January 03, 2008
Despite more than a decade of winter darkness, Saturn's north pole is home
to an unexpected hot spot remarkably similar to one at the planet's sunny
south pole. The source of its heat is a mystery. 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.
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.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency
and the Italian Space Agency. JPL, a division of the California Institute of Technology in
Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington.
The Cassini orbiter was designed, developed and assembled at JPL. The science team for
Cassini's composite infrared spectrometer team is based at NASA's Goddard Space Flight
Center, Greenbelt, Md.
Media contacts: Rosemary Sullivant/Carolina Martinez 818-354-5011
Jet Propulsion Laboratory, Pasadena, Calif.
2008-001