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."

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