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High tide on Enceladus The Cassini flyby of 14 July 2005 revealed plumes of water vapour and ice associated with the 'tiger stripe' features on the surface of Saturn's icy moon Enceladus. Since then the challenge has been to explain the nature of the plumes and the forces driving them. Two papers this week offer an explanation that accounts for both the plume characteristics and the presence of hot spots without the need to assume the existence of near-surface liquid water, a requirement of some previous models. Nimmo et al . identify tidally driven lateral fault motions near the tiger stripes as the most likely drivers of heat and vapour production. And Hurford et al . show that as Enceladus orbits Saturn, the parent planet's tides make the satellite's ice flex. This may cause the tiger stripes to open and close periodically, exposing volatile gases and allowing them to be released. Plumes of gas have been seen near the south polar region of Enceladus, a small icy satellite of Saturn, and observations revealed large rifts in the crust, informally called ‘tiger stripes’. Hurford et al. report that during each orbit, every portion of each tiger stripe rift spends about half the time in tension, which allows the rift to open, exposing volatiles, and allowing eruptions: plume activity is expected to vary periodically. In 2005, plumes were detected near the south polar region of Enceladus 1 , a small icy satellite of Saturn. Observations of the south pole revealed large rifts in the crust, informally called ‘tiger stripes’, which exhibit higher temperatures than the surrounding terrain and are probably sources of the observed eruptions 2 . Models of the ultimate interior source for the eruptions are under consideration 1 , 3 , 4 , 5 . Other models of an expanding plume 6 require eruptions from discrete sources, as well as less voluminous eruptions from a more extended source, to match the observations. No physical mechanism that matches the observations has been identified to control these eruptions. Here we report a mechanism in which temporal variations in tidal stress open and close the tiger-stripe rifts, governing the timing of eruptions. During each orbit, every portion of each tiger stripe rift spends about half the time in tension, which allows the rift to open, exposing volatiles, and allowing eruptions. In a complementary process, periodic shear stress along the rifts also generates heat along their lengths 7 , 8 , 9 , which has the capacity to enhance eruptions. Plume activity is expected to vary periodically, affecting the injection of material into Saturn’s E ring 10 and its formation, evolution and structure. Moreover, the stresses controlling eruptions imply that Enceladus’ icy shell behaves as a thin elastic layer, perhaps only a few tens of kilometres thick.

Cycloidal patterns are widely distributed on the surface of Jupiter's moon Europa. Tensile cracks may have developed such a pattern in response to diurnal variations in tidal stress in Europa's outer ice shell. When the tensile strength of the ice is reached, a crack may occur. Propagating cracks would move across an ever-changing stress field, following a curving path to a place and time where the tensile stress was insufficient to continue the propagation. A few hours later, when the stress at the end of the crack again exceeded the strength, propagation would continue in a new direction. Thus, one arcuate segment of the cycloidal chain would be produced during each day on Europa. For this model to work, the tensile strength of Europa's ice crust must be less than 40 kilopascals, and there must be a thick fluid layer below the ice to allow sufficient tidal amplitude.

Non-synchronous rotation of Europa was predicted on theoretical grounds 1 , by considering the orbitally averaged torque exerted by Jupiter on the satellite's tidal bulges. If Europa's orbit were circular, or the satellite were comprised of a frictionless fluid without tidal dissipation, this torque would average to zero. However, Europa has a small forced eccentricity e ≈ 0.01 ( ref. 2 ), generated by its dynamical interaction with Io and Ganymede, which should cause the equilibrium spin rate of the satellite to be slightly faster than synchronous. Recent gravity data 3 suggest that there may be a permanent asymmetry in Europa's interior mass distribution which is large enough to offset the tidal torque; hence, if non-synchronous rotation is observed, the surface is probably decoupled from the interior by a subsurface layer of liquid 4 or ductile ice 1 . Non-synchronous rotation was invoked to explain Europa's global system of lineaments and an equatorial region of rifting seen in Voyager images 5 , 6 . Here we report an analysis of the orientation and distribution of these surface features, based on initial observations made by the Galileo spacecraft. We find evidence that Europa spins faster than the synchronous rate (or did so in the past), consistent with the possibility of a global subsurface ocean.

During late 1999/early 2000, the solid state imaging experiment on the Galileo spacecraft returned more than 100 high-resolution (5 to 500 meters per pixel) images of volcanically active Io. We observed an active lava lake, an active curtain of lava, active lava flows, calderas, mountains, plateaus, and plains. Several of the sulfur dioxide-rich plumes are erupting from distal flows, rather than from the source of silicate lava (caldera or fissure, often with red pyroclastic deposits). Most of the active flows in equatorial regions are being emplaced slowly beneath insulated crust, but rapidly emplaced channelized flows are also found at all latitudes. There is no evidence for high-viscosity lava, but some bright flows may consist of sulfur rather than mafic silicates. The mountains, plateaus, and calderas are strongly influenced by tectonics and gravitational collapse. Sapping channels and scarps suggest that many portions of the upper ∼1 kilometer are rich in volatiles.

The ice crust of Europa probably floats over a deep liquid-water ocean, and has been continually resurfaced by tectonic and thermal processes driven by tides. Tidal working causes rotational torque, surface stress, internal heating, and orbital evolution. The stress patterns expected on such a crust due to reorientation of the tidal bulge by non-synchronous rotation and due to orbital eccentricity, which introduces periodic ('diurnal') variations in the tide, are shown as global maps. By taking into account the finite rate of crack propagation, global maps are generated of cycloidal features and other distinctive patterns, including the crack shapes characteristic of the wedges region and its antipode on the sub-Jovian hemisphere. Theoretical maps of tidal stress and cracking can be compared with observed tectonics, with the possibility of reconstructing the rotational history of the satellite.[PUBLICATION ABSTRACT]

Theoretical predictions of non-synchronous rotation and of polar wander on Europa have been tested by comparing tectonic features observed in Voyager and Galileo spacecraft images with tidal stresses. Evidence for non-synchronous rotation comes from studying changes in global scale lineaments formed over time, from the character of strike-slip faults, and from comparison of distinctively shaped cycloidal cracks with the longitudes at which such shapes should have formed, in theory. The study of cycloids constrains the rotation period (relative to the direction of Jupiter) to less than 250 000 years, while direct comparison of the orientation of Europa in Voyager and Galileo images shows the rotation is slow, with a period of >12 000 years. Comparison of strike-slip faults with their theoretical locations of formation provides evidence for substantial polar wander, supported by the distribution of various thermally produced features.[PUBLICATION ABSTRACT]