Mechanism of Muong Nong-type tektite formation and speculation on the source of Australasian tektites

Abstract The source crater of the youngest and largest of the tektite strewnfields, the Australasian strewnfield, has not been located. A number of lines of evidence indicate that the Muong Nong-type tektites, primarily found in Indochina, are more primitive than the much more abundant and widespread splash-form tektites, and are proximal to the source. In this study the spatial distribution of Muong Nong-type tektite sites and chemical character have been used to indicate the approximate location of the source. The variation of Muong Nong-type tektite chemical composition appears to be caused by mixing of two silicate rock end-members and a small amount of limestone, and not by vapor fractionation. The variation in composition is not random, and does not support in-situ melting or multiple impact theories. The distribution of both Muong Nong and splash-form tektite sites suggest the source is in a limited area near the southern part of the Thailand-Laos border.     

Changes in abundance and nature of microimpact craters on the surfaces of Australasian microtektites with distance from the proposed source crater location

Abstract– Over 4600 Australasian microtektites from 11 sediment cores along an N–S transect in the Central Indian Ocean have been investigated optically for microimpact features on their surfaces. Detailed scanning electron microscope examination of 68 microtektites along this transect shows 4091 such features. These samples are located between approximate distances of 3900–5000 km from the suggested impact site in Indochina and therefore constitute distal ejecta. The morphology of the microimpacts seems to show distinct variations with distance from the source crater. The total number of microcraters on each microtektite decreases drastically from North to South indicating systematic decrease in the spatial density of the ejecta, and decrease in collisional activity between microtektites with distance from the proposed source crater location. Closer to the proposed source crater location, the microcraters are predominantly small (few μm), pit bearing with radial and concentric cracks, suggestive of violent interparticle collisions. The scenario is reverse farther from the source crater with smaller numbers of impacted microtektites due to increased dispersion of the ejecta and the microcraters are large and shallow, implying gentle collisions with larger particles. These observations provide systematic ground truth for the processes that take place as the ejecta of a large oblique impact which generated the Australasian tektite strewn field is emplaced. The microimpacts appear to take place during the descent of the ejecta and their intensity and number density decrease as a function of the spatial density of the ejecta at any given place and with distance from the source region. These features could help understand processes that take place during ejecta emplacement on planets with substantial atmosphere such as Mars and Venus.     

The search for fullerenes in rocks from the Ries impact crater

Abstract— Since their discovery, fullerenes have been reported from various geological environments. One group of these findings has been related to bolide impacts, e.g., the Sudbury crater and the K‐T and P‐T boundaries. Impact rocks of the Ries crater, Germany, including samples of suevites, metamorphosed crystalline clasts, and glass bombs, have been collected in the Otting, Altebürg, and Seelbronn quarries. No fullerenes in concentrations above 1 ppb have been found in analyzed samples. Laser desorption time‐of‐flight mass spectrometry (LD‐TOF‐MS) confirmed the absence of fullerenes in the analyzed samples. These results support the concept that the Ries impactor was a stony meteorite.     

The Aouelloul crater,Mauritania: On the problem of confirming the impact origin of a small crater

Abstract— The impact origin of small craters in sedimentary rocks is often difficult to confirm because of the lack of characteristic shock metamorphic features. A case in point is the 3.1 Ma Aouelloul crater (Mauritania), 390 m in diameter, which is exposed in an area of Ordovician Oujeft and Zli sandstone. We studied several fractured sandstone samples from the crater rim for the possible presence of shock metamorphic effects. In thin section, a large fraction of the quartz grains show abundant subplanar and planar fractures. Many of the fractures are healed and are evident only as fluid inclusion trails. A few grains showed sets of narrow and densely spaced fluid inclusions trails in one (rarely two) orientations per grain, which could be possible remnants of planar deformation features (PDFs), although such an interpretation is not unambiguous. In contrast, an impact origin of the crater is confirmed by Re-Os isotope studies of the target sandstone and glass found around the crater rim, which show the presence of a distinct extraterrestrial component in the glass.     

Floor-fractured crater models of the Sudbury Structure,Canada: Implications for initial crater size and crater modification

Abstract The pattern of radial and concentric offset dikes at Sudbury strongly resembles fracture patterns in certain volcanically modified craters on the Moon. Since the Sudbury dikes apparently formed shortly after the impact event, this resemblance suggests that early endogenic modification at Sudbury was comparable to deformation in lunar floor-fractured craters. Although regional deformation has obscured many details of the Sudbury Structure, such a comparison of Sudbury with lunar floor-fractured craters provides two alternative models for the original size and surface structures of the Sudbury basin. First, the Sudbury date pattern can be correlated with fractures in the central peak crater Haldane (36 km in diameter). This comparison indicates an initial Sudbury diameter of between 100 and 140 km but requires loss of a central peak complex for which there is little evidence. Alternatively, comparison of the Sudbury dikes with fractures in the two-ring basin Schrödinger indicates an initial Sudbury diameter of at least ～ 180 km, which is in agreement with other recent estimates for the size of the Sudbury Structure. In addition to constraining the size and structure of the original Sudbury crater, these comparisons also suggest that crater modification may reflect different deformation mechanisms at different sizes. Most lunar floor-fractured craters are attributed to deformation over a shallow, crater-centered intrusion; however, there is no evidence for such an intrusion at Sudbury. Instead, melts from the evolving impact melt sheet probably entered fractures formed by isostatically-induced flexure of the crater floor. Since most of the lunar floor-fractured craters are too small (<100-km diameter) to induce significant isostatic adjustment, crater modification by isostatic uplift apparently is limited to only the largest of craters, whereas deformation over igneous intrusions dominates the modification of smaller craters.     

Scattering in the radiation source and the fundamental-harmonic hypothesis

A model for the radiation source for type III solar radio bursts which includes random density fluctuations is reviewed. This methodology is applied to the burst of 28 September, 1973, 03:19 UT which is an archetype fundamental-harmonic pair. It is found that for scattering inhomogeneities consistent with those necessary to explain the observed sizes of the sources, it is impossible to amplify fundamental radiation in a source with a spatially uniform energy density in plasma waves; i.e., it is impossible to interpret this burst as a fundamental-harmonic pair from such a source. However, the supposed fundamental has fine structure similar to type IIIb bursts and since it is very difficult to explain these features except as fundamental radiation, it is concluded that there must be small clumps of intense plasma waves in the source which allow the fundamental to be amplified. These results are also applied to the hectometric burst of 19 July, 1971 for which a steep rise in brightness is observed between 10 and 50 R ₀. It is argued that the most plausible explanation of this rise is that the density inhomogeneities become sufficiently weak to allow the fundamental to be amplified in this range.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Australasian microtektites in the South China Sea and the West Philippine Sea: Implications for age,size, and location of the impact crater

Abstract— Microtektites from two deep‐sea cores in the South China Sea and the West Philippine Sea are identified as belonging to the Australasian tektite strewn field based on the morphology, chronostratigraphic occurrence, and geographical location of these microtektites. The higher concentrations of microtektites (>1000/cm²) in the marginal seas of the western Pacific, with the peak concentration in the South China Sea, support the hypothesis of a large impact crater in Indochina. These two new occurrences lead to a more precise dating of the impact event at 793 ka, whereas the size of the Australasian source crater on the Indochina Peninsula is estimated to be 90–116 km.     

An examination of an interstellar hypothesis for the source of comets

The Sun's gravity tends to pull in nearby an excess of low-energy particles, as compared with the distribution of energies at infinity. The size of this effect is calculated in an attempt to determine if it is large enough to lend support to the hypothesis of an interstellar origin for comets. The effect appears to be too small; the greatest possible reduction of observed mean energy below intrinsic mean energy is a factor $\text{1}\text{2}$, if the true velocity distribution of the particles (comets) at infinity is Maxwellian. Non-Maxwellian distributions, however, could violate this restriction.     

The Carancas meteorite impact crater,Peru: Geologic surveying and modeling of crater formation and atmospheric passage

Abstract— The recent Carancas meteorite impact event caused a worldwide sensation. An H4–5 chondrite struck the Earth south of Lake Titicaca in Peru on September 15, 2007, and formed a crater 14.2 m across. It is the smallest, youngest, and one of two eye‐witnessed impact crater events on Earth. The impact violated the hitherto existing view that stony meteorites below a size of 100 m undergo major disruption and deceleration during their passage through the atmosphere and are not capable of producing craters. Fragmentation occurs if the strength of the meteoroid is less than the aerodynamic stresses that occur in flight. The small fragments that result from a breakup rain down at terminal velocity and are not capable of producing impact craters. The Carancas cratering event, however, demonstrates that meter‐sized stony meteoroids indeed can survive the atmospheric passage under specific circumstances. We present results of a detailed geologic survey of the crater and its ejecta. To constrain the possible range of impact parameters we carried out numerical models of crater formation with the iSALE hydrocode in two and three dimensions. Depending on the strength properties of the target, the impact energies range between approximately 100–1000 MJ (0.024–0.24 t TNT). By modeling the atmospheric traverse we demonstrate that low cosmic velocities (12–14 kms^?1) and shallow entry angles (<20 °) are prerequisites to keep aerodynamic stresses low (<10 MPa) and thus to prevent fragmentation of stony meteoroids with standard strength properties. This scenario results in a strong meteoroid deceleration, a deflection of the trajectory to a steeper impact angle (40–60 °), and an impact velocity of 350–600 ms^?1, which is insufficient to produce a shock wave and significant shock effects in target minerals. Aerodynamic and crater modeling are consistent with field data and our microscopic inspection. However, these data are in conflict with trajectories inferred from the analysis of infrasound signals.     

The origin of lunar crater rays

Lunar rays are filamentous, high-albedo deposits occurring radial or subradial to impact craters. The nature and origin of lunar rays have long been the subjects of major controversies. We have determined the origin of selected lunar ray segments utilizing Earth-based spectral and radar data as well as FeO, TiO₂, and optical maturity maps produced from Clementine UVVIS images. These include rays associated with Tycho, Olbers A, Lichtenberg, and the Messier crater complex. It was found that lunar rays are bright because of compositional contrast with the surrounding terrain, the presence of immature material, or some combination of the two. Mature “compositional” rays such as those exhibited by Lichtenberg crater, are due entirely to the contrast in albedo between ray material containing highlands-rich primary ejecta and the adjacent dark mare surfaces. “Immaturity” rays are bright due to the presence of fresh, high-albedo material. This fresh debris was produced by one or more of the following: (1) the emplacement of immature primary ejecta, (2) the deposition of immature local material from secondary craters, (3) the action of debris surges downrange of secondary clusters, and (4) the presence of immature interior walls of secondary impact craters. Both composition and state-of-maturity play a role in producing a third (“combination”) class of lunar rays. The working distinction between the Eratosthenian and Copernican Systems is that Copernican craters still have visible rays whereas Eratosthenian-aged craters do not. Compositional rays can persist far longer than 1.1 Ga, the currently accepted age of the Copernican-Eratosthenian boundary. Hence, the mere presence of rays is not a reliable indication of crater age. The optical maturity parameter should be used to define the Copernican-Eratosthenian boundary. The time required for an immature surface to reach the optical maturity index saturation point could be defined as the Copernican Period.     

The detrital zone in the shorty crater cores

The lower part of lunar cores 74002/1 contains pure fine-grained black soil grading upward to orange soil. The section, however, between 10 cm and the lunar surface contains a mixture of orange and dark soil with a clast-in-matrix texture and some agglutinates. Therefore, this upper section is interpreted as a detrital zone. Although Shorty Crater was formed approximately 30 m.y. ago, all indicators of soil age give a much shorter time for residence of the detrital zone. Both absolute agglutinate content and authigenic agglutinate content indicate a surface residence of less than 8 m.y. for the detrital portion of the core. Most calculated ages of the detrital zone cluster are around 3 m.y. Grain size distribution is characteristic of an immature soil and there is little evidence, indicated by lack of upward fining and decrease in coarsest grain sizes, ofin situ maturation of the section. Mixing with adjacent soils is very low, even though such soils lie only 0.5 M from the sampling site. Four of the five sub-strata in the upper 10 cm could have been produced by the impact event that produced the 20 M wide boulder field near the sampling site on the Shorty Crater rim. This event would distribute perched clasts over the sampling site. Thickness of the detrital part of the section is in keeping with its being ejecta from the boulder bed crater. The thickness of the agglutinate-rich zone, 1.5 cm, is reasonable for a less-than 4 m.y. residence time.     

The terrestrial cratering record: II. The crater production rate

The terrestrial cratering record for the Phanerozoic has a size-frequency distribution of NαD^?2.05 for D > 22.6 km and NαD^?0.24 for D < 11.3 km. This shallowing of the distribution slope at D > 22.6 km reflects the removal of small terrestrial craters by erosion. The number of large craters on the North American and East European cratons provide estimated terrestrial crater production rates for D > 20 km of 0.36 ± 0.1 and 0.33 ± 0.2 × 10^?14 km^?2 year^?1, respectively. These rates are in good agreement with previous estimates and astronomical observations on Apollo bodies. Comparisons with the lunar rate, taking account of the effects of variations in impact velocity, surface gravity, and gravitational cross section, indicate that the lunar and terrestrial rates overlap, if the cratering flux has been constant during the last 3.4 by. If the early (pre 4.0 by) high-flux rate did not decay to a constant value until 3.0 to 2.5 by then the rates differ by a factor of 2 and the Phanerozoic can be interpreted as a period of higher than normal cratering.     

Millochau crater, Mars: Infilling and erosion of an ancient highland impact crater

The geology and stratigraphy of Millochau crater (21.4° S, 275° W), located in the highlands of Tyrrhena Terra, Mars, are documented through geomorphic analyses and geologic mapping. Crater size-frequency distributions and superposition relationships are used to constrain relative ages of geologic units and determine the timing and duration of the geologic processes that modified Millochau rim materials and emplaced deposits on Millochau's floor. Crater size-frequency distributions show a Middle Noachian age for rim materials and Middle Noachian to Early Hesperian ages for most of the interior deposits. Valley networks and gullies incised within Millochau's rim materials and interior wall, respectively, indicate fluvial activity was an important erosional process. Millochau contains an interior plateau, offset northeast of Millochau's center, which rises up to 400 m above the surrounding crater floor and slopes downward to the south and west. Layers exposed along the northern and eastern scarp boundaries of the plateau are tens to hundreds of meters thick and laterally continuous in MOC images. These layers suggest most materials within Millochau were emplaced by sedimentary processes (e.g., fluvial or eolian), with the potential for lacustrine deposition in shallow transient bodies of water and contributions of volcanic airfall. Mass wasting may have also contributed significant quantities of material to Millochau's interior, especially to the deposits surrounding the plateau. Superposition relationships combined with impact crater statistics indicate that most deposition and erosion of Millochau's interior deposits is ancient, which implies that fluvial activity in this part of Tyrrhena Terra is much older than in the eastern Hellas region. Eolian processes mobilized sediment to form complicated patterns of long- and short-wavelength dunes, whose emplacement is controlled by local topography. These deposits are some of the youngest within Millochau (Amazonian) and eolian modification may be ongoing.     

Review of the Barringer crater studies and views on the crater’s origin

The first scientific studies of Barringer crater, Arizona, USA (also known as the Coon Butte crater), began more than a century ago; however, views on the crater’s origin have been contradictory. At the beginning of the 20th century, D.M. Barringer, a mining engineer, became interested in the possibility of finding large useable iron masses in this crater and searched for these masses for more than 25 years, standing up for the idea of the crater’s meteoric origin, contrary to the objections of opponents who tried to indicate that the crater was caused by terrestrial geological processes. Mining, accompanied by different scientific works, made it possible to obtain reliable data on the structure and impact origin of the crater; however, attempts to find meteorite iron deposits in this crater were unsuccessful. Barringer crater was the first object on the Earth where purposeful studies were performed for many decades and made it possible to develop many criteria of the impact origin of circular geological structures and mechanisms of formation of these structures, as well as to compare this crater with similar morphostructures on the surfaces of other planets. These studies have played an important role in the formation and development of the theory of impact cratering, which has been generally acknowledged in present-day science.     

Petrography and geochemistry of the LaPaz Icefield basaltic lunar meteorite and source crater pairing with Northwest Africa 032

Abstract— We report on the bulk composition and petrography of four new basaltic meteorites found in Antarctica—LAP (LaPaz Icefield) 02205, LAP 02224, LAP 02226, and LAP 02436—and compare the LAP meteorites to other lunar mare basalts. The LAP meteorites are coarse‐grained (up to 1.5 mm), subophitic low‐Ti basalts composed predominantly of pyroxene and plagioclase, with minor amounts of olivine, ilmenite, and a groundmass dominated by fayalite and cristobalite. All of our observations and results support the hypothesis that the LAP stones are mutually paired with each other. In detail, the geochemistry of LAP is unlike those of any previously studied lunar basalt except lunar meteorite NWA (Northwest Africa) 032. The similarities between LAP and NWA 032 are so strong that the two meteorites are almost certainly source crater paired and could be two different samples of a single basalt flow. Petrogenetic modeling suggests that the parent melt of LAP (and NWA 032) is generally similar to Apollo 15 low‐Ti, yellow picritic glass beads, and that the source region for LAP comes from a similar region of the lunar mantle as previously analyzed lunar basalts.     

The age of the Saltpan impact crater,South Africa

Abstract— The 1.13-km-diameter Pretoria Saltpan impact crater is located about 40 km NNW of Pretoria, South Africa. The crater is situated in 2.05 Ga old Nebo granite of the Bushveld Complex that is locally intruded by about 1.3 Ga old volcanic rocks. In 1988, a borehole was drilled in the center of the crater. At depths >90 m, breccias were found that contained minerals with characteristic shock-metamorphic features, thus confirming the impact origin of the crater. Fragments of impact glass were recovered from the melt breccias and several hundred sub-millimeter-sized glass fragments were subjected to fission track analysis. The measurements were complicated by the inhomogeneous composition of the impact glasses, but analysis of a large number of tracks yielded an age of 220 ± 52 ka for the Saltpan crater.     

The Sirente crater,Italy: Impact versus mud volcano origins

Abstract— The Sirente crater is a circular structure with a diameter of ˜80 m. The rim deposit is an inverse‐graded, matrix‐supported breccia. Sedimentological features of the rim deposit suggest that the crater is not related to an explosion or violent mechanical displacement. The structure and texture of the deposit exhibit a primary sedimentary character. The rim deposits do not contain artifacts and do not show evidence of reworking. A multistage formation is reconstructed for the rim growth and associated deposits. The geometry and sedimentology of the deposits indicate that they were produced by the extrusion and accumulation of mudflow deposits. The dominant ejection mechanism was low mud fountains and the transport medium was water. Petrographic and geochemical evidence does not indicate any physical or cryptic trace of an extraterrestrial body. The most realistic agent that explains the observed effects is a rapid local emission of mud and/or water. Geological processes capable of producing these features include piping sinkholes or, more probably, “caldera”‐type mud volcanoes, which may result from underground water‐table perturbation and/or decompression of deep CO₂/hydrocarbon gas reservoirs due to tectonic deformation or faulting activity during a seismic event. In both cases, the name “crater” for this geological form may be maintained, but there is no compelling evidence for an impact origin. In this paper, the scientific literature on the Sirente crater is reconsidered in the light of new morphological, sedimentological, geochemical, and archaeological data. A new mechanism is proposed involving mud‐fountaining.     

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.