There's a
strange moon whizzing around Saturn that's shaped, oddly, like a walnut.
Now
astronomers find that Iapetus got its nutty shape from a super-fast spin that was
frozen into place early in the solar system's formation.
When the
Cassini spacecraft snapped close-ups
of Saturn's moons in 2005, it revealed a bulging waistline of rock along the
equator of the now slowly spinning Iapetus. Astronomers think this
characteristic shape persists because Iapetus was cryogenically frozen in time
about 3 billion years ago, during the moon's "teen" years.
"Iapetus
spun fast, froze young and left behind a body with lasting curves," said
Julie Castillo, a Cassini scientist at NASA's Jet Propulsion Laboratory (JPL)
in Pasadena, Calif.
Unlike any
other moon in the solar system, Iapetus (eye-APP-eh-tuss) has retained its
immature figure. Running exactly along its midsection, a chain of mountains 808
miles (1,300 kilometers) long and 12 miles (19 kilometers) high adds to the
moon's walnut-like
appearance.
"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," said
Dennis Matson, another Cassini project scientist at JPL. However, Mason said,
"we've modeled how Iapetus formed its big, spin-generated bulge and why
its rotation slowed down to its present, nearly 80-day period."
Iapetus
originally spun once every five to 16 hours, which was fast enough to buckle
its surface at the equator, according to the new model detailed in an upcoming
online edition of the journal Icarus.
Scientists
think radioactive elements heated the moon's interior to permit the crust to
stretch and buckle, yet quickly froze the moon into shape as fuel ran out.
"Iapetus'
development literally stopped in its tracks," Castillo said of the 4.564
billion-year-old hunk of rock. In order to slow the young moon down its present
once-per-80-days rotational speed, Castillo explained, "its interior had
to be much warmer, close to the melting point for water ice."
The finding
should help astronomers better understand how the planets and their moon
systems formed in the early solar system.
"This
is the first direct evidence of the early spin history for a satellite in the
outer solar system," Matson said. "It ... broadens our knowledge of the
early history of outer planet satellites."
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