Saturn's white spot: driven by the sun?

A puzzling white spot mars Saturn's normally featureless face about every 30 years. Astronomers now report data suggesting that a seasonal change in solar heating may trigger storms that produce this short-lived phenomenon.

The most recent giant spot erupted over Saturn's equator last October, catching many astronomers by surprise (SN: 10/13/90, p.228). But Agustin Sanchez-Lavega and his colleagues were ready. They had predicted the spot's occurrence based on the frequency of the last four to blemish the ringed planet. By June 1990, the researchers had one telescope in Japan and three more in France trained on Saturn, awaiting the spot's debut.

Their 48 nights of observations, made over a range of visible-light and near-infrared wavelengths from late September through November, provide the most detailed information yet on the phenomenon, asserts Sanchez-Lavega, of the University of Pais Vasco in Bilbao, Spain.

One widely accepted theory attributes the spot to storms that shoot a blob of warm gas up from Saturn's lower atmosphere and through a thick upper mantle of old, smog-stained ammonia ice. As the gas expands in the upper atmosphere, fresh crystals of ammonia condense on the cooling vapor, forming the white region visible from Earth.

Observations made during early October 1990 -- soon after the spot appeared -- uphold that idea, the researchers report in the Oct. 3 NATURE. For example, the spot remained bright in blue and violet light -- even though the old ammonia clouds in Saturn's hazy atmosphere readily absorb these wavelengths. This seems to indicate that the spot sits above the older ammonia clouds, where the theory says such a storm plume should emerge. Observations last November with the Hubble Space Telescope confirm these and other ground-based findings by Sanchez-Lavega's team, says Chris Barnet of NASA's Goddard Space Flight Center. His team will report its findings in an upcoming ICARUS.

Sanchez-Lavega's team was the first to link the spot's appearance to Saturn's orbital period around the sun -- 30 Earth years. Although spots appear alternately over the equator and at higher latitudes, they always form during summer in Saturn's northern hemisphere.

This suggests the sun may drive the formation of Saturn's spots, which typically last several months, the astronomers say. Indeed, they note, the northern summer season comes some 15 Earth years after the equator receives its least sunlight and soon after the sun begins warming the upper atmosphere over the equator. These factors may conspire to create an unusually strong temperature gradient between the upper and lower layers of Saturn's atmosphere at the equator, triggering the sudden updrafts of gas believed responsible for the spots.

If so, Barnet says, spots also should form 30 degrees south of the equator, where computer simulations suggest an even higher temperature gradient occurs. A full test of the theory, however, awaits exploration by spacecraft, since Saturn's rings would block the view of such spots.