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University of Iowa News Release

June 28, 2004

Scientists Find That Saturn's Rotation Period Is A Puzzle

Scientists studying the rotational modulation of radio emissions from Saturn with the Cassini spacecraft are puzzled by apparent long-term variations in the radio rotational period of Saturn.

University of Iowa Space Physicist Don Gurnett (left), head of the team that is analyzing these radio emissions, says: "The rotational modulation of radio emissions from distant astronomical objects has long been used to provide very accurate measurements of their rotation period. The technique is particularly useful for the giant gas planets, such as Jupiter and Saturn, which have no surfaces and are covered by clouds that make direct visual measurements impossible."

The currently accepted rotation period of Saturn came from radio measurements obtained during the Voyagers 1 and 2 flybys of Saturn in 1980-81, and is 10 hour 39 minutes and 24 seconds plus or minus 7 seconds.

The first hint of something strange at Saturn came in 1997 when Alain Lecacheux, Patrick Galopeau, and Monique Aubier, from Observatoire de Paris, published a paper in the Austrian Academy of Science Press reporting that Saturn's radio rotation period was about one percent longer than the value obtained from Voyager. Now, during the Cassini approach to Saturn, where the radio signals from the planet are very strong and well defined, Gurnett and his colleagues have used the Radio and Plasma Wave Science (RPWS) instrument on Cassini to determine a new current value for the radio rotation period. The new value is 10 hours 45 minutes and 45 seconds plus or minus 36 seconds.

Michael D. Desch, Cassini RPWS team member, a scientist at NASA/Goddard Space Flight Center in Greenbelt, Md., who carried out the detailed analysis of the Cassini RPWS data, was also lead author of the earlier Voyager measurements. He says that in order to determine the rotation period he analyzed the recurring pattern of radio bursts from the planet for a little over a year from April 29, 2003, to June 10, 2004. "We all agree that the radio rotation period of Saturn is longer today than it was in during the Voyager flyby in 1980," he says.

Commenting on the new Cassini findings, Gurnett says: "Although Saturn's radio rotation period has clearly shifted by a substantial amount over the 24 years since the Voyager measurements, I don't think any of us could conceive of any process that would cause the rotation of the entire planet to actually slow down. So it appears that there is some kind of slippage between the deep interior of the planet and the magnetic field, which controls the charged particles responsible for the radio emission." He suggests that the solution may be tied to the fact that Saturn's rotational axis is nearly identical to its magnetic axis, differing by an angle of only one degree. Jupiter, which has its magnetic axis tilted by a more substantial 10 degrees relative to its rotational axis shows no comparable irregularities in its radio rotation period.

Writing in the May 1985 issue of "Geophysical Research Letters," Alex J. Dessler, a senior research scientist at the Lunar and Planetary Laboratory, University of Arizona, argued that the magnetic fields of gaseous giant planets, such as Saturn and Jupiter, are more like that of the sun than of the Earth. The sun's magnetic field does not rotate as a solid body. Instead, its rotation period varies with latitude. Commenting on the work of Gurnett and his team, Dessler said, "This finding is very significant because it demonstrates that the idea of a rigidly rotating magnetic field is wrong. Saturn's magnetic field has more in common with the sun than the Earth. The measurement can be interpreted as showing that the part of Saturn's magnetic field that controls the radio emissions has moved to a higher latitude during the last two decades."

"I think we will be able to unravel the puzzle, but it's going to take some time. With Cassini in orbit around Saturn for four years or more, we will be in an excellent position to monitor long-term variations in the radio period, as well as investigate the rotational period using other techniques," Gurnett says.

The radio sounds of Saturn's rotation -- resembling a heartbeat -- and other sounds of space can be heard by visiting Gurnett's Web site at: http://www-pw.physics.uiowa.edu/space-audio.

Cassini, carrying 12 scientific instruments, is on its way to the June 30, 2004 planetary rendezvous, when it will become the first spacecraft to orbit Saturn and begin a four-year study of the planet, its rings and its 31 known moons. The $1.4 billion spacecraft (part of the $3.3 billion Cassini-Huygens Mission) recently flew past Phoebe, Saturn's 137-mile-diameter outer moon, where it captured spectacular images as well as data on its mass and composition. Cassini also carries the Huygens probe, a six-instrument European Space Agency probe, scheduled to land on Titan, Saturn's largest moon, in January 2005.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology, Pasadena, Calif. manages the Cassini-Huygens mission for NASA's Office of Space Science, Washington, D.C. JPL designed, developed and assembled the Cassini orbiter. For the latest images and information about the Cassini-Huygens mission, visit: http://www.nasa.gov/cassini

STORY SOURCE: University of Iowa News Services, 300 Plaza Centre One, Suite 301, Iowa City, Iowa 52242-2500.

CONTACTS: Research: Don Gurnett, 818-393-0345 (JPL Office); 319-400-3156 (cell phone); mailto:donald-gurnett@uiowa.edu; Media: Gary Galluzzo, 319-384-0009, gary-galluzzo@uiowa.edu.