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1.
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   

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

3.
The formation mechanism of layered ejecta craters on Mars has remained a topic of intense debate since their discovery. In this study, we perform a global morphological analysis of Martian layered ejecta craters using Thermal Emission Imaging System (THEMIS) images and Mars Orbiter Laser Altimeter (MOLA) data. The study focuses on the ejecta morphologies and well‐defined distal rampart characteristics associated with 9945 layered ejecta craters with a diameter greater than 1.5 km distributed across the entire Martian surface. Data analysis based on the new database provides new information on the distribution and morphological details of the three major layered ejecta morphologies (single layer ejecta [SLE], double layer ejecta [DLE], and multiple layer ejecta [MLE]). Global analysis is applied to the latitudinal distribution of characteristic parameters, including the ejecta mobility, lobateness values, and onset diameter. Our survey of the distribution and characteristics of layered ejecta craters reveals that strong correlations exist between ejecta mobility and latitude, and there is a latitudinal dependence of onset diameter. Our study of Martian layered ejecta craters provides more detailed information and insights of a connection between the layered ejecta morphologies and the subsurface volatiles.  相似文献   

4.
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   

5.
The relation between the size and velocity of impact crater ejecta has been studied by both laboratory experiments and numerical modeling. An alternative method, used here, is to analyze the record of past impact events, such as the distribution of secondary craters on planetary surfaces, as described by Vickery (Icarus 67 (1986) 224; Geophys. Res. Lett. 14 (1987) 726). We first applied the method to lunar images taken by the CLEMENTINE mission, which revealed that the size-velocity relations of ejecta from craters 32 and 40 km in diameter were similar to those derived by Vickery for a crater 39 km in diameter. Next, we studied the distribution of small craters in the vicinity of kilometer-sized craters on three images from the Mars Orbiter Camera (MOC) on board the Mars Global Surveyor (MGS). If these small craters are assumed to be secondaries ejected from the kilometer-sized crater in each image, the ejection velocities are of hundreds of meters per second. These data fill a gap between the previous results of Vickery and those of laboratory studies.  相似文献   

6.
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   

7.
Abstract— We are testing the idea of Squyres et al. (1992) that rampart craters on Mars may have formed over a significant time period and therefore the onset diameter (minimum diameter of a rampart crater) only reflects the ground ice depth at a given time. We measured crater size frequencies on the layered ejecta of rampart craters in three equatorial regions to derive absolute model ages and to constrain the regional volatile history. Nearly all rampart craters in the Xanthe Terra region are ?3.8 Gyr old. This corresponds to the Noachian fluvial activity that region. Rampart crater formation declines in the Hesperian, whereas onset diameters (minimum diameter) increase. No new rampart craters formed after the end of the Hesperian (?3 Gyr). This indicates a lowering of the ground ice table with time in the Xanthe Terra region. Most rampart craters in the Valles Marineris region are around 3.6 Gyr old. Only one large, probably Amazonian‐aged (?2.5 Gyr), rampart crater exists. These ages indicate a volatile‐rich period in the Early Hesperian and a lowering of the ground ice table with time in the Valles Marineris study region. Rampart craters in southern Chryse Planitia, which are partly eroded by fluvial activity, show ages around 3.9 Gyr. Rampart craters superposed on channels have ages between ?1.5 and ?0.6 Gyr. The onset diameter (3 km at ?1.5 Gyr) in this region may indicate a relatively shallow ground ice table. Loss of volatiles due to diffusion and sublimation might have lowered the ground ice table even in the southern Chryse Planitia region afterwards. In general, our study implies a formation of the smallest rampart craters within and/or shortly after periods of fluvial activity and a subsequent lowering of the ground ice table indicated by increasing onset diameter to the present. These results question the method to derive present equatorial ground ice depths from the onset diameter of rampart craters without information about their formation time.  相似文献   

8.
The about 10.5 km diameter Bosumtwi impact crater is one of the youngest large impact structures on Earth. The crater rim is readily noticed on topographic maps or in satellite imagery. It defines a circular basin filled by water (Lake Bosumtwi) and lacustrine sediments. The morphology of this impact structure is also characterized by a circular plateau extending beyond the rim and up to 9–10 km from the center of the crater (about 2 crater radii). This feature comprises a shallow ring depression, also described as an annular moat, and a subdued circular ridge at its outer edge. The origin of this outermost feature could so far not be elucidated based on remote sensing data only. Our approach combines detailed topographic analysis, including roughness mapping, with airborne radiometric surveys (mapping near‐surface K, Th, U concentrations) and field observations. This provides evidence that the moat and outer ring are features inherited from the impact event and represent the partially eroded ejecta layer of the Bosumtwi impact structure. The characteristics of the outer ridge indicate that ejecta emplacement was not purely ballistic but requires ejecta fluidization and surface flow. The setting of Bosumtwi ejecta can therefore be considered as a terrestrial analog for rampart craters, which are common on Mars and Venus, and also found on icy bodies of the outer solar system (e.g., Ganymede, Europa, Dione, Tethys, and Charon). Future studies at Bosumtwi may therefore help to elucidate the mechanism of formation of rampart craters.  相似文献   

9.
Pangboche crater (17.2°N, 226.7°E; 10.4 km dia.) lies close to the summit of Olympus Mons volcano, Mars, at an elevation of ~20.9 km above the datum. Given a scale height of 11.1 km for the atmosphere, this relatively large fresh crater most likely formed at an atmospheric pressure <1 mbar in essentially volatile‐free young lava flows. Detailed analysis of Pangboche crater from High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) images reveals that volatile‐related features (e.g., fluidized ejecta layers and pitted floor material) are absent. In contrast, abundant impact melt occurs on the floor, inner walls, and rim of the crater, and there is an extensive field of secondary craters that extend up to approximately 45 km from the rim crest. All of these attributes argue that it was the absence of volatiles in the target rocks at the time of crater formation, rather than the thin atmosphere, which had a controlling influence on crater morphology. Digital elevation data derived from the CTX images reveal that Pangboche crater has a depth of about 954 m (depth/diameter = approximately 0.092) and that uplifted target rocks comprise about 58% of the relief of the 180 m‐high north rim. As the target material comprised a sequence of layered lava flows, Pangboche crater may well represent the best crater on Mars for direct comparison with craters formed on the Moon (permitting variations in gravitational effects to be investigated) or on Mercury (allowing the role of the atmosphere to be studied).  相似文献   

10.
Abstract— The northern lowland plains, such as those found in Acidalia and Utopia Planitia, have high percentages of impact craters with fluidized ejecta. In both regions, the analysis of crater geometry from Mars Orbiter Laser Altimeter (MOLA) data has revealed large ejecta volumes, some exceeding the volume of excavation. Moreover, some of the crater cavities and fluidized ejecta blankets of these craters are topographically perched above the surrounding plains. These perched craters are concentrated between 40 and 70°N in the northern plains. The atypical high volumes of the ejecta and the perched craters suggest that the northern lowlands have experienced one or more episodes of resurfacing that involved deposition and erosion. The removal of material, most likely caused by the sublimation of ice in the materials and their subsequent erosion and transport by the wind, is more rapid on the plains than on the ejecta blankets. The thermal inertia difference between the ejecta and the surrounding plains suggests that ejecta, characterized by a lower thermal inertia, protect the underneath terrain from sublimation. This results in a decreased elevation of the plains relative to the ejecta blankets. Sublimation and eolian erosion can be particularly high during periods of high obliquity.  相似文献   

11.
Abstract— We have surveyed Martian impact craters greater than 5 km in diameter using Viking and thermal emission imaging system (THEMIS) imagery to evaluate how the planform of the rim and ejecta changes with decreasing impact angle. We infer the impact angles at which the changes occur by assuming a sin2θ dependence for the cumulative fraction of craters forming below angle θ. At impact angles less than ?40° from horizontal, the ejecta become offset downrange relative to the crater rim. As the impact angle decreases to less than ?20°, the ejecta begin to concentrate in the cross‐range direction and a “forbidden zone” that is void of ejecta develops in the uprange direction. At angles less than ?10°, a “butterfly” ejecta pattern is generated by the presence of downrange and uprange forbidden zones, and the rim planform becomes elliptical with the major axis oriented along the projectile's direction of travel. The uprange forbidden zone appears as a “V” curving outward from the rim, but the downrange forbidden zone is a straight‐edged wedge. Although fresh Martian craters greater than 5 km in diameter have ramparts indicative of surface ejecta flow, the ejecta planforms and the angles at which they occur are very similar to those for lunar craters and laboratory impacts conducted in a dry vacuum. The planforms are different from those for Venusian craters and experimental impacts in a dense atmosphere. We interpret our results to indicate that Martian ejecta are first emplaced predominantly ballistically and then experience modest surface flow.  相似文献   

12.
Abstract— We present numerical simulations of crater formation under Martian conditions with a single near‐surface icy layer to investigate changes in crater morphology between glacial and interglacial periods. The ice fraction, thickness, and depth to the icy layer are varied to understand the systematic effects on observable crater features. To accurately model impact cratering into ice, a new equation of state table and strength model parameters for H2O are fitted to laboratory data. The presence of an icy layer significantly modifies the cratering mechanics. Observable features demonstrated by the modeling include variations in crater morphometry (depth and rim height) and icy infill of the crater floor during the late stages of crater formation. In addition, an icy layer modifies the velocities, angles, and volumes of ejecta, leading to deviations of ejecta blanket thickness from the predicted power law. The dramatic changes in crater excavation are a result of both the shock impedance and the strength mismatch between layers of icy and rocky materials. Our simulations suggest that many of the unusual features of Martian craters may be explained by the presence of icy layers, including shallow craters with well‐preserved ejecta blankets, icy flow related features, some layered ejecta structures, and crater lakes. Therefore, the cratering record implies that near‐surface icy layers are widespread on Mars.  相似文献   

13.
Abstract— We measured the depth, wall steepness, and ejecta roughness and surveyed the wall and floor morphology of all craters 10–25 km in diameter within the typical Vastitas Borealis Formation in the northern lowlands of Mars north of 52°N. Two of the 130 craters have unusually rough ejecta; they are deep, have steep walls, and are apparently the youngest in the population. Icy mantles filling the local subkilometer‐scale topographic lows is the main contribution to ejecta smoothing, which occurs at a time scale on the order of tens of Myr. Wall degradation and crater shallowing generally occur at longer time scales, comparable to the duration of the Amazonian period. Many craters are shallow due to filling of the crater with specific ice‐rich material of uncertain origin. We use our collected data to infer the nature of the past climate back through the Amazonian, a period prior to ?10–20 Myr ago, when orbital parameter solutions are chaotic and one must rely on geological data to infer climate conditions. We conclude that moderately high obliquity and wide obliquity variations were probable during the last 40–160 Myr. We tentatively conclude that high obliquity peaks (>40–45°) may have occurred episodically through the last 210–430 Myr. A sharp step in the frequency distribution of wall steepness at 20° may indicate a geologically long period prior to that time where obliquity never exceeded 40–45°.  相似文献   

14.
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   

15.
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   

16.
Abstract— Mars Global Surveyor (MGS) and Mars Odyssey data are being used to revise the Catalog of Large Martian Impact Craters. Analysis of data in the revised catalog provides new details on the distribution and morphologic details of 6795 impact craters in the northern hemisphere of Mars. This report focuses on the ejecta morphologies and central pit characteristics of these craters. The results indicate that single‐layer ejecta (SLE) morphology is most consistent with impact into an ice‐rich target. Double‐layer ejecta (DLE) and multiple‐layer ejecta (MLE) craters also likely form in volatile‐rich materials, but the interaction of the ejecta curtain and target‐produced vapor with the thin Martian atmosphere may be responsible for the large runout distances of these ejecta. Pancake craters appear to be a modified form of double‐layer craters where the thin outer layer has been destroyed or is unobservable at present resolutions. Pedestal craters are proposed to form in an icerich mantle deposited during high obliquity periods from which the ice has subsequently sublimated. Central pits likely form by the release of vapor produced by impact into ice‐soil mixed targets. Therefore, results from the present study are consistent with target volatiles playing a dominant role in the formation of crater morphologies found in the Martian northern hemisphere.  相似文献   

17.
The depths of 109 impact craters 2–16 km in diameter, located on the ridged plains materials of Hesperia Planum, Mars, have been measured from their shadow lengths using digital Viking Orbiter images (orbit numbers 417S–419S) and the PICS computer software. On the basis of their pristine morphology (very fresh lobate ejecta blankets, well preserved rim crests, and lack of superposed impact craters), 57 of these craters have been selected for detailed analysis of their spatial distribution and geometry. We find that south of 30°S, craters <6.0 km in diameter are markedly shallower than similar-sized craters equatorward of this latitude. No comparable relationship is observed for morphologically fresh craters >6.0 km diameter. We also find that two populations exist for older craters <6.0 km diameter. When craters that lack ejecta blankets are grouped on the basis of depth/diameter ratio, the deeper craters also typically lie equatorward of 30° S. We interpret the spatial variation in crater depth/diameter ratios as most likely due to a poleward increase in volatiles within the top 400 m of the surface at the times these craters were formed.  相似文献   

18.
We review the methods and data sets used to determine morphometric parameters related to the depth (e.g., rim height and cavity depth) and diameter of Martian craters over the past ~45 yr, and discuss the limitations of shadow length measurements, photoclinometry, Earth-based radar, and laser altimetry. We demonstrate that substantial errors are introduced into crater depth and diameter measurements that are inherent in the use of 128th-degree gridded Mars Orbiter Laser Altimeter (MOLA) topography. We also show that even the use of the raw MOLA Precision Engineering Data Record (PEDR) data can introduce errors in the measurement of craters a few kilometers in diameter. These errors are related to the longitudinal spacing of the MOLA profiles, the along-track spacing of the individual laser shots, and the MOLA spot size. Stereophotogrammetry provides an intrinsically more accurate method for measuring depth and diameter of craters on Mars when applied to high-resolution image pairs. Here, we use 20 stereo Context Camera (CTX) image pairs to create digital elevation models (DEMs) for 25 craters in the diameter range 1.5–25.6 km and cover the latitude range of 25° S to 42° N. These DEMs have a spatial scale of ~24 m per pixel. Six additional craters, 1.5–3.1 km in diameter, were studied using publically available DEMs produced from High-Resolution Imaging Science Experiment (HiRISE) image pairs. Depth/diameter and rim height were determined for each crater, as well as the azimuthal variation of crater rim height in 1-degree increments. These data indicate that morphologically fresh Martian craters at these diameters are significantly deeper for a given size than previously reported using Viking and MOLA data, most likely due to the improvement in spatial resolution provided by the CTX and HiRISE data.  相似文献   

19.
Abstract— A model for emplacement of deposits of impact craters is presented that explains the size range of Martian layered ejecta craters between 5 km and 60 km in diameter in the low and middle latitudes. The impact model provides estimates of the water content of crater deposits relative to volatile content in the aquifer of Mars. These estimates together with the amount of water required to initiate fluid flow in terrestrial debris flows provide an estimate of 21% by volume (7.6 × 107km3) of water/ice that was stored between 0.27 and 2.5 km depth in the crust of Mars during Hesperian and Amazonian time. This would have been sufficient to supply the water for an ocean in the northern lowlands of Mars. The existence of fluidized craters smaller than 5 km diameter in some places on Mars suggests that volatiles were present locally at depths less than 0.27 km. Deposits of Martian craters may be ideal sites for searches for fossils of early organisms that may have existed in the water table if life originated on Mars.  相似文献   

20.
Abstract— In oblique impacts with an impact angle under 45°, the bilateral shape of the distal ejecta blanket is used as the strongest indicator for an impact vector. This bilateral symmetry is attenuated and is superimposed by radial symmetry towards the crater rim, which remains circular for impact angles down to 10–15°. The possibility that remnants of bilateral symmetry might still be present in the most proximal ejecta, the overturned flap and the crater rim was explored with the intention of deducing an impact vector. A model is presented that postulates bilateral patterns using proximal ejecta trajectories and predicts these patterns in the orientation of bedding planes in the crater rim. This model was successfully correlated to patterns described by radial grooves in the proximal ejecta blanket of the oblique Tooting crater on Mars. A new method was developed to detect structural asymmetries by converting bedding data into values that express the deviation from concentric strike orientation in the crater rim relative to the crater center, termed “concentric deviation.” The method was applied to field data from Wolfe Creek crater, Western Australia. Bedding in the overturned flap implies an impactor striking from the east, which refines earlier publications, while bedding from the inner rim shows a correlation with the crater rim morphology.  相似文献   

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