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NASA Spacecraft Will Probe Saturn's Winds

Posted on: Tuesday, 17 June 2003, 06:00 CDT

By WILLIAM McCALL

Associated Press -- Astronomers say the winds of Saturn appear to be slowing dramatically just as NASA's Cassini spacecraft approaches the ringed gas planet. Other researchers who study giant planets say the finding is surprising because little change has been detected in the winds of neighboring planets like Jupiter.

A comparison of images taken from one of the Voyager missions in the early 1980s and photos taken by the Hubble Space Telescope from 1996 to 2002 indicate that winds have slowed by about 40 percent at the equator of Saturn.

Saturn is the solar system's windiest planet, with wind speeds peaking at 1,000 mph. With the sudden change, the peak winds now are whipping around the planet at about 600 mph.

By comparison, the highest surface wind ever recorded on Earth was a gust of 231 mph clocked on Mount Washington in New Hampshire on April 12, 1934.

The researchers who performed the Saturn analysis say they do not know why its winds are slowing, unlike the steady winds on Jupiter.

"I think most atmospheric scientists would be willing to bet a substantial amount of money that a giant planet's winds don't do things like this," said the study's co-author, Richard French, a Wellesley College professor who has been a principal Hubble researcher for the past seven years.

More than a century of recorded observations of Jupiter, including Voyager and Galileo spacecraft surveys, have shown its winds move at a relatively constant speed without changing.

Recent studies of the other two gas giants in the outer solar system, Uranus and Neptune, also indicate wind speed is fairly constant, although their winds circulate in the opposite direction of Jupiter and Saturn.

In the new study published in the June 5 issue of the journal Nature, French and Agustin Sanchez-Lavega of the Universidad del Pais Vasco in Bilbao, Spain, suggest that the unique rings of icy particles that encircle Saturn may influence the wind speed by casting shadows on the planet's surface during the 30 Earth years it takes Saturn to make a single orbit around the sun.

Researchers who did not participate in the study agree that shadows could cool the Saturn atmosphere enough to slow the winds.

"The ring shadow moves from the northern hemisphere to the southern hemisphere and back again during the Saturn seasons," said Andy Ingersoll, a Caltech astronomer and expert on the atmospheric dynamics of other planets.

"That's a pretty unusual environment, and it probably has an effect," Ingersoll said.

Heat drives the winds on planets. But the two main heat sources - the sun and geothermal energy - are fairly weak on Saturn and the other distant gas giants. Temperatures in Saturn's upper atmosphere are as low as minus 288 degrees Fahrenheit.

Another artist representation of the Cassini-Huygens mission. Credit: NASA

And unlike the inner planets such as Earth that have cores of molten rock, the gas giants have only small cores that are likely made of hydrogen that is compressed into a metallic form by the enormous pressure of their atmospheres.

French and Sanchez-Lavega estimated the reduction in Saturn's wind speeds by comparing motions at the top of its deep cloud layer, which is 155 miles thick.

They tracked the movement of clouds or prominent storm systems on the Voyager photos from 1980-81, then compared them to the speed of cloud and storm system motion from the Hubble's higher-resolution images taken in 1996-02.

The Cassini spacecraft is due to arrive next year for a long mission orbiting Saturn, and may provide more direct measurements to help explain the mystery of the diminishing winds, Ingersoll said.

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On the Net:

Cassini-Huygens Mission

Nature magazine

Jet Propulsion Laboratory

NASA

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