春山 純一

Processes of water (OH and H2O) migration on the Moon remain unclear, prompting active research. Understanding lunar water migration requires investigation of the trapping and diffusion properties of water at various latitudes and local times. This study analyzed visible to near-infrared spectral data obtained by the Spectral Profiler (SP) onboard SELENE for shadowed regions at various local times and latitudes, not limited to the polar permanently shadowed regions. We assessed SP data for shadowed regions in 60 areas, each spanning a 10° × 10° latitude–longitude grid. Of the 1,061,907 analyzed shadowed-region data, 41,385 at various latitudes exhibited significant absorption in the 1.25 and 1.5 µm bands, indicating water ice particles. Data with the two absorption features suggest the presence of a water ice frost layer covering the lunar surface or suspended water ice particles above the lunar surface, at various latitude shadowed regions. Our spectral simulations have quantified the ice particles as being 0.1–1 µm in diameter, with a column density of 10–4–10–3 kg/m2. The spectral parameters for band absorption at the 1.5 µm band show symmetry between morning and evening sides, which is potentially attributed to the absence of variations in ice grain size and quantity. The 1.5 µm band absorption shows an increasing trend toward terminator regions, indicating variation in the water ice distribution and likely reflecting temperature conditions for water retention. The latitudinal trend of ice grain size and quantity remains uncertain because of the observed noise levels. Observations of water ice particles in shadowed regions at various latitudes and local times can provide new constraints on trapping and diffusion processes of lunar water migration.

Vertical holes of several tens of meters in diameter and depth have been discovered on the Moon, potentially serving as skylights into subsurface volcanic caverns resembling lava tubes. Because of their scientific importance and potential for use as future lunar bases, these lunar holes and caverns are expected to be targets of intensive exploration in the near future. Using numerical simulations, this study investigates the light environment within a directly and/or indirectly sunlit subsurface cavern accessed through a skylight hole. We specifically analyze the Mare Tranquillitatis Hole (MTH), one of the largest lunar vertical holes, situated near the Moon's nearside meridian. The floor and walls of the hole, and the walls and ceiling of its associated subsurface cavern are lit by reflected sunlight from other parts of the hole and cavern, such as the floors, depending on the solar elevation angle. Furthermore, this paper presents estimation results for camera imaging for a case in which solar elevation angle is 45° in the morning, which is appropriate timing for exploration. Assuming reflectance of 0.1 for the lunar hole and cavern surfaces, we estimate the incident energy onto each pixel of a camera as it descends to the hole's floor. We specifically simulate conditions for a camera with a 12-bit dynamic range, similar to the wide-angle optical navigation camera (ONC-W) onboard the Hayabusa 2 spacecraft. The results suggest that a camera with a fixed gain would struggle to capture both directly and indirectly sunlit areas simultaneously, without saturating bright areas and ensuring minimum incident energy resolution (say, a 10 digital number) for darker areas. To overcome this challenge, adjusting the camera gain based on the hole's and cavern's illumination conditions is necessary. Graphical abstract: (Figure presented.)

We studied the lunar mantle composition based on the characteristics of low-Ca pyroxene (LCP)-rich and olivine-rich rocks exposed on the lunar surface. Using data mining with SELENE (Kaguya) hyperspectral data, we found 531 sites with spectra dominated by an ultramafic LCP end-member mineral, most of which are located in the Imbrium basin and the South Pole-Aitken (SPA) basin. Stratigraphic analysis of each site revealed that LCP-rich rock bodies are exposed on fresh geological features that are less affected by space-weathering, such as steep slopes at peaks, and crater walls and ejecta deposits at smaller craters. We also found that, in the SPA and Imbrium basins, LCP-rich bodies are more numerous and more widely distributed than olivine-rich rocks, suggesting that LCP-rich materials deep in the mantle were excavated during the formation of these huge basins. However, olivine-rich rocks were abundant, and no LCP-rich rocks were found in the Moscoviense, Crisium, and Humboldtianum basins, which are known to have almost-zero crustal thicknesses, indicative of mantle excavation. Thus, our results suggest that the composition of rocks derived from the lunar mantle varies with the impact basin. Such a difference might indicate a layered structure of mantle composition, with the olivine-rich upper mantle overlying the LCP-rich mantle, a horizontal heterogeneity in the mantle composition, a regional heterogeneity in early lunar basaltic magmatism, or an impact melt origin. The layered structure or horizontal heterogeneity might have resulted from a mantle overturn caused by gravitational instability in the early stages of the lunar magma ocean.

The Committee on Space Research (COSPAR) Sample Safety Assessment Framework (SSAF) has been developed by a COSPAR appointed Working Group. The objective of the sample safety assessment would be to evaluate whether samples returned from Mars could be harmful for Earth's systems (e.g., environment, biosphere, geochemical cycles). During the Working Group's deliberations, it became clear that a comprehensive assessment to predict the effects of introducing life in new environments or ecologies is difficult and practically impossible, even for terrestrial life and certainly more so for unknown extraterrestrial life. To manage expectations, the scope of the SSAF was adjusted to evaluate only whether the presence of martian life can be excluded in samples returned from Mars. If the presence of martian life cannot be excluded, a Hold & Critical Review must be established to evaluate the risk management measures and decide on the next steps. The SSAF starts from a positive hypothesis (there is martian life in the samples), which is complementary to the null-hypothesis (there is no martian life in the samples) typically used for science. Testing the positive hypothesis includes four elements: (1) Bayesian statistics, (2) subsampling strategy, (3) test sequence, and (4) decision criteria. The test sequence capability covers self-replicating and non-self-replicating biology and biologically active molecules. Most of the investigations associated with the SSAF would need to be carried out within biological containment. The SSAF is described in sufficient detail to support planning activities for a Sample Receiving Facility (SRF) and for preparing science announcements, while at the same time acknowledging that further work is required before a detailed Sample Safety Assessment Protocol (SSAP) can be developed. The three major open issues to be addressed to optimize and implement the SSAF are (1) setting a value for the level of assurance to effectively exclude the presence of martian life in the samples, (2) carrying out an analogue test program, and (3) acquiring relevant contamination knowledge from all Mars Sample Return (MSR) flight and ground elements. Although the SSAF was developed specifically for assessing samples from Mars in the context of the currently planned NASA-ESA MSR Campaign, this framework and the basic safety approach are applicable to any other Mars sample return mission concept, with minor adjustments in the execution part related to the specific nature of the samples to be returned. The SSAF is also considered a sound basis for other COSPAR Planetary Protection Category V, restricted Earth return missions beyond Mars. It is anticipated that the SSAF will be subject to future review by the various MSR stakeholders.

This paper presents an investigation of the size-frequency distribution (SFD) of sub-kilometer-sized projectiles in the Main Belt based on the crater distribution on Ceres. Using image data of 35 m/pixel obtained by the Dawn mission with an onboard Framing Camera during the Low Altitude Mapping Orbit (LAMO), we counted craters having diameter larger than 1 km on the entire surface of Ceres. Based on the crater counting, we investigated the crater size-frequency distribution (CSFD) for the entire surface of Ceres and specifically for floors of 15 named impact craters of various ages. Distinct secondary craters were excluded for derivation of the CSFDs for the 15 crater floors. The CSFDs for the entire surface and those for 8 out of the 15 crater floors show good agreement with the crater production function (PF) for the Lunar Derived Model (LDM), which scales the well-investigated lunar cratering record to the impact environment of Ceres. Although the CSFDs for the other 7 crater floors show somewhat deviations from the PF, the deviations are less likely to show projectile information but are more likely to show the contamination with unidentified secondary craters. Our results indicate that the projectile SFDs on Ceres during 220 Ma – 1.9 Ga are invariably consistent with that found for the Moon. On the other hand, the derived SFDs of sub-kilometer-sized projectiles on Ceres are apparently different from those for the Main Belt Asteroids that have been observed telescopically, which might result from subsurface discontinuity or the Yarkovsky effect.

We examined the global distribution and geological context of lunar sites where olivine- and plagioclase-rich materials co-exist. These sites are areas showing plagioclase-rich spectra adjacent to areas showing olivine-rich spectra, and they extend over several hundreds of meters on the lunar surface. From an analysis of the high-spatial-resolution data obtained from the SELENE (Kaguya) multiband imager, we identified 14 co-existing occurrences among 49 olivine-rich sites located around large impact basins. We found that the co-existing occurrences are limited to a few sites among the olivine-rich sites in the Moscoviense, Crisium, and Imbrium basins, while six of seven olivine-rich sites in the Schrödinger basin show co-existing occurrence. The geological features between the co-existing occurrences and the olivine-rich sites without plagioclase-rich materials showed no clear difference, but both types of site are found at fresh geological features located around the peak rings of impact basins. These results may suggest that the co-existing occurrences are closely related to the material heterogeneity of the peak ring regions. The heterogeneity could be formed by mixing of the olivine-rich mantle and the anorthosite-rich crust during impact basin formation, or it might be a reflection of the compositional heterogeneity of the lunar lower crust. For future sample return missions, the detailed information on the co-existing occurrences presented here might be useful for a sampling strategy to select a “one-stop” site that provides important information for the overall characterization of lunar mantle and crustal materials.

Impact melts are ubiquitous across the Moon, occurring in settings ranging from massive basin deposits to flows and ponds in and around small craters. Recent high spatial resolution imaging and spectroscopy datasets for the Moon have enhabled the identification and study of impact melt units at increasingly small spatial scales, including on the central peaks of complex craters. Lunar impact melts have unique physical properties at many scales (e.g. smooth appearances in visual imagery, low rock concentration implying smoothness and regolith cover at meter scale, high S-band radar returns implying roughness at decimeter scale, and possible anomalously low OMAT values), which may be due to their unique mode of emplacement or post-emplacement modification processes. We isolate impact melt regions in the crater interior of Jackson and Tycho as well as in the continuous ejecta blanket of Tycho using the geologic maps of Dhingra et al. (2017) and analyze their spectral, compositional, and physical properties utilizing datasets from the SELENE Multiband Imager and Terrain Camera, the LROC Narrow Angle Camera, and Diviner Lunar Radiometer. We leverage the unique high-slope setting of impact melts on the central peaks of these two craters to assess the influence of slope on post-emplacement modification processes, compared with unique physical properties of impact melts, on rock concentration and optical maturation. We find that slope is the primary control on optical maturity, while rock concentration plays a secondary role. We also find that melt units are generally more optically mature than their non-melt counterparts, and that this difference attenuates with distance from the crater center until no difference is noted in the continuous ejecta blanket. This suggests that melt units have different surface properties than the non-melt units in similar settings potentially due to the increased degree of shock they experienced during crater formation.

The lunar surface is directly and continuously exposed to Galactic Cosmic ray (GCR) particles and Solar energetic particles (SEPs) due to the lack of atmosphere and lunar magnetic field. These charged particles interact with the lunar surface materials producing secondary radiations such as neutrons and gamma rays. In a departure from precise GCR and SEP data, we estimated the effective dose equivalent at the lunar surface and in a lunar lava tube in this paper by using PHITS, a Monte Carlo simulation tool. The effective dose equivalent due to GCR particles at the lunar surface reached 416.0 mSv yr-1 and that due to SEPs reached 2190 mSv/event. On the other hand, the vertical hole of the lava tube provides significant radiation protection. The exposure by GCR particles at the bottom of the vertical hole with a depth of 43 m was found to be below 30 mSv yr-1 while inside a horizontal lava tube, the value was less than 1 mSv yr-1 which is the reference value for human exposure on the Earth. We expect that the lunar holes will be useful components in the practical design of a lunar base to reduce radiation risk and to expand mission terms.

When the Apollo astronauts collected samples from Tranquility Base in 1969, they provided an unprecedented window into the processes that shaped the Moon. Ever since, the Moon has served as a 'keystone' for understanding planetary geological processes throughout the Solar System. Like all samples that have been returned from the Moon, the Apollo 11 samples were collected from the lunar regolith, a layer of jumbled and pulverized rocks that blankets and obscures the Moon's bedrock geology. When geologists reconstruct the history of the Moon, these samples are like scattered puzzle pieces, each representing important information, but removed from the context of their formation and isolated from the bigger picture of how the Moon's crust was formed. The goal of Moon Diver is to return to Mare Tranquillitatis, taking advantage of the discovery of a natural pit cave entrance exposing a deep cross-section through both the lunar regolith as well as tens of meters of bedrock lava layers. Collecting information on the chemistry, mineralogy, and morphology of these intact bedrock layers would allow us to investigate where rocky crusts come from, how they are emplaced, and the process by which they are transformed into the regolith layer that we see from space. In doing so, the mission would combine the deep knowledge gained by Apollo with the unprecedented in situ access to secondary crust granted by the lunar mare pit to understand these fundamental processes on the Moon, and to use this knowledge as a keystone for understanding the same processes across the Solar System. The success of the Moon Diver concept hinges on accessing the subsurface. The existence of the mare pit provides a cross-section through the lunar maria. Access to the record exposed in the wall of this pit is provided by two critical space technologies: pinpoint landing (allowing the delivery of the payload close to the pit) and extreme terrain mobility (allowing the delivery of capable instruments to the cliff wall). Pinpoint landing is a closed-loop guidance and navigation capability that repeatedly matches visual features from a downward-facing camera to a priori acquired terrain maps. This body-relative navigation is then used with closed-loop control to guide the spacecraft toward its landing target, yielding a tight landing ellipse. Once on the surface, an extreme-terrain robotic explorer, called Axel, would egress from the lander and traverse tens of meters to the pit. The lander provides mechanical support, power and communication to the rover through its umbilical tether. Anchored to the lander, the two-wheeled, tail-dragger rover would pay out its tether as it traverses toward the pit. With the aid of its 300-meter tether, the rover can traverse the steep slopes of the pit funnel and rappel its vertical walls. The rover carries a surface preparation tool together with a suite of three instrument types: (a) a trio of high-resolution cameras (Mars 2020's EECAMs) for acquiring context images of the near and far walls with the near-wall pair in a stereoscopic configuration, (b) an alpha-particle-X-ray Spectrometer (MSL's APXS) for elemental composition, and (c) a multi-spectral microscopic imager (MMI) that uses controlled lighting for minerology. The surface-preparation tool removes dust and patina that may be present on the rock wall by grinding a small area. The surface-preparation tool, the MMI and the APXS would be deployed from their instrument bays in one of the wheel wells. The rover would independently point each of its instruments at the same target of interest on the wall with millimeter-level repeatability. Confidence in the technologies of pinpoint landing and extreme-terrain access is based on helicopter testing of terrain-relative navigation and field testing of extreme terrain mobility respectively. The latter was tested using Axel rover prototypes with integrated science instruments at multiple terrestrial analog sites including a basaltic pit in Arizona. Landing shortly after sunrise, the surface mission timeline is just shy of a lunar day (14 Earth days). Upon landing, the rover would egress from the lander, traverse toward the pit, descend along the pit funnel and rappel down its wall. Throughout its traverse, the rover would acquire multiple measurements of both regolith and mare layers. After descending to the bottom of the layers, the rover will reach a significant overhang. This void space may open into a large cave or lava tube, which could someday provide a protected location for a lunar base. For these reasons, lunar pits provide an exciting new target for lunar exploration.

Intact lunar lava tubes offer a pristine environment to conduct scientific examination of the Moon's composition and potentially serve as secure shelters for humans and instruments. We investigated the SELENE Lunar Radar Sounder (LRS) data at locations close to the Marius Hills Hole (MHH), a skylight potentially leading to an intact lava tube, and found a distinctive echo pattern exhibiting a precipitous decrease in echo power, subsequently followed by a large second echo peak that may be evidence for the existence of a lava tube. The search area was further expanded to 13.00–15.00°N, 301.85–304.01°E around the MHH, and similar LRS echo patterns were observed at several locations. Most of the locations are in regions of underground mass deficit suggested by GRAIL gravity data analysis. Some of the observed echo patterns are along rille A, where the MHH was discovered, or on the southwest underground extension of the rille.

The South Pole-Aitken (SPA) basin is the largest basin on the Moon. The basin-forming impact likely melted the upper part of the mantle and formed an impact melt sheet. Impact melt of large terrestrial craters differentiated following a general magmatic differentiation sequence. However, it is still debated whether or not the SPA melt sheet underwent differentiation. To address this, we investigated the vertical and lateral variations in mineral composition of the SPA impact melt sheet area by analyzing the surface mineralogy including 277 post-SPA craters using remote sensing reflectance data. We identified a 7 km thick, high-Ca pyroxene-rich layer and below that, a >8 km thick, low-Ca pyroxene-rich layer. Both filled the entire impact melt sheet area (630 km in diameter). Considering the vast distribution of these two layers, their thicknesses, their FeO abundances (around 13.5 wt %), which are lower than most of the lunar mare basalt but within the estimated range of the differentiated SPA impact melt, and that the observed lithologies are consistent with a differentiation model, we conclude that the SPA impact melt was differentiated. Moreover, based on the lunar magma ocean differentiation model, Al2O3 and CaO abundances of the upper part of the mantle would be much lower after the mantle overturn. If the SPA impact occurred after overturn, these elemental abundances of the initial impact melt would be much lower than that of the before-overturn impact case and result in different solidified layers. Our observation matches only the after-overturn case, which may imply that the SPA impact occurred after overturn.

Linné is a simple crater, with a diameter of 2.23 km and a depth of 0.52 km, located in northwestern Mare Serenitatis. Recent high-resolution data acquired by the Lunar Reconnaissance Orbiter Camera revealed that the shape of this impact structure is best described by an inverted truncated-cone. We perform morphometric measurements, including slope and profile curvature, on the Digital Terrain Model of Linné, finding the possible presence of three subtle topographic steps, at the elevation of +20, −100, and −200 m relative to the target surface. The kink at −100 m might be related to the interface between two different rheological layers. Using the iSALE shock physics code, we numerically model the formation of Linné crater to derive hints on the possible impact conditions and target physical properties. In the initial setup, we adopt a basaltic projectile impacting the Moon with a speed of 18 km s−1. For the local surface, we consider either one or two layers, in order to test the influence of material properties or composite rheologies on the final crater morphology. The one-layer model shows that the largest variations in the crater shape take place when either the cohesion or the friction coefficient is varied. In particular, a cohesion of 10 kPa marks the threshold between conical- and parabolic-shaped craters. The two-layer model shows that the interface between the two layers would be exposed at the observed depth of 100 m when an intermediate value (~200 m) for the upper fractured layer is set. We have also found that the truncated-cone morphology of Linné might originate from an incomplete collapse of the crater wall, as the breccia lens remains clustered along the crater walls, while the high-albedo deposit on the crater floor can be interpreted as a very shallow lens of fallout breccia. The modeling analysis allows us to derive important clues on the impactor size (under the assumption of a vertical impact and collision velocity equal to the mean value), and on the approximate, large-scale preimpact target properties. Observations suggest that these large-scale material properties likely include some important smaller scale variations, disclosed as subtle morphological steps in the crater walls. Furthermore, the modeling results allow advancing some hypotheses on the geological evolution of the Mare Serenitatis region where Linné crater is located (unit S14). We suggest that unit S14 has a thickness of at least a few hundreds of meters up to about 400 m.

Most impact craters observed on planetary bodies are the results of oblique impacts of meteoroids. To date, however, there have only been very few laboratory oblique impact experiments for analogue targets relevant to the surfaces of extraterrestrial bodies. In particular, there is a lack of laboratory oblique impact experiments into brittle targets with a material strength on the order of 1 MPa, with the exception of ice. A strength on the order of 1 MPa is considered to be the corresponding material strength for the formation of craters in the 100 m size range on the Moon. Impact craters are elliptical if the meteoroid's trajectory is below a certain threshold angle of incidence, and it is known that the threshold angle depends largely on the material strength. Therefore, we examined the threshold angle required to produce elliptical craters in laboratory impact experiments into brittle targets. This work aims to constrain current interpretations of lunar elliptical craters and pit craters with sizes below a hundred meters. We produced mortar targets with compressive strength of 3.2 MPa. A spherical nylon projectile (diameter 7.14 mm) was shot into the target surface at a nominal velocity of 2.3 km/s, with an impact angle of 5°‐90° from horizontal. The threshold angle of this experiment ranges from 15° to 20°. We confirmed that our experimental data agree with previous empirical equations in terms of the cratering efficiency and the threshold impact angle. In addition, in order to simulate the relatively large lunar pit craters related to underground cavities, we conducted a second series of experiments under similar impact conditions using targets with an underground rectangular cavity. Size and outline of craters that created a hole are similar to those of craters without a hole. Moreover, when observed from an oblique angle, a crater with a hole has a topography that resembles the lunar pit craters. The relation between the impact velocity of meteoroids on the Moon and the probability of elliptical crater formation was investigated based on our experimental results and an existing empirical equation. The results suggest a distinct possibility that most craters in the 100 m size range on the Moon, given their elliptical shape, originated as secondary craters.

The formation ages of tectonic structures and their spatial distributions were studied in the northwestern Imbrium and Sinus Iridum regions using images obtained by Terrain Camera and Multiband Imager on board the SELENE spacecraft and the images obtained by Narrow Angle Camera on board LRO. The formation ages of mare ridges are constrained by the depositional ages of mare basalts, which are either deformed or dammed by the ridges. For this purpose, we defined stratigraphic units and determined their depositional ages by crater counting. The degradation levels of craters dislocated by tectonic structures were also used to determine the youngest limits of the ages of the tectonic activities. As a result, it was found that the contractions to form mare ridges lasted long after the deposition of the majority of the mare basalts. There are mare ridges that were tectonically active even in the Copernican Period. Those young structures are inconsistent with the mascon tectonics hypothesis, which attributes tectonic deformations to the subsidence of voluminous basaltic fills. The global cooling or the cooling of the Procellarum KREEP Terrane region seems to be responsible for them. In addition, we found a graben that was active after the Eratosthenian Period. It suggests that the global or regional cooling has a stress level low enough to allow the local extensional tectonics.

We revisit the constraints on the deep lunar interior with a possible low-viscosity zone at the core-mantle boundary obtained from our previous forward modeling of the tidal response of the Moon, by comparing a numerical model with several tidal parameters (i.e., k2, k3, h2, and Q) that have been improved or are newly determined by recent geodetic observations and analyses from GRAIL (gravity), LRO (shape), and LLR (rotation). Our results are in principle consistent with these data and suggest a low-viscosity layer (with an outer radius of about 540-560 km) which possibly extends inside the region where deep moonquakes occur.

We present an improved lunar digital elevation model (DEM) covering latitudes within ±60°, at a horizontal resolution of 512 pixels per degree (~60 m at the equator) and a typical vertical accuracy ~3 to 4 m. This DEM is constructed from ~4.5×109 geodetically-accurate topographic heights from the Lunar Orbiter Laser Altimeter (LOLA) onboard the Lunar Reconnaissance Orbiter, to which we co-registered 43,200 stereo-derived DEMs (each 1°×1°) from the SELENE Terrain Camera (TC) (~1010 pixels total). After co-registration, approximately 90% of the TC DEMs show root-mean-square vertical residuals with the LOLA data of <5 m compared to ~ 50% prior to co-registration. We use the co-registered TC data to estimate and correct orbital and pointing geolocation errors from the LOLA altimetric profiles (typically amounting to <10 m horizontally and <1 m vertically). By combining both co-registered datasets, we obtain a near-global DEM with high geodetic accuracy, and without the need for surface interpolation. We evaluate the resulting LOLA + TC merged DEM (designated as "SLDEM2015") with particular attention to quantifying seams and crossover errors.

The correlation between the spatial patterns of surface Thorium (Th) abundance measured by SELENE GRS data and the crustal thickness from the GRAIL gravity field and LRO LOLA data is investigated in the lunar highland area. Our analysis reveals that there are several areas of local minima for Th abundance exhibiting similar values but different crustal thicknesses. To explain the result, we propose a two-stage scenario for crustal formation. In the first stage, plural thin plateaus form on the surface of the lunar magma ocean (LMO), which corresponds to the observed surface Th distribution. In the second stage, a global crust with dichotomy forms by solidification of the LMO under the plateaus.

We report the global distribution of areas exhibiting no absorption features (featureless or FL) on the lunar surface, based on the reflectance spectral data set obtained by the Spectral Profiler onboard Kaguya/SELENE. We found that FL sites are located in impact basins and large impact craters in the Feldspathic Highlands Terrane, while there are no FL sites in the Procellarum regions nor the South Pole-Aitken basin. FL sites in each impact basin/crater are mainly found at the peak rings or rims, where the purest anorthosite (PAN) sites are also found. At the local scale, most of the FL and PAN points are associated with impact craters and peaks. Most of the FL spectra show a steeper (redder) continuum than the PAN spectra, suggesting the occurrence of space weathering effects. We propose that most of the material exhibiting a FL spectrum originate from space weathered PAN. Taking into account all the occurrence trends of FL sites on the Moon, we propose that both the FL and PAN materials were excavated from the primordial lunar crust during ancient basin formations below the megaregolith in the highlands. Since the FL and PAN sites are widely distributed over the lunar surface, our new data may support the existence of a massive PAN layer below the lunar surface.

We present details of the global distribution of high-Ca pyroxene (HCP)-rich sites in the lunar highlands based on the global data set of hyperspectral reflectance obtained by the SELENE Spectral Profiler. Most HCP-rich sites in the lunar highlands are found at fresh impact craters. In each crater, most of the detection points are distributed on the ejecta, rim, and floor of the impact craters rather than the central peaks, while the central peaks are dominated by purest anorthosite (PAN). This indicates that HCP-rich materials originate from relatively shallower regions of the lunar crust than PAN. In addition, while all ray craters with sizes larger than 40km possess HCP-rich materials, small fresh craters with sizes less than 6-10km do not, indicating that the uppermost mixing layers in the lunar crust are not dominated by HCP. Based on these results, we propose that in the upper lunar crust, a HCP-rich zone overlying the PAN layer exists below the uppermost mixing layer. This HCP-rich zone may originate from interstitial melt during the formation of the flotation anorthositic cumulate, while an impact ejecta origin, impact melt origin, and/or magmatic intrusion into the upper lunar crust may also account for the occurrence of HCP-rich sites in the highlands.

We performed impact experiments with granular targets to reveal the formation process of crater "rays", the non-uniform ejecta distributions around some fresh craters on the Moon and planets. We found mesh patterns, loosely woven with spaces like a net, as ejecta. A characteristic length of spaces between meshes was evaluated, and an angle, defined as the ratio of the characteristic length to the distance from the ejection point, was obtained as ~a few degrees. These features are similar to the results of the analyses of the ray patterns around two lunar craters, Glushko and Kepler. Numerical simulations of granular material showed that clear mesh pattern appeared at lower coefficients of restitution between particles but was less clear at larger one, suggesting that the inelastic collisions between particles cause the clear mesh-pattern formation of impact ejecta.

Lunar mare basalts are spatially unevenly distributed, and their abundances differ between the nearside and farside of the Moon. Although mare asymmetry has been attributed to thickness variations in the low-density anorthositic crust, the eruptive mechanism of lunar magma remains unknown. In this study, we investigate the relationship between mare distribution and crustal thickness using geological and geophysical data obtained by the SELENE (Kaguya) and the Gravity Recovery and Interior Laboratory spacecraft, and quantitatively re-evaluate the influence of the anorthositic crust on magma eruption. We identify a lateral heterogeneity in the upper limit of crustal thickness that allows magma extrusion to the surface. In the Procellarum KREEP Terrane, where the surface abundances of heat-producing elements are extremely high, magmas can erupt in regions of crustal thickness below about 30 km. In contrast, magma eruptions are limited to regions of crustal thickness below about 20 km in other nearside regions, around 10 km in the South Pole-Aitken Basin and approximately 5 km in the farside Felspathic Highland Terrane. Such heterogeneity may result from lateral variations in magma production in the lunar mantle and/or crustal density.

When humans explore the Moon, lunar caves will be an ideal base to provide a shelter from the hazards of radiation, meteorite impact, and extreme diurnal temperature differences. In order to ascertain the existence of caves on the Moon, it is best to visit the Moon in person. The Google Lunar X Prize(GLXP) competition started recently to attempt lunar exploration missions. Ones of those groups competing, plan to land on a pit of Lacus Mortis and determine the existence of a cave inside this pit. In this pit, there is a ramp from the entrance down to the inside of the pit, which enables a rover to approach the inner region of the pit. In this study, under the assumption of the existence of a cave in this pit, a 3D model was developed based on the optical image data. Since this model simulates the actual terrain, the rendering of the model agrees well with the image data. Furthermore, the 3D printing of this model will enable more rigorous investigations and also could be used to publicize lunar exploration missions with ease.

The Spectral Profiler (SP) is a visible-near infrared spectrometer onboard the Japanese Selenological and Engineering Explorer (SELENE), which was launched in 2007 and observed the Moon until June 2009. The SP consists of two gratings and three linear-array detectors: VIS (0.5-1.0 μm), NIR 1 (0.9-1.7 μm), and NIR 2 (1.7-2.6 μm). In this paper, we propose a new method for radiometric calibration of NIR 2, specifically for the dark output (background) estimate, which is different from the previous method used for VIS and NIR 1. We show that the reflectance spectra of NIR 2 derived from the new radiometric calibration show less noise than those of the previous method. Based on an analysis of the reflectance spectra at exposure sites of the end-member minerals on the lunar surface, we demonstrated that the spectral features of the 2- μm band in the NIR 2 spectra are consistent with those expected from the minerals inferred from the features of the 1- μm band in the VIS and NIR 1 spectra. Finally, we examined the repeatability of the radiometric calibration of NIR 2 using the SP data near the Apollo 16 landing site observed at four different times. The typical difference in the reflectance at wavelengths <~2.1 μm was a few percent, which is within the uncertainty due to the error in the background estimate, suggesting that there was no significant change in the sensitivity of NIR 2 over the mission period. © 1980-2012 IEEE.

The Sinus Iridum region is one of the important candidate landing areas for the future Chinese lunar robotic and human missions. Considering its flat topography, abundant geomorphic features and complex evolutionary history, this region shows great significance to both lunar science and landing exploration, including powered descent, surface trafficability and in-situ exploration. First, we use Lunar Reconnaissance Orbiter (LRO) Altimeter (LOLA) and Camera (LROC) data to characterize regional topographic and geomorphological features within Sinus Iridum, e.g.; wrinkle ridges and sinuous rilles. Then, we deduce the iron and titanium content for the mare surface using the Clementine ultraviolet-visible (UVVIS) data and generate mineral absorption features using the Chandrayaan-1 Moon Mineralogy Mapper (M3) spectrometer data. Later, we date the mare surface using crater size-frequency distribution (CSFD) method. CSFD measurements show that this region has experienced four major lava infilling events with model ages ranging from 3.32 Ga to 2.50 Ga. The regional magmatic activities evolved from Imbrian-aged low-titanium to Eratosthenian-aged medium-titanium. The inner Sinus Iridum is mainly composed of pyroxene-rich basalts with olivine abundance increasing with time, while the surrounding highlands have a feldspar-dominated composition. In the northern wall of Sinus Iridum, some potential olivine-rich materials directly excavated from the lunar mantle are visible. The Sinus Iridum region is an ideal target for future landing exploration, we propose two candidate landing sites for the future Chinese robotic and human missions. © 2014 Elsevier Ltd.

Tidal heating of a solid planetary body occurs by viscous dissipation, depending on its internal structure and thermal and orbital states. Calculations of the response of the Moon to tidal forces have considered lunar interior structure, but have not reproduced the geodetically observed dependence of dissipation on the lunar tidal period. The attenuation of seismic waves in the deep lunar interior is expected to be consistent with a low-viscosity layer at the core-mantle boundary, which may explain the observed frequency dependence. Here we numerically simulate the viscoelastic tidal response of a Moon that contains a low-viscosity layer at the core-mantle boundary and compare with geodetic observations. In our simulations, a layer with a viscosity of about 2 × 1016 Pa s leads to frequency-dependent tidal dissipation that matches tidal dissipation measurements at both monthly and annual periods. Compared with the lunar asthenosphere, the calculated viscosity is extremely low, and suggests partial melting at the lunar core-mantle boundary. We also find that tidal dissipation is not evenly distributed in the lunar interior, but localized within the low-viscosity layer, which implies that this layer may act as a thermal blanket on the lunar core and influence the Moon's thermal evolution. © 2014 Macmillan Publishers Limited.

High-resolution optical images returned from recent lunar missions provide a new chance for estimation of lunar regolith thickness using morphology and the size-frequency distribution of small impact craters. In this study, regolith thickness over the Sinus Iridum region is estimated using Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Cameras (NACs) images. A revised relationship between crater geometry and regolith thickness is proposed based on old experimental data that takes into considering the effect of the illumination angle of the images. In total, 227 high-resolution LROC NAC images are used, and 378,556 impact craters with diameters from 4.2 to 249.8 m are counted, and their morphologies are identified. Our results show that 50% of the Sinus Iridum region has a regolith thickness between 5.1 and 10.7 m, and the mean and median regolith thicknesses are 8.5 and 8.0 m, respectively. There are substantial regional variations in the regolith thickness, with its median value varying from 2.6 to 12.0 m for most regions. Local variations of regolith thickness are found to be correlated with the lunar surface age: the older the surface, the greater the thickness. In addition, sporadically distributed impact ejecta and crater rays are associated with relatively larger regolith thickness, which might result from excavation and transport of materials during the formation of the secondaries of Copernican-aged craters. Our estimated regolith thickness can help with future analysis of Chang'E-3 lunar penetrating radar echoes and studies of the subsurface stratigraphic structure of the Moon. Key PointsA revised relation between crater geometry and regolith thickness is obtainedRegolith thickness over Sinus Iridum is estimatedRegolith thickness correlates well with surface age

High-resolution images reveal that numerous pit craters exist on the surface of Mars. For some pit craters, the depth-to-diameter ratios are much greater than for ordinary craters. Such deep pit craters are generally considered to be the results of material drainage into a subsurface void space, which might be formed by a lava tube, dike injection, extensional fracturing, and dilational normal faulting. Morphological studies indicate that the formation of a pit crater might be triggered by the impact event, and followed by collapse of the ceiling. To test this hypothesis, we carried out laboratory experiments of impact cratering into brittle targets with variable roof thickness. In particular, the effect of the target thickness on the crater formation is studied to understand the penetration process by an impact. For this purpose, we produced mortar targets with roof thickness of 1-6 cm, and a bulk density of 1550 kg/m3 by using a mixture of cement, water and sand (0.2 mm) in the ratio of 1:1:10, by weight. The compressive strength of the resulting targets is 3.2±0.9 MPa. A spherical nylon projectile (diameter 7 mm) is shot perpendicularly into the target surface at the nominal velocity of 1.2 km/s, using a two-stage light-gas gun. Craters are formed on the opposite side of the impact even when no target penetration occurs. Penetration of the target is achieved when craters on the opposite sides of the target connect with each other. In this case, the cross section of crater somehow attains a flat hourglass-like shape. We also find that the crater diameter on the opposite side is larger than that on the impact side, and more fragments are ejected from the crater on the opposite side than from the crater on the impact side. This result gives a qualitative explanation for the observation that the Martian deep pit craters lack a raised rim and have the ejecta deposit on their floor instead. © 2014 Elsevier Ltd. All rights reserved.

The South Pole-Aitken (SPA) basin is the largest clearly recognized basin on the lunar surface. Determining the composition and structure of the SPA basin interior provides critical constraints on the deep crustal and/or mantle composition of the Moon and improves our understanding of large-basin-forming impact processes. Here we present a new mineralogical map of the SPA basin interior, based on high-spatial-resolution reflectance spectra using the SELENE (Kaguya) multiband imager, which is combined with topographic data in order to interpret the geologic context. The derived mineralogical map suggests extensive distribution of ejected low-Ca pyroxene-dominant mantle material with the presence of purest anorthosite crustal materials surrounding a possible melt pool of 0.26 to 0.33 of the basin diameter near the basin center, which is significantly smaller than that suggested by the crater-scaling law. The absence of clear evidence of lower crustal material is consistent with recent impact simulation results. Key Points Stratigraphy of the lunar mantle was revealed within the Moon's largest basin A smaller melt pool was suggested than that derived by the crater-scaling law Absence of clear evidence of lower crustal material is suggested by mineralogy ©2014. American Geophysical Union. All Rights Reserved.

We report the surface roughness analysis of the lunar highlands for the baseline range 0.15-100 km. We use the Median Differential Slope αm to investigate the scale dependency of the roughness and derive the global αm distribution from SELENE Laser Altimeter and Terrain Camera data. While αm(l) versus baseline l (km) plots vary among different highland types, all highlands commonly show a peak at 3-30 km. The Pre-Nectarian surface shows a relatively large αm(20-30 km). Our analysis is supported by the simulation of synthetic surface cratering models and crater statistics. In our simulation, a peak of αm(30 km) is successfully reproduced. The actual crater density shows good correlation with an empirical roughness indicator. However, a large part of the Nectarian surface shows a peak at 6-9 km baseline. This peak may be caused by secondary craters and ejecta deposit textures from the Nectarian system basins. Key Points The Median Differential Slope (MDS) is measured on lunar highland surfaces The Pre-Nectarian surface shows a relatively large MDS at 20-30 km baseline A large part of the Nectarian surface shows a peak MDS at 6-9 km baseline ©2014. American Geophysical Union. All Rights Reserved.

Our project aims to search for methane-oxidizing microbes on the Mars surface. The project is in preparation under the scheme of the MELOS working group. Martian soil will be sampled from a depth of about 5 - 10 cm below the surface, where organisms are expected to be protected from the harsh hyper-oxidative environment of the Mars surface. The soils will be stained with a cocktail of fluorescent reagents, and examined by fluorescence microscopy. A combination of fluorescent dyes has been selected to identify life forms in samples. A combination of dyes will be used to detect membranes surrounding the "cell". A substrate dye that emits fluorescence upon cleavage by a catalytic reaction will be used to detect the catalytic activity of the "cell". This combination will also be useful for detecting pre-biotic organic material as well as remnants of ancient Martian life. Hydrolysis of the polymers in the "cell" followed by HPLC for amino acid analysis will be effective for examining whether Martian life is identical to or different from terrestrial life. The number and type of the amino acids as well as their chirality will be analyzed to distinguish whether the polymers are contaminants from Earth.

We present details of the identification of sites that show an absorption band at visible wavelengths and a strong 2 μm band using the SELENE Spectral Profiler. All the sites exhibiting the visible feature are found on the regional dark mantle deposit (DMD) at Sinus Aestuum. All the instances of the visible feature show a strong 2 μm band, suggestive of Fe- and Cr-rich spinels, which are different from previously detected Mg-rich spinel. Since no visible feature is observed in other DMDs, the DMD at Sinus Aestuum is unique on the Moon. The occurrence trend of the spinels at Sinus Aestuum is also different from that of the Mg-rich spinels, which are associated with impact structures. This may suggest that the spinel at Sinus Aestuum is a different origin from that of the Mg-rich spinel. Key Points The existence of sites exhibiting visible absorption band on the Moon A unique DMD with Fe- and Cr-rich spinels at Sinus Aestuum Two types of origin of spinel on the lunar surface ©2013. American Geophysical Union. All Rights Reserved.

Observations of the lunar surface within the past 10years have been made with various lunar remote sensing instruments, the Moon Mineralogy Mapper (M3) onboard the Chandrayaan-1 mission, the Spectral Profiler (SP), the Multiband Imager (MI), the Terrain Camera (TC) onboard the SELENE mission, and the ground based USGS Robotic Lunar Observatory (ROLO) for some of them. The lunar phase functions derived from these datasets, which are used in the photometric modeling to correct for the various illumination conditions of the data, are compared to assess their differences and similarity in order to improve interpretations of lunar surface spectra. The phase functions are found to be similar across various phase angles except in the 0-20° range. Differences across the 0-20° range likely result from two different inputs in the photometric modeling of the M3 and SP data: (1) M3 has larger emission angles due to the characteristics of the instrument and the attitude of the spacecraft, and (2) M3 viewing geometry was derived from the local topography whereas SP used a spherical Moon (no topography). The combination of these two different inputs affects the phase function at small phase angles where shadows play a more substantial role, with spatial resolution differences between M3 and SP being another possible source for the differences. SP data are found to be redder (i.e., steeper slope with increasing wavelengths) than MI, M3 and ROLO. Finally, the M3 overall reflectance is also found to be lower than that the other instruments (i.e., MI, SP, and ROLO), generally at least 10% darker than MI. These differences can be observed at local scales in specific examples at hundreds of meters resolutions. At regional and global scales, the same differences are found, which demonstrates the overall stability of the various datasets. The observations from M3, TC, SP and MI are very stable and agree well; however caution should be used when making interpretations based on the spectral slope of SP data or on the absolute reflectance of M3 data. © 2013 Elsevier Inc.

Remote-sensing datasets obtained by each instrument aboard Selenological and Engineering Explorer (SELENE) and Chandrayaan-1 have not been compared directly, and the characteristics of each instrument's data, which may reflect the observation conditions of each instrument and/or residual error in instrument calibration, are unknown. This paper describes the basic characteristics of the data derived by each instrument, briefly describes the data-processing conversion from radiance to reflectance, and demonstrates what we can achieve by combining data obtained by different instruments on different missions (five remote-sensing instruments and an Earth-based telescope). The results clearly demonstrate that the spectral shapes of the instruments are comparable and thus enable us to estimate the composition of each geologic unit, although absolute reflectances differ slightly in some cases. © 2013 Elsevier Inc.

Several modern optical instruments orbited the Moon during 2008 and 2009 onboard the SELENE and Chandrayaan-1 spacecraft and provided a welcomed feast of spectroscopic data to be used for scientific analyses. The different spatial and spectral resolutions of these sensors along with diverse illumination geometry during data acquisition make each set of data unique, and each instrument contributes special value to integrated science analyses. In order to provide the maximum science benefit, we have undertaken a careful cross-validation of radiance data among these orbital instruments and also a set of systematic data acquired using Earth-based telescopes. Most radiance values at 750nm fall between 0 and 100W/(m2μmsr), but a small important fraction can be up to ×2 to ×3 that value, with the largest values occurring at the highest spatial resolution. All instruments are in agreement about overall spectral properties of lunar materials, but small systematic differences are documented between instruments. Lunar radiance values measured with remote sensors for landing sites are all not as high as that estimated from laboratory measurements of returned soil. This is largely because laboratory measurements of lunar soils cannot retain or duplicate the fine structure of lunar regolith found in the natural space environment. © 2013 Elsevier Inc.

Whether water molecules of cometary and/or solar wind origin migrated to and accumulated in cold permanently shadowed areas at the lunar poles has long been debated from the perspective of scientific interest and expectations for future utilization. Recently, high reflectance condition was observed inside the lunar South Pole Shackleton Crater for the 1064.4 nm of the Lunar Orbiter Laser Altimeter on the Lunar Reconnaissance Orbiter, and the high reflectance was explained to perhaps be due to a surface frost layer in excess of 20% water-ice. Here we investigate the crater with the Selenological Engineering Explorer Multi-band imager that has nine bands in the visible to near-infrared range, including a 1050 nm band (62 m/pixel resolution). Part of the illuminated inner wall of Shackleton Crater exhibits high reflectance at 1050 nm but also exhibits the diagnostic 1250 nm spectral absorption, a signature that is consistent with naturally bright purest anorthosite. © 2013. American Geophysical Union. All Rights Reserved.

We present a study of the Lavoisier lunar crater combining photometric data from the AMIE camera (SMART-1 mission) and hyperspectral data from the Moon Mineralogy Mapper M3 (Chandrayaan-1 mission), with a special emphasis on the pyroclastic deposits considered to be present on the crater floor. The photometric parameters are in agreement with the general photometric behaviors of the lunar regolith, especially the backscattering properties. The assumed pyroclastic materials within Lavoisier present at first order a rather homogeneous photometric behavior, in favor of their surface state homogeneity. However, they are not significantly different from other "non-dark" patches on the crater's floor, whereas the assumed pyroclastic deposit of Lavoisier F displays clearly different photometric parameters, indicative of distinct physical surface properties from the pyroclastic materials within Lavoisier. Using laboratory data to get hindsight on the reliability of results from orbital datasets, we show that the use of more or less depleted phase curves for photometric inversions has a clear impact on the photometric parameters that are derived. The hyperspectral analysis of Lavoisier crater shows that the various pyroclastic deposits present the same mineralogical composition, distinct from the floor of the crater and the mare basalts. M3 spectra do not differentiate between the pyroclastic deposits within Lavoisier and Lavoisier F. They have the same spectral signatures, share a similar mineralogical composition, and probably the same volcanic origin. Therefore, the differences seen in the photometric analysis from the AMIE observations are indicative of variations in grain sizes, and/or roughness, and/or particles scattering properties, and/or compaction state. The combined mineralogical and photometric analysis is a very useful approach to document the nature of the pyroclastic deposits of the Moon, and possibly of other objects of the Solar System (e.g., Mercury) as the combination of the mineralogy and the physical properties sets constraints on the origin and mode of emplacement of the deposits, and characterizes the eruption styles. © 2013 Elsevier Inc.

We have investigated the geological conditions below two lava flow units through determining the bulk permittivity and porosity in the uppermost basalt layer to depths of a few hundred meters. We use a newly developed method based on three data sets obtained by the Lunar Radar Sounder (LRS), Multiband Imager (MI), and Terrain Camera (TC) onboard the Selenological and Engineering Explorer (SELENE; Kaguya) spacecraft. The bulk permittivity of the uppermost basalt layer is calculated as the ratio of the apparent radar depth to the thickness of the uppermost basalt layer. Its thickness can be constrained from the excavation depths of two types of craters (haloed and nonhaloed craters). These craters are identified on the basis of FeO and/or TiO2 maps created from the MI data. These excavation depths are determined based on the measurement of the crater diameter using the TC data. The apparent radar depth is derived from the time delay between the surface echo and subsurface echo measured by LRS near the craters. The bulk permittivities are estimated to be 2.8-5.5 in a lava flow unit of Mare Humorum and 4.2-18.0 in a lava flow unit of Mare Serenitatis. These bulk permittivities are indicative of porous basalt layers with the porosities of 19%-51% in the unit of Humorum and 0%-33% in the unit of Serenitatis. The estimated porosities would be mainly explained by two different sources: intrinsic voids of lava and impact-induced cracks. Key Points The permittivitties of lunar basalt layers were estimated by a new method The permittivities estimated in the two mare regions were 2.8-5.5 and 4.2-18.0 Intrinsic voids and impact-induced cracks were main porosity sources ©2013. American Geophysical Union. All Rights Reserved.

This paper investigates the detectability performance of a breadboard model of a ground penetrating radar (GPR) for subsurface sounding of solid bodies in the Solar System up to a depth of tens of meters. The developed GPR uses a linear FM chirp signal whose frequency is linearly swept from 300 MHz to 900 MHz for 330 μs and can produce a narrower pulse of greater peak amplitude by applying a pulse compression technique. Vivaldi antennas are selected as the antennas of the radar system and designed by a particle swarm optimization (PSO) method to minimize the S11 parameter in the operational frequencies. A laboratory experiment for the radar electronics demonstrates that amplitude modulation of a transmitted signal by specific window functions could increase its dynamic range. The S11 parameter of the developed antenna is found to be very close to the desired value. Finally, subsurface sounding is simulated by modeling the two-layer subsurface structure. The simulation results reveal that the GPR could observe the subsurface structures up to a depth of 10-20 m from the air and a depth of 15-25 m from the ground. © 2013 Antenna Society of the Chinese Institute of Electronics.

We present a new global survey of the purest anorthosite (PAN) rock using the Spectral Profiler onboard Kaguya. We found that PAN rocks are widely distributed over the Moon, including the Feldspathic Highland Terrain and the south and north polar regions. All PAN sites are associated with huge impact structures with diameters larger than 100 km. Based on the global distributions of PAN and olivine-rich sites, we propose the existence of a massive PAN layer with a thickness of ̃50 km below an uppermost mafic-rich mixed layer with a thickness of ̃10 km. Below the PAN layer, a lower crustal layer with olivine-rich materials may be present on the nearside, but not on the far side of the Moon. The existence of a PAN layer with a thickness of ̃50 km suggests an Al2O3 abundance of 33 to 34 wt.% in the lunar crust, which is higher than previous estimates of <32 wt.%. Our data indicate the massive production event of PAN during the early stage of the formation of the Moon, supporting the lunar magma ocean scenario. © 2012. American Geophysical Union.

The Moon's nearside and farside differ in topography 1, crustal thickness 2, mare volcanic activity 3 and elemental concentrations 4. The origin of this dichotomy is still unclear 5-7. It is also unknown whether the characteristics of the oldest crust, the anorthositic lunar highlands, reflect a different magmatic evolution of nearside and farside crust. Based on analyses of nearside highland rocks 8,9, it has been suggested that nearside crustal growth occurred from an evolved, iron-rich magma ocean 10, but information from the farside highlands is lacking. Here we apply an empirical algorithm to lunar reflectance spectra 11 from the Kaguya Spectral Profiler and report that magnesium contents relative to iron of primitive crustal highland rocks on the farside are higher than on the nearside. Our findings indicate that the farside crust consists of rocks that crystallized from less-evolved magma than the nearside crust. We conclude that the lunar dichotomy is directly linked to crystallization of the magma ocean and suggest that the composition of the magma ocean was more primitive at the time of crustal growth than previously estimated. © 2012 Macmillan Publishers Limited. All rights reserved.

The crater retention ages of the mare deposits within the Orientale multi-ring impact basin are investigated using 10-m resolution images obtained by the SELENE (Kaguya) spacecraft, in order to constrain the volcanic history of the Moon around the nearside-farside boundary. Precise crater-counting analyses reveal that mare deposits in the Orientale region are much younger than previously estimated: ∼2.9 Ga mare basalt in the eastern part of Mare Orientale and ∼1.8-2.2 Ga mare deposits in Lacus Veris and Lacus Autumni, maria along the northeastern rings of the basin. The latter age estimates indicate that the Orientale region experienced volcanic activities ∼2 billion years after the basin-formation impact. The dominance of a uniform surface age across the mare deposits in the peripheral regions strongly suggests that these volcanic eruptions are contemporary with the elevated volcanic activity episode proposed for the Procellarum KREEP Terrane on the lunar nearside at ∼2 Ga and that this activity peak is much more widespread than previously estimated. The longevity of mare volcanism in the Orientale region further suggests high initial temperatures and/or high content of heat-producing elements in the underlying mantle of this region. © 2012. American Geophysical Union. All Rights Reserved.