When NASA’s Cassini mission arrived at Saturn’s moon Enceladus, it found geysers of material spewing from the southern hemisphere. Four long, parallel scratches, dubbed tiger stripes, scarred Enceladus’s south pole, venting material from the moon’s interior into space. But the moon’s north pole remained surprisingly unblemished, a mystery that has continued to puzzle planetary scientists for more than a decade.
Now new research supports an idea that many have long considered: Ice at the northern pole could be thicker than its striped southern companion, preventing cracking. The paper, published in Earth and Planetary Science Letters in August, also examines the tiger stripes in context with other scars on Enceladus, such as shallower scratches at the equator.
The idea of thinner ice around the southern pole is certainly not new. Observations from Cassini revealed that the moon’s ice is thicker around the equator and thinner at the poles, especially the southern pole. But previous studies have failed to simulate the formation of cracks that resemble the tiger stripes. Those studies focused on what happened when gravitational tugs on the moon were at their strongest. The new study considers the breaking point of the ice itself.
“One of the challenges of Enceladus is, if you can make fractures at the south pole with low stress, you should make fractures at the north pole with the same stress,” said first author Alyssa Rhoden, a planetary scientist at the Southwest Research Institute in Colorado. “You have to have a reason that you don’t see it at the north pole.”
As Enceladus orbits Saturn, it experiences a gravitational pull not only from the giant planet but also from its fellow moons. These tidal stresses can dramatically increase when the moons reach specific alignments, exerting a powerful tug on Enceladus’s icy shell. But even when the moons don’t line up, Enceladus continues to feel their presence through tidal stresses.
Rhoden and her colleagues modeled a variety of icy shell structures to see how they would react to the pull of lunar tides. They found that the ever present tugs created stripes most effectively when the ice at the southern pole was less than 5 kilometers thick. Ice at the northern pole most likely reaches a thickness of more than 10 kilometers, preventing it from breaking.
Rhoden compared Enceladus’s icebreaking process to stretching a rubber band until it snaps. “When it breaks, it doesn’t matter how much you would or could have pulled it; it only matters what the failure strength of the rubber band was,” said Rhoden.
Previous studies that focused on what happened to ice at the point of the strongest tidal stress often created fractures, but the simulations could not point the tiger stripes in the appropriate direction. But by simulating ice cracking at its failure strength, rather than at the maximum point of pressure, Rhoden and her colleagues were able to accurately model the features.
“When you hit that breaking point, you get the right orientations,” Rhoden said.
Mikael Beuthe, a researcher at the Royal Observatory of Belgium, remained cautious about the results. According to Beuthe, who was not involved with the new research, the forces Rhoden and her team modeled are very small, smaller than the typical yield strength of pure ice seen in the lab. At the same time, he acknowledged that researchers don’t know the resistance of Enceladus’s ice in real conditions. It’s possible that it could be weak or porous, filled with debris from Saturn’s rings rather than made up completely of water ice.
“There are a lot of things we don’t know and have to clear up to understand these worlds,” Beuthe said, referring to all of the icy moons of Saturn and Jupiter.
An Evolving Ocean
Enceladus’s peculiar ice patterns are among the most intriguing puzzles for planetary scientists to solve: Why is the ice on Enceladus’s north pole so much thicker than the ice on its south pole? How did the series of cracks observed along the moon’s equator form?
Scientists have explored whether an impact event may have cracked Enceladus’s shell or whether a tidal process thinned the ice gradually over time. “This is a big mystery and an area of active research now,” Beuthe said.
Rhoden suspects the moon’s global ocean could provide insight into its ice formations. As Enceladus orbits Saturn, the changing stresses could cause the ocean to routinely freeze and melt, putting pressure on different points in the ice at different points in time.
“Eventually something breaks,” Rhoden said.
Although there is currently no mission planned to visit Enceladus, Beuthe said the upcoming Europa Clipper mission to Jupiter’s moon Europa may provide some insight into Enceladus. Like Enceladus, Europa hides a global ocean beneath an icy crust. Some faint, transient plumes have been identified spouting from Europa, although they are nothing like the mighty tiger stripes.
“For the ice itself, we don’-t expect much difference [between Enceladus and Europa],” Beuthe said. Until data extrapolated from Europa Clipper or a future Enceladus mission present new evidence, Beuthe and Rhoden will continue to model the moon’s features. “We’re going to work on simulations until we’re contradicted by reality,” Beuthe said, “until something tells us it works otherwise.”
—Nola Taylor Redd (@NolaTRedd), Science Writer