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A Correction to this paper has been published: [ABSTRACT FROM AUTHOR]

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. • New geological maps of southwest Mare Tranquillitatis, centered on the Apollo 11 landing site, were produced. • New N (1) values of the geological unit at the landing site were compared with a survey of radiometric sample ages. • The Apollo 11 landing site is best represented by the Group A basalts. • Updated N(1) values are consistent with the Neukum (1983) and Neukum et al. (2001) chronology functions. [ABSTRACT FROM AUTHOR]

Highlights • We mapped the Ac-2 Coniraya Quadrangle of Ceres at a scale of 1:500,000. • We derived CSFD measurements for all major surface units and obtained theirAMAs. • The youngest features are the fresh craters like Oxo (∼4 Ma) or FC IV (∼12 Ma), while the oldest unit is the cratered terrain (3.1 Ga to 3.5 Ga). • We identified smooth impact melt-like deposits with pits in and around Ikapati crater. • Possible ice in the crust is heterogeneously distributed, as lobate flows, bright spots, and pits are only found at certain locations such as Ikapati and Oxo. Abstract Since its arrival at Ceres, Dawn's Framing Camera has been imaging the dwarf planet at different altitudes, using 8 different filters. Based on these images, global clear filter mosaics, digital terrain models, and global color mosaics were produced. These datasets are basis for the derived photo-geologic map of the Ac-2 Coniraya quadrangle, located between 0° and 90° eastern longitude and between 20° and 40° northern latitude. Crater size-frequency distribution (CSFD) measurements have been applied to derive the stratigraphic history of the identified geologic units in the Ac-2 Coniraya quadrangle. Based on these investigations, we found that the Ac-2 Coniraya quadrangle shows evidence for a long geologic history lasting over 3.5 Ga. Most of the identified geologic units are related to impact cratering at different periods of time. While the oldest unit identified in the Ac-2 Coniraya quadrangle, the cratered terrain, shows an absolute model age (AMA) of 3.3–3.5 Ga, the youngest units, including fresh impact craters and lobate flows, show AMAs of < 100 Ma. The large number of different sized impact craters excavated a variety of materials with different colors and albedo, indicative of a heterogeneous crustal composition, including water or volatiles. Pits identified at Ikapati crater's smooth deposits might further indicate the presence of volatiles in the subsurface. Furthermore, our mapping of the Ac-2 Coniraya quadrangle shows that possible H 2 O ice in the crust is heterogeneously distributed, as lobate flows, bright spots, and pits are only found at certain locations, related to relatively fresh impact craters. [ABSTRACT FROM AUTHOR]

Multi-temporal spacecraft observations are key to detect and understand surface changes and time-critical processes on Mars. Since the 1970s, the number of orbital observations of Mars has increased to over one million images. The observations have revealed that the surface of Mars is changing due to exogenic processes, including eolian activity, mass movement, seasonal ice and frost cover, and crater-forming impacts. The increasing number of observations highlights the importance of efficient and convenient tools for planetary image data management and change detection analyses. To support the identification of surface changes we developed the Multi-Temporal Database of Planetary Image Data (MUTED), which is accessible at “ http://muted.wwu.de ”. The database enables scientists to quickly identify the spatial and multi-temporal coverage of planetary image data from Mars. As a basis for various change detection analyses, the location, number, and time range of acquisitions of overlapping images taken by the Viking Orbiter (VO), the Mars Orbiter Camera (MOC), the High Resolution Stereo Camera (HRSC), the Thermal Emission Imaging Instrument (THEMIS), the High Resolution Imaging Science Experiment (HiRISE), the Compact Reconnaissance Imaging Spectrometer of Mars (CRISM) and the Context Camera (CTX) as well as future instruments (e.g., Colour and Stereo Surface Imaging System (CaSSIS)) can be identified. The database will assist and optimize image data searches to support the analysis and understanding of short-term, seasonal, and long-term processes at the surface and in the atmosphere of Mars. To demonstrate the capability and scientific potential of the database, we analyzed the occurrence of dark slope streaks and observed their formation within a time interval of <∼5 days in different regions on Mars. [ABSTRACT FROM AUTHOR]

Recent work on dating Copernican-aged craters, using Lunar Reconnaissance Orbiter (LRO) Camera data, re-encountered a curious discrepancy in crater size-frequency distribution (CSFD) measurements that was observed, but not understood, during the Apollo era. For example, at Tycho, Copernicus, and Aristarchus craters, CSFDs of impact melt deposits give significantly younger relative and absolute model ages (AMAs) than impact ejecta blankets, although these two units formed during one impact event, and would ideally yield coeval ages at the resolution of the CSFD technique. We investigated the effects of contrasting target properties on CSFDs and their resultant relative and absolute model ages for coeval lunar impact melt and ejecta units. We counted craters with diameters through the transition from strength- to gravity-scaling on two large impact melt deposits at Tycho and King craters, and we used pi-group scaling calculations to model the effects of differing target properties on final crater diameters for five different theoretical lunar targets. The new CSFD for the large King Crater melt pond bridges the gap between the discrepant CSFDs within a single geologic unit. Thus, the observed trends in the impact melt CSFDs support the occurrence of target property effects, rather than self-secondary and/or field secondary contamination. The CSFDs generated from the pi-group scaling calculations show that targets with higher density and effective strength yield smaller crater diameters than weaker targets, such that the relative ages of the former are lower relative to the latter. Consequently, coeval impact melt and ejecta units will have discrepant apparent ages. Target property differences also affect the resulting slope of the CSFD, with stronger targets exhibiting shallower slopes, so that the final crater diameters may differ more greatly at smaller diameters. Besides their application to age dating, the CSFDs may provide additional information about the characteristics of the target. For example, the transition diameter from strength- to gravity-scaling could provide a tool for investigating the relative strengths of different geologic units. The magnitude of the offset between the impact melt and ejecta isochrons may also provide information about the relative target properties and/or exposure/degradation ages of the two units. Robotic or human sampling of coeval units on the Moon could provide a direct test of the importance and magnitude of target property effects on CSFDs. [ABSTRACT FROM AUTHOR]

Thermochemical models have predicted that Ceres, is to some extent, differentiated and should have an icy crust with few or no impact craters. We present observations by the Dawn spacecraft that reveal a heavily cratered surface, a heterogeneous crater distribution, and an apparent absence of large craters. The morphology of some impact craters is consistent with ice in the subsurface, which might have favored relaxation, yet large unrelaxed craters are also present. Numerous craters exhibit polygonal shapes, terraces, flowlike features, slumping, smooth deposits, and bright spots. Crater morphology and simple-to-complex crater transition diameters indicate that the crust of Ceres is neither purely icy nor rocky. By dating a smooth region associated with the Kerwan crater, we determined absolute model ages (AMAs) of 550 million and 720 million years, depending on the applied chronology model. [ABSTRACT FROM AUTHOR]

The Hellas basin on Mars is the second-largest topographically well-defined impact structure in the Solar System and has repeatedly been interpreted as a major sink of volcanic, glacio-fluvial and eolian materials. Based on established guidelines for planetary mapping, we compiled a comprehensive photogeological map of Hellas Planitia, i.e., the Hellas basin floor (1:2,000,000; ∼1.8 × 10 6 km 2 ; see Supplementary online material ), using the Thermal Emission Imaging System (THEMIS-IR) day-time mosaic as basemap (supplemented by several other datasets). We identified 33 units, which were categorized into a “Rim Assemblage”, containing “Dissected units”, a “Layered rim sequence”, and “Other basin rim units”, as well as a “Floor Assemblage”, containing the “Honeycomb formation”, an “Interior formation”, and a “Plains sequence”. Relative dating of units was performed wherever contacts revealed stratigraphic relationships and was complemented by absolute model ages (AMAs) of all units that lend themselves to reliable crater-size frequency distribution (CSFDs) measurements. On the basis of our results, as well as AMAs of circum-Hellas volcanic provinces by previous authors, we compiled a chronostratigraphic model of the Hellas basin floor. The northern basin rim shows evidence (vast layered, hydrous mineral-bearing deposits containing meandering, channel-like valleys), that the early history of the basin until ∼3.8 Ga ago experienced extended periods of low-energy fluvial, and possibly lacustrine, activity. Superposing the layered rim sequence, the majority of the Hellas basin infill (∼1.5–1.7 × 10 6 km 3 ) consists of volcanic material (a lower and an upper wrinkle-ridged plains unit), which was shortened by a compressive stress field relatively soon after its emplacement. Based on their ages and stratigraphic considerations, we identified Malea Patera (and possibly also Tyrrhena Patera) as a suitable source for the older, lower plains (∼3.8 Ga), and Hadriaca and/or Amphitrites Paterae for the younger, upper plains (∼3.7 Ga). Shortly after these volcanic episodes, the entire basin floor was covered by large amounts of deposits (>10 6 km 3 ) containing aqueously altered mafic materials. We interpret the deposits to have originated from Hesperia Planum and the Hesperia-Hellas trough, where they might have been removed by glacio-fluvial processes, which also sculpted the Hellas rim sections along northern Promethei Terra and Malea Planum around the same time. Such an apparent correlation between nearby volcanic activity and increased erosion/deposition is in agreement with previous models, which suggest outgassing by a maximum in global volcanism at that time caused warmer episodes enabling repeated subaerial water run-off. After the emplacement of these deposits, intense erosion, likely deflation along a rim-parallel annulus, began to exhume the outer parts of the wrinkle-ridged plains again, forming an interior formation of more elevated erosional remnants including the Alpheus Colles. Since ∼3.7 Ga ago, fluvial activity of Dao/Niger and Harmakhis Valles dissected the eastern plains and emplaced pancake-shaped deposits in the basin center. Later, deflation and abrasion by katabatic winds moving through the Hellas basin in a clock-wise direction parallel to the basin floor outline carved out the northwestern Hellas Planitia trough (called Peneus Palus), possibly within a time span of few hundreds of Ma or less within the Amazonian. Being consistent with erosional patterns we observed within the basin, circulation models predict such winds to be ongoing today and might, thus, have recently exhumed the so called “honeycomb” formation on parts of Peneus Palus. This enigmatic, unique terrain was possibly formed by ductile deformation of pre-3.8 Ga material and appears to be partially covered by another unique landform, the “banded terrain”. Although we cannot rule out that it is the surface expression of a thicker unit, the banded terrain might represent flows of volatile-rich airfall deposits emplaced along the edges of the Hellas wind alley by turbulent, marginal wind currents, which currently/recently prevail on both sides of the Hellas Planitia trough. In summary, our investigations enabled us to draw an updated, comprehensive and self-consistent picture of the geologic evolution of the Hellas basin floor, including volcanic, (peri-)glacial, fluvial and eolian processes, their possible interactions, and the implications on the climatic and geologic development of the circum-Hellas region and the entire planet. [ABSTRACT FROM AUTHOR]

Boguslawsky crater (72.9°S, 43.3°E, ~100 km in diameter) is a primary target for the Luna-Glob mission. The crater has a morphologically smooth (at the resolution of WAC images), flat, and horizontal floor, which is about 55–60 km in diameter. Two ellipses were selected as specific candidate landing areas on the floor: the western ellipse is centered at 72.9°S, 41.3°E and the eastern ellipse is centered at 73.9°S, 43.9°E. Both ellipses represent areas from which Earth is visible during the entire year of 2016 and lack permanently shadowed areas. Boguslawsky crater is located on or near the rim of the South Pole–Aitken basin, which provides the unique possibility to sample some of the most ancient rocks on the Moon that probably pre-date the SPA impact event. The low depth/diameter ratio of Boguslawsky suggests that the crater has been partly filled after its formation. Although volcanic flooding of the crater cannot be ruled out, the more likely process of filling of Boguslawsky is the emplacement of ejecta from nearby and remote large craters/basins. Three morphologically distinctive units are the most abundant within the selected landing ellipses: rolling plains (rpc), flat plains (fp), and ejecta from crater Boguslawsky-D (ejf), which occurs on the eastern wall of Boguslawsky. The possible contribution of materials from unknown sources makes the flat and rolling plains less desirable targets for landing. In contrast, ejecta from Boguslawsky-D represents local materials re-distributed by the Boguslawsky-D impact from the wall onto the floor of Boguslawsky. Thus, this unit, which constitutes about 50% of the eastern landing ellipse, represents a target of clearer provenance and a higher scientific priority. [ABSTRACT FROM AUTHOR]

The howardite, eucrite and diogenite (HED) meteorites are genetically related and represent the most voluminous group of achondrites. They are the closest analog materials to Vesta and V-type asteroids. Many of these meteorites were the focus of intense petrologic and visible to near infrared spectral studies. As ground-based and orbital observations of basaltic asteroids have increased, an improved understanding of HEDs is needed. For this study, we investigated 24 HED samples, mainly new finds from Northwest Africa (NWA). Visible to near infrared (up to 2.5 μm) spectral measurements under varying illumination and observation geometries were acquired for 4 samples. Phase reddening and bluing (i.e., increase and decrease in spectral slope) is observed for the visible slope as phase angle increase. Monotonic phase reddening can occur for the near infrared slope as phase angle increase. Non-systematic changes with phase angle are found for the Band Area Ratio parameter. At phase angles higher than ∼60°, the decrease of reflectance and decrease of pyroxene bands depth are undistinguishable from admixture of low albedo material to HED samples. To assess the precision of empirical equations relating spectral properties and composition, the pyroxenes, feldspar, and olivine chemistry of the samples was determined. Using previous calibrations, systematic overestimations of the ferrosilite (Fs) and wollastonite (Wo) contents are found, especially in the 15–40 Fs range. To overcome such discrepancies, a new set of empirical equations is proposed. For an application of the new calibration, we selected two compositional end-member areas on Vesta on the basis of their iron content. For the iron-poor terrain we found an average pyroxene composition of Fs 30 Wo 5 and for the iron-rich terrain an average of Fs 47 Wo 14 . [ABSTRACT FROM AUTHOR]

Utopia and Acidalia Planitiae show a set of similar morphologic features indicative of the presence of large reservoirs of water/mud in these regional topographic lows. The most important of these features are: (1) impact crater ejecta morphology that progressively changes from ballistic/rampart at the basins periphery to strongly degraded pancake-like in the central portions of the basins, and (2) etched flows (interpreted as mudflows) that are concentrated in those regions of the central portions of both basins where the pancake-like ejecta prevail. The Vastitas Borealis Formation (VBF) in Utopia and Acidalia Planitiae represents the uppermost stratigraphic limit of possible reservoirs in both basins and has practically the same age in these regions; ∼3.57 (±0.02) Ga in Utopia Planitia and ∼3.61 (+0.05/−0.08) Ga in Acidalia Planitia. The southern contact of the VBF closely follows specific contour lines in Utopia Planitia (∼−3.6 km ± 54 m, over a distance of ∼1500 km) and Acidalia Planitia (∼−3.9 km ± 70 m, over the distance of ∼6000 km). The presence of the large water/mud reservoirs and the age and topographic configuration of the VBF in Utopia and Acidalia Planitiae are consistent with the existence of an extensive single body of water (an ocean) in the northern plains of Mars. [ABSTRACT FROM AUTHOR]

Highlights: [•] Flows of mud were detected in the SW portion of Utopia Planitia. [•] Mud flows are spatially associated with giant polygons. [•] Rampart craters are concentrated at the periphery of Utopia Planitia. [•] Craters with pancake-like ejecta prevail in the center of Utopia Planitia. [•] A large body of water/ice likely existed in the center of Utopia Planitia. [Copyright &y& Elsevier]

Abstract: We present new insights into possible formation mechanisms and implications for previously identified landforms of putative glacio-fluvial origin along the southern rim of the Argyre basin on Mars. We compiled a detailed geomorphologic map of the study area and conducted morphometric and stratigraphic analyses of specific features, e.g., esker-like sinuous ridges on layered terrain. Based on their morphology and orientations, we subdivided the sinuous ridges on the southern Argyre basin floor into two populations, which could reflect changing conditions of glacial retreat. With the transition and oblique path methods we quantified the ice thickness of the glacier under which the first, lesser degraded, population of ridges probably formed. Our results imply an ice sheet thickness of ~2km and at least ~100,000–150,000km³ of ice on the southern floor of the Argyre basin during the time those ridges were deposited (>30× the volume of Vatnajökull, Iceland). The second population of ridges is more degraded and shows layers occasionally extending into the surrounding layered terrain. Comparisons with the morphology surrounding the Piedmont-style Malaspina Glacier in Alaska show similarities, suggesting population II formed during a glacial retreat involving back- and downwasting of stagnant ice lying beneath fresh outwash sediments, creating degraded and layered lag deposits around the emerging eskers. If outwash sediments were fed by the same drainage source as the eskers, sections of layers can extend from a given ridge into the surrounding deposits. The differences between the two ridge populations are probably a result of the subglacial drainage direction changing from northward to north-eastward around 3.6Ga ago. This was likely coupled with the deposition of less or no outwash sediments resulting in a decrease of lag deposits. A subsequent phase of stagnant glacial retreat left no terminal moraines and largely preserved the population I ridges, thus implying sufficient glacial thinning in order for the ice flow to stop. This, in turn, may have been caused either by sublimation in a cold but increasingly dry climate, or by melting and increased glacier surface runoff due to rising temperatures. [Copyright &y& Elsevier]

Abstract: The Moon is key to understanding both Earth and our Solar System in terms of planetary processes and has been a witness of the Solar System history for more than 4.5Ga. Building on earlier telescopic observations, our knowledge about the Moon was transformed by the wealth of information provided by Apollo and other space missions. These demonstrated the value of the Moon for understanding the fundamental processes that drive planetary formation and evolution. The Moon was understood as an inert body with its geology mainly restricted to impact and volcanism with associated tectonics, and a relative simple composition. Unlike Earth, an absence of plate tectonics has preserved a well-defined accretion and geological evolution record. However recent missions to the Moon show that this traditional view of the lunar surface is certainly an over simplification. For example, although it has long been suspected that ice might be preserved in cold traps at the lunar poles, recent results also indicate the formation and retention of OH− and H2O outside of polar regions. These volatiles are likely to be formed as a result of hydration processes operating at the lunar surface including the production of H2O and OH by solar wind protons interacting with oxygen-rich rock surfaces produced during micrometeorite impact on lunar soil particles. Moreover, on the basis of Lunar Prospector gamma-ray data, the lunar crust and underlying mantle has been found to be divided into distinct terranes that possess unique geochemical, geophysical, and geological characteristics. The concentration of heat producing elements on the nearside hemisphere of the Moon in the Procellarum KREEP Terrane has apparently led to the nearside being more volcanically active than the farside. Recent dating of basalts has shown that lunar volcanism was active for almost 3Ga, starting at about 3.9–4.0Ga and ceasing at ∼1.2Ga. A recent re-processing of the seismic data supports the presence of a partially molten layer at the base of the mantle and shows not only the presence of a 330km liquid core, but also a small solid inner core. Today, the Moon does not have a dynamo-generated magnetic field like that of the Earth. However, remnant magnetization of the lunar crust and the paleomagnetic record of some lunar samples suggest that magnetization was acquired, possibly from an intrinsic magnetic field caused by an early lunar core dynamo. In summary, the Moon is a complex differentiated planetary object and much remains to be explored and discovered, especially regarding the origin of the Moon, the history of the Earth–Moon system, and processes that have operated in the inner Solar System over the last 4.5Ga. Returning to the Moon is therefore the critical next stepping-stone to further exploration and understanding of our planetary neighborhood. [Copyright &y& Elsevier]

Abstract: We have mapped the area of Isidis Planitia (1–27°N, 75–103°E) in order to assess the geologic history of this region using modern data sets such as MOLA topography and the high-resolution images provided by the HRSC, CTX, and HiRISE cameras. Results of our mapping show that the geologic history of Isidis Planitia consists of three principal episodes. (1) Impact dominated episode (Noachian, until ∼3.8Ga): During this time, the oldest materials in the study area were formed mostly by impact reworking and mass-wasting. Other processes (e.g., volcanism and fluvial/glacial activity) likely operated at this time but played a subordinate role. (2) An episode related to volcanic and fluvial/glacial activities (late Noachian–early Amazonian, ∼3.8–2.8Ga): Volcanism appears as the most important process at the beginning of this episode (∼3.8–3.5Ga) and was responsible for the formation of a large circum-Isidis volcanic province by the early Hesperian epoch. Volcanic materials covered large portions of the Isidis rim, almost completely buried the previous crater record on the floor of the Isidis basin, and probably were the major contributors to the filling of the basin. Fluvial/glacial processes prevailed closer to the end of the episode (early Hesperian–early Amazonian, ∼3.5–2.8Ga) and were responsible for widespread resurfacing in the Isidis Planitia region, mostly at ∼3.1–3.4Ga. Glaciers and/or ice sheets probably resulted in a massive glaciation of the rim and the floor of the Isidis basin. The total volume of material eroded from the Isidis rim by glacial and fluvial activity is estimated to be about 35,000–50,000km3, which is equivalent to a composite layer about 40–60m thick on entire floor of the basin. More important, however, is that the eroded materials were likely saturated with ice/water and could form wet deposits on the floor. (3) Wind-dominated episode (since early Amazonian, ∼2.8Ga): Wind activity dominated the later geologic history of Isidis Planitia but resulted only in minor modification of the surface. [Copyright &y& Elsevier]

Abstract: We present results of our morphologic and stratigraphic investigations in the Amenthes region for which our observations suggest a complex spatial and temporal interrelation between volcanic and possibly water-related processes. We have produced a series of self-consistent geological maps and a stratigraphic correlation chart that show the spatial and temporal distribution of volcanic, fluvial and tectonic processes. The Amenthes region consists of a broad trough-like topographic depression that has served as a path for the supply of materials from Hesperia Planum to Isidis Planitia. It is most likely that Hesperia Planum and, in particular the area north of Hesperia Planum, including Tinto Vallis, Palos crater and the surrounding dissected highlands have acted as a source region for materials that were transported into the Amenthes trough and farther into the Isidis basin. The Amenthes trough, as well as the graben of Amenthes Fossae were formed after the Isidis impact in the Noachian and represent likely the oldest features in the Amenthes region. Dendritic valley networks, that bear evidence for surface runoff, have dissected the highlands adjacent to Amenthes Planum and within the Tinto Vallis and Palos crater region before ∼3.7Ga. The ridged volcanic plains located near the Palos crater and Tinto Vallis region, within Amenthes Planum as well as within the Isidis transitional plains were formed between ∼3.5 and 3.2Ga and represent the volcanic activity which resulted in the flooding of the Amenthes trough. The sinuous channel of Tinto Vallis was formed in the Hesperian (≲3.5Ga) and shows characteristics, which are consistent with both ground water sapping and igneous processes. The Palos crater outflow channel was formed nearly at the same time as Tinto Vallis, between ∼3.5Ga and ∼3.2Ga and postdates the volcanic flooding of the Amenthes trough in the Hesperian. Small valleys (∼3.4–2.8Ga) incised into the ridged plains of Amenthes Planum appear also within the transitional plains located between the Amenthes plains and the Isidis interior plains. Our model ages show that Tinto Vallis, the Palos crater outflow channel as well as the small valleys are unlikely formed at the same time and by the same processes as the dendritic valley networks and represent an episode that clearly postdates the volcanic activity. [Copyright &y& Elsevier]

Abstract: Scheduled for launch on board the BepiColombo Mercury Planetary Orbiter (MPO) in 2014, the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) is an innovative instrument for studying the surface composition and mineralogy of planet Mercury. MERTIS combines an uncooled grating push broom IR-spectrometer (TIS) with a radiometer (TIR), which will operate in the wavelength region of 7–14 and 7–40μm, respectively. The spatial resolution of the MERTIS observations will be about 500m globally and better than 500m for approximately 5–10% of the surface. The thermal infrared range offers unique diagnostic capabilities to study the surface composition of Mercury. In particular, feldspars can easily be detected and characterized, because they show several diagnostic spectral signatures in the 7–14μm range: the Christiansen feature, reststrahlen bands, and the transparency feature. In addition, MERTIS will allow the identification and mapping of elemental sulfur, pyroxenes, olivines, and other complex minerals. The scientific objectives of MERTIS include: (1) characterization of Mercury''s surface composition, (2) identification of rock-forming minerals, (3) mapping of the surface mineralogy, and (4) study of surface temperature variations and the thermal inertia. In preparation for the MERTIS data interpretation, we are performing spectral measurements of appropriate analogue materials in the Planetary Emissivity Laboratory (PEL) and are building a spectral library (Berlin Emissivity Database (BED)) of these materials for a variety of grain sizes. [Copyright &y& Elsevier]

Abstract: The observation of gullies on Mars raised questions about the presence of liquid water in the recent past. In some regions like Hale and Bond crater, gullies occur in one crater (Hale) but do not in another crater nearby (Bond). These regional differences have been interpreted as an argument for a formation of the gullies related to groundwater. The formation of gullies on Earth depends on rainfall and/or melting of snow as well as on several parameters such as the presence of steep slopes and sufficient amounts of fines and debris. We investigated the Hale/Bond region for differences in crater wall morphology and texture, slopes, and thermal properties to determine whether the gully formation is dependent on factors such as steep slope angles and availability of fine-grained material. Morphologically there exist two kinds of gullies in the Hale crater: Gullies on the south- and east-facing crater slopes have a pristine appearance with deep channels eroded into the talus material and well-preserved aprons. Gully-like features on the north- and west-facing slopes are degraded and superposed by craters, indicating that they are old in comparison to the pristine ones. However, their formation process is unclear and might be due to debris flows, surface runoff or dry mass wasting processes or a combination of these processes. The crater walls of Bond do not show gullies. Their morphology is most likely consistent with a degraded mantle deposit. Slope measurements reveal that the gullies in Hale crater occur on slopes between ∼20° and ∼30° in contrast to the slopes without gullies in Bond that are between ∼10° and ∼20° steep. Mean thermal inertia values on slopes with younger gullies are ∼175Jm−2 K−1 s−1/2 corresponding to higher amounts of fine-grained material. At slopes with older gully-like features mean thermal inertia values are ∼315Jm−2 K−1 s−1/2 corresponding to higher amounts of bedrock or possibly indurated grain sizes. Mean thermal inertia values of the Bond crater walls are ∼230Jm−2 K−1 s−1/2 indicating more consolidated terrain possibly due to the cementation of the dissected mantle material. From our investigation we conclude that the occurrence of gullies in the Hale/Bond region most likely depends on the distribution of unconsolidated material and steep slopes. The regional and local gully distribution on Mars likely varies due to differences in topography and surface material properties. Their proposed clustered distribution on Mars is not an argument for a groundwater formation mechanism of the gullies. [Copyright &y& Elsevier]

Pitted impact deposits on Vesta show higher reflectance and pyroxene absorption band strengths compared to their immediate surroundings and other typical Vestan materials. We investigated whether heating to different temperatures for different durations of Vestan regolith analog materials can reproduce these spectral characteristics using mixtures of HEDs, the carbonaceous chondrite Murchison, and terrestrial analogs. We find no consistent spectral trend due merely to temperature increases, but observed that the interiors of many heated samples show both higher reflectance and pyroxene band I strength than their heated surfaces. With electron probe microanalysis, we additionally observe the formation of hematite, which could account for the higher reflectance. The presence of hematite indicates oxidation occurring in the sample interiors. In combination with heat, this might cause the increase of pyroxene band strengths through migration of iron cations. The effect grows larger with increasing temperature and duration, although temperature appears to play the more dominant role. A higher proportion of Murchison or the terrestrial carbonaceous chondrite analog within our mixtures also appears to facilitate the onset of oxidation. Our observations suggest that both the introduction of exogenic material on Vesta as well as the heating from impacts were necessary to enable the process (possibly oxidation) causing the observed spectral changes. [ABSTRACT FROM AUTHOR]

This review paper summarizes the observations and results of the Mars Express Mission and its application in the analysis of geological processes and landforms on Mars during the last 20 years. The Mars Express observations provided an extended data base allowing a comparative evaluation of different geological surface landforms and their time-based delimitation. High-resolution imagery and digital elevations models on a local to regional scale and spectral measurements are the basis for geological analyses of water-related surface processes on Mars. This includes the nature and discharges of valley networks, formation timescale of deltas, volumina of sedimentary deposits as well as estimating the age of geological units by crater size–frequency distribution measurements. Both the quantifying of geological processes and the determination of absolute model ages allows to constraint the evolution of Martian water-related activity in space and time. Comparative age estimation of fluvial, glacial, and lacustrine deposits, as well as their timing and episodicity, has revealed the nature and evolution of the Martian surface hydrological cycle. Fluvial and lacustrine activity phases are spread over a time span from Noachian until Amazonian periods, but detailed studies show that they have been interrupted by multiple and long-lasting phases of cessation and quiescent. In addition, evidence of glacial activity shows discrete phases of enhanced intensity correlating with increased spin-axis obliquity amplitude. The episodicity of geological processes, erosion, deposition, and glaciation on Mars demonstrate a close correlation between individual surface processes and endogenic activity as well as spin-axis/orbital variations and changing climate condition. [ABSTRACT FROM AUTHOR]

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. • Using recent data, a new geological map is produced for the Apollo 12 landing site region. • The N (1) values of the mapped geological units were compared with recently determined radiometric sample ages. • Updated calibration points are consistent with the lunar cratering chronology of Neukum (1983) and Neukum et al. (2001). [ABSTRACT FROM AUTHOR]

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. • Reconstruction of geologic history of the Acidalia Mensa region is presented. • Acidalia Mensa likely represents a volcanic province of Noachian age. • Absolute model ages of the Vastitas Borealis Formation southward of Acidalia Mensa are ~3.58 Ga. • A reservoir of fine-grained materials underlies Vastitas Borealis Formation in the Acidalia Mensa region. [ABSTRACT FROM AUTHOR]

Launched onboard the BepiColombo Mercury Planetary Orbiter (MPO) in October 2018, the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) is on its way to planet Mercury. MERTIS consists of a push-broom IR-spectrometer (TIS) and a radiometer (TIR), which operate in the wavelength regions of 7-14 μm and 7-40 μm, respectively. This wavelength region is characterized by several diagnostic spectral signatures: the Christiansen feature (CF), Reststrahlen bands (RB), and the Transparency feature (TF), which will allow us to identify and map rock-forming silicates, sulfides as well as other minerals. Thus, the instrument is particularly well-suited to study the mineralogy and composition of the hermean surface at a spatial resolution of about 500 m globally and better than 500 m for approximately 5-10% of the surface. The instrument is fully functional onboard the BepiColombo spacecraft and exceeds all requirements (e.g., mass, power, performance). To prepare for the science phase at Mercury, the team developed an innovative operations plan to maximize the scientific output while at the same time saving spacecraft resources (e.g., data downlink). The upcoming fly-bys will be excellent opportunities to further test and adapt our software and operational procedures. In summary, the team is undertaking action at multiple levels, including performing a comprehensive suite of spectroscopic measurements in our laboratories on relevant analog materials, performing extensive spectral modeling, examining space weathering effects, and modeling the thermal behavior of the hermean surface. [ABSTRACT FROM AUTHOR]

Results of NASA's Dawn mission indicate that Ceres, the biggest object in the main asteroid belt, may be geologically active because it shows changes in its morphology that might have happened in geologically recent times. Juling is a ∼2.5 Ma old, 20-km diameter impact crater on this dwarf planet, which has an extensive ejecta blanket of spectrally bright bluish material and water ice exposures on its steep northern inner wall. The crater floor is dominated by a spectrally reddish material, which is distinct from other surface types in this region. No other crater of Juling's size on Ceres, shows such a reddish floor. In addition, the floor has a complex morphology characterized by lobate flows and indications of a north-south directed mass wasting possibly leading to the elongated, ∼16 km long and several hundred-meter-high central structure. Here we describe the characteristics of the material that constitutes the floor, and we present a geological map of the crater, using the Framing Camera (FC) imagery. From the analysis of data acquired by the Visible and Infrared Spectrometer (VIR), we did not find evidence for the presence of organic rich materials in Juling at the available data resolution. The spectrum of the floor material seems to be a combination of mineralogy and physical properties of the regolith. Our findings suggest that the processes leading to the reddish material and the peculiar morphology of the crater floor, must have occurred after the formation of Juling crater. • Juling and its neighbor Kupalo are young craters that share remarkably bright ejecta. • Water ice may have triggered and supported landslides that reshaped the crater. • Material filling Juling's floor is spectrally distinct from the surrounding terrain. • The infrared data does not indicate presence of organic-rich material in Juling. • Physical properties and mineralogy likely cause the red spectral slope of the Juling floor. [ABSTRACT FROM AUTHOR]

The results of our regional and local studies of the Fecunditatis basin and Mare Fecunditatis, and areas adjacent to the Luna 16 landing site suggest the following. If the Fecunditatis basin is an impact structure, which is consistent with the crustal thickness models and gravity data, it predates the Nectaris basins (∼4.17 Ga). The unusually flat topographic profile of the Fecunditatis basin can be explained by viscous relaxation of the original topography of the basin and the subsequent filling of the basin by ejecta from the Nectaris and Crisium basins. The results of our crater size-frequency measurements within the mare domain of the Fecunditatis basin yield an absolute model age (AMA) of ∼3.81 Ga. Thus, the population of all recognizable craters within Mare Fecunditatis denotes the existence of an extensive episode/episodes of resurfacing that has erased the ancient crater population accumulated after the Fecunditatis impact event but before the mare emplacement. Our determinations of AMAs of the surface of Mare Fecunditatis (∼3.45 Ga) are consistent with most radiometric age determinations from the Luna 16 samples that are clustered at ages of ∼3.5–3.4 Ga. The current Mare Fecunditatis surface is likely to represent the latter episodes of volcanism, the main phase of which was active during this time interval. Although older radiometric ages have not yet been reported, the presence of older basalts in the Luna 16 sample should not be ruled out, taking into account the likely multiple sources of its materials and the complex nature of the emplacement of lavas in Mare Fecunditatis. The thickness of mare basalts in Mare Fecunditatis are highly variable. At the regional scale, thicknesses are most likely at least ∼1.5 km, in order to be able to hide a significant number of ancient larger craters. In the areas of partly or completely flooded craters, the mare thickness increases up to ∼2–2.7 km. In the vicinity of the Luna 16 landing site, however, the thickness of the most than a few hundred meters. • The unusual topographic profile of the Fecunditatis basin can be explained by viscous relaxation of the original topography. • Recognizable craters within Mare Fecunditatis suggest the existence of an extensive episode of resurfacing before the mare emplacement. • Our determinations of AMAs of the surface of Mare Fecunditatis (∼3.45 Ga) are consistent with most radiometric age determinations from the Luna 16 samples (∼3.5–3.4 Ga). • Older radiometric age of Mare Fecunditatis materials should not be ruled out. [ABSTRACT FROM AUTHOR]

Abstract Slope streaks are gravity-driven dark or light-toned features that form throughout the martian year in high-albedo and low-thermal inertia equatorial regions on Mars. These distinctive features have never been observed in a terrestrial environment although some analogs have been proposed. Based on diverse orbital observations, a number of mechanisms including granular and aqueous flows have been proposed to explain their formation. Using multi-temporal images taken by the Context Camera (CTX) aboard the Mars Reconnaissance Orbiter (MRO), we identified newly formed streaks in multiple martian years and estimated seasonal streak formation rates at intermediate latitudes, as well as at the equator. We found seasonal variations in streak activity in multiple consecutive martian years, including high formation rates during autumn (solar longitude L S ~190°). During this time, slope streak activity exceeds the long-term formation rate multiple times. The highest seasonal formation rate of 0.16% per streak per martian day was observed in the Olympus Mons aureole in autumn (L S ~210°) of Mars year 30. In some sites, slope streak formation rate at slopes of opposite orientation peaks at different seasons. The seasonal variations of the formation rate are inconsistent with sporadic trigger mechanisms and revealed that changing conditions at the slopes affect the formation of the streaks throughout the martian year. Modelled environmental parameters at the streak-bearing slopes indicate a correlation between seasonal streak activity and surface temperature, as well as wind velocity. Seasonal variations of streak activity could be a result of varying intensities of both dry and wet mechanisms or could be explained by the interaction of multiple mechanisms. [ABSTRACT FROM AUTHOR]

Fan-shaped accumulations of debris flow deposits are common landforms in polar regions such as Svalbard. Although depositional processes in these environments are of high interest to climate as well as Mars-analog research, several parameters, e.g., debris flow recurrence periods, remain poorly constrained. Here, we present an investigation based on remote sensing as well as in situ data of a ~ 0.4 km 2 large colluvial fan in Hanaskogdalen, central Spitsbergen. We analyzed high resolution satellite and aerial images covering five decades from 1961 to 2014 and correlated them with lichenometric dating as well as meteorological data. Image analyses and lichenometry deliver consistent results and show that the recurrence period of large debris flows (≥ 400 m 3 ) is about 5 to 10 years, with smaller flows averaging at two per year in the period from 2008 to 2013. While this is up to two orders of magnitude shorter than previous estimates for Svalbard (80 to 500 years), we found the average volume of ~ 220 m 3 per individual flow to be similar to previous estimates for the region. Image data also reveal that an avulsion took place between 1961 and 1976, when the active part of the fan moved from its eastern to its western portion. A case study of the effects of a light rain event (~ 5 mm/day) in the rainy summer of 2013, which triggered a large debris flow, further shows that even light precipitation can trigger major flows. This is made possible by multiple light rain events or gradual snow melt pre-saturating the permafrost ground and has to be taken into account when predicting the likelihood of potentially hazardous mass wasting in polar regions. Furthermore, our findings imply a current net deposition rate on the colluvial fan of ~ 480 m 3 /year, which is slightly less than the integrated net deposition rate of 576 to 720 m 3 /year resulting from the current fan volume divided by the 12,500 to 10,000 years since the onset of fan build-up after the area's deglaciation. However, the actual deposition rate, which should increase in a warmer climate including more rain, cannot be constrained due to effects like ongoing toe-cutting of the debris fan and some flows only causing internal redistributions. [ABSTRACT FROM AUTHOR]

In this paper, we present the results of our detailed study of morphology, topography, and age of the Deuteronilus contact that outlines Vastitas Borealis Formation (VBF) in the northern plains and the Isidis Planitia unit. The Deuteronilus contact represents a sharp and distinct geological boundary that can be traced continuously for many hundreds to thousands of kilometers. In the northern plains, segments of the Deuteronilus contact occur at two distinct topographic levels. In the northern plains, the long-wavelength topography of the Deuteronilus contact occur at two distinct topographic levels. In the Tempe, Chryse, Acidalia, and Cydonia-Deuteronilus regions (the total length is ∼14,000 km), the contact is at the mean elevation of about −3.92 km (the decile range is 180 m, from −4.01 km to −3.83 km). In the Pyramus-Astapus, Utopia, and Western Elysium regions (the total length is ∼7700 km), the mean elevation of the contact is about −3.58 km (the decile range is 270 m, from −3.73 km to −3.46 km). These levels to large extent (but not completely) correspond to the model geoids that may have been characterized the shape of Mars at the time of the VBF emplacement. Largest deviations of the actual topographic position of the contact from the model geoids occur in the Tantalus and Phlegra regions where the deviations are due to the post-VBF changes of the regional topography. The fact that the model geoids satisfactory describe the shape of the largest portion of the contact provides additional evidence for both the emplacement of the VBF edges near an equipotential surface and for relative stability of the shape of Mars during a long time interval of about 3.6 Ga. Within the northern plains in the Tempe Terra, Acidalia Planitia, Cydonia-Deuteronilus, Pyramus-Astapus, and the southern Utopia regions, the absolute model ages of the VBF surface near the Deuteronilus contact are tightly clustered around the age of ∼3.6 Ga, which we interpret as the age of the VBF emplacement. The surface of the VBF-like Isidis Planitia unit is distinctly younger, ∼3.50 ± 0.01 Ga, which suggests that this unit formed independently. Neither volcanic nor glacial modes of emplacement are consistent with the topographic configuration and the shape of the Deuteronilus contact within both the northern plains and in Isidis Planitia. The broad flooding and formation of extensive water/mud reservoirs remains to be the most plausible mode of formation of the VBF in the northern plains and the VBF-like unit on the floor of the Isidis basin. [ABSTRACT FROM AUTHOR]

Kupalo is a ∼4 ​Ma old, 26-km diameter impact crater on dwarf planet Ceres, which exhibits extensive areas of bright bluish material. Here we describe, for the first time, the geology of Kupalo on a regional and local scale in detail, based on Dawn Framing Camera (FC) imagery. We find the crater has a complex geology consistent with a brittle and heterogeneous crust in this area. Through analyses of the FC colour data, we identify a correlation between the geologic units and the spectral variations, which can be explained by a mixture of subsurface materials in response to the impact. The brightest sites of Kupalo, located at the upper west wall and the central ridge, show similar FC colour and spectral IR data, which suggest that the bright material in these locations likely has the same origin. To explain the distribution of the bright bluish material in the crater and its vicinity, we propose two scenarios for the structure of the upper Cerean crust. Both require deep-seated brine or salt reservoirs, possibly connected to a brine ocean at the crust-mantle transition. • Geology of Kupalo suggests that Ceres' crust is rather heterogeneous (69). • Mixture of impacted subsurface materials explains correlation of spectra and geology (84). • Bright material in Kupalo would have originally been deposited at a depth of ∼3 ​km (82). • The brightest exposures are linked to subsurface reservoirs of hydrated carbonates (82). [ABSTRACT FROM AUTHOR]

Using SELENE/Kaguya Terrain Camera and Lunar Reconnaissance Orbiter Camera (LROC) data, we produced a new, high-resolution (10 m/pixel), geomorphological and impact melt distribution map for the lunar crater Tycho. The distal ejecta blanket and crater rays were investigated using LROC wide-angle camera (WAC) data (100 m/pixel), while the fine-scale morphologies of individual units were documented using high resolution (∼0.5 m/pixel) LROC narrow-angle camera (NAC) frames. In particular, Tycho shows a large coherent melt sheet on the crater floor, melt pools and flows along the terraced walls, and melt pools on the continuous ejecta blanket. The crater floor of Tycho exhibits three distinct units, distinguishable by their elevation and hummocky surface morphology. The distribution of impact melt pools and ejecta, as well as topographic asymmetries, support the formation of Tycho as an oblique impact from the W-SW. The asymmetric ejecta blanket, significantly reduced melt emplacement uprange, and the depressed uprange crater rim at Tycho suggest an impact angle of ∼25–45°. [ABSTRACT FROM AUTHOR]

Images of Mars taken by spacecraft in the last few decades indicate that the landscape has changed and that current processes are continuously changing the surface. The modifications of the landscape are caused by exogenic processes including eolian activity, mass movement, the growth and retreat of the polar caps, glacial processes and crater-forming impacts. In particular the High Resolution Stereo Camera (HRSC) on board Mars Express (MEx) and the Context Camera (CTX) on board the Mars Reconnaissance Orbiter (MRO) cover large areas at high resolution and thus are particularly well-suited to detect the extent and origin of surface changes on Mars. Multi-temporal observations of variable features on Mars became possible by the increasing number of repeated image acquisitions of the same surface areas. To support the investigation of surface changes that represents a key element in martian research, we developed MUTED, the “Multi-Temporal Database of Planetary Image Data”, which is a tool for the identification of the spatial and multi-temporal coverage of planetary image data from Mars. Using MUTED, scientists are able to identify the location, number, and time range of acquisitions of overlapping images from, for example, HRSC and CTX. MUTED also includes images from other planetary datasets such as those of the Mars Orbiter Camera (MOC), the Thermal Emission Imaging System (THEMIS), and the High Resolution Imaging Science Experiment (HiRISE). The database supports the identification and analysis of surface changes and short-lived surface processes on Mars based on fast automatic planetary image database queries. From the multi-temporal planetary image database and investigations based on multi-temporal observations we will better understand the interactions between the surface of Mars and external forces, including the atmosphere. MUTED is available for the planetary scientific community via the webpage of the Institut für Planetologie (IfP) Muenster. [ABSTRACT FROM AUTHOR]

Two of the most widely studied landforms that are associated with a putative ocean that filled the northern hemisphere of Mars are (1) the Vastitas Borealis Formation (VBF), plain units that cover a larger portion of the northern lowlands of Mars, and (2) a candidate paleoshoreline, e.g., the Deuteronilus contact, which represents the outer margin of the VBF. The VBF and the Deuteronilus contact are interpreted to result from a short-lived Late Hesperian ocean that readily froze and sublimated. Similar landforms are also present in the impact basin of Isidis Planitia and suggest formation processes comparable to those that formed the VBF and the Deuteronilus contact in the northern lowlands. Our study of the Deuteronilus contact in Isidis revealed geologic evidence that possibly supports the existence of a Late Hesperian/Early Amazonian Isidis Sea. For example, numerous valleys that are incised into the plains of the southern Isidis basin rim between 82°/90°E and 3°/6°N and trend a few tens of kilometers to the north following the general topographic gradient toward the center of Isidis Planitia. A few of them reach the Deuteronilus contact and continue as sinuous ridges in the Isidis Interior Plains (IIP). Based on our findings we conclude that the geologic setting along the Deuteronilus contact, including the valleys and ridges is a result of (1) Late Hesperian short-term fluvial activity, (2) a Late Hesperian/Early Amazonian short-lived Isidis Sea that readily froze, (3) subglacial drainage and esker formation, and (4) subsequent sublimation of the proposed Isidis ice sheet. Although the fluvio-glacial model we introduce in our manuscript cannot fully explain the geologic setting, possible alternative formation models, including relief inversion and fluvio-volcanic scenarios are even less capable in explaining the observed geologic setting along the Deuteronilus contact. [ABSTRACT FROM AUTHOR]

BepiColombo is a joint mission between the European Space Agency, ESA, and the Japanese Aerospace Exploration Agency, JAXA, to perform a comprehensive exploration of Mercury. Launched on 20 th October 2018 from the European spaceport in Kourou, French Guiana, the spacecraft is now en route to Mercury. Two orbiters have been sent to Mercury and will be put into dedicated, polar orbits around the planet to study the planet and its environment. One orbiter, Mio, is provided by JAXA, and one orbiter, MPO, is provided by ESA. The scientific payload of both spacecraft will provide detailed information necessary to understand the origin and evolution of the planet itself and its surrounding environment. Mercury is the planet closest to the Sun, the only terrestrial planet besides Earth with a self-sustained magnetic field, and the smallest planet in our Solar System. It is a key planet for understanding the evolutionary history of our Solar System and therefore also for the question of how the Earth and our Planetary System were formed. The scientific objectives focus on a global characterization of Mercury through the investigation of its interior, surface, exosphere, and magnetosphere. In addition, instrumentation onboard BepiColombo will be used to test Einstein's theory of general relativity. Major effort was put into optimizing the scientific return of the mission by defining a payload such that individual measurements can be interrelated and complement each other. [ABSTRACT FROM AUTHOR]

Abstract: We describe the results of our morphologic, stratigraphic and mineralogic investigations of fluvial landforms, paleolakes and possible shoreline morphologies at the Libya Montes/Isidis Planitia boundary. The landforms are indicative of aqueous activity and standing bodies of water, including lakes, seas and oceans, that are attributed to a complex hydrologic cycle that may have once existed on Mars in the Noachian (>3.7Ga) and perhaps also in the Hesperian (>3.1Ga). Our observations of the Libya Montes/Isidis Planitia boundary between 85°/86.5°E and 1.8°/5°N suggest, that (1) the termination of valley networks between roughly −2500 and −2800m coincide with lake-size ponding in basins within the Libya Montes, (2) an alluvial fan and a possible delta, layered morphologies and associated Al-phyllosilicates identified within bright, polygonally fractured material at the front of the delta deposits are interpreted to be the results of fluvial activity and discharge into a paleolake, (3) the Arabia “shoreline” appears as a series of possible coastal cliffs at about −3600 and −3700m indicating two distinct still stands and wave-cut action of a paleosea that temporarily filled the Isidis basin the Early Hesperian, and (4) the Deuteronilus “shoreline” appears at −3800m and is interpreted to be a result of the proposed sublimation residue of a frozen sea that might have filled the Isidis basin, similar to the Vastitas Borealis Formation (VBF) identified in the northern lowlands. We interpret the morphologic–geologic setting and associated mineral assemblages of the Libya Montes/Isidis Planitia boundary as results of fluvial activity, lake-size standing bodies of water and an environmental change over time toward decreasing water availability and a cold and dry climate. [Copyright &y& Elsevier]

Abstract: We investigated gullies and their relationships to the atmospherically derived dust–ice mantle and aeolian features in the northwestern part of the Argyre basin. A detailed morphologic map of the Argyre study region allowed us to constrain the stratigraphic relationships and relative ages of gullies. In addition, we investigated the morphologic characteristics and orientations of all gullies in the Argyre study region. Maximum absolute ages for gullies were determined with crater size–frequency distribution measurements of the dust–ice mantle, which is the source material of gullies in the study area. Gullies only evolve from this mantle probably by melting of its ice content. Two different morphologies of pristine and degraded gullies were identified, mostly occurring on pole- and equatorward-facing slopes, respectively. We conclude that the morphologies and orientations were initiated either by a more rapid and extensive erosion of equatorward-facing gullies or by at least two generations of gullies with generally older gullies on equatorward-facing slopes and younger ones on pole-facing slopes. Different intensities of solar insolation on equator- and pole-facing slopes might be responsible for the different development of pristine and degraded gullies. Gullies in the study area generally have ages ≲20Ma. Some uncratered (and thus very young) aeolian dunes are superposed by a few gullies in some locations, indicating another even younger generation of gullies with an upper limit absolute model age of about <500ka. [Copyright &y& Elsevier]

Abstract: We report on the first observations of bright dust devil tracks (BDDTs) on Earth, observed in the Turpan depression desert in northwestern China, where raindrop impacts on sand surfaces form aggregates of sand, silt and clay resulting in rough surface textures, which are destroyed by passages of dust devils leading to smooth surface textures within the tracks. The differences in photometric properties between the track and outside the tracks cause the albedo differences leading to the formation of BDDTs and similar processes might lead to BDDTs on Mars in areas with thick dust covers. [ABSTRACT FROM AUTHOR]

Abstract: “Dendritic valley networks” and broad “longitudinal valleys” are widespread within the eastern Libya Montes region. The morphologic characteristics of the studied dendritic valleys, especially their occurrence at local summits, provide evidence for the initial formation by surface runoff due to atmospheric precipitation whereas the longitudinal valley systems are thought to have been formed and modified by groundwater-induced processes. Our crater counts and model ages show, that the dendritic valley networks (average age ∼4.0 Ga) formed earlier than the longitudinal valley systems (average age ∼3.7 Ga). Both fluvial forms indicate a temporal change of the erosive environment at the Noachian/Hesperian transition at ∼3.8 Ga. Our results suggest that the fluvial activity within our investigation area lasted for ∼800 My, from ∼4.1 Ga to ∼3.3 Ga. The dendritic valleys show valley densities between 0.12 km−1 and 0.57 km−1, indicative of highly mature and integrated valley systems that we interpret to be characteristic of warmer and wetter conditions on Mars. [Copyright &y& Elsevier]

Abstract: It is widely accepted that lunar volcanism started before the emplacement of the mare fills ( b.p.) and lasted for probably more than 3.0Ga. While the early volcanic activity is relatively easy to understand from a thermal point of view, the late stages of volcanism are harder to explain, because a relatively small body like the Earth''s Moon is expected to cool rapidly and any molten layer in the interior should solidify rather quickly. We present several thermal evolution models, in which we varied the boundary conditions at the model surface in order to evaluate the influence on the extent and lifetime of a molten layer in the lunar interior. To investigate the influence of a top insulating layer we used a fully three-dimensional spherical shell convection code for the modelling of the lunar thermal history. In all our models, a partial melt zone formed nearly immediately after the simulation started (early in lunar history), consistent with the identification of lunar cryptomare and early mare basalt volcanism on the Moon. Due to the characteristic thickening of the Moon''s lithosphere the melt zone solidified from above. This suggests that the source regions of volcanic rock material proceeded to increasing depth with time. The rapid growth of a massive lithosphere kept the Moon''s interior warm and prevented the melt zone from fast freezing. The lifetimes of the melt zones derived from our models are consistent with basalt ages obtained from crater chronology. We conclude that an insulating megaregolith layer is sufficient to prevent the interior from fast cooling, allowing for the thermal regime necessary for the production and eruption of young lava flows in Oceanus Procellarum. [Copyright &y& Elsevier]

Ceres' regolith contains water ice that has receded in response to insolation‐driven sublimation. Specially targeted, high spatial‐resolution measurements of hydrogen by Dawn's Gamma Ray and Neutron Detector (GRaND) reveal elevated hydrogen concentrations in and around Occator, a young, 90 km diameter, complex crater located at 19.82°N where near‐surface ice is not expected. The excess hydrogen can be explained by impact excavation of water‐rich outer crustal materials and their emplacement in the crater floor and ejecta blanket. This is supported by thermophysical models that show water ice could survive at sub‐meter depths, given Occator's relatively young age (∼20 Myr). We hypothesize that the regolith can be replenished with ice from large impacts and that this process partially controls the distribution and depth of near surface ice. This is supported by results from Occator and similarities in the global distribution of hydrogen and the pattern of large craters (20–100 km diameter). Plain Language Summary: The outermost meter of dwarf planet Ceres contains water ice that is gradually sublimating in response to heating of the surface by sunlight. Since Ceres' axis of rotation is nearly perpendicular to the Sun's rays, ice has receded to greater depths at the equator than the poles. The distribution of subsurface ice within this outer layer was inferred from measurements of hydrogen by Dawn's Gamma Ray and Neutron Detector. Special operations during Dawn's last mission phase brought the spacecraft close to the surface, enabling measurements within and around a large, young crater called Occator. Anomalously high concentrations of hydrogen were detected, suggesting the impact that formed Occator excavated water rich materials from the crust and deposited them on the surface. Comparison of the global distribution of hydrogen with the pattern of large craters on Ceres further supports excavation of crustal ice by impacts as a partial control on the depth of ice near the surface. Results confirm that Ceres' crust is rich in water ice and show that ice can survive in materials ejected by impacts into airless, icy bodies. Key Points: Neutron spectroscopy reveals enhanced hydrogen concentrations in the outermost meter of the surface of a prominent young, complex craterResults confirm Ceres outer crust is ice rich and support retention of water ice within impact ejecta on airless, icy bodiesThe data imply partial control of regolith ice content by large impacts, relaxing constraints on surface age and regolith grain size [ABSTRACT FROM AUTHOR]

Slope streaks are gravity-driven landforms that occur throughout the martian year in dust-rich equatorial regions on Mars. The distinctive streaks predominantly originate from point sources showing no indication for a specific trigger mechanism. Rare observations revealed dust devil-related triggering of multiple slope streaks and thus indicate a dry triggering process. • Multiple examples for dust devil-triggered slope streaks indicate a dry triggering process. • Dust devil tracks crossing older streaks confirm fading by airfall dust. • Most slope streaks originate from point sources and show no indication for a specific trigger mechanism. [ABSTRACT FROM AUTHOR]

To detect the mineral diversity of a planet's surface, it is essential to study the spectral variations over a broad wavelength range at relevant simulated laboratory conditions. The MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) mission to Mercury discovered that irrespective of its formation closest to the Sun, Mercury is richer in volatiles than previously expected. This is especially true for sulfur (S), with an average abundance of 4 wt%. It has been proposed that sulfur in the interior of Mercury can be brought to the surface through volcanic activity in the form of sulfides as slag deposits in Mercury hollows and pyroclastic deposits. However, comprehensive spectral library of sulfide minerals measured under vacuum conditions in a wide spectral range (0.2–100 μm) was lacking. This affects the detectability and understanding of the distribution, abundance, and type of sulfides on Mercury using remote-sensing spectral observations. In the case of Mercury, the effect of thermal weathering affecting the spectral behavior of these sulfides must be studied carefully for their effective detection. In this study, we present a spectral library of synthetic sulfides including MgS, FeS, CaS, CrS, TiS, NaS, and MnS. For each sample, we performed emissivity measurements in the thermal infrared range (TIR: ∼7–14 μm) for sample temperatures from 100 °C–500 °C, covering the daytime temperature cycle on Mercury's surface. In addition, for each sample we measured the spectral reflectance of fresh and thermally processed sulfides over a wide spectral range (0.2–100 μm) and at four different phase angles, 26°, 40°, 60°, 80°. This spectral library facilitates the detection of sulfides by past and future missions to Mercury by any optical spectrometer of any spectral range. Specifically, the emissivity measurements in this study will support the Mercury Radiometer and Thermal Imaging Spectrometer (MERTIS) instrument on the ESA/JAXA BepiColombo mission, which will study the surface mineralogy over a wavelength range of 7–14 μm at a spatial resolution of 500 m/pixel. The measured reflectance of these sulfides in 0.2–100 μm at various phase angles will support the interpretation of measurements from past (MDIS, MASCS on MESSENGER) and future missions (SIMBIO-SYS on BepiColombo). • Emissivity of sulfides under simulated Mercury Daytime conditions is studied. • Reflectance of fresh and thermally processed sulfides at wide spectral range (0.2–100 μm) is measured. • Sulfides show strong and distinct spectral features in thermal IR (7–14 μm) spectral region. • UVVIS (0.3–0.6 μm) spectral region is immune to thermal weathering for sulfides. [ABSTRACT FROM AUTHOR]

Highlights • Banded terrain covers 30,000 km2 and occurs up to 240 km away from honeycomb terrain. • Grid map shows no regional band pattern and no correlation with local slope. • Buckled cusps and thermokarst do not support interpretation as deeply rooted material. • Esker- and sandur-like landforms indicative of formation in subglacial pressure field. Abstract Northwestern Hellas Planitia hosts landforms that are unique on Mars, e.g., the so called honeycomb and banded (aka "taffy pull") terrains. Recently, robust formation models for the ∼6 km large honeycomb depressions involving salt or ice diapirism have been formulated. However, the nature of the banded terrain, a ∼30,000 km² area characterized by a decameter- to kilometer-scale pattern of curvilinear troughs, has remained elusive. While previous interpretations range from deep-seated, honeycomb-related outcrops to a younger veneer, recent reports of putative periglacial features (e.g., potential thermokarst) strongly indicate it to be a relatively thin, volatile-related surface unit. In order to further constrain the origin and nature of the banded terrain, we investigated the northwestern Hellas basin floor employing various datasets. We mapped the banded terrain's extent at high precision, showing that it partially superposes the honeycomb terrain, but also occurs up to ∼240 km away from it. Via stratigraphic analyses and crater size-frequency measurements, we bracketed the age of the banded terrain between ∼1.9 and ∼3.7 Ga. Furthermore, the banded terrain can be differentiated into two types, ridged and creviced, with the former predominantly occurring among the lowest reaches of the terrain's ∼2 km topographic extent. We also produced a grid map (2 × 2 km box size) of the entire banded terrain and identified no large-scale (> 25 km) band pattern and no correlation between local slope and band orientation. Because of this, we submit that regional tectonics or gravity-driven flow down modern topography are unlikely to have played decisive roles for banded terrain formation. Instead, we observed numerous locations, where band slabs appear to have broken off and subsequently rotated, as well as "cusps" that seem to have resulted from buckling. Based on this, we suggest that the banded terrain experienced both, ductile deformation as well as brittle failure on or near the surface. Despite certain similarities, neither salt (as salt glaciers), lava sheets, or land-based glaciers are in agreement with the extensive curvilinear texture and topographic/geologic setting of the banded terrain. Ice shelf margins, on the other hand, can produce surface textures akin to the banded terrain in both form and scale, even including cusps and broken off, rotated blocks. However, an ice-covered sea between 1.9 and 3.7 Ga ago is not indicated by the geologic inventory of the Hellas basin, which previous investigations found to lack any landforms indicative of a standing body of water. Instead, we identified several sinuous ridges terminating at plains covered by smaller, braiding ridges, which we interpret as eskers and glacial sandurs, respectively. As both are embayed and partially covered by the banded terrain, we tentatively propose an alternative, subglacial model of the banded terrain having formed as wet till that was viscously deformed according to the stress fields created by the ice overburden pressure in conjunction with bed topography. Although this formation model remains inconclusive, it is in agreement with climate models suggesting obliquity excursions and a denser, early Amazonian atmosphere to have caused ice accumulation in the adjacent northwestern Hellas basin rim, thus potentially enabling flow onto the floor entailing subglacial banded terrain formation. [ABSTRACT FROM AUTHOR]

Highlights • Vinalia and Cerealia Faculae on Ceres have a common carbonate composition and a variety of morphologies and topographies. • We considered the ejection and deposition of carbonate grains by three possible materials: ice, gas and brine. • We find that brine eruption is the most likely formation mechanism explaining bright mantling and central structures. Abstract Vinalia and Cerealia Faculae are bright and salt-rich localized areas in Occator crater on Ceres. The predominance of the near-infrared signature of sodium carbonate on these surfaces suggests their original material was a brine. Here we analyze Dawn Framing Camera's images and characterize the surfaces as composed of a central structure, either a possible depression (Vinalia) or a central dome (Cerealia), and a discontinuous mantling. We consider three materials enabling the ascent and formation of the faculae: ice ascent with sublimation and carbonate particle lofting, pure gas emission entraining carbonate particles, and brine extrusion. We find that a mechanism explaining the entire range of morphologies, topographies, as well as the common composition of the deposits is brine fountaining. This process consists of briny liquid extrusion, followed by flash freezing of carbonate and ice particles, particle fallback, and sublimation. Subsequent increase in briny liquid viscosity leads to doming. Dawn observations did not detect currently active water plumes, indicating the frequency of such extrusions is longer than years. [ABSTRACT FROM AUTHOR]

Highlights • Absolute model formation ages of Occator crater range between 1.6 and 63.7 Ma. • Previous studies of Occator crater suggest age errors that are underestimated. • Impactor/crater scaling parameters have the largest influence on age dating results. • Ballistics on low gravity, quick rotating bodies allow for self-secondary cratering. Abstract The 90.5-km Occator crater, with its peculiar and unique bright spots, is one of the most prominent and renowned feature on Ceres. Occator attracted broad public attention in scientific media as it is proposed to exhibit signs of post-impact cryovolcanic activity. In order to understand the time sequence of deposition, several attempts were made during DAWN's primary mission by different research groups to date geomorphologic key units using superposed crater densities. Resulting absolute model formation ages for Occator's ejecta and its interior lobate deposits range from 200 Ma to 78 Ma and about 100 Ma to 6.9 Ma, but were based on different cratering chronology models, measurements on image data at varying resolution, and different statistical and methodological approaches. Here we present the results of a comprehensive approach of determining absolute model formation ages for Occator. This is achieved by using the best resolved Framing Camera image data, by careful treatment of secondary crater admixture and the natural variability of crater detection and sizes by different crater analysts, and by applying appropriate, objective criteria for count area selection. In this context, we evaluate previously published model ages and explain why our results are likely to yield more consistent and robust information about the formation age of Occator. We also show that, in contrast to previous publications, CSFDs measured on Occator's less competent ejecta blanket, and its more competent cryovolcanic- or impact melt related interior lobate deposits (ILDs), are so similar to each other, that the slight differences might rather be explained by different scaling parameters in different target materials or that the ILDs have formed almost contemporaneously or only shortly after the Occator forming impact. [ABSTRACT FROM AUTHOR]

Highlights • Four morphologic classifications of bright spots are mapped across Ceres' surface. • Most bright spots form via upwelling or excavation in association with impacts. • Most bright spots disappear from the surface over timescales of several hundred Ma. • Ceres' surface remains active and the near surface may still support brines. Abstract The otherwise homogeneous surface of Ceres is dotted with hundreds of anomalously bright, predominantly carbonate-bearing areas, termed "faculae," with Bond albedos ranging from ∼0.02 to >0.5. Here, we classify and map faculae globally to characterize their geological setting, assess potential mechanisms for their formation and destruction, and gain insight into the processes affecting the Ceres surface and near-surface. Faculae were found to occur in four distinct geological settings, associated predominantly with impact craters: (1) crater pits, peaks, or floor fractures (floor faculae), (2) crater rims or walls (rim/wall faculae), (3) bright ejecta blankets, and (4) the mountain Ahuna Mons. Floor faculae were identified in eight large, deep, and geologically young (asteroid-derived model (ADM) ages of <420 ± 60 Ma) craters: Occator, Haulani, Dantu, Ikapati, Urvara, Gaue, Ernutet, and Azacca. The geometry and geomorphic features of the eight craters with floor faculae are consistent with facula formation via impact-induced heating and upwelling of volatile-rich materials, upwelling/excavation of heterogeneously distributed subsurface brines or their precipitation products, or a combination of both processes. Rim/wall faculae and bright ejecta occur in and around hundreds of relatively young craters of all sizes, and the geometry of exposures is consistent with facula formation via the excavation of subsurface bright material, possibly from floor faculae that were previously emplaced and buried. A negative correlation between rim/wall facula albedo and crater age indicates that faculae darken over time. Models using the Ceres crater production function suggest initial production or exposure of faculae by large impacts, subsequent dissemination of facula materials to form additional small faculae, and then burial by impact-induced lateral mixing, which destroys faculae over timescales of less than 1.25 Gyr. Cumulatively, these models and the observation of faculae limited to geologically young craters indicate relatively modern formation or exposure of faculae, indicating that Ceres' surface remains active and that the near surface may support brines in the present day. [ABSTRACT FROM AUTHOR]

Highlights • We produced a geologic map Ceres' Kerwan quadrangle to determine regional geologic history. • Kerwan is the oldest, largest (∼284 km) undisputed impact crater on Ceres. • It marks a major cerean time-stratigraphic event and between Kerwanan and Pre-Kerwanan units. • A smooth material in and around Kerwan suggests impact melting of a crust rich in water ice. Abstract We conducted a geologic mapping investigation of Dawn spacecraft data to determine the geologic history of the Kerwan impact basin region of dwarf planet Ceres, which is mostly located in the Ac-7 Kerwan Quadrangle. Geological mapping was applied to Dawn Framing Camera images from the Low Altitude Mapping Orbit (LAMO, 35 m/pixel) and supplemented by digital terrain models and color images from the High Altitude Mapping Orbit (HAMO, 135 m/pixel), as well as preliminary Visible and Infrared Spectrometer (VIR) and gravity data. The 284-km diameter Kerwan impact basin is the oldest unequivocal impact crater on Ceres, and has a highly discontinuous, polygonal, degraded rim and contains a 'smooth' unit that both fills the basin floor and surrounds the degraded rim to the west, south, and east. Although there are some subtle topographic features in the Kerwan basin that could be interpreted as flow boundaries, there is no indisputable evidence of cryovolcanic features in or around the basin (however if such features existed they could be buried). Nevertheless, all data point to impact-induced melting of a cerean crust enriched in a volatile, likely water ice, to produce the Kerwan smooth material. Subsequent geologic activity in this region includes emplacement of impact craters such as Dantu, which produced a variety of colorful deposits, and rayed craters such as Rao and Cacaguat. Based on the crater size-frequency distribution absolute model ages of the Kerwan smooth material in and around the basin, marking a minimum age for the Kerwan basin, our mapping defines this as the oldest boundary within the cerean geologic timescale, separating the Pre-Kerwanan and Kerwanan Periods at > 1.3 Ga (Lunar-derived chronology model) or > 230–850 Ma (Asteroid-derived chronology model, depending on strength of target material). [ABSTRACT FROM AUTHOR]

Abstract Twelve landing ellipses (15 × 30 km each) were previously selected during the preliminary science and safety assessment of details of the Russian Luna-Glob mission. In our work, we have conducted a photogeological and topographical study of the top three most desirable landing ellipses (ellipses 1, 4, and 6) and modelled the thickness of the lunar basin ejecta at these locations in order to characterize their safety conditions and possible sources of materials. For all three potential landing sites large (>0.3–0.5 m) boulders do not represent a significant threat to the mission. The major safety concerns are small but abundant craters whose walls are steeper than the technical constraint of 7°. We have collected the data on density, areal fraction, and the mean distance between the neighboring steep-wall craters, which allow us to rank the safety of terrains within each landing ellipse. A set of morphologically and topographically distinctive units make up the surface of the landing ellipses. The units form two groups, the hilly units and the flat plains. The hilly units within the ellipses 1 and 6 are among the safest terrains. The flat plains of ellipses 1 and 4 represent the most unsafe surfaces. The ejecta of the South Pole-Aitken basin (SPA) strongly dominate the Luna-Glob landing zone and constitute ∼95% (by volume) of materials than can be encountered and analyzed within the landing ellipses. For example, the hilly units in the ellipses 1 and 6 likely represent materials of the SPA ejecta. In the ellipse 4, the hilly unit corresponds to the ejecta of Moretus crater and may represent materials excavated from below the SPA ejecta blanket. However, the small area and configuration of the hilly unit in ellipse 1 and abundant small steep-wall craters on the hilly unit in ellipse 4 make them either a difficult or an unsafe target to land. The flat plains in ellipses 1 and 4 are the most unsafe terrains to land because of the abundant steep-wall craters on their surfaces. Materials ejected by the large craters Boguslawsky and Boussingault from the lower portions of the SPA ejecta blanket make up the majority of the hilly unit in the landing ellipse 6. This unit is among the safest terrains in all top three landing ellipses of the Luna-Glob mission. Highlights • The sources of materials in the top-three (1, 4, 6) Luna-Glob landing sites were analyzed. • The ejecta of the SPA basin dominate the entire Luna Glob landing area. • Flat plains in ellipses 1 and 4 may represent materials ejected from below the SPA ejecta. • Flat plains in ellipses 1 and 4 are the most unsafe units to land. • Units in ellipse 6 may represent materials from the lower portions of the SPA ejecta. [ABSTRACT FROM AUTHOR]

Abstract: The geologic context of red organic‐rich materials (ROR) found across an elongated 200 km region on Ceres is evaluated with spectral information from the multispectral framing camera (FC) and the visible and near‐infrared mapping spectrometer (VIR) of Dawn. Discrete areas of ROR materials are found to be associated with small fresh craters less than a few hundred meters in diameter. Regions with the highest concentration of discrete ROR areas exhibit a weaker diffuse background of ROR materials. The observed pattern could be consistent with a field of secondary impacts, but no appropriate primary crater has been found. Both endogenic and exogenic sources are being considered for these distinctive organic materials. [ABSTRACT FROM AUTHOR]

Impact crater size-frequency measurements are key to date and investigate surface processes on terrestrial planetary bodies. Using high-resolution images of numerous space missions, various regional and global crater datasets of the Moon and other planetary bodies have been created in recent years. In addition, machine-learning techniques are increasingly being used to automatically detect craters and to create extensive crater catalogs. Accurate map projection-independent measurement of crater sizes is essential for an accurate dating of the investigated surface. Thus, based on the work of Kneissl et al. (2011) on CraterTools for the geographic information system ArcGIS, we developed the OpenCraterTool as an open source QGIS extension. Our tool allows a map projection-independent measurement and comparison of craters. Results can be visualized within the tool or exported for further external analysis. With our new tool, we compared five global lunar crater datasets; three datasets based on manual crater measurements and two machine-learning products. The comparison revealed substantial differences in both the number of craters and crater diameters. In particular, two catalogs have about three times as many craters as the two smaller catalogs in the analyzed crater diameter range (>5 ​km). The compared craters revealed mean relative differences of 0.4 and 2.3% between the respective catalogs. As expected, the best agreements were found between the manual and automatically generated crater catalogs, where the manual counts were used as training datasets. However, the comparison of the machine-learning products revealed the lowest agreements between themselves. Although the mean relative differences between all the catalogs are small, the number of outliers, as well as the number of under- and overcounted craters over the entire diameter range, are high between all compared catalogs. The differences in crater diameter, as well as the number of over- and undercounted craters have noticeable effects on the age determination of the surfaces. • The OpenCraterTool is an open source extension for QGIS to measure and compare craters. • Our tool enables a map projection-independent measurement of crater sizes as basis for accurate dating of planetary surfaces. • We compared five global crater catalogs of the Moon that result from both manual and machine-learning based methods. • The comparison revealed substantial differences in both the number of craters and crater diameters. [ABSTRACT FROM AUTHOR]

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

Crater size-frequency distributions on the ejecta blankets of Aristarchus and Tycho Craters are highly variable, resulting in apparent absolute model age differences despite ejecta being emplaced in a geologic instant. Crater populations on impact melt ponds are a factor of 4 less than on the ejecta, and crater density increases with distance from the parent crater rim. Although target material properties may affect crater diameters and in turn crater size-frequency distribution (CSFD) results, they cannot completely reconcile crater density and population differences observed within the ejecta blanket. We infer from the data that self-secondary cratering, the formation of impact craters immediately following the emplacement of the continuous ejecta blanket by ejecta from the parent crater, contributed to the population of small craters (< 300 m diameter) on ejecta blankets and must be taken into account if small craters and small count areas are to be used for relative and absolute model age determinations on the Moon. Our results indicate that the cumulative number of craters larger than 1 km in diameter per unit area, N (1), on the continuous ejecta blanket at Tycho Crater, ranges between 2.17 × 10 −5 and 1.0 × 10 −4 , with impact melt ponds most accurately reflecting the primary crater flux ( N (1) = 3.4 × 10 −5 ). Using the cratering flux recorded on Tycho impact melt deposits calibrated to accepted exposure age (109 ± 1.5 Ma) as ground truth, and using similar crater distribution analyses on impact melt at Aristarchus Crater, we infer the age of Aristarchus Crater to be ∼280 Ma. The broader implications of this work suggest that the measured cratering rate on ejecta blankets throughout the Solar System may be overestimated, and caution should be exercised when using small crater diameters (i.e. < 300 m on the Moon) for absolute model age determination. [ABSTRACT FROM AUTHOR]