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1.
The location, size, and principal characteristics of the currently known proven and probable terrestrial impact structures are tabulated. Of the 78 known probable structures, only 3 are Precambrian and the majority are <300 my in age. A survey of the variation in preservation with size and age indicates that, unless protected by sedimentary cover, a structure <20 km in diameter has a recognizable life of <600 my. The depth-diameter relationships of terrestrial structures are similar to lunar craters; however, it is believed that terrestrial craters were always shallower than their lunar counterparts. Complex structures formed in sedimentary targets are shallower than those in crystalline targets, and the transition from simple to complex crater morphology occurs in sedimentary strata at approximately one-half the diameter of the morphology transition in crystalline rocks. This is a reflection of target strength. Although observations indicate that crater size, target strength, and surface gravity are variables in the formation of complex craters, they do not permit an unequivocal choice between collapse and rebound processes for the formation of complex structures. It may be that both processes act together in the modification of crater morphology during the later stages of excavation. The major emphasis of recent shock metamorphic studies has been toward the development of models of cratering processes. An important contribution has been the identification, through meteoritic contamination in the melt rocks, of the type of bolide at a number of probable impact structures. This has served to strengthen the link between the occurrence of shock metamorphic effects and their origin by hypervelocity meteorite impact. 相似文献
2.
Abstract— It has been known for some time that the volume of impact melt (Vm) relative to that of the transient cavity (Vtc) increases with the magnitude of the impact event. This paper investigates the influence that this phenomenon has on the nature of terrestrial impact craters. A model of impact melting is used to estimate the volume of melt produced during the impact of chondritic projectiles into granite targets at velocities of 15, 25, and 50 km S?1. The dimensions of transient cavities formed under the same impact conditions are calculated from current crater-scaling relationships, which are derived from dimensional analysis of data from cratering experiments. Observed melt volumes at terrestrial craters are collated from the literature and are paired with the transient-cavity diameters (Dtc) of their respective craters; these diameters were determined through an established empirical relationship. The model and observed melt volumes have very similar trends with increasing transient-cavity diameter. This Vm-Dtc relationship is then used to make predictions regarding the nature of the terrestrial cratering record. In particular, with increasing size of the impact event, the depth of melting approaches the depth of the transient cavity. As a consequence, the base of the cavity, which ultimately would appear as an uplifted central structure in a complex crater, will record shock stresses that will increase up to a maximum of partial melting. Examination of the terrestrial record indicates a general trend for higher recorded shock levels in central structures at larger diameters; impact structures in the 100-km size range record partially melted and vesiculated parautochthonous target rocks in their centers. In addition, as the depth of melting approaches a depth equivalent to that attained by the base of the transient cavity, the floor of the transient cavity will have progressively less strength, with the result that cavity modification and uplift will not produce topographic central peaks. Again, the observed terrestrial record is not inconsistent with this prediction, and we offer differential melt scaling as a possible mechanism for the transition from central topographic peaks to rings with increasing crater diameter. Among other implications is the likelihood that impact basins in the 1000-km size range on the early Earth would not have the same multi-ring form as observed on the moon. 相似文献
3.
The terrestrial impact record contains currently ~145 structures and includes the morphological crater types observed on the other terrestrial planets. It has, however, been severely modified by terrestrial geologic processes and is biased towards young (≤ 200 Ma) and large (≥ 20 km) impact structures on relatively well-studied cratonic areas. Nevertheless, the ground-truth data available from terrestrial impact structures have provided important constraints for the current understanding of cratering processes. If the known sample of impact structures is restricted to a subsample in which it is believed that all structures ≥ 20 km in diameter (D) have been discovered, the estimated terrestrial cratering rate is 5.5±2.7 × 10?15km?2a?1 for D ≥ 20 km. This rate estimate is equivalent to that based on astronomical observations of Earth-crossing bodies. These rates are a factor of two higher, however, than the estimated post-mare cratering rate on the moon but the large uncertainties preclude definitive conclusions as to the significance of this observation. Statements regarding a periodicity in the terrestrial cratering record based on time-series analyses of crater ages are considered unjustified, based on statistical arguments and the large uncertainties attached to many crater age estimates. Trace element and isotopic analyses of generally siderophile group elements in impact lithologies, particularly impact melt rocks, have provided the basis for the identification of impacting body compositions at a number of structures. These range from meteoritic class, e.g., C-1 chondrite, to tentative identifications, e.g., stone?, depending on the quality and quantity of analytical data. The majority of the identifications indicate chondritic impacting bodies, particularly with respect to the larger impact structures. This may indicate an increasing role for cometary impacts at larger diameters; although, the data base is limited and some identifications are equivocal. To realize the full potential of the terrestrial impact record to constrain the character of the impact flux, it will be necessary to undertake additional and systematic isotopic and trace element analyses of impact lithologies at well-characterized terrestrial impact structures. 相似文献
4.
The terrestrial impact record contains currently ~145 structures and includes the morphological crater types observed on the other terrestrial planets. It has, however, been severely modified by terrestrial geologic processes and is biased towards young ( 200 Ma) and large ( 20 km) impact structures on relatively well-studied cratonic areas. Nevertheless, the ground-truth data available from terrestrial impact structures have provided important constraints for the current understanding of cratering processes. If the known sample of impact structures is restricted to a subsample in which it is believed that all structures 20 km in diameter (D) have been discovered, the estimated terrestrial cratering rate is 5.5±2.7 × 10–15km–2a–1 for D 20 km. This rate estimate is equivalent to that based on astronomical observations of Earth-crossing bodies. These rates are a factor of two higher, however, than the estimated post-mare cratering rate on the moon but the large uncertainties preclude definitive conclusions as to the significance of this observation. Statements regarding a periodicity in the terrestrial cratering record based on time-series analyses of crater ages are considered unjustified, based on statistical arguments and the large uncertainties attached to many crater age estimates. Trace element and isotopic analyses of generally siderophile group elements in impact lithologies, particularly impact melt rocks, have provided the basis for the identification of impacting body compositions at a number of structures. These range from meteoritic class, e.g., C-1 chondrite, to tentative identifications, e.g., stone?, depending on the quality and quantity of analytical data. The majority of the identifications indicate chondritic impacting bodies, particularly with respect to the larger impact structures. This may indicate an increasing role for cometary impacts at larger diameters; although, the data base is limited and some identifications are equivocal. To realize the full potential of the terrestrial impact record to constrain the character of the impact flux, it will be necessary to undertake additional and systematic isotopic and trace element analyses of impact lithologies at well-characterized terrestrial impact structures. 相似文献
5.
The terrestrial cratering record for the Phanerozoic has a size-frequency distribution of NαD?2.05 for D > 22.6 km and NαD?0.24 for D < 11.3 km. This shallowing of the distribution slope at D > 22.6 km reflects the removal of small terrestrial craters by erosion. The number of large craters on the North American and East European cratons provide estimated terrestrial crater production rates for D > 20 km of 0.36 ± 0.1 and 0.33 ± 0.2 × 10?14 km?2 year?1, respectively. These rates are in good agreement with previous estimates and astronomical observations on Apollo bodies. Comparisons with the lunar rate, taking account of the effects of variations in impact velocity, surface gravity, and gravitational cross section, indicate that the lunar and terrestrial rates overlap, if the cratering flux has been constant during the last 3.4 by. If the early (pre 4.0 by) high-flux rate did not decay to a constant value until 3.0 to 2.5 by then the rates differ by a factor of 2 and the Phanerozoic can be interpreted as a period of higher than normal cratering. 相似文献
6.
Clark R. Chapman 《Icarus》1974,22(3):272-291
Computerized cratering-obliteration models are developed for use in interpreting planetary surface histories in terms of the diameter-frequency relations for craters classified by morphology. An application is made to a portion of the lunar uplands, revealing several episodes of blanketing, presumably due to the formation of some of the major basins.Application to Martian craters leads to the following picture of Martian cratering and obliteration history. During a probable period of intense early bombardment, craters were degraded by two processes: a depositional-type process connected with the declining cratering rate, and a process tending to flatten the largest craters (e.g., isostatic adjustment). During late stages of the early bombardment, or subsequent to it, there occurred a major relative episode of obliteration (probably atmosphere related), but it ceased concurrently with the massive (presumably volcanic) resurfacing of the cratered plains. Subsequent resurfacing episodes have occurred in the smooth plain terrains, but obliteration processes have been virtually absent in the low-latitude cratered terrains.Recent global Martian cratering interpretations of Hartmann and Soderblom are compared. Absolute cratering chronologies are only so good as knowledge of the absolute cratering flux on Mars. The crater data of Arvidson, Mutch, and Jones do not confirm the basis, whereby Soderblom requires the dominant Martian crater obliteration process to be coincident in time with the early bombardment. If the asteroidal-cometary impact flux on Mars has averaged five times the lunar flux during post-lunar-mare epochs, then the obliterative episode lasted about half a billion years and occurred about 1.5 × 109 yr ago. 相似文献
7.
Gordon R. Osinski 《Meteoritics & planetary science》2006,41(10):1571-1586
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
8.
The observed record of impact craters on the surface of the planet Venus can be used to calculate the contribution of fine materials generated by impact processes to the global sedimentary cycle. Using various methods for the extending the population of impact craters with diameters larger than 8 km observed on the northern 25% of the Venus to the entire surface area of the planet, we have estimated how materials ejected from the integrated record of impact cratering over the past 0.5 to 1.0 æ might have been globally distributed. Relationships for computing the fraction of ejected materials from impact craters in a given size range originally developed for the Moon (and for terrestrial nuclear explosion cratering experiments) were scaled for Venus conditions, and the ejecta fragments with sizes less than 30 m were considered to represent those with the greatest potential for global transport and eventual fallout. A similar set of calculations were carried out using the observed terrestrial cratering record, corrected for the missing population of small craters and oceanic impacts that have either been eroded or are unobserved (due to water cover). Our calculations suggest that both Venus and the Earth should have experienced approximately 6000 impact events over the past 0.5 to 1 æ (in the size range from 1 km to about 180 km). The cumulative global thickness of impact-derived fine materials that could have produced from this record of impacts in this time period is most likely between 1–2 mm for Venus, and certainly no more than 6 mm (assuming an enhanced population of large 150–200 km scale impact events). For Earth, the global cumulative thickness is most likely 0.2 to 0.3 mm, and certainly no more than 2 to 3 mm. The cumulative volume of impact ejecta (independent of particle size) for Venus generated over the past 1 æ, when spread out over the global surface area to form a uniform layer, would fall between 2 and 12 meters, although 99% of this material would be deposited in the near rim ejecta blanket (from 1 to 2.3 crater radii from the rim crest), and only 0.02% would be available for global transport as dust-sized particles. Thus, our conclusion is that Venus, as with the Earth, cannot have formed a substantial impact-derived regolith layer over the past billion years of its history as is typical for smaller silicate planets such as the Moon and Mercury. This conclusion suggests that there must be other extant mechanisms for sediment formation and redistribution in the Venus environment, on the basis of Venera Lander surface panoramas which demonstrate the occurrence of local sediment accumulations.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci. Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT, Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena). 相似文献
9.
Abstract— Asteroids tens to hundreds of meters in diameter constitute the most immediate impact hazard to human populations, yet the rate at which they arrive at Earth's surface is poorly known. Astronomic observations are still incomplete in this size range; impactors are subjected to disruption in Earth's atmosphere, and unlike the Moon, small craters on Earth are rapidly eroded. In this paper, we first model the atmospheric behavior of iron and stony bodies over the mass range 1–1012 kg (size range 6 cm‐1 km) taking into account deceleration, ablation, and fragmentation. Previous models in meteoritics deal with rather small masses (<105–106 kg) with the aim of interpreting registered fireballs in atmosphere, or with substantially larger objects without taking into account asteroid disruption to model cratering processes. A few earlier attempts to model terrestrial crater strewn fields did not take into account possible cascade fragmentation. We have performed large numbers of simulations in a wide mass range, using both the earlier “pancake” models and also the separated fragments model to develop a statistical picture of atmosphere‐bolide interaction for both iron and stony impactors with initial diameters up to ?1 km. Second, using a compilation of data for the flux at the upper atmosphere, we have derived a cumulative size‐frequency distribution (SFD) for upper atmosphere impactors. This curve is a close fit to virtually all of the upper atmosphere data over 16 orders of magnitude. Third, we have applied our model results to scale the upper atmosphere curve to a flux at the Earth's surface, elucidating the impact rate of objects <1 km diameter on Earth. We find that iron meteorites >5 times 104 kg (2.5 m) arrive at the Earth's surface approximately once every 50 years. Iron bodies a few meters in diameter (105–106 kg), which form craters ?100 m in diameter, will strike the Earth's land area every 500 years. Larger bodies will form craters 0.5 km in diameter every 20,000 years, and craters 1 km in diameter will be formed on the Earth's land area every 50,000 years. Tunguska events (low‐level atmospheric disruption of stony bolides >108 kg) may occur every 500 years. Bodies capable of producing hazardous tsunami (?200 m diameter projectiles) should strike the Earth's surface every ?100,000 years. This data also allows us to assess the completeness of the terrestrial crater record for a given area over a given time interval. 相似文献
10.
Abstract— Environmental conditions on Mars are conducive to the modification and erosion of impact craters, potentially revealing the nature of their substructure. On Earth, postimpact erosion of complex craters in a wide range of target rocks has revealed the nature and distribution of craterrelated fault structures and a complex array of breccia and pseudotachylyte dikes, which range up to tens of meters in width and tens of kilometers in length. We review the characteristics of fault structures, breccia dikes, and pseudotachylyte dikes on Earth, showing that they occur in complex network‐like patterns and are often offset along late‐stage crater‐related faults. Individual faults and dikes can undulate in width and can branch and bifurcate along strike. Detailed geological analyses of terrestrial craters show that faults and breccia dikes form during each of the major stages of the impact‐cratering process (compression, excavation, and modification). We report here on the discovery of prominent, lattice‐like ridge networks occurring on the floor of a highly modified impact crater 75 km in diameter near the dichotomy boundary of the northern lowland and southern upland. Interior fill and crater‐floor units have been exhumed by fluvial and eolian processes to reveal a unit below the crater floor containing a distinctive set of linear ridges of broadly similar width and forming a lattice‐like pattern. Ridge exposures range from ?1–4 km in length and ?65–120 m in width, are broadly parallel, straight to slightly curving, and are cross‐cut by near‐orthogonal ridges, forming a box or lattice‐like pattern. Ridges are exposed on the exhumed crater floor, extending from the base of the wall toward the center. On the basis of the strong similarities of these features to terrestrial crater‐related fault structures and breccia dikes, we interpret these ridges to be faults and breccia dikes formed below the floor of the crater during the excavation and modification stages of the impact event, and subsequently exhumed by erosion. The recognition of such features on Mars will help in documenting the nature of impact‐cratering processes and aid in assessment of crustal structure. Faults and breccia dikes can also be used as data for the assessment of post‐cratering depths and degrees of landform exhumation. 相似文献
11.
Studies of the internal structure of asteroids, which are crucial for understanding their impact history and for hazard mitigation, appear to be in conflict for the S-type asteroids, Eros, Gaspra, and Ida. Spacecraft images and geophysical data show that they are fractured, coherent bodies, whereas models of catastrophic asteroidal impacts, family and satellite formation, and studies of asteroid spin rates, and other diverse properties of asteroids and planetary craters suggest that such asteroids are gravitationally bound aggregates of rubble. These conflicting views may be reconciled if 10-50 km S-type asteroids formed as rubble piles, but were later consolidated into coherent bodies. Many meteorites are breccias that testify to a long history of impact fragmentation and consolidation by alteration, metamorphism, igneous and impact processes. Ordinary chondrites, which are the best analogs for S asteroids, are commonly breccias. Some may have formed in cratering events, but many appear to have formed during disruption and reaccretion of their parent asteroids. Some breccias were lithified during metamorphism, and a few were lithified by injected impact melt, but most are regolith and fragmental breccias that were lithified by mild or moderate shock, like their lunar analogs. Shock experiments show that porous chondritic powders can be consolidated during mild shock by small amounts of silicate melt that glues grains together, and by friction and pressure welding of silicate and metallic Fe,Ni grains. We suggest that the same processes that converted impact debris into meteorite breccias also consolidated asteroidal rubble. Internal voids would be partly filled with regolith by impact-induced seismic shaking. Consolidation of this material beneath large craters would lithify asteroidal rubble to form a more coherent body. Fractures on Ida that were created by antipodal impacts and are concentrated in and near large craters, and small positive gravity anomalies associated with the Psyche and Himeros craters on Eros, are consistent with this concept. Spin data suggest that smaller asteroids 0.6-6 km in size are unconsolidated rubble piles. C-type asteroids, which are more porous than S-types, and their analogs, the volatile-rich carbonaceous chondrites, were probably not lithified by shock. 相似文献
12.
Nonuniform cratering of the Moon and a revised crater chronology of the inner Solar System 总被引：1，自引：0，他引：1
We model the cratering of the Moon and terrestrial planets from the present knowledge of the orbital and size distribution of asteroids and comets in the inner Solar System, in order to refine the crater chronology method. Impact occurrences, locations, velocities and incidence angles are calculated semi-analytically, and scaling laws are used to convert impactor sizes into crater sizes. Our approach is generalizable to other moons or planets. The lunar cratering rate varies with both latitude and longitude: with respect to the global average, it is about 25% lower at (±65°N, 90°E) and larger by the same amount at the apex of motion (0°N, 90°W) for the present Earth-Moon separation. The measured size-frequency distributions of lunar craters are reconciled with the observed population of near-Earth objects under the assumption that craters smaller than a few kilometers in diameter form in a porous megaregolith. Varying depths of this megaregolith between the mare and highlands is a plausible partial explanation for differences in previously reported measured size-frequency distributions. We give a revised analytical relationship between the number of craters and the age of a lunar surface. For the inner planets, expected size-frequency crater distributions are calculated that account for differences in impact conditions, and the age of a few key geologic units is given. We estimate the Orientale and Caloris basins to be 3.73 Ga old, and the surface of Venus to be 240 Ma old. The terrestrial cratering record is consistent with the revised chronology and a constant impact rate over the last 400 Ma. Better knowledge of the orbital dynamics, crater scaling laws and megaregolith properties are needed to confidently assess the net uncertainty of the model ages that result from the combination of numerous steps, from the observation of asteroids to the formation of craters. Our model may be inaccurate for periods prior to 3.5 Ga because of a different impactor population, or for craters smaller than a few kilometers on Mars and Mercury, due to the presence of subsurface ice and to the abundance of large secondaries, respectively. Standard parameter values allow for the first time to naturally reproduce both the size distribution and absolute number of lunar craters up to 3.5 Ga ago, and give self-consistent estimates of the planetary cratering rates relative to the Moon. 相似文献
13.
Gordon R. OSINSKI Johanna KIENIEWICZ Jennifer R. SMITH Mark B. E. BOSLOUGH Mark ECCLESTON Henry P. SCHWARCZ Maxine R. KLEINDIENST Albert F. C. HALDEMANN Charles S. CHURCHER 《Meteoritics & planetary science》2008,43(12):2089-2107
Abstract— Impact cratering is a ubiquitous geological process on the terrestrial planets. Meteorite impact craters are the most visible product of impact events, but there is a growing recognition that large aerial bursts or airbursts should occur relatively frequently throughout geological time. In this contribution, we report on an unusual impact glass‐the Dakhleh Glass (DG)–which is distributed over an area of ?400 km 2of the Dakhleh Oasis, Egypt. This region preserves a rich history of habitation stretching back to over 400,000 years before the emergence of Homo sapiens. We report on observations made during recent fieldwork and subsequent analytical analyses that strengthen previous suggestions that the DG formed during an impact event. The wide distribution and large size of DG specimens (up to ?50 cm across), the chemistry (e.g., CaO and Al2O3 contents up to ?25 and ?18 wt, respectively), the presence of lechatelierite and burnt sediments, and the inclusion of clasts and spherules in the DG is inconsistent with known terrestrial processes of glass formation. The age and other textural characteristics rule out a human origin. Instead, we draw upon recent numerical modeling of airbursts to suggest that the properties of DG, coupled with the absence of a confirmed crater, can best be explained by melting of surficial sediments as a result of a large airburst event. We suggest that glass produced by such events should, therefore, be more common in the rock record than impact craters, assuming that the glass formed in a suitable preserving environment. 相似文献
14.
Michael H. POELCHAU Thomas KENKMANN Klaus THOMA Tobias HOERTH Anja DUFRESNE Frank SCHÄFER 《Meteoritics & planetary science》2013,48(1):8-22
Abstract– The MEMIN research unit (Multidisciplinary Experimental and Modeling Impact research Network) is focused on analyzing experimental impact craters and experimental cratering processes in geological materials. MEMIN is interested in understanding how porosity and pore space saturation influence the cratering process. Here, we present results of a series of impact experiments into porous wet and dry sandstone targets. Steel, iron meteorite, and aluminum projectiles ranging in size from 2.5 to 12 mm were accelerated to velocities of 2.5–7.8 km s?1, yielding craters with diameters between 3.9 and 40 cm. Results show that the target’s porosity reduces crater volumes and cratering efficiency relative to nonporous rocks. Saturation of pore space with water to 50% and 90% increasingly counteracts the effects of porosity, leading to larger but flatter craters. Spallation becomes more dominant in larger‐scale experiments and leads to an increase in cratering efficiency with increasing projectile size for constant impact velocities. The volume of spalled material is estimated using parabolic fits to the crater morphology, yielding approximations of the transient crater volume. For impacts at the same velocity these transient craters show a constant cratering efficiency that is not affected by projectile size. 相似文献
15.
B. A. Ivanov 《Solar System Research》2005,39(5):381-409
Multi-ring impact basins have been found on the surfaces of almost all planetary bodies in the Solar system with solid crusts. The details of their formation mechanism are still unclear. We present results of our numerical modeling of the formation of the largest known terrestrial impact craters. The geological and geophysical data on these structures accumulated over many decades are used to place constraints on the parameters of available numerical models with a dual purpose: (i) to choose parameters in available mechanical models for the crustal response of planetary bodies to a large impact and (ii) to use numerical modeling to refine the possible range of original diameters and the morphology of partially eroded terrestrial craters. We present numerical modeling results for the Vredefort, Sudbury, Chicxulub, and Popigai impact craters and compare these results with available geological and geophysical information. 相似文献
16.
Plagioclase feldspar is one of the most abundant minerals on the surface of the Earth, the Moon, and Mars, and is also commonly found in meteorites. Studying shock effects in feldspar thus provides us with fundamental information about impact cratering processes on planetary bodies. In this study, plagioclase from monomict and polymict breccias, impact melt rocks, and shock‐metamorphosed target rocks, from throughout the Mistastin Lake impact structure, Canada, was examined using 514 nm laser Raman spectroscopy. As one of the very few impact structures with anorthosite in the target rocks, the Mistastin Lake impact structure provides a unique opportunity to study shocked plagioclase displaying progressive shock metamorphic features. A series of microscopic features was observed within plagioclase, including twins, needle‐like inclusions, planar features, and alteration. The lack of planar deformation features is notable. Raman spectra of these features suggest that this technique is capable of differentiating and classifying shock features in low to moderately shocked rocks. Caution should be exercised, however, as Raman spectra collected from unshocked plagioclase references with known compositions indicate that peak width and peak ratio of the Raman peaks in lower wave number region (<350 cm?1) and the main signature peaks around 500 cm?1 vary with chemical composition and crystal orientation. Data collected from diaplectic glass suggest that Raman features are efficient in distinguishing crystalline plagioclase and diaplectic glass. We also observed significant variations in the Raman intensities collected from diaplectic glass, which we ascribe to the localized disorder or inhomogeneity of shock pressure and temperature throughout the target. 相似文献
17.
W. U. Reimold 《Earth, Moon, and Planets》1995,70(1-3):21-45
Only since several decades has impact cratering been recognized as an important surface process on all planetary bodies in the Solar System. However, as the process has not yet been effectively introduced into geological curricula, it is necessary to inform a wider public about its importance for (i) planetary formation and (ii) evolution, (iii) the understanding of this process as a geological process, (iv) the terrestrial impact crater record and its limitations, and (v) the recognition criteria for terrestrial impact structures, as well as (vi) the need of improvement of the impact cratering record in the light of the potential danger of an impact catastrophe on this planet. It is, particularly for developing countries, of interest to examine the economic and educational-environmental potential of impact structures. That it is possible to carry out an effective, low-budget geological investigation of impact structures within a Second World environment is demonstrated by the discussion of the progress that has been made in recent years with regard to the Southern African impact crater record. Several recommendations on how to improve, on the one hand, the terrestrial impact crater record and, on the other, their general working situation by activation of workers in Developing Countries are discussed. 相似文献
18.
Abstract— Hypervelocity impact involves the near instantaneous transfer of considerable energy from the impactor to a spatially limited near‐surface volume of the target body. Local geology of the target area tends to be of secondary importance, and the net result is that impacts of similar size on a given planetary body produce similar results. This is the essence of the utility of observations at impact craters, particularly terrestrial craters, in constraining impact processes. Unfortunately, there are few well‐documented results from systematic contemporaneous campaigns to characterize specific terrestrial impact structures with the full spectrum of geoscientific tools available at the time. Nevertheless, observations of the terrestrial impact record have contributed substantially to fundamental properties of impact. There is a beginning of convergence and mutual testing of observations at terrestrial impact structures and the results of modeling, in particular from recent hydrocode models. The terrestrial impact record provides few constraints on models of ejecta processes beyond a confirmation of the involvement of the local substrate in ejecta lithologies and shows that Z‐models are, at best, first order approximations. Observational evidence to date suggests that the formation of interior rings is an extension of the structural uplift process that occurs at smaller complex impact structures. There are, however, major observational gaps and cases, e.g., Vredefort, where current observations and hydrocode models are apparently inconsistent. It is, perhaps, time that the impact community as a whole considers documenting the existing observational and modeling knowledge gaps that are required to be filled to make the intellectual breakthroughs equivalent to those of the 1970s and 1980s, which were fueled by observations at terrestrial impact structures. Filling these knowledge gaps would likely be centered on the later stages of formation of complex and ring structures and on ejecta. 相似文献
19.
We use conventional numerical integrations to assess the fates of impact ejecta in the Saturn system. For specificity we consider impact ejecta launched from four giant craters on three satellites: Herschel on Mimas, Odysseus and Penelope on Tethys, and Tirawa on Rhea. Speeds, trajectories, and size of the ejecta are consistent with impact on a competent surface (“spalls”) and into unconsolidated regolith. We do not include near-field effects, jetting, or effects peculiar to highly oblique impact. Ejecta are launched at velocities comparable to or exceeding the satellite's escape speed. Most ejecta are swept up by the source moon on time-scales of a few to several decades, and produce craters no larger than 19 km in diameter, with typical craters in the range of a few km. As much as 17% of ejecta reach satellites other than the source moon. Our models generate cratering patterns consistent with a planetocentric origin of most small impact craters on the saturnian icy moons, but the predicted craters tend to be smaller than putative Population II craters. We conclude that ejecta from the known giant craters in the saturnian system do not fully account for Population II craters. 相似文献
20.
We estimate the impact flux and cratering rate as a function of latitude on the terrestrial planets using a model distribution of planet crossing asteroids and comets [Bottke, W.F., Morbidelli, A., Jedicke, R., Petit, J.-M., Levison, H.F., Michel, P., Metcalfe, T.S., 2002. Icarus 156, 399-433]. After determining the planetary impact probabilities as a function of the relative encounter velocity and encounter inclination, the impact positions are calculated analytically, assuming the projectiles follow hyperbolic paths during the encounter phase. As the source of projectiles is not isotropic, latitudinal variations of the impact flux are predicted: the calculated ratio between the pole and equator is 1.05 for Mercury, 1.00 for Venus, 0.96 for the Earth, 0.90 for the Moon, and 1.14 for Mars over its long-term obliquity variation history. By taking into account the latitudinal dependence of the impact velocity and impact angle, and by using a crater scaling law that depends on the vertical component of the impact velocity, the latitudinal variations of the cratering rate (the number of craters with a given size formed per unit time and unit area) is in general enhanced. With respect to the equator, the polar cratering rate is about 30% larger on Mars and 10% on Mercury, whereas it is 10% less on the Earth and 20% less on the Moon. The cratering rate is found to be uniform on Venus. The relative global impact fluxes on Mercury, Venus, the Earth and Mars are calculated with respect to the Moon, and we find values of 1.9, 1.8, 1.6, and 2.8, respectively. Our results show that the relative shape of the crater size-frequency distribution does not noticeably depend upon latitude for any of the terrestrial bodies in this study. Nevertheless, by neglecting the expected latitudinal variations of the cratering rate, systematic errors of 20-30% in the age of planetary surfaces could exist between equatorial and polar regions when using the crater chronology method. 相似文献