Scientists have found evidence that Saturn’s moon Enceladus may have been tipped over at some point in its past. The culprit may have been a collision with an asteroid or another small body.
Using data from NASA’s Cassini mission, currently in orbit around Saturn, researchers led by Cornell University in Ithaca, New York, found what appear to be remnants of a previous equator and poles. Their findings are published in the journal Icarus.
This suggests the moon may now be tipped by up to 55 degrees from its original axis, more than halfway to rolling on its side. This could explain why the two poles of Enceladus today look so different.
"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 and lead author of the paper, in a statement.
The idea is based on something called the “true polar wander” (TPW), where the axis of a body shifts as its mass is moved around. Last year, it was suggested something similar may have happened on Mars due to volcanism. On Enceladus, an impact looks more likely.
The south pole of the moon has a series of streaks across the surface known as tiger stripes. These are thought to be formed in part by water seeping out from the vast ocean under the surface. On numerous occasions, Cassini has seen jets of water spewing from the south pole of Enceladus.
No such activity takes place at the north pole, though. So it may be that an asteroid struck the region where the tiger stripes appear now. This made the moon’s rotation unsteady and wobbly, and after a million years it settled in its new position with its axis being reoriented.
This explains why the poles do not look similar, with only one ejecting water from the ocean below. The south pole is active and young, but the north pole appears much older. It’s likely that they once looked similar, but the impact reshaped what the new south pole looks like.
“The polar asymmetry seen today remains peculiar; nonetheless, our results show that Enceladus before TPW had polar symmetry, with topographic and geological features that can be explained through plausible geophysical processes,” the researchers conclude at the end of their paper.