Repository of works by Caltech published authors.
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The Cassini Cosmic Dust Analyzer - Srama, R.; Ahrens, T. J.
The Cassini-Huygens Cosmic Dust Analyzer (CDA) is intended to provide direct observations of dust grains with masses between 10^(−19) and 10^(−9) kg in interplanetary space and in the jovian and saturnian systems, to investigate their physical, chemical and dynamical properties as functions of the distances to the Sun, to Jupiter and to Saturn and its satellites and rings, to study their interaction with the saturnian rings, satellites and magnetosphere. Chemical composition of interplanetary meteoroids will be compared with asteroidal and cometary dust, as well as with Saturn dust, ejecta from rings and satellites. Ring and satellites phenomena which might be...
Shock-induced superheating and melting curves of geophysically important minerals - Luo, Sheng-Nian; Ahrens, Thomas J.
Shock-state temperature and sound-speed measurements on crystalline materials, demonstrate superheating-melting behavior distinct from equilibrium melting. Shocked solid can be superheated to the maximum temperature, T_c′. At slightly higher pressure, P_c, shock melting occurs, and induces a lower shock temperature, T_c. The Hugoniot state, (P_c,T_c), is inferred to lie along the equilibrium melting curve. The amount of superheating achieved on Hugoniot is, Θ_H+=T_c′/T_c−1. Shock-induced superheating for a number of silicates, alkali halides and metals agrees closely with the predictions of a systematic framework describing superheating at various heating rates [Appl. Phys. Lett. 82 (12) (2003) 1836]. High-pressure melting curves are constructed...
Assessment of shock effects on amphibole water contents and hydrogen isotope compositions: 2. Kaersutitic amphibole experiments - Minitti, Michelle E.; Leshin, L. A.; Dyar, M. Darby; Ahrens, Thomas J.; Guan, Yunbin; Luo, Sheng-Nian
To constrain the influence of impact shock on water and hydrogen isotope signatures of Martian meteorite kaersutites, we conducted shock recovery experiments on three terrestrial kaersutite crystals. Homogeneous impact shock to 32 GPa, commensurate with shock levels experienced by Martian meteorite kaersutites, led to increases in kaersutite water contents (ΔH_2O = 0.25–0.89 wt.%), decreases in Fe^3+/ΣFe (4–20%), and enrichments in hydrogen isotope composition (ΔD = + 66 to + 87‰) relative to pre-shock values. The latter values represent the largest shock-induced hydrogen isotope fractionations measured to date. These observations are explained most completely by a two-step shock process. First, shock-induced...
Phase diagram of iron, revised-core temperatures - Ahrens, Thomas J.; Holland, Kathleen G.; Chen, George Q.
Shock-wave experiments on iron preheated to 1573 K from 14 to 73 GPa, yield sound velocities of the γ- and liquid-phases. Melting is observed in the highest pressure (∼71 ± 2 GPa) experiments at calculated shock temperatures of 2775 ± 160 K. This single crossing of the γ-liquid boundary agrees with the γ-iron melting line of Boehler , Saxena et al. , and Jephcoat and Besedin . This γ-iron melting curve is ∼300°C lower than that of Shen et al.  at 80 GPa. In agreement with Brown  the discrepancy between the diamond cell melting data and the iron...
Dynamic tensile strength of terrestrial rocks and application to impact cratering - Ai, Huirong-Anita; Ahrens, Thomas J.
Dynamic tensile strengths and fracture strengths of 3 terrestrial rocks, San Marcos gabbro, Coconino sandstone, and Sesia eclogite were determined by carrying out flat-plate (PMMA and aluminum) impact experiments on disc-shaped samples in the 5 to 60 m/sec range. Tensile stresses of 125 to 300 MPa and 245 to 580 MPa were induced for gabbro and eclogite, respectively (with duration time of ~1 μs). For sandstone (porosity 25%), tensile stresses normal to bedding of ~13 to 55 MPa were induced (with duration times of 2.4 and ~1.4 μs). Tensile crack failure was detected by the onset of shock-induced (damage) P...
Shock properties of H_2O ice - Stewart, Sarah T.; Ahrens, Thomas J.
To understand the mechanics and thermodynamics of impacts on, and collisions between, icy planetary bodies, we measured the dynamic strength and shock states in H2O ice. Here, we expand upon previous analyses and present a complete description of the phases, temperature, entropy, and sound velocity along the ice shock Hugoniot. Derived from shock wave measurements centered at initial temperatures (T_0) of 100 K and 263 K, the Hugoniot is composed of five regions: (1) elastic shocks in ice Ih, (2) ice Ih deformation shocks, and shock transformation to (3) ice VI, (4) ice VII, and (5) liquid water. In each...
Mass spectrometer calibration of Cosmic Dust Analyzer - Ahrens, Thomas J.; Gupta, Satish C.; Jyoti, G.; Beauchamp, J. L.
The time-of-flight (TOF) mass spectrometer (MS) of the Cosmic Dust Analyzer (CDA) instrument aboard the Cassini spacecraft is expected to be placed in orbit about Saturn to sample submicrometer-diameter ring particles and impact ejecta from Saturn's satellites. The CDA measures a mass spectrum of each particle that impacts the chemical analyzer sector of the instrument. Particles impact a Rh target plate at velocities of 1-100 km/s and produce some 10^(−8) to 10^(−5) times the particle mass of positive valence, single-charged ions. These are analyzed via a TOF MS. Initial tests employed a pulsed N2 laser acting on samples of kamacite,...
Decreased values of cosmic dust number density estimates in the Solar System - Willis, M. J.; Burchell, M. J.; Ahrens, T. J.; Krüger, H.; Grün, E.
Experiments to investigate the effect of impacts on side-walls of dust detectors such as the present NASA/ESA Galileo/Ulysses instrument are reported. Side walls constitute 27% of the internal area of these instruments, and increase field of view from 140° to 180°. Impact of cosmic dust particles onto Galileo/Ulysses Al side walls was simulated by firing Fe particles, 0.5–5 μm diameter, 2–50 km s^(−1), onto an Al plate, simulating the targets of Galileo and Ulysses dust instruments. Since side wall impacts affect the rise time of the target ionization signal, the degree to which particle fluxes are overestimated varies with velocity. Side-wall...
Hypervelocity microparticle impact studies using a novel cosmic dust mass spectrometer - Austin, Daniel E.; Grimm, Ronald L.; Manning, Heidi L. K.; Bailey, Carl L.; Farnsworth, James E.; Ahrens, Thomas J.; Beauchamp, J. L.
Micron-sized iron and copper particles accelerated to 2–20 km/s in a 2 MV van de Graaff electrostatic accelerator were used to test the performance of our recently developed cosmic dust mass spectrometer. This compact in situ dust analyzer, known as the Dustbuster, is designed to determine the elemental composition of cosmic dust particles through impact ionization and subsequent time-of-flight mass spectrometry. Results from 750 laboratory impacts show high mass resolution, typically 150–350 (m/Δm) for projectile components and 300–600 for the target material (tantalum). Peaks corresponding to H, C, O, Na, and K ions are also observed, consistent with previous microparticle...
Effects of shock-induced cracks on the ultrasonic velocity and attenuation in granite - Ai, Huirong A.; Ahrens, Thomas J.
Measurements of the compressional wave velocity and the attenuation coefficients of 1-cm cubes were conducted. Samples were taken at various radii and depths beneath a 20 × 20 × 15 cm San Marcos granite block, impacted by a lead bullet at a velocity of 1200 m/s. The damage parameters of the cubes are calculated from the measured preimpact and postimpact P wave velocities, V_(p0) and V_p, and the crack density is inverted from the measured P wave velocities. The anisotropic orientation of cracks is more obvious from the attenuation than crack density and damage parameters calculated from the ultrasonic velocity....
Unstable kinetic relations and the dynamics of solid-solid phase transitions - Rosakis, Phoebus; Knowles, James K.
In recent continuum-mechanical models of phase transitions in solids, the kinetic relation for a transition is usually assumed to be such that the driving force acting on a phase boundary is a monotonically increasing function of phase boundary velocity. The present paper explores the implications of relinquishing this assumption in the dynamics of one-dimensional elastic bars undergoing stress-induced transitions. Among other results, it is found that, for a class of non-monotonic kinetic relations, models of the kind discussed here permit stick-slip motions of a phase boundary, as observed in certain experiments.
Shock Hugoniot of H_2O ice - Stewart, Sarah T.; Ahrens, Thomas J.
The outcome of impacts onto and between icy planetary bodies is controlled by the material response defined by the shock Hugoniot. New Lagrangian shock wave profile measurements in H_2O ice at initial temperatures (T_0) of 100 K, together with previous T_0 = 263 K data, define five distinct regions on the ice Hugoniot: elastic shocks in ice Ih, ice Ih deformation shocks, and shock transformation to ices VI, VII and liquid water. The critical pressures required to induce incipient melting (0.6, 4.5 GPa) and complete melting (3.7, >5.5 GPa) upon isentropic release from the shock state (for T_0 = 263,...
On the kinetics of an austenite → martensite phase transformation induced by impact in a Cu-Al-Ni shape-memory alloy - Abeyaratne, Rohan; Knowles, James K.
The kinetics of a propagating phase boundary in a single crystal of Cu-Al-Ni is determined. As particles cross this moving interface, they transform from austenite to β'_1-martensite; the former phase is cubic, while the latter is monoclinic. The data that are used in this paper were obtained by Escobar and Clifton (Escober, J. C. and Clifton, R. J., On pressure-shear plate impact for studying the kinetics of stress-induced phase transformations. J. Mater. Sci. Engng, 1993, A170, 125–142; Escober, J. C. and Clifton, R. J. Pressure-shear impact-induced phase transitions in Cu-14.4Al-4.19Ni single crystals. SPIE, 1995, 2427, 186–197) and Escobar (Escobar, J....
Dynamic compression of SiO_2: A new interpretation - Akins, Joseph A.; Ahrens, Thomas J.
In light of recent discoveries of post-stishovite phases of SiO_2, with the CaCl_2 and α-PbO_2 structures, we have reassigned the regimes along the Hugoniots for initial quartz, coesite, cristobalite, porous coesite, and fused silica. Calculated Hugoniots for fused silica, cristobalite and porous coesite indicate transition to stishovite, and then melt. Hugoniots for crystal quartz and coesite indicate that transition occurs to stishovite, then the CaCl2 structure and finally to melt.
Impact induced damage beneath craters - Xia, Kaiwen; Ahrens, Thomas J.
Ackermann et al. (1975) described the subsurface structure of Meteor Crater and identified a fractured rock zone extending to about 1 km deep. The depth of the fractured/damage zone can be used to extract information about the impact cratering process. We impacted rock samples (San Marcos gabbro) in the laboratory and imaged the damage structure using both dicing and tomography methods. We propose a simple model to describe the damage zone depth based on the laboratory measurements. The model agrees well with other methods for the estimation of the projectile size of Meteor Crater and it may be used for...
Piezometer Performance at Wildlife Liquefaction Site, California - Scott, Ronald F.; Hushmand, B.
In response to an urgent need for field data from instrumented liquefaction sites, the U.S. Geological Survey in 1982 selected and instrumented a site in southern California called the Wildlife site. Two accelerometers (one at ground surface and one at a depth of 7.5 m) and six electrical pore-pressure transducers (five in a liquefiable silty sand layer) were placed at the site. The November 1987 Superstition Hills earthquake triggered sand boils and the desired instrumental response by generating excess pore-water pressure that approximately equaled the initial effective overburden pressure. These records are the first from a field site to trace...
In situ dust measurements in the inner Saturnian system - Srama, R.; Ahrens, T. J.
In July 2004 the Cassini–Huygens mission reached the Saturnian system and started its orbital tour. A total of 75 orbits will be carried out during the primary mission until August 2008. In these four years Cassini crosses the ring plane 150 times and spends approx. 400 h within Titan's orbit. The Cosmic Dust Analyser (CDA) onboard Cassini characterises the dust environment with its extended E ring and embedded moons. Here, we focus on the CDA results of the first year and we present the Dust Analyser (DA) data within Titan's orbit. This paper does investigate High Rate Detector data and...
Shock compression of preheated molybdenum to 300GPa - Asimow, Paul D.; Sun, Daoyuan; Ahrens, Thomas J.
Shock compression experiments on samples at elevated initial temperature provide a test of the Mie-Grüneisen method of predicting off-principal Hugoniot states. Pure molybdenum, preheated to 1673 K, was shocked to peak pressures up to 300 GPa, double the compression range previously studied for this material at elevated temperature. The data lie strictly below the cold Hugoniot of Mo in shock velocity vs. particle velocity space, with some downward curvature. Previous approximations and extrapolations from lower-compression data fail to match these results, but the data are well fit by a Mie-Grüneisen correction to the cold Hugoniot. However, the data are insufficiently...
Bugbuster — survivability of living bacteria upon shock compression - Willis, M. J.; Ahrens, T. J.; Bertani, L. E.; Nash, C. Z.
Shock recovery experiments were conducted on suspensions of 10^6/ml E. coli bacteria contained in a water-based medium that is emplaced within stainless steel containers. The water is shocked and recovered. These experiments simulate the environment of bacteria residing either in surface bodies of water or in subsurface water-filled cracks in rocks. Early Earth life is likely to have existed in such environments. However, the E. coli are not believed to be representative of early life and are merely used here for initial experiments. Some 10^(− 2) to 10^(− 4) of the bacteria population survived initial (800 ns duration) shock pressures...
Molecular dynamics modeling of stishovite - Luo, Sheng-Nian; Çağin, Tahir; Strachan, Alejandro; Goddard, William A., III; Ahrens, Thomas J.
A Morse-stretch potential charge equilibrium force field for silica system has been employed to simulate the thermodynamics of stishovite with the molecular dynamics (MD) method. The equation of state, thermal expansivity and melting curve of stishovite have been obtained. This simple force field yielded results in accordance with the static and dynamic experiments. The stishovite melting simulation appears to validate the interpretation of superheating of the solid along the Hugoniot in the shock melting experiments. MD simulations show that the thermal expansivity of stishovite at lowermost mantle conditions is a weak function of temperature. The phase diagram of silica up...