Saturn's South Pole Decagon: A Ten Sided Shape That Was Not There Before
Cassini spent thirteen years staring at that pole and saw nothing. The planet turned away for a decade, and when it turned back, there was a decagon waiting.
By Brian Bullock | Everyone Knows | brianbullockwriter.com
On the second of September, an international team led by Agustin Sanchez-Lavega of the University of the Basque Country published a paper in Science Advances confirming something nobody expected to find. A giant ten sided atmospheric wave is wrapped around Saturn's south pole. It sits at about sixty three degrees south latitude, it runs deep enough to show up in several layers of the atmosphere rather than only in the cloud tops, and it is getting sharper every year.
The reason this is a real discovery and not a pretty photograph is that we had already checked. Thoroughly. For decades. The answer we got every time was that there was nothing there.
The Shape at the Other Pole
Saturn's north pole has a hexagon.
Voyager One and Voyager Two photographed it during the flybys in nineteen eighty and nineteen eighty one. Nobody quite understood what they were looking at until David Godfrey worked through the images later that decade and identified the full six sided outline. When Cassini arrived and began mapping the planet up close from two thousand four onward, the hexagon was still there, unchanged.
The scale of it is difficult to hold in your head. It measures roughly thirty thousand kilometers across, which is wide enough to set two Earths side by side inside it. Each of the six sides is longer than the diameter of our planet. It reaches down about three hundred kilometers into the atmosphere. The jet stream that traces its outline moves at something over two hundred miles an hour.
It has held that shape for more than forty years. Saturn's year runs about twenty nine and a half Earth years, so each season lasts roughly seven of ours, and the north pole spent a long stretch of that in total winter darkness where nothing could see it. When the light came back, the hexagon was exactly where it had been.
Amy Simon of NASA Goddard, who is a co-author on the new paper, put the point simply. The northern hexagon has been there every time anyone has looked for it.
For most of the time we have known about the hexagon, we have also had a reason to think it was a fluke.
The south pole was the control. Cassini orbited Saturn for thirteen years, from two thousand four until the mission ended in two thousand seventeen, and it mapped the southern pole in detail. What it found down there was a monstrous vortex at the center, something like a hurricane the size of a continent, and around it nothing but ordinary circular bands. No polygon. No corners. No sides.
Two poles on the same planet, one with a geometric figure locked in place for four decades and the other with plain circles. The sensible conclusion was that the hexagon was a one off, some accident of the northern jet stream that had no counterpart.
The Answer Came Out of a Water Tank
The most interesting work on why a planet's atmosphere would produce corners at all was not done at a telescope. It was done in a laboratory at the University of Oxford by Ana Claudia Barbosa Aguiar and Peter Read.
They built a spinning tank. A cylinder of water rotating to stand in for the planet, and inside it a ring turning at its own separate speed, playing the part of the polar jet stream. Then they varied how fast the ring turned relative to the tank and watched what the fluid did.
At low speed differences the jet stayed roughly circular, which is what fluids normally do. As they increased the difference, small eddies began forming along the edge of the ring. Those eddies grew, they strengthened, and they started pushing the flow inside into straight segments with corners between them.
The team could produce ovals. They could produce triangles. They could produce squares. Read described it as being able to create almost anything they liked.
The controlling factor turned out to be the size of the speed difference between the spinning body and the jet. A large difference produced a shape with few sides. A smaller one gave the figure more of them.
That result explained the hexagon. Saturn's northern polar jet happens to run at a rate relative to the rest of the atmosphere that favors six sides. It also carried a prediction inside it, one that nobody could test at the time. If the count of sides depends on that gap in speed, then a different jet running at a different relative rate should produce a different polygon. Not a hexagon somewhere else. A different shape entirely.
Nobody had a place to test that. There was only the one polygon, on the one pole, on the one planet.
The Decade Nobody Could Look
Saturn is tilted on its axis by roughly twenty seven degrees, which means it has seasons in the same way Earth does, only far longer. That tilt also decides which parts of the planet we on Earth are able to see at any given time.
Between two thousand twelve and two thousand twenty three, the south pole was turned away from us. Cassini had already ended by twenty seventeen, and no spacecraft has orbited Saturn since. So for eleven years there was no way to get a good look at that pole from anywhere. Not from the ground, not from Hubble, and not from a probe, because there was no probe.
The decagon appeared during that window.
Researchers can put rough brackets on it. Cassini saw nothing through twenty seventeen. Hubble picked up the first faint traces in twenty twenty three, once the pole came back into view. That means the shape formed somewhere in a six year gap when nobody on this planet was in a position to watch it happen.
That is worth sitting with. A structure larger than the Earth assembled itself in the atmosphere of another world, and the entire event took place while we were looking the other way.
How It Was Found
The people who noticed first were not running a space telescope.
In images taken in twenty twenty four, Sanchez-Lavega and two amateur astronomers, Trevor Barry and Jean-Paul Oger, spotted a faint wavy band near the southern pole. It was not much. It was enough to be worth another look.
Ground based observations through twenty twenty five sharpened the picture and suggested the wave had a regular repeating structure to it. At that point the team went to Hubble, because Hubble can do something no ground telescope can. It sits above the atmosphere, so it can hold a sharp image across a complete rotation of Saturn without the blur that our own air puts on everything.
The Hubble images came from a program called the Outer Planet Atmospheres Legacy, OPAL for short, which has photographed Jupiter, Saturn, Uranus and Neptune once a year for more than a decade. That archive is what allowed the team to run the discovery backward and find the feature already present, faintly, in the twenty twenty three data.
Mike Wong of the University of California at Berkeley, another co-author, made the point that OPAL was built expecting surprises without knowing which surprises. A lot of what comes out of it, he said, rests not on a single observation but on years and years of them.
An amateur with a backyard telescope saw it first. A space telescope confirmed it. A ten year archive dated it. No one of those three would have been enough on its own.
Why Ten Is the Interesting Number
A second hexagon at the south pole would have been a curiosity. It would have said the same conditions can occur twice on the same planet.
A decagon says something different, and it lines up with what came out of that water tank in Oxford. If a wider gap between jet and atmosphere means fewer corners, then ten sides at the south pole means a gentler gap down there than the six sided north.
The laboratory work predicted that varying that one factor would vary the shape. Saturn has now handed over a second polygon with a different number of sides on the opposite pole of the same planet, which is about as close to a controlled experiment as a gas giant is ever going to give anybody.
It also changes what the hexagon means. For forty years it was the only one of its kind anywhere, which made it look like an oddity. Two of them, at different latitudes with different geometry, suggests that polygonal jet patterns are something rapidly rotating atmospheres tend to produce when the conditions line up, rather than a freak event that happened once.
Nobody is claiming that as settled. The paper does not claim it. The team is explicit that they cannot yet say what caused the decagon to form when it did, and one of the ideas they intend to test with computer modeling is whether a nearby storm disturbed the jet enough to start it.
What Happens Next
The decagon is not sitting still. It has grown more defined every year since twenty twenty three, which is the opposite of what the northern hexagon does. The hexagon is the stable one, holding its shape through decades and through polar winters. The southern feature is still developing, and nobody knows whether it settles into something permanent or comes apart.
Hubble is scheduled to look at Saturn again later this month. Those images will show whether the sharpening has continued.
After that the team wants the James Webb Space Telescope on it. Webb sees in infrared wavelengths that Hubble's instruments cannot reach, which would let researchers work out how deep the structure goes, what temperatures run through it, and what it is made of at different altitudes.
The modeling runs alongside the observation. If a simulation can be made to produce a ten sided wave out of Saturn's southern wind profile, that would go a long way toward explaining why this one exists and why it arrived when it did.
There is no spacecraft at Saturn to help with any of it. Cassini burned up in the atmosphere in twenty seventeen and nothing replaced it. Everything we learn about that decagon for the foreseeable future comes from telescopes hundreds of millions of miles away, which is precisely why an eleven year gap in the record was possible in the first place.
Saturn had been the planet with the shape at the top. It now has one at the bottom too, with four more sides than the first, and it built the thing while our backs were turned.
THE END
— Brian Bullock / Everyone Knows Podcast | Starborne Studios | brianbullockwriter.com | X @EveryoneKnws1