The evolution and compositional structure of the lunar mantle has been extensively modeled but insufficiently constrained by observations. Here, we identify and characterize mantle materials exposed by the Moon's largest impact basin to better understand the composition, stratigraphy, and evolution of the upper mantle. The vast South Pole-Aitken Basin (SPA) exhibits a broad, crescent-shaped thorium and potassium distribution. These incompatible elements are predicted to be concentrated in the dregs of the lunar magma ocean during end-stage crystallization. Through consideration of basin formation models convolved with subsequent geologic evolution, we demonstrate that the distribution and implied stratigraphy of Th-and K-bearing materials across SPA are consistent with an upper mantle ejecta origin. The most pristine exposures of these materials are confined to northwest SPA and also exhibit elevated Ti and Fe (relative to the farside highlands) in association with a gabbronoritic mineralogy. This is consistent with latestage magma ocean assemblages predicted by petrologic models. In contrast, SPA impact melt derived from greater depths is associated with a low-Ca pyroxene-dominated assemblage. Together, these compositional patterns are evidence for a stratified ancient upper mantle. Importantly, the incompatible-element-enriched, ilmenite-bearing ferroan gabbronoritic cumulates evidently had not participated in gravitational overturn at the time of SPA formation. Contrary to recent hypotheses invoking nearside sequestration of incompatible elements to explain hemispherical differences in crustal building and volcanic resurfacing, it follows that incompatible elements were globally distributed in the magma ocean at the time of SPA formation. Plain Language Summary Like the Earth, the Moon is layered into a crust and mantle. The Moon's layering was shaped by an early global melting event known as the "Lunar Magma Ocean." As the magma ocean solidified, dense minerals sank to form the mantle, while less-dense minerals floated to form the crust. Elements such as thorium are not easily incorporated into mineral structures, and remain in the liquid. Because of this, a thorium-rich dreg layer was sandwiched between the crust and mantle. These dregs are very dense and are expected to sink into the underlying mantle during or soon after crystallization. We demonstrate that the Moon's largest and oldest impact basin excavated material from this dense, thorium-rich layer before it sank. The exposed material was then diluted and obscured by four billion years of impact cratering and volcanic eruptions. However, we identify several pristine exposures created by recent craters. The impact basin also melted rocks from greater depths than the rocks it ejected. These melted rocks exhibit a much different composition. This indicates that the lunar upper mantle included two compositionally distinct layers that were exposed in different ways by this large impact event. These results have important implications for understanding the ...

We use Lunar Reconnaissance Orbiter Camera Narrow Angle Camera images to characterize boulder populations around six small (<1 km), young (<200 Ma) impact craters near spacecraft landing sites. The Narrow Angle Camera boulder counts are used to analyze how boulder distributions vary around craters of different sizes and ages. These comparisons inform how various properties affect the distance to which boulders are ejected and the size and density of boulders produced by an impact event. The counts show that boulder population densities decrease with crater age, with few boulders remaining at craters older than a few hundred million years, consistent with results of other studies of boulder degradation rates on the Moon. Variations in boulder distributions around younger craters may provide information regarding impact conditions; South Ray crater has a larger population of small boulders than the larger North Ray crater, which could be explained by variations in impact velocity. Large craters generally excavate more boulders than smaller craters, and the size of the largest boulder ejected is related to crater size by a power‐law function. Larger boulders occur closer to the crater rim (within 2–4 crater radii), whereas smaller boulders occur at all distances. The density of boulders is greater near the crater rim and decreases with increasing radial distance; this data can aid in establishing safe landing zones for future missions. Analyzing boulder distributions across craters of varying ages allows us to test models of boulder breakdown rates, with implications for understanding the Moon's regolith production rate.

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The Lunar Science for Landed Missions workshop was convened at the National Aeronautics and Space Administration Ames Research Center on 10–12 January, 2018. Interest in the workshop was broad, with 110 people participating in person and 70 people joining online. In addition, the workshop website (https://lunar-landing.arc.nasa.gov) includes video recordings of many of the presentations. This workshop defined a set of targets that near‐term landed missions could visit for scientific exploration. The scope of such missions was aimed primarily, but not exclusively, at commercial exploration companies with interests in pursuing ventures on the surface of the Moon. Contributed and invited talks were presented that detailed many high priority landing site options across the surface of the Moon that would meet scientific goals in a wide variety of areas, including impact cratering processes and dating, volatiles, volcanism, magnetism, geophysics, and astrophysics. Representatives from the Japan Aerospace Exploration Agency and the European Space Agency also presented about international plans for lunar exploration and science. This report summarizes the set of landing sites and/or investigations that were presented at the workshop that would address high priority science and exploration questions. In addition to landing site discussions, technology developments were also specified that were considered as enhancing to the types of investigations presented. It is evident that the Moon is rich in scientific exploration targets that will inform us on the origin and evolution of the Earth‐Moon system and the history of the inner Solar System, and also has enormous potential for enabling human exploration and for the development of a vibrant lunar commercial sector.

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It is often necessary to identify a pattern of observed craters in a single image of the lunar surface and without any prior knowledge of the camera’s location. This so-called “lost-in-space” crater identification problem is common in both crater-based terrain relative navigation (TRN) and in automatic registration of scientific imagery. Past work on crater identification has largely been based on heuristic schemes, with poor performance outside of a narrowly defined operating regime (e.g., nadir pointing images, small search areas). This work provides the first mathematically rigorous treatment of the general crater identification problem. It is shown when it is (and when it is not) possible to recognize a pattern of elliptical crater rims in an image formed by perspective projection. For the cases when it is possible to recognize a pattern, descriptors are developed using invariant theory that provably capture all of the viewpoint invariant information. These descriptors may be pre-computed for known crater patterns and placed in a searchable index for fast recognition. New techniques are also developed for computing pose from crater rim observations and for evaluating crater rim correspondences. These techniques are demonstrated on both synthetic and real images.

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