Bulletin of the American Physical Society
15th APS Topical Conference on Shock Compression of Condensed Matter
Volume 52, Number 8
Sunday–Friday, June 24–29, 2007; Kohala Coast, Hawaii
Session M4: Geophysics and Planetary Physics |
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Chair: Ricky Chau, Lawrence Livermore National Laboratory Room: Fairmont Orchid Hotel Plaza II |
Wednesday, June 27, 2007 10:30AM - 11:00AM |
M4.00001: The Deep Impact Oblique Impact Experiment Invited Speaker: The Deep Impact experiment represents a unique challenge. Without being able to see clearly the final crater, properties of the target requires comparing the ballistic ejecta with analytical and theoretical models for crater excavation. But the appropriate excavation model needs to be identified first. Consequently, each critical stage of cratering observed for the DI collision (initial coupling, late-stage ejection) is described and compared with a wide range of laboratory experimental results. The early-time flash and vapor plume rapidly evolve along the trajectory: an initial faint ``first light'' uprange from the projected point of impact; a fading source along the trajectory that moves downrange ($\sim$100-170m) over the next 0.125s after impact; gradual brightening over the next 0.62s; and then a sudden ``flash'' (saturated pixels) around 0.25s after the ``first light.'' This evolution is consistent with a high-porosity, layered target, which is also inferred from the high-resolution imaging of the impact point. Because of the low impact angle for DI (between 25$^{\circ}$ and 35$^{\circ}$ from the surface horizontal), changing styles of ejecta with time are mapped out spatially by the ballistic ejecta. Such changing styles provide qualitative but critical clues for scaling including initial coupling (plume evolution, shallow versus deep coupling) and excavation stages (symmetric versus asymmetric, non-radial rays). Two different approaches are used to constrain the final crater size: backward ray traces to the surface and estimates derived from the total ejected mass from earth-based telescopic observations. Ejecta ray traces indicate a diameter of about 175m. The total ejected mass based on Earth-based observations (107 kg dust and water ice) should be 50 times less than the total displaced mass for the crater (neglecting the contribution by ices). Based on this (and other considerations), the crater diameter could be a maximum of 250m. Nevertheless, the excavated mass observed from the earth (or other probes) most likely was derived from a very small fraction (and likely the upper surface) due to the oblique trajectory. The crater may very well be a nested crater, i.e., a deep penetration funnel surrounded by a shallow excavation crater. [Preview Abstract] |
Wednesday, June 27, 2007 11:00AM - 11:30AM |
M4.00002: Impact Cratering Physics al Large Planetary Scales Invited Speaker: Present understanding of the physics controlling formation of $\sim$10$^3$ km diameter, multi-ringed impact structures on planets were derived from the ideas of Scripps oceanographer, W. Van Dorn, University of London's, W, Murray, and, Caltech's, D. O'Keefe who modeled the vertical oscillations (gravity and elasticity restoring forces) of shock-induced melt and damaged rock within the transient crater immediately after the downward propagating hemispheric shock has processed rock (both lining, and substantially below, the transient cavity crater). The resulting very large surface wave displacements produce the characteristic concentric, multi-ringed basins, as stored energy is radiated away and also dissipated upon inducing further cracking. Initial calculational description, of the above oscillation scenario, has focused upon on properly predicting the resulting density of cracks, and, their orientations. A new numerical version of the Ashby--Sammis crack damage model is coupled to an existing shock hydrodynamics code to predict impact induced damage distributions in a series of 15--70 cm rock targets from high speed impact experiments for a range of impactor type and velocity. These are compared to results of crack damage distributions induced in crustal rocks with small arms impactors and mapped ultrasonically in recent Caltech experiments (Ai and Ahrens, 2006). [Preview Abstract] |
Wednesday, June 27, 2007 11:30AM - 11:45AM |
M4.00003: Shock compression of iron foils to Earth core conditions with GEKKO-HIPER laser Keisuke Shigemori, Tetsuo Irifune, Daigo Ichinose, Kazuto Otani, Tatsuhiro Sakaiya, Hiroshi Azechi, Kunioki Mima, Justin Wark, Bruce Remington We have been developing an experiment to create the Earth core condition ($>$300 GPa, $\sim $ 6000 K) with intense laser. Experiments were done on GEKKO-HIPER laser facility which has twelve beams from one direction for planar target experiments. We irradiated iron foils with a shaped pulse by stacking laser beams with certain time delay between the beams. Shock parameters (shock velocity, shocked temperature) were measured by velocity interferometer for any reflector (VISAR) and optical spectral pyrometer. We also measured the sound velocity of the shock compressed iron foils with side-on x-ray backlighting technique. The measured sound velocity ($>$ 300 GPa, $\sim $ 5000K) shows solid sound velocity ($\sim $ 11 km/s) in good agreements with previous seismic data. We also started to measure the crystal conditions of the shocked iron with x-ray diffraction technique. Preliminary results were obtained at a pressure of $>$15 GPa. [Preview Abstract] |
Wednesday, June 27, 2007 11:45AM - 12:00PM |
M4.00004: Ejection mechanisms for Martian meteorites Paul S. De Carli, Ahmed El Goresy, Zhidong Xie, Thomas G. Sharp At least 34 meteorites have been identified by their characteristic isotope signatures as originating on Mars. The Martian origin of these meteorites is not in dispute. It is generally accepted that the meteorites were ejected from Mars as a result of asteroid or comet impacts. However, there is no agreement on the detailed mechanism by which these meteorites were accelerated to the Martian escape velocity of 5 km/s. The simplest mechanism, that the meteorites were accelerated by a strong shock, implies a minimum shock pressure of about 65 Gpa. Evidence from the meteorites themselves implies that none of them have been subjected to shock pressures in excess of about 40 GPa. Measurements of the magnetic properties of Martian meteorite ALH 84001 indicate that the ejection event did not heat it above its curie temperature of about 40 C, implying a maximum shock pressure during ejection of less than 13 GPa. We have not been able to reproduce recent calculations that predict high velocity low pressure spalls. We explore the possibility that Martian meteorites are accelerated to escape velocity in a high velocity vapor or ejecta cloud. [Preview Abstract] |
Wednesday, June 27, 2007 12:00PM - 12:15PM |
M4.00005: Improved Strength and Damage Modeling of Geologic Materials Sarah Stewart, Laurel Senft Collisions and impact cratering events are important processes in the evolution of planetary bodies. The time and length scales of planetary collisions, however, are inaccessible in the laboratory and require the use of shock physics codes. We present the results from a new rheological model for geological materials implemented in the CTH code [1]. The `ROCK' model includes pressure, temperature, and damage effects on strength, as well as acoustic fluidization during impact crater collapse. We demonstrate that the model accurately reproduces final crater shapes, tensile cracking, and damaged zones from laboratory to planetary scales. The strength model requires basic material properties; hence, the input parameters may be benchmarked to laboratory results and extended to planetary collision events. We show the effects of varying material strength parameters, which are dependent on both scale and strain rate, and discuss choosing appropriate parameters for laboratory and planetary situations. The results are a significant improvement in models of continuum rock deformation during large scale impact events. [1] Senft, L. E., Stewart, S. T. Modeling Impact Cratering in Layered Surfaces, J. Geophys. Res., submitted. [Preview Abstract] |
Wednesday, June 27, 2007 12:15PM - 12:30PM |
M4.00006: ABSTRACT WITHDRAWN |
Wednesday, June 27, 2007 12:30PM - 12:45PM |
M4.00007: Heterogeneous Thermal Emission from Shocked Basalt Sarah Stewart, Achim Seifter, Gregory Kennedy, Michael Furlanetto, Andrew Obst Natural flaws in geologic materials result in heterogeneous pressure and temperature distributions upon shock compression. The effects of flaws are apparent in the thermal emission from shocked samples. We present emission temperature measurements from Columbia River Basalt using multi-band pyrometry (0.65 nm to 4.8 $\mu $m) and gated infrared imaging. After release from peak shock pressures between 9.5 and 45 GPa, free surface thermal emission temperatures range from 450 to $>$1250 K. The emission measurements show a departure from a quasi-single temperature surface between 10 and 14 GPa, where, at pressures well below that required for bulk melting of basalt, emission temperatures $>$1600 K are detected. In this pressure range, partial melting in fractures and pore spaces produce a bimodal temperature distribution comprised of a continuum and hot spots. The inferred hot spot distributions are in excellent agreement with petrographic studies of localized melting and generation of high pressure phases in basaltic meteorites from Mars shocked to similar pressures. However, the measured continuum temperatures in Columbia River basalts are 100 to 400 K higher than inferred for Martian meteorites. [Preview Abstract] |
Wednesday, June 27, 2007 12:45PM - 1:00PM |
M4.00008: Soft X-ray -- Induced Shock Loading of Meteorite and Planetary Materials John Remo, Michael Furnish The response of meteorite and planetary materials to high- intensity $<$1 keV x-rays from Z-pinch sources is described. These materials include iron and stony meteorites, magnesium rich olivine (dunite), and Al and Fe calibration samples. Input stresses varied from 6.1 to 12.4 GPa, attenuating to $\sim$ 1.4 to 2.5 GPa for the iron meteorites, $\sim$ 0.3 to 1.9 GPa for the stony meteorites, and 1.64 to 1.91 GPa for dunite. The calibration (pure) metals showed less attenuation than the highly inhomogeneous natural materials: 9.5 to $\sim$ 5 GPa for Fe and 12.4 to 10.6 GP for Al. Putative equations of state are computed from Hugoniot pressure and shock velocity as a function of particle velocity. These data are useful for planetary and astrophysical modeling and for near-Earth object mitigation studies requiring momentum coupling, and momentum enhancement coefficients. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000. [Preview Abstract] |
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