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."

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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.