Small Craters Research Articles

Cratering and age of the small Saturnian satellites

Context. The small (≤135 km mean radius) satellites of Saturn are closely related to its rings and together they constitute a complex dynamical system where formation and destruction mechanisms compete against each other. The Cassini-Huygens mission provided high-resolution images of the surfaces of these satellites and therefore allowed for the calculation of observational crater counts. Aims. We model the cratering process by Centaur objects on the small Saturnian satellites, and compare our results with the observational crater counts obtained from the Voyager and Cassini missions. Methods. Using a theoretical model previously developed we calculate the crater production on these satellites considering two slopes of the size-frequency distribution (SFD) for the smaller objects of the Centaur population and compare our results with the available observations. In addition, we consider the case of catastrophic collisions between these satellites and Centaur objects and calculate the age of formation of those satellites that suffer one or more disruptions. Results. In general we find that the observed crater distributions are best modeled by the crater size distribution corresponding to the s2 = 3.5 index of the SFD of impactors with diameters smaller than 60 km. However, for crater diameters D ≲ 3–8 km (which correspond to impactor diameters d ~ 0.04–0.15 km), the observed distributions become flatter and deviate from our results, which may evidence processes of erosion and/or crater saturation at small crater sizes or a possible break in the SFD of impactors at d ~ 0.04–0.15 km to a much shallower differential slope of approximately − 1.5. Our results suggest that Pan, Daphnis, Atlas, Aegaeon, Methone, Anthe, Pallene, Calypso, and Polydeuces suffered one or more catastrophic collisions over the age of the solar system, the younger being associated to arcs with ages of ~108 yr. We have also calculated surface ages for the satellites, which indicate ongoing resurfacing processes.

Subsurface water piping transports plankton and prevents meromixis in a deep volcanic crater lake (Dominica, West Indies)

Boeri Lake—a small (3.6 ha) but deep (39.6 m) crater lake on Morne Micotrin in Dominica, West Indies—presents a limnological enigma; it exhibits strong morphometric and circumstantial evidence for meromixis, yet it is not stratified. We tested the hypothesis that water seepage from Boeri Lake overcomes morphometric drivers of stratification and prevents the onset of meromixis. We compared water chemistry and plankton community composition in Boeri Lake to perennial streams on Morne Micotrin to assess if water discharging from these springs originates in Boeri Lake. Lacustrine phytoplankton and zooplankton taxa were detected in nearby streams, which also had similar water chemistry to that of Boeri Lake. In contrast, two other streams that drain Morne Micotrin and one neighboring reference stream had little in common with waters from Boeri Lake. This suggests that Boeri Lake’s anomalous limnology is explained by hydrologic connectivity to nearby flanking streams, and supports our hypothesis that subsurface water piping, combined with high annual rainfall, stymies the onset of meromixis. We provide an explanation for how holomictic lakes can persist and transport organisms through the ground in tropical mountain ecosystems and discuss implications of consistent water piping for plankton community assembly on island lakes.

Impact cratering in and around the Orientale Basin: Results from recent high-resolution remote sensing datasets

Impact craters are the predominant geological features on the lunar surface. Data about the craters are crucial for inferring information about surface age, the generation processes of the geological units, and the sequences of geological events. The higher-resolution remote sensing datasets collected by recent lunar missions enable the investigation of impact craters of smaller size and provide more accurate information. This paper presents an investigation of the distribution and population characteristics of impact craters in and around the Orientale Basin based on high-resolution datasets. First, an update to the crater catalogue for the Orientale Basin and its surrounding area is provided, including craters as small as 1 km in diameter. Based on the updated crater catalogue, the crater densities and depth-to-diameter ratios in and around the Orientale Basin are investigated. The inclusion of small craters enables a crater density map with higher resolution, revealing a significantly higher crater density of the study area. Also, the surface age of Orientale Basin is estimated from the size–frequency distribution (SFD) using the new crater catalogue, showing an age of 3.75 Ga using the production function of Neukum et al. (2001), which is in good agreement with previous studies. Finally, distribution patterns of secondary craters around the Orientale Basin are investigated, indicating the Orientale Basin may be caused by an oblique impact with a downrange direction of about 235°–260° and an offset strength towards the direction of about 305°–350°.

Shadow–highlight feature matching automatic small crater recognition using high-resolution digital orthophoto map from Chang’E Missions

This paper introduces a new method of small lunar craters’ automatic identification, using digital orthophoto map (DOM) data. The core of the approach is the fact that the lunar exploration DOM data reveal contrasting highlight and shadow characteristics of small craters under sunlight irradiation. This research effort combines image processing and mathematical modeling. Overall it proposes a new planetary data processing approach, to segment and extract the highlight and shadow regions of small craters, using the image gray frequency (IGF) statistical method. IGF can also be applied to identify the coupling relationships between small craters’ shape and their relative features. This paper presents the highlight and shadow pair matching (HSPM) model which manages to perform high-precision automatic recognition of small lunar craters. Testing was performed using the DOM data of Chang’E-2 (CE-2). The results have shown that the proposed method has a high level of successful detection rate. The proposed methodology that uses DOM data can complement the drawbacks of the digital elevation model (DEM) that has a relatively high false detection rate. A hybrid fusion model (FUM) that combines both DOM and DEM data, was carried out to simultaneously identify small, medium, and large-sized craters. It has been proven that the FUM generally shows stronger recognition ability compared to previous approaches and it can be adapted for high precision identification of craters on the whole lunar surface. The results meet the requirements for a reliable and accurate exploration of the Moon and the planets.

The effect of atmospheric interaction on impact ejecta dynamics and deposition

Fine dust from impact ejecta settles slowly in an atmosphere, whereas large chunks of ejecta easily traverse the surrounding gas on nearly parabolic trajectories. In order to study the effects of the interaction of impact ejecta with an atmosphere, we implemented a “representative particle” approach into the iSALE-2D shock physics code. We verify the modelling approach using analytical equilibrium sedimentation velocities, and successfully benchmark the results against simulations with the shock physics code SOVA. We study vertical dust settling scenarios of different initial mass and dust size (100 μm – 10 cm), and find three different regimes of settling – sedimentation, formation of density currents, and the free-fall. Based on this, we model natural ejecta curtains for craters from 200 m – 4 km in radius, and small craters in the laboratory for different atmospheric pressures from 1 kPa – 6 MPa. We assess the range of ballistic ejecta deposition within an atmosphere in terms of ejecta thickness and deposition velocity.

Geology of the Morasko craters, Poznań, Poland—Small impact craters in unconsolidated sediments

AbstractConfirmed small impact craters in unconsolidated deposits are rare on Earth, and only a few have been the subjects of detailed investigations. Consequently, our knowledge of indicators permitting unambiguous identification of such structures is limited. In this work, detailed geological mapping was performed in the area of the Morasko craters, of which the largest crater is of about 96 m diameter. These craters were formed in the mid‐Holocene (~5000 yr ago) in unconsolidated sediments of a glacial terminal moraine. Fragments of the impactor—an iron meteorite—have been found in the craters’ vicinity for many decades. Despite numerous studies of the meteorite, no detailed research concerning the geological structure around the craters and of the ejecta deposits has been undertaken. The new data, including evaluation of over 52 sediment cores and 260 shallow drillings, permit the identification of four main sediment types: Neogene clays, diamicton with Neogene clay clasts containing charcoal pieces, diamicton without clasts, and sand with locally preserved paleosoil and charcoal pieces. Based on sedimentological properties, the ejecta deposits are mainly identified as diamicton with Neogene clay clasts, described as lithic impact breccia, covering locally preserved pre‐impact soil. Moreover, crater sections characterized by inverse stratigraphy of sediments are identified as belonging to overturned flaps.

A New Global Database of Lunar Impact Craters >1–2 km: 1. Crater Locations and Sizes, Comparisons With Published Databases, and Global Analysis

AbstractThis paper presents a new, global database of lunar impact craters, estimated to be a complete census of all craters with diameters larger than 1–2 km. The database contains over 2 million craters, making it larger in number than any previously published lunar effort by more than a factor of 10. Of those craters, 1.3 million have diameters ≥1 km, approximately 83,000 are ≥5 km, and 6,972 craters are ≥20 km. How the database was constructed along with the reliability of features is described in detail. Comparisons are made with past published databases, demonstrating good agreement for crater size and location. An ellipticity analysis is conducted, illustrating there is no dominant direction for elliptical crater orientation based on location, diameter range, or ellipticity amount, consistent with randomness for craters ≥10 km. A spatial density analysis is described, comparing the spatial density of small versus large craters, and numerous observations about the nonuniformity of the size distributions of craters across the Moon are made. The spatial density is also used in a discussion about kilometer‐scale secondary impact craters and clearly shows that they dominate the crater population in some areas of the lunar surface. This paper presents just a tiny sample of the scientific investigations that could be done with this new crater database.

Space Weathering Induced Via Microparticle Impacts: 2. Dust Impact Simulation and Meteorite Target Analysis

AbstractThe role of micrometeorite bombardment in space weathering on asteroid surfaces was studied using a 2 MV Van de Graaff accelerator. About 90,000‐ to 100,000‐micron‐ to submicron‐sized copper particles with a mass and velocity distribution similar to the interplanetary dust population were fired onto the surfaces of polished Allende CV3 chondrite and eucrite NWA 6966 samples at speeds between 1 to 70 kilometers per second. We find a clear relationship between microparticle bombardment, infrared reflectance decrease, and overall spectral reddening. Differences in impact effects due to variable particle speed, size, and structure are observed. Some Cu particles form large clusters that break up upon impact and disperse. Other impactors leave imprints on the surface, and implant or generate typical craters with rims and spallation features. Very small, fast particles generate small craters without spallation or significant crater rim. Mid‐infrared (IR) spectra (bulk and microscopic measurements of individual components), 3‐D laser microscopic images and XRD spectra from the processed and unprocessed samples were collected. Mid‐IR spectra (700–6,000 cm−1) over the entire sample surface show overall darkening of features. Microscopic IR spectra show the damage seen as reflectance decrease and spectral reddening, which is variable in the micrometer range, depending on impact density and target properties (mineralogic composition). The fine‐grained Allende matrix with predominantly Fe‐rich olivine seems less affected than coarse‐grained chondrules with Mg‐rich silicates, where darkening can reach 60%. X‐ray diffraction analysis also suggests chemical and crystallographical differences in the bombarded sample, due to impact shock.

Young wrinkle ridges in Mare Imbrium: Evidence for very recent compressional tectonism

Wrinkle ridges are common landforms widely distributed in the lunar maria. Many young wrinkle ridges were found inside Mare Imbrium using Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) high-resolution images. They represent very recent tectonic activity on the lunar surface. Determining their formation times and morphology is important in understanding the Moon's late evolution. The young ridges were mapped, and they have a winding and arched morphology. Based on their small scale and crisp appearance, crosscutting relations with very young small craters whose youth was concluded from spatial boulder densities within their ejecta blankets, these young wrinkle ridges were estimated to be as young as 10 or a few 10s of Ma, and may still be active today. The young ridges were mainly distributed in the inner region of Imbrium basin, which may suggest that the thermal and structural evolution of the inside area of Imbrium basin was protracted.

Craters, boulders and regolith of (101955) Bennu indicative of an old and dynamic surface

Small, kilometre-sized near-Earth asteroids are expected to have young and frequently refreshed surfaces for two reasons: collisional disruptions are frequent in the main asteroid belt where they originate, and thermal or tidal processes act on them once they become near-Earth asteroids. Here we present early measurements of numerous large candidate impact craters on near-Earth asteroid (101955) Bennu by the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) mission, which indicate a surface that is between 100 million and 1 billion years old, predating Bennu’s expected duration as a near-Earth asteroid. We also observe many fractured boulders, the morphology of which suggests an influence of impact or thermal processes over a considerable amount of time since the boulders were exposed at the surface. However, the surface also shows signs of more recent mass movement: clusters of boulders at topographic lows, a deficiency of small craters and infill of large craters. The oldest features likely record events from Bennu’s time in the main asteroid belt.

The martian daytime convective boundary layer: Results from radio occultation measurements and a mesoscale model

We investigate the behavior of the Martian daytime convective boundary layer (CBL) through a combination of data analysis and modeling. This study relies on two subsets of Mars Express radio occultation (RO) measurements that sounded the atmosphere in northern spring of successive Mars years. Only the first year of observations has been examined previously (Hinson et al., 2008); the second year provides complementary spatial coverage and greatly increases the total number of observations. Analysis of the RO profiles yields basic characteristics of the CBL, such as its depth D and the average potential temperature of the mixed layer θm. We also combine RO retrievals of surface pressure with surface temperatures from infrared sounding to characterize the surface forcing, expressing the result as a potential temperature θs. These observations are at local times in early afternoon for θs and late afternoon for θm and D, when each parameter is near its diurnal maximum. We use measurements at mid-to-low latitudes, which sample a wide range of θs (227–294 K), to determine the response of the lower atmosphere to spatial variations in surface forcing. The depth of the CBL ranges from less than 3 km in the midlatitude topographic basins to more than 9 km above elevated terrain in the tropics. The dependence of θm on θs is linear, with a characteristic slope of about 0.7 in both years. We gain further insight by performing a simulation with the Oregon State University Mars Mesoscale Model in a region centered on Isidis Planitia, which includes two potential landing sites for the Mars 2020 Rover. As expected from previous modeling of much smaller craters, the arc of steep topography along the western and southern margins of Isidis produces a distinctive, diurnally varying, mesoscale circulation. The simulation captures key features of the observations, such as the wide variations in θm and D — by 34 K and 9 km, respectively — that occur within this region. The model also accounts for peculiar features of RO profiles on the rim of Isidis, where the wind field strongly influences the depth and diurnal evolution of the CBL. Detailed comparisons with the observations validate the general performance of the model and confirm several aspects of the simulated wind field.

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.

Determining patterns of stratification and mixing in tropical crater lakes through intermittent water-column profiling: A case study in western Uganda

In tropical lakes, deep water-column mixing exerts great influence on nutrient dynamics and food-web structure, and on the probability for hazardous accumulation of bio- or geogenic gases in the hypolimnion. However, conventional criteria to infer a lake's mixing regime in temperate regions, such as the changing seasonal position of the thermocline or the depth to anoxia, often lead to faulty conclusions when applied to stratifying tropical lakes. The water column of these lakes tends to exhibit multiple thermoclines, and deep-water anoxia tends to spread quickly throughout the hypolimnion in all but the least productive lakes, complicating the interpretation of isolated temperature and dissolved-oxygen profiles. Here, multiple depth profiles of temperature, dissolved oxygen and conductivity from a diverse selection of 11 small tropical crater lakes in western Uganda are combined with time series of temperature-logger data to determine the occurrence and maximum depth of water-column mixing within a given time period. We propose three reliable indicators of past mixing: 1) presence of oxygen below the photic zone; 2) evidence of recent cooling at a given depth; and 3) disappearance of previously documented thermo- and chemoclines. The greatest depth of mixing can only be inferred with a fair degree of certainty from a chronosequence of at least two sets of profiles.

Embrittlement mechanism in a low-carbon steel at intermediate temperature

Embrittlement behaviors were studied in a cast steel that bears a relatively large proportion of alloying elements (Mn, Cu, Nb, Ni) at intermediate temperatures. A low-carbon cast steel was heated to 1673 K (1,400 °C) of 10 K/s, then cooled to 673–873 K of 10 K/s. Tensile tests were performed at the target temperature. Reduction of area decreased abruptly; it was smallest at 823 K (550 °C), then increased gradually as the tensile testing temperature decreased. At 823 K, the fracture surface showed intergranular fracture with small craters. Further study on the fracture behavior determined that upper bainite structure existing cementite at the prior austenite grain boundaries is crucial for the embrittlement. Segregated phosphorus triggers initial micro-crack generation at the grain boundary cementite/matrix interface. We suggest that intergranular fracture of a low-carbon steel at intermediate temperature occurs by formation of cementite forms at prior austenite grain boundaries, then active segregation of P at the GB cementite/matrix interface; microcracks then form starting at this interface; S segregates at the resulting free surfaces and thereby nucleates formation of micro-cracks, which grow and coalesce until a sufficient number of them link, then fracture occurs. To fabricate a crack-free slab, its temperature should be controlled to avoid the upper bainite temperature range at the unbending zone where stress is applied during continuous casting.

Terra-mare comparison of small young craters on the Moon

Several characteristics of very small impact craters of lunar surface have recently been improved by high resolution datasets. Lunar impact craters with diameter below 100 m and especially below 10 m were surveyed and compared at mare and terra surfaces in this work, altogether at 0.6 km2 area with 4866 craters there. The surface density values (of all craters/km2) at the mare regions were 1.5–2.0 times larger than at the terra regions, and this difference came mainly from <10 m diameter craters. Although the crater retention and stability might have been a reason for such a difference causing shorter crater lifetime on steeper terra units, similar spatial density was found at sloping and nearly horizontal parts of the terra area, and no signature of partly destroyed craters was observed there, leaving the resolution of the difference open. The difference in the properties of target material is a relevant candidate reason for it, however based on shadow length measurements no depth difference was identified between terra and mare craters.There is an ongoing debate on the reliability and usage of small craters for age estimation (Xiao and Strom, 2012), this work provides some further information to evaluate this aspect. No correlation was found between the areal density values of craters and ages determined from size-frequency distributions, confirming (using examples of smaller craters than analysed in most earlier works), that large sized craters dominate the age estimation despite the larger number of <10 m craters. Characteristic age values for <10 m diameter crater size category ranged between 65 and 100 Ma, while for <100 m craters the estimated ages were 76–250 Ma. Substantial fluctuations in age within terra and mare regions were observed in the absence of nearby secondary craters, suggesting that the role of nearby secondaries might be minor in the production of the observed areal heterogeneity. Although the role of distant secondaries could not be excluded, the results suggest natural random fluctuations should be more carefully considered in the future. Analysis of the depth of regolith layer that different size groups of craters influence, roughly the top 1–2 meter layer could be mixed during 30–60 Ma age, and this age range is close to recent estimations by other authors.

Deep‐Learning‐Based Bughole Detection for Concrete Surface Image

Bugholes are surface imperfections that appear as small pits and craters on concrete surface after the casting process. The traditional measurement methods are carried out by in situ manual inspection, and the detection process is time‐consuming and difficult. This paper proposed a deep‐learning‐based method to detect bugholes on concrete surface images. A deep convolutional neural network for detecting bugholes on concrete surfaces was developed, by adding the inception modules into the traditional convolution network structure to solve the problem of the relatively small size of input image (28 × 28 pixels) and the limited number of labeled examples in training set (less than 10 K). The effects of noise such as illumination, shadows, and combinations of several different surface imperfections in real‐world environments were considered. From the results of image test, the proposed DCNN had an excellent bughole detection performance and the recognition accuracy reached 96.43%. By the comparative study with the Laplacian of Gaussian (LoG) algorithm and the Otsu method, the proposed DCNN had good robustness which can avoid the interference of cracks, color‐differences, and nonuniform illumination on the concrete surface.

Tick, Days Three Through Thirty

Tick, Days Three Through Thirty Rosanna N. Henderson (bio) I was sifting through my daughter's tangled sweat-curls when I found a tiny brittle clump of black legs hanging out of her neck. The tick's body was almost completely buried in her creamy nape fat. Dizzying, but not as horrifying as the ticks you don't find, the ones that have time to swell up and fade to grey like polished pebbles. My best friend lost half of seventh grade to a tick she never found. My uncle lost his mind. In the new forests of the apocalypse, we will shave all our hair so ticks have nowhere to hide. Midway through his decline, my uncle paused to look at an empty tree branch. "She watched us here." He cocked his head, remembering the cat's gaze on his neck but forgetting cougar. His forest walks spiraled out longer and further until his wife had to call the neighbors and then search and rescue, and then, when he grew violent, the police. Years ago there'd been warning headaches, unexplained fevers, joint pain. But this was up on the butte, where mail only comes three times a week and you can cure almost anything with a long enough ramble in the woods. By the time they diagnosed his Lyme disease, hours of intravenous antibiotics couldn't root it out. The tick released from my daughter's neck with a steady pull of [End Page 161] the tweezers, leaving a small bloodless crater behind. I swabbed it with disinfectant and flushed the tick down the toilet and washed and washed and washed my hands, sterilized the tweezers. How long had it taken to burrow so deep? I counted the hours, back through the night while we slept curled together in the cabin—could I have missed it during the cursory bathtime tick-check, hurrying the kids to bed after an evening of chasing wild turkeys—could the tick have clung, feeding since the afternoon when we played Owl-and-Mouse in the meadow? Please no earlier. The internet says ticks start transmitting diseases after thirty-six to forty hours, time enough for them to start swelling. The signs of infection—rashes, headaches, fevers—can appear three to thirty days later, and the sooner you get the antibiotics, the better your chances. In the new forests of the end, there will be no antibiotics for anyone's chances. Even so, we will all want land like my uncle's when time runs out. The solar panels run over with light, the light drying the hills' south faces into sagebrush desert, yet the new well spurted thirty feet into the air and the spring in the garden never freezes under the snow. My uncle loved the place for how it might have saved us all; for how, skate-skiing down off the butte, he caught air on the long-abandoned ditches of Chinese laborers, though he'd forgotten the words for waxes and bindings and gaiters, for gold and flumes and slag-heaps, for ponderosas and sagebrush slowly burying the truth: the end has been here before. Have you seen the smoke? Ask those Chinese laborers—they shoveled thirty miles of rock, no pay until their year was up. My god, ask the Nez Perce, the Northern Paiute. Back among sidewalks and fences a week after tickbite, my daughter has learned to shrug my fingers from her neck. The tick's burrow grows shallower but I keep my watch. Is this the moment I will come to regret, when more vigilance might have prevented disaster? Or has it already passed? Mostly the rashes appear seven days after infection, the internet says. My daughter says there's a coover in the closet and she doesn't want it to eat her. She means cougar, and usually they just eat deer and rodents, the very creatures whose burgeoning populations nurture the ticks of the apocalypse. [End Page 162] Twenty-one days after tickbite, the burrow has tightened into a sandy little scar, pale and rash-free. But here's a speckly rash lower on my daughter's back, the sort that I would...

Long-Term Hydrologic Fluctuations and Dynamics of Primary Producers in a Tropical Crater Lake

Aquatic ecosystems in tropical regions remain understudied and their long-term dynamics poorly understood. In East Africa, a better understanding of how natural communities of primary producers in small freshwater ecosystems respond to climatic variability is needed to improve management and conservation of aquatic resources. This study explored the response of algae and bacteria communities to marked hydrological variation over the past 1500 years in a small western Ugandan crater lake, Lake Nkuruba. We analyzed sedimentary algal and bacterial pigments to evaluate the magnitude and direction of change in the autotrophic community in response to severe climatic perturbations in the region. The lithology of the Lake Nkuruba sediment core indicated that external forcing in the form of a major drought, associated with the Medieval Climate Anomaly, caused a heavy, short-lived detrital pulse to the basin that led to a brief but substantial disruption of the lake system in the second half of the 13th century. The system appears to have recovered rapidly, and then transitioned to a more productive state than the one preceding the drought. The considerable variation observed in the sedimentary pigment biomarkers is likely linked with climatically-induced changes in the water column structure of this small crater lake. Our results highlight the challenge of defining appropriate baselines or reference conditions in climatically-sensitive East African aquatic ecosystems and disentangling long-term anthropogenic impacts from the strong regional hydrological flux at the decadal to centennial scale.

Floor‐Fractured Craters on Ceres and Implications for Interior Processes

AbstractSeveral of the impact craters on Ceres have sets of fractures on their floors. These fractures appear similar to those found within a class of lunar craters referred to as floor‐fractured craters (FFCs). We have cataloged the Ceres FFCs according to the classification scheme designed for the Moon. An analysis of the depth to diameter ratio for Ceres craters shows that, like lunar FFCs, the Ceres FFCs are anomalously shallow. Large (&gt;50 km) Ceres FFCs are most consistent with Class 1 lunar FFCs, while smaller craters on Ceres are more consistent with Class 4 lunar FFCs. This suggests that Ceres FFCs may similarly be undergoing fracturing due to the intrusion of a low‐density material below the craters. While on the Moon (and Mars) the intrusive material is hypothesized to be silicate magma, cryomagmatic intrusions are more likely responsible for the formation of the Ceres FFCs. However, new models suggest that at least some of the FFC fractures may have formed due to the solid‐state flow of a low‐viscosity and low‐density material into the crater wall.

Evolution of Escarpments, Pediments, and Plains in the Noachian Highlands of Mars

AbstractExtensive Noachian‐aged intercrater planation surfaces comprise much of the southern highlands of Mars. We mapped aggradational and stable to degradational surfaces in three study areas with diverse relief elements and ages: the high and rugged relief of Libya Montes, the well‐preserved intercrater plains of Noachis Terra, and the rolling relief with more drainage development in Terra Cimmeria. Here we describe four major geomorphic features that formed in these regions: debris‐mantled escarpments, regolith pediments, sloping aggradational surfaces, and depositional plains. We interpret that with tectonic stability and an arid paleoclimate, these features supported slow pedogenesis, sediment transport, and diagenesis over hundreds of millions of years during heavy impact bombardment. Slow aqueous weathering generated primarily fine‐ or medium‐grained particles from basaltic surfaces of impact ejecta and megabreccia. These sediments were collected in local lows, reducing surface roughness, permeability, and populations of small craters. Larger crater walls and structural escarpments retreated radially or linearly as ~5–20° slopes, indicating efficient removal of fine‐ or medium‐grained debris but little downslope transport of coarse material by fluvial erosion or creep. Gently to moderately sloping, composite intercrater planation surfaces evolved as regolith pediments with tectonic stability and little fluvial dissection. Noachian impact craters degraded in place on pediments and became embayed or buried on basin floors. The concentration of aggradational surfaces in low‐lying areas, lack of coarse‐grained alluvial fans in most locations, and resistance to later eolian deflation suggest intermittent low‐magnitude (hypo‐)fluvial erosion with aqueous cementation or development of a lag in basins.