Repository of works by Caltech published authors.
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Shock compression and release of polycrystalline magnesium oxide - Duffy, Thomas S.; Ahrens, Thomas J.
Release wave profile and Hugoniot equation of state data for low‐porosity polycrystalline MgO are reported to 36 and 133 GPa, respectively. At low pressure, the Hugoniot lies above single‐crystal data, reflecting material strength of 3.1±0.8 GPa. Unloading wave velocities to 27 GPa are consistent with extrapolated compressional sound velocities. The unloading profiles can be fit using a viscoplastic model, but there are significant differences between waveforms for different experimental geometries.
Free surface velocity profiles in molybdenum shock compressed at 1400 °C - Duffy, Thomas S.; Ahrens, Thomas J.
The equation of state, constitutive properties and unloading wave velocities of molybdenum have been determined from free surface velocity profiles on samples shock compressed from a 1400 °C initial state. The equation of state of 1400 °C molybdenum agrees with previous streak camera measurements and the combined equation of state between 12 and 96 GPa is: U_S =4.78 (0.02)+1.42 (0.02)u_p . Unloading wave velocities measured between 12 and 81 GPa range from 6.30 to 7.91 km/s and are 4–8% below extrapolated 25 °C compressional velocities. The yield strength, Y, was found to be 0.79–0.94 GPa, compared with values of 1.3–1.6 GPa from ambient‐temperature...
Oblique impact jetting of geological materials - Yang, Wenbo; Ahrens, Thomas J.
To understand jetting of earth materials, gabbro slabs (5 mm thick) were accelerated to 1.5–2 km/s and impacted inclined gabbro (5–10 mm thick), novaculite (10 mm thick) and porous sandstone (12 mm thick) targets at angles of 30°–60°. The ejecta were collected using a catcher box filled with styrofoam and the particles were extracted using chloroform. The mass of the ejected particles per unit area (∼50 mg/cm^2) remains almost independent of impact velocity, inclination angle, thickness of the target and sample mineralogy and density. Based on this result, we predict that a 500 m diameter asteroid will produce only ∼10^7...
Stress wave attenuation in shock damaged rock - Luo, Cangli; Ahrens, Thomas J.
The attenuation of ultrasonic stress waves in samples of gabbroic rock subjected to shock loading in the 11 Gpa range were studied. We determined the damage deficits, D_p, and attenuation coefficients, α_p, for the samples with different damage deficits under dynamic strains of 2×10^(−7) and at frequencies around 2 MHz using ultrasonic pulse-echo method. A fit to the data yields the P-wave spatial attenuation coefficient versus damage deficit: α_p=40.9D_p−30.5D^2_p (db/cm). Basing on the O’Connell-Budiansky theory, the relation between attenuation coefficient and crack density is given. The predictions of α_p from Walsh’s theory agrees well with the experiment results for the samples...
Application of shock compression science to Earth and planetary physics - Ahrens, Thomas J.
After the development of shock compression methods for obtaining pressure-density Hugoniot curves, it became clear that these could be applied to both determining the equations-of-state and investigation of polymorphic phase changes in silicate minerals of planetary mantles and crusts, as well as, the iron alloys of the metallic cores of terrestrial planets. These data, when taken with seismological models of the Earth, yield constraints on the composition of the Earth’s mantle and core. Shock data for molten silicates provide a basis for understanding the initial layering of a cooling terrestrial magma ocean. Application of shock-wave data is critical to delineating...
Characteristic of dynamic tensile fracture in augite-peridotite - He, Hongliang; Jin, Xiaogang; Jing, Fuqian; Ahrens, Thomas J.
Planar impact experiments were carried out to induce controlled dynamic tensile fracture in augite-peridotite. Samples, backed with PMMA buffer and windows, were impacted with PMMA impactor at velocities of 30 to 160 m/s. This resulted in maximum tensile stresses were in the range of ∼50 to 290 MPa. Spall strength was determined to be ∼58.1 MPa from a particle velocity profile measurement. The spall strength/HEL ratios for augite-peridotite and several other rocks were discussed based on the Griffith’s yield criterion and the experimental measurements.
Shock Temperature of Stainless Steel and a High Pressure - High Temperature Constraint on Thermal Diffusivity of Al_2O_3 - Gallagher, Kathleen G.; Bass, Jay D.; Ahrens, Thomas J.; Fitzner, M.; Abelson, J. R.
Time dependent shock temperatures were measured for stainless steel (SS) films in contact with transparent anvils. The anvil/window material was the same as the driver material so that there would be symmetric heat flow from the sample. Inferred Hugoniot temperatures, T_h , of 5800–7500 K at 232–321 GPa are consistent with previous measurements in SS. Temperatures at the film‐anvil interface (T_i ), which are more directly measured than T_h , indicate that T_i did not decrease measurably during the approximately 250 ns that the shock wave was in Al_2O_3 or LiF anvils. Thus an upper bound is obtained for the...
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...
Theory of shock magnetization of asteroids Gaspra and Ida - Chen, George Q.; Ahrens, Thomas J.; Hide, Raymond
The observed magnetism of asteroids such as Gaspra and Ida (and other small bodies in the solar system including the Moon and meteorites) may have resulted from an impact-induced shock wave producing a thermodynamic state in which iron-nickel alloy, dispersed in a silicate matrix, is driven from the usual low-temperature, low-pressure, α, kaemacite, phase to the paramagnetic, ε (hcp), phase. The magnetization was acquired upon rarefaction and reentry into the ferromagnetic, α, structure. The degree of re-magnetization depends on the strength of the ambient field, which may have been associated with a solar-system-wide magnetic field. A transient field induced by...
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...
Free-Surface light emission from shocked Teflon - Gallagher, Kathleen G.; Yang, Wenbo; Ahrens, Thomas J.
Shock initiated light emission experiments were performed on Teflon shock loaded to pressures up to ∼17 GPa. Radiances up to 600×10^6W⋅m^(−2)/(ster ⋅nm), were measured over a range of 390 to 820 nm. We have measured the spectra of light emitted upon reflection of the shock at the free surface and observed it to be distinctly non‐thermal in nature. The lights appears to result from bond destruction such as observed in shock recovery experiments on Teflon and in quasistatic experiments conducted on other polymers.
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...
Shock compression and isentropic release of rhyolite - Yang, W.; Chen, G.; Anderson, W. W.; Ahrens, Thomas J.
A series of shock compression experiments have been conducted on rhyolite at pressure ranging from 6 to 33 GPa. A velocity interferometer (VISAR) was employed to monitor the particle velocity of an aluminum reflector with a diffused surface bonded to the rhyolite sample. In the forward ballistic experiments, a slow rise shock wave front is observed at 6 GPa. While in the forward experiments their release waves are smeared, in a reverse ballistic experiment, the particle velocity variation at the shock wave plateau and the isentropic release wave arrival have been clearly observed. Using Swegle’s mixed phase model, we simulated...
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....