Images provided by NASA's Cassini spacecraft in 2005 reveal an almost straight-line equatorial mountain range that towers upwards of 12 miles and spreads as wide as 60 miles, encircling more than 75 percent of Iapetus, the ringed planet's third-largest moon, and causing it to resemble a walnut.
"There's nothing else like it in the solar system," said Andrew Dombard, associate professor of earth and environmental sciences at the University of Illinois at Chicago. "It's something we've never seen before and didn't expect to see."
Some scientists have hypothesized that Iapetus's mountains were formed by internal forces such as volcanism, but Dombard, along with Andrew Cheng, chief scientist in the space department at the Johns Hopkins University Applied Physics Laboratory, 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 or mini-moon orbiting Iapetus, which 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," said McKinnon. "At first the debris would have made holes to form a groove that eventually filled up."
Dombard and his collaborators 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 the 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's sub-satellite. Dombard said 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," said Cheng.
This lends plausibility to the rain of debris along the equator hypothesis, Dombard said, but he adds that more sophisticated computer modeling and analysis is planned in the coming years to back it up.
Other explanations have been proposed by scientists as to what caused this odd formation of mountains on Iapetus, but Dombard said they all have shortcomings.
"There are three critical observations that you need to explain," he said. "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 Dec. 15 at the American Geophysical Union's fall meeting in San Francisco.
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