GRAVITY RECONNAISSANCE AT THREE MAURITANIAN CRATERS OF EXPLOSIVE ORIGIN

We have carried out reconnaissance gravity surveys across three Mauritanian craters: Aouelloul, an undoubted meteorite crater; Tenoumer, a probable meteorite crater with a unique array of concentric dikes on its outer rim flanks containing xenoliths of country rock showing abundant shock artifacts; and Temimichat Ghallaman, a crater of possible meteorite impact origin. All three have residual negative gravity anomalies associated with their interiors. In all cases the gravity values return to “normal” immediately outside their rims. At Tenoumer the anomaly has the form and magnitude expected for a meteorite crater which has been subsequently in-filled with unconsolidated sediments to the level of the surrounding country. Maximum depth from the present crater floor to the bottom of the sedimentary fill (top of the original crater floor) is at least 750 feet. With a rim-rim diameter of 6,300 feet, the origin depth/diameter ratio of about 1:8 is virtually identical with that of Meteor Crater, Arizona. Temimichat, with a rim-rim diameter of 2,100 to 2,400 feet, is somewhat larger than has been previously reported. If it is meteoritic in origin the gravity data dictate a surprisingly shallow structure, with a depth from the present floor to the original crater floor of 150 feet maximum and an original depth/diameter ratio of 1:15. No positive evidence for an impact origin has yet been found for Temimichat. Aouelloul is also larger than generally reported, with a rim-rim diameter averaging 1,275 feet. As for Temimichat the gravity data dictate a remarkably shallow structure having a depth/diameter ratio of about 1:13. The combination of a shallow depth and a reasonably high rim apparently requires a scaled depth of burst for the impact event substantially in excess of 0.50, a value previously considered a maximum for explosive impacts. The morphological resemblance between Temimichat and Aouelloul is striking but, without additional evidence, this fact alone cannot be used to infer a similar origin.     

The Whitecourt meteorite impact crater,Alberta, Canada

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

Radial variation of lunar crater rim topography

The variation of rim topography as a function of range from the crater rim has been determined for a group of morphologically fresh lunar craters (D = 10–140 km) using the recent series of Lunar Topographic Orthophotomaps. The rate at which exterior crater topography converges with the surrounding surface is highly variable along different radial directions at individual craters as well as between different craters. At several craters, oblique impact appears to have contributed to azimuthal elevation/range variations. The topographic expression of a crater above the surrounding surface typically decreases to one-tenth of the estimated rim height at a range of 1.3R–1.7R, well within the rough-textured ejecta deposit surrounding the crater. Comparisons with terrestrial craters suggest that the topographic crater rim is predominantly a structural feature. In most craters large portions of the hummocky facies and virtually all of the radial facies, in spite of their rough appearance and local topographic variations, provide no significant net topographic addition to the preexisting surface. The extreme variability of crater rim topography strongly suggests that ejecta thicknesses are highly variable and that a unique power-law expression cannot truly represent the radial variation of ejecta deposit thickness.     

Geological and geochemical data from the proposed Sirente crater field: New age dating and evidence for heating of target

Abstract— The proposed Sirente crater field consists of a slightly oblong main structure (main crater) 120 m in width and about 30 smaller structures (satellite craters), all in unconsolidated but stiff carbonate mud. Here we focus on the subsurface structure of the satellite craters and compare the Sirente field with known meteorite crater fields. We present a more complete outline of the crater field than previously reported, information on the subsurface morphology of a satellite crater (C8) 8 m in width, radiocarbon and thermoluminescence (TL) ages of material from this crater, and evidence for heated material in both crater C8 and the rim of the main crater. Crater C8 has a funnel shape terminating downwards, and evidence for soil injection from the surface to a depth of 9 m. The infill contained dispersed charcoal and small, irregular, porous fragments of heated clay with a calibrated age of b.p. 1712 (¹³C‐corrected radiocarbon age: b.p. 1800 ± 100) and a TL age of b.p. 1825 (calculated error ± 274). Together with previous radiocarbon age (b.p. 1538) of the formation of the main crater (i.e., target surface below rim), a formation is suggested at the beginning of the first millennium a.d. Although projectile vaporization is not expected in Sirente‐sized craters in this type of target material, we used geochemistry in an attempt to detect a meteoritic component. The results gave no unequivocal evidence of meteoritic material. Nevertheless, the outline of the crater field, evidence of heated material within the craters, and subsurface structure are comparable with known meteorite crater fields.     

LAGUNA GUATAVITA: NOT METEORITIC,PROBABLE SALT COLLAPSE CRATER

Laguna Guatavita (Colombia), a crater 700 m across and 125 m deep containing a central lake, appears not to be a meteorite crater as widely supposed. The tectonic style is not that of an impact site and there is no raised rim or ejected debris. We could find no impactite, shock metamorphic effects or shock fractures (shatter cones). Most likely it is a collapsed crater caused by the solution and withdrawal of salt from an underlying anticline     

Bosumtwi impact structure,Ghana: Evidence for fluidized emplacement of the ejecta

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.     

Mass-Wasting Processes on the Surface of Phobos

This paper considers morphologic signatures of mass-wasting processes on the surface of Phobos. Two types of downslope movement of material are distinguished: (i) intracrater volume landslides inside impact craters and (ii) downslope near-surface movement of material. Crater statistics for the Stickney area (based on new images of Phobos) showed that the landslide in the crater Stickney could have been formed after resurfacing of the outer rim of the crater in the process of meteorite bombardment. An estimate of the volume of the landslide in Stickney (1–2 km³) and simulation of its movement allowed us to classify the landslide as a long-runout one. The possibility of forming a hummocky topographic relief to the east of Stickney due to the crater ejecta and the emplacement of the frontal part of the long-runout landslide is discussed.     

A meteorite impact crater field in eastern Bavaria? A preliminary report

Abstract— Numerous circular depressions north of Burghausen in eastern Bavaria, with diameters ranging from meters to tens of meters in size and dispersed over an area of at least 11 times 7 km, are suspected to have an extraterrestrial origin since they resemble other small meteorite impact craters. The depressions are bowl‐shaped, have high circularity and a characteristic rim. Most of them were formed in unconsolidated glacial gravels and pebbles intermixed with fine‐grained sand and clay. Magnetic investigations reveal weak anomalies with amplitudes of less than ±10 nano Tesla (nT). In some cases, the origins of the anomalies are suspected to be due to human activity within the structures. So far, no traces of meteoritic material have been detected. An evident archaeological or local geological explanation for the origin of the craters does not exist. A World War I and II explosive origin can be excluded since trees with ages exceeding 100 years can be found in some craters. One crater was described in 1909. Carbon‐14 dating of charcoal found in one crater yielded an age of 1790 ± 60 years. Hence, a formation by meteorite impacts that occurred in Celtic or early medieval times should be considered. A systematic archaeological excavation of some structures and an intensified search for traces of meteoritic material are planned.     

Crater dimensions from apollo data and supplemental sources

A catalog of crater dimensions that were compiled mostly from the new Apollo-based Lunar Topographic Orthophotomaps is presented in its entirety. Values of crater diameter, depth, rim height, flank width, circularity, and floor diameter (where applicable) are tabulated for a sample of 484 craters on the Moon and 22 craters on Earth. Systematic techniques of mensuration are detailed. The lunar craters range in size from 400 m to 300 km across and include primary impact craters of the main sequence, secondary impact craters, craterlets atop domes and cones, and dark-halo craters. The terrestrial craters are between 10 m and 22.5 km in diameter and were formed by meteorite impact.     

Kgagodi Basin: The first impact structure recognized in Botswana

Abstract— The 3.4 km wide, so‐called Kgagodi Basin structure, which is centered at longitude 27°34.4′ E and latitude 22°28.6′ S in eastern Botswana, has been confirmed as a meteorite impact structure. This crater structure was first recognized through geophysical analysis; now, we confirm its impact origin by the recognition of shock metamorphosed material in samples from a drill core obtained close to the crater rim. The structure formed in Archean granitoid basement overlain and intruded by Karoo dolerite. The crater yielded a gravity model consistent with a simple bowl‐shape crater form. The drill core extends to a depth of 274 m and comprises crater fill sediments to a depth of 158 m. Impact breccia was recovered only between 158 and 165 m depth, below which locally brecciated basement granitoids grade into fractured and eventually undeformed crystalline basement, from ～250 m depth. Shock metamorphic effects were only found in granitoid clasts in the narrow breccia zone. This breccia is classified as suevitic impact breccia due to the presence of melt and glass fragments, at a very small abundance. The shocked grains are exclusively derived from granitoid target material. Shock effects include multiple sets of planar deformation features in quartz and feldspar; diaplectic quartz, and partially and completely isotropized felsic minerals, and rare melt fragments were encountered. Abundances of some siderophile elements and especially, Ir, in suevitic breccia samples are significantly elevated compared to the contents in the target rocks, which provides evidence for the presence of a small meteoritic component. Kgagodi is the first impact structure recognized in the region of the Kalahari Desert in southern Africa. Based on lithological and first palynological evidence, the age of the Kgagodi structure is tentatively assigned to the upper Cretaceous to early Tertiary interval. Thus, the crater fill has the potential to provide a long record of paleoclimatic conditions.     

IMPACT AND IMPACT-LIKE STRUCTURES IN ALGERIA PART I FOUR BOWL-SHAPED DEPRESSIONS

From orbital, aviation and geologic documents, four circular depressions on the Sahara sedimentary platform were selected for field investigation because of their possible impact origin. Our results can be summarized as follows: Amguid Crater (26° 05′N; 004° 23.5′E; 450 m diameter, 30 m deep) is perfectly circular, with a steep wall, a raised rim and an ejecta blanket. The strata are uplifted, outward dipping, dislocated and locally overturned at the rim crest. Large blocks are scattered around the rim. There is petrological evidence of shock by planar elements in quartz. Amguid is a well preserved impact crater probably no older than 100,000 years. Talemzane (33° 19′N; 004° 02′E; 1.7 km diameter, 70 m deep) is also perfectly circular and displays a raised rim. The strata are uplifted, outward dipping, and locally highly fractured. Numerous breccia veins are clearly exposed in the crater wall. Consolidated ejected debris form a continuous blanket more than 500 m outward from the rim. Reworked mixed breccias are exposed at the base of the crater wall. Planar elements are observed in quartz clasts in the mixed breccia. Talemzane is an impact crater on the order of 0.5 to 3 million years old. El Mouilah (33° 51′N; 002° 03′E; 4.5 km diameter, 130 m deep) is almost perfectly circular, the walls are steep and there is a central dome. In spite of a promising morphology, there is no field evidence of impact. El Mouilah is possibly a recent collapse structure due to dissolution in the thick underlying limestone and gypsum formations or purely erosional in origin. Aflou (34° 00′N; 002° 03′E) is not circular (3 × 5 km) but was selected because it appears in the literature as a probable impact crater, the main argument being the existence of fused materials in the center (Marks et al., 1972). We found no evidence of impact, but several occurrences of igneous rocks along an E-W direction suggest a structurally controlled volcanic activity. A volcanic activity is also supported by the existence of a local magnetic anomaly centered on the depression. Aflou is neither an impact structure nor a crater. Located on a probable structural dome, at the intersection of several structural trends, the formation of the depression can be due to erosion and/or dissolution in the thick underlying limestone and gypsum formations.     

Catalogue of large alluvial fans in martian impact craters

We conducted a systematic, global survey using Thermal Emission Imaging System Infrared (THEMIS IR) coverage (∼100 m/pixel) to search for large alluvial fans in impact craters on Mars. Our survey has focused on large fans (apron areas greater than ∼40 km², usually located in craters greater than 20 km in diameter) due to the resolution of the THEMIS images and Mars Orbiter Laser Altimeter (MOLA) coverage. We find that the host craters are found to have a distinctive diameter range from 30-150 km. The fans generally cluster in three geographic areas—southern Margaritifer Terra, southwestern Terra Sabaea, and southwestern Tyrrhena Terra, however several outliers do exist. The alluvial fans do not form in a particular orientation along the crater rim nor are they associated with the location of current high rim topography. Fan area magnitude and variability increase with crater diameter while fan concavity magnitude and variability increase with decreasing crater diameter. Smaller fan aprons in general have higher, more variable concavity. The source of the water forming these fans is uncertain given the challenges of accommodating the global distribution pattern and formation patterns within the craters.     

Bosumtwi impact structure,Ghana: Geochemistry of impactites and target rocks,and search for a meteoritic component

Abstract— Major and trace element data, including platinum group element abundances, of representative impactites and target rocks from the crater rim and environs of the Bosumtwi impact structure, Ghana, have been investigated for the possible presence of a meteoritic component in impact‐related rocks. A comparison of chemical data for Bosumtwi target rocks and impactites with those for Ivory Coast tektites and microtektites supports the interpretation that the Bosumtwi structure and Ivory Coast tektites formed during the same impact event. High siderophile element contents (compared to average upper crustal abundances) were determined for target rocks as well as for impactites. Chondrite‐normalized (and iron meteorite‐normalized) abundances for target rocks and impactites are similar. They do not, however, allow the unambiguous detection of the presence, or identification of the type, of a meteoritic component in the impactites. The indigenous siderophile element contents are high and possibly related to regional gold mineralization, although mineralized samples from the general region show somewhat different platinum‐group element abundance patterns compared to the rocks at Bosumtwi. The present data underline the necessity of extensive target rock analyses at Bosumtwi, and at impact structures in general, before making any conclusions regarding the presence of a meteoritic component in impactites.     

The Sirente crater field,Italy

Abstract— We propose the Sirente crater field to be the first discovered impact craters in Italy. They are located in the Sirente plain within the mountains of the Abruzzo region, central Italy. The craters are distributed in a field 450 m long and 400 m wide. This field consists of ?17 smaller craters close to a larger main crater. The main crater is located in the southern end of the crater field and is 140 m long and 115 m wide, measured rim‐to‐rim. It has a well‐developed, saddle‐shaped rim that rises at a maximum 2.2 m above the surrounding plain. Radiocarbon dating of the target surface preserved below the rim gave a calibrated age of formation at about a.d. 412 (1650 ± 40 radiocarbon years b.p.). This young age is consistent with the apparent little modification of the rim. The morphology of the main crater and its relation to a crater field strongly points to its origin by impact from a projectile that broke up during its passage through the atmosphere. Quartz is very rare in the target and no planar deformation features have been found so far. The rim material and the upper 4 m of the main crater infill are impregnated with ferric oxides, which gives a more reddish colour compared to the other sediments of the plain. Rusty crusts with high Fe and Mn content occur in the rim material, but have not been found in the plain's sediments. Some of these crusts can be separated by magnet, and have sporadic micron‐sized Ni‐rich granules. The main crater is in the size range of the craters with explosive dispersion of the projectile and has many features comparable to both large experimental and meteoritic impact craters formed in loose sediments. We suggest that this crater represents a rare example of well‐preserved, small impact crater formed in unconsolidated target materials.     

Authentication controversies and impactite petrography of the New Quebec Crater

Abstract— A meteoritic origin was proposed for the New Quebec Crater in 1949 on the basis of an aerial photograph showing its unique circularity and raised rim amid Precambrian gneisses of the Canadian Shield. At that time, only those few craters associated with meteorites were generally accepted as of impact origin. When the earliest field expeditions failed to find meteorites or impact products, two leading meteoriticists, Frederick C. Leonard and Lincoln LaPaz, cited the “Chubb” Crater as a flagrant example for which claims of meteoritic origin were advanced without valid proof. They also listed the Lake Bosumtwi Crater in Ashanti (now Ghana) among crater-like features, clearly of non-meteoritic origin, misidentified as meteorite craters. Controversy over the origin these two craters continued for decades. In Part I of this paper, we trace the investigations that led to the current acceptance of New Quebec as an authentic impact crater. We note that, for reasons that are not entirely clear, a meteoritic origin for the New Quebec Crater achieved wider acceptance at an earlier date than for the Lake Bosumtwi Crater, where petrographic and chemical evidence is more abundant and compelling. In Part II, we describe the petrography of two impact melt samples from the New Quebec Crater and present new evidence on the degrees of shock metamorphism affecting the accessory minerals: apatite, sphene, magnetite and zircon. Zircon, in particular, shows a range from euhedral grains with no signs of alteration to those decomposed to baddeleyite plus silica.     

Gebel Kamil: The iron meteorite that formed the Kamil crater (Egypt)

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

The effect of target properties on crater morphology: Comparison of central peak craters on the Moon and Ganymede

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.     

Mineralogy,petrology, and distribution of meteorites at the Whitecourt crater,Alberta, Canada

The Whitecourt meteorite impact crater, Alberta, Canada is a rare example of a well‐preserved small impact structure, with which thousands of meteorite fragments are associated. As such, this crater represents a unique opportunity to investigate the effect of a low‐energy impact event on an impacting iron bolide. Excellent documentation of meteorite fragment locations and characteristics has generated a detailed distribution map of both shrapnel and regmaglypted meteorite types. The meteorites' distribution, and internal and external characteristics support a low‐altitude breakup of the impactor which caused atmospherically ablated (regmaglypted) meteorites to fall close to the crater and avoid impact‐related deformation. In contrast, shrapnel fragments sustained deformation at macro‐ and microscales resulting from the catastrophic disruption of the impactor. The impactor was significantly fragmented along pre‐existing planes of weakness, including kamacite lamellae and inclusions, resulting in a bias toward low‐mass (<100 g) fragments. Meteorite mineralogy was investigated and the accessory minerals were found to be dominated by sulfides and phosphides with rare carlsbergite, consistent with other low‐Ni IIIAB iron meteorites. Considerations of the total mass of meteoritic material recovered at the site relative to the probable fraction of the impactor that was preserved based on modeling suggests that the crater was formed by a higher velocity, lower mass impactor than previously inferred.     

Stratigraphy, mineralogy, and origin of layered deposits inside Terby crater, Mars

The 174 km diameter Terby impact crater (28.0°S-74.1°E) located on the northern rim of the Hellas basin displays anomalous inner morphology, including a flat floor and light-toned layered deposits. An analysis of these deposits was performed using multiple datasets from Mars Global Surveyor, Mars Odyssey, Mars Express and Mars Reconnaissance Orbiter missions, with visible images for interpretation, near-infrared data for mineralogical mapping, and topography for geometry. The geometry of layered deposits was consistent with that of sediments that settled mainly in a sub-aqueous environment, during the Noachian period as determined by crater counts. To the north, the thickest sediments displayed sequences for fan deltas, as identified by 100 m to 1 km long clinoforms, as defined by horizontal beds passing to foreset beds dipping by 6-10° toward the center of the Terby crater. The identification of distinct sub-aqueous fan sequences, separated by unconformities and local wedges, showed the accumulation of sediments from prograding/onlapping depositional sequences, due to lake level and sediment supply variations. The mineralogy of several layers with hydrated minerals, including Fe/Mg phyllosilicates, supports this type of sedimentary environment. The volume of fan sediments was estimated as >5000 km³ (a large amount considering classical martian fan deltas such as Eberswalde (6 km³)) and requires sustained liquid water activity. Such a large sedimentary deposition in Terby crater is characteristic of the Noachian/Phyllosian period during which the environment favored the formation of phyllosilicates. The latter were detected by spectral data in the layered deposits of Terby crater in three distinct layer sequences. During the Hesperian period, the sediments experienced strong erosion, possibly enhanced by more acidic conditions, forming the current morphology with three mesas and closed depressions. Small fluvial valleys and alluvial fans formed subsequently, attesting to late fluvial processes dated as late Early to early Late Hesperian. After this late fluvial episode, the Terby impact crater was submitted to aeolian processes and permanent cold conditions with viscous flow features. Therefore, the Terby crater displays, in a single location, geologic features that characterize the three main periods of time on Mars, with the presence of one of the thickest sub-aqueous fan deposits reported on Mars. The filling of Terby impact crater is thus one potential “reference geologic cross-section” for Mars stratigraphy.     

Erosional modification and gully formation at Meteor Crater, Arizona: Insights into crater degradation processes on Mars

Hydrogeological modification of Meteor Crater produced a spectacular set of gullies throughout the interior wall in response to rainwater precipitation, snow melting, and possible groundwater discharge. The crater wall has an exceptionally well-developed centripetal drainage pattern consisting of individual alcoves, channels, and fans. Some of the gullies originate from the rim crest and others from the middle crater wall where a lithologic transition occurs; broad gullies occur along the crater corner radial faults. Deeply incised alcoves are well developed on the soft Coconino Sandstone exposed on the middle crater wall, beneath overlying dolomite. In general, the gully locations are along crater wall radial fractures and faults, which are favorable locales of erosion due to preferential rock breakup from faulting, and groundwater flow/discharge; these structural discontinuities are also the locales where the surface runoff from rain precipitation and snow melting can preferentially flow, causing erosion and crater degradation. Channels are well developed on the talus deposits and alluvial fans on the periphery of the crater floor. Caves exposed on the lower crater level point to percolation of surface runoff and selective discharge through fractures on the crater wall. In addition, lake sediments on the crater floor provide significant evidence of a past pluvial climate, when the water table was higher, and groundwater may have seeped from springs on the crater wall. Although these hydrological processes continue at Meteor Crater today, conditions at the crater are much more arid than they were soon after impact, reflecting a climatic shift. This climate shift and the hydrological modifications observed at Meteor Crater provide insights for landscape sculpturing on Mars during various parts of its history.