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Saturn's
Old Moon Iapetus Retains Its Youthful Figure
Tuesday, July 17, 2007
Saturn's
moon Iapetus.
Credit:
NASA/JPL/Space Science Institute
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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.
Source:
NASA / JPL
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