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.     

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.     

Reconstruction Of The Subsurface Structure Of The Marquez Impact Crater In Leon County,Texas, Usa,Based On Well‐Log And Gravity Data

Abstract— In Leon County, Texas, USA, the Marquez Dome, an approximately circular 1.2 km diameter zone of disturbed Cretaceous rocks surrounded by shallow dipping Tertiary sediments, has been interpreted by Gibson and Sharpton (1989) and Sharpton and Gibson (1990) as the surface expression of a buried complex impact crater. New gravity and magnetic anomaly data collected over the Marquez Dome have been combined with well‐log and seismic reflection information to develop a better estimate of the overall geometry of the structure. A three‐dimensional model constructed to a depth of 2000 m from all available information indicates a complex crater 13 km in diameter with an uplift in the center of at least 1120 m. The zone of deformation associated with the cratering event is limited to a depth of <1720 m. No impact breccias were recovered in drilling at two locations, 1.1 and 2 km from the center of the structure, and the central uplift may be the only prominent remnant of this impact into unconsolidated, water‐rich sediments. The magnetic anomaly field shows no correlation with the location and extent of the structure.     

Formation of resurge gullies at impacts at sea: The Lockne crater,Sweden

Abstract— The Lockne crater in Sweden is a marine‐target crater, formed in a shelf sea, approximately 460 Ma ago. The crater structure consists of an inner crater surrounded by an outer, inclined surface that extends to almost 12 km from the center. Marine craters differ in several respects from craters formed on land. One special feature is the formation of resurge gullies excavated by the erosional force of the resurging sea water after the impact. The formation of these gullies strongly depends on the ratio crater‐rim height to water depth, as well as on the size of the impact structure. Such gullies are known from very few marine‐target craters. At the Lockne impact site, four gullies are identified, each of which cuts radially through the rim of the outer crater. The rapid collapse of that part of the crater cavity, which formed in the seawater, resulted in forceful flooding of the crater. The resurging seawater not only contained fallback‐ejecta; on its way towards the cavity on the sea‐bottom it incorporated fractured lithologies from the sea‐bottom as well. This entrained material disintegrated during transport and constitutes today the dominantly monomict lower part of the resurge sequence. The resurge flood was highly turbulent, highly erosive, and developed to a probable hyperconcentrated flow or a possible water flood. The erosion in the gullies proceeded as headward erosion down to the transition zone between the brecciated and the less disintegrated crystalline basement.     

Predicted and observed magnetic signatures of martian (de)magnetized impact craters

The current morphology of the martian lithospheric magnetic field results from magnetization and demagnetization processes, both of which shaped the planet. The largest martian impact craters, Hellas, Argyre, Isidis and Utopia, are not associated with intense magnetic fields at spacecraft altitude. This is usually interpreted as locally non- or de-magnetized areas, as large impactors may have reset the magnetization of the pre-impact material. We study the effects of impacts on the magnetic field. First, a careful analysis is performed to compute the impact demagnetization effects. We assume that the pre-impact lithosphere acquired its magnetization while cooling in the presence of a global, centered and mainly dipolar magnetic field, and that the subsequent demagnetization is restricted to the excavation area created by large craters, between 50- and 500-km diameter. Depth-to-diameter ratio of the transient craters is set to 0.1, consistent with observed telluric bodies. Associated magnetic field is computed between 100- and 500-km altitude. For a single-impact event, the maximum magnetic field anomaly associated with a crater located over the magnetic pole is maximum above the crater. A 200-km diameter crater presents a close-to-1-nT magnetic field anomaly at 400-km altitude, while a 100-km diameter crater has a similar signature at 200-km altitude. Second, we statistically study the 400-km altitude Mars Global Surveyor magnetic measurements modelled locally over the visible impact craters. This approach offers a local estimate of the confidence to which the magnetic field can be computed from real measurements. We conclude that currently craters down to a diameter of 200 km can be characterized. There is a slight anti-correlation of −0.23 between magnetic field intensity and impact crater diameters, although we show that this result may be fortuitous. A complete low-altitude magnetic field mapping is needed. New data will allow predicted weak anomalies above craters to be better characterized, and will bring new constraints on the timing of the martian dynamo and on Mars’ evolution.     

Geophysical signature of Målingen,the minor crater of the Lockne–Målingen doublet impact structure

Målingen is the 0.7 km wide minor crater associated to the 10 times larger Lockne crater in the unique Lockne–Målingen doublet. The craters formed at 458 Ma by the impact of a binary asteroid related to the well-known 470 Ma Main Belt breakup event responsible for a large number of Ordovician craters and fossil meteorites. The binary asteroid struck a target sequence including ~500 m of sea water, ~80 m of limestone, ~30 m of dark mud, and a peneplainized Precambrian crystalline basement. Although the Lockne crater has been extensively studied by core drillings and geophysics, little is known about the subsurface morphology of Målingen. We performed magnetic susceptibility and remanence, as well as density, measurements combined with gravity, and magnetic field surveys over the crater and its close vicinity as a base for forward magnetic and gravity modeling. The interior of the crater shows a general magnetic low of 90–100 nT broken by a clustered set of high-amplitude, short wavelength anomalies caused by bodies of mafic rock in the target below the crater and as allogenic blocks in the crater infill. The gravity shows a general −1.4 mgal anomaly over the crater caused by low-density breccia infill and fractured crystalline rocks below the crater floor. The modeling also revealed a slightly asymmetrical shape of the crater that together with the irregular ejecta distribution supports an oblique impact from the east, which is consistent with the direction of impact suggested for the Lockne crater.     

Resurge gullies and “inverted sombrero” morphology,Flynn Creek impact structure,Tennessee

The Flynn Creek impact structure is an approximately 3.8 km diameter, marine‐target impact structure, which is located in north central Tennessee, USA. The target stratigraphy consists of several hundreds of meters of Ordovician carbonate strata, specifically Knox Group through Catheys‐Leipers Formation. Like other, similarly sized marine‐target impact craters, Flynn Creek's crater moat‐filling deposits include, in stratigraphic order, gravity‐driven slump material, aqueous resurge deposits, and secular (postimpact) aqueous settling deposits. In the present study, we show that Flynn Creek also possesses previously undescribed erosional resurge gullies and an annular, sloping surface that comprises an outer crater rim surrounding an inner, nested bowl‐shaped crater, thus forming a concentric crater structure. Considering this morphology, the Flynn Creek impact structure has a crater shape that has been referred to at other craters as an “inverted sombrero.” In this paper, we describe the annular rim and the inner crater at Flynn Creek using geographic information system technology. We relate these geomorphic features to the marine environment of crater formation, and compare the Flynn Creek impact structure with other marine‐target impact structures having similar features.     

Geophysical signature of the Pretoria Saltpan impact structure and a possible satellite crater

Abstract— The genesis of the 1.13-km-diameter Pretoria Saltpan crater has long been the focus of a controversy. Its origin has been explained by either meteorite impact or “cryptoexplosive” volcanic activity, but it was recently confirmed, through detailed petrographic and chemical analysis of a breccia layer forming part of the crater fill, that the crater was formed by impact. As the limited previous geophysical work failed to support an impact origin, a more detailed gravity and magnetic study was conducted. A possible 400-m-diameter circular crater located 3 km to the southwest of the main crater was also investigated with geophysical methods, including resistivity, seismics and ground-probing radar. The gravity signature of the main crater is compatible with that of a simple impact crater and the magnetic signature (no magnetic anomaly could be detected) rules out the possibility of a central magnetic volcanic body below the crater-fill sediments. The results for the possible twin or satellite crater are inconclusive. As it is the only such feature in the entire region, it should not be overlooked. A drilling program may reveal interesting results.     

Geology of the Morasko craters,Poznań, Poland—Small impact craters in unconsolidated sediments

Confirmed small impact craters in unconsolidated deposits are rare on Earth, and only a few have been the subjects of detailed investigations. Consequently, our knowledge of indicators permitting unambiguous identification of such structures is limited. In this work, detailed geological mapping was performed in the area of the Morasko craters, of which the largest crater is of about 96 m diameter. These craters were formed in the mid‐Holocene (~5000 yr ago) in unconsolidated sediments of a glacial terminal moraine. Fragments of the impactor—an iron meteorite—have been found in the craters’ vicinity for many decades. Despite numerous studies of the meteorite, no detailed research concerning the geological structure around the craters and of the ejecta deposits has been undertaken. The new data, including evaluation of over 52 sediment cores and 260 shallow drillings, permit the identification of four main sediment types: Neogene clays, diamicton with Neogene clay clasts containing charcoal pieces, diamicton without clasts, and sand with locally preserved paleosoil and charcoal pieces. Based on sedimentological properties, the ejecta deposits are mainly identified as diamicton with Neogene clay clasts, described as lithic impact breccia, covering locally preserved pre‐impact soil. Moreover, crater sections characterized by inverse stratigraphy of sediments are identified as belonging to overturned flaps.     

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.     

Density and magnetic susceptibility of rocks from the Lockne and Tvären marine impact structures

Abstract— The Lockne and Tvären impact craters in Sweden formed in a marine environment during the Ordovician The contrast in density between the impact breccias and the surrounding target rock of these two craters is significantly lower than what has been found in craters formed in crystalline targets on land. Another marine‐target structure, the Estonian Kärdla structure, demonstrates intermediate contrast in impact breccia and target rock, which we attribute to the interpreted shallowness of the sea at the Kärdla impact site. We conclude that the main cause for these low‐density contrasts is pore and fracture filling of calcite with subordinate quartz and fluorite. Calcite is the most abundant cement, and its density differs most from that of fractured and brecciated bedrock with a low degree of cementation. Furthermore, from the studied cases, it is concluded that the target rock to impact rock contrast is generally the highest in craters formed on land in crystalline targets and the lowest in craters formed at sea, while craters formed on land in sedimentary targets are intermediate. The low density contrasts should decrease the negative gravity anomalies of marine craters.     

Numerical modeling of oblique hypervelocity impacts on strong ductile targets

Abstract– The majority of meteorite impacts occur at oblique incidence angles. However, many of the effects of obliquity on impact crater size and morphology are poorly understood. Laboratory experiments and numerical models have shown that crater size decreases with impact angle, the along‐range crater profile becomes asymmetric at low incidence angles, and below a certain threshold angle the crater planform becomes elliptical. Experimental results at approximately constant impact velocity suggest that the elliptical threshold angle depends on target material properties. Herein, we test the hypothesis that the threshold for oblique crater asymmetry depends on target material strength. Three‐dimensional numerical modeling offers a unique opportunity to study the individual effects of both impact angle and target strength; however, a systematic study of these two parameters has not previously been performed. In this work, the three‐dimensional shock physics code iSALE‐3D is validated against laboratory experiments of impacts into a strong, ductile target material. Digital elevation models of craters formed in laboratory experiments were created from stereo pairs of scanning electron microscope images, allowing the size and morphology to be directly compared with the iSALE‐3D craters. The simulated craters show excellent agreement with both the crater size and morphology of the laboratory experiments. iSALE‐3D is also used to investigate the effect of target strength on oblique incidence impact cratering. We find that the elliptical threshold angle decreases with decreasing target strength, and hence with increasing cratering efficiency. Our simulations of impacts on ductile targets also support the prediction from Chapman and McKinnon (1986) that cratering efficiency depends on only the vertical component of the velocity vector.     

Comparison of the crater morphology-size relationship for Mars,Moon, and Mercury

New central peak-crater size data for Mars shows that a higher percentage of relatively unmodified Martian craters have central peaks than do fresh lunar craters below a diameter of 30 km. For example, in the diameter range 10 to 20 km, 60% of studied Martian craters have central peaks compared to 26% for the Moon. Gault et al. (1975, J. Geophys. Res.80, 2444–2460) have demonstrated that central peaks occur in smaller craters on Mercury than on the Moon, and that this effect is due to the different gravity fields in which the craters formed. Similar differences when comparing Mars and the Moon show that gravity has affected the diameter at which central peaks form on Mars. Erosion on Mars, therefore, does not completely mask differences in crater interior structure that are caused by differences in gravity. Effects of Mars' higher surface gravity when compared to the Moon are not detected when comparing terrace and crater shape data. The morphology-crater size statistics also show that a full range of crater shapes occur on Mars, and craters tend to become more morphologically complex with increasing diameter. Comparisons of Martian and Mercurian crater data show differences which may be related to the greater efficacy of erosion on Mars.     

Geophysical survey of the proposed Tsenkher impact structure,Gobi Altai,Mongolia

Abstract– We have performed forward magnetic and gravity modeling of data obtained during the 2007 expedition to the 3.7 km in diameter, circular, Tsenkher structure, Mongolia, in order to evaluate the cause of its formation. Extensive occurrences of brecciated rocks, mainly in the form of an ejecta blanket outside the elevated rim of the structure, support an explosive origin (e.g., cosmic impact, explosive volcanism). The host rocks in the area are mainly weakly magnetic, silica‐rich sandstones, and siltstones. A near absence of surface exposures of volcanic rocks makes any major volcanic structures (e.g., caldera) unlikely. Likewise, the magnetic models exclude any large, subsurface, intrusive body. This is supported by an 8 mGal gravity low over the structure indicating a subsurface low density body. Instead, the best fit is achieved for a bowl‐shaped structure with a slight central rise as expected for an impact crater of this size in mainly sedimentary target. The structure can be either root‐less (i.e., impact crater) or rooted with a narrow feeder dyke with relatively higher magnetic susceptibility and density (i.e., volcanic maar crater). The geophysical signature, the solitary appearance, the predominantly sedimentary setting, and the comparably large size of the Tsenkher structure favor the impact crater alternative. However, until mineralogical/geochemical evidence for an impact is presented, the maar alternative remains plausible although exceptional as it would make the Tsenkher structure one of the largest in the world in an unusual setting for maar craters.     

Marine‐target craters on Mars? An assessment study

Abstract— Observations of impact craters on Earth show that a water column at the target strongly influences lithology and morphology of the resultant crater. The degree of influence varies with the target water depth and impactor diameter. Morphological features detectable in satellite imagery include a concentric shape with an inner crater inset within a shallower outer crater, which is cut by gullies excavated by the resurge of water. In this study, we show that if oceans, large seas, and lakes existed on Mars for periods of time, marine‐target craters must have formed. We make an assessment of the minimum and maximum amounts of such craters based on published data on water depths, extent, and duration of putative oceans within “contacts 1 and 2,” cratering rate during the different oceanic phases, and computer modeling of minimum impactor diameters required to form long‐lasting craters in the seafloor of the oceans. We also discuss the influence of erosion and sedimentation on the preservation and exposure of the craters. For an ocean within the smaller “contact 2” with a duration of 100,000 yr and the low present crater formation rate, only ?1–2 detectable marine‐target craters would have formed. In a maximum estimate with a duration of 0.8 Gyr, as many as 1400 craters may have formed. An ocean within the larger “contact 1‐Meridiani,” with a duration of 100,000 yr, would not have received any seafloor craters despite the higher crater formation rate estimated before 3.5 Gyr. On the other hand, with a maximum duration of 0.8 Gyr, about 160 seafloor craters may have formed. However, terrestrial examples show that most marine‐target craters may be covered by thick sediments. Ground penetrating radar surveys planned for the ESA Mars Express and NASA 2005 missions may reveal buried craters, though it is uncertain if the resolution will allow the detection of diagnostic features of marine‐target craters. The implications regarding the discovery of marine‐target craters on Mars is not without significance, as such discoveries would help address the ongoing debate of whether large water bodies occupied the northern plains of Mars and would help constrain future paleoclimatic reconstructions.     

Chesapeake Bay impact structure: Morphology,crater fill,and relevance for impact structures on Mars

Abstract— The late Eocene Chesapeake Bay impact structure (CBIS) on the Atlantic margin of Virginia is one of the largest and best‐preserved “wet‐target” craters on Earth. It provides an accessible analog for studying impact processes in layered and wet targets on volatile‐rich planets. The CBIS formed in a layered target of water, weak clastic sediments, and hard crystalline rock. The buried structure consists of a deep, filled central crater, 38 km in width, surrounded by a shallower brim known as the annular trough. The annular trough formed partly by collapse of weak sediments, which expanded the structure to ?85 km in diameter. Such extensive collapse, in addition to excavation processes, can explain the “inverted sombrero” morphology observed at some craters in layered targets. The distribution of crater‐fill materials in the CBIS is related to the morphology. Suevitic breccia, including pre‐resurge fallback deposits, is found in the central crater. Impact‐modified sediments, formed by fluidization and collapse of water‐saturated sand and silt‐clay, occur in the annular trough. Allogenic sediment‐clast breccia, interpreted as ocean‐resurge deposits, overlies the other impactites and covers the entire crater beneath a blanket of postimpact sediments. The formation of chaotic terrains on Mars is attributed to collapse due to the release of volatiles from thick layered deposits. Some flat‐floored rimless depressions with chaotic infill in these terrains are impact craters that expanded by collapse farther than expected for similar‐sized complex craters in solid targets. Studies of crater materials in the CBIS provide insights into processes of crater expansion on Mars and their links to volatiles.     

Crater morphology in sandstone targets: The MEMIN impact parameter study

Abstract– Hypervelocity (2.5–7.8 km s^?1) impact experiments into sandstone were carried out to investigate the influence of projectile velocity and mass, target pore space saturation, target‐projectile density contrast, and target layer orientation on crater size and shape. Crater size increases with increasing projectile velocity and mass as well as with increasing target pore space saturation. Craters in water‐saturated porous targets are generally shallower and larger in volume and in diameter than craters from equivalent impacts into dry porous sandstone. Morphometric analyses of the resultant craters, 5–40 cm in diameter, reveal features that are characteristic of all of our experimental craters regardless of impact conditions (I) a large central depression within a fragile, light‐colored central part, and (II) an outer spallation zone with areas of incipient spallation. Two different mechanical processes, grain fragmentation and intergranular tensile fracturing, are recorded within these crater morphologies. Zone (I) approximates the shape of the transient crater formed by material compression, displacement, comminution, and excavation flow, whereas (II) is the result of intergranular tensile fracturing and spallation. The transient crater dimensions are reconstructed by fitting quadric parabolas to crater profiles from digital elevation models. The dimensions of this transient and of the final crater show the same trends: both increase in volume with increasing impact energy, and with increasing water saturation of the target pore space. The relative size of the transient crater (in percent of the final crater volume) decreases with increasing projectile mass and velocity, signifying a greater contribution of spallation on the final crater size when projectile mass and velocity are increased.     

Gravity and Magnetic Investigations in the Haughton Impact Structure,Devon Island,Canada

Abstract— The results of a new gravity survey show that the Haughton impact structure is associated with a 24 km diameter negative Bouguer gravity anomaly with a maximum amplitude of ?12 mgal. A local minimum with a half-width of 2 km and an amplitude of ?4 mgal is located at the center of the structure. A positive magnetic total field anomaly with a half-width of 0.6 km and an amplitude of 700 nT coincides with the local central gravity anomaly. The overall negative gravity anomaly is explained by lowered rock densities due to impact-related fracturing in the crater area. The central gravity and magnetic anomalies are believed to be due to highly shocked and heated sedimentary and crystalline basement rocks forming the unexposed peak of the central uplift in the Haughton impact structure.     

A survey of possible impact structures on 25143 Itokawa

We determined the morphologies and dimensions of possible impact craters on the surface of Asteroid 25143 Itokawa from images taken by the Hayabusa spacecraft. Circular depressions, circular features with flat floors or convex floors, and circular features with smooth surfaces were identified as possible craters. The survey identified 38 candidates with widely varying morphologies including rough, smooth and saddle-shaped floors, a lack of raised rims and fresh material exposures. The average depth/diameter ratio was 0.08±0.03: these craters are very shallow relative to craters observed on other asteroids. These shallow craters are a result of (1) target curvature influencing the cratering process, (2) raised rim not being generated by this process, and (3) fines infilling the craters. As many of the crater candidates have an unusual appearance, we used a classification scheme that reflects the likelihood of an observed candidate's formation by a hypervelocity impact. We considered a variety of alternative interpretations while developing this scheme, including inherited features from a proto-Itokawa, spall scars created by the disruption of the proto-Itokawa, spall scars following the formation of a large crater on Itokawa itself, and apparent depressions due to random arrangements of boulders. The size-frequency distribution of the crater candidates was close to the empirical saturation line at the largest diameter, and then decline with decreasing diameter.     

Laboratory simulation of the herringbone pattern associated with lunar secondary crater chains

V-shaped ridge components of the herringbone pattern associated with lunar secondary crater chains have been simulated by simultaneous and nearly simultaneous impact of two projectiles near one another. The impact velocities and angles of the projectiles were similar to those of the fragments that produced secondary craters found at various ranges from large lunar craters.Variables found to affect the included angles of the V-shaped ridges are: relative time of impact of the projectiles, impact angle, relative projectile mass, and azimuth angle of the crater chain relative to the projection of the flight line onto the target surface. The functional relationships between the forms of the ridges and many of these variables are similar to those observed for lunar V-shaped ridges.Comparison of the magnitudes of the ridge angles of both laboratory crater pairs and secondary crater chains of the crater Copernicus implies that material was ejected from Copernicus at angles in excess of 60°, measured from the normal, to form many of Copernicus' satellitic craters. Moreover, other independent calculations presented indicate that many of the fragments that produced secondary craters also ricocheted to produce tertiary craters.Application of the study to identification of isolated secondary craters and to the determination of the origin of large lunar craters is discussed.