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Above the South Pole, jets at Saturn's moon Enceladus shoot ice crystals into a vast plume along four separate fractures known as "tiger stripes." As Enceladus orbits Saturn, the tiger stripes undergo variations in tidally generated shear and normal traction. When NASA's Cassini mission analyzed the samples from these plumes, it was found that the chemical conditions believed to be necessary for life may exist in the ocean deep beneath Enceladus's surface. In a new study, Caltech scientists used a detailed geophysical model to characterize the motion of these tiger-stripe faults. The study offers fresh insights into the geophysical processes controlling jet activity. A comprehensive understanding of these and other aspects is essential to obtaining a full image of the moon's potential habitability over time. These aspects include the amount to which the jet material represents the subsurface ocean, the duration of jet activity, the topology of its ice shell, and so on. Throughout Enceladus's 33-hour orbit around Saturn, the plume over its south pole waxes and wanes, producing two prominent brilliant peaks in emission. According to a theory, tidal forces drive the tiger-stripe faults to open and close like an elevator door, causing them to release material in cycles that match the tides. However, these models cannot precisely forecast when plume brightness peaks will occur. Even more worrisome is that the energy needed for this fault-opening mechanism is more significant than what would be anticipated from tidal force alone. In this new study, scientists created a sophisticated numerical model to simulate strike-slip motion along Enceladus' faults. The model simulates slip along the tiger stripes to match the variations in plume brightness and spatial variations in surface temperature, suggesting that the jets are indeed controlled by the strike-slip motion over Enceladus's orbit. Scientists in this new study suggest that the movement of tiger-stripe faults in a strike-slip fashion influences Enceladus's plume strength. The energy required for such fault motion is considerably less than needed for the opening/closing mechanism. According to the researchers ' theory, the individual jets are thought to occur at "pull-apart" in the faults, which are bending fault sections that open due to regional strike-slip motion. The tiger-stripe region was the subject of recent independent research from JPL, which also discovered geological evidence for pull-apart along the faults, precisely where the jets are located. Graduate student Alexander Berne said, "We now appear to have geologic and geophysical reasons to suspect that jet activity occurs at pull-apart along Enceladus's tiger stripes." Mark Simons, the John W. and Herberta M. Miles Professor of Geophysics and director of the Brinson Exploration Hub at Caltech, said, "For life to evolve, the conditions for habitability must be right for a long time, not just an instant. On Enceladus, you need a long-lived ocean. Geophysical and geological observations can provide key constraints on the core and the crust dynamics as well as the extent to which these processes have been active over time." Berne said, "Detailed motion measurements along the tiger stripes are needed to confirm the hypotheses laid out in our work. For instance, we now can image fault slips, such as earthquakes, on Earth using radar measurements from satellites in orbit. Applying these methods at Enceladus should allow us to understand better the transport of material from the ocean to the surface, the thickness of the ice crust, and the long-term conditions which may enable life to form and evolve on Enceladus." Journal Reference:
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