Swelling, Freezing Seas May Have Given Enceladus Its Tiger Stripes

With a lunar entourage that now numbers 82, Saturn has its fair share of weird moons. But few have flummoxed scientists more than Enceladus—a volcanically active body streaked with four “tiger stripes” etched into its southern pole.

First spotted by the Cassini spacecraft in 2005, the stripes are actually fissures through which Enceladus spews geysers of salty water vapor. And since their discovery, these briny burps from the vast ocean beneath the moon’s icy crust have generated a fair bit of buzz. Erupting from the stripes are jets of water that appear to be teeming with organic molecules and hydrogen, ingredients that hint at the potential for life on Enceladus.

In the journal Nature Astronomy, researchers propose a possible origin story for Enceladus’ tiger stripes. The team suggests that as waters froze and expanded beneath the moon’s surface during a cooling period, they strained Enceladus’ icy shell until it broke, producing the parallel quartet of southerly cracks we see today.

“It’s like your pipes freezing on a cold day,” Francis Nimmo, a planetary scientist at the University of California, Santa Cruz who was not involved in the study, told Adam Mann at Science last month, when the paper was first posted on the preprint server arXiv.

Study author Doug Hemingway, a planetary scientist at the Carnegie Institution for Science, and his colleagues arrived at their conclusions after extensively modeling the physical evolution of Enceladus’ icy shell.

The first fissure that formed, they argue, was the largest—an 80-mile-long feature named Baghdad, reports Nadia Drake for the New York Times. Water vapor from the sea below began to vent out of Baghdad, freezing in the atmosphere and drifting back down to the moon’s surface in a form of snow. The weight of the material began to strain the moon from the outside, forcing the formation of two more cracks, Cairo and Damascus, bookending Baghdad by about 20 miles each.

A similar sequence, the researchers say, eventually yielded the final major crack called Alexandria, another 20 miles away from Cairo, as well as a minor, stripe-like feature called E. (Unlike its larger siblings, E doesn’t usually headline as one of Enceladus’ primary tiger stripes; it’s also the only fissure not to bear a name deriving from the One Thousand and One Nights collection of Middle Eastern folktales.)

Eventually, the stripes’ domino effect petered out, leaving just four giant features raked across Enceladus’ south pole.

Enceladus' striped were earned over time. In their model, Hemingway and his colleagues suggest Enceladus acquired its stripes over 100,000 to 1 million years. The moon’s relatively low gravity may have helped things along; without the crushing weight of surface ice to weld burgeoning cracks shut, fissures can more easily crack the moon’s surface and persist. That may be why other icy moons in our solar system, like Europa, Callisto and Ganymede, remain fracture-free.

Enceladus’ ice is thinnest at its poles, making both likely spots for the first fissures to appear. “It’s kind of a coin toss whether that first fracture happens at the north pole or the south pole,” Hemingway tells Drake. For whatever reason, the south succumbed to the pressure first, perhaps relieving the strain that might have driven more splintering in the north, Mann reports.

“[This study] corrals a host of observations of Enceladus’ south polar terrain and its geysering activity with a rather simple idea,” Carolyn Porco, a planetary scientist at the Space Science Institute and Cassini team member who wasn’t involved in the study, tells Monica Young at Sky & Telescope. Understanding the stripes better, Porco adds, may also help scientists understand other mysterious aspects of Enceladus’ geology, including more about its tantalizing geysers.

“Another puzzle awaits the intrepid!” Porco tells Young. And in the meantime, we’ve got plenty of cool facts about Enceladus to tide us over. Spewing those, after all, is always a good way to break the ice.

Katherine J. Wu | | READ MORE

Katherine J. Wu is a Boston-based science journalist and Story Collider senior producer whose work has appeared in National Geographic, Undark magazine, Popular Science and more. She holds a Ph.D. in Microbiology and Immunobiology from Harvard University, and was Smithsonian magazine's 2018 AAAS Mass Media Fellow.