The planet Saturn is famous for its magnificent system of rings, visible in even the smallest of atronomical telescopes. But even more mysterious is the system of high-altitude winds sculpting the clouds into a gigantic hexagon around the giant planet’s north pole at a latitude of roughly 76°. First discovered by the Voyager 2 spacecraft in 1982, astronomers had to wait till the arrival of the Cassini spacecraft in 2006 to get more data to unravel the mystery.Since its original discovery, the standard explanation has been that there must be a large jet-stream atmospheric current circling the planet’s north pole. The winds in this system are moving at speeds of about 320 kilometers per hour, and the system has a diameter of about 30,000 kilometers. The current does not flow in a straight line, but swings between higher and lower latitudes, and these variations form a standing wave that resonates along the circumference of the system at six nodes, which naturally forms the hexagon shape that we can see so clearly. This remains the standard explanation, but it has a problem in that no such jetstream has been observed by Cassini. It also doesn’t explain why there’s a hexagon at the north pole, but not at the south.
One frustration to solving the mystery was that Saturn’s northern hemisphere was in the middle of winter when Cassini arrived. Because of Saturn’s long year, its seasons last more than seven Earth years, the north pole has been shrouded in darkness all this time, limiting observations to infrared only. But late in 2012, the seasons changed and the long arctic winter came to an end. “As we approach Saturn’s summer solstice in 2017, lighting conditions over its north pole will improve, and we are excited to track the changes that occur both inside and outside the hexagon boundary,” said Scott Edgington, Cassini deputy project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California.
Cassini has now managed to record high-resolution images of the swirling currents, and a video of the results in different imaging bands can be seen in the video clip below. Because the images are intended for scientific study, the colours have been enhanced and exaggerated to make subtle details and features stand out more clearly – they do not represent what Saturn would actually look like if you could travel out there and see it with your own eyes.
One interesting revelation from these new data is that the mix of different sized haze particles changes according to whether you look inside the hexagon or outside. “Inside the hexagon, there are fewer large haze particles and a concentration of small haze particles, while outside the hexagon, the opposite is true,” says 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.”
Personally, I’m waiting for the new data to provide confirmation of an alternative theory to the cause of the hexagon feature around Saturn’s north pole. According to Ana Aguar, from Universidade de Lisboa, Portugal, the hexagon is simply the result of turbulence between two currents moving past each other at very different speeds. Like Jupiter, Saturn has equatorial belts, vast currents of wind which ring the planet at a fixed range of latitudes. When she charted the different wind speeds against latitude, she found that although the winds near 76°N were not unusually fast, there was a particularly sharp difference between winds above and below that latitude, leading to very strong turbulence. When she tested the effects of such turbulence between rotating currents in a laboratory experiment, using specially constructed tanks, she found that by varying the relative speeds she could not only recreate the Saturn hexagon, but any shape – heptagons, triangles, even an oval!
Aguar’s model has the benefit of accurately predicting the behaviour of the Saturn hexagon, including the series of smaller eddies and cyclones observed moving around the apexes of the hexagon (these “small” features are comparable to Earth’s hurricanes, although they are several times larger and last for months, instead of days) and the lack of a similar hexagon around the South pole. But it will require more testing before it becomes widely accepted amongst planetary scientists as the correct explanation for what is being observed. With luck, the better data that we can expect over the next few years will resolve the question once and for all.