Image: NASA

As North America endures its annual battering from hurricanes with names like Georges, the Galileo spacecraft is returning data about their distant cousins on Jupiter. In the images above, lightning from huge low-pressure storms flashes through the dark clouds on the night side of Jupiter. The lightning originates in Jupiter's water cloud, which is 50 to 75 kilometers (30 to 45 miles) below the ammonia cloud. The individual strikes emit as much light energy as 30 million 100-watt lightbulbs burning for one second--hundreds of times brighter than lightning on Earth. Each of the three fast-moving storms shown above was photographed at two-minute intervals by the space probe.

Even so, the latest data returned by Galileo, which has been orbiting Jupiter and its moons for 2-1/2 years, indicate that Jovian weather is controlled by processes very similar to those on Earth--although on a much greater scale. Low pressure systems spawn violent storms while high pressure areas tend to be more stable and slow moving.

"Lightning is an indicator of convection and precipitation," says Andrew Ingersoll, a professor of planetary science at the California Institute of Technology and member of the Galileo Imaging Team. "These processes are the main sources of atmospheric energy, both on Earth and on Jupiter."

Image: NASA

"WHITE OVAL." This newly formed high pressure storm on Jupiter is the biggest storm in the solar system.

As on Earth, Jupiter's low pressure storms, or cyclones, rotate in a counterclockwise direction in the Northern Hemisphere and clockwise in the Southern; high pressure storms, known as anticyclones, have the opposite rotation. Whereas terrestrial hurricanes are powered by warm, moist air drawn up from the ocean surface, those on Jupiter obtain their energy from the warm interior of the planet. On Jupiter these cyclones are amorphous, turbulent regions that are spread out in the east-west direction. "We caught one of these bright clouds on the day side and saw it flashing away on the night side less than two hours later," says Ingersoll.

In contrast, Jupiter's high pressure storms are larger, more stable, and longer lived. Three of these storms, called "white ovals" because of their color and egg shapes, have been observed in one band around Jupiter's midsection for half a century. Two of them--each about two-thirds the diameter of Earth--recently combined to form a feature as large as Earth.

Image: NASA

RAIN CLOUD? Meteorologists say this extremely deep cloud must contain liquid water.

"The newly merged white oval is the strongest storm in our solar system, with the exception of Jupiter's 200-year-old 'Great Red Spot' storm," says Glenn Orton, a senior research scientist at NASA's Jet Propulsion Laboratory, which manages the Galileo mission. "This may be the first time humans have ever observed such a large interaction between two storm systems."

The white ovals are cold storms--the center of the newly formed one has a temperature of minus 157 degrees Celsius (minus 251 Fahrenheit) at its center, about one degree colder than its surroundings. The NASA scientists believe that they obtain their energy from small, warm storms thrown off by the more violent cyclones.

It's axiomatic to meteorologists that where there is lightning, there is precipitation. The question with Jupiter is whether it is snow or rain. "Models of terrestrial lightning suggest that to build up electrical charge, both liquid water and ice have to be present," says Ingersoll.

The recent Galileo data may have provided an answer. Galileo's imaging system captured a cloud so deep that it almost certainly contains water, according to Galileo imager Don Banfield of Cornell University. Now, says Ingersoll, "we know the water is there, and we know where it's raining. This is a big step toward understanding how Jupiter's weather gets its energy."