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Enceladus Geysers Mask the Length of Saturn's Day
03/23/2007
From nasa.gov
(Additional comments below)
Pasadena, Calif. -- In a David and
Goliath story of Saturnian proportions, the little moon Enceladus is
weighing down giant Saturn's magnetic field so much that the field
is rotating slower than the planet. This phenomenon makes it nearly
impossible to measure the length of the Saturn day using techniques
that work at the other giant planets.
"No one could have predicted that the little moon Enceladus would
have such an influence on the radio technique that has been used for
years to determine the length of the Saturn day," said Dr. Don
Gurnett of the University of Iowa, Iowa City. Gurnett is the
principal investigator on the radio and plasma wave science
experiment onboard NASA's Cassini spacecraft. The radio technique
measures the rotation of the planet by taking its radio pulse rate
-- the rhythm of natural radio signals from the planet.
A new study of Cassini data reported
this week in the online version of the journal Science determined
that Saturn's magnetic field lines, invisible lines originating from
the interior of a magnetized planet, are being forced to slip
relative to the rotation of the planet by the weight of electrically
charged particles originating from geysers spewing water vapor and
ice from Enceladus. These results are based on joint observations by
two Cassini instruments -- the radio and plasma wave instrument and
the magnetometer.
The neutral gas particles ejected from the geysers on Enceladus form
a donut-like torus around Saturn. As these particles become
electrically charged, they are captured by Saturn's magnetic field,
forming a disk of ionized gas, or plasma, which surrounds the planet
near the equator. The particles weigh down the magnetic field so
much that the rate of rotation of the plasma disk slows down
slightly. This slippage causes the radio period, controlled by the
plasma disk rotation, to be longer than the planet's actual rotation
period.
Scientists conclude the period Cassini has been measuring from radio
emission is not the length of the Saturn day, but rather the
rotation period of the plasma disk. At present, because of Saturn's
cloud motion, no technique is known that can accurately measure the
planet's actual internal rotation.
Finding out the length of Saturn's day has been a challenge because
the gaseous planet has no surface or fixed point to clock its
rotation rate. Initially, the approach was to use periodic regular
radio signals, as has been done for Jupiter, Uranus and Neptune.
However, Saturn's radio period has turned out to be troubling in two
ways. It seems to be a pulsed signal rather than a rotating,
lighthouse-like beam. Secondly, the period seems to be slowly
changing over months to years. The day measured by Cassini is some
six minutes longer than the day recorded by NASA's Voyager
spacecraft in the early 1980s, a change of nearly 1 percent.
"We have linked the pulsing radio signal to a rotating magnetic
signal. Once each rotation of Saturn's magnetic field, an asymmetry
in the field triggers a burst of radio waves," said Dr. David
Southwood, co-author, Imperial College London, and director of
science at the European Space Agency. "We have then linked both
signals to material that has come from Enceladus."
Based on the new observations, scientists now think there are two
possible reasons for the change in radio period. The first theory is
that the geysers on Enceladus could be more active now than in
Voyagers' time. The second is that there may be seasonal variations
as Saturn orbits the sun once every 29 years.
"One would predict that when the geysers are very active, the
particles load down the magnetic field and increase the slippage of
the plasma disk, thereby increasing the radio emission period even
more. If the geysers are less active, there would be less of a load
on the magnetic field, and therefore less slippage of the plasma
disk, and a shorter period," said Gurnett.
"The direct link between radio, magnetic field and deep planetary
rotation has been taken for granted up to now. Saturn is showing we
need to think further," said Michele Dougherty, principal
investigator on Cassini's magnetometer instrument, Imperial College
London.
The Saturn radio emissions detected by Cassini have been converted
into an audio file available at:
http://www.nasa.gov/cassini
and http://saturn.jpl.nasa.gov.
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 Science Mission Directorate, Washington. The Cassini orbiter
was designed, developed and assembled at JPL. The radio and plasma
wave science experiment team is based at the University of Iowa,
Iowa City. The magnetometer team is based at Imperial College
London.
Full article
here
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