Cumulative Size-frequency Distribution Research Articles

The Population of Young Craters on the Moon: New Catalog and Spatial and Temporal Analysis

We present a new catalog of the Moon’s rayed and rocky craters from 5 to 130 km in diameter. Using data from the Lunar Reconnaissance Orbiter and the JAXA Kaguya/Selene missions, we identify 571 unique craters with albedo, maturity, and/or thermophysical rays, or rocky ejecta, or all of these, located between 70° S and 70° N at all longitudes. We analyze the cumulative size–frequency distribution (CSFD) and spatial distribution of these craters and find that in general the albedo-rayed population and the rocky population are consistent with an aggregate age of ∼1.2 Gyr, and both groups show deviations from the CSFD predicted by canonical production and chronology functions at diameters >30 km. Strikingly, we also find that although the rocky craters and craters with thermophysical rays show uniform spatial distributions with both longitude and latitude, the albedo-rayed craters show a strong equatorial concentration that deviates significantly from uniform and that is too large to be explained by analytical models of orbital motion.

A New View of the Lunar South Pole from the Lunar Orbiter Laser Altimeter (LOLA)

Upcoming missions to the lunar south pole require detailed maps over large areas to fully characterize landing sites locally and regionally and to place their data into proper geologic context. To that end, we enhanced the Lunar Orbiter Laser Altimeter altimetry data set for the south polar region, from which we produced new maps of topography, topographic roughness, and permanently shadowed regions (PSRs). The roughness maps reveal a diversity of terrains characterized by hectometer-scale roughness that is controlled in this region primarily by cratering and downslope mass transport. The south polar region is littered with linear roughness features of order ∼1–10 km wide and approximately tens to hundreds of kilometers long hypothesized to be secondary impacts within extended ejecta rays. Nonuniformities in these features could reflect variations in secondary impactor properties and/or target terrain properties. Poleward of 80° S, the PSR cumulative size–frequency distribution (CSFD) shows an approximately power-law behavior whose exponent has a spatial variation of ∼10%. PSRs with areas <1 km2 contain 15% ± 5% of the total PSR area. Finally, we studied the effect of false positives and false negatives on the accuracy of the measured PSR CSFD and on the area for any individual PSR. The new maps presented here have many applications in the science and exploration of the lunar south polar region, such as geologic mapping and traverse planning.

Identification of a 4.3 billion year old asteroid family and planetesimal population in the Inner Main Belt

Context. Understanding the conditions that lead to the formation of planetesimals – the building blocks of planets - and their initial size distribution is a central problem of modern planetology. While most of these original planetesimals were accreted onto the terrestrial planets and the cores of the giant planets, some were also stranded in the main belt, where 4.5 Gyr of collisional evolution broke most of them into families of collisional asteroid fragments. However, some planetesimals survived, and are still hidden amongst asteroid fragments in the main belt. Aims. We make use of astronomical data to identify these leftover planetesimals amongst all other asteroids. Our search is based on separating planetesimal survivors from families of asteroids generated by collisions. Namely, we aim to identify and “clean” the main belt of collisional family members: by doing so, we would be left with the surviving members of the original planetesimals. Methods. We focus here on the inner portion of the main belt for asteroids with intermediate to high albedo. It is known that current asteroid family catalogs are not suitable for the aforementioned cleaning; they are conservative and only one-quarter of the known asteroids are associated with the approximately 120 distinct asteroid families. We therefore developed methods to inclusively link asteroids to known collisional families in order to better capture their extent. Namely, we apply a hierarchical clustering method (HCM) on asteroids filtered according to the V-shape of the Yarkovsky drift of each family in order to reassess family membership (V-shape-constrained HCM). The identified families were removed and the remaining background population was searched for previously undetected collisional families. Results. We succeed in using our V-shape-constrained HCM to link family “halos” to their cores. After removing these reassessed families from the asteroid population, our V-shape search reveals a previously unknown collisional family of S-type asteroids in the inner main belt with an age of 4.3 ± 1.7 Gyr and a significance level of 3.4σ. When this ancient collisional family is removed, 34 planetesimals are identified and their size-frequency distribution is presented. Conclusions. The asteroid belt has two components: planetesimals and collisional fragments. The cumulative size-frequency distribution of planetesimals has a steep power-law index for bodies larger than 100 km in diameter and a much smaller power-law index for planetesimals smaller than 100 km.

A 2022τ-Herculid meteor cluster from an airborne experiment: automated detection, characterization, and consequences for meteoroids

Context. The existence of meteor clusters has long since been a subject of speculation and so far only seven events have been reported, among which two involve less than five meteors, and three were seen during the Leonid storms.Aims. The 1995 outburst of Comet 73P/Schwassmann-Wachmann was predicted to result in a meteor shower in May 2022. We detected the shower, proved this to be the result of this outburst, and detected another meteor cluster during the same observation mission.Methods. Theτ-Herculids meteor shower outburst on 31 May 2022 was continuously monitored for 4 h during an airborne campaign. The video data were analyzed using a recently developed computer-vision processing chain for meteor real-time detection.Results. We report and characterize the detection of a meteor cluster involving 38 fragments, detected at 06:48 UT for a total duration of 11.3 s. The derived cumulative size frequency distribution index is relatively shallow:s= 3.1. Our open-source computer-vision processing chain (named FMDT) detects 100% of the meteors that a human eye is able to detect in the video. Classical automated motion detection assuming a static camera was not suitable for the stabilized camera setup because of residual motion.Conclusions. From all reported meteor clusters, we crudely estimate their occurrence to be less than one per million observed meteors. Low heliocentric distance enhances the probability of such meteoroid self-disruption in the interplanetary space.

Pre-encounter Predictions of DART Impact Ejecta Behavior and Observability

We overview various efforts within the DART Investigation Team’s Ejecta Working Group to predict the characteristics, quantity, dynamical behavior, and observability of DART impact ejecta. We discuss various methodologies for simulation of the impact/cratering process with their advantages and drawbacks in relation to initializing ejecta for subsequent dynamical propagation through and away from the Didymos system. We discuss the most relevant forces acting on ejecta once decoupled from Dimorphos’s surface and highlight various software packages we have developed and used to dynamically simulate ejecta under the action of those forces. With some additional software packages, we explore the influence of additional perturbing effects, such as interparticle collisions within true N-body codes and nonspherical and rotating particles’ interplay with solar radiation pressure. We find that early-timescale and close-proximity ejecta evolution is highly sensitive to some of these effects (e.g., collisions) while relatively insensitive to other factors. We present a methodology for turning the time-evolving size- and spatially discretized number density field output from ejecta simulations into synthetic images for multiple platforms/cameras over wide-ranging vantage points and timescales. We present such simulated images and apply preliminary analyses to them for nominal and off-nominal cases bracketing realistic total mass of ejecta and ejecta cumulative size–frequency distribution slope. Our analyses foreshadow the information content we may be able to extract from the actual images taken during and after the DART encounter by both LICIACube and Earth-vicinity telescopes.

Population of Degrading Small Impact Craters in the Chang’E-4 Landing Area Using Descent and Ground Images

The landing camera (LCAM) of Chang’e-4 lander provides a series of low (46 cm/pixel) to high (2.3 cm/pixel) resolution images, which are suitable for centimeter-scale craters. In this paper, we analyze the degradation of those small-sized craters to provide detailed information on the local geological evolution of the lunar surface. From the mosaicked descent image, 6316 craters were extracted and classified into four degradation levels based on their morphology on the image: fresh, slightly degraded, moderately degraded, and severely degraded. The ground terrain camera (TCAM) image and the DEM of the Yutu-2 panoramic camera (PCAM) validate the crater degradation levels from a qualitative and quantitative perspective, respectively. The results show that the smaller the size of the craters, the more easily they are degraded. The crater populations in equilibrium in the four study areas indicate that the cumulative size–frequency distribution (SFD) slope is different from previous research results, and the smaller the craters, the more difficult to reach an equilibrium state (for craters smaller than a given size, the production rate is exactly balanced by the removal rate), which may be due to secondary cratering and surface resurfacing caused by the burial of ejecta from neighboring craters.

New Craters on Mars: An Updated Catalog

AbstractWe present a catalog of new impacts on Mars. These craters formed in the last few decades, constrained with repeat orbital imaging. Crater diameters range from 58 m down to &lt;1 m. For each impact, we report whether it formed a single crater or a cluster (58% clusters); albedo features of the blast zone (88% halos; 64% linear rays; 10% arcuate rays; majority dark‐toned; 4% light‐toned; 14% dual‐toned); and exposures of ice (4% definite; 2% possible). We find no trends in the occurrences of clusters with latitude, elevation, or impact size. Albedo features do not depend on atmospheric fragmentation. Halos are more prevalent at lower elevations, indicating an atmospheric pressure dependence; and around smaller impacts, which could be an observational bias. Linear rays are more likely to form from larger impacts into more consolidated material and may be enhanced by lower atmospheric pressure at higher elevations. Light‐ and dual‐toned blast zones occur in specific regions and more commonly around larger impacts, indicating excavation of compositionally distinct material. Surfaces covered with bright dust lacking cohesion are favored to form detectable surface features. The slope of the cumulative size frequency distribution for this data set is 2.2 for diameters &gt;8 m (differential slope 2.9), significantly shallower than the slope of new lunar craters. We believe that no systematic biases exist in the Martian data set sufficient to explain the discrepancy. This catalog is complete at the time of writing, although observational biases exist, and new discoveries continue.

Impacts of thrusting, extensional faulting, and glaciation on cratering records of Pluto’s largest moon Charon: Implications for the evolution of Kuiper belt objects

A first-order question in the studies of the Solar System is how its outer zone known as the Kuiper belt was created and evolved. Two end-member models, involving coagulation vs. streaming instability, make different predictions—testable by the cratering history of Kuiper Belt Objects (KBOs)—about the cumulative size-frequency distribution (SFD) of the KBOs. Among all of the imaged KBOs, Pluto’s largest icy moon, Charon, appears to preserve the largest size range of seemingly undisturbed craters, their diameters (<italic>D</italic>) on Charon ranging from &lt; 1 km to &gt; 220 km. Current work shows that Charon’s craters with <italic>D</italic> &lt; 10−20 km are fewer than those expected by the coagulation mechanism, but whether this is an artifact of post-cratering modification of smaller craters is unknown. We address this issue by conducting systematic photogeological mapping and performing detailed landform analysis using the highest resolution images obtained by the New Horizons spacecraft, which reveal a range of differentiable terrains on Charon. The most important findings of our work include (1) truncation and obliteration of large craters (diameters &gt; 30−40 km) and their crater rim ridges along the eastern edges of several north-trending, eastward-convex, arcuate ranges in Oz Terra of the northern encountered hemisphere, (2) lobate ridges, lobate knob trains, and lobate aprons resembling glacial moraine landforms on Earth, (3) dendritic channel systems containing hanging valleys, and (4) locally striated surfaces defined by parallel ridges, knob trains, and grooves that are &gt; 40−50 km in length. The above observations and the topographic dichotomy of Charon’s encountered hemisphere can be explained by a landscape-evolution model that involves (i) a giant impact that created the Vulcan Planitia basin and the extensional fault zone along its northern rim, (ii) a transient atmosphere capable of driving N₂-ice glacial erosion of the water-ice bedrock and transporting water-ice debris to sedimentary basins, (iii) regional glacial erosion and transport of earlier emplaced impact ejecta deposits from the highlands of Oz Terra into the lowland basin of Vulcan Planitia, (iv) syn-glaciation north-trending thrusting, interpreted to have been induced by Charon’s despinning, and (v) the development of a water-ice debris cover layer over subsurface N₂ ice below Vulcan Planitia during global deglaciation. The infilling of the Vulcan Planitia could have been accompanied by cryovolcanism. The extensive modification of impact craters means that the size-frequency distributions of Charon’s craters should serve only as a lower bound when used to test formation mechanisms proposed for Kuiper belt objects.

Degradation of Small Simple and Large Complex Lunar Craters: Not a Simple Scale Dependence

AbstractThe crater record of a planetary surface unit is often analyzed by its cumulative size‐frequency distribution (CSFD). Measuring CSFDs involves traditional approaches, such as traditional crater counting (TCC) and buffered crater counting (BCC), as well as geometric corrections, such as nonsparseness correction (NSC) and buffered nonsparseness correction (BNSC). NSC and BNSC consider the effects of geometric crater obliteration on the CSFD. On the Moon, crater obliteration leads to two distinct states in which obtained CSFDs do not match the production CSFD—crater equilibrium and nonsparseness. Crater equilibrium occurs when each new impact erases a preexisting crater of the same size. It is clearly observed on lunar terrains dominated by small simple craters with steep‐sloped production CSFDs, such as Imbrian to Eratosthenian‐era mare units. Nonsparseness, on the other hand, is caused by the geometric overlap of preexisting craters by a new impact, which is also known as “cookie cutting.” Cookie cutting is most clearly observed on lunar terrains dominated by large craters with shallow‐sloped production CSFDs, such as the pre‐Nectarian lunar highlands. We use the Cratered Terrain Evolution Model (CTEM) to simulate the evolution of a pre‐Nectarian surface unit. The model was previously used to simulate the diffusion‐induced equilibrium for small craters of the lunar maria. We find that relative to their size, large craters contribute less to the diffusion of the surrounding landscape than small craters. Thus, a simple scale dependence cannot account for the per‐crater contribution to degradation by small simple and large complex craters.

Morphological and morphometric analysis of a topographic depression near Huygens basin, Mars: Identification of a putative endorheic playa

Inter-crater depressions on Mars, which may be host to candidate lakes, have not been explored in much detail. Even if a small percentage of these depressions were actually filled with water, it would mean that Mars was hydrologically more active than previously thought. The present study examined valley networks within the equatorial region of Mars. Analysis of the drainage networks around Huygens crater revealed a topographically flat basin towards it north. The basin was divided into two geological units on the basis of their morphological differences. Several features including wrinkled ridges, dike, different types of polygonal desiccation patterns (PDPs) and craters with layered ejecta blankets were observed within the two units. Estimation of lobateness, ejecta mobility and depth to diameter ratio of craters within these units suggest climate controlled fluctuations of volatiles and multiple episodes of resurfacing. Geological history of the basin was reconstituted and a possible stratigraphy was formulated after integrating basin morphology with Cumulative Size Frequency Distribution (CSFD) analysis. From the study, we infer the basin to be an ephemeral, endorheic playa that underwent several geodynamic processes and is a geologically rich terrain for future exploration missions.

The equilibrium size-frequency distribution of small craters reveals the effects of distal ejecta on lunar landscape morphology

Small craters of the lunar maria are observed to be in a state of equilibrium, in which the rate of production of new craters is, on average, equal to the rate of destruction of old craters. Crater counts of multiple lunar terrains over decades consistently show that the equilibrium cumulative size-frequency distribution (SFD) per unit area of small craters of radius >r is proportional r−2, and that the total crater density is a few percent of so-called geometric saturation, which is the maximum theoretical packing density of circular features. While it has long been known that the primary crater destruction mechanism for these small craters is steady diffusive degradation, there are few quantitative constraints on the processes that determine the degradation rate of meter to kilometer scale lunar surface features. Here we combine analytical modeling with a Monte Carlo landscape evolution code known as the Cratered Terrain Evolution Model to place constraints on which processes control the observed equilibrium size-frequency distribution for small craters. We find that the impacts by small distal ejecta fragments, distributed in spatially heterogeneous rays, is the largest contributor to the diffusive degradation which controls the equilibrium SFD of small craters. Other degradation or crater removal mechanisms, such cookie cutting, ejecta burial, seismic shaking, and micrometeoroid bombardment, likely contribute very little to the diffusive topographic degradation of the lunar maria at the meter scale and larger.

Giordano Bruno: Small crater populations––Implications for self-secondary cratering

Giordano Bruno is a lunar farside Copernican-age crater. The relatively few superposed impact craters on its floor and ejecta blanket and extensive bright rays indicate its youth. High-resolution Lunar Reconnaissance Orbiter Camera images reveal that the frequency and the characteristics of the cumulative size-frequency distributions of the small-diameter impact craters and their morphology vary across the clastic ejecta and impact melt covered surfaces. Crater frequencies (N(10) defined as ≥10 m km−2) vary by a factor of 10 s–100 s (N(10) 2–700) across the ejecta blanket, and between the ejecta and the melt deposits. Numerous craters on the ejecta blanket are degraded and buried by debris and impact melt, indicating that these partially buried craters formed during the deposition of the ejecta and prior to or during the emplacement of the impact melt. From geologic relations and crater statistics we conclude that a significant fraction of the craters observed on the ejecta blanket and the melt were formed during the cratering process itself and represent "self-secondaries." Further, we conclude that these craters do not represent an extra-lunar primary impact production population. Self-secondary craters are formed by material launched into near-vertical trajectories and having velocities such that their flight time is sufficiently long that the bulk of the clastic ejecta and impact melt are deposited before that material impacts the surface. The presence of a significant number of self-secondary craters on the ejecta makes the determination of relative and absolute age dates problematic. Crater counts would indicate an inappropriately old age. Using data for craters on the melt surfaces and for small-diameter bright ejecta craters, an absolute model age of 1 Ma is estimated. This age is considerably younger than that estimated by other studies and probably represents a maximum age.

Geomorphological and spectrophotometric analysis of Seth's circular niches on comet 67P/Churyumov–Gerasimenko using OSIRIS images

We provide a detailed geomorphological and spectrophotometric analysis of the circular niches located on the Seth region of 67P using OSIRIS images. The features can be related to landslide events that occurred on 67P and shaped its surface, as the recent Aswan cliff collapse detected in the same region. We therefore provide an analysis of the area pre- and post-perihelion suggesting that no specific changes have been observed. To assess this, after performing a geomorphological map of the area that allows us to identify different terrain units, we computed the boulders cumulative size frequency distribution (SFD) of the niches, before and after the perihelion passage. The niches SFDs are characterized by a similar trend with two different power-law indices within the same deposit: lower power-law value (between −2.3 and −2.7) for boulders smaller than 5 m and steeper power-law value (between −4.7 and −5.0) for boulders larger than 5 m. These trends suggest that smaller boulders have evolved and progressively have been depleted (lower power-law index), while bigger boulders are more representative of the event that generated the deposit and are less degraded. Then, we perform the spectrophotometric analysis of this region comparing pre- and post-perihelion results. We found colour changes within the area, in particular brighter patches related to the presence of exposed water ice mixed to the refractory materials have been detected in the post-perihelion images.

Physical properties and dynamical relation of the circular depressions on comet 67P/Churyumov-Gerasimenko

Aims. We aim to characterize the circular depressions of comet 67P/Churyumov-Gerasimenko and investigate whether such surface morphology of a comet nucleus is related to the cumulative sublimation effect since becoming a Jupiter family comet (JFC). Methods. The images from the Rosetta/OSIRIS science camera experiment are used to construct size frequency distributions of the circular depression structures on comet 67P and they are compared with those of the JFCs 81P/Wild 2, 9P/Tempel 1, and 103P/Hartley 2. The orbital evolutionary histories of these comets over the past 100 000 yr are analyzed statistically and compared with each other. Results. The global distribution of the circular depressions over the surface of 67P is charted and classified. Descriptions are given to the characteristics and cumulative size frequency distribution of the identified features. Orbital statistics of the JFCs visited by spacecraft are derived. Conclusions. The size frequency distribution of the circular depressions is found to have a similar power law distribution to those of 9P/Tempel 1 and 81P/Wild 2. This might imply that they could have been generated by the same process. Orbital integration calculation shows that the surface erosion histories of 81P/Wild 2, and 9P/Tempel 1 could be shorter than those of 67P, 103 P/Hartley 2 and 19P/Borrelly. From this point of view, the circular depressions could be dated back to the pre-JFC phase or the transneptunian phase of these comets. The north-south asymmetry in the distribution of the circular depressions could be associated with the heterogeneous structure of the nucleus of comet 67P and/or the solar insolation history.

Size-frequency distribution of boulders ≥10 m on comet 103P/Hartley 2

Aims. We derive the size-frequency distribution of boulders on comet 103P/Hartley 2, which are computed from the images taken by the Deep Impact/HRI-V imaging system. We indicate the possible physical processes that lead to these boulder size distributions.Methods. We used images acquired by the High Resolution Imager-Visible CCD camera on 4 November 2010. Boulders ≥10 m were identified and manually extracted from the datasets with the software ArcGIS. We derived the global size-frequency distribution of the illuminated side of the comet (~50%) and identified the power-law indexes characterizing the two lobes of 103P. The three-pixel sampling detection, together with the shadowing of the surface, enables unequivocally detection of boulders scattered all over the illuminated surface.Results. We identify 332 boulders ≥10 m on the imaged surface of the comet, with a global number density of nearly 140/km2 and a cumulative size-frequency distribution represented by a power law with index of −2.7 ± 0.2. The two lobes of 103P show similar indexes, i.e., −2.7 ± 0.2 for the bigger lobe (called L1) and −2.6+ 0.2/−0.5 for the smaller lobe (called L2). The similar power-law indexes and similar maximum boulder sizes derived for the two lobes both point toward a similar fracturing/disintegration phenomena of the boulders as well as similar lifting processes that may occur in L1 and L2. The difference in the number of boulders per km2 between L1 and L2 suggests that the more diffuse H2 O sublimation on L1 produce twice the boulders per km2 with respect to those produced on L2 (primary activity CO2 driven). The 103P comet has a lower global power-law index (−2.7 vs. −3.6) with respect to 67P. The global differences between the two comets’ activities, coupled with a completely different surface geomorphology, make 103P hardly comparable to 67P. A shape distribution analysis of boulders ≥30 m performed on 103P suggests that the cometary boulders show more elongated shapes when compared to collisional laboratory fragments as well as to the boulders present on the surfaces of 25 143 Itokawa and 433 Eros asteroids. Consequently, this supports the interpretation that cometary boulders have different origins with respect to the impact-related asteroidal boulders.

Size-frequency distribution of boulders ≥7 m on comet 67P/Churyumov-Gerasimenko

We derive for the first time the size-frequency distribution of boulders on a comet, 67P/Churyumov-Gerasimenko (67P), computed from the images taken by the Rosetta/OSIRIS imaging system. We highlight the possible physical processes that lead to these boulder size distributions. We used images acquired by the OSIRIS Narrow Angle Camera, NAC, on 5 and 6 August 2014. The scale of these images (2.44−2.03 m/px) is such that boulders ≥7 m can be identified and manually extracted from the datasets with the software ArcGIS. We derived both global and localized size-frequency distributions. The three-pixel sampling detection, coupled with the favorable shadowing of the surface (observation phase angle ranging from 48◦ to 53◦ ), enables unequivocally detecting boulders scattered all over the illuminated side of 67P. We identify 3546 boulders larger than 7 m on the imaged surface (36.4 km 2 ), with a global number density of nearly 100/km 2 and a cumulative size-frequency distribution represented by a power-law with index of −3.6 +0.2/−0.3. The two lobes of 67P appear to have slightly different distributions, with an index of −3.5 +0.2/−0.3 for the main lobe (body) and −4.0 +0.3/−0.2 for the small lobe (head). The steeper distribution of the small lobe might be due to a more pervasive fracturing. The difference of the distribution for the connecting region (neck) is much more significant, with an index value of −2.2 +0.2/−0.2. We propose that the boulder field located in the neck area is the result of blocks falling from the contiguous Hathor cliff. The lower slope of the size-frequency distribution we see today in the neck area might be due to the concurrent processes acting on the smallest boulders, such as i) disintegration or fragmentation and vanishing through sublimation; ii) uplifting by gas drag and consequent redistribution; and iii) burial beneath a debris blanket. We also derived the cumulative size-frequency distribution per km 2 of localized areas on 67P. By comparing the cumulative size-frequency distributions of similar geomorphological settings, we derived similar power-law index values. This suggests that despite the selected locations on different and often opposite sides of the comet, similar sublimation or activity processes, pit formation or collapses, as well as thermal stresses or fracturing events occurred on multiple areas of the comet, shaping its surface into the appearance we see today.

Impact spallation processes on the Moon: A case study from the size and shape analysis of ejecta boulders and secondary craters of Censorinus crater

Impact spallation is a fundamental process responsible for formation of ejecta boulders from impact craters. Although theoretical spallation models were developed about three decades ago, only limited geological observations have been made so far to test these models. The 3.8km Censorinus crater on the Moon provides an excellent opportunity for studying the impact spallation processes associated with a fresh simple crater formed by oblique impact. Using the Lunar Reconnaissance Orbiter Narrow Angle Camera images, we prepared the ejecta boulder distribution map of Censorinus crater and measured the boulder sizes and shapes. Mapping of about 242,000 ejecta boulders enabled us to document the size distribution of boulders both radial and concentric to the impact crater. Larger size boulders dominate the crater rim areas, while they become smaller away from the crater. The boulder distribution exhibits a radial asymmetry suggesting Censorinus is a oblique impact, in which the uprange ejecta have smaller ranges with larger concentration of boulders near the southwestern crater rim, while the downrange ejecta are in general characterized by smaller boulders with high spatial dispersion. The cumulative size–frequency distribution (CSFD) of boulders shows a highly variable fragmentation history in which the uprange boulders suffered more complex fragmentation. The ejecta boulders also exhibit a variety of shapes that are gleaned from their axial ratios and edge angle characteristics. There is a general decrease of axial ratios away from the crater rim. Rectangular boulders dominate the crater rim and they become more equant away from the crater. In addition to the boulder sizes, the boulder shape distribution also exhibits a mild asymmetry in response to the oblique impact. Small size fresh impact craters (84,000 craters) are abundant on the Censorinus ejecta and post-date Censorinus. These craters are found in two morphologic types in which a large majority of craters have subdued ejecta (rayless craters), while some possess bright-rayed ejecta (bright-rayed craters). The CSFD of rayless craters show a steep power-law slope with a b-value of −4.0, similar to the secondary craters produced by the impact of ejecta from primary craters. We therefore interpret the rayless craters as the secondary craters of Censorinus. On the other hand, the CSFD of bright-rayed craters have smaller power-law slope (b value −2.7) which is a characteristic of primary craters, and thus provide 3Ma age for Censorinus crater. When the characteristics of Censorinus boulders are compared with the theoretical spallation models that are sensitive to the petrophysical properties of the target (lunar highland), the models generally agree with the Censorinus boulders. However, the observed shape and size characteristics of the Censorinus boulders are found to be more complex than the theoretical spallation models. The ejecta boulders suffered more complex fragmentation and asymmetric distribution in response to the oblique impact. The spallation models accounting oblique impacts have not yet been developed. Therefore, our Censorinus boulder observations can be used to develop and validate the new theoretical spallation models for the effects of oblique impacts.

The kilometer-sized Main Belt asteroid population revealed bySpitzer

Aims. Multi-epoch Spitzer Space Telescope 24 μm data is utilized from the MIPSGAL and Taurus Legacy surveys to detect asteroids based on their relative motion. Methods. Infrared detections are matched to known asteroids and average diameters and albedos are derived using the near Earth asteroid thermal model (NEATM) for 1865 asteroids ranging in size from 0.2 to 169 km. A small subsample of these objects was also detected by IRAS or MSX and the single wavelength albedo and diameter fits derived from these data are within the uncertainties of the IRAS and/or MSX derived albedos and diameters and available occultation diameters, which demonstrates the robustness of our technique. Results. The mean geometric albedo of the small Main Belt asteroids in this sample is pV = 0.134 with a sample standard deviation of 0.106. The albedo distribution of this sample is far more diverse than the IRAS or MSX samples. The cumulative size-frequency distribution of asteroids in the Main Belt at small diameters is directly derived and a 3σ deviation from the fitted size-frequency distribution slope is found near 8 km. Completeness limits of the optical and infrared surveys are discussed.

Measuring and localizing acoustic emission events in snow prior to fracture

Acoustic emissions (AE) are transient elastic waves produced by a sudden redistribution of stress in a material caused by changes in the internal structure. In other natural, heterogeneous materials monitoring AE has proven to be a valuable tool for stability estimation and failure prediction. After studying the characteristics of ultrasonic wave propagation in snow, we measured the acoustic emission signals during snow loading experiments in a cold laboratory. Using snow columns we found that most energy of an artificial acoustic signal was transmitted at 31kHz. Best coupling to snow was achieved by attaching the AE sensors with silicone adhesive to thin aluminium plates which were then frozen to the snow. Localizing AE events during fracture of layered snow samples showed that the AE originated within the weakest layer, i.e. the relevant layer for snow failure. For finding an indication of imminent failure, we analysed the exponent β of the cumulative size-frequency distribution (‘survival curve’) of event energy. At the occurrence of instabilities, the β-curve deviated from steady behaviour and exhibited distinct ‘drops’, indicating that the power law behaviour of the distribution was not fulfilled anymore. Studying the temporal evolution of the exponent β might therefore provide useful information about snowpack stability also in the field—provided that the AE signals are not too strongly attenuated and can be detected in time before catastrophic failure occurs.

Volcanism on farside of the Moon: New evidence from Antoniadi in South Pole Aitken basin

The lunar surface is characterized by asymmetric distribution of its volcanic deposits. The nearside contains about 90% of the mare basalts, while there are a few on the farside with some are in the South Pole Aitken (SPA) basin, which is the largest and oldest impact basin on the Moon. Although the mare volcanism on the nearside of the Moon has been very well characterized, our understanding of the farside is relatively poor. Our geological mapping using Lunar Reconnaissance Orbiter (LRO) and Chandrayaan-1 Moon Mineralogy Mapper (M3) data of 143km Antoniadi impact basin provides new insights into volcanic processes in the southern SPA. Antoniadi has excavated the SPA floor as deep as 9km. Distribution of secondary craters around Antoniadi shows that the basin was formed by oblique impact with the direction of 50° counter-clockwise from 178°E longitude. Our geological mapping has found, for the first time, several volcanic cones (0.2–2.5km diameter) in association with the mare basalt flows. As per morphological classification, these are C, E, N and P type volcanic cones with abundant volcanic fragments in their summit areas. The base and summit diameters of these cones are similar to those of Marius Hills on the nearside of the Moon. The cumulative size frequency distributions of the summit fragments provide a range of power index values between −3 and −9, suggesting the involvement of dynamic fragmentation of the volcanic materials during the eruptive processes. The grabens in the basin floor show evidence for fissure eruption of the mare basalts. The graben near the central peak exposes about 140m thick basalt sequence containing a minimum of five individual lava layers. The crater size–frequency distributions of the mare deposits suggest an age of 1.6Ga for the flows in the central part of the basin, while 2.6Ga for those on the outer floor near the basin wall, indicating the episodic volcanism in the basin. The young volcanism of 1.6Ga is reported for the first time from SPA. The M3 data reveal that the basin materials contain abundant low-Ca pyroxene, while the mare basalts with high-Ca pyroxene. We calculated the liquidus density of mare basalts based on FeO and TiO2 content, which suggest a range of density from 2.7 to 2.8g/cm3, which would be lower than a gross lunar crust. This would imply the emplacement of basaltic lavas through dikes by simple magma buoyancy or over-pressurization of magma chambers, if they present at the crust-mantle boundary when the magma density is greater than the host crust. The 1.6Ga old volcanism in Antoniadi suggests that the mantle beneath the SPA has been conducive for young magmatism, possibly due to presence of KREEP like material in the mantle.