Cassini spacecraft images show the scars of collisions with many space rocks but astronomers say they are focusing on a curving, narrow fracture known as a graben, a block of ground lower than its surroundings and bordered by cliffs on either side.
This feature appears remarkably recent, cutting most of the craters it crosses with only a few small craters superimposed, a release from NASA's Jet Propulsion Laboratory in Pasadena, Calif., reported Monday.
Flying by Rhea at an altitude of 620 miles, Cassini's camera's captured 12 images of Rhea's rough and icy surface, researchers said.
It was Cassini's fourth close encounter with Rhea. The spacecraft will pass the moon again in a few years, NASA said, but at a much greater distance.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency.