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Wellesley College

CONTACT:
Mary Ann Hill, mhill{at}wellesley.edu, 781-283-2376

For immediate release: June 4, 2003

Saturn's Equatorial Winds Decreasing

Spanish-American Team's Findings Raise Questions About Planet's
Atmosphere

WELLESLEY, Mass. -- Saturn, one of the windiest planets, has recently
had an unexpected and dramatic change in weather: its equatorial
winds have subsided from a rapid 1700 km/hr during the Voyager
spacecraft flybys in 1980-81 to a modest 990 km/hr from 1996 to 2002.
This slow-down in the winds has been detected by a Spanish-American
team of scientists, including Richard French of Wellesley College in
Massachusetts, who report their findings in the June 5 issue of the
journal, Nature. (5 June 2003, Vol. 423, pp. 623-625) 

Using Hubble Space Telescope (HST) images of the ringed giant planet,
the scientists (A. Sanchez-Lavega, S. Perez-Hoyos, J. F. Rojas, and
R. Hueso from Universidad Pais Vasco in Bilbao, Spain, and French
from Wellesley College), measured the motions of cloud features and
storm systems on the ringed giant planet. 

"One of the major mysteries in atmospheric sciences is why the giant
planets Jupiter and Saturn -- huge spheres composed mainly of
hydrogen and helium -- have an alternating pattern of east-west
winds, which vary in direction with latitude," explains French.
"Unlike winds on terrestrial planets like Earth, which are powered
primarily by sunlight, winds on the giant planets have an additional
energy source in the heat that escapes from their deep interiors.
Even though the strength of this interior heat is a mere fraction of
the sunlight on Earth, the giant planets' winds are ten times more
intense than terrestrial winds."

The role of these interior energy sources in sustaining these strong
winds in giant planets and understanding why the maximum speed is
reached at the equator constitute major challenges to theories of
atmospheric motion in planets and stars. 

There currently are two quite different explanations for the system
of jets on giant planets. At one extreme, the winds are thought to
extend very deep into the interior of the planet, tapping the heat
released from the planet to drive their motions. At the other
extreme, the atmospheric circulation is modelled as on the
terrestrial planets, driven by the solar heat deposited in a shallow
upper atmospheric layer. Both explanations have important drawbacks,
and neither can account for the strong equatorial winds.

One way to test these models is to analyse the long-term behaviour of
the winds by measuring their sensitivity to changes in the amount of
sunlight due to seasonal effects or to other influences. Previous
studies showed that Jupiter's winds are quite stable, and not
sensitive to seasonal changes, but little was known about Saturn,
whose muted cloud features are much harder to measure. 

Using the high-resolution capability of the Wide Field Planetary
Camera onboard the HST, the Spanish-American team has been able to
track enough cloud elements in Saturn to measure the wind velocity
over a broad range of latitudes. The equatorial winds measured in
1996-2001 are only half as strong as was found in 1980-81, when the
Voyager spacecraft visited the planet. In contrast, the windy jets
far from the equator have remained stable and show a strong
hemispheric symmetry not found in Jupiter.

The different behaviour of Saturn's winds could have a simple
explanation, note the scientists. The long seasonal cycle in Saturn's
atmosphere (one Saturn year is about thirty terrestrial years) and
the equatorial shadowing by the planet's giant rings could account
for the sudden slowdown in the equatorial winds. Rather than being
tied to the deep interior of Saturn, driven primarily by internal
heat, the equatorial winds could be in part a shallow surface
phenomenon, affected as well by seasonal variations in sunlight. In
fact, Saturn's equatorial region has been the location of giant storm
systems, such as those seen in 1990 and 1994. These storms may have
induced strong dynamical changes, perhaps resulting in the observed
weakening of the equatorial winds. 

Another possibility is that the winds measured by the team are at
higher altitudes where the winds are likely to decrease in speed. In
the Nature article, the team notes that Saturn's non-equatorial winds
have remained unchanged during this period, resembling Jupiter in
this respect, which hints that these winds could be more deeply
rooted. 

New HST observations by the Spanish-American team are planned for the
end of this year. The new data and the high-resolution imaging to be
obtained by the NASA-ESA Cassini orbital mission expected to arrive
at Saturn in mid-2004 will enable them and other scientists to learn
whether the current wind pattern will persist or will change over the
course of Saturn's seasonal cycle. In either case, notes French, "
these results will be important tests of our theoretical
understanding of winds on the giant planets." 

***

Richard French, professor of astronomy at Wellesley College, is
available for interviews. He can be reached at 781-283-3747 or
781-444-4660.

Wellesley College is a prominent liberal arts college and has been a
leader in the education of women for more than 125 years. The
College's 500-acre campus near Boston is home to about 2,300
undergraduate students.

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