Cassini Finds Mingling Moons May Share a Dark Past

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Carolina Martinez 818-354-9382
Jet Propulsion Laboratory, Pasadena, Calif.
carolina.martinez@jpl.nasa.gov

NEWS RELEASE: 2008-028 Feb. 19, 2008

Cassini Finds Mingling Moons May Share a Dark Past

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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