Cassini Finds Saturn Sends Mixed Signals

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Jia-Rui Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jccook@jpl.nasa.gov

Feature: 2011-091 March 22, 2011

Cassini Finds Saturn Sends Mixed Signals

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2011-091&cid=release_2011-091

Like a petulant adolescent, Saturn is sending out mixed signals.

Recent data from NASA's Cassini spacecraft show that the variation in radio waves
controlled by the planet's rotation is different in the northern and southern hemispheres.
Moreover, the northern and southern rotational variations also appear to change with the
Saturnian seasons, and the hemispheres have actually swapped rates. These two radio
waves, converted to the human audio range, can be heard in a new video available online
at: http://www.nasa.gov/multimedia/videogallery/index.html?media_id=74390781

"These data just go to show how weird Saturn is," said Don Gurnett, Cassini's radio and
plasma wave science instrument team lead and professor of physics at the University of
Iowa, Iowa City. "We thought we understood these radio wave patterns at gas giants,
since Jupiter was so straightforward. Without Cassini's long stay, scientists wouldn't have
understood that the radio emissions from Saturn are so different."

Saturn emits radio waves known as Saturn Kilometric Radiation, or SKR for short. To
Cassini, they sound a bit like bursts of a spinning air raid siren, since the radio waves vary
with each rotation of the planet. This kind of radio wave pattern had been previously
used at Jupiter to measure the planet's rotation rate, but at Saturn, as is the case with
teenagers, the situation turned out to be much more complicated.

When NASA's Voyager spacecraft visited Saturn in the early 1980s, the radiation
emissions indicated the length of Saturn's day was about 10.66 hours. But as its clocking
continued by a flyby of the joint ESA-NASA Ulysses spacecraft and Cassini, the radio
burst varied by seconds to minutes. A paper in Geophysical Research Letters in 2009
analyzing Cassini data showed that the Saturn Kilometric Radiation was not even a solo,
but a duet, with two singers out of sync. Radio waves emanating from near the north pole
had a period of around 10.6 hours; radio waves near the south pole had a period of
around 10.8 hours.

A new paper led by Gurnett that was published in Geophysical Research Letters in
December 2010 shows that, in recent Cassini data, the southern and northern SKR
periods crossed over around March 2010, about seven months after equinox, when the
sun shines directly over a planet's equator. The southern SKR period decreased from
about 10.8 hours on Jan. 1, 2008 and crossed with the northern SKR period around
March 1, 2010, at around 10.67 hours. The northern period increased from about 10.58
hours to that convergence point.

Seeing this kind of crossover led the Cassini scientists to go back into data from previous
Saturnian visits. With a new eye, they saw that NASA's Voyager data taken in 1980,
about a year after Saturn's 1979 equinox, showed different warbles from Saturn's
northern and southern poles. They also saw a similar kind of effect in the Ulysses radio
data between 1993 and 2000. The northern and southern periods detected by Ulysses
converged and crossed over around August 1996, about nine months after the previous
Saturnian equinox.

Cassini scientists don't think the differences in the radio wave periods had to do with
hemispheres actually rotating at different rates, but more likely came from variations in
high-altitude winds in the northern and southern hemispheres. Two other papers involving
Cassini investigators were published in December, with results complementary to the
radio and plasma wave science instrument -- one by Jon Nichols, University of Leicester,
U.K., in the same issue of Geophysical Research Letters, and the other led by David
Andrews, also of University of Leicester, in the Journal of Geophysical Research.

In the Nichols paper, data from the NASA/ESA Hubble Space Telescope showed the
northern and southern auroras on Saturn wobbled back and forth in latitude in a pattern
matching the radio wave variations, from January to March 2009, just before equinox.
The radio signal and aurora data are complementary because they are both related to the
behavior of the magnetic bubble around Saturn, known as the magnetosphere. The paper
by Andrews, a Cassini magnetometer team associate, showed that from mid-2004 to mid-
2009, Saturn's magnetic field over the two poles wobbled at the same separate periods as
the radio waves and the aurora.

"The rain of electrons into the atmosphere that produces the auroras also produces the
radio emissions and affects the magnetic field, so scientists think that all these variations
we see are related to the sun's changing influence on the planet," said Stanley Cowley, a
co-author on both papers, co-investigator on Cassini's magnetometer instrument, and
professor at the University of Leicester.

As the sun continues to climb towards the north pole of Saturn, Gurnett's group has
continued to see the crossover trend in radio signals through Jan. 1, 2011. The period of
the southern radio signals continued to decrease to about 10.54 hours, while the period of
the northern radio signals increased to 10.71 hours.

"These papers are important in helping to explain the complicated dance between the sun
and Saturn's magnetic bubble, something normally invisible to the human eye and
imperceptible to the human ear," said Marcia Burton, a Cassini fields and particles
scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif., who was not involved in
the work. "Cassini will continue to keep an eye on these changes."

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 in Pasadena, manages the mission for NASA's Science Mission Directorate,
Washington, D.C. The Cassini orbiter and its two onboard cameras were designed,
developed and assembled at JPL. The radio and plasma wave science team is based at the
University of Iowa, Iowa City, where the instrument was built. The magnetometer team is
based at Imperial College, London, U.K.

The Hubble Space Telescope is a project of international cooperation between NASA and
the European Space Agency. NASA's Goddard Space Flight Center manages the
telescope. The Space Telescope Science Institute conducts Hubble science operations.
STScI is operated for NASA by the Association of Universities for Research in
Astronomy, Inc., in Washington, D.C.

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