A giant white spot hurtling around the equator of Saturn and recently photographed by the Hubble Space Telescope may be the result of a rare "burp" of gas from the ringed planet's interior into its frigid upper atmosphere, scientists said yesterday.
The rapidly changing spot -- 6,000 miles wide, hundreds of miles thick and propelled by an eastbound jet stream at more than 1,000 miles per hour -- shows similarities to Jupiter's Great Red Spot, a massive, hurricane-type storm that has raged uninterrupted for at least 300 years.
But the white spot on Saturn may dissipate within months of its late-September appearance as it continues to spread out along the equator, said Andrew Ingersoll, professor of planetary science at the California Institute of Technology.
"The last time Saturn did anything of this magnitude was in 1933," he said, "and that one only hung around for a month or two." Another of the rare equatorial spots was observed in the 1870s; smaller spots at higher latitudes of the planet are more common.
Their evolution is still uncertain, but scientists know from ground observations and data from the Voyager spacecraft that Saturn -- second-largest of the solar system's nine planets -- is fluid to its core, a bubbling caldron of hydrogen, helium and other trace compounds.
"If you like, Saturn burped" and sent a huge geyser of gases high into the atmosphere, Dr. Ingersoll said. The white spot is "probably a high-altitude cloud of ammonia ice crystals" that formed as the gases cooled and spread in the fierce winds.
Although the long intervals between "burps" are a mystery, he suggested seasonal heating effects on the planet's atmosphere during its 30-year orbit of the sun.
"It's now summer in the northern hemisphere, and in some mysterious way, Saturn burps in the northern summer."
The 64 photos of the spot taken by Hubble's wide-field/planetary camera on Nov. 9 and 11 were the first real test of the orbiting telescope's highly touted ability to make repetitive observations of a target.
Although Hubble's main mirror contains a flaw that blurs its vision, bright objects such as planets can be effectively photographed and the data enhanced by computer to equal or surpass that from the biggest telescopes on Earth.
Dr. Ingersoll said scientists would continue space telescope studies of Saturn after the planet returns to good viewing conditions in three to four months, and later with the Cassini spacecraft, scheduled to orbit Saturn for four years and send a probe to the surface of its largest moon, Titan.