Researchers Present Cassini Findings at Saturn

http://www.eurekalert.org/pub_releas...-rpc121604.php

Public release date: 16-Dec-2004

Contact: Gary Galluzzo

319-384-0009
University of Iowa

Researchers present Cassini findings at Saturn

As NASA's Cassini spacecraft approached Saturn last July, it found
evidence that lightning on Saturn is roughly one million times stronger
than lightning on Earth.

That's just one of several Cassini findings that University of Iowa
Space Physicist Don Gurnett will present in a paper to be published
Thursday, Dec. 16 in Science Express, an online version of the journal
Science, and in a talk to be delivered Friday, Dec. 17 at a meeting of
the American Geophysical Union in San Francisco.

Other findings include:

o Cassini impacted dust particles as it traversed Saturn's rings.
o Saturn's radio rotation rate varies.

The comparison between Saturn's enormously strong lightning and Earth's
lightning began several years ago as the Cassini spacecraft prepared
for
its journey to Saturn by swinging past the Earth to receive a
gravitational boost. At that time, Cassini started detecting radio
signals from Earth's lightning as far out as 89,200 kilometers from the
Earth's surface. In contrast, as Cassini approached Saturn, it started
detecting radio signals from lightning about 161 million kilometers
from
the planet. "This means that radio signals from Saturn's lightning are
on the order of one million times stronger than Earth's lightning.
That's just astonishing to me!" says Gurnett, who notes that some radio
signals have been linked to storm systems observed by the Cassini
imaging instrument.

Earth's lightning is commonly detected on AM radios, a technique
similar
to that used by scientists monitoring signals from Cassini.

Regarding Saturn's rings, Gurnett says that the Cassini Radio and
Plasma
Wave Science (RPWS) instrument detected large numbers of dust impacts
on
the spacecraft. Gurnett and his science team found that as Cassini
approached the inbound ring plane crossing, the impact rate began to
increase dramatically some two minutes before the ring plane crossing,
then reached a peak of more than 1,000 per second at almost exactly the
time of the ring plane crossing, and finally decreased to pre-existing
levels about two minutes later. Gurnett notes that the particles are
probably quite small, only a few microns in diameter, otherwise they
would have damaged the spacecraft

Finally, variations in Saturn's radio rotation rate came as a surprise.
Based upon more than one year of Cassini measurements, the rate is 10
hours 45 minutes and 45 seconds, plus or minus 36 seconds. That's about
six minutes longer than the value recorded by the Voyager 1 and 2
flybys
of Saturn in 1980-81. Scientists use the rotation rate of radio
emissions from the giant gas planets such as Saturn and Jupiter to
determine the rotation rate of the planets themselves because the
planets have no solid surfaces and are covered by clouds that make
direct visual measurements impossible.

Gurnett suggests that the change in the radio rotation rate is
difficult
to explain. "Saturn is unique in that its magnetic axis is almost
exactly aligned with its rotational axis. That means there is no
rotationally induced wobble in the magnetic field, so there must be
some
secondary effect controlling the radio emission. We hope to nail that
down during the next four to eight years of the Cassini mission."

One possible scenario was suggested nearly 20 years ago. 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 earlier
this year 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."

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, on June 30, 2004 became
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
is
part of the $3.3 billion Cassini-Huygens Mission that includes 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.

RESEARCH CONTACT: Don Gurnett,

MEDIA CONTACT: Gary Galluzzo, Writer, 319-384-0009,