Bulletin of the American Physical Society
18th Biennial Intl. Conference of the APS Topical Group on Shock Compression of Condensed Matter held in conjunction with the 24th Biennial Intl. Conference of the Intl. Association for the Advancement of High Pressure Science and Technology (AIRAPT)
Volume 58, Number 7
Sunday–Friday, July 7–12, 2013; Seattle, Washington
Session Z5: GP2: Planetary II |
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Chair: Dylan Spaulding, Harvard University Room: Cascasde I |
Friday, July 12, 2013 11:00AM - 11:30AM |
Z5.00001: DFT modeling of chemistry on the Z machine Invited Speaker: Thomas R. Mattsson Density Functional Theory (DFT) has proven remarkably accurate in predicting properties of matter under shock compression for a wide-range of elements and compounds: from hydrogen to xenon via water. Materials where chemistry plays a role are of particular interest for many applications. For example the deep interiors of Neptune, Uranus, and hundreds of similar exoplanets are composed of molecular ices of carbon, hydrogen, oxygen, and nitrogen at pressures of several hundred GPa and temperatures of many thousand Kelvin. High-quality thermophysical experimental data and high-fidelity simulations including chemical reaction are necessary to constrain planetary models over a large range of conditions. As examples of where chemical reactions are important, and demonstration of the high fidelity possible for these both structurally and chemically complex systems, we will discuss shock- and re-shock of liquid carbon dioxide (CO2) in the range 100 to 800 GPa, shock compression of the hydrocarbon polymers polyethylene (PE) and poly(4-methyl-1-pentene) (PMP), and finally simulations of shock compression of glow discharge polymer (GDP) including the effects of doping with germanium. Experimental results from Sandia's Z machine have time and again validated the DFT simulations at extreme conditions and the combination of experiment and DFT provide reliable data for evaluating existing and constructing future wide-range equations of state models for molecular compounds like CO2 and polymers like PE, PMP, and GDP. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Friday, July 12, 2013 11:30AM - 11:45AM |
Z5.00002: First-principles simulations of planetary ices S. Hamel, A. Correa, M. Bethkenhagen, R.N. Mulford, D.C. Swift A large fraction of the transiting exoplanets observed are similar in mass and radii to the Ice Giants in our Solar System. The structure of such planets is heavily dependent on the equation-of-state properties of mixtures of water, ammonia and methane (referred to as ``planetary ices'') at high pressures and temperatures. Many observables properties of Uranus and Neptune, such as gravitational moments and magnetic fields, are thought to be determined by the physical and chemical properties of matter within this ice layer. Of particular interest is the impact of the complex organic chemistry on the fluid properties at these extreme conditions. To cover the wide range of pressure and temperatures relevant to the description of these planets, different approaches are used to generate the EOS data. One of these approaches is quantum molecular dynamics, which we use to address the high-temperature high-pressure part of the EOS. Here we report our QMD results for the structure, composition and properties of the high-pressure planetary ices. As well as producing usable EOS in their own right, QMD can validate chemical equilibrium calculations, which are based on assumed functional forms for thermodynamic potentials. Where valid, these calculations can be performed much faster than QMD. [Preview Abstract] |
Friday, July 12, 2013 11:45AM - 12:00PM |
Z5.00003: Complex prebiotic chemistry within a simple impacting icy mixture Nir Goldman We present results of prebiotic molecule synthesis in shock compressed mixtures of simple ices from quantum molecular dynamics (MD) simulations. Given the likelihood of a CO$_{2}$-rich primitive atmosphere, it is possible that impact processes of comets or other icy bodies were partially responsible for the creation of prebiotic chemical compounds on early Earth. We have conducted simulations of the chemical reactivity within an oxidized astrophysical icy mixture to close to equilibrium using a density functional tight binding (DFTB) approach. We observe that moderate shock pressures and temperatures (35 GPa and 2800 K) produce a number of functionalized polycyclic aromatic hydrocarbons (PAHs), which remain intact upon expansion and cooling to lower conditions. At higher shock pressures and temperatures (48-62 GPa, 3700-4700 K), we observe the synthesis of a variety of short-lived, exotic C---C and C---N bonded oligomers which decompose upon expansion and cooling to form precursors to amino acids and other prebiotic compounds, such as long chain alkanes, HCN, CH$_{4}$ and formaldehyde. Our results provide a mechanism for shock synthesis of prebiotic molecules at realistic impact conditions that is independent of external features such as the presence of a catalyst, illuminating UV radiation, or pre-existing conditions on a planet. [Preview Abstract] |
Friday, July 12, 2013 12:00PM - 12:15PM |
Z5.00004: Nature of the interiors of Uranus and Neptune William Nellis, N. Ozaki, R. Ahuja, T. Mashimo, M. Ramzan, T. Kaewmaraya Ever since the spacecraft flyby missions to Uranus and Neptune the nature of the interiors of these similar planets have been puzzles. Planetary materials are H-He; ``ice,'' hydrogenous molecular and ionic fluids; rock (oxides); and Fe. Measured gravitational moments cannot resolve mass distribution between 3-layer and 2-layer models, the former with sharp mass discontinuities and the latter with mass varying continuously. Also a puzzle is the material distribution that would produce the spherical annulus proposed to explain a dynamo that would generate the tilted magnetic fields. A mass distribution needs to be identified that is consistent with both the gravitational and magnetic data. If all materials become conductors then miscibility and dynamos are both possible. Gd$_{3}$Ga$_{5}$O$_{12}$ is a strong insulator with Gd-O and Ga-O bond strengths similar to Mg-O and Si-O. We have measured optical reflectivities of shock fronts in melted Gd$_{3}$Ga$_{5}$O$_{12}$ from 0.5 to 2 TPa at the Osaka laser facility. Measured reflectivities are $\sim$ 0.1, in reasonable agreement with optical properties of amorphous Gd-Ga-O calculated in the corresponding density range. Thus, ``ices'', rock, decomposed hydrogenous molecules, pure H, and Fe are probably all poor metals at conditions in the deep planetary interiors and thus miscible to a significant degree. A qualitative picture of the interiors with radially continuous mass distributions will be proposed. $^{\mathrm{1}}$Harvard University, $^{\mathrm{2}}$Osaka University, $^{\mathrm{3}}$Uppsala University, $^{\mathrm{4}}$Kumamoto University [Preview Abstract] |
Friday, July 12, 2013 12:15PM - 12:30PM |
Z5.00005: Hypervelocity Impacts of Ice Grains into a Titanium Spacecraft Instrument Chamber James D. Walker, Sidney Chocron, J. Hunter Waite, Tim G. Brockwell The Cassini spacecraft, currently in orbit around Saturn, has an Ion and Neutral Mass Spectrometer (INMS). There have been some unexpected readings of the instrument in flybys of the moon Enceladus. These flybys range from 7 to 18 km/s, and it has been suggested that ice grain impacts in the instrument could have a velocity-dependent response that influences the materials that the instrument records. To explore the physics of the impacts, computations were performed with CTH. Small ice grains (1 micron across) were impacted into a titanium alloy at a range of speeds of interest. Initial results indicate the formation of a titanium vapor plume begins at impact velocities of 16 km/s. Efforts have been made to quantify the titanium vapor and titanium solid and liquid ejecta at various impact speeds, as all of these may influence chemistry in the instrument's antechamber and thus affect what ions or molecules are seen by the INMS. [Preview Abstract] |
Friday, July 12, 2013 12:30PM - 12:45PM |
Z5.00006: Planetary structure and impact calculations using new mixture equations of state A. Correa, D.C. Swift, R.N. Mulford, S. Hamel Studies of the structure of icy planets and exoplanets, and of comet impacts, are hampered by limited high-pressure data on ices. We have recently predicted equations of state (EOS) for water-methane-ammonia mixtures using quantum molecular dynamics and equilibrium chemistry based on empirically-derived potentials. Here we use these EOS to predict astrophysical mass-radius relations with a firmer theoretical footing. We previously developed a hydrocode model of heterogeneous mixtures using stress and thermal equilibration among a set of homogeneous components, with each component described by its own EOS and constitutive model. We have extended this model to treat multiphase flow more completely, by including a particle velocity for each component, drag, and the evolution of particle sizes. An externally-applied bulk acceleration from hydrodynamics induces different accelerations in each component, according to the differences in mass density. This model is suitable for simulating the dynamic loading of asteroids and comet nuclei, where one component such as an ice may be vaporized, resulting in the acceleration of embedded particles of a less volatile material. The same model can be used to simulate some aspects of planetary evolution, such as differentiation. [Preview Abstract] |
Friday, July 12, 2013 12:45PM - 1:00PM |
Z5.00007: Volatile Loss during Collisional Growth of Planets Sarah Stewart, Sujoy Mukhopadhyay During the end stage of planet formation, rocky planets grow by collisions with planetary embryos and planetesimals. Impact velocities are typically 1 to 4 times the escape velocity of the largest bodies (up to about 40 km/s). The collision velocities are large enough to induce substantial vaporization of the projectile. Using the collision history of growing planets from recent N-body simulations, we present estimates of the loss of volatiles by vaporization of impacting planetesimals and erosion of the atmosphere during the end stage of planet formation. We propose that major differences in the noble gas signatures of the atmospheres of Venus, Earth and Mars are a result of the different outcomes of late impact events on each planet. [Preview Abstract] |
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