Miércoles 17 de Mayo de 2006, Ip nº 153

So, exactly how long is a day on Saturn?
Por Ker Than

How long is a day on Saturn? According to a new study, it's 10 hours, 47 minutes and 6 seconds -- give or take 40 seconds.

This value differs from previous estimates by up to 8 minutes and is based on measurements of a mysterious magnetic field signal emanating from the planet. If confirmed, the finding could help scientists gain a better understanding of Saturn's turbulent atmosphere and its shrouded interior.

The study, led by Giacomo Giampieri of NASA's Jet Propulsion Laboratory, is detailed in the May 4 issue of the journal Nature.

A tricky art
The rotation period of rocky worlds like Earth can be calculated through simple observations of the motions of a particular spot on the planet in relation to other celestial objects. This method doesn't work, however, for the likes Jupiter and Saturn because the solid cores of these gas giants are completely obscured by thick clouds that make direct visual measurements impossible.

Instead, the typically stated rotation periods for these planets are those of their magnetic fields, which scientists believe are closely tied to the rotations of their solid interiors.

But measuring the rotation period of Saturn's magnetic field is difficult because its rotational axis -- the imaginary line around which the planet rotates -- is nearly identical to the axis around which its magnetic field revolves.

In contrast, the magnetic and rotational axes of Jupiter are separated by a few degrees and scientists can use this difference to calculate its rotation period.

It's like measuring the spin rate of a CD that has a distinct label on it versus a blank one that doesn't, Giampieri explained. "It's very difficult to tell if a blank CD is rotating at all."

For Saturn, the accepted workaround had been to analyze distortions in radio signals emitted by the planet -- the signals are thought to be linked to the planet's magnetic field.

Analysis of Saturnian radio emissions by NASA's Voyager spacecrafts during the 1980s revealed a rotation period of about 10 hours, 39 minutes and 22 seconds. But more recent radio data collected by the agency's Cassini spacecraft in 2003 and 2004 got a different number: 10 hours, 45 minutes and 45 seconds.

This discrepancy has long puzzled scientists. One explanation is that Saturn's rotation rate actually slowed down by about 6 minutes, but many scientists think this is highly unlikely.

"You can't say that Saturn itself has changed its rotation," said David Stevenson, a researcher at the California Institute of Technology who was not involved in the study. "It's just way too big."

Another possible explanation is that the radio emissions never reflected the planet's rotation period at all, but something else.

In any case, using radio signals to measure Saturn's rotation period is like relying on a substitute for a substitute: the radio signals were linked to magnetic fields that were in turn linked to the rotation of Saturn's solid core.

The new method potentially cuts out one of these steps.

Cutting out a middleman
Using magnetic field data collected by Cassini over a period of 14 months beginning in summer 2004, Giampieri and his team teased out a signal that they think is probably linked to Saturn's magnetic field rotation and thus to the planet's true rotation period.

"We saw a small spot on the blank CD," Giampieri told SPACE.com. "By looking at that, we could measure the rotation period of the magnetic field."

More studies will be needed to confirm the link between the magnetic field signal and Saturn's rotation period, Giampieri said, but so far the results look promising. Unlike the radio emissions, the magnetic field signal appears stable over time.

A precise value of Saturn's rotation period will help scientists accurately determine the speed of its atmospheric winds and the size of its solid core, which is believed to be composed of rock and ice.

"The amount of rotation gives you the amount of centrifugal force acting on the interior of the planet," Giampieri said. "That is a very important ingredient for planetary modeling."

  04/05/2006. CNN.