COMPUTER SIMULATION SUGGESTS MECHANISMS THE DRIVE JOVIAN JET STREAMS
From Lori Stiles, University Communications, UA, 520-621-1877
September 6, 2005

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Contact Information
Adam Showman, 520-621-4021
Related Web site
http://www.lpl.arizona.edu/people/faculty/showman.html/
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Turbulence driven by sunlight and thunderstorm activity may explain the
multiple east-west jet streams on Jupiter and Saturn and even produce
strong
winds extending hundreds or thousands of kilometers into the interior,
far
below the altitudes where the jets are driven.

Scientists have been trying to understand the mechanisms that form the
jet
streams and control their structure since the first high-resolution
images
of Jupiter were returned by the Pioneer and Voyager spacecraft in the
1970s.

On Earth, the jet streams -- narrow currents of air flowing from west
to
east in the midlatitudes -- form a major component of our planet's
global
circulation, and they control much of the large-scale weather
experienced by
the United States and other countries outside of the tropics. Similar
east-west jet streams dominate the circulation of the giant planets
Jupiter,
Saturn, Uranus, and Neptune, reaching up to 400 miles per hour on
Jupiter
and nearly 900 miles per hour on Saturn and Neptune. The question of
what
causes these jet streams and how deep they extend into the interior of
the
giant planets remain some of the most important unsolved problems in
the
study of planetary atmospheres.

Adam Showman and Yuan Lian of The University of Arizona in Tucson and
Peter
Gierasch of Cornell University in Ithaca, New York, explained how
cloud-layer turbulence can drive deep jets at the 37th annual meeting
of the
Division of Planetary Sciences of the American Astronomical Society,
held in
Cambridge, England.

Lian, Showman, and Gierasch performed computer simulations showing
that
horizontal temperature contrasts -- generated by sunlight or
differences in
thunderstorm activity -- can produce multiple jet streams that
penetrate
deep into the interior of a giant planet. In the simulations, the
temperature contrasts induce deep-penetrating circulation cells that in
turn
drive the deep jets. The study, which uses an advanced
three-dimensional
computer model, is among the first that allows an assessment of how
jets
formed near the top of the atmosphere interact with the interior.

"Most planetary scientists have assumed that jets pumped near the top
of
the atmosphere will remain confined to those shallow layers, and we've
shown
that this is not a valid assumption," Showman said.

NASA's Galileo Probe, which parachuted through Jupiter's atmosphere in
1995, was intended in part to help answer the question of how deep the
jet
streams extend. The probe found strong winds extending at least 150
kilometers (almost 100 miles) below the clouds. Planetary scientists
have
widely interpreted this measurement as evidence that the jets are
driven
from deep inside Jupiter's interior. The new study challenges this
interpretation.

"We still don't know whether the jets on the giant planets are driven
from
the top or within the deep interior," Showman said. "But our study
shows
that the deep winds measured by the Galileo probe could just as easily
result from shallow cloud-layer turbulence as from turbulence deep
inside
Jupiter's interior."

"This result contradicts a long-standing assumption on the part of
many
planetary scientists."

The new study also shows that, under realistic conditions, the
turbulence
can produce not only numerous jet streams but a strong eastward flow at
the
equator, as observed on Jupiter and Saturn. Such flows are notoriously
difficult to produce in atmospheric models, Showman noted.