(See description below)
SATURN'S MOON IAPETUS SHOWS A BULGING WAISTLINE
Images returned by the Cassini spacecraft cameras during a New
Year's Eve flyby December 31, 2004 have unveiled several unique
surface features on Iapetus that have modified discussions about
the origin of the extremely dark region on Iapetus, known as
Cassini Regio.
One of these features is a startling, long narrow ridge that lies
almost exactly on the equator of Iapetus, bisects its entire dark
hemisphere and reaches 20 kilometers high (12 miles). It extends
over 1,300 kilometers (808 miles) from side to side, along its
midsection. No other moon in the solar system has such a striking
geological feature. In places the ridge is comprised of
mountains. Some are bright white, while others have exactly the
same brownish color as the dark material. It has even been suggested
that the remarkable features of Iapetus are signs of intelligent life there some
time in the past.
Dr. Tilmann Denk, imaging team associate at Freie Universitaet in
Berlin, Germany, and the planetary satellite expert responsible
for planning the Iapetus imaging observations, was delighted with
the results of the flyby and amazed at the new findings.
"When we first saw hints of an equatorial feature within the
dark terrain in early images, we did not realize that this would
turn into one of the major discoveries on Iapetus. This is the
prize after more than two years of detailed planning. The Iapetus
images alone will keep us busy for years."
The flyby images, which revealed a region of Iapetus never before
seen, show feathery-looking black streaks at the boundary between
dark and bright hemispheres, that indicate dark material has
fallen onto Iapetus. They also show craters near this boundary
with bright walls facing towards the pole and dark walls facing
towards the equator.
"We can now say with reasonable certainty that the leading
hemisphere of Iapetus was coated by a dark material falling onto
it from a particular direction," said Dr. Carolyn Porco,
imaging team leader and director of the Cassini Imaging Central
Laboratory for Operations in Boulder, Colo. "What isn't
clear yet is where the material came from. However, that
equatorial ridge is looking awfully suspicious."
The leading theories are material originating from either within
or outside Iapetus. Opinions differ on which is the correct
explanation.
Dr. Gerhard Neukum, a professor at Freie Universitaet and an
imaging team member said, "I think there is evidence now
that a genetic relationship exists between the dark material in
Cassini Regio and the newly detected equatorial bulge. The
process responsible for placement of the dark coating could be
plume-style eruptions at the equatorial bulge where dark material
accumulated at the surface as fallout."
Dr. Alfred McEwen, a professor in the Department of Planetary
Sciences at the University of Arizona, Tucson, takes the opposing
view. "There are no clear volcanic landforms on
Iapetus," he said. "The surface is entirely heavily
cratered, so any volcanism must have been minor, there is no
known source of energy for volcanism, and the body has an
irregular shape, so it probably isn't differentiated from
internal heat. This is about the least likely place in the solar
system to find volcanism, especially post-dating all the
cratering."
Other scientists who thought the material originated from the
outside of Iapetus are now waffling as a result of the new
images. "A week or so ago, I would have said
"outside" without batting an eyelid," said Dr.
Paul Helfenstein, an imaging team associate and an expert in icy
satellites from Cornell University, Ithaca, New York. "I'm
still inclined to lean that way, but now with the discovery of
the bizarre equatorial ridge I'm non-plussed! I kind of like it
that way... makes the investigation all the more fun. I suspect
that our data will soon reveal some more secrets."
The features on Iapetus are so remarkable that it has been suggested that they are signs of intelligent life there some time in the past. Below are close-ups of the "wall," rectilinear (not round) structures, and a mile-high tower.
