Search

We show that bulk amorphous silica recovered from shock-wave experiments on quartz to 57 GPa is not a true glass but rather keeps a large degree of long-range structural information that can be recovered by static cold recompression to 13 GPa. At this pressure, shock-retrieved silica assumes the structure of crystalline stishovite. A minor amount of material recovers the structure of a recently discovered new silica polymorph. Keywords: Meteorite, shock, phase transition, amorphous to solid, XRD data, stishovite, high pressure

We present the discovery of a novel dense silica polymorph retrieved from shock-wave and diamond-anvil cell experiments. This polymorph is the first observed silicate composed of face-sharing polyhedra and it has a density similar to stishovite. Sterical constraints on the bond angles induce an intrinsic disorder of Si positions, such that the Si-coordination is transitional between four- and sixfold. The structure provides a mechanism for this coordination change in silica and other silicates at high temperature that is fundamentally different from mechanisms at 300 K. The new polymorph also illustrates how the face-sharing polyhedra, naturally occurring along previously proposed compression mechanisms for dense silicate melts, can be constructed without inferring unphysically small bond angles.

The elastic properties of stainless steel 304 by subjecting the alloy to wave shocks measured at approximately 7 and 80 GPa. Velocity interferometry is used to compile data on shock states and time histories while a 40 mm bore propellant gun is used for shock loading. Particle velocity measurement results show consistency with impedance match computations by up to 2%.

A semianalytic model for the breakup of fragments of Comet Shoemaker-Levy 9 upon entry into Jupiter's atmosphere is presented. The model assumes that the impacting fragments behave as bodies of viscous fluid and that the dominant process in their breakup is the growth of hydrodynamic instabilities. It is shown that the size of the smallest instabilities that contribute to mass loss largely determines the depth of penetration in a way which is consistent with the changes in penetration depth obtained using numerical models with different resolutions. If the diameter of the impactor corresponds to 8 resolution elements then the penetration depths obtained are about [10.sup.2] km too large. To obtain penetration depths within one scale height ([approximately equal to]25 km) of the viscosity limited value, at least 25 resolution elements are required across the diameter of the impactor. This result is in agreement with the numerical studies of K. Zahnle and M.-M. Mac Low (1994, Icarus 108, 1-17). It is also shown that two different regimes of hydrodynamic mass loss exist, one caused by Kelvin-Helmholtz (KH) type instabilities and a later one caused by the onset of Rayleigh-Taylor (RT) type instabilities. These regimes can be identified in the numerical results of D. A. Crawford et al. (1994, Shock Waves 4, 47-50), where KH instabilities appear to be the major mass loss mechanism between 100 and 200 km below I bar and RT instabilities become dominant below 200 km below 1 bar. The upward velocity of material behind the shock caused by the expansion of the superheated gas in the Comet's wake is then calculated and shown to be about 12 km [sec.sup.-1] and, to a first approximation, independent of the size of the impacting fragment provided that the fragment is not significantly decelerated before it reaches the tropopause (100 mbar). This upward velocity implies a plume height of 3000 km above the 1-bar level, which agrees with Hubble Space Telescope observations. It is shown that for no significant deceleration to occur before the tropopause the impacting fragments that produced plumes must have had diameters larger than 0.3 km. This, in turn, implies a progenitor diameter of 1.6 km. It is then estimated that the time interval between impacts of 0.3 km diameter comets on Jupiter is approximately 500 years, whereas the interval between the impact of 1.6 km comets is about 6000 years.

Numerical results of impact of Comet Shoemaker-Levy 9 (SL9) on Jupiter are compared with observational phenomena to investigate the size and origin of SL9 and impact features. Plume evolution is simulated numerically for 0.4- and 2.0-km-diameter (D) impactors. Analytical estimates indicate that the maximum plume heights are proportional to [(D/In D).sup.3]. This relation is also confirmed by the extrapolation of numerical simulations. From the correlation between maximum plume heights and impactor diameters, the diameters of large SL9 fragments and the parent body are estimated as [approximately]2 and 4-5 km, respectively. The plume induced by the impact of fragments of SL9 consists of atmospheric gas and impactor material. The chemical abundances of the plumes, especially their high CO abundances, indicate that they contain primitive materials from fragments and thus SL9 is probably of cometary origin. The lateral expansion of plume is also examined. Our simulations suggest that the energy source required to produce the observable waves should be located in the stratospheric region, rather than in the deep troposphere. Finally, the ejecta pattern observed by Hubble Space Telescope [Hammel et al. (1995) Science 267, 1288-1296] is simulated by a simple particle model. The results suggest that the ejection of atmospheric gas and cometary materials from a narrow [approximately]30 [degrees] cone region along the trajectory spreads out and yields the ejecta patterns observed by Hubble Space Telescope overlying the clouds.

We use the smoothed particle hydrodynamics method to simulate catastrophic collisions on silicate bodies whose impact response is dominated by gravity rather than material strength. Encounter speeds of 3, 5, and 7 km [sec.sup.-1], impact angles of 15 [degrees], 45 [degrees], and 75 [degrees], and target diameters of 10 to 1000 km are investigated. The projectile and target materials are modelled using the Tillotson equation of state for granite. Our model treats gravity rigorously, but neglects strength and fracture effects. We calculate the initial hydrodynamic phase of each event; after the impact shock wave crosses the target, particle motions are nearly ballistic and can be treated analytically. Material that does not escape may reaccrete [approximately]1 hr to [approximately]1 yr after the impact. The partitioning of impact energy into heat and motion of projectile and target material favors kinetic energy at higher speeds and larger projectile:target diameter ratios, but does not depend on the absolute size scale of the event. After the impact, most of the kinetic energy is carried by a small amount of fast ejecta. Particle velocity distributions are not sensitive to size scale and have complex, evolving shapes that are poorly represented by simple approximations. The catastrophic threshold (impact energy per unit target mass required to permanently eject 50% of the target against gravity) ranges from 8 x [10.sup.3] J [kg.sup.-1] at 10 km diameter to 1.5 x [10.sup.6] J [kg.sup.-1] at 1000 km, varying as target diameter to the 1.13 [+ or -] 0.01 power. Extrapolating these results suggests that gravity dominance extends to stony bodies as small 250 [+ or -] 150 m in diameter, smaller than previously believed. This result implies that asteroids as small as a few hundred meters across may be 'rubble piles.' Nearly catastrophic impacts can exhume target core material and catapult surface rocks to the antipodes ('scrambling' the target), but selective removal of the outer layers is inefficient. Most material strongly heated in these impacts escapes, limiting globally averaged heating from a single collision to [less than or equal to]50 [degrees] C for asteroids [less than or equal to]1000 km in diameter.

A study of the collision between two gabbro slabs shows that high-velocity particles are released during collisions between geological materials. The mass of particles ejected per unit area of the collider is basically independent of impact velocity, target thickness, inclination angle, density and type of material. That the ejected particles have a crystal structure is seen from X-ray diffraction studies. This is in agreement with thermodynamical calculations. Hydrodynamical models are used to study collisions of geological materials and impact jetting.

Researchers have advanced a hypothesis that magnetism in asteroids and other small bodies in the solar system including the moon was caused by a shock wave generated by a hypervelocity impact. The shock wave induced a thermodynamic state which led iron-nickel alloy to be driven from its low-temperature, low-pressure phase to the paramagnetic phase. Magnetization occurred when the iron-nickel alloy rarefied and reentered the ferromagnetic structure. Magnetism in the asteroids Ida and Gaspra may have resulted from this process.

Scientists have long tried to explain the origin of the planet Earth. However, many problems and questions, especially on the composition of the Earth's interior, remain.

Analysis of shock compressed-pure molybdenum, heated at 1400 degrees C, reveals a shock wave pattern between peak free surface velocity and peak Hugoniot elastic limit (HEL) velocity. HEL amplitude ranges from 1.5 to 1.7 GPa. Variations in unloading wave velocities and HEL velocity depend on the influence of temperature on compressional elastic wave velocity.

The smoothed particle hydrodynamics (SPH) method uses three-dimensional simulations to analyze the impact of fragments of Comet Shoemaker-Levy 9 on Jupiter and the resultant plume evolution. The atmosphere received most of the initial kinetic energy of the fragment and the atmospheric gas exhibited dissociation and partial ionization. Upward expansion of resulting plume led to an explosive release of energy and density gradient of the ambient atmosphere forced upward expansion of plume.

Near-Earth objects (NEOs) such as asteroids and comet nuclei that might smash into the Earth could be warded off either by firing a rocket-borne mass at them or by setting off nuclear warheads near them. The destruction and terrible loss of life that an NEO impact would cause on Earth should prompt the human race to consider preventive measures. NEOs smaller than 100 meters across could be steered away by striking them with heavy objects, but larger NEOs would require a nuclear detonation. A radiative nuclear explosion would be more effective than a surface detonation designed to break apart the NEO.