JOVIAN DREAMSSeventy-Day Jupiter Movie Pulls Patterns Out Of Chaos
A kaleidoscopic movie made from about 1,200 Jupiter images taken by NASA's Cassini spacecraft reveals unexpectedly persistent polar weather patterns on the giant planet.
Long-lived storms and globe-circling belts of clouds are familiar features around Jupiter's midsection, easily seen even in still pictures. Closer to the poles, though, still images show widespread mottling that appears chaotic.
"You'd expect chaotic motions to go with the chaotic appearance, but that's not what we see," said the planetary scientist who put the movie together, Dr. Ashwin Vasavada of the California Institute of Technology, Pasadena. "The movie shows that the small spots last a long time and move in organized patterns."
Cassini shot the images in infrared light to cut through Jupiter's upper haze and show the clouds underneath in black and white. The movie clip combines those images taken over a span of 70 days into a sequence less than a minute long. The version centered on the north pole and another version showing the entire planet are available online
Caltech planetary scientist Dr. Andrew Ingersoll, a member of the Cassini imaging team, said the movie also gives insight into storms' duration in Jupiter's high latitudes. "There are thousands of storms there the size of the biggest storms on Earth," he said.
"Until now, we didn't know the lifetime of those storms." The movie shows thousands of spots bumping into each other but generally moving together within each band of latitude. The spots occasionally change bands or merge with each other, but usually they last for the entire 70 days. Each spot is an active storm in Jupiter's atmosphere.
"The smaller and more numerous storms at high latitude share many of the properties of their larger cousins like the Great Red Spot at lower latitudes," Ingersoll said.
The mystery of Jupiter's weather is why the storms last so long. Storms on Earth last a week before they break up and are replaced by other storms. The new data heighten the mystery because they show long-lived storms at the highest latitudes, where the weather patterns are more disorganized than at low latitudes.
"Perhaps we should turn the question around and ask why the storms on Earth are so short lived," Ingersoll said. "We have the most unpredictable weather in the solar system, and we don't know why."
Dr. Carolyn Porco, Cassini imaging team leader and a planetary scientist at the Boulder, Colo., office of Southwest Research Institute, presented the movie at a meeting of Jupiter scientists in Boulder recently. "This is the first movie ever made of the motions of Jupiter's clouds near the poles, and it seems to indicate that one notion concerning the nature of the circulation on Jupiter is incomplete at best, and possibly wrong," she said.
The model in question suggests that Jupiter's alternating bands of east-west winds are the exposed edges of deeper, closely-packed rotating cylinders that extend north-south through the planet. In this laboratory-tested model, Porco said, "many such cylinders sit side-by-side, girdling the planet like rings of narrow solid-rockets strapped to the outside of a larger rocket."
At the planet's surface, one would see only east and west winds, alternating with latitude and symmetric about the equator.
"However, the east-west winds that the movie shows in the polar regions don't fit that model," Porco said. Jupiter's wind pattern may involve a mix of rotation-on-cylinders near the equator and some other circulation mechanism near the poles.
The movie required processing of images that Cassini took through an infrared filter during the last three months of 2000. The position of the spacecraft slightly north of the planet's equatorial plane gave an oblique view of Jupiter's north pole.
The images were projected into maps of the northern hemisphere as if viewed from directly above the pole. In that view, the high-latitude mottling becomes a concentric series of circular bands, each rotating in the opposite direction as adjacent bands.
Cassini, launched in 1997, passed Jupiter on Dec. 30, 2000, on its way toward its ultimate destination, Saturn. It will begin orbiting Saturn on July 1, 2004, and drop its piggybacked Huygens probe onto the haze-wrapped moon Titan about six months later.
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