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       Waves of up-and-down winds that span great ranges in air pressure may explain the surprisingly clear, dry areas near Jupiter's equator, new research based on data from NASA's Galileo entry probe indicates.

       Scientists have been trying to understand the stability of these clear "hot spots" ever since the probe plunged into one of them nearly five years ago.

       "If you could ride in a balloon coming into one of the hot spots, you would experience a vertical drop of 100 kilometers (about 62 miles) -- more than 10 times the height of Mount Everest," explained Dr. Andrew Ingersoll, a Galileo science team member at the California Institute of Technology in Pasadena.

       An explanation of how these deep holes in Jupiter's clouds could persist is reported in today's edition of the journal Science by Dr. Adam Showman, of NASA's Ames Research Center, Moffett Field, Calif., and Dr. Timothy Dowling, director of the University of Louisville's Comparative Planetology Laboratory in Kentucky.

       "This helps answer one of the big puzzles we ended up with after the probe entry," said Dr. Torrence Johnson, Galileo project scientist at NASA's Jet Propulsion Laboratory in Pasadena.

       Showman and Dowling propose that air moving west to east just north of Jupiter's equator is also moving dramatically up and down every few days. Water and ammonia vapors condense into clouds in Jupiter's white equatorial plumes as the vapors rise. Then the wrung-out air drops, forming the clear patches. After crossing those hot spots, the air rises again and returns to its normal cloudy state.

       The researchers developed a computer simulation that recreates known traits of the hot spots and plumes when the simulation starts with a large-scale pressure difference. Dowling said smaller pressure differences do not produce stable patterns.

       "There are no wimpy hot spots, only strong ones," he quipped.

       During the Galileo probe's hour-long descent on Dec. 7, 1995, it returned the only direct measurements ever made from within Jupiter's atmosphere. Scientists quickly realized the entry point was a special place. On a planet mostly wrapped in high clouds, the probe hit the southern rim of a clear spot where infrared radiant energy from the planet's interior shines through.

       The computer simulation reveals that the probe's entry site is probably even more unusual than previously thought. Both the probe and the computer model show that the head winds on the southern rim of a hot spot get stronger and stronger with depth into the planet. But in the model, this trend is reversed on the northern rim. "These results underscore the importance of future multi-probe missions to Jupiter," said Dowling.

       The hot spots were known previously, but their depth was a surprise, noted Ingersoll. A better name for them might be bright spots, since the temperature at their visible depth is only about 32 degrees Fahrenheit (zero Celsius), though that is relatively balmy compared to the neighborhood of minus 200 degrees Fahrenheit (minus 130 degrees Celsius) at surrounding cloud tops.

       Each hot spot is about the size of North America and lasts for months. The hot spots alternate with larger cloudy plumes in a band near Jupiter's equator. In some ways, the dry areas where wrung-out air masses are descending resemble subtropical deserts on earth, Ingersoll said. But, unlike Earth, Jupiter has no firm surface to stop the air's fall.

       All the hot spots combined make up less than one percent of Jupiter's global area, but understanding how they remain stable is important for understanding the whole planet's atmospheric dynamics, Dowling said.

       Also, the hot spots have "mathematical cousins" in some equatorial movements in Earth's oceans and atmosphere, he said. "How distantly or closely related they are is a question we are just beginning to study."

       The Galileo mission includes an orbiter that has been studying Jupiter and its moons since it finished relaying information from the atmospheric probe nearly five years ago. The mission is managed by JPL for NASA's Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology.

Images of a Jupiter hot spot are available at: . Additional information about the Galileo mission is available at . Information on the University of Louisville's Comparative Planetology Laboratory is available at .

9/8/00 GW/JP