Showing papers by "Olivier S. Barnouin published in 2010"

Initial observations from the Lunar Orbiter Laser Altimeter (LOLA)

Abstract: As of June 19, 2010, the Lunar Orbiter Laser Altimeter, an instrument on the Lunar Reconnaissance Orbiter, has collected over 2.0 × 10^9 measurements of elevation that collectively represent the highest resolution global model of lunar topography yet produced. These altimetric observations have been used to improve the lunar geodetic grid to ~10 m radial and ~100 m spatial accuracy with respect to the Moon's center of mass. LOLA has also provided the highest resolution global maps yet produced of slopes, roughness and the 1064-nm reflectance of the lunar surface. Regional topography of the lunar polar regions allows precise characterization of present and past illumination conditions. LOLA's initial global data sets as well as the first high-resolution digital elevation models (DEMs) of polar topography are described herein.

Visualization of the failure of quartz under quasi‐static and dynamic compression

Abstract: [1] Quasi-static and dynamic compression experiments were performed on natural α quartz single crystal specimens at strain rates ranging from 10 ―3 to 10 3 s ―1 using a high-speed camera for visualization of failure In one set of experiments, the specimens were compressed until catastrophic failure occurred, shattering the specimen into many small pieces The results of the experiments show little strain rate dependence of the compressive strength of quartz for the range of strain rates applied in this study In a second set of experiments, referred to here as interrupted compression, the specimens were compressed to a stress level of about half of the failure strength and then unloaded For times up to when the peak load is achieved, images of the specimen recorded during the experiment show no crack initiation or propagation However, in these experiments, the growth of large planar cracks was observed during (and only during) the unloading phase The real-time visualization demonstrated that behavior of failure during unloading occurs in both the quasi-static and dynamic interrupted compression experiments The crystallographic indices of the failure planes were identified to be of the {1101} and {1010} families, indicating cleavage failure on the positive and negative rhombohedral surfaces, respectively

Mechanical Structure of Mercury's Lithosphere from MESSENGER Observations of Lobate Scarps

Abstract: OBSERVATIONS OF LOBATE SCARPS. J. Andreas Ritzer, Steven A. Hauck, II, Olivier S. Barnouin, Sean C. Solomon, Thomas R. Watters. Geological Sciences, Case Western Reserve University, Cleveland, OH 44106, andreas.ritzer@case.edu, Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015 Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington, DC 20560.

Dynamic strength measurements of l5 chondrite macalpine hills 88118.

Abstract: Introduction: Limited data on the compressive strength of meteorites exist in the literature. Previous researchers have performed laboratory measurements on stony meteorites [1, 2, 3, 4, 5, 6, 7], as well as iron meteorites [8, 9]. These studies were mostly designed to determine the strength of meteorites with regard to understanding the breakup of meteors entering the Earth’s atmosphere. The process of traversing the Earth’s atmosphere takes place on the time scale of seconds, and the strength under low rate (quasistatic) loading conditions is expected to control the failure process. Thus the laboratory measurements performed were conducted at quasistatic loading rates. The only measurements performed at high strain rates to date were performed on specimens of an iron meteorite [9]. For applications where the loading conditions are more dynamic (e.g. impact cratering), the quasistatic strength may not provide an accurate description of material response. Terrestrial rock specimens often exhibit an increased strength when compressed at strain rates higher than 10 s [10, 11, 12, 13]. Stony meteorites are expected to exhibit similar rate dependence, but no measurements exist. To begin to fill this gap in data we have chosen to investigate the compressive strength of the meteorite MacAlpine Hills 88118 (MAC 88118) at strain rates ranging from 10 to 10 s. Experimental Method: MAC 88118 has been characterized as a L5 chondrite with microstructural features consistent with a shock state S1. Cube shaped specimens with edge lengths of approximately 5mm were cut from larger slabs of the meteorite specimen. Specimens were compressed in the direction normal to the surface of the parent plate to isolate any effects of specimen anisotropy [6]. Loading faces of the specimen were polished to ensure that the faces were parallel to within 5 μm across the specimen. The quasistatic compression experiments were conducted using a MTS servohydraulic uniaxial testing machine, while the dynamic compression experiments were conducted using a Kolsky (split-Hopkinson) bar [10, 14] as shown in Fig. 1. In all of the experiments the specimens were loaded until material failure. For all experiments images were recorded in real time to capture the evolution of failure in the specimen. Experimental Results: The stress-strain response of a specimen subjected to quasistatic compression (strain rate 10 s) is shown in Fig. 2. Here the specimen stress was calculated from the recorded load data, and the specimen cross sectional area. The strain was measured with an electrical resistance strain gage bonded to the surface of the specimen. For strains below 0.01, the stress–strain curve follows a linear trend. The slope of this portion of the data corresponds to the Young’s modulus, E, of the meteorite which is determined to be 3.2 GPa. For strains greater than 0.01 we observe a large increase in stress without a corresponding increase in strain. We interpret this as a failure of the strain gage. Here we see that the specimen stress increases to a peak value of 50 MPa before dropping off to zero. This peak corresponds to the quasistatic compressive strength of the meteorite.

Emerging Perspectives on Mercury's Internal Structure from MESSENGER Flyby Observations and Geophysical Modeling

Abstract: FLYBY OBSERVATIONS AND GEOPHYSICAL MODELING. Maria T. Zuber, David E. Smith, Roger J. Phillips, Sean C. Solomon, Gregory A. Neumann, Frank G. Lemoine, Stanton J. Peale, Jean-Luc Margot, Steven A. Hauck, II, James W. Head, Catherine L. Johnson, Michael E. Purucker, Jurgen Oberst10, Grant T. Farmer, Jiangning Lu, Youshun Sun, M. Nafi Toksoz, Olivier S. Barnouin, Mark E. Perry, Dipak K. Srinivasan, Mark H. Torrence, Massachusetts Institute of Technology, Cambridge, MA 02129 (zuber@mit.edu); Southwest Research Institute, Boulder, CO 80302; Carnegie Institution of Washington, Washington, DC 20015; NASA Goddard Space Flight Center, Greenbelt, MD 20771; University of California, Santa Barbara, CA 93106; University of California, Los Angeles, CA 90095; Case Western Reserve University, Cleveland, OH 44106; Brown University, Providence, RI 02912; University of British Columbia, Vancouver, BC, Canada V6T 1Z4; DLR, Berlin, Germany; Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723; SGT, Inc., 7701 Greenbelt Rd., Greenbelt, MD 20770.

The Dynamic Fracture of Rocky Bodies: Applications to Planetary Impact Problems

Abstract: 242 formation of an impact crater will provide new constraints on the collisional evolution of the asteroid belt. To first order, higher strain rates yield smaller fragment sizes. As Fig. 1 shows, the relationship is really more complex. As a first step toward better understanding the failure of geological materials at fairly moderate strain rates— more akin to what a planetary body typically encounters during a planetary-scale impact—we embarked on an experimental investigation to probe the response of a model geological material (single-crystal quartz) under uniaxial compression. Dynamic compression experiments were conducted using a Kolsky bar, which allowed a detailed history of the stress–time response on the microsecond time scale to be obtained. Ultra-high-speed photography recorded the evolution of damage and the propagation of cracks. Experiments also were done at quasi-static loading rates to further determine the effect of loading rate. Again, images detailing the evolution of the failure process were recorded. An increase in compressive strength was observed with increased loading rate. In specimens that were not loaded to (catastrophic) failure, significant crack growth was observed during the mechanical unloading of the specimen. The mechanism (or mechanisms) responsible for generating and propagating these “unloading cracks” is currently under investigation. A parallel modeling effort is in progress (Fig. 2). Impacts initiate dynamic fracturing on macroand micro-scales, and the resulting fragmentation can be related to strain rate. Dynamic fracture has been directly observed at low strain rates (~10 −2 to 10−3 s−1, during earthquakes) and at high strain rates (~105 to 106 s−1, during laboratory-scale hypervelocity impact experiments). On the basis of firstorder estimates of the strain rate in the event (approximate impact velocity/projectile diameter) and numerical results such as those shown, the strain rates encountered in a typical planetary-scale impact range from ~100 to 102 s−1. These intermediate values lie within a strain rate regime that can be observed with the Kolsky bar, but are not easy to observe during typical small-scale hypervelocity (<2 km/s) impact experiments. Combining our numerical simulations with new dynamic fragmentation models derived from Kolsky bar experiments, we have begun to Figure 1. Factors that contribute to the failure of materials on the basis of classical dynamic fracture mechanics. Most of the region cratered by a bolide actually undergoes moderate rather than high strain rates, which result in different mechanical and failure behaviors of the impacted body. The Dynamic Fracture of Rocky Bodies: Applications to Planetary Impact Problems