Pan isn’t the only Saturnian moon with a ridge. Before scientists even spotted Pan’s tutu-shaped equatorial fringe, they knew about the one on Iapetus, Saturn’s third-largest moon.
With a diameter of 1400 kilometers, Iapetus looks like any old, almost spherically shaped moon made of rock and ice—except for the seamlike ridge around its slightly bulging middle. Unlike Pan’s ridge, Iapetus’s ridge may not call to mind images of pasta or meat-filled delicacies. But scientists are just as intrigued because they have no idea how it formed.
In 2004, NASA’s Cassini spacecraft revealed Iapetus’s ridge to be some 20 kilometers high (that’s more than two Mount Everests on something about half the size of our Moon) and 15 kilometers wide in some places. The 1300-kilometer-long ridge circles almost the entirety of Iapetus’s equator, and huge mountains stand in spaces where the ridge breaks up.
“It’s something that we hadn’t seen anywhere else in the solar system,” at least before Pan, said Angela Stickle, a planetary scientist at Johns Hopkins University’s Applied Physics Laboratory in Laurel, Md.
Although Pan’s ridge formation is fairly straightforward—the moon has slowly siphoned off Saturnian ring material for many years—the ridge on Iapetus offers no obvious origin story.
However, Stickle, who studies impact dynamics throughout the solar system, and her colleagues have come up with one scenario. She thinks that millions of small impacts could explain the ridge. Stickle presented the research on 21 March at the 48th Lunar and Planetary Science Conference in The Woodlands, Texas.
Building a Ridge from the Outside In
Impacts create weird features on bodies all over the solar system, including rings of mountains, huge basins, and central peaks. An impact is even thought to be responsible for the formation of our own Moon. So it’s not unreasonable to think that an impact could have played a role in creating Iapetus’s ridge, Stickle said. Iapetus is covered in large craters, so scientists know that at one point, impacts were common.
To explain Iapetus’s unique ridge, Stickle and her colleagues suggest a double whammy: A long time ago, something slammed into Iapetus, launching enough material upward to form a ring of debris that circled space around the planet, like a mini Saturn ring. Over time, the light pull of Iapetus’s weak gravity caused the debris to fall back onto the moon in tens of millions of impacts that piled up material into an immense ridge.
The researchers tested the second part of this hypothesis—the many impacts from a debris disk—using an impact model. They set parameters such as size of impactors (in this case, between 1-meter- and 1-kilometer-sized chunks), the speed of impact (400 meters per second), and a low-angle entry.
Most people think of impacts as coming from straight above, which allows the impactor to focus all its energy into forming a massive crater and hurling debris into the sky. However, impacts from a disk of debris usually impact at a low, “grazing angle,” Stickle said. This is because as the orbit of the debris disk decays, it sends chunks of ice and rock swirling into Iapetus like marbles circling a drain. These bits of debris would skim the surface as they descended.
A grazing impact means that the chunk’s momentum, instead of being transferred immediately into the ground, would keep propelling the chunk forward. When a rock slams into a body at a low angle, its top “decapitates,” Stickle said, and the momentum in the sheared-off piece sends it speeding away, scouring the surface as it slides.
Because the disk’s material was likely orbiting in a rough plane, over time these decapitated bits would have run into each other, Stickle suggests, creating a traffic jam of meteorite heads. Enough pieces could have built up the near-complete ridge seen on Iapetus today, Stickle said.
Building a Ridge from the Inside Out
A series of impacts isn’t the only origin possibility for Iapetus’s ridge, Stickle said. Some scientists have proposed that the ridge formed from some internal process on Iapetus.
For example, a 2013 paper published in Earth, Planets and Space suggests that the ridge formed as Iapetus cooled and its interior separated into distinct layers. The authors suggest that in the moon’s early days, temperatures could have been hot enough to melt its interior, causing the formation of a dense core and a layer of low-density material between the core and the primitive crust.
This situation would have been unstable, triggering large-scale overturning motion, explained Leszek Czechowski, a geophysicist at the University of Warsaw and coauthor of the 2013 paper. During the overturn, buoyant, icy material would have moved upward at the equator while denser rock fell inward at the poles, he explained.
In Czechowski’s hypothesis, low-density material eventually formed a large bulging ring on the equator. Iapetus’s weak gravity then allowed the buoyant material to poke out as a ridge.
Stickle’s impact origin hypothesis “represents a good piece of science,” Czechowski said. However, he remains unswayed and prefers to think of Iapetus’s ridge as an internally created feature.
A Lingering Mystery
Until scientists can get a closer look at Iapetus to search for telltale scour marks or other evidence from low-angle impacts, the origins of its ridge will remain shrouded in mystery, Stickle said.
And the mystery is unlikely to be solved anytime soon. As Cassini’s mission comes to an end, its orbit moves closer and closer to Saturn itself—and away from its outer moons. The spacecraft is now far from Iapetus and has no plans to get any closer.
So Stickle and other scientists curious about Iapetus’s ridge will have to await the next mission to Saturn…whenever that may be.
—JoAnna Wendel (@JoAnnaScience), Staff Writer