While combing through data gathered by NASA’s Cassini spacecraft during flybys of Enceladus, the sixth-largest of Saturn’s moons, researchers have found the first evidence that the frozen moon’s axis has reoriented, possibly due to a collision with a smaller body.
This enhanced-color view of Enceladus is largely of the southern hemisphere and includes the south polar terrain at the bottom of the image. Ancient craters remain somewhat pristine in some locales, but have clearly relaxed in others. Northward-trending fractures, likely caused by a change in the moon’s rate of rotation and the consequent flattening of the moon’s shape, rip across the southern hemisphere. The south polar terrain is marked by a striking set of ‘blue’ fractures and encircled by a conspicuous and continuous chain of folds and ridges, testament to the forces within Enceladus that have yet to be silenced. The mosaic was created from 21 false-color frames taken during Cassini’s close approaches to Enceladus on March 9 and July 14, 2005. Images taken using filters sensitive to UV, visible and IR light (spanning wavelengths from 338 to 930 nm) were combined to create the individual frames. Image credit: NASA / JPL / Space Science Institute.
Examining Enceladus’ geological features, the scientists showed how Enceladus appears to have tipped away from its original axis by about 55 degrees.
“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 Cassini imaging team member Dr. Radwan Tajeddine, of Cornell University.
“Their pattern reflects spatial variations in the icy shell, consistent with a variety of geological features visible in Cassini images.”
At Enceladus’ current southern end, active jets discharge water vapor — as well as organic compounds, gases, salts and silica — through vents from an ocean deep beneath the moon’s icy-crust surface.
It’s a place technically known as the south polar terrain, and researchers have nicknamed the long, geologically active fractures ‘tiger stripes’ — each about 80 miles long and a little over a mile wide.
Dr. Tajeddine and co-authors believe an asteroid may have struck the moon’s current south polar region when it was closer to the equator in the past.
“The geological activity in this terrain is unlikely to have been initiated by internal processes,” Dr. 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.”
Tajeddine et al have found evidence that Enceladus may have tipped over, reorienting itself so that terrain closer to its original equator was relocated to the poles. Image credit: NASA / JPL-Caltech / Space Science Institute / Cornell University.
Wobbly, rickety and unsteady after an asteroid’s smack, the physics of Enceladus’ rotation would have eventually re-established stability, a process that likely took over a million years.
To do that, the north-south axis needed to change — a mechanism called ‘true polar wander.’
Enceladus’ topographic and geological features can be explained through geophysical processes, but the icy moon’s north and south poles are quite different.
The south is active and geologically young, while the north is covered in craters and appears much older.
“The differences Cassini has observed between the north and south poles remains peculiar,” Dr. Tajeddine said.
“Originally, the poles of Enceladus would have been the same, more or less, before true polar wander occurred.”
“The true polar wander hypothesis seems very plausible when we take a combined look at the patterns of highs and lows across the moon’s surface, the physical appearance of surface features and the differences between the current poles.”
The research is published in the journal Icarus.
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Radwan Tajeddine et al. 2017. True polar wander of Enceladus from topographic data. Icarus 295: 46-60; doi: 10.1016/j.icarus.2017.04.019
