Iapetus Is Coated With Foreign Dust

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http://www.news.cornell.edu/stories/Dec09/CassiniIapetus.html

Dark side of the moon: Saturnian satellite Iapetus is coated with foreign dust
By Anne Ju (am...@cornell.edu)
Cornell University
Dec. 10, 2009

Iapetus is often called Saturn's most bizarre
moon, due to its starkly contrasting hemispheres
-- one black as coal, the other white as snow.

Images taken by the Cassini-Huygens spacecraft,
orbiting Saturn since 2004, offer the most
compelling evidence to date of why and how the
moon got its yin-yang appearance, as well as
clues to how other such satellites might have
formed in the early universe. Analyzed by a
research team that includes Cornell scientists,
the images are detailed in the Dec. 10 online
edition of the journal Science.

"This is not the most fundamental problem in the
world," said research team member Joseph A.
Burns, Cornell's Irving Porter Church Professor
of Engineering and professor of astronomy. "But
it's an enigma that's been puzzling astronomers
for centuries."

Since pictures of Iapetus from the Voyager
mission 30 years ago confirmed its intriguing
color scheme, scientists have puzzled over
whether Iapetus' dark-light contrast was the
result of external debris hitting some of the
moon, or whether the dark dust was the result of
interior activity. Now they know the dust came
from elsewhere.

Using pictures taken by Cassini, particularly
during a September 2007 close fly-by, the
scientists assert that Iapetus' darker half,
called Cassini Regio, is the result of the
planet's leading side getting bombarded by dusty
debris from another Saturnian moon, Phoebe, which
orbits in the opposite direction beyond Iapetus.

It is a longstanding theory, but in a paper
published in the journal Nature in October, three
Cornell-trained astronomers announced the
discovery of an enormous ring of debris --
10,000 times the area of Saturn's famous main
ring system -- around Saturn and near Phoebe,
pointing to it as the ring's source. Burns calls
this ring the "smoking gun" supporting dust
hitting Iapetus and other moons around Saturn.

"The ring of collisional debris that has come off
Phoebe is out there, and its companion moons are
out there, and now we understand the process
whereby the stuff is coming in," Burns said.
"When you see the coating pattern on Iapetus, you
know you've got the right mechanism for producing
it."

Small, white craters that dot Iapetus' darker
half indicate a veneer of dark dust, only meters
deep, covering a white, icy surface that matches
the rest of the satellite. The imaging data also
revealed that all the materials on the leading
side are much redder than the shielded and
brighter trailing side -- another indication that
the leading side's dust came from elsewhere.

Other pictures showed that the transition from
the dark to light hemisphere is not a solid line,
but rather a mottled, patchy array of bright and
dark spots.

The pattern, the scientists say, supports a
previous theory described in a companion paper in
Science that the darker parts of the moon tend to
heat up when struck by sunlight, causing the ice
to evaporate underneath. This causes any dark
spots to get even darker, creating the mottled
look.

The research team includes Paul Helfenstein and
Peter C. Thomas, both senior research associates
at Cornell. The paper's first author is Tilmann
Denk, Cassini imaging scientist at the Free
University in Berlin, Germany. The Cassini
program is an international cooperative effort
involving NASA, the European Space Agency and the
Italian space agency.

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