So, researchers at NASA's Jet Propulsion Laboratory, and colleagues at Imperial College London and the University of California, Los Angeles, set out to solve the mystery.

Reporting in the May 4 issue of Nature, the team described how they used the Cassini spacecraft's magnetometer instrument to identify a clear period in the planet's magnetic field that indicates a Saturnian day is 10 hours and 47 minutes long.

This measurement is about eight minutes slower than was calculated by the Voyager spacecraft in the early 1980s, and slower than previous estimates from another Cassini instrument. The magnetometer results provide the best estimate of the Saturn day to date, however, because it can probe deeply inside Saturn.

Planets rotate around their spin axes as they orbit the Sun. Rocky planets such as Earth and Mars have rotation periods that remain quite constant and are easy to measure based on surface features.

Gaseous planets do not have solid surfaces to track and are not as rigid as rocky planets. Therefore, their periods may change more than those of rocky planets and are less easy to measure.

Scientists have sought to use proxy measures such as the repetition rate of radio signals or the period of the rotation of the direction of the magnetic axis of the planet. For Saturn, however, this has proven difficult, because previous missions could not detect a period in the magnetic field measurements. Some radio data have shown a period, but it has changed in the time between previous missions and Cassini.

Since the Voyager days, for example, scientists have seen changes in the period of radio observations - but they knew it was virtually impossible to slow down or speed up a mass as large as Saturn.

As Cassini's measurements of the rhythms of natural radio signals from the planet continued to vary, scientists began to realize the signals probably did not track Saturn's rotation rate exactly. Now, measurements of the planet's magnetic field might have finally solved this puzzle.

"Making this measurement has been one of team's most important science goals," said co-author Michele Dougherty of Imperial College London. "Finding a period in the magnetic field rotation helps us to understand the internal structure of Saturn's magnetic fields and from that, of Saturn itself, which will help us understand how the planet formed."

Lead author Giacomo Giampieri of JPL said calculating Saturn's rotation has posed a great challenge to scientists.

"Imagine you want to check whether the CD is playing," Giampieri said. "If your CD has a label it is easy to see at a glance that it is spinning very fast in the CD player. But if the CD has no label, you would not be able to tell whether the CD is moving or not because it would look static".

He said Saturn's magnetic field is similar to a blank CD. "If you just look at it, it seems that it is not rotating at all," he said.

In the past, Pioneer 11 and two Voyager spacecraft encountered Saturn during brief fly-bys and collected data, but scientists could find a clear periodic signal in their magnetic-field data. Then, in July 2004, Cassini entered orbit at Saturn and now has completed many circuits around the gas-giant planet.

Based on the data collected over this extended period of time, and the use of appropriate algorithms, scientists finally have detected a small but regular periodic signature in the magnetic field close to the planet, with a period of 10 hours 47 minutes and 6 seconds (plus or minus 40 seconds).

The team likens the discovery to finding a small spot on a CD that reveals how fast it is spinning. The result was somewhat surprising, however.

"The period we found from the magnetic field measurements has remained constant since Cassini entered orbit almost two years ago," Giampieri explained, "while radio measurements since the Voyager era have shown large variability. By monitoring the magnetic field over the rest of the mission, we will be able to solve this puzzle".

The periodic signal of Saturn's magnetic field does not fit simple models for planetary magnetic fields. Giampieri added. "Saturn's periodic magnetic field differs from that found at Jupiter, which can be modeled as a dipole field tilted with respect to the rotation axis," he said.

Giampieri said the study opens a new perspective on the internal structure and dynamics of Saturn, and how it affects the source of the magnetic field. "We now know that the internal rotation of Saturn and its connection to the external magnetic field is very complex," he said. "Our study is the first step in breaking the code."

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