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Saturn's hexagon emerges from winter darkness
Posted: December 10, 2009

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After a long wait for Saturn's north pole to emerge from winter, Cassini has captured the most detailed images yet of the curious rotating hexagon structure dancing around the planet's high northern latitudes.

The shape marks out the location of a jet stream flowing around the north pole and reveals concentric circles, walls and streamers not seen before; the last visible light image of the structure was taken by the Voyager spacecraft around 30 years ago, and the hexagon lives on.

The images were taken in visible light on 3 January 2009. The smallest features resolved in the image have a horizontal scale of about 100 kilometres. Image: NASA/JPL/Space Science Institute.

“The longevity of the hexagon makes this something special, given that weather on Earth lasts on the order of weeks,” says 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 jet stream powers around the ringed planet at about 100 metres per second (220 mile per hour) at a latitude of about 77 degrees north, and spans a diameter wider than the equivalent of two Earths. While the hexagon has been hidden from visible light observations, Cassini's infrared instruments studied its heat patterns in 2006, but those images showed the structure to be nearly stationary over a 12-day period, and extending deep into the atmosphere. Further study will allow scientists to understand what causes the jet stream to take on a hexagonal shape and how it is powered.

The new image, which was taken in January as Saturn approached equinox, is composed of 55 images that reveal new wave-like structures that appear to radiate from the corners of the hexagon, and a multi-walled structure that extends to the top of Saturn's cloud layer along each side of the hexagon. Another point of interest is a large dark spot that has moved to a new position inside the hexagon since it was last observed by Cassini's infrared cameras; in a previous Voyager image a dark spot was seen outside of the hexagon, which could be related.

“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,” says Baines. “Solving these unanswered questions about the hexagon will help us answer basic questions about weather that we’re still asking about our own planet.”

Unlike the Earth, Saturn has no land masses or oceans to complicate its weather system, so planetary scientists hope that the conditions here should present more of an elementary model of circulation patterns.