MR23A-1021
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.
MR23A-1022
Investigation of the High P-T Behavior of Binary Hydrogen-Helium Mixtures
The gas giant planets are currently thought to be composed primarily of hydrogen-helium mixtures, with the interiors of these planets potentially composed of metallic hydrogen. The phase diagram for one such binary H2-He mixture (70% He and 30% H2) was investigated in the pressure-temperature region of 300- 500 K and 2-10 GPa with the goal of developing a satisfactory experimental methodology for mapping the temperature-pressure-composition space of the H2-He system. The gas mixture was loaded into a diamond anvil cell, with both resistance laser heating methods employed to access high temperatures. Raman spectroscopy was then used to monitor the temperature and pressure dependence of the H2 vibron. We find that the H2 vibron frequency of the H2-He mixture (4305 cm-1) is significantly higher than that of pure H2 (4200 cm-1) at 6 GPa, indicating that He dilutes H2 and disrupts the intermolecular coupling. Preliminary studies of the data at higher temperatures (> 370 K) indicate that the vibron frequencies tend to follow the same general trend with pressure as observed at lower temperature. The results of this study agree well with similar studies carried out in different pressure and temperature regimes.
MR23A-1023
The local structure of compressed water
The structural properties of liquid water are known to undergo significant changes upon compression, yet a clear description of these changes in terms of local structural properties has been lacking. We have used a combination of molecular dynamics simulations and high-pressure diamond anvil cell experiments to characterize the changes that occur in the hydrogen bonding network of water as it moves from ambient to high- pressure conditions. In particular, first-principles electronic structure theory has been used to make a direct comparison between structural data obtained from simulations to various experimental measurements, including x-ray diffraction and inelastic x-ray scattering. Our results provide new insights into the hydrogen bonding environment in compressed liquid water and ice. This work was performed under the auspices of the U.S. Dept. of Energy at the University of California/Lawrence Livermore National Laboratory under contract no. W-7405-Eng-48. http://www.llnl.gov/qsg
MR23A-1024
Phase Stability and Melting Temperature of Epsilon Oxygen at Half a Megabar
Measurements of the chemistry and physics of the pure endmembers of the C-O-H-N-S system are important building blocks in the study of planetary ices. To that end, we performed x-ray diffraction measurements on pure oxygen using the laser-heated diamond anvil cell. Using a novel experimental setup and a combined interpretation of the diffraction patterns in conjunction with spectroradiometric observations, the temperature and pressure stability field of the ε -phase has been studied. We also present measurements of the melting temperature of oxygen at 47 and 55 GPa, pressures more than twice as high as previously achieved. The ensemble of our experimental evidence highlights the remarkable thermodynamic stability of the O8 cluster structure with respect to lower pressure molecular arrangements and points to non-magnetic ordering as one of the main reasons for that.
MR23A-1025
High-Pressure Raman Spectroscopy of Protonic Diffusion in ice VII
Raman spectroscopy studies of mutual diffusion of protons and deuterons in ice VII were carried out at 473 K and pressures up to 25 GPa in a diamond anvil cell (DAC). Despite the fact that the diffusion model proposed for water ices is fifty five years old, the experimental verification of this mechanism remains extremely challenging. Present Raman spectroscopy measurements of two ice layers (H2O and D2O, accordingly) loaded without mixing in a gasket-insert assemblage signify another major advancement in experimental investigation of this phenomenon. Previous infrared spectroscopy studies of H2O/D2O ice bilayer at high pressure did not take into account variations of pressure in the DAC during the heating cycles, and, thus, might not be that accurate. The use of Raman scattering spectroscopy for measurements of mutual diffusion of protons and deuterons provides us with an opportunity to monitor pressure in situ. Our results show that pressure dependence of diffusion coefficients is negative with activation volume of ~0.001 eV/GPa. On the basis of these estimates a lower boundary of superionic (protonic diffusion coefficient is in the order of 10-8 m2/s) state of water ice will be presented on the phase diagram of water.
MR23A-1026
High Pressure Viscosity Measurements of Planetary Fluids
We have measured the viscosity of argon to 1.5 GPa and 353 Kelvin, 4% sodium-chloride water solution to 1.1 GPa at room temperature, and water up to 4.0 GPa and 500 Kelvin using an externally-heated, rolling-sphere diamond-cell viscometer. The viscosity of argon increases by a factor of three between 0.5 GPa and the crystallization pressure of 1.2 GPa. The viscosity of the sodium-chloride solution exhibits the same relative change as pure water, about a factor of 2 over 1 GPa. Surprisingly, we were able to compress water past the crystallization pressure at room temperature, and measure viscosity of metastable liquid water up to 1.4 GPa. The maximum viscosity attained was ~ 3 mPa*s, and appears to be consistent with simple extrapolations from both the low-pressure data (< 1 GPa) and the high-temperature, high-pressure data (> 323 K, ~1.5 GPa). Virtually no data previously existed for aqueous solutions at high pressures, and our experiments increase the pressure range for extensive viscosity measurements on H2O and Ar by 50%. Measurements using the diamond-cell viscometer with heating capability provide significant tests of theoretical models of fluid-transport properties at conditions existing deep inside planets.
MR23A-1027
Similar Sea Ice Behavior Dynamics on Earth and Mars
Sea ice formation on Mars was postulated by Murray et al 2005, based partially on images showing analogous structures to those reported for the Earth's Polar Regions. Further examination of additional high-resolution imagery from HRSC and HiRise have shown features that resemble sea ice pressure ridges and pile-ups that also have been observed in the Earth's sea ice cover. Ridging features with linear, sinuous, and rectilinear characteristics examined, border darker boundaries that are suggested to be older material. Side-by-side comparisons of Earth and Mars image pairs have indicated highly similar structures in both sets of imagery. From these, we have also made characteristic measurements on for example, the lengths and widths of rafting thrust structures, the "wavelengths" of sinuous ridges, and floe sizes. From these statistics of characteristic structures, we suggest strong similarities in the apparent material behavior on the two planets. Particularly important and more characteristic of sea ice than other materials is that small temperature changes in the material near the melting point (<-5 C) cause large phase changes that result in the highly dynamic transition from ductile to brittle behavior. The ridging comparisons indicate that although the measurements differ, these features require the same two-phase processes of ductile to brittle behavior to form these features. Pile-up features for both planets were evaluated by determining the ratio of the pile-up area to the cleared area to give an indication of the pile-up height. A discrete element pile-up simulation model conducted by Hopkins et al., 1999, proposes that although similar mechanisms are attributed to creating these pile-up features, lower gravity and normal force affect the potential energy, which is proportional to the increased pile-up height on Mars. This unique behavior on both planets provides further evidence that sea ice formed on Mars in an earlier epoch.
MR23A-1028
Physico-Chemical Models of Cometary Comae
Physico-chemical modeling is important to understand the nature and processes relevant to comets. Photochemistry is the major source of radicals and plasma that further initiate key gas-phase reactions, leading to the plethora of molecules and atoms seen in their comae. The effects of photoelectrons that react via electron impact reactions are important to the overall ionization. Within this modeling framework, important physico- chemical processes can be identified to interpret observations and \it in situ measurements of comets and to provide valuable insights into the intrinsic properties of their nuclei. Details of these processes are presented, from the collision-dominated inner coma to the solar wind interaction region. The results include temperature and velocity structures, and photo- and gas-phase chemistry, composition, gas and electron energetics throughout the cometary atmosphere. This model successfully accounted for the Halley water-group chemistry and composition. Prior model results are generally consistent with \it in situ measurements of the PEPE instrument onboard the Deep Space 1 Mission to Comet Borrelly, S2 in Comet Hyakutake, and observations of C2, C3, and NS in Comet Hale-Bopp. This extensive modeling effort to investigate these important cometary processes is highly relevant to past, on going, and future spacecraft missions comets and Earth-based observations of these primitive objects. Acknowledgements: We acknowledge funding and support from the NASA Discovery Data Analysis Program and the NSF Planetary Astronomy Program.
MR23A-1029
Simulation of Water Detection in Lunar Polar Regions With Different Types of Neutron Detectors
Permanently shadowed craters in the polar regions of the Moon can probably contain water ice. It is possible to search for ice deposits by measuring variations of lunar neutron albedo caused by the permanent bombardment of the Moon by Galactic Cosmic Rays. Different types of neutron detectors can be used to measure variations of the neutron albedo. We consider a set of models of neutron detectors with equal mass. All instruments use 3He proportional counters that are designed to detect variations of epithermal neutron flux. A lunar map of hydrogen distribution could be developed by analyzing those variations. If the lunar polar regions contain water ice deposits then hydrogen abundance in lunar soil in those regions increases, and thus such spots can be detected. For the purpose of achieving better spatial resolution some instruments use collimators. Our models which have detailed 3D-geometry are intended for the Monte-Carlo simulation with MCNPX code. Besides, we consider a set of lunar soil models in our simulations. For each of these models we calculate the neutron background flux. A special procedure was developed for simulations of neutron flux detected by different instruments. Different lunar soil models were produced by varying such parameters as density, water abundance, temperature and geometry of possible region containing water ice. As a result for each type of detector we obtained data on variation of counting rate corresponding to variation of lunar soil models. These models will be eventually tested against observations of the future lunar missions.