Exploring the attenuation of ice at elevated confining pressure

The anelastic response of planetary materials to dynamic forcing and the associated dissipation of elastic strain energy influence a wide range of geophysical processes across seismic to tidal to isostatic rebound timescales, such as the attenuation of seismic waves in Earth’s mantle and tidal heating of icy satellites orbiting large host planets.  Most attenuation experiments on ice and other planetary materials are conducted at relatively low stresses, to mimic seismic attenuation and to prevent fracturing of experimental samples in the absence of confining pressure. Typically, these experiments explore a linear, amplitude-independent regime, primarily governed by mechanisms associated with grain boundaries. At sufficiently high stresses, nonlinear attenuation in planetary materials is predicted to occur above some critical strain amplitude from the motion of dislocations and becomes important, for example, in exoplanetary systems wherein highly eccentric orbits of planets with close proximity to host stars can yield enormous tidal stresses. To study the onset of nonlinear dissipation behavior of ice and the associated amplitude dependence, we conducted cyclic deformation experiments in a high-pressure gas apparatus at elevated confining pressures. Cylindrical ice samples with a uniform initial grain size of ~0.3 mm were sealed in indium jackets, pressurized to 30 MPa and loaded to a median differential stress in the range 4 to 8 MPa.  The samples were then subjected to sinusoidal stress oscillations from 0.5 to 3 MPa about the median stress at frequencies of 10-3 to 5x10-3 Hz.  The resulting sample strain was measured with a strain gauge sandwiched between the outer surface of the ice cylinder and the 0.4 mm-thick indium jacket. The phase lag between the applied stress and the resulting strain response was used to calculate the inverse quality factor, Q-1. For experiments conducted at 260 K, we observe a marked dependence of Q-1 on stress amplitude at three different frequencies.  We also observed that Q-1 decreases, and Young’s modulus increases, with increasing forcing frequency.  Our experimental results will be evaluated in the context of existing theoretical models of nonlinear attenuation.