Friction in Cold Ice Within Outer Solar System Satellites With Reference to Thermal Weakening at High Sliding Velocities

Abstract

The icy shells of Enceladus and Europa consist from top down of cold (~100 K) ice at low (<0.1 MPa) pressure, cold ice at high pressure up to ~10 MPa, and warm ice near 273 K the base. The pressure ~10 MPa and temperature near 273 K of basal ice within Enceladus and Europa are similar to that within terrestrial glaciers that are known to have seismicity (icequakes). Warm ice easily melts during sliding so its icequakes are qualitatively explained and expected on these satellites if the macroscopic strain rates are comparable to those within terrestrial glaciers subjected to oceanic tides. However, cold ice at high pressures does not readily macroscopically melt during sliding. A dynamic weakening mechanism for crustal faults in rock may be applicable to Enceladus and Europa. Micron‐scale real contacts support ~0.4‐GPa shear tractions and normal tractions on rapidly sliding ice faults. At sliding velocities above ~0.1 m/s, the asperity tips of the contacts become hot and weak in ice. The macroscopic friction depends on the average strength of the asperity tips during the lifetimes of contact. The strength of the asperity tips self‐organizes so that frictional heating balances the heat lost from the asperity tip by conduction. The macroscopic coefficient of friction at coseismic sliding velocities decreases to a modest fraction of the low‐velocity coefficient of friction, but does not approach zero. This velocity‐weakening mechanism likely allows major icequakes within the cold interiors of Europa and Enceladus.

Frictional Strength, Stability, and Potential Shear Heating on Icy Satellite Faults

“…To estimate frictional heating, we use a solution for heat diffusion during sliding on a fault of finite thickness (Lachenbruch, 1986) where deformation is assumed to be homogenous in the fault zone and heat is transferred only by conduction. We choose a relatively high sliding velocity of 1 m/s based on potentially coseismic velocities described by Sleep (2019), but we explore the model sensitivity to sliding velocity, as well as slip magnitude, slip duration, and fault half‐width given that they are largely unconstrained at icy satellite conditions.…”

“…10.1029/2023JE008215 (Lachenbruch, 1986) where deformation is assumed to be homogenous in the fault zone and heat is transferred only by conduction. We choose a relatively high sliding velocity of 1 m/s based on potentially coseismic velocities described by Sleep (2019), but we explore the model sensitivity to sliding velocity, as well as slip magnitude, slip duration, and fault half-width given that they are largely unconstrained at icy satellite conditions. Temperature, T, was calculated as a function of time, t, during/after slip, and distance, x, away from the fault center, expressed as…”

Surviving in Ocean Worlds: Experimental Characterization of Fiber Optic Tethers across Europa-like Ice Faults and Unraveling the Sliding Behavior of Ice

“…The calculation of potential slip rates used here is discussed in Appendix A and is on the order of 10 −7 to 10 −4 m s −1 . For these tests, the sample assembly was pushed together under feedback-controlled normal stress (∼100 kPa), and the central sliding block was forced down under controlled velocities that represent a potential range of fault sliding behavior on Europa and Enceladus (Nimmo et al 2007;Rhoden et al 2012;Sleep 2019), from creeping (ramps of 0.5, 1, and 10 μm s −1 ) to seismic, stick-slip events (100, 200, and 300 μm s −1 ; complete range of 3.0 × 10 −4 to 5.0 × 10 −7 m s −1 ), each following a zero-velocity hold. Even without knowing Europa's slip rates exactly, these tests provide a measure of the robustness of tethers to shear.…”