Distribution Of Ejecta Research Articles

The Surface of Deimos: Contribution of Materials and Processes to Its Unique Appearance

Among the well-imaged small satellites and asteroids, Deimos displays a unique surface: very smooth with global-scale albedo features. We have examined the disk-resolved photometry of Deimos using Viking Orbiter images for clues to its distinctive appearance. Hapke parameters were fit to characterize the phase behavior and to compute normal reflectance. The opposition surge amplitude (B0) is smaller for Deimos than for Phobos. Outside the range of the opposition effect the two martian satellites have similarly shaped phase curves, but Deimos is about 20–30% brighter than Phobos from 10°–80° phase. The calculated mean normal reflectance of Deimos (λeff= 0.54 μm) is 0.068 ± 0.007. The brighter and darker areas on Deimos exhibit constant contrast between 0.6° and 81° phase; this characteristic allows a calculation of the range of normal reflectances over most of its surface, nearly all of which values are between 0.06 and 0.09. The trailing side of Deimos has a larger relative distribution of the brighter material, and is on average about 10% brighter than the leading side. The mean normal reflectance cannot be formally distinguished from that of Phobos (0.071 ± 0.012; Simonelli, D. P., M. Wisz, A. Switala, D. Adinolfi, J. Veverka, P. C. Thomas, and P. Helfenstein 1996. Submitted toIcarus). Although the statistical distribution of normal reflectances on the two satellites is similar, the geography of the albedo variations is very different. Deimos has gradational changes in albedo downslope from ridge crests, primarily manifested in long albedo “streamers.” On Phobos there is a more patchy global distribution of albedos, apparently related to ejecta from the large crater Stickney. Because of the similarity of mean density, spectral properties, mean normal reflectance, the range of normal reflectance, and phase function outside the opposition effect, Deimos appears to be made of materials with compositions very similar to those on Phobos, although the apparent wider distribution of ejecta on Deimos has been cited as indicating a greater role for strength scaling in cratering on Deimos (Lee, S. W., P. Thomas, and J. Veverka 1986.Icarus68, 77–86). Simple modeling of the formation of the albedo patterns by gardening, creep, and “weathering” of bright material from crater rims suggests that impact gardening contributes very little to the motion of the material downslope, and that vertical mixing and/or “weathering” must be important in addition to an unspecified creep process. The distinction of Deimos is primarily in the smooth surface that allows a particularly large scale of downslope movement of regolith on very gentle slopes. This smoothness is most easily explained by the effects from impact formation of a 10-km concavity at high southern latitudes in the latter half of Deimos' surface history. This impact scar is relatively much larger than is the crater Stickney on Phobos. The effects of this large impact probably include blanketing by an average of nearly 200 m of ejecta, but also may include seismic erasing of craters similar to that proposed for Ida by Asphauget al. (Asphaug, E., J. M. Moore, D. Morrison, W. Benz, and R. A. Sullivan 1996.Icarus120, 158–184).

Erosion and Ejecta Reaccretion on 243 Ida and Its Moon

Galileo images of Asteroid 243 Ida and its satellite Dactyl show surfaces which are dominantly shaped by impact cratering. A number of observations suggest that ejecta from hypervelocity impacts on Ida can be distributed far and wide across the Ida system, following trajectories substantially affected by the low gravity, nonspherical shape, and rapid rotation of the asteroid. We explore the processes of reaccretion and escape of ejecta on Ida and Dactyl using three-dimensional numerical simulations which allow us to compare the theoretical effects of orbital dynamics with observations of surface morphology.The effects of rotation, launch location, and initial launch speed are first examined for the case of an ideal triaxial ellipsoid with Ida's approximate shape and density. Ejecta launched at low speeds (V⪡Vesc) reimpact near the source craters, forming well-defined ejecta blankets which are asymmetric in morphology between leading and trailing rotational surfaces. The net effect of cratering at low ejecta launch velocities is to produce a thick regolith which is evenly distributed across the surface of the asteroid. In contrast, no clearly defined ejecta blankets are formed when ejecta is launched at higher initial velocities (V∼Vesc). Most of the ejecta escapes, while that which is retained is preferentially derived from the rotational trailing surfaces. These particles spend a significant time in temporary orbit around the asteroid, in comparison to the asteroid's rotation period, and tend to be swept up onto rotational leading surfaces upon reimpact. The net effect of impact cratering with high ejecta launch velocities is to produce a thinner and less uniform soil cover, with concentrations on the asteroids' rotational leading surfaces.Using a realistic model for the shape of Ida (P. Thomas, J.Veverka, B. Carcich, M. J. S. Belton, R. Sullivan, and M. Davies 1996,Icarus120, 000–000), we find that an extensive color/albedo unit which dominates the northern and western hemispheres of the asteroid can be explained as the result of reaccretion of impact ejecta from the large and evidently recent crater “Azzurra.” Initial ejection speeds required to match the color observations are on the order of a few meters per second, consistent with models (e.g., M. C. Nolan, E. Asphaug, H. J. Melosh, and R. Greenberg 1996,Icarus, submitted; E. Asphaug, J. Moore, D. Morrison, W. Benz, and R. Sullivan 1996,Icarus120, 158–184) that multikilometer craters on Ida form in the gravity-dominated regime and are net producers of locally retained regolith. Azzurra ejecta launched in the direction of rotation at speeds near 10 m/sec are lofted over the asteroid and swept up onto the rotational leading surface on the opposite side. The landing locations of these particles closely match the distribution of large ejecta blocks observed in high resolution images of Ida (P. Lee, J. Veverka, P. Thomas, P. Helfstein, M. J. S. Belton, C. Chapman, R. Greeley, R. Pappalardo, R. Sullivan, and J. W. Head 1996,Icarus120, 87–105).Ida's shape and rotation allow escape of ejecta launched at speeds far below the escape velocity of a nonrotating sphere of Ida's volume and presumed density. While little ejecta from Ida is captured by Dactyl, about half of the mass ejected from Dactyl at speeds of up to 20 m/sec eventually falls on Ida. Particles launched at speeds just barely exceeding Dactyl's escape velocity can enter relatively long-term orbit around Ida, but few are ultimately reaccreted by the satellite. Because of its low gravity, erosion of Dactyl would take place on exceedingly short time scales if unconsolidated materials compose the satellite and crater formation is in the gravity regime. If Dactyl is a solid rock, then its shape has evolved from a presumably irregular initial fragment to its present remarkably rounded figure by collision with a population of impactors too small to be detected by counting visible craters. As the smallest solar system object yet imaged by a spacecraft, the morphology of Dactyl is an important clue to the asteroid population at the smallest sizes.

Size distributions of circumplanetary dust

Dust rings have been observed around each of the giant planets and may also exist around Mars. The particles comprising these rings have short lifetimes due to a number of processes including exospheric and plasma drag, Poynting-Robertson drag, sputtering, collision with other circumplanetary particles, and the Lorentz force for charged grains. The supply of dust is maintained by collisions between macroscopic ring particles and bombardment of moons and ring particles by interplanetary impactors. All of the processes that act to remove or alter the circumplanetary dust grains are functions of particle size, so the initial size distribution of the grains released from an impact onto a moon or ring particle is modified. The size distribution of the impact ejecta can be described by a power-law of the form n(r)dr χ r −qdr where n(r)dr is the number of particles in the size range [ r,r+ dr] and q is the power-law index. For hypervelocity impact excavation, q3.5. Drag acts more efficiently on smaller grains resulting in a reduction in q of 1. Other dynamical processes can lead to particle-size dependent collision rates with other circumplanetary objects. These processes can lead to local steepening of the size distribution (increase in q) and to truncation of the dust size distribution to a narrow range of sizes.

Explosive volcanism on Venus: Transient volcanic explosions as a mechanism for localized pyroclast dispersal

It is proposed that transient volcanic explosions of the vulcanian type may provide a mechanism for the generation and dispersal of pyroclastic material on Venus. The influence of the Venusian high atmospheric pressure environment implies that continuous discharge plinian eruptive activity is relatively uncommon: the tendency for suppression of exsolution and expansion of magmatic gases favors effusive eruptions. However, it may be possible for explosive activity to occur, in a fashion analogous to vulcanian eruptions on Earth, as a result of the accumulation of hot, pressurized gas under a coherent rock “lid”. The explosion may be initiated by the failure of this retaining caprock, causing the catastrophic release of the high‐pressure gas, which expands out of the vent driving the fragmented caprock material ahead of it and displacing the surrounding atmosphere. On Earth the driving gas may originate either from vaporization of groundwater or from degassing of a stalled magma body in the near‐surface crust, whereas on Venus, where the presence of crustal stores of volatile compounds is uncertain, the latter option only is favored: prolonged degassing may lead to an accumulation of gas sufficient to initiate an explosion. This paper presents the results of a numerical model describing the explosion process under boundary conditions representing the Venusian physical environment. This involves treatments of the acceleration of the driving gas, caprock and displaced atmospheric gas out of the vent and the subsequent motions and aerodynamic interactions between the atmosphere and the ejected blocks of fragmented caprock. In this way, predictions of the eruption velocities and of the resulting distribution of (large) solid ejecta can be obtained for likely conditions on Venus. Deposits of large blocky debris are predicted to range up to a maximum distance of the order of 1 km from the vent on Venus, compared with distances of several kilometers commonly attained by ejecta from transient explosions on Earth. More typical blocky deposits may extend for only a few hundred meters, which implies that they would not be detected in the Magellan radar data. However, the possible presence of associated pyroclastic flow and fine‐grained ashfall deposits may constitute aids to the identification of sites of vulcanian eruptions on Venus.

A General Formulation for the Distribution of Impacts and Ejecta from Small Planetary Satellites

Small planetary satellites and ring moons are the source of dust rings around each of the giant planets, and presumably Mars, by cratering impacts from interplanetary material. The relative velocities of planetary satellites and interplanetary impactors lead to an anisotropic flux of impactors onto planetary satellites. The anistropy in the impactor flux seen from the frame of the satellite results in an anisotropic production of dust from the surface of the satellite. This anisotropy can have observable consequences for the distribution of material in dust rings. In this paper, an expression for the angular distribution of meteoroid ejecta from impacts onto a single moon or ring particle orbiting a planet with arbitrary obliquity is derived. I allow for any distribution of orbits of the impactors by using a Monte Carlo approach for the micrometeoroid orbits. Ejecta distributions are prograde due to the enhanced impact velocity on the leading face of the target. Ejecta orbits are therefore concentrated in the region exterior to the source satellite. Orbitally averaged ejecta distributions are weakly dependent on the planetary obliquity. Ejecta distributions vary with orbital season for planets with a significant obliquity. The orbitally averaged ejecta distribution is weakly dependent on the orbital distribution of the impactor population, largely due to the smearing effect of the assumed ejects "phase function." The impact velocity of micrometeoroids in the frame of the satellite is correlated with the orbital element distribution of the micrometeoroid population, allowing the orbital elements of interplanetary dust to be inferred from dust impact experiments, such as those on Galileo and Cassini. Dust rings derived from micrometeoroid ejecta from small planetary satellites may show seasonal variations in the vertical distribution of dust as the planet orbits the Sun.

Size Distributions of Satellite Dust Ejecta: Effects of Radiation Pressure and Planetary Oblateness

We examine the steady-state size distribution of dust ejecta in the vicinity of a satellite which acts as both a source and a sink for dust particles. The orbital motion of the dust particles is modified by radiation pressure and planetary oblateness which together produce periodic eccentricity perturbations. Our numerical simulations show that these perturbations can lead to a steepening of the steady-state power-law size distribution such that the index increases by 1 from the index of the source distribution in a specific size regime. This regime is determined by the perturbation strength, and the size and orbital radius of the satellite. The steady-state size distribution is reached in 10-20 years for the conditions considered here. In general, the perturbations discussed here affect dust particle orbits on shorter time scales than Poynting-Robertson, exospheric, or plasma drag forces.

Atmospheric dust dispersal analyzed by granulometry of the Misers Gold Event

Granulometric analysis of ejecta from Misers Gold high explosive cratering experiment demonstrates that atmospheric dust dispersal can be evaluated by particle‐size distribution data. From size analyses of the Misers Gold preshot test bed alluvium, ejecta, and sweep‐up materials collected out to 35 crater radii (1227 m), we find that approximately 5.9 × 106 kg (∼11% of the total crater ejecta mass) was depleted from the crater ejecta deposits and likely represents the portion of the cratered mass initially lofted into atmospheric suspension. The dominant size range of this lofted dust was 88 to 2000 μm with a mean diameter by mass of 384 μm,. In addition to the dust lofted in the explosion column, dust in the size range of 100–800 μm was swept up from the ground by the explosive air blast and base surge dominantly between ranges of 10 and 18 crater radii (360 and 650 m from ground zero). This sweep‐up dust was convectively drawn into the column and contributed up to 2% of the total mass of lofted. Based on the measured abundance of coarse (>250 μm diameter) dust particles in the estimated lofted dust, it is likely that about 70% of this lofted mass fell out within about 1 hour, such that the remaining dust cloud mass was ∼1.8 × 106 kg, which is equivalent to a dust lofted per unit blast yield of 0.5 kT/kT or about 4% of the crater ejecta mass. This study supports the hypothesis that if initial distributions can be constrained, the volume of dust lofted into atmospheric suspension from large surface explosions can be estimated from analysis of particle‐size distributions of ejecta deposited near the explosion. This result may have particular applications to study of the atmospheric effects of historic and prehistoric volcanic eruptions.

Ballistic transport in planetary ring systems due to particle erosion mechanisms: II. Theoretical models for Saturn's A- and B-ring inner edges

Ballistic transport is the net radial transport of mass and angular momentum caused by exchanges of meteoroid impact ejecta. The detailed numerical simulations and analytic arguments in this paper demonstrate that many of the common morphological features in Saturn's A- and B-ring inner edge regions can be understood as products of ballistic transport. In particular, the sharp edges themselves are maintained at their present observed widths by a balance between sharpening by ballistic transport and broadening by viscous transport; and the nearly linear ramps in optical depth interior to the sharp edges are created by ballistic transport alone. Hump-like structures on the high optical depth side and undulations on both sides of the edge are also possible. We suggest that the observed 100-km undulatory structure in the inner B-ring arises through ballistic transport echoing of the inner edge. Features in the edge regions are generally well fit by using a strongly prograde ejecta distribution function, such as that of Cuzzi and Durisen (1990, Icarus 84, 467–501), combined with an inverse cube power-law ejecta speed distribution ranging from ⪅2 m/sec up to ⪆70 m/sec . For plausible values of various uncertain input parameters, such as meteoroid influx rate, ring viscosity, and hypervelocity impact yield, we deduce ages for the inner edge features of 10 7 to 2 × 10 8 years.

Impact cratering on Venus: Physical and mechanical models

Results are presented on laboratory, theoretical and numerical simulations of various phenomena related to impact cratering processes specific to the Venusian environment. Such processes are influenced mainly by two factors: the high atmospheric density and the high surface temperature. Model results are presented for the influence of the atmosphere on high‐velocity meteoroids, the influence of high‐velocity meteoroids on the atmosphere, the influence of atmospheric shock waves on the Venusian surface, and the influence of high crustal temperature on impact melting and melt cooling. New modeling techniques based on the two‐dimensional Free‐Lagrangian method have been employed and show that an icy body of 1‐km radius may reach the bottom of the Venusian atmosphere in a partially dispersed state and form an impact crater approximately 20 km in diameter. A significant amount of energy is transferred to the Venusian atmosphere during meteoroid transit, and the gas dynamic flow will be similar to a strong explosion. Modeling shows that a significant wake of low density will form behind the meteoroid and have an important influence on crater formation and ejecta distribution. According to our estimates, the boundary of the flow of dense gas may correspond to the region of hummocky ejecta deposition observed for Venusian impact craters. The collapse of the gas‐rock debris column on the boundary between the low‐ and high‐density regions may result in a turbiditelike flow outside the hummocky ejecta area. We carried out preliminary laboratory simulations of the interaction of meteoroid shock waves with the surface. Considerable surface disruption is possible, even under the trajectory of an oblique impact. The possibility of “back venting” caused by shock wave pressuring and depressuring in ground pores may provide an explanation for the origin of dark margins. The differences between Venus and Earth for impact melting and melt cooling were examined. Melt generation on Venus should be 20% to 40% greater than on Earth for the same crater diameter. Melt volumes should stay molten about an order of magnitude longer on Venus, leading to substantial melt flows and smooth crater floors.

Wind interaction with falling ejecta: Origin of the parabolic features on Venus

Unusual parabolic features associated with impact craters have been observed by Magellan on Venus. A strong correlation exists between the orientation of the features and the zonal winds on Venus. We propose a quantitative model in which the parabolic features are produced by the interaction of the zonal winds with material ejected ballistically from the impact crater. As the ejecta particles fall through the atmosphere, the winds transport them downwind from their entry point, smaller particles being transported a greater distance. Since the ejecta distribution is initially axially symmetric and smaller particles are thrown farther from the crater, the winds blow the particles on the upwind side back upon one another, leading to a pile‐up of material. On the downwind side, the winds disperse the ejecta particles and no pile‐up occurs. The resulting thickness distribution on the Venusian surface matches the observed parabolic features closely. The dual parabolic feature associated with the crater Carson is also explained by this model.

Micrometeorite erosion of the main rings as a source of plasma in the inner Saturnian plasma torus

Micrometeorite bombardment is thought to erode the main rings of Saturn and to produce a plasma locally, accounting for transient ring phenomena and the supply of water molecules to Saturn's atmosphere. This process is shown here to be also a significant source of water molecules and molecular ions in the region between the outer edge of the main rings and Enceladus, adding to those neutrals and plasma produced by sputtering of the icy satellites. However, the magnitude and distribution of the ring source depends on the micrometeorite flux and the energy distribution of the neutral ejecta, both of which are uncertain. Limiting cases are examined, their implications in the Cassini division are calculated, and their relevance to the neutral and plasma clouds in this region is discussed.

A statistical study of impact ejecta distribution around Phobos and Deimos

Impact-ejecta dynamics around Phobos and Deimos are considered for dust particles of the order of 1 mm and greater. Only the gravitational forces of the planet and the satellite as well as the centrifugal and Coriolis forces are taken into account. In the case of Phobos the ejecta generally either reimpact the surface in tens of minutes after their launch or escape rapidly away from the gravitational attraction of the satellite. Deimos, being subjected to a relatively smaller perturbation force from Mars, can keep its ejecta on reimpacting and temporary satellite orbits for hours. Phobos shows large variability in its escape velocity as a function the launch point and the direction of launch. The statistical properties of large numbers of particles moving around the satellites are determined in a series of Monte Carlo simulations. Two factors, important for the regolith-formation process, are investigated: the escape probability from a given site on the surface and the reimpact probability. Both of these functions vary appreciably over the surfaces of Phobos and Deimos. The differences of the escape probabilities between adjoining regions are greater for Phobos than for Deimos, while the reimpact probability variations are substantial for both satellites. The density patterns for Phobos exhibit an interesting dependence on the particles' launch velocities. For small velocities (4–5 m sec −1) we observe two curved tubes of enhanced density that start from the sub-Mars and anti-Mars points on the surface. The density tends to become isotropic for greater launch velocities. Deimos shows a rather regular, ellipsoidal dust halo. The overall density of dust particles—with the meteoroid flux intensity, cratering laws, and velocity distribution of ejecta taken into account—is estimated to be of about a few hundred particles per kilometer cubed close to the surface of Phobos and drops down to about tens of particles per kilometer cubed at a distance of 40 km from the satellite's center. The density of dust around Deimos is two to three times larger.

Bombardment of planetary rings by meteoroids: General formulation and effects of Oort cloud projectiles

We present a general solution for the angular distribution of the intensity and velocity of interplanetary projectiles as they impinge on a planetary ring system, given the original distribution in inertial space. Our solution lends itself immediately to three results of importance. First, we demonstrate a variation with orbital longitude of an impact-velocity-weighted impact rate of cometary meteoroids in planetary rings. The largest rate of energetic impacts occurs at orbital longitudes near solar midnight, consistent in location and form with observed creation rates of “spokes” in Saturn's rings. This provides support for spoke formation hypotheses which rely on meteroid impact. Second, we determine the angular distribution of the intensity of ejecta which result from the bombardment of a planetary ring composed of centimeter- to meter-sized particles by interplanetary meteroids in the subcentimeter-size range. We do this using a radiative transfer formalism. The angular distribution of ejecta resulting from the bombardment of a single target particle is first obtained as a function of angle from a single incident direction. In this step, we incorporate results from laboratory studies of nondisruptive hypervelocity impacts into granular and powdery surfaces. This single particle “phase function” is part of a standard expression for the net scattering function of the layer as a whole, which accounts for the escape probabilities of ejecta emerging from impacts occurring at different depths within the ring layer. We assume that multiple scattering terms resulting from impacts into nearby particles by the more slowly moving ejecta are negligible. Finally, the net scattering function of the layer is integrated over the angular distribution of the incoming meteoroids, which is determined in the frame of an orbiting ring particle by accounting for aberration effects arising from the orbital motion of the ring particle and its parent planet. Our calculated ejecta distributions are incorporated into evolutions of ring mass density and radial structure in a companion paper (Durisen et al. 1989, Icarus, 80 136–166). Third, we calculate the radial drift velocity of a planetary ring of arbitrary optical depth which results from two factors: simple mass loading, and aberration-induced asymmetry in the impact rate. Bombardment of ring systems at the currently accepted rate of interplanetary meteoroid flux leads to an inward radial drift of several cm yr −1 in regions of moderate optical depth. This rate is large enough for the Uranian α and β rings, and the entire C ring of Saturn, to fall into the atmosphere of the parent planet in about 10 8 years under only the known flux of projectiles on Oort cloud orbits. These effects can vary significantly with the orbital distribution of the projectile population. In this paper, we present results for projectiles with “Oort cloud” orbits. In a subsequent paper, we will present the results anticipated for projectiles in “local family” prograde, low inclination orbits.

The Early Evolution of Supernova Remnants

AbstractThe density distribution of the supernova ejecta and that of the surrounding medium are the most important parameters for the early evolution of supernova remnants. The distribution of the ejecta depends on the detailed hydrodynamics of the explosion, but the outer parts of a supernova can probably be represented by a steep power law density distribution with radius. Self-similar solutions are especially useful for modeling the interaction of a supernova with its surrounding. The supernova first interacts with mass loss from the progenitor star. Evidence for circumstellar interaction is present in a number of extragalactic supernovae, including SN1987a. The explosions of massive stars probably interact with circumstellar gas for a considerable time while Type Ia supernovae interact more directly with the interstellar medium. X-ray spectroscopy is a good diagnostic for the physical conditions in young supernova remnants and for the composition of the supernova gas.

Variation in ejecta size with ejection velocity

A knowledge of the size‐velocity distribution of ejecta from impact craters is important for a number of processes that occur or may have occurred in the solar system, such as planetary accretion, collisional evolution of asteroids, the ejection of meteorites from parent bodies larger than asteroids (such as the lunar meteorites and the SNC meteorites, which may be from Mars), and massive biological extinctions on the Earth. In order to estimate the size‐velocity distribution of impact crater ejecta, the sizes and ranges from the primary crater were measured for the secondary craters of twelve large craters on Mercury, the Moon and Mars. The ballistic equation for spherical bodies was used to convert the ranges to velocities, and the velocities and crater sizes were used in the appropriate Schmidt‐Holsapple scaling relation to estimate ejecta sizes. Assuming that d ⧜ v− β, lines were fitted to the log (maximum size) vs. log (ejection velocity) using the least squares method to determine the velocity exponent. Certain problems with data collection, such as partial burial of some craters and interference from other craters and topographic features, made it impossible to determine a simple, unique relation between size and velocity. The velocity exponents are generally between −1 and −3, with an average value of approximately −1.9. The complicating effect of secondaries produced by clusters of impactors implies that the calculated lines are probably steeper than the slopes of the “true” fragment size‐velocity distributions.

Point source solutions and coupling parameters in cratering mechanics

Many phenomena of interest in impact cratering occur on a time and distance scale large compared to those of the impactor. As a consequence, the details of the impactor may be of little consequence, and the effects of the impactor can be replaced by an equivalent point source of energy and momentum. This point source assumption is developed and proved to define a single scalar “coupling parameter” measure, C ∝ αUμδν of the impactor radius α, velocity U, and density δ. Idealized examples are given where this measure can range from the cube root of the impactor kinetic energy, with μ = 2/3, to cases where it is the cube root of its momentum with μ = 1/3. For real materials the correct choice of μ depends on the material of the target. Materials with little dissipation have the exponent μ ≃ 0.6; while materials with large dissipation have μ ≃ 0.4. The assumption of such a measure is shown to lead to a unified theory of crater scaling in terms of the exponent μ. Results are given for crater size, ejecta distributions, growth histories, time of formation, melt volume, and shock decay. The results are compared to experiments and code calculations. (Craters, impacts, scaling, point source, self‐similar, coupling parameter).

Effect of an impact-generated gas cloud on entrained surface boulders and spall ejecta

The purpose of this investigation is to study the effect of an impact-generated gas cloud on the size-velocity distribution of large, solid ejecta fragments that become entrained in it, with particular reference to the possible Martian origin of the SNC meteorites. The entrainment both of loose surface boulders and of the early-time, large-size, high-velocity spall component of the crater ejecta is modeled numerically. Surface boulders from as far as 40 km from the center of impact can be accelerated by the high velocity leading edge of the gas cloud to velocities in excess of Martian escape velocity (5 km/s), but are generally crushed by the acceleration. Spall fragments become entrained later and nearer the center of the gas cloud, where gas velocities are much less. High velocity spalls are decelerated by the gas and low velocity spalls are accelerated by the gas, but no spalls ejected at < 5km/s are accelerated to velocities 5km/s. An impact-generated gas cloud is thus expected to scour the pre-existing surface of loose material and to change the size-velocity distribution of spall ejecta, but does not sufficiently enhance the velocities of crater ejecta to explain the Martian origin of SNC meteorites as large rocks.

Phobos, Deimos, and the Moon: Size and distribution of crater ejecta blocks

The size and radial distributions of ejecta blocks around craters ( D = 0.8 to 10 km) on Phobos and Deimos have been compared to those around lunar craters ( D = 0.2 to 3.5 km). The radial distribution of blocks was found to be similar on Phobos and the Moon, but more dispersed on Deimos. For the best imaged crater on Deimos ( D = 800 m), the size distributions of blocks and the fraction of excavated volume present as blocks are similar to those on the Moon. The wider dispersal of blocks on Deimos is consistent with other findings on the spread of finer ejecta over the satellite.

Size-velocity distribution of large ejecta fragments

The size-range distributions of secondary craters around three large primary craters on the Moon and Mars were measured. Ejection velocities of the fragments that produced the secondaries were estimated from the ranges of the secondaries from the primary. These velocities were then used in the Schmidt-Holsapple scaling relations to estimate the size of the ejecta fragments. Since it is unclear whether the secondary cratering events lie in the strength-dominated regime or in the gravity-dominated regime (or in the transition between the two), the data were reduced separately for both regimes in order to bracket the “true” answer. Furthermore, Schmidt and Holsapple have defined two scaling relations for the gravity regime, one appropriate for porous rock or regolith, and the other for nonporous target rocks. Since the nature of the target rock is unknown, both gravity scaling relations were used to reduce the data. The inferred size-velocity distributions of ejecta are compared to the predictions of the Melosh spallation model for 10 and 20 km/sec impacts of andesite onto andesite. The maximum inferred ejecta sizes are much greater than the maximum predicted spall sizes. This is consistent with the corresponding secondary craters having been produced by clustered impactors, with the inferred impactor diameter approximating the effective diameter of the cluster (Schultz and Gault, 1985). The inferred velocities show cutoffs at ⪅1 km sec . Whether this cut-off is real or is due to the inability to recognize secondary craters at greater ranges is unknown.

Fine fragments in high‐velocity impact experiments

Ejection angle dependences of size and kinetic energy of high‐velocity fine fragments ejected by high‐velocity impact into basalt targets were studied by exposing secondary targets to the ejecta. According to the properties of the fine ejecta the ejection angles were divided into four regions: outer region (region I, 10° &lt; θ &lt; 40°), outer belt region (region II, 40° &lt; θ &lt; 45°), inner belt region (region III, 45° &lt; θ &lt; 55°) and inner region (region IV, θ &gt; 55°), where θ was measured from the target surface. The size distributions of the fine ejecta were expressed by power law functions with power indices of about −3.5, except for region IV (about −3) in the size range of 1–70 μm. The secondary microcraters smaller than 100 μm in diameter were most abundant in region II, and their diameter distributions were also expressed by power law functions with power indices of about −3. Comminution energy, which was estimated from surface energy of the fine ejecta, was about 10% of the impact energy, and kinetic energy of the high‐velocity fine ejecta, which was estimated from formation energy of the secondary microcraters, was about 40% of the impact energy.