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The Mars Reconnaissance Orbiter and its Context Camera (CTX) have acquired more than 100,000 separate panchromatic images that capture nearly the entire surface of Mars at ∼5–6 m/pixel. This paper describes a data processing workflow used to generate the first contiguous global mosaic of CTX data, which represents a large improvement in spatial resolution over existing 100 m/pixel contiguous global mosaics. We describe the overarching strategy for the mosaic's construction, which was to maximize the scientific utility of a continuous mosaic that is 5.7 trillion pixels in size. The pipeline used for data processing prioritized traceability and reproducibility of the final mosaic, such that the provenance of all pixels is reported, equipping scientists with information to differentiate mosaic artifacts from surface landforms and to incorporate critical image metadata into their analyses. The CTX data set synthesized into a global CTX mosaic facilitates ready analysis and provides a new capability in transitioning global studies of Mars from high‐resolution investigations of individual images to systematic studies of the entire Martian surface at outcrop‐resolving quality without regard to image boundaries. Plain Language Summary We generated a global mosaic of Mars using Context Camera satellite imagery at 5.0 m/pixel, a substantial improvement in spatial resolution over the previous global mosaic at 100 m/pixel. This paper describes a new technique for merging overlapping images together in a way that preserves all information from each component image and produces a map of all image boundaries that is included with the mosaic. This makes scientific analyses that are facilitated by the mosaic fully traceable and reproducible. With this mosaic, scientists can now readily analyze landforms without the overhead of downloading individual images and perform complete global analyses of Mars at a resolution capable of seeing rock outcrops on the surface. Key Points A global mosaic of Mars at 5.0 m/pixel has been created from 86,571 Context Camera images that are co‐registered to each other and seam‐corrected Non‐destructive image processing was used to preserve all image metadata and map seams The new mosaic will facilitate systematic studies of Mars at 5.0 m/pixel spatial resolution

Valley networks, regional drainage patterns suggesting liquid water stability at the surface, are confined to early in the history of Mars (the Noachian/Hesperian boundary and before), prior to a major climate transition to the hyperarid cold conditions of the Amazonian. Several later fluvial valley systems have been documented in specific Hesperian and Early Amazonian environments, and are thought to have formed due to local conditions. Here we describe fluvial valley systems within Lyot crater that have the youngest well‐constrained age reported to date (Middle or Late Amazonian) for systems of this size (tens of km). These valleys are linked to melting of near‐surface ice‐rich units, extend up to ∼50 km in length, follow topographic gradients, and deposit fans. The interior of Lyot crater is an optimal micro‐environment, since its low elevation leads to high surface pressure, and temperature conditions at its location in the northern mid‐latitudes are sufficient for melting during periods of high‐obliquity. This micro‐environment in Lyot apparently allowed melting of surface ice and the formation of the youngest fluvial valley systems of this scale yet observed on Mars.

Boulder‐sized clasts are common on the surface of Mars, and many are sufficiently large to be resolved by the high resolution imaging science experiment (HiRISE) camera aboard the Mars reconnaissance orbiter. The size, number, and location of boulders on the surface and their spatial distribution can reveal the processes that have operated on the surface, including boulder erosion, burial, impact excavation, and other mechanisms of boulder transport and generation. However, quantitative analysis of statistically significant boulder populations, which could inform these processes, entails prohibitively laborious manual segmentation, granulometry, and morphometry measurements over large areas. Here, we develop, describe, and validate an automated tool to locate and measure boulders on the Martian surface: the Martian Boulder Automatic Recognition System (MBARS). Our open‐source Python‐based toolkit automatically measures boulder diameter and height in HiRISE images enabling rapid and accurate assessments of boulder populations. We compare our algorithm with existing boulder‐counting methods, manual analyses, and objects of known size to verify accuracy and precision. Additionally, we test how MBARS quantitatively characterizes boulders around an impact crater in the Martian northern lowlands. We compare this to previous work on rock excavation during impact cratering using manually counted boulders around lunar craters. Plain Language Summary Large boulders (>1 m diameter) are widely distributed on the Martian surface. They are easily observed from orbit, making them visible with high‐resolution imaging. Mapping the location, number, and size of boulders is helpful for understanding which geological processes bring boulders to the surface, move them around, and fragment them into smaller rocks and soil. Here, we describe and validate the Martian Boulder Automatic Recognition System (MBARS), a set of tools that automatically locates and measures boulders in high‐resolution images of the Martian surface. We compare results generated by MBARS with results from other automated boulder‐measuring tools as well as with results from manual boulder measurements to ensure accuracy. We also use MBARS to map boulders around an impact crater on Mars and compare the boulder distribution to a similar‐sized crater on the Moon. Key Points The Martian Boulder Automatic Recognition System (MBARS) is a new tool to detect and measure boulders on the Martian surface MBARS is comparably or more accurate than prior published algorithms that measure boulders MBARS readily reproduces manually measured results

Impact basin formation is a fundamental process in the evolution of the Moon and records the history of impactors in the early solar system. In order to assess the stratigraphy, sequence, and ages of impact basins and the impactor population as a function of time, we have used topography from the Lunar Orbiter Laser Altimeter (LOLA) on the Lunar Reconnaissance Orbiter (LRO) to measure the superposed impact crater size‐frequency distributions for 30 lunar basins (D ≥ 300 km). These data generally support the widely used Wilhelms sequence of lunar basins, although we find significantly higher densities of superposed craters on many lunar basins than derived by Wilhelms (50% higher densities). Our data also provide new insight into the timing of the transition between distinct crater populations characteristic of ancient and young lunar terrains. The transition from a lunar impact flux dominated by Population 1 to Population 2 occurred before the mid‐Nectarian. This is before the end of the period of rapid cratering, and potentially before the end of the hypothesized Late Heavy Bombardment. LOLA‐derived crater densities also suggest that many Pre‐Nectarian basins, such as South Pole‐Aitken, have been cratered to saturation equilibrium. Finally, both crater counts and stratigraphic observations based on LOLA data are applicable to specific basin stratigraphic problems of interest; for example, using these data, we suggest that Serenitatis is older than Nectaris, and Humboldtianum is younger than Crisium. Sample return missions to specific basins can anchor these measurements to a Pre‐Imbrian absolute chronology. Key Points New measurements of crater statistics and stratigraphy for 30 lunar basins Any transition in lunar impactor populations occurred before the mid‐Nectarian The oldest lunar basins are likely cratered to saturation equilibrium

The Lunar CRater Observations and Sensing Satellite (LCROSS) impacted a Centaur rocket stage into a permanently shadowed region (PSR) in Cabeus crater, excavating water ice and other volatiles. We used the Miniature Radio Frequency (Mini‐RF) instrument on the Lunar Reconnaissance Orbiter and the ShadowCam instrument on the Korean Pathfinder Lunar Orbiter to detect the probable 22‐m diameter crater that resulted from the LCROSS impact. The crater formed superposed upon a dense small crater population along a crater ray from a larger pre‐existing crater. From its geologic context, the ice and regolith excavated by LCROSS were likely modified within the last 0.1–0.5 Gyr. An upper limit for the excavated volatiles is ~0.9 Gyr, as the location was not a PSR prior to that time. A young age for the LCROSS‐detected volatiles supports the idea that they were mostly emplaced by an exogenic mechanism, such as from comets or the solar wind. Plain Language Summary The LCROSS experiment formed an impact crater in an area of permanent shadow on the Moon, striking the surface at 2.5 km/s with a 2,300 kg spent rocket body on 9 October 2009. The impact ejecta from this cratering event included detectable amounts of water and other volatiles, which is perhaps the most direct evidence for significant water deposits on the Moon. However, since the impact location is in permanent shadow (no direct solar illumination), it proved hard to observe definitively the crater that LCROSS formed. Here, we use data from Mini‐RF, which illuminated the surface with S‐band radar, combined with ShadowCam, which acquires images within permanent shadows, to find the probable LCROSS impact crater. The impact crater is 22‐m in diameter, a bit smaller than was inferred indirectly after LCROSS. We also present new evidence that the volatiles in the ejecta likely got there in the last 20% of lunar history, which is important for understanding their origin and evolution. Key Points We used Mini‐RF and ShadowCam to locate the 22‐m crater formed by LCROSS within a permanently shadowed region near the Moon's south pole Given the crater's size, the regolith that was excavated, including volatiles, likely came from approximately the upper 2 m The geologic context of the LCROSS crater suggests that the volatiles it excavated were relatively young, from the Copernican epoch

Analysis of images from the Messenger MDIS narrow angle camera imply that at least part of the radial graben of the Pantheon Fossae structure, and probably the structure as a whole, predate the deformation that led to circumferential ridges on the Caloris interior plains. This follows from structural analysis and comparison with similar geological relationships on Venus and the Moon, where graben are known to both postdate and predate ridges. Observations suggest that the Pantheon Fossae radial graben (extension) formed first, pre-dating observed circumferential graben (also extension), with ridges (compression) formed in between. This scenario puts constraints on the models for the deformation of the Caloris basin and its vicinity. Our observations and analysis are consistent with Pantheon Fossae having formed in a similar manner to Venusian astra/novae, where radial dikes that propagate away from a magmatic center led to graben formation. Our results also have implications for the length of time between the emplacement of the basin volcanic fill and the onset of the compressional stresss regime that led to ridge-formation. If the Pantheon Fossae structure formed before the emplacement of ridges, as we suggest, this means that compressional stresses took some time to develop sufficiently to deform the volcanic plains. Since the Caloris interior plains had to have been already in place when Pantheon Fossae formed, and since these plains represented a significant load to the underlying lithosphere, it is striking that compression took some time to develop. These observations may provide new information about the rigidity of the basin-filling material and will help constrain models for the mechanisms and timing of events within and around the Caloris basin.

Impact cratering is likely a primary agent of regolith generation on airless bodies. Regolith production via impact cratering has long been a key topic of study since the Apollo era. The evolution of regolith due to impact cratering, however, is not well understood. A better formulation is needed to help quantify the formation mechanism and timescale of regolith evolution. Here, we propose an analytically derived stochastic model that describes the evolution of regolith generated by small, simple craters. We account for ejecta blanketing as well as regolith infilling of the transient crater cavity. Our results show that the regolith infilling plays a key role in producing regolith. Our model demonstrates that, because of the stochastic nature of impact cratering, the regolith thickness varies laterally, which is consistent with earlier work. We apply this analytical model to the regolith evolution at the Apollo 15 site. The regolith thickness is computed considering the observed crater size-frequency distribution of small, simple lunar craters (< 381 m in radius for ejecta blanketing and < 100 m in radius for the regolith infilling). Allowing for some amount of regolith coming from the outside of the area, our result is consistent with an empirical result from the Apollo 15 seismic experiment. Finally, we find that the timescale of regolith growth is longer than that of crater equilibrium, implying that even if crater equilibrium is observed on a cratered surface, it is likely the regolith thickness is still evolving due to additional impact craters.

Impact basin formation is a fundamental process in the evolution of the Moon and records the history of impactors in the early solar system. In order to assess the stratigraphy, sequence, and ages of impact basins and the impactor population as a function of time, we have used topography from the Lunar Orbiter Laser Altimeter (LOLA) on the Lunar Reconnaissance Orbiter (LRO) to measure the superposed impact crater size‐frequency distributions for 30 lunar basins (D ≥ 300 km). These data generally support the widely used Wilhelms sequence of lunar basins, although we find significantly higher densities of superposed craters on many lunar basins than derived by Wilhelms (50% higher densities). Our data also provide new insight into the timing of the transition between distinct crater populations characteristic of ancient and young lunar terrains. The transition from a lunar impact flux dominated by Population 1 to Population 2 occurred before the mid‐Nectarian. This is before the end of the period of rapid cratering, and potentially before the end of the hypothesized Late Heavy Bombardment. LOLA‐derived crater densities also suggest that many Pre‐Nectarian basins, such as South Pole‐Aitken, have been cratered to saturation equilibrium. Finally, both crater counts and stratigraphic observations based on LOLA data are applicable to specific basin stratigraphic problems of interest; for example, using these data, we suggest that Serenitatis is older than Nectaris, and Humboldtianum is younger than Crisium. Sample return missions to specific basins can anchor these measurements to a Pre‐Imbrian absolute chronology. New measurements of crater statistics and stratigraphy for 30 lunar basins Any transition in lunar impactor populations occurred before the mid‐Nectarian The oldest lunar basins are likely cratered to saturation equilibrium

The spatial structure of Mercury's thermal lithosphere depends on the balance between internal heat and surface temperature as controlled by solar insolation. For bodies not in a spin-orbital resonance, observed spatial temperature variations are due to internal heating variations. However, for Mercury's present 3:2 spin-orbit coupling a notable difference of ~150 K exists for the sub-skin depth temperature of the crust as a function of longitude (Fig 1B.; [1,2]). Mercury's longitudinal \"hot poles\" and \"cold poles\", in addition to the standard poles (i.e. North and South), have large temperature contrasts that could lead to systematic differences in crater size, morphology, or morphometry, especially for large impacts.