A mountain range like no other in the solar system encircles the equator of Iapetus up to 12.4 miles high and 124 miles wide, forming a ridge that makes the moon resemble a walnut.
No other planet or moon in the solar system has this kind of ridge, a feature that has long puzzled astronomers.
"I would love to stand at the base of this wall of ice 20 kilometers tall that heads off straight in either direction until it dips below the horizon," Andrew Dombard, a planetary scientist at the University of Illinois at Chicago, told SPACE.com.
Dombard and fellow researchers suggest the ridge could be the remains of a dead moon, created by a giant impact more than 4.5 billion years ago.
Rubble blasted from Iapetus in the impact could have coalesced around Iapetus as a "sub-satellite," a moon of a moon, they said.
Then the gravitational pull of Iapetus could have eventually torn the companion moon back into pieces, creating an orbiting ring of debris that then rained down onto Iapetus, forming the ridge that now exists.
The ridge could have formed in a short time, in cosmic terms, "probably on a scale of centuries," Dombard said.