March 28, 2007—Everyone knows Saturn's rings, but what about the planet's hexagon?
A new image from NASA's Cassini spacecraft offers the first direct view of a six-sided feature that encircles Saturn's north pole. The 15,000-mile-wide (25,000-kilometer-wide) cloud formation was initially spied during the Voyager missions in the 1980s. But scientists remain baffled by the atmospheric forces driving the unusual feature.
"Nobody understands what it is," said Kevin Baines, a member of Cassini's visual and infrared mapping spectrometer team at NASA's Jet Propulsion Laboratory. "Until Cassini no one had seen any other dynamic feature last more than a year on Saturn," he added. "But this thing has been there for over a quarter century."
Voyager's images, taken from near the equator when the pole was illuminated by sunlight, could only reveal the upper layers of the hexagon and saw the shape at an angle. So this time scientists used Cassini's infrared camera to snap the clouds from almost directly overhead during polar night.
"We used Saturn's own internal glowing heat" as the light source, Baines said. "What we're seeing is the clouds in silhouette" and the bright regions where heat shines through the clearings.
The image indicates that the complex of clouds extends deep into the planet's atmosphere, at least 60 miles (100 kilometers) below the layers seen by Voyager. It also appears to have remained in the same position for more than 25 years.
"It's locked with the spinning of the planet, but clouds are moving inside the hexagon like cars around a racetrack," Baines said. This suggests that the feature could be a planetary wave similar to polar vortices on Earth. It also suggests that the hexagon could be a key tool for pinning down Saturn's actual rotation rate.
Gas giants are mostly fluids, Baines explained, but "50,000 kilometers [31,000 miles] down you'd finally hit something solid." The fluids spin at a different rate than the core, and without the ability to send a craft to the surface, scientists have relied on magnetic fields to estimate the planet's rotation.
Because the hexagonal formation extends so deep into the atmosphere, its dynamics could be "a manifestation of the true rotation of Saturn," Baines said. "It's just speculation, but there's hope here."
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