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, or 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’s fall meeting in San Francisco.