Cassini stares deep into the swirling hurricane-like vortex at Saturn's south pole, where the vertical structure of the clouds is highlighted by shadows. Such a storm, with a well-developed eye ringed by towering clouds, is a phenomenon never before seen on another planet.
Credit: NASA/JPL/Space Science Institute
A massive whirling vortex recently discovered over Saturn's south pole has features that are similar to hurricanes on Earth and unlike anything astronomers have seen before, a new study finds.
The polar vortex was first discovered by NASA's Cassini spacecraft on Oct. 11, 2006, as it flew over the gas giant's south pole. The mass of swirling clouds took scientists by surprise.
"This is something we have never seen before," said study team leader Ulyana Dyudina of Caltech. "Before Cassini, we didn?t know such a feature could exist on the poles."
Dyudina and her colleagues used more than three hours of observations of the vortex to examine its dynamics and structure. False color images of cloud heights showed a dark, red central eye similar to those at the center of terrestrial hurricanes, indicating that the upper atmosphere in the eye was nearly cloud-free.
Two eye walls encircle the eye and their clouds rotate in the same direction as the planet does, just as they do in hurricanes. Eyes and eye walls have never been observed anywhere else except on Earth.
Saturn's polar vortex is much bigger than any hurricane found on Earth though: Its eye alone measures about 2,500 miles (4,000 kilometers) in diameter, Dyudina told SPACE.com. The eye of a typical terrestrial hurricane is often just 2 or 3 miles wide.
Exactly what drives the vortex is uncertain, but it's not the warm ocean moisture that fuels hurricanes on Earth. Dyudina said that it could be supported by the moisture-driven motion of clouds in the lower atmosphere, but that scientists have no way to tell just yet.
The vortex also differs from hurricanes because it is stationary, constantly spinning over the same portion of the south pole. Polar vortices have been observed on other planets (they cover the North and South Pole of Earth), but unlike any of these vortices, Saturn's core eye is warm. In contrast, the polar vortices of Earth have cold cores.
As Saturn's year (about 29.5 Earth-years) shifts so that the sun no longer shines on the south pole and begins to illuminate the north pole, Dyudina hopes that Cassini's extended mission will allow scientists to observe indications of a north pole vortex and to continue observing the south pole vortex to see if they are permanent features and how the vortices fit into the planet's overall atmospheric circulation.
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