Characterizing Laboratory-Prepared Specimens of Snow and Firn with Electron Backscatter Diffraction

Abstract:

A great deal of information about snow microstructures pertinent to better understanding both seasonal and perennial snow cover processes, such as firn densification, can be derived from advanced and classical materials characterization techniques, including X-ray computed microtomography (micro-CT) and cross-polarized light imaging. However, there currently exists no standardized technique for ascertaining the full microstructural detail of snow, including both c-axis and a-axis crystallographic orientations of individual snow grains. This missing piece of information is relevant to better understanding a variety of physical snow processes including its mechanical behavior, wet and dry snow metamorphism, sintering, and its dielectric response to incident electromagnetic radiation. In this study, we present a novel combination of existing field and laboratory techniques that allow for the efficient extraction and full characterization of both c-axis and a-axis crystallographic orientations of individual snow grains from a snowpack using electron backscatter diffraction (EBSD). From within a controlled cold laboratory environment, various snow samples have been produced similar to that found in natural environments, including depth hoar, surface hoar, and polar firn. Once created, each snow sample is imaged with micro-CT before being cast with dimethyl phthalate and cut to approximately 40 x 15 x 5 mm (lwh). Sample surfaces are prepared with a biological sledge microtome before being loaded into a Field Emission Scanning Electron Microscope (FE-SEM) for EBSD analysis. After pressure cycling within the FE-SEM to reduce unintended water vapor deposition on the sample surface, we have been able to successfully produce electron backscatter diffraction patterns from these various types of snow grains. Although still ongoing, our preliminary results have shown promise in further applying this technique to field-collected snow samples as well, including firn cores, that can be used for better understanding firn densification processes on ice sheets.