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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]

Ejecta blankets around craters on a km-scale show stark differences in the ejecta mobility (maximal extent of ejecta divided by the crater radius) with latitude and types of plains on Mars, which has been known for a long time (Barlow and Pollak, 2002; Li et al., 2015) but is not well-understood. Our main goal is to explore the idea that the subsurface rheology drives ejecta dynamics and leaves characteristic, observable imprints in the spatial distributions of ejecta. We conduct numerical simulations of impact cratering into a variety of subsurface models and study the impact of varied subsurface material (porous basalt, water ice, water), as well as structure and layering on impact sites and properties of impact ejecta. Our models consider variations in the depth of water or ice covering basalt; the thickness of buried ice; and the depth of burial of ice. We find that ejection angles and velocities depend on these quantities and configurations, e.g. velocities decrease and angles of ejection increase with increasing water layer thickness, buried ice thickness, or thickness of basaltic debris over an ice sheet. As a result, several ejecta characteristics derived from ejecta thickness profiles carry information about the underlying rheology. Specifically, the ejecta mobility, as well as the radial position of the maximum and the slope of the ejecta thickness profiles are diagnostic. The power-law function of the thickness profile of the ejecta blanket right after formation is a good indicator of ejecta from water- or ice-covered targets but not from more complex multi-layered structures, hence we caution against using scaling relations to describe ejecta from such targets. As any surface water/ice volatile mass is unstable on Mars, we also consider the geomorphology of craters and ejecta after the loss of volatiles. Our main findings include the observation that after the loss of surface volatiles, craters which formed in water-covered targets would appear shallower than those formed in basalt. The ejecta mobilities observed today would also be smaller if craters formed in volatile-covered targets than in basalt. Our findings can aid the interpretation of data returned from cameras onboard multiple spacecraft orbiting Mars such as HiRISE, CaSSIS, or CTX, and constitute a proof of concept of the hypothesis that information on the subsurface, including where ice is or was present, is encoded in the ejecta profile and spatial distribution. • Ejecta blankets around km-scale craters are sensitive to subsurface rheology. • Ejection velocities and angles depend on the layering of basalt and water/ice. • Post-sublimation ejecta from volatile-rich subsurfaces are smaller than from basalt. • Craters in volatile-covered targets sublime & become shallower than those in basalt. • These differences can be resolved by spacecraft imaging systems. [ABSTRACT FROM AUTHOR]

The Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) imaged two newly formed impact craters on the South Polar Layered Deposits (SPLD) of Mars in 2018 and 2020. These two new craters, the first detected on the SPLD, measure ∼17 m and ∼48 m in diameter. Follow-up observations were conducted with the High Resolution Imaging Science Experiment (HiRISE), showing seasonal and interannual changes, and providing stereo coverage for the production of digital terrain models (DTMs). Mars Climate Sounder (MCS) data were obtained over the region of these new impacts, giving surface temperature information for the time interval before and after the impacts were detected. Taken together, the optical and infrared observations of these sites reveal craters largely consistent with the morphologies of other small, dated impact craters on Mars, and crater ejecta patterns that suggest a more dust/regolith-dominated upper few meters of the SPLD in contrast to mid-latitude buried ice and lobate debris aprons (LDAs). This supports previous conclusions that the SPLD may have an upper surface depleted in water ice relative to the North PLDs, possibly the result of a widespread deflation event. • 17- and 48-m diameter new (formed 2018–2020) craters were found on the South Polar Layered Deposits. • These new craters are morphologically similar to <100 m craters elsewhere on Mars. • Ejecta composition suggests the SPLD's upper meters differ from midlatitude ices. [ABSTRACT FROM AUTHOR]

Impact-generated hydrothermal systems are a potential habitat for life on Earth and other planetary bodies. The ubiquity of impact craters on Mars makes them of particular interest for the search for life on this planet although their viability as a habitat is not well understood. To better understand such systems, two analogue impact structures on Earth were investigated, as their dominantly basaltic target rock makes them similar to impact structures in the Martian crust: the Vista Alegre and Vargeão Dome impact structures in Brazil. The goals of this paper are to 1) better understand the Vista Alegre and Vargeão Dome impact structures, and 2) understand how impact-generated hydrothermal systems on Earth can be compared with similar systems on Mars. To enable comparison, the software HYDROTHERM is used to reconstruct the hydrothermal evolution of both impact systems on Earth. Permeability, porosity, thermal conductivity, and initial temperature distributions are varied to simulate slow--, medium-, and fast-cooling models. The medium cooling rate models are then adapted to Martian conditions and scaled to 1×, 2×, 4×, and 8× the size on Earth, also adapting the impact energy and initial heat distribution, to check the comparability using similar target rocks. Finally, target rocks are changed to a basaltic composition only, to check how a lower-permeability Martian surface composition would affect the hydrothermal system. The models indicate between 150 and 650 thousand years (ka) of hydrothermal activity for both the Vista Alegre and Vargeão Dome impact structures. Due to the limited availability of water in the initial, high-temperature situation in the crater centre, water flux first increases and then decreases, although after 650 kyr the water flux towards the crater centre remains active in Vargeão Dome only. This sustained water flux is likely related to the low relative pressure in the crater centre due to the lack of overlying rock in comparison to outside the crater, as well as the direct connection of an underlying sandstone aquifer with high-permeability, fractured basalt in the centre of the structure. The Martian models with the same lithologies as on Earth show overall lower water flux and contain permafrost, which prevents the water flux within the aquifer from outside the crater towards the crater centre as seen in Vargeão Dome. The cooling times increase with scaling from 1× to 8× the size to ∼0.2 and 5 Myr, respectively. With only basalts in the target, cooling times reach ∼0.2 to 9 Myr. However, in that case, the water flow only remains active for up to the first Myr, and a minimum crater diameter of approximately 40 km is needed to achieve surface water fluxes close to those in Vargeão Dome and Vista Alegre. Below the Martian surface, hydrothermal system activity can persist for millions of years, providing potential for supporting life. Unfortunately, these parts of the system are currently unreachable for both sampling and spectroscopic techniques, and the short lifetime and low water fluxes at the surface related to the permafrost layers do not favour impact-generated hydrothermal systems on present-day Mars as targets for the search for life. However, these systems are highly dependent on the rock types and related permeabilities and could have provided habitats before 3.5 Ga, when the planet was warmer and liquid water was present at the surface. • This is the first fluid-flow model of terrestrial impact craters in basaltic targets. • This is the first direct comparison of fluid-flow models for impact-generated hydrothermal systems on Earth and Mars. • This is the first time a (semi)quantitative limit is set for impact-generated hydrothermal systems on Mars. • The paper provides new insights into impact-generated hydrothermal systems on Mars and their potential as a habitat. [ABSTRACT FROM AUTHOR]

The formation of layered ejecta craters (LECs) is generally thought to be related to the subsurface water-ice. The distribution of LECs in the northern regions of Mars (0 to 70° N) features both local and latitudinal variations, e.g., there are generally few LECs within volcanic areas, and the observed minimum and median diameters of the LECs decreases with increasing latitude. The observed minimum diameters of the LECs in the six planitiae within the northern lowlands suggest the roof depth of the LECs-forming layer has clear lateral variability, ranging between ∼90 m in the Arcadia Planitia and ∼280 m in the Isidis Planitia. The observed LECs are suggested to mainly form during the Amazonian, implying that the LEC-forming water-ice-bearing strata might be still present. The variation might reflect the comprehensive influence on the subsurface water-ice by the climate conditions and geologic events during the Amazonian period. The proposed subsurface stratigraphies in the six planitiae suggest that the total thickness of the LECs-related strata in the Chryse Planitia is the largest (∼4.6 km) and in the Arcadia Planitia is the smallest (∼1.6 km). There could be as much as ∼3.5–16 million km3 subsurface water stored in these strata, resulting in a ∼24–110 m global equivalent layer (GEL) of water. Given that water is a fundamental ingredient to life, its migration from surface to subsurface shifts the focus of its exploration. Both Arcadia and Utopia Planitiae represent ideal candidates as locations where both shallow and deep subsurface water-ice might be abundant, they represent high-priority targets for future missions at mid-latitudes for biosignature searching and in-situ resource utilization. • The distribution of LECs in northern Mars (0 to 70° N) has a regional and latitudinal variation. • The stratigraphy of LECs-related strata is constructed, with roof depth having lateral variability. • The possible (current) volume of the subsurface water stored in the LECs-related strata is estimated. • Arcadia and Utopia Planitiae are proposed as high-priority regions for future missions. [ABSTRACT FROM AUTHOR]

Hundreds of new impact craters have been observed to form on Mars since spacecraft began imaging that planet. New impact craters produced visible ejecta deposits and many of them also have visible rays, similar to lunar and mercurian craters. However, some of the new martian impact craters have a circular feature of relatively low reflectance that we call a "halo." This feature is distinct from the usual visible ejecta deposits or ray patterns. In this paper we present an observational study of this halo feature and we discuss the results of this study with respect to the nature of the halos: what they are and how they may have formed. To address these questions, we measured diameters of both halos and their central craters. We found a strong correlation between halo diameter and crater diameter, which indicates that the nature of the halos is fundamentally governed by the amount of impact energy available at their formation. Specifically, halo size is controlled by impact energy according to the non-linear relationship D H ∝ E2/3, where D H is the diameter of the halo and E is the impact energy. We also found that certain factors may influence the formation of the halos: a thicker dust layer and lower elevations are both correlated with larger halos. From these correlations we conclude that the local surface characteristics as well as local atmospheric pressure influence the formation of the halos. Our description and analysis of the martian halo features provide a framework upon which specific halo formation mechanisms can be developed and tested in the future. • Some new martian impact craters are surrounded by a dark circular feature: a halo. • Halo diameter strongly correlates with crater diameter. • Halo formation depends on the amount of available impact energy. • Halo formation depends on local surface properties and atmospheric pressure. [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]

Highlights • We analyzed by structural modeling three lobate scarps in Aonia Terra, Mars. • Fault dips (23°−35°) and fault slips (1700–3100 m) have been estimated. • The depth of faulting indicates values of BDT ranging from 18 to 45 km. • The heat flow values calculated from the BDT depths range from 25 to 51 mW/m2. Abstract Martian lobate scarps are prominent tectonic structures interpreted to be the topographic expression of large thrust faults generated under compression. Here, we present a structural modeling performed on three large lobate scarps (Ogygis Rupes, Bosporos Rupes and Phrixi Rupes) located in the heavily cratered highlands of Mars, specifically in Aonia Terra, between Thaumasia Montes and Argyre impact basin. These lobate scarps, formed in the Late Noachian/Early Hesperian, strike parallel to the edge of the Thaumasia Montes, and were generated by ESE-vergent thrust faults. Structural analysis of craters deformed by these lobate scarps gives minimum estimates for the faults slip of ∼1700–3100 m. We applied two structural methods in order to constrain the geometry of these thrust faults at depth, area balanced cross sections and forward mechanical dislocation modeling, obtaining a depth of faulting in this area between ∼18 and ∼45 km, and dip angles between 23° and 35°. These results are consistent with previous studies of lobate scarps on Mars. The depth of faulting gives an estimation of the depth of the brittle-ductile transition at the time of its formation giving a range of depth in which the state of the lithosphere change from brittle to ductile-dominated deformation. The heat flow values calculated from the obtained depths of the brittle–ductile transition range from 25 to 51 mW m−2. We show that the brittle-ductile transition depth in Aonia Terra is set in 18–27 km at a larger distance from the basin center, while it is deeper closer to the Argyre rim (∼33–45 km). This difference seems to indicate a thickening of the brittle domain under Argyre main rim with respect to the external area but, attending to regional geology and heat flow values calculated, this high value (∼33–45 km) might be an overestimation of the depth of faulting caused by the presence of the crater rim elevation before the formation of the lobate scarps. [ABSTRACT FROM AUTHOR]

Highlights • The bulk permittivity and crack distribution were investigated around two laboratory-scale craters. • The crack volume fraction showed clear inverse correlations with the bulk permittivity. • The effective medium model could explain the permittivity distribution around the crater. • The suitable effective medium models succeeded in explaining the measurement results of the Apollo sample. • Application of an effective medium model in radar observations was proposed. Abstract The impact of meteorites forms craters on celestial surfaces, below which cracks are formed. Lunar geological conditions around such craters have been estimated based on permittivity surveys by radar exploration. The relationship between density and permittivity was empirically determined using Apollo soil and rock samples. In this manner, radar exploration enabled the geological survey of the moon. However, based on the effective medium theory, even for equal densities, crack direction can change the permittivity. Here, we show the relationship between crack direction and physical parameters (i.e., the bulk density, bulk permittivity, and loss tangent) around craters based on impact experiments. We characterize three types of cracks (concentric, radial, and corner cracks) around a ∼9.1-cm-diameter crater. The crack volume fraction was gradually decreased as the distance from the crater center increased. The decrease was particularly remarkable in the area within the crater radius, arising from the concentric and radial cracks. The bulk density, bulk permittivity, and loss tangent showed clear inverse correlations with the crack volume fraction. We found that the effective medium model, considering the effect of crack direction, explained the bulk permittivity distribution around the crater. In addition, the effective medium models, assuming the regolith contains spherical basalt grains and the rock includes aligned cracks, could successfully explain the results of the density and permittivity of Apollo sample. The estimated permittivity of the pore-free lunar basalt was ∼15. These results provide suggestions for application of the effective medium model in the derivation of the bulk density from the bulk permittivity determined by radar observations. (1) If the lunar mare subsurface layer has a permittivity lower than ∼4–5 and is in regolith condition, its bulk density should be estimated from an effective medium model isotropically containing spherical pore-free basalt grains with a permittivity of ∼15 in a vacuum. (2) If the lunar mare subsurface layer has a permittivity larger than ∼4–5 and is in rock condition, its bulk density should be estimated from an effective medium model, assuming the void distribution or crack direction against the electric field of radar pulses in basalt rock with a permittivity of ∼15. [ABSTRACT FROM AUTHOR]

Highlights • How three fan-shaped deposits formed at Bradbury crater, Mars was investigated. • Quantitative stratigraphic methods were applied to the northernmost deposit. • The stratal dip of 6.1° is consistent with alluvial/fluvial or debris flow fan. • Without a deltaic architecture, no evidence for a standing water body was observed. Abstract The stratigraphic architectures of three fan-shaped deposits at Bradbury crater, Mars were investigated to test the hypothesis that the deposits were formed in standing bodies of water. Quantitative stratigraphic methods were applied to the northernmost deposit, which displays meter-thick layering. Planes fit to exposed strata display a constant dip throughout the section, with no obvious slope breaks and a mean dip of 6.1° to the northwest. We conclude that the stratal geometries of this outcrop are most consistent with formation as an alluvial/fluvial or debris flow fan. As this deposit lacks the diagnostic stratigraphic architecture of a delatic system, it presents no conclusive evidence for a sustained standing body of water at Bradbury crater. [ABSTRACT FROM AUTHOR]

Highlights • We built a 1D model of crater obliteration. • We document the time-dependence of the crater obliteration rates on Mars continuously decreased between 3.8 Ga and 3 Ga (Hartmann, 2005 timescale). • Mars never experienced a dramatic climatic shift, but rather it gradually dried out over time. Abstract A change in the global climate on early Mars is suspected from the geologic evidence of warmer wetter conditions that prevailed during the Noachian but stands in contrast to the dry and cold conditions observed currently. However, the timing and evolution of this climatic change represents a gap in our understanding of the history of Mars. Here, we document the time-dependence of the crater obliteration rates on Mars that decreased continuously between 3.8 Ga and 3 Ga. Our results indicate that the erosion responsible for crater modification and obliteration declined gradually through the Noachian and into the Hesperian periods. These results suggest that the climatic conditions also changed gradually and the amount of water available on the surface of Mars slowly decreased over time. [ABSTRACT FROM AUTHOR]

Highlights • We find a wide diversity of crater morphologies in the Sintana quadrangle. • Gravity-driven mass wasting plays an important role in forming Ceres' surface and their characteristics in the Sintana quadrangle support an ice-rich subsurface. • We observe no morphological structures that can be related to endogenic activity in the Sintana quadrangle as observed for other parts of Ceres. • The young absolute age of the cratered terrain in the Sintana quadrangle suggests resurfacing by Kerwan impact melt. Abstract A geologic map at a scale of 1:1,000,000 of the Ac-11 Sintana quadrangle (21–66°S and 0–90°E) from the dwarf planet Ceres and based on data from NASA's Dawn spacecraft, was produced as part of a geologic mapping campaign in order to investigate the surface structure, stratigraphy, composition and surface modifying processes. The Sintana quadrangle is geologically dominated by cratered terrain with morphologically distinct crater types. The most prominent craters, Sintana, Hamori, Jarimba, Tupo, Annona and Darzamat, have complex structures including smooth crater material covering their floors, collapsed crater walls forming terraces, and central ridges. Multiple gravity-driven blocky mass wasting features indicate post-impact crater modification and are indicative of an ice-rich subsurface, in which there is no topographic or structural evidence of endogenic activity visible in the quadrangle. Absolute age estimates suggest that the entire surface in the quadrangle was resurfaced ∼1.1 Ga ago, possibly by Kerwan-related impact-induced melting. [ABSTRACT FROM AUTHOR]

Calcium sulfate veins have been found in Gale crater and Endeavour crater as indicators of Martian fluid events. The presence of mixed-cation sulfates has been suggested because a wide variety of sulfates containing different cation elements have been detected in in-situ exploration targets (e.g., soils, drilled materials, calcium sulfate veins, and sandstones). In order to establish a spectroscopic library of mixed-cation sulfates, five calcium sulfate double salts (CSDS) were successfully synthesized using high-temperature solid phase reaction and aqueous solution precipitation methods. The phase and homogeneity of these samples were confirmed by X-ray diffraction (XRD). Raman, mid-infrared (MIR), visible and near-infrared (VNIR), and Laser-induced breakdown spectrometry (LIBS) spectra were also collected to study vibrational features and elemental emission properties. All these spectral data are valuable for the mixed-cation sulfate detections by those payloads with similar spectroscopic technologies empolyed on Mars. We also studied the conversion relationships among five CSDS, providing constraints for their origins in sedimentary (e.g., calcium sulfate veins) and volcanic environments on Mars. • Five pure calcium sulfate double salts (CSDS) relevant to Mars are synthesized. • The Raman, MIR, VNIR, and LIBS spectral data are obtained to support Mars missions. • The five CSDS could be indicators of aqueous and volcanic environments. [ABSTRACT FROM AUTHOR]

Abstract The Martian climate is governed by an annual cycle, that results in the condensation of CO 2 ice during winter, up to a meter thick at the pole and thousands of kilometers in extension. Water and dust may be trapped during the condensation and freed during the sublimation. In addition, ice may be translucent or granular depending on the deposition process (snow vs direct condensation), annealing efficiency, and dust sinking process. The determination of ice translucency is of particular interest to confirm or reject the cold jet model (also known as Kieffer model). This work is focused on the dune field of Richardson Crater (72° S, 180° W)in which strong interactions between the water, dust and CO 2 cycles are observed. We analyzed CRISM hyperspectral images in the near IR using radiative transfer model inversion. We demonstrate that among the states of CO 2 ice, the translucent state is observed most frequently. The monitoring of surface characteristics shows a decrease in the thickness of the ice during the spring consistently with climate models simulations. We estimate a very low dust content of a few ppmv into the CO 2 ice, consistent with the formation scenario of cold jets. The water impurities is around 0.1%v, almost stable during the spring, suggesting a water escape from the surface of subliming CO 2 ice layer. The water ice grain size varies in a range 1 to 50 microns. From these results, we propose the following new mechanism of small water ice grain suspension: as a cold jet occurs, water ice grains of various sizes are lifted from the surface. These jets happen during daytime, when the general upward gas flux from the subliming CO 2 ice layer is strong enough to carry the smaller grains, while the bigger fall back on the CO 2 ice layer. The smaller water grains are carried away and integrated to the general atmospheric circulation. [ABSTRACT FROM AUTHOR]

Bakhuysen Crater is a Late Noachian to Early Hesperian age ∼150 km-diameter impact structure located in the heavily cratered southern highlands of Mars. It is one of the largest craters on Mars with well-preserved ejecta deposits. Impact ejecta deposits allow an understanding of crustal composition, volatile content, and evolution of terrestrial planets, yet the emplacement of ejecta deposits on bodies throughout the solar system is still debated. Here, we present the first detailed geomorphologic map of Bakhuysen Crater. We distinguish two distinct ejecta units: (1) a largely hummocky ejecta unit (Be1) that comprises the continuous ejecta blanket; and (2) an uppermost, discontinuous unit (Be2) with a distinct ponded morphology, pitted texture, and previously unreported lobate and flow deposits. Pitted materials are present in the crater interior deposits as well as the ejecta. Bakhuysen is the largest Martian crater thus far identified with crater-related pitted material, which has been suggested previously to represent volatile-rich primary impact melt-bearing deposits. Unit Be1 is interpreted as the continuous ejecta blanket emplaced through ballistic sedimentation and radial flow. Unit Be2 shares many attributes with impact melt-rich ejecta seen on other planetary bodies (e.g., Earth, Moon, and Mercury) and is interpreted as having been emplaced as ground-hugging flows during the later modification stage of crater formation. Channels are observed as having emanated from Bakhuysen impactites and represents a clear example of the role of volatiles in the impact cratering process on Mars. Bakhuysen ejecta deposits and its associated features suggests that, if indeed a substantial volatile source is sampled by Martian impact events, then the formation of ejecta-associated channels on Mars are likely genetically linked to the formation of pitted materials where present. The preservation and exposure of the impact deposits of Bakhuysen present a unique opportunity to study these features and their relationship to the impact process on Mars. [ABSTRACT FROM AUTHOR]

Impact craters across Mars, that are otherwise similar, commonly exhibit vastly different thermal inertia values on their rims. We focused on the Tisia Valles region because the terrain is mostly flat, and craters of various ages are present within the same target geologic unit. We present an investigation of two hypotheses for why these thermal inertia variations exist. The first hypothesis is that crater rim thermal inertia is affected by crater degradation state, and the second hypothesis is that thermal inertia is affected by the amount of regolith mantling the rims, regardless of degradation state. To investigate the first hypothesis, we used a series of multilevel regression analyses to test for correlations between crater rim thermal inertia and depth-diameter ratio, rim irregularity, and radii variation, which are indicators of crater degradation state. To investigate the second hypothesis, we tested for a correlation between thermal inertia and mantled crater rim percentage, which is indicative of the amount of regolith atop the rims. Our results did not support the first hypothesis but did support the second hypothesis. Therefore, we concluded that regolith rim mantling affects crater rim thermal inertia variations in the Tisia Valles region. Crater degradation may not have an effect on rim thermal inertia because regolith is transported down the crater rim slopes faster than local regolith is produced from erosion in this region. An implication of this work is that the spatial extent of regolith-rich regions near Tisia Valles can be mapped using the spatial extent of proximal craters with low thermal inertia rims. Additionally, the timing of mantling events can be quantified by comparing counts of craters with high thermal inertia rims to those of low thermal inertia rims within a region. [ABSTRACT FROM AUTHOR]

Impact craters in the mid-latitudes of Mars are commonly filled to variable degrees with some combination of ice, dust, and rocky debris. Concentric surface features visible in these craters have been linked to debris transportation and glacial and periglacial processes. Concentric crater fill (CCF) observed today are interpreted to be the remains of repeated periods of accumulation and sublimation during the last tens to hundreds of million years. Previous work suggests that during phases of high obliquity, ice accumulates in crater interiors and begins to flow down steep crater slopes, slowly filling the crater. During times of low obliquity ice is protected from sublimation through a surface debris layer consisting of dust and rocky material. Here, we use an ice flow line model to understand the development of concentric crater fill. In a regional study of Utopia Planitia craters, we address questions about the influence of crater size on the CCF formation process, the time scales needed to fill an impact crater with ice, and explore commonly described flow features of CCF. We show that observed surface debris deposits as well as asymmetric flow features can be reproduced with the model. Using surface mass balance data from global climate models and a credible obliquity scenario, we find that craters less than 80 km in diameter can be entirely filled in less than 8 My, beginning as recently as 40 Ma ago. Uncertainties in input variables related to ice viscosity do not change the overall behavior of ice flow and the filling process. We model CCF for the Utopia Planitia region and find subtle trends for crater size versus fill level, crater size versus sublimation reduction by the surface debris layer, and crater floor elevation versus fill level. [ABSTRACT FROM AUTHOR]

We present the results of a systematic mapping of seasonally sculpted terrains on the South Polar region of Mars with the Planet Four: Terrains (P4T) online citizen science project. P4T enlists members of the general public to visually identify features in the publicly released Mars Reconnaissance Orbiter Context Camera (CTX) images. In particular, P4T volunteers are asked to identify: (1) araneiforms (including features with a central pit and radiating channels known as ‘spiders’); (2) erosional depressions, troughs, mesas, ridges, and quasi-circular pits characteristic of the South Polar Residual Cap (SPRC) which we collectively refer to as ‘Swiss cheese terrain’, and (3) craters. In this work we present the distributions of our high confidence classic spider araneiforms and Swiss cheese terrain identifications in 90 CTX images covering 11% of the South polar regions at latitudes ≤ − 75° N. We find no locations within our high confidence spider sample that also have confident Swiss cheese terrain identifications. Previously spiders were reported as being confined to the South Polar Layered Deposits (SPLD). Our work has provided the first identification of spiders at locations outside of the SPLD, confirmed with high resolution HiRISE (High Resolution Imaging Science Experiment) imaging. We find araneiforms on the Amazonian and Hesperian polar units and the Early Noachian highland units, with 75% of the identified araneiform locations in our high confidence sample residing on the SPLD. With our current coverage, we cannot confirm whether these are the only geologic units conducive to araneiform formation on the Martian South Polar region. Our results are consistent with the current CO 2 jet formation scenario with the process exploiting weaknesses in the surface below the seasonal CO 2 ice sheet to carve araneiform channels into the regolith over many seasons. These new regions serve as additional probes of the conditions required for channel creation in the CO 2 jet process. [ABSTRACT FROM AUTHOR]

The modern climate of Mars has been well characterized from over a decade of orbiting spacecraft, in situ measurements via landers/rovers, and theoretical advances in climate modeling. Nonetheless, important questions remain unanswered, including the present-day mass balance of the north polar residual cap and its icy outliers. Exposed water-ice mounds are found in craters, and extend as far equatorward as 70.2°N. Due to their southerly location, these ice mounds are likely more sensitive to ongoing changes in climate. We analyze high-resolution images of the Louth crater ice mound, and employ a coupled 1-D thermal and atmospheric model to estimate annual mass balance of both Louth and Korolev water ice. We incorporate the effects of shallowly-sloping surfaces and seasonally-dependent water ice albedo. No clear trend in the advance or retreat of Louth crater water ice is observed in over 4 Mars years of repeat, high-resolution images. Secular changes are either sufficiently small as to not be detected, or the ice is in equilibrium. Modeled mass balance ranges from −6 to +2 mm of water ice per Mars year at both sites, with nominal cases being in near-equilibrium (<0.5 mm of ice loss per Mars year). [ABSTRACT FROM AUTHOR]

The internal gravity and self-gravitational energy of a comet, asteroid, or small moon have applications to their geophysics, including their formation, evolution, cratering, and disruption, the stresses and strains inside such objects, sample return, eventual asteroid mining, and planetary defense strategies for potentially hazardous objects. This paper describes the relation of an object’s self-energy to its collisional disruption energy, and shows how to determine an object’s self-energy from its internal gravitational potential. Any solid object can be approximated to any desired accuracy by a polyhedron of sufficient complexity. An analytic formula is known for the gravitational potential of any homogeneous polyhedron, but it is widely believed that this formula applies only on the surface or outside of the object. Here we show instead that this formula applies equally well inside the object. We have used these formulae to develop a numerical code which evaluates the self-energy of any homogeneous polyhedron, along with the gravitational potential and attraction both inside and outside of the object, as well as the slope of its surface. Then we use our code to find the internal, external, and surface gravitational fields of the Platonic solids, asteroid (216) Kleopatra, and comet 67P/Churyumov–Gerasimenko, as well as their surface slopes and their self-gravitational energies. We also present simple spherical, ellipsoidal, cuboidal, and duplex models of Kleopatra and comet 67P, and show how to generalize our methods to inhomogeneous objects and magnetic fields. At present, only the self-energies of spheres, ellipsoids, and cuboids (boxes) are known analytically (or semi-analytically). The Supplementary Material contours the central potential and self-energy of homogeneous ellipsoids and cuboids of all aspect ratios, and also analytically the self-gravitational energy of a “duplex” consisting of two coupled spheres. The duplex is a good model for “contact binary” comets and asteroids; in fact, most comets seem to be bilobate, and might be described better as “dirty snowmen” than as “dirty snowballs”. [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 implementation of the Planet Weather Research and Forecasting (PlanetWRF) model, MarsWRF, is used here to simulate the atmospheric conditions at Gale Crater for different seasons during a period coincident with the Curiosity rover operations. The model is first evaluated with the existing single-point observations from the Rover Environmental Monitoring Station (REMS), and is then used to provide a larger scale interpretation of these unique measurements as well as to give complementary information where there are gaps in the measurements. The variability of the planetary boundary layer depth may be a driver of the changes in the local dust and trace gas content within the crater. Our results show that the average time when the PBL height is deeper than the crater rim increases and decreases with the same rate and pattern as Curiosity's observations of the line-of-sight of dust within the crater and that the season when maximal (minimal) mixing is produced is L s 225°–315° (L s 90°–110°). Thus the diurnal and seasonal variability of the PBL depth seems to be the driver of the changes in the local dust content within the crater. A comparison with the available methane measurements suggests that changes in the PBL depth may also be one of the factors that accounts for the observed variability, with the model results pointing towards a local source to the north of the MSL site. The interaction between regional and local flows at Gale Crater is also investigated assuming that the meridional wind, the dynamically important component of the horizontal wind at Gale, anomalies with respect to the daily mean can be approximated by a sinusoidal function as they typically oscillate between positive (south to north) and negative (north to south) values that correspond to upslope/downslope or downslope/upslope regimes along the crater rim and Mount Sharp slopes and the dichotomy boundary. The smallest magnitudes are found in the northern crater floor in a region that comprises Bradbury Landing, in particular at L s 90° when they are less than 1 m s −1 , indicating very little lateral mixing with outside air. The largest amplitudes occur in the south-western portions of the crater where they can exceed 20 m s −1 . Should the slope flows along the crater rims interact with the dichotomy boundary flow, which is more likely at L s 270° and very unlikely at L s 90°, they are likely to interact constructively for a few hours from late evening to nighttime (∼17–23 LMST) and from pre-dawn to early morning (∼5–11 LMST) hours at the norther crater rim and destructively at night (∼22–23 LMST) and in the morning (∼10–11 LMST) at the southern crater rim. We conclude that a better understanding of the PBL and circulation dynamics has important implications for the variability of the concentration of dust, non-condensable and trace gases at the bottom of other craters on Mars as mixing with outside air can be achieved vertically, through changes in the PBL depth, and laterally, by the transport of air into and out of the crater. [ABSTRACT FROM AUTHOR]

We investigated the area to the northwest of the Isidis impact basin (hereby referred to as “NW Isidis”) using thermal infrared emission datasets to characterize and quantify bulk surface mineralogy throughout this region. This area is home to Jezero crater and the watershed associated with its two deltaic deposits in addition to NE Syrtis and the strong and diverse visible/near-infrared spectral signatures observed in well-exposed stratigraphic sections. The spectral signatures throughout this region show a diversity of primary and secondary surface mineralogies, including olivine, pyroxene, smectite clays, sulfates, and carbonates. While previous thermal infrared investigations have sought to characterize individual mineral groups within this region, none have systematically assessed bulk surface mineralogy and related these observations to visible/near-infrared studies. We utilize an iterative spectral unmixing method to statistically evaluate our linear thermal infrared spectral unmixing models to derive surface mineralogy. All relevant primary and secondary phases identified in visible/near-infrared studies are included in the unmixing models and their modeled spectral contributions are discussed in detail. While the stratigraphy and compositional diversity observed in visible/near-infrared spectra are much better exposed and more diverse than most other regions of Mars, our thermal infrared analyses suggest the dominance of basaltic compositions with less observed variability in the amount and diversity of alteration phases. These results help to constrain the mineralogical context of these previously reported visible/near-infrared spectral identifications. The results are also discussed in the context of future in situ investigations, as the NW Isidis region has long been promoted as a region of paleoenvironmental interest on Mars. [ABSTRACT FROM AUTHOR]

Impact melt flows are observed within the continuous and discontinuous ejecta blanket of the 9 km lunar crater Pierazzo, from the crater rim to more than 40 km away from the center of the crater. Our mapping, fractal analysis, and thermal modeling suggest that melt can be emplaced ballistically and, upon landing, can become separated from solid ejecta to form the observed flow features. Our analysis is based on the identification of established melt morphology for these in-ejecta flows and supported by fractal analysis and thermal modeling. We computed the fractal dimension for the flow boundaries and found values of D = 1.05–1.17. These are consistent with terrestrial basaltic lava flows (D = 1.06–1.2) and established lunar impact melt flows (D = 1.06–1.18), but inconsistent with lunar dry granular flows (D = 1.31–1.34). Melt flows within discontinuous ejecta deposits are noted within just 1.5% of the mapping area, suggesting that the surface expression of impact melt in the extended ejecta around craters of this size is rare, most likely due to the efficient mixing of melts with solid ejecta and local target rocks. However, if the ejected fragments (both, molten and solid) are large enough, segregation of melt and its consequent flow is possible. As most of the flows mapped in this work occur on crater-facing slopes, the development of defined melt flows within ejecta deposits might be facilitated by high crater-facing topography restricting the flow of ejecta soon after it makes ground contact, limiting the quenching of molten ejecta through turbulent mixing with solid debris. Our study confirms the idea that impact melt can travel far beyond the continuous ejecta blanket, adding to the lunar regolith over an extensive area. [ABSTRACT FROM AUTHOR]

The Jezero crater open-basin lake contains two well-exposed fluvial sedimentary deposits formed early in martian history. Here, we examine the geometry and architecture of the Jezero western delta fluvial stratigraphy using high-resolution orbital images and digital elevation models (DEMs). The goal of this analysis is to reconstruct the evolution of the delta and associated shoreline position. The delta outcrop contains three distinct classes of fluvial stratigraphy that we interpret, from oldest to youngest, as: (1) point bar strata deposited by repeated flood events in meandering channels; (2) inverted channel-filling deposits formed by avulsive distributary channels; and (3) a valley that incises the deposit. We use DEMs to quantify the geometry of the channel deposits and estimate flow depths of ∼7 m for the meandering channels and ∼2 m for the avulsive distributary channels. Using these estimates, we employ a novel approach for assessing paleohydrology of the formative channels in relative terms. This analysis indicates that the shift from meandering to avulsive distributary channels was associated with an approximately four-fold decrease in the water to sediment discharge ratio. We use observations of the fluvial stratigraphy and channel paleohydrology to propose a model for the evolution of the Jezero western delta. The delta stratigraphy records lake level rise and shoreline transgression associated with approximately continuous filling of the basin, followed by outlet breaching, and eventual erosion of the delta. Our results imply a martian surface environment during the period of delta formation that supplied sufficient surface runoff to fill the Jezero basin without major drops in lake level, but also with discrete flooding events at non-orbital (e.g., annual to decadal) timescales. [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]

Orientale Basin is one of the youngest and best-preserved multi-ring impact basins in the Solar System. The structure is ∼950 km across and is located on the western edge of the nearside of the Moon. The interior of the basin, which possesses three distinct rings and a post-impact mare fill, has been studied extensively using modern high-resolution datasets. Exterior to these rings, Orientale has an extensive ejecta blanket that extends out radially for at least 800 km from the basin rim in all directions and covers portions of both the nearside and farside of the Moon. These deposits, known as the Hevelius Formation, were first mapped using photographic data from the Lunar Orbiter IV probe. In this study, we map in detail the morphology of each distinct facies observed within the Orientale ejecta blanket using high resolution Lunar Reconnaissance Orbiter (LRO) Wide Angle Camera (WAC) and Narrow Angle Camera (NAC) images and Lunar Orbiter Laser Altimeter (LOLA) elevation data. We identified 5 unique facies within the ejecta blanket. Facies A is identified as a region of hummocky plains located in a low-lying topographic region between the Outer Rook and Cordillera rings. This facies is interpreted to be a mix of crater-derived impact melt and km-scale blocks of ballistic ejecta and host rock broken up during the modification stage and formation of the Cordillera ring. Facies B is an inner facies marked by radial grooves extending outward from the direction of the basin center. This facies is interpreted as the continuous ballistic ejecta blanket. Facies C consists of inner and outer groupings of flat smooth-surfaced deposits isolated in local topographic lows. Facies D displays characteristic sinuous ridges and lobate extensions. Facies C and D are interpreted to be impact melt-rich materials, which manifest as flows and ponds. Our observations suggest that these facies were emplaced subsequent to the ballistic ejecta blanket – most likely during the modification stage of crater formation – and flowed and ponded in topographically low-lying regions. Facies E consists of distinct crater chains emanating radially from the basin center and extending from ∼700 to ∼1000 km from the center of Orientale. This facies is considered to be chains of secondary craters formed from large blocks of ballistic ejecta. Our mapping effort shows that the individual ejecta facies were influenced and controlled to varying degrees by pre-existing slopes and topography. At the basin scale, the overall downslope direction toward the lunar lowlands to the east and southeast of the basin center resulted in large impact melt flows 100's of kilometers in length, while the regional upslope trends in the west and northwest inhibited the development of extensive impact melt flows. On a smaller scale it can be observed that ground-hugging ejecta collected behind and flowed around high topographic obstacles while diverting into topographic low regions (e.g., around uplifted pre-existing crater rims, but down into pre-existing crater floors). The dispersion of the various ejecta facies mapped here also indicates both a direction and an angle for the impact event that formed Orientale Basin. The bilateral distribution of both ballistic and impact melt-rich ejecta deposits across a line running northeast – southwest suggests the impact occurred from the northeast toward the southwest. Careful mapping of the secondary impact crater chains (Facies E) shows the development of a “forbidden zone” lacking these impacts to the northeast as well as a concentration of the longest secondary crater chains to the northwest and southeast, perpendicular to the aforementioned line of bilateral ejecta distribution. This distribution of secondary impact craters contrasts with the circularity of the basin and suggests that Orientale Basin was formed by ∼ 25–45° impact from the northeast. [ABSTRACT FROM AUTHOR]

The Dorsa Argentea Formation (DAF), a set of geomorphologic units covering ∼1.5 million square kilometers in the south circumpolar region of Mars, has been interpreted as the remnants of a large south polar ice sheet that formed near the Noachian-Hesperian boundary and receded in the early Hesperian. Determining the extent and thermal regime of the DAF ice sheet, as well as the mechanism and timing of its recession, can therefore provide insight into the ancient martian climate and the timing of the transition from a presumably thicker CO 2 atmosphere to the present climate. We used the Laboratoire de Météorologie Dynamique (LMD) early Mars global climate model (GCM) and the University of Maine Ice Sheet Model (UMISM) glacial flow model to constrain climates allowing development of a south polar ice sheet of DAF-like size and shape. In addition, we modeled basal melting of this ice sheet in amounts and locations consistent with observed glaciofluvial landforms. A large, asymmetric region of ice stability surrounding the south pole is a robust feature of GCM simulations with spin-axis obliquity of 15° or 25° and a 600–1000 mb CO 2 atmosphere. The shape results from the large-scale south polar topography of Mars and the strong dependence of surface temperature on altitude under a thicker atmosphere. Of the scenarios considered in this study, the extent of the modeled DAF ice sheet in UMISM simulations most closely matches that of the DAF when the surface water ice inventory of Mars is a ∼137 m global equivalent layer (GEL) and spin-axis obliquity is 15°. In climates warmed only by CO 2 , significant basal melting does not occur except when the ice inventory is larger than plausible estimates for early Mars. In this case, the extent of the south polar ice sheet is also much larger than that of the DAF, and basal melting is more widespread than observed landforms indicate. When an idealized greenhouse gas warms the surface by at least 20°C near the poles relative to CO 2 alone, the stable extent of the ice sheet is less than that of the DAF units, but widespread basal melting occurs, with maxima in the locations where eskers are currently observed. We therefore conclude that warming by a gas other than CO 2 alone was necessary to enable the construction of glaciofluvial landforms in the DAF. Previously published crater exposure ages of eskers in the DAF indicate that eskers were being exposed as activity was ceasing in the equatorial valley networks, suggesting that the warming that allowed basal melting at the edges of the DAF ice sheet were broadly contemporaneous with those in which the valley networks were carved. Finally, elevated Tharsis topography is required to produce an ice sheet with the shape of the DAF. Thus, our results are not consistent with the DAF (and the valley networks) forming before the emplacement of Tharsis, as recently suggested. [ABSTRACT FROM AUTHOR]

We use topographic data to show that impact craters with pitted floor deposits are among the deepest on Mars. This is consistent with the interpretation of pitted materials as primary crater-fill impactite deposits emplaced during crater formation. Our database consists of 224 pitted material craters ranging in size from ∼1 to 150 km in diameter. Our measurements are based on topographic data from the Mars Orbiter Laser Altimeter (MOLA) and the High-Resolution Stereo Camera (HRSC). We have used these craters to measure the relationship between crater diameter and the initial post-formation depth. Depth was measured as maximum rim-to-floor depth, ( d r ), but we also report the depth measured using other definitions. The database was down-selected by refining or removing elevation measurements from “problematic” craters affected by processes and conditions that influenced their d r /D, such as pre-impact slopes/topography and later overprinting craters. We report a maximum (deepest) and mean scaling relationship of d r = ( 0.347  ±  0.021 ) D 0.537  ±  0.017 and d r = ( 0.323  ±  0.017 ) D 0.538  ±  0.016 , respectively. Our results suggest that significant variations between previously-reported MOLA-based d r vs. D relationships may result from the inclusion of craters that: 1) are influenced by atypical processes ( e.g. , highly oblique impact), 2) are significantly degraded, 3) reside within high-strength regions, and 4) are transitional (partially collapsed). By taking such issues into consideration and only measuring craters with primary floor materials, we present the best estimate to date of a MOLA-based relationship of d r vs. D for the least-degraded complex craters on Mars. This can be applied to crater degradation studies and provides a useful constraint for models of complex crater formation. [ABSTRACT FROM AUTHOR]

The lunar crater record features ∼ 50 basins. The radiometric dating of Apollo samples indicates that the Imbrium basin formed relatively late — from the planet formation perspective — some ≃ 3. 9 Ga. Here we develop a dynamical model for impactors in the inner solar system to provide context for the interpretation of the lunar crater record. The contribution of cometary impactors is found to be insignificant. Asteroids produced most large impacts on the terrestrial worlds in the last ≃ 3 Gyr. The great majority of early impactors were rocky planetesimals left behind at ∼ 0. 5 –1.5 au after the terrestrial planet accretion. The population of terrestrial planetesimals was reduced by disruptive collisions in the first t ∼ 20 Myr after the gas disk dispersal. We estimate that there were ∼ 4 × 1 0 5 diameter d > 10 km bodies when the Moon formed (total planetesimal mass ∼ 0. 015 M Earth at t ∼ 50 Myr). The early bombardment of the Moon was intense. To accommodate ∼ 50 known basins, the lunar basins that formed before ≃ 4. 35 –4.41 Ga must have been erased. The late formation of Imbrium occurs with a ∼ 15 –35% probability in our model. About 20 d > 10 -km bodies were expected to hit the Earth between 2.5 and 3.5 Ga, which is comparable to the number of known spherule beds in the late Archean. We discuss implications of our model for the lunar/Martian crater chronologies, Late Veneer, and noble gases in the Earth atmosphere. • The leftover planetesimals produced most lunar impacts in the first 1 Gyr. • The early lunar crater record in the first 200 Myr must have been erased. • The Imbrium basin formation happens with a 15%–35% probability in our model. • Applying the lunar chronology to Mars can lead to incorrect age estimates. • The model explains the terrestrial spherule beds found in the late Archean. [ABSTRACT FROM AUTHOR]

The area surrounding Lomonosov crater on Mars has a high density of seemingly organised boulder patterns. These form seemingly sorted polygons and stripes within kilometre scale blockfields, patches of boulder strewn ground which are common across the Martian high latitudes. Several hypotheses have been suggested to explain the formation of clastic patterned ground on Mars. It has been proposed that these structures could have formed through freeze-thaw sorting, or conversely by the interaction of boulders with underlying fracture polygons. In this investigation a series of sites were examined to evaluate whether boulder patterns appear to be controlled by the distribution of underlying fractures and test the fracture control hypotheses for their formation. It was decided to focus on this suite of mechanisms as they are characterised by a clear morphological relationship, namely the presence of an underlying fracture network which can easily be evaluated over a large area. It was found that in the majority of examples at these sites did not exhibit fracture control. Although fractures were present at many sites there were very few sites where the fracture network appeared to be controlling the boulder distribution. In general these were not the sites with the best examples of organization, suggesting that the fracture control mechanisms are not the dominant geomorphic process organising the boulders in this area. [ABSTRACT FROM AUTHOR]

Deimos, one of the Martian moons, has numerous impact craters. However, it is unclear whether crater saturation has been reached on this satellite. To address this issue, we apply a statistical test known as nearest-neighbor analysis to analyze the crater distribution of Deimos. When a planetary surface such as the Moon is saturated with impact craters, the spatial distribution of craters is generally changed from random to more ordered. We measured impact craters on Deimos from Viking and HiRISE images and found (1) that the power law of the size-frequency distribution of the craters is approximately −1.7, which is significantly shallower than those of potential impactors, and (2) that the spatial distribution of craters over 30 m in diameter cannot be statistically distinguished from completely random distribution, which indicates that the surface of Deimos is inconsistent with a surface saturated with impact craters. Although a crater size-frequency distribution curve with a slope of −2 is generally interpreted as indicating saturation equilibrium, it is here proposed that two competing mechanisms, seismic shaking and ejecta emplacement, have played a major role in erasing craters on Deimos and are therefore responsible for the shallow slope of this curve. The observed crater density may have reached steady state owing to the obliterations induced by the two competing mechanisms. Such an occurrence indicates that the surface is saturated with impact craters despite the random distribution of craters on Deimos. Therefore, this work proposes that the age determined by the current craters on Deimos reflects neither the age of Deimos itself nor that of the formation of the large concavity centered at its south pole because craters should be removed by later impacts. However, a few of the largest craters on Deimos may be indicative of the age of the south pole event. [ABSTRACT FROM AUTHOR]

An estimate is made of the thickness of the radar-bright deposits in craters near to Mercury’s north pole. To construct an objective set of craters for this measurement, an automated crater finding algorithm is developed and applied to a digital elevation model based on data from the Mercury Laser Altimeter onboard the MESSENGER spacecraft. This produces a catalogue of 663 craters with diameters exceeding 4 km, northwards of latitude + 55 ∘ . A subset of 12 larger, well-sampled and fresh polar craters are selected to search for correlations between topography and radar same-sense backscatter cross-section. It is found that the typical excess height associated with the radar-bright regions within these fresh polar craters is (50 ± 35) m. This puts an approximate upper limit on the total polar water ice deposits on Mercury of ∼ 3 × 10 15 kg. [ABSTRACT FROM AUTHOR]

The majority of hydrated silicate occurrences on Mars are associated with impact craters (Ehlmann et al., 2011; Carter et al., 2013). Three formation mechanisms have been suggested to account for this correlation: (1) aqueous alteration occurred pre-impact, and was subsequently exposed via the impact (pre-impact; Bibring et al., 2006; Ehlmann et al., 2011), (2) heat generated from the impact facilitated the formation of a hydrothermal system, leading to alteration products (syn-impact; e.g. Marzo et al., 2010; Osinski et al., 2013), and/or (3) altered materials were deposited after crater formation, or formed within the crater well after the impact had taken place (post-impact). In this study, we analyze the central uplift of Elorza Crater, a ∼40 km diameter impact crater located ∼300 km north of Valles Marineris. To determine whether hydrated minerals found within the uplift were generated pre-, syn-, or post-impact, we used a data synthesis approach, utilizing High Resolution Imaging Science Experiment (HiRISE), Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), Context Camera (CTX), and Thermal Emission Imaging System (THEMIS) imagery. Opaline silica is observed in two locations on the southwestern side of the uplift and is interpreted to have been pre-existing or formed via hydrothermal alteration due to stratigraphic relationships with the overlying impact melt unit. Both Fe/Mg smectite and low-calcium pyroxene (LCP) are found throughout the uplift. Bedrock exposures on the northern wall of Coprates Chasma containing Fe/Mg smectite and LCP suggest an uplifted origin for these units. In all cases, although a pre-existing origin is probable, it is difficult to rule out the possibility of an impact-generated hydrothermal origin. Using the observed stratigraphy exposed in Coprates Chasma and bedrock exposures analyzed in nearby craters, we were able to constrain the pre-impact stratigraphy around Elorza. The near-subsurface consists of Hesperian-aged, discontinuous lava/ash deposits that may be interposed with opaline silica-bearing deposits, overlying Noachian basement consisting of smectite-bearing bedrock and LCP- bearing light-toned fractured bedrock. [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]

We conclude our radar imaging survey of Mars, which maps spatial variations in depolarized radar reflectivity using Arecibo S-band (λ12.6 cm) observations from 2005–2012. Whereas our earlier paper (Harmon et al., 2012, Arecibo radar imagery of Mars: the major volcanic provinces. Icarus 220, 990–1030) covered the volcanic regions of Tharsis, Elysium, and Amazonis, this paper includes non-volcanic regions where hydrologic and impact processes can be the dominant resurfacing agents affecting radar backscatter. Many of the more prominent and interesting radar-bright features outside the major volcanic provinces are located in and around Chryse Planitia and Xanthe Terra. These features are identified with: a basin in northeast Lunae Planum containing the combined deposits from Maja Vallis and Ganges Catena outflows; channel outwash plains in western and southern Chryse basin; plateaus bordering chasma/chaos zones, where surface modification may have resulted from hydrologic action associated with incipient chaos formation; and some bright-ejecta craters in Chryse basin, of a type otherwise rare on Mars. Dark-halo craters have also been identified in Chryse and elsewhere that are similar to those seen in the volcanic provinces. Although the cratered highlands are relatively radar-bland, they do exhibit some bright depolarized features; these include eroded crater rims, several unusual ejecta flows and impact melts, and terrain-softened plains. The rims of large impact basins (Hellas, Argyre, Isidis) show a variety of radar-bright features provisionally identified with massif slopes, erosion sediments, eroded pyroclastics, impact melts, and glacial deposits. The interiors of these basins are largely radar-dark, which is consistent with coverage by rock-free sediments. Tempe Terra and Acheron Fossae show bright features possibly associated with rift volcanism or eroded tectonic structures, and northwest Tempe Terra shows one very bright feature associated with glacial or other ice processes in the dichotomy boundary region. The first delay-Doppler images of the radar-bright features from the north and south polar icecaps are presented. Both poles show the circular polarization inversion and high reflectivity characteristic of coherent volume backscatter from relatively clean ice. The south polar feature is primarily backscatter from the residual CO 2 icecap (with a lesser contribution from the polar layered deposits), whose finite optical depth probably accounts for the feature's strong S/X-band wavelength dependence. Conversely, the north polar radar feature appears to be mostly backscatter from the H 2 O-ice-rich polar layered deposits rather than from the thin residual H 2 O cap. The north polar region shows additional radar-bright features from Korolev Crater and a few other outlying circumpolar ice deposits. [ABSTRACT FROM AUTHOR]

A variety of hydrothermal environments have been documented in terrestrial impact structures. Due to both past water interactions and meteoritic bombardment on the surface of Mars, several authors have predicted various scenarios that include the formation of hydrothermal systems. Geological and mineralogical evidence of past hydrothermal activity have only recently been found on Mars. Here, we present a geological and mineralogical study of the Auki Crater using the spectral and visible imagery data acquired by the CRISM (Compact Reconnaissance Imaging Spectrometer for Mars), CTX (Context Camera) and HiRISE (High Resolution Imaging Science Experiment) instruments on board the NASA MRO mission. The Auki Crater is a complex crater that is ∼38 km in diameter located in Tyrrhena Terra (96.8°E and 15.7°S) and shows a correlation between its mineralogy and morphology. The presence of minerals, such as smectite, silica, zeolite, serpentine, carbonate and chlorite, associated with morphological structures, such as mounds, polygonal terrains, fractures and veins, suggests that the Auki Crater may have hosted a post impact-induced hydrothermal system. Although the distribution of hydrated minerals in and around the central uplift and the stratigraphic relationships of some morphological units could also be explained by the excavation and exhumation of carbonate-rich bedrock units as a consequence of crater formation, we favor the hypothesis of impact-induced hydrothermal circulation within fractures and subsequent mineral deposition. The hydrothermal system could have been active for a relatively long period of time after the impact, thus producing a potential transient habitable environment. [ABSTRACT FROM AUTHOR]

Past observations of the martian exosphere have given a wide range of values for the mean temperature and number density of the hydrogen population that occupies this uppermost layer. More recently, observations by HST and MEX have found large variations over short timescales exhibited by this layer, which have been attributed to seasonal effects. Here we present an analysis of the modeling techniques used to study the martian exosphere and their related uncertainties, and discuss the sensitivity of various modeling parameters for any remote observations of the martian hydrogen exosphere. Degeneracy between the two free parameters in the model, the exobase temperature and density of hydrogen at Mars introduces difficulty in accurately characterizing the properties of the martian exosphere. An independent measurement of at least one parameter is required in order to positively identify the other. The likely presence of a superthermal component of H adds another uncertainty to the modeling process, with large changes in the resulting escape flux. A study of the latitudinal symmetry of the martian exosphere found the radial emission profiles to be asymmetric below 2.5 martian radii, and then more uniform at high altitudes. Comparisons between simulated spacecraft and HST intensity profiles with altitude suggest that a larger coverage of intensity profiles is important to better determine the characteristics of the martian exosphere. [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]

When the Hale impact crater penetrated the martian cryosphere 1Ga, landforms indicating post-impact volatile mobilisation were generated. We have found landforms in the ejecta blanket of Hale Crater similar to 'permafrost molards' found in periglacial environments on Earth, and probably related to the past or present presence of volatiles at/near the surface. Permafrost molards are conical mounds of debris associated with landslide deposits, resulting from the degradation of blocks of ice-rich material mobilised by a landslide in periglacial terrains. Here we analyse the spatial and topographic distribution of conical mounds around the Hale crater at regional and local scales, and compare them to those of molards on the deposits of the Mount Meager debris avalanche in Canada. Hale Crater's conical mounds are located at the distal boundary of the thickest ejecta blanket, which is the closest to the main crater. We observe a similar spatial arrangement of molards along the distal parts of the terminal lobe of the Mount Meager debris avalanche. We then compare the morphology and morphometrics of the conical mounds on Hale Crater to those of terrestrial molards on the Paatuut and Niiortuut rock avalanches in western Greenland. We find that morphology and setting of conical mounds within Hale Crater ejecta are consistent with the formation pathway of molards on Earth. We infer that they originated from blocks of ice-cemented regolith that were produced by the Hale-crater-forming impact, transported by the ejecta flows, and finally degraded to cones of debris (molards) on loss of the interstitial ice. The similarities in distribution between the ejecta flows of Hale and Mount Meager debris avalanche on Earth suggest that the mounds resulted from the rheological separation of the ejecta flows, with a relatively fluid-poor phase that allowed the volatile-rich blocks to survive transport. This supports the prevailing hypothesis that the Hale impact event penetrated the martian cryosphere, providing important constraints on the rheology of martian ejecta deposits. • Conical mounds reveal the history of the cryosphere near the Argyre impact basin. • Conical mounds within the ejecta of Hale Crater are similar to terrestrial molards. • Spatial distribution and morphometry of terrestrial and martian molards are compared. • Conical mounds in Hale reveal volatile-rich ejecta at the time of emplacement. [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]

We simulate the long-term deformation of Martian craters and investigate the role of lower crustal flow in the evolution of surface and subsurface topography. Using the finite element method and a viscoelastic rheological model, we model the deformation of more than 30 large craters and Quasi-Circular Depressions (QCDs), in the diameter range of ∼200–500 km, in both the Northern Lowlands and Southern Highlands. We determine the most appropriate background heat fluxes that produce the current topography beneath the impacts at the crust–mantle boundary (ranges from 40 to ∼90 mW m −2 ). Our study shows that a higher background heat flux leads to more relaxation at the surface and subsurface. By applying various viscous creep parameters for hydrous and anhydrous rheologies, we demonstrate that Mars's interior is wet to a certain degree, which is consistent with other estimates. Since craters and QCDs are distributed fairly equally on the surface of the Red Planet, this study provides a less regionally biased picture of the thermal history of early Mars than in previous studies. Based on our results, the ancient average background heat flux in the Northern Lowlands was higher than that of the Southern Highlands, which could indicate that whatever process formed the crustal dichotomy had a thermal signature at least through the middle Noachian. [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 report on line-of-sight extinction in northern Gale Crater, Mars as seen by the Mars Science Laboratory (MSL) rover, Curiosity from sol 100 to sol 900; a little more than an entire martian year. Navcam images oriented due north, which show the distant crater rim, the near ground and the sky allow the extinction due to dust within the crater to be determined. This line-of sight extinction is compared to a complementary dataset of column extinctions derived from Mastcam. The line-of-sight extinction within the crater is less than the column extinction for the majority of the martian year. This implies that the relatively low mixing ratio of dust within the crater as compared to the atmosphere above the crater rim persists through most of the year. This suggests relatively little mixing between the atmosphere above the crater and the atmosphere inside the crater and suggests that northern Gale Crater is a net sink of dust in the current era. The data does however show a yearly convergence of the line-of-sight extinction and the column-averaged extinction around L s = 270–290°. This suggests that air above the crater mixes with air in the crater at this time, as predicted by mesoscale models. Matching line-of-sight and column extinction values are also seen around L s ≈ 135°, a season that has only been observed once in this dataset, this is particularly interesting as the Rover Environmental Monitoring Station onboard Curiosity reports increased convective boundary layer heights in the same season. [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]

In this paper we investigate Charon's craters size distribution using a deep learning model. This is motivated by the recent results of Singer et al. (2019) who, using manual cataloging, found a change in the size distribution slope of craters smaller than 12 km in diameter, translating into a paucity of small Kuiper Belt objects. These results were corroborated by Robbins and Singer (2021), but opposed by Morbidelli et al. (2021), necessitating an independent review. Our MaskRCNN-based ensemble of models was trained on Lunar, Mercurian, and Martian crater catalogues and both optical and digital elevation images. We use a robust image augmentation scheme to force the model to generalize and transfer-learn into icy objects. With no prior bias or exposure to Charon, our model find best fit slopes of q = −1.47 ± 0.33 for craters smaller than 10 km, and q = −2.91 ± 0.51 for craters larger than 15 km. These values indicate a clear change in slope around 15 km as suggested by Singer et al. (2019) and thus independently confirm their conclusions. Our slopes however are both slightly flatter than those found more recently by Robbins and Singer (2021). Our trained models and relevant codes are available online on github.com/malidib/ACID. [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]