Carolina Martinez (818) 354-9382        
Jet Propulsion Laboratory, Pasadena, Calif.

Gary Galluzzo (319) 384-0009       
University of Iowa, Iowa City

News Release: 2004-164        June 28, 2004

Scientists Find That Saturn's Rotation Period Is A Puzzle

On approach to Saturn, data obtained by the Cassini spacecraft are already posing a puzzling question: How long is the day on Saturn?

Cassini took readings of the day-length indicator regarded as most reliable, the rhythm of natural radio signals from the planet. The results give 10 hours, 45 minutes, 45 seconds (plus or minus 36 seconds) as the length of time it takes Saturn to complete each rotation. Here's the puzzle: That is about 6 minutes, or one percent, longer than the radio rotational period measured by the Voyager 1 and Voyager 2 spacecraft, which flew by Saturn in 1980 and 1981.

Cassini scientists are not questioning Voyager's careful measurements. And they definitely do not think the whole planet of Saturn is actually rotating that much slower than it did two decades ago. Instead, they are looking for an explanation based on some variability in how the rotation deep inside Saturn drives the radio pulse.

The radio sounds of Saturn's rotation, which are also the first sounds from Saturn studied by Cassini, are like a heartbeat and can be heard by visiting <http://jpl.convio.net/site/R?i=t0fAYyrZUyVO-3BCLCXxIg..>http://www.jpl.nasa.gov/videos/cassini/0604/ and <http://jpl.convio.net/site/R?i=4MLHKpoQ7_xO-3BCLCXxIg..>http://www-pw.physics.uiowa.edu/space-audio

"The rotational modulation of radio emissions from distant astronomical objects has long been used to provide very accurate measurements of their rotation period," said Dr. Don Gurnett, principal investigator for the Cassini Radio and Plasma Wave Science instrument, University of Iowa, Iowa City. "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 first hint of something strange about that type of measurement at Saturn was in 1997, when a researcher from Observatoire de Paris reported that Saturn's radio rotation period differed substantially from Voyager.

Dr. Michael D. Desch, Cassini Radio Plasma Wave Science team member, and scientist at NASA's Goddard Space Flight Center in Greenbelt, Md., has analyzed Saturn radio data collected by Cassini 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 said.

Gurnett said, "Although Saturn's radio rotation period has clearly shifted substantially 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 the solution may be tied to the fact that Saturn's rotational axis is nearly identical to its magnetic axis. Jupiter, with a more substantial difference between its magnetic axis and its rotational axis, shows no comparable irregularities in its radio rotation period.

"This finding is very significant. It demonstrates that the idea of a rigidly rotating magnetic field is wrong," said Dr. Alex Dessler, a senior research scientist at the University of Arizona, Tucson. In that way, the magnetic fields of gas giant planets may resemble that of the Sun. The Sun's magnetic field does not rotate uniformly. Instead, its rotation period varies with latitude. "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," said Dressler.

"I think we will be able to unravel the puzzle, but it's going to take some time," said Gurnett. "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."

Cassini, carrying 12 scientific instruments, is just two days from its planetary rendezvous with Saturn. On June 30 it will become the first spacecraft to orbit Saturn, when it begins a four-year study of the planet, its rings and its 31 known moons. The spacecraft recently flew past Saturn's cratered moon Phoebe, where it captured spectacular images as well as data on its mass and composition.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, 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 more information about the Cassini-Huygens mission, visit http://www.nasa.gov/cassini .

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Everything I need to know I learned through Googling.

Couldn't they have measured rotation rates in different depths? From what I know, Saturn exhibits differential rotation.

Man, if the woo-woos get this into their hands: Saturn's rotation slowed down by 1 percent in 20 years...

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"Flying in space is risky business, but just staying on this planet is risky business too." - John Young, astronaut

Differential with latitude. The question is why the source of the radio emissions has drifted in latitude. Lots of fun science will come from this.

Actually, in hindsight it may not have been a surprise, because for Jupiter we have a system 1, 2 and 3 coordinate system (and an on-againg off-again system 4). System 1 and 2 describe the rotation period of the gas planet at different latitudes:
system I, equatorial clouds: 9h 50m 30.00s = 9.842 hours
system II, circumpolar clouds: 9h 55m 40.63s = 9.928 hours
system III, magnetic field: 9h 55m 29.71 s = 9.925 hours
So it all depends where and how you measure. But if Dessler says it is significant, then I will believe him, he is a great expert.
Another nice thing about the Kronian magnetic field is that it is almost perfectly dipolar!!
Cassini is going to be soooo much fun, too bad I am not working on that project at the moment
Martin

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