A POROUS SILICATE INTERIOR FOR ENCELADUS, BUT LIMITED GEOLOGICAL ACTIVITY AT THE SEAFLOOR OF EUROPA

Multiple icy satellites of Jupiter and Saturn are astrobiological targets because of the prospect that chemical reactions at their seafloors, enhanced by porous and fractured rock, might support the development of chemoautotrophic habitats there. To estimate the mechanical and geological properties of their silicate interiors, we combine rock mechanics techniques with remotely sensed geophysical data for Enceladus and Europa. We find that the very low gravitational acceleration inside Enceladus results in modest porosity (e.g., 5%) at the rock–water interface remaining above zero at the center of the rocky interior. Depending on the effective porosity of the interior, it is possible that the entire rock volume of Enceladus is saturated with water and, as a result, may even be fully serpentinized. Additionally, although present-day diurnal stresses (~800 Pa) are much less than those needed to initiate frictional sliding (~3 MPa), a small radius decrease (≤15 m) of the rocky interior from secular cooling would be capable of driving thrust faulting. But the situation within Europa is dramatically different. The deeper ocean and greater mass of this moon results in even high porosities (>20%) at the seafloor reducing to zero within but a few kilometers; fluid-filled pore space likely does not prevail below this depth. The overburden pressure from the ocean results in a seafloor that is mechanically much stronger than that of Enceladus, with differential stresses of 50 MPa and 280 MPa required to initiate normal and thrust faulting, respectively. We further calculate the probable depth of the brittle–ductile transition (BDT) within Europa—the depth to which faults can penetrate before strain is accommodated via crystal plasticity and other ductile deformation mechanisms—for end-member thermal gradients of 2 K/km and 20 K/km and a strain rate of 10^–20 s^–1. For these values, the Europan BDT lies at a depth of between 16 and 150 km. Together, our results indicate that, although rock–water interactions within Enceladus are possible, even likely, the existence of a pervasive fracture network within the seafloor of Europa through which seawater, hydrothermal fluids, or magma might circulate could be severely limited.