RUI: A laboratory study of ultrasonic scattering attenuation by possible microstructures in Earth's inner core

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Earth's inner core is elastically anisotropic, with seismic waves propagating parallel to the rotation axis about 3% faster than those parallel to the equatorial plane. The inner core also exhibits an attenuation anisotropy, with the faster waves having smaller amplitudes. There is now evidence that the pattern of both anisotropies is more complex, exhibiting depth dependence, hemispherical variations, and smaller scale regional variations. The isotropic seismic velocity may also exhibit regional variations. These puzzling seismic inferences are clues as to the structure and evolution of the solid iron alloy inner core. This grant will allow the PI to investigate the causes of inner core attenuation and its anisotropy, and thus help to better understand the most remote part of our planet. In particular, in laboratory studies the PI will use ultrasonic waves to probe metallic alloys with a variety of microstructures that have been suggested for the inner core, employing ratios of wavelengths to grain and sub-grain lengthscales that are relevant to the inner core. By comparison with seismic data, this will allow the PI to quantify the relative importance of scattering attenuation versus intrinsic attenuation (viscoelasticity). As an RUI grant, the project will involve diverse undergraduates in all aspects of the research, providing them with training and experience in doing science.Most explanations for the elastic anisotropy rely on a lattice preferred orientation (texturing) of hexagonal close-packed iron crystals, the most likely stable phase of iron under inner core conditions. Explanations for the texturing fall broadly into two classes, that due to solidification and that due to deformation. Directional solidification of metallic alloys typically results in columnar grains formed by primary and secondary dendrites of the primary compositional phase, with the secondary phase along grain boundaries and between dendrites, i.e., intragranular. Such microstructure has been proposed for the inner core, and scattering off grain boundaries of elongated crystals has been suggested as a cause for the attenuation anisotropy. Solidification microstructure is not thermodynamically stable, however, and annealing can result in coarsening of secondary dendrites, while maintaining the primary dendrites and columnar crystals, or possibly, in recrystallization and polygonal grain growth. The latter typically occurs in deformed materials exposed to high temperature. The PI will examine the ultrasonic scattering attenuation of three possible microstructures likely in Earth's inner core: directional solidified columnar grains composed of primary and secondary dendrites; directionally solidified and then annealed grains; and polygonal grains that result from recrystallization due to deformation and annealing. He will use Pb-Sn because of its simple eutectic phase diagram, ease of use, and relatively small single crystal elastic anisotropy; and microstructures with relative wavelength/scatterer dimensions thought to be similar to those in the inner core. The PI will use the shape and decay of the ultrasonic coda to determine the quality factor Q, and will compare the ultrasonic waveforms with seismic data to infer regional microstructure in the inner core, which will give insight into the evolution of the inner core.