MR11A-01
CH3OH in High-Pressure Phases of H2O: Implications for Ice-Rich Planets
A significant body of research exists on the structure, lattice parameters, and density of high-pressure ice polymorphs, namely Ice VI and Ice VII, as these ices may make up a considerable part of the interior of large icy satellites and select extra solar planets; though most research thus far has been constrained to the pure H2O system. Salty subsurface oceans are also believed to exist within some of these icy bodies which may have prolonged interaction with the Ice VII phase present, incorporating foreign ions or molecules into the lattice of high-pressure ices. Recent research concerning the effects that charged ions have on Ice VII has shown that the presence of these ions notably affects the structure, increasing the Ice VII molar density at any given pressure relative to pure Ice VII (Frank et al., 2006, PEPI, 155, 152-162). This study focused on the incorporation of CH3OH into Ice VII to determine if the change in density was predominantly a result of charge-induced partial ordering of the hydrogen in Ice VII (as outlined in Frank et al., 2006) or if it was controlled solely by the addition of large foreign molecules into the lattice structure. Solutions of 1.60, 5.00 and 10.0 mol% CH3OH in H2O were loaded into a diamond anvil cell. The experiments were performed at GSECARS 13-BM-D at the Advanced Photon Source at Argonne National Laboratory. The unit cell parameters were measured using monochromatic X-ray radiation, 0.3344 Å, and a MAR 345 online imaging system. Powder diffraction patterns were collected in ~1 GPa increments up to ~31, ~48, and ~35 GPa, respectively. The volume-pressure relations (at 300 K) were used to determine an equation of state (EOS) for select compositions in the CH3OH - H2O system. Diffraction data indicate that the unit cell volume of Ice VII formed from a 1.60 mol% CH3OH aqueous solution did not deviate significantly from that of Ice VII formed from pure H2O. Conversely, the volumes of Ice VII formed from 5.00 and 10.0 mol% solutions had reduced unit cell volumes relative to pure Ice VII. Zero-pressure volumes and bulk moduli were calculated with a pressure derivative fixed (4.40): 1.6 mol% CH3OH - H2O had values of 40.4±0.2 Å3 and 24.4±0.7 GPa, respectively, whereas 5.0 mol% CH3OH - H2O had values of 39.2±0.2 Å3 and 26.2±0.5 GPa, respectively. We hypothesize that charged ions will have a greater influence at lower concentrations on the properties of Ice VII than neutral species due to electrostatic interactions within the unit cell and crystallographic constraints imposed by the body-centered cubic structure of Ice VII. In conclusion, these results suggest that ice-phases formed in a solute-rich environment (or by reaction with other phases within the interior of an icy body) will most likely have a greater molar density than ice formed from pure H2O.
MR11A-02 INVITED
Characterizaton of Materials in Jupiter's Interior with First-Principles Computer Simulations
Diamond anvil cell experiments have allowed one to make great progress in characterizing planetary ices at high pressure, new phases have been discovered and transformations in chemical bond networks have been identified. However, our understanding is far from complete because conditions near the center of giant planets (<40 Mbar and <20000K for Jupiter) remain beyond the reach of current experimental techniques. A recently developed experimental technique allows one to probe fluids at higher densities than previously attainable. This is achieved by combining static compression in a diamond anvil cell with dynamic laser shock wave experiments. A brief review of theoretically predicted results of such experiments will be given and their importance for planetary interior studies will be discussed. Results from an extensive set of density-functional molecular dynamics simulations will be presented and an equation of state that spans Jupiter's interior is derived. Estimates for the stability of planetary ices will be given. Furthermore interaction effects of dense hydrogen and helium will be analyzed and the validity of commonly assumed linear mixing approximation will be studied. It will be discussed how helium affects the molecular-to- metallic transition in hydrogen and why the presence of helium stabilizes the molecular phase. Furthermore, an updated model for the interior of Jupiter will be described. We discuss our estimates for the heavy element enrichment as well as for the size of Jupiter's core and compare them with previous models based on the Saumon-Chabrier-Van Horn equation of state. Our results will eventually aid in interpretation of data expected from the Juno orbiter mission. Supported by NASA PGG Grants NAG5-13775 and PGG04-0000-0116 and NSF Grant 0507321.
MR11A-03 INVITED
X-ray induced dissociation of H2O and formation of an O2- H2 compound at high pressure
We observed unexpected radiation chemistry in the ‘simple' H2O system at high pressure. Beginning with an H2O sample contained in a diamond anvil cell at high pressure, we found that exposure to the moderately high energy (~10 keV) x-rays used in our x-ray Raman measurements resulted in cleaving of the H2O molecules, formation of O-O and H-H bonds, and conversion of the O and H framework in ice VII into a new molecular compound composed of O2 and H2 molecules. Using a suite of integrated techniques (i.e. x-ray Raman spectroscopy, x-ray diffraction, and optical Raman spectroscopy) we established that this new crystalline solid differs from previously known phases. This new compound remained stable with respect to variations in pressure, temperature, and further x-ray and laser exposure, thus opening new possibilities for studying molecular interactions in the fundamental O2-H2 system.
MR11A-04
Simple Molecular Systems Under Simultaneous Conditions of High Pressure and Temperature
Knowledge of the elastic, optical and vibrational properties of materials under extreme conditions of high pressure and temperature is crucial for interpreting the results of seismological and planetary observations, for materials science, and for improving our understanding of fundamental physics and chemistry under such conditions. Here, I will present the results of Raman and optical spectroscopy (in pulsed and continuous modes), and also x-ray diffraction measurements of simple molecular materials under conditions of high static pressure and temperature in the diamond anvil cell. High temperatures were generated mainly by laser heating (including a pulsed laser heating), but also using internal and external resistive heating. The behavior of hydrogen, deuterium, nitrogen, oxygen and water under simultaneous conditions of high temperature and high static pressure up to 120 GPa and 4000 K will be reported. The results reveal unexpected routes of transformation to nonmolecular materials and establish the behavior of interatomic interactions in molecular materials. I thank the following individuals for contributing to this work: J. Crowhurst, N. Goldman, L. Fried, C. Mundy, J. Zaug, R. J. Hemley, V. V. Struzhkin, P. Beck, C.-S. Zha, E. Gregoryanz, C. Sanloup, M. Somayazulu, Y. Meng, N. Guignot, M. Mezouar. I acknowledge support by DOE/BES, DOE/NNSA (CDAC), NSF, and the W. M. Keck Foundation.
MR11A-05
Stability of H2O ice polymorphs at high pressure
We employ the generalized gradient approximation within the density functional theory as implemented in the ABINIT package [1] to investigate the high-pressure behavior of H2O ice. We find that ice VIII is dynamically stable to at least 100 GPa. From static (0K) calculations we show that ice X is more stable than ice VIII above 80- 100 GPa. Ice X is dynamically stable in the 120-400 GPa pressure range. Below 120 GPa it is characterized by one unstable flat phonon band, which leads to the amorphization of the structure. In its dynamical stability field, ice X shows a softening of the lowest phonon mode in M (=1/2 1/2 0) under compression, which becomes unstable above 400 GPa. The instability corresponds to the bending of the O-H-O angle. The structure that results after the lock-in of the unstable mode agrees well with the Pbcm orthorhombic structure obtained from molecular-dynamics calculations [2]. We propose a high-pressure low-temperature phase-transition sequence as ice VIII - amorphous - ice X - ice Pbcm. [1] Gonze et al., Comp. Mat. Sci. 25, 478 (2002); Z. Kristall. 220, 558 (2005). [2] Benoit et al., Phys. Rev. Lett., 76, 2934, 1996.
MR11A-06
New Developments in the Understanding of the Deformation Behaviour of ice
Single crystals undergo plastic deformation as soon as there is a component of shear stress on the basal plane. Basal slip is observed for shear stresses in the basal plane lower than 0.02 MPa and takes place by the motion of basal dislocations. Non-basal slip associated with the cross slip of basal screw dislocations is invoked for the multiplication of dislocations. But, its contribution to the deformation of the ice crystal is very small. Due to the low lattice friction, dislocations glide cooperatively and long-range internal stresses develop with deformation. The plastic deformation takes place through isolated bursts or dislocation avalanches as soon as diffusion processes are not significant. Then, the plastic deformation of the ice crystal is characterized by large spatio- temporal fluctuations with scale-invariant patterns. In polar ice sheets, the viscous deformation is essentially produced by basal slip. The other deformation modes required for compatibility reasons can be non-basal slip or the climb of dislocations. These deformation modes give a stress exponent of about 3 provided that dynamic recrystallization is not very active. At low deviatoric stresses, typically lower than 0.1 MPa, the stress exponent can be slightly lower than 2. Basal slip is still the dominant deformation mode. But, it can be accommodated by grain boundary sliding and/or grain boundary migration. A grain size effect is expected for this deformation regime. At the both scales of the ice crystal and the polycrystal, the deformation is highly heterogeneous. This behaviour is associated with the anisotropy of the ice crystal and the mismatch of slip at grain boundaries for the polycrystal. Strain rates fluctuations within grains increase with the development of textures whereas stress fluctuations decrease. The viscoplastic deformation induces the development of lattice preferred orientations (textures) giving a non-random orientation of the c-axes in the largest part of ice sheets and making ice strongly anisotropic. Recrystallization textures associated with dynamic recrystallization are generally found near the bottom where temperature is the highest.
MR11A-07
Anelastic Response of Ice-I/Magnesium Sulfate Hydrate Eutectic Aggregates Obtained from Creep and Cyclic-Loading Experiments
The large icy satellites of the giant planets are subjected to periodic stress due to their resonance-effected orbital eccentricity. The dissipation of this tidal elastic energy represents a potentially significant internal energy source, the magnitude of which depends on the anelastic properties of the various material layers (ice, rock, etc.). In an icy satellite incorporating an internal ocean, as is suggested for Europa, most of the tidal dissipation (attenuation) occurs in the outer ice layer. Current models for attenuation in the icy shell rely on the inversion of a Maxwell model of steady-state rheology because, to date, no direct dynamic measurements of energy absorption in polycrystalline ice at low-stress/low-frequency planetary conditions have been reported. We are pursuing transient and steady-state creep as well as direct attenuation measurements on polycrystalline ice-I and on eutectic aggregates of ice-I and salt hydrates, such as those suggested to be present on the surface of Europa by the near-infrared spectral data. We focus primarily on system H2O-MgSO4 because it represents the best (binary-system) fit to the spectral data. Samples are fabricated using a misting/sifting technique in order to obtain fine grain-size, or, more accurately, eutectic colony-size (< 25 μm). Transformation of the time- domain data suggest the Maxwell model of steady state is inappropriate for understanding attenuation in pure ice, and wildly inappropriate for a ice-I/magnesium sulfate hydrate eutectic material, which demonstrates far higher attenuation (e.g., a factor of 102 at 1 Hz) than that predicted from the model. The absorption behavior of solid-state heterophase boundaries are argued as crucial in this behavior. Attenuation (cyclic compression- compression) experiments have commenced; we will report initial results, comparing these to earlier studies of the static response. Information gleaned from these experiment can help constrain models of crustal thickness and surface dynamics on Europa and icy satellites in general.
MR11A-08
Experimental and Computational Studies of Planetary Ices.
Planetary bodies are essentially crystalline aggregates. Underlying the structure and evolution of all such bodies is the microscopic behaviour of the component crystals – their equilibrium structures, elasticity, and transport properties, for example. Determining the magnitudes of these properties for the different ‘ices' and for the highly hydrated phases thought to exist in the interiors of outer solar-system moons requires a coordinated multidisciplinary approach involving a combination of experimental and computational techniques. All of the substances of interest exhibit complex polymorphism over the range of pressures and temperatures found in the largest icy moons; however, apart from the case of water ice, little is known about the existence and stability fields of these numerous polymorphs. We employ traditional piston-cylinder methods to map the loci of phase boundaries, whilst simultaneously measuring density, bulk modulus and shear modulus. In addition, high-pressure powder neutron diffraction data provide the basis for the solution of high-pressure crystal structures as well as the measurement of thermal expansion tensors and incompressibilities. If the crystal structures are known, it is possible to carry out quantum mechanical calculations to determine a range of physical properties. We present new calculations of the elastic properties of triclinic meridianiite (MgSO4.11H2O), and use the example of epsomite (MgSO4.7H2O) to illustrate our confidence in the accuracy of the calculations as well as the synergy between the experimental and computational methods. Determining the rheology of planetary ices at very low strain rates is critical to the success of planetary evolution modeling. We describe experimental techniques using neutron diffraction which may allow the diffusion creep regime of water ice to be observed, and discuss the status of quantum mechanical calculations of this process.