High- and low-velocity impact experiments on porous sintered glass bead targets of different compressive strengths: Outcome sensitivity and scaling

Impact experiments on porous targets consisting of sintered glass beads have been performed at different impact velocities in order to investigate the disruption impact energy threshold (also called Q^∗) of these targets, the influence of the target compressive strength on this threshold and a scaling parameter of the degree of fragmentation that takes into account material strength. A large fraction of small bodies of our Solar System are expected to be composed of highly-porous material. Depending on their location and on the period considered during the Solar System history, these bodies collide with each other at velocities which cover a wide range of values from a few m/s to several km/s. Determining the impact response of porous bodies in both high- and low-velocity regimes is thus crucial to understand their collisional evolution over the entire Solar System history, from the early stages of planetary formation through collisional accretion at low impact velocities to the current and future stages during which impact velocities are much higher and lead to their disruption. While these problems at large scale can only be addressed directly by numerical simulations, small scale impact experiments are a necessary step which allows the understanding of the physical process itself and the determination of the small scale behavior of the material used as target. Moreover, they are crucial to validate numerical codes that can then be applied to larger scales.Sintered glass beads targets of different shapes and porosity have been built and their main material properties, in particular their compressive strength and their porosity, have been measured. The outcomes of their disruptions both at low and high impact velocities have then been analyzed.We then found that the value of Q^∗ strongly depends on the target compressive strength. Measuring the particle velocities as a function of their distance to the impact point, we first found that the attenuation rate of the stress wave in our sintered glass bead targets does not depend on the impact velocity regime. Ejecta velocities as a function of the distance from the impact point can thus be well fitted by a power law with an exponent about −2 in both velocity regimes. We then looked for a scaling parameter that can apply to both regimes. We found that the scaling parameter PI, which is related to the initial peak pressure and to the stress wave attenuation can be used to represent the outcome in a general way. Future investigations will be performed to determine whether these results can be generalized to other kinds of porous materials.     

Numerical simulations of impacts involving porous bodies: II. Comparison with laboratory experiments

In this paper, we compare the outcome of high-velocity impact experiments on porous targets, composed of pumice, with the results of simulations by a 3D SPH hydrocode in which a porosity model has been implemented. The different populations of small bodies of our Solar System are believed to be composed, at least partially, of objects with a high degree of porosity. To describe the fragmentation of such porous objects, a different model is needed than that used for non-porous bodies. In the case of porous bodies, the impact process is not only driven by the presence of cracks which propagate when a stress threshold is reached, it is also influenced by the crushing of pores and compaction. Such processes can greatly affect the whole body's response to an impact. Therefore, another physical model is necessary to improve our understanding of the collisional process involving porous bodies. Such a model has been developed recently and introduced successfully in a 3D SPH hydrocode [Jutzi, M., Benz, W., Michel, P., 2008. Icarus 198, 242-255]. Basic tests have been performed which already showed that it is implemented in a consistent way and that theoretical solutions are well reproduced. However, its full validation requires that it is also capable of reproducing the results of real laboratory impact experiments. Here we present simulations of laboratory experiments on pumice targets for which several of the main material properties have been measured. We show that using the measured material properties and keeping the remaining free parameters fixed, our numerical model is able to reproduce the outcome of these experiments carried out under different impact conditions. This first complete validation of our model, which will be tested for other porous materials in the future, allows us to start addressing problems at larger scale related to small bodies of our Solar System, such as collisions in the Kuiper Belt or the formation of a family by the disruption of a porous parent body in the main asteroid belt.     

Collisional disruption experiments of porous targets

More and more small bodies are found to be porous. Laboratory impact experiments of various porous materials have been performed, which increased our understanding of the impact process of porous bodies. In this review, we classify porous targets according to the internal structure, describe the physical properties that characterize these materials, and summarize some results of impact strength and the general tendency upon impact condition and material properties. The material property presented here includes a relation between applied load and distension although measured in static condition. Further investigation is required to clarify the role of crack growth and compaction in the time evolution of physical properties of porous materials upon impact. Laboratory experiments with detailed material properties will be useful as a database that can serve as a reference for numerical modeling and theoretical scaling considerations.     

Catastrophic disruption experiments: recent results

This paper presents a review of the progress in the field of catastrophic disruption experiments over the past 4 years, since the publication of the review paper by Fujiwara et al. (Asteroids II, pp. 240–265, University of Arizona Press, Tucson, 1989). We describe the development of new techniques to produce shattering impacts relevant to the study of the collisional evolution of the asteroids, and summarise the results from numerous experiments which have been performed to date, using a variety of materials for both the impactor and the targets. Some of these, such as ice-on-ice, loose aggregates and pressurised targets, are quite new and have provided novel and exciting results. Some of the gaps existing previously in the data on fragment ejection-angle distributions, as well as translational and rotational velocity fields (including fine fragments) have been filled, and these new results will be surveyed.     

Numerical simulations of catastrophic disruption: recent results

Numerical simulations have been used to study high velocity two-body impacts. In this paper a two-dimensional Lagrangian finite difference hydrocode and a three-dimensional smooth particle hydrocode (SPH) are described and initial results reported.

The 2D hydrocode has successfully reproduced both the fragment size distribution and the mean fragment velocities from laboratory impact experiments using basalt and cement mortar. Further, the hydrocode calculations have determined that the energy needed to fracture a body has a much stronger dependence on target size than predicted from most scaling theories. In addition, velocity distributions obtained (using homogeneous targets at impact velocities around 2 km s⁻¹) indicate that mean ejecta speeds resulting from large-body collisions do not generally exceed escape velocities.

The SPH model provides a fully three-dimensional framework for studying impacts, so that phenomena such as oblique collisions or impacts into non-spherical targets may be studied. The gridless code allows for arbitrary levels of distortion, and is hence appropriate for modeling the large-scale deformations which accompany most impact events. Because fragments are modeled explicitly, greater numerical accuracy is achieved in the regions of large fragments than with the purely statistical approach of the 2D model. Of course, this accuracy comes at the expense of significantly greater computational requirements.

These codes can be, and have been, used to make specific predictions about particular objects in our solar system. But more significantly, they allow us to explore a broad range of collisional events. Certain parameters (size, time) can be studied only over a very restricted range within the laboratory; other parameters (initial spin, low gravity, exotic structure or composition) are difficult to study at all experimentally. The outcomes of numerical simulations lead to a more general and accurate understanding of impacts in their many forms.     

A dust cloud of Ganymede maintained by hypervelocity impacts of interplanetary micrometeoroids

A dust cloud of Ganymede has been detected by in situ measurements with the dust detector onboard the Galileo spacecraft. The dust grains have been sensed at altitudes below five Ganymede radii (Ganymede radius=2635 km). Our analysis identifies the particles in the dust cloud surrounding Ganymede by their impact direction, impact velocity, and mass distribution and implies that they have been kicked up by hypervelocity impacts of micrometeoroids onto the satellite's surface. We calculate the radial density profile of the particles ejected from the satellite by interplanetary dust grains. We assume the yields, mass and velocity distributions of the ejecta obtained from laboratory impact experiments onto icy targets and consider the dynamics of the ejected grains in ballistic and escaping trajectories near Ganymede. The spatial dust density profile calculated with interplanetary particles as impactors is consistent with the profile derived from the Galileo measurements. The contribution of interstellar grains as projectiles is negligible. Dust measurements in the vicinities of satellites by spacecraft detectors are suggested as a beneficial tool to obtain more knowledge about the satellite surfaces, as well as dusty planetary rings maintained by satellites through the impact ejecta mechanism.     

Numerical modeling of oblique hypervelocity impacts on strong ductile targets

Abstract– The majority of meteorite impacts occur at oblique incidence angles. However, many of the effects of obliquity on impact crater size and morphology are poorly understood. Laboratory experiments and numerical models have shown that crater size decreases with impact angle, the along‐range crater profile becomes asymmetric at low incidence angles, and below a certain threshold angle the crater planform becomes elliptical. Experimental results at approximately constant impact velocity suggest that the elliptical threshold angle depends on target material properties. Herein, we test the hypothesis that the threshold for oblique crater asymmetry depends on target material strength. Three‐dimensional numerical modeling offers a unique opportunity to study the individual effects of both impact angle and target strength; however, a systematic study of these two parameters has not previously been performed. In this work, the three‐dimensional shock physics code iSALE‐3D is validated against laboratory experiments of impacts into a strong, ductile target material. Digital elevation models of craters formed in laboratory experiments were created from stereo pairs of scanning electron microscope images, allowing the size and morphology to be directly compared with the iSALE‐3D craters. The simulated craters show excellent agreement with both the crater size and morphology of the laboratory experiments. iSALE‐3D is also used to investigate the effect of target strength on oblique incidence impact cratering. We find that the elliptical threshold angle decreases with decreasing target strength, and hence with increasing cratering efficiency. Our simulations of impacts on ductile targets also support the prediction from Chapman and McKinnon (1986) that cratering efficiency depends on only the vertical component of the velocity vector.     

The effect of target lithology on the products of impact melting

Abstract— Impact cratering is an important geological process on the terrestrial planets and rocky and icy moons of the outer solar system. Impact events generate pressures and temperatures that can melt a substantial volume of the target; however, there remains considerable discussion as to the effect of target lithology on the generation of impact melts. Early studies showed that for impacts into crystalline targets, coherent impact melt rocks or “sheets” are formed with these rocks often displaying classic igneous structures (e.g., columnar jointing) and textures. For impact structures containing some amount of sedimentary rocks in the target sequence, a wide range of impact‐generated lithologies have been described, although it has generally been suggested that impact melt is either lacking or is volumetrically minor. This is surprising given theoretical constraints, which show that as much melt should be produced during impacts into sedimentary targets. The question then arises: where has all the melt gone? The goal of this synthesis is to explore the effect of target lithology on the products of impact melting. A comparative study of the similarly sized Haughton, Mistastin, and Ries impact structures, suggests that the fundamental processes of impact melting are basically the same in sedimentary and crystalline targets, regardless of target properties. Furthermore, using advanced microbeam analytical techniques, it is apparent that, for the structures under consideration here, a large proportion of the melt is retained within the crater (as crater‐fill impactites) for impacts into sedimentary‐bearing target rocks. Thus, it is suggested that the basic products are genetically equivalent but they just appear different. That is, it is the textural, chemical and physical properties of the products that vary.     

Impacts into quartz sand: Crater formation,shock metamorphism,and ejecta distribution in laboratory experiments and numerical models

We investigated the ejection mechanics by a complementary approach of cratering experiments, including the microscopic analysis of material sampled from these experiments, and 2‐D numerical modeling of vertical impacts. The study is based on cratering experiments in quartz sand targets performed at the NASA Ames Vertical Gun Range. In these experiments, the preimpact location in the target and the final position of ejecta was determined by using color‐coded sand and a catcher system for the ejecta. The results were compared with numerical simulations of the cratering and ejection process to validate the iSALE shock physics code. In turn the models provide further details on the ejection velocities and angles. We quantify the general assumption that ejecta thickness decreases with distance according to a power‐law and that the relative proportion of shocked material in the ejecta increase with distance. We distinguish three types of shock metamorphic particles (1) melt particles, (2) shock lithified aggregates, and (3) shock‐comminuted grains. The agreement between experiment and model was excellent, which provides confidence that the models can predict ejection angles, velocities, and the degree of shock loading of material expelled from a crater accurately if impact parameters such as impact velocity, impactor size, and gravity are varied beyond the experimental limitations. This study is relevant for a quantitative assessment of impact gardening on planetary surfaces and the evolution of regolith layers on atmosphereless bodies.     

Asteroidal catastrophic collisions simulated by hypervelocity impact experiments

We report the results of six impact fragmentation experiments carried out with free-falling macroscopic targets of different compositions and shapes, and with projectile velocities close to 9 km/sec, i.e., significantly higher than the sound velocity in the target materials. The data have been examined by deriving the mass and shape distributions of the fragments, by reconstructing two of the shattered targets in order to study the geometry of the fracture surfaces, and by analyzing the properties of the fine-grained high-velocity ejecta. The fragment mass distributions show clearly that the degree of target fragmentation depends strongly on the impact parameter. Apart from the few largest fragments, these distributions are well represented by two power laws with different exponents, connected at a size of about 1 cm. The fragment shapes are generally in good agreement with those observed in previous experiments, and no significant shape vs size dependence has been found down to sizes of the order of 0.1 mm. The fragments tend to become larger and possibly more irregular in shape when they are generated farther from the impact point. The fracture surfaces are oriented roughly along meridians and parallels (with the pole at the impact point) when the target is spherical, but are clustered around the symmetry planes when the target is ellipsoidal. Fine-grained particles, with typical sizes and velocities of 0.01 cm and 1 km/sec, respectively, are ejected at low-elevation angles and in a rather collimated way, starting both from the neighborhood of the impact point and from regions of incipient cracking. Particular attention has been paid to a comparison between these results and the observed properties of the outcomes of asteroidal catastrophic collisions, like the dynamical families and the small inner planet crossing objects. While the collisional theory for the origin of families is fully consistent with the experimental results (with some indication for a significant role of the parent asteroid's self-gravitation), the elongated shapes of several Apollo-Amor objects are much rarer among the laboratory fragments, and thus appear to require a different explanation.     

Ejecta from impacts at 0.2-2.3 m/s in low gravity

Collisions between planetary ring particles and in some protoplanetary disk environments occur at speeds below 10 m/s. The particles involved in these low-velocity collisions have negligible gravity and may be made of or coated with smaller dust grains and aggregates. We undertook microgravity impact experiments to better understand the dissipation of energy and production of ejecta in these collisions. Here we report the results of impact experiments of solid projectiles into beds of granular material at impact velocities from 0.2 to 2.3 m/s performed under near-weightless conditions on the NASA KC-135 Weightless Wonder V. Impactors of various densities and radii of 1 and 2 cm were launched into targets of quartz sand, JSC-1 lunar regolith simulant, and JSC-Mars-1 martian regolith simulant. Most impacts were at normal or near-normal incidence angles, though some impacts were at oblique angles. Oblique impacts led to much higher ejection velocities and ejecta masses than normal impacts. For normal incidence impacts, characteristic ejecta velocities increase with impactor kinetic energy, KE, as approximately KE^0.5. Ejecta masses could not be measured accurately due to the nature of the experiment, but qualitatively also increased with impactor kinetic energy. Some experiments were near the threshold velocity of 0.2 m/s identified in previous microgravity impact experiments as the minimum velocity needed to produce ejecta [Colwell, J.E., 2003. Icarus 164, 188-196], and the experimental scatter is large at these low speeds in the airplane experiment. A more precise exploration of the transition from low-ejecta-mass impacts to high-ejecta-mass impacts requires a longer and smoother period of reduced gravity. Coefficient of restitution measurements are not possible due to the varying acceleration of the airplane throughout the experiment.     

Cosmogenic activation in germanium and copper for rare event searches

Experiments looking for rare events like the direct detection of dark matter particles or the nuclear Double Beta Decay are operated in deep underground locations, to suppress or very effectively reduce the effect of cosmic rays; but cosmogenic activation produced at sea level in detectors and other materials can become a serious hazard for them. Copper and germanium are very frequently used in this kind of experiments requiring an ultra-low radioactive background and therefore have been chosen as activation targets in this work. First, the excitation functions for relevant induced long-lived radioactive isotopes have been estimated; special care has been taken in using those calculations giving the best agreement with measured production cross-sections and in distinguishing production by neutrons or by protons when relevant and possible. Then, the corresponding rates of production of the nuclides in natural and enriched (86% ⁷⁶Ge and 14% ⁷⁴Ge) germanium and copper have been evaluated considering two different cosmic ray spectra. Comparison of the obtained activation yields at surface with all the known previous results (based either on calculations or experiments) has allowed to draw conclusions on the general methodology for evaluating cosmogenic activation.     

Measurement of Impact Ejecta from Regolith Targets in Oblique Impacts

This paper reports the results of experiments on projectile impact into regolith targets at various impact angles. Copper projectiles of 240 mg are accelerated to 197 to 272 m s⁻¹ using an electromagnetic gun. The ejecta are detected by thin Al foil targets as secondary targets, and the resulting holes on the foil are measured to derive the spatial distribution of the ejecta. The ejecta that penetrated the foil are concentrated toward the downrange azimuths of impacting projectiles in oblique impacts. In order to investigate the ejecta velocity distribution, the nondimensional volume of ejecta with velocities higher than a given value is calculated from the spatial distribution. In the case of the vertical impact of the projectile, most ejecta have velocities lower than 24% of the projectile speed (∼50 m s⁻¹), and there are only several ejecta with velocities higher than 72 m s⁻¹. This result confirms the existence of an upper limit to the ejection velocity in the ejecta velocity distribution (Hartmann cutoff velocity) (W. K. Hartmann, 1985, Icarus63, 69-98). On the other hand, it is found that, in the oblique impacts, there are a large number of ejecta with velocities higher than the Hartmann cutoff velocity. The relative quantity of ejecta above the Hartmann cutoff velocity increases as the projectile impact angle decreases. Taking these results with the results of S. Yamamoto and A. M. Nakamura (1997, Icarus128, 160-170) from impact experiments using an impact angle of 30°, it can be concluded that the ejecta from these regolith targets exhibit a bimodal velocity distribution. Below a few tens of m s⁻¹, we see the expected velocity distribution of ejecta, but above this velocity we see a separate group of high-velocity ejecta.     

Impacts into porous foam targets: possible implications for the disruption of comet nuclei

We have conducted a series of impact experiments to examine the response of very porous foam targets to various impacts. Under near-vacuum conditions, closed-pore and open-pore foam targets were subjected to ∼1 km s⁻¹ impacts from aluminum and foam projectiles. We found that open-pore targets absorbed the impacts with little or no global fragmentation or noticeable cratering, exhibiting only local damage along the path of the projectile, which tunneled through the target. Closed-pore targets exhibited nearly explosive disruption, apparently resulting from stresses built up within the target due to internal pressurization from air that could not escape the target interior during evacuation of the impact chamber. These results suggest that build-up of internal volatile pressure within the nuclei of collisionally or dynamically unevolved comets could allow comparatively small impacts onto their surfaces to result in disproportionately disruptive outcomes.     

Constraints on the role of impact heating and melting in asteroids

Abstract— Imaging of asteroids Gaspra and Ida and laboratory studies of asteroidal meteorites show that impacts undoubtedly played an important role in the histories of asteroids and resulted in shock metamorphism and the formation of breccias and melt rocks. However, in recent years, impact has also been called upon by numerous authors as the heat source for some of the major geological processes that took place on asteroids, such as global thermal metamorphism of chondrite parent bodies and a variety of melting and igneous events. The latter were proposed to explain the origin of ureilites, aubrites, mesosiderites, the Eagle Station pallasites, acapulcoites, lodranites, and the IAB, IIICD, and HE irons. We considered fundamental observations from terrestrial impact craters, combined with results from laboratory shock experiments and theoretical considerations, to evaluate the efficiency of impact heating and melting of asteroids. Studies of terrestrial impact craters and relevant shock experiments suggest that impact heating of asteroids will produce two types of impact melts: (1) large-scale whole rock melts (total melts, not partial melts) at high shock pressure and (2) localized melts formed at the scale of the mineral constituents (mineral specific or grain boundary melting) at intermediate shock pressures. The localized melts form minuscule amounts of melt that quench and solidify in situ, thus preventing them from pooling into larger melt bodies. Partial melting as defined in petrology has not been observed in natural and experimental shock metamorphism and is thermodynamically impossible in a shock wave-induced transient compression of rocks. The total impact melts produced represent a minuscule portion of the displaced rock volume of the parent crater. Internal differentiation by fractional crystallization is absent in impact melt sheets of craters of sizes that can be tolerated by asteroids, and impact melt rocks are usually clast-laden. Thermal metamorphism of country rocks by impact is extremely minor. Experimental and theoretical considerations suggest that (1) single disruptive impacts cannot raise the average global temperature of strength- or gravity-dominated asteroids by more than a few degrees; (2) cumulative global heating of asteroids by multiple impacts is ineffective for asteroids less than a few hundred kilometers in diameter; (3) small crater size, low gravity, and low impact velocity suggest that impact melt volume in single asteroidal impacts is a very small (0.01–0.1%) fraction of the total displaced crater volume; (4) total impact melt volume formed during the typical lifetime of an asteroid is a small fraction (<0.001) of the volume of impact-generated debris; and (5) much of the impact melt generated on asteroidal targets is ejected from craters with velocities greater than escape velocity and, thus, not retained on the asteroid. The inescapable conclusion from these observations and calculations is that impacts cannot have been the heat source for the origin of the meteorite types listed above, and we must turn to processes other than impact, such as decay of short-lived radionuclides or electromagnetic induction during an early T-tauri phase of the Sun to explain heating and melting of the parent bodies of these meteorites.     

Degree of impactor fragmentation under collision with a regolith surface—Laboratory impact experiments of rock projectiles

Some meteorites consist of a mix of components of various parent bodies that were presumably brought together by past collisions. Impact experiments have been performed to investigate the degree of target fragmentation during such collisions. However, much less attention has been paid to the fate of the impactors. Here, we report the results of our study of the empirical relationship between the degree of projectile fragmentation and the impact conditions. Millimeter‐sized pyrophyllite and basalt projectiles were impacted onto regolith‐like sand targets and an aluminum target at velocities of up to 960 m s^?1. Experiments using millimeter‐sized pyrophyllite blocks as targets were also conducted to fill the gap between this study and the previous studies of centimeter‐sized rock targets. The catastrophic disruption threshold for a projectile is defined as the energy density at which the mass of the largest fragment is the half of the original mass. The thresholds with the sand target were 4.5 ± 1.1 × 10⁴ and 9.0 ± 1.9 × 10⁴ J kg^?1, for pyrophyllite and basalt projectiles, respectively. These values are two orders of magnitude larger than the threshold for impacts between pyrophyllite projectiles onto aluminum targets, but are qualitatively consistent with the fact that the compressive and tensile strengths of basalt are larger than those of pyrophyllite. The threshold for pyrophyllite projectiles and the aluminum target agrees with the threshold for aluminum projectiles and pyrophyllite targets within the margin of error. Consistent with a previous result, the threshold depended on the size of the rocks with a power of approximately ?0.4 (Housen and Holsapple 1999). Destruction of rock projectiles occurred when the peak pressure was about ten times the tensile strength of the rocks.     

Search for radio pulsations in four anomalous X-ray pulsars and discovery of two new pulsars

We have performed deep searches for radio pulsations from four southern anomalous X-ray pulsars (AXPs) to investigate their physical nature in comparison with the rotation powered pulsars. The data were acquired using the Parkes radio telescope with the 1.4 GHz multibeam receiver. No pulsed emission with periodicity matching the X-ray ephemeris have been found in the observed targets down to a limit of ∼0.1 mJy. A blind search has also been performed on all the 13 beams of the multibeam receiver (the central beam being pointed on the target AXP), leading to the serendipitous discovery of two new radio pulsars and to the further detection of 18 pulsars. Also a search for single dispersed pulses has been performed in the aim to detect signals similar to those of the recently discovered rotating radio transients.      

SETI in star clusters: a theoretical approach

For several decades, the search for extraterrestrial intelligence (SETI) has proceeded using advanced astronomical techniques. Different strategies have been proposed for target selection for targeted searches with goals of improving the chances of successful detection of signals from technological civilizations that may inhabit planets around solar-type stars, and to minimize the chances of missing signals from unexpected sites. In this paper we demonstrate that these goals are best achieved by observing star clusters. We show that standard open clusters are not appropriate for SETI scans because their disruption time scale is shorter than the characteristic time scale for the development of a protective atmospheric layer on a habitable planet. However, the old open clusters, those older than some Gy are optimal candidates for SETI surveys as their ages are older than the likely time for intelligent civilizations to emerge and the probability of catastrophic orbital modification as a result of close encounters with other cluster stars is, in general, rather negligible. The final performance of the proposed survey can be significantly increased by using initially a radio telescope beam larger than the cluster apparent size so that the entire cluster can be observed simultaneously. Globular clusters are also good candidates from the statistical point of view but only if hypothetical civilizations located in these clusters have been able to develop astronomical engineering technologies or have been involved in (rather speculative) cosmic colonization. This revised version was published online in July 2006 with corrections to the Cover Date.     

An analysis of differential impact melt-crater scaling and implications for the terrestrial impact record

Abstract— It has been known for some time that the volume of impact melt (V_m) relative to that of the transient cavity (V_tc) increases with the magnitude of the impact event. This paper investigates the influence that this phenomenon has on the nature of terrestrial impact craters. A model of impact melting is used to estimate the volume of melt produced during the impact of chondritic projectiles into granite targets at velocities of 15, 25, and 50 km S^?1. The dimensions of transient cavities formed under the same impact conditions are calculated from current crater-scaling relationships, which are derived from dimensional analysis of data from cratering experiments. Observed melt volumes at terrestrial craters are collated from the literature and are paired with the transient-cavity diameters (D_tc) of their respective craters; these diameters were determined through an established empirical relationship. The model and observed melt volumes have very similar trends with increasing transient-cavity diameter. This V_m-D_tc relationship is then used to make predictions regarding the nature of the terrestrial cratering record. In particular, with increasing size of the impact event, the depth of melting approaches the depth of the transient cavity. As a consequence, the base of the cavity, which ultimately would appear as an uplifted central structure in a complex crater, will record shock stresses that will increase up to a maximum of partial melting. Examination of the terrestrial record indicates a general trend for higher recorded shock levels in central structures at larger diameters; impact structures in the 100-km size range record partially melted and vesiculated parautochthonous target rocks in their centers. In addition, as the depth of melting approaches a depth equivalent to that attained by the base of the transient cavity, the floor of the transient cavity will have progressively less strength, with the result that cavity modification and uplift will not produce topographic central peaks. Again, the observed terrestrial record is not inconsistent with this prediction, and we offer differential melt scaling as a possible mechanism for the transition from central topographic peaks to rings with increasing crater diameter. Among other implications is the likelihood that impact basins in the 1000-km size range on the early Earth would not have the same multi-ring form as observed on the moon.     

Interferometry: The tool to study giant, supergiant and Mira stars

Late-type giant stars have been traditional targets for infrared interferometers. They are bright and big and are therefore easy targets to resolve and to detect. Considerable progress has been and is being made on the spatial structure of these objects thanks to existing interferometers. Beyond the classical measurement of their diameters, pulsations have been directly detected, spatial intensity distributions are more and more understood and more important, consistent scenarios for both spectroscopic and interferometric measurements are on the verge to be validated. All this has been possible with prototype instruments having a small number of baselines and very limited spectral capabilities. AMBER and MIDI will surely open a new era with high spectral resolution, high efficiency and imaging capabilities. This is not an exhaustive review of all the work done in the field but rather a presentation of the context. A recent review of Mira star observations with interferometers was recently written (Scholz, 2003).A brief science case is first introduced in this paper. Achievements with high angular resolution single-telescope techniques are presented. The contributions of optical and infrared interferometers are then explained. Eventually, some hints about the possible progress with VLTI are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.