[meteorite-list] Saturn's Old Moon Iapetus Retains Its Youthful Figure

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 17 Jul 2007 16:20:25 -0700 (PDT)
Message-ID: <200707172320.QAA05461_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.cfm?release=2007-079

Saturn's Old Moon Iapetus Retains Its Youthful Figure
Jet Propulsion Laboratory
July 17, 2007

Saturn's distinctive moon Iapetus (eye-APP-eh-tuss) is cryogenically
frozen in the equivalent of its teenage years. The moon has retained the
youthful figure and bulging waistline it sported more than three billion
years ago, scientists report.

"Iapetus spun fast, froze young, and left behind a body with lasting
curves," said Julie Castillo, Cassini scientist at NASA's Jet Propulsion
Laboratory, Pasadena, Calif.

Unlike any other moon in the solar system, Iapetus is the same shape
today as it was when it was just a few hundred million years old; a
well-preserved relic from the time when the solar system was young.
These results appear in the online version of the journal Icarus.
Cassini flew by Iapetus in early 2005 and discovered the moon had a
walnut shape, bulging at its midsection. On top of that it has a chain
of mountains located exactly along its equator.

Scientists now think the moon's bulging midriff and slow spin rate point
to heating from long-extinct radioactive elements present when the solar
system was born.

"We've modeled how Iapetus formed its big, spin-generated bulge and why
its rotation slowed down to its present nearly 80-day period. As an
unexpected bonus, Iapetus also told us how old it was," said Dennis
Matson, Cassini project scientist at JPL. "You would expect a very
fast-spinning moon to have this bulge, but not a slow-spinning moon,
because the bulge would have been much flatter."

Scientists calculate Iapetus originally rotated much faster--at least
five hours, but less than 16 hours per revolution. The fast spin gave
the moon an oblate shape that increased the surface area (in the same
way the surface area of a round balloon stretches when the balloon is
pressed into an oblate shape). By the time the rotation slowed down to a
period of 16 hours, the outer shell of the moon had frozen. Furthermore,
the surface area of the cold moon was now smaller. The excess surface
material was too rigid to go back smoothly into the moon. Instead, it
piled up in a chain of mountains at the equator.

"Iapetus' development literally stopped in its tracks," said Castillo.
"In order for tidal forces to slow Iapetus to its current spin rate, its
interior had to be much warmer, close to the melting point for water
ice." The challenge in developing a model of how Iapetus came to be
"frozen in time" has been in deducing how it ever became warm enough to
form a bulge in the first place, and figuring out what caused the heat
source to turn off, leaving Iapetus to freeze.

The heat source had to have a limited life span, to allow the moon's
crust to rapidly become cold and retain its immature shape. After
looking at several models, scientists concluded that the heat came from
its rocks, which contain short-lived radioactive isotopes aluminum-26
and iron-60 (which decay very rapidly on a geologic timescale). Since
these elements decay at a known rate, this allowed scientists to "carbon
date" Iapetus by using aluminum-26 instead of carbon. Scientists
calculate the age of Iapetus to be roughly 4.564 billion years old.

Evidence for these same isotopes (aluminum-26 and iron-60) has been
found in meteorites formed in the inner solar system. Therefore, there
is a possibility of comparing the early chronology of the outer solar
system with other objects in the inner solar system, such as Earth,
Earth's moon and asteroids.

"This is the first direct evidence of the early spin history for a
satellite in the outer solar system. It teaches us more about how the
speed of a body's rotation influenced its evolution, and broadens our
knowledge of the early history of outer planet satellites," said Matson.

Cassini's next close encounter with Iapetus will occur on Sept. 10,
2007, at 1,000 kilometers (620 miles) from the surface.

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, D.C.

More information on the Cassini mission is available at:
http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini.

------------------------------------------------------------------------

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

2007-079
Received on Tue 17 Jul 2007 07:20:25 PM PDT