Jets along Enceladus’s famous tiger stripes may be punching through the moon’s icy shell because strike-slip motion periodically tears the features. New research explains how tides between the moon and Saturn force the edges of the scab-like fractures back and forth until they split.
Strike-slip motion regulated by tides could regulate not just the timing and intensity of the moon’s jets but also the geological evolution of Enceladus’s south polar terrain, explained Alexander Berne, a geophysics graduate student at the California Institute of Technology in Pasadena and lead researcher on the study.
Enceladus’s four tiger stripes—named Alexandria, Cairo, Baghdad, and Damascus after locations in Arabian Nights—are roughly 135-kilometer-long fractures in the moon’s south polar terrain. Infrared measurements have shown that the stripes are much hotter than their surrounding areas, suggesting thinner or broken crust at those spots or an unknown heating mechanism.
Images from NASA’s Cassini mission revealed intermittent plumes of water, ice, and organic compounds venting from the stripes. The jets are thought to be tapping material from a subsurface ocean.
From Tides to Jets
“Tidal processes have played an important role in Enceladus’ evolution, the current presence of a subsurface liquid water ocean, and the spectacular plume activity,” explained Sarah Fagents, a planetary scientist at the University of Hawai‘i at Mānoa in Honolulu who was not involved with the research. But it has been difficult to explain how tides influence the timing and brightness of Enceladus’s jets, she said.
The moon’s eccentric 33-hour orbit stretches and contracts the icy crust, but there is a 6- to 7-hour lag between times of maximum crust stress and when a plume’s outflow (and thus its brightness) intensifies. Moreover, past simulations have shown that if tidal stress were opening the tiger stripes straight apart, like divergent plate boundaries on Earth, peak jet brightness would occur only once each orbit, but the jets instead peak twice.
To unriddle the mystery, the researchers simulated the tidal forces experienced at Enceladus’s south pole, inspired by newer earthquake models. “In the last 20 years, very sophisticated numerical models have been developed to model earthquake sequences on Earth along the San Andreas Fault and other places to try to understand the structure of the crust,” Berne said. “We adapted that [approach] for Enceladus.”
Their simulations revealed that for every orbit Enceladus makes around Saturn, the tiger stripes experience two periods of high shear stress from tides. The stress drives strike-slip motion along the two edges of each tiger stripe: Tectonic movement causes blocks of ice to slide past each other, rather than separate or crash together.
Strike-slip motion happened roughly 6–7 hours after the peak tidal stress, matching the lag in observed jet activity. What’s more, strike-slip motion occurred twice each orbit, a characteristic of jet activity that other tidal simulations had not been able to replicate.
Fagents found the research “persuasive,” adding that “the model elegantly explains a key and puzzling aspect of Enceladus’ plume dynamics.”
“The work made a compelling case that tidally driven strike-slip motion is broadly consistent with the plume activity,” said Miki Nakajima, a planetary scientist at the University of Rochester in New York who was not involved with the research.
The simulation showed that areas that slip more match observed hot spots. “This may be indicating that the strike-slip motion helps open the tiger stripes and encourage heat production,” Nakajima said.
Geologic Evolution
One curious feature of Enceladus’s jets is that one of the outflow peaks is brighter than the other. The new simulation reproduced the asymmetry and suggested that the edges of the tiger stripes tend to slip more in one direction than the other during each orbit. More slipping leads to more intense jets.
Berne cautioned that the team has not yet pinned down a mechanism that links the strike-slip activity to jet brightness. The team proposed that strike-slip motion weakens areas along the stripes that have already been stretched and morphed by tides until they eventually pull apart. They plan to explore this possible mechanism more fully in future work.
“It’s exactly the type of tricky modeling work we need to understand a body like Enceladus.”
“This work is really exciting,” said Hamish Hay, a planetary scientist at the University of Oxford in the United Kingdom who was not involved with the study. “It’s exactly the type of tricky modeling work we need to understand a body like Enceladus,” he said. “The correlation the authors find between fault activity and plume brightness is very intriguing.”
Features such as chasmata, rift zones that radiate like bicycle wheel spokes from the south polar terrain, could also have been created by this asymmetrical motion.
“This is how geology is created around tectonic faults, like the San Andreas Fault,” Berne said. “Our models, even though they only treat the short-term deformation problem, have implications for the long-term evolution of the south polar terrain.”
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer