Small Craters Research Articles

The conservation status of the Galapagos MartinProgne modesta: Assessment of historical records and results of recent surveys

SummaryThe endemic Galapagos MartinProgne modestais a rare species. Population sizes in the past are unknown, but the few reports available suggest that it has become rarer in some sites. To obtain a better understanding of its population status, a simultaneous survey was conducted in 2017 around the coastline of 14 islands and 23 islets resulting in sightings of only 26 individuals. A simultaneous survey on Sierra Negra volcano on Isabela Island added five more individuals. However, observations from early expeditions and expeditions since 2015 have shown that the volcanoes of Isabela Island are important feeding areas and probable breeding areas for the Galapagos Martin, but these were largely excluded from the survey in 2017 due to logistical reasons. Historical and recent nesting sites include Tagus Cove on Isabela Island and Daphne Major Island. During our 2017 count, three possible new nesting sites were discovered: Daphne Minor, Santiago and a small crater on Punta Cristóbal on southwestern Isabela Island. From 2015 to 2018, 15 nests were also observed on the cliff in Tagus Cove. Two accessible nests were collected and revealed the first records ofPhilornis downsiin Galapagos Martin nests. The impact of parasitism byP. downsiis unknown but potentially severe. Given the lack of general knowledge about the biology of this species and its apparent low population sizes, further studies are urgently needed.

40Ar/39Ar systematics of melt lithologies and target rocks from the Gow Lake impact structure, Canada

The age of the Gow Lake impact structure (Saskatchewan, Canada) is poorly constrained, with previous estimates ranging from 100 to 250 Ma. Using a combination of step-heating and UV laser in situ40Ar/39Ar analyses we have sought to understand the 40Ar/39Ar systematics of this small impact crater and obtain a more precise and accurate age. This structure is challenging for 40Ar/39Ar geochronology due to its small size (∼5 km diameter), the silicic composition of the target rock, and the large difference in age between the impact event and the target rock (∼1.2 Ga). These factors can serve to inhibit argon mobility in impact melts, leading to retention of ‘extraneous’ 40Ar and anomalously older measured ages. We mitigated the undesirable effects of extraneous 40Ar retention by analysing small volume aliquots of impact glass using step-heating and even smaller volumes via the UV laser in situ40Ar/39Ar technique. Although primary hydration of impact-generated glasses enhanced the diffusivity of 40Ar inherited from silica-rich melts, data still had to be corrected for extraneous 40Ar by using isotope correlation plots to define the initial trapped 40Ar/36Ar components. Our inverse isochron age of 196.8 ± 9.6/9.9 Ma (2σ, analytical/external precision) demonstrates that the Gow Lake event occurred within uncertainty of the Triassic-Jurassic boundary, but there is no evidence that it was part of an impact cluster.

An endogenic origin of cerean organics

Localized deposits of aliphatic organic-rich material have been found on the surface of dwarf planet Ceres, and based on mineralogical context, it has been suggested that these organics formed within Ceres (De Sanctis et al., 2017). However, no obvious source and exposure mechanism can be inferred from the geological settings of the organic-rich terrains. The alternative hypothesis is that the organics were delivered from an exogenic source via impact (Pieters et al., 2018). We numerically simulate the thermal degradation of aliphatic organics delivered to Ceres during hypervelocity impacts, as well as the thermal degradation and excavation of organics from the subsurface. Our models show that exogenic delivery of aliphatic organics is inefficient, as most of the spectral signature of the organic species would be thermally degraded and diluted by mixing with target material in the ejecta blanket of a given crater. Instead, our models suggest that the formation of Ernutet crater excavated a subsurface reservoir of aliphatic organics from a depth of 3-6 km, bringing it close to the surface, where it could be subsequently exhumed by small craters and landslides, compatible with observations.

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.

Investigations of surface defects during laser polishing of tool steel

During laser polishing of tool steel surfaces produced by electric discharge machining with NIR continuous wave laser, small craters were observed on the polished surface. Their formation was observed in situ with high-speed camera. Post mortem investigation of the composition of the craters with energy dispersive X-ray spectroscopy (SEM) has shown that they were caused by copper inclusions on the steel surface. The inclusions are probably coming from the electrode employed during the electric discharge machining process. The low absorption of copper in NIR signifies that the bigger copper particles do not melt at high speed which leads to the formation of the craters.

The subsurface structure and stratigraphy of the Chang’E-4 landing site: orbital evidence from small craters on the Von Kármán crater floor

Chang’E-4 (CE-4) successfully landed on the floor of the Von Kármán crater within the South Pole-Aitken basin (SPA). One of its scientific objectives is to determine the subsurface structure and the thickness of lunar regolith at the landing site and along the traverse route of the Yutu-2 rover. Using orbital data, we employed small craters (diameters <1 km) on the floor of the Von Kármán crater as probes to investigate the subsurface structure and stratigraphy of the CE-4 landing site. In this study, 40 dark-haloed craters that penetrate through the surface Finsen ejecta and excavate underlying mare deposits were identified, and 77 bright ray craters that expose only the underlying fresh materials but do not penetrate through the surface Finsen ejecta were found. The excavation depths of these craters and their distances from the Finsen crater center were calculated, and the thickness distribution of Finsen ejecta on the Von Kármán floor was systematically investigated. The boundary between Finsen ejecta and underlying mare basalt at the CE-4 landing site is constrained to a depth of 18m.We have proposed the stratigraphy for the CE-4 site and interpreted the origins of different layers and the geological history of the Von Kármán crater. These results provide valuable geological background for interpreting data from the Lunar Penetrating Radar (LPR) and Visible and Near-infrared Imaging Spectrometer (VNIS) on the Yutu-2 rover. The CE-4 landing site could provide a reference point for crater ejecta distribution and mixing with local materials, to test and improve ejecta thickness models according to the in situ measurements of the CE-4 LPR.

Evolución espacio-temporal de las bocas eruptivas de los volcanes Irazú, Arenal, Turrialba y Poás en tiempo histórico (Costa Rica)

Historical research was performed analyzing the morphologic and temporary evolution of the craters and small eruptive vents (lateral craters, intracraters, some as pit craters and small craters or “mouths”) of the volcanoes Irazu, Arenal, Turrialba and Poas. The establishment of the small intracrateric eruptive mouths was preceded by fumarolic activity in the summit, where location seems to be controlled by the same tectonic guidelines that predominate at the volcanic edifice or massif: NW-SE, E-W and N-S (Irazu), E-W (Arenal), NE-SE (Turrialba) and N-S (Poas). The initial formation of the intracraters at Irazu and its coalescing was ruled dominated by phreatic activity with phreatomagmatic phases (1917-1921, 1924, 1928-1930, 1933, 1939-1940), which culminated with eruptive activity from 1962 to 1965, in presence of abundant juvenile pyroclasts (strombolian and vulcanian activity), to finally establishing a main crater that encompasses the smaller craters. On Arenal’s western flank, three new craters formed in 1968 in the E-W fissure. Between Irazu’s eruptive period in 1723 and its reactivation in 1917, 194 years passed, similar to Turrialba’s 144 years of relevant inactivity between 1866 and 2010. Turrialba showed the formation of minor vents between 2010 and 2013, to culminate with the enlargement and deepening of the active crater (2014-2019) with various minor active intracraters. The 2017 activity at Poas volcano was also preceded by phreatic activity with the formation of mud volcanoes, which ended with the volcanic dome’s destruction by phreatomagmatic explosions. The birth of a new spatter cone at the same place of the former dome, as well as various sulfur cones and mud volcanoes oriented approximately N-S. In this paper, certain patterns in the succession of craters in the cases mentioned and their relationship with the tectonic trends may have short-term implications for the volcanic hazard once the preferential sectors of evolution and formation are known, as well as the evolution from small crater to larger ones, related to more vigorous eruptive periods.

Crater morphology and evolution at the Phoenix landing site: Insights into timing of modern geological processes and subsurface ice

We report results of an in-depth study of the morphology of small (km-scale and smaller) craters over a 4000 km2 region on Mars that includes the Phoenix landing ellipse, focused on the nature and timing of geological processes that have been active in the region. The study seeks to constrain regolith depositional rates, thickness of ground ice layer, abundance of ice in the subsurface, and the rate of crater degradation. Measurements of the size of ejecta blocks around craters show that the region has not been a net-depositional site over the last billion years. Most craters smaller than 200–350 m lack ejecta blocks regardless of age, indicating that there is a layer approximately 20–35 m deep that either does not form or does not retain ejecta blocks. This layer is expressed both within and outside the extended ejecta blanket of Heimdal crater, implying that it is not associated with the ejecta blanket. We propose that this unconsolidated layer consists dominantly of ice-cemented soil, which raises questions about the emplacement of ice at depth. We also note clear progression in crater degradation morphology, which allows us to place constraints on the rate and style of crater degradation. Small craters (20 to 200 m in diameter) appear to lose their relief due to slow infilling of the bowl over time scales of 5–100 kyr. The infilling of small craters, perhaps by a combination of ice and dust, presents a mechanism by which ice is buried down to a few tens of meters. On the other hand, larger craters (~200 m to 1 km) do not exhibit evidence for infilling, but do exhibit concentric sets of fractures within and outside the rim. We propose that loss of relief of the larger craters is due to viscous creep over time scales of a few to tens of Myr. This latter interpretation would require the presence of water ice in abundances greater than pore-filling down to depths of at least tens of meters.

Boulder Distributions Around Young, Small Lunar Impact Craters and Implications for Regolith Production Rates and Landing Site Safety

AbstractWe use Lunar Reconnaissance Orbiter Camera Narrow Angle Camera images to characterize boulder populations around six small (&lt;1 km), young (&lt;200 Ma) impact craters near spacecraft landing sites. The Narrow Angle Camera boulder counts are used to analyze how boulder distributions vary around craters of different sizes and ages. These comparisons inform how various properties affect the distance to which boulders are ejected and the size and density of boulders produced by an impact event. The counts show that boulder population densities decrease with crater age, with few boulders remaining at craters older than a few hundred million years, consistent with results of other studies of boulder degradation rates on the Moon. Variations in boulder distributions around younger craters may provide information regarding impact conditions; South Ray crater has a larger population of small boulders than the larger North Ray crater, which could be explained by variations in impact velocity. Large craters generally excavate more boulders than smaller craters, and the size of the largest boulder ejected is related to crater size by a power‐law function. Larger boulders occur closer to the crater rim (within 2–4 crater radii), whereas smaller boulders occur at all distances. The density of boulders is greater near the crater rim and decreases with increasing radial distance; this data can aid in establishing safe landing zones for future missions. Analyzing boulder distributions across craters of varying ages allows us to test models of boulder breakdown rates, with implications for understanding the Moon's regolith production rate.

Crust stratigraphy and heterogeneities of the first kilometers at the dichotomy boundary in western Elysium Planitia and implications for InSight lander

InSight landed on Mars on November 26, 2018, in western Elysium Planitia. The Mars crust beneath the lander is subject to complex geologic history next to the great topographic and crustal dichotomy of Mars. Understanding this part of the Martian crust in the subsurface would aid future investigations of the internal structure of the planet based on seismic datasets collected by the Seismic Experiment for Interior Structure (SEIS) instrument. Here, we investigate the subsurface structure and composition from the analysis of mineralogy and morphology of exposures in impact craters as well as on scarps and knobs in the general region of Elysium Planitia. Using a combination of orbital datasets, we identify exposures of subsurface materials with distinct composition and physical properties. We find olivine and pyroxene detections associated with small impact craters (1.5–7 km) rim in the vicinity of InSight lander, as well as in the transition unit and Elysium volcanic unit. Fe/Mg phyllosilicates have been identified in the central peak of the 51-km diameter Kalpin crater and on knobs in the transition unit between the dichotomy and the plains. In addition, eroded meter-scale layered unit subject to erosion has been identified in six impact craters to the north-east of the landing site, including Kalpin crater. Massive bedrock and layered, weak materials co-occur in the transition unit, indicating a complex origin. These results together suggest both materials with altered composition and layered deposit occur as distinct geologic units beneath the basaltic lava flow units. Through analogy with terrestrial sedimentary rocks or clay-bearing sediments, we suggest physically weak materials exist beneath the lava flow units in the general region of Elysium Planitia. The spatial distribution and continuity of these materials are unclear due to the lack of exposures within the lava flow unit where the InSight lander is located. These subsurface materials of distinct physical properties may result in increased attenuation or reverberations of seismic waves, to be collected by SEIS. The findings suggest a close investigation of the potential effects of subsurface stratigraphy on seismic data would help inform future data interpretation and understanding of the internal structure of Mars.

The Tsiolkovskiy crater landslide, the moon: An LROC view

Evidence suggests that the lobate flow feature that extends ~72 km outward from the western rim of Tsiolkovskiy crater is a long runout landslide. This landslide exhibits three (possibly four) morphologically different parts, likely caused by local conditions. All of these, plus the ejecta of Tsiolkovskiy crater, and its mare fill are approximately of the same crater model age, i.e., ~3.55 ± 0.1 Ga. The enormous size of this landslide is unique on the Moon and is a result of a combination of several geometric factors (e.g., its location relative to Fermi crater), and that Tsiolkovskiy crater was an oblique impact that produced an ejecta forbidden zone on its western side (Schultz, 1976). The landslide formed in this ejecta free zone as the rim of Tsiolkovskiy collapsed and its debris flowed across the relatively smooth, flat floor of Fermi crater. In this location, it could be easily identified as a landslide and not ejecta. Its mobility and coefficient of friction are similar to landslides in Valles Marineris on Mars, but less than wet or even dry terrestrial natural flows. This suggests that the Mars landslides may have been emplaced dry. The high density of small craters on the landslide is likely an illusion caused by the effects of age related differences in regolith thickness on crater morphology, and the presence of the abundant young, circular secondary craters produce by debris ejected from distant fresh craters.

Time‐Dependent Production Functions of Lunar Simple Craters on Layered Targets With Consideration of Topographic Degradation

AbstractImpact bombardment changes target properties, and topographic degradation increases crater diameters. These effects potentially change the shape of lunar production function (PF), which is usually assumed to be constant with time. Using crater scaling laws, we model PFs (MPFs) formed by near‐Earth objects on two‐layer target models that have increasing regolith thickness with time. Model‐derived MPFs show a significantly elevated production rate for small lunar craters, which is consistent with a high regolith production rate during the last ~1 Ma. Further integrating the effect of topographic degradation, we model the time‐dependent observable PFs (MOPFs) from the MPFs. Comparing among MOPFs, MPFs, and the prevailing Neukum PF suggests that, for D = 10–1,000 m simple craters, applying MPFs and Neukum PF may cause the derived cumulative density of D &gt; 1 km craters (i.e., N(1)) to differ from its real value by a factor up to ~3 and ~10, respectively.

Abstracts from the 7th Congress of the World Federation for Laser in Dentistry European Division June 20–22, 2019 Parma, Italy

Abstracts from the 7th Congress of the World Federation for Laser in Dentistry European Division June 20–22, 2019 Parma, Italy

Analyzing the ages of south polar craters on the Moon: Implications for the sources and evolution of surface water ice.

Surface water ice in the permanently shadowed polar regions of the Moon has a patchy surficial distribution and is not found within all available cold-trapping areas. To date it is not well understood when the ice was delivered, which has important implications for the surficial characteristics of the ice as well as for possible delivery mechanisms. Here we present absolute model ages for 20 south polar craters that host surface water ice, providing maximum estimates of the ages of surface ice contained within these craters. We quantify the amount of available cold-trapping surface area that is occupied by water ice in order to examine the relationship between the patchiness of ice within each crater and the age of each host crater. The majority of surface ice is contained in old craters ≥∼3.1 Gyr, where the majority of cold-trapping area on the pole exists. The ice is these ancient craters is very patchy in surficial distribution, occupying <11.5% of cold-trapping surface area available in individual craters. This patchy distribution of ice in old craters is likely to be due to impact bombardment and regolith overturn within the polar regions. Interestingly, surface ice is also located within smaller craters (<15 km in diameter), whose sharp crater rim crest morphologies suggest that they may be relatively young. Ice in fresh-looking craters suggests that ice has been delivered to the lunar surface more recently, perhaps from micrometeorites or through solar wind interactions with the lunar regolith. Finally, we also analyze a group of ancient craters that does not host surface water ice, even though these craters are present-day cold traps. These specific ancient craters would not have been thermally stable for the cold-trapping of water ice before the onset of true polar wander suggested by Siegler et al. (2016). If true polar wander did occur on the Moon, then the ages of ice-bearing craters presented here set an upper limit for the age of post-true polar wander hydrogen emplacement of 4.1 ± 0.1 Gyr.

Impact cratering rate consistency test from ages of layered ejecta on Mars

Ages of geological units of planetary bodies are determined from impact crater counts on their surface. These ages are model-dependent, and several models largely used in the community assume a constant production function and a constant cratering rate over the last 3 Ga. We have mapped the population of small impact craters (>200 m in diameter) formed over a population of large impact craters (>5 km in diameter) with layered ejecta on Acidalia Planitia, Mars. We have deduced the age of each large impact crater under the assumption of a constant impact rate and constant production function. The impact rate inferred from this set of ages is, however, not constant and show a significant increasing during the last ~1 Ga compared to chronology models commonly used. We interpret this inconsistency as an evidence for temporal variations in the size-frequency distribution (SFD) of impactors in the main belt, consistent with recent studies argued for a late increasing of the large impactor flux on Earth and the Moon.

Composition, mineralogy and chronology of mare basalts and non-mare materials in Von Kármán crater: Landing site of the Chang’E−4 mission

As the first lunar farside landed mission, China’s Chang’E−4 (CE-4) has safely landed on the volcanically resurfaced floor of Von Kármán crater in South Pole-Aitken (SPA) basin. The CE-4 mission will conduct rover exploration by Yutu-2 similar to what Chang’E−3 did in the northern Imbrium basin. Here we present a comprehensive study of the composition, mineralogy, and chronology of the basaltic and nonmare units in this crater, with the intent to provide context for the compositional properties of the landing site. Compositional maps (FeO, TiO2, Mg#, and Th) demonstrate the heterogeneous distribution of mare and nonmare materials. Using a spectral survey of 1899 small impact craters (<1 km in diameter), we present a mineral map of high-Ca to low-Ca pyroxene ratio that is useful to untangle the effects of spectral mixing of these materials. Mare basalts with low-Ti composition and model ages of ~3.7 Ga and ~3.6 Ga have distinct mineral compositions that differ from the Chang’E−3 landing site, including no olivine enrichment. The non-mare materials have Mg-rich pyroxene, and Finsen crater materials (and Finsen ejecta) have higher Ca/Fe ratios than other parts of the rim of Von Kármán crater, indicating compositional anomaly of the SPA basin. The Von Kármán mare basalts show obvious compositional heterogeneity and complicated mixing effects with non-mare ejecta from subsequent impacts (e.g., the Eratoshenian-aged Finsen impact). We also present a geological map of the landing site, which reveals the geologic history of the landing region and will help to guide the surface exploration route of the CE-4’s Yutu-2 rover. We also put forward some science questions to be answered by Yutu-2, to aid mission planning and scientific research.

Potential Lunar Base on Mons Malapert: Topographic, Geologic and Trafficability Considerations

Polar areas of the Moon are prospective sites for construction of a lunar base due to the near constant illumination conditions and the potential presence of water ice in the regolith of cold traps. The mountain Mons Malapert (MM) near the South pole of the Moon is a key candidate for the location of such a base. MM is an ~30 × 50 km mountain elongated in a WNW-ESE direction with a NNE extension. Its summit stands ~5 km above the 1838 km datum, has constant visibility from Earth and long periods of sunlight (87 to 91% of the year). In this analysis we consider the topographic, geologic and trafficability characteristics of Mons Malapert, which need to be taken into account in the further consideration of MM as a lunar base location. The topography and its derivatives were studied using LROC WAC images and the LOLA-based DTM. South of MM lie the ~50 km craters Haworth and Shoemaker whose floors are in permanent shadow and show a neutron spectrometric signature of high water-ice content that may be a source of water for the base. The geology of the MM region is defined by its position on the rim of the South-Pole-Aitken basin, the largest and most ancient impact basin on the Moon. The ancient age of this area is confirmed by crater spatial density which shows ages of ~4.2 Ga. The MM slopes are mostly rather steep: from ~20 to 30°, while slopes on its summit and base are more gentle. LROC NAC images of this area show that while the summit and base of MM are covered by numerous small craters, its steep slopes show a deficit of craters and are complicated by low ridges appoximately perpendicular to the downslope direction. These characteristics of the steep slopes suggest effective downslope movement of the regolith material that, in turn, suggests that the mechanical properties of the surface layer here are relatively weak. The siting, building and operation of a lunar base implies activity not only in-base and close proximity, but also traversing to other distant sites of interest for resources and scientific investigations. So planning the Mons Malapert base requires the detailed analysis of the trafficability of the region. To consider this issue we return to experience gained by the operations of Soviet Lunokhod 1, 2 and the US Apollo Lunar Roving Vehicles. On the basis of new and evolving technology, rovers designed for the MM lunar base may significantly differ from earlier rovers, but consideration of trafficability of the earlier rovers is important for future planning. Our analysis shows that neither Lunokhods nor the Apollo LRV could successfully climb most of the slopes of Mons Malapert. The acceptable trafficability appears to be only possible along the WNW crest of the mountain. For emergency cases wheel-walking rovers may be considered. Mons Malapert seems to be a good locality for the lunar base but more studies are needed.

The environmental effects of very large bolide impacts on early Mars explored with a hierarchy of numerical models

The origin of the presence of geological and mineralogical evidence that liquid water flowed on the surface of early Mars is now a 50-year-old mystery. It has been proposed (Segura et al., 2002, 2008, 2012) that bolide impacts could have triggered a long-term climate change, producing precipitation and runoff that may have altered the surface of Mars in a way that could explain (at least part of) this evidence. Here we use a hierarchy of numerical models (a 3-D Global Climate Model, a 1-D radiative-convective model and a 2-D Mantle Dynamics model) to test that hypothesis and more generally explore the environmental effects of very large bolide impacts (Dimpactor > 100 km, or Dcrater > 600 km) on the atmosphere, surface and interior of early Mars.Using a combination of 1-D and 3-D climate simulations, we show that the environmental effects of the largest impact events recorded on Mars are characterized by: (i) a short impact-induced warm period (several tens of Earth years for the surface and atmosphere to be back to ambient conditions after very large impact events); (ii) a low amount of hydrological cycling of water (because the evaporation of precipitation that reached the ground is extremely limited). The total cumulative amount of precipitation (rainfall) can be reasonably well approximated by the initial post-impact atmospheric reservoir of water vapour (coming from the impactor, the impacted terrain and from the sublimation of permanent ice reservoirs heated by the hot ejecta layer); (iii) deluge-style precipitation (∼2.6 m Global Equivalent Layer of surface precipitation per Earth year for our reference simulation, quantitatively in agreement with previous 1-D cloud free climate calculations of Segura et al., 2002), and (iv) precipitation patterns that are uncorrelated with the observed regions of valley networks.However, we show that the impact-induced stable runaway greenhouse state predicted by Segura et al. (2012) should not be achievable if convection and water vapour condensation processes are considered. We nevertheless confirm the results of Segura et al. (2008) and Urata and Toon (2013) that water ice clouds could in theory significantly extend the duration of the post-impact warm period, and even for cloud coverage significantly lower than predicted in Ramirez and Kasting (2017). However, the range of cloud microphysical properties for which this scenario works is very narrow.Using 2-D Mantle Dynamics simulations we find that large bolide impacts can produce a strong thermal anomaly in the mantle of Mars that can survive and propagate for tens of millions of years. This thermal anomaly could raise the near-surface internal heat flux up to several hundreds of mW/m2 (i.e. up to ∼10 times the ambient flux) for several millions years at the edges of the impact crater. However, such internal heat flux is largely insufficient to keep the martian surface above the melting point of water.In addition to the poor temporal correlation between the formation of the largest basins and valley networks (Fassett and Head, 2011), these arguments indicate that the largest impact events are unlikely to be the direct cause of formation of the Noachian valley networks. Our numerical results support instead the prediction of Palumbo and Head (2018) that very large impact-induced rainfall could have caused degradation of large craters, erased small craters, and formed smooth plains, potentially erasing much of the previously visible morphological surface history. Such hot rainfalls may have also led to the formation of aqueous alteration products on Noachian-aged terrains, which is consistent with the timing of formation of clays.

Vision‐based crater and rock detection using a cascade decision forest

Both crater and rock detection are components of the autonomous landing and hazard avoidance technology (ALHAT) sensor suite, as craters and rocks represent the majority of landing hazards. Furthermore, places with scientific values are very probable next to craters and rocks. Unsupervised approaches, which potentially use the pattern recognition techniques of ring threshold finding, perform quickly; however, they suffer from handling small craters. The supervised pattern recognition method is more powerful but is time‐consuming. To address these issues, here, a simultaneous multi‐size crater and rock detection algorithm is studied. The authors propose a new supervised machine‐learning framework using a cascade decision forest. Sliding windows are utilised in order to search basic features, and a multi‐grained cascade structure is introduced to enhance the framework's ability to learn the representations of the features. The training time of the proposed algorithm on a PC is comparable to that of deep neural networks, and the efficiency is enhanced for a large‐scale database. The outputs of the simulation verify the effectiveness and validity of the introduced technique.

Global mapping of lunar refractory elements: multivariate regression vs. machine learning

Context.The quantitative estimation of elemental concentrations at the spatial resolution of hyperspectral near-infrared (NIR) images of the lunar surface is an important tool for understanding the processes relevant for the origin and evolution of the Moon.Aims.We aim to map the abundances of the elements Fe, Ca, and Mg at a typical accuracy of about 1 wt.% at the spatial resolution of the Moon Mineralogy Mapper (M3) instrument on-board Chandrayaan-1 lunar mission.Methods.The NIR reflectance of the lunar regolith is an integrated response to the presence of refractory elements and soil alteration processes. Our approach was to define a combination of spectral parameters that are robust with respect to the effects of soil maturity. We calibrated the spectral parameters with respect to elemental abundances measured by the Lunar Prospector Gamma Ray Spectrometer (LP GRS) and the Kaguya GRS (KGRS). For this purpose, we compared a classical multivariate linear regression (MLR) approach and the machine learning based support vector regression (SVR) technique applied to M3global observations.Results.The M3-based global elemental maps are consistent in distribution and range with the LP GRS and KGRS elemental maps and do not show artifacts in immature areas such as small fresh craters. The results derived using MLR and SVR are compared to sample-based ground truth data of the Apollo and Luna sample-return sites, where the root-mean-square deviations obtained by the two regression models are similar.Conclusions.The main advantage of the proposed new algorithm is its ability to minimize artifacts due to space-weathering effects. The elemental maps of Mg and Ca provide additional information and reveal structures not always visible in the Fe map. The global elemental abundance maps derived for the fully calibrated M3observations might thus serve as important tools to investigate the lunar geology and evolution.