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1.
Double-layered ejecta (DLE) craters are distinctive among the variety of crater morphologies observed on Mars, but the mechanism by which they form remains under debate. We assess two ejecta emplacement mechanisms: (1) atmospheric effects from ejecta curtain-induced vortices or a base surge and (2) ballistic emplacement followed by a landslide of ejecta assisted by either surface- or pore-ice. We conduct a morphological analysis of the ejecta facies for three DLE craters which impacted into irregular pre-existing topography. We find that the unique topographic environments affected the formation of grooves and the inner facies, and thus appear to be inconsistent with an atmospheric-effects origin but are supportive of the landslide hypothesis. We distinguish between the two landslide models (lubrication by either surface- or pore-ice) by assessing relationships between DLE crater ejecta and morphologic features indicative of buried ice deposits, including sublimation pits, ring-mold craters, expanded secondary craters, and excess ejecta craters. The association of DLE craters with these features suggests that surface ice was present at the time of the impacts that formed the DLE craters. We also compare the Froude numbers of DLE crater ejecta to landslides, and find that the ejecta of DLE craters are kinematically and frictionally similar to terrestrial landslides that overran glaciers. This suggests that the grooves on DLE craters may plausibly form through the same shear/splitting mechanism as the landslides. In summary, our analysis supports the hypothesis that DLE craters form through meteoroid impacts into decameters-thick surface ice deposits (emplaced during periods of higher obliquity) followed by ejecta sliding on the ice.  相似文献   

2.
Abstract— We have developed a quantitative model for predicting characteristics of ejecta deposits that result from basin‐sized cratering events. This model is based on impact crater scaling equations (Housen, Schmitt, and Holsapple 1983; Holsapple 1993) and the concept of ballistic sedimentation (Oberbeck 1975), and takes into account the size distribution of the individual fragments ejected from the primary crater. Using the model, we can estimate, for an area centered at the chosen location of interest, the average distribution of thicknesses of basin ejecta deposits within the area and the fraction of primary ejecta contained within the deposits. Model estimates of ejecta deposit thicknesses are calibrated using those of the Orientale Basin (Moore, Hodges, and Scott 1974) and of the Ries Basin (Hörz, Ostertag, and Rainey 1983). Observed densities of secondary craters surrounding the Imbrium and Orientale Basins are much lower than the modeled densities. Similarly, crater counts for part of the northern half of the Copernicus secondary cratering field are much lower than the model predicts, and variation in crater densities with distance from Copernicus is less than expected. These results suggest that mutual obliteration erases essentially all secondary craters associated with the debris surge that arises from the impacting primary fragments during ballistic sedimentation; if so, a process other than ballistic sedimentation is needed to produce observable secondary craters. Regardless, our ejecta deposit model can be useful for suggesting provenances of sampled lunar materials, providing information complementary to photogeological and remote sensing interpretations, and as a tool for planning rover traverses (e.g., Haskin et al. 1995, 2002).  相似文献   

3.
Before the Apollo 16 mission, the material of the Cayley Formation (a lunar smooth plains) was theorized to be of volcanic origin. Because Apollo 16 did not verify such interpretations, various theories have been published that consider the material to be ejecta of distant multiringed basins. Results presented in this paper indicate that the material cannot be solely basin ejecta. If smoothplains are a result of formation of these basins or other distant large craters, then the plains materials are mainly ejecta of secondary craters of these basins or craters with only minor contributions of primary-crater or basin ejecta. This hypothesis is based on synthesis of knowledge of the mechanics of ejection of material from impact craters, photogeologic evidence, remote measurements of surface chemistry, and petrology of lunar samples. Observations, simulations, and calculations presented in this paper show that ejecta thrown beyond the continuous deposits of large lunar craters produce secondary-impact craters that excavate and deposit masses of local material equal to multiples of that of the primary crater ejecta deposited at the same place. Therefore, the main influence of a large cratering event on terrain at great distances from such a crater is one of deposition of more material by secondary craters, rather than deposition of ejecta from the large crater. Examples of numerous secondary craters observed in and around the Cayley Formation and other smooth plains are presented. Evidence is given for significant lateral transport of highland debris by ejection from secondary craters and by landslides triggered by secondary impact. Primary-crater ejecta can be a significant fraction of a deposit emplaced by an impact crater only if the primary crater is nearby. Other proposed mechanisms for emplacement of smooth-plains formations are discussed, and implications regarding the origin of material in the continuous aprons surrounding large lunar craters is considered. It is emphasized that the importance of secondary-impact cratering in the highlands has in general been underestimated and that this process must have been important in the evolution of the lunar surface.  相似文献   

4.
A. Mantz  R. Sullivan  J. Veverka 《Icarus》2004,167(1):197-203
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.
We compare three previously independently studied crater morphologies - excess ejecta craters, perched craters, and pedestal craters - each of which has been proposed to form from impacts into an ice-rich surface layer. Our analysis identifies the specific similarities and differences between the crater types; the commonalities provide significant evidence for a genetic relationship among the morphologies. We use new surveys of excess ejecta and perched craters in the southern hemisphere in conjunction with prior studies of all of the morphologies to create a comprehensive overview of their geographic distributions and physical characteristics. From these analyses, we conclude that excess ejecta craters and perched craters are likely to have formed from the same mechanism, with excess ejecta craters appearing fresh while perched craters have experienced post-impact modification and infilling. Impacts that led to these two morphologies overwhelmed the ice-rich layer, penetrating into the underlying martian regolith, resulting in the excavation of rock that formed the blocky ejecta necessary to armor the surface and preserve the ice-rich deposits. Pedestal craters, which tend to be smaller in diameter, have the same average deposit thickness as excess ejecta and perched craters, and form in the same geographic regions. They rarely have ejecta around their crater rims, instead exhibiting a smooth pedestal surface. We interpret this to mean that they form from impacts into the same type of ice-rich paleodeposit, but that they do not penetrate through the icy surface layer, and thus do not generate a blocky ejecta covering. Instead, a process related to the impact event appears to produce a thin, indurated surface lag deposit that serves to preserve the ice-rich material. These results provide a new basis to identify the presence of Amazonian non-polar ice-rich deposits, to map their distribution in space and time, and to assess Amazonian climate history. Specifically, the ages, distribution and physical attributes of the crater types suggest that tens to hundreds of meters of ice-rich material has been episodically emplaced at mid latitudes in both hemispheres throughout the Amazonian due to obliquity-driven climate variations. These deposits likely accumulated more frequently in the northern lowlands, resulting in a larger population of all three crater morphologies in the northern hemisphere.  相似文献   

6.
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   

7.
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   

8.
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   

9.
Patrica Mutch  Alex Woronow 《Icarus》1980,41(2):259-268
Relationships between crater size and ejecta-blanket areal extents imply a maximum ejecta-blanket thickness or maximum rim height for Martian rampart craters. The limiting thickness is encountered only for craters exceeding 6 km diameter. Although smaller rampart craters have ejecta which appears to have undergone flow during emplacement, the larger craters have an additional component of flow, namely, internal flow of the ejecta caused by the greater weight of their thicker ejecta deposits. Pedestal craters most likely result from impacts into less volatile-rich substrates which produce a less fluidized ejecta and, consequently, no flow lobes.  相似文献   

10.
The spatial distribution of volatiles in the Martian hydrolithosphere   总被引：1，自引：0，他引：1  
In order to quantify the spatial distribution of volatiles on Mars, 2600 fluidized ejecta craters have been systematically measured, classified and mapped over the planet Mars, using 1 : 2 M scale USGS photomosaics. The latitudinal distribution of ejecta craters reveals that flower ejecta deposits (Type 1), together with low mobility ejecta, are frequently observed in the equatorial region and on ridged plains. Rampart craters (Type 2), with high mobility ejecta, occur at mid latitudes and exhibit a spatial relationship with polygonal patterns and pseudocrater areas. The increase of ejecta mobility with latitude attests for a concentration of volatiles at high latitudes. Statistical analysis shows that cratered uplands and ridged plains contain less volatile material near the surface than the underlying materials. In Chryse Planitia and Utopia Planitia the statistical study and the spatial relationships between polygonally fractured patterns, pseudocraters and the great number of high mobility ejecta deposits suggest the presence of a water-rich alluvial deposit close to the surface near the mouth of Chryse and Elysium channels. This result explains, on a more quantitative basis, the idea that fractured patterns were preferentially developed in a volatile-rich sedimentary deposits. The behaviour of volatiles, at 41 S, 257 W near Reull Vallis, exhibits a strong anomaly, with the presence of an abnormally volatile rich layer close to the surface.  相似文献   

11.
Palimpsests are large, circular, low-relief impact scars on Ganymede and Callisto. These structures were poorly understood based on Voyager-era analysis, but high-resolution Galileo images allow more detailed inspection. We analyze images of four Ganymedean palimpsests targeted by Galileo: Memphis and Buto Faculae, Epigeus, and Zakar. Ganymedean craters and Europan ring structures are used as tools to help better understand palimpsests, based on morphologic similarities. From analysis of Galileo images, palimpsests consist of four surface units: central plains, an unoriented massif facies, a concentric massif facies, and outer deposits. Using as a tie point the location in these structures where secondary craters begin to appear, outer deposits of palimpsests are analogous to the outer ejecta facies of craters; the concentric massif facies of palimpsests are analogous to the pedestal facies of craters; and the unoriented massif facies and central plains are analogous to crater interiors. These analogies are supported by the presence of buried preexisting structure beneath the outer two and absence of buried structure beneath the inner two units. Our observations indicate that palimpsest deposits represent fluidized impact ejecta, rather than cryovolcanic deposits or ancient crater interiors.  相似文献   

12.
Abstract— The geometry of simple impact craters reflects the properties of the target materials, and the diverse range of fluidized morphologies observed in Martian ejecta blankets are controlled by the near‐surface composition and the climate at the time of impact. Using the Mars Orbiter Laser Altimeter (MOLA) data set, quantitative information about the strength of the upper crust and the dynamics of Martian ejecta blankets may be derived from crater geometry measurements. Here, we present the results from geometrical measurements of fresh craters 3–50 km in rim diameter in selected highland (Lunae and Solis Plana) and lowland (Acidalia, Isidis, and Utopia Planitiae) terrains. We find large, resolved differences between the geometrical properties of the freshest highland and lowland craters. Simple lowland craters are 1.5–2.0 times deeper (≥5s?o difference) with >50% larger cavities (≥2s?o) compared to highland craters of the same diameter. Rim heights and the volume of material above the preimpact surface are slightly greater in the lowlands over most of the size range studied. The different shapes of simple highland and lowland craters indicate that the upper ?6.5 km of the lowland study regions are significantly stronger than the upper crust of the highland plateaus. Lowland craters collapse to final volumes of 45–70% of their transient cavity volumes, while highland craters preserve only 25–50%. The effective yield strength of the upper crust in the lowland regions falls in the range of competent rock, approximately 9–12 MPa, and the highland plateaus may be weaker by a factor of 2 or more, consistent with heavily fractured Noachian layered deposits. The measured volumes of continuous ejecta blankets and uplifted surface materials exceed the predictions from standard crater scaling relationships and Maxwell's Z model of crater excavation by a factor of 3. The excess volume of fluidized ejecta blankets on Mars cannot be explained by concentration of ejecta through nonballistic emplacement processes and/or bulking. The observations require a modification of the scaling laws and are well fit using a scaling factor of ?1.4 between the transient crater surface diameter to the final crater rim diameter and excavation flow originating from one projectile diameter depth with Z = 2.7. The refined excavation model provides the first observationally constrained set of initial parameters for study of the formation of fluidized ejecta blankets on Mars.  相似文献   

13.
Twenty-one lunar craters have radar bright ring appearances which are analogous to eleven complete ring features in the earth-based 12.5 cm observations of Venus. Radar ring diameters and widths for the lunar and Venusian features overlap for sizes from 45 to 100 km. Radar bright areas for the lunar craters are associated with the slopes of the inner and outer rim walls, while level crater floors and level ejecta fields beyond the raised portion of the rim have average radar backscatter. We propose that the radar bright areas of the Venusian rings are also associated with the slopes on the rims of craters.The lunar craters have evolved to radar bright rings via mass wasting of crater rim walls and via post impact flooding of crater floors. Aeolian deposits of fine-grained material on Venusian crater floors may produce radar scattering effects similar to lunar crater floor flooding. These Venusian aeolian deposits may preferentially cover blocky crater floors producing a radar bright ring appearance.We propose that the Venusian features with complete bright ring appearances and sizes less than 100 km are impact craters. They have the same sizes as lunar craters and could have evolved to radar bright rings via analogous surface processes.  相似文献   

14.
Hale crater formed in the Early to Middle Amazonian and is one of the best preserved large craters on Mars. We focus on the emplacement of previously mapped distal continuous ejecta and newly recognized discontinuous distal ejecta deposits reaching up to 450 km northeast of Hale. The distal continuous ejecta deposits are typically tens of meters thick, likely water-rich, and subsequent dewatering of some resulted in flow along gradients of 10 m km-1 for distances of tens of kilometers. The discontinuous distal ejecta are typically <10 m thick with volumes generally <0.5 km3 and embay Hale secondaries, which occur up to ~600 km from Hale. Both continuous and discontinuous distal ejecta deposits are typically smooth at scales of tens to hundreds of meters, relatively dark-toned, devoid of eolian bedforms, inferred to be mostly fine-grained, and were likely emplaced within hours to 1–2 days after impact. The occurrence of well-preserved discontinuous distal ejecta at Hale is unusual compared to other large Martian craters and could be due to impact into an ice-rich substrate that enabled their formation and (or) their survival after minimal postimpact degradation relative to older craters. The pristine nature of distal continuous and discontinuous distal deposits at Hale and the preservation of associated secondaries imply (1) low erosion rates after the Hale impact, comparable to those estimated elsewhere during the Amazonian; (2) the impact did not significantly influence long-term global or regional scale geomorphic activity or climate; and (3) the Hale impact occurred after late alluvial fan activity in Margaritifer Terra.  相似文献   

15.
The Luna-24 site is situated in Mare Crisium at a range of 18.4 km from Fahrenheit, an Eratosthenian-aged crater 6.4 km in diameter. Fahrenheit's ejecta deposits have been degraded to such an extent that secondary craters and rays cannot be unambiguously identified in the vicinity of the Luna-24 site. On the basis of an analogy between Fahrenheit and Lichtenberg B (a much younger crater of comparable size located in northern Oceanus Procellarum) Fahrenheit ejecta deposits near the sample site are inferred to have consisted of secondary crater clusters, subradially aligned secondary crater chains, and lineated terrain furrowed by fine-scale radial grooves. At the range of the Luna-24 site more than 80% of the mare surface should have been morphologically disturbed by the ballistic deposition of Fahrenheit ejecta. Blocks and fragment clusters of primary Fahrenheit ejecta ranging up to 5–20 m in diameter are inferred to have impacted the local surface at velocities of 165–230 m s–1 forming secondary craters ranging up to 100 m in diameter. The maximum depth of excavation of primary Fahrenheit ejecta deposited near the sample site is estimated to be at least 100 m. Primary Fahrenheit ejecta is expected to constitute a substantial fraction of the exterior deposits emplaced at the range of the Luna-24 site. Microgabbro and monomineralic fragments discovered in the Luna-24 drill core may have been derived from gabbroic rocks transported to the sample site by the Fahrenheit cratering event. This hypothesis is consistent with the widespread occurrence and characteristics of Fahrenheit ejecta anticipated in the vicinity of the Luan-24 site. Current interpretations of the drill core sample suggest that the Luna-24 regolith was deposited in its present configuration sometime during the last 0.3 AE implying that at least one local cratering event has occurred since the emplacement of Fahrenheit ejecta 2.0±0.5 AE ago.  相似文献   

16.
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   

17.
The morphologies of Tycho secondary craters and their ejecta deposits were studied using full-Moon, Lunar-Orbiter, and Apollo panoramic photographs. These data were compared with similar data for the secondary craters and light mantle of the Apollo 17 landing site. The results indicate that (1) the central crater cluster and the light mantle can be attributed to Tycho, (2) the dominant mechanism for emplacement of the light mantle was impact by secondary craters that threw material across the valley floor, and (3) level sheets of material may be emplaced locally by secondary impact. Analysis of returned samples confirms that secondary impacts rework mostly local material.  相似文献   

18.
Alex Woronow 《Icarus》1981,45(2):320-330
Measurements of extents of rampart ejecta deposits as a function of the size of the parent craters support models which, for craters larger than about 6 km diameter, constrain ejecta blankets to all have a similar maximum thickness regardless of the crater size. These volatile-rich ejecta blankets may have failed under their own weights, then flowed radially outward. Assuming this to be so, we can then determine some of the physicomechanical properties of the ejecta deposits at the time of their emplacement. Finite-element studies of the stress magnitudes, distributions, and directions in hypothetical Martian rampart ejecta blankets reveal that the material most likely failed when the shear stresses were less than 500 kPa and the angle of internal friction was between 26 and 36°. These figures imply that the ejecta has a water content between 16 and 72%. Whether the upper limit or the lower limit is more appropriate depends on the mode of failure which one presumes; namely, viscous flow or plastic deformation.  相似文献   

19.
Wind erosion seems to be the dominant process eroding crater ejecta deposits and sorrounding materials on Mars. In the equatorial zone, ejecta deposits are eroded back by scarp recession, where scarp heights appear to be approximately equivalent to ejecta thickness. In mantled areas, escarpments develop by relatively rapid deflation of sorrounding aeolian debris, leaving the ejecta deposit (continuous deposit and zone of high density of secondary craters) standing high above sorrounding terrain. If the rate of scarp recession is controlled by the rate of aeolian undercutting of escarpment bases, then recession rates may scale roughly as the inverse with respect to scarp height. Thus, preferential preservation of ejecta deposits emplaced in thickest aeolian debris may occur. An empirical model developed for wind erosion of ejecta deposits in nonmantled areas suggests that removal of ejecta materials on the average is exceedingly slow (～10?5m/yr for 10m high scarp). On the other hand, rapid deflation of aeolian debris around crater ejecta is implied. Results suggest high differential aeolian erosion rates that are a function of both grain sizes and large-scale surface roughness. Aeolian activity on Mars has probably been dominated by rapid recycling of fine-grained debris, the bulk of which formed under more erosive conditions prevalent in the early history of Mars.  相似文献   

20.
Cover          下载免费PDF全文
Cover: This oblique view of the lunar crater Pierazzo (3.3°N, 100.2°W, D≈9km) was taken by NASA’s Lunar Reconnaissance Orbiter Camera’s Narrow Angle Camera in late 2017. The camera was pointed off-nadir to provide this oblique view which, coupled with the moon’s curvature, provides an observation angle of 74°. This young crater features many large deposits of impact melt, typically dark material that is seen strewn throughout the image not only outside the crater (and is found over 40 km from the impact site), but in numerous deposits inside the crater. An extensive analysis of the impact melt was recently published by Veronica Bray et al. (2018, Icarus 201, p. 26–36). Small, bright splotches litter the ejecta and are mostly new craters that post-date the larger Pierazzo impact, though some might be caused by ejected blocks from the crater hitting its own ejecta. The crater is named in honor of Elisabetta (“Betty“) Pierazzo (1963–2011), who studied impact craters, including the production of impact melt material. We selected this image for the cover of this special issue because we think that it presents a good overview of this issue: rather than emphasizing any one study or type of paper in this special issue, it, at a simple glance, shows the force of an impact, the intriguing complexity inherent to their structure, and that even relatively young features are prone to modifi cation by the ongoing process of impact cratering. Credit: NASA/GSFC/ASU  相似文献   

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