Crater Size-frequency Distribution Research Articles

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.

The Gardner volcanic complex on the Moon: Geological characteristics and its volcanic history

Lunar volcanic complexes are prominent features with a high concentration of various volcanic characteristics. They are ideal objects to study the lunar nearside volcanic activity and its thermal evolution. The Gardner volcanic complex is located in the nearside of the Moon, between Mare Serenitatis and Mare Tranquillitatis. It has a series of volcanic features, such as domes and rilles, which make it an ideal object to study the lunar volcanic history. In this study, we make use of recent high-resolution lunar images (Terrain Camera, TC) and multispectral data (Clementine UVVIS) to study the topography, geomorphologic features, iron and titanium contents, and the geochronology of the Gardner volcanic complex. We identified and analyzed different geomorphologic features, such as impact craters, domes, massifs, scarps, and a suspected fault. Spectral analyses indicated that the Gardner region is covered by low-Ti basalt. Absolute model ages of the different units in the Gardner region were obtained using crater size-frequency distribution (CSFD) measurement. The results show that Gardner may have experienced three periods of volcanic activities, which were aged between Nectarian and Imbrium. The volcanic eruption may decrease with time. The relatively high crustal density and the positive Bouguer gravity anomaly of Gardner show that the Gardner volcanic complex is composed of dense volcanic materials on the surface and in the subsurface. An irregular steep-sided depression is located in the central area of the Gardner plateau, which is different from the depression caused by impacting. We suggest that this depression could be a caldera and is once a possible vent responsible for the emplacement of surrounding lava flows. The long history of volcanic activities in this region makes it an ideal area to study the lunar nearside volcanism and thermal evolution, and we suggest it as a candidate landing site for China’s future lunar exploration missions.

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.

A Bayesian Approach to Subkilometer Crater Shape Analysis Using Individual HiRISE Images

The ages of terrains on other planetary bodies are chiefly determined using crater size–frequency distributions. However, primary impacts can generate numerous secondary craters that can affect the crater population. Classifying impact craters as primary or secondary is commonly done via time-consuming manual inspection, which limits the areas that can be analyzed at high resolution. We present a parametric model for characterizing small (100–600 m diameter) impact craters, where the model parameters have implications for describing the physical processes involved in their formation and modification. We infer these parameters from craters in images captured by the high-resolution imaging science experiment (HiRISE) camera onboard the Mars Reconnaissance Orbiter. For each crater within the appropriate size range, our algorithm creates a 3-D surface for a parametrically modeled crater and a 2-D rendering using illumination metadata, including emission, phase, and solar incidence angles at the time when the image was captured. A function describes the likelihood of each set of model parameters in terms of the geometry of craters in a given HiRISE image. These values are then optimized using a Metropolis–Hasting Markov chain Monte Carlo sampler. We evaluated three different prior probability distributions over the parameter space and two different likelihoods: one for digital terrain models and the other for images. We show that after applying t-distributed stochastic neighbor embedding (t-SNE) over the inferred crater parameters, t-SNE is able to project the multidimensional crater parameters into a 2-D space where secondary craters cluster together and are separable from primary craters.

The geology of the Nawish quadrangle of Ceres: The rim of an ancient basin

Herein we present the geology of the Nawish quadrangle, located in the equatorial region of dwarf planet Ceres, named after one of the most prominent craters of the area. Geologic mapping was based on the image mosaics and digital terrain models derived from Dawn Framing Camera data. Interpretation of geologic units was supported by supplemental data, such as the multi spectral color images from the Framing Camera, and the spectral parameters derived from the Visible and Infrared Spectrometer (VIR) data, as well as Dawn gravity data. There is not a primary feature that dominates the geology of Nawish quadrangle, but rather several terrains overlap, and their relations explain the geology of the area. Crater size frequency distributions show that Nawish quadrangle is dominated by two distinct time domains. The central and eastern part of the quadrangle is topographically elevated, which we define as cratered highlands, and contains the older domain. The western lowlands show two younger domains related to impact craters Kerwan and Dantu, including the Kerwan smooth material and Dantu ejecta. This variation of elevation within the Nawish quadrangle is more than the half of the global topographic altitude variation on Ceres. Analysis and comparison of the topography of the Nawish quadrangle with surrounding ones shows that this quadrangle is dominated by the topography of the rim sector of a large, >800 km ancient impact basin, most likely the putative Vendimia Planitia. The Nawish quadrangle thus represents a sector of Ceres which has not undergone large-scale, post-Kerwan, intermediate age-events, but rather represents a place on Ceres where a well-preserved relict of old cerean crust can be studied, together with ejecta from more recent impact events.

Lunar Crater Detection Based on Terrain Analysis and Mathematical Morphology Methods Using Digital Elevation Models

Lunar impact craters are the most typical geomorphic feature on the moon and are of great importance in studies of lunar terrain features. This paper presents a crater detection algorithm (CDA) that is based on terrain analysis and mathematical morphology methods. The proposed CDA is applied to digital elevation models (DEMs) to identify the boundaries of impact craters. The topographic and morphological characteristics of impact craters are discussed, and detailed steps are presented to detect different types of craters, such as dispersal craters, connective craters, and con-craters. The DEM from the Lunar Reconnaissance Orbiter, which has a resolution of 100 m, is used to verify the proposed CDA. The results show that the boundaries of impact craters can be detected. The results enable increased understanding of surface processes through the characterization of crater morphometry and the use of crater size–frequency distributions to estimate the ages of planetary surfaces.

Morphometric and rheological study of lunar domes of Marius Hills volcanic complex region using Chandrayaan-1 and recent datasets

Marius Hills volcanic complex is one of the most important regions on the lunar surface having an abundant number of volcanic features like domes and cones. Systematic mapping of 106 domes/cones in the Marius Hills region was carried out in this study using high-resolution orthoimage and digital elevation models of Chandrayaan-1 and Kaguya missions. Various morphometric parameters like diameter, height, volume, flank slope, circularity index and form factor are derived for all the mapped domes. The rheological parameters, such as viscosity and eruption rate are estimated for isolated domes and cones superimposed over low domes. The morphometric and rheological properties of these domes are comparable to those located in the area near to Hortensius crater and other mare regions. Surface ages derived for a selected region in NW portion of the Marius Hills volcanic complex using crater size-frequency distribution technique yields ages of 2.98 and 1.91 Ga. It suggests that the domes in this region formed at about 2.98 Ga ago, and then, the younger mare basalts likely embayed this region about 1.98 Ga ago. Stratigraphic sequence of rilles, wrinkle ridge and domes shows that wrinkle ridges are the oldest, while the rilles are younger than the domes.

Timings of early crustal activity in southern highlands of Mars: Periods of crustal stretching and shortening

Extensional and compressional structures are globally abundant on Mars. Distribution of these structures and their ages constrain the crustal stress state and tectonic evolution of the planet. Here in this paper, we report on our investigation over the distribution of the tectonic structures and timings of the associated stress fields from the Noachis-Sabaea region. Thereafter, we hypothesize possible origins in relation to the internal/external processes through detailed morphostructural mapping. In doing so, we have extracted the absolute model ages of these linear tectonic structures using crater size-frequency distribution measurements, buffered crater counting in particular. The estimated ages indicate that the tectonic structures are younger than the mega impacts events (especially Hellas) and instead they reveal two dominant phases of interior dynamics prevailing on the southern highlands, firstly the extensional phase terminating around 3.8 Ga forming grabens and then compressional phase around 3.5–3.6 Ga producing wrinkle ridges and lobate scarps. These derived absolute model ages of the grabens exhibit the age ca. 100 Ma younger than the previously documented end of the global extensional phase. The following compressional activity corresponds to the peak of global contraction period in Early Hesperian. Therefore, we conclude that the planet wide heat loss mechanism, involving crustal stretching coupled with gravitationally driven relaxation (i.e., lithospheric mobility) resulted in the extensional structures around Late Noachian (around 3.8 Ga). Lately cooling related global contraction generated compressional stress ensuing shortening of the upper crust of the southern highlands at the Early Hesperian period (around 3.5–3.6 Ga).

Phoebe: A Surface Dominated by Water

Abstract The Saturnian irregular satellite, Phoebe, can be broadly described as a water-rich rock. This object, which presumably originated from the same primordial population shared by the dynamically excited Kuiper Belt Objects (KBOs), has received high-resolution spectral imaging during the Cassini flyby. We present a new analysis of the Visual Infrared Mapping Spectrometer observations of Phoebe, which critically, includes a geometry correction routine that enables pixel-by-pixel mapping of visible and infrared spectral cubes directly onto the Phoebe shape model, even when an image exhibits significant trailing errors. The result of our re-analysis is a successful match of 46 images, producing spectral maps covering the majority of Phoebe’s surface, roughly a third of which is imaged by high-resolution observations (<22 km per pixel resolution). There is no spot on Phoebe’s surface that is absent of water absorption. The regions richest in water are clearly associated with the Jason and south pole impact basins. Phoebe exhibits only three spectral types, and a water–ice concentration that correlates with physical depth and visible albedo. The water-rich and water-poor regions exhibit significantly different crater size frequency distributions and different large crater morphologies. We propose that Phoebe once had a water-poor surface whose water–ice concentration was enhanced by basin-forming impacts that exposed richer subsurface layers. The range of Phoebe’s water–ice absorption spans the same range exhibited by dynamically excited KBOs. The common water–ice absorption depths and primordial origins, and the association of Phoebe’s water-rich regions with its impact basins, suggests the plausible idea that KBOs also originated with water-poor surfaces that were enhanced through stochastic collisional modification.

Compositional and chronological characterization of mare crisium using Chandrayaan-1 and LROC-WAC data

The surface of the Moon has been observed by human being since time immemorial and it appears distinctly different from that of the earth. Moon is geologically inactive for quite long time and therefore it's an ideal place to study the early solar system history. The lunar surface is dominated by various morphological features such as craters, rilles, impact melts, swirls, faults, lineaments and wrinkle ridges. The mare crisium (sea of crisis) is a multi-ring mare basin in the eastern nearside of the moon. Compositional analysis and crater counting based chronological studies conducted in the present work by using Chandrayaan-1 Moon Mineralogical Mapper data (M3), Terrain Mapping Camera (TMC) and Lunar Reconnaissance Orbiter Camera - Wide Angle Camera (LROC -WAC) data. The M3 data has been used for FeO and TiO2 mapping and to measure 1 μm, 2 μm integrated band depth to detect the olivine and pyroxene minerals. Ch-1 TMC and LROC WAC data were used for the chronological characterization through Crater Size Frequency Distribution (CSFD). The CSFD analysis by using TMC data have provided the ages for the southwest, southcentral and southeast regions of the crisium basin as 3.4Ga, 3.6Ga, and 3.45Ga respectively. The spectral characterizations of the selected Crisium features were done to map the various lithological features such as olivine, pyroxene, plagioclase, ilmenite and olivine – pyroxene mixtures which are present in the basalt, gabbroic and anorthositic gabbro rocks from the mare crisium.

A New Tool to Account for Crater Obliteration Effects in Crater Size‐Frequency Distribution Measurements

AbstractThe analysis of crater size‐frequency distributions (CSFDs) is a widely used technique to date and investigate planetary surface processes. There are two well‐established crater measurement techniques, traditional crater counting and buffered crater counting, and two new geometric corrections, nonsparseness correction and buffered nonsparseness correction. The new techniques consider the effects of crater obliteration and subsequent recratering while measuring CSFDs in areas of high crater density. Currently, the ArcGIS add‐in CraterTools can be used to apply the well‐established techniques. The tool relies on Esri's ArcGIS environment and is restricted to 32 bit and single‐core computing. These limitations make the implementation of the new geometric corrections in CraterTools inefficient, as the new techniques are computationally more intensive than the well‐established ones. To this end, we developed CSFD Tools, an application to conduct CSFD measurements from shapefiles. It supports 64 bit and multicore data processing and uses existing open geospatial libraries. Open libraries, however, conduct spatial measurements on a Cartesian plane and do not take a curved planetary surface into account. Therefore, we implemented methods for geodesic measurements and workarounds for the geodesic modification of polygon data to minimize map distortion effects during CSFD measurements. As a result, the new nonsparseness correction and buffered nonsparseness correction techniques can be applied through a software tool.

Investigating the volcanic versus aqueous origin of the surficial deposits in Eastern Elysium Planitia, Mars

The Elysium Volcanic Province consists of numerous overlapping flow units and may include the youngest lava flows on Mars. However, it is possible that these volcanic units have been modified or overprinted by aqueous processes. Understanding the timing of the igneous and aqueous events in this region is therefore essential for constraining the geological and environmental history of Mars during the Amazonian Period. We investigate the geologic evolution of Eastern Elysium Planitia to determine the relationship between major units, with the support of a geological map and chronological constraints from crater size–frequency distributions. We also evaluate the hypothesized origin of these units via volcanic, fluvial, and/or fluvioglacial processes using a detailed facies-mapping approach. The study area includes the Eastern Cerberus Fossae, Rahway Valles, and Marte Vallis. The surficial deposits in Rahway Valles were formerly interpreted to be modified by fluvial and fluvioglacial processes. However, our facies map reveals that the surface of Eastern Elysium Planitia includes nineteen morphologically distinct regions (i.e., facies), which are interpreted to be the products of flood lava volcanism, including: ʻaʻā, pāhoehoe, and transitional lava flow types. In contrast to previous studies, which determined that Rahway Valles and Marte Vallis consist of two distinct geologic units with Middle to Late Amazonian ages, the results of this work show that the region was resurfaced by at least two volcanic flows with much younger ages of 20.0 Ma and 8.8 Ma. Furthermore, by coupling results of our geologic and facies mapping with chronological constraints as well as subsurface information provided by Shallow Radar reflectors, we show that there is an erosional unconformity located between the two youngest lava flow units in Marte Vallis. We interpret that this unconformity was generated by a catastrophic aqueous flooding event that occurred only 8.8 − 20.0 Ma ago. This implies alternating episodes of volcanism and aqueous flooding that have continued into the geologically recent past on Mars, and may again occur within Elysium Planitia.

The age of lunar mare basalts south of the Aristarchus Plateau and effects of secondary craters formed by the Aristarchus event

Using crater size-frequency distributions (CSFDs) and careful discrimination of primary and secondary impact craters in the mare region south of Aristarchus Crater, we reexamined the age of unit ‘P60’, determined previously by Hiesinger et al. (2003) to be one of the youngest basaltic surfaces on the Moon. Owing to the apparently young age of these basalts, this region is a potential site for future sample return missions. We investigated this 55,000 km2 region with Lunar Reconnaissance Orbiter Camera (LROC) images to assess potential variations in CSFDs across the unit, particularly in light of heavy contamination by secondary craters from Aristarchus Crater, and to determine the age(s) of P60 over its full areal extent. We are able to match, within one standard error, the previously determined age using approximately the same counting area. In addition, we defined twelve regions between the rays of secondaries to determine if and how the ages of P60 might vary across the unit. For these inter-ray regions, we find a systematic progression of ages from west to east, ranging from youngest (∼1 Ga) in the west, to 1.8 Ga southwest of Aristarchus, to 2.2 Ga south of Aristarchus, and finally to 2.7 Ga southeast of Aristarchus. This variation in ages is not solely attributable to secondary cratering, indicating it must be at least partially due to volcanic resurfacing. The northwestern-most extension of P60 may belong to a different unit owing to topographic and crater distribution differences. Analysis of the summed CSFD for P60 provides evidence for emplacement of the younger basalts (<2.5 Ga) on top of an older surface of ∼3.6 Ga. We observe that ∼2–3 km diameter craters within an older mare unit are embayed by the younger lava flows, some of which are ‘ghost’ craters, with barely visible rims. In addition, we identified six volcanic vents that are possible sources for the younger P60 flow(s). This work indicates that volcanism spanned a significant range, from ∼3.6 Ga to 1.0 Ga ago across this region.

Ancient Bombardment of the Inner Solar System: Reinvestigation of the “Fingerprints” of Different Impactor Populations on the Lunar Surface

AbstractThe lunar cratering record provides valuable information about the late accretion history of the inner solar system. However, our understanding of the origin, rate, and timing of the impacting projectiles is far from complete. To learn more about these projectiles, we can examine crater size‐frequency distributions (CSFDs) on the Moon. Here we reinvestigate the crater populations of 30 lunar basins (≥ 300 km) using the buffered nonsparseness correction technique, which takes crater obliteration into account, thus providing more accurate measurements for the frequencies of smaller crater sizes. Moreover, we revisit the stratigraphic relationships of basins based on N(20) crater frequencies, absolute model ages, and observation data. The buffered nonsparseness correction‐corrected CSFDs of individual basins, particularly at smaller crater diameters are shifted upward. Contrary to previous studies, the shapes of the summed CSFDs of Pre‐Nectarian (excluding South Pole‐Aitken Basin), Nectarian (including Nectaris), and Imbrian (including Imbrium) basins show no statistically significant differences and thus provide no evidence for a change of impactor population.

How old are lunar lobate scarps? 1. Seismic resetting of crater size-frequency distributions

Previous studies have estimated the ages of lunar lobate scarps, some of the youngest tectonic landforms on the Moon, based on the estimated life-times of their fresh morphologies and associated small graben, using crater degradation ages, or via buffered and traditional crater size-frequency distribution (CSFD) measurements. Here, we reexamine five scarps previously dated by Binder and Gunga (1985) with crater degradation ages to benchmark the evaluation of both the buffered and traditional CSFD approaches for determination of absolute model ages (AMAs) at scarps. Both CSFD methods yield similar ages for each individual scarp, indicating that fault activity not only can be measured on the scarp itself, but also in the surrounding terrain – an indication that tectonic activity causes surface renewal both adjacent to and even kilometers distant from scarps. Size-frequency variations in the regions surrounding the scarps are thus useful for studying the extent and severity of the ground motion caused by coseismic slip events during scarp formation. All age determination approaches continue to indicate that lunar lobate scarps were active in the late Copernican, with some scarps possibly experiencing activity within the last 100 Ma.

Dune-Yardang Interactions in Becquerel Crater, Mars.

Isolated landscapes largely shaped by aeolian processes can occur on Earth, while the majority of Mars' recent history has been dominated by wind-driven activity. Resultantly, Martian landscapes often exhibit large-scale aeolian features, including yardang landforms carved from sedimentary-layered deposits. High-resolution orbital monitoring has revealed that persistent bedform activity is occurring with dune and ripple migration implying ongoing abrasion of the surface. However, little is known about the interaction between dunes and the topography surrounding them. Here we explore dune-yardang interactions in Becquerel crater in an effort to better understand local landscape evolution. Dunes there occur on the north and south sides of a 700 m tall sedimentary deposit, which displays numerous superposed yardangs. Dune and yardang orientations are congruent, suggesting that they both were formed under a predominantly northerly wind regime. Migration rates and sediment fluxes decrease as dunes approach the deposit and begin to increase again downwind of the deposit where the effect of topographic sheltering decreases. Estimated sand abrasion rates (16-40 μm yr-1) would yield a formation time of 1.8-4.5 Myr for the 70 m deep yardangs. This evidence for local aeolian abrasion also helps explain the young exposure ages of deposit surfaces, as estimated by the crater size-frequency distribution. Comparisons to terrestrial dune activity and yardang development begin to place constraints on yardang formation times for both Earth and Mars. These results provide insight into the complexities of sediment transport on uneven terrain and are compelling examples of contemporary aeolian-driven landscape evolution on Mars.

Low crater frequencies and low model ages in lunar maria: Recent endogenic activity or degradation effects?

AbstractRecently a number of studies have identified small lunar geologic structures to be &lt;100 Ma in age using standard remote sensing techniques. Here we present new crater size frequency distributions (CSFDs) and model ages using craters D &gt; 10 m for five small target units: one irregular mare patch (IMP) in Mare Nubium and four regions located on lunar wrinkle ridges in Mare Humorum. For comparison we also date another IMP found in a recent study in Mare Tranquillitatis (Braden et al. ). Absolute model age (AMA) derivation corresponds to 46 ± 5 Ma and 22 ± 1 Ma for Nubium and Sosigenes IMP, respectively. We show that for IMPs and in nearby control mare regions, similar production‐like cumulative log–log SFD slopes of −3 are observed. In contrast, control mare regions in Mare Humorum exhibit shallower equilibrium slopes from −1.83 to −2. Three out of four wrinkle ridges appear to be in equilibrium but with crater lifetimes lower than on the corresponding maria. Low crater frequencies on one wrinkle ridge result in an age of 8.6 ± 1 Ma. This study region contains 80% fresh craters, which suggests that the crater population is still in production indicative of a recent resurfacing event.

Usability of small impact craters on small surface areas in crater count dating: Analysing examples from the Harmakhis Vallis outflow channel, Mars

The availability of very high-resolution images has made it possible to extend crater size-frequency distribution studies to small, deca/hectometer-scale craters. This has enabled the dating of small and young surface units, as well as recent, short-time and small-scale geologic processes that have occurred on the units. Usually, however, the higher the spatial resolution of space images is, the smaller area is covered by the images. Thus the use of single, very high-resolution images in crater count age determination may be debatable if the images do not cover the studied region entirely.Here we compare the crater count results for the floor of the Harmakhis Vallis outflow channel obtained from the images of the ConTeXt camera (CTX) and High Resolution Imaging Science Experiment (HiRISE) aboard the Mars Reconnaissance Orbiter (MRO). The CTX images enable crater counts for entire units on the Harmakhis Vallis main valley, whereas the coverage of the higher-resolution HiRISE images is limited and thus the images can only be used to date small parts of the units. Our case study shows that the crater count data based on small impact craters and small surface areas mainly correspond with the crater count data based on larger craters and more extensive counting areas on the same unit. If differences between the results were founded, they could usually be explained by the regional geology. Usually, these differences appeared when at least one cratering model age is missing from either of the crater datasets. On the other hand, we found only a few cases in which the cratering model ages were completely different.We conclude that the crater counts using small impact craters on small counting areas provide useful information about the geological processes which have modified the surface. However, it is important to remember that all the crater counts results obtained from a specific counting area always primarily represent the results from the counting area—not the whole unit. On the other hand, together with crater count results from extensive counting areas and lower-resolution images, crater counts on small counting areas but by using very high-resolution images is a very valuable tool for obtaining unique additional information about the local processes on the surface units.

Size-Frequency Distribution of Small Lunar Craters: Widening with Degradation and Crater Lifetime

The review and new measurements are presented for depth/diameter ratio and slope angle evolution during small (D < 1 km) lunar impact craters aging (degradation). Comparative analysis of available data on the areal cratering density and on the crater degradation state for selected craters, dated with returned Apollo samples, in the first approximation confirms Neukum’s chronological model. The uncertainty of crater retention age due to crater degradational widening is estimated. The collected and analyzed data are discussed to be used in the future updating of mechanical models for lunar crater aging.

Geology, tectonism and composition of the northwest Imbrium region

The objective of this study is to explore the regional geology of the northwest Imbrium region in which the Chang'E-3 (CE-3) landing site is located. CE-3 successfully landed on December 14, 2013 on the unsampled Eratosthenian basalts whose study is important for understanding the evolution of the Moon. New geologic and structural maps of the research area were produced through the integrated analysis of diverse datasets. The highlands surrounding Imbrium differ from typical Farside Highlands Terrain (FHT). The Iridum highland region (as well as the surrounding Imbrium region) exhibits elevated concentrations of Fe, and abundant local exposures of low-Ca pyroxene and olivine bearing lithologies. In this study these highlands are named as mafic highlands (MH). Our dating results using crater size-frequency distributions (CSFDs) show that the Iridum basin (hosting Sinus Iridum) was formed ∼3.8 Ga, shortly following the Imbrium basin formation and before the last large multiringed basin, Orientale. The Eratosthenian period of lunar basalt eruptions, which lasted longer than other stratigraphic units, is suggested to divide into the Lower Eratosthenian mare (LEm) and Upper Eratosthenian mare (UEm) units. This subdivision is based on whether lava fronts can be clearly seen or not and the age separating the units is 2.35 Ga. The mafic mineralogy of the mare basalts in Imbrium is characterized by abundant olivine in the Eratosthenian-aged basalts and average pyroxene compositions near pigeonite to sub-calcic augite in the Imbrian and Em1 units. The thickness of individual lava for UEm units is 8–11 m, indicative of high effusion rates. The thickness of the Em3 unit ranges from ∼17 m to ∼45 m with lesser thickness to the west and greater thickness in the interior and to the east. The estimated volume and average flux of the Eratosthenian-aged basalts are greater than previously thought. The presence of these youngest basalts in the Procellarum-KREEP terrain (PKT) is hypothesized to be a causal relationship, with the PKT terrain reducing the thickness of the lithosphere and permitting preferential dike emplacement and extrusion there. We speculate that high-Ti and olivine-rich composition in late stage basalts may be consistent with low Si and high Ti and low degrees of partial melting. Large numbers of sinuous rilles and small ridges are identified and mapped. Many young ridges were found inside Imbrium, suggesting a very extended period (at least as young as the last 50 Ma) of the Moon's tectonic activity. The distinct compositions of both highlands and mare basalts and extended tectonism emphasize how the Imbrium basin is an important area for understanding the Moon.