[meteorite-list] Cassini is on the Trail of a Runaway Mystery (Iapetus)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 8 Oct 2007 16:08:54 -0700 (PDT)
Message-ID: <200710082308.QAA23545_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.cfm?release=2007-113

Cassini is on the Trail of a Runaway Mystery
Jet Propulsion Laboratory
October 08, 2007

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 .

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.

------------------------------------------------------------------------

Media contact: Carolina Martinez 818-354-9382
Jet Propulsion Laboratory, Pasadena, Calif.
carolina.martinez at jpl.nasa.gov

2007-113
Received on Mon 08 Oct 2007 07:08:54 PM PDT