Saturn's mysterious hexagon.An odd, six-sided, honeycomb-shaped feature circling the entire north pole of Saturn has captured the interest of scientists with NASA's Cassini mission. The hexagon is similar to Earth's polar vortex, which has winds blowing in a circular pattern around
the polar region. On Saturn,
the vortex has a hexagonal rather than circular shape.
The hexagon is nearly 25,000 kilometers (15,000 miles) across. Nearly four Earths could fit inside it.
Here, brightness indicates the amount of 5-micron (seven times the wavelength visible to the human eye) radiation, or heat, generated in the depths of the warm interior of Saturn that escapes the planet. Clouds at a depth equivalent to 3-Earth-atmospheres pressure block the light radiating from below, revealing themselves in dark silhouette against the background thermal glow of the planet. These deep clouds lie some 75 kilometers (47 miles) underneath the typical ammonia hazes and clouds seen in visual imagery and are likely composed of ammonia-hydrosulfide, although some may be composed of water, as on Earth. A prominent feature seen in this polar view is a strange hexagon wave feature circumscribing the north pole. This nighttime movie was acquired over a one-hour period on Nov. 10, 2006, from an average distance of 1.03 million kilometers (621,000 miles) above Saturn's clouds. The six-sided shape remains a mystery. Scientists think the hexagon is a meandering jet stream at 77 degrees north latitude, but they don't know what controls the path the stream takes. These images also show new phenomena for scientists to decipher, such as waves that can now be seen radiating from the corners of the hexagon where the jet takes its hardest turns. These images confirm the presence of a multi-walled structure in each of the hexagon's six sides, and the structure now can be seen extending to the top of Saturn's cloud layer. The images show that the inside of the hexagon is darker than the outside. The new images also show a large spot inside the hexagon that could be related to a dark spot seen inside the hexagon in 2006 in an image taken by Cassini's VIMS instrument. An earlier Voyager mosaic showed a large spot outside the hexagon. That spot existed at least until 1991 before disappearing into the long winter polar night.NASA's Voyager 1 and 2 spacecraft imaged
the feature over two decades ago.
The fact that it has appeared in Cassini images indicates that it is a long-lived feature. A second hexagon, significantly darker than
the brighter historical feature, is also visible in
the Cassini pictures.
The spacecraft's visual and infrared mapping spectrometer is
the first instrument to capture
the entire hexagon feature in one image.
"
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."Scientists are still trying to figure out what causes
the hexagon, where it gets and expels its energy and how it has stayed so organized for so long. They plan to search
the new images for clues, taking an especially close look at
the newly identified waves that radiate from
the corners of
the hexagon -- where
the jet takes its hardest turns -- and
the multi-walled structure that extends to
the top of Saturn's cloud layer in each of
the hexagon's six sides. Scientists are also particularly intrigued by a large dark spot that appeared in a different position in a previous infrared image from Cassini. In
the latest images,
the spot appears in
the 2 o'clock position.
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