Hot cyclones churn at both Saturn's polar regions

Now, the first detailed views of the gas giant's high latitudes from the Cassini spacecraft reveal a matched set of hot cyclonic vortices, one at each pole.
While scientists already knew about the hot spot at Saturn's south pole from previous observations by the W. M. Keck Observatory in Hawaii, the north pole vortex was a surprise. The researchers report their findings in the Jan. 4 issue of Science.

"We had speculated that the south pole hot spot was connected to the southern, sunlit conditions," said Glenn Orton, a senior research scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif., and co-investigator on Cassini's composite infrared spectrometer. "Since the north pole has been deprived of sunlight since the arrival of winter in 1995, we didn't expect to find a similar feature there."

The infrared data show that the shadowed north pole vortex shares much the same structure and temperature as the one at the sunny south pole. The cores of both show a depletion of phospine gas, an imbalance probably caused by air moving downward into the lowest part of Saturn's atmosphere, the troposphere. Both polar vortices appear to be long-lasting and intrinsic parts of Saturn and are not related to the amount of sunlight received by one pole or the other.

"The hot spots are the result of air moving polewards, being compressed and heated up as it descends over the poles into the depths of Saturn," said Leigh Fletcher, a planetary scientist from the University of Oxford, England, and the lead author of the Science paper. "The driving forces behind the motion, and indeed the global motion of Saturn's atmosphere, still need to be understood."

Though similar, the two polar regions differ in one striking way. At the north pole, the newly discovered vortex is framed by the distinctive, long-lived and still unexplained polar hexagon. This mysterious feature encompassing the entire north pole was first spotted in the 1980s by NASA's Voyager 1 and 2 spacecraft. Cassini's infrared cameras also detected the hexagon in deep atmospheric clouds early in 2007. (See related story)

Image Credit: NASA/JPL/GSFC/Oxford University

This image shows the unexpected "hot spot" at Saturn's north pole. Scientists were surprised to find that the north pole, despite being in winter darkness for more than a decade, is home to a hot, cyclonic vortex very similar to that found on Saturn's much sunnier south pole.

Created with data from the Cassini spacecraft's composite infrared spectrometer, this image, centered on the north pole, shows temperatures in Saturn's northern hemisphere near its 100-millibar tropopause, the top of its convective layer.

The false color denotes temperatures from 72 to 84 Kelvin (about 330 to 310 degrees below zero Fahrenheit). Latitudes are displayed from 30 degrees N at the edges to the north pole in the center. The hot pole is clear at the center of the projection. The distinctive polar hexagon is also evident in the initial warm "ring" around the pole between 75 and 80 degrees North latitude.

Although there is a similar hot pole in the southern hemisphere, there is no hexagon and the atmosphere is otherwise much warmer than in the north, having been heated during Saturn's southern summer for over a decade.

In their paper, Fletcher and his colleagues report that the bright, warm hexagon is much higher than previous studies had shown. "It extends right to the top of the troposphere," says Fletcher. "It is associated with downward motion in the troposphere, though the cause of the hexagonal structure requires further study."

Winter lasts about 15 years on Saturn. Researchers anticipate that when the seasons change in the coming years and Saturn's north pole is once again in sunlight, they will be able to see a swirling vortex with high eye walls and dark central clouds like the one now visible at the south pole. "But Saturn may surprise us again," says Fletcher.

"The fact that Neptune shows a similar south polar hot spot whets our appetite for the strange dynamics of the poles of the other gas giants," Fletcher says.

More information about Jupiter's poles will come from NASA's Juno mission, currently scheduled for launch in 2011 and arrival in 2016.

Fletcher's research was funded by the United Kingdom's Science and Technology Facilities Council.

Source: Jet Propulsion Laboratory
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