Earth, we have a hexagon

How a storm on Saturn gets shaped into a more or less permanent hexagon


Photo: Nasa
Photo: Nasa

Meanwhile on Saturn, they have found a hexagon.

Yes, I mean a six-sided shape like you would find plenty of if you peered into the nearest beehive. Not to my knowledge do these pop up in too many other places in nature, but there’s one on Saturn. Though let’s be clear: There are no bees on the ringed planet. Even if there were, and if for a moment we thought this was part of a beehive, those saturnine bees would have to be enormous. For this hexagon is—hold your breath—about 32,000km wide. How big is that? Well, you would have to place three planet Earths in a row to stretch across the roiling expanse of this thing.

Not your average hexagon.

Not that they found it recently, either. When Nasa’s Voyager spacecraft flew past Saturn in 1980, it sent home images of its experience. In 1988, scientists noticed, while sifting through Voyager data, that there was a massive hexagon at the planet’s north pole. In 1997, Nasa launched the Cassini spacecraft on a mission to Saturn. It started orbiting Saturn in 2004, and has sent plenty of images of the planet, including of its polar regions. Today, we know from Cassini that the hexagon has a tight spiral at its centre, it is about 100km thick, and it has changed colour slowly over the years: from generally blue up to 2012 to mostly yellow by this year.

There’s a possible explanation for the colour change, at any rate. Saturn takes 30 years to go around the Sun, which means its “seasons” are correspondingly longer than ours. When Cassini arrived at Saturn in 2004, that was about the end of Saturn’s “winter” and the beginning of “spring” (in its northern hemisphere). In the years since, “spring” has given way to “summer”. The north pole gets more sunlight now than it did a few years ago, and that’s why it looks yellow. But still, why this unnatural-looking pattern? What could possibly produce a Brobdingnagian hexagon at Saturn’s north pole?

We know it is actually a vast storm system—the mother, sisters, cousins and aunts, if you will, of all typhoons we have ever known on Earth. That spiral, in effect, rotates around the eye of the storm. Now a storm on Saturn is not surprising by itself. But how did this one get shaped into a more or less permanent hexagon?

We have clues. Scientists noticed that the hexagon rotates at nearly the same speed that Saturn itself rotates on its own axis—so it appears almost stationary on Saturn. The images from Cassini also suggest the presence of a powerful current of air—what we call a jetstream here on Earth—flowing east, along the borders of the hexagon, at about 350km per hour. That’s speedy, but it’s not the highest wind speed detected on Saturn—so wind speed alone does not cause this shape.

In 2010, a team of Portuguese scientists led by Ana Aguiar wrote a paper explaining that at the hexagon’s border, there’s a dramatic change in wind speeds. Meaning that right there, you’ll find adjacent segments of Saturn’s atmosphere moving at very different speeds. The scientists theorized that it’s the way these segments move against each other that’s responsible for the hexagon.

They experimented with liquids in concentric containers, moving them at different speeds and observing what happened. At certain speed differences, a distinct wavy motion emerged at the boundary between the liquids. On Saturn, such waves would circle the planet at the latitude where the hexagon forms (about 78 degrees North), meeting themselves after going all the way round. The latitude and the length of that circumnavigation determine how many such waves form—in this case, six. This is why we get a hexagon: if the wind speed change happened further north, Voyager and Cassini might have stumbled upon a pentagon instead.

If all this sounds complicated, don’t worry. Think of this analogy: The swirls and eddies you see when you flush a toilet, or when you vigorously stir your vodka and tonic, are the kinds of shapes moving fluids can produce. Under certain conditions, they can form other kinds of shapes—like hexagons.

In the years since Aguiar’s paper, other scientists have proposed models that more closely fit what’s on Saturn. In 2015, a team headed by Raúl Morales-Juberías of the New Mexico Institute of Mining and Technology simulated in its lab the stream of air that flows around Saturn’s pole. But it introduced occasional small disturbances, as you might get when jetstreams collide and bounce off each other. This produced a “meandering jet” which settled into a stable hexagon, closely matching the behaviour of the one on Saturn.

Such simulation and theorizing reminds us: There may be an uncannily unnatural shape on Saturn, but it has been produced by entirely natural phenomena. As The New York Times report put it, that we have been able to model the hexagon “provid[es] reassurance that there is nothing supernatural going on at Saturn.”

That’s a relief. I think.

Incidentally, right now is a good time to think about everything we know about this gigantic hurricane on Saturn. Cassini, the intrepid spaceship that has sent us so many spectacular images of Saturn over the years, is running out of fuel and has started on its final orbit around that planet this week. Next September, it will dive to destruction on Saturn.

I don’t know where on Saturn that will happen. But I kind of like this possible epitaph: “Cassini: now and forever at peace with my beloved hexagon.”

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. A Matter Of Numbers explores the joy of mathematics, with occasional forays into other sciences. Read Dilip’sMint columns at www.livemint.com/dilipdsouza

Comments are welcome at dilip@livemint.com

READ MORE