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Sunday, 4 September 2016

Saturn's polar hexagon...

Straight edges are a sign of something artificial… nature works in curves.... OK, that’s not entirely true – check out the Giant’s Causeway in Ireland - but when it comes to clouds it’s very nearly true. 

Seriously, you have got to check out the Giant's Causeway. I'm not being paid by the Irish tourist board. Honest
Planets are also very curvy, and they often have clouds. None of the above says ‘will form a huge spinning polygon’, not to my tiny brain anyway. 

So I wouldn’t expect the planet Saturn to have a ten thousand kilometre wide hexagon, made of clouds, spinning around it’s north pole. 

Which it does. 

This thing.

The great Saturn hexagon is odd, in much the same way that elephants are plus sized. What is it? Well… the answer involves the weird physics of fluid flows, and a motorised bucket. First a bit of background: Like Jupiter, Saturn is all atmosphere, with the planet's air getting thicker with depth until it becomes a supercritical fluid (don’t ask)*. There’s nothing solid, at all, on the whole planet. So the huge hexagon, bizarrely, must be made of clouds, wind currents, and vapour. 

Incidentally, the idea that it’s a sign of an incredibly powerful alien intelligence doesn’t really hold water, because it leads to awkward questions like: “What kind of alien would bother to do that?” 

So it’s essentially got to be some kind of cloud. Ruling out aliens, super man, and the flying spaghetti beast… how does the giant hexagon work? 

Oh that flying spaghetti monster... he is a scamp...
It turns out that you can create a vortex with hexagonal, square, or even star shaped sides here on Earth. 

In a motorised bucket, in fact. 

A team from the Technical University of Denmark proved this using transparent buckets with metal bottoms that rotated at high speed. They filled the bucket with water and spun the bottom to whip up the liquid into a whirlpool. The researchers found that, when their buckets got up to serious speeds, the whirlpool formed wasn't circular - as the bucket sped up, it became elliptical, then double eyed*, then a three-sided star, a square, a pentagon, and at the highest speeds (about seven spins of the base a second), a hexagon.
Above: They dropped some ink into the bucket, and lit it from underneath to get the photos. Courtesy of the Technical University of Denmark.
The problem with this is that Saturn is a huge ringed planet, not a huge bucket with a spinning bottom***. But it proves that spinning fluids can create straight sided shapes, so there doesn’t need to be anything fundamentally weird about Saturn’s north pole for the hexagon to happen. 

A better answer, using the some similar fluid physics laws to the bucket experiment, came from a team at the New Mexico Institute of Mining and Technology: A series of computer simulations of a jet stream – a high altitude wind current - around Saturn’s pole were run and, to their surprise, with the right start conditions the simulated jet naturally settled into a stable hexagonal shape that rotates around the pole. 

So the hexagon is actually the result of the right start conditions and a few natural perturbations? 

That might be the answer – but hold on: The Mexico team’s simulation doesn’t replicate all the features of the hexagon… and it’s just a simulation, however well made and carefully run. 
A definitive answer will need a return mission to Saturn itself… 

*If you’re asking the question anyway: Supercritical fluid happens when the pressure and temperature of a fluid is so high it has properties of both a gas and a liquid - it’s a bizarre state, but deeper into a giant planets core things get weirder, with endless oceans of liquid metallic hydrogen, and possibly diamond bergs lurking I the murk.

** Both these first two have also been seen in hurricanes at Venus’ poles. 

*** If that's your personal theory I respect your opinion. Except I don't because it's not, and that's nuts.

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