Cassini Findings Suggest Complex Story of Venting at the South Pole of Enceladus

http://ciclops.org/view.php?id=1414&flash=0

CASSINI FINDINGS SUGGEST COMPLEX STORY OF VENTING AT THE SOUTH POLE OF
ENCELADUS


MEDIA RELATIONS OFFICE
CASSINI IMAGING CENTRAL LABORATORY FOR OPERATIONS (CICLOPS)
SPACE SCIENCE INSTITUTE, BOULDER, COLORADO
http://ciclops.org


Preston Dyches (720) 974-5859
CICLOPS/Space Science Institute, Boulder, Colo.

Mary Nehls-Frumkin (734) 763-7305
University of Michigan

Sascha Kempf (011) 49 6221 516-247
Max-Planck-Institut f?r Kernphysik, Heidelberg



For Immediate Release: August 30, 2005

CASSINI FINDINGS SUGGEST COMPLEX STORY OF VENTING AT THE SOUTH POLE OF
ENCELADUS

Evidence is mounting that the atmosphere of Enceladus, first detected
by
the Cassini Magnetometer instrument, is the result of venting from
ground fractures close to the moon's south pole. New findings from the
close flyby of Enceladus by Cassini this past July add to the emerging
picture of a small icy body, unusual in its past and present level of
activity, and very different from all other icy Saturnian moons.

Within a minute of closest approach to Enceladus on July 14th, 2005,
two
instruments aboard the Cassini spacecraft detected material coming from
the surface of the moon. The Ion and Neutral Mass Spectrometer (INMS)
measured a large peak in the abundance of water vapor at approximately
35 seconds before closest approach to Enceladus, as it flew over the
south polar region at an altitude of 270 kilometers (168 miles).

The High Rate Detector (HRD) of the Cosmic Dust Analyzer (CDA) observed
a peak in the number of fine, powder-sized icy particles coming from
the
surface approximately a minute before reaching closest approach at an
altitude of 460 kilometers (286 miles).

The character of these detections is very similar to the venting of
vapor and fine, icy particles from the surfaces of comets when they are
warmed by sunlight as they near the Sun. On Enceladus, however, it is
believed that internal heat, possibly from tidal forces, is responsible
for the activity. The close but different occurrences of the two
detections are yielding important clues to the location of the vents
and
even the venting process.

"And so the plot thickens," said Dr. Carolyn Porco, Imaging Team leader
at the Space Science Institute in Boulder, Colo. "Enceladus is
surprisingly warm, internally fractured and active, and we're not sure
how it comes to be that its South pole is the warmest, most active
place. The fact that Enceladus is so alive, and Mimas, the moon next
door and roughly the same size, is so dead, is really testing our
understanding of the internal workings of planetary satellites. But
we're not complaining. We like it this way."

Acting on the timing information, the Cassini imaging team examined
images of Enceladus acquired during the same July flyby, tracing the
ground track of the spacecraft across the south pole during the
encounter. They noted that the maximum water vapor abundance detected
by
INMS occurred when Cassini was directly over one of the mid-latitude
tectonic gashes that circumscribe the south polar region, suggesting at
first the sub-spacecraft fracture as being the source of the vapor
detected by INMS.

However, the mid-latitude fracture system is not presently a source of
anomalous heat on Enceladus. Previously reported, combined results from
the Composite Infrared Spectrometer (CIRS) and the Imaging Science
instrument (ISS) instead indicated that an array of narrow linear
cracks
straddling the south pole, called "tiger stripes", is the site of at
least some of greatest concentrations of anomalous heat at the surface,
and that mid-latitude regions are normal and cooler.

Furthermore, analysis of the timings of the INMS and CDA observations
seems to indicate that the vapor and fine material are originating from
the hot polar cap region, some distance away from the spacecraft's
flight path, and that the two processes - production of water vapor and
ejection of fine material - are connected, as they are in a comet.

Recent numerical simulations by members of the CDA team at the
University of Potsdam, Germany, aimed at reproducing the timing of the
CDA observations, have shown that the majority of the icy grains
originate from a source at the moon's south pole that is distributed
along the surface, like the cracks seen in the images, and not from
globally distributed impacts by Saturn-system dust particles as had
been
previously assumed. If this is the case, then the sharpness of the peak
detected in the water vapor abundances by INMS is better explained by
passage of the spacecraft through the edge of a cloud of vapor that
hovers over the south pole, and not through a sub-spacecraft source as
suggested by the images.

The CDA analyses also shed light on the origin of the broad, diffuse E
ring through which Enceladus orbits. "This new finding might explain
why
Enceladus is so efficient in replenishing the E ring with fresh
particles," said Dr. Frank Spahn of the University of Potsdam.

To arrive at a more complete and accurate picture of the surface
environment on Enceladus will require scientists from the various
Cassini instrument teams to combine their findings - something that
will
happen in the days ahead.

"The exploration of the mysteries of Enceladus with Cassini's combined
set of instrumentation," said Dr. Hunter Waite, team leader of the INMS
investigation at the University of Michigan in Ann Arbor, "clearly
demonstrates the virtue of exploring a system as complex as Saturn's
using a very well-equipped flagship like Cassini."

Cassini will encounter Enceladus again at very close range in March,
2008.

New image products showing the types of tectonic features found in the
south polar region of Enceladus, as well as the Cassini ground track
and
the INMS and CDA results, can be found at http://ciclops.org,
http://saturn.jpl.nasa.gov, and http://www.nasa.gov/cassini. More
information on the INMS and CDA observations can be found at
http://aoss.engin.umich.edu/news/04-05_news/Cassini.php.

The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. The Jet Propulsion
Laboratory (JPL), 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 and its two
onboard
cameras were designed, developed and assembled at JPL. The imaging team
consists of scientists from the US, England, France, and Germany. The
imaging operations center and team leader (Dr. C. Porco) are based at
the Space Science Institute in Boulder, Colo. The Ion and Neutral Mass
Spectrometer and team leader (Dr. H. Waite) are based at the University
of Michigan. The Cosmic Dust Analyzer and principal investigator (Dr.
R.
Srama) are based at the Max Planck Institute for Nuclear Physics in
Heidelberg, Germany, and the High Rate Detector (HRD) was designed and
built by the University of Chicago (team leader Dr. A. Tuzzolino).

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