Microstructural study of micron‐sized craters simulating Stardust impacts in aluminum 1100 targets

Abstract— Various microscopic techniques were used to characterize experimental microcraters in aluminum foils to prepare for the comprehensive analysis of the cometary and interstellar particle impacts in aluminum foils to be returned by the Stardust mission. First, scanning electron microscopy (SEM) and energy dispersive X‐ray spectroscopy (EDS) were used to study the morphology of the impact craters and the bulk composition of the residues left by soda‐lime glass impactors. A more detailed structural and compositional study of impactor remnants was then performed using transmission electron microscopy (TEM), EDS, and electron diffraction methods. The TEM samples were prepared by focused ion beam (FIB) methods. This technique proved to be especially valuable in studying impact crater residues and impact crater morphology. Finally, we also showed that infrared microscopy (IR) can be a quick and reliable tool for such investigations. The combination of all of these tools enables a complete microscopic characterization of the craters.     

Capture of particles in hypervelocity impacts in aerogel

Abstract— The capture in aerogel of 106 μm diameter glass beads is investigated for impact speeds of 1 to 7.5 km s^?1. Three different aerogel densities were used, 60,96 and 180 kg m^?3. It was found that the length of the penetration track in the aerogel increases with speed until a maximum is reached. Above the maximum speed the track length decreases. This behaviour is similar to that which has previously been observed for particles impacting polystyrene foams and porous alumina. Whilst track length was not found to be an unambiguous indicator of impact speed, the excavated track volume was found to be a suitable indictor of speed. Further, it was possible to estimate the original particle size by measurements of the track volume and entrance hole size. In addition sub‐100 μm diameter particles composed of various minerals were fired into aerogel and the characterisation of the particles in situ by use of a Raman spectrometer was evaluated. This was found to work well, giving vibrational spectra essentially similar to those of the bulk minerals, thus providing a mineralogical rather than an elemental signature for the captured particles.     

FTIR 2–16 micron spectroscopy of micron‐sized olivines from primitive meteorites

Abstract— Infrared spectra of mineral grains from primitive meteorites could be useful for comparison with astronomical infrared spectra since some of their grains might be similar to those formed in the planet‐forming disks around young stars or in the envelopes surrounding late‐type stars. To assess the usefulness of meteorite spectra, olivine grains separated from primitive meteorites have been analyzed using FTIR microscope techniques in the 2–16 μm wavelength range. The sub‐micron sizes of the grains made a complex preparation process necessary. Five characteristic bands were measured near 11.9, 11.2, 10.4, 10.1, and 10.0 μm. The results of 59 analyses allow the calculation of band positions for meteoritic olivines as a function of their iron and magnesium contents. Comparison of the meteoritic results with astronomical data for comets and dust around young and old stars, which exhibit bands similar to the strongest infrared bands observed in the grains (at 11.2 μm), show that the spectral resolution of the astronomical observations is too low to ascertain the exact iron and magnesium (Mg: Fe) ratio of the dust in the 8–13 μm wavelength range.     

Time evolution and temperatures of hypervelocity impact‐generated tracks in aerogel

Abstract— Aerogel collectors have been used to capture cometary, interplanetary, and interstellar dust grains by NASA's Stardust mission, highlighting their importance as a scientific instrument. Due to the fragile and heterogeneous nature of cometary dust grains, their fragments are found along the walls of tracks that are formed during the capture process. These fragments appear to experience a wide range of thermal alteration and the causes of this variation are not well understood at a theoretical level as physical models of track formation are not well developed. Here, a general model of track formation that allows for the existence of partially and completely vaporized aerogel material in tracks is developed. It is shown that under certain conditions, this general track model reduces to the kinetic “snowplow” model that has previously been proposed. It is also shown, based on energetic considerations, that track formation is dominated by an expansion that is snowplow‐like in the later stages of track formation. The equation of motion for this snowplow‐like stage can be solved analytically, thus placing constraints on the amount of heating experienced by cometary dust fragments embedded in track walls. It is found that the heating of these fragments, for a given impact velocity, is expected to be greater for those embedded in larger tracks. Given the expected future use of aerogels for sample return missions, the results presented here imply that the choice of aerogel compositions can have a significant effect on the modification of samples captured and retrieved by these collectors.     

A study of the observed shift in the peak position of olivine Raman spectra as a result of shock induced by hypervelocity impacts

Kuebler et al. (2006) identified variations in olivine Raman spectra based on the composition of individual olivine grains, leading to identification of olivine composition from Raman spectra alone. However, shock on a crystal lattice has since been shown to result in a structural change to the original material, which produces a shift in the Raman spectra of olivine grains compared with the original unshocked olivine (Foster et al. 2013). This suggests that the use of the compositional calculations from the Raman spectra, reported in Kuebler et al. (2006), may provide an incorrect compositional value for material that has experienced shock. Here, we have investigated the effect of impact speed (and hence peak shock pressure) on the shift in the Raman spectra for San Carlos olivine (Fo₉₁) impacting Al foil. Powdered San Carlos olivine (grain size 1–10 μm) was fired at a range of impact speeds from 0.6 to 6.1 km s^?1 (peak shock pressures 5–86 GPa) at Al foil to simulate capture over a wide range of peak shock pressures. A permanent change in the Raman spectra was found to be observed only for impact speeds greater than ~5 km s^?1. The process that causes the shift is most likely linked to an increase in the peak pressure produced by the impact, but only after a minimum shock pressure associated with the speed at which the effect is first observed (here 65–86 GPa). At speeds around 6 km s^?1 (peak shock pressures ~86 GPa), the shift in Raman peak positions is in a similar direction (red shift) to that observed by Foster et al. (2013) but of twice the magnitude.     

Nondestructive imaging of hypervelocity impact-induced damage zones beneath laboratory-created craters by means of ultrasound travel-time tomography

Since the 1960s, hypervelocity impact experiments have been conducted to study the complex deformation mechanisms which occur in the subsurface of meteorite craters. Here, we present ultrasound tomography measurements of the damage zone underneath seven experimentally produced impact craters in sandstone cubes. Within the framework of the Multidisciplinary Experimental and Modeling Impact Research Network and the NEOShield Project, decimeter-sized sandstone targets were impacted by aluminum and steel projectiles with radii of 2.5, 4, and 5 mm at velocities between ~3.0 and ~7.4 km s⁻¹. The 2-D ultrasound tomography clearly shows a correlation between impact energy and the damaged volume within the target blocks. When increasing impact energies from 805 to 2402 J, a corresponding increase in the damage radius from ~13.1 cm to ~17.6 cm was calculated. p-Wave velocity reductions up to 18.3% (for the highest impact energy) were observed in the vicinity of the craters. The reduction in seismic velocity decreased uniformly and linearly with increasing distance from the impact point. The damage intensities correspond to peak damage parameters of 0.4–0.51 compared to undamaged target blocks. In addition to the damage zone below the crater, we could identify weakened zones at the sandstone walls which represent precursors of spalling. The volume of the damaged subsurface beneath experimentally produced craters determined through ultrasound tomography is larger than that obtained from previously reported p-wave velocity reductions or to microscopic and microcomputed tomography observations of crack densities in experimentally produced craters.     

Interpretation of Wild 2 dust fine structure: Comparison of Stardust aluminum foil craters to the three‐dimensional shape of experimental impacts by artificial aggregate particles and meteorite powders

Abstract— New experimental results show that Stardust crater morphology is consistent with interpretation of many larger Wild 2 dust grains being aggregates, albeit most of low porosity and therefore relatively high density. The majority of large Stardust grains (i.e. those carrying most of the cometary dust mass) probably had density of 2.4 g cm^?3 (similar to soda‐lime glass used in earlier calibration experiments) or greater, and porosity of 25% or less, akin to consolidated carbonaceous chondrite meteorites, and much lower than the 80% suggested for fractal dust aggregates. Although better size calibration is required for interpretation of the very smallest impacting grains, we suggest that aggregates could have dense components dominated by μm‐scale and smaller sub‐grains. If porosity of the Wild 2 nucleus is high, with similar bulk density to other comets, much of the pore space may be at a scale of tens of micrometers, between coarser, denser grains. Successful demonstration of aggregate projectile impacts in the laboratory now opens the possibility of experiments to further constrain the conditions for creation of bulbous (Type C) tracks in aerogel, which we have observed in recent shots. We are also using mixed mineral aggregates to document differential survival of pristine composition and crystalline structure in diverse finegrained components of aggregate cometary dust analogues, impacted onto both foil and aerogel under Stardust encounter conditions.     

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.     

High‐resolution studies of double‐layered ejecta craters: Morphology,inherent structure,and a phenomenological formation model

The ejecta blankets of impact craters in volatile‐rich environments often possess characteristic layered ejecta morphologies. The so‐called double‐layered ejecta (DLE) craters are characterized by two ejecta layers with distinct morphologies. The analysis of high‐resolution image data, especially HiRISE and CTX, provides new insights into the formation of DLE craters. A new phenomenological excavation and ejecta emplacement model for DLE craters is proposed based on a detailed case study of the Martian crater Steinheim—a well‐preserved DLE crater—and studies of other DLE craters. The observations show that the outer ejecta layer is emplaced as medial and distal ejecta that propagate outwards in a debris avalanche or (if saturated with water) a debris flow mode after landing, overrunning previously formed secondary craters. In contrast, the inner ejecta layer is formed by a translational slide of the proximal ejecta deposits during the emplacement stage that overrun and superimpose parts of the outer ejecta layer. Based on our model, DLE craters on Mars are the result of an impact event into a rock/ice mixture that produces large amounts of shock‐induced vaporization and melting of ground ice, leading to high ejection angles, proximal landing positions, and an ejecta curtain with relatively wet (in terms of water in liquid form) composition in the distal part versus dryer composition in the proximal part. As a consequence, basal melting of ice components in the ejecta at the transient crater rim, which is induced by frictional heating and the enhanced pressure at depth, initiates an outwards directed collapse of crater rim material in a translational slide mode. Our results indicate that similar processes may also be applicable for other planetary bodies with volatile‐rich environments, such as Ganymede, Europa, and the Earth.     

Astronomical evidence for the rapid growth of millimeter‐sized particles in protoplanetary disks

Abstract– I summarize recent surveys of protoplanetary disks at millimeter wavelengths and show that the distribution of luminosity, equivalent to the mass in small dust grains, declines rapidly. This contrasts with statistics on the lifetime of disks from infrared observations and the high occurrence of planets from radial velocity and transit surveys. I suggest that these disparate results can be reconciled if most of the dust in a disk is locked up in millimeter and larger‐sized particles within about 2 Myr. This statistical result on disk evolution agrees with detailed modeling of a small number of individual disks and with cosmochemical measurements of rapid planetesimal formation.     

Facies distribution of post‐impact sediments in the Ordovician Lockne and Tvären impact craters: Indications for unique impact‐generated environments

Abstract— The Lockne and Tvären craters formed in the Late Ordovician Baltoscandian epicontinental sea. Both craters demonstrate similarities concerning near‐synchronous age, target seabed, and succeeding resurge deposits; however, the water depths at the impact sites and the sizes of the craters were not alike. The post‐impact sedimentary succession of carbonates, i.e., the Dalby Limestone, deposited on top of the resurge sediments in the two craters, is nevertheless similar. At least three main facies of the Dalby Limestone were established in the Lockne crater, depending on sea‐floor topography, location with respect to the crater, and local water currents. The dominating nodular argillaceous facies, showing low values of inorganic carbon (IC), was distributed foremost in the deeper and quiet areas of the crater floor and depressions. At the crater rim, consisting of crushed crystalline basement ejecta, a rim facies with a reef‐like fauna was established, most certainly due to topographical highs and substrate‐derived nutrients. Between these facies are occurrences of a relatively thick‐bedded calcilutite rich in cephalopods (cephalopod facies). In Tvären, the lower part of the succession consists of an analogous argillaceous facies, also showing similar low IC values as in Lockne, followed by calcareous mudstones with an increase of IC. Occasionally biocalcarenites with a distinctive fauna occur in the Tvären succession, probably originating as detritus from a facies developed on the rim. They are evident as peaks in IC and lows in organic carbon (Corg). The fauna in these biocalcarenites corresponds very well with those of erratic boulders derived from Tvären; moreover, they correspond to the rim facies of Lockne except for the inclusion of photosynthesizing algae, indicating shallower water at Tvären than Lockne. Consequently, we suggest equivalent distribution patterns for the carbonates of the Dalby Limestone in Lockne and Tvären.     

Characterization of micron‐sized Fe,Ni metal grains in fine‐grained rims in the Y‐791198 CM2 carbonaceous chondrite: Implications for asteroidal and preaccretionary models for aqueous alteration

Abstract— –The presence of apparently unaltered, micron‐sized Fe,Ni metal grains, juxtaposed against hydrated fine‐grained rim materials in the CM2 chondrite Yamato (Y‐) 791198 has been cited as unequivocal evidence of preaccretionary alteration. We have examined the occurrence, composition, and textural characteristics of 60 Fe,Ni metal grains located in fine‐grained rims in Y‐791198 using scanning electron microscopy (SEM) and electron microprobe analysis. In addition, three metal grains, prepared by focused ion beam (FIB) sample preparation techniques were studied by transmission electron microscopy (TEM). The metal grains are heterogeneously distributed within the rims. Electron microprobe analyses show that all the metal grains are kamacite with minor element contents (P, Cr, and Co) that lie either within or close to the range for other CM2 metal grains. X‐ray maps obtained by electron microprobe show S, P, and/or Ca enrichments on the outermost parts of many of the metal grains. Z‐contrast STEM imaging of FIB‐prepared Fe,Ni metal grains show the presence of a small amount of a lower Z secondary phase on the surface of the grains and within indentations on the grain surfaces. Energy‐filtered TEM (EFTEM) compositional mapping shows that these pits are enriched in oxygen and depleted in Fe relative to the metal. These observations are consistent with pitting corrosion of the metal on the edges of the grains and we suggest may be the result of the formation of Fe(OH)₂, a common oxidation product of Fe metal. The presence of such a layer could have inhibited further alteration of the metal grains. These findings are consistent with alteration by an alkaline fluid as suggested by Zolensky et al. (1989), but the location of this alteration remains unconstrained, because Y‐791198 was recovered from Antarctica and therefore may have experienced incipient terrestrial alteration. However, we infer that the extremely low degree of oxidation of the metal is inconsistent with weathering in Antarctica and that alteration in an extraterrestrial environment is more probable. Although the presence of unaltered or incipiently altered metal grains in these fine‐grained rims could be interpreted as evidence for preaccretionary alteration, we suggest an alternative model in which metal alteration was inhibited by alkaline fluids on the asteroidal parent body.     

Micron‐scale hypervelocity impact craters: Dependence of crater ellipticity and rim morphology on impact trajectory,projectile size,velocity, and shape

The interstellar collector on NASA's Stardust mission captured many particles from sources other than the interstellar dust stream. Impact trajectory may provide a means of discriminating between these different sources, and thus identifying/eliminating candidate interstellar particles. The collector's aerogel preserved a clear record of particle impact trajectory from the inclination and direction of the resultant tracks. However, the collector also contained aluminum foils and, although impact crater studies to date suggest only the most inclined impacts (>45° from normal) produce crater morphologies that indicate trajectory (i.e., distinctly elliptical), these studies have been restricted to much larger (mm and above) scales than are relevant for Stardust (μm). It is unknown how oblique impact crater morphology varies as a function of length scale, and therefore how well Stardust craters preserve details of impactor trajectory. Here, we present data from a series of impact experiments, together with complementary hydrocode modeling, that examine how crater morphology changes with impact angles for different‐sized projectiles. We find that, for our smallest spherical projectiles (2 μm diameter), the ellipticity and rim morphology provide evidence of their inclined trajectory from as little as 15° from normal incidence. This is most likely a result of strain rate hardening in the target metal. Further experiments and models find that variation in velocity and impactor shape complicate these trends, but that rim morphology remains useful in determining impact direction (where the angle of impact is >20° from normal) and may help identify candidate interstellar particle craters on the Stardust collector.     

Thermochemical stability of low‐iron,manganese‐enriched olivine in astrophysical environments

Abstract– Low‐iron, manganese‐enriched (LIME) olivine grains are found in cometary samples returned by the Stardust mission from comet 81P/Wild 2. Similar grains are found in primitive meteoritic clasts and unequilibrated meteorite matrix. LIME olivine is thermodynamically stable in a vapor of solar composition at high temperature at total pressures of a millibar to a microbar, but enrichment of solar composition vapor in a dust of chondritic composition causes the FeO/MnO ratio of olivine to increase. The compositions of LIME olivines in primitive materials indicate oxygen fugacities close to those of a very reducing vapor of solar composition. The compositional zoning of LIME olivines in amoeboid olivine aggregates is consistent with equilibration with nebular vapor in the stability field of olivine, without re‐equilibration at lower temperatures. A similar history is likely for LIME olivines found in comet samples and in interplanetary dust particles. LIME olivine is not likely to persist in nebular conditions in which silicate liquids are stable.     

FIB‐TEM analysis of cometary material in 10 Stardust foil craters

Aluminum foils from the Stardust cometary dust collector contain impact craters formed during the spacecraft's encounter with comet 81P/Wild 2 and retain residues that are among the few unambiguously cometary samples available for laboratory study. Our study investigates four micron‐scale (1.8–5.2 μm) and six submicron (220–380 nm) diameter craters to better characterize the fine (<1 μm) component of comet Wild 2. We perform initial crater identification with scanning electron microscopy, prepare the samples for further analysis with a focused ion beam, and analyze the cross sections of the impact craters with transmission electron microscopy (TEM). All of the craters are dominated by combinations of silicate and iron sulfide residues. Two micron‐scale craters had subregions that are consistent with spinel and taenite impactors, indicating that the micron‐scale craters have a refractory component. Four submicron craters contained amorphous residue layers composed of silicate and sulfide impactors. The lack of refractory materials in the submicron craters suggests that refractory material abundances may differentiate Wild 2 dust on the scale of several hundred nanometers from larger particles on the scale of a micron. The submicron craters are enriched in moderately volatile elements (S, Zn) when normalized to Si and CI chondrite abundances, suggesting that, if these craters are representative of the Wild 2 fine component, the Wild 2 fines were not formed by high‐temperature condensation. This distinguishes the comet's fine component from the large terminal particles in Stardust aerogel tracks which mostly formed in high‐temperature events.     

Bulk Mg isotopic compositions of Ca‐Al‐rich inclusions and amoeboid olivine aggregates

Abstract— High‐precision Mg isotopic compositions of Ca‐Al‐rich inclusions (CAIs) from both Ningqiang (ungrouped) and Allende (CV3) carbonaceous chondrites and amoeboid olivine aggregations (AOAs) from Allende were analyzed by multicollector inductively coupled plasma mass spectrometry (MC‐ICP‐MS). The CAIs from Allende plot on a line, with an inferred initial ²⁶Al/²⁷Al ratio of (4.77 ± 0.39) × 10^?5 close to the canonical value. This indicates a relatively closed Al‐Mg system in the CAIs and no significant Mg isotope exchange with ambient materials, although two of the CAIs are severely altered. The AOAs contain excess ²⁶Mg and plot close to the CAI regression line, which is suggestive of their contemporary formation. The CAIs from Ningqiang define a different line with a lower inferred (²⁶Al/²⁷Al)₀ ratio of (3.56 ± 0.08) × 10^?5. None of the CAIs and AOAs studied in this work shows significant mass fractionation with enrichment of the heavier Mg isotopes, arguing against an evaporation origin.     

Convective activity in a Martian magma chamber recorded by P‐zoning in Tissint olivine

The Tissint Martian meteorite is an unusual depleted olivine‐phyric shergottite, reportedly sourced from a mantle‐derived melt within a deep magma chamber. Here, we report major and trace element data for Tissint olivine and pyroxene, and use these data to provide new insights into the dynamics of the Tissint magma chamber. The presence of irregularly spaced oscillatory phosphorous (P)‐rich bands in olivine, along with geochemical evidence indicative of a closed magmatic system, implies that the olivine grains were subject to solute trapping caused by vigorous crystal convection within the Tissint magma chamber. Calculated equilibration temperatures for the earliest crystallizing (antecrystic) olivine cores suggest a Tissint magma source temperature of 1680 °C, and a local Martian mantle temperature of 1560 °C during the late Amazonian—the latter being consistent with the ambient mantle temperature of Archean Earth.     

The ion-composition of the plasma produced by impacts of fast dust particles

The composition of the impact plasma produced by fast dust particles (v > 1 km/sec) hitting an Au or W target was measured both with a model of the HELIOS micrometeoroid experiment (low electric field at the target) and a high field detector. The plasma composition and the total plasma charge depend strongly on the impact velocity and the electric field strength at the target. Spectra of 9 different projectile-target combinations were analysed. Two types of spectra could be observed, depending on the projectile material. (1) Spectra of metals and hard dielectrics (Mohs' hardness ? 5). Particle constituents of low ionisation energy (e · u ? 7eV, e.g. Na, K, Al) dominate the spectra of these materials at impact velocities below 10 km/sec. At higher speed the relative intensities change and new ions with higher ionisation energies appear. (2) Spectra of soft dielectrics (Mohs' hardness < 3). Below 9 km/sec these materials produced less total charge than did the others. The highest masses were detected at 74 amu. The relative abundance of ions with low ionization energies such as Li, Na, K, etc. is comparatively small. Negative ions were also observed in the impact plasma. Their total number was found to be approximately 3–6% of that of the positive ions at 6 km/sec particle speed.     

The planforms of low‐angle impact craters in the northern hemisphere of Mars

Abstract— We have surveyed Martian impact craters greater than 5 km in diameter using Viking and thermal emission imaging system (THEMIS) imagery to evaluate how the planform of the rim and ejecta changes with decreasing impact angle. We infer the impact angles at which the changes occur by assuming a sin²θ dependence for the cumulative fraction of craters forming below angle θ. At impact angles less than ?40° from horizontal, the ejecta become offset downrange relative to the crater rim. As the impact angle decreases to less than ?20°, the ejecta begin to concentrate in the cross‐range direction and a “forbidden zone” that is void of ejecta develops in the uprange direction. At angles less than ?10°, a “butterfly” ejecta pattern is generated by the presence of downrange and uprange forbidden zones, and the rim planform becomes elliptical with the major axis oriented along the projectile's direction of travel. The uprange forbidden zone appears as a “V” curving outward from the rim, but the downrange forbidden zone is a straight‐edged wedge. Although fresh Martian craters greater than 5 km in diameter have ramparts indicative of surface ejecta flow, the ejecta planforms and the angles at which they occur are very similar to those for lunar craters and laboratory impacts conducted in a dry vacuum. The planforms are different from those for Venusian craters and experimental impacts in a dense atmosphere. We interpret our results to indicate that Martian ejecta are first emplaced predominantly ballistically and then experience modest surface flow.     

Remote sensing of terrestrial impact craters: The TanDEM‐X digital elevation model

With the TanDEM‐X digital elevation model (DEM), the terrestrial solid surface is globally mapped with unprecedented accuracy. TanDEM‐X is a German X‐band radar mission whose two identical satellites have been operated in single‐pass interferometer configuration over several years. The acquired data are processed to yield a global DEM with 12 m independent posting and relative vertical accuracies of better than 2 m and 4 m in moderate and mountainous terrain, respectively. This DEM provides new opportunities for space‐borne remote‐sensing studies of the entire sample of terrestrial impact craters. In addition, it represents an interesting repository to aid in the search for new impact crater candidates. We have used the TanDEM‐X DEM to investigate the current set of confirmed impact structures. For a subsample of the craters, including small, midsized, and large structures, we compared the results with those from other DEMs. This quantitative analysis demonstrates the excellent quality of the TanDEM‐X elevation data. Our findings help to estimate what can be gained by using the TanDEM‐X DEM in impact crater studies. They may also be beneficial in identifying the regions and morphologies where the search for currently unknown impact structures might be most promising.