MADISON, Wisconsin—Images of Jupiter's night side taken by the Galileo spacecraft reveal that the planet's lightning is controlled by the large-scale atmospheric circulation and is associated with low-pressure regions.
The new findings were reported October 13, 1998 by Andrew Ingersoll at the 30th annual meeting of the American Astronomical Society's Division for Planetary Sciences.
"Lightning is an indicator of convection and precipitation," says 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."
In a terrestrial hurricane, Ingersoll explains, the low pressure at the center draws air in along the ocean surface, where it picks up moisture. Energy is relased when the moisture condenses and falls out as rain.
On Jupiter, energy is transferred from the warm interior of the planet to the visible atmosphere in a similar process. The new findings show that lightning occurs in the low-pressure regions on Jupiter, too.
"On both planets, the air spins counterclockwise around a low in the northern hemisphere and clockwise around a low in the southern hemisphere," Ingersoll says. "The lows are called cyclones and the highs are called anticyclones."
On Jupiter the cyclones are amorphous, turbulent regions that are spread out in the east-west direction. In the Voyager movies they spawn rapidly expanding bright clouds that look like huge thunderstorms. The Galileo lightning data confirm that convection is occurring there.
"We even 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, the Jovian anticyclones tend to be long-lived, stable, and oval-shaped. The Great Red Spot is the best example (it is three times the size of Earth and has been around for at least 100 years), but it has many smaller cousins. No lightning was seen coming from the anticyclones.
"That probably means that the anticyclones are not drawing energy from below by convection," says Ingersoll. "They are not acting like Jovian hurricanes."
Instead, the anticyclones maintain themselves by merging with the smaller structures that get spun out of the cyclones. "That's what we see in the Voyager movies, and the Galileo lightning data bear it out. Whether the precipitation is rain or snow is uncertain," says Ingersoll.
"Models of terrestrial lightning suggest that to build up electrical charge, both liquid water and ice have to be present. Rain requires a relatively wet Jupiter, and that's a controversial subject.
"Water is hard to detect from the outside because it is hidden below the ammonia clouds. And the Galileo probe hit a dry spot where we didn't expect much water."
Fortunately the Galileo imaging system caught glimpses of a cloud so deep it has to be water, according to findings to be reported at the conference by Dr. Don Banfield of Cornell University and an imaging team affiliate. Banfield showed images of the water cloud near the convective centers in the cyclonic regions.
These results appear in the September issue of Icarus, the International Journal of Solar System Studies.
"We know the water is there, and we know where it's raining," says Ingersoll. "This is a big step toward understanding how Jupiter's weather gets its energy."