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1.
The surface of Venus viewed in Arecibo radar images has a small population of bright ring-shaped features. These features are interpreted as the rough or blocky deposits surrounding craters of impact or volcanic origin. Population densities of these bright ring features are small compared with visually identified impact craters on the surface of the Moon and volcanic craters on Io. However, they are comparable to the short-lived radar-bright haloes associated with ejecta deposits of young craters on the Moon. This suggests that bright radar signatures of the deposits around Venusian craters are obliterated by an erosional or sedimentary process. We have evaluated the hypothesis that bright radar crater signatures were obliterated by a global mantle deposited after impacts of very large bolides. The mechanism accounts satisfactorily for the population of features with internal diameters greater than 64 km. The measured population of craters with internal diameters between 32 and 64 km is difficult to account for with the model but it may be underestimated because of poor radar resolution (5 to 20 km). Other possible mechanisms for the removal of radar bright crater signatures include in situ chemical weathering of rocks and mantling by young volcanic deposits. All three alternatives may be consistent with existing radar roughness and cross-section data and Venera 8, 9, and 10 data. However, imaging observations from a lander on the rolling plains or lowlands may verify or disprove the proposed global mantling. New high-resolution ground-based radar data can also contribute new information on the nature and origin of these radar bright ring features. 相似文献
2.
Terrestrial ring dike structures are features consisting of one or more series of concentric fracture systems along which the central block often subsided and up through which lavas intruded and extruded and other volcanic features formed. Before the lunar probe satellites, a search for lunar features that showed characteristics of terrestrial ring dikes was conducted using the LAC charts andKuiper Atlas photographs. More recently the search was extended on the nearside features and to the farside features using the Lunar Orbiter series of photographs resulting in a catalog of 559 nearside candidates and 82 farside. Features exhibiting one or more of the following four criteria were included as lunar analogs to terrestrial ring dikes: (1) inner ridge(s) approximately concentric with the crater wall, (2) inner rill(s) approximately concentric with the crater wall, (3) outer ridge(s) and/or rill(s) approximately concentric with the crater wall, and (4) interior and exterior slopes of the crater wall approximately equal (implying extrusion of lava along a ring fracture). Equal slopes are in contradistinction to a central source eruptive feature or an impact feature both of which usually produce craters with walls whose inner slopes are about twice as steep as their outer flanks, which characterize the vast majority of lunar craters. Features exhibiting each of the four criteria were found and some had combinations of two or more including rills merging into ridges, e.g., in Taruntius and Posidonius. Gambart is an example of equal inner and outer slopes, while Hesiodus A and Marth are two of the best examples of complete inner rings concentric with the outer rings. Ten percent of the candidates were probable impact craters but had subsequent volvanic activity of a ring dike nature. The initial search showed a distribution of the possible lunar ring dikes that was non-random and strongly associated with the margins of the maria, further implying that they are volcanic features. This relation was upheld when extended by the recent survey. The anticipated dearth of farside ring dikes was corroborated in our study and their distribution is restricted to those few mare-like areas on the farside, further supporting the volcanic nature of these features 相似文献
3.
Radar imaging results for Mercury's non-polar regions are presented. The dual-polarization, delay-Doppler images were obtained from several years of observations with the upgraded Arecibo S-band (λ12.6-cm) radar telescope. The images are dominated by radar-bright features associated with fresh impact craters. As was found from earlier Goldstone-VLA and pre-upgrade Arecibo imaging, three of the most prominent crater features are located in the Mariner-unimaged hemisphere. These are: “A,” an 85-km-diameter crater (348° W, 34° S) whose radar ray system may be the most spectacular in the Solar System; “B,” a 95-km-diameter crater (343° W, 58° N) with a very bright halo but less distinct ray system; and “C,” an irregular feature with bright ejecta and rays distributed asymmetrically about a 125-km source crater (246° W, 11° N). Due south of “C” lies a “ghost” feature (242° W, 27° S) that resembles “A” but is much fainter. An even fainter such feature is associated with Bartok Crater. These may be two of the best mercurian examples of large ejecta/ray systems observed in an intermediate state of degradation. Virtually all of the bright rayed craters in the Mariner 10 images show radar rays and/or bright rim rings, with radar rays being less common than optical rays. Radar-bright craters are particularly common in the H-7 quadrangle. Some diffuse radar albedo variations are seen that have no obvious association with impact ejecta. In particular, some smooth plains regions such as the circum-Caloris plains in Tir, Budh, and Sobkou Planitiae and the interiors of Tolstoj and “Skinakas” basins show high depolarized brightness relative to their surroundings, which is the reverse of the mare/highlands contrast seen in lunar radar images. Caloris Basin, on the other hand, appears dark and featureless in the images. 相似文献
4.
Images of Eros from the NEAR Shoemaker spacecraft reveal bright and dark albedo features on steep crater walls unlike markings previously observed on asteroids. These features have been attributed to the downslope movement of space-weathered regolith, exposing less weathered material (Science 292 (2001) 484; Meteor. Planet. Sci. 36 (2001) 1617; Icarus 155 (2002) 145). Here we present observations of the interiors of large craters (>1 km in diameter) to test this hypothesis and constrain the origin of the features. We find that bright regions in these craters correspond to steep slopes, consistent with previous work. The geographic distribution of craters with albedo variations shows no pattern and does not resemble the distribution of ponds, another phenomenon on Eros attributed to regolith movement. Shadows and other indications of topography are not observed at feature boundaries, implying that the transported layer is ?1 m thick. The presence of multiple bright and dark units on long slopes with sharp boundaries between them suggests that mobilized regolith may be halted by frictional or other effects before reaching the foot of the slope. Features on crater walls should darken at the same rate as bright ejecta deposits from crater formation; the lack of observed, morphologically fresh craters with bright interiors or ejecta suggests that the albedo patterns are younger than the most recently formed craters greater than about 100 m in diameter. Smaller or micrometeorite impacts, which would not necessarily leave evident deposits of bright ejecta, remain possible causes of albedo patterns. Although their effectiveness is difficult to assess, electrostatic processes and thermal creep are also candidates. 相似文献
5.
M. S. Robinson P. C. Thomas J. Veverka S. L. Murchie B. B. Wilcox 《Meteoritics & planetary science》2002,37(12):1651-1684
Abstract— The global high‐resolution imaging of asteroid 433 Eros by the Near‐Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft has made it possible to develop the first comprehensive picture of the geology of a small S‐type asteroid. Eros displays a variety of surface features, and evidence of a substantial regolith. Large scale facets, grooves, and ridges indicate the presence of at least one global planar structure. Directional and superposition relations of smaller structural features suggest that fracturing has occurred throughout the object. As with other small objects, impact craters dominate the overall shape as well as the small‐scale topography of Eros. Depth/diameter ratios of craters on Eros average ～0.13, but the freshest craters approach lunar values of ～0.2. Ejecta block production from craters is highly variable; the majority of large blocks appear to have originated from one 7.6 km crater (Shoemaker). The interior morphology of craters does not reveal the influence of discrete mechanical boundaries at depth in the manner of craters formed on lunar mare regolith and on some parts of Phobos. This lack of mechanical boundaries, and the abundant evidence of regolith in nearly every high‐resolution image, suggests a gradation in the porosity and fracturing with depth. The density of small craters is deficient at sizes below ～200 m relative to predicted slopes of empirical saturation. This characteristic, which is also found on parts of Phobos and lunar highland areas, probably results from the efficient obliteration of small craters on a body with significant topographic slopes and a thick regolith. Eros displays a variety of regolith features, such as debris aprons, fine‐grained “ponded” deposits, talus cones, and bright and dark streamers on steep slopes indicative of efficient downslope movement of regolith. These processes serve to mix materials in the upper loose fragmental portion of the asteroid (regolith). In the instance of “ponded” materials and crater wall deposits, there is evidence of processes that segregate finer materials into discrete deposits. The NEAR observations have shown us that surface processes on small asteroids can be very complex and result in a wide variety of morphologic features and landforms that today seem exotic. Future missions to comets and asteroids will surely reveal still as yet unseen processes as well as give context to those discovered by the NEAR Shoemaker spacecraft. 相似文献
6.
Richard E. D&#x;Alli 《Icarus》1977,31(1):146-156
Mariner 9 photographs of the southern hemisphere of Mars taken during the 1971 planet-wide dust storm display circular bright spots at a time when all near-surface features were totally obscured. Correlating the positions and diameters of these spots with topography shows that they correspond to craters. About half of all the large craters in thestudy area were brightened. The associated craters are large and flat-floored, have significant rim uplift, and contain dark splotches on their floors. The depth/diameter relationship of the bright spot craters is comparable to that of a planet-wide sample. Depth may not be important in selectively brightening certain craters. The visibility of bright spots in A-camera photographs is strongly dependent on the wavelength of the filter used during exposure. It is proposed that bright spots result from the multiple scattering of incident light in dust clouds entrained within craters during dust storms. The appearance of the dust clouds is a function of the availability of a dust supply and, perhaps, air turbulence generated by winds flowing over upraised rims and rough crater floors. Bright spots persist during the final stage of the planet-wide dust storm. If bright spots are dust clouds, this persistence demonstrates that crater interiors are the last regions of clearing. 相似文献
7.
Raymond E. Arvidson 《Icarus》1974,21(1):12-27
Crater morphology and size play a major role in determining whether wind-blown streaks emanating from craters or dark splotches within craters will form. Both bright and dark streaks emanate almost exclusively from bowl-shaped craters. Dark splotches are found mainly in flat-floored craters, especially those that are deep and have high rim relief. Trends of dark splotches in the northern to southern midlatitudes closely follow those of bright streaks, suggesting both were formed by similar winds. In the high southern latitudes, on the other hand, dark splotch trends closely follow those of dark streaks.Qualitative models of streak and splotch formation have been derived from these data and results of Sagan et al. (1972, 1973). Bright streaks probably form by trapping and simultaneous streaming of bright dust downwind. Dark splotched craters in regions with bright streaks usually have upwind bright patches, suggesting these features form by dumping of bright dust over crater rims with some minor redistribution of dark materials toward the downwind sides of craters. Data are consistent with dark streaks forming by erosion or nondeposition of bright material or by trapping of dark material. Dark splotches in these regions are probably mainly the result of trapping of dark sand in the downwind sides of crater floors. Craters with dark splotches and dark streaks are usually rimless and shallow. This is consistent with ponded dark sands easily washing over crater walls and extending downwind.Plots of streak length versus crater diameter suggest a complex history of streak formation for most regions.Bright streak trends and latitudinal distributions are consistent with return flow of dust to the southern hemisphere. Some dark streaks may be direct relics of passing sand and dust storms. Trends of dark streaks and splotches away from the south pole are consistent with the spreading of a debris mantle from the polar regions toward the equator. 相似文献
8.
Veronica J. BRAY Gareth S. COLLINS Joanna V. MORGAN Paul M. SCHENK 《Meteoritics & planetary science》2008,43(12):1979-1992
Abstract— We examine the morphology of central peak craters on the Moon and Ganymede in order to investigate differences in the near‐surface properties of these bodies. We have extracted topographic profiles across craters on Ganymede using Galileo images, and use these data to compile scaling trends. Comparisons between lunar and Ganymede craters show that crater depth, wall slope and amount of central uplift are all affected by material properties. We observe no major differences between similar‐sized craters in the dark and bright terrain of Ganymede, suggesting that dark terrain does not contain enough silicate material to significantly increase the strength of the surface ice. Below crater diameters of ?12 km, central peak craters on Ganymede and simple craters on the Moon have similar rim heights, indicating comparable amounts of rim collapse. This suggests that the formation of central peaks at smaller crater diameters on Ganymede than the Moon is dominated by enhanced central floor uplift rather than rim collapse. Crater wall slope trends are similar on the Moon and Ganymede, indicating that there is a similar trend in material weakening with increasing crater size, and possibly that the mechanism of weakening during impact is analogous in icy and rocky targets. We have run a suite of numerical models to simulate the formation of central peak craters on Ganymede and the Moon. Our modeling shows that the same styles of strength model can be applied to ice and rock, and that the strength model parameters do not differ significantly between materials. 相似文献
9.
We present an updated survey of Mercury’s putative polar ice deposits, based on high-resolution (1.5-km) imaging with the upgraded Arecibo S-band radar during 1999-2005. The north pole has now been imaged over a full range of longitude aspects, making it possible to distinguish ice-free areas from radar-shadowed areas and thus better map the distribution of radar-bright ice. The new imagery of the south pole, though derived from only a single pair of dates in 2005, improves on the pre-upgrade Arecibo imagery and reveals many additional ice features. Some medium-size craters located within 3° of the north pole show near-complete ice coverage over their floors, central peaks, and southern interior rim walls and little or no ice on their northern rim walls, while one large (90 km) crater at 85°N shows a sharp ice-cutoff line running across its central floor. All of this is consistent with the estimated polar extent of permanent shading from direct sunlight. Some craters show ice in regions that, though permanently shaded, should be too warm to maintain unprotected surface ice owing to indirect heating by reflected and reradiated sunlight. However, the ice distribution in these craters is in good agreement with models invoking insulation by a thin dust mantle. Comparisons with Goldstone X-band radar imagery indicate a wavelength dependence that could be consistent with such a dust mantle. More than a dozen small ice features have been found at latitudes between 67° and 75°. All of this low-latitude ice is probably sheltered in or under steep pole-facing crater rim walls, although, since most is located in the Mariner-unimaged hemisphere, confirmation must await imaging by the MESSENGER orbiter. These low-latitude features are concentrated toward the “cold longitudes,” possibly indicating a thermal segregation effect governed by indirect heating. The radar imagery places the corrected locations of the north and south poles at 7°W, 88.35°N and 90°W, 88.7°S, respectively, on the original Mariner-based maps. 相似文献
10.
A survey of craters in the vicinity of Newton Basin, using high-resolution images from Mars Global Surveyor and Mars Odyssey, was conducted to find and analyze examples of gullies and arcuate ridges and assess their implications for impact crater degradation processes. In the Phaethontis Quadrangle (MC-24), we identified 225 craters that contain these features. Of these, 188 had gullies on some portion of their walls, 118 had arcuate ridges at the bases of the crater walls, and 104 contained both features, typically on the same crater wall. A major result is that the pole-facing or equator-facing orientation of these features is latitude dependent. At latitudes >44° S, equator-facing orientations for both ridges and gullies are prevalent, but at latitudes <44° S, pole-facing orientations are prevalent. The gullies and arcuate ridges typically occupy craters between ∼2 and 30 km in diameter, at elevations between −1 and 3 km. Mars Orbiter Laser Altimeter (MOLA) elevation profiles indicate that most craters with pole-facing arcuate ridges have floors sloping downward from the pole-facing wall, and some of these craters show asymmetry in crater rim heights, with lower pole-facing rims. These patterns suggest viscous flow of ice-rich materials preferentially away from gullied crater walls. Clear associations exist between gullies and arcuate ridges, including (a) geometric congruence between alcoves and sinuous arcs of arcuate ridges and (b) backfilling of arcuate ridges by debris aprons associated with gully systems. Chronologic studies suggest that gullied walls and patterned crater floor deposits have ages corresponding to the last few high obliquity cycles. Our data appear consistent with the hypothesis that these features are associated with periods of ice deposition and subsequent erosion associated with obliquity excursions within the last few tens of millions of years. Arcuate ridges may form from cycles of activity that also involve gully formation, and the ridges may be in part due to mass-wasted, ice-rich material transported downslope from the alcoves, which then interacts with previously emplaced floor deposits. Most observed gullies may be late-stage features in a degradational cycle that may have occurred many times on a given crater wall. 相似文献
11.
T. W. Thompson W. J. Roberts W. K. Hartmann R. W. Shorthill S. H. Zisk 《Earth, Moon, and Planets》1979,21(3):319-342
Radar, infrared, and photogeologic properties of lunar craters have been studied to determine whether there is a systematic difference in blocky craters between the maria and terrae and whether this difference may be due to a deep megaregolith of pulverized material forming the terra surface, as opposed to a layer of semi-coherent basalt flows forming the mare surface. Some 1310 craters from about 4 to 100 km diameter have been catalogued as radar and/or infrared anomalies. In addition, a study of Apollo Orbital Photography confirmed that the radar and infrared anomalies are correlated with blocky rubble around the crater.Analysis of the radar and infrared data indicated systematic terra—mare differences. Fresh terra craters smaller than 12 km were less likely to be infrared and radar anomalies than comparable mare craters: but terra and mare craters larger than 12 km had similar infrared and radar signatures. Also, there are many terra craters which are radar bright but not infrared anomalies.Our interpretation of these data is that while the maria are rock layers (basaltic flow units) where craters eject boulder fields, the terrae are covered by relatively pulverized megaregolith at least 2 km deep, where craters eject less rocky rubble. Blocky rubble, either in the form of actual rocks or partly consolidated blocks, contributes to the radar and infrared signatures of the crater. However, aging by impacts rapidly destroys these effects, possibly through burial by secondary debris or by disintegration of the blocks themselves, especially in terra regions.PSI Contribution No. 110. 相似文献
12.
The origin of lunar crater rays 总被引：5，自引：0，他引：5
Lunar rays are filamentous, high-albedo deposits occurring radial or subradial to impact craters. The nature and origin of lunar rays have long been the subjects of major controversies. We have determined the origin of selected lunar ray segments utilizing Earth-based spectral and radar data as well as FeO, TiO2, and optical maturity maps produced from Clementine UVVIS images. These include rays associated with Tycho, Olbers A, Lichtenberg, and the Messier crater complex. It was found that lunar rays are bright because of compositional contrast with the surrounding terrain, the presence of immature material, or some combination of the two. Mature “compositional” rays such as those exhibited by Lichtenberg crater, are due entirely to the contrast in albedo between ray material containing highlands-rich primary ejecta and the adjacent dark mare surfaces. “Immaturity” rays are bright due to the presence of fresh, high-albedo material. This fresh debris was produced by one or more of the following: (1) the emplacement of immature primary ejecta, (2) the deposition of immature local material from secondary craters, (3) the action of debris surges downrange of secondary clusters, and (4) the presence of immature interior walls of secondary impact craters. Both composition and state-of-maturity play a role in producing a third (“combination”) class of lunar rays. The working distinction between the Eratosthenian and Copernican Systems is that Copernican craters still have visible rays whereas Eratosthenian-aged craters do not. Compositional rays can persist far longer than 1.1 Ga, the currently accepted age of the Copernican-Eratosthenian boundary. Hence, the mere presence of rays is not a reliable indication of crater age. The optical maturity parameter should be used to define the Copernican-Eratosthenian boundary. The time required for an immature surface to reach the optical maturity index saturation point could be defined as the Copernican Period. 相似文献
13.
The origin of the multiple concentric rings that characterize lunar impact basins, and the probable depth and diameter of the transient crater have been widely debated. As an alternative to prevailing “megaterrace” hypotheses, we propose that the outer scarps or mountain rings that delineate the topographic rims of basins—the Cordilleran at Orientale, the Apennine at Imbrium, and the Altai at Nectaris—define the transient cavities, enlarged relatively little by slumping, and thus are analogous to the rim crests of craters like Copernicus; inner rings are uplifted rims of craters nested within the transient cavity. The magnitude of slumping that occurs on all scarps is insufficient to produce major inner rings from the outer. These conclusions are based largely on the observed gradational sequence in lunar central uplifts:. from simple peaks through somewhat annular clusters of peaks, peak and ring combinations and double ring basins, culminating in multiring structures that may also include peaks. In contrast, belts of slump terraces are not gradational with inner rings. Terrestrial analogs suggest two possible mechanisms for producing rings. In some cases, peaks may expand into rings as material is ejected from their cores, as apparently occurred at Gosses Bluff, Australia. A second process, differential excavation of lithologically diverse layers, has produced nested experimental craters and is, we suspect, instrumental in the formation of terrestrial ringed impact craters. Peak expansion could produce double-ring structures in homogeneous materials, but differential excavation is probably required to produce multiring and peak-in-ring configurations in large lunar impact structures. Our interpretation of the representative lunar multiring basin Orientale is consistent with formation of three rings in three layers detected seismically in part of the Moon—the Cordillera (basin-bounding) ring in the upper crust, the composite Montes Rook ring in the underlying, more coherent “heald” crust, and an innermost, 320-km ring at the crust-mantle interface. Depth-diameter ratios of are consistent with this interpretation and suggest that volumes of transient cavities and hence of basin ejecta may be considerably greater than commonly assumed. 相似文献
14.
In November, 1981, NASA's first Shuttle Imaging Radar mission (SIR-A) began producing maplike photographic strips of Earth scenes from orbital altitude. A Saharan radar image acquired over Algeria clearly delineates two sedimentary basins, Erg Occidental and Erg Oriental, separated by an elongated zone of exposed bedrock, the M'Zab Chebka. At the NE margin of the Chebka, rimrocks, slopes, and ejecta deposits of Talemzane meteorite impact crater appear as a distinct two km wide radar-bright ring. This unique circle of strong radar backscatter distinguishes the solitary impact structure from numerous dayas (similarly appearing karstic depressions) which characterize the region. The crater is prominant on radar, but is obscure on optically obtained satellite and aircraft images, as are partly buried fluvial drainage systems and fault-block traces developed in bedrocks of the Chebka. Radar detection of an annular drainage system indicates possible presence of a ring graben at the crater. Brightest radar signals on the image are cultural features at recently developed gas fields near Hassi er R'Mel. 相似文献
15.
Floor-fractured lunar craters 总被引：1，自引：0，他引：1
Peter H. Schultz 《Earth, Moon, and Planets》1976,15(3-4):241-273
Numerous lunar craters (206 examples, mean diameter = 40km) contain pronounced floor rilles (fractures) and evidence for volcanic processes. Seven morphologic classes have been defined according to floor depth and the appearance of the floor, wall, and rim zones. Such craters containing central peaks exhibit peak heights (approximately 1km) comparable to those within well-preserved impact craters but exhibit smaller rim-peak elevation differences (generally 0–1.5km) than those (2.4km) within impact craters. In addition, the morphology, spatial distribution, and floor elevation data reveal a probable genetic association with the maria and suggest that a large number of floor-fractured craters represent pre-mare impact craters whose floors have been lifted tectonically and modified volcanically during the epochs of mare flooding. Floor uplift is envisioned as floating on an intruded sill, and estimates of the buoyed floor thickness are consistent with the inferred depth of brecciation beneath impact craters, a zone interpreted as a trap for the intruding magma. The derived model of crater modification accounts for (1) the large differences in affected crater size and age; (2) the small peak-rim elevation differences; (3) remnant central peaks within mare-flooded craters and ringed plains; (4) ridged and flat-topped rim profiles of heavily modified craters and ringed plains; and (5) the absence of positive gravity anomalies in most floor-fractured craters and some large mare-filled craters. One of the seven morphologic classes, however, displays a significantly smaller mean size, larger distances from the maria, and distinctive morphology relative to the other six classes. The distinctive morphology is attributed, in part, to the relatively small size of the affected crater, but certain members of this class represent a style of volcanism unrelated to the maria - perhaps triggered by the last major basin-forming impacts. 相似文献
16.
Material is ejected from impact craters in ballastic trajectories; it impacts first near the crater rim and then at progressively greater ranges. Ejecta from craters smaller than approximately 1 km is laid predominantly on top of the surrounding surface. With increasing crater size, however, more and more surrounding surface will be penetrated by secondary cratering action and these preexisting materials will be mixed with primary crater ejecta. Ejecta from large craters and especially basin forming events not only excavate preexisting, local materials, but also are capable of moving large amounts of material away from the crater. Thus mixing and lateral transport give rise to continuous deposits that contain materials from within and outside the primary crater. As a consequence ejecta of basins and large highland craters have eroded and mixed highland materials throughout geologic time and deposited them in depressions inside and between older crater structures.Because lunar mare surfaces contain few large craters, the mare regolith is built up by successive layers of predominantly primary ejecta. In contrast, the lunar highlands are dominated by the effects of large scale craters formed early in lunar history. These effects lead to thick fragmental deposits which are a mixture of primary crater material and local components. These deposits may also properly be named regolith though the term has been traditionally applied only to the relatively thin fine grained surficial deposit on mare and highland terranes generated during the past few billion year. We believe that the surficial highland regolith - generated over long periods of time - rests on massive fragmental units that have been produced during the early lunar history. 相似文献
17.
The composition of small areas within three large lunar highland craters are compared using near-infrared, Earth-based telescopic data. Spectra from many areas in the walls and central peaks of the craters indicate the presence of crystalline components which include olivine, two types of pyroxene, and sometimes Fe-bearing feldspar. Spectra from other regions suspected of being impact melt deposits, located on the floors and walls of the three craters, have remarkably similar, yet anomalous features. These features are interpreted to indicate the presence of pyroxene, Fe-bearing glass, and Fe-bearing feldspar. Pyroxene and feldspar are believed to occur primarily in the form of lithic clasts and rapidly recrystallized impact melt. The Fe-bearing glass is interpreted as impact melt glass. 相似文献
18.
Structural uplift and ejecta thickness of lunar mare craters: New insights into the formation of complex crater rims 
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下载免费PDF全文 We investigate the elevated crater rims of lunar craters. The two main contributors to this elevation are a structural uplift of the preimpact bedrock and the emplacement of ejecta on top of the crater rim. Here, we focus on five lunar complex mare craters with diameters ranging between 16 and 45 km: Bessel, Euler, Kepler, Harpalus, and Bürg. We performed 5281 measurements to calculate precise values for the structural rim uplift and the ejecta thickness at the elevated crater rim. The average structural rim uplift for these five craters amounts to SRU = 70.6 ± 1.8%, whereas the ejecta thickness amounts to ET = 29.4 ± 1.8% of the total crater rim elevation. Erosion is capable of modifying the ratio of ejecta thickness to structural rim uplift. However, to minimize the impact of erosion, the five investigated craters are young, pristine craters with mostly preserved ejecta blankets. To quantify how strongly craters were enlarged by crater modification processes, we reconstructed the dimensions of the transient crater. The difference between the transient crater diameter and the final crater diameter can extend up to 11 km. We propose reverse faulting and thrusting at the final crater rim to be one of the main contributing factors of forming the elevated crater rim. 相似文献
19.
Peter Mouginis-Mark 《Icarus》1981,45(1):60-76
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
20.
Verne R. Oberbeck 《Earth, Moon, and Planets》1971,2(3):263-278
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