On August 11, 2023, the Luna-25 spacecraft was launched with the task of landing in the southern polar region of the Moon and conducting research on the soil and near-surface exosphere. It flew safely to the Moon vicinity and settled into the orbit of Moon satellite. The landing of the spacecraft was scheduled for August 21. In accordance with the flight program, on August 19, a braking impulse was issued to form a pre-landing orbit. But the braking engine worked longer than planned, and the spacecraft crashed into the lunar surface. The team of the LROC television camera of the Lunar Reconnaissance Orbiter, having received information from Roscosmos about the crash site of Luna-25, photographed this site and on August 24 received an image showing a morphologically fresh crater with a diameter of about 10 m, which was not present in previous images of this site. The paper describes the regional topographic and geological characteristics of the site. A photogeological analysis of LROC images of the impact site was performed. An estimate has been made of the expected diameter of the crater formed as a result of the impact of Luna-25. From our examination, it follows that the 10-meter crater described in the NASA message appears to have actually been formed as a result of the impact of Luna-25. Its size corresponds to estimates calculated from impact parameters. The absence of a bright halo of emissions, typical of very young lunar craters, is likely due to the fact that the impact was relatively low-velocity, and in this case the crater is more likely an indentation depression and/or due to the fact that there was about half a ton of unspent fuel in the spacecraft “smeared” the surface near the crater.

The enigma of the apparently young crater retention age for the large lunar volcanic Irregular Mare Patch Ina (∼33.2 Myr) is critically analyzed through the review of: 1) experimental data on high velocity impacts into porous targets, 2) observed small impact crater morphology, and 3) the possible variation in small impact crater morphology with age. While we find that these different data sets and approaches could not unequivocally resolve the enigma, our analysis provides directions for new studies in several disciplines that are designed to help resolve the age conundrum and improve upcoming exploration mission goals and objectives.

The three largest impact craters, the remains of which have been found on Earth to date, had diameters of about 200 km immediately after formation. The search for traces of larger impact structures continues. This paper presents the results of numerical modeling of the formation of terrestrial impact craters larger than those already found. It is shown that the inferred geothermal gradient significantly influences the initial geometry of the impact melt region, which may facilitate the search for the remains of deeply eroded ancient impact structures.

Introduction: Irregular Mare Patches (IMPs) [1] have a rough floor unit (FU) and meniscus-like mounds (MD). Initial Ina CSFD measurements produced ages of ~59 Ma; but Ina is closely associated with an ~3.5 Ga edifice [2]. Could IMP mounds be formed by extrusion of ancient highly vesicular magmatic foams causing impacts to be smaller by ~3X due to formation in compressible magmatic foam [2-3]. Our goal The goal is to distinguish among IMP formation theories [4] by assessing the nature, physical properties, stratigraphic relationships, and ages of Ina units (Fig. 1). Major origin/age questions?: 1) Nature of superposed impact craters?: Formation mode, sampling depth, substrate effects on energy partitioning/subsequent degradation, influence on CSFD ages. 2) Predicted and observed regolith thicknesses?: ~59 Ma lava flows impact-generated regolith should be 5 m. We focus on an extremely fresh ~75m-diameter crater (Fig. 1) on the border between Ina floor and mound units. Nature of Fresh Crater (FC): The 75 m crater rim crest outline is circular where it intersects the mound to the N-NW, slightly indented to the S-SW, and significantly indented to the SE (Fig. 1a). The crater is characterized by a central floor mound ~1-3 m high, and ranges from MD ~2-15 m deep (max d/D of 0.2, typical of small fresh mare craters) to SW FL, ~2 m deep (d/D 0.0266). The crater floor/lower walls/rim are littered with m-scale boulders. Relative and Absolute: Boulders superposed on the main MD, the floor unit and the small SE MD indicates that the crater postdates formation of all these features and initial formation of Ina interior FL and MD units. The median survival time of lunar meter-scale ejecta boulders [5], is ~40-80 Ma, and ~150-300 Ma for ~99% of m-scale blocks. The crater absolute age is clearly in the younger range, placing one of the youngest craters in Ina close to, or greater than the ~59 Ma AMA [1]. Stratigraphy of Target Substrate: The main mound is ~10-13 m above the floor, sloping down to the E. An apparent boundary between the mound and underlying more coherent FL material is seen along the lower crater wall where boulders are exceptionally abundant. Extending adjacent floor unit topographic level laterally into the main mound, we interpret the FL unit to stratigraphically underlie the MD unit. Characteristics of Ejecta Deposit of the Young Crater: Pre-impact target reconstruction suggests the impact point was centered on the NW MD-FL unit boundary; expanding transient cavity intersected the main MD to NW, the FL to SW and the small MD to SE, offering insights into substrate properties, regolith thicknesses/post-impact degradation. FC shows little evidence of typical fresh young maria craters ejecta deposits. Physical Properties of Impacted Substrate: Abundance of crater rim/lower interior boulders, presence of central mound suggest that the crater excavated down into a coherent substrate [13] at 0-13 m depth across the sloping mound surface: this value exceeds both regolith thickness model estimates. The nature of the major mound surface and upper crater wall, particularly along the major mound (NNW) wall, suggest a much more incoherent, regolith-like substrate ranging up to ~13 m thick, but retarded excavation on SW FL unit (d/D = 0.0266). Post-Formation Modification of Main Mound and Crater: A similar-sized degraded crater is observed on the main MD just W of the FC. Its shallow depth (~2 m) and lack of circum-rim boulders suggest that it is >several 100 Ma [5]. Initial analysis of the small SE MD (Fig 1a) appears to indicate that it embays the FC to its NW, indenting its rim circularity, making its formation stratigraphically younger than FC, despite its clearly higher crater density/scattered superposed surface FC boulders. Detailed examination of opposite-side imaging/DTMs, however, reveals a convex-outward topographic mound indentation consistent with FC circularity, exposed boulders along its base, and evidence for mass-wasting of SW mound material down into the FC. Tentative Conclusions About Ina Age/Mode of Origin: There are multiple contradictions in implied ages of the FL/MD units/stratigraphic relationships: 1) FL regolith thickness (thin; 5 m) and optical maturity suggest very ancient age: yet the stratigraphic relationships indicate the MD overlie and are stratigraphically younger than the FL, and CSFD AMAs on both units indicate an extremely young age (~58 Ma). 2) MD appears composed of friable regolith-like material throughout its 0-13 m thickness. 3) Both unit fresh craters lack typical distinctive fresh mare crater ejecta. 4) Distribution of fresh crater boulders suggest a young age; possibly as old as several hundred Ma [5], clearly >58 Ma [1]. 5) Extremely degraded mound crater NW of FC much too degraded to have formed

Venus’ thick atmosphere is capable of destroying kilometer-sized bodies such as asteroids, creating various types of traces on the surface. While larger cosmic bodies are able to reach the surface, creating impact craters or crater dispersion fields, smaller bodies effectively transfer the initial kinetic energy into the atmosphere, resulting in an “atmospheric explosion” at some altitude. In these cases, the most visible marks on the surface of Venus are created by atmospheric shock waves and the flow of gas behind the shock fronts reflected from the solid surface. The transitional sizes of impactors that break up in the atmosphere but reach the surface give rise to clusters of craters. The paper presents the first results of three-dimensional calculations of the destruction of rocky asteroids in the atmosphere of Venus, indicating significant differences from simple two-dimensional axisymmetric calculations.

Приведены результаты трехмерных расчетов деформации, фрагментации и торможения астероида размером 1.5 км в атмосфере Венеры. Сравнение с результатами проведенных ранее двумерных расчетов показало, что используемая в двумерных расчетах осевая симметрия приводит к более сильному торможению фрагментов разрушенного астероида, чем в трехмерной геометрии, более соответствующей реальности. Полученная в трехмерных расчетах структура разрушенного астероида, состоящего из нескольких крупных и множества мелких фрагментов, по-видимому, может объяснить причину образования кратерных кластеров и темных и ярких пятен (splotches), наблюдаемых на поверхности Венеры. The results of calculations of deformation, fragmentation and deceleration of asteroids 1.5–5 km in size in the atmosphere of Venus are presented. The fraction of energy lost by asteroids during their passage through the atmosphere and the effective dimensions of an asteroid (or a cloud of its fragments) at the moment of impact on a solid surface are determined. It is shown that asteroids 1–2 km in size reach the surface of Venus in the form of a cloud of fragments with a diameter of 5–20 km, the kinetic energy of which is 10– 1000 times less than the initial energy of the asteroid. Such impacts do not appear to result in the formation of classical single craters or crater fields, but may be responsible for the formation of dark and bright spots (splotches) observed on the surface of Venus.

Detailed geological mapping of Phoebe Regio on Venus, located near 10°S, 282°W, at a scale of 1:500,000, provided the basis for the discovery of a linear trend with 12 splotches, herein termed the Phoebe Regio Splotch Chain. The formation of splotches on the surface is associated with an explosion that occurs as a result of the interaction of a cosmic body penetrating through the dense atmosphere of Venus. The shockwave from such an air-blast could affect the surface in different ways, leading to both radar darkening and brightening. It was found that the 12 splotches differ in size but have a similar morphology and include a dark center and a radar-bright ring. The presence of such characteristics makes it possible to distinguish splotches from other geological features such as volcano-tectonic features (volcanic edifices, pyroclastic mantles, portions of lava flows and associated structures), which, when analyzed on a broad scale, can look similar. Splotches are known to be among the youngest geological features on the surface of Venus, and the elongated distribution of 12 splotches is suggested to have been formed by a stream of individual fragments of a parent body that disrupted well before entering the atmosphere of Venus.

Our team has discovered first impact craters under the thick Venusian atmosphere with radar images during the Venera 15/16 mission. Later Magellan radar images of a better quality allowed us to count all impact craters and to find amazing features, splotches, resulted most probably from “airbursts” - total explosive disruption in flight of small celestial bodies. Splotches could have a central feature (possibly caused by terminal impacts of fragments), or could be diffusive patches of increased (bright) or decreased (dark) areas of changed radar reflectivity. The main explanation so far is that atmospheric shock waves, generated by airbursts, somehow change the surface radar reflectivity, e.g. creating smoother (radar dark) or more rough (radar bright) zones due to reflection of shocks. Size of splotches vary from ~10 km to ~200 km, being comparable with the characteristic atmosphere thickness. The exact mechanisms of air shock wave interaction with the surface is still under debates, but promises to help us better understand the presence of dust/sand/pebbles/boulders at the surface of Venus as well as to estimate mechanical properties of surface rocks. We start a small project to support the issue. The project includes the numerical modeling of atmospheric shock waves on Venus due to cratering impacts and due to airbursts. Our modeling is compared with results published in 1990s-2000s. Airbursts are modeled as a hot spheric volume gas explosion 10 to 40 km above the surface in the Venusian stratified atmosphere. In addition to trivial parameters like maximum pressure, dynamic pressure and the wind speed behind the shock front, necessary for the following analysis of a possible “aeolian” motion of surface’s fines, we try to formulate a general picture of shock wave propagation in the atmosphere after an airburst. We find that the large-scale hot gas bubble from the source zone creates a n x 10 km plume (a kind of a classical “mushroom”), which effectively expands laterally at high altitudes, pushing forward an enhanced shock wave. This wave is looking like a gradual conversion of the main shock wave from a hemispheric one to a conic front, returning back to surface. The other trivial (but not discussed quantitatively) phenomenon is the seismic wave, created by an air shock, but finally overrun the atmospheric shock front. It means that the surface air shock front at large distances arrive after the seismic wave shakes the surface. We plan to investigates all these phenomena and compare models with observations. An interesting possibility seems to be satellite observation of rare meteoroid entry to the Venusian atmosphere, as it now available for terrestrial bolides.

Abstract—Modeling the relative motion of large crustal blocks vitally depends on an adequate description of the forces acting between the blocks. To describe motion at high strain rates, it proved necessary to assume a dry friction to depend on the shear strain rate and, in some cases, on the amplitude of fault side displacement. The best known examples of large-scale motions with dynamically decreasing friction are long-runout rock avalanches and gravity-driven collapsing of the transitional cavities of large impact craters. In this work, the experience is discussed of using the model of acoustic fluidization as a factor of a temporary decrease in friction to quantitatively simulate impact crater shape on the Earth and other planetary bodies. Immediate promising ways to find more adequate models are outlined.

Introduction: The prominent feature of large impact craters is a set of terrasses at the inner crater slope – see a review by [1]. The presence of terrasses has been interpreted as circular or arc-like landslides along inclined faults, giving a possibility to estimate the pre-landslide crater shape [2]. The classic soil mechanics approach results in estimates of rock cohesion values [3, 4]. Available today numerical models poorly reproduce the origin of terraces in complex craters, mostly giving smooth crater profiles. It seems that partially the lack of terraces in models results form (1) low spatial resolution of numerical grids, and (2) relatively crude acoustic fluidization (AF) model [5], controlling the rock dry friction to reproduce the transient crater floor uplift. We start a small project attempting to improve available models. It is relatively simple to reproduce the localization of rock deformations (“faults”) in a model where rock strength gradually decreases from a high initial value to a dry friction level typical for crashed (granular) rocks – see, e.g., fig. 6 in [6]. For a “static” model of a transient crater collapse deep faults have been modeled with a set of assumptions [7, 8].. Strain-rate softening: In a complete model run (impact-transient cavity-final collapsed crater) the deep circular fault has been reproduced in [9]. The approach we plan to use in the current project has been previously described by Senft and Stewart [10]. They have tested the model where the dynamic friction in damaged tock is controlled with slip velocity and distance, originally proposed in strain-rate softening presentation of a block over block friction sliding [11, 12]. In our approach we have tested the simplified equation for strain-rate dependence of the friction coefficient, previously used in [13]: f = 0.1 + 0.5−0.1 1+e′ e′′ ⁄ (1)

The terrestrial planets, the Earth's Moon, Mars,Venus, Mercury and the asteroids Gaspra and Ida showstriking similarities in their production crater size distribution characteristics. The investigation of hte crater populations in Gaspra and Ida yield information on the crater size distribution in the source region of the bodieslargely responsibe for cratering the inner solar system planets. Comparison of these data especially with the lunar impact record, which is our most reliable data base, confirms the complex shape of the crater size distribution curve (standarddistibution), and lends strong support to the idea of a common population of bodies impacting the inner planets and largely stemming from the asteroid belt. The steepening of the production crater size distribution at sizes D>= 1 km is confirmed to be due to the characteristics of the primary impactor production size distribution and not to an admixture of objects from secondary cratering processes. The impact hazard for craters, or objects, is assessed for the Earth and the other terrestrial planets relative to the lunar case through application of the lunar production size-frequency distribution which is well known for the crater diameter range 10m>D>1000km. current or past impact rates can be calculated for any size of crater or projectile and are given for specific crater sizes and respective projektile sizes as well as for projectile energies. The impact hazard for projectiles, e.g., capableof forming 1-km craters on solid surfacesof the Earth (if the atomsphere were absent)is assessed for the present and results in a production rate of one crater every 1600 yr for the surface area of the whole Earth, or every 6000 yr for the area of the continents; for 100-km craters, e.g. the corresponding number is one event every 27 Myr for the whole Earth.

We continue the analysis of HiRISE high resolution images of Mars to understand properties of dust covering the surface. The data on dust devils observed with Mars landers and surface traces of dust devils could be expanded with elongated albedo features imaged near “new” impact sites (“new” means that we have orbital images before and after the meteoroid impact, which give us an estimate of the impact date and the age of a feature). The age of these features is from 0.5 to 12 terrestrial years. From geometric reasons we could assume that the most possible mechanism of this elongated albedo details is the “footprint” of two or more colliding air shock waves, generated at the impact site. Of ~1200 “new” impacts known today, in 18 cases crater pairs or clusters, created with fragments of the same “parent” meteoroid, we recognize 24 thin “parabolas” with a width of 1 to 10 m (0.2 to 10 main crater diameters, D), extended to 100 – 400 m (3 to 100 D) from the impact site. In ~30 cases near a single crater we observe a curved albedo feature nick-named “scimitar”. These features have width, growing with a distance from the impact point. The length varies from 10 to 100 D, the width varies from 1 to 10 D. Our working hypothesis is that “scimitars” are footprints of ballistic and spherical air shock wave collision at the surface. Both “parabolas” and “scimitars” have an exact bilateral symmetry, which allows us to reconstruct the flight direction of projectiles.We estimate the equivalent energy of spherical air blasts with two different assumptions for “parabolas” and “scimitars” formation. For parabolas we assume a mechanism, similar to dust devil track formation – the negative pressure excurse uplifts the upper fine dust layer. The main assumption is that the dark parabolic strip width corresponds the wave length of the negative pressure phase in the air shock wave. It gives us the minimum energy estimate as in reality the negative phase could be longer. The negative pressures here along the parabola length decay from about 10 to 5 Pa with the phase duration of a few milliseconds. Such a suction pulse is able to mobilize dust particles 50 to 100 microns in size.For scimitars, which in contrast to “dark” parabolas are typically “brighter” than surrounding area, we have no a good mechanical explanation of origin. However, with limits of our current model, the spherical “explosion” air blast should be enough energetic, to overrun the ballistic shock wave. From non-linear motion of the shock wave front we can estimate the fraction of meteoroid’s kinetic energy, converted to the air blast energy. The model is able to reproduce approximately the scimitar’s curvature.

The idea of visualizing shock wave passage along a dusty (sooty) surface was first proposed and tested by Ernst Mach. High resolution HiRISE images of new impact craters on dusty areas of Mars gave in many cases revealed dark “fresh” halos around craters. In ~7% of cases they have low albedo/color contrasting curved strips near craters referred to as “parabolas” and “scimitars”. We analyze these albedo details as the possible surface footprints of atmospheric shock waves generated during atmospheric passage and shocks from impact cratering by small meteoroids and their fragments. In this approach “parabolas” are the trace of two colliding air shocks propagated from a pair of neighboring craters formed after a meteoroid fragmented during the atmosphere passage. The mechanism of the “scimitar’s” formation is more enigmatic and tentatively could be related to the interaction of the ballistic cone wave and a spherical wave from the point of impact. The study of images is accompanied by numerical modeling of impact of small projectiles at the atmosphere/rock boundary. This modeling constrains the minimum efficiency of an impact to generate the air shock wave in the rarified Martian atmosphere below of 0.1% of the kinetic energy for non-volatile targets. Targets with near surface volatiles could amplificated the air blast (if volatiles are presented in the shocked zone). The study is intended to estimate the air-shock wave parameters along the visible surface traces around impact craters. By constraining shock wave parameters opens new possibilities for investigating the mechanical properties of the Martian surface. The work is supported by RAS program 12 “Universe Origin and Evolution from Earth-based Observations and Space Missions” (BAI), and a grant from the NASA Mars Data Analysis Program, number 80NSSC18K1368.

The impact crater formation on the surface of the Earth and other planetary bodies is accompanied by the action of shock waves on rocks and their displacement into a new position. The shock-wave compression results in the occurrence of the remanent heating of rocks (up to melting and evaporation during the unloading). The direct mechanical action of the shock compression and the remanent heating change the magnetic properties and magnetization, which leads to arising of the magnetic anomaly above the crater area. This work presents an example of the complex analysis of the magnetic anomaly above the well-studied impact crater Bosumtwi (Ghana, a diameter of about 10 km), including the numerical simulation of the crater formation process and the magnetic anomaly model on the basis of simulated parameters of shock compression. The complex model demonstrates a good agreement with data of direct measurements.

The numerical model of the Puchezh-Katunki formation processes is described. The model results are preceded with a short description of all model components: the concept of hydrocodes, rock equation of states, brittle/ductile rock damaging, and the acoustic fluidization (AF) model.

Introduction: Irregular Mare Patches (IMPs) [1, 2, 3] are an unusual and distinctive manifestation of lunar volcanism [4, 5, 6]. The largest IMPs (e.g. Ina and Sosigenes) are characterized by isolated smooth bleb-like mounds surrounded by flatter topographically rough and optically relatively immature terrain; they are large enough to obtain Neukum-chronology impact crater model ages. Initial estimates resulted in very young age interpretations for extrusive basaltic flow activity (<100 Ma) [3], while others [4-8] expressed concern about these estimates on the basis of the close association of these IMPs with ancient (~3.5 Ga) edifices and structures. It was proposed instead that the IMP mounds were formed by ancient extrusion of porous highly vesicular magmatic foams predicted theoretically [4] to form in pit craters, and that the young ages were caused by substrate effects with superposed impacts smaller by a factor of ~3 due to their formation in compressible magmatic foam [7,8]. The intensive multidisciplinary characterization and analysis of Ina, Sosigenes and other IMPs provides a basis for further IMP assessment [7, 8]. The goal of the work presented here is to assess the experimental and theoretical basis for impact crater formation in porous targets as a basis for distinguishing among theories for the formation of IMPs and to describe some open questions in the crater retention age determination on IMP mounds and other candidate porous substrates. Cratering in Porous Targets: Recent progress in studying crater formation in porous targets was motivated by assessment of cratering on asteroids and an artificial impact on a comet nucleus (e.g. [9, 10]). These results were cited as helping to explain the relative rarity of larger impact craters accumulated on IMPs mounds, interpreted to be due to a factor of 3 smaller diameter for craters formed on porous mounds in comparison with “normal” lunar sites [7, 8]. Here we use recently published experimental data to test the interpretation of Ina’s mounds as being composed of porous basalts and magmatic foams [4]. Fig. 1 presents data compiled from selected experimental datasets describing impacts in porous targets. The “base line” is dry sand scaling (DSS), plotted with rim crest diameters from [14], and craters for artificial lunar impacts (Ranger and SIVB – see review in [15]). The relevant useful range of lunar craters 40<D(m)<400 is shown with red dots, assuming an impact velocity of 18 km s. Natural pumice and sintered porous targets [11, 12] have high enough strength to maintain the elongated crater shape with a small crater diameter (lower group of points in Fig. 1). Special efforts were undertaken in [13] to obtain highly porous targets without macro-cohesion. In these targets, a relatively deep transient cavity (“bulb” or “carrot-shaped” crater [16]) collapses in a gravity field, forming cone-like craters. The diameters of final craters are shown in Fig. 1. Due to the relatively

Several Snowball Earth periods, in which the Earth has been (almost) totally glaciated, are known from Earth history. Neither the trigger for the initiation, nor the reason for the ending of such phases, are well understood. Here we discuss some mechanical effects of the impact of asteroids 5–10 km in diameter on the Snowball Earth environment. An impact of this scale is the largest impact that is statistically predictable for 10–60 Myr time periods. The impact cratering itself (shock waves, impact crater formation) is not powerful enough to change the natural climate evolution path on Earth. However, the products of impact (mainly—water vapor) can be quickly distributed over a substantial part of the globe, influencing the global circulation (e.g., facilitating cloud formation). It is a question for future studies to confirm if such an event (which is possible statistically during this interval) may or may not have influenced the global climate of the Snowball Earth, and/or contributed to deglaciation.

Hundreds of new impact craters have been observed to form on Mars since spacecraft began imaging that planet. New impact craters produced visible ejecta deposits and many of them also have visible rays, similar to lunar and mercurian craters. However, some of the new martian impact craters have a circular feature of relatively low reflectance that we call a "halo." This feature is distinct from the usual visible ejecta deposits or ray patterns. In this paper we present an observational study of this halo feature and we discuss the results of this study with respect to the nature of the halos: what they are and how they may have formed. To address these questions, we measured diameters of both halos and their central craters. We found a strong correlation between halo diameter and crater diameter, which indicates that the nature of the halos is fundamentally governed by the amount of impact energy available at their formation. Specifically, halo size is controlled by impact energy according to the non-linear relationship D-H proportional to E-2/3, where D-H is the diameter of the halo and E is the impact energy. We also found that certain factors may influence the formation of the halos: a thicker dust layer and lower elevations are both correlated with larger halos. From these correlations we conclude that the local surface characteristics as well as local atmospheric pressure influence the formation of the halos. Our description and analysis of the martian halo features provide a framework upon which specific halo formation mechanisms can be developed and tested in the future.