Saturn's icy, ocean-bearing moon Enceladus may have tipped over in the distant past, possibly due to a collision with a smaller body such as an asteroid, according to new data from NASA's Cassini spacecraft.
Researchers found evidence that the moon's spin axis - the line through the north and south poles - has reoriented. Examining the moon's features, the team showed that Enceladus appears to have tipped away from its original axis by about 55 degrees - more than halfway toward rolling completely onto its side.
"We found a chain of low areas, or basins, that trace a belt across the moon's surface that we believe are the fossil remnants of an earlier, previous equator and poles," said Radwan Tajeddine, a Cassini imaging team associate at Cornell University in the US.
The area around the icy moon's current south pole is a geologically active region where long, linear fractures referred to as tiger stripes slice across the surface. Researchers speculate that an asteroid may have struck the region in the past when it was closer to the equator.
"The geological activity in this terrain is unlikely to have been initiated by internal processes," Tajeddine said.
"We think that, in order to drive such a large reorientation of the moon, it's possible that an impact was behind the formation of this anomalous terrain," he said.
In 2005, Cassini discovered that jets of water vapour and icy particles spray from the tiger stripe fractures – evidence that an underground ocean is venting directly into space from beneath the active south polar terrain.
Whether it was caused by an impact or some other process, researchers said that the disruption and creation of the tiger-stripe terrain caused some of Enceladus' mass to be redistributed, making the moon's rotation unsteady and wobbly. The rotation would have eventually stabilised, likely taking more than a million years.
By the time the rotation settled down, the north-south axis would have reoriented to pass through different points on the surface - a mechanism called "true polar wander."
The polar wander idea helps to explain why Enceladus' modern-day north and south poles appear quite different. The south is active and geologically young, while the north is covered in craters and appears much older. The moon's original poles would have looked more alike before the event that caused Enceladus to tip over and relocate the disrupted tiger-stripe terrain to the moon's south polar region.