User:Ryans1286/Tectonics on Icy Moons

Observations

Europa

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. A subduction mechanism is a key to the ice tectonics hypothesis on Europa. For subduction to occur, convection within or below the ice crust must exert stresses that exceed the strength of the overlying ice crust. But to hold a tenable tectonics hypothesis, one must explain how ice sinks below the surface. If the crustal ice porosity exceeds ~1%, subduction is unlikely, but the high concentrations of salt within the ice make subduction possible with porisities up to 10%. Subduction may occur if differences in salt content exceed 5% between the overriding plate and the subducting plate. However, the processes and conditions that initiate subduction are still poorly explained.

Europa’s ice crust may be fractured by tidal stresses from Jupiter, and it has been hypothesized that liquid water could reach the surface through these cracks. 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. Furthermore, liquid water within any cracks will rapidly freeze. Therefore, a source other than tidal forcing must place the crust under tension for cracks to propagate deeply. Tides may force strike-slip motion along cracks, and this lateral motion would produce heat within the crack and make the ice more susceptible to ductile flow. The warmer and less viscous ice along the cracks is less dense than the surrounding ice, and may flow upwards to the surface. Melt generated within these fractures may briefly exist near the surface before percolating downward to the subsurface ocean over thousand year timescales.

Chaos terrain on the left side of this image of Europa transitions to smooth terrain.

Truncated surface features suggest that subduction on Europa may occur along tabular zones. 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. Instead, it is most likely incorporated into the ice composing the overriding plate. Surface features that intersect tabular zones do not continue on the other side, unlike across strike-slip and dilational faults.

Strike-slip faults in the northern hemisphere of Europa are predominantly left-lateral, while those in the southern hemisphere are predominantly right-lateral. This dichotomy becomes more pronounced the further the fault is from the equator. To explain this, the shell tectonics hypothesis describes a mechanism for strike-slip motion along faults driven by tidal forces from Jupiter. Numerical simulations of shell tectonics strike-slip faulting agrees closely with observations. However, the shell tectonics model requires that a substantial number of fractures or faults already exist on the surface.

Convection and advection within the liquid ocean can transport and freeze liquid water into the ice crust, and that ocean-origin material may potentially reach the surface. 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.

Ridges and fractures on Europa.

The Phaidra Linea region on Europa.

Ganymede

Ganymede has two principle geologic units termed “dark” terrain and “bright” terrain. Bright terrain is hypothesized to be younger because it has fewer craters than the dark terrain. The topography of bright terrain has many linear grooves in some regions, while it appears smooth in others. The appearance of smooth terrain may be an artifact of low resolution Voyager 2 imagery. Bright bands are hypothesized to form by tectonic spreading, possibly analogous to mid-ocean ridge spreading or terrestrial rift spreading. In some regions, dark terrain patches are found within light terrain. Parmentier et al. (1982) suggests that the light terrain material flooded into the dark terrain, leaving dark topographic highs as the observed dark patches surrounded by lower elevation light terrain. Parmentier et al. (1982) find that mid-ocean ridge-like spreading does not occur on Ganymede, citing observations of poorly matched crater remnants and poorly fitting polygonal terrain in regions split by rifts. Instead, offset features and evidence of flooding suggest finite lithospheric rifting produces the bright terrain. Parmentier et al. (1982) infer that the dark terrain is an ice-silicate mixture that is slightly more dense than pure water ice. Extension in the dark terrain causes less dense water-ice to extrude upwards, forming linear and curve rifts of bright terrain. Long, narrow grooves appear in both bright and dark terrains, but are more abundant in light terrain. Grooves are typically symmetrical, which suggests that they are extensional features, rather than compressional features like folds or thrust faults.

Head et al. (2002) reexamine possible formation mechanisms of bright and dark terrains on Ganymede using higher resolution Galileo mission imagery, with particular interest in whether the smooth areas described in Parmentier et al. (1982) are produced by cryovolcanic infilling. Many of the smooth regions observed in Voyager 2 imagery appear that way due to low image resolution. Instead, these “smooth” regions hold smaller linear ridges and troughs. The presence of smooth terrain was key to the cryovolcanic infilling hypothesis, and the presence of ridges and troughs within these regions poses a substantial challenge to that hypothesis. Galileo imagery reveals no lobate features or vents indicative of cryovolcanic flow. Furthermore, in regions with both bright and dark terrain, the bright terrain is topographically higher. These observations demand a tectonic deformation, possibly in addition to cryovolcanism, to explain bright regions. 

Linear grooves and furrows thousands of kilometers in length form concentric arcs on Ganymede’s surface. 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. Right lateral faulting produces sigmoidal structures in the shear zone, where extensional forces create linear grooves and furrows.

Abundant evidence of strike-slip faulting on Ganymede exists in both bright and dark terrain types. 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.