HR: 0800h
AN: P31A-0955    [Abstracts]
TI: Microstructure and Physical Properties of Sulfate Hydrate/Ice Eutectic Aggregates in the Binary System Sodium-Sulfate/Water at Planetary Conditions
AU: * McCarthy, C M
EM: christine\_mccarthy@brown.edu
AF: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: * McCarthy, C M
EM: christine\_mccarthy@brown.edu
AF: Brown University, Geoscience Dept., Box 1846, Providence, RI 02912 United States
AU: Kirby, S
EM: skirby@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Durham, W
EM: durham1@llnl.gov
AF: U.C. Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94450 United States
AU: Stern, L
EM: lstern@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AB: Reflectance spectra data from Mars Odyssey, Galileo and potentially from Cassini suggest the presence of hydrated salts on numerous satellites in environments such as evaporate beds or combined with water ice. Improved mission data on these occurrences indicate that grain structures and properties of such materials merit a closer look using laboratory methods. Here we report the synthesis of a two-phase aggregate of sodium sulfate hydrate and water ice made by eutectic solidification from solution, characterization of its microstructure using cryogenic SEM, and comparison of its physical properties to those of its end-member components. Samples are crystallized from solution using a precision cryobath and seeded growth. The reaction is a "simple" one meaning that there is no solid solution formation in either of the two solid phases. The eutectic composition we studied for the sodium sulfate hydrate is 4wt% Na$_2$SO$_4$, which corresponds to about .06 volume fraction of Na$_2$SO$_4$$\cdot$10H$_2$O, mirabilite, and .94 ice I. The eutectic microstructure observed with this volume fraction, which is termed "broken lamellar", consists of fairly uniform blade-like mirabilite grains arranged in roughly parallel columns within a water ice matrix. The blades and matrix material form a lamella that alternates with lamellae of pure ice. Energy dispersive spectroscopy of these eutectic mixtures confirms the presence of the two crystalline phases. Also, we find that lamellar spacing decreases with increasing growth rate. Constant-strain-rate tests in compression are carried out in the cryogenic gas deformation apparatus at LLNL in a pressure-temperature range appropriate to the icy satellites. We report the rheology of the two-phase aggregate and compare it to the strength properties of pure water ice and pure mirabilite. With the aid of numerous studies on similar structures in the literature on metals, we analyze the deformation mechanics from the perspective of defect and crack propagation between the two phases in the eutectic structure. This ongoing investigation is the first of several planned experimental studies of sulfate-hydrate binaries with ice I that are likely to be important in the icy satellites.
DE: 5112 Microstructure
DE: 5120 Plasticity, diffusion, and creep
DE: 5754 Physical properties of materials
DE: 6218 Jovian satellites
DE: 3902 Creep and deformation
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting