Mapping deeper within Saturn's clouds.

Mapping deeper within Saturn's clouds

The world knows the spectacular whorls, streaks and stripes that bedeck the atmospheres of Jupiter, Saturn and Neptune from the photos taken by the Voyager and Pioneer spacecraft. But are those complexions only skin deep, overlying smoother features farther down in the atmosphere? Three space scientists report signs that, in the case of Saturn at least, not all the atmospheric action is on top.

The researchers, from the California Institute of Technology in Pasadena, recorded the planet's microwave radio emissions at wavelengths of 2 and 6 centimeters in 1986. They used the National Radio Astronomy Observatory's Very Large Array near Socorro, N.M., a complex of 27 separate dishshaped radio antennas. Other scientists have studied Saturn's microwave emissions in the past, but the newly reported data represent what the Caltech scientists call "the ultimate in resolution and sensitivity obtainable from Earth-based radio telescopes."

The emissions come from ammonia in Saturn's atmosphere, most of it near the top. The new results, however, indicate that the atmosphere below the clouds has a region depleted in ammonia, revealing an area of warmer temperatures across the planet, says Arie W. Grossman, who reports the findings with Duane O. Muhleman and Glenn L. Berge in the Sept. 15 SCIENCE.

The band is well known at visible wavelengths from Voyager photographs, Grossman points out, but the new data "suggest that the bands which you see in the visible aren't just variations in the clouds." The authors maintain that the dark band detected in the radio data, centered at about 35[deg.] latitude in the northern hemisphere, represents "a region of ammonia clearing, which allows us to see to deeper, hotter temperatures."

"Actually," Grossman says, "we were quite surprised to observe this. Until these observations, the general notion was that these features were all cloudto features." During much of the three years since the observations, the researchers have labored to reduce the "noise" in their data, due in part to the equipment and in part to scintillations in Earth's atmosphere. Berge says this is "the first radio map of Saturn that shows detailed latitude variations. Previous observations had indicated only a slight north-polar warming."

Grossman, Muhleman and others say they hope Congress funds a proposed spacecraft called Cassini, to be launched in 1996 and to arrive in orbit around Saturn with a microwave radiometer in 2002.