Huge "tiger stripe" fault lines seen on Saturn's moon Enceladus raise hopes that a "long-lived" ocean containing potential alien life may lurk beneath the moon's icy shell.
Fresh investigations have unveiled the lateral side-by-side movement along distinct "tiger stripes" on Enceladus, Saturn's moon, is correlated with bursts of ice crystals erupting from its frozen crust. These discoveries could shed light on the attributes of this icy satellite's subsurface ocean and, consequently, its potential for harboring life.
Enceladus' tiger stripes comprise four parallel fissures at the moon's southern pole initially spotted by NASA's Cassini probe in 2005. In this region, "cryovolcanism" propels ice crystals believed to stem from Enceladus' concealed ocean through these cracks, generating an expansive plume of material above the Saturnian moon's southern pole.
Both the luminosity of this plume and the emanating jets display fluctuations that align with Enceladus' roughly 33-hour orbit around Saturn, the solar system's second most massive planet. This has led researchers to propose that the jet activity intensifies as tidal forces act upon the tiger stripes.
Nevertheless, this hypothesis fails to elucidate why Enceladus' jets reach their brightest hours after tidal stresses peak or why there's a secondary, albeit smaller, peak observed shortly after Enceladus' closest approach to Saturn. A novel computational simulation of Enceladus' tidal forces and the movement of its tiger stripe fractures pinpoints a phenomenon akin to what's observed at the San Andreas fault, corresponding with the jet activity pattern.
"We devised an intricate computational model to replicate tide-driven lateral motion along Enceladus' faults. These simulations take into account the influence of friction, which causes the degree of slippage on the faults to be responsive to both compressive and shearing stresses," remarked Alexander Berne, head of the research team behind the simulation and a doctoral candidate at the California Institute of Technology (Caltech), to Space.com.
"The computational model successfully replicated slippage along Enceladus' faults in a manner consistent with observed fluctuations in plume brightness as well as spatial variances in surface temperature, indicating that the jets and plume brightness variations are governed by lateral motion over Enceladus' orbital path."
The San Andreas fault in space
Berne and his team uncovered that frictional dynamics govern the movement in interfaces along the tiger stripes of Enceladus, where both edges of the fractures converge. This implies that during Enceladus's orbital journey, the tiger strips undergo periodic sliding and locking. This lateral, or "strike-slip," movement aligns with the activity of the jets.
The correlation between strike-slip motion and jet luminosity in the simulation led the researchers to propose that fluctuations in jet activity are regulated by the presence of 'pull-aparts' along the faults. These are curved segments of the fractures that open up during extensive strike-slip motion, enabling water to ascend from the subsurface ocean through the icy shell to fuel the cryovolcanic jets.
"A terrestrial comparison would be motion along pull-apart basin formations along extensive strike-slip faults subject to tectonic strains. One instance of such motion occurs over the Salton Basin – a sizable pull-apart situated on the San Andreas fault, a lateral fault, in Southern California," Berne explained. "Regional lateral movement induces localized crustal extension and volcanism over the Salton Basin. This process mirrors tidally-induced extension along pull-aparts at Enceladus, which may govern the moon's cryovolcanic activity.
"Before embarking on this research, we didn't anticipate such a strong correlation between modeled lateral motion and jet activity."
The team's findings suggest that the tiger stripes of Enceladus open differently than previously envisioned.
"This discovery was unexpected as most past studies on the topic propose extensive opening along the tiger stripes, akin to a sliding and closing motion like an elevator door, as the primary mechanism regulating variations in plume brightness," Berne noted.
The researcher from Caltech added that the team's models indicate that tides play a pivotal role in the evolution of Enceladus and its ocean across multiple time frames.
"On the orbital scale, tides seem to govern the volume of material flowing from a subsurface ocean through the tiger stripe fractures," Berne stated. "Over longer time scales, tides may induce frictional tiger stripes to fracture in a net right-lateral manner."
He went on to suggest that this prolonged right-lateral movement might propel the formation of geological features observed across the terrain of Enceladus's southern pole. This encompasses a fracture emanating from the South Pole on the trailing Hemisphere of Enceladus.
Researchers have posited that Enceladus, with its hidden global ocean, could serve as a prime target for the quest for extraterrestrial life within the solar system. This study and the team's model could provide additional backing for that conjecture.
"Comprehending subsurface material transport pathways through pull-apart or extensive rift zones is critical for determining whether ice particles in Enceladus's jets accurately represent the moon's potentially hospitable global ocean. Our study furnishes a framework for comprehending such transport pathways and their progression over time," Berne affirmed.
"Evidence for the enduring influence of tides on the evolution of Enceladus, which also heat up the interior, implies that the moon's ocean has persisted for a substantial duration, which bears implications for the potential evolution of life within."
Currently, the team's conclusion relies on a computer simulation and thus necessitates validation through actual observations.
"Geophysical measurements on Enceladus using radar would enable us to verify or refute the hypotheses outlined in our paper. More broadly, such observations of the surface movement of Enceladus over time can furnish vital constraints on the dynamics of the core and the crust, as well as the extent of activity of these processes across time," Berne concluded. "We aim to persist in exploring ways we can leverage geophysical measurements to better grasp the conditions that may foster the formation and evolution of life on Enceladus."
The team's research was published on Monday (April 29) in the journal Nature Geoscience.
