Tectonics on icy moons

Igneous activity on icy moons can be defined as the melting, ascension, and solidification of liquids, particularly water and its ice polymorphs. Tectonic features on icy lithospheres occur by global and regional stresses acting on the moon's interior. Fractures in the icy lithosphere influence the mechanisms by which the lithosphere reacts to stress. An unfractured ice lithosphere has a greater shear strength than tensile strength, and accordingly, compressional deformation must occur by shear failure and cause thrust and strike-slip faulting. Conversely, prefractured ice has much less shear strength, and extensional stress will produce normal faults and graben. Residual heat from accretion is one possible source of internal heat for icy moons. But only moons with radii greater than about 2000 km are thought to be massive enough to melt pure water-ice in the outer layers. Tidal heating and the decay of radioactive elements are another possible source of internal heat on icy moons. Warming of a cold interior would cause the satellite to expand and undergo tensional stress on the surface. Cooling, on the other hand, would cause contraction and compression. Mantle convection likely occurred within most icy moons, but is not an important source of lithospheric stress. Asteroid and comet impacts are another source of thermal and seismic energy on icy moons. Impacts could produce melt pools, reactivation of older faults and/or cracks, and deformation to the region antipodal to the impact site. Impacts may impart three general fracture patterns on the icy moon: (1) a global system of radially symmetric fractures originating from the impact site, (2) concentric and radial fractures, and (3) collapse of an impact basin with radial and concentric troughs. Most icy satellites rotate synchronously. If the satellite rotated more rapidly during formation, rotation becomes synchronous within 1,000-1,000,000 years due to tidal friction. A decrease in rotational speed decreases the oblateness of the icy moon, which reduces principle stress in the north–south direction, thereby creating east–west trending dikes. If tidal friction causes the lithosphere to fail, east west extensional features should be expected near the poles, northeast/northwest strike slip features at mid-latitudes, and north–south compressional features at the equator. The transfer of angular momentum from the planet to the orbiting moon causes the moon's orbital distance to increase with time. As a consequence of increasing orbital distance, the tidal bulge decreases. These stresses should produce compression at the planet facing and antipode positions, extension at the poles, and strike-slip faults oriented northeast/northwest elsewhere. == Plate tectonics ==

Some of the satellites of Jupiter have features that may be related to plate-tectonic style deformation, although the materials and specific mechanisms may be different from plate-tectonic activity on Earth. On 8 September 2014, NASA reported finding evidence of plate tectonics on Europa, a satellite of Jupiter—the first sign of subduction activity on another world other than Earth. Titan, the largest moon of Saturn, was reported to show tectonic activity in images taken by the Huygens probe, which landed on Titan on January 14, 2005. The mechanisms of plate tectonics on icy moons, particularly Earth-like plate tectonics are not widely agreed upon or well understood. Plate tectonics on Earth is hypothesized to be driven by “slab pull,” where the sinking of the more dense subducting plate provides the spreading force for mid-ocean ridges. “Ridge push” is comparatively weak in Earth's plate tectonics. Extensional features are abundant on icy moons, but compressional features are sparse. Furthermore, subducting less dense ice into a more dense fluid is difficult to explain. Force balance modeling suggests that subduction is likely to create large scale topographic forcing across icy moons, because the buoyant force is orders of magnitude greater than subducting forces. Fracturing and plate-like motion is more easily explained by volume changes and ice-shell motion that is decoupled from interior motion. == Tectonic and volcanic features ==

Trough and Scarp Sets Linear troughs, chains of pits, and scarps in coherent orientations have been observed on Mimas, Tethys, Rhea, Iapetus, Umbriel, Europa, and Ganymede. These features are thought to be formed from impacts or tidal forcing. . Scarps and troughs traversing older material These features are similar in appearance to trough and scarp sets, but appear geologically distinct from the terrain in which they traverse. It is thought that the troughs are younger material. These features are considered normal faults and rifts formed by extensional tectonics. However, on Dione and Tethys, large impacts may have produced traversing scarps and troughs. Linear and curvilinear ridges Ridges are uncommon, but have been observed on Rhea, Dione, and Ganymede. Ridges are thought to form by compression or transpression. Concentric and radial scarps and furrows Collapsed impact basins are thought to form concentric and radial scarps. The Valhalla ring system on Callisto is one of the most well-preserved examples of these features. Concentric furrows on Ganymede's dark terrain appear, but only as troughs and without scarps. Volcanism Four processes may produce volcanic activity on icy moons: (1) mantle convection, (2) negative diapirism, (3) impact cratering, and (4) antipodal fracturing in response to a large impact. The strongest evidence for volcanism is found in the polygonal coronae on Miranda, a large, fractured and resurfaced region embedded within a heavily cratered region. Grooved terrain Grooved terrain refers to features that are parallel or subparallel, dissect older terrain, are often associated with lighter colored terrain, and are negative relief structures rather than raised. The negative topography suggests that these features formed from global expansion of the icy moon, although some suggest the features formed by reactivation of older structures. . North is to the top of the mosaic and the Sun illuminates the surface from the left. The smallest details that can be discerned in this picture are knobs and small impact craters about across. The resolution is per picture element, and the mosaic covers an area approximately across. A prominent fault scarp crosses the mosaic. This scarp is one of many structural features that form the Valhalla multi-ring structure. == Observations ==

Europa Voyager 2 and Galileo mission imagery revealed a highly fractured surface on Europa devoid of cratering, suggesting that the surface is regularly young and subject to resurfacing. Dilational bands appear morphologically similar to spreading ridges on Earth, and therefore suggest that warm ice ascends upwards to form the bands. However, compressional deformation features are sparse and too small to accommodate spreading from the dilational bands. However, the ice overburden pressure within the crust exceeds tidal stresses at depths greater than 35 m below the ice surface, thereby limiting the depth at which tidally-induced cracks can propagate. Unlike subduction on Earth, differences in the strengths and relative densities of Europan ice, it is unlikely that the subducting ice plate is “pulled” into the subsurface ocean. This dichotomy becomes more pronounced the further the fault is from the equator. However, the forces that drive extension in the ice crust are not well known. Slab pull, where a subducting ice plate pulls the crust apart at divergent boundaries is unlikely to drive extension because ice is less dense than liquid water, and therefore unable to sink into the subsurface ocean. The topography of bright terrain has many linear grooves in some regions, while it appears smooth in others. Many of the smooth regions observed in Voyager 2 imagery appear that way due to low image resolution. Rossi et al. (2018) undertook a detailed tectonic survey of Ganymede, using a combination of Voyager 2 and Galileo mission imagery, to inform an evolutionary tectonic model for the Uruk Sulcus region. Such faulting may expose fresh, light ice within dark terrains. The fields of mapped faults may give evidence of how stress patterns shifted through time to produce the terrain. == References ==