Scientists Find That Saturn's Rotation Period Is A Puzzle

MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA 91109. TELEPHONE (818) 354-5011
http://www.jpl.nasa.gov

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://www.jpl.nasa.gov/videos/cassini/0604/ and
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 .

-end-

Could it be that Saturn is going through a "pole shift" of some description
?

Could it be that Saturn is going through a "pole shift" of some description
?