Illustration of Galileo

Jupiter has both wet and dry regions, just as Earth has tropics and deserts, according to new images and data from the Galileo spacecraft released today. The data may explain why Galileo's atmospheric probe found much less water than scientists had anticipated when it dropped into the Jovian atmosphere in December 1995.

"We had suspected that the probe landed in the Sahara Desert of Jupiter," said Dr. Andrew Ingersoll, a professor at the California Institute of Technology and member of the Galileo science team. "But the new data show there is moisture in the surrounding areas. Jupiter is not as dry overall as we thought."

The area where the probe entered was a clearing in the clouds -- a dry spot through which deeper, warmer layers can be seen. By studying various areas, including those resembling the probe entry site, the Galileo orbiter has helped scientists understand the probe results. In fact, the air around a dry spot has 100 times more water than the dry spot itself, according to Dr. Robert Carlson, of NASA's Jet Propulsion Laboratory, principal investigator for the imaging spectrometer instrument onboard Galileo.

Such dry spots cover less than 1 percent of the Jovian atmosphere, and they appear to be regions where the winds converge and create a giant downdraft, according to Caltech graduate student Ashwin Vasavada. In fact, the water content of the giant, gaseous planet varies at least as much as the moisture varies from place to place on Earth.

"Winds rise from the deep atmosphere and lose water and ammonia," explained Dr. Glenn Orton, a Galileo interdisciplinary scientist at JPL and co-investigator on the photopolarimeter- radiometer instrument. "At the top, when they converge and drop back down, nothing is left to condense back into clouds, and a dry clearing is created. These dry spots may grow and diminish, but they recur in the same places, possibly because of the circulation patterns on Jupiter."

Ingersoll said the dry spots are found in a band in the northern hemisphere at 5 to 7 degrees latitude. When the Galileo probe was released near the tops of the clouds, it found dry air underneath. But at other locations, the weather might be rather Earth-like.

In the months since the probe's descent, Galileo mission scientists have debated whether the dry conditions it encountered were due to the downdraft concept, or whether Jupiter's water had somehow been concentrated deep in the gas planet's interior as it formed and evolved four billion years ago. "There was a cosmo- chemical explanation and a meteorological explanation, and our latest analysis clearly favors the idea that the dry spots are a consequence of weather-related activity," Ingersoll said.

"Fifty miles below the cloud tops, we could expect thunderstorms, lightning and rain," Ingersoll added. "But in contrast to Earth, individual Jovian storms and weather systems sometimes last for months, years or even centuries. The Great Red Spot, for example, has existed for at least the 300 years that we've been aware of it."

Despite the relatively warm temperatures and the presence of water on Jupiter, Ingersoll said it is "highly unlikely" that the planet could sustain life in its thick, gaseous environment without any solid surface. He expressed the opinion that any Jovian life forms would have to hover, and "while we might imagine an advanced life form that could adapt, pre-biotic compounds would not survive in that environment and, therefore, evolution could not take place there."

The latest data from Galileo also shed new light on the auroras which glow in a narrow ring around the poles of Jupiter. The auroras, essentially a light show stretching like a ribbon around the planet, are created when charged particles collide with atmospheric particles, causing them to light up. The new images show the night-side aurora for the first time.

Dr. Scott Bolton of JPL, co-investigator for the Galileo plasma and plasma wave instruments, said the latest findings show "Jupiter's auroras are a lot like the auroras we see on Earth as the northern lights. However, we now know that the auroral arc on Jupiter is thin and patchy, and we can also estimate its altitude is between 300 and 600 kilometers (185 to 370 miles)."

These auroras had previously been viewed in ultraviolet light by the Hubble Space Telescope, and in infrared light by Earth-based telescopes, but Galileo was able to capture images of the auroras in visible light and from a closer vantage point.

Galileo was launched in 1989 and entered orbit around Jupiter on Dec. 7, 1995. The Galileo mission is managed by JPL for NASA's Office of Space Science, Washington, DC. Images and other data received from Galileo are posted on the Galileo home page on the World Wide Web at

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