Cassini Is on the Trail of a Runaway Mystery
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.
Image right: Cassini captures the first high-resolution glimpse of the bright trailing hemisphere of Saturn's moon Iapetus in this false-color mosaic.
This false-color mosaic shows the entire hemisphere of Iapetus. Image credit:
NASA/JPL/Space Science Institute
+ Full image and caption
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
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.
Image left: This image compares midday temperatures on Saturn's moon Iapetus, recorded by the composite infrared spectrometer instrument during Cassini's close Sept. 10, 2007 flyby, with images of the same region recorded during the same flyby by the Cassini imaging science subsystem. Image credit: NASA/JPL/GSFC/SwRI/SSI+ Full image and caption
"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://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.