HR: 0800h
AN: P31A-0954    [Abstracts]
TI: Rheology of ice II at low differential stresses
AU: * Kubo, T
EM: kubotomo@geo.kyushu-u.ac.jp
AF: Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, 1-10-6 Hakozaki, Higashi-ku, Fukuoka, 812-8581 Japan
AU: Durham, W B
EM: durham1@llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550 United States
AU: Stern, L L
EM: lstern@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, CA 94025 United States
AU: Kirby, S H
EM: skirby@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, CA 94025 United States
AB: Knowledge of the rheology of high-pressure phases of water ice is crucial for understanding the thermal structure and internal dynamics within medium- and large-size icy moons of the outer planets. In previous studies of ice at relatively high differential stresses, the flow laws of the several high-pressure phases were inferred to lie mainly in the dislocation creep regime. In this study, we have carried out creep experiments of fine-grained ice II at confining pressures of 200-250 MPa and temperatures of 200-220 K using a gas-medium triaxial deformation apparatus to measure rheology at lower stress conditions. We make fine-grained ice II by multiple I-II transitions at lower temperatures and larger overpressure conditions from the equilibrium boundary. Microstructures of deformed ice II aggregates were examined by cryogenic SEM. Clear differences in creep behavior between ice II made by single and triple I-II transitions were observed at stresses of 4-18 MPa and strain rates of 10$^{-8}$-10$^{-7}$ s$^{-1}$. In the former case, the stress exponent is about 5, which is consistent with the flow law previously obtained at differential stresses above 20 MPa. In the latter case, the ice II is much weaker and the stress exponent is about 2.4. Preliminary estimates of the grain size of ice II are about 50 micron and 10 micron, respectively. These experimental results imply that the grain-size sensitive creep of ice II dominates plastic strain at low stress conditions, making ice II the second phase of ice (after ice I) to manifest a change from a grain-size insensitive rheology to a weaker grain-size sensitive rheology at low stresses and finer grain sizes. The implications for icy moons are towards more convective instability, and lower internal stresses, temperatures, and/or grain sizes.
DE: 5430 Interiors (8147)
DE: 5460 Physical properties of materials
DE: 6020 Ice
DE: 6062 Satellites
DE: 3902 Creep and deformation
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting