Saturn's Old Moon Iapetus Retains Its Youthful Figure
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.
Image right: Saturn's moon Iapetus. Image credit: NASA/JPL/Space Science Institute + Larger view
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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
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
"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:
Media contact: Carolina Martinez 818-354-9382
Jet Propulsion Laboratory, Pasadena, Calif.