When the Cassini spacecraft swung by Saturn’s moon Enceladus in 2005, its cameras glimpsed a particularly arresting feature of the alien world: tiger stripes. Where fractures in Europa’s icy shell race across the surface at random angles, Enceladus’s surface hosts five epic fissures running in parallel across its southern pole, each stretching roughly 80 miles in length.
Now a team of physicists suggests they’ve cracked the origin of these stripes: an unruly ocean burst into space and fell as snow that overloaded ice shelves enough to create the handful of rather uniform cracks, according to results published Monday in Nature Astronomy. The theory also has implications for how water escapes from the moon into orbit, where Cassini was able to taste it and determine that the liquid has most of the chemical ingredients needed to support life as we know it.
“Since it is thanks to these fissures that we have been able to sample and study Enceladus’s subsurface ocean, which is beloved by astrobiologists, we thought it was important to understand the forces that formed and sustained them,” said Douglas Hemingway, a physicist at the Carnegie Institution for Science and paper coauthor, in a press release.
It’s a tidy tale that builds upon previous work to explain why the stripes occurred at a pole, and why they run side by side, but some geologists wonder if it asks for too many miracles. “They need so many assumptions to line up,” says An Yin, a planetary scientist at the University of California, Los Angeles who was not involved with the study. “It’s like three or four perfect [scenarios] that have to come together.”
The story starts with an ocean that sloshes beneath an ice shell. As Enceladus cooled in the distant past, a layer of seawater froze and expanded. Locked underwater by the crust, this expansion pressurized the ocean until, like a glass bottle left too long in the freezer, something had to give.
Saturn’s gravity occasionally squeezes Enceladus as it orbits, a process that’s thinned the crust at the moon’s poles. As the pressure built, it was the south pole that happened to give way first, Hemingway and colleagues contend. The icy crust was ripped apart, exposing the ocean below, and a long crevasse now known as Baghdad ran straight through the world’s southernmost point.
Oceanic tides on the moon caused by Saturn’s gravitational tugs keeps the crack from freezing over, previous research has suggested, by repeatedly ripping it apart and forcing it back together. “You’re actually flushing water out and pumping it back in,” Hemingway says, which churns the water and keeps the crevice warm. Eternally exposed to the vacuum of space, significant quantities of water continuously leach into the sky, generating plumes like the one Cassini dove through.
Ice pellets from those plumes rained down on either side of the Baghdad fissure. They built up over time and weighed each ice sheet down until they cracked, creating Cairo and Damascus (the stripes take their names from places featured in One Thousand and One Nights, a collection of Middle Eastern and Indian folk tales) at about 20 miles to either side. Yin likens the situation to putting a child on the tip of a thin diving board and then feeding them until the diving board buckles.
The process then played out, giving rise to the Alexandria fissure, and one more partial crack too shallow to reach the subsurface ocean, which goes by the unofficial nickname “E.” Hemingway and his colleagues estimated that it took between one hundred thousand and one million years for each fissure to spew enough seawater and rain down enough ice to produce each subsequent pair of cracks. No more crevasses should form, however, because as they stray farther from the pole the ice gets too thick.
Yet some geologists remain skeptical that the new theory really describes Enceladus’s history. “I personally think that the chances are very small,” Yin said. He views the assumption that the global ocean melted and froze uniformly as unrealistic, because picture-perfect craters in the moon’s northern hemisphere show little sign of past trauma from whole-world heating or cooling.
Instead, he suggests that a burst of heat from the moon’s interior liquified part of a half-mile high plateau, melting it like a block of cheese in a pan. As this ice slide cascaded sideways into the moon’s frozen shell, it pushed up wrinkles that remain visible. These rolls then would have frozen in place, and mounting horizontal pressure from the ice slide, he says, would have torn open all five tiger stripes at the same time. Yin and his colleagues detailed their theory, which he says explains more features with fewer assumptions, in a pair of publications in 2015 and 2016.
Yin welcomes the competition from the new ocean-bursting theory—which he agrees is not impossible—because it makes testable predictions. The plumes of evaporating water should have put down layer after layer of ice as they weighted down the ice shelves over hundreds of thousands of years, and the stripes should have exposed these layers. Hemingway’s chasms should also feature deep cuts with exposed seawater along the entire fault, where Yin’s fissures would have exposed seawater that shoots off plumes only at certain points. Current imaging technology can’t distinguish between the two cases, but future exploration of Enceladus will make the origin of the tiger stripes much clearer. Cassini alumni have proposed a follow up “Enceladus Life Finder” mission to trek back to the ice world, but it has not yet received funding.
“In the distant future if we have spacecraft in the orbit, or landers,” Hemingway says, “we can get a much more detailed understanding of the interior, which can put some ideas to the test.”
