion kilometers or so and just let Hubble spy the ringed planet for you, because it has an excellent view when Saturn lights up:
Cool, huh? This image is composed of four separate observations: Three were in colors more or less like what our eyes see in red, green, and blue — these show Saturn looking as we’re used to seeing it — and the fourth was in the far ultraviolet, where the aurorae shine.
What a cool shot! Saturn has an aurora for pretty much the same reason the Earth does. The Sun blows out a relatively steady wind of subatomic particles, some of which have an electric charge. Saturn has a magnetic field that acts like a giant net, catching these particles. Like Earth, the magnetic poles of Saturn are very close to the geographic (spin) poles, and its magnetic field funnels the particles down to them at high speed.
When the particles slam into Saturn’s atmosphere they blast electrons off of atoms and molecules, kind of like shrapnel. Eventually the electrons recombine with atoms, and when they do they emit light at various wavelengths, depending on the atom or molecule.
But here’s where Saturn is different: The usual colors of Earth’s aurorae are due to oxygen and nitrogen, which make up the majority of our air. But Saturn’s atmosphere is mostly hydrogen. When an electron is blasted off a hydrogen atom and then recombines, it emits light at an ultraviolet wavelength of 121.6 nanometers, well outside what our eyes can see.
At first that’s what I thought these observations showed, but then I noticed that on the Hubble page for this image it says the wavelength was at 148 nm. Hmmm, that surprised me. But as it happens I worked on STIS, the camera used to take these observations, so I have some familiarity with how it operated. I poked around the web a bit and found a paper about observations of Saturn’s aurora using STIS, and it says it used a filter on STIS that specifically blocked the 121.6 nm emission from hydrogen atoms … because they wanted to look at hydrogen molecules which emit at different wavelengths. AHA! I looked at the paper figures, and sure enough an observation they took on Aug. 14, 2017, matches the aurora shape in the image. Also, the Hubble page says the STIS observations were taken in 2017, so I’m confident we have a match here.
[UPDATE (May 4, 2019): The STIS observations were made in 2017, but the visible light observations were made much later, in June 2018; the two were then composited to make this final image. Tip o' the medium resolution grating to SpaceGeck on Twitter for pointing this out to me.]
An image of Saturn in the far ultraviolet using STIS, a camera on Hubble, from 1998 shows the glow of its aurora due to hydrogen atoms zapped by the solar wind. Credit: J.T. Trauger (Jet Propulsion Laboratory) and NASA
The observations were taken to look for weaker emission from different processes going on in Saturn’s atmosphere once the solar wind particles hit it. They timed their observations so they would be taken simultaneously with observations made by the Cassini spacecraft, which was still orbiting Saturn at the time. I had to laugh when I saw how much power was in the aurora, too: No joke, it was 120 gigaWatts. Doc Brown would’ve vaporized his DeLorean using Saturn’s aurora.
In the end these observations tell scientists about the properties of Saturn’s atmosphere and how it couples with the magnetic field, and that in turn can lead to all sorts of surprising insights, including, amazingly, learning more about Saturn’s rings, too.
I think, more directly for you and me — people who (probably) don’t study Saturn for a living — what this image shows us is that Saturn, as alien a world as it is, with huge rings, frigid temperatures, bizarre atmospheric components, storms bigger than some planets, and all that, still has some features that remind us of home.
And that’s good. That means that by studying Saturn we learn more about the Earth, too. I like Earth (it’s where all my friends are for the moment), so the more we understand it, the better.