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Lunar soils are discussed within the framework of a three-group classification scheme. Group I comprises North Ray Crater soils, group II contains light soils, and group III is made up of dark soils. It is suggested that group I soils originated from the light friable unit, one of the three units inside North Ray Crater. Group II soils were probably derived from the light matrix breccia unit; group II soils are mixtures of materials from all three units. It is concluded that soils with group III properties have been at the surface continuously for long periods. Group II soils show a record of solar wind exposure in the distant past (i.e., they have large (Ar-40/Ar-36)T ratios). Thus the regolith at the Apollo 16 site, which was observed to have marked layers of dark against light soils, contains sizeable 'pockets' or horizons at depth which are the sources of the group II soils.

A catalog of crater dimensions that were compiled mostly from the new Apollo-based Lunar Topographic Orthophotomaps is presented in its entirety. Values of crater diameter, depth, rim height, flank width, circularity, and floor diameter (where applicable) are tabulated for a sample of 484 craters on the Moon and 22 craters on Earth. Systematic techniques of mensuration are detailed. The lunar craters range in size from 400 m to 300 km across and include primary impact craters of the main sequence, secondary impact craters, craterlets atop domes and cones, and dark-halo craters. The terrestrial craters are between 10 m and 22.5 km in diameter and were formed by meteorite impact.

Sixteen samples of Boulder 1 from Station 2 at the Apollo 17 site were analyzed by radiochemical neutron activation analysis for Ag, Au, Bi, Br, Cd, Cs, Ge, Ir, Ni, Rb, Re, Sb, Te, Tl, U, and Zn. Two clast samples contam no meteoritic material and appear to consist of relatively pristine igneous rocks: an unusual, KREEP-rich pigeonite basalt of very high Ge content, and an alkali-poor coarse norite. Nine grey or black breccia samples contain a unique, Group 3 meteoritic component of Ir/Au ratio 0.65–0.82, which appears to separate into subgroups 3H and 3L on the basis of Ni, Ge, and Re content. It is quite distinct from the Group 2 component (Ir/Au - 0.46–0.54) that dominates at the Apollo 17 site. The unique black-rimmed clasts from this boulder show striking compositional zoning. The cores of anorthositic breccia are very low in Rb, Cs, and U, and have a distinctive 5L meteoritic component (Ir/Au≈1.1). The black rinds are 5- to 10-fold richer in Rb, Cs, and U and have a Group 3 meteoritic component. The cores may represent breccias formed in an earlier impact that became coated with alkali-rich ejecta during the event that produced the boulder. Because of the rarity of the Group 3 meteoritic component at the Apollo 17 site, this boulder cannot represent ordinary Serenitatis ejecta, with their characteristic admixture of the Group 2 Serenitatis projectile. It may represent pre-Serenitatis material excavated from the fringes of the crater during late stages of the Serenitatis impact, but only lightly shocked and hence uncontaminated by the Serenitatis projectile.

Grooved and hilly terrains occur at the antipode of major basins on the Moon (Imbrium, Orientale) and Mercury (Caloris). Such terrains may represent extensive landslides and surface disruption produced by impact-generatedP-waves and antipodal convergence of surface waves. Order-of-magnitude calculations for an Imbrium-size impact (1034 erg) on the Moon indicateP-wave-induced surface displacements of 10 m at the basin antipode that would arrive prior to secondary ejecta. Comparable surface waves would arrive subsequent to secondary ejecta impacts beyond 103 km and would increase in magnitude as they converge at the antipode. Other seismically induced surface features include: subdued, furrowed crater walls produced by landslides and concomitant secondary impacts; emplacement and leveling of light plains units owing to seismically induced ‘fluidization’ of slide material; knobby, pitted terrain around old basins from enhancement of seismic waves in ancient ejecta blankets; and perhaps the production and enhancement of deep-seated fractures that led to the concentration of farside lunar maria in the Apollo-Ingenii region.

Between 1000 and 2000 infrared (eclipse) and radar anomalies have been mapped on the nearside hemisphere of the Moon. A study of 52 of these anomalies indicates that most are related to impact craters and that the nature of the infrared and radar responses is compatible with a previously developed geologic model of crater aging processes. The youngest craters are pronounced thermal and radar anomalies; that is, they have enhanced eclipse temperatures and are strong radar scatterers. With increasing crater age, the associated thermal and radar responses become progressively less noticeable until they assume values for the average lunar surface. The last type of anomaly to disappear is radar enhancement at longer wavelengths. A few craters, however, have infrared and radar behaviors not predicted by the aging model. One previously unknown feature - a field strewn with centimeter-sized rock fragments - has been identified by this technique of comparing maps at the infrared, radar, and visual wavelengths.

The crater populations of 18 lunar light plains (Cayley plains) show a variation in relative ages by a factor of about 4 in crater frequency in regions in the surroundings of the Orientale and Imbrium basin, and by a factor of greater than 25 for more distant sites. Thus the idea of a moon-wide synchronism in the emplacement of the lunar light plains with the formation of the basins Imbrium or Orientale cannot be supported. Some light plains are younger than the youngest basin Orientale. Since these plains cannot have been emplaced by any other basin-forming event and local impact-derived origin can certainly be excluded, an endogenic (magmatic) origin is proposed for these plains. Age determination data (D sub L values) by Soderblom and Lebofsky (1972) and Soderblom and Boyce (1972) are shown to be correlated with own cumulative crater frequency data (N) for surfaces younger than about 3.8 b.y. It is found that D sub L is proportional to the 0.6 power of N. For ages greater than 3.8 b.y., the D sub L data by those authors, especially their light plains data, are incompatible with the present crater frequency data.

Dense Doppler tracking coverage of the Apollo 15 and 16 subsatellites over ten and eighteen day periods when periapsis altitudes were 15-50 km has provided detailed gravity mapping of the lunar frontside. Many new gravity features are revealed, including one that does not correlate with any visible topographic structure. All unfilled craters sampled are negative anomalies. The mascons consistently produce gravity heights that load the surface with about 800 kg/sq cm excess mass. The Orientale region is represented with a solution grid of 177 point masses that clearly show the ringed structure. The eastern limb is also displayed with a solution grid of point masses. The gravity variations over the central portion of the frontface are shown as line-of-sight acceleration contours in milligals.

The Imbrium sculpture texture, a distinctive ridged and furrowed pattern radial to the Imbrium basin and seen in other basins, has long been debated as to its origin (internal, formed by basin-related fractures; external, related to ejecta patterns). To test for the presence of deep radial fractures on the moon, the azimuth and length of linear rim segments of twenty-four post-Imbrium-basin craters were measured. Linear segments of crater rims parallel preexisting fracture patterns in terrestrial craters floored in an indurated substrate. Craters forming in a terrain containing pervasive fractures radial to Imbrium should show evidence of this tectonic influence by forming rim crest segments (terrace scarps) preferentially along these directions. No systematic relation of these segments with Imbrium radial structure was found. This suggests that the surface radial grooves may not extend to depth. The relatively young Orientale basin shows two types of radial structures: (1) pervasive subparallel ridges and furrows formed by a spectrum of sizes of secondary crater chains emanating from the main crater, and from flowage of material during secondary cratering; (2) parallel, generally radial ridges which appear to have formed on top of outward flows of debris. These types of radial textures (both depositional and erosional) appear unrelated to major faults or fractures. Therefore, these two lines of evidence suggest that much of the Imbrium-type sculpture surrounding major lunar basins is sedimentary, rather than tectonic, in origin.

This investigation uses results obtained in an earlier Monte Carlo study by Oberbeck et al. (1973) to shed light on the consequences of mare-regolith origin by impact comminution of primarily local basaltic rocks by evaluating regolith growth and mixing as a function of time. The study demonstrates that regolith growth is self-regulated and has the same trend and nearly the same terminal growth rates whatever the history of bombardment: rapid initial accumulation followed by diminishing rates of growth. Mixing and all other processes investigated are growth regulated. Mixing increases as growth slows, but never to the extent that the regolith is homogenized. Because the average regolith is never homogenized, products of growth-regulated processes are preserved in the stratigraphy. This model indicates that deeper levels contain thinner depositional units, lesser quantities of meteoritic and exotic components, and more debris derived from shallow levels in the mare basalts than material in near-surface layers.

Many lunar craters greater than 10 km in diam exhibit a variety of morphological characteristics which are not produced by meteorite impact or meteorite erosion. Most such craters are located in or near the margins of the maria. Although some could have resulted from processes such as cauldron resurgence, caldera formation, or ring dike emplacement, most have formed by modification of impact craters by endogenic processes including erosion by flowing lava, fissure volcanism, plutonism and uplift of crater floors along ring fractures of impact origin.

Discrepancies in published crater frequency data prompted this study of lunar crater distributions. Effects modifying production size distributions of impact craters such as surface lava flows, blanketing by ejecta, superposition, infilling, and abrasion of craters, mass wasting, and the contribution of secondary and volcanic craters are discussed. The resulting criteria have been applied in the determination of the size distributions of unmodified impact crater populations in selected lunar regions of different ages. The measured cumulative crater frequencies are used to obtain a general calibration size distribution curve by a normalization procedure. It is found that the lunar impact crater size distribution is largely constant in the size range 0.3 km ⩽D ⩽ 20 km for regions with formation ages between ≈ 3 × 109 yr and ≳ 4 × 109 yr. A polynomial of 4th degree, valid in the size range 0.8 km ⩽D ⩽ 20 km, and a polynomial of 7th degree, valid in the size range 0.3 km ⩽D ⩽ ⩽ 20 km, have been approximated to the logarithm of the cumulative crater frequencyN as a function of the logarithm of crater diameterD. The resulting relationship can be expressed asN ∼D α(D) where α is a function depending onD. This relationship allows the comparison of crater frequencies in different size ranges. Exponential relationships with constant α, commonly used in the literature, are shown to inadequately approximate the lunar impact crater size distribution. Deviations of measured size distributions from the calibration distribution are strongly suggestive of the existence of processes having modified the primary impact crater population.

Before the Apollo 16 mission, the material of the Cayley Formation (a lunar smooth plains) was theorized to be of volcanic origin. Because Apollo 16 did not verify such interpretations, various theories have been published that consider the material to be ejecta of distant multiringed basins. Results presented in this paper indicate that the material cannot be solely basin ejecta. If smoothplains are a result of formation of these basins or other distant large craters, then the plains materials are mainly ejecta of secondary craters of these basins or craters with only minor contributions of primary-crater or basin ejecta. This hypothesis is based on synthesis of knowledge of the mechanics of ejection of material from impact craters, photogeologic evidence, remote measurements of surface chemistry, and petrology of lunar samples. Observations, simulations, and calculations presented in this paper show that ejecta thrown beyond the continuous deposits of large lunar craters produce secondary-impact craters that excavate and deposit masses of local material equal to multiples of that of the primary crater ejecta deposited at the same place. Therefore, the main influence of a large cratering event on terrain at great distances from such a crater is one of deposition of more material by secondary craters, rather than deposition of ejecta from the large crater. Examples of numerous secondary craters observed in and around the Cayley Formation and other smooth plains are presented. Evidence is given for significant lateral transport of highland debris by ejection from secondary craters and by landslides triggered by secondary impact. Primary-crater ejecta can be a significant fraction of a deposit emplaced by an impact crater only if the primary crater is nearby. Other proposed mechanisms for emplacement of smooth-plains formations are discussed, and implications regarding the origin of material in the continuous aprons surrounding large lunar craters is considered. It is emphasized that the importance of secondary-impact cratering in the highlands has in general been underestimated and that this process must have been important in the evolution of the lunar surface.

New detailed gravity measurements were obtained over a 10- to 70-km surface strip from − 70° to + 70° long. during low-altitude orbits (≈ 12 km at periapsis). The trajectory path (Figure 1) went over the centers of both Maria Serenitatis and Crisium, providing a complete center gravity profile of two large mascons. Consistent with the previous results for Mare Nectaris and Mare Humorum, both Serenitatis and Crisium mascons are approximately disk-shaped near-surface mass anomalies of net uncompensated loading, 800 kg cm−2. This strengthens Booker's contention that all mascons are approximately the same thickness. Also revealed for the first time are significant positive gravity measurements over mountain ranges - Apennines (near Hadley Mountain) and the Marius Hills. The data suggests that the Apennines have undergone some isostatic compensation, whereas the Marius Hills have not. The crater anomalies detected are all consistently negative as observed before, implying loss of mass from the impact event which formed them.

Microcraters were formed in heated soda-lime glass by the normal incidence of spheres of plastic or fused silica with diameters between 0.8 and 4.5μm and velocities between 2.5 and 10 km s−1. The morphology of the craters in targets at temperatures up to 800°C is little different from those formed in unheated glass. Spallation still occurs to the same extent and above the same velocity threshold, but the spalls sag and sharp edges become dull in a few seconds at temperatures above the softening point. There is a small increase in the flow of glass from the central pit into a narrow lip at the higher temperatures, but this lip is often removed by spallation, especially at the higher velocities of impact. There is no evidence of a splashed lip with strings of melt overlying the spalled area. The results in conjunction with other evidence suggest that most lunar craters of micrometer size with a smooth central pit, splashed lip, and a spallation zone are the result of primary impacts.

The lunar Orientale basin is a 900 km diam circular topographic depression covering an area of over 700,000 sq km on the western limb of the moon. Three major rings surround the central Mare Orientale. Orientale basin structures are considered along with Orientale basin deposits and the sequence of formation of structures and deposits. It is found that the structures and facies are related in time and mode of origin to the formation of a major impact crater approximately 620 km in diam. The study suggests that the Orientale basin configuration is very nearly the same as its geometry at its time of formation. The formation of multiringed basins such as Orientale provides a mechanism for an instantaneous production of tremendous volumes of melted lunar crystal material.

A thermal and mechanical history of lunar gabbroic anorthosite 15418 (1140g) has been deduced from petrographic examination of both exterior and interior thin sections and electron microprobe analysis and transmission electron microscopy of interior thin sections. We suggest that the rock underwent two major shock events - an early brecciation and annealing that produced a recrystallized breccia, followed by a second shock event that melted the surface of the rock, vitrified the interior plagioclase and heavily deformed the mafic phases. This latter shock even was also followed by annealing which crystallized the shock-produced glass and promoted recovery and recrystallization of the deformed crystalline phases. The complex mechanical and thermal history of 15418 compared with other ANT suite rocks at Spur Crater suggests that it had a different provenance.

Numerous lunar craters (206 examples, mean diameter = 40km) contain pronounced floor rilles (fractures) and evidence for volcanic processes. Seven morphologic classes have been defined according to floor depth and the appearance of the floor, wall, and rim zones. Such craters containing central peaks exhibit peak heights (approximately 1km) comparable to those within well-preserved impact craters but exhibit smaller rim-peak elevation differences (generally 0–1.5km) than those (2.4km) within impact craters. In addition, the morphology, spatial distribution, and floor elevation data reveal a probable genetic association with the maria and suggest that a large number of floor-fractured craters represent pre-mare impact craters whose floors have been lifted tectonically and modified volcanically during the epochs of mare flooding. Floor uplift is envisioned as floating on an intruded sill, and estimates of the buoyed floor thickness are consistent with the inferred depth of brecciation beneath impact craters, a zone interpreted as a trap for the intruding magma. The derived model of crater modification accounts for (1) the large differences in affected crater size and age; (2) the small peak-rim elevation differences; (3) remnant central peaks within mare-flooded craters and ringed plains; (4) ridged and flat-topped rim profiles of heavily modified craters and ringed plains; and (5) the absence of positive gravity anomalies in most floor-fractured craters and some large mare-filled craters. One of the seven morphologic classes, however, displays a significantly smaller mean size, larger distances from the maria, and distinctive morphology relative to the other six classes. The distinctive morphology is attributed, in part, to the relatively small size of the affected crater, but certain members of this class represent a style of volcanism unrelated to the maria - perhaps triggered by the last major basin-forming impacts.

Analysis of terrain in the Apollo 16 Descartes landing region shows a series of features that form a stratigraphic sequence which dominates the history and petrogenesis at the site. An ancient 150 km diam crater centered on the Apollo 16 site is one of the earliest recognizable major structures. Nectaris ejecta was concentrated in a regional low at the base of the back slope of the Nectaris basin to form the Descartes Mountains. Subsequently, a 60 km diam crater formed in the Descartes Mountains centered about 25 km to the west of the site. This crater dominates the geology and petrogenetic history of the site. Stone and Smoky Mountains represent the degraded terraced crater walls, and the dark matrix breccias and metaclastic rocks derived from North and South Ray craters represent floor fallback breccias from this cratering event. Subsequent major cratering occurred in the region (Dollond B, etc.) prior to the Imbrium and Orientale basin-forming events but had minor effect on the site. Based on this interpretation, contributions from Imbrium at the Apollo 16 site are minor and those from Orientale negligible. The petrology of the Apollo 16 rocks supports this stratigraphic and process model of a local crater-dominated history for this region.

Cosmic-ray exposure ages of lunar samples have been used to date surface features related to impact cratering and downslope movement of material. Only when multiple samples related to a feature have the same rare-gas exposure age or when a single sample has the same Kr-81 -Kr and track-exposure age can a feature be considered as reliably dated. Based on these criteria, there are only five well-dated lunar features: Cone Crater (Apollo 14), 26 m.y,; North Ray Crater (Apollo 16), 50 m.y.; South Ray Crater (Apollo 16), 2 m.y.; the emplacement of the Station 6 boulders (Apollo 17), 22 m.y.; and the emplacement of the Station 7 boulder (Apollo 17), 28 m.y. Other features are tentatively dated or have limits set on their ages: Bench Crater (Apollo 12), upper limit of 99 m.y.; Baby Ray Crater (Apollo 16), upper limit of 2 m.y.; Shorty Crater (Apollo 17), approximately 30 m.y.; Camelot Crater (Apollo 17) upper limit of 140 m.y.; the emplacement of the Station 2 boulder 1 (Apollo 17), 45 to 55 m.y.; and the slide which generated the light mantle (Apollo 17), lower limit of 50 m.y.

New lunar soils, freshly deposited as impact ejecta, evolve into more mature soils by a complex set of processes involving both near-surface effects and mixing. Poor vertical mixing statistics and interregional exchange by impact ejection complicate the interpretation of soil maturization. Impact ejecta systematics are developed for the smaller cratering events which, with cumulative crater populations observed in young mare regions and on Copernicus ejecta fields, yield rates and a range distribution for the horizontal transport of material by impact processes. The deposition rate for material originating more than 1 m away is found to be about 8 mm m.y.−1 Material from 10 km away accumulates at a rate of about 0.08 mm m.y.−1, providing a steady influx of foreign material. From the degradation of boulder tracks, a rate of 5±3 cm m.y.−1 is computed for the filling of shallow lunar depressions on slopes. Mass wastage and downslope movement of bedrock outcroppings on Hadley Rille seems to be proceeding at a rate of about 8 mm m.y.−1 The Camelot profile is suggestive of a secondary impact feature.

Craters and associated aeolian features visible on Mariner 9 photographs have been examined. An automated system used to gather, reduce, and store data may have application to other types of planet-wide geological studies. A brief description of the method used to accumulate data is presented, along with some preliminary results for A-frames.

The region including the Aristarchus Plateau and Montes Harbinger is probably the most diverse, geologically, of any area of comparble size on the Moon. This part of the northwest quadrant of the lunar near side includes unique dark mantling material; both the densest concentration and the largest of the sinuous rilles; apparent volcanic vents, sinks, and domes; mare materials of various ages and colors; one of the freshest large craters (Aristarchus) with ejecta having unique colors and albedos; and three other large craters in different states of flooding and degradation (krieger, Herodotus, and Prinz). The three best-authenticated lunar transient phenomena were also observed here. This study is based principally on photographic and remote sensing observations made from Earth and Apollo orbiting space craft. Results include (1) delineation of geologic map units and their stratigraphic relationships; (2) discussion of the complex interrelationships between materials of volcanic and impact origin, including the effects of excavation, redistribution and mixing of previously deposited materials by younger impact craters; (3) deduction of physical and chemical properties of certain of the geologic units, based on both the remote-sensing information and on extrapolation of Apollo data to this area; and (4) development of a detailed geologic history of the region, outlining the probable sequence of events that resulted in its present appearance. A primary concern of the investigation has been anomalous red dark mantle on the Plateau. Based on an integration of Earth- and lunar orbit-based data, this layer seems to consist of fine-grained, block-free material containing a relatively large fraction of orange glass. It is probably of pyroclastic origin, laid down at some time during the Imbrian period of mare flooding.

A model for shock-lithification of terrestrial and lunar regolith is proposed that accounts for: (1) observed petrographic properties and densities of shock-lithified material from missile impact craters at White Sands, New Mexico and from Meteor Crater, Arizona; (2) observed petrographic textures of lunar soil and lunar soil analogues experimentally shocked to known pressures in laboratory experiments; (3) theoretical calculations of the behavior of air and water under shock compression; and (4) measured Hugoniot and release adiabat data on dry and wet terrestrial soils and lunar regolith. In this model it is proposed that air or an air-water mixture initially in the pores of terrestrial soil affects the behavior of the soil-air-water system under shock-loading. Shock-lithified rocks found at Meteor Crater are classified as ‘strongly lithified’ and ‘weakly lithified’ on the basis of their strength in hand specimen; only weakly lithified rocks are found at the missile impact craters. These qualitative strength properties are related to the mechanisms of bonding in the rocks. The densities of weakly lithified samples are directly related to the pressures to which they were shock-loaded. A comparison of the petrographic textures and densities of weakly lithified samples with textures and densities of ‘regolith’ shock-loaded to known pressures suggests that weakly lithified terrestrial samples formed at pressures well under 100 kb, probably under 50 kb. If terrestrial soils are shock-loaded to pressures between 100 and 200 kb by impact events of short duration, the pore pressure due to hot air or air-water mixtures exceeds the strength of the weak lithification mechanisms and fragmentation, rather than lithification, occurs. At pressures above 200 kb, lithification can occur because the formation of glass provides a lithification mechanism which has sufficient strength to withstand the pore pressure. During shock-lithification of lunar regolith at pressures below 50 kb, the material is compressed to intrinsic crystal density and remains at approximately that density upon release from the shocked state. It is proposed, however, that at pressures in excess of 50 kb, the release of trapped volatiles from lunar soil grains into fractures causes an expansion of the regolith during unloading from the shocked state.

An analysis has been made of the tendency of large lunar craters to lie along circles. A catalog of the craters at least 50 km in diameter was prepared first, noting position, diameter, rim sharpness and completion, nature of underlying surface, and geological age. The subset of those craters 50-400 km in diameter was then used as input to computer programs which identified each 'family' of four or more craters of selected geological age lying on a circular arc. For comparison, families were also identified for randomized crater models in which the crater spatial density was matched to that on the moon, either overall or separately for mare and highland areas. The observed frequency of lunar arcuate families was statistically highly significantly greater than for the randomized models, for craters classified as either late-pre-Imbrian (Nectarian), middle pre-Imbrian, or early pre-Imbrian, as well as for a number of larger age-classes. The lunar families tend to center in specific areas of the moon; these lie in highlands rather than maria and are different for families of Nectarian craters than for pre-Nectarian. The origin of the arcuate crater groupings is not understood.

The Apollo 17 ALSE (Apollo Lunar Sounder Experiment) VHF radar provided imagery and continuous profiling data around the moon during two revolutions. The imagery data are used to derive depth and diameter measurements of small craters (diameters less than 30 km). The profiling data are used to study the topography of a few large craters: the bulged floors in Hevelius, Neper, and Aitken; central peaks in Neper and Buisson; and the depressed floor of Maraldi. The same data provided accurate (better than 25 m) profiles of Mare Crisium and Mare Serenitatis.

Material is ejected from impact craters in ballastic trajectories; it impacts first near the crater rim and then at progressively greater ranges. Ejecta from craters smaller than approximately 1 km is laid predominantly on top of the surrounding surface. With increasing crater size, however, more and more surrounding surface will be penetrated by secondary cratering action and these preexisting materials will be mixed with primary crater ejecta. Ejecta from large craters and especially basin forming events not only excavate preexisting, local materials, but also are capable of moving large amounts of material away from the crater. Thus mixing and lateral transport give rise to continuous deposits that contain materials from within and outside the primary crater. As a consequence ejecta of basins and large highland craters have eroded and mixed highland materials throughout geologic time and deposited them in depressions inside and between older crater structures.

The lunar cratering rate is known reasonably well from comparison of observed crater frequencies with radiometric ages. Attempts to obtain a cratering rate for Mars have usually been based on calculation of the relative flux of asteroidal and cometary bodies on Mars and the Moon. The asteroidal flux on Mars cannot be obtained in a simple way from the observed number of Mars-crossing asteroids, i.e. those asteroids with perihelia within the orbit of Mars. Calculations of the secular perturbations of these asteroids by several authors, particularly williams, has shown that most of these bodies rarely come near even to Mars' aphelion when they are in the vicinity of the ecliptic plane, and their contribution to the Martian meteoroid flux is much smaller than has been commonly stated. Ring asteroids in the vicinity of the ‘secular resonances’ discovered by Williams, high velocity fragments of asteroids on the inner edge of the asteroid belt, and possibly objects obtained from the 2:1 Kirkwood gap by a process described by Zimmerman and Wetherill are probably of greater importance in the 103-106 g meteoroid size range but are much less important in the production of large craters. Calculations of the Martian asteroidal and cometary impact rate are made, but the present unavoidable uncertainties in the results of these calculations result in their being of little value in establishing a Martian chronology. Suggestions for improving this situation are discussed.

Evidence for very recent emission of volatiles on the Moon is primarily of four types: (1) transient lunar optical events observed by Earth-based astronomers; (2) excursions on Apollo SIDE and mass spectrometer instruments; (3) localized Rn222/Po210 enhancements on the lunar surface detected by Apollo 15 and 16 orbital alpha spectrometers; (4) presence in lunar fines of retrapped Ar40 and other volatiles. Available evidence indicates that the release rate of volatile substances into the lunar atmosphere is not steady, but instead sporadic and episodic. Rn222/Po210 anomalies are at locations that are among those from which transient events have most often been reported (edges of maria, certain specific craters), and are probably related to them. Volatiles emitted at maria rims may originate in the Moon's fluid core, reaching the surface through deep cylindrical fault systems that ring the maria borders. The sources of volatiles emitted at craters such as Aristarchus or Tsiolkovsky, which possess floors which are cracked or filled with dark lava and possess central peaks, are more likely to be local pockets of magma or trapped gas at shallower depths. The volatiles are produced directly by radioactive decay (He4, Ar40, Rn) and by heating (other volatiles). The release by heating can occur either during melting or by ‘bakeout’ of unmelted materials. Release of gas into the lunar atmosphere is probably triggered by buildup of its own pressure. This may be assisted by tidal forces exerted on the Moon by the Earth. In addition to independent release, volatile emission is also expected to accompany other lunar activity, such as ash flows, if any lunar volcanism is presently active.

This paper considers the origin of certain tongues of lava-like material in Chenier Crater, a meteorite crater located about 63 km northeast of the major crater, Tsiolkovsky, on the lunar far side. The author contends that the tongues originated from subsurface movement of magma generated as a result of the meteorite impact which created Tsiolkovsky Crater. The impact produced lines of weakness which were further enhanced by the impact forming Chenier. Magma then moved from Tsiolkovsky through the zones of weakness to Chenier Crater, extruding on the surface to form the first stringer. Following the extrusion, magnetic movement stopped and a cap formed over the vent. Enough heat was left in place under Chenier, however, to cause crustal melting and consequently the extrusion of a second and possible third lava-like stringer before the magma chamber under Chenier cooled and a cap over the vents permanently formed. Confirmation of the theory depends upon whether magma can move through weak zones in the lunar subsurface. Indications of this possibility have been suggested in findings dealing with the floors of Tycho and Aristarchus craters and in a study of the effects of artificial cratering.

Details for identifying the first 66 reference points of a selenodetic control network are presented. These reference points consist of craters having diameters larger than 8 km and which are therefore detectable on most plates used so far in lunar photogrammetric studies. The distribution of the adopted 211 reference points on Blagg-Mueller (1935) maps is given along with the Blagg-Mueller catalog number of each crater. The correspondence is indicated among the Blagg-Mueller number, the Lunar and Planetary Laboratory Catalog number, and the frame number of Lunar Orbiter IV photographs on which the craters have been studied.

Central peak features in 580 craters on the lunar near side were systematically studied, and detailed studies were made of several features of interest, in an attempt to define the mechanism of central peak formation. The peaks tend to be elongated along the preferred directions of the lunar grid. A weak correlation exists between peak size and crater size. Some peaks appear to have been formed or modified by volcanic processes, though strictly mechanical processes are known to produce central peaks on Earth. A more detailed knowledge of impact mechanics is required to account for the observations of the lunar central peaks.

The data of three-times repeated magnetic survey of the section of Lunokhod-2 route 1.5 km long are analyzed. The linear size of the regions of magnetic field anomalies disclosed is 200–300 m. The results of magnetic survey near the tectonic break of Straight Rille and near the south rim of crater Le Monnier were used for estimation of rock magnetization in situ. It is shown that mare basalts in south-east region of crater Le Monnier have oblique magnetization (at the angle c30° to horizon). The magnitude of magnetization is × 5 × 10−5 G cm g−1. The south-east slope of the crater Le Monnier is magnetized roughly vertically, the upper limit of magnetization of the rocks of the rim is c 1 × 10−5 G cm3 g−1. The results of an analysis of 160 magnetic field variations recorded by Lunokhod-2 indicate that the horizontal components of variations have nearly linear polarization. The principal axes of hodographs stretch in the direction north-west-south-east. Such a polarization of variations may be due to an increase of the thickness of the upper isolated layer under Mare Serenitatis.

A search for lunar features that showed characteristics of terrestrial ring dikes was conducted, using the Lunar Orbiter series of photographs. Features exhibiting one or more of the following four criteria were included as lunar analogs to terrestrial ring dikes: (1) inner ridges approximately concentric with the crater wall, (2) inner rills approximately concentric with the crater wall, (3) outer ridges and/or rills approximately concentric with the crater wall, and (4) interior and exterior slopes of the crater wall approximately equal. Features exhibiting each of the four criteria were found, and some had combinations of two or more including rills merging into ridges - e.g., in Taruntius and Posidonius. Gambart is an example of equal inner and outer slopes, while Hesiodus A and Marth are two of the best examples of complete inner rings concentric with the outer rings. Ten per cent of the candidates were probable impact craters but had subsequent volcanic activity of a ring dike nature. The initial search showed a distribution of the possible lunar ring dikes that was nonrandom and strongly associated with the margins of the maria, further implying that they are volcanic features.

Relative age schemes of crater degradation are calibrated to radiometric dates obtained from lunar samples, changes in morphologic features are analyzed, and the style and rate of lunar surface degradation processes are modeled in relation to lunar geologic time. A comparison of radiometric age scales and the relative degradation of morphologic features for craters larger than about 5 km in diameter shows that crater degradation can be divided into two periods: Period I, prior to about 3.9 billion years ago and characterized by a high meteoritic influx rate and the formation of large multiringed basins, and Period II, from about 3.9 billion years ago to the present and characterized by a much lower influx rate and a lack of large multiringed basins. Diagnostic features for determining the relative ages of craters are described, and crater modification processes are considered, including primary impacts, lateral sedimentation, proximity weathering, landslides, and tectonism. It is suggested that the fundamental degradation of early Martian craters may be associated with erosional and depositional processes related to the intense bombardment characteristics of Period I.

A correlation study based on Al/Si intensity ratios, broad-spectrum visible-albedo measurements, color-difference photography, color-ratio images, and spectral-reflectivity curves was undertaken to understand why the Tacquet Formation in southwestern Mare Serenitatis has unusually low visible albedos for the Al/Si intensity ratios measured by the Apollo 15 X-ray fluorescence experiment. The statistical analysis of the Al/Si data is described, general correlations among all the remote-sensing data are noted, and correlations in the area of the Tacquet Formation are examined. It is shown that the discrepancy between visible albedos and Al/Si intensities may be due primarily to the presence of unevenly deposited highland-type ejecta from Menaleus crater, although the possibility cannot be excluded that the Tacquet Formation is more complex than a simple mixture of highland and mare materials.

Multivariate analyses were performed on certain linear dimensions of six genetic types of craters. A total of 320 craters, consisting of laboratory fluidization craters, craters formed by chemical and nuclear explosives, terrestrial maars and other volcanic craters, and terrestrial meteorite impact craters, authenticated and probable, were analyzed in the first data set in terms of their mean rim crest diameter, mean interior relief, rim height, and mean exterior rim width. The second data set contained an additional 91 terrestrial craters of which 19 were of experimental percussive impact and 28 of volcanic collapse origin, and which was analyzed in terms of mean rim crest diameter, mean interior relief, and rim height. Principal component analyses were performed on the six genetic types of craters. Ninety per cent of the variation in the variables can be accounted for by two components. Ninety-nine per cent of the variation in the craters formed by chemical and nuclear explosives is explained by the first component alone.

Reduction of doppler radio tracking of the orbiting spacecraft has shown consistency with Apollo 14 data results and has revealed new gravity anomalies. Large craters are negative anomalies while wrinkle ridge regions are positive. The Central highlands are mostly a positive anomaly except for the Apollo 16 landing site, which is in a negative area. A gravity high northwest of Theophilus is not easily explained.

A thermodynamic model of meteorite impact is used to investigate the possibility that the creation of two impact craters of the same apparent diameter may have been accompanied by dissimilar shock heating regimes. For small craters, the idea may be concisely represented by the question of which event produces more glass, a large slow meteorite or a small fast one, if the resulting craters are of the same diameter. So that the results may be most apparent, the only parameters varied in this study are meteorite size and impact velocity. Identical basaltic material is assumed for meteorite and target. For impact velocities between 10 and 70 km/sec, an available model determines the radius of a spherical meteorite which would produce a crater diameter of 20 cm.

Making use of Orbiter and Apollo photographs, frequency counts of craters down to 2 km diam as indicators of the relative ages of lunar features, have been made on 264 areas, including 15 terrae, 27 recognized maria, 174 flat-floored craters and 48 lava-covered areas with indefinite boundaries designated as ‘marets’. Analysis of frequency counts on flat-floored craters on the basis of this data and re-assessment of former results, combined with the relatively restricted age range of lunar samples, make it unlikely that the present observations are able to reach back in time to impacts on an assumed primordial floating crust. The range of crater frequencies on the marets, together with their wide distribution over the lunar surface, suggest lava migrations to the surface within autonomous domains each with its own chronology, covering an extensive period of lunar history. The close association of marets with flat-floored craters provides a reasonable origin for the floor material of these latter objects. The lava migrations associated with the marets suggest that internal heating may be a more important factor in the origin of lunar surface features than had formerly been supposed. Kopal's views on the origin of the moon's multiple moments of intertia (1972) are considered to support the concept of autonomous domains. It is considered that the time sequence of separate lava flows represented by the marets may be a reflection of physical processes within the moon responsible for the successive lava flows associated with the larger maria.

The processes of movement and turnover of the lunar regolith are described by a Monte Carlo model. The movement of material by the direct cratering process is the dominant mode, but slumping is also included for angles exceeding the static angle of repose. Using a group of interrelated computer programs, a large number of properties are calculated, including topography, formation of layers, depth of the disturbed layer, nuclear-track distributions, and cosmogenic nuclides. In the most complex program, the history of a 36-point square array is followed for times up to 400 million years. The histories generated are complex and exhibit great variety. Because a crater covers much less area than its ejecta blanket, there is a tendency for the height change at a test point to exhibit periods of slow accumulation followed by sudden excavation. In general, the agreement with experiment and observation seems good, but two areas of disagreement stand out. First, the calculated surface is rougher than that observed. Second, the observed bombardment ages, of the order 400 million are shorter than expected (by perhaps a factor of 5).

It is shown that endogenic lava flow processes can be identified by their characteristic effects on lunar crater size distributions without necessarily being able to recognise individual flows on the photographs studied. The thickness of lava flows or a series of flows can be estimated from these crater size distribution characteristics. The lava flow histories of the Apollo landing sites 11, 12 and 15 are discussed in detail. The thicknesses of the most recent (3–3.4 × 109 years ago) flows there and of the youngest flows in an area in south-west Mare Imbrium (3 × 109 years) are found to range between 30 and 60 m. The subsequent flow episodes at the landing sites showing up in the crater size distributions can be related to differences in the radiometric ages of the respective lunar rocks.

A review is presented of four hypotheses concerning the origin of lunar craters, taking into account the bubble hypothesis, the tide hypothesis, the volcanic hypothesis, and the impact hypothesis. A description is given of a series of experiments on impact craters and studies of a meteorite crater in Arizona are considered. It is concluded that the typical lunar craters can best be interpreted as impact craters.

V-shaped ridge components of the herringbone pattern associated with lunar secondary crater chains have been simulated by simultaneous and nearly simultaneous impact of two projectiles near one another. The impact velocities and angles of the projectiles were similar to those of the fragments that produced secondary craters found at various ranges from large lunar craters. Variables found to affect the included angles of the V-shaped ridges are: relative time of impact of the projectiles, impact angle, relative projectile mass, and azimuth angle of the crater chain relative to the projection of the flight line onto the target surface. The functional relationships between the forms of the ridges and many of these variables are similar to those observed for lunar V-shaped ridges. Comparison of the magnitudes of the ridge angles of both laboratory crater pairs and secondary crater chains of the crater Copernicus implies that material was ejected from Copernicus at angles in excess of 60 deg, measured from the normal, to form many of Copernicus' satellite craters.

The intensity distribution of lunar radar echoes has been mapped for two-thirds of the earth-visible lunar surface at a wavelength of 70 cm. The depolarizing effects of the lunar surface were observed by simultaneously receiving the radar echoes in opposite polarizations. These echoes were mapped with areal resolutions of 25–100 km2. Mappings with this resolution confirmed that the young craters have enhanced returns. A few craters were found to have enhanced echoes only from their rims. Backscattering differences were also observed between various areas within a mare, between different highland areas, and between maria and adjacent highlands. These scattering differences were interpreted with a simple model, which assumed that the surface backscattered with varying amounts of quasi-specular and diffuse power. Only an increase in the diffuse power was needed to give the numerical values of the enhancements.

Lunar position differences between thirteen craters in Mare Serenitatis were computed from VHF radar-imagery obtained by the Lunar Sounder instrument flown on the Apollo 17 command module. The radar-derived position differences agree with those obtained by conventional photogrammetric reductions of Apollo metric photography. This demonstrates the feasibility of using the Apollo Lunar Sounder data to determine the positions of lunar features along the Apollo 17 orbital tracks. This will be particularly useful for western limb and farside areas, where no Apollo metric camera pictures are available.