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We carried out a petrological, mineralogical, and geochemical study of fragmental and regolith breccia clasts separated from two Chang'e‐5 (CE‐5) soil samples, CE5C0000YJYX03501GP and CE5C0400, which provide an opportunity to investigate the compositional change of regolith at the landing site through time. Fragmental breccia CE‐5‐B3 contains a diverse range of basaltic clasts and basaltic mineral fragments, and some rare Mg‐suite‐like minerals. Regolith breccias CE‐5‐B006, CE‐5‐B007, CE‐5‐B010‐08, CE‐5‐B010‐09, CE‐5‐B011‐07, and CE‐5‐B016‐03 contain mare basaltic fragments, mare vitrophyric clasts, rare Mg‐rich fragments possibly derived from the Mg‐suite rocks, and impact‐derived glass spherules. Pb‐isotope data obtained for baddeleyite grains found both inside some of the basaltic clasts identified in breccia fragments and in the breccia matrices yield Pb/Pb dates similar to the 2 Ga crystallization age of the CE‐5 basalt fragments, extracted directly from the soil sample. Seventy‐four Pb isotope analyses of Ca‐phosphate grains also indicate that the majority of these grains have Pb/Pb dates of 2 Ga, suggesting that they originate from the CE‐5 basalts. In addition, a Pb–Pb isochron drawn through analyses of four Ca‐phosphates in breccia CE5‐B006 yielded an intercept corresponding to a date of 3871 ± 46 Ma, which is the best possible estimate of the formation age of these four grains. Electron probe microanalysis shows that the breccias contain components similar to CE‐5 mare basalt fragments extracted directly from the soil sample, implying that the fragmental and regolith breccia fragments are mostly composed of material sourced from the underlying basalts. The general absence of impact melt breccia clasts, along with the general lack of Fe–Ni metal and absence of added meteoritic debris all suggest that the regolith at the CE‐5 landing site is immature and dominated by material mixed together by small local impact cratering events. Trace element analyses show that the glass beads in the regolith breccias have a Th abundance of 4.06–5.28 μg g−1. This is similar to the Th content of the regolith above the Em4 unit at the landing site as measured from orbit, as well as the estimated bulk Th content of CE‐5 basalts, suggesting that Th of the local regolith is predominantly sourced from the underlying mare basalts, without significant Th addition from Th‐rich exotic clasts sourced from evolved lunar lithologies. [ABSTRACT FROM AUTHOR]

Mare domes (interpreted to be a type of shield volcano) represent one important endmember of a variety of volcanic edifices occurring across the volcanic plains on the Moon, whereas Ring‐Moat Dome Structures (RMDSs) are interpreted to reflect the thermodynamic behavior of plain‐forming mare flows during their emplacement and cooling. A comprehensive study of the direct association between mare domes and RMDSs can not only provide deep insights into their formation mechanisms but also yield key information on the nature of mantle sources. We characterized a total of 200 mare domes and more than 6,400 RMDSs within Mare Tranquillitatis using multiple sets of imagery and topography data. RMDS‐bearing domes (80 out of 200) are on average larger than those hosting no RMDSs (average diameter 10.2 vs. 5.5 km) and have lower height/diameter (H/D) ratios (0.01 vs. 0.02) and flank slopes (1.2° vs. 2°). We attribute the presence of RMDSs on some domes to be due to relatively higher effusion rates forming longer thinner flows, producing favorable conditions for the formation of RMDSs. The average diameter of the RMDSs on mare domes (226 m, n = 1,027) appears to be slightly smaller than those located in mare plains (256 m, n = 527). This may be due to slope effects and that the relatively thicker off‐dome part of flows undergoes a relatively higher degree inflation process, producing slightly larger RMDSs. We adapt the RMDS‐formation theoretical model to shallow subcrustal magma reservoir model to account for the Tranquillitatis dome‐RMDS associations. Plain Language Summary: Ring‐moat dome structures (RMDSs) are common features across a large expanse of the lunar mare surface, but their nature is poorly understood. A key recent observation is that RMDSs have a close relationship with mare dome‐style volcanism, which represents a typical eruption style on the Moon, providing a useful window into the composition and geophysical evolution of the lunar interior. In this study, a total of 80 mare domes in Mare Tranquillitatis were found to be superimposed by varying numbers of RMDSs on their tops and flanks. RMDSs were also found in the maria surrounding these domes. The nature, correlation, and morphologic properties of the RMDSs and their host mare domes were measured. Based on new observations, a basalt flow emplacement scenario for the formation of both mare domes and associated volcanic features (e.g., RMDSs) related to dike‐closing in the late stages of eruptions is reviewed and updated to account for these observed associations. Key Points: The presence of RMDSs on some mare domes is attributed to be due to relatively higher effusion rates forming longer thinner flowsDike‐fed volcanic eruptions from the shallow subcrustal magma reservoirs are likely to favor the production of domes around the ventThe Tranquillitatis dome‐RMDS association can be explained by adapting the RMDS theoretical model to the shallow subcrustal magma reservoir model [ABSTRACT FROM AUTHOR]

HiRISE images and digital elevation models (DEMs) of outcrops in candidate Martian glaciovolcanoes provide more detailed evidence for glaciovolcanic processes than has previously been available for Mars. A group of ridges in the Pavonis Mons fan‐shaped glacial deposit features pervasive layering, evidence for local collapse and slumping, and steeper faces in the direction of paleoglacier flow inferred from other features in the deposit. After comparison with terrestrial analogs, we conclude that these ridges are excellent candidates for tephra‐dominated tindar, formed in phreatomagmatic subglacial eruptions. The englacial meltwater lakes required for a phreatomagmatic origin represent a rare example of voluminous surface water bodies in the Late Amazonian of Mars. Plain Language Summary: Steep‐sided landforms in glacial deposits next to volcanoes are good places to look for evidence that lava and ice interacted. We studied high‐resolution images of a steep‐sided ridge in a glacial deposit on the Pavonis Mons volcano on Mars, and used two of the images to make a 3‐D model of the ridge. The distinctly layered beds of rock that make up the ridge, the depressions in nearby ridges, the evidence that part of the ridge collapsed while it was being built, and the way the slope of the ridge changes with location are very similar to landforms on Earth called "tephra‐dominated tindar". This suggests that the Martian ridge formed in a long‐lived body of liquid water, which have been very uncommon on Mars for the past few hundred million years. Key Points: A steep‐sided ridge in the Pavonis Mons glacial deposits comprises a fractured lower unit overlain by layers dipping away from the crestIts steepness and layering are similar to effusion‐dominated tuyas and tephra‐dominated tindar. Morphology and context favor the latterThis implies substantial ice melting and that liquid water remained to interact with the growing edifice, unusual for late Amazonian Mars [ABSTRACT FROM AUTHOR]

In the central area of the South Pole‐Aitken (SPA) basin, an intermediate albedo, mafic compositional anomaly (SPA Compositional Anomaly, SPACA) has been documented by previous studies, but its origin remains uncertain. We conducted an investigation of stratigraphic units defined based on morphology and composition and their relative ages, and placed these in the context of basin topography and the observed sequence of geological events, all helping to distinguish between SPACA origins from: (a) SPA impact melt, (b) volcanism induced by the SPA event and (c) lunar cryptomaria. We conclude that SPACA represents extensive traditional cryptomare deposits overlying the SPA impact melt. We interpret the basin center to be filled with cryptomare deposits at least one km thick (>1 × 105 km3 in volume) with ages not younger than Early Imbrian. We attribute the relatively high albedo of SPACA to lateral mixing of ejecta from nearby highlands craters and basins, and conclude that the cryptomaria basalts are likely to be very similar to basalts on the nearside. Our findings imply a 0.5%–1.8% increase in the total volume of global lunar mare and cryptomare deposits. These results show that mare volcanism was common only in areas of thinnest crust on the lunar farside, a factor important in understanding lunar nearside‐farside asymmetries. Despite this significant increase in total cryptomare volume in the SPA basin center, SPA remains underfilled relative to nearside mascon basins. Return of mare basalts from the SPA region by Chang'E‐6 will help determine potential mantle source region differences and petrogenetic pathways. Plain Language Summary: On the enigmatic farside of the Moon, a huge impact basin called SPA basin occupies an immense area (∼2,500 km diameter). Previous spectroscopic investigations identified four compositional zones across SPA, and the central one shows a distinctive high‐Ca pyroxene anomaly, designated the "South Pole‐Aitken compositional anomaly" (SPACA). SPACA has a higher albedo compared to typical maria but lower than lunar highlands and has a paucity of impact craters relative to other SPA units. Previous studies concluded that SPACA is either associated with (1) the SPA impact melt sheet or (2) subsequent volcanic resurfacing events of a special composition induced by the SPA impact. Our reassessment favors the hypothesis that SPACA represents cryptomaria erupted into the basin center during a period of higher impact flux, resulting in lateral dusting of the dark mare surfaces by highland material, thereby raising their albedos. Consequently, the SPACA is similar to other lunar basins in that cryptomaria were deposited in the basin center, followed by maria when the impact flux declined and lateral dusting was minimal. Our SPA cryptomare volume estimates represent a 0.5%–1.8% increase in the total volume of global lunar mare and cryptomare deposits, but SPA remains underfilled relative to nearside giant basins. Key Points: The central South‐Pole Aitken basin compositional anomaly is formed of cryptomare deposits rather than a differentiated impact melt sheetSouth‐Pole Aitken basin resembles other lunar basins having mare/cryptomare infilling the center, adding 0.5%–1.8% to the global basalt inventoryCrustal thickness variations may account for the concentration of farside maria in the South‐Pole Aitken basin, yet it remains underfilled [ABSTRACT FROM AUTHOR]

The Ismenius Lacus region of Mars has a diverse geological history, and we present the first high‐resolution map of Deuteronilus Cavus (36.2°N; 14.0°E, ∼120 km diameter) in the fretted terrain south of the dichotomy boundary. Strong evidence suggests a volcanic origin of the regional plains, based on the ∼50 m thick volcanic bed underlying 180–300 m of sublimation residue associated with Amazonian plateau glaciation. Pervasive external volcanic flooding, internal erosional modification, and enlargement of a pre‐existing crater by up to 175%–200% resulted in the cavus' present shape. The phyllosilicates detected within Deuteronilus Cavus could be primary materials associated with the surficial aqueous activity, subsurface alteration products excavated by impacts, or a combination of both. We observe branching fluvial channels that are more recent than the traditional valley networks and may be related to fretted terrain resurfacing during the waning period of a high‐obliquity glaciation phase. This is consistent with our interpretation of the ∼600 m thick lobate and lineated deposits, which are remnants of receding glaciers. The glacial ice, protected by a 15–20 m insulating layer of debris cover, is of significant interest for future landing missions because of its potential to preserve biological and climatological signatures, to provide a critical test of Amazonian plateau glaciation, and to be used for in situ resource utilization. With our detailed geological mapping, we improved our understanding of the geological evolution and climatic conditions in the enigmatic fretted terrain near the dichotomy boundary. Plain Language Summary: The ∼120 km long Deuteronilus Cavus was initiated by an impact event. The resulting impact crater was modified by glacial erosional and fluvial processes, leading to the enlargement of 175%–200% of the pre‐existing crater. In addition, we find strong evidence for recent glaciation (<1 Ga) that left 180–300 m of sublimation residue on the plateau superimposed on a ∼50 m thick volcanic bed, suggesting a volcanic origin of the regional plains. During the waning period of a high‐glacial phase, the meltwater ponded on the surface of the cavus, altered surface rocks to produce phyllosilicates, formed channels (now observed as inverted sinuous ridges), and locally distributed branched fluvial channels that are more recent than the traditional valley networks. Glacial landforms still contain up to 600 m of remnant ice from the retreating glaciers at the end of the last glacial period. The relatively pure ice, protected by a 15–20 m insulating layer of debris cover, is critical for future landing missions because of its potential to preserve biological and climatological signatures and to be used for in situ resource utilization. Overall, this research enhances our understanding of the geological evolution and climatic history of Mars. Key Points: We have produced the first high‐resolution map of Deuteronilus Cavus in the fretted terrain south of the Martian dichotomy boundaryThe region records a complex erosional and depositional history, including fluvial and glacial processes in the Amazonian periodThis study provides a framework for exploration of high‐obliquity mid‐latitude plateau glaciation [ABSTRACT FROM AUTHOR]

Global magmatic trends inferred from gamma‐ray, visible/near‐infrared, and thermal infrared spectrometers on Mars‐orbiting spacecraft have been used to constrain planetary petrogenetic processes and global thermal evolution models. Inferred magmatic trends include temporal variations in the relative proportions of low‐Ca and high‐Ca pyroxenes, and in the abundances of potassium (and total alkalis), silica, FeO* (total iron expressed as FeO), and thorium. These patterns are evaluated for consistency with the compositions of surface igneous rocks of different ages analyzed by Mars rovers and of martian meteorites. Trends of decreasing low‐Ca pyroxene/total pyroxene ratios and of decreasing potassium (and total alkalis), with time are generally supported by surface rock analyses. However, significant differences in the GRS‐measured silica in Amazonian volcanoes and in martian meteorites of equivalent age result from contamination by silica‐rich dust and are problematic for a silica trend. Comparison of FeO* in Noachian and Amazonian surface data shows no decrease. An inferred temporal trend in thorium is in conflict with the complex enrichment and depletion patterns of incompatible trace elements in martian meteorites of various ages. A dearth of analyses of Hesperian‐age surface rocks precludes a firm evaluation of inferred Noachian‐Hesperian trends and Hesperian‐Amazonian trends, but abundant Noachian rocks and a few Hesperian rocks at rover sites, and Amazonian martian meteorites, collectively representing at least 16 surface locations, afford useful comparisons with orbital remote‐sensing data. [ABSTRACT FROM AUTHOR]

Lunar surface disturbances by spacecraft engine jets are of particular concern in the current new phase of lunar exploration, when dozens of landing missions and permanent bases are being planned. Some of these exploration efforts will involve multiple landings and liftoffs around the same lunar site; thus, it is essential to evaluate their effect on astronauts and assets on the lunar surface. Here, we assess the surface disturbances during the Chang'e‐5 landing and liftoff procedures through the photometric analysis of high‐resolution multi‐temporal surface and orbital images. Centimeter‐scale surface images reveal a four‐stage evolution of the landing plume impingement over a period of ∼50 s, which involves phenomena such as dust devils and streaks, and displacement of cobbles. Temporal‐ratio calculation of orbital images (including one acquired between landing and liftoff) enables the first direct observation of ascent plume effects. The ascent blast zone consists of two separated sub‐areas (∼3,400 km2 in total), which is nearly twice larger than that of the landing blast zone. The final disturbed surface is characterized by a central main zone (∼2,300 m2) surrounded by a marginal diffuse zone (∼15,300 m2). Phase‐ratio analyses suggest that plume impingement destroys the micro‐porous structure of the uppermost regolith. We estimate that future lunar landers (e.g., SpaceX's Starship) may cause significant lunar surface disturbances over an area of square kilometers. Our results provide unique insights into Chang'e‐5 mission activities, and instructive references for the planning of future lunar endeavors, including the design and construction of surface experiment packages and permanent lunar bases. Plain Language Summary: When spacecraft land on or liftoff from the lunar surface, the release of huge volumes of gases from retrorocket engines significantly disturbs the lunar surface beneath it. These surface disturbance effects are of particular concern as several landing missions and permanent lunar bases are being planned. It is necessary to evaluate how people and instruments will be affected by these disturbances. Here, we study the surface modifications at the Chang'e‐5 mission landing site using high‐resolution surface and orbital images. Centimeter‐scale Chang'e‐5 images reveal dramatic surface phenomena during the landing procedure over a period of ∼50 s, including dust devils and movements of cobbles. Orbital images obtained at different times enable the first direct observation of surface disturbances during liftoff, which are unexpectedly about twice more extensive than those during landing. The final disturbed surface is composed of a central main zone and a marginal diffuse zone. We predict that future lunar landers (e.g., SpaceX's Starship) may disturb the lunar surface over an area of several square kilometers. Our results provide unique insights into an understudied aspect of the Chang'e‐5's mission, and important references for future lunar explorations, including the design and emplacement of lunar surface instruments and permanent lunar bases. Key Points: High‐resolution multi‐temporal surface and orbital images reveal extensive surface disturbance at the Chang'e‐5 landing siteLunar surface disturbed during liftoff is nearly twice larger than that during landing, much more extensive than previously assumedThis provides a unique insight into Chang'e‐5 mission activities and instructive references for future lunar base design and construction [ABSTRACT FROM AUTHOR]

Inverted fluvial channels are widespread on Mars, and along with valley networks, they provide evidence of flowing liquid water in the early period of the planet's geologic history. We previously described tributary ridge networks within a closed‐source drainage basin crater in the southern highlands, which we interpreted as inverted fluvial channels that were fed by meltwater from the surface of cold‐based glaciers located in crater wall alcoves. Here, we estimate both the paleodischarge within the inverted channels and the necessary melting rates for discharge sources in glacial alcoves. We find that paleodischarges of up to ∼7,000 m3/s were possible with melting rates exceeding 2,000 mm/hr in an extreme warming scenario, with lower rates potentially representing more gradual glacial ice loss. These end‐member estimates can provide key constraints for further early Mars climate and landform evolution modeling. Plain Language Summary: Eroded river deposits are widespread on Mars, and along with river valleys, they provide evidence of flowing liquid water in the early period of the planet's geologic history. We previously described similar river deposits within a crater in the southern highlands that we interpreted to have been fed by meltwater from the surface of glaciers located on the crater wall. Here, we estimate the amount of liquid water flow and glacial melting that would have been necessary to form the river deposits in the crater. Very high flow and melting rates may have been possible if the climate warmed quickly, with lower rates potentially representing more gradual glacial ice loss. These estimates can provide key constraints for further modeling of the early Mars climate. Key Points: Paleodischarge estimates for individual intracrater inverted channel segments vary from ∼2 to 7,000 m3/sHigh end discharges would require supraglacial melting rates of >2,000 mm/hrPredicted range of paleodischarges and melting rates corresponds to either gradual or cataclysmic end‐member warming scenarios [ABSTRACT FROM AUTHOR]

We used imaging, laser altimeter and thermal infrared data to map a 0.5 by 0.5° area containing the landing site of the Zhurong rover of the Tianwen‐1 mission. We analyzed the regional topography and thermophysical characteristics, and identified and characterized cones, impact craters, polygonal troughs, ridges and aeolian bedforms. An absolute model age of 757 ± 66 Ma was obtained for the major unit dominating the study area, younger than previous geological mapping results. We propose several targets of high scientific interest for Zhurong and a stratigraphic model composed of five layers (from bottom to top: Noachian basement, Early Hesperian volcanic ridged plains, Vastitas Borealis Formation materials, rocky materials and loose materials). The geomorphic features identified, the proposed stratigraphy and the regional geological evolution scenario set the stage for the Zhurong rover to continue to carry out its exploration of Mars. Plain Language Summary: Tianwen‐1 is China's first independent Mars mission. It consists of an orbiter and a lander and a rover (Zhurong). The Zhurong rover successfully touched down on the surface of Mars on May 15, 2021. The landing site lies in southern Utopia Planitia, in the northern lowlands of Mars. We identified various geomorphologic features within an area of 0.5 by 0.5° centered around the landing site of the Zhurong rover using image, elevation and temperature remote sensing data. We propose a history and geological evolution of the region to set the stage for the Zhurong rover exploration and analysis. Key Points: We conducted detailed geological mapping of the Zhurong landing region on MarsHigh scientific interest targets for Zhurong rover including cones, pitted‐wall craters, aeolian bedforms, polygons, and ridges are proposedA middle Amazonian surface age is suggested and a five‐layer stratigraphic model of the Zhurong landing region is proposed [ABSTRACT FROM AUTHOR]

Chang'e‐5 successfully returned ∼1,731 g of lunar samples on December 17, 2020. We systematically studied the morphology and morphometry of craters surrounding the Chang'e‐5 site based on high‐resolution images. A key ejecta source crater database was produced, incorporating their morphometrical parameters, excavation depth, target rock types, and ages from either crater counting, crater morphology or degradation methods. We found that (a) the regolith at the Chang'e‐5 site is ∼4–6 m thick, (b) ejecta are primarily from five proximal craters, including Xu Guangqi, dominantly with ages <200 Ma; (c) distal materials are mainly from Harpalus, Copernicus, Aristarchus, and Mairan G craters; (d) the drill samples are likely to be mainly from Impact Crater (IC)‐261 and IC‐265 particles at the top and the contribution of Xu Guangqi increases with depth; (e) the ejecta of Xu Guangqi would buried preexisting materials but probably overturned by postimpacts. Plain Language Summary: The Chang'e‐5 mission, China's first lunar sample‐return mission, successfully landed on the Moon on December 1, 2020, at 43.06°N, 51.92°W, and returned 1,731 g of lunar soil samples. Because of the potentially large contribution from local impact craters to the collected samples, we conducted this research to study these craters based on high‐resolution orbital data. Our results support the hypothesis that local craters are the major ejecta sources, mainly from Impact Crater (IC)‐259, IC‐261, IC‐265, IC‐266, and Xu Guangqi crater. Of these, Xu Guangqi is the most critical ejecta source crater and Chang'e‐5 landed on its ejecta deposit. Its ejecta would bury preexisting materials, such as those from distant craters Harpalus, Copernicus, Aristarchus, and Mairan G, but subsequent craters may redistribute buried materials again, and represented in the regolith sampled by Chang'e‐5. Key Points: An impact crater database surrounding the Chang'e‐5 site was producedCrater ages are constrained either by CSFD or crater morphology or degradation methodsEjecta are mainly from local craters, delivered by Copernican‐aged (>200 Ma) craters [ABSTRACT FROM AUTHOR]

Mare domes, small shield volcanoes typically <∼30 km diameter, are part of the spectrum of lunar volcanic features that characterize extrusive basalt deposits. We used new spacecraft data to document these in Mare Tranquillitatis, among the oldest maria and the site commonly interpreted as an ancient degraded non‐mascon impact basin. We found 283 known and suspected mare domes, with the majority (n = 229) concentrated on a broad, ∼450 km circular topographic rise in eastern Mare Tranquillitatis. The domes (median diameter 5.6 km, height 68 m, volume 0.7 km3) contain summit pits (74%; median diameter 0.8 km), and exhibit minor compositional variability between domes and surrounding flows, suggesting that domes both supply and are embayed by these flows. Based on their characteristics and associations, we interpret the small shield volcanoes to have been built from individual low‐volume (<∼10–100 km3), low volatile content, short duration, cooling‐limited eruptions. The ∼450 km broad volcanic rise is ∼920 m high (volume ∼1.6 × 105 km3) and is interpreted to be built from multiple occurrences of small shield eruptions, a shield plains volcanism style. This implies a shallow mantle source region capable of supplying distributed dike‐emplacement and eruption events over an area of 1.75 × 105 km2 early in mare volcanism history (∼3.7 Ga). The difference between Mare Tranquillitatis and younger mare‐filled mascon basins is attributed to the more ancient thermal state and crustal structure of the viscously relaxed Tranquillitatis basin, and a shallower broad magma source region present in earlier lunar thermal history. Plain Language Summary: Lunar mare volcanic activity spans several billion years in early middle lunar history and involves melting in the mantle, ascent in blade‐like cracks (dikes) and eruption to the surface to form basaltic lava flows. The features surrounding the eruption vent (e.g., flows, channels, sinuous rilles, cones, domes, pyroclastics, etc.) provide important information about eruption conditions (e.g., magma volume, rate of eruption, cooling behavior, composition, volatile content, etc.). The array of these features in specific mare locations and with differing ages provides critical information on the evolution of mantle source regions and the thermal evolution of the Moon. We studied ∼3.7 Ga‐aged lava deposits in Mare Tranquillitatis and found over 200 small shield volcanoes clustered in a ∼450 km broad volcanic rise, and formed from a series of eruptions in a distinctive shield plains volcanism style. Missing or rare were other types of volcanic features (sinuous rilles, steep flow fronts, pyroclastics, cones, lava channels). Individual small shield volcanos are interpreted to have formed from relatively low‐volume, low‐volatile content, short‐duration, cooling‐limited eruptions. This unusual concentration in eastern Mare Tranquillitatis implies broad, relatively shallow source regions below this possible ancient, non‐mascon impact basin, in contrast to later mascon‐basin maria (Crisium, Serenitatis, and Imbrium). Key Points: The distribution of over 200 mare domes in Mare Tranquillitatis shows a concentration in a broad rise in eastern TranquillitatisThe broad volcanic rise was formed by shield plains volcanism, differing from volcanism in younger mascon basinsDifferences between Mare Tranquillitatis and younger maria are due to greater ages of the basin and mare, and a shallower source region [ABSTRACT FROM AUTHOR]

Ring‐moat dome structures (RMDSs) are small circular mounds of diameter typically about 200 m and ∼3–4 m in height, surrounded by narrow, shallow moats. They occur in clusters, are widespread in ancient Imbrian‐aged mare basalt host units and show mineralogies comparable to those of their host units. Based on these close associations and similarities, a model has been proposed for the formation of RMDS as the result of late‐stage flow inflation, with second boiling releasing quantities of magmatic volatiles that migrate to the top of the flow as magmatic foams and extrude through cracks in the cooled upper part of the flow to produce the small RMDS domes and surrounding moats. In contrast to this model advocating a contemporaneous emplacement of RMDSs and their host lava flows, a range of observations suggests that the RMDS formed significantly after the emplacement and cooling of their host lava flows, perhaps as recently as in the Copernican Period (∼1.1 Ga to the present). These observations include: (a) stratigraphic embayment of domes into post‐lava flow emplacement impact craters; (b) young crater degradation age estimates for the underlying embayed craters; (c) regolith development models that predict thicknesses in excess of the observed topography of domes and moats; (d) landform diffusional degradation models that predict very young ages for mounds and moats; (e) suggestions of fewer superposed craters on the mounds than on the adjacent host lava flows, and (f) observations of superposed craters that suggest that the mound substrate does not have the properties predicted by the magmatic foam model. Together, these observations are consistent with the RMDS formation occurring during the period after the extrusion and solidification of the host lava flows, up to and including the geologically recent Late Copernican, that is, the last few hundreds of millions of years of lunar history. We present and discuss each of these contradictory data and interpretations and summarize the requirements for magma ascent and eruption models that might account for young RMDS ages. We conclude with a discussion of the tests and future research and exploration that might help resolve the RMDS age and mode of emplacement conundrum. Plain Language Summary: The research reported in this study provides multiple lines of evidence for the discovery of very young mare volcanism on the Moon, thus extending the lunar volcanic history into the Copernican period (∼1.1 Ga to present). Current lunar thermal evolution models have placed an upper limit for the cessation of large‐scale mare volcanism at ∼2.0 Ga ago. In this study, we used both morphometric analysis and topographic diffusion models to date some small craters (100–300 m in scale) which are partially overlapped by ring‐moat dome structures (RMDSs, basaltic lava flow surface features characterized by a domical profile encircled by a ring moat). Our results lead to a conclusion that lunar mare volcanism accounting for the emplacement of these RMDS‐bearing basalts may have occurred several hundreds of millions of years ago (i.e., 130–1,500 Ma), thus challenging the present consensus on the thermal history of the Moon. To address this issue, we also list a series of outstanding questions and robotic/human exploration strategies that could provide conclusive evidence for or against our hypotheses, thus increasing our understanding of the duration and flux of lunar volcanism and the thermal evolution of the Moon. Key Points: Clusters of unusual features, ring moat dome structures (RMDSs), occur extensively in the lunar MariaMultiple lines of evidence suggest two alternative origins, contemporaneous with Imbrian flows, or later, in the CopernicanWe list outstanding questions and suggest future research and exploration scenarios to resolve the age conundrum [ABSTRACT FROM AUTHOR]

China's Chang'e-5 (CE-5) mission recently returned samples from a young intermediate-Ti mare unit (Em4/P58, ~1.5 Ga) in Northern Oceanus Procellarum. Rima Sharp, previously mapped as the longest lunar sinuous rille, is the most prominent volcanic feature associated with the landing region. Our analysis shows that Rima Sharp is not a single rille, but instead is composed of two separate rilles (Rima Sharp, originating from the North Vent, and Rima Mairan from the South Vent), meeting at ~40.40°N, 48.38°W. Both vent have characteristics suggesting relatively low magma volatile contents. Rima Mairan and associated lavas (southeast of Em4/P58), embay and are slightly younger than Rima Sharp. Rille formation is largely influenced by pre-existing topography (earlier mare surface, proto-wrinkle ridges, highlands); rilles and deposits experienced post-formation deformation (wrinkle ridges, mare subsidence). CE-5 samples probably originate mainly from Rima Sharp's source vent, but may represent deposits from both rilles. Plain Language Summary A major unanswered question in lunar science is the age and nature of the youngest lunar lava flows, located in the northwest nearside of the Moon but unsampled by Apollo or Luna missions. How long did internal activity and eruption of lavas continue on the Moon? Also very poorly known is the impact flux in the last third of Solar System history. Craters counted on a geologically young lava flow radiometrically dated in the laboratory would answer this question. China's Chang'e-5 (CE-5) mission recently returned samples from just such a young mare unit. But how was this unit emplaced and how does this help us understand the nature of the lava source regions? Rima Sharp, previously thought to be the longest lunar sinuous rille, crosses the young unit. We find that Rima Sharp is actually formed by two rilles, the 320 km-long Rima Sharp and the 150 km-long Rima Mairan, that meet in the middle of the unit. Detailed analysis shows that Rima Sharp was first, forming most of the young basalt unit, followed closely by Rima Mairan, embaying Rima Sharp. Samples returned by CE-5 are most likely to be from the Rima Sharp eruption. [ABSTRACT FROM AUTHOR]

Advances in origins of life research and prebiotic chemistry suggest that life as we know it may have emerged from an earlier RNA World. However, it has been difficult to reconcile the conditions used in laboratory experiments with real‐world geochemical environments that may have existed on the early Earth and hosted the origin(s) of life. This challenge is due to geologic resurfacing and recycling that have erased the overwhelming majority of the Earth's prebiotic history. We therefore propose that Mars, a planet frozen in time, comprised of many surfaces that have remained relatively unchanged since their formation > 4 Gya, is the best alternative to search for environments consistent with geochemical requirements imposed by the RNA world. In this study, we synthesize in situ and orbital observations of Mars and modeling of its early atmosphere into solutions containing a range of pHs and concentrations of prebiotically relevant metals (Fe2+, Mg2+, and Mn2+) spanning various candidate aqueous environments. We then experimentally determine RNA degradation kinetics due to metal‐catalyzed hydrolysis (cleavage) and evaluate whether early Mars could have been permissive toward the accumulation of long‐lived RNA polymers. Our results indicate that a Mg2+‐rich basalt sourcing metals to a slightly acidic (pH 5.4) environment mediates the slowest rates of RNA cleavage, though geologic evidence and basalt weathering models suggest aquifers on Mars would be near neutral (pH ~ 7). Moreover, the early onset of oxidizing conditions on Mars has major consequences regarding the availability of oxygen‐sensitive metals (i.e., Fe2+ and Mn2+) due to increased RNA degradation rates and precipitation. Overall, (a) low pH decreases RNA cleavage at high metal concentrations; (b) acidic to neutral pH environments with Fe2+ or Mn2+ cleave more RNA than Mg2+; and (c) alkaline environments with Mg2+ dramatically cleaves more RNA while precipitates were observed for Fe2+ and Mn2+. [ABSTRACT FROM AUTHOR]

Since its recognition as an impact structure 60 years ago, no volcanics were anticipated in the circular depression of the 14.8 Ma old Nördlinger Ries. Here, we describe for the first time a volcanic ash‐derived clinoptilolite‐heulandite‐buddingtonite bed within the 330 m thick Miocene lacustrine crater fill. Zircon U‐Pb ages of 14.20 ± 0.08 Ma point to the source of the volcanic ash in the Pannonian Basin, 760 km east of the Ries. The diagenetically derived zeolite‐feldspar bed occurs in laminated claystones of the Ries soda‐lake stage and represents the first unequivocal stratigraphic marker bed in this basin, traceable from marginal surface outcrops to 218 m below surface in the crater center. These relationships demonstrate a deeply bowl‐shaped geometry of crater fill sediments, not explainable by sediment compaction and corresponding stratigraphic backstripping alone. Since most of the claystones formed at shallow water depths, the bowl‐shaped geometry must reflect 134 +23/−49 m of sagging of the crater floor. We attribute the sagging to compaction and closure of the dilatant macro‐porosity of the deeply fractured and brecciated crater floor during basin sedimentation and loading, a process that lasted for more than 0.6 Myr. As a result, the outcrop pattern of the lithostratigraphic crater‐fill units in its present erosional plane forms a concentric pattern. Recognition of this volcanic ash stratigraphic marker in the Ries crater provides insights into the temporal and stratigraphic relationships of crater formation and subsidence that have implications for impact‐hosted lakes on Earth and Mars. Plain Language Summary: We describe for the first time a volcanic ash layer from the lake sediment fill of the 14.8 million years old asteroid impact crater Nördlinger Ries. Radiometric age and trace element characteristics of this ash layer are identical to that of a volcanic field in Hungary, so that the ash reflects a volcanic eruption 760 km east of the Ries basin. Recognition of this ash layer enables its use as a marker bed. The ash layer can be traced from surface outcrops to 218 m depth in drillings. This indicates that the strata are significantly inclined toward the crater center. Calculations of sediment compaction by further sediment load and burial only partially explain the observed deeply bowl‐shaped geometry. We attribute the additional sagging to the subsidence of the crater floor substrate, formed of rocks highly shattered by the impact event. Both effects cause a concentric pattern of outcropping strata in the partially eroded crater fill. The presence of the ash layer and its use to help disentangle the source and timing of subsidence (due to compaction of lake sediments, and closure of deeper, impact‐induced fractures), has important implications for lakes formed in impact craters on Earth and Mars. Key Points: A critical question in the evolution of impact‐crater‐hosted lakes is the origin and timing of post‐impact floor subsidenceWe describe a volcanic ash layer from the Ries impact crater that demonstrates a deeply bowl‐shaped geometry of its lacustrine crater fillThis geometry, leading to a concentric outcrop pattern, requires significant crater floor sagging, in addition to sediment compaction [ABSTRACT FROM AUTHOR]

Water ice has been delivered to the lunar poles from different sources over billions of years, but this accumulation was punctuated by large impacts that excavated dry regolith from depth and emplaced it in layers over the poles. Here, we model the resulting stratigraphies of ice and ejecta deposits in the lunar polar regions. Large polar craters were age dated, and their ejecta distributions calculated with standard scaling relations. We then created a Monte Carlo model for ice deposition and ejecta emplacement. Typical model runs showed that deposits in older cold traps (>4 Ga) are divided into two zones: buried ice‐rich gigaton deposits and younger more gardened mantles. The latter are consistent with small crater morphometry measurements, but the existence of substantial ice buried at great depths is more difficult to confirm. Rare outlier model runs included Mercury‐like cases with significant deposition events in recent history (<200 Ma). Plain Language Summary: The polar regions of Earth's Moon have topographic depressions that are never directly exposed to the Sun, so they are cold enough for deposits of ice to exist. Water can get into these regions by water‐bearing asteroids colliding with the Moon, or from lunar volcanoes erupting gases that travel to the poles. At the same time, large impact craters that form at the poles eject an enormous amount of soil and rock that could bury existing ice. It is not well understood how these two processes work together to build up deposits that may have alternating layers of ice‐rich and ice‐poor soil. In this study, we used computer simulations to predict what these layered deposits may look like. We found it is likely that large amounts of relatively pure ice are buried at depth in the oldest deposits, covered with thinner layers hosting less ice. Key Points: Large polar craters emplaced enough ejecta to cover areas of the poles with centimeter‐ to tens of meter‐thick layers, affecting ice in surrounding cold trapsTypical simulated stratigraphies showed thick gigaton ice deposits at depth in the oldest cold traps, with a gardened mantle aboveThe majority of ice was delivered to the lunar poles before most of the large, present‐day cold traps would have formed, limiting ice accumulation [ABSTRACT FROM AUTHOR]

Lunar mare regolith is traditionally thought to have formed by impact bombardment of newly emplaced coherent solidified basaltic lava. We use new models for initial emplacement of basalt magma to predict and map out thicknesses, surface topographies and internal structures of the fresh lava flows, and pyroclastic deposits that form the lunar mare regolith parent rock, or protolith. The range of basaltic eruption types produce widely varying initial conditions for regolith protolith, including (1) autoregolith, a fragmental meter‐thick surface deposit that forms upon eruption and mimics impact‐generated regolith in physical properties, (2) lava flows with significant near‐surface vesicularity and macroporosity, (3) magmatic foams, and (4) dense, vesicle‐poor flows. Each protolith has important implications for the subsequent growth, maturation, and regional variability of regolith deposits, suggesting wide spatial variations in the properties and thickness of regolith of similar age. Regolith may thus provide key insights into mare basalt protolith and its mode of emplacement. Plain Language Summary: Following recent studies of how lava eruptions are emplaced on the lunar surface, we show that solid basalt is only one of a wide range of starting conditions in the process of forming lunar soil (regolith). Gas present in the lavas during eruption also produced bubbles, foams, and explosive products, disrupting the lava and forming other starting conditions for mare soil parent material. Key Points: New basalt magma emplacement models explore fresh lava flows that form the lunar mare regolith parent rock, or protolithSome conditions predict immediate formation of a fragmental meter‐thick "autoregolith" that mimics impact‐generated regolithLava flows with significant near‐surface vesicularity, macroporosity, and magmatic foams will create nontraditional regolith growth rates [ABSTRACT FROM AUTHOR]

Impact craters are the dominant features on the Moon, and their degradation with time is the most common geological process on this body. This work is aimed at detailed quantitative characterization of this process. We quantitatively characterize crater materials by (1) the topographic roughness calculated from Lunar Orbiter Laser Altimeter and (2) rock abundances and nighttime soil temperatures derived from Diviner measurements. In contrast to preceding works, we separately consider crater material subunits: central peaks, floors, walls, and continuous ejecta to study their degradation in detail. We mapped totally 4,770 individual crater material subunits. All subunits of the youngest craters are characterized by increased roughness, rock abundance, and soil temperature. These parameters decrease with age and tend toward equilibrium, stable state; the continuous ejecta reach equilibrium more rapidly than other subunits. Kilometer‐baseline roughness of crater walls in the lunar maria is higher than in the highlands. Rock abundances and soil temperatures of the walls and floors of simple craters in the maria are also higher than in the highlands. We attribute these trends to differences in the mechanical properties of the target materials. Although the properties studied are not exact proxies for age, they can be used to assess individual age predictions; for example, several craters that were erroneously classified as Copernican have been detected. We also found that the unusual thermophysical signature of the walls of the Late Imbrian crater Bonpland D is due to recent regolith flows that could have been caused by a strong, shallow seismic event. Plain Language Summary: Impact craters have been forming on the Moon by meteoritic impacts throughout billions of years of its history. To assess their changes with time (degradation), we mapped many such craters in detail and studied their quantitative characteristics using data from the Lunar Reconnaissance Orbiter. In particular, we used data characterizing surface topography and rockiness. We found that the youngest craters are the roughest and the rockiest. With age, craters become smoother and rocks disappear. This happens for all parts of the craters; these changes occur more rapidly, however, for the area around the craters covered with material excavated by the impact. We found several craters that had been marked as very young on older geological maps of the Moon but are very smooth and lack rocks; we believe that they were misclassified. We also found one moderately old, but extremely rocky crater. In zoomed‐in images we found very fresh, unusual landslides on its slopes; we suggest that these landslides were caused by strong, geologically recent moonquakes. Key Points: Young craters are characterized by high roughness, rock abundance, soil temperature; these parameters decrease toward equilibrium with ageContinuous ejecta reach roughness equilibrium faster than other subunits; small craters located in the maria form more rocksUnusual young regolith flows in Late Imbrian crater Bonpland D are interpreted to be caused by a recent strong shallow seismic event [ABSTRACT FROM AUTHOR]

A transient lunar atmosphere formed during a peak period of volcanic outgassing and lasting up to about ~70 Ma was recently proposed. We utilize forward‐modeling of individual lunar basaltic eruptions and the observed geologic record to predict eruption frequency, magma volumes, and rates of volcanic volatile release. Typical lunar mare basalt eruptions have volumes of ~102–103 km3, last less than a year, and have a rapidly decreasing volatile release rate. The total volume of lunar mare basalts erupted is small, and the repose period between individual eruptions is predicted to range from 20,000 to 60,000 years. Only under very exceptional circumstances could sufficient volatiles be released in a single eruption to create a transient atmosphere with a pressure as large as ~0.5 Pa. The frequency of eruptions was likely too low to sustain any such atmosphere for more than a few thousand years. Transient, volcanically induced atmospheres were probably inefficient sources for volatile delivery to permanently shadowed lunar polar regions. Plain Language Summary: Could gas emitted from volcanic eruptions during the most intense and voluminous period of lunar mare volcanism produce a temporary lunar atmosphere? Could the presence of such an atmosphere enable volatiles to reach the cold traps in the permanently shadowed regions at the lunar poles? We use information from lunar geology and sample analyses to predict the number of eruptions with time, the volume of individual eruptions, the rates of volcanic gas release during each eruption, and the time between eruptions. We find that only under rare circumstances could a single eruption or two eruptions closely spaced in time release enough gas to create a transient atmosphere with a pressure as large as ~0.5 Pa. Furthermore, it is difficult to sustain such an atmosphere for more than a few thousand years. These results suggest that volcanically produced atmospheres are inefficient source mechanisms for delivery of volatiles to form deposits in permanently shadowed polar regions of the Moon; this favors volatile‐rich impactors as the major source of polar ice. Key Points: A transient lunar atmosphere from peak volcanic degassing lasting up to ~70 Ma was recently proposed as a source of lunar polar volatilesWe forward‐model individual eruption volume, degassing patterns, and duration of periods between eruptions (repose periods)Transient, volcanically induced atmospheres are inefficient sources for volatile delivery to permanently shadowed lunar polar regions [ABSTRACT FROM AUTHOR]

The roughness properties of impact craters are valuable indicators of crater degradation and can provide insight into crater ages. We evaluate the roughness of lunar craters from different geologic eras, confirming that young, Copernican craters are distinctly rougher than older craters. We evaluate the potential age of small (less than ~15 km) craters that are thought to host surface ice by quantifying the roughness inside these craters, as well as outside. Interior roughness may be subdued by slope processes or the presence of volatiles. The distribution of ice‐bearing craters is skewed toward roughness values higher than those of pre‐Imbrian craters, although no ice‐bearing craters are within the Copernican‐only domain in roughness space. All of the 15 rough, permanently shadowed craters that are found within the Copernican‐only domain lack water‐ice detections, suggesting that either ice has not been delivered to these young craters or that it has since been destroyed. Plain Language Summary: When a planetary surface is struck by an impactor, the preexisting surface is disturbed as rocks and boulders scatter from the high‐energy collision. Over time, the surface texture becomes smoother as these rocks and boulders are broken up into smaller pieces and begin to diffuse away. Surface texture is thus a powerful tool for evaluating the degradation of impact craters, given that young, fresh craters tend to be rougher than old craters due to the presence of rocks and boulders on the recently disturbed surface. Here we quantify the surface texture of lunar craters of different geologic eras and establish a relationship between the roughness and age of craters. We evaluate the roughness of small craters that host near‐polar surface ice and find a population of craters likely to be post‐Nectarian in age. However, the youngest polar craters lack surface ice detections. Key Points: Copernican craters have distinctly rougher interiors and exteriors than older cratersSome small ice‐bearing craters are rougher than pre‐Imbrian craters and surface ice within them was likely delivered post‐NectarianThe roughest polar craters appear to be Copernican and lack surface ice detections [ABSTRACT FROM AUTHOR]

One of the most mysterious lunar features discovered during the Apollo era was Ina, a ~2 × 3‐km depression composed of bleb‐like mounds surrounded by hummocky and blocky terrains. Subsequent studies identified dozens of similar features in lunar maria, describing them as Irregular Mare Patches (IMPs). Due to the unusual and complex characteristics of IMPs, their specific formation mechanism is debated. To improve our understanding of the nature and origin of IMPs, we undertook an updated search and geological characterization of all IMPs and established a classification approach encompassing the full spectrum of IMPs. We present an updated catalog of 91 IMPs and survey the detailed characteristics of each IMP. We find that the majority of IMPs occur in maria emplaced over three billion years ago, contemporaneous with the peak period of global lunar volcanism. We utilized geologic context information and characteristics to establish two classification schemes for lunar IMPs: (1) geologic context: IMPs are categorized into (a) small shield volcano summit pit floor and flank, (b) linear/sinuous rille interior and adjacent exterior, and (c) typical maria; (2) characteristics: IMPs are classified into (a) "mound + floor" and (b) "pit only" types. We showed the range of characteristics of lunar IMPs was consistent with the waning‐stage magmatic foam formation and extrusion scenario in different environments. Our updated catalog and classification raise several outstanding questions concerning the nature and origin of lunar IMPs. Assessing these questions will improve our knowledge of lunar thermal and geologic evolution. Plain Language Summary: Composed of fresh‐looking bulbous‐shaped mounds surrounded by a hummocky and blocky floor, the 2 × 3‐km Ina depression is one of the most enigmatic and poorly understood features discovered during early lunar exploration. Later studies identified dozens of similar features in the lunar maria and named them Irregular Mare Patches (IMPs). To achieve an improved knowledge of IMPs, we undertook an updated search, geologic analysis, and classification study of all currently known IMPs, presenting a combined catalog of 91 IMPs. We find that the majority of lunar IMPs occur in mare units emplaced over three billion years ago, coinciding with the peak period of global lunar volcanism. We then classified the entire IMP population on the basis of their geologic settings and characteristics and documented and classified the detailed surface texture of the floor terrains at each IMP site. These new and updated detailed characterization and classification of the entire IMP population provide important new information about their nature and origin. Association with volcanic vent areas and ancient volcanic structures and deposits suggests that late stage volcanic degassing processes during the period of mare volcanism >3 Ga ago should be considered in more detail. Key Points: We present an updated catalog of 91 lunar Irregular Mare Patches (IMPs) and survey the geologic settings and characteristics of each IMPTwo classification schemes for the entire IMP population are proposed on the basis of their geologic settings and characteristicsThe characteristics of lunar IMPs are consistent with the waning‐stage magmatic foam extrusion origin model in different environments [ABSTRACT FROM AUTHOR]

Ring‐moat dome structures (RMDSs) are positive morphologic features found clustered across many mare regions on the Moon, of which only a few isolated examples have been previously reported. Our continuing survey has expanded the known locations of the RMDSs from ~2,600 to over 8,000, indicating that RMDSs are more common geological features than previously thought. This work presents a detailed geomorphological analysis of 532 RMDSs identified in several mare basins. The combination of detailed elemental mapping, morphological and morphometric analyses, spatial distribution relationships with other geologic structures, and comparison with terrestrial analogs lead us to conclude that (1) RMDSs represent low circular mounds with diameters of a few hundred meters (average about 200 m) and a mean height of 3.5 m. The mounds are surrounded by moats ranging from tens to over 100 m in width and up to several meters in depth; (2) there is a wide variation of titanium abundances, although RMDSs are more commonly found in mare regions of moderate‐to‐high titanium content (>3 wt% TiO2); (3) RMDSs are found to occur on or around fractures, graben, and volcanic edifices (small shields and cones); (4) a spatial association between RMDSs and Irregular Mare Patches (see Braden et al., 2014, https://doi.org/10.1038/ngeo2252) is observed, suggesting that both may form from related lava flows; (5) comparisons between RMDSs and lava inflationary structures on Earth support an inflation‐related extrusive nature and a genetic relationship with host lava flow processes; and (6) RMDS embayment relationships with craters of different degradation ages superposed on the host mare, and regolith development models, produces conflicting age relationships and divide theories of RMDS origin into two categories, (1) synchronous with the emplacement and cooling of the host lava flows ~3–4 Ga and (2) emplaced substantially after the host mare lava unit, in the period ~0–3 Ga. We outline the evidence supporting this age conundrum and implications for the different theories of origin and describe future research avenues to help resolve these outstanding questions. Plain Language Summary: The research reported in this paper is focused on the statistical, compositional, and morphological characterization of a newly documented lunar lava flow surface feature characterized by very low, circular‐ or ellipse‐shaped mounds typically several hundred meters across and surrounded by ring depressions. Detailed measurement of 532 mounds shows that their diameters range from 68 to 645 m, and heights from 0.4 to 14 m. The surrounding ring depressions are tens to more than 100 m in width and have depths up to several meters. These mounds have complex relationships with circular and irregular depressions or degraded craters. Compositional mapping results show that this type of mound preferentially forms in lava flow fields of moderate to high titanium content (>3 wt% TiO2). Flow features, fractures, graben, and small shields and cones associated with ring‐moat dome structures support their extrusive nature and a genetic relationship with late stages of host lava flow emplacement and cooling. Some embayment relationships suggest, however, that ring‐moat dome structures could have formed many hundreds of millions of years to billions of years after emplacement of their host lava units. The documentation of these features has important implications for the understanding of mare basalt eruption and lava‐emplacement mechanisms, and for the age and duration of mare basalt volcanism, one of the most important indicators of lunar thermal evolution and mantle geodynamics. Key Points: Ring‐moat dome structures in the lunar maria are characterized quantitativelyGreater than 8,000 RMDSs are mapped, more commonly occur in moderate‐high Ti mareCandidate origins are derived: (1) contemporaneous with host mare (3–4 Ga) and (2) recent (0–3 Ga) [ABSTRACT FROM AUTHOR]

Over the extreme temperature variations experienced in a single lunar day (∆T ≈ 300 K), particular minerals common to the lunar surface show spectral changes at discrete near‐infrared wavelengths in laboratory settings (Roush & Singer, 1986, 1987, https://10.1111/10.1029/JB091iB10p10301, https://10.1111/10.1016/0019-1035(87)90026-1; Singer & Roush, 1985, https://10.1111/10.1029/JB090iB14p12434). Variations in temperature can cause variations in the size and shape of crystallographic sites, which control the position, shape, and depth of crystal field absorptions. At an observation wavelength of 1,064 nm, the Lunar Orbiter Laser Altimeter (LOLA) should be highly sensitive to temperature‐dependent changes of orthopyroxene. Here we analyze temperature‐dependent spectral changes of the lunar surface as measured from orbit by LOLA. We couple LOLA measurements of normal albedo with measurements of surface temperature from the Diviner Lunar Radiometer Experiment, analyzing the maria and highlands between ±50°. We provide the first evidence of temperature‐dependent spectral changes on the lunar surface from orbital observations, finding that the majority of the surface between ±50° demonstrates a small, yet measurable, negative change in 1,064‐nm albedo with temperature (−∆R/∆T). The measurable effect is on the order of a few percent change in reflectance per ~80 K, indicating that temperature changes do not have a large effect on measurements of albedo at the sensitivity of the LOLA instrument. Stronger −∆R/∆T values tend to be associated with regions with elevated orthopyroxene, and the maria typically have higher levels of orthopyroxene and stronger −∆R/∆T values than the highlands. Our results suggest that single‐wavelength lasers may be powerful tools for understanding the distribution of particular minerals on the lunar surface. Plain Language Summary: In a single day, the surface temperature of the Moon varies over 300 K (540 °F). This massive swing in temperature can affect the ways in which we measure light that is scattered from the lunar surface. The properties of scattered light are important tools for understanding the color and mineralogy of the Moon's surface; therefore, understanding the influence of temperature on these properties is particularly important in interpreting the presence of different rock‐forming minerals or ices on the Moon. Here we measure the effect that temperature has on measurements of surface reflectance specifically for the Lunar Orbiter Laser Altimeter, which is an instrument onboard the Lunar Reconnaissance Orbiter and currently orbiting the Moon. We find that there is a small yet measurable effect where surfaces tend to appear brighter when the surface is colder. This change is consistent with previous laboratory studies of lunar minerals and soils that detected a temperature‐dependent change in reflectance properties. Key Points: The Lunar Orbiter Laser Altimeter is sensitive to temperature‐dependent changes of normal albedoAt 1,064 nm, the maria show a slightly stronger negative change in surface reflectance with temperature than the highlandsThe measured temperature‐dependent changes in normal albedo are consistent with the presence of pyroxene at the lunar surface [ABSTRACT FROM AUTHOR]

It has been proposed that groundwater was released onto the early martian surface in several locations, including Meridiani Planum, NE Hellas, Terra Sirenum, the northern lowlands, and Gale crater. We assume that groundwater was released in a manner similar to a groundwater‐fed lake on Earth and then use a combination of a global climate model and mathematical relationships to place lower limit estimates on the early climate and subsurface water inventory that are required for groundwater release to occur at all of these locations. We find that the minimum required conditions are global mean annual temperature >284 K and a subsurface water inventory >496 m global equivalent layer. Additionally, thermal models predict that if such warm conditions occurred from punctuated heating in an otherwise "cold and icy" climate, the warm conditions must have persisted for thousands of years in order to remove the cryosphere and permit groundwater release. Plain Language Summary: Geologic evidence has been interpreted to mean that groundwater was released from the subsurface to the early martian surface at multiple locations. Groundwater release requires both a warm climate and sufficient water in the subsurface. In this work, we estimate the climate and subsurface water inventory that would be required for groundwater to be released at these locations. The plausibility of the predicted climate and subsurface water inventory can act as a test for these groundwater‐related interpretations. We find that early Mars would be required to have had average global temperatures similar to present‐day Earth and a very large subsurface water inventory. Key Points: Several locations on Mars have been proposed to have undergone groundwater release to the surface in the Noachian/HesperianWe estimate the climate, subsurface water inventory, and duration of warm conditions required for such groundwater releaseRequirements for release are global mean annual temperature >284 K for approximately thousands of years and >496 m global equivalent layer of groundwater [ABSTRACT FROM AUTHOR]

The lunar shield volcano Cauchy 5, sitting at the low diameter‐height‐volume end of the population, is the only known example containing two different types of Irregular Mare Patches (IMPs) in very close association: (1) the pit crater interior Type 1 IMP composed of bleb‐like mounds surrounded by a hummocky and blocky floor unit and (2) Type 2 IMPs, small, often optically immature pits less than ~5 m deep, located on the generally block‐deficient shield flanks. A four‐phase lunar magma ascent/eruption model predicts that during a relatively brief eruption, low magma rise rates maximize volatile exsolution in lava filling the pit crater. Bubble‐rich magmas overtop the pit crater and form extremely vesicular flows on the shield flanks. Exposure of the flanking flows to vacuum produces a fragmental layer of exploded glassy bubble walls. Subsequent second boiling upon cooling of the flanking flow interiors releases additional volatiles which migrate and collect, forming magmatic foams and gas pockets. As magma rise rates slow, trapped gas and magmatic foam build up below the cooling pit crater floor. Magmatic foams are extruded to form Type 1 IMP deposits. Type 2 IMPs on the flanks are interpreted to be due primarily to subsequent impacts causing collapse of the flow surface layer into the extremely vesicle‐ and void‐rich flow interior. Anomalously young pit crater floor/shield flank crater retention ages compared with surrounding maria ages may be due to effects of Cauchy 5 substrate characteristics (extreme micro‐ and macroporosity, foamy nature, and glassy auto‐regolith) on superposed crater formation and retention. Plain Language Summary: A group of distinctive and unusual features in the lunar maria known as "irregular mare Patches" (IMPs) are of two types: Type 1 ("mound + floor") usually occurring in volcanic pit craters and related depressions, and dated to less than 100 Ma old and Type 2 ("pit only") occurring as scattered pits in localized areas of the lunar maria and too small to obtain ages. We investigated Cauchy 5, a small lava shield that is anomalous in that both Type 1 and Type 2 IMPs occur in very close association. Models of magma ascent and eruption in small‐volume, low‐volume‐flux mare basalt eruptions show that gas exsolution is optimized. Gas release patterns and pit crater lava lake behavior produce Type 1 IMPs on the lava lake floor and Type 2 IMPs on the shield volcano flanks from void collapse and subsequent impacts. The extremely vesicular, void‐rich, and foam‐like nature of the lava lake floor and shield flank flows forms a substrate whose characteristics are predicted to significantly influence the formation and degradation of superposed impact craters. This potentially causes the IMPs to appear to be much younger than the adjacent mare units. Key Points: Cauchy 5 small shield volcano displays two types of IMPs: Type 1 (mound + floor) in its summit pit and Type 2 (pit only) on its flanksSmall edifice volume maximizes volatile exsolution and favors strombolian lava lake activity and emplacement of vesicular flank lavasRelationships at Cauchy 5 summit and flanks provide a link to understanding the genetic relationship between the two IMP subtypes [ABSTRACT FROM AUTHOR]

The current goals of the astrobiology community are focused on developing a framework for the detection of biosignatures, or evidence thereof, on objects inside and outside of our solar system. A fundamental aspect of understanding the limits of habitable environments (surface liquid water) and detectable signatures thereof is the study of where the boundaries of such environments can occur. Such studies provide the basis for understanding how a once inhabitable planet might come to be uninhabitable. The archetype of such a planet is arguably Earth's sibling planet, Venus. Given the need to define the conditions that can rule out bio-related signatures of exoplanets, Venus provides a unique opportunity to explore the processes that led to a completely uninhabitable environment by our current definition of the term. Here we review the current state of knowledge regarding Venus, particularly in the context of remote-sensing techniques that are being or will be employed in the search for and characterization of exoplanets. We discuss candidate Venus analogs identified by the Kepler and TESS exoplanet missions and provide an update to exoplanet demographics that can be placed in the potential runaway greenhouse regime where Venus analogs are thought to reside. We list several major outstanding questions regarding the Venus environment and the relevance of those questions to understanding the atmospheres and interior structure of exoplanets. Finally, we outline the path toward a deeper analysis of our sibling planet and the synergy to exoplanetary science. [ABSTRACT FROM AUTHOR]

Ina, a distinctive ~2 × 3 km D‐shaped depression, is composed of unusual bulbous‐shaped mounds surrounded by optically immature hummocky/blocky floor units. The crisp appearance, optical immaturity, and low number of superposed impact craters combine to strongly suggest a geologically recent formation for Ina, but the specific formation mechanism remains controversial. We reconfirm that Ina is a summit pit crater/vent on a small shield volcano ~3.5 billion years old. Following detailed characterization, we interpret the range of Ina characteristics to be consistent with a two‐component model of origin during the waning stages of summit pit eruption activities. The Ina pit crater floor is interpreted to be dominated by the products of late‐stage, low‐rise rate magmatic dike emplacement. Magma in the dike underwent significant shallow degassing and vesicle formation, followed by continued degassing below the solidified and highly microvesicular and macrovesicular lava lake crust, resulting in cracking of the crust and extrusion of gas‐rich magmatic foams onto the lava lake crust to form the mounds. These unique substrate characteristics (highly porous aerogel‐like foam mounds and floor terrains with large vesicles and void space) exert important effects on subsequent impact crater characteristics and populations, influencing (1) optical maturation processes, (2) regolith development, and (3) landscape evolution by modifying the nature and evolution of superposed impact craters and thus producing anomalously young crater retention ages. Accounting for these effects results in a shift of crater size‐frequency distribution model ages from <100 million years to ~3.5 billion years, contemporaneous with the underlying ancient shield volcano. Plain Language Summary: Among the most outstanding questions in lunar evolution is the end of extrusive volcanic activity, commonly thought to be at least a billion years old. A recent study found evidence for volcanic activity within the last 100 million years, in the form of irregular mare patches dated by size‐frequency distributions of superposed craters. The most prominent irregular mare patch is Ina, an ~2 × 3‐km depression composed of bulbous‐shaped mounds surrounded by fresh, hummocky and blocky floor units, both supporting the geologically very recent age. We undertook a detailed characterization of the setting of Ina and its interior and found that its location on the summit of a 3.5 billion years old small shield volcano, together with Hawaiian field analogs and theoretical analyses of the ascent and eruption of magma, provided new clues to its origin and age. Late‐stage magma extrusion in the summit pit characterized by gas‐rich Strombolian activity produced a very vesicular crust and concentration of underlying magmatic foams; cracking of the crust caused magmatic foam extrusions to produce the mounds. The very porous nature of Ina deposits decreases the superposed crater diameters and shifts the crater size‐frequency distribution to ~3.5 billion years, coincident with the ancient age of the volcano. Key Points: Ina feature occurs as a summit pit crater/vent atop a broad ~22‐km‐diameter, ~3.5‐Ga‐old shield volcanoThe range of geologic characteristics of Ina is most consistent with an ancient origin of waning‐stage lava lake magmatic foam extrusionHighly vesicular nature of the magmatic foam mounds and lava lake crust floor substrate results in anomalously young crater retention ages [ABSTRACT FROM AUTHOR]

Potential water ice concentrated within the permanently shadowed regions (PSRs) near lunar poles is both scientifically significant and of value for future explorations. However, after decades of observations, the existence and characteristics of PSR water ice remain controversial. The 1,064‐nm laser reflectance measurements collected by the Lunar Orbiter Laser Altimeter (LOLA) onboard the Lunar Reconnaissance Orbiter (LRO) provide a unique opportunity to detect and characterize PSR water ice. In this work, we focus on all major PSRs on the flat floors of lunar polar craters and analyze their detailed LOLA 1,064‐nm albedo and then compare this with the adjacent flat non‐PSRs. We find that the LOLA albedo of the majority of these PSRs is systematically higher than their adjacent non‐PSRs. Potential contributions of various factors to the observed LOLA albedo are individually quantitatively evaluated; we show that each of them is unable to account for the observed LOLA albedo anomalies and that the presence of surface water ice is the most likely explanation. Combined characterization of LOLA albedo and substrate impact cratering records (crater populations and depths) reveals that the inferred PSR water ices are in very small quantity (probably in the form of a surface frost layer or admixture with regolith) and are laterally heterogeneous in model ice concentration, ranging from negligible to ~6%. We recommend that these PSRs as priority targets for future surface in situ exploration endeavors, and a case assessment of Amundsen crater is presented. Plain Language Summary: The possible presence of surface water ice within the permanently shadowed regions (PSRs) in lunar polar areas is important for both science and future lunar resource exploration. However, significant uncertainty still exists concerning both the presence and properties of water ice in lunar PSRs. A recent Moon‐orbiting laser instrument measures the laser brightness of the lunar surface, including areas that are permanently shadowed in visible light. These brightness data are very useful for studying the possible presence of water ice in PSRs, as water ice is much brighter than typical lunar surface soil. Our study finds that most of the PSRs on flat surface are generally brighter than the surrounding surface. We analyze many candidate reasons for this difference but find that the presence of water ice is the only explanation that can successfully explain the observations. We thus suggest that water ice very likely exists in many lunar PSRs. The surface water ice within PSRs is very thin and has different ice contents. These candidate ice‐containing PSRs on flat surfaces are optimal landing sites for future lunar exploration and are an important reference base for future lunar mission site selection. Key Points: Permanently shadowed regions (PSRs) on flat floors of polar crater have systematically higher 1,064‐nm albedo than the adjacent terrainsExposed surface water ice, not other factors, serves as the most plausible explanation for the elevated 1,064‐nm albedo of these PSRsThe inferred PSR water ice exposures are in very small quantity and laterally heterogeneous in model water ice abundance [ABSTRACT FROM AUTHOR]

Abstract: Lunar irregular mare patches (IMPs) comprise dozens of small, distinctive, and enigmatic lunar mare features. Characterized by their irregular shapes, well‐preserved state of relief, apparent optical immaturity, and few superposed impact craters, IMPs are interpreted to have been formed or modified geologically very recently (<~100 Ma; Braden et al. ). However, their apparent relatively recent formation/modification dates and emplacement mechanisms are debated. We focus in detail on one of the major IMPs, Sosigenes, located in western Mare Tranquillitatis, and dated by Braden et al. ( ) at ~18 Ma. The Sosigenes IMP occurs on the floor of an elongate pit crater interpreted to represent the surface manifestation of magmatic dike propagation from the lunar mantle during the mare basalt emplacement era billions of years ago. The floor of the pit crater is characterized by three morphologic units typical of several other IMPs, i.e., (1) bulbous mounds 5–10 m higher than the adjacent floor units, with unusually young crater retention ages, meters thick regolith, and slightly smaller subresolution roughness than typical mature lunar regolith; (2) a lower hummocky unit mantled by a very thin regolith and significantly smaller subresolution roughness; and (3) a lower blocky unit composed of fresh boulder fields with individual meter‐scale boulders and rough subresolution surface texture. Using new volcanological interpretations for the ascent and eruption of magma in dikes, and dike degassing and extrusion behavior in the final stages of dike closure, we interpret the three units to be related to the late‐stage behavior of an ancient dike emplacement event. Following the initial dike emplacement and collapse of the pit crater, the floor of the pit crater was flooded by the latest‐stage magma. The low rise rate of the magma in the terminal stages of the dike emplacement event favored flooding of the pit crater floor to form a lava lake, and CO gas bubble coalescence initiated a strombolian phase disrupting the cooling lava lake surface. This phase produced a very rough and highly porous (with both vesicularity and macroporosity) lava lake surface as the lake surface cooled. In the terminal stage of the eruption, dike closure with no addition of magma from depth caused the last magma reaching shallow levels to produce viscous magmatic foam due to H2O gas exsolution. This magmatic foam was extruded through cracks in the lava lake crust to produce the bulbous mounds. We interpret all of these activities to have taken place in the terminal stages of the dike emplacement event billions of years ago. We attribute the unusual physical properties of the mounds and floor units (anomalously young ages, unusual morphology, relative immaturity, and blockiness) to be due to the unusual physical properties of the substrate produced during the waning stages of a dike emplacement event in a pit crater. The unique physical properties of the mounds (magmatic foams) and hummocky units (small vesicles and large void space) altered the nature of subsequent impact cratering, regolith development, and landscape evolution, inhibiting the typical formation and evolution of superposed impact craters, and maintaining the morphologic crispness and optical immaturity. Accounting for the effects of the reduced diameter of craters formed in magmatic foams results in a shift of the crater size–frequency distribution age from <100 Myr to billions of years, contemporaneous with the surrounding ancient mare basalts. We conclude that extremely young mare basalt eruptions, and resulting modification of lunar thermal evolution models to account for the apparent young ages of the IMPs, are not required. We suggest that other IMP occurrences, both those associated with pit craters atop dikes and those linked to fissure eruptions in the lunar maria, may have had similar ancient origins. [ABSTRACT FROM AUTHOR]

Abstract: Ancient valley networks (VNs) and related open‐ and closed‐basin lakes are testimony to the presence of flowing liquid water on the surface of Mars in the Late Noachian and Early Hesperian. Uncertain, however, has been the mechanism responsible for causing the necessary rainfall and runoff and/or snowfall and subsequent melting. Impact cratering has been proposed (e.g., Segura et al. 2002) as a process for temporarily raising temperatures and inducing conditions that would produce rainfall, snowmelt, runoff, and formation of the VNs and associated lacustrine features. We refer to the collective effects of this process as the ICASE model (impact cratering atmospheric/surface effects). In this contribution, we assess the proposed impact cratering mechanism in order to understand its climatic implications for early Mars: we outline the step‐by‐step events in the cratering process and explore the predictions for atmospheric and surface geological consequences. For large and basin‐scale impacts, rainfall should be globally and homogeneously distributed and characterized by very high temperatures. Rainfall rates are predicted to be high, ~2 m yr−1, similar to rates in tropical rainforests on Earth, and runoff rates are correspondingly very high. These predicted characteristics do not seem to be consistent with the observed VNs, which are mainly equatorial and not homogeneous in their distribution. Prior to the Late Noachian, however, we predict that basin‐scale impact effects are very likely to contribute significantly to degradation of crater rims and regional smoothing of terrain, implying vast resurfacing and resetting of crater ages following large crater and basin‐scale impacts. Furthermore, the high temperatures of impact‐induced rainwater and snowmelt and the pervasive penetration of heat into the regolith substrate are predicted to have a significant influence on the mineralogical alteration of the crust and its resulting physical properties. We conclude by describing a case example (Isidis basin) and describe how the ICASE model provides an alternative scenario for the interpretation of the layered phyllosilicates in the Nili Fossae and NE Sytris regions. We outline specific conclusions and recommendations designed to improve the ICASE model and to promote further understanding of its implications for the geological, mineralogical, and climate history of early Mars. [ABSTRACT FROM AUTHOR]

Abstract: Double‐layered ejecta (DLE) craters are distinctive among the variety of crater morphologies observed on Mars, but the mechanism by which they form remains under debate. We assess two ejecta emplacement mechanisms: (1) atmospheric effects from ejecta curtain‐induced vortices or a base surge and (2) ballistic emplacement followed by a landslide of ejecta assisted by either surface‐ or pore‐ice. We conduct a morphological analysis of the ejecta facies for three DLE craters which impacted into irregular pre‐existing topography. We find that the unique topographic environments affected the formation of grooves and the inner facies, and thus appear to be inconsistent with an atmospheric‐effects origin but are supportive of the landslide hypothesis. We distinguish between the two landslide models (lubrication by either surface‐ or pore‐ice) by assessing relationships between DLE crater ejecta and morphologic features indicative of buried ice deposits, including sublimation pits, ring‐mold craters, expanded secondary craters, and excess ejecta craters. The association of DLE craters with these features suggests that surface ice was present at the time of the impacts that formed the DLE craters. We also compare the Froude numbers of DLE crater ejecta to landslides, and find that the ejecta of DLE craters are kinematically and frictionally similar to terrestrial landslides that overran glaciers. This suggests that the grooves on DLE craters may plausibly form through the same shear/splitting mechanism as the landslides. In summary, our analysis supports the hypothesis that DLE craters form through meteoroid impacts into decameters‐thick surface ice deposits (emplaced during periods of higher obliquity) followed by ejecta sliding on the ice. [ABSTRACT FROM AUTHOR]

Abstract: We use the global distribution of the steepest slopes on crater walls derived from Mars Orbiter Laser Altimeter profile data to assess the magnitudes of degradational processes with latitude, altitude, and time. We independently confirm that Amazonian polar/high‐latitude crater slope modification is substantial, but that craters in the low latitudes have essentially escaped significant slope modification since the Early Hesperian. We find that the total amount of crater wall degradation in the Late Noachian is very small in comparison to the circumpolar regions in the Late Amazonian, an observation that we interpret to mean that the Late Noachian climate was not characterized by persistent and continuous warm and wet conditions. A confirmed elevational zonality in degradation in the Early Hesperian is interpreted to mean that the atmosphere was denser than today. [ABSTRACT FROM AUTHOR]

We report on a newly discovered morphological feature on the lunar surface, here named Ring-Moat Dome Structure (RMDS). These low domes (a few meters to ~20 m height with slopes <5°) are typically surrounded by narrow annular depressions or moats. We mapped about 2,600 RMDSs in the lunar maria with diameters ranging from tens to hundreds of meters. Four candidate hypotheses for their origin involving volcanism are considered. We currently favor a mechanism for the formation of the RMDS related to modification of the initial lava flows through inflated flow squeeze-ups and/or extrusion of magmatic foams below a cooling lava flow surface. These newly discovered features provide new insights into the nature of emplacement of lunar lava flows, suggesting that in the waning stages of a dike emplacement event, magmatic foams can be produced, extrude to the surface as the dike closes, and break through the upper lava flow thermal boundary layer (crust) to form foam mounds and surrounding moats. [ABSTRACT FROM AUTHOR]

The Mercury Laser Altimeter (MLA) measured surface reflectance, r s, at 1064 nm. On Mercury, most water-ice deposits have anomalously low r s values indicative of an insulating layer beneath which ice is buried. Previous detections of surface water ice (without an insulating layer) were limited to seven possible craters. Here we map r s in three additional permanently shadowed craters that host radar-bright deposits. Each crater has a mean r s value >0.3, suggesting that water ice is exposed at the surface without an overlying insulating layer. We also identify small-scale cold traps (<5 km in diameter) where r s >0.3 and permanent shadows have biannual maximum surface temperatures <100 K. We suggest that a substantial amount of Mercury's water ice is not confined to large craters but exists within microcold traps, within rough patches and intercrater terrain. [ABSTRACT FROM AUTHOR]