JOHNS HOPKINS / WASHINGTON U.-ST. LOUIS (US) — A strange mountain range girding the equator of Saturn’s third-largest moon may have been formed not by geological forces but rather by the explosive breakup of an orbiting mini-moon, scientists suggest.
The strange geological landmark on Iapetus, revealed in images provided by NASA’s Cassini spacecraft, is an almost straight-line mountain range that towers higher than 12 miles and spreads as wide as 60 miles.
The formation make Iapetus look something like a walnut with a ridge between the halves of its shell. The Iapetus ridge, however, is higher than Mount Everest and extends for thousands of miles, almost completely around the equator of the 900-mile diameter satellite.
“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. “It’s something we’ve never seen before and didn’t expect to see.”
While other scientists have hypothesized that the Iapetus mountains were formed by internal forces such as volcanism, Dombard, along with researchers at Johns Hopkins University and Washington University in St. Louis, think the mountains resulted from icy debris raining down from a sub-satellite, a mini-moon orbiting Iapetus that burst into bits under the influence of tidal forces from 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 William McKinnon, professor of earth and planetary sciences at Washington University in St. Louis. “At first, the debris would have made holes to form a groove that eventually filled up.”
The scientists think the mini-moon was created by what planetary scientists call a “giant impact.” Such crashes and coalescing debris during the solar system’s formation more than 4 billion years ago formed other satellites such as Earth’s moon and Pluto’s largest satellite, Charon.
The team has done a preliminary analysis demonstrating the plausibility of impact formation and subsequent evolution of Iapetus’ sub-satellite. Dombard says that Iapetus has the largest Hill Sphere of any moon in the solar system. The Hill Sphere is the zone surrounding a moon where its own gravitational force is stronger than that of the planet it circles.
Iapetus “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,” says Andrew Cheng, chief scientist in the space department at Johns Hopkins University.
This fact lends plausibility to the team’s suggestion that a later rain of debris along the equator formed the Iapetus ridge, Dombard says. He adds that more sophisticated computer modeling and analysis is planned in the coming years.
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 are presenting details of their model on Dec. 15 at the American Geophysical Union Fall Meeting in San Francisco.