3:00 PM - 3:15 PM
[PPS01-06] Europa’s Seafloor is Likely Mechanically Strong and Geologically Inert
Introduction: Europa is strongly suspected to have a subsurface ocean, and is a body of major astrobiological interest. Water–rock interactions at the moon’s seafloor have been hypothesized to support a habitable, chemoautotrophic environment there. On Earth, such environments are enabled by continuous fracturing that exposes fresh rock to seawater, allowing for the production of redox couples and the release of dissolved metals into the water column, but the tectonic state of Europa’s seafloor is unknown.
Here, we combine rock mechanics techniques with remotely sensed geophysical data for Europa to first derive lithospheric strength profiles and then determine the differential stresses necessary to drive faulting at the moon’s seafloor. We further calculate the brittle lithospheric thickness of Europa’s rocky interior, and assess whether convection of the silicate mantle could overcome the strength of that lithosphere.
Findings: The differential normal and thrust fault strengths at a depth of 100 m below the seafloor for a scenario under which the seafloor rock is unaltered peridotite scenario are 410 kPa and 1.3 MPa, respectively. The corresponding values for a scenario under which the seafloor comprises hydrothermally altered serpentinite are 480 kPa and 880 kPa, respectively.
There are several mechanisms that could plausibly act on pre-existing fractures in the seafloor, such as stresses raised by the diurnal tide. However, the maximum tidal stress experienced by the Europan seafloor is 53 kPa, only about 13% that needed to drive slip along a favorably oriented normal fault at a depth of 100 m under a peridotite seafloor. Although repeated cyclic loading can weaken rock, fatigue strength of ~25% that of the normal fault strength, i.e., ~100 kPa, is still a factor of two greater than the diurnal tidal stress, and becomes rapidly greater with depth. Global contraction from secular interior cooling is another means by which the surface of a silicate body can be fractured (by thrusting), but the Europan interior would need to contract by more than 1 km before thrust faults could even start to develop. By this measure, no mechanism obviously capable of driving faulting can match the 100 m-depth strength of Europa’s seafloor.
Brittle–Plastic Transition Depth: With increasing depth, brittle failure gives way to plastic deformation. The intersections of brittle and ductile strength profiles give the depths of the brittle–plastic transition (BPT) and, functionally, the thickness of the elastic lithosphere. For a scenario under which the seafloor is mechanically weak and the heat flux is high, the BPT is at a depth of 30 km; should the seafloor be mechanically strong with low heat flux, the BPT is as deep as 180 km.
Mantle Convective Stresses: Stresses from mantle convection can drive deformation at the surface. With scaling laws for mantle motion, we compare convective tractions to the peak lithospheric strength values we earlier calculated. Convective stresses are dependent on mantle viscosity, but even over three orders of magnitude (1018–1020 Pa s) those stresses never exceed ~1 MPa—substantially less than even the lowest peak lithospheric strength of ~40 MPa (for normal faults under the “weak” scenario).
Implications for Europan Habitability: Other processes that might expose fresh rock to the Europan ocean include volume increase during serpentinization and thermal expansion anisotropy, which we have yet to consider. But neither diurnal tides, cycling loading, nor global contraction seem obviously capable of driving slip along pre-existing fractures, at least in the present epoch, likely limiting any major geological activity at the seafloor and the replenishment of redox reactants into the ocean. It may be that specific circumstances involving seawater flow through seafloor rocks may be required for sustained habitable conditions at the moon’s seafloor.
Here, we combine rock mechanics techniques with remotely sensed geophysical data for Europa to first derive lithospheric strength profiles and then determine the differential stresses necessary to drive faulting at the moon’s seafloor. We further calculate the brittle lithospheric thickness of Europa’s rocky interior, and assess whether convection of the silicate mantle could overcome the strength of that lithosphere.
Findings: The differential normal and thrust fault strengths at a depth of 100 m below the seafloor for a scenario under which the seafloor rock is unaltered peridotite scenario are 410 kPa and 1.3 MPa, respectively. The corresponding values for a scenario under which the seafloor comprises hydrothermally altered serpentinite are 480 kPa and 880 kPa, respectively.
There are several mechanisms that could plausibly act on pre-existing fractures in the seafloor, such as stresses raised by the diurnal tide. However, the maximum tidal stress experienced by the Europan seafloor is 53 kPa, only about 13% that needed to drive slip along a favorably oriented normal fault at a depth of 100 m under a peridotite seafloor. Although repeated cyclic loading can weaken rock, fatigue strength of ~25% that of the normal fault strength, i.e., ~100 kPa, is still a factor of two greater than the diurnal tidal stress, and becomes rapidly greater with depth. Global contraction from secular interior cooling is another means by which the surface of a silicate body can be fractured (by thrusting), but the Europan interior would need to contract by more than 1 km before thrust faults could even start to develop. By this measure, no mechanism obviously capable of driving faulting can match the 100 m-depth strength of Europa’s seafloor.
Brittle–Plastic Transition Depth: With increasing depth, brittle failure gives way to plastic deformation. The intersections of brittle and ductile strength profiles give the depths of the brittle–plastic transition (BPT) and, functionally, the thickness of the elastic lithosphere. For a scenario under which the seafloor is mechanically weak and the heat flux is high, the BPT is at a depth of 30 km; should the seafloor be mechanically strong with low heat flux, the BPT is as deep as 180 km.
Mantle Convective Stresses: Stresses from mantle convection can drive deformation at the surface. With scaling laws for mantle motion, we compare convective tractions to the peak lithospheric strength values we earlier calculated. Convective stresses are dependent on mantle viscosity, but even over three orders of magnitude (1018–1020 Pa s) those stresses never exceed ~1 MPa—substantially less than even the lowest peak lithospheric strength of ~40 MPa (for normal faults under the “weak” scenario).
Implications for Europan Habitability: Other processes that might expose fresh rock to the Europan ocean include volume increase during serpentinization and thermal expansion anisotropy, which we have yet to consider. But neither diurnal tides, cycling loading, nor global contraction seem obviously capable of driving slip along pre-existing fractures, at least in the present epoch, likely limiting any major geological activity at the seafloor and the replenishment of redox reactants into the ocean. It may be that specific circumstances involving seawater flow through seafloor rocks may be required for sustained habitable conditions at the moon’s seafloor.