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.
Carolina Martinez 818-354-9382
Jet Propulsion Laboratory, Pasadena, Calif.