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The multiple-layered ejecta surrounding crater Kotka (east of Elysium Mons) are studied using imagery and physical modelling. This particular crater was chosen not only because its ejecta are well preserved, but more importantly because the impact area is surrounded by mounds, which provide a means of determining the velocity of the ejecta based on run-up criteria. If the ejecta passed over a mound of a certain height, the velocity was greater than that necessary to rise up that height, while the presence of a shadow beyond the mound indicates a velocity lower than that limit. Top ejecta flow velocities were found to vary between 25 m/s and 80 m/s. Velocities are also determined based on the length of the jump against craters rims, a criterion that provides an estimate of the velocity, rather than a limit, and are found to be compatible with those estimated with run-up criteria. We find that a first train of ejecta travelling at high velocity was capable of overcoming many mounds. A peculiar rampart often visible at the foot of many of the mounds is interpreted as a frozen hydraulic jump indicating a phase in which the ejecta were about to stop. The velocity of the ejecta was found to decrease with distance from the rim but not as fast as a constant friction model would suggest, indicating effective friction that increases with distance, and more complex rheology than pure frictional behavior. The velocities indicate a rheology for the fluidized ejecta in which the debris material was completely fluidized, to the point that the friction coefficient decreased by one to two orders of magnitude compared to the one of fragmented rock. Our conceptual model is that the ejecta material initially contained a large amount of solid ice that was fluidized and vaporized by the impact. The chains of pits visible in the ejecta, interpreted as fossilized bubbles of volatiles released through the hot fluidized material, confirm that high temperatures were reached during impact, as also indicated by analytical estimates. Fluidization altered the rheology of the ejecta in a way that has yet to be understood. • The run-ups on mounds of ejecta surrounding the multiple-layered ejecta crater Kotka on Mars are studied. • Velocities estimated on mounds an with leaps lengths along craters are found in the range 25–80 m/s. • The dynamics indicate initial fluidized phase due to hot vapor followed by frictional behavior of ejecta. [ABSTRACT FROM AUTHOR]

• The lake stages in Jezero are consistent with semiarid climates • The Jezero lake was a throughflow lake driven by significant surface runoff • Hydrological conditions do not favor large-scale evaporite deposition • Similar climates at Jezero and Gale produce contrasting lacustrine environments Jezero crater, the landing site of the Perseverance rover, hosts sedimentary fan deposits, aqueously altered sedimentary rock, and an outlet breach of the crater rim, all of which indicates that a long-lived lake (or lakes) once existed within the crater. The deltaic deposits in the crater may preserve evidence of habitable conditions within the past lake system, which would have been influenced by the climate and conditions of the past hydrological system within the watersheds and basin. However, the aqueously altered sedimentary deposits within Jezero are less extensive than those found in other crater lakes on Mars, like Gale with a crater filling mound of cemented sediment. In this work we use hydrological models of the past Jezero lake system to constrain the climatic and hydrological conditions by comparing model predictions to the proposed lake stands and mineralogy in the crater observed from orbit and by the Perseverance rover thus far. We then compare these results to Gale crater to understand the influence of basin hydrology on the depositional setting found within past crater lakes. We show that a lake forming at the outlet breach elevation or higher in Jezero requires a semiarid or wetter-arid climate, consistent with previous estimates. An integrated hydrologic system with seasonal variations similar to arid and semiarid climates on Earth results in a lake system at Jezero characterized by both surface and subsurface outflow from the lake. Outflow to the subsurface and through the outlet breach will remove dissolved solids from the lake system, mitigating large-scale evaporite cementation within the Jezero basin, which is in contrast to Gale crater, where subsurface inflow constituted a larger percentage of the total inflow to the lake and all loss was through evaporation due to the setting in a deeper impact crater. Furthermore, at hyperarid climatic and/or groundwater-driven hydrological conditions, lake formation within Jezero would have ceased while a lacustrine depositional environment would have continued at Gale. This highlights the importance of considering basin hydrology, in addition to climate, when interpreting lacustrine sedimentary deposits in craters on Mars. [ABSTRACT FROM AUTHOR]

Investigating the relationship between thermal inertia (TI) and aerosol optical depth (AOD) is significant in giving insights into the seasonality of dust deposition and lifting phenomenon. The present study focuses on establishing a relationship of AOD with TI and different particle sizes over different Martian seasons. Two different Martian landforms (exposed rock and sand dunes) have been used to establish these relationships. TI layer was generated using THEMIS nighttime images for different seasons, whereas Curiosity Rover measured AOD values and Mars Climate database (MCD) visible column dust optical depth were used to derive rover equivalent AOD. An inverse relation was observed between AOD and TI for exposed rock and sand dune regions for all the seasons with low to moderate coefficient of determination (R2). A similar inverse trend was observed between rover equivalent AOD and particle size with R2 values ranging from 0.8 in the case of sand dunes (winter) to 0.93 in exposed rock (autumn). The results were further compared within the AOD obtained from orbiter image (HRSC) derived using Shadow method for spring season (Shaheen et al., 2022). The same inverse relation was found within TI having good R2 values of 0.61 for exposed rock and 0.76 for the sand dunes. Error estimation using Mean Absolute Error (MAE), Root Mean Square Error (RMSE), Normalized Mean Square Error (NMSE), Fractional Bias (FB), Index of agreement errors was carried out for TI vs. AOD and particle size vs. AOD. Excellent statistical significance was obtained for AOD and particle size, in the case of sand dunes it was 0.96 for autumn and 0.99 in case of exposed rock for spring season, respectively. • An inverse trend was observed between TI and Rover equivalent AOD over Gale crater for all the seasons. • A similar inverse trend was observed between particle size and Rover equivalent AOD for all the seasons. • Inverse trend matched well with dust lifting and deposition phenomenon over Gale crater. [ABSTRACT FROM AUTHOR]

The analysis of high-resolution images provided by the Perseverance rover indicates the presence of a Gilbert-type deltaic complex in the western part of the Jezero crater, which includes fluvial topsets, as well as subaqueous clinoforms. While previously considered to be genetically related, the topsets and the foresets are separated by subaerial unconformities and belong to different sedimentation cycles (depositional sequences) and systems tracts. Based on the stratal stacking patterns observed at Kodiak butte, the clinoforms belong to falling-stage systems tracts, as indicated by the downstepping trajectory of the clinoform rollovers, the occasional preservation of offlap, and the sharp-based nature of the clinoforms. In contrast, the topsets are part of the lowstand systems tracts of the overlying depositional sequences, and are separated from the underlying foresets by truncation surfaces (i.e., subaerial unconformities). The possible environments for the deposition of clinoforms in the Jezero crater range from lacustrine to marine. In the latter case, a connection between the Jezero Basin and a northern ocean can be inferred, allowing for the possibility of tidal processes influencing the patterns of deposition within the deltaic complex. This may explain the rhythmites observed in the deltaic foresets, although other explanations for the cyclic changes in lithology and energy conditions, such as seasonal variations in fluvial discharge and sediment load, are also possible. The location of the Jezero crater in the shoreline area of the Isidis Basin, which is a bay of the northern ocean, may have created a configuration of interconnected embayments able to amplify the otherwise small tidal range expected from the two moons of the planet. Clarification of the paleogeography at the time of deltaic progradation (c. 3.6–3.8 Ga) has major implications for the strategy of exploration for early life forms on Mars. The acquisition of rock samples from the Jezero Basin and the northern ocean are essential to gain further insight into the early land-ocean interaction and possible co-evolution of life and environments on Mars. • The Jezero crater on Mars hosts a river-dominated deltaic complex which includes fluvial topsets and subaqueous clinoforms. • The observation of stratal stacking patterns affords the construction of the sequence stratigraphic framework even in the absence of rock samples. • The topsets and the clinoforms are separated by subaerial unconformities and belong to lowstand and falling-stage systems tracts, respectively. • As the sedimentological composition of systems tracts is unconstrained, the origin of rhythmites observed in the deltaic foresets is currently unknown. • A connection between the Jezero crater and a northern ocean could facilitate a tidal influence in the foresets of the deltaic complex. [ABSTRACT FROM AUTHOR]

Impact crater cataloging is an important tool in the study of the geological history of planetary bodies in the Solar System, including dating of surface features and geologic mapping of surface processes. Catalogs of impact craters have been created by a diverse set of methods over many decades, including using visible or near infra-red imagery and digital terrain models. I present an automated system for crater detection and cataloging using a digital terrain model (DTM) of Mars — In the algorithm craters are first identified as rings or disks on samples of the DTM image using a convolutional neural network with a UNET architecture, and the location and size of the features are determined using a circle matching algorithm. I describe the crater detection algorithm (CDA) and compare its performance relative to an existing crater dataset. I further examine craters missed by the CDA as well as potential new craters found by the algorithm. I show that the CDA can find three–quarters of the resolvable craters in the Mars DTMs, with a median difference of 5–10% in crater diameter compared to an existing database. A version of this CDA has been used to process DTM data from the Moon and Mercury (Silburt et al., 2019). The source code for the complete CDA is available at https://github.com/silburt/DeepMoon, and Martian crater datasets generated using this CDA are available at https://doi.org/10.5683/SP2/MDKPC8. • Trained a neural network CDA to identify and locate craters. • Applied the neural network to the MOLA/HRSC blended DTM. • Performance of the CDA is comparable to expert human classifiers. • Reproduced human-generated crater distribution on a global DTM. [ABSTRACT FROM AUTHOR]

Abstract Channel networks in the mid-latitude during the late Noachian to early Hesperian Period have invoked opinions in favor of presence of water on Martian surface. Disregarding the controversy of the presence of water, this study aims to probe into the controlling factors involved in the development of the channel pathways. These apparently curvilinear channels may have certain similarities in map pattern though their origin and development might differ in time and space with subtle differences, cryptically engraved within their geometries. Our study of the geometries of the channels and craters with the aid of appropriate statistical methods help us to group channels with different geometrical attributes, implying the control of pre-existing tectonic lineaments of the terrain on the channel course development. This research uses such high precision statistical analyses on THEMIS-IR Day Global Mosaic and detailed observations from MOLA colorized elevation map, HRSC images, HRSC MOLA Blended DEM within the area, Pyrrhae Fossae region, Noachis Terra. Graphical abstract Image 1 Highlights • Geometrical similarity among the channels of Pyrrhae Fossae region. • Similarity between straight segments of polygonal impact craters and channels. • Statistical approach to classify the geometrically similar channels into groups. • Trend analysis indicating structural control on channel courses. [ABSTRACT FROM AUTHOR]

Abstract Calcium sulfates, including gypsum, bassanite, and anhydrite, have been identified on Mars by orbital and in situ remote sensing observations. A comprehensive spectroscopic study of three calcium sulfates in laboratory was performed. Raman, mid-infrared (mid-IR), and near infrared (NIR) spectra of the samples were acquired. The assignments for fundamental modes are made in the Raman and the mid-IR spectra and for overtones and combinational modes in the NIR spectra. Finally, we performed a case study to derive the abundance of gypsum in Columbus crater on Mars using the spectral unmixing analysis method. Our results show high abundance of gypsum and moderate abundance of hydrated sulfate without any basaltic glass or amorphous weathering products, which indicates these deposits could have formed through precipitation and evaporation and is consistent with the deep lake hypothesis proposed by previous studies. Highlights • Laboratory Raman, mid-IR, and NIR spectroscopic study of gypsum, bassanite, and anhydrite were performed. • Detailed assignments for peaks in Raman and mid-IR and absorption bands at NIR were made. • Gypsum abundances at Columbus crater on Mars were mapped and analyzed with CRISM data. [ABSTRACT FROM AUTHOR]

The dynamics of Mars' obliquity are believed to be chaotic, and the historical ∼3.5 Gyr (late-Hesperian onward) obliquity probability density function (PDF) is highly uncertain and cannot be inferred from direct simulation alone. Obliquity is also a strong control on post-Noachian Martian climate, enhancing the potential for equatorial ice/snow melting and runoff at high obliquities (≳40°) and enhancing the potential for desiccation of deep aquifers at low obliquities (≲25°). We developed a new technique using the orientations of elliptical craters to constrain the true late-Hesperian-onward obliquity PDF. To do so, we developed a forward model of the effect of obliquity on elliptic crater orientations using ensembles of simulated Mars impactors and ∼3.5 Gyr-long Mars obliquity simulations. In our model, the inclinations and speeds of Mars crossing objects bias the preferred orientation of elliptic craters which are formed by low-angle impacts. Comparison of our simulation predictions with a validated database of elliptic crater orientations allowed us to invert for the best-fitting obliquity history. We found that since the onset of the late Hesperian, Mars' mean obliquity was likely low, between ∼10° and ∼30°, and the fraction of time spent at high obliquities >40° was likely <20%. [ABSTRACT FROM AUTHOR]

The existence of a wet period lasting long enough to allow the development of elementary forms of life on Mars has always been a very interesting issue. Given this perspective, the research for geological markers of such occurrences has been continually pursued. Once a favorable site is detected, effort should be spent to get as much information as possible aimed at a precise assessment of the genesis and evolution of the areas showing the selected markers. In this work, we discuss the recent finding of possible deposits pointing to the past existence of liquid water in Tikhonravov crater located in Arabia Terra. Comparison of CRISM spectra and those of laboratory minerals formed by aqueous alteration has led us to the conclusion that the studied areas within the impact crater host phyllosilicates deposits. In addition, analysis of the CRISM spectra has resulted in the tentative identification of carbonates mixed with phyllosilicates. [ABSTRACT FROM AUTHOR]

Many of the sedimentary basins of Mars show patterns of faults and off-horizontal layers that, if correctly understood, could serve as a key to basin history. Sediment compaction is a possible cause of these patterns. We quantified the possible role of differential sediment compaction for two Martian sedimentary basins: the sediment fill of Gunjur crater (which shows concentric graben), and the sediment fill of Gale crater (which shows outward-dipping layers). We assume that basement topography for these craters is similar to the present-day topography of complex craters that lack sediment infill. For Gunjur, we find that differential compaction produces maximum strains consistent with the locations of observed graben. For Gale, we were able to approximately reproduce the observed layer orientations measured from orbiter image-based digital terrain models, but only with a >3 km-thick donut-shaped past overburden. It is not immediately obvious what geologic processes could produce this shape. [ABSTRACT FROM AUTHOR]

Inspection of geological material is one of the main goals of the Perseverance rover during its journey across the landscape of the Jezero crater in Mars. NASA's rover integrates SuperCam, an instrument capable of performing standoff characterization of samples using a variety of techniques. Among those tools, SuperCam can perform laser-induced breakdown spectroscopy (LIBS) studies to elucidate the chemical composition of the targets of interest. Data from optical spectroscopy can be supplemented by simultaneously-produced laser-produced plasma acoustics in order to expand the information acquired from the probed rocks thanks to the SuperCam's microphone (MIC) as it can be synchronized with the LIBS laser. Herein, we report cover results from LIBS and MIC during Perseverance's first 380 sols on the Martian surface. We study the correlation between both recorded signals, considering the main intrasample and environmental sources of variation for each technique, to understand their behavior and how they can be interpreted together towards complimenting LIBS with acoustics. We find that louder and more stable acoustic signals are recorded from rock with compact surfaces, i.e., low presence loose particulate material, and harder mineral phases in their composition. Reported results constitute the first description of the evolution of the intensity in the time domain of shockwaves from laser-produced plasmas on geological targets recorded in Mars. These signals are expected contain physicochemical signatures pertaining to the inspected sampling positions. As the dependence of the acoustic signal recorded on the sample composition, provided by LIBS, is unveiled, the sound from sparks become a powerful tool for the identification of mineral phases with similar optical emission spectra. [Display omitted] • Acoustics from laser-produced sparks on geological targets on Mars is analyzed. • The target surface is highlighted as a key factor of the emerging acoustic signal. • Acoustics spectroscopy may add information on the physics of geological targets. • Acoustics spectroscopy may complement the chemical features provided by LIBS. • Results encourage using laser-driven acoustic responses to identify mineral phases. [ABSTRACT FROM AUTHOR]

Evidence for mid-high latitude glacial episodes existing within the Late Amazonian history of Mars has been reported from analysis of variety of glacial/periglacial landforms and their stratigraphic relationships. In this study, using the Context Camera (CTX) images, we have surveyed the interior of craters within the Alba Mons region of Mars (30°–60°N; 80°–140°W) to decipher the presence of ice-related flow features. The primary goals of this study are to (1) suggest from observations that the flow features identified in the interior of Alba Mons craters have flow characteristic possibly different from concentric crater fill (CCF) landforms and (2) interpret the extent of glacial activity that led to formation of flow features with respect to previously described mid-latitude ice-related landforms. Our geomorphic investigation revealed evidence for the presence of tongue-like or lobate shaped ice-related flow feature from the interior of ∼346 craters in the study region. The presence of ring-mold crater morphologies and brain-terrain texture preserved on the surface of flow features suggests that they are possibly formed of near-surface ice-rich bodies. We found that these flow features tend to form inside both the smaller (<5 km) and larger (>5 km) diameter craters emplaced at a wide range of elevation (from ∼ −3.3 km to 6.1 km). The measurement of overall length and flow direction of flow features is suggestive that they are similar to pole-facing small-scale lobate debris apron (LDA) formed inside craters. Crater size-frequency distribution of these small-scale LDAs reveals a model age of ∼10–100 Ma. Together with topographic and geomorphic observations, orientation measurements, and distribution within the study region, we suggest that the flow features (identified as pole-facing small-scale LDAs in the interior of craters) have flow characteristic possibly different from CCF landforms. Our observations and findings support the results of previous analyses that suggests Mars to have preserved records of multiple debris-covered glacial episodes occurred in the Late Amazonian. [ABSTRACT FROM AUTHOR]

A global and uniformly distributed spike of secondary impact craters on Phobos with diameters (D) <0.6 km and a portion of craters up to D 2 km were produced by Stickney Crater ejecta, including secondary craters within the surface area of Stickney Crater. The global exposure of Phobos to Stickney secondary impacts was facilitated by the desynchronized orbital/rotational period of Phobos that was produced by the impulse of the Stickney impact event. In our model we apply the Tsiolkovsky rocket equation to calculate the total available Stickney impact acceleration impulse delta-v (Δv) and further calculate the effective impulse by incorporating the energy conversion inefficiencies of the crater formation process. We also calculate the pre- and post-impact Phobos moment of inertia that further contributes to the desynchronizing effect. The majority of the Stickney ejecta that exited from Phobos was trapped in orbits around Mars until it later accumulated back onto Phobos over a period of <1000 years. However, Phobos de-spun back to a synchronous rotation after a much longer period of at least 5000 years. Therefore, a sufficient period of desynchronized rotation exposed the entire surface of Phobos to ejecta that returned from martian orbits. In view of how all or most craters observed inside Stickney Crater approximate the size/frequency distribution (SFD) of Stickney secondary impacts, it is infeasible to derive an age for Stickney Crater based on an assumption of background impacts (~2.8–4.2 Ga according to Schmedemann et al. (2014)). In view of how crater-counting is unworkable for age-dating Stickney Crater we conclude an alternate age for Stickney Crater of 0.1–0.5 Ga that is constrained instead by the boulder evidence of Thomas et al. (2000), the boulder destruction rate analysis of Basilevsky et al. (2013, 2015), and the observed space weathering of Phobos regolith (Cipriani et al., 2011; Pieters et al., 2014). Assessing several implications of our model we 1) summarize the crater SFD and temporal nature of the Stickney secondary impact spike on Phobos, 2) predict the global equivalent thickness of deposits on Phobos from Stickney ejecta and subsequent secondary impact gardening, 3) examine the hypothesis that the Stickney impact was a trailing hemisphere event on Phobos that reoriented Phobos to its present-day synchronous “tidal lock” longitude, 4) set limits on the volume of low-velocity Stickney ejecta that is available to produce Phobos grooves from rolling boulders, and 5) estimate the crater SFD of a meteor spike on Mars from a trailing hemisphere Stickney impact. [ABSTRACT FROM AUTHOR]

We examine the stratigraphic architecture and mineralogy of the western fan deposit in the Jezero crater paleolake on Mars to reassess whether this fan formed as a delta in a standing body of water, as opposed to by alluvial or debris flow processes. Analysis of topography and images reveals that the stratigraphically lowest layers within the fan have shallow dips (<2°), consistent with deltaic bottomsets, whereas overlying strata exhibit steeper dips (∼2–9°) and downlap, consistent with delta foresets. Strong clay mineral signatures (Fe/Mg-smectite) are identified in the inferred bottomsets, as would be expected in the distal fine-grained facies of a delta. We conclude that the Jezero crater western fan deposit is deltaic in origin based on the exposed stratal geometries and mineralogy, and we emphasize the importance of examining the stratigraphic architecture of sedimentary fan deposits on Mars to confidently distinguish between alluvial fans and deltas. Our results indicate that Jezero crater contains exceptionally well-preserved fluvio-deltaic stratigraphy, including strata interpreted as fine-grained deltaic bottomsets that would have had a high potential to concentrate and preserve organic matter. Future exploration of this site is both geologically and astrobiologically compelling, and in situ analyses would be complementary to the ongoing in situ characterization of fluvio-lacustrine sediment in the Gale crater paleolake basin by the Curiosity rover. [ABSTRACT FROM AUTHOR]

Numerous craters showing floor fractures have been identified on the Moon and Mars. Whereas on the Moon, the floor morphology, crater geology and fracture characteristics suggest that the floor deformation is caused by underlying magmatic intrusions, the origin of fractures and floor modifications of Martian floor-fractured craters (FFCs) remains debated. Here, we focus on FFCs displaying hierarchical crack patterns. We use different quantitative fracture characteristics to investigate the physical processes at the origin of cracks and compare their variations to scaling laws derived theoretically and from desiccating laboratory experiments. We show that cracks on a large set of Martian FFCs close to Valles Marineris differ from that of Lunar FFCs: they may be explained by tensile stresses caused by the shrinkage of a material adhering on the crater floor, similar to desiccating cracks. Other Martian FFCs close to Syrtis Major show cracks more similar to those caused by a magmatic intrusion. • Quantitative characteristics of hierarchical cracks in crater floors are determined. • These characteristics are compared to analytical and experimental predictions. • Hierarchical cracks on Martian FFCs' floors originate from two different processes. • Crack characteristics for a set of Martian FFCs are similar to Lunar ones. • Cracks from a second set show characteristics evolving as desiccating ones. [ABSTRACT FROM AUTHOR]

Tyrrhena Terra, an area in the cratered highlands south of the Martian equator, contains two unnamed craters. The mineralogical composition of light-toned deposits (LTDs) within these craters is unknown. We performed morphologic and morphometric analyses of the surfaces of these LTDs through an integrated study of images available through the Reconnaissance Mars Orbiter High-Resolution Imaging Science Experiment. Our analysis revealed that the Martian landforms contain dolines of polygenetic origin. These dolines are similar to karst landforms observed in different karst terrains on Earth and other regions on Mars. The observed karst landforms might highlight the possible evaporitic origin of these materials, suggesting a climatic change and the presence of liquid water caused by ice melting during the Amazonian age. [ABSTRACT FROM AUTHOR]

Noachian (prior to ca. 3700 Ma) terranes are the oldest and most heavily cratered landscapes on Mars, with crater densities comparable to the ancient highlands of the Moon and Mercury. Intense early cratering affected Mars by melting and fracturing its crust, draping large areas in impact ejecta, generating regional-scale hydrothermal systems, and increasing atmospheric pressure (and thereby, temperature) to periodically re-start an otherwise moribund hydrological cycle. Post primary-accretionary bombardment scenarios that shaped early Mars can be imagined in two ways: either as a simple exponential decay with an approximately 100 Myr half-life, or as a “sawtooth” timeline characterized by both faster-than-exponential decay from primary accretion and relatively lower total delivered mass. Indications are that a late bombardment spike was superposed on an otherwise broadly monotonic decline subsequent to primary accretion, of which two types are investigated: a classical “Late Heavy Bombardment” (LHB) peak of impactors centered at ca. 3900 Ma that lasted 100 Myr, and a protracted bombardment typified by a sudden increase in impactor flux at ca. 4100–4200 Ma with a correspondingly longer decay time (≤400 Myr). Numerical models for each of the four bombardment scenarios cited above show that the martian crust mostly escaped exogenic melting from bombardment. We find that depending on the chosen scenario, other physical effects of impacts were more important than melt generation. Model output shows that between 10 and 100% of the Noachian surface was covered by impact craters and blanketed in resultant (hot) ejecta. If early Mars was generally arid and cold, impact-induced heating punctuated this surface state by intermittently destabilizing the near-subsurface cryosphere to generate regional-scale hydrothermal systems. Rather than being deleterious to the proclivity of Noachian Mars to host an emergent biosphere, this intense early impact environment instead enhanced the volume and duration of its surface/subsurface geophysical habitable zone. [ABSTRACT FROM AUTHOR]

The precise mechanism for the formation and evolution of crater valley networks in the Martian southern highlands remains under debate, with precipitation, groundwater flow, and melting induced by impact being suggested. We studied valley networks within four craters of the Noachis Terra highlands that were representative of similar features in Noachis Terra and where orbital data existed for analysis in order to characterise their morphology and infer possible processes involved in their formation and evolution. We found evidence for valleys carved by liquid water and ice-related processes. This included strong evidence of liquid water-based valley formation through melting of ice-rich deposits throughout our study area, suggesting an alternative to previously suggested rainfall or groundwater-based scenarios. The location of these valleys on steeply sloping crater walls, as opposed to the shallow slopes of the highlands where Martian valleys are typically found, suggested that our ‘fluvial’ valleys had not evolved a more structured fluvial morphology as valley networks found on the Martian plains. Our studied valleys' association with ice-rich material and abundant evidence for erosion caused by downslope flow of ice-rich material are consistent with a cold, wet Mars hypothesis where accumulation, flow, and melting of ice have been dominant factors in eroding crater valleys. Additionally, analysis of valley morphology with slope and aspect suggested a greater dependence on local geology and presence of volatiles than larger valley networks, though ice-related valleys were consistently wider for their length than valleys assessed as fluvial carved. We assessed that local conditions such as climate, geology, and availability of ice-rich material played a major role in the erosion of crater valleys at our study site. [ABSTRACT FROM AUTHOR]

An improved description of magnetic fields of terrestrial bodies has been obtained from recent space missions, leading to a better characterization of the internal fields including those of crustal origin. One of the striking differences in their crustal magnetic field is the signature of large impact craters. A comparative analysis of the magnetic characteristics of these structures can shed light on the history of their respective planetary-scale magnetic dynamos. This has motivated us to identify impact craters and basins, first by their quasi-circular features from the most recent and detailed topographic maps and then from available global magnetic field maps. We have examined the magnetic field observed above 27 complex craters on the Earth, 34 impact basins on Mars and 37 impact basins on the Moon. For the first time, systematic trends in the amplitude and frequency of the magnetic patterns, inside and outside of these structures are observed for all three bodies. The demagnetization effects due to the impact shock wave and excavation processes have been evaluated applying the Equivalent Source Dipole forward modeling approach. The main characteristics of the selected impact craters are shown. The trends in their magnetic signatures are indicated, which are related to the presence or absence of a planetary-scale dynamo at the time of their formation and to impact processes. The low magnetic field intensity at center can be accepted as the prime characteristic of a hypervelocity impact and strongly associated with the mechanics of impact crater formation. In the presence of an active internal field, the process of demagnetization due to the shock impact is associated with post-impact remagnetization processes, generating a more complex magnetic signature. [ABSTRACT FROM AUTHOR]

We have performed a combined geological and spectral analysis of two impact craters on Mercury: the 15 km diameter Waters crater (106°W; 9°S) and the 62.3 km diameter Kuiper crater (30°W; 11°S). Using the Mercury Dual Imaging System (MDIS) Narrow Angle Camera (NAC) dataset we defined and mapped several units for each crater and for an external reference area far from any impact related deposits. For each of these units we extracted all spectra from the MESSENGER Atmosphere and Surface Composition Spectrometer (MASCS) Visible-InfraRed Spectrograph (VIRS) applying a first order photometric correction. For all the mapped units, we analyzed the spectral slope in two wavelength ranges, 350–450 nm and 450–650 nm, and the absolute reflectance in the 700–750 nm range. Normalized spectra of Waters crater display a generally bluer spectral slope than the external reference area over both wavelength windows. Normalized spectra of Kuiper crater generally display a redder slope than the external reference area in the 350–450 nm window, while they display a bluer slope than the external reference area in the 450–650 nm wavelength range. The combined use of geological and spectral analyses enables reconstruction of the local scale stratigraphy beneath the two craters, providing insight into the properties of the shallower crust of Mercury. Kuiper crater, being ~4 times larger than Waters crater, exposes deeper layers with distinctive composition, while the result for Waters crater might indicate substantial compositional homogeneity with the surrounding intercrater plains, though we cannot exclude the occurrence of horizontal compositional heterogeneities in the shallow sub-surface. [ABSTRACT FROM AUTHOR]

The Kings Bowl lava field, associated with the Craters of the Moon National Monument and Preserve on the Great Rift in Idaho, U.S.A., contains many explosive pits, the largest of which has a substantial field of ejecta blocks. These features were formed when lava supply diminished, the fissure was evacuated, and ground water entered. We have documented the properties of the ejecta block field: crater shape, the ejecta field to the west and the tephra field to the east, the vesicularity and density of the blocks, the block size distribution and the number of blocks per unit area as a function of distance from the crater, and the aspect ratios of the blocks. To the degree possible we compare the data to those of ejecta blocks at other phreatomagmatic craters on rifts especially, Deception Island, South Shetland Islands, and White Island, New Zealand, and find them similar. Many of the Kings Bowl ejecta characteristics can be described by relationships that have previously been published to describe ejecta blocks around impact craters. We have also performed ballistic and energy calculations to derive ejection velocities for the blocks consistent with our data. The "velocity exponent" b in the relationship d ​∝ ​v i -b where d is ejecta size and v i is the initial ejecta velocity for our data is around −3.0 and in the range previously observed for impact ejecta (−3.7 to −0.32), although at the small end. Using the calculated ejection velocities (20–52 ​m/s) and the calculated total mass ejected (1.2 ​× ​108 ​kg), and correcting for the energy consumed in the fragmentation of the blocks, we calculate that the total energy of the explosion that produced the crater and the ejecta field is about 1012 ​J, roughly half used in fragmenting the blocks and half in their dispersal, and that about 4 ​× ​107 ​kg of water was involved in the explosion. Although subject to a number of assumptions, this method of calculating subsurface water from crater and ejecta block data could be used remotely to provide an indication of the amount of subsurface water in the vicinity of phreatomagmatic craters on Mars. • We document the ejecta block field at Kings Bowl phreatomagmatic crater Idaho. • The blocks obey many of the empirical equations for ejecta from impact craters. • We calculate ejection velocities (20–52 ​m/s) and mass ejected (1.2 ​× ​108 ​kg). • Thus the water/magma ratio was ∼0.30, which is near optimum for this process. • Subsurface water can be determined from ejecta blocks near such craters on Mars. [ABSTRACT FROM AUTHOR]

There are many buried craters on Mars invisible in imaging or topographic data; however, the residual gravity anomaly separated from the Mars Bouguer gravity anomaly by minimum curvature separation method can reveal these buried craters. In our research, gravity separation results exposed 39 residual gravity anomaly highs (RGAHs), which may represent deeply buried impact craters with 300–500 ​km diameter. The RGAHs are widely distributed across the surface of Mars, but are especially concentrated in volcanic areas like Tharsis and Elysium, which indicates that there might be more craters buried in these regions. Older craters in volcanic regions were likely buried by subsequent volcanism coming from surrounding volcanoes, so the retention age of craters in all areas where RGAHs are located will be older than reported in the existing research based on topographic data alone. This study will help improve the chronological model of Mars. • The impact craters of Mars with diameter of 300–500 km appear central positive residual gravity anomalies. • The gravity separation results exposed 39 anomaly highs, that is inferred to be buried impact craters. • The most RGAHs are concentrated in volcanic areas just like Tharsis and Elysium. [ABSTRACT FROM AUTHOR]

Craters are topographic structures resulting from impactors striking the surface of planetary bodies. This paper proposes a novel way of simultaneously utilizing optical images, digital elevation maps (DEMs), and slope maps for automatic crater detection on the lunar surface. The proposed system utilizes Mask R–CNN by tuning it for the crater detection task. Two catalogs, namely, Head-LROC and Robbins, are used for performance evaluation, and extensive analysis of detection results for the lunar surface is performed for both of these catalogs. A recall value of 93.94% is obtained for the Head-LROC catalog, which has relatively strict crater markings. For the Robbins catalog, an exhaustive crater catalog based on relatively liberal marking, F 1 -score of the proposed system ranges from 64.27% to 81.33%, for different crater size ranges. The proposed system's generalization capability for crater detection on different terrains with different input data types is also evaluated. Experimental results show that the proposed system trained on the lunar surface can also detect craters on the Martian surface. This model is trained by simultaneously using lunar surface's optical images and DEMs with their corresponding slope maps; however, it is tested on an entirely different input data type, thermal IR images from the Martian surface. • Crater detection by simultaneously using optical images, DEMs, and slope data. • Evaluation of crater detection and localization on conservative and liberal catalogs. • Proposed system either outperforms or matches the current state-of-the-art. • For the first time exploration of training deep learning system on Robbins catalog. • Generalization capability explored on Martian surface with different input data type. [ABSTRACT FROM AUTHOR]

Mars and Deimos are tidally locked so one would always get to view the same side of Deimos from Mars. Most of the contemporary functional satellites orbiting Mars viz. Mars Reconnaissance Orbiter , Mars Express, Mars Odyssey, MAVEN etc. are positioned between Mars and Deimos, hence they always view only one side of Deimos i.e the Mars-side, while the far-side is always looking outward with respect to all these satellites. However the maiden Indian Mars Orbiter Mission (MOM) is an exception as it has a very large elliptical orbit and goes behind the orbit of Deimos. This has enabled MOM to view the far-side of Deimos which has not been viewed for last few decades by any Mars orbiter. Mars Colour Camera (MCC), onboard MOM acquired four image-frames of the Deimos, the farthest of two natural satellites of Mars, on October 14, 2014 at around 13:05 UT. These four images of MCC have been used to generate an High Dynamic Range (HDR) product so as to enhance the image details. The two well-known craters of near-side are missing in this HDR image and the shape of far-side, recently updated and reported models of Deimos proposed by other researchers, matches with the shape of Deimos in MCC images thus confirming that the far-side view of Deimos has been captured by MCC of Mars Orbiter Mission. The magnitude of Deimos has also been computed from this far-side image, which varies by a small margin from the known magnitude of Deimos. This could probably be due to possible difference in the near and far side surface characteristics, which needs further detailed investigation and more imaging of far-side in future. However, there could be reasons other than the physical characteristics of the Deimos. Thus far-side sighting has generated a scientific interest to understand it holistically. This note is a brisk or rapid communication regarding sighting of the far-side of the Deimos by MCC after a long spell of more than three decades by any other Mars mission, including the contemporary operational Mars orbiters. It highlights the imaging capabilities of MCC-MOM and possible research to come in future. [ABSTRACT FROM AUTHOR]

The presence of valley networks and the highly degraded state of Noachian highland craters has led to the interpretation that Mars was once warmer and wetter. Recent climate models have suggested, however, that the extremely cold climate in the Noachian would be unlikely to support liquid water precipitation. The presence of a thicker atmosphere thermally coupled to the surface is predicted instead to concentrate surface snow and ice deposits in the higher-altitude southern highlands, producing a Late Noachian Icy Highlands (LNIH) characterized by hundreds of meters of relatively continuous ice cover. In this study we test this hypothesis by reevaluating the degradation state of Noachian highland craters to assess whether their degradation state might be attained in such a cold and icy climate. We review the characteristics of Amazonian-aged impact craters hypothesized to form in surface snow and ice layers (excess ejecta, EE; double-layered ejecta, DLE; and pedestal, Pd, craters) to provide the potential initial conditions of craters forming in Late Noachian surface snow and ice layers. We then examine modification processes active in the Amazonian that may have played a role in crater degradation in the Late Noachian. In addition, we examine the potential morphometric effects of impacting into a thick surface ice deposit, and the potential erosive effects of backwasting, top-down melting, basal ice melting, and atmospheric warming pulses on the morphology of Noachian highland craters. We find that several aspects of the highly degraded state of Noachian craters could be accounted for in the context of a cold and icy climate, and we outline further tests of the hypothesis. [ABSTRACT FROM AUTHOR]

The “ponds” on asteroid (433) Eros are fine-grained deposits approximating flat (quasi-equipotential) surfaces with respect to local topographic depressions (e.g., craters) in spacecraft images. These ponds are discussed in the context of laboratory simulation experiments, crater-related ponded and pitted deposits observed on Mars and Vesta, terrestrial phreatic craters, and degassing features associated with eroded impact craters on Earth. While the details of formation of these features on Mars, Vesta and the Earth are thought to be different, they all include mechanisms that require the interactions between surface materials and volatiles (e.g., water vapor). Indeed, analogous features similar to the Eros ponds can be reproduced in the laboratory by the release of vapor (ice sublimation, water evaporation, or N 2 ) through an unconsolidated regolith (independent of regolith composition). Eros is widely thought to be dry, but the discovery of exogenic water on Vesta, and recent arguments that subsurface water might be present in the inner asteroid belt suggest that endogenic water might also be present and serve as a source of the gases produced in the ponds. The amount of water required is comparable to the amount of water observed in little-metamorphosed ordinary chondrites (a few wt%). The primary morphologic characteristics of the Eros ponds can be explained in this model. The heat source for degassing could have been solar heating following transfer from a main belt orbit to a near Earth orbit. Although other hypotheses (e.g., electrostatic levitation, seismic shaking, and comminution of boulders) can account for most of the features of the ponds, recent observations regarding the role of volatiles on planetary surfaces, our laboratory experiments, and fluidization deposits on active comets suggests that degassing is a reasonable hypothesis to be considered and further tested for explaining the Eros ponds, and similar features on other bodies. [ABSTRACT FROM AUTHOR]

The 250 m-deep, Ubehebe Crater is the youngest (< 1000 years) and largest (800 m-wide) phreatomagmatic crater of the Ubehebe maar field of northern Death Valley, California, USA. The crater is blanketed by loose, unconsolidated ejecta debris and prominent tongue-shaped, steep-fronted debris lobes up to 200 m long occur at the base of the crater's steep northeast-facing interior slope. Geomorphological observations, sedimentological analysis of outcrops and a ground penetrating radar survey, show the largest lobe to be composed of downslope-dipping progradational beds of ejecta resulting from repeated downslope avalanching (‘grain flow’) and cohesive debris flow inconsistent with an origin of the lobe as a singular coherent slump or flow. Large oval depressions on the upper surface of lobes are interpreted as relict nivation hollows occupied by former seasonal snow patches during recent Little Ice Age cooling (c. ~ 700 year BP) when numerous rock glaciers and other periglacial landforms formed at high elevations in surrounding mountains. Lobes are interpreted as protalus ramparts formed when loose, non-cohesive ejecta from the steep unstable backwall of the crater moved downslope across snow patches and accumulated around their outer margins. Snow patch and lobe formation is considered to be a function of the local aspect-related meteorology of the crater's interior involving shading of northeast-facing interior slopes, radiative cooling and ponding of cold air, enhanced snow accumulation in the lee of the steep backwall combined with the availability of large volumes of loose ejecta. Ubehebe Crater and its lobes expand the geomorphic record of cool climate processes in Death Valley, the most well-known being the winter-ice facilitated ‘sliding boulders’ of Racetrack Playa. Having formed in an extreme environment, lobes offer an accessible terrestrial analog for poorly-understood debris lobes within Martian impact craters. [ABSTRACT FROM AUTHOR]

Missions to Mars progressively reveal the past and present habitability of the red planet. The current priority for Mars science is the recognition of definitive biosignatures related to past or present life. Success of life detection missions requires choices of the best mission design, location on Mars and particular sample to be analyzed. It is essential therefore to incorporate as much information as possible into the mission planning stages to maximize the precious opportunities provided by robotic operation on Mars. Bayesian statistics allow us to accommodate the many unknowns associated with a mission that has yet to take place. We have used Bayesian statistics to reveal that although in situ missions are less complex the overall probabilities of a successful mission to detect biosignatures on Mars are higher for sample return. If a mission has been designed with safe landing and operation as a priority, recognizing and avoiding those samples that do not contain the target biosignature is the most important characteristic, while for a mission where the best possible samples have been targeted the probability that the sample contains the target biosignature and that it can be correctly detected is the most dominant issue. Usefully, Bayesian statistics can be used to evaluate the chances of detecting past or present life for missions to different landing sites on Mars. A comparative assessment of Eberswelde Crater and Gale Crater indicates a higher probability of success for the latter and the probabilities of success are consistently higher for the sample return mission variant. Bayesian statistics, therefore, can inform future Mars mission planning steps to help maximize the possibility of success. [ABSTRACT FROM AUTHOR]

Eberswalde crater, Mars, contains a well-preserved fluvial distributary network in a likely deltaic setting. The meandering inverted paleochannels and closed drainage basin of this deposit support relatively well constrained estimates of channel-forming discharge (over an individual event flood timescale), runoff production (event and annual timescales), and longevity of deposition (geologic timescale) during the Late Hesperian to Early Amazonian Epochs. The width and meander dimensions of two inverted paleochannels reflect the channel-forming discharge from event floods (~ 200 to 400 m 3 /s), the deposit surface indicates the level (− 1400 to − 1350 m) and surface area (410 to 810 km 2 ) of the likely paleolake, and the topography and mapped extent of tributaries constrain the watershed area (5000 to 17,000 km 2 ). Based on these results and terrestrial empirical constraints on evaporation and sediment concentration, we evaluated three hypothetical water sources: meltwater liberated by the nearby Holden crater impact (continuous deposition over ~ 10 1 –10 2 years), intermittent rainfall or snowmelt during finite periods controlled by orbital evolution (deposition over ~ 10 4 –10 6 years), and highly infrequent runoff or melting of accumulated snowpacks following distant impacts or secular changes in orbital parameters. Local impact-generated runoff and highly infrequent rainfall or snowmelt require unreasonably high and low rates of evaporation, respectively, to maintain the paleolake level. The local impact hypothesis alternatively depends on one flooding episode with very high concentrations of fluvial sediment that are inconsistent with morphologic considerations. Multiple primary impact craters in the area postdate Holden ejecta but were later dissected, indicating fluvial erosion long after the Holden impact. Intermittent rainfall of ~ 1 cm/day and seasonal snowmelt are both consistent with our results over a deposition timescale totaling ~ 10 4 –10 6 years. [ABSTRACT FROM AUTHOR]

Spectacular floods were spawned on Mars when large craters filled to overtopping and catastrophically breached. The wealth of data now available permits quantitative analysis of these ancient floods. Galilaei Crater is examined here, along with the likely sources of its water and the processes that formed Tana Vallis, its 1-km-deep outflow channel. Laser altimetry data, stereo-photogrammetry, and scaled images are used to reconstruct the lake volume and the geometry of the preserved breach. It is theorized that surface flows from Hydaspis Chaos eroded the rim and floor of Galilaei Crater and partly filled the basin, leading to degradation of the underlying cryosphere. Groundwater released from a confined aquifer then slowly filled Galilaei Crater until the enclosed lake overtopped its rim. The resulting flood breached the crater wall, rapidly drained the lake, and eroded Tana Vallis. The drainable lake volume was ≥ 1.110 × 10 13 m 3 , based on laser altimetry data from Mars Global Surveyor. This volume was 90% of the volume in Lake Superior in the Great Lakes. Using the reconstructed breach and channel geometry, methods used to analyze terrestrial dam breaches were applied to produce hydrographs of lake stage and discharge. Peak flood discharge in Tana Vallis likely ranged from 1.5 to 4.7 × 10 7 m 3 s − 1 . Mean channel incision rates are estimated at 0.333, 0.133, and 0.0667 m min − 1 for scenarios in which Tana Vallis formed in 2, 5, and 10 days. For the 5-day breach erosion scenario, stream power per unit streambed area at the time of peak discharge was estimated at > 3.5 × 10 5 W m − 2 . The volume of material eroded to form the vallis was only ~ 3% of the lake volume that drained in 5 days. Open-channel calculations were performed to compare with the dam breach hydrographs for Tana Vallis. The channel as seen today never flowed bank full or even at three-fourths its maximum depth because the implied flows would have exceeded peak discharges from the crater breach. I also evaluate an alternative model (catastrophic ice lake collapse) for the morphology and breach of Aram Chaos, a large crater near Galilaei Crater. That model has fundamental difficulties because of improbable assumptions and inadequate consideration of the knowledge base of terrestrial basalt hydrology. The model of ice lake collapse fails to explain the breach and outflow of crater lakes like those that existed in Aram Chaos and Galilaei Crater. The present analysis supports a simpler model of groundwater inflow that fills a crater to overtopping. This model harmonizes with the low elevations of Aram Chaos and Galilaei Crater and is more consistent with extensive evidence of fluid breakouts in this region and with theoretical and terrestrial knowledge. [ABSTRACT FROM AUTHOR]

Craters are distinctive features on the surfaces of most terrestrial planets. Craters reveal the relative ages of surface units and provide information on surface geology. Extracting craters is one of the fundamental tasks in planetary research. Although many automated crater detection algorithms have been developed to exact craters from image or topographic data, most of them are applicable only in particular regions, and only a few can be widely used, especially in complex surface settings. In this study, we present a machine learning approach to crater detection from topographic data. This approach includes two steps: detecting square regions which contain one crater with the use of a boosting algorithm and delineating the rims of the crater in each square region by local terrain analysis and circular Hough transform. A new variant of Haar-like features (scaled Haar-like features) is proposed and combined with traditional Haar-like features and local binary pattern features to enhance the performance of the classifier. Experimental results with the use of Mars topographic data demonstrate that the developed approach can significantly decrease the false positive detection rate while maintaining a relatively high true positive detection rate even in challenging sites. [ABSTRACT FROM AUTHOR]

The source area of the Yellow River (SAYR) in the northeastern Tibetan Plateau (TP) has a series of lakes with an outflowing stream that crosses an active left-lateral strike-slip fault. How and when these lakes began to flow into the Yellow River remains an open question. Here, using geologic maps and a digital elevation model (DEM), we map the drainage network and relate it to tectonically active structural features to explore the role of tectonic processes in drainage integration. We also date paleo-shorelines and other geomorphological features around the modern Gyaring Lake and Ngoring Lake using optically stimulated luminescence (OSL) to illuminate the climate-driven processes leading to lake overspill. Our results show that: (1) drainage reversal in Duoshi Gorge was driven by subsidence in a pull-apart basin, likely during Marine Isotope Stage (MIS) 3; (2) Gyaring Lake and Ngoring Lake experienced two simultaneous overspill highstand periods: one during the last deglaciation (∼14-12 ka) and a second during the early Holocene (∼9 ka), similar to records from closed-basin lakes in the northeastern TP; (3) alluvial fan aggradation was likely enhanced during cold-to-warm transitions, including MIS 5b-a, MIS 4-3, and the last deglaciation; (4) aeolian sediments blanketing the landscape accumulated since the middle Holocene (∼6 ka), consistent with the interpretation of improved trapping efficiency due to increasing moisture availability and denser vegetation. Based on the geomorphic features and dating results, we propose a lake overspill model to explain the land surface processes, reconciling our new observations with previous data on the SAYR. Together, our results indicate a complex history of drainage integration, initiated by a tectonic-driven capture event likely during MIS 3 and punctuated by episodic climate-driven lake overspill and river incision. The lake overspill model may be widely applicable across the Tibetan Plateau, and potentially to Mars in the high plateau of its southern hemisphere. • Drainage reversal in a subsidence basin triggered river headwater initial integration. • Climatically episodic overspill integrated Gyaring & Ngoring Lakes into Yellow River. • Simultaneous highstands in the above two lakes occurred at 14-12 ka and ∼9 ka by OSL. • Alluvial fan aggradation enhanced in transitions of MIS 5b-a, MIS 4-3, & deglaciation. • Our proposed geomorphic lake overspill model provides an analog for craters on Mars. [ABSTRACT FROM AUTHOR]

Hellas basin is a large impact basin situated in the southern highlands of Mars. The north-western part of the basin has the lowest elevation (−7.5 km) on the planet and contains a possibly unique terrain type, which we informally call “banded terrain”. The banded terrain is made up of smooth-looking banded deposits that display signs of viscous behavior and a paucity of superimposed impact craters. In this study, we use newly acquired high spatial resolution images from the High Resolution Imaging Science Experiment (HiRISE) in addition to existing datasets to characterize the geomorphology, the morphometry and the architecture of the banded terrain. The banded terrain is generally confined to the NW edge of the Alpheus Colles plateau. The individual bands are ~3–15 km-long, ~0.3 km-wide and are separated by narrow inter-band depressions, which are ~65 m-wide and ~10 m-deep. The bands display several morphologies that vary from linear to concentric forms. Morphometric analysis reveals that the slopes along a given linear or lobate band ranges from 0.5° to 15° (average ~6°), whereas the concentric bands are located on flatter terrain (average slope ~2–3°). Crater-size frequency analysis yields an Amazonian-Hesperian boundary crater retention age for the terrain (~3 Gyr), which together, with the presence of very few degraded craters, either implies a recent emplacement, resurfacing, or intense erosion. The apparent sensitivity to local topography and preference for concentrating in localized depressions is compatible with deformation as a viscous fluid. In addition, the bands display clear signs of degradation and slumping at their margins along with a suite of other features that include fractured mounds, polygonal cracks at variable size-scales, and knobby/hummocky textures. Together, these features suggest an ice-rich composition for at least the upper layers of the terrain, which is currently being heavily modified through loss of ice and intense weathering, possibly by wind. [ABSTRACT FROM AUTHOR]

Abstract: Impact craters (“craters”) are ubiquitous across most solid surfaces in the Solar System. The most common use of crater populations (populations as defined by diameter- or “size-” frequency) is to estimate relative and absolute model surface ages based on two assumptions: Craters will form spatially randomly across the planetary body, and craters will form following a random distribution around a known or assumed temporal flux. Secondary craters – craters that form from the ejecta of a crater formed by an extraplanetary-sourced impactor – belie both of these assumptions and so will affect crater-based ages if not removed from crater counts. A question unanswered with observational data to this point has been, what is the population of primary versus secondary craters on a given planet? We have answered this question for Mars for craters larger than 1 km in diameter by using a recently published global crater database, classifying craters as primary or secondary, and creating maps of the population statistics. Our approach was to err on the side of a crater being primary by default and hence our work is a conservative measurement. We show that, globally, secondary craters are at least 24% as numerous as primary craters (comprising 19% of the total population) for diameters . However, there are many “hot spots” across the globe where secondary craters are more numerous than primary craters for diameters as large as 9 km. This is the first time such a study has been conducted globally for any body and it shows that, not only are secondary craters numerous, but they can significantly affect crater populations in a non-uniform way across a planetary surface. [Copyright &y& Elsevier]

Abstract: Basin formation ages for Moon, Mars and Mercury are determined by cratering statistics, compared and evaluated with respect to their maximum surface ages and available isotope ages, and two possible Solar System evolution models. Both Mars and Mercury appear to have undergone significant resurfacing, so that at least the first 200–400 million years are not recorded on their surfaces. Basin frequency and crater frequencies below 150 km indicate that the Moon has the oldest surface, Mercury has an intermediate age, and Mars has the youngest preserved terrain. An offset between the basin size-frequency distribution, the smaller crater size-frequency distribution and the main belt asteroid size-frequency distribution is observed in all three cases, suggesting an age difference of about 150 Ma between basin and smaller crater distribution-based ages. I interpreted this in terms of lack of understanding of the basin formation process, and suggest that one possible explanation for the apparently under-representative basin frequency could be a different (lower) average impact velocity compatible with the ‘Nice’ flux model. The basin formation pattern derived with the standard monotonically decaying or the sawtooth-like Nice-model flux does not reveal a coherent picture according to the late heavy bombardment idea. This is here attributed to an incomplete understanding of the cratering rate ratios between the planetary bodies considered here. Because of the Moon's unique formation history, I also suggest that it is questionable whether the Moon is a suitable analogue for the formation, evolution and cratering record of the other terrestrial bodies. [Copyright &y& Elsevier]

Abstract: Floor-Fractured Craters (FFCs) represent an impact crater type, where the infilling is separated by cracks into knobs of different sizes and shapes. This work focuses on the possible processes which form FFCs to understand the relationship between location and geological environment. We generated a global distribution map using new High Resolution Stereo Camera and Context Camera images. Four hundred and twenty-one potential FFCs have been identified on Mars. A strong link exists among floor fracturing, chaotic terrain, outflow channels and the dichotomy boundary. However, FFCs are also found in the Martian highlands. Additionally, two very diverse craters are used as a case study and we compared them regarding appearance of the surface units, chronology and geological processes. Five potential models of floor fracturing are presented and discussed here. The analyses suggest an origin due to volcanic activity, groundwater migration or tensile stresses. Also subsurface ice reservoirs and tectonic activity are taken into account. Furthermore, the origin of fracturing differs according to the location on Mars. [Copyright &y& Elsevier]

Abstract: High-resolution images reveal that numerous pit craters exist on the surface of Mars. For some pit craters, the depth-to-diameter ratios are much greater than for ordinary craters. Such deep pit craters are generally considered to be the results of material drainage into a subsurface void space, which might be formed by a lava tube, dike injection, extensional fracturing, and dilational normal faulting. Morphological studies indicate that the formation of a pit crater might be triggered by the impact event, and followed by collapse of the ceiling. To test this hypothesis, we carried out laboratory experiments of impact cratering into brittle targets with variable roof thickness. In particular, the effect of the target thickness on the crater formation is studied to understand the penetration process by an impact. For this purpose, we produced mortar targets with roof thickness of 1–6cm, and a bulk density of 1550kg/m3 by using a mixture of cement, water and sand (0.2mm) in the ratio of 1:1:10, by weight. The compressive strength of the resulting targets is 3.2±0.9MPa. A spherical nylon projectile (diameter 7mm) is shot perpendicularly into the target surface at the nominal velocity of 1.2km/s, using a two-stage light-gas gun. Craters are formed on the opposite side of the impact even when no target penetration occurs. Penetration of the target is achieved when craters on the opposite sides of the target connect with each other. In this case, the cross section of crater somehow attains a flat hourglass-like shape. We also find that the crater diameter on the opposite side is larger than that on the impact side, and more fragments are ejected from the crater on the opposite side than from the crater on the impact side. This result gives a qualitative explanation for the observation that the Martian deep pit craters lack a raised rim and have the ejecta deposit on their floor instead. [Copyright &y& Elsevier]

Abstract: The Sample Analysis at Mars (SAM) instrument suite is the largest scientific payload on the Mars Science Laboratory (MSL) Curiosity rover, which landed in Mars׳ Gale Crater in August 2012. As a miniature geochemical laboratory, SAM is well-equipped to address multiple aspects of MSL׳s primary science goal, characterizing the potential past or present habitability of Gale Crater. Atmospheric measurements support this goal through compositional investigations relevant to martian climate evolution. SAM instruments include a quadrupole mass spectrometer, a tunable laser spectrometer, and a gas chromatograph that are used to analyze martian atmospheric gases as well as volatiles released by pyrolysis of solid surface materials (Mahaffy et al., 2012). This report presents analytical methods for retrieving the chemical and isotopic composition of Mars׳ atmosphere from measurements obtained with SAM׳s quadrupole mass spectrometer. It provides empirical calibration constants for computing volume mixing ratios of the most abundant atmospheric species and analytical functions to correct for instrument artifacts and to characterize measurement uncertainties. Finally, we discuss differences in volume mixing ratios of the martian atmosphere as determined by SAM (Mahaffy et al., 2013) and Viking (Owen et al., 1977; Oyama and Berdahl, 1977) from an analytical perspective. Although the focus of this paper is atmospheric observations, much of the material concerning corrections for instrumental effects also applies to reduction of data acquired with SAM from analysis of solid samples. [Copyright &y& Elsevier]

Abstract: There is significant geomorphologic evidence for the past presence of longitudinally widespread, latitudinally zoned deposits composed of ice-rich material at the northern and southern mid latitudes on Mars (lobate debris aprons, lineated valley fill, concentric crater fill, pedestal craters, etc.). Among these features, pedestal craters (Pd) are impact craters interpreted to have produced a protective layer on top of decameters-thick ice deposits now missing in intercrater regions. The time during which these various deposits were present is still highly debated. To address this question we have analyzed the distribution and characteristics of pedestal craters; here, we use a population of 2287 pedestal craters (Pd) to derive a crater retention age for the entire population, obtaining a minimum timescale of formation of ~90Myr. Given that the ice-rich deposit has not been continuously present for this duration, the timescale of formation is necessarily longer than ~100Myr. We then compiled impact crater size-frequency distribution dates for 50 individual pedestal craters in both hemispheres to further assess the frequency distribution of individual ages. We calculated pedestal crater ages that ranged from ~1Myr to ~3.6Gyr, with a median of ~140Myr. In addition, 70% of the pedestal ages are less than 250Myr. During the 150Myr period between 25Ma and 175Ma, we found at least one pedestal age every 15Myr. This suggests that the ice-rich paleodeposit accumulated frequently during that time period. We then applied these results to the relationship between obliquity and latitudinal ice stability to suggest some constraints on the obliquity history of Mars over the past 200Myr. Atmospheric general circulation models indicate that ice stability over long periods in the mid latitudes is favored by moderate mean obliquities in the ~35° range. Models of spin-axis/orbital parameter evolution predict that the average obliquity of Mars is ~38°. Our data represent specific observational evidence that ice-rich deposits accumulated frequently during the past 200Myr, supporting the prediction that Mars was characterized by this obliquity range during an extensive part of that time period. Using these results as a foundation, the dating of other non-polar ice deposits will permit the specific obliquity history to be derived and lead to an assessment of volatile transport paths in the climate history of Mars. [Copyright &y& Elsevier]

Crater and ejecta morphology provide insight into the composition and structure of the target material. Fluidized ejecta surrounding Martian rampart craters are thought to result from the addition of water to the ejecta during impact into a water-rich (ice or liquid) regolith. Here we test experimentally an alternate hypothesis. We propose that the decompression of a rock–water mixture across the water vaporization curve during the excavation stage of impact cratering results in an increased proportion of fines in the ejecta. This enables the ejecta to flow with little or no liquid water present. To test this hypothesis, fragmentation experiments were conducted on sandstone (28 vol% open porosity) from the northern Eldorado Mountains, Nevada, using a shock-tube apparatus at the LMU Munich, Germany. Rock samples with 0–92% of their open pore space filled with water were pressurized to 15 MPa at 177 °C or 300 °C and rapidly decompressed. As the water vaporization curve is crossed, the water in the pore space rapidly flashes to steam causing, together with the expanding gas in the water-free pore space, the sample to fragment. The presence of water has a significant effect on the grain size distribution and grain shape of the fragmented rock samples. In comparison with (dry) control samples, samples with water with 15–50% open pore space exhibit much smaller grain sizes. The predominant grain shape of dry as well as partially water-saturated samples is bladed, reflecting fracturing parallel to the decompression front. Samples with water in open pore space had an increase in fines and larger particles but less intermediate sized particles. Fragments from experiments with water in open pore space also displayed a more equant grain shape, indicating that the decompression of water caused fracturing independent of the orientation of the decompression front. These results may provide insight into the morphology of Martian rampart craters. We propose here that even relatively low water contents in the target ( ) may be sufficient to produce a significant increased proportion of fines allowing the ejecta to flow with little or no water present. [ABSTRACT FROM AUTHOR]

Abstract: Peak-ring basins are important in understanding the formation of large impact basins on planetary bodies; however, debate still exists as to how peak rings form. Using altimetry and image data from the MESSENGER and LRO spacecraft in orbit around Mercury and the Moon, respectively, we measured the morphometric properties of impact structures in the transition from complex craters with central peaks to peak-ring basins. This work provides a comprehensive morphometric framework for craters and basins in this morphological transition that may be used to further develop and refine various models for peak-ring formation. First, we updated catalogs of craters and basins ≥50km in diameter possessing interior peaks on Mercury and the Moon. Crater degradation states were assessed and morphometric measurements were made on the freshest examples, including depths to the crater floor, areas contained within the outlines of the rim crest and floor, crater volumes, and rim-crest and floor circularity. There is an abrupt decrease in crater depth in the crater to basin transition on both Mercury and the Moon. Peak-ring basins have larger floor area/interior area ratios than complex craters; this ratio is larger in craters on Mercury than on the Moon. The dimensions of central peaks (heights, areas, and volumes exposed above the surface) increase continuously up to the transition to basins. Compared with central peaks, peak rings have reduced heights; however, all interior peaks are typically >1km below the rim-crest elevations. Topographic profiles of peak-ring basins on Mercury and the Moon are distinct from complex craters and exhibit interior cavities or depressions that are bounded by the peak ring with outer annuli that are at higher elevations. We interpret the trends in floor and interior area to be largely due to differences in impact melt production and retention, although variations in types and thicknesses of impactites, including proximal ejecta, could also contribute to the observed trends. Our trends illustrate that the transition from craters to basins is characterized by an abrupt change in morphology, implying a change in process for the formation of peak rings. Refinement of models for peak-ring formation through improved quantitative predictions of crater morphometry and cross-validation with our morphometric framework are needed to better constrain the processes that form peak rings on the terrestrial planets. [Copyright &y& Elsevier]

Abstract: The use of numerical datasets of the same type derived from remote sensing instruments is often treated as equivalent in value and utility, within the limits of their spatial resolution. Surface topography is a good example of this. We used gridded digital terrain models (DTMs) derived from both laser altimetry and stereo pairs as well as the original laser data points to quantify the topography of impact craters, which have well-studied morphometry. The primary purpose was to test the accuracy of laser data given an image-based DTM with approximately a factor of 10 better spatial resolution; we used the Mars Orbiter Laser Altimeter and High-Resolution Stereo Camera datasets for this work. We found that the ability to derive accurate topographic information in laser altimetry diminished well before the ability to visually resolve the feature in the dataset, but we also found that laser point and gridded data-based results were in good agreement down to their resolution limits. Relative to the image-based DTM, pixel-for-pixel, the laser data were more useful in their ability to visibly resolve the impact craters. This work has implications for remote sensing in general, but specifically the application to limited sources of data for planetary surface topography, such as Mercury where the northern hemisphere topography is being measured with a laser altimeter but the southern hemisphere topography is based only on stereo pairs. [Copyright &y& Elsevier]

Abstract: During its year-long orbital mission, the Dawn spacecraft has mapped the surface of main-belt asteroid Vesta multiple times at different spatial resolutions and illumination and viewing angles. The onboard Framing Camera has acquired thousands of clear filter and narrow band images, which, with the availability of high-resolution global shape models, allows for a photometric characterization of the surface in unprecedented detail. We analyze clear filter images to retrieve the photometric properties of the regolith. In the first part of the paper we evaluate different photometric models for the global average. In the second part we use these results to study variations in albedo and steepness of the phase curve over the surface. Maps of these two photometric parameters show large scale albedo variations, which appear to be associated with compositional differences. They also reveal the location of photometrically extreme terrains, where the phase curve is unusually shallow or steep. We find that shallow phase curves are associated with steep slopes on crater walls and faults, as calculated from a shape model. On the other hand, the phase curve of ejecta associated with young impact craters is steep. We interpret these variations in phase curve slope in terms of physical roughness of the regolith. The lack of rough ejecta around older craters suggests that initially rough ejecta associated with impact craters on Vesta are smoothed over a relatively short time of several tens of Myr. We propose that this process is the result of impact gardening, and as such represents a previously unrecognized aspect of Vesta space weathering (Pieters et al., 2012). If this type of space weathering is common, we may expect to encounter this photometric phenomenon on other main belt asteroids. [Copyright &y& Elsevier]

Abstract: Early extrusive volcanism from mantle melting marks the transition from primary to secondary crust formation. Detection of secondary crust is often obscured by the high impact flux early in solar system history. To recognize the relationship between heavily cratered terrain and volcanic resurfacing, this study documents how volcanic resurfacing alters the impact cratering record and models the thickness, area, and volume of volcanic flood deposits. Lunar Orbiter Laser Altimeter (LOLA) data are used to analyze three different regions of the lunar highlands: the Hertzsprung basin; a farside heavily cratered region; and the central highlands. Lunar mare emplacement style is assumed to be similar to that of terrestrial flood basalts, involving large volumes of material extruded from dike-fed fissures over relatively short periods of time. Thus, each region was flooded at 0.5km elevation intervals to simulate such volcanic flooding and to assess areal patterns, thickness, volumes, and emplacement history. These simulations show three primary stages of volcanic flooding: (1) Initial flooding is largely confined to individual craters and deposits are thick and localized; (2) basalt flows breach crater rim crests and are emplaced laterally between larger craters as thin widespread deposits; and (3) lateral spreading decreases in response to regional topographic variations and the deposits thicken and bury intermediate-sized and larger craters. Application of these techniques to the South Pole-Aitken basin shows that emplacement of ∼1−2km of cryptomaria can potentially explain the paucity of craters 20–64km in diameter on the floor of the basin relative to the distribution in the surrounding highlands. [Copyright &y& Elsevier]

Abstract: Demonstration of the existence of clay minerals and sulfates of Hellas brings us closer to the finding that water on Mars might have existed earlier and it may even exist now. Hellas Crater is a potential area where episodes of liquid water may appear. Infrared data from OMEGA and PFS installed on the Mars Express probe have been used for the investigation of the surface of Hellas for evidence of water. There are emissivity spectra from PFS and shortwave reflectance spectra from OMEGA. Only the most characteristic spectra indicating water have been chosen for presented studies. Additionally, high-resolution images from HiRISE installed on Mars Recconaissance Orbiter probe have been used for comparison. Some images from the HiRISE apparatus disclosed Hellas'' area weathered surface possibly created by water erosion, that is confirmed by infrared spectra of the OMEGA apparatus. Wave number of 1160cm−1 has been identified in the spectra of the PFS Mars Express probe, that fits very well to the band of sulfates. The band responsible for the occurrence of clay minerals associated with the presence of water in Hellas has been also found in the spectra obtained with the OMEGA instrument. [Copyright &y& Elsevier]

Abstract: Palimpsest, ghost, or degraded craters have so far been identified on Mercury, the Earth, the Moon, Mars, Ganymede, Callisto, and possibly even Titan. We have identified at least 10 palimpsests on HiRISE image ESP_016526_2415 of Mars and developed an algorithm to automatically characterize these structures on high resolution images. Since the genesis of palimpsests may be related to freeze–thaw cycles these features might have provided habitable environments. [Copyright &y& Elsevier]

Abstract: Radiation levels expected at the surface of Mars are known to have an impact on life form as well as biological and organic materials. Therefore the ionizing radiation must be studied in order to evaluate the potential organic preservation on Mars. MarsREC (Keating et al., 2005) and dMEREM (Gonçalves et al., 2009) are ionizing radiation environment characterization models based on GEANT4 (Agostinelli et al., 2003) Monte Carlo applications developed for the European Space Agency. Enabling the interface with geophysical databases, these models allow the evaluation of Martian radiation environment as a function of climatological and geological variables. This work defines critical geophysical parameters associated with geological evolution and assesses their impact in terms of radiation environment. Results show that the erosion of the Martian atmosphere and the regolith, due to the LHB era, led to an important increase of the radiation environment, consequently radiation hazard, and led to the degradation of habitability conditions at the surface of Mars. [Copyright &y& Elsevier]

Abstract: Impact craters cover a large portion of the surface of Mars and could constitute a significant exobiology research target as their formation provided heat sources for aqueous processes. To date, only rare examples of hydrothermal alteration in craters have been reported on Mars while many studies have focused on modeling their effect. Using data from the Mars Reconnaissance Orbiter and Mars Express probes, we report the presence of hydrated minerals, mainly Fe/Mg phyllosilicates, with vermiculite as best-fit, that are found in an alluvial fan. This fan is located inside a crater located in NE Hellas region and dated to the Late Hesperian by crater counts and crosscutting relationships. The stratigraphic position of the hydrated minerals and presence of small domes interpreted as hydrothermal vents indicate that the alteration occurred in the lower level of the alluvial fan and was triggered by bottom-up alteration. These observations are best explained by a combination of snow deposition and subsequent melting eroding crater rims and forming the fan, with impact warming, which triggered the alteration at the base of the fan. This example shows that phyllosilicates are able to form late in the Martian history, especially in local niches of strong exobiological interest. It also suggests that a similar process was possible in alluvial fans of other large impact craters including those at Gale crater. [Copyright &y& Elsevier]

Abstract: Hydrated silicates occur widespread in thousands of locations in the Martian Noachian-aged southern highlands. If an ocean existed on the northern lowlands of Mars during the Noachian, phyllosilicates are likewise expected to be present in the primeval crust of the Martian plains, since on Earth they are common products of continental fluvial transportation and oceanic sedimentation. Here we analyze data from spaceborne imaging spectrometers searching for Noachian Mg/Al-phyllosilicates in 198 impact craters in the northern lowlands of Mars, as impact-excavated materials have already been demonstrated as qualifying samples of preexisting subsurface deposits. Our results indicate that only 15 impact craters in the Martian lowlands show evidence of excavated Mg/Al-phyllosilicates (<10%, consistent with previous estimates). We have also analyzed 88 craters in the highlands region located between −1000 and −3000m, and in this case the number of craters showing excavated Mg/Al-phyllosilicates was 64 (>70%). Therefore, hydrated silicates form a mineralogical dichotomy noticeable in the ancient Noachian crust on a global scale. This is indicative of little or no significant fluvial transportation of phyllosilicate-rich, southern highland materials into the northern basins. But the Noachian phyllosilicate-rich terrains in the southern highlands are heavily dissected by ∼80,000 valley networks, revealing sustained surface runoff expected to be efficient at remobilizing sediments into the northern plains. The presence of widespread ice masses rimming cold glacial Noachian oceans is a plausible explanation for the lack of large-scale phyllosilicate minerals in the primeval basement of the lowlands of Mars. [Copyright &y& Elsevier]

Abstract: Gale Crater, the landing site of the 2011 Mars Science Laboratory mission, formed in the Late Noachian. It is a 150km diameter complex impact structure with a central mound (Mount Sharp), the original features of which may be transitional between a central peak and peak ring impact structure. The impact might have melted portions of the substrate to a maximum depth of ∼17km and produced a minimum of 3600km3 of impact melt, half of which likely remained within the crater. The bulk of this impact melt would have pooled in an annular depression surrounding the central uplift, creating an impact melt pool as thick as 0.5–1km. The ejecta blanket surrounding Gale may have been as thick as ∼600m, which has implications for the amount of erosion that has occurred since Gale Crater formed. After the impact, a hydrothermal system may have been active for several hundred thousand years and a crater lake with associated sediments is likely to have formed. The hydrothermal system, and associated lakes and springs, likely caused mineral alteration and precipitation. In the presence of S-rich host rocks, the alteration phases are modelled to contain sheet silicates, quartz, sulphates, and sulphides. Modelled alteration assemblages may be more complex if groundwater interaction persisted after initial alteration. The warm-water environment might have provided conditions supportive of life. Deep fractures would have allowed for hydraulic connectivity into the deep subsurface, where biotic chemistry (and possibly other evidence of life) may be preserved. [Copyright &y& Elsevier]