Limited Prospect for Geological Activity at the Seafloors of Europa, Titan, and Ganymede; Enceladus OK

Several icy satellites of Jupiter and Saturn feature known or suspected subsurface oceans situated atop rocky interiors, and are of substantial astrobiological significance because chemical reactions at the ocean floors, enhanced by fractured and porous rock, might support the development of chemoautotrophic habitats there. Here, we combine rock mechanics techniques with remotely sensed geophysical data for Europa, Titan, Ganymede, and Enceladus, to assess to first order the likelihood of tectonic deformation at their rock-water/ice interfaces. We find that the rocky interior of Enceladus is probably porous, and thus possibly hydrated, through to the center. Although present-day diurnal stresses ( 800 Pa) are far below those needed to initiate frictional sliding (about 3 MPa), a small radius decrease (c. 15 m) of the rocky interior from secular cooling would be capable of driving thrust faulting. For Ganymede, however, the incredible pressure at the base of the ocean precludes any pore space in the seafloor; normal or thrust faulting would require differential stresses of at least 560 MPa and 3.2 GPa, respectively. The depth of the brittle-ductile transition (BDT) within Ganymede may be as little as about 8 km (for an end-member maximum thermal gradient of 20 K/km) to as much as around 80 km (for a minimum gradient of 2 K/km). A comparable situation exists at Titan: negligible porosity and stresses of 290 MPa and 1.6 GPa necessary for normal and thrust faulting, respectively, and a BDT at a depth of 15-140 km for the same range of heat fluxes as for Ganymede. For Europa, the depth of the water column makes for an ocean floor mechanically weaker than at Ganymede and Titan, although the stresses required for normal faulting (50 MPa) and thrust faulting (280 MPa) are still substantial. This moon's BDT lies at a depth of between 16 and 150 km. Together, our results indicate that, although rock-water interactions within Enceladus are possible, the prospect of pervasive fracture networks within the ocean floors of Ganymede, Titan, and Europa through which seawater, hydrothermal fluids, or even magma might circulate could be severely limited.