Planetary ices up close: imaging by cryogenic SEM.

Insight into the mechanical response of icy minerals and mixtures relevant to outer solar system environments comes not only from data and images of the solar-system objects themselves but also from laboratory study of these materials. Over the past several decades, we have conducted experimental research on the deformation behavior and microphysics of some of the principal rock-forming cryominerals within the C-O-H-N-S chemical systems and the multiphase rocks composed of them ( e.g. Durham et al. 1987; 1992; 1993; 1996; 1997; 2000; 2001; 2003a,b; 2005a,b,c; Durham & Stern 2001; Kubo et al. 2006; McCarthy et al. 2007; and references therein.) Our experimental approach has been to make reproducible samples with well-characterized grain sizes and textures for controlled deformation experiments, and then conduct post-test analyses to determine how deformation alters sample texture, phase composition, and phase distribution. Such characterization down to the microscopic level is necessary to evaluate the microphysics of flow, and is now routinely performed in our laboratory by means of cryogenic scanning electron microscopy (CSEM). With this technique we can resolve surface features down to a fraction of a micron and see evidence for processes that occur both during sample growth as well as during subsequent deformation. This technique also permits investigation into the physics of flow of two-phase solid alloys by directly observing how the phases articulate and interact. Phase identification is then determined by energy dispersive x-ray spectroscopy (EDS), or for bulk analysis, by cryogenic powder x-ray diffraction (XRD). Here, we present an overview of the cryominerals that we have synthesized, tested, and observed by CSEM to date, and the key results revealed or verified by the CSEM imaging process. These materials include: (1) H2O ice I; including grain growth experiments, hydrostatic compaction of cold (< 120 K) ice I, and flow of fine-grained ice I with entrained impurities; (2) ice II in the regime of grain-size-sensitive (GSS) creep, (3) sI and sII gas clathrate hydrates and clathrates mixed with particulates; and (4) hydrated salts in the NaCl-H2O, MgSO4-H2O, and Na2SO4-H2O systems, and associated mixtures or eutectic compositions with water ice.