Definitely my Astronomy Picture of the Day this one. Why is Saturn’s north polar jet stream hexagonal? Answers below in comments please. 🙂
Image Courtesy of Carolyn Porco carolynporco.com. Click to visit ciclops.org article
Take the Mickey-Pedia tells us:
Saturn’s hexagon is a persisting hexagonal cloud pattern around the north pole of Saturn, located at about 78°N.[1][2] The sides of the hexagon are about 13,800 km (8,600 mi) long, which is longer than the Earth‘s diameter.[3] It rotates with a period of 10h 39m 24s, the same period as Saturn’s radio emissions from its interior.[4] However, the hexagon does not shift in longitude like other clouds in the visible atmosphere.[5]
Saturn’s south pole does not have a hexagon, according to Hubble observations.[6] But it does have a vortex, and there is also a vortex inside the northern hexagon.
Saturn’s polar hexagon discovery was made by the Voyager mission, and it was revisited since 2006 by the Cassini mission.[7] Cassini was only able to take thermal infrared images of the hexagon, until it started to become visible by light in January of 2009.[8]
And that’s all it tells us… 😉
I can add that:
“From one side, over the top through the pole, to the other side, it is about 2 Earth diameters across” – Carolyn Porco
Source: Hulton Archive/Getty Images
Tallbloke's Talkshop 
‘Why is Saturn’s north polar jet stream hexagonal?’
The short answer might be here…
‘Saturn’s hexagon recreated in the laboratory’
http://www.planetary.org/blogs/emily-lakdawalla/2010/2471.html
Looks more like 6 waves on a ring than a hexagon. Maybe a spatially fixed gradient in some key location + a natural spatiotemporal resonance.
What does mainstream conventional wisdom have to say about it? Anyone know? (I haven’t checked.)
I linked this in suggestions a couple of months back. Then, they were talking about it being a hurricane but if it’s spinning with Saturn it can’t really be called that…probably another completely fabricated quote from another pesky NASA email, Rog. 😉
Did I hear they have HAL on board?
Fascinating. Question raising.
I suspect that the same physical forces which produce hexagonal columns in basalt (Giant’s causeway for instance) are at work. These basalt structures are formed when cooling in the rock occurs in multiple columns whose centres are spaced equally apart, forming an hexagonal fracture/jointing pattern. The problem of course is that here we have only one giant hexagon formed, so not immediately obvious how we can extend the analogy. And we think earth’s atmosphere is complex – nothing compared to the gas giants of our solar system!
Nice image but nothing odd here, hexagonal cell formation is common in fluids, one of the forms for Rayleigh-Benard convection.
Try eg. hexagonal convection cell
That should lead to lots of stuff as well as images.
Example for weather
Click to access stageverslag_Noteboom.pdf
Intuitively, if you pack circles together there are spaces between them. If these turn into circles and size is equalised, what shape do you get?
In some ways, the gas giant’s atmospheres are simpler than Earth’s. No continental masses, smaller moons, smaller obliquities. Some clue might be gained by looking at Ray Tomes’ annotation of a shot from above Jupiter’s pole: Video
A few more links here:
http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=4875
Folks; It’s NOT a hexagon, and it’s not any `weirder’ or mysterious than the Earth’s jet stream. It is a stream of moving air that has a wavenumber 6 pattern superimposed on it. That’s all. If it weren’t wrapped around a planet, it wouldn’t look like it had straight sides. Check out this video showing the Earth’s jet stream, and you’ll see it is more or less a wavenumber 6 pattern, though far more irregular and temporally variable. http://www.youtube.com/watch?v=C_HiBj0teRY The m=6 has to do with a number of factors: the size of the planet, the speed of rotation, the depth and thermal state of the atmosphere, etc. Those obviously work out to be m=6 for both Earth and Saturn. The Earth’s jet stream is far more variable and discontinuous, because there’s more friction owing to the atmosphere/solid-ground interaction, which the giant planets have none of. Also, the role of smaller eddies and vortices is important and that is the greatest source of uncertainty in understanding any meteorological system, be it Earth or Saturn.
I hope this settles the mystics among you who are hoping for crystal energy on the cosmic scale. The truth is: there isn’t any!
Hurricane Isabel – September 2003 –
Click to access BAMS_KosSch.pdf
My article :
http://translate.google.it/translate?sl=it&tl=en&js=n&prev=_t&hl=it&ie=UTF-8&u=http%3A%2F%2Fdaltonsminima.altervista.org%2F%3Fp%3D18827
Neil Armstrong’s plane is an X-15, if I recall correctly. I read articles about it in National Geographic when I was at school. NASA did more interesting stuff in those days!
OT, but it brought back memories.
Oh, and what produces the TRUE hexagonal structure in cooling basalts — like the Giant’s Causeway in Ireland — has NOTHING to do with the 6 waves comprising Saturn’s polar jet stream. The first has to do with the cooling and contraction process in lavas, for example. On Saturn, it is a manifestation of particular instabilities in fluid flow.
Fascinating. My first impulse was that maybe it had to do with some sort of minimization criterion as found in a honeycomb. But, then, oldbrew and Carolyn Porco showed it is just one of many polygonal patterns which could exist.
Carolyn: Welcome, and thanks for calling by. Do you think those instabilities near the polar vortex might help explain the harmonic stepping noticed by Ray Tomes on Jupiter (see video above). If so, does this tell us anything about the propagation of weather driving energy, perhaps in the form of planetary waves?
Thanks
I don’t know what Tomes means by harmonics, but certainly there are wave structures in Jupiter’s atmosphere, most likely driven by instabilities in the flow. And to say weather drives energy in the form of planetary waves, I am presuming you mean the idea that eddies become consumed by the flow and contribute their energy to it, rather than being derived from the flow. We have found for both Jupiter and Saturn that eddies are consumed by the jets and supply energy to them. That has been one of the main meteorological results from the Voyager and Cassini missions.
The sun has a sub-polar hexagon as shown in this NASA graphic.
OB: Good spot. Need to think why the polar jet would be at the same latitude on both bodies, given their very different composition. Fluid dynamics only part of the answer I suspect. What latitude is ‘average’ for the Earth’s polar jet streams? Any N-S difference?
Stab in the dark: something to do with the Coriolis effect?
Basalt hexagonals:
Structural failure of rock, basic properties. Don’t know why, but 60*/120* failure is THE way in which rock fails under compressive forces. Fault patterns in the subsurface, the basic craton, are 60*/120*, as are simple jointing patterns in limestone, down to fractures in a hand specimen those rock carvers of us pick up from metasomatized silicates (soapstones, prophyllites etc.).
I don’t know why such a diverse group of materials should fail as it does. But it does! The basalt cooling is, of course, a cause, through contraction, of failure through extensional rather than compressive forces. Then there are the mud cracks, dessication features that can also be hexagonal, although polygonal is generally the case. Same with tundra/high alpine polygonal surface features in frozen ground, where top couple of feet melt in the summer: you get water and sand or rock pushed up by freeze-melt cycles along straight edges (not always, but often).
Material failure in rock is commonly along 60*/120* boundaries. Of course there is mineral failure along 60*/120* planes, as minerals are built at an atomic or molecular level in certain styles, but a rock has no orientation to its mineralogy/crystalography.
A material science person would be best to answer this.
The fact that fluids with a “6-wave” phenomena show hexagonal ….
The Greeks saw similar structures in the universe. They just said that that is how the universe is made. We have to have a reason why. It would be nice to know, but of course that is the challenge of finding the ultimate equation that answers everything.
What I wondered about this photo:
At the centre, it looks to be counterclockwise, going in and down. On the outside are smaller spirals (some that look two armed like some galaxies) that appear to be clockwise rotating. Are we looking at a “drainhole” or “spout” in which there is vertical movement down and in or up and out of the lower atmosphere? And what about the smaller but similar vortices outside the hexagonal?
OB: I’m unconvinced about the coriolis effect for big weather systems. More to do with convection and harmonic resonance between ‘bands’ of weather systems defined by turbulent boundaries creating drag. In summary: Fluid dynamics as Carolyn said. But something more too perhaps, the seasonality induced by eccentric orbits and obliquity coupling with heat capacity to produce multiple of 4 harmonics for Jupiter? This still leaves the R6 nature of the polar jets. Maybe a simple winter/summer consequence of dividing the 12 of the next band out by 2?
Dunno, just some thoughts.
Doug: I achieved distinctions in materials science and fluid dynamics at college. I still hesitate to jump to standard solutions for conceptualising planetary atmosphere boundary conditions though. Carolyn had to return to work. I hope she returns later to tell us more about the way her team arrived at their meteorological insight.
oldbrew says: August 5, 2013 at 10:30 pm
“Stab in the dark: something to do with the Coriolis effect?”
That ‘was’ a dark stab OB, but close. The fluid dynamic of a rotating body dictates that the configuration includes a ‘planar centrifuge’ at the points of ‘axial rotation’ (poles) and a ‘radial centrifuge’ at the ‘equatorial’ region. As it ‘appens, ‘Coriolis effect’ dictates that there is an ‘unforced’ region at mid latitudes in each hemisphere which tends to ‘roll’ as a region of ‘unforced’ fluid between each hemisphere’s radial and planar centrifuges.
Re: Polar Climate Cell = planar centrifuge, Hadley Climate Cell = radial centrifuge, and the Ferrel Climate Cell = unforced dead space in between.
This should make it clearer:
http://sageography.myschoolstuff.co.za/geogwiki/grade-11-caps/grade-11-caps-climate-and-weather/2-global-air-circulation/3-tri-cellular-circulation/
You can read into the behaviour of Coriolis as it exerts its effect at different latitudes. Please remember that Earth is a ‘rock type’ planet and planetary protuberances act as ‘impellers’ to fluid, thus, the ‘wavy architecture’ of Earth’s NH Polar jet (see the pressure differences either side of land masses).
Reference is always made to ‘heating from insolation’. If the rotating planet had a viable atmosphere without insolation, the Climate Cell configuration would be the same, but if the planet was without rotation and enjoyed ‘heating from insolation’ only the Hadley Climate Cell (of sorts) would exist across each hemisphere.
For Gas Giant planets, there must be a ‘boundary’ that duplicates the ‘boundary layer’ for a ‘rock type’ planet. Surely this ‘boundary layer duplicate’ must dictate the shape of both the Polar Vortex and the Polar Jets?
Best regards, Ray.
OK, I get to speak for the lunatic fringe!!
In a recent paper Miles Mathis explains about the winds of Venus speeding up and the rotation slowing with his charge and spins, then the temperature of the poles of Mercury (too cold) Venus (too hot), earth and Saturn, explaining it with charge spin cancellation and augmentation and charge that passes through the poles rather than mostly spinning out the equator and lower latitudes. He then throws in a bone about the hexagonal shape of the vortices being due to the hexagonal shape of the nucleus (in his atomic world) scaling up if there is sufficient sphericity and homogeneity in the area.
Click to access venus2.pdf
Roger,
The 60/120 failure in thick (>1000m) sections of mixed limestone, dolomite, sandstone, siltstone and shale is really peculiar to my thinking. It is reasonably easy to see in our geological mapping with 1 datapoint (well) even per square mile. There is always a conjugate set, a stress release “other”. Near vertical, though “scoop”, in compressional situation, but lateral set obvious .
Odd. As if you’d expect standard fracture pattern in a plywood (though plywood does go at right angles layer to layer, so not really the same).
I should look at plate tectonic maps, see what the angles are there.
Doug, I doubt Gas giant’s interior, super compressed ‘solid gas’ cores act like amorphous rock, I would expect expect more of a glassy flow produced by convection like Tim C suggested. Even if that produced hexagonal cells (unlikely on a fast spinning planet, unless the core revolves much more slowly) I can’t see it affecting upper atmosphere dynamics. Those will be driven by rotation and turbulence primarily.
@ suricat
NASA says: “Though Saturn’s magnetic field is not as huge as Jupiter’s, it is still 578 times as powerful as the Earth’s.”
One would imagine this plays some part in the workings of the hexagonal vortex, which is 3 times the width of the Earth and apparently never goes away.
In response to my request, Carolyn Porco has kindly posted some links to her meteorological results for Jupiter and Saturn on her facebook page:
https://www.facebook.com/carolynporco
Everyone,
I’ve been asked to provide references for the findings by my team members that eddies in the atmospheres of both Jupiter and Saturn are consumed by the jets and supply energy to them, and not vice versa. Well here they are:
The Jupiter results were reported in
Salyk, C., A.P. Ingersoll, J. Lorre, A. Vasavada, and A.D. Del Genio, 2006: Interaction between eddies and mean flow in Jupiter’s atmosphere: Analysis of Cassini imaging data. Icarus, 185, 430-442, doi:10.1016/j.icarus.2006.08.007.
Unfortunately, we do not have this paper on the CICLOPS website at this time.
However, for Saturn we do. Go to….
http://www.ciclops.org/sci/papers.php
… and find the following papers under the sections entitled `2007′, ‘2012’, and `2009′ respectively.
2007: First Saturn results:
Del Genio, A.D., J.M. Barbara, J. Ferrier, A.P. Ingersoll, R.A. West, A.R. Vasavada, J. Spitale, and C.C. Porco, 2007: Saturn eddy momentum fluxes and convection: First estimates from Cassini images. Icarus, 189, 479-492, doi:10.1016/j.icarus.2007.02.013.
2012: Updated Saturn results:
Del Genio, A.D., and J.M. Barbara, 2012: Constraints on Saturn’s tropospheric general circulation from Cassini ISS images. Icarus, 219, 689-700, doi:10.1016/j.icarus.2012.03.035.
2009: A more general general discussion of how the eddies supply momentum to the jets rather than taking momentum out of them can be found in the last part of section 6.3.3 of the following article in the book “Saturn from Cassini-Huygens”:
Del Genio, A.D., R.K. Achterberg, K.H. Baines, F.M. Flasar, P.L. Read, A. Sánchez-Lavega, and A.P. Showman, 2009: Saturn atmospheric structure and dynamics. In Saturn from Cassini-Huygens. M.K. Dougherty, L.W. Esposito, and S.M. Krimigis, Eds. Springer-Verlag, 113-159, doi:10.1007/978-1-4020-9217-6_6.
That ought to keep you all busy for a while!
oldbrew says: August 6, 2013 at 3:48 pm
“@ suricat
NASA says: “Though Saturn’s magnetic field is not as huge as Jupiter’s, it is still 578 times as powerful as the Earth’s.”
One would imagine this plays some part in the workings of the hexagonal vortex, which is 3 times the width of the Earth and apparently never goes away.”
I concur OB. This would require a ‘modification’ to Newtonian Mechanics. There’s something else in play.
Best regards, Ray.
What a great photo!
Hexagons at small scales are also not uncommon. The carbon compounds benzene and cyclohexane are examples, as is the common but beautiful snowflake. Perhaps something significant about the hexagons internal angle of 120 degrees?
I would be surprised if there were no harmonic patterns on the gas giants or the sun.
More about the properties of hexagons.
‘we can use any circle to construct on it a hexagon and an equilateral triangle. Joining three pairs of points then reveals a line and its golden section point’
http://www.maths.surrey.ac.uk/hosted-sites/R.Knott/Fibonacci/phi2DGeomTrig.html#equilPhi
[…] Sta di fatto che l’immagini ripresa dalla sonda Cassini è decisamente suggestiva. Qui per altre info. […]
Scientists analyzed these images in false color, a rendering method that makes it easier to distinguish differences among the types of particles suspended in the atmosphere — relatively small particles that make up haze — inside and outside the hexagon.
“Inside the hexagon, there are fewer large haze particles and a concentration of small haze particles, while outside the hexagon, the opposite is true,” said Kunio Sayanagi, a Cassini imaging team associate at Hampton University in Virginia. “The hexagonal jet stream is acting like a barrier, which results in something like Earth’s Antarctic ozone hole.”
The Antarctic ozone hole forms within a region enclosed by a jet stream with similarities to the hexagon. Wintertime conditions enable ozone-destroying chemical processes to occur, and the jet stream prevents a resupply of ozone from the outside. At Saturn, large aerosols cannot cross into the hexagonal jet stream from outside, and large aerosol particles are created when sunlight shines on the atmosphere. Only recently, with the start of Saturn’s northern spring in August 2009, did sunlight begin bathing the planet’s northern hemisphere.
Story:
http://www.nasa.gov/jpl/cassini/saturn-north-pole-hexagon-20131204.html#.UqEAJcRdX4a