CONTACT: Lawrence A. Sromovsky, (608) 263-6785

UW EXPERIMENT POISED TO SAMPLE JUPITER'S STORMY WEATHER MADISON--Completing a six-year journey through the solar system, a University of Wisconsin-Madison experiment is poised to take the first direct radiation measurements of the stormy atmosphere of Jupiter.

The experiment, known as the Net Flux Radiometer, is one of a cluster of sensors aboard a capsule rocketing toward the largest planet in the solar system.

Jettisoned by the Galileo spacecraft July 13, the capsule or entry probe will slam into Jupiter's atmosphere Dec. 7 at 115,000 miles per hour and for 75 minutes will sample its stormy atmosphere, descending 372 miles before it is crushed and vaporized by the planet's atmospheric extremes.

The UW-Madison experiment is part of NASA's $1.5 billion Galileo mission, considered the most ambitious interplanetary undertaking in the history of space flight. Galileo, a 2.5-ton spacecraft consisting of an orbiting vehicle and the instrument-laden atmospheric probe, will be the first to orbit Jupiter and the first to directly investigate its atmosphere.

The $6-million Net Flux Radiometer, built at UW-Madison's Space Science and Engineering Center, is designed to measure the radiation that reaches Jupiter from the Sun as well as the thermal radiation or heat generated by the planet itself, a relic of its formation.

From measurements of how radiated energy--from both Jupiter and the Sun--is exchanged in the atmosphere, a team headed by UW-Madison scientist Lawrence A. Sromovsky hopes to determine where the atmosphere is being heated and cooled, and the roles of solar and internal heat sources in driving atmospheric motions

. "The main purpose of the Net Flux Radiometer is to define the heat engine that runs the planet's circulation," Sromovsky said.

The radiometer measurements will also help define the location and characteristics of cloud layers, and the amount of water vapor in Jupiter's atmosphere.

When combined with measurements from other probe experiments that measure winds, temperature, pressure, chemical composition, cloud particles and lightning, data from the Net Flux Radiometer will give scientists a new understanding of weather that is unlike anything on Earth.

The bizarre weather of Jupiter is characterized by giant storms, including one more than twice as big as the Earth and that has raged for more than 300 years, several layers of chemically distinct clouds, and alternating bands of high-speed winds--up to 300 miles an hour--that seem to change abruptly and uniformly with latitude.

Moreover, the planet's composition--90 percent hydrogen and 10 percent helium with traces of other chemicals--still retains clues about the cloud of dust and gas from which the sun and planets formed. The first direct measurements of Jupiter's atmosphere may thus yield a better idea of what the early solar system was like and how planet formation took place, said Sromovsky.

The 745-pound capsule with the Net Flux Radiometer on board is on a trajectory aimed at a spot just north of Jupiter's equator. On Dec. 7, the capsule will slice into the ammonia ice clouds of Jupiter's upper atmosphere. Within minutes friction will slow the craft to 100 miles per hour and the capsule will deploy a parachute and drift through multiple cloud layers to the dense atmosphere below.

"The probe will be entering in a region where the cloud structure can be quite variable," Sromovsky said.

Beneath the uppermost ammonia cloud layer, scientists think they may find at least two other layers of clouds, one composed of ammonium hydrosulfide and a layer composed of a mixture of ice and water. Finding the location of clouds, and the location and amount of water vapor will be an important diagnostic of Jovian weather, said Sromovsky.

"This will be our first look beneath Jupiter's clouds and we are bound to get a few surprises," Sromovsky said. "The real challenge will be to relate this one detailed vertical profile of observations to conditions elsewhere on a very large and complex planet. It's a problem akin to figuring out the Earth's weather from observations made by a single weather balloon."

--Terry Devitt, (608) 262-8282