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1.
We report on the microscopic impactor debris around Kamil crater (45 m in diameter, Egypt) collected during our 2010 geophysical expedition. The hypervelocity impact of Gebel Kamil (Ni‐rich ataxite) on a sandstone target produced a downrange ejecta curtain of microscopic impactor debris due SE–SW of the crater (extending ~300,000 m2, up to ~400 m from the crater), in agreement with previous determination of the impactor trajectory. The microscopic impactor debris include vesicular masses, spherules, and coatings of dark impact melt glass which is a mixture of impactor and target materials (Si‐, Fe‐, and Al‐rich glass), plus Fe‐Ni oxide spherules and mini shrapnel, documenting that these products can be found in craters as small as few tens of meters in diameter. The estimated mass of the microscopic impactor debris (<290 kg) derived from Ni concentrations in the soil is a small fraction of the total impactor mass (~10 t) in the form of macroscopic shrapnel. That Kamil crater was generated by a relatively small impactor is consistent with literature estimates of its pre‐atmospheric mass (>20 t, likely 50–60 t).  相似文献   

2.
Abstract– The <1,100 yr old Whitecourt meteorite impact crater, located south of Whitecourt, Alberta, Canada, is a well‐preserved bowl‐shaped structure having a depth and diameter of approximately 6 and 36 m, respectively. There are fewer than a dozen known terrestrial sites of similar size and age. Unlike most of these sites, however, the Whitecourt crater contains nearly all of the features associated with small impact craters including meteorites, ejecta blanket, observable transient crater boundary, raised rim, and associated shock indicators. This study indicates that the crater formed from the impact of an approximately 1 m diameter type IIIAB iron meteoroid traveling east‐northeast at less than approximately 10 km s?1, striking the surface at an angle between 40° and 55° to horizontal. It appears that the main mass survived atmospheric transit relatively intact, with fragmentation and partial melting during impact. Most meteoritic material has a jagged, shrapnel‐like morphology and is distributed downrange of the crater.  相似文献   

3.
Most impacts occur at an angle with respect to the horizontal plane. This is primarily reflected in the ejecta distribution, but at very low angle structural asymmetries such as elongation of the crater and nonradial development of the central peak become apparent. Unfortunately, impact craters with pristine ejecta layers are rare on Earth and also in areas with strong past or ongoing surface erosion on other planetary bodies, and the structural analysis of central peaks requires good exposures or even on‐site access to outcrop. However, target properties are known to greatly influence the shape of the crater, especially the relatively common target configuration of a weaker layer covering a more rigid basement. One such effect is the formation of concentric craters, i.e., a nested, deeper, inner crater surrounded by a shallow, outer crater. Here, we show that with decreasing impact angle there is a downrange shift of the outer crater with respect to the nested crater. We use a combination of (1) field observation and published 3‐D numerical simulation of one of the best examples of a terrestrial, concentric impact crater formed in a layered target with preserved ejecta layer: the Lockne crater, Sweden; (2) remote sensing data for three pristine, concentric impact craters on Mars with preserved ejecta layers further constraining the direction of impact; as well as (3) laboratory impact experiments, to develop the offset in crater concentricity into a complementary method to determine the direction of impact for layered‐target craters with poorly preserved ejecta layers.  相似文献   

4.
Abstract— We surveyed the impact crater populations of Venus and the Moon, dry targets with and without an atmosphere, to characterize how the 3‐dimensional shape of a crater and the appearance of the ejecta blanket varies with impact angle. An empirical estimate of the impact angle below which particular phenomena occur was inferred from the cumulative percentage of impact craters exhibiting different traits. The results of the surveys were mostly consistent with predictions from experimental work. Assuming a sin2θ dependence for the cumulative fraction of craters forming below angle θ, on the Moon, the following transitions occur: >?45 degrees, the ejecta blanket becomes asymmetric; >?25 degrees, a forbidden zone develops in the uprange portion of the ejecta blanket, and the crater rim is depressed in that direction; >?15 degrees, the rim becomes saddle‐shaped; >?10 degrees, the rim becomes elongated in the direction of impact and the ejecta forms a “butterfly” pattern. On Venus, the atmosphere causes asymmetries in the ejecta blanket to occur at higher impact angles. The transitions on Venus are: >?55 degrees, the ejecta becomes heavily concentrated downrange; >?40 degrees, a notch in the ejecta that extends to the rim appears, and as impact angle decreases, the notch develops into a larger forbidden zone; >?10 degrees, a fly‐wing pattern develops, where material is ejected in the crossrange direction but gets swept downrange. No relationship between location or shape of the central structure and impact angle was observed on either planet. No uprange steepening and no variation in internal slope or crater depth could be associated with impact angle on the Moon. For both planets, as the impact angle decreases from vertical, first the uprange and then the downrange rim decreases in elevation, while the remainder of the rim stays at a constant elevation. For craters on Venus >?15 km in diameter, a variety of crater shapes are observed because meteoroid fragment dispersal is a significant fraction of crater diameter. The longer path length for oblique impacts causes a correlation of clustered impact effects with oblique impact effects. One consequence of this correlation is a shallowing of the crater with decreasing impact angle for small craters.  相似文献   

5.
Abstract– The 45 m in diameter Kamil impact crater was formed <5000 yr ago in the eastern Sahara, close to the southern border of modern Egypt. The original features of this structure, including thousands of fragments of the meteorite impactor, are extremely well preserved. With the exception of a single 83 kg regmaglypted individual, all specimens of Gebel Kamil (the iron meteorite that formed the Kamil crater) are explosion fragments weighing from <1 g to 34 kg. Gebel Kamil is an ungrouped Ni‐rich (about 20 wt% Ni) ataxite characterized by high Ge and Ga contents (approximately 120 μg g?1 and approximately 50 μg g?1, respectively) and by a very fine‐grained duplex plessite metal matrix. Accessory mineral phases in Gebel Kamil are schreibersite, troilite, daubréelite, and native copper. Meteorite fragments are cross‐cut by curvilinear shear bands formed during the explosive terrestrial impact. A systematic search around the crater revealed that meteorite fragments have a highly asymmetric distribution, with greater concentrations in the southeast sector and a broad maximum in meteorite concentration in the 125–160° N sector at about 200 m from the crater rim. The total mass of shrapnel specimens >10 g, inferred from the density map compiled in this study is 3400 kg. Field data indicate that the iron bolide approached the Earth’s crust from the northwest (305–340° N), travelling along a moderately oblique trajectory. Upon hypervelocity impact, the projectile was disrupted into thousands of fragments. Shattering was accompanied by some melting of the projectile and of the quartz‐arenite target rocks, which also suffered shock metamorphism.  相似文献   

6.
Abstract— Crater‐ejecta correlation is an important element in the analysis of crater formation and its influence on the geological evolution. In this study, both the ejecta distribution and the internal crater development of the Jurassic/Cretaceous Mjølnir crater (40 km in diameter; located in the Barents Sea) are investigated through numerical simulations. The simulations show a highly asymmetrical ejecta distribution, and underscore the importance of a layer of surface water in ejecta distribution. As expected, the ejecta asymmetry increases as the angle of impact decreases. The simulation also displays an uneven aerial distribution of ejecta. The generation of the central high is a crucial part of crater formation. In this study, peak generation is shown to have a skewed development, from approximately 50–90 sec after impact, when the peak reaches its maximum height of 1‐1.5 km. During this stage, the peak crest is moved about 5 km from an uprange to a downrange position, ending with a final central position which has a symmetrical appearance that contrasts with its asymmetrical development.  相似文献   

7.
Abstract— We use Mars Orbiter Laser Altimeter (MOLA) topographic data and Thermal Emission Imaging System (THEMIS) visible (VIS) images to study the cavity and the ejecta blanket of a very fresh Martian impact crater ?29 km in diameter, with the provisional International Astronomical Union (IAU) name Tooting crater. This crater is very young, as demonstrated by the large depth/diameter ratio (0.065), impact melt preserved on the walls and floor, an extensive secondary crater field, and only 13 superposed impact craters (all 54 to 234 meters in diameter) on the ?8120 km2 ejecta blanket. Because the pre‐impact terrain was essentially flat, we can measure the volume of the crater cavity and ejecta deposits. Tooting crater has a rim height that has >500 m variation around the rim crest and a very large central peak (1052 m high and >9 km wide). Crater cavity volume (i.e., volume below the pre‐impact terrain) is ?380 km3 the volume of materials above the pre‐impact terrain is ?425 km3. The ejecta thickness is often very thin (<20 m) throughout much of the ejecta blanket. There is a pronounced asymmetry in the ejecta blanket, suggestive of an oblique impact, which has resulted in up to ?100 m of additional ejecta thickness being deposited down‐range compared to the up‐range value at the same radial distance from the rim crest. Distal ramparts are 60 to 125 m high, comparable to the heights of ramparts measured at other multi‐layered ejecta craters. Tooting crater serves as a fresh end‐member for the large impact craters on Mars formed in volcanic materials, and as such may be useful for comparison to fresh craters in other target materials.  相似文献   

8.
Kamil is a 45 m diameter impact crater identified in 2008 in southern Egypt. It was generated by the hypervelocity impact of the Gebel Kamil iron meteorite on a sedimentary target, namely layered sandstones with subhorizontal bedding. We have carried out a petrographic study of samples from the crater wall and ejecta deposits collected during our first geophysical campaign (February 2010) in order to investigate shock effects recorded in these rocks. Ejecta samples reveal a wide range of shock features common in quartz‐rich target rocks. They have been divided into two categories, as a function of their abundance at thin section scale: (1) pervasive shock features (the most abundant), including fracturing, planar deformation features, and impact melt lapilli and bombs, and (2) localized shock features (the least abundant) including high‐pressure phases and localized impact melting in the form of intergranular melt, melt veins, and melt films in shatter cones. In particular, Kamil crater is the smallest impact crater where shatter cones, coesite, stishovite, diamond, and melt veins have been reported. Based on experimental calibrations reported in the literature, pervasive shock features suggest that the maximum shock pressure was between 30 and 60 GPa. Using the planar impact approximation, we calculate a vertical component of the impact velocity of at least 3.5 km s?1. The wide range of shock features and their freshness make Kamil a natural laboratory for studying impact cratering and shock deformation processes in small impact structures.  相似文献   

9.
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.  相似文献   

10.
Confirmed small impact craters in unconsolidated deposits are rare on Earth, and only a few have been the subjects of detailed investigations. Consequently, our knowledge of indicators permitting unambiguous identification of such structures is limited. In this work, detailed geological mapping was performed in the area of the Morasko craters, of which the largest crater is of about 96 m diameter. These craters were formed in the mid‐Holocene (~5000 yr ago) in unconsolidated sediments of a glacial terminal moraine. Fragments of the impactor—an iron meteorite—have been found in the craters’ vicinity for many decades. Despite numerous studies of the meteorite, no detailed research concerning the geological structure around the craters and of the ejecta deposits has been undertaken. The new data, including evaluation of over 52 sediment cores and 260 shallow drillings, permit the identification of four main sediment types: Neogene clays, diamicton with Neogene clay clasts containing charcoal pieces, diamicton without clasts, and sand with locally preserved paleosoil and charcoal pieces. Based on sedimentological properties, the ejecta deposits are mainly identified as diamicton with Neogene clay clasts, described as lithic impact breccia, covering locally preserved pre‐impact soil. Moreover, crater sections characterized by inverse stratigraphy of sediments are identified as belonging to overturned flaps.  相似文献   

11.
Cosmogenic He, Ne, and Ar as well as the radionuclides 10Be, 26Al, 36Cl, 41Ca, 53Mn, and 60Fe have been determined on samples from the Gebel Kamil ungrouped Ni‐rich iron meteorite by noble gas mass spectrometry and accelerator mass spectrometry (AMS), respectively. The meteorite is associated with the Kamil crater in southern Egypt, which is about 45 m in diameter. Samples originate from an individual large fragment (“Individual”) as well as from shrapnel. Concentrations of all cosmogenic nuclides—stable and radioactive—are lower by a factor 3–4 in the shrapnel samples than in the Individual. Assuming negligible 36Cl decay during terrestrial residence (indicated by the young crater age <5000 years; Folco et al. 2011 ), data are consistent with a simple exposure history and a 36Cl‐36Ar cosmic ray exposure age (CRE) of approximately (366 ± 18) Ma (systematic errors not included). Both noble gases and radionuclides point to a pre‐atmospheric radius >85 cm, i.e., a pre‐atmospheric mass >20 tons, with a preferred radius of 115–120 cm (50–60 tons). The analyzed samples came from a depth of approximately 20 cm (Individual) and approximately 50–80 cm (shrapnel). The size of the Gebel Kamil meteoroid determined in this work is close to estimates based on impact cratering models combined with expectations for ablation during passage through the atmosphere (Folco et al. 2010 , 2011 ).  相似文献   

12.
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.  相似文献   

13.
In the western hemisphere of Mars Amazonian volcanism from Arsia Mons produced the smooth surfaces of Daedalia Planum and masks older rocks. Close to the southern termination of Daedalia Planum basement rocks are exposed in which are preserved craters that escaped or were only partially filled by this most recent volcanism. Pickering Crater is an approximately 130 km diameter crater. The youngest lavas flowed into this crater from Daedalia Planum by way of a NW rim breach, covering its western part. East of a well-defined flow front an older lava sequence with a distinctive platy surface and derived from a more proximal unestablished source to the northeast is exposed. Several units are identified within this sequence on the basis of surface texture, which is more subdued in progressively older rocks. Only local mapping of the flow front boundaries of these units is possible because of incomplete coverage by high resolution imagery. During emplacement of the older lavas a NE-SW striking en echelon graben system and parallel smaller troughs and dikes formed under inferred regional NW-SE extension. A much earlier strike-slip regime pre-dating the lavas exposed in the crater floor is postulated, based on the highly fretted nature of the rim of Pickering Crater and an elongated smaller crater to its northeast, approximately 40 km long in the NE-SW direction. The rims of these craters contrast with that of a smoother rimmed impact crater in the southeast that was excavated subsequent to strike-slip deformation but prior to the emplacement of platy surfaced lavas.  相似文献   

14.
From an analysis of 1173 craters possessing single (Type I) and double (Type 2) concentric ejecta deposits, Type 2 craters are found to occur most frequently in areas that have also been described as possessing periglacial features. The frequency of occurence of central peaks and wall failure (terraces plus scallops) within the craters indicate that, by analogy with previous analyses, Type 1 craters form in more fragmental targets than Type 2 craters. The maximum range of the outer ejecta deposits of Type 2 craters, however, consistently extends ～0.8 crater radii further than ejecta deposits of Type 1 craters, suggesting a greater degree of ejecta fluidization for the twin-lobed Type 2 craters. Numerous characteristics of Ries Crater, West Germany, show similarities to craters on Mars, indicating that Martian fluidized ejecta craters may be closer analogs to this terrestrial crater than are lunar craters.  相似文献   

15.
The Deep Impact oblique impact cratering experiment   总被引：1，自引：0，他引：1  
The Deep Impact probe collided with 9P Tempel 1 at an angle of about 30° from the horizontal. This impact angle produced an evolving ejecta flow field very similar to much smaller scale oblique-impact experiments in porous particulate targets in the laboratory. Similar features and phenomena include a decoupled vapor/dust plume at the earliest times, a pronounced downrange bias of the ejecta, an uprange “zone of avoidance” (ZoA), heart-shaped ejecta ray system (cardioid pattern), and a conical (but asymmetric) ejecta curtain. Departures from nominal cratering evolution, however, provide clues on the nature of the impact target. These departures include: fainter than expected flash at first contact, delayed emergence of the self-luminous vapor/dust plume, uprange-directed plume, narrow early-time uprange ray followed by a late-stage uprange plume, persistence of ejecta asymmetries (and the uprange ZoA) throughout the approach sequence, emergence of a downrange ZoA at late times, detachment of uprange curved rays, very long lasting non-radial ejecta rays, and high-angle ejecta plume lasting over the entire encounter. The first second of crater formation most closely resembles the consequences of a highly porous target, while later evolution indicates that the target may be layered as well. Experiments also reveal that impacts into highly porous targets produce a vapor/dust plume directed back up the incoming trajectory. This uprange plume is attributed to cavitation within a narrow penetration funnel. The observed lateral expansion speed of the initial vapor plume downrange provides an estimate for the total vaporized mass equal to ∼5mp (projectile masses) of water ice or 6mp of CO2. The downrange plume speed is consistent with the gas expansion added to the downrange horizontal component of the DI probe. Based on high-speed spectroscopy of experimental impacts, the observed delay in brightening of the DI plume may be the result of delayed condensation of carbon, in addition to silicates. Observed molecular species in the initial self-luminous vapor plume likely represent recombination products from completely dissociated target materials. The crater produced by the impact can be estimated from Earth-based observations of total ejected mass to be 130-220 m in diameter. This size range is consistent with a 220 m-diameter circular feature at the point of impact visible in highly processed, deconvolved HRI images. The final crater, however, may resemble an inverted sombrero-hat, with a deep central pit surrounded by a shallow excavation crater. Excavated distal material observed from the Earth was likely from the upper few meters contrasted with ballistic ejecta observed from the DI flyby, which included deep materials (10-30 m) within the diffuse plume above the crater and shallower (5-10 m) materials within the ejecta curtain.  相似文献   

16.
The Lonar impact crater, India, is one of the few known terrestrial impact craters excavated in continental basaltic target rocks (Deccan Traps, ~65 Ma). The impactites reported from the crater to date mainly include centimeter‐ to decimeter‐sized impact‐melt bombs, and aerodynamically shaped millimeter‐ and submillimeter‐sized impact spherules. They occur in situ within the ejecta around the crater rim and show schlieren structure. In contrast, non–in situ glassy objects, loosely strewn around the crater lake and in the ejecta around the crater rim do not show any schlieren structure. These non–in situ fragments appear to be similar to ancient bricks from the Daityasudan temple in the Lonar village. Synthesis of existing and new major and trace element data on the Lonar impact spherules show that (1) the target Lonar basalts incorporated into the spherules had undergone minimal preimpact alteration. Also, the paleosol layer as preserved between the top‐most target basalt flow and the ejecta blanket, even after the impact, was not a source component for the Lonar impactites, (2) the Archean basement below the Deccan traps were unlikely to have contributed material to the impactite parental melts, and (3) the impactor asteroid components (Cr, Co, Ni) were concentrated only within the submillimeter‐sized spherules. Two component mixing calculations using major oxides and Cr, Co, and Ni suggest that the Lonar impactor was a EH‐type chondrite with the submillimeter‐sized spherules containing ~6 wt% impactor components.  相似文献   

17.
Abstract— Terrestrial impact structures provide field evidence for cratering processes on planetary bodies that have an atmosphere and volatiles in the target rocks. Here we discuss two examples that may yield implications for Martian craters: 1. Recent field analysis of the Ries crater has revealed the existence of subhorizontal shear planes (detachments) in the periphery of the crater beneath the ejecta blanket at 0.9–1.8 crater radii distance. Their formation and associated radial outward shearing was caused by weak spallation and subsequent dragging during deposition of the ejecta curtain. Both processes are enhanced in rheologically layered targets and in the presence of fluids. Detachment faulting may also occur in the periphery of Martian impacts and could be responsible for the formation of lobe‐parallel ridges and furrows in the inner layer of double‐layer and multiple‐layer ejecta craters. 2. The ejecta blanket of the Chicxulub crater was identified on the southeastern Yucatán Peninsula at distances of 3.0–5.0 crater radii from the impact center. Abundance of glide planes within the ejecta and particle abrasion both rise with crater distance, which implies a ground‐hugging, erosive, and cohesive secondary ejecta flow. Systematic measurement of motion indicators revealed that the flow was deviated by a preexisting karst relief. In analogy with Martian fluidized ejecta blankets, it is suggested that the large runout was related to subsurface volatiles and the presence of basal glide planes, and was influenced by eroded bedrock lithologies. It is proposed that ramparts may result from enhanced shear localization and a stacking of ejecta material along internal glide planes at decreasing flow rates when the flow begins to freeze below a certain yield stress.  相似文献   

18.
Observations of high resolution photographs of part of one of the prominent rays of the lunar crater Copernicus show that there is a concentration of small bright rayed and haloed craters within the ray. These craters contribute to the overall ray brightness; they have been measured and their surface distribution has been mapped. Sixty-two percent of the bright craters can be identified from study of high resolution photographs as concentric impact craters. These craters contain in their ejecta blankets, rocks from the lunar substrate that are brighter than the adjacent mare surface. It is concluded that the brightness of the large ray from the crater Copernicus is due to the composite effect of many small concentric impact craters with rocky ejecta blankets. If this is the dominant mechanism for the production of other rays from Copernicus and other large lunar craters, then rays may not contain significant amounts of ejecta from the central crater or from large secondary craters. They may in fact only reflect local excavation of mare substrate material by myriads of small secondary or tertiary impact craters.  相似文献   

19.
Abstract– The Lonar crater in Maharashtra state, India, has been completely excavated on the Deccan Traps basalt (approximately 65 Ma) at approximately 570 ± 47 ka by an oblique impact of a possible chondritic asteroid that struck the preimpact target from the east at an angle of approximately 30–45o to the horizon where the total duration of the shock event was approximately 1 s. It is shown by our early work that the distribution of ejecta and deformation of target rocks around the crater rim are symmetrical to the east–west plane of impact ( Misra et al. 2010 ). The present study shows that some of the rock magnetic properties of these shocked target basalts, e.g., low‐field anisotropy of magnetic susceptibility (AMS), natural remanent magnetization (NRM)/bulk susceptibility (χ), and high‐coercivity and high‐temperature (HC_HT) magnetization component, are also almost symmetrically oriented with reference to the plane of impact. Studies on the relative displacements of K3 (minimum) AMS axes of shocked basalts from around the crater rim and from the adjacent target rocks to the approximately 2–3 km west of the crater center suggest that the impact stress could have branched out into the major southwestward and northwestward components in the downrange direction immediately after the impact. The biaxial distribution of AMS axes in stereographic plots for the unshocked basalts transforms mostly into triaxial distribution for the shocked basalts, although transitional type distribution also exists. The degree of anisotropy (P′) of AMS ellipsoids of the shocked basalts decreases by approximately 2% when compared with those of the unshocked target (approximately 1.03). The NRM/χ (Am?1) values of the shocked basalts on the rim of the Lonar crater do not show much change in the uprange or downrange direction on and close to the east–west plane of impact, and the values are only approximately 1.5 times higher on average over the unshocked basalts around the crater. However, the values become approximately 1.4–16.4 times higher for the shocked basalts on the crater rim, which occur obliquely to the plane of impact. The target basalts at approximately 2–3 km west of the crater center in the downrange also show a significant increase (up to approximately 26 times higher) in NRM/χ. The majority of the shocked basalt samples (approximately 73%) from around the crater rim, in general, show a lowering of REM, except those from approximately 2–3 km west of the crater center in the downrange, where nearly half of the sample population shows a higher REM of approximately 3.63% in average. The shocked target basalts around the Lonar crater also acquired an HC_HT magnetization component due to impact. These HC_HT components are mostly oriented in the uprange direction and are symmetrically disposed about the east–west plane of impact, making an obtuse angle with the direction of impact. The low‐coercivity and low‐temperature (LC_LT) components of both the unshocked and shocked basalts are statistically identical to the present day field (PDF) direction. This could be chemical and/or viscous remanent magnetization acquired by the target basalts during the last 570 ± 47 ka, subsequent to the formation of the Lonar crater. The shocked Lonar target basalts appear to have remagnetized under high impact shock pressure and at low temperature of approximately 200–300 °C, where Ti‐rich titanomagnetite was the main magnetic remanence carrier.  相似文献   

20.
Abstract— Impact cratering is an important geological process on Mars and the nature of Martian impact craters may provide important information as to the volatile content of the Martian crust. Terrestrial impact structures currently provide the only ground‐truth data as to the role of volatiles and an atmosphere on the impact‐cratering process. Recent advancements, based on studies of several well‐preserved terrestrial craters, have been made regarding the role and effect of volatiles on the impact‐cratering process. Combined field and laboratory studies reveal that impact melting is much more common in volatile‐rich targets than previously thought, so impact‐melt rocks, melt‐bearing breccias, and glasses should be common on Mars. Consideration of the terrestrial impact‐cratering record suggests that it is the presence or absence of subsurface volatiles and not the presence of an atmosphere that largely controls ejecta emplacement on Mars. Furthermore, recent studies at the Haughton and Ries impact structures reveal that there are two discrete episodes of ejecta deposition during the formation of complex impact craters that provide a mechanism for generating multiple layers of ejecta. It is apparent that the relative abundance of volatiles in the near‐surface region outside a transient cavity and in the target rocks within the transient cavity play a key role in controlling the amount of fluidization of Martian ejecta deposits. This study shows the value of using terrestrial analogues, in addition to observational data from robotic orbiters and landers, laboratory experiments, and numerical modeling to explore the Martian impact‐cratering record.  相似文献   

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