[meteorite-list] Cassini Finds Mingling Moons May Share a Dark Past

[Ron Baalke]

http://www.jpl.nasa.gov/news/news.cfm?release=2008-028

Cassini Finds Mingling Moons May Share a Dark Past
Jet Propulsion Laboratory
February 19, 2008

Despite the incredible diversity of Saturn's icy moons, theirs is a
story of great interaction. Some of them are pock-marked, some seemingly
dirty, others pristine, one spongy, one two-faced, some still spewing
with activity and some seeming to be captured from the far reaches of
the solar system. Yet many of them have a common thread -- black "stuff"
coating their surfaces.

"We are beginning to unravel the mysteries of these different and
strange moons," said Rosaly Lopes, Cassini scientist at NASA's Jet
Propulsion Laboratory, Pasadena, Calif. She coordinated a special
section of 14 papers about Saturn's icy moons that appears in the
February issue of the journal Icarus.

Taken together, the papers bring an idea that Cassini scientist Bonnie
Buratti calls "the ecology of the Saturn system" to the forefront.
"Ecology is about your entire environment - not just one body, but how
they all interact," said Buratti. "The Saturn system is really
interesting, and if you look at the surfaces of the moons, they seem to
be altered in ways that aren't intrinsic to them. There seems to be some
transport in this system."

Though the details of that transport are not yet clear, mounting
evidence suggests that some mechanism has spread the mysterious dark
material found on several of the moons from one to another; the material
may even have a common cometary origin. Along those lines, several of
the new papers focus on similarities between the dark material found on
different moons - on Hyperion and Iapetus, for example, or between
Phoebe and Iapetus.

Roger Clark of the U.S. Geological Survey in Denver goes further,
saying, "We see the same spectral signature on all the moons that have
coatings of dark material." Clark is lead author of one of the new
papers, which focuses on Saturn's moon Dione. His team found the dark
material there to be extremely fine-grained, making up only a very thin
layer on the moon's trailing side. Its distribution and composition, as
measured by the Cassini visual and infrared mapping spectrometer,
indicate that the dark material is not native to Dione. And scientists
see many of the same signatures there that appear on the moons Phoebe,
Iapetus, Hyperion and Epimetheus, and also in Saturn's F-ring.

As for where this material comes from and what the dark material is,
Clark said, "It's a mystery, which makes it intriguing. We're still
trying to find the exact match." The visual and infrared spectrometer
detected unique absorption bands in the dark material within the Saturn
system, which scientists have not seen anywhere else in the solar
system. "The data keep getting better and better," he said. "We're
ruling things out and figuring out pieces." So far, the team has
identified bound water and, possibly, ammonia in the dark material.

Ongoing geologic activity is another component of Saturn's ecology as
some of the moons continue to feed the planet's rings, which in turn
affect many of the moons.

Clark's team reports tentative evidence to support the hypothesis
presented earlier this year that Dione is still geologically active. In
one series of observations, the infrared spectrometer detected a cloud
of methane and water ice encircling Dione in its orbit within the outer
portions of Saturn's E-ring.

Of course the big story is the icy plumes spewing from the warm, south
polar region of Enceladus. These plumes are believed to be feeding the
E-ring. A paper led by Frank Postberg of the Max Planck Institute for
Nuclear Physics in Heidelberg, Germany, says there are traces of organic
compounds or silicate materials within the water ice-dominated E-ring,
close to Enceladus. This implies that the moon's rocky core and liquid
water are dynamically interacting. The finding could bolster a theory
that Dennis Matson and Julie Castillo of JPL put forth this year, which
said that a warm, organic brew might lie just below Enceladus' surface.

Cassini's next close study of an icy moon is the highly-anticipated
flyby of Enceladus scheduled for March 12. During that flyby, Cassini
will pass by the active moon at a distance of only 50 kilometers (30
miles) at its point of closest approach, and at a distance of around 200
kilometers (120 miles) when it passes through the plumes.

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-Huygens mission for NASA's Science Mission Directorate,
Washington. The Cassini orbiter was designed, developed and assembled at
JPL.

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

2008-028