"They're so giant that they still have retained some of their heat of formation," Ingersoll said. "So they have their own internal heat that can generate these giant storms."
The moisture requirement explains why gigantic, hurricane-like storms don't seem to occur on Venus or Mars, he added.
"The giant planets have moisture down below the clouds," Ingersoll said. "But Venus doesn't. Venus is dry as a bone, hot and dry. It's not comparable. And Mars is cold and dry."
Saturn storm mysteries
Saturn's Great White Spot tends to erupt every few decades, shattering long periods of calm and quiescence. Scientists still aren't sure why some storms on the giant planets should be so big, and so infrequent.
"For some reason, they store up that energy for a long time, then let it loose in a violent, huge storm," Ingersoll said. "It didn't have to work out that way; they could let off a little popcorn now and then. But they don't do that."
He's hoping that Cassini will help resolve this question — just one of many that scientists are grappling with as they try to understand the weather systems on other planets.
"We're working right now on this giant Saturn storm, with that exact question in mind," Ingersoll said.
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