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Wolfe Creek crater lies in northwestern Australia at the edge of the Great Sandy Desert. Together with Meteor Crater, it is one of the two largest craters on Earth from which meteorite fragments have been recovered. The age of the impact is poorly constrained and unpublished data places the event at about 300,000 years ago. In comparison, Meteor Crater is well constrained by exposure dating. In this paper, we present new ages for Wolfe Creek Crater from exposure dating using the cosmogenic nuclides 10Be and 26Al, together with optically stimulated luminescence ages (OSL) on sand from a site created by the impact. We also present a new topographic survey of the crater using photogrammetry. The exposure ages range from ~86 to 128 ka. The OSL ages indicate that the age of the impact is most likely to be ~120 ka with a maximum age of 137 ka. Considering the geomorphic setting, the most likely age of the crater is 120 ± 9 ka. Last, we review the age of Meteor Crater in Arizona. Changes in production rates and scaling factors since the original dating work revise the impact age to 61.1 ± 4.8 ka, or ~20% older than previously reported. [ABSTRACT FROM AUTHOR]

Ray craters are good indicators for deciphering the most recent cratering rate and surface alteration rate on a solar system solid body. We utilize Voyager and Galileo images to examine the distribution of ray craters (D > 10 km) on Callisto. We identify a total of 57 ray craters and 279 rim deposit craters and find that ray craters (D > 25 km) on Callisto show a pronounced decreasing apex‐antapex asymmetry (i.e., crater density decreases from the center of the leading hemisphere to the center of the trailing hemisphere), which would be consistent with a heliocentric origin, but the degree of the asymmetry is much lower than the theoretical expectation for ecliptic comets. A possible explanation for this is the preferential surface alteration rate on the leading hemisphere. The retention times of ray craters on Callisto are slightly longer than those on the dark terrain of Ganymede; this is possibly because of the substantially weaker micrometeorite gardening or/and sputtering on Callisto than those on Ganymede. The slope of the size‐frequency distribution of ray craters on Callisto is shallower than that of all impact craters, which is due to the higher erasure rate for the smaller craters' rays. Plain Language Summary: Distributions of impact craters can give insights into the sources of impactors and geological processes on a planetary body. However, the records of all impact craters on Callisto might have lost crucial information due to crater saturation. Instead, ray craters could serve as better indicators because they are much younger and unsaturated. This study examines the distributions of ray craters on Callisto and conducts comparisons of the ray crater distributions on Callisto and Ganymede. We find that large ray craters on Callisto exhibit a pronounced decreasing apex‐antapex asymmetry similar to those on the bright terrain of Ganymede, which is inconsistent with the previous view that Jovian impactors are mainly from ecliptic comets. Multiple explanations may be responsible for these trends, one being that differential surface alteration rate may preferentially erase rays on the leading hemisphere. Key Points: The distributions of ray craters on Callisto by using both Voyager and Galileo images are examinedLarge ray craters on Callisto exhibit a pronounced apex‐antapex asymmetry, similar to those on the bright terrain of GanymedeDifferential surface alteration rate may preferentially erase rays on the leading hemisphere [ABSTRACT FROM AUTHOR]

Carbonaceous meteorites provide clues with regard to prebiotic chemistry and the origin of life. Geological Survey of India recorded a carbonaceous chondrite meteorite fall in Mukundpura, India, on June 6, 2017. We conducted a study to investigate the microbial community that survived the meteorite impact. 16S rRNA metagenomic sequencing indicates the presence of Actinobacteria, Proteobacteria, and Acidobacteria in meteorite impact soil. Comparative phylogenetic analysis revealed an intriguing abundance of class Bacilli in the impact soil. Bacillus thermocopriae IR-1, a moderately thermotolerant organism, was isolated from a rock, impacted by the Mukundpura meteorite. We investigated the resilience of B. thermocopriae IR-1 to environmental stresses and impact shock in a Reddy shock tube. Bacillus thermocopriae IR-1 survived (28.82% survival) the effect of shock waves at a peak shock pressure of 300 kPa, temperature 400 K, and Mach number of 1.47. This investigation presents the first report on the effect of impact shock on B. thermocopriae IR-1. The study is also the first report on studying the microbial diversity and isolation of bacteria from impact crater soil immediately after meteorite impact event. [ABSTRACT FROM AUTHOR]

Virtual citizen science platforms allow nonscientists to take part in scientific research across a range of disciplines, including planetary science. What is required of the volunteer can vary considerably in terms of task type, variety, judgment required, and autonomy—even when the overall goal is unchanged. Through analysis of our live Zooniverse Planet Four: Craters citizen science platform, the effects of task workflow design factors including volunteer autonomy, task variety, task type, and judgment required on volunteer engagement and crater marking performance were investigated. Website analytics showed volunteers using the Full interface (most autonomy and variety) were more likely to return to the platform, although the amount of time spent per visit was unaffected by the interface used. However, analysis of performance suggested that how this time was used did differ. The interface involving the least complex task resulted in the greatest amount of data and rate of collection, although this also coincided with a greater number of false positives when compared with the expert. Performance in terms of agreement, both between participants and with the expert judgment, was significantly improved when using the Stepped interface for crater position and the Ramped (Mark) when measuring diameter—interfaces that both directly measured the metric with a specific, delineated task. The implications for planetary scientists considering the citizen science route is that there is a balancing act to perform, weighing the importance of volunteer engagement with scientists' data needs and the resources that can be committed to data validation. [ABSTRACT FROM AUTHOR]

The Sariçiçek howardite meteorite shower consisting of 343 documented stones occurred on September 2, 2015 in Turkey and is the first documented howardite fall. Cosmogenic isotopes show that Sariçiçek experienced a complex cosmic‐ray exposure history, exposed during ~12–14 Ma in a regolith near the surface of a parent asteroid, and that an ~1 m sized meteoroid was launched by an impact 22 ± 2 Ma ago to Earth (as did one‐third of all HED meteorites). SIMS dating of zircon and baddeleyite yielded 4550.4 ± 2.5 Ma and 4553 ± 8.8 Ma crystallization ages for the basaltic magma clasts. The apatite U‐Pb age of 4525 ± 17 Ma, K‐Ar age of ~3.9 Ga, and the U,Th‐He ages of 1.8 ± 0.7 and 2.6 ± 0.3 Ga are interpreted to represent thermal metamorphic and impact‐related resetting ages, respectively. Petrographic; geochemical; and O‐, Cr‐, and Ti‐isotopic studies confirm that Sariçiçek belongs to the normal clan of HED meteorites. Petrographic observations and analysis of organic material indicate a small portion of carbonaceous chondrite material in the Sariçiçek regolith and organic contamination of the meteorite after a few days on soil. Video observations of the fall show an atmospheric entry at 17.3 ± 0.8 km s−1 from NW; fragmentations at 37, 33, 31, and 27 km altitude; and provide a pre‐atmospheric orbit that is the first dynamical link between the normal HED meteorite clan and the inner Main Belt. Spectral data indicate the similarity of Sariçiçek with the Vesta asteroid family (V‐class) spectra, a group of asteroids stretching to delivery resonances, which includes (4) Vesta. Dynamical modeling of meteoroid delivery to Earth shows that the complete disruption of a ~1 km sized Vesta family asteroid or a ~10 km sized impact crater on Vesta is required to provide sufficient meteoroids ≤4 m in size to account for the influx of meteorites from this HED clan. The 16.7 km diameter Antionia impact crater on Vesta was formed on terrain of the same age as given by the 4He retention age of Sariçiçek. Lunar scaling for crater production to crater counts of its ejecta blanket show it was formed ~22 Ma ago. [ABSTRACT FROM AUTHOR]

Abstract The Sirente crater field (Central Italy) consists of a main, ≈130 m wide, droplet-shaped in plan view depression with a prominently elevated rim, and 30 smaller depressions in its close vicinity. It was discovered in 2002 and proposed to be of meteorite impact origin based on its geological and geomorphological similarities to known crater fields. It is developed in non-lithified lake sediments (i.e. carbonate mud). Given the age of formation in the 3rd to 5th centuries A.D., the inferred catastrophic origin was, in the media, soon related to the celestial sign ("Chi Rho") said to have been seen by Emperor Constantine in 312 A.D. and suggested to have changed the course of both Roman and Christian history. However, the meteoritic origin is not yet confirmed and has been contested. This work presents new geophysical data on the subsurface structure of the main crater, which provides further clues around the controversy of its origin. Two roughly transversal seismic reflection profiles across the main crater reveal a deep (53 m on average), rootless, bowl-shaped structure with a poorly developed, deep-seated central mound, as well as different seismic facies representing a crater infill of disturbed material resembling the breccia lens in craters formed in rock. This survey also allowed the recognition of possible compaction-fissures and upturned strata below the rim, similar to what is known from explosion craters in porous, unconsolidated targets. Some of these structures had already been noted in published resistivity surveys (ERT). The structural features noted in this geophysical survey are consistent with the impact hypothesis and do not support other proposed mechanisms of formation such as karst or mud volcano. Highlights • A deep, rootless, bowl-shaped structure with a deep-seated central mound. • A well-developed chaotic infill which can be translated as "breccia lens". • Possible compaction-fissures and upturned strata below and just outside the rim. • Structural features consistent with the impact hypothesis. • Structural features not supporting other proposed mechanisms of formation. [ABSTRACT FROM AUTHOR]

Planetary noble gases in chondrites are concentrated in an unidentified carrier phase, called "Q." Phase Q oxidized at relatively low temperature in pure oxygen is a very minor part of insoluble organic matter (IOM), but has not been separated in a pure form. High‐pressure (HP) experiments have been used to test the effects of thermal metamorphism on IOM from the Orgueil (CI1) meteorite, at conditions up to 10 GPa and 700 °C. The effect of the treatment on carbon structural order was characterized by Raman spectroscopy of the carbon D and G bands. The Raman results show that the IOM becomes progressively more graphite‐like with increasing intensity and duration of the HP treatment. The carbon structural transformations are accompanied by an increase in the release temperatures for IOM carbon and 36Ar during stepped combustion (the former to a greater extent than the latter for the most HP treated sample) when compared with the original untreated Orgueil (CI1) sample. The 36Ar/C ratio also appears to vary in response to HP treatment. Since 36Ar is a part of Q, its release temperature corresponds to that for Q oxidation. Thus, the structural transformations of Q and IOM upon HP treatment are not equal. These results correspond to observations of thermal metamorphism in the meteorite parent bodies, in particular those of type 4 enstatite chondrites, e.g., Indarch (EH4), where graphitized IOM oxidized at significantly higher temperatures than Q (Verchovsky et al.). Our findings imply that Q is less graphitized than most of the macromolecular carbonaceous material present during parent body metamorphism and is thus a carbonaceous phase distinct from other meteoritic IOM. [ABSTRACT FROM AUTHOR]

In the history of the study of Meteor Crater, there is an interlude between its initial geologic exploration in early 1890s (finalized by the seminal publication of the volcanic model by Gilbert in 1896) and the spectacular research-prospective activity of Barringer, completed with a meteoritic concept published in 1905. Here it is shown that the period between 1896 and 1905 did not represent a conceptual stasis in solving the mystery of the Arizona crater. Popular interest in the attractive theme was promoted by known scientists, such as Crookes, Holder, and Meyer. However, it was primarily exemplified by the work of Joseph A. Munk. In unnoticed publications from 1903 to 1906, he presented Coon Mountain indubitably as a meteorite crater, along with a proposal to adopt the appropriate name of Meteorite Mountain for this natural curiosity. His proimpact geologic arguments were widely repeated in later papers. In addition, Munk was a visionary pioneer in the propagation of the geotourist attraction represented by this geologic wonder, as manifested in his well-read guidebook Arizona Sketches. Furthermore, a surprising but significant issue is the hidden stimulating role of Fred W. Volz, known hitherto as a local businessmen and meteorite dealer. Especially in the light of Munk's publications, Volz is shown to have been an insightful observer of geologic phenomena, including a progressive revival of Gilbert's incipient conception of a buried "stellar body" under Coon Mountain (rejected by Gilbert himself). Even if this activity can be downgraded as only popular writing, Munk and (to some degree) Volz ought to be known at least as inspiring propagators and therefore pioneers of the meteorite hypothesis regarding Meteor Crater, independently of the widely known Barringer engagement. [ABSTRACT FROM AUTHOR]

Abstract The dwarf planet Ceres is a heavily cratered rocky body, and complex craters with a central peak are widely observed on its surface. These types of craters form when a large body impacts the surface, generating extreme temperatures and pressures. During the impact event a large volume of rock is raised from the subsurface and a central uplift is formed. The material composing the central uplift is called crater central peak material (ccp) and the spectral analysis of such geologic areas can provide information about the composition of Ceres' subsurface. Reflectance spectra of 32 ccps, acquired by the VIR spectrometer on board the NASA/Dawn spacecraft, were analyzed and shows absorption bands located at about 2.7, 3.1, 3.4 and 4.0 µm which are also common on the Cerean surface. These absorptions are related, respectively, to Mg-phyllosilicates, NH 4 -phyllosilicates and Mg/Ca-carbonates. The spectral parameters considered in this work are: spectral slopes estimated between 1.2 µm and 1.9 µm, band depths at 2.7-, 3.1-, 3.4- and 4.0-µm, and band centers near 4.0-µm. The ccps spectral parameters were analyzed in conjunction with other Cerean parameters, such as the estimated depth of excavation of the material composing the central peak, in order to search for correlations and information about Ceres' subsurface. Central peak material located polewards show stronger 2.7- and 3.1-µm band depths with respect to those at the equatorial region, suggesting that subsurface deposits closer to poles are probably richer in Mg- and NH 4 -phyllosilicates. The 3.4-µm spectral feature is also deeper in ccps located at poleward latitudes, similar to the phyllosilicates. Conversely, the 4.0-µm band does not show this trend with latitude. An increase in both 3.1- and 3.4-µm band depths with the estimated depth of excavation indicates that the spectral feature at 3.4-µm is the result of different contributions from carbonates and NH 4 -phyllosilicates, as expected. However, depending on their relative influence, the shape of the 3.4-µm spectral feature can vary. Phyllosilicates and carbonates are the resulting products of aqueous alteration of chondritic material and, given the increasing abundance of such minerals (in particular ammoniated phyllosilicates) with depth of excavation, it is likely that our investigation involved subsurface layers nearby the boundary between the volatile-rich crust and the silicate-rich mantle. Na-carbonate is found in the crater central peak material of Ernutet, Haulani and Ikapati, characterized by an estimated depth of excavation of about 6–9 km, where deposits of sodium carbonates could be locally present. [ABSTRACT FROM AUTHOR]

Highlights • Spectral analysis of Rongo quadrangle reveals a heterogeneous composition. • The two band depths at 2.7 and 3.1 µm display an overall low correlation. • Localized Na-carbonates units have been observed. • A correlation between the main geologic units and the Mg-phyllosilicates is found. Abstract Quadrangle Ac-H-10 'Rongo' (Lat 22°S to 22°N, Lon 288°–360°E) shows a fairly homogeneous topography, with the presence of notable elevations such as Ahuna Mons, Liberalia Mons , and part of Samhain and Uhola Catenae. The deepest areas correspond to the Rongo crater region, the areas between Samhain and Uhola catenae, and the region of the quadrangle south of Ahuna Mons. A substantial variability in the 2.7-µm band depth distribution is observed across the Rongo quadrangle, indicating an east-west gradient in the abundance of Mg-phyllosilicates. The NH 4 -phyllosilicates distribution appears quite homogeneous, except some localized regions, such as crater Haulani's ejecta, the flanks of Ahuna Mons, and crater Begbalel. The two band depths at 2.7 and 3.1 µm display an overall low correlation, suggesting a variable degree of mixing between Mg-phyllosilicates and NH 4 -phyllosilicates. At the local scale, mineralogical phases other than phyllosilicates are observed. Quadrangle Rongo includes sodium carbonate-rich regions, such as the flanks of Ahuna Mons, the ejecta of Xevioso crater located in the southern edge of Liberalia Mons, and crater Begbalel, which often display a reduction in both the 2.7- and 3.1-µm band depths, associated with an increased band depth at ∼4 µm, related to the presence of Na-rich carbonate phases. This suggests recent hydrothermal activity in this area, due to several episodes of cryovolcanism, or impacts that unveiled a peculiar composition in the shallow subsurface. Alternatively, the crust in this region might show a variable degree of compactness, such that the formation of Na-carbonates is favored only in specific locations (De Sanctis et al., 2016; Ruesch et al., 2016; Zambon et al., 2017). From a geological standpoint, quadrangle Ac-H-10 Rongo shows a correlation between its two main geologic units (Platz et al., 2017) and the distribution of Mg-phyllosilicates, suggesting a link between geology and mineralogy in this area. [ABSTRACT FROM AUTHOR]

Highlights • Spectral parameters of Urvara and Yalode quadrangles are mapped. • Band depths at 2.7 and 3.1 µm are shallower than western latitudes. • Correlation between geology and mineralogy arises only in the Yalode quadrangle. • The central peak of Urvara crater is enriched in ammonium. • Hummocky crater floors are darker because of their different roughness. Abstract We studied the distribution in the Urvara–Yalode region of Ceres (latitudes 21–66°S, longitudes 180–360°E) of main spectral parameters derived from the VIR imaging spectrometer onboard the NASA/Dawn spacecraft, as an overall study of Ceres mineralogy reported in this special issue. In particular, we analyzed the distribution of reflectance at 1.2 µm, band depth at 2.7 and 3.1 µm, ascribed to magnesium and ammoniated phyllosilicates, respectively. Whereas the average band depths of this region are lower than eastern longitudes, reflecting the E-W dichotomy of abundance of phyllosilicates on Ceres, spectral variations inside this region are observed in the following units: (a) the central peak of the Urvara crater (45.9°S, 249.2°E, 170 km in diameter), showing a deep 3.1 µm band depth, indicating an ammonium enrichment; (b) the cratered terrain westwards of the Yalode basin (42.3°S, 293.6°E, 260 km in diameter), where absorption bands are deeper, probably due to absence of phyllosilicates depletion following the Yalode impact; (c) the hummocky cratered floor of Yalode and Besua (42.4°S, 300.2°E) craters, characterized by lower albedo and band depths, probably due to different roughness; (d) Consus (21°S 200°E) and Tawals (39.1°S, 238°E) craters, whose albedo and band depths decreasing could be associated to different grain size or abundance of dark materials. Twenty-two small scale (i.e., lower than 400 m) bright spots are observed: because their composition is similar to the Ceres average, a strong mixing may have occurred since their formation. [ABSTRACT FROM AUTHOR]

Highlights • We present a method for thermal removal from Ceres spectra. • Most of the crater floors are devoid of volatiles due to the impact events. • Na-carbonates on Ikapati crater are likely preexistent to impact event. • We present the map of abundance of organic aliphatic material in Ernutet crater. • Organic rich-regions show different compositions pointing to a complex environment. Abstract Ceres has been explored by NASA/Dawn spacecraft, which allowed for the discovery of the main mineralogical and compositional characteristics of Ceres' surface. Here, we use mainly data from the Visible and InfraRed imaging spectrometer (VIR) in order to investigate the main spectral characteristics of the quadrangle Ac-H-2 Coniraya, one of the 15 quads in which Ceres' surface has been divided. Coniraya quadrangle is characterized by the presence of mostly highly degraded impact craters of diameters between 50 and 200 km and clusters of small to midsize impact craters. Although the composition over the quadrangle appears to be quite uniform, significant differences have been detected between different craters by spectral parameters analysis and spectral modeling. Ernutet crater presents two regions with very peculiar band at 3.4 µm, typical of organics aliphatic material. One region result to be correlated with larger amount of carbonates, the other region does not present such correlation. Ikapati crater shows strong absorption bands at 4.0 µm, indicating the presence of Na-carbonates in the floor and ejecta. Ikapati, Gaue and other craters present smaller spectral features of NH 4 and/or OH stretching, suggesting a volatile depletion process induced by the heating of the impact event. [ABSTRACT FROM AUTHOR]

Highlights • Dantu is a key feature in the investigation of Ceres' surface composition. • Dantu's surface composition provides insight into the composition of Ceres' crust. • Carbonates on Dantu's floor exhibit a similar composition as Occators' bright spots. Abstract Mapping Ceres' surface composition in the Dantu region, located between 21°–66°N and 90°–180°E, offers the unique possibility to investigate changes in the surface composition related to different stratigraphic levels of Ceres' crust. Dantu is located in a huge depression named Vendimia Planitia, which possibly represents a completely degraded impact basin formed in the beginning of Ceres' geological history. Most parts of this depression are characterized by strong phyllosilicate absorptions, which are stronger than elsewhere on Ceres' surface. This spectral signature possibly is related to the material emplaced at the time of the Vendimia impact event excavating material from deeper regions of Ceres' crust. Subsequent impacts in this basin reach far deeper into Ceres' crust than any impact events outside of Vendemia Planitia, which could explain the spectral signature of Dantu, possibly pointing to a higher concentration of ammonium-bearing phyllosilicates in Ceres' deeper crust. Spectral differences with respect to the small fresh craters on Dantu's floor are probably related to grain size effects causing a bluish visible slope as observed by fresh impact craters on other places on Ceres. The local enrichment of carbonates in the Dantu area could also be associated with the impact event and may have been formed by additional impact-triggered and/or post-impact alteration processes. [ABSTRACT FROM AUTHOR]

We have generated sub-millimetre-resolution DEMs of weathered rock surfaces using SfM photogrammetry techniques. We apply a close-range method based on structure-from-motion (SfM) photogrammetry in the field and use it to generate high-resolution topographic data for weathered boulders and bedrock. The method was pilot tested on extensively weathered Triassic Moenkopi sandstone outcrops near Meteor Crater in Arizona. Images were taken in the field using a consumer-grade DSLR camera and were processed in commercially available software to build dense point clouds. The point clouds were registered to a local 3-D coordinate system (x , y , z), which was developed using a specially designed triangle-coded control target and then exported as digital elevation models (DEMs). The accuracy of the DEMs was validated under controlled experimental conditions. A number of checkpoints were used to calculate errors. We also evaluated the effects of image and camera parameters on the accuracy of our DEMs. We report a horizontal error of 0.5 mm and vertical error of 0.3 mm in our experiments. Our approach provides a low-cost method for obtaining very high-resolution topographic data on weathered rock surfaces (area < 10 m 2). The results from our case study confirm the efficacy of the method at this scale and show that the data acquisition equipment is sufficiently robust and portable. This is particularly important for field conditions in remote locations or steep terrain where portable and efficient methods are required. [ABSTRACT FROM AUTHOR]

Reflectance spectra (0.3–4 μm) of powder samples of the third largest stone C0002 returned from the asteroid Ryugu by JAXA Hayabusa2 mission were measured and compared with proximity observation data taken by the Optical Navigation Camera - Telescopic (ONC-T) and the Near-Infrared Spectrometer (NIRS3) onboard the Hayabusa2 spacecraft. Although the characteristics of the 2.7 μm absorption band due to structural hydroxyl of the Ryugu powder samples are consistent with CI1 chondrite meteorites, NIRS3 spectra of the global surface of Ryugu exhibit a wavelength shift of +6 nm in the 2.7 μm absorption band center in comparison with those of both the powder samples and an artificial crater on the asteroid formed by the Small Carry-on Impactor (SCI). This is consistent with effects of solar wind-induced space weathering being present across the global surface of asteroid Ryugu. The darkness of the Ryugu surface in comparison with the returned samples and an upturn in reflectance observed in the average ONC-T spectrum at ultraviolet wavelengths may also be due to space weathering. • Analysis of the 2.7 μm band of asteroid Ryugu samples confirms its composition as CI1 chondrite. • A shift in the 2.7 μm band position of Ryugu surface indicates space weathering by solar wind. • The rate of space weathering on Ryugu is faster than the lifetime of its optical surface material. [ABSTRACT FROM AUTHOR]

We use observations of Bennu's impact craters, and measurements of particle sizes seen in images from the OSIRIS-REx (the Origins, Spectral Interpretation, Resource Identification Regolith Explorer sample-return mission) sampling event, to conclude there is a ∼ global sub-surface layer of finer-grained material on Bennu. The layer is evident in particle measurements from areas inside and around craters, roughness assessments of crater interiors vs. surrounding terrain, crater colors, crater depth-to-diameter (d/D) ratios, and the global crater size-frequency distribution. All data sets show a transition in behavior at crater diameters between 20 m and 40 m, with the most abrupt changes at ∼25 m diameter. Based on the depth of ∼25 m diameter craters, we infer that the layer is 1–4 m thick. The reddest craters are the youngest craters, and there is a trend to older ages as redness decreases. The decrease in redness is characterized by an initial, rapid fade, followed by a slower fade. The reduction in expected crater abundance with diameters ∼40–80 m, when extrapolating from larger sizes, is consistent with impact armoring caused by deeper, large sub-surface boulders depressing crater diameter. The existence of the layer appears to be a natural byproduct of impacts across a wide energy range and can be described as a "punctuated equilibrium": small, frequent impacts contribute to the local production of finer-grained particles and locally mix smaller particles into the subsurface; larger, less-frequent large impacts eject the fine-grained material, exposing interior boulders, and resetting the finer-grained particle population in that area. Other processes, e.g. thermal fracturing, could also contribute to the production of fine-grained material on rapid timescales. Extrapolating to the broader asteroid population and considering the production and preservation of small particles is related to target strength, surface acceleration, and impact speeds, we expect to find such layers on other asteroids, and that the layer thickness and depth will vary with composition, size, and orbital parameters. • Impact crater features on asteroid (101955) Bennu indicate subsurface layering. • Particles visible during the sampling event at Bennu support this conclusion. • A near-surface layer consists of finer-grained particles, and is 1–4 m thick. • We predict a layer of finer-grained particles should exist on other asteroids. • Layering has implications for the evolution of asteroid surfaces and interiors. [ABSTRACT FROM AUTHOR]

The evolution of Earth's atmosphere over billions of years provides important constraints on the geological history of our planet. However, major challenges in studying this have been finding samples in which ancient atmospheric signals are reliably preserved and precisely determining the age of the trapped atmosphere. Hydrothermal minerals can preserve fluid inclusions that formed from air-saturated waters. However, the use of hydrothermal minerals as paleo-atmospheric proxies is often limited by contamination by deeply-sourced crustal gases (e.g., radiogenic noble gases) and the difficulty of providing precise age constraints on the crystallization of the hydrothermal minerals. In this study, we tested the hypothesis that meteorite impact craters could provide an untapped archive of atmospheric evolution by investigating noble gases (Ne, Ar, Kr and Xe) contained in fluid inclusions within post-impact hydrothermal minerals from the Rochechouart impact structure (France). Impact craters can be dated using a variety of methods and because meteorite impacts cause localized fracturing of country rocks and the development of minor thermal anomalies, they are often associated with short-lived (up to a few Ma, depending on the size of the crater) and shallow hydrothermal systems dominated by surface-derived hydrothermal fluids. We found that the elemental ratios of noble gases in quartz from Rochechouart are close to the elemental ratios of modern Air. Furthermore, most noble gas isotope ratios are close to modern atmosphere, except for rare deviations caused by radiogenic ingrowth, nucleogenic effects and/or minor mass fractionation. After a minor correction for post-entrapment decay of 40K, an upper limit for the 40Ar/36Ar ratio of the ∼200 Ma atmospheric component trapped in Rochechouart quartz is determined as 292.7 ± 3.6 (2 σ s.d.), which is significantly lower than the modern value of 298.6 ± 0.3. A Mesozoic 40Ar/36Ar ratio lower than 292.7 ± 3.6 is in line with the existing paleo-atmospheric dataset and existing models of the temporal evolution of the atmospheric 40Ar/36Ar ratio and conclusively proves the preservation of an ancient atmospheric component in rocks from Rochechouart. Given that meteorite impacts have occurred throughout the entire Earth's history they might collectively provide a unique, yet unexplored, repository of the atmospheric evolution. The same approach might also be applicable to investigating the atmospheric evolution of other terrestrial planets. • Challenging to find reliable good proxies of the composition of the ancient atmosphere. • Investigation of post-impact hydrothermal minerals of the Rochechouart impact structure. • Identification of a paleo-atmospheric signal with a low 40Ar/36Ar ratio. • Rocks from impact craters can be scrutinized to find paleo-atmospheric signals. [ABSTRACT FROM AUTHOR]

This should have read: "Hasnaa during the post-conference field trip to Meteor Crater for the 2007 I Meteoritical Society Meeting i ." I Meteoritics & Planetary Science i , 58: 1171-1172. https://doi.org/10.1111/maps.14029 Figure 2 legend "Hasnaa during the post-conference field trip for the 2014 I Meteoritical Society Meeting i ." was incorrect. Bishop, J.L., Reimold, W.U., Baratoux, D. and Ferrière, L. (2023), Meteoritical Society Service Award Citation for Hasnaa Chennaoui Aoudjehane. [Extracted from the article]

Abstract: The Ghubara meteorite contains abundant trapped gases in voids of highly retentive phases that can be released by stepwise crushing and thermal degassing. Their composition is dominated by the solar wind component and by radiogenic argon. We favor a scenario in which a large impact event on L-chondrite asteroid 470 Ma ago caused release, mobilization, fractionation and redistribution of accumulated gases on the Ghubara parent body. The Ghubara breccia was formed at that event and occluded trapped gases into the voids. The uncommonly high 20Ne/36Ar ratios of the analysed samples compared to the solar composition is considered to be due to trapping of gases released from surrounding rocks that lost light noble gases preferentially over the heavy ones. The 4He/20Ne and 4He/36Ar ratios, being as usually lower than in solar wind, gradually increase during stepped crushing, indicating non equilibrium distribution of the gases between the voids of different sizes that can be caused by the dynamics of the shock metamorphism process. The neon isotopic composition released by stepwise crushing and combustion is a mixture of two components: solar dominating trapped and cosmogenic Ne. The former component is mainly degassed in the initial crushing steps opening the large inclusions/voids, while the relative contribution of the latter, likely released from galactic cosmic ray produced tracks, increases with progressive crushing. During stepwise combustion the same trend in the release of the Ne components with increasing temperature is observed. The nitrogen and carbon abundances as well as their isotopic compositions in Ghubara are usual for ordinary chondrites. Most of nitrogen is chemically bounded and associated with carbon. The delivery time of Ghubara from the parent body asteroid to the Earth calculated from its exposure age is 9-28 Ma. [ABSTRACT FROM AUTHOR]

Abstract Venus lacks an internally generated magnetic field today. Whether one existed in the past is unknown, but critical to atmospheric evolution and potential habitability. Canonical models assume the core of Venus is cooling too slowly for convection and thus a magnetic dynamo to occur today. Core/mantle heat flow is suppressed in these models after a putative transition in mantle dynamics associated with widespread, volcanic resurfacing. However, recent studies of impact craters and other surface features support more steady heat loss over geologic time. Precipitation of MgO and/or SiO 2 from the core can also drive compositional convection even with slow cooling. Here we reevaluate the likelihood that Venus has an "Earth-like" (at least partially liquid and chemically homogeneous) core using numerical simulations of the coupled atmosphere–surface–mantle–core evolution. An Earth-like core is only compatible with the modern lack of a dynamo if the thermal conductivity of core material is towards the higher end of modern estimates (i.e., >100 W m−1 K−1). If lower estimates like ∼40–50 W m−1 K−1 are actually correct, then we favor recent proposals that the core has completely solidified or preserved primordial stratification. Any simulation initialized with a homogeneous, liquid core predicts a global magnetic field with Earth-like surface strength for >2–3 billion years after accretion—consistent with all available observations—and also sporadic activity within the surface age while temperatures remain below the Curie point of magnetite. Therefore, future spacecraft missions should prioritize the first-ever magnetometer measurements below the ionosphere to search for crustal remanent magnetism. Highlights • Venus is the only major planet without known evidence for a magnetic dynamo. • We simulate dynamo activity over geologic time using atmosphere/interior models. • High thermal conductivity is needed to stop a dynamo today in an "Earth-like" core. • Maybe Venus does not have an "Earth-like" (liquid, chemically homogeneous) core. • Crustal remanent magnetism may await detection by future spacecraft missions. [ABSTRACT FROM AUTHOR]

The Decorah structure, recently discovered in northeastern Iowa, now appears as an almost entirely subsurface, deeply eroded circular basin 5.6 km in diameter and ~200 m deep, that truncates a near-horizontal series of Upper Cambrian to Lower Ordovician platform sediments. Initial analysis of geological and well-drilling data indicated characteristics suggestive of meteorite impact: a circular outline, a shallow basin shape, discordance with the surrounding geology, and a filling of anomalous sediments: (1) the organic-rich Winneshiek Shale, which hosts a distinctive fossil Lagerstätte, (2) an underlying breccia composed of fragments from the surrounding lithologies, and (3) a poorly known series of sediments that includes shale and possible breccia. Quartz grains in drill samples of the breccia unit contain abundant distinctive shock-deformation features in ~1% of the individual quartz grains, chiefly planar fractures (cleavage) and planar deformation features (PDFs). These features provide convincing evidence that the Decorah structure originated by meteorite impact, and current models of meteorite crater formation indicate that it formed as a complex impact crater originally ~6 km in diameter. The subsurface characteristics of the lower portion of the structure are not well known; in particular, there is no evidence for the existence of a central uplift, a feature generally observed in impact structures of comparable size. The current estimated age of the Decorah structure (460-483 Ma) suggests that it may be associated with a group of Middle Ordovician impact craters (a terrestrial "impact spike") triggered by collisions in the asteroid belt at ca. 470 Ma. [ABSTRACT FROM AUTHOR]

Abstract Spire-like pinnacles and fins are common features in silicic ignimbrite sheets that represent the fluxing of water through the cooling deposit. Fumarolic pinnacles at Crater Lake, Oregon (Mt. Mazama; 7.7 ka) and in the Valley of Ten Thousand Smokes, Alaska (VTTS; 1912) represent the iconic images of these processes. Glasses within these structures remain fresh despite experiencing high temperatures and extensive water–rock interaction with infiltrating groundwater. These glasses can be used to interrogate the thermal and hydrologic conditions of pinnacle formation, which are not well-understood. Higher diffusivity of water at elevated temperatures facilitates rapid secondary hydration of glass. This allows for δ D of pinnacle glasses to be used a “snapshot” proxy for the isotopic composition of the hydrating meteoric water with applications to paleoaltitude and paleoclimate. We present comprehensive H and O isotope data for volcanic glasses from pinnacles in the VTTS and from Crater Lake, which includes bulk δ 18 O glass and δ 17 O glass analyses and a new parameter, δ 18 O of water-in-glass (δ 18 O wig). The δ D and δ 18 O values of lake waters from the Katmai region are used to constrain the likely composition of VTTS hydration waters. The VTTS rhyolitic glasses from fumarolic mounds in the uppermost portion of the ignimbrite have depleted δ D glass (≥ − 152 ‰) but minimally depleted δ 18 O glass in high H 2 O (4.2 wt.%) pumices. These trends are best explained by the simple addition of modern meteoric waters from the Katmai region, thereby providing a snapshot of the meteoric water composition in the years to decades following the 1912 AD Novarupta-Katmai eruption. Glasses from Crater Lake pinnacles are used to estimate the δ D and δ 18 O of their hydration water shortly following the eruption of Mt. Mazama as well as the temperature conditions of their formation. Mt. Mazama glasses contain a narrow range of 1.6–1.9 wt.% H 2 O with depleted δ D and δ 18 O glass values as low as −149‰ and 0.95‰, respectively. All measured δ D glass and δ 18 O glass values fall between 75–100 °C based on estimated δ D and δ 18 O equilibrium fractionation between glass and water, as do Δ'17O glass compositions. The new δ 18 O glass — δ 18 O wig approach gives a wider range of temperatures up to ∼150 °C, but overall are in good agreement with δ 18 O glass isotope data. These compositions indicate 75–150 °C hydration temperatures and record extensive isotope exchange with meteoric waters. The near-boiling hydration temperatures strongly suggest that cooling ignimbrites remain dry above these temperatures. Cooling ignimbrite sheets likely have enough magmatic vapor overpressure to drive off vapor from ambient hydrosphere surface waters until they approach ∼100 °C, at which point external waters are able to percolate into the deposit. Pinnacles therefore reflect late-stage cooling features rather than vigorous fumaroles. These results are consistent with previous modeling work on cooling ignimbrites that drive water away until sub-boiling temperatures, but contrast with the near-magmatic temperatures previously suggested by depletions in δ 18 O in ignimbrite groundmass. Reconstructed δ D and δ 18 O values of paleo-meteoric waters at Mt. Mazama are −130 and −120‰ and −17.5 and −16.3‰ respectively, notably lighter than modern spring water compositions. This is interpreted to reflect a low- δ D and low- δ 18 O groundwater source fed by high-altitude glaciers or snowfields that originated from >3 km on the pre-collapse of Mt. Mazama edifice, more than 1 km above the modern lake level. Highlights • Ignimbrite-hosted pinnacles provide a snapshot view of paleoclimate or paleoaltitude. • Low- δ D and δ 18 O of pinnacle glasses record hydration temperature around 100 °C. • They are common late stage cooling features of ignimbrite–hydrosphere interaction. • Rapidly hydrated VTTS glasses record simple addition of modern meteoric water. • The Mt. Mazama edifice had glaciers above 3 km prior to caldera collapse. [ABSTRACT FROM AUTHOR]

The ∼20-m sized asteroid that entered the Earth's atmosphere at 19 km/s over the Chelyabinsk Region of Russia on February 15, 2013, broke and deposited ∼500 kt of kinetic energy at 45–25 km altitude, causing an airburst strong enough to create widespread glass damage. The shockwave hit a densely populated area. More than a thousand people asked for medical assistance at hospitals. In this paper, we analyze the available information about how many and what type of injuries were sustained. We combine previously collected data from government reports and from phone and internet surveys shortly after the event with newly collected data from local hospitals. As expected, the percentage of injuries was highest near the asteroid trajectory, but surprisingly the type of injury (cuts or bruises) do not show dependence on the distance from the asteroid trajectory. Results are compared to asteroid impact risk assessment models. The results provide insight for first responders in future asteroid impacts and help to rеfine these models. [ABSTRACT FROM AUTHOR]

Abstract For the first time, 60 large diamond aggregates were found inside the Popigai meteorite crater during washing of alluvial deposits along the Dogoi river crossing the crater. These aggregates are similar in appearance to yakutites from the placers of Northern Yakutia (YPY), and we regard them as yakutites from the Popigai crater (YPC). The structure and optical properties of Popigai impact diamonds from the impact melt rocks (tagamites) in the crater (PIDT) and yakutites YPC/YPY were compared in detail. In all these cases, a polycrystalline structure consisting of nanoscale grains of cubic and twinned cubic diamond (lonsdaleite) was found. This is the result of a solid-phase graphite-diamond transition due to an impact event 35 million years ago. The diamond aggregates show the following features: a red shift of the short-wave edge of the transmission, broadening of the diamond Raman peaks, signals from other diamond polytypes and numerous inclusions of other minerals in the Raman spectra, and a dominant broadband photoluminescence (PL). PL in the N3 system associated with N 3 V centers in PIDT diamonds indicates a high-temperature annealing of these aggregates with resulting aggregation of impurities during the prolonged cooling of large impact melt pockets and pools. It is assumed that some of the impact diamonds were ejected from the crater during the impact event and experienced rapid cooling. Some of these diamonds fell back into the crater (YPC yakutites), others have been deposited outside the crater and displaced during erosion (YPY yakutites). Difference in size and shape between the PIDTs and yakutites YPC/YPY is due to the difference in size of original graphite flakes or aggregates and/or due to the fundamentally different technologies of diamond extraction. Graphical abstract Unlabelled Image Highlights • 60 large diamond aggregates were found in placers on Dogoi river in the Popigai crater; • Analysis of physical properties allowed us to relate these aggregates to impact diamonds; • N3 system found only in diamonds from tagamites indicates their high-temperature annealing; • These diamond aggregates were thrown out from the crater and then fell back to the crater; • Morphology difference between impact diamonds is a result of different extraction techniques. [ABSTRACT FROM AUTHOR]

In this paper, we present a global database of craters larger than 0.7 km in diameter on the surface of (4) Vesta, which we created based on a global mosaic of the Dawn spacecraft's high-altitude mapping orbit (HAMO) images at a resolution of ∼60 m/pixel and cross-checked using the images from the low-altitude mapping orbit (LAMO) mission phase. The method to produce the crater database consists of visual crater identification and mathematical shape determination. To give precise measurements, topographic corrections have also been considered by combining the digital elevation model (DEM) data in this global survey. We calculated the morphometric parameters of the craters, which consist of diameter, depth, depth-to-diameter ratio, ellipticity, and azimuth of the major axis. Our results show that the craters on (4) Vesta are very shallow, with a mean depth-to-diameter ratio of ∼0.065, which is similar to that of the craters on (1) Ceres and (25143) Itokawa but lower than for craters on other solid bodies. The results also show that the fractions of craters with an ellipticity greater than 1.1 or 1.2 are smaller than those of craters on the Martian surface, which is probably because few secondaries exist on the surface of (4) Vesta. In addition, the azimuths of the major axis are almost uniformly distributed in different directions. Our globally consistent crater database has many potential applications for future studies such as geologic mapping and surface age dating. [ABSTRACT FROM AUTHOR]

Abstract: Ceres’ surface has commonly been linked with carbonaceous chondrites (CCs) by ground‐based telescopic observations, because of its low albedo, flat to red‐sloped spectra in the visible and near‐infrared (VIS/NIR) wavelength region, and the absence of distinct absorption bands, though no currently known meteorites provide complete spectral matches to Ceres. Spatially resolved data of the Dawn Framing Camera (FC) reveal a generally dark surface covered with bright spots exhibiting reflectance values several times higher than Ceres’ background. In this work, we investigated FC data from High Altitude Mapping Orbit (HAMO) and Ceres eXtended Juling (CXJ) orbit (~140 m/pixel) for global spectral variations. We found that the cerean surface mainly differs by spectral slope over the whole FC wavelength region (0.4–1.0 μm). Areas exhibiting slopes <−10% μm−1 constitute only ~3% of the cerean surface and mainly occur in the bright material in and around young craters, whereas slopes ≥−10% μm−1 occur on more than 90% of the cerean surface; the latter being denoted as Ceres’ background material in this work. FC and Visible and Infrared Spectrometer (VIR) spectra of this background material were compared to the suite of CCs spectrally investigated so far regarding their VIS/NIR region and 2.7 μm absorption, as well as their reflectance at 0.653 μm. This resulted in a good match to heated CI Ivuna (heated to 200–300 °C) and a better match for CM1 meteorites, especially Moapa Valley. This possibly indicates that the alteration of CM2 to CM1 took place on Ceres. [ABSTRACT FROM AUTHOR]

Abstract: Thermal evolution modeling has yielded a variety of interior structures for Ceres, ranging from a modestly differentiated interior to more advanced evolution with a dry silicate core, a hydrated silicate mantle, and a volatile‐rich crust. Here we compute the mass and hydrostatic flattening from more than one hundred billion three‐layer density models for Ceres and describe the characteristics of the population of density structures that are consistent with the Dawn observations. We show that the mass and hydrostatic flattening constraints from Ceres indicate the presence of a high‐density core with greater than a 1σ probability, but provide little constraint on the density, allowing for core compositions that range from hydrous and/or anhydrous silicates to a mixture of metal and silicates. The crustal densities are consistent with surface observations of salts, water ice, carbonates, and ammoniated clays, which indicate hydrothermal alteration, partial fractionation, and the possible settling of heavy sulfide and metallic particles, which provide a potential process for increasing mass with depth. [ABSTRACT FROM AUTHOR]

Over the past few decades, important fossils of dinosaurs, crocodylomorphs, and other latest Cretaceous vertebrates have been recovered from the Bauru Group of Brazil, giving unique insight into how Gondwanan faunas were evolving before the end-Cretaceous asteroid impact. Most of these records, however, come from a few regions, most notably Mato Grosso, western São Paulo, and Triângulo Mineiro. Here we describe a fossil reptile assemblage from the Late Cretaceous of the Adamantina and Marília formations (Bauru Group, Paraná Basin) in a region that has gone largely unexplored: southern Goiás State. Although these fossils are isolated and largely incomplete, they record a high diversity of vertebrates, including podocnemidoid turtles, crocodyliforms, and titanosaurid dinosaurs. As previous records from this region were limited to a single indeterminate sauropod bone, these new fossils are the first diagnostic members of their groups from southern Goiás State. Despite the fragmentary condition of the new fossils, they demonstrate that the local fauna was diverse during the latest Cretaceous, and generally similar to Bauru Group faunas from other parts of Brazil. These fossils lend further support to the hypothesis that a diverse terrestrial fauna, dominated by dinosaurs and crocodylomorphs but also including turtles, was widespread across South America (and possibly Gondwana) during the very end of the Cretaceous Period. [ABSTRACT FROM AUTHOR]

Abstract: This paper reviews major findings of the Multidisciplinary Experimental and Modeling Impact Crater Research Network (MEMIN). MEMIN is a consortium, funded from 2009 till 2017 by the German Research Foundation, and is aimed at investigating impact cratering processes by experimental and modeling approaches. The vision of this network has been to comprehensively quantify impact processes by conducting a strictly controlled experimental campaign at the laboratory scale, together with a multidisciplinary analytical approach. Central to MEMIN has been the use of powerful two‐stage light‐gas accelerators capable of producing impact craters in the decimeter size range in solid rocks that allowed detailed spatial analyses of petrophysical, structural, and geochemical changes in target rocks and ejecta. In addition, explosive setups, membrane‐driven diamond anvil cells, as well as laser irradiation and split Hopkinson pressure bar technologies have been used to study the response of minerals and rocks to shock and dynamic loading as well as high‐temperature conditions. We used Seeberger sandstone, Taunus quartzite, Carrara marble, and Weibern tuff as major target rock types. In concert with the experiments we conducted mesoscale numerical simulations of shock wave propagation in heterogeneous rocks resolving the complex response of grains and pores to compressive, shear, and tensile loading and macroscale modeling of crater formation and fracturing. Major results comprise (1) projectile–target interaction, (2) various aspects of shock metamorphism with special focus on low shock pressures and effects of target porosity and water saturation, (3) crater morphologies and cratering efficiencies in various nonporous and porous lithologies, (4) in situ target damage, (5) ejecta dynamics, and (6) geophysical survey of experimental craters. [ABSTRACT FROM AUTHOR]

Abstract: The oldest known large bolide impacts onto Earth are represented by approximately 3.47–3.2 Ga old Archean spherule layers of the Barberton Greenstone Belt (BGB) in South Africa and the Pilbara craton in West Australia. These layers were recognized as impact deposits by their excessively high platinum group element (PGE) contents that are indicative of an extraterrestrial component. This was followed by measurements of extraterrestrial Cr isotopic ratios, in some cases. Recently, the extraterrestrial PGE signature in Archean spherule layers from the BGB was localized and positively associated with the presence of submicrometer PGE alloy micronuggets associated with Ni,Cr‐rich spinel. The actual formation of these platinum group mineral (PGM) phases has, however, not yet been resolved. Primary meteoritic particles from the impacting body, the products of impact melting, or condensation from impact vapor plumes have all been proposed as possible genetic process. Resolving this requires detailed microanalytical investigation of the internal microchemical and microstructural compositions, textural characteristics, and crystallographic relationships between the different phases. Here, we report the results of a first transmission electron microscopy (TEM) study of six such PGE microparticles enclosed in Ni‐Cr spinel or occurring in groundmass of Barberton spherule layers from the BARB5 ICDP drill core and from the CT3 exploration core. Results include a variety of chemical and structural PGM compositions that are difficult to explain by a single process, leading to the conclusion that several processes may have been involved in the formation of PGMs in Archean spherule layers from the BGB. There is evidence supporting formation of these PGMs by exsolution from the spinel host phase, precipitation from a melt phase, and condensation from a gas phase (of the impact vapor plume). [ABSTRACT FROM AUTHOR]

Abstract: 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. [ABSTRACT FROM AUTHOR]

Viscously relaxed craters provide a window into the thermal history of Ganymede, a satellite with copious geologic signs of past high heat flows. Here we present measurements of relaxed craters in four regions for which suitable imaging exists: near Anshar Sulcus, Tiamat Sulcus, northern Marius Regio, and Ganymede's south pole. We describe a technique to measure apparent depth, or depth of the crater with respect to the surrounding terrain elevation. Measured relaxation states are compared with results from finite element modeling to constrain heat flow scenarios [see companion paper: Bland et al. (2017)]. The presence of numerous, substantially relaxed craters indicates high heat flows—in excess of 30–40 mW m −2 over 2 Gyr, with many small (<10 km in diameter) relaxed craters indicating even higher heat flows. Crater relaxation states are bimodal for some equatorial regions but not in the region studied near the south pole, which suggests regional variations in Ganymede's thermal history. [ABSTRACT FROM AUTHOR]

Dwarf planet Ceres, the largest object in the Main Asteroid Belt, has a surface that exhibits a range of crater densities for a crater diameter range of 5–300 km. In all areas the shape of the craters’ size-frequency distribution is very similar to those of the most ancient heavily cratered surfaces on the terrestrial planets. The most heavily cratered terrain on Ceres covers ∼15% of its surface and has a crater density similar to the highest crater density on <1% of the lunar highlands. This region of higher crater density on Ceres probably records the high impact rate at early times and indicates that the other 85% of Ceres was partly resurfaced after the Late Heavy Bombardment (LHB) at ∼4 Ga. The Ceres cratering record strongly indicates that the period of Late Heavy Bombardment originated from an impactor population whose size-frequency distribution resembles that of the Main Belt Asteroids. [ABSTRACT FROM AUTHOR]

The interactions between a katabatic flow on a plain and a circular basin cut into the plain and surrounded by an elevated rim were examined during a 5-h steady-state period during the Second Meteor Crater Experiment (METCRAX II) to explain observed disturbances to the nocturnal basin atmosphere. The approaching katabatic flow split horizontally around Arizona's Meteor Crater below a dividing streamline while, above the dividing streamline, an ~50-m-deep stable layer on the plain was carried over the 30-50-m rim of the basin. A flow bifurcation occurred over or just upwind of the rim, with the lowest portion of the stable layer having negative buoyancy relative to the air within the crater pouring continuously over the crater's upwind rim and accelerating down the inner sidewall. The cold air intrusion was deepest and coldest over the direct upwind crater rim. Cold air penetration depths varied around the inner sidewall depending on the temperature deficit of the inflow relative to the ambient environment inside the crater. A shallow but extremely stable cold pool on the crater floor could not generally be penetrated by the inflow and a hydraulic jump-like feature formed on the lower sidewall as the flow approached the cold pool. The upper nonnegatively buoyant portion of the stable layer was carried horizontally over the crater, forming a neutrally stratified, low-wind speed cavity or wake in the lee of the upwind rim that extended downward into the crater over the upwind sidewall. [ABSTRACT FROM AUTHOR]

Using high-resolution analytical electron microscopy, heterogeneous fragments of cosmogenic substances, such as nickelphosphide Ni3P and the unnamed intermetallide ZnAl2, are detected for the first time for a large meteorite crater, the Zhamanshin circular structure. Due to the impossibility of simultaneous finding these phases in the same meteorite, an assumption is made on the cometary nature of the impactor of the Zhamanshin crater. [ABSTRACT FROM AUTHOR]

Fragments of heterogeneous cosmonegic substance (nickelphosphide NiP and ZnAl) were found using high resolution analytical electron microscopic techniques, for the first time in samples from a large meteorite crater: the Zhamanshin astrobleme in Kazakstan. Inasmuch as such fragments cannot simultaneously occur in meteorite of any one type, we suggest that the impactor of the Zhamanshin crater was of comet nature. [ABSTRACT FROM AUTHOR]