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10/23/1996 07:00:00

Thundercloud Photos, Theory Suggest That Jupiter Is "Wet" After All

PASADENA— The Galileo probe that dropped into Jupiter's atmosphere last December detected a surprisingly small amount of water. But scientists at the California Institute of Technology have new thundercloud photographs and a theory to suggest that the solar system's largest planet may be "wet" after all.

According to Andrew Ingersoll, a professor of planetary science at Caltech, the probe didn't find much water because it dropped into a cloud-free area of the Jovian atmosphere. Because water would likely be manifested in clouds, Ingersoll thinks the probe might have detected the predicted amount of water had it fallen into another region. As it stands, the probe data suggest that water is about 10 times more scarce than most scientists once thought.

"This was the most cloud-free area on the planet," Ingersoll says, "so the probe went into an anomalous region. A low cloud abundance means that there's less of the condensable substances, including water."

At the 28th annual meeting of the Division for Planetary Sciences of the American Astronomical Society, Ingersoll and colleagues are offering photographic evidence and a theory to back up their assertion that water on Jupiter was merely hidden from the probe. The photographic evidence is a series of near-infrared images taken by the Galileo orbiter of the Great Red Spot. The images were released as part of a report just published in Science by the Galileo imaging team under the leadership of Dr. Michael Belton of Kitt Peak National Observatory near Tucson, Arizona.

The infrared images show several dozen thunderclouds to the northeast of the Great Red Spot. These round clouds jut about 20 miles above the cloud cover, and are of similar diameter.

The researchers are not sure yet whether the clouds are made of water. But they do know that they are high and that they evolve rapidly, like terrestrial thunderstorms. Ingersoll and Caltech graduate student Ashwin Vasavada documented how the features changed over a 70-minute interval.

"Water is the most likely cause of these explosive convective events," says Ingersoll. "Other gases present in Jupiter's atmosphere just don't have the energy."

The new theory, which Ingersoll developed jointly with graduate student Adam Showman, argues that the cloudless region the probe dropped into is part of a huge downdraft that extends deep into Jupiter's fluid interior.

Large-scale downdrafts, like those over Earth's deserts, are dry because the moisture falls out in the neighboring updraft regions, Ingersoll explains. "The trick here is to keep the downdraft going to the level where the Galileo probe failed, which is 40 miles below the cloud base. You have to keep the dry descending air from mixing with the moist air from Jupiter's interior.

"That can happen if the descending air is less dense (lighter) than the surroundings, but that violates another rule that says less dense air wants to rise.

"We are saying that the region where the probe went in is a heat engine running backward, like a refrigerator. It takes energy from the convection at neighboring latitudes, and uses it to stuff the less dense air downward. The energy released in the neighboring latitudes is not dissipated, so it has to go somewhere."

Thus, the researchers believe that the Galileo probe could have missed the water, and that Jupiter can still be viewed as a planet of violent thunderstorms, lightning, hurricanes, and rain.

"But if they get a good sample of different places on the planet and still fail to find water," Ingersoll says, "then I'll throw in the towel."

Written by Robert Tindol