Crater Size-frequency Distribution Research Articles

Absolute model age of lunar Finsen crater and geologic implications

As the primary source of materials measured inside of Von Kármán crater by the Chang'e-4 rover, the absolute model age (AMA) of Finsen crater is derived by the crater size-frequency distribution (CSFD) measurements. Both cumulative and differential fits reveal an AMA of ~3.5 Ga, indicating Finsen crater is Imbrium-aged. Based on thickness estimation of the Finsen crater ejecta-sourced fine-grained regolith at Chang'e-4 landing site (~12 m) (Lai et al., 2019; Li et al., 2020), the average regolith growth rate at the landing site is estimated to be about 3.4 m/Gyr. The current depths of 25 largest craters on the floor of Finsen crater are calculated by the profile-average-depth method, and their original depths are estimated under the assumption that the initial depth-to-diameter (d/D) ratio is 0.2. Depth differences between current and estimated original depths, together with the AMA of Finsen crater, derive the average crater degradation/infilling rate within Finsen crater is about 21 ± 3 m/Gyr.

A Multi-Annotator Survey of Sub-km Craters on Mars

We present here a dataset of nearly 5000 small craters across roughly 1700 km2 of the Martian surface, in the MC-11 East quadrangle. The dataset covers twelve 2000-by-2000 pixel Context Camera images, each of which is comprehensively labelled by six annotators, whose results are combined using agglomerative clustering. Crater size-frequency distributions are centrally important to the estimation of planetary surface ages, in lieu of in-situ sampling. Older surfaces are exposed to meteoritic impactors for longer and, thus, are more densely cratered. However, whilst populations of larger craters are well understood, the processes governing the production and erosion of small (sub-km) craters are more poorly constrained. We argue that, by surveying larger numbers of small craters, the planetary science community can reduce some of the current uncertainties regarding their production and erosion rates. To this end, many have sought to use state-of-the-art object detection techniques utilising Deep Learning, which—although powerful—require very large amounts of labelled training data to perform optimally. This survey gives researchers a large dataset to analyse small crater statistics over MC-11 East, and allows them to better train and validate their crater detection algorithms. The collection of these data also demonstrates a multi-annotator method for the labelling of many small objects, which produces an estimated confidence score for each annotation and annotator.

Geological mapping and chronology of lunar landing sites: Apollo 12

The lunar cratering chronology is fundamentally important, because it is not just used to determine ages for unsampled lunar surfaces, but is commonly applied to other planetary bodies. As part of a systematic study of the calibration of the lunar cratering chronology at the Apollo landing sites, we produced a new detailed geological map of the Apollo 12 landing site, using recent orbital data including Lunar Reconnaissance orbiter (LRO) Narrow Angle (NAC) and Wide Angle (WAC) images, and a Lunar Orbiter Laser Altimeter (LOLA)/SELENE merged digital elevation model (DEM), as well as Chandrayaan M3 spectral data. Using high-resolution LRO NAC images and NAC-derived DTMs, new crater size-frequency distributions (CSFDs) were measured on the updated geological units to determine crater retention ages or N(1) values (i.e., the cumulative number of craters with diameters ≥1 km). Since the Apollo 12 landing site is located in Oceanus Procellarum on a ray of Copernicus crater, it provides two possible calibration points for the lunar cratering chronology: (1) a mare basalt surface age, and (2) an approximate age for Copernicus crater via its ray material. We calibrated our new N(1) values with recently determined radiometric ages of Apollo 12 basalt samples, and compare our results with work done by Hiesinger et al. (2012) for Copernicus crater, and the Neukum (1983) chronology. The updated calibration points are consistent with the lunar chronology of Neukum (1983), which indicates that no updates to this function are currently necessary.

Interpreting the Cratering Histories of Bennu, Ryugu, and Other Spacecraft-explored Asteroids

Abstract Asteroid crater retention ages have unknown accuracy because projectile–crater scaling laws are difficult to verify. At the same time, our knowledge of asteroid and crater size–frequency distributions has increased substantially over the past few decades. These advances make it possible to empirically derive asteroid crater scaling laws by fitting model asteroid size distributions to crater size distributions from asteroids observed by spacecraft. For D > 10 km diameter asteroids like Ceres, Vesta, Lutetia, Mathilde, Ida, Eros, and Gaspra, the best matches occur when the ratio of crater to projectile sizes is f ∼ 10. The same scaling law applied to 0.3 < D < 2.5 km near-Earth asteroids such as Bennu, Ryugu, Itokawa, and Toutatis yield intriguing yet perplexing results. When applied to the largest craters on these asteroids, we obtain crater retention ages of ∼1 billion years for Bennu, Ryugu, and Itokawa and ∼2.5 billion years for Toutatis. These ages agree with the estimated formation ages of their source families and could suggest that the near-Earth asteroid population is dominated by bodies that avoided disruption during their traverse across the main asteroid belt. An alternative interpretation is that f ≫ 10, which would make their crater retention ages much younger. If true, crater scaling laws need to change in a substantial way between D > 10 km asteroids, where f ∼ 10, and 0.3 < D < 2.5 km asteroids, where f ≫ 10.

The Acidalia Mensa region on Mars: A key element to test the Mars ocean hypothesis

We present the results of a detailed photogeological study of a region surrounding Acidalia Mensa (43–50°N, 15–35°W) based on analysis of a mosaic of the CTX images (nominal resolution ~5 m/px). The main result of the study is the unfolding of the geologic history of the study region and determination of absolute model ages of its major episodes. The documented history begins with the formation of a volcanic plateau of Acidalia Mensa ~4 Ga ago. The surface of the Acidalia plateau was strongly modified since its formation. The crater size-frequency distribution (CSFD) on the plateau indicates that the resurfacing was likely consisted of multiple episodes. The latest such episode recognizable by the crater statistics occurred ~3.65 Ga ago. The vast majority of the surface around Acidalia Mensa is covered by materials of the background plains that correspond to the interior unit of the Vastitas Borealis Formation (VBF). The CSFD on the background plains corresponds to an absolute model age (AMA) of ~3.58 Ga, which is completely within the range of AMAs determined for the VBF along its boundary with the southern uplands. Small-scale features such as bright cones/mounds, polygonal troughs, and flow-like mantling materials postdate the emplacement of the background plains and manifest the post-formational evolution of the VBF. The majority of crater ejecta in the study area have higher albedo in the CTX and HiRISE images, but appear as dark spots in the THEMIS-IR nighttime images. These wavelength-dependent properties of the ejecta indicate the presence of light-toned and fine-grained material beneath the surface of the background plains. This material was the source of numerous bright cones/mounds. We interpret the formation of this reservoir as the result of sorting of large amounts of water/mud deposited in the study area and, by implication, within the northern lowlands. The final episodes of the ancient geologic history of the study area were related to the emplacement of materials of the thick flows at topographically high levels within the Acidalia plateau ~3.47 Ga ago. The morphology of the flows is inconsistent with lava and we interpret the occurrences of the thick flows as the result of displacement of water-saturated regolith in the form of either soli- or gelifluction flows, or ice-cored rock glaciers.

Automated precision counting of very small craters at lunar landing sites

Automated analysis of multiple LROC NAC images of the same area on the lunar surface, with each image exposed under different illumination conditions, can result in confident detection rates for craters as small as 3 m in diameter. The use of automatically generated templates improves the measurement accuracy of crater diameters and centre locations, relative to expertly counted benchmarks. By matching craters across multiple images, false positives can be eliminated and false negatives included. The requirement that all craters be matched in at least two images establishes a uniform detection threshold that enables different areas of the Moon to be compared quantitatively. Crater Size Frequency Distribution (CSFD) curves from a number of landing/impact sites on the nearside maria reveal significant differences in crater counts that require explanations in terms of different production and/or erosion rates. At some sites, crater populations can be rationalised in terms of just two distinct components, namely craters up to a few tens of metres in diameter formed within the last few hundred million years and an ancient component that has survived for billions of years. Some sites are more complex than this, their crater populations having been affected by secondary impacts.

A catalogue of impact craters larger than 200 m and surface age analysis in the Chang'e-5 landing area

Chang'e-5 (CE-5) is the first sample-return mission of China's lunar exploration and will be launched in 2020. The planned landing area is near Mons Rümker in Oceanus Procellarum. It is important to have a detailed geological analysis of the landing area prior to the lunar sample collection. The crater size-frequency distribution curve, combined with radioisotope dating of components of the returned sample by CE-5, may allow a refinement of the lunar chronology function. Based on a digital orthophoto map with a pixel size of 1.5 m generated from more than 700 Lunar Reconnaissance Orbiter Camera Narrow Angle Camera images of the CE-5 landing area, all impact craters larger than 200 m in diameter were extracted. Using the resultant crater catalogue of 32,277 craters, a systematic analysis of the landing area including the crater distribution, morphology characteristics, and geologic model ages is conducted. The results show that craters smaller than 1,000 m are denser in the east and the west parts of the region, while those larger than 1,000 m tend to be more concentrated in the central part, particularly within the range of 61.5°W–64.5°W. Within nine geologic units divided using the spectral characteristics, the geologic unit of Em1 (Eratosthenian-aged mare, Em) near the Mons Rümker is the youngest with a model age of 2.02−0.16+0.16 Ga. The three Rümker plateau (IR) units are the oldest parts of the landing area with the largest model age of 3.49−0.12+0.073 Ga. The crater catalogue and the surface model age analysis results can contribute to the CE-5 mission as well as further scientific research.

Refined model age for Orientale Basin derived from zonal crater dating of its ejecta

The Orientale basin is the most well-preserved multi-ring impact basin on the Moon and it has been used to define the end of the Early Imbrian Epoch. Thus a precise determination of the absolute model age (AMA) of Orientale Basin is important for lunar geology study. The method of crater size–frequency distribution (CSFD) has been used to date Orientale Baisn in previous studies, and the resultant AMA ranges from 3.68 Ga to 3.8 Ga. The inconsistence may be attributed to the choice of counting area and identified superposed craters. In this research we have mapped 27,093 craters larger than 0.7 km in diameter on the entire ejecta of the Orientale Basin, and derived that the AMAs of the Orientale Basin and the pre-existing luanr suface are 3.80−0.0079+0.0074 Ga and ~4.14−0.022+0.019 Ga, respectively. And the knee point of the CSFD of the entire counting area is ~10 km, corresponding to ~ 2 km of the average thickness of the Orientale Basin ejecta. To further analyze the variation of the ejecta thickness of Orientale Basin, the counting area was divided into eight 45-degree subsections. The analysis of the knee points of the CSFD in the subdivided counting areas demonstrates that Orientale Basin was formed by an oblique impact from the south-west, and the impact incidence was between 45° and 20°.

The minimum confidence limit for diameters in crater counts

Size-frequency distribution (i.e., SFD) of impact craters are frequently used to study relative and absolute model ages for planetary surfaces. It is well accepted that crater statistics are incomplete and crater recognition is inaccurate at diameter ranges approaching a few pixels of the images used, but there is no benchmark about the minimum confidence limit for diameter (Dmin). Here we resolve this problem by comparing the SFD of each studied crater population using a same image dataset that has been down-sampled by different folds. A systematical study of several different-aged lunar crater populations using various image datasets found that crater statistics can be regarded as completed and crater rim identification can be regarded as accurate at diameters larger than about 10 pixels of the base images. Integrating other potential difficulties in crater counts (e.g., illumination conditions, personal biases, topography roughness, size of counting areas) would indicate that 10 pixels should be considered as a conservative suggestion. Using 10 pixels as Dmin could substantially enhance the reliability of crater counts, especially for prevalent high-resolution regional geological studies that employ populations of small craters.

The spatial signature of a changing ancient impactor population for Mars

Ancient solar system processes are recorded by impact crater populations on Mars. We investigated the spatial distribution of Noachian-aged (3.8–4.0 Ga) craters in order to test hypotheses for the shortage of <32 km diameter craters. Using (i) a global database of Mars impact craters and (ii) angular two-point correlation statistics to quantify local to regional crater clustering, we found that degraded craters on low-lying middle-Noachian-highland terrains were subject to spatially-patchy crater obliteration by surface processes, while older, higher-standing craters on the early Noachian highlands remain consistent with being drawn from a spatially uniform distribution. This result supports the hypothesis that the multi-sloped Noachian crater size-frequency distribution results from an early impactor population that changed during the Noachian, rather than from extensive obliteration of craters <32 km in diameter by surface processes such as fluvial erosion, volcanic flooding, and aeolian infilling. The cause of the change in impactor populations remains unknown.

Mars in ice ages for ∼25% of post-Noachian geologic history

Mars is currently a hyperarid, hypothermal desert planet, whose surface inventory of water is primarily confined to the north and south polar ice caps. Previous studies have shown that Mars has undergone massive shifts in its spin-axis obliquity (present-day is 25.2°) due to secular spin orbit resonances. During periods of higher obliquity, water-ice from the polar caps is mobilized to the mid-latitudes (∼35° obliquity) and even the equator (≥45° obliquity), where it is deposited as snow and accumulates over time to form thick regional surface ice sheets. Abundant evidence exists today for the remnants of these ice ages in the form of debris-covered glaciers and ice deposits, but due to the chaotic nature of orbital simulations beyond ∼20 Ma, it has remained unclear to what temporal extent Mars has experienced such ice ages. Recent developments have suggested that impact events which formed in martian surface ice deposits exhibit a distinctive double-layered ejecta morphology. In tandem with cratering statistics, this observation offers the potential to better our understanding of the history of ice ages on Mars. This work explores the timing of ice age events by evaluating the size-frequency distribution of craters forming in surface ice. Using Hartmann isochron model ages, this work shows that Mars has experienced mid-latitude ice ages for up to a cumulative ∼680 Myr out of the past 3.6 Ga, and experienced equatorial ice ages for up to a cumulative ∼250 Myr within the same time period. The results of this study indicate that Mars has experienced mid-latitude/equatorial ice age states for up to approximately 25% of its post-Noachian geologic history, emphasizing that much of the geologic history of Mars is dominated by the presence of widespread non-polar surface ice sheets.

Age determination for valley networks on Mars using tectonic-fluvial interaction

Two areas on Mars in the old Thaumasia region were analysed, where interaction between fluvial and tectonic landforms provided conditions to improve age estimation for valley network formation. An Earth analog terrain was also involved in this work at Villány Hills (Hungary) for reference and to see the relations in tectonic-fluvial interaction. On Mars such interaction was observed by morphology (parallel curvatures of neighbour valleys) and by the comparison of statistical analyses of the fluvial and tectonic lineaments’ azimuthal orientation. While in the case of Mars the fluvial network elements’ distribution closely follows that of the tectonic elements’ distribution, in the case of the Earth the connection is weaker. The comparison gives an interesting example for tectonic landforms controlling fluvial valley orientations, occasionally producing resembling features in valley tracks, influencing the sediment transport and the locations of deposits.Because the tectonic faults crossed a much larger area than the fluvial valleys they influenced, crater size frequency distribution based ages could be better measured with them. Using this approach, we found the minimal age of the fluvial network formation to be around 3.7 Ga (or up to 3.9 Ga). This result demonstrates that the small, scattered, and poorly integrated valleys might also have formed at the same period as the larger, branching systems with well determined ages: around the Noachian/Hesperian transition. Thus valley network formation even in the case of small, non-integrated systems might not necessarily extend into the early or middle Noachian ages and might have happened at a potential climatic optimum around 3.7 Ga ago. However the most difficult task remains the exact dating of old and small valleys at other locations on Mars, to confirm this possibility.

Investigation of target property effects on crater populations in long lava flows: A study in the Cerberus region, Mars, with implications for magma source identification

Young volcanism on Mars is exemplified within the Cerberus region, which enables examination of relatively pristine lava flow surfaces. In this investigation, we derived model ages within the circum-Cerberus channelized lavas for sites proximal, medial, and distal to the lava flow sources, finding that these ages decrease with increasing distance from the inferred source. Investigating multiple possible explanations for this downstream decrease in model ages, we conclude that the most likely cause is changes in the rheological properties of the lava during emplacement. Analogous terrestrial lava flows exhibit rheological changes along their length during emplacement, supporting the hypothesis that target properties affect crater size frequency distributions (CSFDs) on extraterrestrial bodies. Using scaling methods, we investigated how possible material property changes in the Cerberus channelized lavas affected CSFDs. To derive a single model age for each channel, we arithmetically size-scaled the distal and proximal crater sizes. We also used pi-group scaling to estimate the effect of material strength and porosity on the final CSFDs. We infer that the Athabasca, Grjótá, and Marte Valles lavas underwent minor, moderate, and major rheological changes, respectively, to account for the progressively larger age discrepancies along their respective flow lengths. The arithmetic size-scaling yields relative lava emplacement ages, from oldest to youngest, respectively, as Grjótá, Marte, and Athabasca. This lack of a consistent progressive directional emplacement (east-to-west or west-to-east) of lava model ages implies that the magma source was spatially distributed beneath the Cerberus region, although magma contributions from the two bordering volcanic provinces cannot be ruled out. The results of this study support the findings of previous lunar and martian studies that inferred target property changes affected CSFDs, which may have some bearing on age dating of young lava surfaces across the inner Solar System.

Geological mapping and chronology of lunar landing sites: Apollo 11

Crater size-frequency distribution (CSFDs) measurements allow the derivation of absolute model ages (AMAs) for geological units across various terrestrial bodies in the Solar System based on body-specific adjustments to the lunar chronology (e.g., Hartmann, 1970; Neukum et al., 1975, 1983, 2001; Stöffler and Ryder, 2001; Stöffler et al., 2006; Hiesinger et al., 2012; Robbins, 2014). Thus, it is important to revisit and test the accuracy of the lunar chronology using data from recent lunar missions (e.g., Hiesinger et al., 2000, 2012, 2015; Rajmon and Spudis, 2004; Stöffler et al., 2006), as well as newer analyses of lunar samples (e.g., Gaffney et al., 2011, Meyer, 2012; Snape et al., 2016; Welsh et al., 2018). We generated a new detailed geological map of the Apollo 11 landing region based on spectral characteristics, topography, and albedo maps, which shows several mare units adjacent to the lunar module. Lunar Reconnaissance Orbiter Camera (LROC) images were used to measure new CSFDs and derive the cumulative number of craters with diameters ≥1 km or N(1) for the Apollo 11 landing site. The newly derived N(1) values are consistent with the presence of only one surficial unit at the landing site: the Group A, High-K (high potassium) “young” mare basalt as described in Meyer (2012). We reviewed the radiometric ages for Apollo 11 samples that have been determined since the calibration of the lunar cratering chronology, used our new geological map to reinterpret their provenance, and correlated them with the new N(1) values. These are plotted and compared with the lunar chronology of Neukum et al. (1983). Our calibration point for the Apollo 11 landing site is consistent with the earlier values, thus, confirming Neukum's 1983 and Neukum and Ivanov's 2001 lunar chronology curves.

Geology of the northeastern flank of Apollinaris Mons, Mars: Constraints on the erosional history from morphology, topography, and crater populations

We have utilized THEMIS, MOLA, CTX and HiRISE data sets to investigate the morphologic and topographic characteristics of the northeast flank of Apollinaris Mons, a region with evidence for significant erosion of volcanic deposits. Using ArcGIS software, we mapped the geology of the northeast flank and obtained age estimates using crater size-frequency distributions from crater counts. Of the eight mapped units, three (Apollinaris Mons upper, mid-, and lower flank units) represent volcanic flank materials in various states of preservation. The Late Noachian-Early Hesperian upper flank unit is massive and contains joint-like vertical structures along the scarp faces of two large plateaus whose upper surfaces likely represent the original surface or near-surface deposits of Apollinaris Mons. Downslope-facing plateau scarps have up to 500 m of relief. Positioned below this unit are the mid-flank and lower flank units, each representing eroded surfaces within Apollinaris Mons flank materials that have stabilization ages between the Late Hesperian to Early Amazonian. Mantling material in the region contains cavities and boxwork-like textures that resemble patterns in terrestrial eroded pyroclastic flow deposits. Dome-shaped mounds west of the mapping region have morphologic similarities to terrestrial fumarolic mounds or possibly inverted impact craters. Both the texture and features suggest widespread pyroclastic deposits adjacent to Apollinaris Mons. Using MOLA gridded topography, we estimate that ~308 km3 of materials have been eroded along the flanks of the volcano. Statistical and histogram data from the thickness values of eroded materials shows that up to ~300 m of material has been removed from the majority of flank surfaces. Assuming steady state erosion of flank surfaces, we estimate an area-normalized loss rate of ~0.859 nm/yr for northeast Apollinaris Mons. This erosion rate is within the long-term range for Mars (~0.01–10 nm/yr) as estimated from MER landing site geology by Golombek et al. [2006].

Hypervelocity Impacts and Exposed Lithospheric Mantle: A Way to Recognize Large Terrestrial Impact Basins?

On the Moon and Mars olivine of probable mantle origin is detected at rims of large Late Heavy Bombardment (LHB) age impact basins for which excavation depth estimates exceed crustal thickness estimates. But lunar Crisium size impact basins are not recognized on Earth nor expected in the Phanerozoic from conventional interpretations of crater size frequency distributions. In this study several large circular to elliptical basin structures on Earth, for which hypothesized impact excavation depth would greatly exceed crustal thickness, are examined for the presence of exposed lithospheric mantle, expressed as ophiolite, at the rims. Three Phanerozoic impact basins, modified by plate tectonics and tentatively correlated with “ophiolite obduction” plus global extinction events, are proposed here. These tentatively suggested Phanerozoic impact basins are: (1) Yucatan Basin/Puerto Rico Trench with a Greater Antilles ophiolite rim. Cretaceous-Paleogene Boundary global extinction may correlate with Maastrichtian ophiolite obduction in Southeast Cuba. (2) Loyalty Basin with a New Caledonia ophiolite plus d’Entrecasteaux Ridge rim. Late Eocene global extinction may correlate with obduction of the New Caledonia Peridotite Nappe. (3) Sulu Sea Basin with a Palawan, Sabah etc. ophiolite rim. The Middle Miocene Disruption Event may correlate with ophiolite obduction plus ophiolitic melange emplacement in Sabah and in Palawan. These originally circular to elliptical belts of exposed lithospheric mantle may serve as strain markers for relative plate motions in the vicinity of plate boundaries during post-impact geologic times. It is further speculated that plate boundaries may be initiated and/or modified by such impacts.

Impacts and Exposed Lithospheric Mantle: A Way to Recognize large Terrestrial Impact basins?

AbstractOrientale size craters are not recognized on Earth nor expected for Phanerozoic and Proterozoic eons from conventional crater size frequency distributions (Ivanov et al., 2002). Here suggested are three such Phanerozoic craters, modified by plate tectonics, and tentatively correlated with extinction and “ophiolite obduction” events. Hypothesis testing is proposed and plate tectonics implications are discussed. Such basins might manifest: circular to elliptical rims (or rim segments), with exposed lithospheric mantle, as strain markers for plate boundary motion; thick ejecta near rim expressed as “ophiolitic melange”; power law decay of ejecta thickness with radial distance from rim (McGetchin et al., 1973) and/or systematic azimuthal variation of ejecta thickness for low angle impacts (Schultz, 1999); weathering resistant shocked mantle minerals (Bohor et al., 1990) in ejecta;• global spherule layer with PGE anomalies (Alvarez et al., 1980); rim structures consistent with cratering mechanics (Melosh, 1989; Kenkmann, 2014); impact melt basement (Grieve et al., 1992; Pierazzo et al. 2000) recording uniform cooling age and Earth's magnetic polarity of the time. Tentatively suggested Phanerozoic impact basins: Yucatan Basin: Greater Antilles ophiolite rim – KPg Boundary? Maastrichtian ophiolite obduction in southeast Cuba (Iturralde‐Vinent et al., 2006). Sulu Sea Basin: Palawan, Sabah etc. ophiolite rim – Middle Miocene Disruption? MM ophiolitic mélange emplacement in Sabah (Clennell, 1991). Loyalty Basin: New Caledonia ophiolite and d'Entrecasteaux ridge rim – EO Boundary? EO ophiolite obduction in New Caledonia (Cluzel et al., 2012).

Morphological and Chronological Mapping of Manilius Crater Region Using Chandrayaan-1 Data Sets

Fine-resolution morphological mapping aided by ortho-images and digital elevation model from Chandrayaan-1 Terrain Mapping Camera and 3D GIS visualization has revealed scientifically diverse characteristics of lunar surface features, due to unique topographical significance of morphological features, i.e., highlands, basaltic plains and craters, which are very well manifested in 3D GIS environment. The distribution of various morphological features provides insights into the sequential evolution and surface process of the study area. The highland region represented by the Fra Mauro formation in the study area exhibits high albedo with distinct topography. The northern part of the study area falls in the southern part of major basin Serenitatis, and exhibits the dark mantling material with low albedo. The morphological features, i.e., wrinkle ridges and rilles, indicate volcanic flow events consequence to the loading of basaltic materials in the interior of the Serenitatis and Imbrium Basins and related extensional failure. The Manilius crater, which occupies the central part of the study area, is a complex crater with a central peak and asymmetric ejecta deposit. The ages of the major surficial features were determined based on size, frequency and distribution pattern of craters using crater size-frequency distribution model. Age of the Fra Mauro highland, Manilius crater, Mare Serenitatis and Mare Vaporum is, respectively, 3.9, 3.5, 2.8 and 1.7 Ga years, indicating that the lunar surface of this region evolved in Imbrian to Eratosthenian age of lunar selenological timescale.

Possible (closed system) pingo and ice-wedge/thermokarst complexes at the mid latitudes of Utopia Planitia, Mars

On Earth, hydrostatic or closed-system pingos (CSPs) are perennial ice-cored mounds formed by the freeze-thaw cycling of water when or as thermokarst lakes lose their water by drainage or evaporation. The mounds vary in shape from circular or sub-circular to elongate, are sub-kilometre in their long axes, and may reach decametres in height. Often, the mounds show summit depressions, radial fractures and single tiers or benches. As the CSPs degrade, small-scale debris flows or slumps occur; end-stage degradation typically exhibits debris-laden ramparts elevated metres above the surrounding terrain.Typically, the CSPs originate and develop where deep and continuous permafrost is ice-rich, i.e. infused with excess ice, as it is in the Tuktoyaktuk Coastlands of northern Canada, mid to northern Alaska and northeast Russia. In these areas or regions the terrain often is populated by: a) thermokarst lakes and alases (thermokarst lake basins absent of water); b) fields of small-scale ice-wedge polygons (either low-centred or high-centred relative to the polygon margins); and, c) melt-water filled or emptied polygon-margin troughs or -corner pits. Individual troughs and pits connected by means of the local topographical gradient comprises beaded stream systems or networks.Here, we report, describe and discuss a suite of candidate CSPs at the mid-latitudes of Utopia Planitia (UP) (~35-50° N; ~80-115° E). The morphologies, scale and height of the Martian mounds, as well as the surface textures and characteristics, landscape features and spatially-associated landforms, are consistent with the CSPs observed in ice-rich regions on Earth.The UP mounds are nested in or reside adjacent to: flat-floored, possibly thermokarstic depressions; small-scale polygons (some of which exhibit low or high centres relative to their margins); and, connected polygon-margin depressions or pits that mirror beaded-stream systems in ice-rich terrain on Earth. The size-frequency distribution of craters in our study region precludes a surface younger than ~100 Ma. Thus, the CSP-like mounds in UP and the pre-mound periglacial-revisions of the landscape presented by us may be markers of freeze-thaw cycling that occurred much earlier in the Amazonian Epoch than most of the current discussions about periglaciation in UP suggest.

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.