The study of attenuation in Earth and planetary materials

Speaker: Christine McCarthy
“Understanding the role of grain-boundary dynamics in anelasticity and plasticity is important to the study of materials in general and to terrestrial and planetary tectonics in particular. In icy satellites, for instance, the magnitude of tidal heating produced in a body is dependent on the anelastic properties of the material comprising its icy shell. The disparity between predicted heat flux using simple rheological models and the observed dynamic behavior from recent fly-bys (geysers on Encaeladus, chaotic terrain on Europa and the various features identified as collapse and cryogenic flooding on Titan) presented a need to refine such models with experimentally determined parameters. My doctoral work on the attenuation of ice and ice/salt aggregates illustrates the influence of grain (and phase) boundaries on dissipation and demonstrates the inadequacy of the Maxwell model in estimating the dissipation of tidal energy in icy satellites.

In terrestrial settings, improved attenuation data derived from laboratory experiments help explain shear wave velocities and attenuation and provide better parameters for interpretation of structure and dynamics. With careful extrapolation, the form of attenuation spectra can be used to infer conditions at depth. I will review the various thermally activated mechanisms that have been attributed to dissipation and the mechanical models that have been used historically for extrapolation. I will share recent results from our study (here at ERI) on the frequency, temperature and grain size dependence of attenuation. Through the use of earth analogue materials, we have been able to obtain very precise measurements for compliance and modulus over 5 decades of frequency. Future studies will additionally explore the role of partial melt, which will have significance to both the terrestrial and planetary communities.”