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Cassini
Finds that Storms Power Saturn's Jet Streams
05.08.07
New Cassini research suggests eddies, or giant
rotating storms, are the "engine" powering Saturn's jet
stream winds.
Full
Caption and Hi-Res Image Click Above
The
image shows small-scale, sheared-out cloud features
associated with turbulent eddies in the vicinity of one of
Saturn's eastward flowing jet streams, or "jets."
Image
credit: NASA/JPL/Space Science Institute
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"The new
information about how Saturn's jet streams are powered is exactly
the opposite of what we thought prior to Cassini," said
Anthony Del Genio of NASA's Goddard Institute for Space Studies,
New York, N.Y. Del Genio is a Cassini imaging team member and
lead author of a paper describing this research in press in the
journal Icarus.
Jet streams are motions in an atmosphere
that carry clouds rapidly eastward or westward. The eddies get
fed into the jet streams, in much the same way that rotating
gears can power a conveyor belt.
"While we thought
the conveyor belt--in this case, the jet streams--powered the
rotating eddies, we now think the opposite: the rotating eddies
power the jet streams," said Del Genio.
"Intuition
would say that the eddies take energy out of the jets, because of
the friction and tugging of the storms. Instead, what we find is
that they are pumping energy into the jets," said Andrew
Ingersoll, a Cassini imaging team member with the California
Institute of Technology, Pasadena, Calif. Ingersoll says that
while this process has been known to occur on Earth, it was only
recently shown to operate on Jupiter and is a new idea for
Saturn, where data from the earlier Voyager missions had failed
to detect the eddy-jet interactions.
The Cassini team
analyzed, for the first time, how storms and eddies interact with
Saturn's jet streams. By tracking the movements of these cloud
features in successive images separated by about 10 hours (about
one Saturn rotation), Cassini scientists have confirmed that the
eddies on either side of the jet give up their energy and
momentum, which helps keep the winds in the jet blowing.
"We
knew the eddies were powering the jets because they were pointing
in the same direction and carrying momentum in that direction. If
the eddies had been tapering the other way, we would have
concluded the opposite," added Ingersoll.
The
analysis of Cassini images covering most of Saturn's southern
hemisphere suggests that similar processes are occurring all over
the planet. This explains why Saturn's alternating pattern of
eastward and westward jets has remained constant over most of the
planet during the many decades that scientists have been able to
observe it. The same process was also recently found to occur on
Jupiter, in data obtained when Cassini flew by that planet on its
way to Saturn. The process is a well known feature on Earth in
the two jet streams that circle the globe in the northern and
southern hemisphere.
The findings suggest that
traditional ideas about the banded clouds of Jupiter and Saturn
need to be revised.
"We used to assume that the
bright cloud bands are regions where air rises and the dark bands
are where air sinks. But if the eddies power the jets in the way
we observe, the opposite must be true," said Del Genio. "And
indeed, we find thunderstorms only in the dark bands on both
planets, which has to mean that the air is rising there."
The Cassini-Huygens mission is a cooperative project of
NASA, the European Space Agency and the Italian Space Agency.
NASA's Jet Propulsion Laboratory, a division of Caltech, manages
the Cassini-Huygens mission for NASA's Science Mission
Directorate, Washington. The Cassini orbiter and its two onboard
cameras were designed, developed and assembled at JPL. The
imaging team is based at the Space Science Institute.
Source:
NASA / JPL
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