Impact Craters Research Articles

Sediments and morphology of deep lakes of Central Tunguska Plateau (Krasnoyarsk krai, Evenkia): on the problem of the Tunguska Event at 1908

The shape of the bottom and the thickness of sediments of small lakes Zapovednoye and Peyungda, located 50-60 km from the supposed epicenter of the “Tunguska Event” of 1908, were studied using echo-sounding and seisimo-acoustic profiling, and their age was estimated using 137Cs, 210Pb, 14C. It was revealed that both lakes have funnel shape with a depth of 60 and 34 m, respectively, and are similar in shape and size to Lake Cheko, which, according to some researchers, is an impact crater formed by a celestial body as a result of the “Tunguska Event 1908”. The examples of lakes Zapovednoye and Peyungda show that the shape of Lake Cheko is not unique to this area. The age of the bottom sediments of lakes Zapovednoye and Peyungda exceeds several thousand years. In the sediments of lakes Zapovednoe and Peyungda, a layer with a high content of terrigenous elements was identified, presumably formed due to the input of disturbed soil layer as a result of the massive fall of trees after the “Tunguska 1908”. Thus, the presence of two deep funnel-shaped lakes near the epicenter of the “Tunguska 1908” much older than 1908, casts doubt on the uniqueness of Lake Cheko, and therefore its impact origin. In addition, according to our estimate, the age of Lake Cheko is no less than 300 years, which contradicts the hypothesis of its origin as a result of the “Tunguska 1908”.

Depth-to-diameter Ratios of Fresh Craters on the Moon and Implications for Surface Age Estimates

The depth-to-diameter (d/D) ratios of small lunar craters (D < 400 m) can be used to determine important properties of the upper regolith, specifically material strength or thickness. The d/D is also an important component of topographic diffusion models that describe how different erosive processes influence and change the topography of a surface over time, and these models have been applied to estimate surface ages. These models must make assumptions regarding rates of erosion and the initial d/D of a crater. Previous works investigating d/D of small craters, which use various methodologies to calculate depth, typically assume that a fresh appearing crater is a young crater. Work presented here provides d/D measurements of known—rather than assumed—young, meter-scale craters to provide better constraints on small crater depths and to help further our understanding of lunar surface ages and upper regolith properties. Given the interest in impact crater modification at small, human scales on the Moon and the wide range of assumptions built into topographic diffusion models and their predictions, understanding whether the results for initial d/D from past work hold up under different analyses is critical. We observed no distinct trends in d/D for small, young craters and report a wide range of d/D from 0.08 to 0.215, in contrast with past work that derived different averages based on crater size. The variation in d/D may correspond to heterogeneous regolith properties or be a result of a data source artifact.

Crater Populations of the Saturnian Satellites Mimas, Rhea, and Iapetus

AbstractThe Saturnian system has been explored by four spacecraft: Pioneer 11, Voyager 1 and 2, and Cassini. Only the last three took images suitable for photogeologic analysis of the surfaces of Saturn's moons, and over the decades, several research groups have published data about the crater distributions on the Saturnian satellites. These groups have used those data to draw conclusions about the impactor populations and resurfacing histories of the moons, but no one has examined how well the different data agree between the researchers. We present independent mapping of the crater populations of Saturn's moons Mimas, Rhea, and Iapetus, and compare them with many published crater populations. We found that Mimas data are the most consistent between different researchers, and Rhea data are the least consistent. We attribute these differences to (a) data biases where there are fewer images upon which to map Mimantean craters but a large variety exist for Rhean, and (b) Rhea likely has different terrains with different impact crater populations which have not been generally recognized before. We also found that Iapetus' small craters appear to have a shallow branch, as others have found, and that shallow branch is not attributable to completeness limitations. Other bodies have shallow branches at small diameters, too, but they are not as shallow as Iapetus's, which suggests varying impacting populations as one moves closer to Saturn, in line with others' work on planetocentric impactors.

An automatic anchor-free convolutional neural network method for recognizing small-scale lunar impact craters

An automatic anchor-free convolutional neural network method for recognizing small-scale lunar impact craters

A New Mechanism for Stishovite Formation During Rapid Compression of Quartz and Implications for Asteroid Impacts

AbstractShock‐induced transformations of quartz to high‐pressure polymorphs and diaplectic glass are decisive in identifying impact cratering events. Under shock compression, quartz can melt in local hot spots and crystallization of these silica melts under pressure can yield the high‐pressure mineral stishovite. A solid‐state transition to stishovite in relation to the formation of amorphous lamellae was already suggested in the late 1960s, but this idea was never comprehensively proven. Therefore, the mechanism responsible for such an intracrystalline stishovite formation is unknown to date. Herein, crystallographically oriented single crystals of quartz were compressed and decompressed in a membrane‐driven diamond anvil cell. These experiments aim at simulating the pressure paths of natural impacts on the timescale of seconds using compression rates between 0.2 and 0.6 GPa/s and peak pressures between 20 and 37 GPa. During the compression of quartz, the time‐resolved synchrotron X‐ray diffraction patterns reveal the almost simultaneous formation of two high‐pressure polymorphs, the recently identified rosiaite‐structured silica and stishovite. Transmission electron microscopic observations of recovered samples show that stishovite occurs as arrays of uniformly oriented nanometer‐sized crystals in amorphous intracrystalline lamellae. These observations indicate that the numerous stishovite crystals likely nucleated from the structurally similar rosiaite phase and thus inherited their uniform orientation during compression. During decompression, the metastable and non‐quenchable rosiaite‐structured phase collapsed to the amorphous stishovite‐containing lamellae. These findings attest to a novel mechanism of the formation of stishovite in the solid state and provide an explanation for similar microstructural occurrences of stishovite in impact‐metamorphic rocks and shocked meteorites.

Isotopic Geochronological Constraints on the Formation and Evolution of the Moon

One major task of studying the formation and evolution of the Moon is to construct a timeline of the important events with precise isotopic ages. Here, we review recent major isotopic geochronological progress in the past decade and the unsolved problems in isotopic geochronology. The Moon probably formed between 4.52 and 4.42 Ga. Recent high-precision whole-rock and mineral Sm-Nd isotopic dating results suggested that ferroan anorthosite and highlands magnesian suite rocks formed contemporarily around 4.37 to 4.33 Ga. Although the major mare basaltic volcanism took place from 3.85 to 2.93 Ga, new geochronological data from lunar meteorites and Chang’e-5 basalts suggested that lunar basaltic volcanism took place as old as up to 4.37 Ga and at least as young as 2.0 Ga, respectively. Impact events older than 3.9 Ga have also been revealed based on U-bearing minerals Pb/Pb ages and Ar-Ar ages and can provide important clues to understand the late heavy bombardment hypothesis. However, the reliable isotopic ages for the important events on the Moon are still far from conclusive, due to lack of pristine samples that directly crystallized from Lunar Magma Ocean and samples from impact melt sheets in large impact basins (e.g., the South Pole-Aitken basin). In the future, collection and return of pristine samples of ferroan anorthosite and highlands magnesian suite rocks from the farside, cryptomare basalts and late-stage basalts, quartz monzogabbros, granites/felsites, and rocks from impact melt sheets in large impact basins are required for better understanding the formation and evolution of the Moon.

Evolution of the Lunar Magnetic Field

The Moon currently lacks a global magnetic field; however, both paleomagnetic analyses of lunar rock samples and orbital magnetic measurements indicate that it once possessed a core dynamo. Magnetic field measurements of some datable impact basins suggest that the lunar core dynamo persists to the Nectarian period (~3.9 to 3.8 billion years ago Ga). Investigations of the Apollo samples using modern methods demonstrate that the field overall was active between 4.25 and 1.92 Ga. During the period prior to 3.56 Ga, the field was sometimes comparable to Earth’s but subsequently declined dramatically and ultimately ceased. Several hypotheses have been proposed to explain the dynamo generation and duration. Thermal convection in the lunar core could have provided dynamo energy for the first several hundred million years while core crystallization could have sustained the dynamo for up to 1.5 Ga. Other mechanisms, such as mantle and/or inner core precession, changes in the rotation rate of the lunar mantle caused by impacts, and a basal magma ocean, also hold the potential to power the dynamo during some time of lunar evolutionary history. Impacts related to plasmas are believed to be insufficient for crustal magnetization though they can amplify the pre-existing magnetic field before the impacts. This paper summarizes and reviews the current understanding of lunar magnetic field evolution, including paleomagnetic studies that quantify the timing of the lunar surface strength, global crustal magnetization features derived from recent global magnetic field models based on orbital magnetic measurements, and various models explaining the powering of a lunar dynamo, which can account for most observations. Finally, we propose the outstanding questions and offer guidance for future lunar exploration such as the Chang’E series and lunar scientific observatories.

Evolution of the Lunar Tectonic Framework and Structures

The evolution of the Moon is driven by both endogenic (e.g., magmatism) and exogenic processes (e.g., impact). The lunar surface can be divided into 3 first-order tectonic units based on differences in geochemistry, crustal thickness, topography, and distribution of structures. However, the interpreted boundaries demarcated by different features varied widely. As research progresses, the understanding of the lunar structure is continuously improving, and new types of structures are constantly being discovered. The previous structural classification system needs to be updated. According to the major geological events and dynamic evolution, the evolutionary history of the Moon can be divided into 3 stages and 1 pivotal event. The first stage (4.52 to 4.3 Ga) is the evolution of the magmatic ocean, and it was dominated by endogenic processes. The formation of South Pole-Aitken (SPA) basin (~4.3 Ga) is the pivotal event of crustal evolution and indicates that the lunar crust had cooled and consolidated to sufficient rigidity to preserve structures. Both endogenic and exogenic processes were active in the second stage (4.3 to 3.0 Ga), which results in numerous impact basins, mare basalt flooding, and different types of associated structures. The last stage (3.0 Ga to the present) is late neotectonic activity, which has weak modification of the tectonic framework. Response of tectonic framework and structure to lunar major geological events had been organized and discussed in this paper, and key scientific issues are summarized. This study can provide reference and support for implementation of subsequent lunar exploration projects.

Об одном способе ультразвукового бурения скважин для забора грунта на небесных телах

The article discusses technical and theoretical issues in the development of an ultrasonic drill for drilling soil on celestial bodies. The specific use of the drill requires low energy consumption, an efficient operating cycle and a wide temperature range during operation. One of the promising areas of development is devices that have vibration decoupling from the ground due to the transformation of ultrasonic vibrations into repeated shocks of sound frequency through a specially introduced “free” body between the ultrasonic and impact parts of the device. This allows you to maintain the functionality of the device and carry out drilling regardless of the state of the impact bit, including even its temporary jamming. The main technical solutions of both domestic and foreign authors are considered, an analysis is made and design schemes, operating conditions, capabilities and limitations are evaluated. A number of theoretical works were also analyzed and it was found that the materials presented in them do not contain detailed mathematical models necessary for simulation modeling at the stage of design and optimization of the structure. The theoretical part of the article considers a lumped mathematical model obtained on the basis of equivalent parameters obtained from a distributed description of the drill design elements in numerical calculation systems. The equations of mathematical physics, classical mechanics, elements of impact theory and similarity theory were used. A parametric study was carried out and, in particular, the dependences of the oscillation frequency of a free body on the size of the impact gap, body mass and the amplitude of oscillations of the concentrator were obtained. The considered model can be used for simulation modeling when developing a drill design.

An age-depth model for Lake Bosumtwi (Ghana) to reconstruct one million years of West African climate and environmental change

Situated within a 1.07 million-year-old meteorite crater, Lake Bosumtwi in Ghana stands as a pivotal location for comprehending climatic, ecological and environmental fluctuations within the sub-Saharan region of West Africa. The region's susceptibility to seasonal environmental shifts and climate oscillations is heightened by the annual movements of the tropical rain belt driven by atmospheric circulation. Yet, there is no satisfying age-depth model available for the entire sedimentary sequence strongly limiting our understanding of changes in this circulation pattern and associated (broad-scale) environmental responses during the last million years in the local to regional context of Lake Bosumtwi. To overcome this, we statistically examine the cyclicity in total natural gamma ray (NGR) data on a core from the lake's centre and create a cyclostratigraphic age-depth model. The calculated maximum age of 946 ka agrees well with the meteorite impact age (∼10 % offset). In order to refine this purely statistical approach, we also perform a correlative age-depth model using 33 tie points accounting for the complexity of climatic and environmental imprints to the NGR record that may exceed direct insolation related effects. Special attention is paid to the core's robustly dated (14C, OSL, U/Th) uppermost part covering the last 200 ka. Here, high NGR and co-varying K counts coincide with warm periods (except of the water-saturated and unconsolidated Holocene part) and the inverse for glacials and stadials. Based on this, we define tie points for correlating our NGR data to the age-depth model of a NE Atlantic SST record. Comparing our results to the correlation target, other global climate records and Sahara dust flux data reveals striking similarities and supports a proxy understanding with increased in wash of K-enriched terrigenous material from the crater rims in warm and moist periods (high NGR) and K-depleted dust input in stadials possibly contributing to low NGR values in addition to reduced input of K-enriched sediments from the crater rims. Our correlative age model results in precession amplitudes matching eccentricity well, providing further support especially because an over-tuning is unlikely with the used 33 tie points. Overall we provide crucial chronological context to numerous datasets along with environmental constrains that can be used to study the potential habitat availability of early anatomically modern humans in West Africa.

Alien Algae Species Invasions in Humic Rivers within Weakly Human Impact Basin.

Increasing anthropogenic influence and climate change are leading to significant transformations in living conditions for indigenous representatives of aquatic communities. The problem of alien species invasions is actively discussed in the example of large rivers and water reservoirs, but medium and small rivers with weak anthropogenic influence have been insufficiently studied in this aspect. With the help of analysis of literary data and the results of our own long-term observations of phytoplankton using morphological and molecular genetic methods in some left-bank Volga tributaries, we identified six invasive species of different taxonomic groups of algae, with a predominance of diatoms. The relevance of using both traditional and modern approaches to identifying invasive algae species is revealed. Such invasive species as Thalassiosira incerta, T. faurii, Skeletonema subsalsum, Unruhdinium kevei, and Gonyostomum semen were part of planktonic communities; the benthic species Plagiotropis lepidoptera var. proboscidea sometimes reached a significant level of development (up 6 to 44% from total biomass) in plankton. It was demonstrated that some algae species have firmly taken the position of dominants and subdominants in planktonic algae communities. The expansion of alien representatives of microphytobenthos was noted in the Volga River basin for the first time. For Gonyostomum semen, its European origin was revealed, for plankton and benthic diatom-Ponto-Caspian. Our study showed that the processes of invasion and subsequent development of alien species take place in habitats with weak anthropogenic influence, which is likely determined by the hydrological, hydrochemical, and climatic changes in river basins and the high adaptive capabilities of invasive species.

A Collective Impact approach to improving the physical health of people living with mental illness.

Introduction: Eighty percent of people living with mental illness have a co-existing physical health condition. Yet due to ‘diagnostic overshadowing’ (where the presenting problem overshadows all other concerns and risks), stigma and systemic discrimination, the rate of screening and treatment for mortality-related chronic health conditions in this part of our community is low. Consequently, as a group, people living with mental illness have poor health and have a 13 - 23 year reduced life expectancy. This pattern of poor health and reduced life-expectancy has been consistently reported in research studies across the world for more than 20 years.&#x0D; Aims, Objectives, Theory or Methods: The aim of this paper is to show how the countries of Australia, France, New Zealand and England have applied the collective impact approach to address the issue of the poor health and reduced life-expectancy of people living with mental illness. A collective approach seeks to coordinate organisations to reduce duplication and amplify impact. Co-chaired by consumers and with an emphasis on consumer-led initiatives, a collective impact approach seeks to coalesce multiple agencies/stakeholders through a common goal to address complex personal and system challenges. &#x0D; Highlights or Results or Key Findings: Although each country has adopted a collective impact approach, the way this has been implemented in different contexts has been quite different. The paper compares and contrasts approaches to ‘developing a shared agenda’, ‘mutually reinforcing activities’, and facilitating coordination. Common to each of these collective impact core elements is the need for a strategic approach to communication. This presentation will challenge the collective impact theory of ‘continuous communication’ using examples of the communication strategies from each country.&#x0D; Further, this paper will present how each country measures progress and seeks to demonstrate public value. This is a particularly important issue for intermediary organisations who do not provide direct health care services.&#x0D; Finally, it will highlight how by networking and working together internationally, these countries have successfully initiated reform in policy, and practice changes in areas such as cardiovascular disease management and vaccination for people living with mental illness.&#x0D; &#x0D; Conclusions: Collective impact appears to be an effective way to mobilise diverse organisations and amplify actions converting policy to practice. There is scope to apply a collective impact approach flexibly to different service system contects. Consistent across systems is the need for a backbone team to drive change and maintain momentum. &#x0D; Implications for applicability/transferability, sustainability, and limitations: The processes and methods of a collective impact approach can be applied to a range of health scenarios to facilitate enhanced integration and collaboration. However, sustainability requires a backbone team and providing ongoing evidence of impact and success.&#x0D;

Failure mechanism of 6252-armor steel under hypervelocity impact by 93W alloy projectile

In this study, we analyzed the fracture mechanism of the 6252-armor steel target exposed to the impact of a kinetic energy projectile made of 93W alloy. By examining the impact pressure, crater morphology, and fracture mechanism at various impact velocities, we established the relationship among these factors. As the impact velocity increased, the ratio of penetration depth to crater diameter initially increased, then stabilized, and ultimately approached 0.7 when impact pressures exceeded 49.78 GPa. For impact pressures below 70.71 GPa, macroscopic cracks were predominantly observed on the side of the crater, with no macroscopic cracks found at the bottom. However, if the pressure exceeded 70.71 GPa, the majority of macroscopic cracks occurred at the bottom of the crater. We also found a decrease in spacing between cracks or shear bands and the recrystallized grain size at the interface when the impact velocity increased. Notably, the fracture patterns demonstrated a mixture of brittle and ductile failure modes simultaneously.

Deep subsurface microbial life in impact-altered Late Paleozoic granitoid rocks from the Chicxulub impact crater.

In 2016, IODP-ICDP Expedition 364 recovered an 829-meter-long core within the peak ring of the Chicxulub impact crater (Yucatán, Mexico), allowing us to investigate the post-impact recovery of the heat-sterilized deep continental microbial biosphere at the impact site. We recently reported increased cell biomass in the impact suevite, which was deposited within the first few hours of the Cenozoic, and that the overall microbial communities differed significantly between the suevite and the other main core lithologies (i.e., the granitic basement and the overlying Early Eocene marine sediments; Cockell etal., 2021). However, only seven rock intervals were previously analyzed from the geologically heterogenic and impact-deformed 587-m-long granitic core section below the suevite interval. Here, we used 16S rRNA gene profiling to study the microbial community composition in 45 intervals including (a) 31 impact-shocked granites, (b) 7 non-granitic rocks (i.e., consisting of suevite and impact melt rocks intercalated into the granites during crater formation and strongly serpentinized pre-impact sub-volcanic, ultramafic basanite/dolerite), and (c) 7 cross-cut mineral veins of anhydride and silica. Most recovered microbial taxa resemble those found in hydrothermal systems. Spearman correlation analysis confirmed that the borehole temperature, which gradually increased from 47 to 69°C with core depth, significantly shaped a subset of the vertically stratified modern microbial community composition in the granitic basement rocks. However, bacterial communities differed significantly between the impoverished shattered granites and nutrient-enriched non-granite rocks, even though both lithologies were at similar depths and temperatures. Furthermore, Spearman analysis revealed a strong correlation between the microbial communities and bioavailable chemical compounds and suggests the presence of chemolithoautotrophs, which most likely still play an active role in metal and sulfur cycling. These results indicate that post-impact microbial niche separation has also occurred in the granitic basement lithologies, as previously shown for the newly formed lithologies. Moreover, our data suggest that the impact-induced geochemical boundaries continue to shape the modern-day deep biosphere in the granitic basement underlying the Chicxulub crater.

Preservation of pristine materials under impact craters formed on comet nuclei

Comet nuclei are the most primitive small bodies in the solar system, and would contain highly volatiles and amorphous water ice. However, comet nuclei may lose their primordial properties after high-velocity impacts with other small bodies. Such high-velocity impacts might also cause the characteristic circular structures and pinnacles on the comet-nucleus surface. However, the true origin of these geological structure remains uncertain. We here sought to clarify the shape of impact craters and the effects of post-shock heating on the comet surfaces. We conducted high-velocity impact experiments at velocities of 3–5.8 km s−1 on porous-ice targets with 40%, 50% and 60% porosity, and measured the post-shock temperature around the impact crater. The hard, dense refrozen layer formed on the crater wall resembled the pinnacles and sharp pit rims observed on comet nuclei, suggesting that such structures on comets may originate from impacts. In addition, the crater size scaling law for porous ice targets was successfully obtained in the strength-dominated regime. From the temperature profiles obtained, the energy partition coefficient of the projectile kinetic energy to the post-shock heat, γ, was estimated as follows: γ=0.08, 0.27 and 0.63 for 40%, 50% and 60% porosity at 4.2 km s−1 impact velocity, and γ=0.58 and 0.22 for 50% porosity at 3 km s−1 and 5.8 km s−1 impact velocities. These results, together with our crater size scaling law for porous ice, were used to estimate the temperature distribution induced by post-shock heat on the comet-nucleus surface. Volatile depletion and crystallization of amorphous water ice can occur at distances less than twice the crater radius from the crater center—that is, the post-shock heat effects are confined to the area around the crater. This might suggest that the comet nucleus surface remains relatively primitive after impacts.

Zircon (U-Th)/He impact crater thermochronometry and the effects of shock microstructures on He diffusion kinetics

Accurate age determination of impact cratering events remains difficult and controversial. Less than a quarter of all documented impact craters are regarded as accurately and precisely age dated. Zircon (U-Th)/He (ZHe) dating of impactites has received recent attention as a novel technique to date impact structures. Complicated and localized post-impact hydrothermal systems could also affect the reliability of ZHe impact ages. We quantitatively define the effects of shock-induced microstructures on He diffusion kinetics in zircon with different shock microstructures characterized by Secondary Electron Microscopy (SEM) and electron backscatter diffraction (EBSD). We investigated samples from the Chicxulub and Ries impact craters, to compare diffusion kinetics from structures of different sizes, ages, and hydrothermal activity. Step-heating fractional-release experiments were used to quantify He diffusion kinetics of variably shocked zircon and examine the relationship of the experimentally derived diffusion domain sizes to the EBSD-defined subgrains. The diffusion data show a consistent activation energy independent of shock level, while shock microstructures, such as planar deformation bands (PDBs) and granular textures systemically and dramatically lower the He retentivity due to the microstructural reduction of the effective diffusion domain size. Shocked zircon were dated by ZHe and the less deformed grains yield ages within error of the accepted impact ages as determined by other methods (40Ar/39Ar and U-Pb), whereas the grains with PDBs or granular textures commonly exhibited younger ages. Thus, the characterization of impact-induced zircon microstructures is critical for determining accurate impact ages because shocked grains are less suitable for reliable impact dating, but potentially powerful for exploring temperature–time history of hydrothermal activity.

Lithological Investigation of The Drill Core from a Sedimentary Cover in the Area of the Siljan Ring, Central Sweden

The ring structure of Siljan, located in the central part of Sweden, is considered by many researchers to be a meteorite (impact) crater. Impact craters are among the most complex geological objects on the Earth. The origin and formation of these structures still raises many questions. To find answers to these questions we need reliable geological information about the structure of the crater and the composition of the rocks. Information about the thickness and geological structure of the Siljan Ring area sedimentary cover will help to understand the process of the Siljan Ring structure’s formation as well as other similar geological formations on the Earth. Here, we present the results of laboratory studies of sedimentary rock samples taken from four exploration wells drilled in the vicinity of the Siljan Ring crater, which made it possible to compile their detailed lithological description. The laboratory studies included a structural analysis of the samples, and a texture and mineralogical analysis in thin sections. A structure analysis was carried out visually, while structural and mineralogical analyses were carried out on thin sections using a polarizing microscope and a scanning electron microscope. The main components of the rocks (minerals and fragments), along with their ratio and secondary transformations, were determined. The results of the structural analysis of the samples, and the textural and mineralogical analysis of the rocks in thin sections, showed that the sediments’ composition in the sedimentary cover near the Siljan Ring structure changes in different areas in accordance with their facies and stratigraphic characteristics. Furthermore, a change in the thickness of the sections and the succession sequences of rock units was established. A change of this nature is presumably caused by tectonic disturbances of an endogenous or impact source.

Amateur Astronomer Finds a Possible Crater on Io

The most volcanically active body in the solar system may have an impact crater, a discovery spotted by a curious nonprofessional scientist.

3‐D geophysical modeling of a buried, simple impact crater: Holleford impact structure, Ontario, Canada

AbstractHolleford Crater is a deeply buried, 2.35 km diameter late Proterozoic‐early Cambrian (550 ± 50 Ma) simple impact crater located in southeastern Ontario, Canada. Exploration drilling in the 1950–60s indicated a &gt;450 m deep, simple impact structure with an infill stratigraphy of Cambro‐Ordovician clastic and carbonate sediments and a −2.2 mGal gravity anomaly. We conducted new ground‐based geophysical surveys (magnetics, gravity) and potential field modeling to better resolve the buried impact structure depth, subsurface geometry, and postimpact modification. Geophysical surveys reveal a well‐defined ~3 mGal Bouguer gravity low and &lt;20 nT residual magnetic anomaly over the crater basin. The lack of a well‐defined magnetic anomaly is due to the low magnetic contrast between Mesoproterozoic metasedimentary target rocks and Paleozoic infill sediments. The modeled basement surface shows a deeply buried (&gt;400 m), eroded simple impact structure with a rim‐to‐rim diameter (D) of 1.8–2 km, a residual rim height of about 30 m and a true depth (dt) &gt;400 m. The modeled diameter is smaller than the previous estimate (2.35 km) based on the surface outcrop pattern of Paleozoic strata, which overestimates the buried impact structure dimensions. The modeled basement surface identifies a rim breach and a possible fluvial outflow channel in the southeast impact structure rim. The channel is up to 150 m deep and 400 m in width and has morphology similar to outlet channels produced by fluvial rim dissection of terrestrial impact structures and “tadpole craters” on Mars.

Simulation of the temperatures in the permanently shadowed region of the Moon's south pole and data validation

The permanently shadowed regions (PSRs) at the lunar poles without direct solar illumination maintain low temperatures throughout the year, making them optimal environments for the preservation of water ice. This paper investigates the diurnal and seasonal temperature variations of the PSRs within craters at the lunar south pole. The craters de Gerlache, Shoemaker, Amundsen, and Wiechert E are selected to explore how latitudes and shapes impact PSR temperature. Using the ray tracing approach, the shadow effect of terrain on the scattered sunlight and thermal radiation is calculated. PSR temperatures are simulated with the one-dimensional heat conduction equation. The thermal emissivity of the lunar surface is studied. Simulations with anisotropic emissivity are closer to the Diviner data in PSRs than the simulations with isotropic emissivity. In addition, by comparing the simulations to the Diviner data, it is found that the thermal conductivity at the extremely low temperatures in PSRs might be lower than that at high temperatures outside the PSRs. Simulations of the selected craters are used to study the influence of polar day phenomena occurring at high-latitude craters during the summer and the effect of direct solar illumination at the floors of low-latitude craters on the temperature distribution of PSRs. The influence of crater shape is analyzed as well by investigating the temperature distributions within PSRs of flat-floored and bowl-shaped craters. With the analysis of the heat flux propagation distances, emission angles, and incident angles, this study quantitatively elucidates the spatial and temporal variations in PSR temperatures. To explain why the Diviner data are lower than the simulation results at the solar-illuminated crater walls, an undulating rough surface is generated. The locations of the Sun and the observer on opposite sides of the rough surface may decrease the observed infrared brightness temperature according to the simulations. The PSR peak temperatures simulated with the anisotropic emissivity and thermal conductivity for PSRs are used to search for low-temperature areas suitable for preserving water ice within de Gerlache and Shoemaker.