Saturn’s Equatorial Winds Are 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 Jupiters
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 Saturns winds could have a simple explanation, note
the scientists. The long seasonal cycle in Saturns atmosphere (one Saturn year
is about thirty terrestrial years) and the equatorial shadowing by the planets
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,
Saturns 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 Saturns 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 Saturns 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.