"Cassini is indebted to Voyager for its many fascinating discoveries and for pavingthe way for Cassini," says Linda Spilker, Cassini project scientist at JPL, who started her career working on Voyager from 1977 to 1989. "On Cassini, we still compare our data to Voyager's and proudly build on Voyager's heritage."
But the Voyager Mission left a few mysteries that Cassini has not yet solved. One of the most perplexing mysteries is Saturn's hexagpn. NASA scientists first spotted a hexagonal weather pattern when they stitched together Voyager images of Saturn's north pole. Cassini has obtained higher-resolution pictures of the hexagon – which tells scientists it's a remarkably stable wave in one of the jet streams that remains 30 years later – but scientists are still not sure what forces maintain the object.
'Now that we can see undulations and circular features instead of blobs in the hexagon, we can start trying to solve some of the unanswered questions about one of the most bizarre things we've ever seen in the solar system, said Kevin Baines, Atmospheric scientist at NASA's Jet Propulsion Laboratory after viewing Cassini images in 2009. "Solving these unanswered questions about the hexagon will help us answer basic questions about weather that we're still asking about our own planet."
After waiting years for the sun to illuminate Saturn's north pole again, cameras aboard NASA's Cassini spacecraft captured detailed images yet of the intriguing hexagon four times the size of Earth crowning the planet.
The images of the hexagon, whose shape is the path of a jet stream flowing around the north pole, reveal concentric circles, curlicues, walls and streamers not seen in previous images.
The last visible-light images of the entire hexagon were captured by NASA's Voyager spacecraft nearly 32 years ago, the last time spring began on Saturn.
After the sunlight faded, darkness shrouded the north pole for 15 years. Much to the delight and bafflement of Cassini scientists, the location and shape of the hexagon in the latest images matched up with what they saw in the Voyager pictures.
"The longevity of the hexagon makes this something special, given that weather on Earth lasts on the order of weeks," said Kunio Sayanagi, a Cassini imaging team associate at the California Institute of Technology. "It's a mystery on par with the strange weather conditions that give rise to the long-lived Great Red Spot of Jupiter."
The hexagon was originally discovered in images taken by the Voyager spacecraft in the early 1980s. It encircles Saturn at about 77 degrees north latitude and has been estimated to have a diameter wider than two Earths. The jet stream is believed to whip along the hexagon at around 100 meters per second (220 miles per hour).
Early hexagon images from Voyager and ground-based telescopes suffered from poor viewing perspectives. Cassini, which has been orbiting Saturn since 2004, has a better angle for viewing the north pole. But the long darkness of Saturnian winter hid the hexagon from Cassini's visible-light cameras for years. Infrared instruments, however, were able to obtain images by using heat patterns. Those images showed the hexagon is nearly stationary and extends deep into the atmosphere. They also discovered a hotspot and cyclone in the same region.
Scientists are still trying to figure out what causes the hexagon, where it gets and expels its energy and how it has stayed viable for so long.
Mystery solved:
Saturn's hexagon recreated in the laboratory
A lot of readers have expressed interest in the origin of Saturn's north polar hexagon. The hexagon is a long-lived pattern in the clouds surrounding Saturn's north pole, which has been observed since the Voyagers passed by in 1980 and 1981. Unlike Jupiter, whose cloud bands are obvious in visible light, Saturn's cloud features are more subtle in visible wavelengths. The cloud features pop to life when viewed at longer, thermal wavelengths, as in this Cassini VIMS mosaic: Saturn's north polar hexagon This image of Saturn's north pole was taken by Cassini's VIMS spectrometer at a mid-infrared wavelength of 5 microns. It was winter at Saturn's north pole; all illumination is thermal radiation (heat) welling up from Saturn's depths. Some of the heat radiation is blocked by clouds floating in Saturn's atmosphere at about 75 kilometers below the cloud tops that can be seen in visible wavelengths. The pressure at that level is about three times Earth's atmospheric pressure. The patterns in the image are created by alternating cloudy and clear areas. The image has been contrast-reversed so that the glouds show up as bright spots, while open areas appear dark. |
 | Saturn's north polar hexagon This movie of Saturn's north pole was taken by Cassini's VIMS spectrometer at a mid-infrared wavelength of 5 microns. It was winter at Saturn's north pole; all illumination is thermal radiation (heat) welling up from Saturn's depths. Some of the heat radiation is blocked by clouds floating in Saturn's atmosphere at about 75 kilometers below the cloud tops that can be seen in visible wavelengths. The pressure at that level is about three times Earth's atmospheric pressure. The patterns in the image are created by alternating cloudy and clear areas. The images have been contrast-reversed so that the clouds show up as bright spots, while open areas appear dark. The 37 frames in the animation (only 13 of which are shown in the thumbnail version) were taken over a period of about an hour, as Saturn rotated about 30 degrees. |
In an article published in the April issue of Icarus, Ana Aguiar and her coauthors advance an alternative explanation, and test it in the laboratory. The explanation has to do with the speeds of winds in Saturn's atmosphere. It begins with the following plot, showing how Saturn's wind speeds change with latitude.
 Wind speeds on Saturn Wind speeds on Saturn were derived by tracking the motions of clouds observed by Voyager and Cassini. The fastest winds blow around Saturn's equator; at higher latitudes there are alternating eastward and westward jets. Exactly how fast the winds are blowing depends upon your choice of Saturn's rotation rate, which is generally measured from the rotation rate of its magnetic field. That rotation rate was measured to be slower by Cassini than by Voyager. If you use the Voyager reference rotation rate, most of Saturn's winds are prograde (the air moves eastward relative to Saturn's rotation), shown as the dashed line on this graph. If you use the Cassini reference rotation rate, shown as the solid line, the winds alternate eastward and westward, as they do on Jupiter. |
Aguiar and her coauthors argue that it's not the wind speeds that are important per se; it's the gradient in wind speeds. Where there are steep contrasts in wind speeds -- adjacent parts of Saturn's atmosphere moving at very different speeds -- you can induce unstable behavior in a fluid, including waves, eddies, and swirls. That little prograde peak in wind speeds at around 78 degrees north is actually the narrowest peak on the graph, so that part of Saturn's atmosphere contains one of the steepest wind speed gradients to be found on the whole planet -- a good place to generate weird atmospheric features, including wavelike disturbances.
In the paper, Aguiar et al. run through a mathematical model that shows how the steep gradient in wind speed can set up a wavelike motion of this high-latitude jet, and that there are likely to be exactly six waves encircling the planet, setting up the hexagon (something that I explain in more detail here), and that wave propagates at about exactly the same speed as the jet flows (meaning that the hexagon will appear nearly stationary with respect to Saturn's rotation). Moreover, they show that the observed wind conditions near the south pole are sufficiently different from those near the north pole that the south pole is not predicted to produce a similar wave, which is good, because there's no hexagon at Saturn's south pole.
Having shown that the idea of wind speed gradients driving the formation of the hexagonal wave, they moved to the laboratory. Fun ensued, and science too.
They set up a cylindrical, rotatable tank 10 centimeters deep and 60 centimeters wide. The tank had a lid and base that were split into concentric sections. They could rotate the inner circle of the lid and floor of the tank at a different rate than they rotated the outer circle of the tank and floor, setting up a gradient in the flow speed of the liquid at the joint between the inner and outer circles. Depending on the relative speeds of the two disks, different things happened. At low relative speeds, there was nothing particularly unusual in the flow, just rotation of the water in the tank. But as the gradient between the two rotating sections was increased, wavelike instabilities started forming at the boundary between the two disks. Depending on conditions, the waves evolved chaotically or sometimes quite stably; there might be as few as two or as many as eight waves encircling the axis of rotation. But for a reasonably wide range of experimental parameters, they produced a wavenumber of 6: a hexagon.
 Creating Saturn's hexagon in the laboratory This is a top-down view of a laboratory tank in an experiment designed to reproduce the wind conditions near Saturn's north pole. The whole cylindrical tank is 60 centimeters wide. Its lid and base are split into two concentric sections, with the inner circle being 30 centimeters in diameter. The inner and outer circles are rotated at different rates, which sets up an instability at their boundary, producing a standing wave; the number of waves it takes to encircle the "pole" depends on various experimental parameters, including the rotation speed. In this particular case, the wavenumber is 6, producing a hexagon. Dye has been injected into the tank to make the form of the turbulent flow visible. Eddies are produced outside the wave. |
To track the precise speed at different points within the fluid, they changed the fluid to a mixture of water and glycerol and seeded it with pellets of Pliolite (the glycerol was used so that the tracer particles would have neutral buoyancy within the fluid):
I should note here something that Aguiar wanted me to mention, which is that her work is not at all the first time that such patterns have been created in the laboratory; here, for instance, is another example. She and her coauthors were just first to publish a paper making a case for so explicit a connection between such laboratory experiments and conditions on Saturn.
They did manage to make a hexagon like the one that's seen on Saturn, but with slightly different choices of initial conditions (spin rates) they generated waves with other wavenumbers too, meaning they got polygons of different shapes. I want to thank Ana Aguiar very much for taking the time to dig these other cases out of her files for me, and especially for being willing to share data that doesn't look like the hexagon-near-Saturn's-pole situation she was trying to reproduce with her experiment. I asked to see them because we may see a hexagon near Saturn's pole, but different conditions around different planetary atmospheres could conceivably produce other shapes. And also because it's just so cool to see something as apparently chaotic and unpredictable as the turbulent flow of a liquid produce geometrical shapes like triangles, septagons, and ovals.
Want wavenumber = 7? Here's a septagon.
 Creating Saturn's...septagon(?) in the laboratory This is a top-down view of a laboratory tank in an experiment designed to reproduce the wind conditions near Saturn's north pole. The whole cylindrical tank is 60 centimeters wide. Its lid and base are split into two concentric sections, with the inner circle being 30 centimeters in diameter. The inner and outer circles are rotated at different rates, which sets up an instability at their boundary, producing a standing wave; the number of waves it takes to encircle the "pole" depends on various experimental parameters, including the rotation speed. In this particular case, the wavenumber is 7, producing a septagon -- one more "wave" than the number that surrounds Saturn's pole. Dye has been injected into the tank to make the form of the turbulent flow visible. Eddies are produced outside the wave. |
 Three eddies surrounding a pole In an experiment designed to simulate the conditions near Saturn's north pole, one experimental setup generated not a hexagon but a triangle. |
 Two eddies and a pole With an experimental setup intended to produce Saturn's north polar hexagon, one choice of initial conditions produced two waves circling the pole, creating an oval shape bounded by two eddies. (Corrupt tape in the experiment's recording device caused the funky checkerboards in this image.) |
 Venus' south pole A view of Venus in the thermal infrared, at a wavelength of 5 microns. The brightest part of the image is the uppermost atmosphere reflecting solar radiation on the dayside of Venus. On the nightside, subtle cloud features are visible, especially near the south pole, as thermal radiation is emitted from Venus' upper atmosphere at an altitude of around 60 kilometers (36 miles). This image was captured on April 12, 2006 from a distance of 210,000 kilometers. |
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