[meteorite-list] Mystery on Saturn's Satellite: Icy Debris Formed Iapetus' Ridge?

[Ron Baalke]

The Johns Hopkins University Applied Physics Laboratory
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Diana Lutz, Washington University in St. Louis
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dlutz at wustl.edu

December 13, 2010

FOR IMMEDIATE RELEASE

MYSTERY ON SATURN'S SATELLITE: ICY DEBRIS FORMED IAPETUS' RIDGE?

As space-based probes and telescopes continue to reveal new and 
seemingly unimaginable features of our universe, a geological 
landmark on Saturn's moon Iapetus ranks high on the list of things 
particularly peculiar.

Images provided by NASA's Cassini spacecraft in 2005 reveal an almost 
straight-line mountain range that towers higher than 12 miles and 
spreads as wide as 60 miles, spanning more than 75 percent of the 
equatorial belt on Iapetus - the ringed planet's third-largest moon 
at 900 miles in diameter. It is shaped like a walnut, which has a 
ridge between the halves of its shell, but the Iapetus ridge is 
higher than Mount Everest and extends for thousands of miles - almost 
completely around the moon's equator.

"There's nothing else like it in the solar system," says Andrew 
Dombard, associate professor of earth and environmental sciences at 
the University of Illinois at Chicago (UIC). "It's something we've 
never seen before and didn't expect to see."

While other scientists have hypothesized that Iapetus' mountains were 
formed by internal forces such as volcanism, Dombard, along with 
Andrew Cheng, chief scientist in the Space Department at the Johns 
Hopkins University Applied Physics Laboratory in Laurel, Md., William 
McKinnon, professor of earth and planetary sciences at Washington 
University in St. Louis and Jonathan Kay, a UIC graduate student 
studying with Dombard, think the mountains resulted from icy debris 
raining down from a sub-satellite, a mini-moon orbiting Iapetus that 
burst into bits under tidal forces of the larger moon.

"Imagine all of these particles coming down horizontally across the 
equatorial surface at about 400 meters per second - the speed of a 
rifle bullet, one after another, like frozen baseballs," says 
McKinnon. "At first, the debris would have made holes to form a 
groove that eventually filled up."

The scientists think the phenomenon is the result of what planetary 
scientists call a "giant impact," where crashing and coalescing 
debris during the solar system's formation more than 4 billion years 
ago created satellites such as Earth's moon and Pluto's largest 
satellite, Charon.

They've done a preliminary analysis demonstrating the plausibility of 
impact formation and subsequent evolution of Iapetus' sub-satellite. 
Dombard says Iapetus is the solar system's moon with the largest Hill 
Sphere - the zone surrounding a moon where the gravitational force is 
stronger than that of the planet it circles. "It is the only moon far 
enough from its planet, and large enough relative to its planet, that 
a giant impact may be able to form a sub-satellite," Cheng says.

This fact lends plausibility to the rain of debris along the equator 
hypothesis, Dombard says, but he adds that more sophisticated 
computer modeling and analysis is planned in the coming years to back it up.

Several other models have been proposed by scientists about what 
caused this odd formation of mountains on Iapetus, but Dombard says 
they all have shortcomings.

"There are three critical observations that you need to explain," he 
says "Why the mountains sit on the equator, why it's found only on 
the equator, and why only on Iapetus? Previous models address maybe 
one or two of those critical observations. We think we can explain all three."

The planetary scientists will present details of their model on Dec. 
15 at the American Geophysical Union Fall Meeting in San Francisco.

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