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Lunar sinuous rilles are large (kilometers long; tens of meters wide) channels commonly found on the lunar maria. Although there is broad consensus that sinuous rilles were generated during lava-flow emplacement, disagreement remains regarding the relative contributions of erosional and constructional processes during rille formation. Careful mapping of portions of Rima Marius (centered at 16.37°N, 49.54°W) using Lunar Reconnaissance Orbiter (LRO) Narrow Angle Camera (NAC) images with resolutions as high as 0.5 m/pixel reveal morphologic evidence for the origin of sinuous rilles. Specifically, shallow (

The objective of this study is to explore the regional geology of the northwest Imbrium region in which the Chang'E-3 (CE-3) landing site is located. CE-3 successfully landed on December 14, 2013 on the unsampled Eratosthenian basalts whose study is important for understanding the evolution of the Moon. New geologic and structural maps of the research area were produced through the integrated analysis of diverse datasets. The highlands surrounding Imbrium differ from typical Farside Highlands Terrain (FHT). The Iridum highland region (as well as the surrounding Imbrium region) exhibits elevated concentrations of Fe, and abundant local exposures of low-Ca pyroxene and olivine bearing lithologies. In this study these highlands are named as mafic highlands (MH). Our dating results using crater size-frequency distributions (CSFDs) show that the Iridum basin (hosting Sinus Iridum) was formed ∼3.8 Ga, shortly following the Imbrium basin formation and before the last large multiringed basin, Orientale. The Eratosthenian period of lunar basalt eruptions, which lasted longer than other stratigraphic units, is suggested to divide into the Lower Eratosthenian mare (LEm) and Upper Eratosthenian mare (UEm) units. This subdivision is based on whether lava fronts can be clearly seen or not and the age separating the units is 2.35 Ga. The mafic mineralogy of the mare basalts in Imbrium is characterized by abundant olivine in the Eratosthenian-aged basalts and average pyroxene compositions near pigeonite to sub-calcic augite in the Imbrian and Em1 units. The thickness of individual lava for UEm units is 8–11 m, indicative of high effusion rates. The thickness of the Em3 unit ranges from ∼17 m to ∼45 m with lesser thickness to the west and greater thickness in the interior and to the east. The estimated volume and average flux of the Eratosthenian-aged basalts are greater than previously thought. The presence of these youngest basalts in the Procellarum-KREEP terrain (PKT) is hypothesized to be a causal relationship, with the PKT terrain reducing the thickness of the lithosphere and permitting preferential dike emplacement and extrusion there. We speculate that high-Ti and olivine-rich composition in late stage basalts may be consistent with low Si and high Ti and low degrees of partial melting. Large numbers of sinuous rilles and small ridges are identified and mapped. Many young ridges were found inside Imbrium, suggesting a very extended period (at least as young as the last 50 Ma) of the Moon's tectonic activity. The distinct compositions of both highlands and mare basalts and extended tectonism emphasize how the Imbrium basin is an important area for understanding the Moon.

Because of the impending energy crisis and the environmental Impact of fossil fuels, researchers are actively looking for alternatives, such as Helium-3 on the Moon. Although it remains challenging to explore energies on the Moon due to the long physical distance, the lunar features, such as craters and rilles, can be the hotspots for such energy sources, according to recent studies. Thus, identifying lunar features, such as craters and rilles, can facilitate the discovery of Helium-3 on the Moon, which is enriched in such hotspots. However, previously, no computational method was developed to recognize the lunar features automatically for facilitating space energy discovery. In our research, we aim at developing the first deep learning method to identify multiple lunar features simultaneously for potential energy source discovery. Based on the state-of-the-art deep learning model, High Resolution Net, our model can efficiently extract semantic information and high-resolution spatial information from the input images, which ensures the performance for recognizing the lunar features. With a novel framework, our method can recognize multiple lunar features, such as craters and rilles, at the same time. We also used transfer learning to handle the data deficiency issue. With comprehensive experiments on three datasets, we show the effectiveness of the proposed method. All the datasets and codes are available online. The source code and the data are available at https://github.com/frankchen121212/High-Resolution-MoonNet

We model the ascent and eruption of lunar mare basalt magmas with new data on crustal thickness and density (GRAIL), magma properties, and surface topography, morphology and structure (Lunar Reconnaissance Orbiter). GRAIL recently measured the broad spatial variation of the bulk density structure of the crust of the Moon. Comparing this with the densities of lunar basaltic and picritic magmas shows that essentially all lunar magmas were negatively buoyant everywhere within the lunar crust. Thus positive excess pressures must have been present in melts at or below the crust–mantle interface to enable them to erupt. The source of such excess pressures is clear: melt in any region experiencing partial melting or containing accumulated melt, behaves as though an excess pressure is present at the top of the melt column if the melt is positively buoyant relative to the host rocks and forms a continuously interconnected network. The latter means that, in partial melt regions, probably at least a few percent melting must have taken place. Petrologic evidence suggests that both mare basalts and picritic glasses may have been derived from polybaric melting of source rocks in regions extending vertically for at least a few tens of km. This is not surprising: the vertical extent of a region containing inter-connected partial melt produced by pressure-release melting is approximately inversely proportional to the acceleration due to gravity. Translating the ∼25 km vertical extent of melting in a rising mantle diapir on Earth to the Moon then implies that melting could have taken place over a vertical extent of up to 150 km. If convection were absent, melting could have occurred throughout any region in which heat from radioisotope decay was accumulating; in the extreme this could have been most of the mantle. The maximum excess pressure that can be reached in a magma body depends on its environment. If melt percolates upward from a partial melt zone and accumulates as a magma reservoir, either at the density trap at the base of the crust or at the rheological trap at the base of the elastic lithosphere, the excess pressure at the top of the magma body will exert an elastic stress on the overlying rocks. This will eventually cause them to fail in tension when the excess pressure has risen to close to twice the tensile strength of the host rocks, perhaps up to ∼10 MPa, allowing a dike to propagate upward from this point. If partial melting occurs in a large region deep in the mantle, however, connections between melt pockets and veins may not occur until a finite amount, probably a few percent, of melting has occurred. When interconnection does occur, the excess pressure at the top of the partial melt zone will rise abruptly to a high value, again initiating a brittle fracture, i.e. a dike. That sudden excess pressure is proportional to the vertical extent of the melt zone, the difference in density between the host rocks and the melt, and the acceleration due to gravity, and could readily be ∼100 MPa, vastly greater than the value needed to initiate a dike. We therefore explored excess pressures in the range ∼10 to ∼100 MPa. If eruptions take place through dikes extending upward from the base of the crust, the mantle magma pressure at the point where the dike is initiated must exceed the pressure due to the weight of the magmatic liquid column. This means that on the nearside the excess pressure must be at least ∼19 ± 9 MPa and on the farside must be ∼29 ± 15 MPa. If the top of the magma body feeding an erupting dike is a little way below the base of the crust, slightly smaller excess pressures are needed because the magma is positively buoyant in the part of the dike within the upper mantle. Even the smallest of these excess pressures is greater than the ∼10 MPa likely maximum value in a magma reservoir at the base of the crust or elastic lithosphere, but the values are easily met by the excess pressures in extensive partial melt zones deeper within the mantle. Thus magma accumulations at the base of the crust would have been able to intrude dikes part-way through the crust, but not able to feed eruptions to the surface; in order to be erupted, magma must have been extracted from deeper mantle sources, consistent with petrologic evidence. Buoyant dikes growing upward from deep mantle sources of partial melt can disconnect from their source regions and travel through the mantle as isolated bodies of melt that encounter and penetrate the crust–mantle density boundary. They adjust their lengths and internal pressure excesses so that the stress intensity at the lower tip is zero. The potential total vertical extent of the resulting melt body depends on the vertical extent of the source region from which it grew. For small source extents, the upper tip of the resulting dike crossing the crust–mantle boundary cannot reach the surface anywhere on the Moon and therefore can only form a dike intrusion; for larger source extents, the dike can reach the surface and erupt on the nearside but still cannot reach the surface on the farside; for even larger source extents, eruptions could occur on both the nearside and the farside. The paucity of farside eruptions therefore implies a restricted range of vertical extents of partial melt source region sizes, between ∼16 and ∼36 km. When eruptions can occur, the available pressure in excess of what is needed to support a static magma column to the surface gives the pressure gradient driving magma flow. The resulting typical turbulent magma rise speeds are ∼10 to a few tens of m s−1, dike widths are of order 100 m, and eruption rates from 1 to 10 km long fissure vents are of order 105 to 106 m3 s−1. Volume fluxes in lunar eruptions derived from lava flow thicknesses and surface slopes or rille lengths and depths are found to be of order 105 to 106 m3 s−1 for volume-limited lava flows and >104 to 105 m3 s−1 for sinuous rilles, with dikes widths of ∼50 m. The lower end of the volume flux range for sinuous rilles corresponds to magma rise speeds approaching the limit set by the fact that excessive cooling would occur during flow up a 30 km long dike kept open by a very low excess pressure. These eruptions were thus probably fed by partial melt zones deep in the mantle. Longer eruption durations, rather than any subtle topographic slope effects, appear to be the key to the ability of these flows to erode sinuous rille channels. We conclude that: (1) essentially all lunar magmas were negatively buoyant everywhere within the crust; (2) positive excess pressures of at least 20–30 MPa must have been present in mantle melts at or below the crust–mantle interface to drive magmas to the surface; (3) such pressures are easily produced in zones of partial melting by pressure-release during mantle convection or simple heat accumulation from radioisotopes; (4) magma volume fluxes available from dikes forming at the tops of partial melt zones are consistent with the 105 to 106 m3 s−1 volume fluxes implied by earlier analyses of surface flows; (5) eruptions producing thermally-eroded sinuous rille channels involved somewhat smaller volume fluxes of magma where the supply rate may be limited by the rate of extraction of melt percolating through partial melt zones.

The lunar maria cover approximately 17% of the Moon's surface. Discerning discrete subsurface layers in the mare provides some constraints on thickness and volume estimates of mare volcanism. Multiple types of data and measurement techniques allow probing the subsurface and provide insights into these layers, including detailed examination of impact craters, mare pits and sinuous rilles, and radar sounders. Unfortunately, radar sounding includes many uncertainties about the material properties of the lunar surface that may influence estimates of layer depth and thickness. Because they distribute material from depth onto the surface, detailed examination of impact ejecta blankets provides a reliable way to examine deeper material using orbital instruments such as cameras, spectrometers, or imaging radars. Here, we utilize Miniature Radio Frequency (Mini-RF) data to investigate the scattering characteristics of ejecta blankets of young lunar craters. We use Circular Polarization Ratio (CPR) information from twenty-two young, fresh lunar craters to examine how the scattering behavior changes as a function of radius from the crater rim. Observations across a range of crater size and relative ages exhibit significant diversity within mare regions. Five of the examined craters exhibit profiles with no shelf of constant CPR near the crater rim. Comparing these CPR profiles with LROC imagery shows that the magnitude of the CPR may be an indication of crater degradation state; this may manifest differently at radar compared to optical wavelengths. Comparisons of radar and optical data also suggest relationships between subsurface stratigraphy and structure in the mare and the block size of the material found within the ejecta blanket. Of the examined craters, twelve have shelves of approximately constant CPR as well as discrete layers outcropping in the subsurface, and nine fall along a trend line when comparing shelf-width with thickness of subsurface layers. These observations suggest that surface CPR measurements may be used to identify near-surface layering. Here, we use ejected material to probe the subsurface, allowing observations of near-surface stratigraphy that may be otherwise hidden by layers higher from remote observations.

In general, textures are thought to be some complicated repeated patterns formed by elements, or primitives which are sorted in certain rules. Lunar surfaces record the interactions between its outside environment and itself, thus, based on high-resolution DEM model or image data, there are some topographic features which have different roughness and entropy values or signatures on lunar surfaces. Textures of lunar surfaces can help us to concentrate on typical topographic and photometric variations and reveal the relationships between obvious features (craters, impact basins, sinuous rilles (SRs) and ridges) with resurfacing processes on the Moon. In this paper, the term surface roughness is an expression of the variability of a topographic or photometric surface at kilometer scale, and the term entropy can characterize the variability inherent in a geological and topographic unit and evaluate the uncertainty of predictions made by a given geological process. We use the statistical moments of gray-level histograms in different-sized neighborhoods (e.g., 3, 5, 10, 20, 40 and 80 pixels) to compute the kilometer-scale roughness and entropy values, using the mosaic image from 70°N to 70°S obtained by Lunar Reconnaissance Orbiter (LRO) Wide Angle Camera (WAC). Large roughness and entropy signatures were only found in the larger scale maps, while the smallest 3-pixel scale map had more disorderly and unsystematic textures. According to the entropy values in 10-pixel scale entropy map, we made a frequency curve and categorized lunar surfaces into three types, shadow effects, maria and highlands. A 2D scatter plot of entropy versus roughness values was produced and we found that there were two point clusters corresponding to the highlands and maria, respectively. In the last, we compared the topographic and photometric signatures derived from Lunar Orbiter Laser Altimeter (LOLA) data and WAC mosaic image. On the lunar surfaces, the ridges have obvious multilevel topographic textures which are sensitive to the topographic changes, while the ejecta deposits of fresh craters appear obvious photometric textures which are sensitive to the brightness variations.

In this paper, we estimated the relative and absolute ages of the seven rilles (1–7) in southwest Aristarchus region in order to know when these lunar rilles formed and how they fit into lunar volcanic history. According to their superposition relationships and colors in the color-ratio composite, the geological units and features in the study area were sorted in an order from old to young: the highland materials, the glassy Fe 2+ rich pyroclastics (DMD), the rille 1, 2, 3, 5, and 7, the mature and fresh basaltic units in the east and south of the Aristarchus plateau, the rille 4 and 6, the youngest Aristarchus Craters and its ejecta deposits. Buffered Crater Counting (BCC) method provides a chance to determine ages of linear features that do not have enough surface areas for the traditional crater counting method. The BCC analysis of seven rilles had shown that five of them probably formed in the Imbrium Period and the other two formed in Eratosthenian Period. The absolute age of oldest rille is 3.77 − 0.04 + 0.03 Ga and the youngest rille׳s age is 1.49 − 0.26 + 0.26 Ga . Because their formation is interpreted to lava flowing and eroding of magma eruptions, we can see that the volcanic activities in Aristarchus region lasted for a long time from Pre-Nectarian Period (DMD materials) to Eratosthenian Period (rille 6) and weakened gradually to form small scale rilles as times went on. This may be because of the basaltic volcanic eruptions produced by heating elements in Aristarchus region to partially melt the underlying lunar mantle, which become the materials and source of these rilles.

Chang'e-5, China's first lunar sample return mission, is targeted to land in northern Oceanus Procellarum, within a region selected on the basis of 1) its location away from the Apollo-Luna sampling region, 2) the presence of the Procellarum KREEP Terrane (PKT), 3) the occurrence of one of the youngest lunar mare basalts (Em4), and 4) its association with Rima Sharp. In order to provide context for returned sample analyses, we conducted a comprehensive study of the regional and global settings, geomorphology, composition, mineralogy, and chronology of the Em4 mare basalts. Superposed on Imbrian-aged low-Ti basalts, Em4 covers 37,000 km2 and is composed of Eratosthenian-aged (∼1.53 Ga), high-Ti basalts with a mean thickness of ∼51 m and a volume between ∼1450 and 2350 km3. Minor variations in TiO2 and FeO abundance occur within the unit and the thorium content averages ∼6.7 ppm, typical of PKT mare basaltic regolith. No specific source vents (e.g., fissures, cones, domes) were found within the unit. We show that Rima Sharp is actually composed of three major rilles, whose source vents are located outside of, and which flow into, and merge in Em4, suggesting that they may be among the sources for Em4. Regolith thickness averages ∼7 m and there is abundant evidence for vertical and lateral mixing; the most likely sources of distal ejecta are Aristarchus, Harpalus, and Sharp B craters. Returned samples from local and distant materials delivered by impact will thus provide significant new insights into lunar geochronology, inner Solar System impact fluxes, the age of very young mare basalts, the role of the PKT in the generation of mare basalts, the role of sinuous rilles in lava flow emplacement, and the thermal evolution of the Moon.

Basaltic eruptive fissures of the Great Rift and surroundings on the eastern Snake River Plain of Idaho, USA, and selected volcanic features in Hawai’i, Iceland and northern Africa were surveyed for their relevancy as planetary analogs. Evaluated during field investigations and in satellite imagery for structures, physiography, and geologic setting, fissures were categorized into four broad types: (1) simple, monogenetic fissures with obvious volcanic constructs or deposits, (2) monogenetic fissures now obscured by low shields or relatively large cones, (3) polygenetic volcanic rift zones with multiple vents and deposits, and (4) compound regional fissure systems or dike swarms that comprise major rift zones or large volcanic terrains. Using this classification as an initial base, we surveyed imagery of volcanic features for likely fissure vents in two major geologic settings on the Moon: floor-fractured craters (FFCs) and mare and cryptomare provinces. Two major regions on Mars, the volcanic plains around Alba Mons and the greater Tharsis region, were also surveyed for fissure types and volcanic associations of fissure-like features. The planetary surveys suggest that the proposed classification provides a suitable analog starting point to interpret structures associated with fissure systems on the Moon and Mars. With few exceptions, our survey indicates that each of the studied terrains exhibits a dominant fissure type. Type 1 fissures, most with pyroclastic deposits, prevail in lunar FFCs and mare-like regions; whereas type 2 fissures are ubiquitous in the Tharsis region of Mars and a few exist on the Moon as low shields. Type 3 volcanic rift zones are not common on either the Moon or Mars, although they might become evident in future work on chemically evolved terrains. Type 4 fissures are inferred in mare terrains, often represented as the extensions of major linear rille networks or rimae, with possibly complex dike swarms that were buried beneath voluminous mare basalt lava flows. Likewise, numerous flood lavas on Mars are possibly associated with now-obscured or difficult to define type 4 fissure systems.

Large shield-building volcanic structures (>30 km) with central vents are rare on the Moon. The most popular candidate targets of lunar large shield volcanoes are volcanic complexes on the lunar nearside within the compositionally evolved Procellarum KREEP terrane (PKT). However, there is a unique mass concentrated volcanic complex not within the PKT, namely Gardner (16.1°N, 34.1°E, ∼70 km in diameter) on the northern edge of Mare Tranquillitatis, whose origin is enigmatic. Here we use multi-source remote-sensing data to assess its morphology, composition and stratigraphy to better understand its formation and evolution. Results show that domes, sinuous rilles, and an irregular depression exist in this region. Gardner has experienced four stages of volcanic activities with absolute model ages from 3.90 to 2.96 Ga, contemporaneous with the main volcanic activity periods of the lunar nearside. An area with relatively low albedo, high FeO and TiO2 contents, low rock abundances, and young surface age is identified in the southeast part of the irregular depression at the summit of the plateau, which could have been the result of a long-lived eruptive center. Quasi-circular positive Bouguer-gravity anomaly suggests either sill/laccolith intrusions or large conduit solidifications underneath this area, which could be related to its surrounding radially distributed lava domes, wrinkle ridges and faults. We conclude that Gardner is the largest central-vent volcano identified so far on the Moon, and it provides an essentially new perspective and supplement for the knowledge of the lunar nearside volcanism.

Domes, an analog of the terrestrial shield volcanoes are one of the important volcanic features found on the lunar surface. Such volcanic features are windows to better understanding of the contrasting natures of lunar volcanism, giving an insight into the source and the nature of the basaltic magmas. Marius Hills Complex is one of the most important regions in the entire lunar surface for having a complex setting of volcanic constructs with an abundant number of volcanic features like domes, cones and rilles. As a part of initiation of the study of Marius Hills volcanism, an effusive dome located to the south of Rima Galilaei, near the contact of Imbrian and Eratosthenian geological units is taken for the present study. Inferring from the Terrain Mapping Camera-Digital Elevation Model (TMC-DEM), the morphometric parameters are estimated (350 m in height, 9.62 km in diameter), and accordingly the rheological parameters are also estimated. As the signatures of multiphase eruption are not clear geomorphologically and also in topography, the dome is assumed to evolved in monogenetic eruption. The causative dike parameters of the dome are estimated, which gives upper bounds of true values of the parameters. The estimated feeder dike length (150 km) and width (233 m) implies that the source region is lying most probably in the mantle portion of moon. The crater size frequency distribution (CSFD) is applied to determine the age of the particular dome and also the surrounding mare surface so as to better construct a stratigraphic correlation. It is found that dome belongs to oldest age unit of Marius Hills region while the surrounding units are relatively younger. Using Chandrayaan-I Moon Mineralogy Mapper (M3) data, the surface composition for the study area is also analysed. Thus, the morphometry, rheology, dike parameters, age determination and mineralogy are found to be in good agreement with results of the earlier studies. Such a study, covering all the domes and other volcanic features in Marius Hills using high resolution data sets will provide a clear and better understanding of the volcanic history of the region and the Oceanus Procellarum Basin as well. In such a study, the application potential of high resolution Chandrayaan-I TMC image and its DEM generated from the stereo data has been useful.

Accepted: 2014-06-11, 資料番号: SA1140139000

Lunar sinuous rilles (SRs) are enigmatic features interpreted to have formed as the result of lava erosion into the lunar surface. While specific SRs have been studied in detail over the past few decades, the most recent general survey of these features was conducted in 1969 using Lunar Orbiter IV and V photographs. The current global study updates the catalog of SRs, using higher resolution SELENE and LRO image and topography data to provide detailed observations and measurements of the rilles observed across the lunar surface. The new survey catalogs more than 200 SRs that vary in length from 2 km to 566 km (median length 33.2 km), in width from 160 m to 4.3 km (median width 480 m), in depth from 4.8 m to 534 m (median depth 49 m), in slope from −1.4° to 0.5° (median slope −0.2°), and in sinuosity index from 1.02 to 2.1 (median sinuosity 1.19). Oceanus Procellarum contains 48% of the rilles mapped in this survey, and these rilles are typically associated with the known centers of volcanism within the Procellarum-KREEP Terrain, the Aristarchus Plateau and the Marius Hills. The current study also constrains the timing of the formation of lunar SRs, using the assumptions that the incised unit represents an upper age limit and the terminal or embaying unit represents a lower age limit. Results indicate that the distribution of ages of rille formation is highly correlated with the emplacement ages of mare units, where the majority of rilles are observed to have formed between 3.0 Ga and 3.8 Ga ago, though some of the features associated with the Aristarchus Plateau may have formed as recently as 1.0 Ga to 1.5 Ga ago. The documented observations can be used to better understand how SRs formed; for example, the range of slopes observed for all rilles (−1.4° to 0.5°) indicates that thermal erosion is likely dominant during the formation of the 78% of rilles that are observed to have formed in solidified mare basalt material, though mechanical erosion is likely to have been a more significant process during the formation of the 25% of observed rilles that originate in the highlands (20% of the mapped rilles crossed from the highlands into the mare), where a thicker regolith is expected to have been more easily eroded.

Phase equilibrium experiments have been performed to place constraints on the melting processes in the lunar interior using the Apollo 15 group A ultramafic green glass, the most abundant pyroclastic glass composition found in the polymict regolith breccia (15426) near Hadley Rille at station 7. Experiments on two compositional end members of the 15A green glass trend yield experiments saturated with olivine and orthopyroxene on the liquidus at 1520 °C and 2.1 GPa. Experiments were also performed on high-Al and -Ca analog compositions and on mixtures of the green glass compositions and natural minerals (olivine, orthopyroxene and garnet) in order to obtain experimental liquids multiply saturated with olivine, orthopyroxene, clinopyroxene and garnet. Three liquid compositions in equilibrium with a garnet-lherzolite residual phase assemblage were produced at 2.35 GPa, 1440 and 1420 °C and at 2.6 GPa, 1460 °C. Observed compositional variations in the 15A glasses rule out any simple partial melting process that would involve a olivine + orthopyroxene bearing source region. The results of these experiments are used to test petrogenetic models for the origin of the 15A green glasses. Synergistic evidence from major and trace element geochemistry, geophysical data on the lunar interior structure and results from models of lunar magma ocean differentiation is used to develop a model for the origin of the 15A ultramafic glasses. We propose that a mixing process involving a primary magma, derived by melting primordial lunar mantle, and remelted late-stage magma ocean cumulates led to the origin of the 15A glass. The model uses the 15A green glass multiple saturation point at 1520 °C and 2.1 GPa to constrain the depth of a cumulate remelting process. Decompression partial melting of undifferentiated lunar mantle, produced by adiabatic upwelling of the deep, hot interior of the moon, produced melts that infiltrated and selectively melted late stage magma ocean cumulates that had sunk to the base of a lunar magma ocean. The compositional variations observed in the 15A glasses are consistent with this model.

Typical flow thicknesses of lunar mare basalts were not well constrained in the past, because as craters and rilles age, downslope movement of loose material tends to mix and bury stratigraphy, obscuring the three dimensional nature of the maria. New Lunar Reconnaissance Orbiter Camera high resolution images unambiguously reveal thicknesses of mare basalt layers exposed in impact craters, rilles, and steep-walled pits. Pits up to one hundred meters deep present relatively unmodified, near-vertical sections of mare in three cases, and many young impact craters also expose well preserved sections of mare. Oblique views of each pit and many of these craters reveal multiple layers, 3 to 14 m thick, indicating that eruptions typically produced a series of ∼10 m thick flows (or flow lobes) rather than flows many tens to hundreds of meters thick. Additionally, these images unambiguously show that the floors of two pits extend beneath the mare surfaces, thus revealing sublunarean voids of unknown lateral extent. We also document the occurrence of pits that may be expressions of collapse into subsurface voids in non-mare impact melt deposits. These voids are compelling targets for future human and robotic exploration, with potential as temporary shelters, habitations, or geologic museums.

Ascraeus Mons was one of the first of the Martian volcanoes to be imaged by the High Resolution Stereo Camera (HRSC) experiment onboard the ESA Mars Express spacecraft. These images show much of the volcano at a higher resolution than previously, and details of its lava flows, sinuous rilles, flank vents and tectonic features indicate an unexpected origin for some of these features. We establish the time-stratigraphic sequence for these features, and use a numerical model on HRSC stereo DTMs of the sinuous rilles, and conclude that they were formed by water erosion. Terrestrial analogues for such features are found at Reunion Island and other volcanoes. We then examine the overall structure of the volcano, which is dissimilar to that of large terrestrial volcanoes in important respects, and perform laboratory analogue experiments of its deformation, concluding that the tectonic features were formed by sinking of the volcano into a substratum that was much weaker than the volcanic edifice. An ice-rich substratum melted by a combination of pressure melting and magmatic heating seems the most likely mechanism. Analogous water-escape structures in a similar volcanic situation have been identified at Mt Haddington in the Antarctic. The possible role of a hydrological cycle and a hydrothermal system within the volcano are discussed. Based on field evidence, we propose that much of the broad aprons of lobate flows issuing from the NE and SSW foot of Ascraeus Mons are composed of mudflows rather than lava flows. These different approaches are linked into a coherent history of this volcano. The similarity of Ascraeus Mons to Pavonis Mons and Arsia Mons (though Ascraeus is younger) suggests that some of our conclusions may apply to these volcanoes too.

This study examines a set of lunar domes with very low flank slopes which differ in several respects from the frequently occurring lunar effusive domes. Some of these domes are exceptionally large, and most of them are associated with faults or linear rilles of presumably tensional origin. Accordingly, they might be interpreted as surface manifestations of laccolithic intrusions formed by flexure-induced vertical uplift of the lunar crust (or, alternatively, as low effusive edifices due to lava mantling of highland terrain, or kipukas, or structural features). All of them are situated near the borders of mare regions or in regions characterised by extensive effusive volcanic activity. Clementine multispectral UVVIS imagery indicates that they do not preferentially occur in specific types of mare basalt. Our determination of their morphometric properties, involving a combined photoclinometry and shape from shading technique applied to telescopic CCD images acquired at oblique illumination, reveals large dome diameters between 10 and more than 30 km, flank slopes below 0.9 degrees, and volumes ranging from 0.5 to 50 km(3). We establish three morphometric classes. The first class, In1, comprises large domes with diameters above 25 kin and flank slopes of 0.2 degrees-0.6 degrees, class In2 is made up by smaller and slightly steeper domes with diameters of 1015 kin and flank slopes between 0.4 degrees and 0.9 degrees, and domes of class In3 have diameters of 13-20 km and flank slopes below 0.3 degrees. While the morphometric properties of several candidate intrusive domes overlap with those of some classes of effusive domes, we show that a possible distinction criterion are the characteristic elongated Outlines of the candidate intrusive domes. We examine how they differ from typical effusive domes of classes 5 and 6 defined by Head and Gifford [Head, J.W., Gifford, A., 1980. Lunar mare domes: classification and modes of origin. Moon Planets 22, 235-257], and show that they are likely no highland kipukas due to the absence of spectral contrast to their surrounding. These considerations serve as a motivation for an analysis of the candidate intrusive domes in terms of the laccolith model by Kerr and Pollard [Kerr, A.D., Pollard, D.D., 1998. Toward more realistic formulations for the analysis of laccoliths. J. Struct. Geol. 20(12), 1783-1793], to estimate the geophysical parameters, especially the intrusion depth and the magma pressure, which would result from the observed morphometric properties. Accordingly, domes of class In1 are characterised by intrusion depths of 2.3-3.5 km and magma pressures between 18 and 29 MPa. For the smaller and steeper domes of class In2 the magma intruded to shallow depths between 0.4 and 1.0 km while the inferred magma pressures range from 3 to 8 MPa. Class In3 domes are similar to those of class In1 with intrusion depths of 1.8-2.7 km and magma pressures of 15-23 MPa. As an extraordinary feature, we describe in some detail the concentric crater Archytas G associated with the intrusive dome At1 and discuss possible modes of origin. In comparison to the candidate intrusive domes, terrestrial laccoliths tend to be smaller, but it remains unclear if this observation is merely a selection effect due to the limited resolution of our telescopic CCD images. An elongated outline is common to many terrestrial laccoliths and the putative lunar laccoliths, while the thickness values measured for terrestrial laccoliths are typically higher than those inferred for lunar laccoliths, but the typical intrusion depths are comparable. C) 2009 Elsevier Inc. All rights reserved.

More than 200 venusian channels and valleys have been mapped based on analyses of Magellan SAR images. Sinuous rilles are the most abundant channels among six types of venusian channels, and they are widely distributed on Venus. Morphological characteristics of venusian sinuous rilles include sinuous narrowing reaches, source depressions, and length of several 10s to a few 100s of km. This type of channels is known to exist on the Moon and possibly on Mars. Valley networks on Venus often occur in the vicinity of or in connection to sinuous rilles. Cross-sectional morphologies of sinuous rilles and valley networks are of special importance in discussing their formation processes both qualitatively and quantitatively. We reconstructed cross-sectional profiles of 6 sinuous rilles and 2 valley networks using a new radar clinometric method. Reconstructed cross-sections revealed that floors of the channels and valleys are clearly lower than the surrounding plains. This finding implies that the sinuous rilles and the valley networks have erosional origins. Longitudinal depth profiles of the sinuous rilles show distinct decreasing trends toward the termini. Such decreasing trends of depths are qualitatively in agreement with theoretical models and laboratory experiments of thermal erosion. In order to verify this assertion quantitatively, we conduct simple 1-dimensional model calculations under the assumption that both channel-forming lavas and ground substrate are tholeiitic basalt. For initial lava thicknesses in the range 2–6 m, the model calculations yield good matches to the depth profiles. Estimated duration of lava effusion ranges from several months to a few years. These numerical results support thermal erosion of the sinuous rilles but do not necessarily exclude contributions from mechanical erosion processes. Valley networks seem to have formed under a strong structural control in comparison to sinuous rilles. The valleys vary widely in characteristics of the depth profile and flow directions relative to surface slopes. Therefore valley networks appear to have originated from diverse formation mechanisms.

Arnus Vallis (AV) is a >300-km-long sinuous, rille located on the northeastern flank of the Syrtis Major volcano on Mars. Observational evidence presented here suggests that AV formed as an open lava channel that was at least partly incised into the pre-existing terrain. The lava source area consists of a sub-circular pit at the southwestern end of a 7-km-long straight section of channel. AV trends down slope from this source with an average bottom slope of 0.26% or 0.14°. Width varies from ∼1 km at the source to ∼0.6 km near the distal end, with a mean of 0.76 km. Depth decreases from ∼180 m at the source to ∼25 m near the distal end. The AV terminus is obscured by a large impact crater. We suggest that the material that flowed in AV must have been a relatively high temperature, low viscosity lava dynamically and perhaps compositionally similar to terrestrial komatiite or some lunar basalt lavas. If correct, this finding has implications for the mode of construction of Syrtis Major.

Valley networks on Venus are classified as rectangular, labyrinthic and pitted, or irregular. The venusian valley networks are structurally controlled, as indicated by the morphological patterns of valley branches, consistency between valley and fracture orientations, and associations with the deformed terrains. The morphologies resemble those of terrestrial and martian sapping valleys. Valley networks on Venus probably formed initially from fracture systems and became enlarged by low viscosity lava sapping processes. Subsurface flow of lava may locally have been assisted by surface flows. The lavas probably moved through permeable media and fractures. Venusian valley networks have a higher degree of network integration than do lunar sinuous rilles, but they are less integrated than martian and terrestrial sapping valleys. The viscosity of valley-forming lavas must have been very low, but was not low enough to exploit the permeable media so extensively as to attain a high degree of network integration. The compositions of these lavas may have been mafic to ultramafic or mafic alkaline. Alternatively, the lavas could have had more exotic compositions, such as carbonatite and sulfur. Valley networks are often associated with corona and corona-like features, which are hypothesized to be the surface expressions of mantle plumes. A plume association could mean that the lavas came from the mantle.

The spatial resolution of image data tends to constrain the horizontal length scale of genetic hypotheses that are addressable by those data. No ‘simple’ formula exists when image resolution is sufficient to test a given geomorphic process, which is dependent on what characteristics are diagnostic of the particular process. Genetic hypotheses should be formulated along the lines of the “multiple working hypotheses” concept as described in a classic paper by Chamberlin [J. Geol. 5 (1897) 837]. An essential element of a viable working hypothesis is a clear indication of the characteristics predicted by, or a consequence of, the hypothesis. An untestable hypothesis is not an effective working hypothesis. The history of the study of lunar sinuous rilles is outlined as an illustration of the influence of image resolution and the formulation of genetic hypotheses on the subsequent advancement of understanding of the problem. Sinuous rilles on Venus and Mars, and controversial sinuous ridges on Mars are also reviewed. In the lunar case, the three-order-of-magnitude improvement in spatial resolution provided by Lunar Orbiter photographs over Earth-based telescopic photographs did not result in definitive examination and elimination of published hypotheses for the formation of sinuous rilles. Topographic data obtained from cartographically controlled Apollo orbital photographs, along with important observations and samples obtained by the astronauts on the lunar surface, did test and exclude several hypotheses. The formulation of a genetic hypothesis, including testable consequences of that hypothesis, is a greater determinant of its ultimate utility to the scientific community than is the image resolution available at any given time.

Among the major discoveries made by planetary missions, channel structures, formed by a wide range of fluids such as water and lavas, have proven to be surprisingly common. Recent identification of gigantic flood channels in continental regions and submarine channels on the continental shelves provide new channel categories to an already rich inventory of terrestrial channels. Sinuous rilles on the Moon, once hypothesized to have formed by water action, now constitute an important lava channel type. Mars has abundant evidence of an ancient warm-wet climate recorded in the channels, including an outflow channel which is the widest (∼200 km) known in the Solar System. Some lava channels observed on the flanks of the huge Martian shield volcanoes seem to have been modified by water. The Magellan radar mission revealed a wide range of volcanism on Venus. Most unusual is the great variety of channels produced by large-scale, low-viscosity lava eruptions. This includes potentially the longest channel in the Solar System (6800 km). Venusian atmospheric conditions alone do not produce sufficient differences in the lavas' cooling rates to explain the formation of many Venusian channels. The origin of these channels must be explained by eruption parameters, probable high discharge rates, high temperatures, long duration, and the possible involvement of very fluid silicates or lavas of exotic composition.

Localized volcanic deposits along the linear rille Rima Parry V are evidence for the presence of near-surface magma and are interpreted to be the result of degassing and minor eruptions subsequent to magma emplacement at shallow depth. Theoretical analyses, together with the observed style of emplacement of lunar mare volcanic deposits, strongly suggest that mare volcanic eruptions are fed by vertical dikes from source regions at the base of the crust or deeper in the lunar mantle. Some dikes intrude into the lower crust, while others penetrate to the surface and are the sources for voluminous outpourings of lava. Still others stall near the surface generating an extensional stress field above the dike top. We investigate the hypothesis that some lunar linear rilles (graben) such as Rima Parry V are the near-surface manifestations of dikes intruded to shallow depths and that they from simultaneously with dike emplacement. For two examples (Rima Parry V and Rima Sirsalis) the geometry of the faults implies dike widths (150 and 600 m) and depths to the dike tops ( 650 and 2400 m) that are consistent with other theoretical and observational data on lunar dike geometry. For the shallower of the two dikes (Rima Parry V), degassing and extrusion have produced pyroclastic cones and related deposits along the central part of the graben.

An updated map is presented which shows the distribution of more than 200 channels and valleys on Venus. A large number of channels are concentrated in equatorial regions characterized by highlands, rift and fracture zones, an associated volcanic features. Many channels associated with flow deposits are similar to typical terrestrial lava drainage channels. They are associated with a wide range of volcanic edifices. More than half of the sinuous rilles are associated with coronae, coronalike features, or arachnoids. Corona volcanism driven by mantle plume events may explain this association. Many valley network are observed in highlands and in association with coronae, coronalike features, or arachnoids. This indicates that highlands and coronae provided fractures and flow-viscosity lavas, both of which seem to be required for network formation by lava sapping processes. Canali-type channels have a unique distribution limited to some plains regions.

The source of the lunar magnetic anomaly associated with the Rima Sirsalis linear rille has been modelled using the vector field intensities due to arbitrary uniform magnetization in a rectangular prism. It is shown that in order to match the Apollo 16 subsatellite data, the lunar surface near the rille must have a vertical magnetization of 6–9 × 10 −3 G if the anomaly is due to flux leakage from a gap in the crust with the dimensions of the rille. This is more than one order of magnitude larger than the magnetization of any lunar sample, but is comparable with the high magnetization recently deduced for the Reiner γ formation in Oceanus Procellarum. An alternative explanation is that Rima Sirsalis and its surroundings are the site of a vertical magnetization contrast of ∼10 −5 – 10 −4 G which is at least as wide as the rille and extends to a depth of tens of kilometers in the crust. A wider magnetic source reduces the required magnetization (or depth) proportionately, since to first order the field at high altitude is proportional to the magnetic dipole moment per unit length.

The products of volcanism on the Earth and Moon differ widely in terms of morphology, distribution, composition, and age. These differences result partly from the different thermal histories of the two bodies and partly through the different controls on volcanic eruption conditions. The controls of volcanism are here separated into three groups: (1) controls which remain constant on any one body but which differ from planet to planet: (2) parameters that are controlled by the rheology of the magma: and (3) controls which are intrinsic properties of individual eruptions and are themselves dependent upon the planetary and rheological variables. In terms of planetary variables it can be predicted that lunar volcanic morphologies are influenced by greater tephra range, hemispherical eruption clouds, lesser lithostatic pressures at corresponding depths, slower cooling, slower erosion, lesser horizontal stress differences, and no features typical of hydrosphere-magma interaction. In terms of rheology, the lunar eruptions had different rock and gas compositions, low yield strengths and viscosities, and high densities. Many lunar basalts appear to have been produced at high eruption rates, to have been derived from monogenetic volcanoes, and to have flowed in a turbulent manner. The weight of available data points to the vast majority of lunar craters being of impact origin though some dark halo craters, sinuous rille source craters, rimless pits, and craters atop domes and cones are undoubtedly of volcanic origin. Other impact craters appear to have been modified by volcanic events. Included within this latter group are the floor-fractured craters and the mare basins. Studies of volcanic morphologies on bodies other than the Earth may help isolate the effectiveness of those controls of volcanism which remain constant on any one body. Although volcanology has advanced substantially within the last few decades, much more detailed integrated scientific research is required before we are able to predict confidently the environmental hazards that result from volcanism and to understand the role of volcanism in planetary evolution.

Variations in modal petrology, mineral compositions, and bulk compositions were determined for ten Apollo 15 regolith breccias for comparison with local soils and assessment of the intrasite petrologic variability of the Apollo 15 regolith. Based on the above criteria the breccias are of local origin and mimic the soils from the corresponding sampling stations, with the exception of station 2 breccia 15205. This sample formed from an anomalous regolith, and although not considered exotic to the site is not representative of the soil at the site. KREEP basalt and green glass components vary from trace amounts to dominant in the breccias, evidence that these materials entered the regolith prior to formation of the breccias. Breccias from the edge of Hadley Rille are modally richer in highland fragments than the soils, whereas at the base of Hadley Delta the reverse is true. This is explained by the loss of material into the Rille to be replaced by basalt-derived material, making the soils more basalt-rich. At the base of Hadley Delta highland material is accumulating and the soils are becoming more highland-rich. Over billions of years these processes have developed differences between the present day, evolving soils, and 'fossil' nonevolving soils represented by the regolith breccias. This shows that there has been little change in the geology and the morphology of the Apollo 15 site, probably since the eruption of mare basalts at the site (about 3.3 b.y.).

High-resolution television pictures obtained by Mariner 9 are presented as evidence to show that the ragged dark streaks which appeared behind many dark craters several months after the end of the 1971 global dust storm resulted from wind erosion of a thin surface veneer consisting of dust-storm fallout. The pictures were taken over an area near the border between Mare Serpentis and Pandorae Fretum (approximately 30 deg S, 335 deg W). The high-resolution pictures are compared with low-resolution views of the same area, and it is shown that one very long dark streak is interrupted by a rille. It is concluded that this interruption proves that the dark streaks were produced by the present erosional mechanism.

Measurements of the magnetic fields by the electron reflection method in the neighborhood of the long structural rille Rima Sirsalis show that a magnetic field with a strength of at least 100 nT (100 gammas) is present over a region of the order of 10 km in width and at least 300 km long. The center of the magnetized region closely parallels and is centered on the rille. The linear magnetization feature extends at least to latitude 8 deg S, 60 km beyond the place where the rille disappears at the edge of Oceanus Procellarum. This extension is coincident with the extrapolation of the rille based on photographs. However, the magnetization is much weaker or entirely absent at 5 deg S and has vanished at 0 deg latitude. These results suggest that the rille is indeed a structural feature and has associated with it magnetization, either in the form of intrusive magnetized rock or of a gap in an otherwise more or less uniformly magnetized layer of rock of large extent in two dimensions. Furthermore, the rille structure evidently is present for some distance beneath the lava flows of the Oceanus Procellarum basin.

Lava tubes and channels develop around active sources of low viscosity lava. The channels normally form without erosion; however, sustained flow can result in the incision of a lava channel and simulation of fluvial erosion features. Lava erosion by means of thermal incision was modelled by computer, erosion rates calculated, and these compared with rates observed terrestrially. Lunar sinuous rilles are examined in light of the proposed lava erosion. The mechanism explains many features of lunar rilles that were heretofore puzzling and implies erosion rates comparable to terrestrial rates. Many Mars channels also appear to form by the action of lava; however, the larger, more spectacular Mars channels do not appear to have been formed by the same process.

The paper describes a new method of detecting lunar surface magnetic fields, summarizes electron reflection measurements and correlations of surface field anomalies to moon geologic features, and discusses the constraints on the origin of lunar magnetism. Apollo 15 and 16 measurements of lunar surface magnetic fields by the electron reflection method show patches of strong surface fields distributed over the lunar surface, and a positive statistical correlation is found in lunar mare regions between the surface field strength and the geologic age of the surface. However, there is a lack of correlation of surface field with impact craters indicating that the mare does not have a strong large-scale uniform magnetization as may be expected from an ancient lunar dynamo. Fields were found in lunar highlands which imply that the rille has a strong magnetization associated with it as intrusive, magnetized rock or as a gap in a uniformly magnetic layer of rock.

The lunar “mascons” appear to be too big to be caused primarily by the infalling bodies which created the ringed maria. Because these maria are topographic depressions, any other possible causes require that the moon have differentiated a crust prior to creation of the ringed maria. The mechanism of mass transfer which appears most consistent with the low level of geologic activity is the generation of excess pressures consequent upon the more rapid cooling of the outer parts of the moon. These pressures created the mass concentrations primarily by pushing the dense root past isostatic equilibrium in the weak area caused by the impacting body. The morphological evidences of superficial loading indicate, however, that there may also have been lava flows or sedimentation. In addition to mass transfer, there also could have operated mechanisms of densification, the most significant of which appears to be the outgassing of water after the impact, which resulted in the ringed maria not being serpentinized to the same extent as other parts of the moon. Higher thermal conductivity of the mare material may also lead to a lower temperature gradient and hence some increase in density due to thermal contraction. This dehydration of the ringed maria is most strongly indicated by the distribution of sinous rilles. At present, the moon is essentially rigid to at least 160 km depth, and even more if magnetometry in the moon's wake is correctly interpreted as indicating temperatures below 700 °C. Hence the mechanism of support of the mass concentrations since their creation can be pure elastic strength, in contrast to the earth.

An internal Rb-Sr isochron for the large basalt boulder 15555 returned from the edge of Hadley rille by the Apollo 15 mission yields an age3.32 ± 0.06AE and an initial88Sr/86Sr,I = 0.69934 ± 5. This age andI value fall well within the range obtained for the Apollo 12 basalts from the Ocean of Storms and may indicate that extensive lava flows occurred at ∼ 3.3 AE over widespread areas of the moon. The Sr composition of the anorthosite 15415 is as low as that of plagioclase extracted from the Apollo 11 low K rocks. The initial Sr composition of 15415 for anassumed age of 3.3 to 4.6 AE is extremely primitive and provides further evidence for an extremely short formation interval (3 to 1 × 106 yr) of a nonchondritic moon with respect to an origin in time defined by BABI. The initial87Sr/86Sr for crystalline rocks returned from all lunar missions is correlated with the concentration of Rb and correspondingly K, U and Th. This correlation places distinctive constraints on the evolution of lunar magmas and the internal structure of the moon.

A model of Mare Humorum is presented as a working hypothesis. The model was deduced by combining theories and observations of the origin of the vittae (rays), rilles, wrinkle ridges, and faults. In this model, Mare Humorum consists of a basin which subsided by tilting and normal faulting. The tilting is deduced from the observation of the partial flooding of Gassendi and Dopplemeyer, on the north and south shores of the Mare, respectively. The faulting is deduced from the location of several linear features and the vittae, which are interpreted as being loci of sublimate deposition along infra-mare faults. The basin is filled by volcanic material which behaves incompetently with respect to the basement. Wrinkle ridges are features of the volcanic cover due to compression resulting from the subsidence. Rilles are features of the competent basement material, due to tension stresses occurring at the hinge areas of the subsiding basin and may be grabens and/or superficial dikes. The crustal weakness responsible for Mare Humorum could have originated by impact, but endogenous tectonism is the primary factor responsible for the present structure.

In situ measurements of lunar surface brightness temperatures made as a part of the Apollo Lunar Surface Experiments Package at the Apollo 15 Hadley Rille landing site are reported. Data derived from five thermocouples of the Heat Flow Experiment, which are lying on or just above the surface, are used to examine the thermal properties of the upper 15 cm of the lunar regolith using eclipse and nighttime cool-down temperatures. Application of finite-difference techniques in modeling the lunar soil shows that the thermocouple data are best fit by a model consisting of a low-density and low-thermal conductivity surface layer approximately 2 cm thick overlying a region increasing in conductivity and density with depth. Conductivities on the order of 0.00001 W per cm per deg K are postulated for the upper layer, with conductivity increasing to the order of 0.0001 W per cm per deg K at depths exceeding 20 cm. An increase in mean temperature with depth indicates that the ratio of radiative to conductive transfer at 350 K is 2.7 for at least the upper few centimeters of lunar soil; this value is nearly twice that measured for returned lunar fines.

The lunar rilles, maria ridges, and maria domes are spacially associated with low areas of the lunar surface,the maria, and immediately adjacent terrae. The spacial association implies a genetic relationship. The morphology and structural setting of most of the rilles suggests that they are tectonic features of tensional origin produced by downward bending of maria borders during a stage of maria foundering. The domes and ridges were most likely produced by small central eruptions or by laccolithic intrusions. Most of the ridges appear to be volcanic structures which grew above dike feeders during the last stages of maria filling. Age relationships and structural settings of the rilles and ridges further suggest that the formation of these features took place in sequence, ridges and domes followed by rilles. It is suggested here that this sequence of events occurred repeatedly during a long history of maria growth and that the maria grew to their present sizes by successive stages of volcanic inundation and collapse. Circular maria probably resulted from a simple enlargement of initially circular depressions whereas the irregularly shaped maria resulted from a coalescence of two or more growing basins or depressions.

He, Ne, Ar, Kr and Xe were measured mass spectrometrically in eight bulk fines and one sample of 2–4 mm fines (15603) from stations LM-ALSEP, 2, 6, 6 1 , 7 a , and 9. We have also measured these gases in size fractions of samples 15091 and 15601. These samples come from three morphologically distinct selenographic settings: the Appenine Front, the Mare and Hadley Rille. Measured gas contents in these samples are comparable to those from previous Apollo missions. He 4 and the other trapped gases are inversely proportional to grain diameter in the size fractions of 15091 and 15601. More than 90 per cent of the trapped gas in 15601 is surface correlated; hence is probably directly implanted solar wind. Size fractions of 15091 contain large volume correlated amounts of He 4 , Ne 20 and Ar 36 . He 4 /Ne 20 ranges from about 20 to 60; Ne 20 /Ar 36 from 5 to 8: Ar 36 /Kr 84 from 2400 to 3200; and Kr 84 /Xe 132 from 3·2 to 7·3. The lowest He 4 /Ne 20 ratios occur in samples rich in green glass spherules which have He 4 /Ne 20 ≤ 10 ( Lakatos , Heymann , and Yaniv , unpublished). He 4 /He 3 ranges from about 210 to 2700; Ne 20 /Ne 22 from 12·7 to 13·2; Ne 21 /Ne 22 from 0·035 to 0·041; and Ar 36 /Ar 38 from 5·26 to 5·45. The measured Ar 40 /Ar 36 ratios range from 0·757 to 3·56; when corrected for radiogenic Ar 40 , the range becomes 0·6 to 3·4. The largest corrected Ar 40 /Ar 36 ratios occur in samples from the Apennine Front, the smallest occur in the Mare. This could be due to slope effects between the front as opposed to the mare terrain. An alternative possibility is that the Front fines acquired their atmospheric Ar 40 at a time when the concentration of neutral Ar 40 in the lunar atmosphere was relatively large. Ne 21 radiation ages were calculated for all samples. There is evidence in the landing area for debris from craters with ages less than 100 × 10 6 yr, but these craters cannot be firmly identified from the data.

Depth measurements of loose (or loosely bonded) material are obtained for the three impact areas of Ranger VII, VIII and IX by making use of rilles or multiple dimple formations assumed to be caused by loose material draining or crumbling into underground cavities.

An introduction to the study of the moon is presented from a geological point of view. Lunar physiography, stratigraphy and tectonics are described and discussed. The most important physiographic features are craters, terrae, maria, rilles, wrinkle ridges, rays, haloes, mountain ranges and alpine valleys. The surface of the moon is generally smooth or gently sloping. Steep slopes are common, but occupy a small part of the total area. A very porous layer covering essentially all of the exposed surface to an unknown depth is structurally very complex and rough at a scale of less than one centimetre. This layer is perhaps the result of ‘space weathering’; that is, bombardment by micrometeorites and subatomic particles. Geologic processes acting on the moon include the just-mentioned ‘space weathering’, meteoritic impact, volcanism, transportation and sedimentation accomplished by ejection from impact craters in a ballistic trajectory, possible electrostatic transportation and sedimentation, tectonism, and perhaps others still unknown. The stratigraphy of the moon has been determined by applying the law of superposition to lunar terranes. The stratigraphic column, beginning with the lowest unit, consists of the Pre-Imbrian, Imbrian, Procellarian, Eratosthenian and Copernican Systems. By measuring the number of craters per unit area it is possible to determine the relative ages of exposed terranes. Absolute dating and even interplanetary correlation with the earth's geological column have been attempted. Lunar tectonics are complicated by the lack of an easy method for measuring the equipotential surface, defined on the earth by the oceanic surface. The moon apparently shows a bulge which points toward the earth, but whether this bulge is in isostatic equilibrium or not is still unknown. Endogeneous features on the surface of the moon include rilles, wrinkle ridges, faults, aligned craters, domes, and perhaps isolated volcanic craters. The future of lunar exploration is discussed. The Ranger, Surveyor and Apollo Programmes of the U.S.A. are presented.