New research has found that the side-to-side motion along distinctive 'tiger stripes' on Saturn's moon Enceladus is linked to jets of ice crystals erupting from its icy shell. The findings could help determine the characteristics of this icy moon of Saturn's subsurface ocean, and thus whether Enceladus is favorable for life.
Enceladus' tiger stripes consist of four parallel line breaks in the moon's south pole that were first observed by NASA's Cassini spacecraft in 2005. "Cryovolcanism" in this region shoots out ice crystals believed to come from the buried ocean of Enceladus. a broad plume of material collecting above the south pole of Saturn's moon.
Both the brightness of this plume and the jets that create it appear to vary in a pattern consistent with Enceladus' nearly 33-hour orbit around Saturn, the solar system's second-heaviest planet. This has led scientists to theorize that the jets' activity increases as tidal stress acts on the tiger stripes.
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However, this theory cannot explain why Enceladus's jets reach their maximum brightness hours after tidal stresses reach their maximum, or why a second smaller peak is observed shortly after Enceladus's closest approach to Saturn. A new numerical simulation of Enceladus' tidal stresses and the motion of the Tiger Stripe faults identifies a phenomenon similar to that observed on the San Andreas fault, corresponding to the pattern of jet activity.
"We have developed an advanced numerical model to simulate the tidally driven strike-slip motion along Enceladus' faults. These models take into account the role of friction, making the amount of slip on the faults sensitive to both compression and shear stresses" , says Alexander. Berne, leader of the team behind the simulation and a PhD candidate at the California Institute of Technology (Caltech), told Space.com.
"The numerical model was able to simulate slip along Enceladus' faults in a way that was consistent with observed variations in the plume brightness and with spatial variations in the surface temperature, suggesting that the variations in the jets and the brightness of the plume is controlled by the motion of the beat and the slip along the orbit of Enceladus."
The San Andreas Fault in space
Berne and colleagues discovered that frictional mechanics controls motion at interfaces along Enceladus' tiger stripes, where both sides of the faults meet. This means that during Enceladus' orbital cycle, the tiger strips shift and lock periodically. This side-to-side or 'strike-slip' motion corresponds to jet activity.
The correlation between the strike-slip activity and the brightness of the jets in the simulation led the team to hypothesize that variations in the jet activity are controlled by the presence of 'pulled apart' parts along the fault lines. These are curved parts of the faults that open under a broad strike-slip motion, allowing water from the subsurface ocean to rise through the icy shell to feed the cryovolcanic jets.
"A close terrestrial analogy is the movement along pull-apart basin structures across major faults subject to tectonic stresses. An example of such movement occurs over the Salton Basin - a large pull-apart fault located on the San Andreas Fault, a fault line." slip fault in Southern California," Berne said. 'Regional strike-slip motion is causing local crustal extension and volcanism over the Salton Basin. This process is similar to the tidally driven extension along pull-aparts at Enceladus, which could regulate the moon's cryovolcanic activity.
"Before we conducted the study, we did not expect such a high correlation between the modeled strike-slip motion and jet activity."
The team's research shows that Enceladus' tiger stripes open differently than previously modeled.
"This finding was surprising because most previous studies on this topic invoke a wide opening along the tiger stripes, such as opening and closing like an elevator door, as the primary mechanism regulating variations in plume brightness," Berne said.
The Caltech researcher added that the team's models suggest that tides play a fundamental role in the evolution of Enceladus and its ocean on multiple time scales.
"On the orbital time scale, the tides appear to regulate the amount of material flowing from a subsurface ocean through the tiger stripe faults," Berne said. "On longer timescales, tides can cause friction stripes to break in a net right-lateral sense."
He continued by suggesting that this long-term right-lateral motion could drive the formation of geological features observed around the terrain of Enceladus' south pole. This includes a fault radiating away from the South Pole in Enceladus' trailing hemisphere.
Scientists have suggested that Enceladus, with its buried global ocean, could be a prime target for the search for life elsewhere in the solar system. This research and the team's model could provide additional support for this hypothesis.
"Understanding subsurface material transport routes through pulled-apart or wide rift zones is crucial to determine whether ice grains in Enceladus' jets are representative of the moon's potentially habitable global ocean. Our study provides a framework for understanding such transport routes and their evolution over time," said Bern.
"Evidence for the long-term influence of tides on the evolution of Enceladus, which also warms its interior, implies that the moon's ocean is long-lived, which has implications for the potential evolution of life in its interior."
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At this time, the team's conclusion is based on a computer simulation and thus needs to be confirmed with actual observations.
"Geophysical measurements at Enceladus using radar would allow us to confirm or refute the hypotheses set out in our paper. More broadly, such observations of the motion of Enceladus' surface over time could provide important constraints for the dynamics of the core and crust, as well as the extent to which these processes have been active over time," Berne concluded. "We want to continue exploring ways in which we can use geophysical measurements to better understand the conditions understand what makes life possible and evolves on Enceladus."
The team's research was published Monday (April 29) in the journal Nature Geoscience.
