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S1094b is the largest (155 m-size) and southernmost known ice-exposing fresh crater discovered so far on Mars, revealing a relatively pure and unstable subsurface ice deposit located at the northern Martian mid-latitudes. In this work, we analyze HiRISE images taken on 27 February 2022 and on 5 December 2022 to perform a multi-temporal analysis of its ice-rich ejecta, combining this analysis with geologic mapping, the boulder size frequency distribution (SFD) and thermal modeling. The objective is to provide a multidisciplinary characterization of both the impact and subsequent exposed ice sublimation processes. The boulder SFD of both February and December cases show a power-law best fit with indices −4.68 ± 0.15 and − 3.47 ± 0.10, respectively. In the same timeframe, the density of boulders per km2 ≥ 1.5 m changes from 3908, to 596. This flattening is mainly due to the sublimation and consequent loss of the smaller-size icy boulders. This is confirmed by the ice volume computation performed on the area, which changed from ∼20,274 ± 3997 m3 to ∼7951 ± 1117 m3, i.e. a decrease of ∼60% in 274 Sols. The thermal models showed that the ice in this region is always unstable, leading to a total of 6504.71 sublimation hours from which we estimated a sublimation rate of ∼0.15 ± 0.04 mm/h (i.e. ∼3.60 ± 0.96 mm/Sol). The presence of this amount of accessible ice at such low latitudes could be a valuable resource for potential future human missions. • Conduction of multitemporal, multidisciplinary analyses on Martian crater S1094b. • Preparation of a geological mapping 5 geologic units and 4 landforms. • Observation of sublimation focusing on the smallest icy boulders loss. • Detection of the 60% water ice sublimation over 281 days. • Average sublimation rate detected at ∼1.96 m3/h or ∼ 0.15 ± 0.04 mm/h. [ABSTRACT FROM AUTHOR]

Highlights • We test hypotheses for the formation of craters within martian glacial deposits. • Nine crater types and multiple degradation sequences observed. • There is a small size difference between ring-mold craters and bowl-shaped craters. • Flow lineations cross-cut older ring-mold craters but not other crater types. • Observations most consistent with formation by mantle emplacement and dissection. Abstract Lobate debris aprons (LDA), lineated valley fill (LVF), and concentric crater fill (CCF) on Mars, interpreted to be debris-covered glaciers, possess craters with a suite of distinct interior landforms (called "ring-mold craters") that have been attributed to the presence of glacial ice at depth or surface modification processes. We tested current hypotheses for the formation of ring-mold craters by conducting a comprehensive analysis of the size and morphology of 16,457 impact craters ≥125 m in diameter formed within glacial deposits in Deuteronilus Mensae. Two major groups, bowl-shaped craters and ring-mold craters, are found, with at least nine distinct crater types. While there is statistical difference in median diameters between these crater types, this difference is relatively small and is within the estimated uncertainty in diameter measurements and may be related to more enhanced erosion of the rims of ring-mold craters. Clear degradation sequences are observed, supporting a role for post-impact modification in producing at least some of the diversity in crater landforms. The spatial density of ring-mold craters is also directly correlated with the development of LDA, LVF, and CCF surface textures. Flow lineations cross-cut two ring-mold crater types but they maintain their circular planforms in some cases, suggesting that the craters initially formed completely within mantling layers deposited after glacial flow had ceased. We also find analogous craters in non-glacial units; glacial ice is therefore not required to form the observed morphologic diversity. Our observations are most consistent with formation of crater landforms by emplacement and modification of at least two depositional episodes of icy dust (i.e., "mantle"). This mantle was initially tens of meters in thickness to support crater formation, and has experienced much downwasting and erosion since emplacement. Derived crater retention ages of 460 Ma for LDA, LVF, and CCF features in the region therefore reflect deposition of mantle units and only give a very minimum age for the formation of LDA, LVF, and CCF. [ABSTRACT FROM AUTHOR]

Polygonal networks of patterned ground are a common feature in cold-climate environments. They can form through the thermal contraction of ice-cemented sediment (i.e. formed from fractures), or the freezing and thawing of ground ice (i.e. formed by patterns of clasts, or ground deformation). The characteristics of these landforms provide information about environmental conditions. Analogous polygonal forms have been observed on Mars leading to inferences about environmental conditions. We have identified clastic polygonal features located around Lyot crater, Mars (50°N, 30°E). These polygons are unusually large (>100 m diameter) compared to terrestrial clastic polygons, and contain very large clasts, some of which are up to 15 metres in diameter. The polygons are distributed in a wide arc around the eastern side of Lyot crater, at a consistent distance from the crater rim. Using high-resolution imaging data, we digitised these features to extract morphological information. These data are compared to existing terrestrial and Martian polygon data to look for similarities and differences and to inform hypotheses concerning possible formation mechanisms. Our results show the clastic polygons do not have any morphometric features that indicate they are similar to terrestrial sorted, clastic polygons formed by freeze-thaw processes. They are too large, do not show the expected variation in form with slope, and have clasts that do not scale in size with polygon diameter. However, the clastic networks are similar in network morphology to thermal contraction cracks, and there is a potential direct Martian analogue in a sub-type of thermal contraction polygons located in Utopia Planitia. Based upon our observations, we reject the hypothesis that polygons located around Lyot formed as freeze-thaw polygons and instead an alternative mechanism is put forward: they result from the infilling of earlier thermal contraction cracks by wind-blown material, which then became compressed and/or cemented resulting in a resistant fill. Erosion then leads to preservation of these polygons in positive relief, while later weathering results in the fracturing of the fill material to form angular clasts. These results suggest that there was an extensive area of ice-rich terrain, the extent of which is linked to ejecta from Lyot crater. [ABSTRACT FROM AUTHOR]

The Mars Science Laboratory ( Curiosity rover) Dynamic Albedo of Neutrons (DAN) experiment detects neutrons for the purpose of searching for hydrogen in the shallow subsurface of Mars. DAN has two modes of operation, active and passive. In passive mode, the instrument detects neutrons produced by Galactic Cosmic Ray interactions in the atmosphere and regolith and by the rover's Multi-Mission Radioisotope Thermoelectric Generator. DAN passive data from Yellowknife Bay to Amargosa Valley (sols 201 through 753) are presented and analyzed here. Water equivalent hydrogen (WEH) estimates from this portion of Curiosity's traverse range from 0.0 wt. % up to 15.3 wt. %. Typical uncertainties on these WEH estimates are ∼0.5 wt. % but in some cases can be as high as ∼4.0 wt. % depending on the specific circumstances of a given measurement. Here we also present a new way of reporting results from the passive mode of the experiment, the DAN passive geochemical index (DPGI). This index is sensitive to some key geochemical variations, but it does not require assumptions about the abundances of high thermal neutron absorption cross section elements, which are needed to estimate WEH. DPGI variations in this section of the traverse indicate that the shallow regolith composition is changing on both the local (∼meters) and regional (∼100 s of meters) scales. This variability is thought to be representative of the diverse composition of source regions for sediments within the crater floor. Kolmogorov-Smirnov Tests on the populations of WEH estimates and DPGI values demonstrate there are statistically significant differences between nearly all of the geologic units investigated along the rover's traverse. We also present updated previous DAN passive results from Bradbury Landing to John Klein that make use of revised DAN active mode results for calibration, however, no qualitative changes in the interpretations made in Tate et al. (2015b) are incurred. [ABSTRACT FROM AUTHOR]

Excess ice is widespread throughout the martian mid-latitudes, particularly in Arcadia Planitia, where double-layer ejecta (DLE) craters also tend to be abundant. In this region, we observe the presence of thermokarstically-expanded secondary craters that likely form from impacts that destabilize a subsurface layer of excess ice, which subsequently sublimates. The presence of these expanded craters shows that excess ice is still preserved within the adjacent terrain. Here, we focus on a 15-km DLE crater that contains abundant superposed expanded craters in order to study the distribution of subsurface volatiles both at the time when the secondary craters formed and, by extension, remaining today. To do this, we measure the size distribution of the superposed expanded craters and use topographic data to calculate crater volumes as a proxy for the volumes of ice lost to sublimation during the expansion process. The inner ejecta layer contains craters that appear to have undergone more expansion, suggesting that excess ice was most abundant in that region. However, both of the ejecta layers had more expanded craters than the surrounding terrain. We extrapolate that the total volume of ice remaining within the entire ejecta deposit is as much as 74 km 3 or more. The variation in ice content between the ejecta layers could be the result of (1) volatile preservation from the formation of the DLE crater, (2) post-impact deposition in the form of ice lenses; or (3) preferential accumulation or preservation of subsequent snowfall. We have ruled out (2) as the primary mode for ice deposition in this location based on inconsistencies with our observations, though it may operate in concert with other processes. Although none of the existing DLE formation hypotheses are completely consistent with our observations, which may merit a new or modified mechanism, we can conclude that DLE craters contain a significant quantity of excess ice today. [ABSTRACT FROM AUTHOR]

Fluvial features present around impact craters on Mars can offer insight into the ancient martian climate and its relationship to the impact cratering process. The widespread spatial and temporal distribution of surface ice on Mars suggests that the interaction between impact cratering and surface ice could have been a relatively frequent occurrence. We explore the thermal and melting effects on regional surface ice sheets in this case, where an impact event occurs in regional surface ice deposits overlying a regolith/bedrock target. We provide an estimate for the post-impact temperature of martian ejecta as a function of crater diameter, and conduct thermal modeling to assess the degree to which contact melting of hot ejecta superposed on surface ice deposits can produce meltwater and carve fluvial features. We also evaluate whether fluvial features could form as a result of basal melting of the ice deposits in response to the thermal insulation provided by the overlying impact ejecta. Contact melting is predicted to occur immediately following ejecta emplacement over the course of hundreds of years to tens of kyr. Basal melting initiates when the 273 K isotherm rises through the crust and reaches the base of the ice sheet ∼0.1 to ∼1 Myr following the impact. We assess the range of crater diameters predicted to produce contact and basal melting of surface ice sheets, as well as the melt fluxes, volumes, timescales, predicted locations of melting (relative to the crater), and the associated hydraulic and hydrologic consequences. We find that the heat flux and surface temperature conditions required to produce contact melting are met throughout martian history, whereas the heat flux and surface temperature conditions to produce basal melting are met only under currently understood ancient martian thermal conditions. For an impact into a regional ice sheet, the contact and basal melting mechanisms are predicted to generate melt volumes between ∼10 −1 and 10 5 km 3 , depending on crater diameter, ice thickness, surface temperature, and geothermal heat flux. Contact melting is predicted to produce fluvial features on the surface of ejecta and the interior crater walls, whereas basal melting is predicted to produce fluvial features only on the interior crater walls. Before basal melting initiates, the ice-cemented cryosphere underlying the crater ejecta is predicted to melt and drain downwards through the substratum, generating a source of water for chemical alteration and possibly subsurface clay formation. These candidate melting processes are predicted to occur under a wide range of parameters, and provides a basis for further morphologic investigation. [ABSTRACT FROM AUTHOR]

Endeavour crater (2.28°S, 354.77°E) is a Noachian-aged 22 km-diameter impact structure of complex morphology in southern Meridiani Planum. The degradation state of the crater has been studied using orbital data from the Mars Reconnaissance Orbiter and in situ data from the Opportunity rover. Multiple exposed crater rim segments range in elevation from ∼10 m to over 100 m above the level of the embaying Burns Formation. The crater is 200–500 m deep and the interior wall exposes over ∼300 m of relief around the southern half of the crater. Slopes of 6–16% flank the exterior of the largest western rim segment. On the west side of the crater, both pre-impact rocks (Matijevic Formation) and Endeavour impact ejecta (Shoemaker Formation) are present at Cape York, but only the Shoemaker Formation (up to ∼140 m section) outcrops at Cape Tribulation. Study of similar sized pristine craters Bopolu and Tooting (with complex morphology) and use of metrics for describing the morphometry of martian craters suggest the original rim of Endeavour averaged 410 m in elevation, but relief varied about ±200 m around the circumference. A 250–275 m section of ejecta (±50–60 m) would have comprised a significant fraction of the rim height. The original crater was likely 1.5–2.2 km deep and may have had a central peak (no obvious evidence is present) between 200 and 500 m high. Comparison between the predicted original and current form of Endeavour suggests 100–200 m of rim degradation ranging from nearly complete ejecta removal in some locations to preservation of a thick ejecta section in others. Differences in rim relief are at least partially due to degradation and not just original rim relief and (or) due to offsets along rim faults. Most degradation occurred prior to deposition of the Burns Formation which is ∼200 m thick outside the crater, but likely thicker inside the crater. Aeolian stripping of the Burns Formation continues today via prevailing winds and lesser mass wasting is important on steeper walls. However, analogy with degraded Noachian craters south of Meridiani suggests fluvial processes were most important in early degradation and is consistent with the nearly complete removal of ejecta from some rim segments, gaps in the rim, formation of Marathon Valley, and interpretation of a pediment flanking the western rim. Slope processes likely accompanied incision that may have accounted for tens of metres rim lowering near Marathon Valley to more than 100 m at Cape York. [ABSTRACT FROM AUTHOR]

The predictions of crater chronology models have customarily been evaluated by dividing a crater population into discrete diameter intervals, plotting the crater density for each, and finding a best-fit model isochron, with the uncertainty in the procedure being assessed using 1/√ n estimates, where n is the number of craters in an interval. This approach yields an approximate evaluation of the model predictions. The approximation is good until n becomes small, hence the often-posed question: what is the minimum number of craters for an adequate prediction? This work introduces an approach for exact evaluation of a crater chronology model using Poisson statistics and Bayesian inference, expressing the result as a likelihood function with an intrinsic uncertainty. We demonstrate that even in the case of no craters at all, a meaningful likelihood function can be obtained. Thus there is no required minimum count: there is only varying uncertainty, which can be well described. We recommend that the Poisson timing analysis should be preferred over binning/best-fit approaches. Additionally, we introduce a new notation to make it consistently clear that crater chronology model calibration errors are inseparable from stated crater model ages and their associated statistical errors. [ABSTRACT FROM AUTHOR]

Zumba is a ∼2.9 km diameter rayed crater on Mars located on extensive lava plains in Daedalia Planum to the southwest of Arsia Mons. It is a well-preserved young crater with large ejecta rays that extend for hundreds of kilometers from the impact site. The rays are thermally distinct from the background lava flows in THEMIS daytime and nighttime thermal infrared data, a unique characteristic among martian rayed craters. Concentrated within the rays are solitary or dense clusters of secondary craters with associated diffuse dark-toned deposits along with fewer secondary craters lacking dark-toned deposits. Using CTX images, we have mapped secondary craters with dark-toned deposits, collectively termed “secondary fields”, to investigate their distribution as a function of distance from the impact site. The mapped secondary field was then used to investigate various aspects of the crater-forming event such as the surface angle and direction of the projectile, the effect of secondary craters on surface age estimates, and the number of secondary craters produced by the impact event. From our mapping, a total of 13,064 secondary fields were documented out to a 200 km radial distance beyond a 15 km-wide non-secondary zone around Zumba crater. Results show that the highest areal coverage of secondary fields occurs within 100 km of Zumba and within its rays, decreasing radially with distance to a background scattering of small secondary fields that are <10 km 2 in size (∼91.9% of the total population). The strong east–west asymmetry of crater rays and low areal coverage of fields to the south of Zumba correlate well with studies of Zumba impact melt deposits, indicating a moderately oblique impact projectile coming from the south. Using primary craters in a ∼101 km 2 sample region and all craters (primaries and secondaries) from 43 select secondary fields in two map sectors in the study area, we obtain ages of ∼580 ± 100 Ma and ∼650 ± 70 Ma, respectively, for the lava flows into which Zumba impacted. These ages are consistent with and intermediate to 0.1–1 Ga volcanic flow units within and near Daedalia Planum. For craters within the secondary fields, a log differential plot of the data shows a pronounced downward deflection of the binned points for craters <100 m in diameter. This is an indication that possible aeolian erosion and deposition, in combination with emplacement and remobilization of Zumba ejecta, have modified the lava flow surfaces in Daedalia Planum over time. Our crater count results also show that even in the presence of large numbers of secondary craters, the age of the primary crater surface can still be identified from the data for craters >100 m. We estimate that ∼352,000 secondary craters were produced by the Zumba impact event. [ABSTRACT FROM AUTHOR]

“Blast zones” (BZs) around new martian craters comprise various albedo features caused by the initial impact, including diffuse halos, extended linear and arcuate rays, secondary craters, ejecta patterns, and dust avalanches. We examined these features for changes in repeat images separated by up to four Mars years. Here we present the first comprehensive survey of the qualitative and quantitative changes observed in impact blast zones over time. Such changes are most likely due to airfall of high-albedo dust restoring darkened areas to their original albedo, the albedo of adjacent non-impacted surfaces. Although some sites show drastic changes over short timescales, nearly half of the sites show no obvious changes over several Mars years. Albedo changes are more likely to occur at higher-latitude sites, lower-elevation sites, and at sites with smaller central craters. No correlation was seen between amount of change and Dust Cover Index, relative halo size, or historical regional albedo changes. Quantitative albedo measurements of the diffuse dark halos relative to their surroundings yielded estimates of fading lifetimes for these features. The average lifetime among sites with measurable fading is ∼15 Mars years; the median is ∼8 Mars years for a linear brightening. However, at approximately half of sites with three or more repeat images, a nonlinear function with rapid initial fading followed by a slow increase in albedo provides a better fit to the fading behavior; this would predict even longer lifetimes. The predicted lifetimes of BZs are comparable to those of slope streaks, and considered representative of fading by global atmospheric dust deposition; they last significantly longer than dust devil or rover tracks, albedo features that are erased by different processes. These relatively long lifetimes indicate that the measurement of the current impact rate by Daubar et al. (Daubar, I.J. et al. [2013]. Icarus 225, 506–516. http://dx.doi.org/10.1016/j.icarus.2013.04.009 ) does not suffer significantly from overall under-sampling due to blast zones fading before new impact sites can be initially discovered. However, the prevalence of changes seen around smaller craters may explain in part their shallower size frequency distribution. [ABSTRACT FROM AUTHOR]

We conducted a survey of central mounds within large (>25 km diameter) impact craters on Mars. We use mound locations, mound offsets within their host craters, and relative mound heights to address and extend various mound formation hypotheses. The results of this survey support the hypothesis that mound sediments once filled their host craters and were later eroded into the features we observe today. The majority of mounds are located near the boundaries of previously identified large-scale sedimentary deposits. We discuss the implications of the hypothesis that central mounds are part of previously more extensive sedimentary units that filled and overtopped underlying impact craters. In this scenario, as erosion of the sedimentary unit occurred, the sediment within impact craters was preserved slightly longer than the overlying sediment because it was sheltered by the crater walls. Our study also reveals that most mounds are offset from the center of their host crater in the same direction as the present regional winds ( e.g. , the mounds in Arabia Terra are offset towards the western portion of their craters). We propose that this implies that wind has been the dominant agent causing the erosion of central mounds. [ABSTRACT FROM AUTHOR]

We calculate the present-day impact flux on Mars and its variation over the martian year, using the current data on the orbital distribution of known Mars-crossing minor planets. We adapt the Öpik–Wetherill formulation for calculating collision probabilities, paying careful attention to the non-uniform distribution of the perihelion longitude and the argument of perihelion owed to secular planetary perturbations. We find that, at the current epoch, the Mars crossers have an axial distribution of the argument of perihelion, and the mean direction of their eccentricity vectors is nearly aligned with Mars’ eccentricity vector. These previously neglected angular non-uniformities have the effect of depressing the mean annual impact flux by a factor of about 2 compared to the estimate based on a uniform random distribution of the angular elements of Mars-crossers; the amplitude of the seasonal variation of the impact flux is likewise depressed by a factor of about 4–5. We estimate that the flux of large impactors (of absolute magnitude H < 16 ) within ±30° of Mars’ aphelion is about three times larger than when the planet is near perihelion. Extrapolation of our results to a model population of meter-size Mars-crossers shows that if these small impactors have a uniform distribution of their angular elements, then their aphelion-to-perihelion impact flux ratio would be 11–15, but if they track the orbital distribution of the large impactors, including their non-uniform angular elements, then this ratio would be about 3. Comparison of our results with the current dataset of fresh impact craters on Mars (detected with Mars-orbiting spacecraft) appears to rule out the uniform distribution of angular elements. [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]

Ground ice has been observed in small fresh craters in the vicinity of the Viking 2 lander site (48°N, 134°E). To explain these observations, current models for ground ice invoke levels of atmospheric water of 20 precipitable micrometers – higher than observations. However, surface frost has been observed at the Viking 2 site and surface water frost and snow have been shown to have a stabilizing effect on Antarctic subsurface ice. A snow or frost cover provides a source of humidity that should reduce the water vapor gradient and hence retard the sublimation loss from subsurface ice. We have modeled this effect for the Viking 2 landing site with combined ground ice and surface frost models. Our model is driven by atmospheric output fields from the NASA Ames Mars General Circulation Model (MGCM). Our modeling results show that the inclusion of a thin seasonal frost layer, present for a duration similar to that observed by the Viking Lander 2, produces ice table depths that are significantly shallower than a model that omits surface frost. When a maximum frost albedo of 0.35 was permitted, seasonal frost is present in our model from Ls = 182° to Ls = 16°, resulting in an ice table depth of 64 cm – which is 24 cm shallower than the frost-free scenario. The computed ice table depth is only slightly sensitive to the assumed maximum frost albedo or thickness in the model. [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]

We present a new catalog of 205 candidate closed-basin lakes contained within impact craters across the surface of Mars. These basins have an inlet valley that incises the crater rim and flows into the basin but no visible outlet valley, and are considered candidate closed-basin lakes; the presence of a valley flowing into a basin does not necessitate the formation of a standing body of water. The major geomorphic distinction within our catalog of candidate paleolakes is the length of the inlet valley(s), with two major classes – basins with long (>20 km) inlet valleys (30 basins), and basins with short (<20 km) inlet valleys (175 basins). We identify 55 basins that contain sedimentary fan deposits at the mouths of their inlet valleys, of which nine are fed by long inlet valleys and 46 are fed by short inlet valleys. Analysis of the mineralogy of these fan deposits suggests that they are primarily composed of detrital material. Additionally, we find no evidence for widespread evaporite deposit formation within our catalog of candidate closed-basin lakes, which we conclude is indicative of a general transience for any lakes that did form within these basins. Morphometric characteristics for our catalog indicate that as an upper limit, these basins represent a volume of water equivalent to a ∼1.2 m global equivalent layer (GEL) of water spread evenly across the martian surface; this is a small fraction of the modern water ice reservoir on Mars. Our catalog offers a broader context within which results from the Mars Science Laboratory Curiosity rover can be interpreted, as Gale crater is a candidate closed-basin lake contained within our catalog. Gale is also one of 12 closed-basin lakes fed by both long and short inlet valleys, and so in␣situ analyses by Curiosity can shed light on the relative importance of these two types of inlets for any lacustrine activity within the basin. [ABSTRACT FROM AUTHOR]

Ritchey crater is a ∼79 km diameter complex crater near the boundary between Hesperian ridged plains and Noachian highland terrain on Mars (28.8°S, 309.0°E) that formed after the Noachian. High Resolution Imaging Science Experiment (HiRISE) images of the central peak reveal fractured massive bedrock and megabreccia with large clasts. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) spectral analysis reveals low calcium pyroxene (LCP), olivine (OL), hydrated silicates (phyllosilicates) and a possible identification of plagioclase bedrock. We mapped the Ritchey crater central uplift into ten units, with 4 main groups from oldest and originally deepest to youngest: (1) megabreccia with large clasts rich in LCP and OL, and with alteration to phyllosilicates; (2) massive bedrock with bright and dark regions rich in LCP or OL, respectively; (3) LCP and OL-rich impactites draped over the central uplift; and (4) aeolian deposits. We interpret the primitive martian crust as igneous rocks rich in LCP, OL, and probably plagioclase, as previously observed in eastern Valles Marineris. We do not observe high-calcium pyroxene (HCP) rich bedrock as seen in Argyre or western Valles Marineris. The association of phyllosilicates with deep megabreccia could be from impact-induced alteration, either as a result of the Richey impact, or alteration of pre-existing impactites from Argyre basin and other large impacts that preceded the Ritchey impact, or both. [ABSTRACT FROM AUTHOR]

Craters within Arabia Terra, Mars, contain hundreds of meters of layered strata showing systematic alternation between slope- and cliff-forming units, suggesting either rhythmic deposition of distinct lithologies or similar lithologies that experienced differential cementation. On Earth, rhythmically deposited strata can be examined in terms of stratal packaging, wherein the interplay of tectonics, sediment deposition, and base level (i.e., the position above which sediment accumulation is expected to be temporary) result in changes in the amount of space available for sediment accumulation. These predictable patterns of sediment deposition can be used to infer changes in basin accommodation regardless of the mechanism of deposition (e.g. fluvial, lacustrine, or aeolian). Here, we analyze sedimentary deposits from three craters (Becquerel Crater, Danielson Crater, Crater A) in Arabia Terra. Each crater contains layered deposits that are clearly observed in orbital images. Although orbital images are insufficient to specifically determine the origin of sedimentary deposits, depositional couplets can be interpreted in terms of potential accommodation space available for deposition, and changes in the distribution of couplet thickness through stratigraphy can be interpreted in terms of changing base level and the production of new accommodation space. Differences in stratal packaging in these three craters suggest varying relationships between sedimentary influx, sedimentary base level, and concomitant changes in accommodation space. Previous groundwater upwelling models hypothesize that layered sedimentary deposits were deposited under warm climate conditions of early Mars. Here, we use observed stacking patterns to propose a model for deposition under cold climate conditions, wherein episodic melting of ground ice could raise local base level, stabilize sediment deposition, and result in differential cementation of accumulated strata. Such analysis demonstrates that a first-order understanding of sedimentary deposition and accumulation—despite a lack of textural information that inhibits interpretation of depositional mechanism—can provide insight into potentially changeable depositional conditions of early Mars. [ABSTRACT FROM AUTHOR]

A range of observations indicates widespread subsurface ice throughout the mid and high latitudes of Mars in the form of both pore-filling and excess ice. It is generally thought that this ice was recently emplaced and is not older than a hundred thousand to a few millions of years old based on ice stability and orbital-induced climate change. We analyze the distribution of subsurface ice in Arcadia Planitia, located in the northern mid latitudes, by mapping thermokarstically expanded secondary craters, providing additional evidence for extensive excess ice down to fairly low latitudes (less than 40°N). We further infer the minimum age of this subsurface ice based on the ages of the four primary craters that are thought to be the source of a large portion of these secondaries, which yields estimates on the order of tens of millions of years old – much more ancient than anticipated. This estimated ancient age suggests that ice can be preserved in the shallow subsurface for long periods of time, at least in some parts of Arcadia Planitia where expanded secondary craters are especially abundant. We estimate the amount of ice lost to sublimation during crater expansion based on measurements of expanded secondary craters in HiRISE Digital Terrain Models. The loss is equivalent to a volume of ice between ∼140 and 360 km 3 , which would correspond to a global layer of 1–2.5 mm thick. We further argue that much more ice (at least 6000 km 3 ) is likely preserved beneath the un-cratered regions of Arcadia Planitia since significant loss of this excess ice would have caused extensive terrain dissection and the removal of the expanded secondary craters. Both the loss of ice due to secondary crater expansion and the presence of this ice today have implications for the martian climate. [ABSTRACT FROM AUTHOR]

Martian craters with fluidized ejecta – including single-layered, double-layered and multiple-layered craters – have been studied extensively, with their formation generally suggested to require some presence of volatiles in the subsurface. However, experimental reproduction of these morphologies, impact modelling, and the occurrence of layered ejecta in putative volatile poor regions suggests that other factors may also play important roles. A recent extensive catalogue of martian impact craters (Robbins, S.J., Hynek, B.M. [2012a]. J. Geophys. Res. 117, E05004) classifies crater ejecta along with their location, diameters and ejecta extents, potentially providing new information on the links between these morphologies and the subsurface. We utilise this catalogue to examine the regional variation in ejecta mobility, onset diameter and the correlation between ejecta mobility and diameter for single- and double-layered ejecta craters on Mars. A simple regional stratigraphic model is developed to explain the observed trends through the viscosity of the layers within the target. Using this model, the potential relative thickness and burial depths of low viscosity layers in the martian subsurface are hypothesised, and compared to other observations and models of subsurface volatiles and how they have varied throughout time. [ABSTRACT FROM AUTHOR]

The acquisition of high-resolution imagery for the surface of Mars has enabled mapping of spatially limited (order of <10 3 km 2 ) landforms such as alluvial fans, deltas, and lacustrine deposits that are targets for exploration due to their association with liquid water. It is essential for our understanding of the planet’s geologic and climate history therefore to place these landforms within the global chronostratigraphic context. Here, we analyze both the statistical variability in the cratering pattern as well as the influence of small crater resurfacing on crater counting small landforms. We identified and counted craters (diameter ( D ) > 200 m) on four type terrains using Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) imagery that span the Noachian, Hesperian, and Amazonian epochs. The counts from each location include a region covering 10,000 km 2 , ten 1000 km 2 subsets of that larger area, and approximately one hundred 100 km 2 samples. The data demonstrate significant variation in the crater size frequency and derived model ages across a single terrain type for the 100 km 2 samples. The crater size frequency at this area scale varies across a single, uniform geologic unit by up to a factor of 2–3 on the four different terrains. At 1000 km 2 , the local pattern variations that are relevant at the 100 km 2 scale become less important and the age variations are tighter. In all four terrain cases, the 10,000 km 2 and 1000 km 2 samples capture distinct crater populations (km-sized craters) that formed before and after resurfacing event(s). However, due to the relatively high mean distance between km-sized craters, the 100 km 2 size area samples more commonly than not exclude a statistically significant sample at the kilometer size range, masking important information about the pre-resurfacing history of the terrain. We therefore suggest that due to the effect of pattern variability in cratering over 100 km 2 and the susceptibility of smaller craters to resurfacing, crater counts derived from small area samples are suspect to major uncertainties. [ABSTRACT FROM AUTHOR]

Low-aspect ratio layered ejecta (LARLE) craters are one of the most enigmatic types of martian layered ejecta craters. We propose that the extensive outer layer of these craters is produced through the same base surge mechanism as that which produced the base surge deposits generated by near-surface, buried nuclear and high-explosive detonations. However, the LARLE layers have higher aspect ratios compared with base surge deposits from explosion craters, a result of differences in thicknesses of these layers. This characteristics is probably caused by the addition of large amounts of small particles of dust and ice derived from climate-related mantles of snow, ice and dust in the areas where LARLE craters form. These deposits are likely to be quickly stabilized (order of a few days to a few years) from eolian erosion by formation of duricrust produced by diffusion of water vapor out of the deposits. [ABSTRACT FROM AUTHOR]

A new mapping-based study of the 900-km-long Mangala Valles outflow system was motivated by the availability of new high-resolution images and continued debates about the roles of water and lava in outflow channels on Mars. This study uses photogeologic analysis, geomorphic surface mapping, cratering statistics, and relative stratigraphy. Results show that Mangala Valles underwent at least two episodes of fluvial activity and at least three episodes of volcanic activity during the Late Amazonian. The occurrence of scoured bedrock at the base of the mapped stratigraphy, in addition to evidence provided by crater retention ages, suggests that fluvial activity preceded the deposition of two of the volcanic units. Crater counts performed at 30 locations throughout the area have allowed us to construct the following timeline: (1) formation of Noachian Highlands and possible initial flooding event(s) before ∼1 Ga, (2) emplacement of Tharsis lava flows in the valley from ∼700 to 1000 Ma, (3) a megaflooding event at ∼700–800 Ma sourced from Mangala Fossa, (4) valley fill by a sequence of lava flows sourced from Mangala Fossa ∼400–500 Ma, (5) another megaflooding event from ∼400 Ma, (6) a final phase of volcanism sourced from Mangala Fossa ∼300–350 Ma, and (7) emplacement of eolian sedimentary deposits in the northern portion of the valley ∼300 Ma. These results are consistent with alternating episodes of aqueous flooding and volcanism in the valles. This pattern of geologic activity is similar to that of other outflow systems, such as Kasei Valles, suggesting that there is a recurring, and perhaps coupled, nature of these processes on Mars. [ABSTRACT FROM AUTHOR]

Morphologic characteristics of ice-rich landforms in the martian mid-latitudes record evidence for significant modification of the landscape in response to spin–axis/orbital parameter-driven shifts in the Late Amazonian climate. These landforms are spatially distributed across the mid-latitudes and their co-existing presence has so far not been observed from a single crater to infer how exactly a terrain has been modified while Mars was undergoing major–moderate–minor shifts in its Late Amazonian climate. We have therefore carried out an in-depth investigation of Moreux crater (∼135 km, centered at 41.66°N, 44.44°E in the Protonilus Mensae region) for identification of features associated with recent and episodic glacial events and for emphasizing the role played by these glacial events in the modification of the crater. Evidence for extensive modification of the surfaces over crater rim/wall and around central peak by emplacement of multiple scales of ice-rich landforms that represents large history of glacial activities was found. From our results we document phases of major–moderate–minor glacial activities within the crater as: (1) piedmont lobes/lobate debris aprons/linear valley fills (∼1 Ga–100 Ma), (2) viscous flow features (∼30–0.1 Ma) and (3) gullies/thermal contraction crack polygons (∼2.1–0.4 Ma). The form and distribution of the random valleys observed within Moreux suggests their formation by pressure-induced melting and flow occurring beneath an extensive layer of ice. We also suggest that central peak of Moreux probably acted as the locus for accumulation of ice/snow and the diversity of glacial/periglacial features within the crater was possibly controlled by differences in the amount of accumulated ice/snow and the rate at which the terrain responded to the shifts in climate during subsequent periods of obliquity changes. Taken together, these ice-rich deposits within Moreux suggest that sequential modification of the crater surfaces over the rim/wall and around central peak has occurred over the last tens of millions of years of martian history. This new evidence thus adds another well-documented case to rapidly accumulating evidences for widespread glacial activity in the middle latitudes of Mars in recent martian history. [ABSTRACT FROM AUTHOR]

The process of formation of observed geologically recent gully features on Mars has remained a topic of intense debate since their discovery. In this study, we performed quantitative morphological analysis on certain parameters of gullies from different settings, such as crater walls, terraces, and sand dunes, on the martian surface in addition to the Meteor and Xiuyan craters on the Earth. The morphometric parameters were measured for cross profiles, which were extracted along each gully at certain intervals. Some interesting relationships among the parameters were determined, which could provide us a comprehensive understanding of the morphologies of the gullies’. The results show that strong correlations exist among those parameters, and the gullies are morphometrically similar, except for a scale difference in different geologic settings. The morphometric similarity implies that they were probably formed by some common processes. On the other hand, the morphometric differences indicate that the processes may have played different roles in the formation of the gullies. The formation of gullies on the Earth crater walls was heavily affected by surface flow and slippage, and pre-existing fractures and faults were also very influential in their formation. We propose that gullies in martian crater walls and terraces should have a similar formation mechanism, and they can probably account for most of gullies appearing on crater walls. The morphometric differences between the gullies in sand dunes and other gully types are probably a result of the disparity in lithological settings, which have significant influence on erosion ability even for the same agents. [ABSTRACT FROM AUTHOR]

Milna Crater, Mars (23.4S, 12.3W) exhibits signs of fluvial modification early in Mars history, including a large multi-lobed fan deposit cut by several sinuous valleys. We describe the past hydrologic conditions in Milna and the surrounding area, including a potential lake with a volume of 50 km 3 . We also introduce new methods (i) to calculate the timescale of sediment deposition by considering fluvial sediment input into the entire crater while accounting for non-fluvial input, and (ii) to place improved constraints on the channel dimensions through which sediment was delivered to Milna by comparing to the dimensions of inner channels found in valleys in the region surrounding Milna. By calculating the flux of fluid and sediment into the crater, we find that the crater cavity was flooded for at least months and that the time of active fluvial sediment transport without hiatus is on the order of decades to centuries, with a preferred timescale of centuries. We also calculate the total amount of water required to transport the volume of sediment we measure in Milna and conclude that impacts alone are likely insufficient to deliver enough water to Milna to allow the sedimentary fill we see. Combining the timescales of fluvial activity in the adjacent Paraná Valles with estimates for global Noachian erosion rates, we calculate an intermittency factor for fluvial activity of ∼0.01–0.1% during 10 5 –10 6 yr near the Noachian–Hesperian boundary in the Paraná Valles region. These values are comparable to arid climates on Earth where the majority of fluvial sedimentary transport takes place during floods with multi-year to decadal recurrence intervals. Our calculations of intermittency help to quantitatively reconcile the divergent estimates of the short and long timescales of fluvial activity on Mars reported in the literature. [ABSTRACT FROM AUTHOR]

Highlights: [•] Impact craters that form in layered targets exhibit unique morphologies. [•] This work determines how the dimensions of the crater shape relate to layer depth. [•] This work determines the boundary conditions under which this relationship holds. [•] This work examines the implications of applying this analysis to Mars. [Copyright &y& Elsevier]

Highlights: [•] Very young (∼0.2Ma) and well-preserved deposits of water bearing debris flows in a mid-latitude crater on Mars. [•] Insolation-controlled slope processes are surprisingly efficient on Mars under recent climate conditions. [•] Debris flows postdates the latest dust–ice mantle emplacement. [Copyright &y& Elsevier]

Highlights: [•] We model craters on Mars and the Moon using the observed terrestrial fireball flux. [•] We reproduce martian and lunar crater-count chronometry systems. [•] It may be possible to date young surfaces using small craters on the Moon and Mars. [•] Crater-count chronometry systems are derived from predominately primary craters. [Copyright &y& Elsevier]

Highlights: [•] The Mars Rover Opportunity analyzed Burns Fm ejecta blocks at Santa Maria crater. [•] Many blocks are eolian sandstone, but others are massive, fine-grained and nodular. [•] The fine-grained rocks may be mudstones, formed in a lacustrine setting. [•] This is a unique occurrence of such fine material at Meridiani Planum. [•] Surface water may have created a different environment than previously regarded. [ABSTRACT FROM AUTHOR]

Highlights: [•] Some martian crater high ejecta mobility proposed to result from sliding on ice. [•] Ice vaporization, fragmentation and deceleration, reduces ejecta mobility (EM). [•] Surface ice thickness affects EM: thicker surface ice inhibits EM. [•] Long-runout distances of LARLE craters point to formation in a thin icy substrate. [•] Formation of martian crater classes (MLE, SLE, DLE, Pd, LARLE) is examined. [ABSTRACT FROM AUTHOR]

Highlights: [•] We look at global trends in martian dune fields. [•] Dunes are concentrated in ergs north of 70°N and in craters south of 40°S. [•] Dune types are similar to terrestrial dunes, except bullseye dune fields. [•] Ground-based wind evidence is consistent with some expected wind patterns. [•] Inconsistent wind evidence likely due to effects of topography on surface winds. [ABSTRACT FROM AUTHOR]

Highlights: [•] Wind erosion reveals Saheki crater fan stratigraphy. [•] A distributary network of fluvial channels fed extensive mudflow overbank deposits. [•] The fans are up to 850m thick and contain 550km3 of sediment. [•] Fan-forming discharges derived from annual or episodic melting of crater rim snow. [•] Thousands of years were required to deposit the fans. [ABSTRACT FROM AUTHOR]

Highlights: [•] Martian flat high inertia crater floors formed by volcanic infilling at ∼3.5Ga. [•] Impact events removed portions of the upper crust causing mantle decompression melting. [•] This process was widespread on early Mars and has gone previously undocumented. [Copyright &y& Elsevier]

Highlights: [•] We investigate the fluid-like ejecta of Hale crater in Argyre basin using HiRISE. [•] The ejecta display unique morphologies indicative of volatile-rich composition. [•] The ejecta behave like viscous flow features. [•] The ejecta have been modified by hydrological and glacial/periglacial processes. [Copyright &y& Elsevier]

Highlights: [•] Differential isochron fits are useful for identifying multiple resurfacing events. [•] The range of crater-distribution flattening for a resurfacing episode is limited. [•] A flattened crater distribution covering a range greater than Ω corresponds to an age. [•] The binning bias for differentially/incrementally plotted crater data can be removed. [•] A revised calculation of the martian epoch boundary times is presented. [Copyright &y& Elsevier]

Highlights: [•] We date by crater-counting the terrains of the NW flank of Hecates Tholus, Mars. [•] We highlight and sequence the main volcanic, fluvial and glacial events. [•] Glacial activity played a long and important role in the evolution of the volcano. [•] The last important glacial activity in this volcano could be as young as 440ka. [•] Crater model ages agree with geomorphological observations of geological processes. [Copyright &y& Elsevier]

Closed-system pingos (CSPs) are perennial ice-cored mounds that evolve in relatively deep and continuous permafrost. They occur where thermokarst lakes either have lost or are losing their water by drainage or evaporation and by means of freeze–thaw cycling, permafrost aggradation and pore-water migration. The presence of CSPs on Mars, particularly on late-Amazonian Epoch terrain at near-polar latitudes, would indicate: (1) the antecedent occurrence of ponded water at the mound-formation sites; (2) freeze–thaw cycling of this water; and (3) boundary-conditions of pressure and temperature at or above the triple point of water much more recently and further to the north than has been thought possible. In 2005 we studied two crater-floor landscapes in northern Utopia Planitia and used MOC narrow-angle images to describe mounds within these landscapes that shared a suite of geological characteristics with CSPs on Earth. Here, we show the results of a circum-global search for similar crater-floor landscapes at latitudes >∼55°N. The search incorporates all relevant MOC and HiRISE images released since 2005. In addition to the two periglacially suggestive crater-floor landscapes observed by us earlier, we have identified three other crater floors with similar landscapes. Interestingly, each of the five mound-bearing craters occur within a tight latitudinal-band (∼64–69°N); this could be a marker of periglacial landscape-modification on a regional scale. Just to the north of the crater-based pingo-like mounds Conway et al. have identified large (km-scale) crater-based perennial ice-domes. They propose that the ice domes develop when regional polar-winds transport and precipitate icy material onto the floor of their host craters. Under a slightly different obliquity-solution ice domes could have accumulated at the lower latitudes where the putative CSPs have been observed. Subsequently, were temperatures to have migrated close to or at 0°C the ice domes could have thawed, forming endogenic paleolakes. This region also contains a significant concentration of crater-floor polygons. The polygons are thought to have formed by desiccation (El Maarry et al. [2010]. J. Geophys. Res., 115 (E10006)) or thermal contraction (Soare et al. [2005a]. Icarus, 174 (373–382)); on Earth each of these processes is associated with the end-stage of lake evolution. On the basis of our enhanced image collection, a new map displaying the global distribution of mound-bearing craters and a two new digital-elevation models of a crater-floor with pingo-like mounds, we evaluate the CSP hypothesis anew. We also explore two alternative hypotheses: (1) the mounds are weathered central-uplift complexes; or (2) they are impact-related hydrothermal structures. However, we propose that the CSP hypothesis is much more robust than these alternatives, encompassing geomorphological, cartographical, stratigraphical and climatological observations, and less subject to inconsistencies. [ABSTRACT FROM AUTHOR]

Abstract: We performed a systematic survey of typical surface patterns and small (5–50m) impact craters in a zone between 50° and 70° latitude in the Northern Lowlands of Mars. We found that small-scale (<10m) polygonal pattern formed by narrow fractures is typical in the northern half of the studied zone. We interpret this pattern to be formed by active thermal contraction polygons associated with the present-day shallow ground ice. These fresh small polygons are superposed over a larger-scale (∼15–30m) polygonal network of smooth gentle troughs and mounds. The latter pattern extends southward from the shallow ground ice boundary to the southern edge of the study zone. We interpret this pattern to be a relic from different climate conditions in the geologically recent past (possibly, to the most recent periods of high obliquity of Mars spin axis). The population of small craters is sparse and indicates intensive resurfacing in the recent geological past. The retention age of 5–10m diameter craters in the northern part of our study zone is surprisingly young, on the order of hundreds to thousands of years, which means that quick obliteration of such craters occurs under the present-day spin/orbit configuration of Mars. The crater obliteration time scale increases for larger craters and toward the southern edge of the study area. [Copyright &y& Elsevier]

Highlights: [•] Over 200 new, dated impact sites have been discovered on Mars. [•] We measure a current impact rate of 1.6×10−6/km2/yr for >3.9m effective diameter. [•] Resulting model ages are a factor of 5–10 below predictions based on lunar cratering. [•] Many new impacts form clusters, so D <50m craters should not be used to estimate ages. [Copyright &y& Elsevier]

Abstract: We find net sedimentation rate ≲(13–200)μm/yr in the Aeolis Dorsa region, Mars, using the frequency of crater–river interactions. This sets a lower bound of 1–20Myr on the total interval spanned by fluvial activity, which we correlate to the Noachian–Hesperian transition. The main uncertainty is the impact flux at the time of deposition. This result rules out basin-filling by a single catastrophic episode, such as a single impact-induced water–vapor greenhouse. [Copyright &y& Elsevier]

Highlights: [•] Five percentage of pit craters w/CTX coverage have valleys draining into their central pit. [•] Many valleys in central pit craters are young (Amazonian/Hesperian). [•] Lakes existed in some of the central pits, and valleys deposited deltas. [ABSTRACT FROM AUTHOR]

Highlights: [•] Age-modeled 78 large martian craters/basins including Gale. [•] Discuss uncertainties/problems/issues with crater age-dating. [•] Tie results to overall martian chornostratigraphy. [•] Implications for cessation of martian dynamo and surface erosion. [ABSTRACT FROM AUTHOR]

Abstract: A model for the formation and distribution of sedimentary rocks on Mars is proposed. In this model (ISEE-Mars), the rate-limiting step is supply of liquid water from seasonal melting of snow or ice. The model is run for a O(102) mbar pure CO2 atmosphere, dusty snow, and solar luminosity reduced by 23%. For these conditions snow melts only near the equator, when obliquity and eccentricity are high, and when perihelion occurs near equinox. These requirements for melting are satisfied by 0.01–20% of the probability distribution of Mars’ past spin–orbit parameters. This fraction is small, consistent with the geologic record of metastable surface liquid water acting as a “wet-pass filter” of Mars climate history, only recording orbital conditions that permitted surface liquid water. Total melt production is sufficient to account for observed aqueous alteration. The pattern of seasonal snowmelt is integrated over all spin–orbit parameters and compared to the observed distribution of sedimentary rocks. The global distribution of snowmelt has maxima in Valles Marineris, Meridiani Planum and Gale Crater. These correspond to maxima in the sedimentary-rock distribution. Higher pressures and especially higher temperatures lead to melting over a broader range of spin–orbit parameters. The pattern of sedimentary rocks on Mars is most consistent with a model Mars paleoclimate that only rarely produced enough meltwater to precipitate aqueous cements (sulfates, carbonates, phyllosilicates and silica) and indurate sediment. This is consistent with observations suggesting that surface aqueous alteration on Mars was brief and at low water/rock ratio. The results suggest intermittency of snowmelt and long globally-dry intervals, unfavorable for past life on Mars. This model makes testable predictions for the Mars Science Laboratory Curiosity rover at Gale Crater’s mound (Mount Sharp, Aeolis Mons). Gale Crater’s mound is predicted to be a hemispheric maximum for snowmelt on Mars. [Copyright &y& Elsevier]

Abstract: We propose a numerical model for the formation of the closely-spaced pits found in the thin, ejecta-related deposits superposed on the floors, interior terrace blocks, and near-rim ejecta blankets of well-preserved martian impact craters. Our model predicts the explosive degassing of water from this pitted material, which is assumed to originally be water-bearing, impact melt-rich breccia at the time of deposition. This process is analogous to what occurred in the fall-out suevite deposits at the Ries impact structure in Germany. At Ries, impact heating of water-bearing target material resulted in the rapid degassing of its water and other volatiles. The martian environment plays an important role in enhancing the effects of this degassing by increasing the flow-speed of the escaping gas. The high flow-rate of gas through particulate materials, such as suevite, tends to quickly form segregation channels or vent pipes, similar to those found in the Ries deposits. These pipes act as conduits for the efficient high-speed escape of the gas and small clasts that it entrains. Escaping gas and entrained clasts abraded and eroded the conduit walls, flaring them to form pits above a network of pipes. [Copyright &y& Elsevier]

Abstract: The fan deposit in Eberswalde crater has been interpreted as strong evidence for sustained liquid water on early Mars with a paleolake formed during the Noachian period (>3.7Gy). This location became a key region for understanding the Mars paleo-environment. Eberswalde crater is located 50km north of the rim of the 150km diameter crater Holden. Stratigraphic relationships and chronology obtained using recent Mars Express High Resolution Stereo Camera and Mars Reconnaissance Orbiter Context Camera images show that Eberswalde fluvial activity crosscuts Holden ejecta and thus postdates Holden crater, whose formation age is estimated from crater counts as Late Hesperian (∼3.5Gy, depending on models). Fluvial modeling shows that short term activity (over several years to hundreds of years) involving dense flows (with sediment:water ratio between 0.01 and 0.3) may be as good an explanation of the fluvial landforms as dilute flow over longer durations. Modeling of the thermal effect of the Holden impact in the Eberswalde watershed is used to evaluate its potential role in aqueous activity. The relative timing of the Holden impact and Eberswalde’s fan is a constraint for future studies about the origin of these landforms. Holden ejecta form a weak and porous substrate, which may be easy to erode by fluvial incision. In a cold climate scenario, impact heating could have produced runoff by melting snow or ground ice. Any attempt to model fluvial activity at Eberswalde should take into account that it may have formed as late as in the Late Hesperian, after the great majority of valley network formation and aqueous mineralization on Mars. This suggests that hypotheses for fan formation at Eberswalde by transient and/or localized processes (i.e. impact, volcanism, unusual orbital forcing) should be considered on a par with globally warmer climate. [Copyright &y& Elsevier]

Abstract: Near-horizontal planetary impacts result in elliptical craters. The percentage of elliptical craters on a planet can be used to infer the impact angle at which craters become elliptical. Previous surveys of the Moon, Mars, and Venus indicated that planetary craters become elliptical at more vertical angles than experimental impacts into a strengthless medium, and this was attributed to a higher ratio of crater diameter to projectile diameter. Here we determined the percentage of elliptical craters on the mid-sized saturnian satellites and Mercury, bodies that represent Solar-System extremes of impactor velocity, target density, and target strength. On the saturnian satellites, 7.6% of the craters have ellipticities e (ratio of major to minor axis) greater than 1.2, but only 0.4% have e >1.5, and no craters have e >1.75. On Mercury, 3% of the craters have e >1.2 and 0.5% have e >1.5. The mercurian percentages are slightly lower than the other terrestrial planets, attributable to a higher crater diameter to projectile diameter caused by the higher impact velocities at Mercury. We attribute the high percentage of moderately elliptical craters on the saturnian satellites to the rugged target terrain on those bodies. We interpret enhanced crater collapse on the icy surfaces of the saturnian satellites as preventing craters with extremely high ellipticities like the lunar crater Schiller. Finally, a reexamination of the martian crater population shows its elliptical crater population to be consistent with the other planets, and we see little evidence for a large population of craters formed by inward-spiraling moonlets. [Copyright &y& Elsevier]

Abstract: Recently acquired high-resolution images of martian impact craters provide further evidence for the interaction between subsurface volatiles and the impact cratering process. A densely pitted crater-related unit has been identified in images of 204 craters from the Mars Reconnaissance Orbiter. This sample of craters are nearly equally distributed between the two hemispheres, spanning from 53°S to 62°N latitude. They range in diameter from ∼1 to 150km, and are found at elevations between −5.5 to +5.2km relative to the martian datum. The pits are polygonal to quasi-circular depressions that often occur in dense clusters and range in size from ∼10m to as large as 3km. Pit sizes scale with both the host crater’s diameter and the host deposit size. These pits have subtle raised rims, and unlike primary and secondary impact craters, they lack well-defined ejecta deposits and overlapping stratigraphic relationships. They also lack any sign of any preferential alignment expected of volcanic or tectonic collapse features. Morphologic and stratigraphic evidence in support of an impact origin includes the observation that pitted materials primarily occur as ponded and flow-like deposits on crater floors, behind terraces, and infilling the lowest local topographic depressions atop the ejecta blanket—similar to the distribution of impact melt-bearing bodies on the Moon. Based on the observations and comparisons to terrestrial and lunar analogs, we conclude that the pit-bearing materials are impactite deposits. The presence of these deposits in older craters, where preserved, suggests that they have formed on Mars throughout most of its geologic history; thus, understanding their origin may help to constrain the hydrological and climate history of Mars. [Copyright &y& Elsevier]

Abstract: Impact crater formation is a highly dynamic and complex geological phenomenon. Methods of structural geology are capable of giving insights into deformation processes that occur during cratering. While most deformation observed in craters exhibits a generally radial symmetry, recent discoveries of non-radial structural elements in the central uplifts of terrestrial craters have led to the suggestion that obliquely impacting projectiles can cause non-radial deformation in the central subsurface area of the crater. The lack of ejecta blankets as accepted indicators of an oblique impact on Earth make it difficult to correlate the observed non-radial structures with the impact direction. Therefore, in this study three martian complex impact craters were selected that exhibit an oblique ejecta blanket and layered bedrock in the central uplift. The central uplifts were structurally mapped using High Resolution Imaging Science Experiment (HiRISE) imagery and additional high-resolution digital terrain models (DTMs). Structural data were evaluated and show a correlation between the orientation of at least two structural criteria and the direction of impact: (1) The strike of upturned bedrock layers is on average perpendicular to the impact trajectory and (2) the majority of faults show a preferred trend parallel to the impact trajectory. A fourth martian crater was examined as a counterexample, which had a radial ejecta blanket and thus no indication of an oblique trajectory. Strike in this crater showed no preferred orientation. These results will help to solidify the suggested correlation between non-radial structural features in impact craters and the horizontal component of momentum transferred from an obliquely impacting projectile to the target. [Copyright &y& Elsevier]

Abstract: The mode of formation of gullies on Mars, very young erosional–depositional landforms consisting of an alcove, channel, and fan, is one of the most enigmatic problems in martian geomorphology. Major questions center on their ages, geographic and stratigraphic associations, relation to recent ice ages, and, if formed by flowing water, the sources of the water to cause the observed erosion/deposition. Gasa (35.72°S, 129.45°E), a very fresh 7-km diameter impact crater and its environment, offer a unique opportunity to explore these questions. We show that Gasa crater formed during the most recent glacial epoch (2.1–0.4Ma), producing secondary crater clusters on top of the latitude-dependent mantle (LDM), interpreted to be a layered ice-dust-rich deposit emplaced during this glacial epoch. High-resolution images of a pre-Gasa impact crater ∼100km northeast of Gasa reveal that portions of the secondary-crater-covered LDM have been removed from pole-facing slopes in crater interiors near Gasa; gullies are preferentially located in these areas and channels feeding alcoves and fans can be seen to emerge from the eroding LDM layers to produce multiple generations of channel incision and fan lobes. We interpret these data to mean that these gullies formed extremely recently in the post-Gasa-impact time-period by melting of the ice-rich LDM. Stratigraphic and topographic relationships are interpreted to mean that under favorable illumination geometry (steep pole-facing slopes) and insolation conditions, melting of the debris-covered ice-rich mantle took place in multiple stages, most likely related to variations in spin-axis/orbital conditions. Closer to Gasa, in the interior of the ∼18km diameter LDM-covered host crater in which Gasa formed, the pole-facing slopes display two generations of gullies. Early, somewhat degraded gullies, have been modified by proximity to Gasa ejecta emplacement, and later, fresh appearing gullies are clearly superposed, cross-cut the earlier phase, and show multiple channels and fans, interpreted to be derived from continued melting of the LDM on steep pole-facing slopes. Thus, we conclude that melting of the ice-rich LDM is a major source of gully activity both pre-Gasa crater and post-Gasa crater formation. The lack of obscuration of Gasa secondary clusters formed on top of the LDM is interpreted to mean that the Gasa impact occurred following emplacement of the last significant LDM layers at these low latitudes, and thus near the end of the ice ages. This interpretation is corroborated by the lack of LDM within Gasa. However, Gasa crater contains a robustly developed set of gullies on its steep, pole-facing slopes, unlike other very young post-LDM craters in the region. How can the gullies inside Gasa form in the absence of an ice-rich LDM that is interpreted to be the source of water for the other adjacent and partly contemporaneous gullies? Analysis of the interior (floor and walls) of the host crater suggest that prior to the Gasa impact, the pole-facing walls and floor were occupied by remnant debris-covered glaciers formed earlier in the Amazonian, which are relatively common in crater interiors in this latitude band. We suggest that the Gasa impact cratering event penetrated into the southern portion of this debris-covered glacier, emplaced ejecta on top of the debris layer covering the ice, and caused extensive melting of the buried ice and flow of water and debris slurries on the host crater floor. Inside Gasa, the impact crater rim crest and wall intersected the debris-covered glacier deposits around the northern, pole-facing part of the Gasa interior. We interpret this exposure of ice-rich debris-covered glacial material in the crater wall to be the source of meltwater that formed the very well-developed gullies along the northern, pole-facing slopes of Gasa crater. [Copyright &y& Elsevier]