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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 Jupiterıs
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 Saturnıs winds could have a
simple explanation, note the scientists. The long seasonal
cycle in Saturnıs atmosphere (one Saturn year is about thirty
terrestrial years) and the equatorial shadowing by the planetıs
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, Saturnıs
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 Saturnıs 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 Saturnıs
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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