ICARUS 25, 495-537 (1975)

The Moon after Apollo

PAROUK EL-BAZ National Air and Space Museum, Smithsonian Institution, Washington, D.G- 20560

Received September 17, 1974

The Apollo missions have gradually increased our knowledge of the Moon's chemistry, age, and mode of formation of its surface features and materials. Apollo 11 and 12 landings proved that mare materials are volcanic rocks that were derived from deep-seated basaltic melts about 3.7 and 3.2 billion years ago, respec- tively. Later missions provided additional information on lunar mare basalts as well as the older, anorthositic, highland rocks. Data on the chemical make-up of returned samples were extended to larger areas of the Moon by orbiting geo- chemical experiments. These have also mapped inhomogeneities in lunar surface chemistry, including radioactive anomalies on both the near and far sides. Lunar samples and photographs indicate that the moon is a well-preserved museum of ancient impact scars. The crust of the Moon, which was formed about 4.6 billion years ago, was subjected to intensive metamorphism by large impacts. Although bombardment continues to the present day, the rate and size of impact- ing bodies were much greater in the first 0.7 billion years of the Moon's history. The last of the large, circular, multiringed basins occurred about 3.9 billion years ago. These basins, many of which show positive gravity anomalies (mascons), were flooded by volcanic basalts during a period of at least 600 million years. In addition to filling the circular basins, more so on the near side than on the far side, the basalts also covered lowlands and circum-basin troughs. Profiles of the outer lunar skin were constructed from the mapping camera system, including the laser altimeter, and the radar sounder data. Materials of the crust, according to the lunar seismic data, extend to the depth of about 65 km on the nearside, probably more on the far side. The mantle which underlies the crust probably extends to about 1100 km depth. It is also probable that a molten or par- tially molten zone or core underlies the mantle, where interactions between both may cause the deep-seated moonquakes. The three basic theories of lunar origin—capture, fission, and binary accretion —are still competing for first place. The last seems to be the most popular of the three at this time; it requires the least number of assumptions in placing the Moon in Earth orbit, and simply accounts for the chemical differences between the two bodies. Although the question of origin has not yet been resolved, we are beginning to see the value of interdisciplinary synthesis of Apollo scientific returns. During the next few years we should begin to reap the fruits of attempts at this synthesis. Then, we may be fortunate enough to take another look at the Moon from the proposed Lunar Polar Orbit (LPO) mission in about 1979.

I. INTRODUCTION ledge for better understanding of the scientific data now being collected from Exploration of the Moon has yielded a other planets. There are indications that plethora of information regarding the the forces of nature that created the mater- chemistry, age, and evolution of the surface ial of the Earth were also operative on the materials. A complete and thorough under- Moon and probably the terrestrial planets, standing of these data is important for two Mars, Venus, and Mercury, reasons: first, to decipher the thermal To a lesser degree, a study of the Moon is history and origin of Earth's closest also important in studying the Sun's past, neighbor, and, second, to apply that know- At present, there are numerous new tools Copyright © 1975 by Academic Press, Inc. 495 All rights of reproduction in any form reserved. Printed in Great Britain 496 FAROUK EL-BAZ

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i* cS cS C „-^ < MOON AFTER APOLLO 497 to study with increasing degrees of sophis- hard to acknowledge all contributors to a tication the processes that are acting on given result, theory or manner of present- the Sun. Before the return of lunar samples, ation. However, and at the expense of however, it was not feasible to study the brevity, it is attempted here to give proper past history of the Sun. The lunar materials credit to authors for the sake of readers who have collected and kept a record of the solar may wish to consult original papers dealing particles reaching them from the Sun. with the various subjects or areas of More than fifty spacecraft have flown discussion. Other summary reports of near or landed on the Moon since 1958. lunar science include those by Hinners Twelve American astronauts have walked (1971), EL-Baz (1974), Schmitt (1974), on and sampled its surface, traversing Runcorn (1974), and Taylor (1975). nearly one hundred kilometers. Two un- manned Soviet vehicles have returned II. THE APOLLO MISSIONS samples from two different sites on the Moon, and two others have roamed the A. Mare Tranquillitatis lunar surface telemetering their intelligence It was a giant leap for Neil Armstrong to to the Earth. Five scientific observatories take the first step on the Moon. The left by the astronauts on the Moon continue Apollo 11 mission was also a giant leap for today to transmit information about the science because it provided the first lunar environment. The two Soviet passive returned samples from another body in stations and similar laser retrorefiectors the solar system. Before Apollo, no matter at three Apollo landing sites are being used how sophisticated the instruments were of to measure the Earth-Moon distance, and remote sensing, whether from Earth or also the rate of continental drift on Earth. from lunar orbit, no undisputable conclu- About twenty thousand photographs sions could be drawn as to the nature of the taken from lunar orbit have captured the lunar surface materials. The exact chem- Moon in great detail, permitting better istry of a rock, the mineral species that interpretations of its features. The majority form its fabric, as well as the absolute age of the hard facts, however, were provided of that rock are among the fundamental by the soft-landed spacecraft (Fig. 1). parameters necessary for a full understand- Most prominent among these are some 400 ing of its origins and its history. These kilograms of rock and soil samples that parameters are nearly impossible to define were returned, mostly by the Apollo mis- without having the rock samples to study sions. These samples are being studied by in the laboratory. more than a thousand scientists of diverse The Apollo 11 landing site is in a relative- specialities in nearly twenty countries. ly smooth and level part of Mare Tran- As is customary in scientific research, the quillitatis (Fig. 2a). This part of Tran- findings from lunar studies generated quillitatis is characterized by a low albedo questions that remain unanswered. How- (reflecting only about 9% of incident sun- ever, many questions have been satis- light; Pohn and Wildey, 1970) and a high factorily answered and several conclusions density of craters (Grolier, 1970; Trask, can be drawn. It is my intention to sum- 1970) ranging in diameter from less than marize in this paper the state of the art of one meter to several tens and hundreds of lunar science, with emphasis on what I meters. Materials of the regolith, the upper- think is important among the results of the most fragmental surface layer, range in six Apollo lunar surface exploration size from very fine particles to blocks more missions. than a meter across. The landing was made For the past eight years I have been a about 6 km west-southwest of the center of member of the so-called "lunar science the landing ellipse. The astronauts avoided team." Between team members of this a blocky-rimmed crater named West, "mutual admiration society," intimate which is about 180 meters in diameter and verbal contact is the rule rather than the 30 meters deep. Its hummocky eject a exception. For this reason, it is sometimes extends about 250 meters out from the rim 498 FAROTJK EL-BAZ MOON AFTER APOLLO 499 crest. Ejected blocks, radially arranged 3. Occurrence of fragments, mostly around that crater were photographed by angular, of basaltic rocks and glass or the astronauts, and samples of its finer ray glass-coated rocks (Fig. 2c). materials were probably returned by 4. Aggregation of fragments and par- Apollo 11. ticles back into a coherent rock, breccia Samples from this site consist of (a) (LSPET, 1969). coarse-to-fine grained ophitic basalts of 5. Deformation of the structure of igneous origin, (b) microbreccias, which individual minerals (plagioclase and pyrox- are believed to be a mechanical mixture ene) and the production of fractures, of soil and small rock fragments compacted planar structures, and multiple twinning into a coherent rock and (c) fine-grained (Short, 1970). soil (LSPET, 1969). The soil is a diverse 6. Further deformation causing com- mixture of crystalline fragments and plete disruption of the mineral lattice, glassy fragments (Fig. 2c). It contains a shock-induced thermal melting, and re- small number of crystalline fragments crystallization of the materials (von Engel- which are totally different from any of the hardteW., 1970). sampled igneous rocks. These fragments 7. Smallness of the median particle were interpreted to have been derived size, in the soil, which may represent the from the nearby highlands by Wood et al. average mineral grain size in the crystalline (1970). This interpretation was found to rocks (Duke et al., 1970). Sizes below a few hold true on later Apollo flights. The soil microns dominate (Gold et al., 1971). was also found to contain small fragments 8. Presence of small particles of of iron meteorites (LSPET, 1969). meteoritic debris (composed mostly of Evidence of shock effects due to meteor- iron-nickel) in the soil or aggregated with oid impacts are well preserved in soil sam- lunar particles (Chao et al., 1970). ples and the fabric of returned rocks. The Studies of trace elements in the Apollo shock features in rock fragments are 11 basalts have shown that the samples equivalent to those produced in the lab at are remarkably similar in a wide range of 40 kilobars (Quade et al., 1970), or higher, chemical characteristics. These character- and in the fine dust to those approaching istics distinguish the Apollo 11 rocks from the megabar region (von Engelhart et al., basaltic meteorites and terrestrial basalts. 1970). Evidence that the regolith is the As reported by Oast and Hubbard (1970) product of meteoroid impacts include the they show (1) a high concentration of ti- following. tanium, zirconium, and rare earth elements, 1. Observation of glass coatings on (2) a two- to fourfold depletion of europium blocks (Fig. 2b) as well as rock and soil relative to samarium and gadolinium, samples (McKay et al., 1970); some of the and (3) a low abundance of sodium relative coatings contain extra amounts of elements to the other major elements. Oast and attributed to the impacting meteorites Hubbard (1970) conclude that these basalts (Anders et al., 1971, and Laul et al., 1971). must have formed by the eruption of lavas 2. Presence of minute impact pits, from beneath the surface of the Moon, after also known as "zap craters," on surfaces of segregation of molten rock and its fraction- rocks and soil samples (e.g. Horz et al., ation to produce this particular composi- 1971). tion.

FIG. 2. Apollo 11: (a) Oblique view looking west of the landing site (arrow) in southwestern Mare Tranquillitatis. There are numerous craters, several hundred meters across, in the site area, like the one shown in Fig. 2b. Note the proximity of highland masses to the mare surface in which the LM landed (Apollo 11 frame 6092); (b) A cluster of blocks in the center of the floor of a crater 200m in diameter. Presence of the blocks suggests exposure of bedrock that underlies the regolith (uppermost fragmental layer) by the impact that created the crater (Apollo 11 frame 5954); (c) Fragments of Apollo 11 soil composed mainly of basaltic rock, but including a dark-colored glass fragment (1), the product of impact metamorphism and a light-colored anorthositic fragment (2), probably derived from the nearby highlands (Photograph courtesy of Carl Zeiss Company). 500 FAROUK EL-BAZ

Ages of rock fragments from the Apollo than being impact-produced, near-surface 11 basalts were investigated by various melts). There is also ample chemical radiometric age-dating techniques. The evidence that the original magmas had low rubidium-strontium analyses on total oxidation levels. rocks and minerals appear to produce the 4. Samples of fine dust and soil are in most consistent internal isochrons. Albee most part derived from the basaltic rocks. et al. (1970a) reported precise internal The soil is chemically similar to the rock isochrons of rubidium 87-strontium 87 samples except for minor components in five rocks giving the same age of 3.65 ± which are probably derived from the 0.05 billions years. They considered this nearby (anorthositic) highlands. to be the crystallization time of the Marc 5. Rocks, rock fragments, and fine Tranquillitatis basalts sampled on Apollo dust are most likely the product of repeated 11. Argon 40-argon 39 age-dating tech- impacts on solid bed rock surfaces. There niques of seven crystalline rocks also is ample evidence of modifications of the indicated a similar age of 3.7 billion vears original rock structures by shock meta- (Turner, 1970). morphism, including the formation of Age-dating of the fine dust from Apollo glass and breccias. 11 produced other significant results. 6. The lunar volcanic rocks at the Concentrations of uranium, thorium, and Apollo 11 site are extremely old; they lead in the soil samples are very low: from crystallized from melts 3.7 billion years less than one to a few ppm. However, the ago. (This particular fact contradicts some extremely radiogenic lead allows radio- earlier suggestions that the Moon has been metric dating of the fine soil. Based on the volcanically active in the recent past.) ratios of lead 207: lead 206, lead 206: The model age of the soil, 4.6 billion uranium 238, lead 207: uranium 235, and years, probably represents the age of the lead 206: thorium 232, Tatsumoto and Moon. Rosholt (1970) obtained a concordant age of 4.66 billion years. Silver (1970) also B. Oceanus Procellarum reported that uranium-thorium-lead Since the Apollo 11 mission landed too isotope relations in dust samples yield far west of the planned landing point, one apparent model ages of about 4.6 billion of the main objectives of the Apollo 12 years. He emphasized that the discrepency mission was to develop and exercise the between rock ages and dust ages poses a capability of pinpoint landing. The general problem of rock genesis on the Moon. area of the Surveyor III landing site was In summary, the Apollo 11 rock and soil selected as the target of this mission. samples provided significant new informa- The site is located in the extreme south- tion from which the following are empha- eastern part of Oceanus Procellarum (Fig. sized . 1) and it is covered by a broad ray which is 1. The majority of Mare Tranquilli- believed to have originated by ejection of tatis samples are volcanic rocks of basaltic material from the crater Copernicus some composition. This is a confirmation of 370 km to the north (Fig. 3a). The mare pre-Apollo photogeologic interpretations, material in this site is characterized by a as well as interpretations of data trans- relatively smooth surface and a smaller mitted by the Surveyor missions. number of craters per unit area than the 2. The rocks are remarkably similar Apollo 11 site (Trask, 1970; Pohn, 1971). in composition except for the concentra- The Apollo 12 site is characterized by a tions of a few minor and trace elements. distinctive cluster of 50-400 m craters that By terrestrial standards, they are rich in was named the "Snowman" (Fig. 3b). The iron and titanium, and poor in sodium, lunar module landed on the northwest rim carbon and water. They also do not contain of the 200 meter Surveyor crater, within any evidence that life once existed on the which Surveyor III had landed two years Moon. earlier. The regolith at the site is made up 3. The volcanic rocks are derivatives of microscopic particles to blocks several of differentiation in a melt at depth (rather meters across. In addition to samples of MOON AFTER APOLLO 501

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FIG. 3. Apollo 12: (a) The landing site area (arrow) in southeastern Oceamis Procellarum. Kote that the area is covered by a bright ray from the crater Copernicus near the top of the photograph (Lunar Orbiter IV frame M-126); (b) Close-up view of the landing site showing a cluster of craters named "Snowman" with the Surveyor crater in the middle. The position of the Surveyor III space- craft is shown by a cross and the Apollo 12 LM by an arrow (Lunar Orbiter III, frame H-137); (c) Exposures of white soil in the Apollo 12 site that may be part of the Copernicus ray material (Apollo 12 frame 7052). 502 FAROIXK EL-BAZ

rock and soil, the Apollo 12 astronauts component a foreign one to the basalts of brought back parts of the Surveyor space- Oceanus Procellarum. For this reason, and craft to be studied for effects of the lunar because of the association with the light- environment. colored soil, it is interpreted as part of the Basaltic igneous rocks, breccias, and ejecta from the crater Copernicus, i.e., soil were collected from a variety of the the result of penetration of the mare fill and geologic features in the Oceanus Pro eel - ejection of the underlying pre-mare high- larum site. The igneous rocks show many lands by the crater Copernicus (Fig. 3a). differences from the Tranquillitatis basalts. One Apollo 12 rock, number 13, proved The rocks of Apollo 12 are predominantly to be the most unusual lunar sample yet crystalline, whereas the Apollo 11 rocks obtained. It is a very heterogenous mixture are half crystalline and half microbreccias of light and dark components with various (LSPET, 1970). The Procellarum basalts degrees of mixing between them. The light show greater variation in grain size; some component is granitic in composition, con- are very coarse grained. This was explained sisting predominantly of potassium-rich to be the result of segregation during feldspar and quartz with plagioclase and crystallization in relatively thick lava pyroxene. The dark component is similar flows or shallow intrusive dikes or sills to the basaltic microbreccias of the Apollo (LSPET, 1970). Later, geologic evidence 11 site consisting of angular fragments of was developed indicating that these rocks plagioclase and pyroxene in a fine mixture were samples from several distinct basalt of plagioclase, pyroxene, and ilmenite. units, perhaps at least three (Button and One simple model of rock 13 (Albee et al., Schaber, 1971). This is supported by 1970b) suggests that the dark basaltic petrographic evidence of various types of component was permeated and assimilated sampled basalts, including a porphyritic to varying degrees by the light, granitic basalt that is made up of large crystals component. of olivine and pyrozene in a fine-grained The age of this rock, 4.0 billion years, matrix. Prom the chemical-mineralogical was ascribed to the time when the light point of view, according to Melson and material intruded and reacted with the Mason (1971), the Tranquillitatis basalts dark component (Albee etal., 1970b). Both show a greater abundance of titanium, the chemistry and the age indicate that lower anorthite content of the plagioclase, "rock 13" is foreign to the Procellarum lower hyperstene: diopside ratio, and a basalts. The crystallization ages of these predominantly quartz-normative miner- basalts as obtained by rubidium-strontium alogy when compared with the Procellarum method varied between 3.1 and 3.3 billion basalts. years (Papanastassiou and Wasserburg, Some of the fine-grained material col- 1971). lected at the 12 site is of much higher Among the most important conclusions albedo than the surroundings (Fig. 3c). that could be drawn from the findings of Soil and breccia samples from this high Apollo 12 are the following. albedo unit contain fragments of unusual 1. The chemistry of the two mare composition. According to Mayer et al. landing sites, Apollo 11 and 12, resembles (1971), these occur as ropy and sculptured the findings of Surveyor V and VI (see glasses, glassy matrix breccias, and an- Fig. 1 for locations). This chemistry nealed fragments with a predominance of distinguishes lunar basalts from basaltic orthopyroxene. The composition of these meteorites and terrestrial basalts. materials is essentially basaltic, but with 2. At least two, probably three or greatly enriched trace element contents of more, mare units were sampled on Apollo potassium (K), rare earth elements (REE), 12 based on petrologic and petrographic and phosphorus (P). This resulted in evidence. This confirms photogeologic coining of the acronym KREEP by interpretations that suggest the presence Hubbard ef aZ. (1971). of more than one lava flow in the landing This chemistry makes the KREEP site area. MOON AFTER APOLLO 503

3. The age of the Apollo 12 basalts, large boulders are also exhibited in which is approximately 3.2 billion years, collected samples. attests to the fact that the lunar maria are Inspection of the returned samples and geologically very old. The wide difference study of thin sections (LSPET, 1971; between the ages of Mare Tranquillitatis LSAPT, 1972, 1973) confirmed the brec- and Oceanus Procellarum basalts (600 ciated nature of almost all the returned million years) indicates a prolonged episode samples. Many generations of brecciation of mare filling on the Moon. were observed, that is breccias within 4. The high albedo material collected breccias within breccias. This suggests in the fine samples as well as rock 13 appear that, before excavation of the Imbrium to be foreign to the Procellarum basalts. basin, the host rock had been subjected to These light-colored materials seem to be earlier impacts. The effects of the Imbrium part of the Copernicus ray. KREEP impact on the rock appears to be re crystal- components in the Apollo 12 site may, lization, mineral reactions, and incipient therefore, be part of buried Era Mauro fusion, characteristics that are to be Formation materials. If so, KREEP associated with shock metamorphism due basalts should be an important component to a large impact. This shock meta- in the Apollo 14 rocks, which they are. morphism may be in part due to a base- surge like environment described in vol- G. Fra Mauro Formation canic eruptions on Earth (Moore, 1967). The site selected for Apollo 13 and later A recent study of the Orientale basin Apollo 14 after the former was aborted is in and the surrounding ejecta by Moore et al. the Era Mauro Formation. This unit covers (1974) shows that there are six facies of a substantial part of the near side of the ejecta around its outer rim, the Cordillera Moon as a broad belt that surrounds the Mountains: concentric, radial, smooth- Imbrium basin; the Formation is inter- plained, grooved, secondary impact crat- preted to be an ejecta deposit from the ered, and fissured. The transport of these impact that created the basin (Eggleton, ejecta (based on photogeologic mapping, 1964). It is characterized by broad ridges terrestrial crater analogs, and experimental that form a characteristically hummocky data) occurred by the following processes: terrain (Fig. 4a), with the major orienta- block-riding, landsliding, debris flow tion being radial to the Imbrium basin. (possibly by base-surge), viscous flow, and The thickness of this ejecta blanket is ballistic ejection. In their view of the judged to be several kilometers by the dynamic transport mechanisms, therefore, calculated depth of the craters which are mixing of ejecta and substratum materials almost completely obscured by it (Eggle- must have occurred. Extrapolation of this ton, 1964; Eggleton and Offield, 1960). model to Imbrium suggests a wide distri- The Apollo 14 LEM landed about 1100 bution of ejecta from this basin. Moore et meters west of Cone crater which is a sharp- al. (1974) point out that the locations of rimmed crater, 350 meters in diameter, Apollo 14, 15, 16, and 17 permit the that apparently penetrated one of the presence of Imbrium ejecta of considerable characteristic ridges of the Fra Mauro thicknesses at all these sites. Formation (Fig. 4b). The sampling traverse Chemically, the Apollo 14 samples have was planned with the ejecta of Cone crater consistently lower contents of iron oxide in mind, i.e. to collect fresh exposures of and higher contents of aluminum oxide the ridgy material excavated by this than mare basalts. The iron content is relatively recent event. Study of the more like most terrestrial basalts. The photographs taken on the lunar surface concentration of minor or trace elements, indicates that large blocks in the area are such as potassium, rubidium, uranium, composed of breccias in which dark and thorium, barium, yttrium, and europium light clasts are set in a light-colored matrix are much higher than those found in mare (Fig. 4c). The returned rocks show that basalts. The closest compositions are brecciation and presence of clasts in the those of the Apollo 12 KREEP basalts and 504 FABOUK EL-EAZ

Fio. 4. Apollo 14: (a) Oblique view showing the ridgy Fra Mauro Formation (middle part of the frame) within which the Apollo 14 LM landed (arrow) (Apollo 16 metric frame 2507); (b) Close-up view of the landing site (arrow) southwest of Gone crater (upper right part of photograph). Gone crater, which is about 350 meters in diameter, apparently penetrated one of the Fra Mauro Form- ation ridges (right half of photograph) (Lunar Orbiter III frame H-133); (c) Large blocks, several meters across, near the rim of Cone crater showing dark and light clasts of breccia set in a light- colored matrix. The brecciation appears to be the result of impact metamorphism (Apollo 14 frame 9449). MOON AFTER APOLLO 505 the dark portion of' 'rock 13." The chemical unit area to the mare of Oceanus Procel- composition of the soil closely resembles larum in which Apollo 12 landed, and, that of the fragmental rocks, except for the therefore, it was believed to be of the same fact that some trace elements are somewhat relative age (Carr et al., 1971). Hadley §, depleted in the soil (LSPET, 1971). which is part of the Apennine Mountains The Apollo 14 rocks are dated at 3.95 was believed to include material that was billion years (Tera etal., 1973). These rocks uplifted from the subsurface at the time of (samples of the Fra Mauro Formation) the formation of the Imbrium basin most probably represent ejecta from the (Shoemaker, 1962, among many others). Imbrium basin, where the impact event First to be studied were the large must have reset the radioactive clocks. numbers of photographs returned by the Therefore, the 3.95 billion years age repre- Apollo 15 crew. Both surface and orbital sents the last stage of recrystallization of photographs were used to decipher local the Imbrium basin ejecta. In summary, stratigraphic relationships. For example, important results of the study of the high resolution panoramic camera photo- Apollo 14 samples are: graphy showed meters thick layers on the 1. The complex history of shock walls of Rima Hadley (Fig. 5b). These metamorphism of all the rocks, for which layers, which confirm the theory of filling there are three probable causes: (a) excava- of mare basins by successive lava flows, tion by the Imbrium impact of preexisting were also depicted on high resolution brecciated materials; (b) formation of surface photographs (LSAPT, 1972). Also, breccia within breccia during the Imbrium a highland prominence, informally called event, and the base-surge that resulted Silver Spur, within the Apennine chain of from it; and (c) local cratering events mountains and hills showed several strata superposed on the Imbrium event. (Fig. 5c). Each layer is about 100 meters 2. The composition of the Fra Mauro thick, and the layers may be ejecta from Formation rocks is different from the the Imbrium basin (Swann et al., 1970). mare rocks of Apollo 11 and 12. The high- The mare material returned by Apollo 15 land or KREEP basalts of Apollo 14 are was found to be basalts similar to that characterized by a lower iron oxide content returned from Oceanus Procellarum. and higher aluminium oxide and trace Material of Hadley S was found to be element concentrations than those of mare chemically different from both the mare basalts. basalts and the KREEP basalts of Apollo 3. Since the recrystallization age of 14. It is mostly anorthositic rock; some of the Apollo 14 rocks is about 3.95 billion the returned samples are crystalline rocks years, and if the Fra Mauro Formation is which indicates that brecciation was not as indeed ejecta of the Imbrium basin, then extensive as at the Apollo 14 site. we have established the absolute age of The anorthositic rocks are held by most the Imbrium event which shaped much lunar investigators to represent a large part of the near side of the Moon. of the lunar crust. The chemistry of this material is similar to that of the light- D. Apenni?ie-Hadley colored fine particles collected in the soil The Apollo 15 lunar module landed on from several sites [for example, Apollo 11 the mare surface of Palus Putredinis on (Wood et al., 1970) and Luna 16 (Vino- the eastern part of Mare Imbrium, which gradov, 1971)]. Unlike the Fra Mauro is rimmed by the Montes Apenninus. The KREEP this material does not show high landing site is strategically situated such concentration in radioactive elements. that the astronauts were able to sample the One anorthositic crystalline rock from the mare material, the highland massif Hadley highland regolith samples on Apollo 15 8 or its regolith, and the sinuous Rima and named "Genesis Rock" gives an age of Hadley (Fig. 5a). The mare material of 4.1 billion years (LSAPT, 1972). This is Palus Putredinis is very similar in texture, the oldest rock sampled on that mission. appearance, and number of craters per Study of the samples collected from the FAROUK EL-BAZ

FIG. 5. Apollo 15: (a) Photograph of the landing site (arrow) in Palus Putredinis. The site is west of the Apennine Mountain chain and east of the sinuous Hadley Rille (Apollo 15 frame 12811); (b) High resolution photograph of part of Hadley Rille that is marked by a square in Fig. 5a. Note the ledge of bedrock near the top and the numerous blocks that accumulated at the bottom of the V-shaped rille (AprHo 15 panoramic camera frame 9342); (c) Photograph of the layered Silver Spur (near center of horizon) at Hadley S as viewed from the LM (Apollo 15 photography 11371). MOON AFTER APOLLO 507 rim of Bima Hadley failed to add any flat unit which resembles in gross appear- significant insight into the origin of the ance old mare units but displays a much structure. The morphological character- higher albedo and a denser population of istics of this sinuous depression may best craters, the plains-forming Cayley Form- be explained as the result of drainage of ation. The second is a topographically lava into a conduit whose top part may higher unit which is characterized by hilly, have congealed and later collapsed to form grooved, and furrowed terrain. The latter the rille (Greeley, 1971; Carr et al, 1971; ("Descartes") unit is morphologically simi- Howard et al., 1972). However, the data lar to some terrestrial volcanic deposits. are not conclusive, and the origin of this Hence, it was interpreted as a representa- and other sinuous rilles remain somewhat tive of volcanism in the lunar highlands; enigmatic. relatively viscous volcanic materials could Among the most significant results of be responsible for the formation of the hilly the Apollo 15 mission are the following. topography (Milton, 1968; Wilhelms and 1. The basalts of the Apollo 15 site McCauley, 1971; El-Baz and Roosa, 1972; are similar chemically and texturally to Milton and Hodges, 1972; and others). If the Apollo 12 basalts. Both mare basalts this interpretation is correct, one would are also about 3.2 billion years old. They expect an abundance of crystalline high- constitute the youngest group of sampled land rocks at the Apollo 16 site. basalts. The luna 16 and Apollo basalts are During the mission, however, the astro- 3.5 and 3.7 billion years old, respectively. nauts observed that most of the rocks they The oldest basalt is one of the Apollo 14 encountered were breccias rather than rocks, number 53, which is 3.95 billion crystalline or igneous rocks. Examination years old (LSAPT, 1972). of the returned samples confirmed that the 2. Rocks and soil samples of Hadley § vast majority of the samples are breccias are in general terms anorthositic gabbros and only a few are unmetamorphosed and gabbroic anorthosites. Some of these igneous rocks (LSPET, 1973a). There is an are crystalline rocks, which indicates that abundance of high aluminum and calcium brecciation was not as extensive at the rim rocks: many are monomict breccias ap- of the Imbrium scar as it was farther out on proaching pure calcic plagioclase com- the ejecta blanket (Apollo 14 breccias). position. These crushed anorthosite 3. The lunar highlands are indeed breccias are unlike other lunar breccias, older than the maria, which is in agreement displaying no evidence that they are with photogeologic interpretations. The mechanically produced mixtures of pre- fact that "Genesis Rock" is 4.1 billion years existing rocks. The Apollo 16 breccias old is very significant, in that the rock is probably formed from coarse-grained older than the Imbrian basin, and was not anorthosites by impact metamorphism metamorphosed by that event. (LSPET, 1973a). 4. Although samples and close up Ray materials and ejecta derived from photographs were taken of Rima Hadley, two bright rayed craters in the area, North no new information was gathered on the Ray and South Ray (Fig. 6), appear to origin of that sinuous rille. All existing mantle much of the traversed surfaces, theories of sinuous rille formation are still and, therefore, dominate the sampling competing, and there may be many modes locations. Because of this there is some of origin for these rilles. uncertainty whether the material of the hilly terrain was sampled. It was clear, E. Descartes Highlands however, that premission theories of a Apollo mission 16 was dedicated to volcanic origin were probably incorrect. sampling and exploration of part of the After the mission, several theories were central lunar highlands north of the crater advanced relating the majority of Apollo Descartes (Fig. 1). Two distinctive units 16 rocks to Imbrium basin ejecta (Hodges were recognized and mapped before the et al., 1973), Orientale basin ejecta (Chao mission (Fig. 6). The first is a relatively et al., 1973), material generated by secon- 508 FAROUK EL-BAZ

FK:. 0. High resolution photograph of the Apollo 16 landing site area. The landing point (arrow) is approximately midway between two bright craters, 800 meters in diameter, North Ray (N) and South Ray (S). The latter is the younger and therefore the brighter of the two. Ejecta from both was collected on the mission in addition to materials of the light plains (Cayley Formation) in which the LM had landed (Apollo 16 panoramic camera frame 4623). MOON AFTEK APOLLO 509 dary craters of Imbrium (Oberbeck et a cometary impact may have formed al., 1974), and Nectaris ejecta and locally North Ray crater. generated deposits (Head, 1974). From studies of the Apollo 16 samples, None of these theories fully explains all the following results are most significant. the observations, although all the con- 1. Photogeologic interpretations by sidered impacts may have contributed to many investigators, including myself, of the terra deposits in the Descartes site. landforms in the Descartes region were However, the study of the Orientale basin proved incorrect. The hilly and grooved ejecta by Moore et al. (1974) may provide topography in that area is probably additional support for this theory of unrelated to terra volcanism and most Imbrium basin ejecta origin for the Apollo likely to the accumulation of basin ejecta. 16 materials. Moore et al. (1974) state that 2. Although the Imbrium basin Imbrium-related debris more than 500 appears to be the most likely source of the meters thick at the Apollo 16 site is quite Descartes area deposits, differences be- possible, and is consistent with the photo- tween the Apollo 16 and 14 rocks are still geologic evidence in the Orientale case. not fully understood. This, however, does not preclude the 3. The ages of Apollo 16 rocks (about presence of some Orientale and other basin 3.9 billion years, Tera et al., 1973) are ejecta at the site. Moore et al. (1974) expect consistent with an Imbrium origin, al- a thinner deposit of Orientale materials in though some samples dated by argon the Descartes region. Locally generated 40-argon 39 give crystallization ages of debris must also be present, e.g., ejecta approximately 4.1 to 4.2 billion years blankets and rays of North and South Ray (Husain and Schaeffer, 1973). and older craters. 4. The high aluminum and calcium If we are to accept the theory of Imbrium rocks of the Apollo 16 site are crushed basin ejecta for the Descartes site, two anorthosite breccias, probably derived fundamental questions will remain un- from cataclysmic metamorphism of an answered : First, why are the rocks of the anorthositic complex containing 70 to 90 Apollo 16 site so different morphologically, percent plagioclase. petrologically, and from the metamorphic 5. Although the lunar environment is history point of view than the Apollo 14 completely void of water and is char- breccias (which are also believed to be acterized by exceedingly low oxygen ejecta from Imbrium)? Second, why are the pressure, goethite was formed in the concentrations in KREEP basalts so much Apollo 16 rocks as a mineral reaction rim. higher in the Era Mauro Formation rocks The water vapor may have been part of than in the Descartes highlands, if both are the impacting (cometary) projectile that mostly Imbrium ejecta deposits? formed North Ray crater. Some of the samples of North Ray crater ejecta appear to have a rustlike F. Taurus-Littrow coating. Studies of the opaque mineral The Apollo 17 site is located on a dark assemblage in these rocks revealed the unit that fills a valley between two large presence of an iron hydroxide, goethite, in massifs on the southeast rim of Mare that coating. According to El Goresy et Serenitatis (Figs. 1 and 7). This flat unit is al. (1973), goethite occurs as a thin film among the darkest of all lunar surface around metallic Fe/Ni and infiltrates the materials (albedo value about 6-7%; silicate grain boundaries in the vicinity Pohn and Wildey, 1970). As mapped by of the Fe/Ni metals. These observations Carr (1966) this unit was believed to be lead to the conclusion that goethite was younger than the inner mare fill of the formed on the lunar surface by hydro- Serenitatis basin. Also, according to thermal or similar reactions between Greeley and Gault (1973) this unit is so troilite and water vapor, perhaps during a moderately cratered that it was considered cometary impact, the water being a among the youngest marelike units on the component of the cometary tail. Therefore, near side of the Moon. It displays a very 510 FAROTTK BL-BAZ

FIG. 7. Area of the Apollo 17 exploration site. The LM (arrow) landed in the dark plain between two highland massifs, North Massif (N) and South Massif (S). Samples were collected from the plains, the two massifs, the landslide near South Massif, the scarp between the two massifs, and Shorty crater (circle) (Apollo 17 panoramic camera frame 2314). MOON AFTER APOLLO 511 smooth surface which was interpreted to produced (Roedder and Weiblen, 1973), be the result of a thin pyroclastic mantle most agree that they represent a product (El-Baz, 1972a). This interpretation was of volcanic eruption (Reid et al., 1973, endorsed by the sighting in this vicinity, among others). Orange-colored deposits from the Apollo 15 orbit, of small smooth- and ejecta blankets are apparently not rimmed craters which were interpreted as restricted to the Apollo 17 landing site cinder cones (El-Baz et al., 1972). Part of area; orbital observations on Apollo 17 this unit is covered with a relatively thin indicate that they abound in the Sulpicius layer of light-colored material which is Gallus region on the western Serenitatis believed to be a landslide from the South rim (Evans and El-Baz, 1973; Lucchitta Massif (El-Baz, 1972b; Scott et al., 1972). and Schmitt, 1974). The massif units measure up to two The highland materials collected at the kilometers in height and display relatively Apollo 17 site show several suites of fresh surfaces with numerous blocks. A anorthositic rocks. Although petrologically large population of blocks could also be complex, massif rocks may be divided into seen at the base of the massifs, indicating two chemical suites: (a) noritic breccias unstable slopes and much downslope which include the petrologic varieties of movement of material. Crossing the site in a green-gray, blue-gray, and light gray north-southerly trend is a scarp with mare breccias; and (b) anorthositic gabbros, ridgelike segments. This scarp is inter- which petrologically are brecciated preted as a fault scarp with en-echelon gabbroic rocks. The second group occurs segments producing the ridgelike appear- both as clasts in noritic rocks and as ance. Muehlberger (1974) interprets it to isolated samples. Although the noritic be an asymmetric compressed fault, part breccias are broadly similar to KREEP of postmare structural adjustments in rocks, they contain only one-half the southeastern Serenitatis. minor and trace element content (LSPET, Observations made at the site indicate 1973b). that the dark unit is composed of igneous The ages of Apollo 17 basalts were found rock which is similar to the mare basalts; to be close to those of the Apollo 11 site, a thick pyroclastic mantle was not obvious. about 3.7 billion years old (Tera et al., The observations were confirmed in the 1974). The highland rocks gave crystalliza- laboratory where the basalts showed a tion ages of approximately 4.0 billion years high titanium content in the form of the (Tera et al., 1974). However, age dating of mineral ilmenite much like the Apollo 11 highly crushed dunite fragments of a clast basalts (LSPET, 1973b). This tends to in a South Massif boulder sample (72435) support the studies by Adams and McCord gave a model age of 4.48 billion years (1971) which suggest that the albedo of (Albee et al., 1974). From its composition the mare unit depends partly on chemical and old age, these authors tentatively composition. In this case the darker the conclude that the rock must represent a unit, the higher the titanium content. very early differentiate derived from the At Shorty crater (Fig. 7), the astronauts upper lunar mantle, and that it represents encountered an orange-colored layer of fine a cumulus formed during early lunar grained material which appeared during differentiation and associated differential the mission to be the result of fumarolic gravitational settling. activity, i.e., alteration by oxidation of the Particle-track measurements on target host rock. This observation, however, did materials carried to the lunar surface on not hold in the laboratory and the orange Apollo 17 have provided new data on heavy material was found to be made of very fine ions from solar and galactic cosmic rays grained glass spheres, yellow, orange, in the low energy range (LSAPT, 1973). brown to black in colour with high lead, Previously unknown "quiet time" fluxes copper, zinc and chlorine content (LSAPT, of ions from carbon to iron in the energy 1973). Although some investigators believe range of .05 to 1 million electron volts per that the glass spheres may be impact nucleon were observed. Also, the large 512 FARO UK EL-BAZ

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FIG. 8a. Distribution of mare material on the near side of the- Moon. The mare material is concen- trated in multiringed circular basins (Hartman and Kuiper, 1962) and in their peripheral troughs. Kaula et al. (1973) have determined a 2 to 3 km offset toward the Earth of the Moon's center of mass from its center of figure. They ascribe this to a variable thickness of low-density highland crust, thinner on the near side than on the far side. The maria are therefore concentrated on the near side MOON AFTER APOLLO 513

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st£y%:£ because the mare basalts could more easily penetrate the thinner crust. Numbered are Oceanus Procellarum (1) and the maria of Imbrium (2), Cognitum (3), Humorum (4), Nubium (5), Frigoris (6), Serenitatis (7), Vaporum (8), Tranquillitatis (9), Nectaris (10), Humboldtianum (11), Crisium (12), Fecunditatis (13), Marginis (14), Smythii (15), and Australe (16). (Drawing from Masursky et al., 1975; base map by the National Geographic Society). 514 FAROTJK EL-BAZ

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FIG. 8b. Distribution of mare material on the far side of the Moon. The presence of fewer maro regions on the far side than on the near side is ascribed to a thicker highland crust (Kaula et al., 1973); basaltic lavas would have more difficulty in penetrating a thicker crust to reach the surface. Local thinning of the crust through excavation by basins, and particularly by craters within the MOON AFTER APOLLO 515

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basins, may explain the observed distribution of maria on the far side (see also Fig. 12). Numbered are the maria of Marginis (14), Smythii (15), Australe (16), Moscoviense (17), Ingenii (18), and Orientale (19). (Drawing from Masursky el al., 1975; base map by the National Geographic Society.) 510 FAROTJK BL-BAZ

solar flares of August 1972 produced 3. The highland rocks from the mas- relatively large amounts of radioactivity sifs show more petrologic variety than any in the Apollo 17 samples. These flares were other group collected on previous sites. shown to have a larger average particle These rocks might represent both materials energy and greater intensity than anv flare uplifted at the rim of the Serenitatis basin in the past fifteen years (LSAPT, 1973). as well as ejecta from both Serenitatis and In summary, among the most important Imbrium. findings of the Apollo 17 surface explor- 4. The age of the highland rocks of ation are the following. Apollo 17 (about four billion years) lends 1. The basalts contained in the Taurus support to the idea of a cataclysmic event -Littrow Valley are grossly similar to or events which affected much of the near those from Mare Tranquillitatis in their side of the Moon around 3.9 to 4 billion high content of titanium, as well as in their years ago as suggested by Tera et al., 1973 age. The age of these basalts, about 3.7 and Albee et al., 1974. However, discovery billion years, contradicts earlier expecta- of older dunite fragments (about 4.48 tions that the Apollo 17 basalts would be billion years old) also indicates that earlier young. differentiates may still be preserved in the 2. Orange, brown and black spheres lunar crust. of glass that are probably the product of volcanism are also 3.7 billion years old. III. GLOBAL CHEMISTRY Thev may represent a more complex history of volcanism in that region. High Mare materials form only about one-fifth concentrations of volcanic glass spheres of the lunar surface. The distribution of may be responsible for the apparent maria is an obvious indication of the smoothness of the dark mare unit. asymmetry of the Moon, more mare

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:•:•:•:•: -so TO -TOO MILLIGALS ^H-100+ MILLIGALS FIG. 9. Gravity anomalies on the lunar near side and limb regions. Mascons (circular areas) have mass excesses of 800 kilogram/m2. These occur in multiringcd circular basins and are interpreted by Sjogren et al. (1974), to bo near surface, disc-shaped features. A circular or ring structure is evident at Marc Orientale on the western limb of the Moon; other ring structures are less evident near the other mascons. (Courtesy of W. L. Sjogren, Jet Propulsoin Laboratory.) MOON AFTER APOLLO 517 material on the near side than on the far ials, although suggested by telescopic side (Fig. 8). observations and photogeologic interpre- Large exposures of mare materials can tations, was first indicated by the ^-scat- be divided into two types of occurrences: tering elemental analyses made by (1) the probably thick (tens of kilometers) Surveyors V and VI (Fig. 1) in Mare inner fill of circular basins; and (2) the Tranquillitatis and Sinus Medii, respective- relatively thin (a few kilometers) cover of ly (Turkevich et al, 1969). The widely basin troughs and other low-lands. As first spaced locations of the Surveyor V and VI discovered by Muller and Sjogren (1968), sampling sites, as well as Apollo missions the first occurrences display large positive 11, 12, 15, 17, and Luna 16 sites (Fig. 1), gravity anomalies or mass concentrations leave no doubt that basalts are the major (mascons). The gravimetric data, which component of all the lunar maria. are available for the nearside and limb Although Jackson and Wilshire (1968) regions, show mascons underlying the point out that there are gross similarities maria of Smythii, Crisium, Nectaris, Seren- between lunar basalts and flood basalts on itatis, Imbrium, Humorum, and Orientale Earth (e.g., Columbia River Plateau (Fig. 9). However, large mare expanses of basalts), Melson and Mason (1971) and irregular shape, such as Oceanus Procel- Wood (1974a, b) discuss differences be- larum, do not show a positive gravity tween the lunar basalts, terrestrial basalts anomaly; both topography and gravi- and basaltic meteorites. The basic differ- metrics indicate that the mare material ence between lunar and terrestrial basalts in this region formed only a shallow veneer. is that the former are depleted in sodium, The basaltic composition of mare mater- along with all other volatile elements.

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0— g .45 EH 45-55 |vl .55 Pi .55-.65 na .65 ■I .65-.75 Fiu. 10 Aluminum : silicon concentration ratios as detected by the X-ray experiments on Apollo missions 15 and 16 (Adler et al. 1973). Higher values for aluminum show over highland areas and lower values over maria; intermediate values occur where there is much mixing between the two types of materials. The magnesium: silicon concentration ratios show a converse relationship—higher values over maria and lower values over highlands. (Courtesy of I. Adler, University of Maryland.) 518 FAEOUK EL-BAZ MOON AFTER APOLLO 519

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Therefore, the plagioclase of lunar basalts Apollo and Luna missions originated from contains lesser amounts of the albite differentiates; they have the specialized component. chemical compositions that are character- As discussed above, lunar mare basalts istically produced by magmatic differ- are not compositionally or texturally entiation. This indicates that the part of uniform within a given site or between the Moon that was sampled had been sites. Variations are common in grain size, molten at one time or another. The basic crystallinity, and vesicularity, as well as in pattern of primary differentiation may detailed composition. These variations have been relatively simple, but the are most probably due to minor differences picture has been greatly complicated by in the source materials beneath the surface, the tendency of large cratering events and as well as to variations in thickness and lava eruptions to transport and mix cooling rates of individual lava flows. chemical entities during and after the Mare basalts contain greater amounts original differentiation (LSAPT, 1973). of iron and titanium than do the anortho- The chemical pattern that was developed sitic and feldspathic highland materials, by studying the returned samples was which are enriched in aluminum and confirmed by the orbiting geochemical calcium. These iron-rich basalts are also sensors on Apollo missions 15 and 16, which depleted in radioactive elements. The carried X-ray and y-ray spectrometers. presence of some iron-rich intermediate The X-ray sensor measured the character- rocks, particularly in the Apollo 14 site istic fluorescence signatures of aluminum, (samples 14053 and 14072) prompted Gast silicon, magnesium, and iron that were (1973) to suggest that the typical mare excited by impinging solar radiation basalts may be derived from iron-rich (Adler et al, 1973). The Al/Si intensity source regions that have been subjected to profiles clearly show low Al/Si values over previous partial melting. the lunar maria, and higher values over Highland materials, which make up terra materials, especially on the lunar about four-fifths of the lunar surface are farside (Fig. 10). Intermediate Al/Si chemically and petrologically more com- values usually correspond with boundaries plex than the mare materials. In general, between the basaltic mare units and the the sampled highlands are feldspathic anorthositic highlands. A reverse correla- breccias. The common brecciation, re- tion generally exists in Mg/Si ratios, where crystallization, and apparent partial melt- mare units show a higher value than the ing is consistent with patterns of over- highlands (Adler et al., 1973). lapping ejecta from numerous impacts. The y-ray spectrometer measured con- The major rock types are: feldspar-rich centrations of natural radioactive emis- rocks, especially the type called anorthosite; sions from the lunar surface (Fig. 11). The KREEP basalts with a relatively high highest counts, which correspond roughly concentration of radioactive elements; to about 10 ppm thorium, are confined to "Very High Aluminum" (VHA) basalts the western near side of the Moon, namely (Hubbard et al., 1973) which are common two regions in Mare Imbrium and Oceanus in the Apollo 16 samples; and the anortho- Procellarum (Metzger et al., 1973). Based sitic gabbros of Apollo 17. There are several on the aforementioned discussion of sample variations in rock types, where petro- compositions at Apollo 12 and 14 sites, graphers recognize at least a half dozen this high radioactivity probably is due to categories of highland breccia rocks the presence of KREEP basalts. This leads (LSAPT, 1973). A number of truly igneous to the conclusion that the areas of high rocks have also been collected in the high- radioactivity must contain exposures of lands, but only a few of these may have ejecta from the Imbrium basin. The hitter crystallized from internal lunar melts; most is the largest of the impact basins on the are undoubtedly the product of impact lunar near side, and, therefore, may have melting. penetrated through originally radioactive All the lunar rock and soil returned by parts of the crust. A high radioactive spot MOON AFTER APOLLO 521 in the vicinity of the crater Van de Graaff fidently identified as older equivalents on the far side is also within a very old and of more clearly expressed features. large basin, even larger than Imbrium Several attempts have been made to (Stuart-Alexander, 1975). The localization classify features by relative age according of the highly radioactive rocks supports the to topographic and morphologic char- deduction that they cannot represent acteristics. A stratigraphic sequence was average lunar surface materials. first developed by Shoemaker (1962) and Groundtruth derived from the study of Shoemaker and Hackman (1962) of an returned samples from six Apollo missions area around the crater Copernicus. This and two Luna missions are, therefore, sequence was later modified and applied to extrapolated, by "orbital truth" data, to most of the near side of the Moon by U.S. larger areas of the Moon. Geological Survey mappers as summarized by McCauley (1967), Wilhelms (1970), Wilhelms and McCauley, (1971), and Mutch IV. EVOLUTION OP THE SURFACE (1972). Also, it was recently realized that Before the return of lunar samples, an extension of the nearside lunar strati- geologists were limited to establishment graphy could be employed on the far side as of a relative time scale to describe events well (e.g., EL-Baz, 1974; Wilhelms and and major provinces on the Moon. This El-Baz, 1975). relative age time scale can effectively be At the present time, a Moon-wide time- applied to the entire Moon, most of which stratigraphic sequence exists, where sur- will not be sampled. The absolute time face units belong to one of the following, scale, which is based on age dating measure- in the order of decreasing relative age. ments of the lunar rocks, can now be used 1. Pre-Nectarian. All materials form- to calibrate the relative time scale, as will ed before the Nectaris basin and as far be discussed below. back as the formation of the Moon are classed as pre-Nectarian. The majority of A. Relative Ages pre-Nectarian units are distinguished Global morphology of the Moon is on the lunar far side. These include mater- controlled by the large features of impact ials of very old and subdued basins such origin known as multiring circular basins as Al-Khwarizmi (El-Baz, 1973), and (Fig. 12). From the time of its formation, mantled and subdued craters cuch as the crust of the Moon has probably Pasteur (Wilhelms and El-Baz, 1975). been repeatedly shaped by basin and 2. Sectarian System. This system in- crater formation (Baldwin, 1969; El-Baz, cludes all materials stratigraphically above 1973; Howard et al., 1974). Materials of and including Nectaris basin materials, up those basins and smaller craters compose to and not including Imbrium basin strata most of the lunar highlands. As discussed (Stuart-Alexander and Wilhelms, 1974). above, mare materials, of later volcanic Ejecta of the Nectaris basin that can be origin, are concentrated in the basins and traced near the east limb region allows their peripheral troughs, and the nearby recognition of these materials as a strati- lowlands. graphic datum for the farside highlands. Thus, the main surface sculpting mechan- Younger than Nectaris ejecta, hence isms on the Moon appear to be meteorite grouped under the Nectarian system, are bombardment and the downslope move- materials of such basins as Humbold- ment of fine-grained debris from higher to tianum, Moscoviense, and Mendeleev. lower levels, aided by tectonic and/or Some plains units, particularly on the seismic shaking. Study of fresh-appearing far side, are believed to be Nectarian in age and young-looking features can, therefore, (Wilhelms and El-Baz, 1975). help in understanding relatively older 3. Imbrian System. A large part of the ones. As discussed in detail by Wilhelms lunar surface is occupied by ejecta sur- and McCauley (1971), subtly expressed rounding both the Imbrium and Orientale forms can be recognized and often con- basins. These form the lower and middle 522 FABOUK EL-BAZ MOON AFTER APOLLO 523 parts of the Imbrian System, including One age about which most investigators units such as the Fra Mauro Formation, agree is the model age of the lunar soil, Hevelius Formation, and Cayley Forma- 4.6 billion years. As pointed out by Tatsu- tion, the latter being light-plains material moto and Bosholt (1970), this age is of Imbrian age. Two-thirds of the mare comparable with the age of meteorites materials belong to the Imbrian System, and with the age generally accepted for particularly in the eastern maria of the Earth. The same figure presumably Crisium, Fecunditatis, Tranquillitatis, represents the age of the Moon; however, Nectaris, the dark annulus of Serenitatis, the "Lunatic Asylum" of the California as well as most mare occurrances on the Institute of Technology files a complaint lunar far side. This system also includes that the age of the Earth and the Moon are materials of numerous large craters such as lower than the accepted age of meteorites Plato, Arzachel, Petavius, and Tsiol- by 0.1 billion years (Tera et al., 1974). kovskij. It was shown above that there are 4. Eratosthenian System. This system chemical-petrological indications of an includes materials of ray less craters such as early differentiation resulting in the form- Eratosthenes. Most of these are believed ation of the Moon's crust. Age-dating to have once displayed rays that are no results of highland rocks give indirect longer visible because of mixing due to indications of the same thing. Studies by prolonged micrometeorite bombardment Nyquist et al. (1973) suggest differentation as well as solar radiation. The system also ages of 4.3-4.4 billion years. Tera et al. includes about one-third of the mare (1974) conclude from all the available surface materials on the lunar nearside. absolute age data that planetary differ- These are generally concentrated in entiation processes took place between Oceanus Procellarum, in western Mare 4.6 and 4.3 billion years, and that the crust Ibrium, and possibly the central region of is 4.43 billion years old. Mare Serenitatis. The solid crust must have continued to 5. Copernican System. This is strati- receive impacting projectiles. Reshaping graphically the highest and, hence, the of the lunar physiography, accompanied youngest lunar time-scale unit. It includes by resetting of the radioactive clocks, materials of fresh-appearing, intermediate continued with each major impact erasing to high albedo, bright-rayed craters. The most of the earlier history within its sphere system also includes exposures of very of influence. From the uranium-lead high albedo material on innerwallsof Coper- evolution data on highland rocks from nican and older craters, as well as other the four Apollo sites, 14, 15, 16 and 17, scarps. Brightness in these cases is inter- Tera et al. (1974) confirm that a major preted as the result of fresh exposure by lunar cataclysm occurred at 3.9 to 4.0 mass wasting and downslope movement billion years on the near side of the Moon. along relatively steep slopes. The Coper- In addition to other materials, ejecta of nican System also includes isolated occur- the Imbrium basin has most likely been rences of relatively small dark-halo craters. sampled at all four Apollo sites. The ages Many of these are probably impact craters, of rocks from the Fra Mauro Formation however, the possibility that some may be (Apollo 14), a distinct ejecta unit of the volcanic in origin cannot be discounted. Imbrium basin, cluster at 3.95 billion years (Tera et al., 1973). It is, therefore, B. Absolute Ages probable that the 3.9-4.0 billion year old As previously discussed, many tech- cataclysm is either: (1) the formation of niques have been employed in absolute the Imbrium basin itself, or (2) the forma- dating of lunar rock and soil, sometime tion in quick succession (within 0.1 billion yielding different results. However, some years) of the large nearside basins, i.e., generalizations could be made particularly Serenitatis Nectaris, Imbrium and possibly from the strontium-rubidium and uran- Orientale. ium-lead data. The majority of sampled mare basalts 524 FAROUK EL-BAZ show ages between 3.2 and 3.7 billion about one billion years ago, if the noritic years. The distinction is very clear between glass component of the Apollo 12 soil is at least two ages of mare basalts. One group correctly interpreted as Copernicus ray clusters around 3.2 billion years, repre- material. Cosmic-ray exposure ages of senting samples of Apollo 12 and 15 basalts. rocky debris from Cone crater, the domin- The second group gives ages around 3.7 ant landmark at the 14 site, show that it billion years, representing basalts of was excavated about 25 million years ago. Apollo 11 and 17. The Luna 16 basalts of Similarly, in the four billion year old Mare Fecunditatis are closer to the latter highland units, cosmic-ray exposure ages group, being about 3.5 billion years old. of rocks and boulders ejected from the This quite convincingly correlates with the craters in the Apollo 16 site (Fig. 6) show two major mare episodes discussed earlier that North Ray crater was excavated that belonged to the Eratosthenian and the 50 million years ago while South Ray Imbrian systems. The Apollo 12 and 15 crater occurred onlv 2 million years ago basalts are Eratosthenian in age (3.1 to (LSAPT, 1973). The time of formation 3.3 billion years old) and the Apollo 11 and of Shorty crater in the Apollo 17 site, based 17 and Luna 16 basalts are Imbrian in age on the exposure age of orange and black (3.5 to 3.7 billion years old). soil samples, was determined to be 20 to In addition to rock ages, the times of 30 million years ago (Eberhardt et al., formations of lunar craters have also been 1974). determined radiometrically (LSAPT, Age sequences like these could be used to 1972). For example, the crater Copernicus calibrate the relative age time scale already on the south rim of Imbrium was formed established among time-stratigraphic and

TIME IN BILLIONS OF YEARS 0- COPERNICAN ornatior. cf" smaller ::rat SYSTEM orth Ray, Cons , Shorty ,

'.pact craters id marie. ERATOSTHENIAN SYSTEM

Ycunr, basalts (Apollo ] " ^:,(J -5;

3.5- Old basalts (Luna If) IMBRIAN SYSTEM Older basalts (Apollo i: ?.na 17)

NECTARIAN ij - SYSTEM

PRE-NECTARIAN \ — 4.5 i Oldest rock (Ape

Fie;. 13. Schematic illustration of the evolution of the lunar surface. The absolute time scale in billions of years as deduced from age and exposure date techniques is shown along the median line. On the right, major events and episodes of material emplacement are shown; on the left is the lunar- wide relative age scheme based mainly on superposition relationships of geologic units. MOON AFTER APOLLO 525 rock-stratigraphic units. An attempt is V. LAYERS OF THE MOON made here (Fig. 13) to correlate the relative age scheme derived mainly from strati- A. The Outer Skin graphic relations and photogeologic inter- Details of the broad-scale topographic pretations with the absolute ages of relief of the lunar surface were provided by returned rock and soil samples. laser altimeter measurements made on In a nutshell, the Moon is assumed to Apollo missions 15 through 17. The alti- have formed 4.6 billion years ago; global meter measured precise altitudes of the planetary differentiation processes took orbiting Command Service Module above place between 4.6 and 4.3 billion years ago. the lunar surface (Wollenhaupt and Sjo- As the solid anorthositic crust was formed gren, 1972). Measurements were made at it kept a record of moonlets that struck it points spaced every one to one-and-a-half leaving numerous scars (pre-Nectarian lunar degrees, or every 30 to 45 km on the time). The last of the impact basins were surface. The agreement between measure- formed on the near side about 4.0 to 3.9 ments on the three missions emphasizes the billion years ago (Imbrian System, starting accuracy of the profiles. about 3.95). Volcanic eruptions filled the Figure 14 is one example of an Apollo 15 basins and low lands with basalts from 3.7 profile where the measurements were made to 3.2 billion years ago (Imbrian and in a plane inclined at approximately 26° Eratosthenian time). Postbasalt history with respect to the lunar equator. Lunar included transport and mixing of the lunar elevations were computed relative to an materials, mostly by impact craters, that assumed mean lunar radius of 1738 km. continues to the present day (Copernican It is clear from Fig. 14 that the far side is System). over 4 km higher than the near side on the

Apennines 1738-km mean lunar radius

Longitude, deg

Gagarin

Longitude, deg

FIG. 14. Apollo 15 laser latimeter profiles of the lunar near side (top) and far side (bottom). Eleva- tion data are computed relative to a sphere of 1738 km radius with respect to the lunar center of mass. (After Robertson and Kaula, 1972.) 19 526 FAEOUK EL-BAZ

average. Figure 14 (top) also shows the ID 3 greatly depressed mare basins, on the near side, e.g., Smythii, Crisium, and Seren- ' CO itatis. g a g Another method of generating elevation profiles of the lunar surface was provided s g s s SO'8 by the lunar sounder (Phillips et al., 1973). 9 130 m, and an estimated relative accuracy of 5 m over mare surfaces. The same t- p ft authors compared the radar profiles with the Apollo 17 laser altimeter measure- q,aO.SB . ments. Although these two sets of data < * S were not acquired simultaneously, they ID _J" 3 J3 ■~ CO agree within 150 meters over mare sur- * ^ "H faces. Brown et al. (1974) used a 1738 ^ "H ^H km sphere about the lunar center of mass

c8 and the center of figure, as well as a 1734 g S km sphere about what they termed "center 3 cS of maria" (Fig. 16). To generate the latter, they used the mare surfaces of western Procellarum, Serenitatis and Crisium; they found that Mare Smythii falls within 40 meters of that circle. This suggestion of a center of maria gives ' i—I fT\ !fi TO credence to the idea that lunar lavas rose o •2 g c3 4-, only to a certain level, at least in large TS O 5 El M quantities. Examination of Fig. 16c shows

4-, that the northern floor of the large basin o _fl CD 3 on the far side at about 180° is near the same be c3 mare level. This basin encloses the largest concentration of mare patches on the n ^ £ ■? far side (Gornitz, 1974, and Stuart-Alex- TJ S -§ 3 « c « ^ ander, 1975). As mentioned previously, it o % -p also contains the highest y-ray activity on the far side (Metzger et al., 1973), and the 3 CO ia cr g u O strongest intrinsic magnetic field as mea- £ '3i « sured by the subsatellite magnetometer of a ° 3 CD Apollo 15 (Russell et al., 1973). Lunar •SP.S S 3 ^ <3 n 5 magnetism, its cause and probable origins h a are discussed by Fuller (1974), and its 5 3 X, " > o += —s relationship to the interior of the Moon O U IO CD by DyaleW., (1974). 1-1 r3 &3 3~ From both the laser altimeter and radar CD .2 sounder profiles, it is clear that the impacts h 3 — • S * that formed the multiringed circular basins resulted in an enormous loss of mass from MOON AFTER APOLLO 527

FIG. 16. Radial plots of lunar sounding radar profiles about: (A) the center of mass, where the radius of the sphere is 1738 km; (B) the center of figure, where the radius of the sphere is also 1738 km; and (C) the center of maria, where the radius of the sphere is 1734 km. The radial scale in all three cases is 8 km/cm. (After Brown et al., 1974.) the impact sites. These same basins, as depressions, and also topographically low discussed earlier, display large mascons. plains. The additional weight of tens-of - Some older basins were probably formed kilometers thick, dense basalts would, early enough to be essentially obliterated therefore, contribute to the positive gravity by intense cratering and volcanism (Stuart- anomaly within the isostatically com- Alexander and Howard, 1970). Others pensated circular impact basins (Wise and were formed by the impact of very large Yates, 1970; and Wood, 1970). objects in a solid lunar crust which was underlain by an upper, partially plastic B. The Lunar Interior mantle. The partially plastic mantle may With respect to the shallow lunar inter- have allowed for an upward movement to ior, it was realized from the first seismo- isostatically compensate for the lost mass meter measurements of impacts on the and its redistribution by the impact. At lunar surface that the travel times are a later time, upward moving volcanic lavas relatively low (Latham et al., 1970). The used as channelways the fractures that observed travel times, and the relatively were created by the impacts. The lavas low seismic velocity in lunar near-surface erupted to the surface to fill the circular rocks, are interpreted by Watkins (1971) 528 FAROTJK EL-BAZ to mean either, (1) that lunar near-surface 0" Serenitatis rock consists of cold, participate, unfused material accreted from space; or (2) that lunar near-surface rock has fused at some time during the evolution of the Moon, but subsequently has been brecciated and fractured to considerable depth by meteor- ite impact. Although Watkins favors the cold accretion model, the second model is favored by most investigators: first, there is ample photogeologic and petrographic evidence of an early melting, and second, there is evidence of repeated fracturing and brecciation of the lunar rock to a depth from a few km to 25 km by meteoroid bombardment (Latham et al., 1972). Our knowledge of the deep lunar interior consists of inferences based on (1) the geophysical data provided by the network

of geophysical stations or observatories at ■Center of mass the Apollo sites 12, 14, 15 and 16 (Fig. 1); and (2) the chemistry and mineralogy of FIG. 17. Inferred cross-section of the lunar the surface samples. Of special importance interior. It shows a crust 65 km thick, with a are the recordings of natural moonquakes highly fractured upper 25 km; a mantle 1100 km by the seismometer network which allowed thick; and a core that is either partially molten or includes a molten zone. This cross section the determination of their origin time, represents a segment of the Moon drawn to scale epicenter, and focal depth. Also, seismic from a sphere 1738 km in radius from the center data, mainly from man-made impacts, of mass. Surface features correspond roughly to revealed a major discontinuity at a part of the near side topography shown in depth of between 55 and 70 km in eastern Fig. 14 (top). Oceanus Procellarum (Latham etal., 1971). Additional data were obtained from a large meteoroid impact that occurred on the of velocity down from the surface due to far side of the Moon on 17 July 1972, where pressure effects on dry rocks. There is also direct shear-wave arrivals were not a minor discontinuity at a depth of about observed at some of the Apollo seismic 25 km which may be related to intensive stations. This led to the conclusion by fracturing of the upper crust and/or to petrological differences. The seismic velo- Nakamura et al. (1973) that the material l in the lunar interior at a depth of 1000 to city is nearly constant (6.8 km sec ) 1100 km may be in a partially molten between 25 and 65 km (Toksoz et al., 1972). 2. Mantle: at the major dicontinuity state. From these data and an analogy with at about 65 km depth, there is a large in- the Earth's interior, I previously sum- crease in the velocity to about 9.0 km sec '. marized a model of the lunar interior This interface is comparable to the rise (El-Baz, 1974) where the successive layers of velocity at the Mohorovicic discon- from top downward are as follows (Fig. 17). tinuity on Earth (Toksoz et al., 1972). 1. Crust: a layer enriched in alumin- This layer appears to continue to the depth um and calcium (anorthositic gabbros), of 1100 km. Compositions of the surface approximately 60 km thick in the region of rocks, the measured seismic velocities, the Apollo seismic network, as compared and constraints imposed by the Moon's to the 5 to 35 km thick crust on Earth. In mean density and moments of inertia this layer there are local mass concentra- support a pyroxene-rich composition for tions of iron-rich rocks, and a rapid increase the lunar mantle. MOON AFTER APOLLO 529

3. Core: the moment of inertia and explain chemical differences between the the overall density of the Moon place an Earth and Moon. upper limit of about 500 km on the radius In this theory the Moon is envisioned as of the lunar core, if a core exists (Toksoz a body that was captured from an orbit et al., 1974). A molten or partially molten about the Sun to one around the Earth. As silicate zone or core below about 1100 km discussed by Wood (1974b) some authors from the surface (Nakamura et al., 1973) believe that the Moon was captured may be responsible for deep-seated moon- intact; others suggest that in the capturing quakes, which are focused between 800 and process, disruption and reaccumulation 1100 km deep. The data however, do not occurred. Proponents of this theory have rule out the possibility of a small, 400 km looked for ways to decelerate the approach- radius, iron-rich (Fe-FeS?) core (Solomon ing Moon to allow capture by the Earth. and Toksoz, 1973). This may be staged by tidal interaction between the two bodies, or by collisions between the approaching Moon with VI. QUESTIONS OF ORIGIN smaller objects circling the Earth (Kaula and Harris, 1973). There are three basic theories of origin Other ways were sought for selective of the Moon. She is either Earth's wife, capture of small bodies into geocentric captured from some other orbit; daughter, orbits without resorting to tidal or colli- fissioned directly from "proto" Earth; or sional deceleration (Opik, 1972; Wood and sister, accreted from the same binary Mitler, 1974). According to the latter, system. silicate-rich material can be captured and Before Apollo, it was widely believed metal-rich bodies rejected into the helio- that the lunar exploration missions would centric orbits. However, Wood (1974b) provide the necessary information for a states that the process, being inefficient, final answer to the question of origin. would not result in complete rejection of This did not happen. As discussed above, metal-rich fragments. the primitive lunar crustal materials have been subjected to much metamorphism, B. Fission due to large impacts. The idea that the Moon came directly Thus, the three basic theories of lunar from the Earth was first proposed by Sir origin (Fig. 18) are still competing, espe- George Darwin in 1880. The theory is still cially since all three have been modified to viable and in recent years has been accommodate the new findings. Each supported by Wise (1963 and 1969) and theory has its share of positive points as O'Keefe (1970). well as shortcomings, and these vary The present concept of the fission theory depending on whose paper one reads. starts with a fluid body rotating around However, as discussed by Wood (1974b), the Sun. As heavier components settle at one of the most important constraints on the core and lighter ones float on top, the the theories of origin is the chemical spin is increased to cause rotational makeup of the Moon. Any theory will have instability. The latter causes elongation of to account for the lower metal and volatile the body perpendicular to the spin axis, content, and the higher refractory element until creation of two separate bodies is content in the lunar rocks, as compared to achieved. the compositions of terrestrial rocks and This theory accounts for the chemical meteorites. differences between the two bodies. Fission would have occurred after the formation A. Capture of a core in the part of the body that formed The basic attractiveness of the capture the Earth. The Moon-forming part of the theory is the assumption that the Moon body would then be depleted in metals. was formed somewhere else in the solar Binder (1974) believes that sampled lunar svstem, therefore, one does not have to rock types could be made of materials that 530 FAROUK EL-BAZ

hyperbolic orbits CAPTURE FISSI 'BINARY ACCRETION

FIG. 18. Schematic illustrations of the three theories of lunar origin: (a) capture : selective capture by the Earth of bodies that accreted to form the Moon; fragments in orbits 1 to 5 are lost in hyper- bolic orbits; those in orbits 6 to 10 are captured in elliptical orbits (from Wood and Mitler, 1974); (b) fission of the Moon from "mother" Earth as it was spun to instability during core formation (after Wise, 1963); and (c) binary accretion of both the Earth and Moon from the solar nebula. The Earth was formed first, and heavy metals settled to form a core. Lighter fragments in geocentric orbits accreted by collision to form the Moon (adapted from Ringwood, 1972). are chemically equivalent to those of nebula along with the terrestrial planets. Earth's mantle. In this context, WTood (1974b) discussed Ringwood (1970) added a new dimension the differences between what he termed the to the fission theory by proposing that the American and Russian schools of thought. temperature of the accreting Earth could The first (championed by Cameron, 1973) have been high enough to form a thick, hot pictures a massive solar system nebula in atmosphere of metal and oxide vapors. which rapid accretion of the terrestrial These vapors could have been spun out of planets causes extensive heating and the equator to form the Moon as accretion melting. On the other hand, the Russian proceeds in Earth orbit. school (established by Schmidt, 1958) Whether fission occurred from a solid views the planets as accretionary products body or by escape of hot gases, it appears of slowly accumulating planetismals from that the major objection to the theory is a small solar nebula (Wood, 1974b). The the excessive amount of angular momen- global chemistry of the Moon indicates tum required for the process (Wood, probable melting of at least the outer 100 1974b). km. Therefore, it seems logical to consider a rapid accretionary process to have caused C. Binary Accretion such melting as the Moon formed. This theory envisages both the Earth In simple terms, one can envision a and Moon accreting together from the solar rapidly rotating accretionary system with MOON AFTER APOLLO 531 a dense proto-Earth and a swarm of 4. The low-density highland rocks planetismals, dust and gases in geocentric are exposed everywhere on the lunar crust orbits. Mutual collisions among the mem- except where covered by denser basalts. bers of the swarm tended to round up their These are the products of internally orbits and bring them into a common generated volcanic melts which spread on plane. From these colliding planetismals, the surface during a period of 600 million a moonlet was formed. The moonlet swept years (between 3.7 and 3.2 billion years the circum-terrestrial orbit by attracting ago)- more and more planetismals. The accre- 5. There are many indications of the tionary energy would have resulted in shaping of the Moon by large impacts, melting of the uppermost layers. Also, in basin and crater formation being the major the process much of the volatile elements sculpting mechanism on the Moon. Shock would have been lost from the growing effects on the rocks attest to the violent moonlet. This view of the binary accretion impact history. Although the bombard- theory shares some aspects with the ment is now continuing, the size and precipitation theory of Ringwood (1970 frequency of impacting projectiles were and 1972). much larger in the early history of the This theory requires the least number of Moon. assumptions. It also accounts for all the 6. As confirmed by the orbiting chemical differences between the Earth geochemical experiments, samples col- and Moon. For these reasons, it has been lected at the landing sites most probably the most widely accepted theory among represent the whole Moon. lunar scientists in recent years. 7. The figure of the Moon shows pro- nounced asymmetry; the far side is higher, and the near side is lower, relative to a VII. FUTURE OUTLOOK mean lunar radius of 1738 km. Also, the In the past decade, our knowledge of the center of mass is shifted about 2 km Moon has evolved through the findings of towards the Earth relative to the center Ranger, Surveyor, Lunar Orbiter, Luna of the figure. and Apollo missions. However, six manned 8. There is lateral asymmetry in Apollo lunar exploration missions provided natural y-ray radiation on the Moon. the most tangible of data. Our understand- Pockets of radioactive materials are con- ing of the Moon grew systematically and centrated in the western near side; only a chronologically from the rich harvest of small anomaly in the vicinity of the crater rocks and soil as well as remotely sensed Van de Graaff exists on the lunar far side. information gathered by Apollo. 9. The moon is probably layered into To me, the most important findings from a crust, about 65 km on the near side and Apollo lunar exploration are the following. probably thicker on the far side; a solid 1. The lunar samples indicate that the mantle, about 1100 km thick; and a silicate Moon does not now, nor did it ever, host core, about 500 km thick, that is in part any water or life forms of any kind. molten or contains a partially molten 2. The Moon is made up of the same zone. chemical elements as is the Earth, but in 10. The data so far collected do not varying proportions. The lunar rocks are provide enough proof for final conclusions enriched in refractory elements and de- regarding the origin of the Moon. pleted in volatile elements. Only two years have passed since the last 3. The Moon is a differentiated body: Apollo mission, and only about 10% of the extensive melting may have occurred in lunar rocks and soil have been studied and the upper layers of the Moon, to about 100 analyzed in great detail. Much of the data km depth, at the time of accretion. As the obtained by sensors from lunar orbit is hot magma cooled, light plagioclase cry- yet to be processed to fully extract all its stals floated to form the low-density crust meanings. It will perhaps take two years leaving denser materials below. to process all of these data adequately, and 532 EAROTJK EL-BAZ two additional years for thorough inter- From the geochemical point of view, pretations and correlations. Geophysical there is still much information to be gained data from five ALSEP stations on the about the Moon. Most important among Moon continue to yield new results about these are the lunarwide natural gamma the lunar interior and the lunar environ- radiation fields. The y-ray experiment has ment. All of these stations are operating already shown that there is asymmetry in well and could continue to do so for several natural gamma radiation. The source and more years. Only then could concrete the distribution of this radiation will not conclusions be drawn concerning the be fully understood unless we have data ultimate results of Apollo lunar explor- covering almost the entire Moon. In ation. addition to this, we also need to know After four more years have passed, we more about the global chemistry of the may be fortunate enough to take another Moon, both concerning the distribution of look at the Moon. The National Aero- elements like aluminum, calcium, iron nautics and Space Administration and magnesium in the highlands and in (NASA) is now considering the possibility the maria, and also concerning the distri- of a mission to the Moon. This would be an bution of titanium in the maria. Chemical unmanned Lunar Polar Orbit mission, or mapping of the maria should extend to the LPO. This indeed appears to be perfect lunar far side, perhaps utilizing both X-ray timing; by then, much of the Apollo and multispectral equipment. information would have been assimilated From the geophysical point of view, two and synthesized, at least to a first order types of information appear to be most synthesis. required. First is the definition of an To voice my opinion, some of the things equipotential gravitational surface on the that need yet to be done relate to three Moon. For this, one needs to understand general fields: photogeology, geochemistry, and measure gravitational anomalies or and geophysics. mascons on the lunar far side. Second is the Contrary to widespread belief, the sur- measurements of the figure of the Moon face of the Moon has not yet been adequate- including details of the figure at higher ly photographed. Although Lunar Orbiter latitudes, including the polar regions. IV covered nearly all the near side of the It is hoped that the LPO mission will Moon, this photography can neither be provide such information so that a com- used for measurements nor is it stereo prehensive view of the Moon could be photography. We only have stereometric gained. If we are to apply the knowledge coverage of about 20% of an equatorial gained from lunar exploration to planetary area of the Moon at 30 m resolution. Much exploration, that knowledge should be is yet to be known and learned about the complete or near complete, so that when lunar surface features and their mode of correlations are made they would be formation. One of the most important drawn from clear positions of strength. localities to our understanding of large This is especially relevant since we know impact processes and the transport of from the Mariner 9 data of Mars and the materials on the lunar surface is that of the Mariner 10 data of Mercury, that the Orientale basin. The Orientale basin is not processes that acted on the Moon have covered by stereographic photographs. also been responsible for many features Models of the Moon and of the transport of both planets. of ejecta on the surface have all referred to Orientale because of its freshness as the type of locality to be studied. However, we REFERENCES lack the proper tools for the necessary ADAMS, J. B., AND MCCOBD, T. B. (1971). Optical studies. 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