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Abstract The dwarf planet Ceres is a heavily cratered rocky body, and complex craters with a central peak are widely observed on its surface. These types of craters form when a large body impacts the surface, generating extreme temperatures and pressures. During the impact event a large volume of rock is raised from the subsurface and a central uplift is formed. The material composing the central uplift is called crater central peak material (ccp) and the spectral analysis of such geologic areas can provide information about the composition of Ceres' subsurface. Reflectance spectra of 32 ccps, acquired by the VIR spectrometer on board the NASA/Dawn spacecraft, were analyzed and shows absorption bands located at about 2.7, 3.1, 3.4 and 4.0 µm which are also common on the Cerean surface. These absorptions are related, respectively, to Mg-phyllosilicates, NH 4 -phyllosilicates and Mg/Ca-carbonates. The spectral parameters considered in this work are: spectral slopes estimated between 1.2 µm and 1.9 µm, band depths at 2.7-, 3.1-, 3.4- and 4.0-µm, and band centers near 4.0-µm. The ccps spectral parameters were analyzed in conjunction with other Cerean parameters, such as the estimated depth of excavation of the material composing the central peak, in order to search for correlations and information about Ceres' subsurface. Central peak material located polewards show stronger 2.7- and 3.1-µm band depths with respect to those at the equatorial region, suggesting that subsurface deposits closer to poles are probably richer in Mg- and NH 4 -phyllosilicates. The 3.4-µm spectral feature is also deeper in ccps located at poleward latitudes, similar to the phyllosilicates. Conversely, the 4.0-µm band does not show this trend with latitude. An increase in both 3.1- and 3.4-µm band depths with the estimated depth of excavation indicates that the spectral feature at 3.4-µm is the result of different contributions from carbonates and NH 4 -phyllosilicates, as expected. However, depending on their relative influence, the shape of the 3.4-µm spectral feature can vary. Phyllosilicates and carbonates are the resulting products of aqueous alteration of chondritic material and, given the increasing abundance of such minerals (in particular ammoniated phyllosilicates) with depth of excavation, it is likely that our investigation involved subsurface layers nearby the boundary between the volatile-rich crust and the silicate-rich mantle. Na-carbonate is found in the crater central peak material of Ernutet, Haulani and Ikapati, characterized by an estimated depth of excavation of about 6–9 km, where deposits of sodium carbonates could be locally present. [ABSTRACT FROM AUTHOR]

Highlights • Spectral analysis of Rongo quadrangle reveals a heterogeneous composition. • The two band depths at 2.7 and 3.1 µm display an overall low correlation. • Localized Na-carbonates units have been observed. • A correlation between the main geologic units and the Mg-phyllosilicates is found. Abstract Quadrangle Ac-H-10 'Rongo' (Lat 22°S to 22°N, Lon 288°–360°E) shows a fairly homogeneous topography, with the presence of notable elevations such as Ahuna Mons, Liberalia Mons , and part of Samhain and Uhola Catenae. The deepest areas correspond to the Rongo crater region, the areas between Samhain and Uhola catenae, and the region of the quadrangle south of Ahuna Mons. A substantial variability in the 2.7-µm band depth distribution is observed across the Rongo quadrangle, indicating an east-west gradient in the abundance of Mg-phyllosilicates. The NH 4 -phyllosilicates distribution appears quite homogeneous, except some localized regions, such as crater Haulani's ejecta, the flanks of Ahuna Mons, and crater Begbalel. The two band depths at 2.7 and 3.1 µm display an overall low correlation, suggesting a variable degree of mixing between Mg-phyllosilicates and NH 4 -phyllosilicates. At the local scale, mineralogical phases other than phyllosilicates are observed. Quadrangle Rongo includes sodium carbonate-rich regions, such as the flanks of Ahuna Mons, the ejecta of Xevioso crater located in the southern edge of Liberalia Mons, and crater Begbalel, which often display a reduction in both the 2.7- and 3.1-µm band depths, associated with an increased band depth at ∼4 µm, related to the presence of Na-rich carbonate phases. This suggests recent hydrothermal activity in this area, due to several episodes of cryovolcanism, or impacts that unveiled a peculiar composition in the shallow subsurface. Alternatively, the crust in this region might show a variable degree of compactness, such that the formation of Na-carbonates is favored only in specific locations (De Sanctis et al., 2016; Ruesch et al., 2016; Zambon et al., 2017). From a geological standpoint, quadrangle Ac-H-10 Rongo shows a correlation between its two main geologic units (Platz et al., 2017) and the distribution of Mg-phyllosilicates, suggesting a link between geology and mineralogy in this area. [ABSTRACT FROM AUTHOR]

Highlights • Spectral parameters of Urvara and Yalode quadrangles are mapped. • Band depths at 2.7 and 3.1 µm are shallower than western latitudes. • Correlation between geology and mineralogy arises only in the Yalode quadrangle. • The central peak of Urvara crater is enriched in ammonium. • Hummocky crater floors are darker because of their different roughness. Abstract We studied the distribution in the Urvara–Yalode region of Ceres (latitudes 21–66°S, longitudes 180–360°E) of main spectral parameters derived from the VIR imaging spectrometer onboard the NASA/Dawn spacecraft, as an overall study of Ceres mineralogy reported in this special issue. In particular, we analyzed the distribution of reflectance at 1.2 µm, band depth at 2.7 and 3.1 µm, ascribed to magnesium and ammoniated phyllosilicates, respectively. Whereas the average band depths of this region are lower than eastern longitudes, reflecting the E-W dichotomy of abundance of phyllosilicates on Ceres, spectral variations inside this region are observed in the following units: (a) the central peak of the Urvara crater (45.9°S, 249.2°E, 170 km in diameter), showing a deep 3.1 µm band depth, indicating an ammonium enrichment; (b) the cratered terrain westwards of the Yalode basin (42.3°S, 293.6°E, 260 km in diameter), where absorption bands are deeper, probably due to absence of phyllosilicates depletion following the Yalode impact; (c) the hummocky cratered floor of Yalode and Besua (42.4°S, 300.2°E) craters, characterized by lower albedo and band depths, probably due to different roughness; (d) Consus (21°S 200°E) and Tawals (39.1°S, 238°E) craters, whose albedo and band depths decreasing could be associated to different grain size or abundance of dark materials. Twenty-two small scale (i.e., lower than 400 m) bright spots are observed: because their composition is similar to the Ceres average, a strong mixing may have occurred since their formation. [ABSTRACT FROM AUTHOR]

Highlights • We present a method for thermal removal from Ceres spectra. • Most of the crater floors are devoid of volatiles due to the impact events. • Na-carbonates on Ikapati crater are likely preexistent to impact event. • We present the map of abundance of organic aliphatic material in Ernutet crater. • Organic rich-regions show different compositions pointing to a complex environment. Abstract Ceres has been explored by NASA/Dawn spacecraft, which allowed for the discovery of the main mineralogical and compositional characteristics of Ceres' surface. Here, we use mainly data from the Visible and InfraRed imaging spectrometer (VIR) in order to investigate the main spectral characteristics of the quadrangle Ac-H-2 Coniraya, one of the 15 quads in which Ceres' surface has been divided. Coniraya quadrangle is characterized by the presence of mostly highly degraded impact craters of diameters between 50 and 200 km and clusters of small to midsize impact craters. Although the composition over the quadrangle appears to be quite uniform, significant differences have been detected between different craters by spectral parameters analysis and spectral modeling. Ernutet crater presents two regions with very peculiar band at 3.4 µm, typical of organics aliphatic material. One region result to be correlated with larger amount of carbonates, the other region does not present such correlation. Ikapati crater shows strong absorption bands at 4.0 µm, indicating the presence of Na-carbonates in the floor and ejecta. Ikapati, Gaue and other craters present smaller spectral features of NH 4 and/or OH stretching, suggesting a volatile depletion process induced by the heating of the impact event. [ABSTRACT FROM AUTHOR]

Highlights • Dantu is a key feature in the investigation of Ceres' surface composition. • Dantu's surface composition provides insight into the composition of Ceres' crust. • Carbonates on Dantu's floor exhibit a similar composition as Occators' bright spots. Abstract Mapping Ceres' surface composition in the Dantu region, located between 21°–66°N and 90°–180°E, offers the unique possibility to investigate changes in the surface composition related to different stratigraphic levels of Ceres' crust. Dantu is located in a huge depression named Vendimia Planitia, which possibly represents a completely degraded impact basin formed in the beginning of Ceres' geological history. Most parts of this depression are characterized by strong phyllosilicate absorptions, which are stronger than elsewhere on Ceres' surface. This spectral signature possibly is related to the material emplaced at the time of the Vendimia impact event excavating material from deeper regions of Ceres' crust. Subsequent impacts in this basin reach far deeper into Ceres' crust than any impact events outside of Vendemia Planitia, which could explain the spectral signature of Dantu, possibly pointing to a higher concentration of ammonium-bearing phyllosilicates in Ceres' deeper crust. Spectral differences with respect to the small fresh craters on Dantu's floor are probably related to grain size effects causing a bluish visible slope as observed by fresh impact craters on other places on Ceres. The local enrichment of carbonates in the Dantu area could also be associated with the impact event and may have been formed by additional impact-triggered and/or post-impact alteration processes. [ABSTRACT FROM AUTHOR]

Reflectance spectra (0.3–4 μm) of powder samples of the third largest stone C0002 returned from the asteroid Ryugu by JAXA Hayabusa2 mission were measured and compared with proximity observation data taken by the Optical Navigation Camera - Telescopic (ONC-T) and the Near-Infrared Spectrometer (NIRS3) onboard the Hayabusa2 spacecraft. Although the characteristics of the 2.7 μm absorption band due to structural hydroxyl of the Ryugu powder samples are consistent with CI1 chondrite meteorites, NIRS3 spectra of the global surface of Ryugu exhibit a wavelength shift of +6 nm in the 2.7 μm absorption band center in comparison with those of both the powder samples and an artificial crater on the asteroid formed by the Small Carry-on Impactor (SCI). This is consistent with effects of solar wind-induced space weathering being present across the global surface of asteroid Ryugu. The darkness of the Ryugu surface in comparison with the returned samples and an upturn in reflectance observed in the average ONC-T spectrum at ultraviolet wavelengths may also be due to space weathering. • Analysis of the 2.7 μm band of asteroid Ryugu samples confirms its composition as CI1 chondrite. • A shift in the 2.7 μm band position of Ryugu surface indicates space weathering by solar wind. • The rate of space weathering on Ryugu is faster than the lifetime of its optical surface material. [ABSTRACT FROM AUTHOR]

We use observations of Bennu's impact craters, and measurements of particle sizes seen in images from the OSIRIS-REx (the Origins, Spectral Interpretation, Resource Identification Regolith Explorer sample-return mission) sampling event, to conclude there is a ∼ global sub-surface layer of finer-grained material on Bennu. The layer is evident in particle measurements from areas inside and around craters, roughness assessments of crater interiors vs. surrounding terrain, crater colors, crater depth-to-diameter (d/D) ratios, and the global crater size-frequency distribution. All data sets show a transition in behavior at crater diameters between 20 m and 40 m, with the most abrupt changes at ∼25 m diameter. Based on the depth of ∼25 m diameter craters, we infer that the layer is 1–4 m thick. The reddest craters are the youngest craters, and there is a trend to older ages as redness decreases. The decrease in redness is characterized by an initial, rapid fade, followed by a slower fade. The reduction in expected crater abundance with diameters ∼40–80 m, when extrapolating from larger sizes, is consistent with impact armoring caused by deeper, large sub-surface boulders depressing crater diameter. The existence of the layer appears to be a natural byproduct of impacts across a wide energy range and can be described as a "punctuated equilibrium": small, frequent impacts contribute to the local production of finer-grained particles and locally mix smaller particles into the subsurface; larger, less-frequent large impacts eject the fine-grained material, exposing interior boulders, and resetting the finer-grained particle population in that area. Other processes, e.g. thermal fracturing, could also contribute to the production of fine-grained material on rapid timescales. Extrapolating to the broader asteroid population and considering the production and preservation of small particles is related to target strength, surface acceleration, and impact speeds, we expect to find such layers on other asteroids, and that the layer thickness and depth will vary with composition, size, and orbital parameters. • Impact crater features on asteroid (101955) Bennu indicate subsurface layering. • Particles visible during the sampling event at Bennu support this conclusion. • A near-surface layer consists of finer-grained particles, and is 1–4 m thick. • We predict a layer of finer-grained particles should exist on other asteroids. • Layering has implications for the evolution of asteroid surfaces and interiors. [ABSTRACT FROM AUTHOR]

In this paper, we present a global database of craters larger than 0.7 km in diameter on the surface of (4) Vesta, which we created based on a global mosaic of the Dawn spacecraft's high-altitude mapping orbit (HAMO) images at a resolution of ∼60 m/pixel and cross-checked using the images from the low-altitude mapping orbit (LAMO) mission phase. The method to produce the crater database consists of visual crater identification and mathematical shape determination. To give precise measurements, topographic corrections have also been considered by combining the digital elevation model (DEM) data in this global survey. We calculated the morphometric parameters of the craters, which consist of diameter, depth, depth-to-diameter ratio, ellipticity, and azimuth of the major axis. Our results show that the craters on (4) Vesta are very shallow, with a mean depth-to-diameter ratio of ∼0.065, which is similar to that of the craters on (1) Ceres and (25143) Itokawa but lower than for craters on other solid bodies. The results also show that the fractions of craters with an ellipticity greater than 1.1 or 1.2 are smaller than those of craters on the Martian surface, which is probably because few secondaries exist on the surface of (4) Vesta. In addition, the azimuths of the major axis are almost uniformly distributed in different directions. Our globally consistent crater database has many potential applications for future studies such as geologic mapping and surface age dating. [ABSTRACT FROM AUTHOR]

Viscously relaxed craters provide a window into the thermal history of Ganymede, a satellite with copious geologic signs of past high heat flows. Here we present measurements of relaxed craters in four regions for which suitable imaging exists: near Anshar Sulcus, Tiamat Sulcus, northern Marius Regio, and Ganymede's south pole. We describe a technique to measure apparent depth, or depth of the crater with respect to the surrounding terrain elevation. Measured relaxation states are compared with results from finite element modeling to constrain heat flow scenarios [see companion paper: Bland et al. (2017)]. The presence of numerous, substantially relaxed craters indicates high heat flows—in excess of 30–40 mW m −2 over 2 Gyr, with many small (<10 km in diameter) relaxed craters indicating even higher heat flows. Crater relaxation states are bimodal for some equatorial regions but not in the region studied near the south pole, which suggests regional variations in Ganymede's thermal history. [ABSTRACT FROM AUTHOR]

Dwarf planet Ceres, the largest object in the Main Asteroid Belt, has a surface that exhibits a range of crater densities for a crater diameter range of 5–300 km. In all areas the shape of the craters’ size-frequency distribution is very similar to those of the most ancient heavily cratered surfaces on the terrestrial planets. The most heavily cratered terrain on Ceres covers ∼15% of its surface and has a crater density similar to the highest crater density on <1% of the lunar highlands. This region of higher crater density on Ceres probably records the high impact rate at early times and indicates that the other 85% of Ceres was partly resurfaced after the Late Heavy Bombardment (LHB) at ∼4 Ga. The Ceres cratering record strongly indicates that the period of Late Heavy Bombardment originated from an impactor population whose size-frequency distribution resembles that of the Main Belt Asteroids. [ABSTRACT FROM AUTHOR]

We have simulated in-orbit variations of the impact flux and spatial distributions of >100 km diameter ( D ) crater production for Mercury in its current 3:2 and hypothetical 2:1 and 1:1 spin–orbit resonances. Results show that impact fluxes and D > 100 km cratering are non-uniform for these rotational states when Mercury's orbit is significantly eccentric. Variations in the impact flux and D > 100 km cratering depend on the orbital elements of Mercury and its impactors. The observed spatial distribution of large Mercurian craters is difficult to generate by cratering in Mercury's current 3:2 spin–orbit resonance, but can be produced by cratering in a former 1:1 (as previously proposed by Wieczorek et al., 2012 ) or 2:1 spin–orbit resonance. We have calculated capture probabilities at spin–orbit resonances for a rigid Mercury. If Mercury's initial rotation was prograde, we find that a higher order spin–orbit resonance is the most likely first capture for feasible (low) values of Mercury's past triaxiality. In light of Mercury's crater record, we examined the possibility that impacts have initiated transitions in past spin–orbit resonances. Although the number of craters whose generating impact would have destabilized a spin–orbit resonance is sensitive to the crater scaling procedure, any initial rotational state of Mercury has likely been destabilized by impacts. An initial and permanent 3:2 spin–orbit resonance capture seems untenable. Mercury's tidal torque decelerates Mercury's rotation for the most likely range of Mercury's orbital eccentricity. Only one or two craters are candidate relics of an impact-event that facilitates an instantaneous transition from a former synchronous rotation to the 3:2 spin–orbit resonance, and only for a small crater scaling factor. We propose a rotational evolution trajectory for Mercury with visits to spin–orbit resonances of decreasing order including a substantial period in the 2:1 spin–orbit resonance, which can account for the observed spatial distribution of large craters. [ABSTRACT FROM AUTHOR]

This paper proposes the use of Transfer Learning from the Moon to Ceres for real-time autonomous crater detection operations on asteroids. This approach, based on the use of a U-Net for crater segmentation, starts with the network training on a Moon Digital Elevation Model; the trained model is then refined using two different strategies, one based on data augmentation and one based on fine tuning with small sets of Ceres images. Data from the DAWN mission was used, while the reference crater catalogue for Ceres is the one created by Zeilnhofer from LAMO data, containing 44,594 craters with diameter greater than 1 km in a latitude range of 84.66°S–89.62°N, full longitude. The loss function chosen during network training is the Focal Tversky Loss, which takes into consideration the imbalanced distribution of class pixels in the ground truth (pixels corresponding to crater rims represent approximately only 3% over the whole dataset). For both the Moon and Ceres, the U-Net output is post-processed with a template matching algorithm. The detected craters are counted and compared to the crater catalogue locations in order to compute metrics performances (precision, recall, F1 score) in the crater domain. Post-processing on the Moon reached performance of 84.12% F 1 score on the baseline model. The baseline and augmented models gave a lower performance on Ceres data, although comparable with the outcomes found in other works, with a best F 1 score of 70.17%. However, when the model is completely retrained starting from the baseline, it ends up with a best F 1 score of 79.96%. Finally, a strategy for the actual on-board implementation is proposed considering the NVIDIA Jetson TX2 accelerator. We demonstrated that the inference time of the model on the accelerator is on the order of few milliseconds without loss of performances, thus proving the possibility to consider this approach for future real-time on-board navigation systems. • U-Net training on Moon DEM data and TL on Ceres for autonomous crater detection. • Loss-based imbalanced class training for segmentation performance optimization. • Application of U-Net for asteroid crater detection for the first time. • On-board algorithm implementation in real-time on the Nvidia GPU TX2. [ABSTRACT FROM AUTHOR]

Europa is an attractive target for future lander missions due to its dynamic surface and potentially habitable sub-surface environment. Seismology has the potential to provide powerful new constraints on the internal structure using natural sources such as faults or meteorite impacts. Here we predict how many meteorite impacts are likely to be detected using a single seismic station on Europa to inform future mission planning efforts. To this end, we derive: (1) the current small impactor flux on Europa from Jupiter impact rate observations and models; (2) a crater diameter versus impactor energy scaling relation for icy moons by merging previous experiments and simulations; and (3) scaling relations for seismic signal amplitudes as a function of distance from the impact site for a given crater size, based on analogue explosive data obtained on Earth’s ice sheets. Finally, seismic amplitudes are compared to predicted noise levels and seismometer performance to determine detection rates. We predict detection of 0.002–20 small local impacts per year based on P-waves travelling directly through the ice crust. Larger regional and global-scale impact events, detected through mantle-refracted waves, are predicted to be extremely rare (10 − 8 –1 detections per year), so are unlikely to be detected by a short duration mission. Estimated ranges include uncertainties from internal seismic attenuation, impactor flux, and seismic amplitude scaling. Internal attenuation is the most significant unknown and produces extreme uncertainties in the mantle-refracted P-wave amplitudes. Our nominal best-guess attenuation model predicts 0.002–5 local direct P detections and 6 × 10 − 6 –0.2 mantle-refracted detections per year. Given that a plausible Europa landed mission will only last around 30 days, we conclude that impacts should not be relied upon for a seismic exploration of Europa. For future seismic exploration, faulting due to stresses in the rigid outer ice shell is likely to be a much more viable mechanism for probing Europa’s interior. [ABSTRACT FROM AUTHOR]

Ejecta velocity distribution is an important property for controlling asteroid surface evolution and for changing the size frequency distribution of asteroids and planetary dusts. Recent asteroid explorations revealed that boulders on an asteroid surface had a wide size frequency distribution. On the other hand, many studies on ejecta velocity distribution for cratering experiments used fine-grained homogeneous targets. Thus, to study the ejection process of various-sized boulders on rubble-pile asteroids, we conducted impact experiments using gas guns at impact velocities of 100 m s−1 to 4 km s−1 on targets with various-sized glass beads, and analyzed boulder trajectories in three dimensions to clarify the effect of grain size on ejection velocity distribution. The results showed that the ejection velocity, v 0 , decreased as the bead size increased, and the ejecta velocity scaling law was improved to v 0 gR = k 2 ′ r 0 + a R − 1 μ ′ including the bead radius, a ; r 0 is the initial position of the bead, g is the gravitational acceleration, R is the crater radius, and k 2 ′ and μ ′ are, respectively, 0.58 ± 0.02 and 0.62 ± 0.02 for the low-impact velocity range (<200 m s−1) and 0.61 ± 0.07 and 0.57 ± 0.04 for the high-impact velocity range (>1 km s−1). Using our improved ejecta velocity scaling law, we calculated the landing points of ejected boulders and concluded that boulders with radii >0.34 R could not be ejected outside the final crater. Moreover, when the Urashima crater on asteroid 162173 Ryugu was formed on the surface, boulders up to 64 m in diameter may have been ejected beyond the escape velocity of Ryugu to become tiny monolithic asteroids. • Cratering experiments were done on simulated rubble-pile surface on asteroids. • Ejecta velocity distributions for various sized boulders were obtained. • Crater scaling law for ejecta velocity was improved considering the boulder size. • The size of largest boulder ejected outside of the crater is 0.34 crater radius. • Urashima crater on Ryugu might eject a boulder <70 m to be a tiny asteroid. [ABSTRACT FROM AUTHOR]

In 2016 NASA will launch the InSight discovery-class mission, which aims to study the detailed internal structure of Mars for the first time. Short- and long-period seismometers form a major component of InSight’s payload and have the potential to detect seismic waves generated by meteorite impacts. Large globally detectable impact events producing craters with diameters of ∼ 100 m have been investigated previously and are likely to be rare (Teanby, N.A., Wookey, J. [2011]. Phys. Earth Planet. Int. 186, 70–80), but smaller impacts producing craters in the 0.5–20 m range are more numerous and potentially occur sufficiently often to be detectable on regional scales (≲1000 km). At these distances, seismic waves will have significant high frequency content and will be suited to detection with InSight’s short-period seismometer SEIS-SP. In this paper I estimate the current martian crater production function from observations of new craters (Malin, M.C. et al. [2006]. Science 314, 1573–1577; Daubar, I.J. et al. [2013]. Icarus 225, 506–516), model results (Williams, J.P., Pathare, A.V., Aharonson, O. [2014]. Icarus 235, 23–36), and standard isochrons (Hartmann, W.K. [2005]. Icarus 174, 294–320). These impact rates are combined with an empirical relation between impact energy, source-receiver distance, and peak seismogram amplitude, derived from a compilation of seismic recordings of terrestrial and lunar impacts, chemical explosions, and nuclear tests. The resulting peak seismogram amplitude scaling law contains significant uncertainty, but can be used to predict impact detection rates. I estimate that for a short-period instrument, with a noise spectral density of 10 −8 ms −2 Hz −1/2 in the 1–16 Hz frequency band, approximately 0.1–30 regional impacts per year should be detectable with a nominal value of 1–3 impacts per year. Therefore, small regional impacts are likely to be a viable source of seismic energy for probing Mars’ crustal and upper mantle structure. This is particularly appealing as such impacts should be easily located with orbital imagery, increasing their scientific value compared to other types of events with unknown origins. Finally, comparison of the empirical results presented here with the modelling study of Teanby and Wookey (Teanby, N.A., Wookey, J. [2011]. Phys. Earth Planet. Int. 186, 70–80) provides constraints on the seismic efficiency, suggesting that values of ∼ 5 × 10 −4 may be appropriate for impact generated seismic waves. Comparing explosion and impact datasets indicate that buried explosions are ∼ 10 times more efficient at generating seismic waves than impacts. [ABSTRACT FROM AUTHOR]

CM/CI meteorites range in degree of aqueous alteration suggesting differences in initially accreted materials including water ice and possible spatial heterogeneities within their parent bodies. As alteration progresses, the total abundance and magnesium content of phyllosilicates increases. In this paper we present the results of a coordinated spectral–mineralogical study of a well-characterized suite of CM/CI meteorites that range from 60 to 90% alteration. By acquiring spectra the same meteorite powders as Howard et al. (Howard, K.T., Benedix, G.K., Bland, P.A., Cressey, G. [2009]. Geochim. Cosmochim. Acta 73, 4576–4589; Howard, K.T., Benedix, G.K., Bland, P.A., Cressey, G. [2011]. Geochim. Cosmochim. Acta 75, 2735–2751) and Bland et al. (Bland, P.A., Cressey, G., Menzies, O.N. [2004]. Meteorit. Planet. Sci. 39(1), 3–16), we are able for the first time to directly correlate mineralogy with features in reflectance spectra. At visible/near-infrared wavelength, the presence of a 0.7-μm charge transfer band is indicative of aqueous alteration. However, not all altered CM/CI meteorites exhibit this feature; thus the lack of a 0.7 μm absorption band in asteroids does not necessarily signify a lack of aqueous alteration. Furthermore, the position and depth of 0.7-μm charge transfer band shows no correlation with the mineralogical changes associated with aqueous alteration. Similarly, the near-infrared slope, which is not directly related to the mineralogic progression associated with increasing alteration, is not unambiguously related to degree of alteration in the CM/CI meteorites studied. However, the mid-infrared reflectance spectra of CM/CI meteorites contain a broad absorption feature in the 10–13-μm region, which is a convolution of vibrational features due to Mg-rich phyllosilicates and unaltered olivine. The overall feature continuously changes with total phyllosilicate abundance from a shorter wavenumber/longer wavelength peak (815 cm −1 , 12.3-μm) for less altered meteorites to a longer wavenumber/shorter wavelength (875 cm −1 , 11.4-μm) peak in the highly altered meteorites, with roughly equal spectral contributions producing a doublet in intermediately altered meteorites, Using the results from the mid-infrared analyses of meteorite spectra, it is possible to estimate the degree of alteration on dark primitive asteroids. We find Asteroid (24) Themis to have a 1000–700 cm −1 (10–13-μm) peak at longer wavelengths suggesting Themis has a complex surface mineralogy with approximately 70 vol.% phyllosilicates and 25 vol.% anhydrous silicates. [ABSTRACT FROM AUTHOR]

Over the last decades, a significant number of small asteroids (diameter < 10 km) having a satellite in orbit around them have been discovered. This population of binary asteroids has very specific properties (secondary-to-primary diameter ratio of about 0.3, semi-major axis to primary diameter ratio around 2 and an obliquity of the system close to either 0 ∘ or 180 ∘) pointing at formation by YORP-induced spin-up and rotational fission. When impacting the surface of terrestrial bodies, those exotic objects lead to the formation of binary craters, exhibiting various morphologies depending on the configuration of the system at the moment of the impact. Planetary surfaces constitutes therefore the best (if not the only one) record of binary asteroid population through time. In contrast to the Moon or Mercury, a large fraction of impact craters on Mars exhibits thick ejecta layers due to the presence of volatile material at the moment of the impact (e.g., water ice). The martian surface represents thus the ideal case to survey for the existence of binary craters, as the ejecta morphology can attest of a synchronous impact. From a survey of 87% of Mars surface, we identify 150 binary craters (0.5% of the total), likely formed by the impact of binary asteroids. The properties of these craters contrast with those of the population of binary asteroids: size ratio close to unity, large separation, and isotropic orientation on the surface. We run numerical simulations of impacts to test whether tidal effects on the impact trajectory can explain these discrepancies. Our results suggest that a population of similarly-sized and well-separated binary asteroids with non-zero obliquity remains to be observed. [Display omitted] • First database of Martian binary craters with 150 entries compiled from a survey over 87% of the surface. • Comparison between properties of the binary craters and the population of binary asteroid systems. • Mismatch between observed and simulated properties of binary craters on Mars. • Unobserved population of binary asteroids suspected with large separation, near-similar size, and non-zero obliquity. [ABSTRACT FROM AUTHOR]

As part of systematic global mapping of Vesta using data returned by the Dawn spacecraft, we have produced a geologic map of the north pole quadrangle, Av-1 Albana. Extensive seasonal shadows were present in the north polar region at the time of the Dawn observations, limiting the ability to map morphological features and employ color or spectral data for determination of composition. The major recognizable units present include ancient cratered highlands and younger crater-related units (undivided ejecta, and mass-wasting material on crater floors). The antipode of Vesta’s large southern impact basins, Rheasilvia and Veneneia, lie within or near the Av-1 quadrangle. Therefore it is of particular interest to search for evidence of features of the kind that are found at basin antipodes on other planetary bodies. Albedo markings known as lunar swirls are correlated with basin antipodes and the presence of crustal magnetic anomalies on the Moon, but lighting conditions preclude recognition of such albedo features in images of the antipode of Vesta’s Rheasilvia basin. “Hilly and lineated terrain,” found at the antipodes of large basins on the Moon and Mercury, is not present at the Rheasilvia or Veneneia antipodes. We have identified small-scale linear depressions that may be related to increased fracturing in the Rheasilvia and Veneneia antipodal areas, consistent with impact-induced stresses (Buczkowski, D. et al. [2012b]. Analysis of the large scale troughs on Vesta and correlation to a model of giant impact into a differentiated asteroid. Geol. Soc. of America Annual Meeting. Abstract 152-4; Bowling, T.J. et al. [2013]. J. Geophys. Res. – Planets, 118. http://dx.doi.org/10.1002/jgre.20123 ). The general high elevation of much of the north polar region could, in part, be a result of uplift caused by the Rheasilvia basin-forming impact, as predicted by numerical modeling (Bowling, T.J. et al. [2013]. J. Geophys. Res. – Planets, 118. http://dx.doi.org/10.1002/jgre.20123 ). However, stratigraphic and crater size–frequency distribution analysis indicate that the elevated terrain predates the two southern basins and hence is likely a remnant of the ancient vestan crust. The lack of large-scale morphological features at the basin antipodes can be attributed to weakened antipodal constructive interference of seismic waves caused by an oblique impact or by Vesta’s non-spherical shape, or by attenuation of seismic waves because of the physical properties of Vesta’s interior. A first-order analysis of the Dawn global digital elevation model for Vesta indicates that areas of permanent shadow are unlikely to be present in the vicinity of the north pole. [ABSTRACT FROM AUTHOR]

The Dawn Framing Camera (FC) has imaged the northern hemisphere of the Asteroid (4) Vesta at high spatial resolution and coverage. This study represents the first investigation of the overall geology of the northern hemisphere (22–90°N, quadrangles Av-1, 2, 3, 4 and 5) using these unique Dawn mission observations. We have compiled a morphologic map and performed crater size–frequency distribution (CSFD) measurements to date the geologic units. The hemisphere is characterized by a heavily cratered surface with a few highly subdued basins up to ∼200 km in diameter. The most widespread unit is a plateau (cratered highland unit), similar to, although of lower elevation than the equatorial Vestalia Terra plateau. Large-scale troughs and ridges have regionally affected the surface. Between ∼180°E and ∼270°E, these tectonic features are well developed and related to the south pole Veneneia impact (Saturnalia Fossae trough unit), elsewhere on the hemisphere they are rare and subdued (Saturnalia Fossae cratered unit). In these pre-Rheasilvia units we observed an unexpectedly high frequency of impact craters up to ∼10 km in diameter, whose formation could in part be related to the Rheasilvia basin-forming event. The Rheasilvia impact has potentially affected the northern hemisphere also with S–N small-scale lineations, but without covering it with an ejecta blanket. Post-Rheasilvia impact craters are small (<60 km in diameter) and show a wide range of degradation states due to impact gardening and mass wasting processes. Where fresh, they display an ejecta blanket, bright rays and slope movements on walls. In places, crater rims have dark material ejecta and some crater floors are covered by ponded material interpreted as impact melt. [ABSTRACT FROM AUTHOR]

We used Dawn spacecraft data to identify and delineate geological units and landforms in the Marcia quadrangle of Vesta as a means to assess the role of the large, relatively young impact craters Marcia (∼63 km diam.) and Calpurnia (∼53 km diam.) and their surrounding ejecta field on the local geology. We also investigated a local topographic high with a dark-rayed crater named Aricia Tholus, and the impact crater Octavia that is surrounded by a distinctive diffuse mantle. Crater counts and stratigraphic relations suggest that Marcia is the youngest large crater on Vesta, in which a putative impact melt on the crater floor ranges in age between ∼40 and 60 Ma (depending upon choice of chronology system), and Marcia’s ejecta blanket ranges in age between ∼120 and 390 Ma (depending upon choice of chronology system). We interpret the geologic units in and around Marcia crater to mark a major vestan time-stratigraphic event, and that the Marcia Formation is one of the geologically youngest formations on Vesta. Marcia crater reveals pristine bright and dark material in its walls and smooth and pitted terrains on its floor. The smooth unit we interpret as evidence of flow of impact melts and (for the pitted terrain) release of volatiles during or after the impact process. The distinctive dark ejecta surrounding craters Marcia and Calpurnia is enriched in OH- or H-bearing phases and has a variable morphology, suggestive of a complex mixture of impact ejecta and impact melts including dark materials possibly derived from carbonaceous chondrite-rich material. Aricia Tholus, which was originally interpreted as a putative vestan volcanic edifice based on lower resolution observations, appears to be a fragment of an ancient impact basin rim topped by a dark-rayed impact crater. Octavia crater has a cratering model formation age of ∼280–990 Ma based on counts of its ejecta field (depending upon choice of chronology system), and its ejecta field is the second oldest unit in this quadrangle. The relatively young craters and their related ejecta materials in this quadrangle are in stark contrast to the surrounding heavily cratered units that are related to the billion years old or older Rheasilvia and Veneneia impact basins and Vesta’s ancient crust preserved on Vestalia Terra. [ABSTRACT FROM AUTHOR]

The Quadrangles Av-11 and Av-12 on Vesta are located at the northern rim of the giant Rheasilvia south polar impact basin. The primary geologic units in Av-11 and Av-12 include material from the Rheasilvia impact basin formation, smooth material and different types of impact crater structures (such as bimodal craters, dark and bright crater ray material and dark ejecta material). Av-11 and Av-12 exhibit almost the full range of mass wasting features observed on Vesta, such as slump blocks, spur-and-gully morphologies and landslides within craters. Processes of collapse, slope instability and seismically triggered events force material to slump down crater walls or scarps and produce landslides or rotational slump blocks. The spur-and-gully morphology that is known to form on Mars is also observed on Vesta; however, on Vesta this morphology formed under dry conditions. [ABSTRACT FROM AUTHOR]

Highlights: [•] Craters on 3 terrain types analyzed by 8 crater experts and 1000s of volunteers. [•] Found 10–35% dispersion among experts in number of craters found. [•] Experts are more consistent than volunteers in identifying craters. [•] Many artifacts occur with craters <10px across and near minimum diameters. [•] Minimally trained volunteers as an ensemble can reproduce expert crater counts. [Copyright &y& Elsevier]

Highlights: [•] Pristine crater depth is directly affected by the target heat flow at the time of impact. [•] Europa’s ice crust is estimated at 7km thick, representative over Europa’s global area and recorded geologic history. [•] The crater morphology trends of Ganymede and Europa are simulated. [•] Central melt pools are simulated in the largest craters, possible precursors to floor-pit formation. [•] Summit-pit craters were produced as a result of sub-surface layers at a depth of one third of the final crater diameter. [Copyright &y& Elsevier]

Understanding the geological modification processes on asteroids is fundamental for elucidating their surface evolution. Images of small asteroids from spacecrafts show a depletion in smaller craters. Seismic shaking was considered to be responsible for erasing such small craters and the main driver modifying the geology of asteroids. However, a recent artificial impact experiment on asteroid Ryugu conducted by the Japanese Hayabusa2 spacecraft revealed minimal seismic efficiency on the asteroid. To investigate whether a standard seismic shaking model can reproduce the observed crater record, we analyzed the crater distributions on four asteroids, i.e., Eros, Itokawa, Ryugu, and Bennu, using crater production functions under cohesionless conditions. Crater retention ages were estimated as a function of crater diameter for the four asteroids using the crater size-frequency distribution and crater production function estimated for each asteroid. We obtained the relation between a crater retention age t and crater diameter D in the form of power-law function (t ∝ D a ).We found that the power-law indices a are inconsistent with diffusion processes (e.g., seismic shaking, where a = 2). This result suggests that seismic shaking models based on a linear diffusion equation cannot explain the crater distribution on the small asteroids. Alternative crater obliteration processes include surface flows suggested by geomorphological and spectral features of Ryugu. Using the crater statistics, we also show that the vertical mixing of the Ryugu material at the depths shallower than 1 m occurs in 103–105 yr by cratering. This rapid resurfacing and replacement of the surface layer in the short timescale would account for the decrease in space weathering rate suggested by previous studies. Furthermore, the timescale required for vertically transporting Ryugu materials to the depths of 2–4 m (104–106 yr), where cosmic rays would not reach, can be compared with the cosmic-ray exposure ages of returned samples. This comparison can be used to constrain the distribution of impactors that collide with Ryugu. • Crater obliteration process on Ryugu cannot be explained by standard seismic shaking models • Vertical mixing of materials shallower than 1 m occurs in 103–105 yr on Ryugu • CRE age of returned samples would reflect the mixing timescale to 2–4 m (104–106 yr) [ABSTRACT FROM AUTHOR]

We use Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) images to analyze the boulder distribution around Vaughan crater, a small, fresh crater on the far side of the Moon. Using six similar lunar impact craters of known age for comparison, we determine the approximate age of the crater. Vaughan crater lies in a part of the South Pole-Aitken basin that is compositionally similar to lunar meteorite Dhofar 961, on the basis of Lunar Prospector Gamma Ray Spectrometer data, making Vaughan crater a potential candidate for the source of this meteorite and possibly others paired with it. Boulder distributions reveal that as impact craters age, the boulder population density in their ejecta decreases. This understanding allows us to use size-frequency distributions to compare crater ages on the basis of their boulder population density. We find that Vaughan crater is very young (<25 Ma) owing to its placement within the boulder size-frequency comparison plot and its population of large (>10 m diameter) boulders. Additionally, we compare LROC NAC distributions to thermophysically derived values for rock abundance from the Diviner Lunar Radiometer. We find that our LROC NAC count is consistent with Diviner rock abundance values, which further supports the inference that Vaughan is a very young impact crater and could plausibly be a source of lunar meteorite Dhofar 961. • Vaughan is a very young, rocky, lunar crater located in the northern South Pole-Aitken basin. • Boulder size-frequency distributions indicate an age < 25 Myr. • Imagery-based rock abundance is consistent with thermophysically derived data. • Compositional similarities suggest Vaughan as a potential lunar meteorite source crater in South Pole-Aitken basin. [ABSTRACT FROM AUTHOR]

Abstract: We report on the physical properties of the craters of Asteroid (2867) Steins based on an analysis of images obtained with the OSIRIS instrument during the Rosetta flyby that took place on 5 September 2008. To compensate for the limited spatial resolution of the images and unfavorable illumination conditions, we applied three different filtering techniques designed to enhance the craters detection. The selected images covered 44% of the overall surface of the asteroid and 42 craters were detected. We calculated their diameter and depth using a forward modeling procedure that combines a global shape model with an idealized semi-hemispherical representation of the craters, further incorporating photometric properties to generate simulated images. The resulting images were then compared to the original images. Crater diameters range from 150 to 2100m with depth-to-diameter ratios of 0.04–0.25, a wide range consistent with other small bodies. A striking dichotomy was observed between the two sides of the asteroid reflected by both a ratio of 3.6 between the spatial density and very different power exponents of the size distribution functions, −3.3 versus −1.5. Altogether these peculiar properties most likely reflect the particular history of Steins, in particular the impact that created the largest crater, Diamond, and extensively damaged the “original” Steins. A combination of various processes, including destruction, erasure, direct or indirect infill by seismic shaking, and even the YORP effect can explain the observed physical properties of the craters on Steins. A few craters may be drainage basins connected to a hypothetical fault, itself a product of the Diamond event. [Copyright &y& Elsevier]

Abstract: We present a methodology to remove atmospheric effects from High Resolution Imaging Science Experiment data in order to calculate the albedo of the martian surface in the near infrared (0.8–1 m), red (0.55–0.85 m), and blue green (0.4–0.6 m) spectral bands. The application of our methodology results in corrections of up to 20 in the albedo measured by the satellite. Time evolution of the surface albedo is used to study dark spots observed in Richardson Crater, on Mars south polar region. These dark spots form in late winter and vanish in late spring. They are of high scientific interest because they appear to be caused by the flow of granular material, liquid, or a combination of both. Besides images, the following data are used as ancillary information in our study: dust optical depth derived from measurements by the Thermal Emission Imaging System, the detection of the presence of either CO2 ice or H2O ice by the Compact Reconnaissance Imaging Spectrometers for Mars, surface pressure and temperature values derived from measurements by the Thermal Emission Spectrometer, and finally kinetic calculations and numerical modeling. Our results support the gas venting hypothesis previously proposed to explain the formation of dark spots. We show that the ejection of CO2 gas and accompanying loose material through cracks in the translucent CO2 ice layer, suggested by this hypothesis, is consistent with the time evolution of the surface albedo. Once dark spots form, surface albedo values indicate that they have three distinct areas: a dark core at their center, a brighter zone surrounding them (referred to as bright halo by various authors), and an optically distinct intermediate area separating the dark core from the bright halo. Our analysis indicates that these three areas are physically distinct and that the deposition and sublimation of CO2 and H2O ices are necessary to explain the time evolution of the albedo of the dark spots and the areas surrounding them. However, these conventional mechanisms cannot explain some observed features such as an unexpected decrease and subsequent increase in albedo while the surface temperature raises continuously. We hypothesize that this unexpected decrease in albedo is caused by brine formation and propose a mechanism to explain it. A new ejection of dark material or dust deposition could also explain this decrease. However, we show that these processes are unlikely to be the cause of the albedo decrease. The hypothesis that the dark spots observed in Richardson Crater contain liquid brines can be tested with laboratory experiments or in situ measurements by future landers. [Copyright &y& Elsevier]

Abstract: A database of bedrock exposed in crater central uplifts has been compiled from multiple orbital datasets. In this study we focus on uplifts which show decameter-scale layers within the exposed megablocks derived from the bedrock of the preexisting target. This distinctive morphology, found in 41 craters globally, occurs mostly in regions mapped as Hesperian plains material, generally interpreted as regions of extensive flood lavas. The association with volcanic units coupled with morphology and mineralogy that is consistent with flood lava emplacement suggests that the layers are stacks of dense lava interbedded with weaker materials. Estimates for the uplift of stratigraphic sections coupled with morphologic and morphometric analyses lead to estimated thicknesses of the layered unit. These estimates indicate Hesperian flood lavas are ∼23% thicker than a previous minimum estimate, with a volume of 5.5×107 km3. This increases the known extrusive volcanic production for the history of Mars and volatile release to past martian atmospheres. [Copyright &y& Elsevier]

Abstract: Hydrogeological modification of Meteor Crater produced a spectacular set of gullies throughout the interior wall in response to rainwater precipitation, snow melting, and possible groundwater discharge. The crater wall has an exceptionally well-developed centripetal drainage pattern consisting of individual alcoves, channels, and fans. Some of the gullies originate from the rim crest and others from the middle crater wall where a lithologic transition occurs; broad gullies occur along the crater corner radial faults. Deeply incised alcoves are well developed on the soft Coconino Sandstone exposed on the middle crater wall, beneath overlying dolomite. In general, the gully locations are along crater wall radial fractures and faults, which are favorable locales of erosion due to preferential rock breakup from faulting, and groundwater flow/discharge; these structural discontinuities are also the locales where the surface runoff from rain precipitation and snow melting can preferentially flow, causing erosion and crater degradation. Channels are well developed on the talus deposits and alluvial fans on the periphery of the crater floor. Caves exposed on the lower crater level point to percolation of surface runoff and selective discharge through fractures on the crater wall. In addition, lake sediments on the crater floor provide significant evidence of a past pluvial climate, when the water table was higher, and groundwater may have seeped from springs on the crater wall. Although these hydrological processes continue at Meteor Crater today, conditions at the crater are much more arid than they were soon after impact, reflecting a climatic shift. This climate shift and the hydrological modifications observed at Meteor Crater provide insights for landscape sculpturing on Mars during various parts of its history. [Copyright &y& Elsevier]

Abstract: Almost every meteorite impact occurs at an oblique angle of incidence, yet the effect of impact angle on crater size or formation mechanism is only poorly understood. This is, in large part, due to the difficulty of inferring impactor properties, such as size, velocity and trajectory, from observations of natural craters, and the expense and complexity of simulating oblique impacts using numerical models. Laboratory oblique impact experiments and previous numerical models have shown that the portion of the projectile’s kinetic energy that is involved in crater excavation decreases significantly with impact angle. However, a thorough quantification of planetary-scale oblique impact cratering does not exist and the effect of impact angle on crater size is not considered by current scaling laws. To address this gap in understanding, we developed iSALE-3D, a three-dimensional multi-rheology hydrocode, which is efficient enough to perform a large number of well-resolved oblique impact simulations within a reasonable time. Here we present the results of a comprehensive numerical study containing more than 200 three-dimensional hydrocode-simulations covering a broad range of projectile sizes, impact angles and friction coefficients. We show that existing scaling laws in principle describe oblique planetary-scale impact events at angles greater than 30° measured from horizontal. The displaced mass of a crater decreases with impact angle in a sinusoidal manner. However, our results indicate that the assumption that crater size scales with the vertical component of the impact velocity does not hold for materials with a friction coefficient significantly lower than 0.7 (sand). We found that increasing coefficients of friction result in smaller craters and a formation process more controlled by impactor momentum than by energy. [Copyright &y& Elsevier]

Abstract: In this paper we investigate the formation of the Cretaceous–Paleogene (K–Pg) boundary layer through numerical modeling. The K–Pg layer is widely agreed to be composed of meteoritic material and target rock from the Chicxulub impact site, that has been ejected around the globe and mixed with local material during final deposition. The observed composition and thickness of the K–Pg boundary layer changes with azimuth and distance from the impact site. We have run a suite of numerical simulations to investigate whether we can replicate the observational data, with a focus on the distal K–Pg layer and the impact glasses at proximal sites such as Beloc, Haiti. Previous models of the K–Pg ejecta have assumed an initial velocity distribution and tracked the ejecta to its final destination. Here, we attempt to model the entire process, from impact to the arrival of the ejecta around the globe. Our models replicate the observed ejecta thickness at proximal sites, and the modeled ejecta is composed of sediments and silicate basement rocks, in agreement with observational data. Models that use a 45° impact angle are able to replicate the total ejecta and iridium volume at distal sites, and the majority of the ejecta is composed of meteorite and target sediments. Sub-vertical impacts generate too little iridium, and oblique impacts of ⩽30 degrees generate too much. However, in contrast to observations, models that involve ballistic transport of ejecta lead to ejecta thickness decreasing with increasing distance, and are unable to transport shocked minerals (quartz and zircon) from the Chicxulub basement rocks around the globe. We suggest that much of the K–Pg ejecta is transported non-ballistically, and that the most plausible mechanism is through re-distribution from a hot, expanding atmosphere. The results are important for future investigations of the environmental effects of the Chicxulub impact. [Copyright &y& Elsevier]

Abstract: Viking mission image data revealed the total spatial extent of preserved volcanic surface on Mars. One of the dominating surface expressions is Olympus Mons and the surrounding volcanic province Tharsis. Earlier studies of the global volcanic sequence of events based on stratigraphic relationships and crater count statistics were limited to the image resolution of the Viking orbiter camera. Here, a global investigation based on high-resolution image data gathered by the High-Resolution Stereo Camera (HRSC) during the first years of Mars Express orbiting around Mars is presented. Additionally, Mars Orbiter Camera (MOC) and Thermal Emission Imaging System (THEMIS) images were used for more detailed and complementary information. The results reveal global volcanism during the Noachian period () followed by more focused vent volcanism in three (Tharsis, Elysium, and Circum-Hellas) and later two (Tharsis and Elysium) volcanic provinces. Finally, the volcanic activity became localized to the Tharsis region (about 1.6 Ga ago), where volcanism was active until very recently (200–100 Ma). These age results were expected from radiometric dating of martian meteorites but now verified for extended geological units, mainly found in the Tharsis Montes surroundings, showing prolonged volcanism for more than 3.5 billions years. The volcanic activity on Mars appears episodic, but decaying in intensity and localizing in space. The spatial and temporal extent of martian volcanism based on crater count statistics now provides a much better database for modelling the thermodynamic evolution of Mars. [Copyright &y& Elsevier]

Abstract: We experimentally studied the formation and collapse processes of transient craters. Polycarbonate projectiles with mass of 0.49 g were impacted into the soda-lime glass sphere target (mean diameters of glass spheres are ∼36, 72, and 220 μm, respectively) using a single-stage light-gas gun. Impact velocity ranged from 11 to 329 m s−1. We found that the transient crater collapses even at laboratory scales. The shape (diameter and depth) of the transient crater differs from that of the final crater. The depth–rim diameter ratios of the final and transient craters are 0.11–0.14 and 0.26–0.27, respectively. The rim diameter of both the transient and final crater depends on target material properties; however, the ratio of final to transient crater diameter does not. This suggests that target material properties affect the formation process of transient craters even in the gravity regime, and must be taken into account when scaling experimental results to planetary scales. By observing impacts into glass sphere targets, we show that although the early stage of the excavation flow does not depend on the target material properties, the radial expansion of the cavity after the end of vertical expansion does. This suggests that the effect of target material properties is specifically important in the later part of the crater excavation and collapse. [Copyright &y& Elsevier]

Images of Eros from the NEAR Shoemaker spacecraft reveal bright and dark albedo features on steep crater walls unlike markings previously observed on asteroids. These features have been attributed to the downslope movement of space-weathered regolith, exposing less weathered material (Science 292 (2001) 484; Meteor. Planet. Sci. 36 (2001) 1617; Icarus 155 (2002) 145). Here we present observations of the interiors of large craters (>1 km in diameter) to test this hypothesis and constrain the origin of the features. We find that bright regions in these craters correspond to steep slopes, consistent with previous work. The geographic distribution of craters with albedo variations shows no pattern and does not resemble the distribution of ponds, another phenomenon on Eros attributed to regolith movement. Shadows and other indications of topography are not observed at feature boundaries, implying that the transported layer is &les;1 m thick. The presence of multiple bright and dark units on long slopes with sharp boundaries between them suggests that mobilized regolith may be halted by frictional or other effects before reaching the foot of the slope. Features on crater walls should darken at the same rate as bright ejecta deposits from crater formation; the lack of observed, morphologically fresh craters with bright interiors or ejecta suggests that the albedo patterns are younger than the most recently formed craters greater than about 100 m in diameter. Smaller or micrometeorite impacts, which would not necessarily leave evident deposits of bright ejecta, remain possible causes of albedo patterns. Although their effectiveness is difficult to assess, electrostatic processes and thermal creep are also candidates. [Copyright &y& Elsevier]

Of the impact craters on Earth larger than 20 km in diameter, 10–15% (3 out of 28) are doublets, having been formed by the simultaneous impact of two well-separated projectiles. The most likely scenario for their formation is the impact of well-separated binary asteroids. If a population of binary asteroids is capable of striking the Earth, it should also be able to hit the other terrestrial planets as well. Venus is a promising planet to search for doublet craters because its surface is young, erosion is nearly nonexistent, and its crater population is significantly larger than the Earth''s. After a detailed investigation of single craters separated by less than 150 km and “multiple” craters having diameters greater than 10 km, we found that the proportion of doublet craters on Venus is at most 2.2%, significantly smaller than Earth''s, although several nearly incontrovertible doublets were recognized. We believe this apparent deficit relative to the Earth''s doublet population is a consequence of atmospheric screening of small projectiles on Venus rather than a real difference in the population of impacting bodies. We also examined “splotches,” circular radar reflectance features in the Magellan data. Projectiles that are too small to form craters probably formed these features. After a careful study of these patterns, we believe that the proportion of doublet splotches on Venus (14%) is comparable to the proportion of doublet craters found on Earth (10–15%). Thus, given the uncertainties of interpretation and the statistics of small numbers, it appears that the doublet crater population on Venus is consistent with that of the Earth. [Copyright &y& Elsevier]

Lunar meteorites provide information about areas of the Moon not sampled by the Apollo and Luna missions, so they can be used as more-representative geochemical datasets for evaluating global lunar evolution. However, the launching craters remain unknown for many lunar meteorites. This paper, therefore, aims to identify the launching sites of lunar meteorites with remote-sensing datasets. This work focuses on one lunar-meteorite group, the Northwest Africa 773 (NWA 773) clan, which has the very-low-Ti basaltic composition and one of the youngest crystallization ages (~3 Ga) among the lunar basaltic meteorites. We investigated the source of the NWA 773 clan by comparing their geochemical compositions and geochronological information with the following remote-sensing datasets: 1) the Lunar Prospector gamma-ray elemental maps (FeO, TiO 2 , Th) and the SELENE gamma-ray elemental maps (CaO), 2) the surface ages determined from imaging data by using the crater-counting method, and 3) high-spatial-resolution maps of the FeO and TiO 2 and spectral-ratio images made from the SELENE multi-band imager data. From these comparisons, the NWA 773 clan is most likely to have originated from the north part of Mare Imbrium inside the Procellarum KREEP Terrane (PKT). Furthermore, the eruption age of surface basalt surrounding the Le Verrier D crater in the north part of Mare Imbrium between 3.0 and 3.3 Ga, and the formation of the Le Verrier D crater, are consistent with the duration of igneous activity and timing of meteoroid impact of the NWA 773 clan. There is a fresh crater with bright ejecta on the edge of the Le Verrier D crater, and it is possibly the young launch crater of this meteorite group. • The source region of lunar meteorite is investigated with remote sensing datasets. • The basaltic meteorite group, NWA 773 clan originated from Mare Imbrium. • NWA 773 clan likely launched from young crater within the Le Verrier D crater. [ABSTRACT FROM AUTHOR]

High-resolution mapping of Ceres, Vesta and the icy satellites of Saturn, Uranus and Pluto reveals a rich variety of well-preserved impact crater morphologies on these low gravity bodies. These objects provide a natural laboratory to study effects of composition on crater formation processes under similar surface gravity conditions (though mean impact velocities vary by several factors). Simple craters occur on all these bodies but subtle differences in morphology on Ceres and Vesta are recognized. Immature complex craters (with large floor mounds but not terraces or conical central peaks) occur on Vesta and while smaller than predicted are consistent with its silicate composition. Asymmetric simple craters (with incomplete scarp development) on all bodies are likely related to differential overburden stresses in the rim, and their occurrence is consistent with lower crustal strength on icy bodies including Ceres. Immature and mature complex craters exhibit increasing degrees of complexity, including spiral floor deformation patterns (related to failure in converging floor material), central peaks, and impact melt. Cerean crater morphologic types and simple-complex transition diameters are smaller than on Vesta but similar to those on icy satellites, indicating a much weaker rheology for Ceres' outer layers under impact conditions. These are consistent with geophysical indications of a low-density water ice and probably clathrate rich outer shell. Fluidized floor deposits (impact melt or melt-solid mixtures) are significant in craters >25 km across on Ceres but absent on Saturn satellites. Central pit craters are common on Ceres (at diameters of ~75 to 150 km consistent with gravity scaling from the larger Galilean satellites) but are absent on Saturnian satellites and Charon. The contrasting impact melt and central pit behaviors on Ceres and Saturn's moons is contrary to expectation given the higher impact velocities at Saturn but might be related to lower internal temperatures, or the higher fraction of non-ice material on Ceres. The correlation or scaling of transition diameters to surface gravity is near −0.65 rather than −1, perhaps due to increased porosity on lower gravity bodies. The fundamental similarity of crater morphologies on Ceres and icy satellites, however, indicates that the weaker rheology of water ice results in similar craters even if the non-(ice+clathrate) components are as high as ~30 vol%. • Impact craters on mid-sized asteroids, dwarf planets and icy moons form under similar gravity but in icy to silicate crusts • Complex craters occur at very different diameters on Vesta and Ceres due to their very distinct compositions • Crater morphologies on Ceres are very similar to those on icy moons, indicating ice is the dominant crustal component • Impact melt deposits and central peaks on Ceres (but not mid-sized icy moons) may be related to temperature differences • Asymmetric simple craters are related to local overburden pressure on the rim scarp and crustal strength differences [ABSTRACT FROM AUTHOR]

Using data acquired by the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) mission, we investigate spectral properties of craters on the near-Earth asteroid (101955) Bennu. We compare Bennu's craters with its global average by means of four spectral parameters: (a) minimum position of the band at 2.7 μ m, (b) depth of the hydrated phyllosilicate absorption band at 2.7 μ m, (c) normalized spectral slope from 0.55 to 2.0 μ m, and (d) reflectance factor at 0.55 μ m. We examine 45 craters using spectral data obtained under various observing conditions. For 20 craters, we find a shortward shift of the 2.7- μ m band minimum relative to the global 2.7- μ m band minimum, which we attribute to the presence of relatively fresh (less space-weathered) material excavated from the sub-surface by crater-forming impacts. For three craters, we find an anti-correlation between spectral slopes and reflectance factor for a series of spectra acquired during a specific scan, where we observe that spectra become redder and darker towards the center of the crater. We attribute this to the presence of fine-particulate regolith. Localized spectral heterogeneities are apparent inside a prominent equatorial crater on Bennu, which is one of the asteroid's oldest geological features. We propose that such local spectral heterogeneities could be used as a tracer of mass movement on Bennu. We show that younger craters are redder, brighter, and have deeper 2.7- μ m bands. Comparing global average spectral values of Bennu and crater frequency distributions as a function of the chosen spectral parameters, we find that craters evolve to assume the global average spectral properties of Bennu. A positive correlation identified between the reflectance factor and 2.7- μ m band depth suggests that brighter craters tend to be more hydrated. Finally, we put into context, the results from the Small Carry-on Impactor experiment by the Hayabusa2 spacecraft, which created an artificial crater on the near-Earth asteroid (162173) Ryugu. • The shortward shift of the 2.7-μm band inside craters indicates the presence of freshly excavated sub-surface material. • A scan-dependentant anti-correlation between REFF0.55 μm and spectral slopes hints at the presence of fine particulates. • Younger craters are brighter and have deeper 2.7-μm hydration bands and redder spectral slopes. • Nightingale crater, the primary sample site of the OSIRIS-REx mission, is the reddest and the darkest crater [ABSTRACT FROM AUTHOR]

The near-Earth asteroid 162173 Ryugu, the target of the Hayabusa2 mission, is noted to be a spinning top-shaped rubble-pile. Craters are among the most prominent surface features on Ryugu. Their shapes, particularly their depth-to-diameter ratio (d/D), can provide an important proxy for probing both the internal structure and surface processes of planetary bodies. Here, we report d/D of every impact crater on Ryugu using a shape model derived from stereo-photoclinometry. We found that the average, standard deviation, and observed range of d/D for the entire set of craters are 0.09, 0.02, and 0.03–0.15, respectively. Except for possible pit craters, the maximum d/D of large craters on Ryugu (D > 50 m) is close to 0.13, which is comparable with those of fresh simple craters on rocky asteroids, such as Gaspra and Ida. Conversely, the d/D of small craters (D < 50 m) increases with the crater diameter. This behavior implies that a smaller crater on Ryugu is formed as a shallower crater. As on Itokawa, the surface environment on Ryugu likely inhibits craters becoming deep. This especially affects smaller craters, as their normal small depth decreases in the Ryugu environment and they become still more shallow. As a result, small craters rapidly degrade beyond the point where they can be identified as candidate craters. This is likely responsible for the apparent lack of small craters. The d/D has no reliable relationship with the types of crater classification in Hirata et al. (2020). Examination of latitudinal and longitudinal variation in d/D of craters on Ryugu revealed no statistically significant trends. • We measured the depth-to-diameter ratio (d/D) of 77 possible impact craters on Ryugu. • The max d/D of large craters is comparable to those of fresh craters on asteroids. • We found that the d/D of small craters (D < 50 m) increases with crater diameter. • The d/D has no reliable relationship with the crater type in Hirata et al., 2020. • There are no statistically significant trends in the regional variation of the d/D. [ABSTRACT FROM AUTHOR]

Of the several near-Earth and Main Belt asteroids visited by spacecraft to date, three display a paucity of small craters and an enhanced number of smoothed and degraded craters: 433 Eros, which has a deficit of craters ≲ 100 m in diameter; 2867 Šteins, which has a deficit of craters ≲ 500 m in diameter; and 25143 Itokawa, which has a deficit of craters ≲ 100 m in diameter. The purpose of this work was to investigate and model topographic modification and crater erasure due to impact-induced seismic shaking, as well as impact-driven regolith production and loss, on these asteroid surfaces. To perform this study, we utilized the numerical, three-dimensional, Cratered Terrain Evolution Model (CTEM) initially presented in J. E. Richardson, Icarus 204 (2009), which received a small-body (SB) specific update for this work. SBCTEM simulations of the surface of 433 Eros correctly reproduce its observed cratering record (for craters up to ∼ 3 km in diameter) at a minimum Main Belt exposure age of 225 ± 75 Myr using a 'very weak rock' target strength of 0.5–5.0 MPa, and producing a mean regolith depth of 80 ± 20 m, which agrees with published estimates of an actual regolith layer "tens of meters" in depth. SBCTEM simulations of the surface of 2867 Šteins correctly reproduce its heavily softened cratering record at a minimum Main Belt exposure age of 175 ± 25 Myr using a 'weak rock' target strength of 2.5–5.0 MPa. Extending this simulation produces a mean regolith depth of 155 ± 5 m at a minimum Main Belt exposure age of 475 ± 25 Myr, in good agreement with the 145 ± 35 m actual mean regolith depth estimated from two chains of drainage-pit craters on the surface, with both crater density and regolith depth in an equilibrium state. SBCTEM simulations of the surface of 25143 Itokawa correctly reproduce its extremely sparse cratering record at a minimum Main Belt exposure age of 20 ± 5 Myr, using a 'soft soil' target strength of 10-20 kPa, and producing a mean regolith overturn depth of 5. 0 ± 1. 0 m, given that Itokawa is composed entirely of regolith and boulders. These simulations demonstrate the efficacy of impact-induced seismic shaking to erase small craters on the surface of asteroids ≲ 25 km mean diameter, correctly reproducing their observed cratering records while using a Main Belt impactor population and standard crater scaling relationships. These simulations also show that the observed regolith layer on asteroids in the 5-50 km size range can be generated entirely through impact cratering processes, and indicate that regolith depth can be used as a additional constraint on the surface age of a given asteroid. • Three asteroids show a deficit of small craters: 433 Eros, 2867 Steins, 25143 Itokawa. • This deficit is hypothesized to be caused by impact-induced seismic shaking. • We performed three-dimensional terrain, Monte-Carlo crater modeling for each. • These small crater deficits are correctly reproduced when seismic shaking is included. • Asteroid regolith production is also shown to be due to impact cratering processes. [ABSTRACT FROM AUTHOR]

On asteroids, impact-induced seismic shaking is an important geological process related to the modifications of the surface features that are due to the movement of regolith grains on the asteroid surface. Mass movements caused by seismic shaking (e.g., landslides and crater erasures) were recently observed by several spacecrafts. To elucidate the seismic shaking areas induced by impacts on asteroids, it is necessary to determine the excitation and the decay processes of impact-induced seismic waves. Here we conducted impact cratering experiments using 500-μm quartz sand to simulate an asteroid surface at the Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA). We used 4.75-mm- and 2-mm-diameter projectiles with density from 14.9 to 1.2 g cm−3; the impact velocity ranged from 7 to 0.2 km s−1. We measured the acceleration waveform by using three to five accelerometers setting on the target surface, and we examined three parameters characterizing the impact-induced seismic wave with varying distances from the impact point: the propagation velocity (V prop), the maximum acceleration (g max), and the half period of the first upward peak (T half). The V prop was obtained to be 52.4 ± 7.2 m s−1, regardless of the impact velocity and the projectile properties, and the T half slightly depended on the impact velocity as v i 0.14 but was independent of the projectile properties. The g max had a close relationship with the distance from the impact point normalized by the crater rim radius, x / R rim , regardless of the impact velocity and the projectile properties. The obtained empirical equation was shown as g max = 102.21(x / R rim)−3.18. By using the empirical equations we obtained herein, we observed that the seismic energy normalized by the kinetic energy of a projectile, E s / E k , decreased with the increase of the normalized distance, x / R rim. We also estimated the area of impact-induced seismic shaking on an idealized small (500 m in diameter) body at the impact velocity of 5 km s−1. • We studied impact-induced seismic waves excited during the crater formation. • Propagation velocity in quartz sand was 52 m s−1, independent of impact conditions. • Maximum acceleration decayed with the distance and was scaled as g max ∝ (x / R rim)−3.18. • Half period of first peak depended slightly on the impact velocity as T half ∝ v i 0.14. • E s / E k decreased with the increase of the normalized distance x / R rim within 6. [ABSTRACT FROM AUTHOR]