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Topic Name: Cassini is on the Trail of a Runaway Mystery
Category: STAR (Space, Telecommunications & Radioscience)
Research persons: John Spencer , Tilmann Denk , Amanda Hendrix
Location: 4800 Oak Grove Drive, Pasadena, California 91109, United States
Details
NASA scientists are on the trail of Iapetus' mysterious dark
side, which seems to be home to a bizarre "runaway" process that is transporting
vaporized water ice from the dark areas to the white areas of the
Saturnian moon.
This "thermal segregation" model may explain many details of the
moon's strange and dramatically two-toned appearance, which have been revealed
exquisitely in images collected during a recent close flyby of Iapetus by
NASA's Cassini spacecraft.
Infrared observations
from the flyby confirm that the dark material is warm enough (approximately
minus 230 degrees Fahrenheit or 127 Kelvin) for very slow release of water vapor
from water ice, and this process is probably a major factor in determining the
distinct brightness boundaries.
"The side of Iapetus that faces forward in its orbit around
Saturn is being darkened by some mysterious process," said John Spencer, Cassini
scientist with the composite infrared spectrometer team from the
Southwest Research Institute, Boulder, Colo.
Using multiple instruments on Cassini, scientists are piecing
together a complex story to explain the bright and dark faces of Iapetus. But
yet to be fully understood is where the dark material is coming from. Is it
native or from outside the moon? It has long been hypothesized that this
material did not originate from within Iapetus, but instead was derived from
other moons orbiting at a much greater distance from Saturn in a direction
opposite to Iapetus.
Scientists are now converging on the notion that the darkening
process in fact began in this manner, and that thermal effects subsequently
enhanced the contrast to what we see today.
"It's interesting to ponder that a more than 30-year-old idea
might still help explain the brightness difference on Iapetus," said Tilmann
Denk, Cassini imaging scientist at the
Free University in Berlin, Germany. "Dusty material spiraling in from outer
moons hits Iapetus head-on, and causes the forward-facing side of Iapetus to
look different than the rest of the moon."
Once the leading side is even slightly dark,
thermal segregation can proceed rapidly. A dark surface will absorb more
sunlight and warm up, explains Spencer, so the water ice on the surface
evaporates. The water vapor then condenses on the nearest cold spot, which could
be Iapetus's poles, and possibly bright, icy areas at lower latitudes on the
side of the moon facing in the opposite direction of its orbit. So the dark
stuff loses its surface ice and gets darker, and the bright stuff accumulates
ice and gets brighter, in a
runaway
process.
Scientists say the result is that there are virtually no shades
of gray on Iapetus. There is only white and very dark.
Ultraviolet data also show a non-ice component in the bright,
white regions of Iapetus.
Spectroscopic analysis will reveal whether the composition of the material
on the dark hemisphere is the same as the dark material that is present within
the bright terrain.
"The ultraviolet data tell us a lot about where the water ice is
and where the non-water ice stuff is. At first glance, the two populations do
not appear to be present in the pattern we expected, which is very interesting,"
said Amanda Hendrix, Cassini scientist on the ultraviolet imaging spectrograph
team at NASA's Jet Propulsion Laboratory,
Pasadena, Calif.
Because of the presence of very small craters that excavate the
bright ice beneath, scientists also believe that the dark material is thin, a
result consistent with previous Cassini radar results. But some local areas may
be thicker. The dark material seems to lie on top of the bright region,
consistent with the idea that it is a residual left behind by the sublimated
water ice.
Some other mysteries are coming together. There are more data on
the signature mountain ridge that gives Iapetus its "walnut" appearance. In some
places it appears subdued. One big question that remains is why it does not go
all the way around. Was it partially destroyed after it formed, or did it never
extend all the way around the moon? Scientists have ruled out that it is a
youthful feature because it is pitted with craters, indicating it is old. And
the ridge looks too solid and competent to be the result of an equatorial ring
around the moon collapsing onto its surface. The ring theory cannot explain
features that look like tectonic structures in the new high resolution images.
Over the next few months, scientists hope to learn more about
Iapetus' mysteries.
New Iapetus images, temperature maps and other visuals on
Iapetus are available at:
http://saturn.jpl.nasa.gov and
http://www.nasa.gov/cassini .
Mission:
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 Cassini mission for
NASA's Science Mission Directorate, Washington, D.C.
About scientists :
John Spencer
Southwest Research Institute
1050 Walnut St., Suite 300
Boulder, CO 80302
Phone: (303) 546-9674
Fax: (303) 546-9687
spencer(at)boulder(dot)swri(dot)edu
http://www.boulder.swri.edu/~spencer/
Tilmann Denk
Institution: Institut für Geologische Wissenschaften / Fachrichtung Planetologie
und Fernerkundung
Field of activity: Projekt Cassini
Address: Malteserstr. 74-100, 12249 Berlin-Lankwitz
FU telephone: 030-838 70560
FU email address: Tilmann.Denk@fu-berlin.de
Homepage: http://www.fu-berlin.de/planeten
Free University in Berlin, Germany
Amanda Hendrix
Jet Propulsion Laboratory
M/S 230-250
4800 Oak Grove Drive
Pasadena, CA 91109
Phone:818.393.1628
Fax:838.393.4669
Education
B.S., Aeronautical Engineering, California Polytechnic State University (1991)
M.S., Aerospace Engineering Sciences, University of Colorado (1994)
Ph.D., Aerospace Engineering Sciences, University of Colorado( 1996)
NASA's Jet Propulsion Laboratory, Pasadena, Calif
Relate Important Links:
http://www.jpl.nasa.gov/
http://www.solarviews.com/eng/enceladu.htm
http://www.solarviews.com/eng/dione.htm
http://www.physorg.com/news111078576.html
http://ircatalog.gsfc.nasa.gov/
 
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