Geologic interpretation of lunar data

Earth-Science Reviews - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

GEOLOGIC INTERPRETATION OF LUNAR DATA 1 H. J. M O O R E

U.S. Department of the Interior, Branch of Astrogeological Studies, Menlo Park, Calif. (U.S.A.) ABSTRACT MOORE, H. J., 1971. Geologic Interpretation of lunar data. Earth-Sci. Rev., 7(1): 5-33. The application of geologic concepts such as uniformitarianism and erosion cycles to lunar problems is illustrated by comparing terrestrial features and experiments with lunar features. Such comparisons include boulder tracks, blocky craters, volcanoes, and an experimental simulation of meteor bombardment. Lunar rock-stratigraphic units, lateral continuity, and superposition are illustrated using the crater Aristarchus. Reflection-emission data collected from the earth prior to unmanned and manned lunar exploration were remarkably good and captured the essence of the lunar surface at the fine scale. These data showed that the roughness of the lunar surface increased at the finer scale and that the lunar surface materials were porous and probably fragmental. Unmanned explorations significantly refined our knowledge of the lunar surface and its materials. In particular, unmanned soft-landing spacecraft obtained information on lunar topography and the physical properties and the chemical composition of the near-surface materials.

INTRODUCTION

The purpose of this paper is to review some of the data on the Moon collected from spacecraft and terrestrial observations prior to manned landing and to illustrate the application of some basic geologic concepts to these data. The lunar data fit into three categories: (1) visual-photographic, (2) reflectionemission, and (3) surface experiments by spacecraft. Visual photographic data will be used to illustrate uniformitarianism, erosion cycles, rock-stratigraphic units, lateral continuity, and superposition. As illustrations of uniformitarianism, convincing lunar analogs of terrestrial features are discussed, including (1) tracks produced by rolling boulders, (2) blocky debris ejected from large craters, and (3) volcanic craters. Additionally, experimental data on impact and explosive craters show that a soil-like layer (regolith or epilith) overlies hard rock in many places on the Moon. A lunar erosion cycle is implied by smaller craters whose forms range from fresh appearing to so subdued that they are barely discernible. Experiments in which projectiles repeatedly impact into sand and carborundum powder simulate the lunar erosion cycle, bombardment of the lunar surface by debris from space, and the frequency distribution of 1 Publication authorized by the Director, U.S. Geological Survey. Work performed under U.S. National Aeronautics and Space Administration contract R-66.

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smaller craters on the lunar surface. Lunar rock-stratigraphic units are inferred to underlie surfaces having distinctive topography and other characteristics such as light reflectivity-polarization, infrared emission, and radar reflectivity, while continuity is implied by their extent. Superposition is impled by geometric, topographic, and morphologic relationships between units. These concepts can be used to establish relative ages of certain units that are consistent with interpretations of the reflection-emission data. Reflection-emission data include measurements of the response of the Moon's surface to solar and man-made radiation. These responses (reflectivity and polarization of sunlight, radar echoes, infrared emissions, and radio emissions) are consistent with a surface layer of porous fine-grained materials. Infrared data show that large, bright craters with complex topographies have lower than average thermal inertias, while radar echo data show that these craters have higher than average dielectric constants. These results imply that the concentration of rock in these craters is greater than average and, therefore, that the craters are relatively young. Soft-landing spacecraft provided data on the physical and chemical properties of the materials of the lunar surface and the topography and morphology of fine-scale features of the surface. These data show that much of the surface material is porous and fine grained (10--50/~) and has a cohesion n e a r 1 0 3 - 1 0 4 dynes/cm 2 and angles of internal friction between 30 ° and 40 °. Craters, clods, and some blocks dominate the fine-scale lunar scenes. VISUAL-PHOTOGRAPHIC DATA

Uniformitarianism In order to apply the principle of uniformity to the interpretation of lunar visual-photographic data, it must be defined in a broad sense. Here, the principle will be taken to mean that "The history of the Earth (or Moon) may be deciphered in terms of present observations on the assumption that physical and chemical laws are invariant with time" (after HUBBERT, 1967, p.4; parenthetic insertion mine). Extrapolation of our knowledge of terrestrial geologic processes to the Moon is by no means direct. The Moon has no atmosphere to erode and transport materials or to protect its surface from meteor bombardment and solar radiation. In addition, the acceleration of gravity at the lunar surface is one-sixth that at the Earth's surface. For these reasons, observations must include experimental data collected in vacuum, various acceleration fields, and ur.der other non-terrestrial conditions, as well as under normal terrestrial conditions. Nevertheless, uniformity provided a rational basis for early studies of the Earth, and it seems reasonable to apply the concept in a broad sense to lunar problems while recognizing limitations. Thus, the burden of this section is to show that there are closely analogous Earth-Sci. Rev., 7 (1971) 5-33

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terrestrial a n d l u n a r processes a n d t h a t such processes p r o d u c e features with closely a n a l o g o u s m o r p h o l o g i e s . Boulder tracks. Landslides, rockfalls, a n d individual blocks were set in m o t i o n a n d then m o v e d d o w n s l o p e d u r i n g the H e b g e n Lake, M o n t a n a , e a r t h q u a k e o f 1959 (HADLEY, 1964). I n d i v i d u a l blocks bounced, rolled, skidded, a n d " w a l k e d " d o w n s l o p e a n d p r o d u c e d tracks and i n d e n t a t i o n s in the underlying d e f o r m a b l e soils a n d debris. M o t i o n s can be inferred from the m o r p h o l o g i e s o f the tracks p r o d u c e d by the boulders. F o r example, tracks with periodic elliptical depressions

Fig.l. Vertical photograph showing boulders and tracks on the Madison Slide. Boulder 1.7 m across at end of 75 m track is indicated with an arrow. A second track can be seen above the first track. Tracks can be seen from U.S. Forest Service viewing station near Earthquake Lake, Montana. Bar at lower right is 60 m long. Photograph courtesy of U.S. Forest Service. Earth-Sci. Rev., 7 (1971) 5-33

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were produced by bouncing boulders, while continuous tracks with deep depressions that alternate from side to side indicate that the boulder (usually rectangular) "walked" downslope. Normally, tracks represent a combination of motions. The tracks of boulders dislodged by the Hebgen Lake earthquake are still visible today. One 75-m track on the Madison Slide (Fig.l) was produced in coarse debris by a boulder with a diameter of 1.7 m that bounced and rolled down a slope of about 40 ° at the upper part of the track and a few degrees near the lower part. The boulder came to rest behind a pad it produced by displacement of the coarse cohesionless fragments. Some 300 boulder tracks have been found on Lunar Orbiter photographs (ANoNyMOUS, 1969a), most of them in areas photographed by Lunar Orbiter V

Fig.2. Photograph taken by Lunar Orbiter V showing lunar boulders and tracks in Rima Hyginus. White bar is about 100 m long (.Lunar Orbiter V high resolution frame H-95, part of framelets 960-962, lat. 7°40'N; long. 6°15 ' E). Earth-Sci. Rev., 7 (1971) 5 33

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where steep slopes, rocky source-area and deformable debris combine to favor the generation of tracks. The lunar boulders and tracks are closely similar to terrestrial ones (see, e.g., Fig.2). Tracks with morphologies indicating bouncing, rolling, skidding, and "walking" are seen on the photographs. Some boulders at the end of tracks are approximately 50 m across, but they are commonly about 5-10 m across. Smaller blocks also produce tracks once they are set in motion, and many faint linear features below the threshold of identification resolution on Lunar Orbiter photographs are probably tracks. "Mini-tracks" were produced by the rolling and sliding of fragments set in motion by Surveyors V (SHOEMAKERet al., 1967, pp.23-26) and VI (MORRIS et al., 1968, pp.30-32). Boulder tracks may also have practical applications to lunar exploration because the combination of the boulder and track permit the interpretation that the boulder is resting on the surface. Then, with suitable measurements of bearing surface, boulder size, and an estimate of the density of the boulder, approximate bearing pressures can be calculated (see FILICE, 1967, and EGGLESTONet al., 1968). The existence of terrestrial and lunar boulder tracks demonstrates the applicability of uniformity to the interpretation of visual-photographic data on the Moon. In both cases, a mass was set in motion, moved downslope under the influence of gravity, and produced tracks in a deformable material. There are significant differences. Gravitational acceleration at the lunar surface is one-sixth that at the Earth's surface. Thus, the static force exerted by a lunar boulder is one-sixth as much as it would be on Earth. Additionally, no pore fluid pressures would be expected during the deformation of lunar soil-like materials, whereas such pore pressures, due to either air or water, can be important during deformation of terrestrial soils. Blocky craters. Uniformity is further illustrated by comparison of terrestrial craters and lunar craters produced by single disruptive events, such as explosions and impacts. Experience with craters produced by chemical and nuclear explosives has shown that some of the properties of the material in which the craters were produced are reflected in the character of the ejecta (see POLATTY et al., 1965; NORDYKE and WRAY, 1964; and MACIVER, 1967, B-I-B-21). Ejecta around craters produced by explosives in sparsely fractured basalt flows are characterized by a profusion of blocks and fragments ranging in size from a few centimeters to several meters across. Many of the ejecta, however, are very fine grained. The ejected debris are the result of the combined effects ofcomminution of the rock by shock and stress waves and separation along joints and fractures. Although fine debris are markedly affected by the presence of an atmosphere, the large blocks are affected to a smaller degree (SHERWOOD, 1967). it is this coarse debris which reflects some of the physical properties of the basalt, such as the presence of widely spaced joints and fractures in a material with sufficient strength to permit ejection and impact without disintegration upon impact. One such crater in basalt, Danny Boy, Earth-Sci. Rev., 7 (1971) 5 33

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is shown in Fig.3. It was formed by a single disruptive event that lasted several minutes. The ejecta of many lunar craters resemble those of the crater Danny Boy (Fig.4). The lunar ejecta are characterized by abundant large blocks, some over 10 m in diameter. Many of the blocks have rectangular outlines suggestive of separation along pre-existing fractures. There is no evidence of periodic ejections of debris and blocks; apparently a single disruptive event formed these craters. Evidence for ballistic ejection of debris and blocks is found in the numerous secondary impact craters produced by the impact of the ejected material at some distance from the crater rim. Some secondary impact craters can be matched with the blocks that produced them. If ejection angles of 40°-60 ° are assumed, then some of the blocks that produced secondary impact craters traveled at velocities of 12-40 m/sec. The large ratios between the sizes of secondary impact craters and their blocks, combined with such tow velocities, suggest that the uppermost lunar surface materials are

Fig.3. Photograph of terrestrial crater, Danny Boy, produced by an explosive at the Nevada Test Site showing blocky ejecta produced by crater in indurated, sparsely fractured basalt. Crater is about 80 m across. Largest blocks are near 3-5 m across. Photograph courtesy of the U.S. Atomic Energy Commission.

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Fig.4. P h o t o g r a p h o f lunar impact crater showing blocky ejecta. Crater is about 500 m across. Largest blocks are nearly 15 m across. (Lunar Orbiter III high-resolution frame H-189, crater on framelet 628, lat, 2.60 ° S; long. 43.41 ° W.)

easily deformed and soil-like, although the substrate is indurated and sparsely fractured. The extent of the application of uniformity is that both the blocky terrestrial and lunar craters were produced during single events in materials with widely spaced fractures and joints and with sufficient strength to allow acceleration during ejection and deceleration during impact without disaggregation. Since the lunar crater in Fig.4 is probably an impact crater (QUAIDE and OBERBECK, 1968, p. 5255), comparison between it and the explosive craters cannot be carried much beyond the general character of their ejecta. Although both impact and explosive processes involve the transmission of shock waves through the rocks, explosives are normally buried and then detonated, whereas the projectile producing the Earth-Sci. Rev., 7 (1971) 5-33

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impact crater must pass the boundary between space and target material. Lunar volcanoes. Arguments for lunar vulcanism are based on comparisons of the morphologies of terrestrial volcanic features with lunar features. Such comparisons lack the benefit of studies of lunar rocks from such features, although the recent successful manned landings of Apollo 1 l and 12 resolve part of this problem. In any event, some lunar features are so closely analogous in morphology to terrestrial volcanoes and volcanic features that most geologists consider them as such. In the Merriam crater field near Flagstaff, Arizona (Fig.5), and many other places, there are a number of volcanoes and volcanic features which resemble features on the Moon. Typically, high-rimmed craters formed of cinders occur along with flows and fissures. Floors of the craters are higher than the surroundings or at comparable elevations, and their walls and flanks are steep. The craters commonly occur in pairs or alone.

Fig.5. Photograph showing terrestrial volcanic craters near Flagstaff, Arizona. Rims composed chiefly of cinders. Note floors of craters are nearly at same elevation as the lower flanks of the craters. U.S. Geological Survey Photograph. Earth-Sci. Rev., 7 (1971) 5--33

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Craters similar to those described above are found on the Moon. Fig.6 shows a particularly good example north of the Aristarchus Plateau. The flanks of the craters measure 2.5 x 5 kin, and the diameters of the craters are near 1.2 and 1.4 kin. Shadow measurements indicate that the floors of the craters are 220 and 320 m below their rims, whereas the surrounding surfaces are about 290 m below the rims. Floors that are near or higher than the surrounding surfaces are not characteristic of impact craters. Additionally, the narrow smooth flanks slope 24 °-29 ° outward, unlike the gently sloping flanks of impact craters. Careful inspection of the northern rim of one of these lunar craters reveals that a rough lobate feature extends from the crater onto the surrounding mare surface. This

Fig.6. L u n a r volcanic craters a b o u t 1.2-1.4 k m across. N o t e floors of craters at a b o u t the s a m e elevation as the s u r r o u n d i n g level surfaces, the steep slopes o f the flanks, a n d the possible volcanic flow on the n o r t h flank. W h i t e bar is 5 k m long. ( L u n a r Orbiter IV high-resolution p h o t o g r a p h H-158, on framelets 156 a n d 157.)

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lobate feature by analogy is probably a volcanic flow. Thus, these lunar craters are inferred to have formed by volcanism by comparison of their morphologies with terrestrial features. Volcanoes have also been inferred in other areas of the Moon. Domes with summit craters and maars have been described by SHOEMAKER(1962, pp.301-306), and an extensive volcanic terrain has been mapped in the Marius Hills area (McCAULEY, 1968, pp.1992-1999). In addition, flow-like structures identified on Earth-based telescopic photographs (KuIPER et al., 1966, p.206, fig.131a) have been photographed by Lunar Orbiter V (site 38, H-159-162; see also MER~HELD et al., 1969). Other lunar features. The principle of uniformity has been applied to account for many other lunar features. Workers at Ames Research Center (QvMDE and OBERBECK, 1968) have shown that definite similarities exist between morphologies of (1) small craters produced experimentally by projectile impacts with targets of cohesionless sand underlain by a hard substrate and (2) lunar craters 10-200 m across, and, in addition, that morphologies of impact craters are strongly dependent on the relative sizes of the craters and the thickness of the cohesionless layer. When the ratio of crater diameter to layer thickness is less than 4, normal conical craters form. When this ratio is between 4 and 10, central-mound and flat-floored craters form. When the ratio is greater than 10, craters with internal concentric structures form. Additionally, morphologies of lunar craters a few centimeters to several meters across and the character of their ejecta can be used to make a general appraisal of the properties of lunar near-surface materials, such as cohesion (GAVLT et al., 1966). Lunar rilles have also been studied using the principle of uniformity. Rima Ariad~eus is a graben (FIELDER, 1961, p.214; MORRISand WmHELMS, 1967), whereas other linear rilles, and sinuous rilles, may be the result of degassing along fractures (SCHUMM, 1969, 1970). Lunar erosion cycle The erosion cycle is a well-known geologic concept, and terms such as young, mature, and old have been applied to individual landforms as well as to landscapes (DAviS, 1909). There is good evidence that the same concept applies to lunar surfaces and features. However, the objection to the use of "cycle" applies to the Moon as well as the Earth; that is, young landforms do not necessarily reappear in the identical spot where a previous one, exactly the same size, existed prior to being eroded. A lunar erosion cycle is implied by small craters (10-100 m and less) of different sizes whose forms vary from fresh appearing to subdued almost beyond recognition. The manner in which the erosion may occur will be illustrated by theoretical considerations and, in keeping with the principle of uniformity as Earth-Sci. Rev., 7 (1971) 5-33

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qualified above, by experiment. Finally, the results of the experiments will be compared with the lunar evidence. Predicted distribution o f craters on the lunar surface. Calculations of the frequency

distribution of impact craters produced by the influx of meteoroids and other objects from space suggest that the lunar surface should reach a "steady-state" in which it is almost completely covered with small impact craters of various sizes (MOORE, 1964; SHOEMAKER,1965, pp.122-124). In this "steady-state" condition, craters are destroyed as rapidly as they are formed, so that the general appearance of the surface remains the same, although the details of the surface change continually. The "steady-state" is first attained for the smallest craters and eventually extends to the larger craters. One set of calculations (MOORE, 1964) assumes that the cumulative frequency of meteoroids and other objects from space that impact the lunar surface is inversely proportional to their masses (see WmPpez, 1963, curve B), and then combines cratering theory to establish the size of craters produced by the impacts of the meteoroids and other debris. On the basis of Whipple's curve B, the lunar surface cannot contain the number of craters that would eventually form on it. This requires that the craters be destroyed by subsequent impacts. If the life time of a crater is assumed proportional to its relief and, hence, linear dimension (see also Ross, 1968 and SODERBLOM,1970), then the calculations suggest that the cumulative frequency distribution of craters on the lunar surface should be composed of two parts. The cumulative frequency of the smaller craters, where the "steady-state" has been achieved, should be inversely proportional to the square of their diameters; and the cumulative frequency of the larger craters, where the "steady-state" has not been achieved, should be inversely proportional to the cube of their diameters. For a billion-year-old surface, 34~ of the area should be covered by craters within each decade of crater diameters (D-10 D) from 0.0001-10 m and possibly 100 m across. For this "steady-state", the craters in each of the size intervals would range from fresh and unmodified to partly destroyed by erosion and infilling (see JAFFE, 1965). For this condition, 34~o of the surface is covered by craters in a given decade of diameters that range from fresh craters to those that are barely discernible. Craters in a given decade of diameters that range from fresh craters to those about 3/10 of the way to destruction cover 10~o of the surface. The erosion of the craters results from the ejection of debris during crater formation by subsequent impacts. The infilling results from deposition of debris which is concentrated in depression because of downslope displacements of the debris impacting on slopes. Since crater lifetimes'are considered a function of their initial relief and linear dimensions, craters can be classed into morphological stages. Craters whose initial relief is essentially unchanged are fresh; those with between 31/32 and 3/4 of their original relief are young; those with 3/4-1/2 of their original relief are mature; Earth-ScL Rev., 7 (1971) 5-33

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MOORE

and those with 1/2-0 of their original relief are old. The approximate expectations for relative frequencies of such craters on a "steady-state" surface are shown in Fig.7, where the uppermost line represents the cumulative frequency of all craters (old, mature, young, and fresh), the second line represents the cumulative frequency of mature, young and fresh craters, and so forth. Thus, the craters can be grouped into morphologic stages, like those of terrestrial features (see, e.g., DAVIS, 1909, pp.170 and 176-178): fresh, young, mature and old. Such stage designations do not indicate relative ages of craters of different sizes, but rather relative ages of craters of the same size. The problem of estimating relative ages of craters of different sizes on a "steady-state" surface has been discussed by TRASK (1969, p.8-11). Crater distributions similar to the "steady-state" might also be produced by the impact of fragments along rays ejected from craters such as Copernicus, Tycho,

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Fig.7. Graph showing cumulative frequency of craters of various morphologies and size for a "steady-state" surface. Uppermost line represents approximate cumulative frequency of fresh, young, mature and old craters, i.e., all craters. Second line represents approximate cumulative frequency of fresh, young, and mature craters. Third line represents approximate cumulative frequency of fresh and young craters. Lowermost line represents approximate cumulative frequency of fresh craters. Crater profiles define boundaries between various morphologies.

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or Aristarchus (MOORE,1964, p.49; SHOEMAKER, 1965), as well as around young smaller impact craters.

Experimental simulation of the lunar cratering process. As part of a cooperative program of research, investigators from the U.S. Geological Survey and Ames Research Center 1 (NASA) bombarded surfaces underlain by carborundum powder and sand with projectiles of various sizes (see also GAULT, 1970). The experiment illustrates how repeated impacts produce a "steady-state" surface and what crater morphologies may be expected for a surface that has reached the "steadystate". The surface produced in this experiment was similar to surfaces shown in the highest resolution Ranger photographs and the surfaces around the Surveyors. These surfaces are characterized by craters of various sizes in various states of preservation. In the experiment, craters were produced by dropping and firing projectiles into a box of 244 cm 2 and filled to a depth of 30 cm with non-cohesive sand covered with a layer of coarse carborundum powder about 2 cm thick. The projectiles, the method of projection, the average diameters of craters produced, and the relative frequencies of impact are shown in Table I. TABLE I EXPERIMENTAL CRATERING IN SURFACES UNDERLAIN BY SAND AND CARBORUNDUM POWDER

Projectile

Projector

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Relative Projectile Frequency

30 cal. 22 cal. BB Birdshot # 8 shot #12 shot

Rifle, 30-06 Rifle, 22 BB gun Rifle, 22 (sawed off) 25-foot drop 25-foot drop

30 13 4.2 2.5 1.l 0.52

1.0 10.4 106 1,410 11,200 127,000

The 30-caliber, 22-caliber, and BB projectiles were fired at points selected at random. A grid was used to control aiming. The sequence of firing was not random: one 22-caliber was fired after every ten BB's, and one 30-caliber was fired after every ten 22-caliber projectiles. Precise sequencing of birdshot and drops of # 8 and # 12 shot was not possible since about 5, 13, and 153 craters per 929 cm 2 were produced by each firing or drop of birdshot, # 8 shot, and # 12 shot, 1 Although many individuals contributed to the experiment described here, the principal contributors from Ames Research Center were D. E. Gault, W. L. Quaide, and V. R. Oberbeck.

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respectively. The calculated number of BB's and shot fell only into the central 1.49 m 2 because of the large number of shot required. According to the calculations of MOORE (1964), the product of the relative frequencies of the craters produced in each size class and the cube of their diameters should be constant. The list above shows that this was nearly achieved; however, the craters produced by the BB's were about 20% too small, which caused the bend in the crater frequency curve (Fig.8). The most significant shortcoming of the experiment is the 3J

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Fig.8. Comparison between frequency distributions of small craters on the lunar surface and a predicted frequency distribution and experimental simulation. Upper boundary of shaded area corresponds to condition of 34~ of area covered by craters in each decade of crater diameters (D-10D); lower boundary of shaded area corresponds to condition of 10~o of area covered by craters in each decade of crater diameters (D-10D).

range of sizes of craters produced. On the lunar surface, craters range in size from less than a few centimeters to thousands of meters across, whereas the simulated craters ranged from 1/2-30 cm. In spite o f the small range o f sizes o f craters p r o d u c e d and the undersize

BB craters, the experiment was instructive. As the firing proceeded, the initially smooth surface soon became pocked with craters of all sizes. In the early stages, areas between the smallest craters were smooth and flat where no craters were present. With continued b o m b a r d m e n t the surface reached a "steady-state" for Earth-Sci. Rev., 7 (1971) 5-33

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the smaller craters, and none of the original surface escaped the b o m b a r d m e n t . U p o n reaching the "steady-state", craters were produced approximately as rapidly as they were destroyed. Further b o m b a r d m e n t eventually p r o d u c e d a "steady-state" for all sizes of craters. W h e n the entire surface reached the "steadystate", the general appearance remained essentially the same with continued b o m b a r d m e n t , although the details o f the surface continually changed. During the experiment craters were destroyed b y a combination of erosion and infilling. Fig.9 illustrates this. Two fresh craters with virtually no superposed smaller craters can be seen. The ejecta from these craters have infilled and buried many smaller craters a r o u n d them, and a large number o f smaller craters were

Fig.9. Photograph of experimental simulation of lunar surface illustrating erosion and infilling of craters by projectile impacts. Note the various stages of craters represented by their appearance which ranges from fresh-appearing craters with rims to those barely discernible. Two fresh craters indicated by arrows. Largest crater is about 30 cm across. Earth-Sci. Rev., 7 (1971) 5-33

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destroyed by the excavation. In the lower right-hand corner, the ejecta blankets of two young craters have been somewhat eroded by smaller craters. Many rimless craters have been so eroded by later, smaller impacts that their ejecta blankets are no longer discernible. In addition, there are shallow depressions with virtually no shadows. These represent, in part, craters on the verge of being completely obliterated. N. J. Trask counted craters in a photograph of the central 1.49 m 2 of the final simulated surface. The data are plotted in Fig.8. The final surface will be compared with the lunar surface below. It is also noteworthy that many of the larger craters have smaller craters on their rims, and a few doublets are found among the intermediate-size craters. Other craters appear to be alined. Craters on the lunar surface. Frequency distributions and morphologies of craters on the lunar surface compare well with both the predictions and the experimental simulations. Crater counts (TRASK, 1966) made from Ranger photographs (ANo~YMUS, 1964a, 1964b, 1966), Surveyor photographs (SHOEMAKER et al.,

Fig.10. Photograph of floor of crater Alphonsus taken by Ranger IX. Largest craters are near 500 m across. Note morphologies of small craters which range from fresh through old.

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Fig.llA. Oblique view of the simulated surface. Light is to the left. B. Photograph of cratered lunar surface taken by Surveyor I. Sun is to the right. Young rimmed crater near upper left is 1 m across.

1969, pp.65-71), a n d L u n a 9 p h o t o g r a p h s ( A k a d e m i y a N a u k S.S.S.R., 1966) exhibit the p r e d i c t e d distributions a n d m o r p h o l o g i e s (see Fig.9, 10, l l ) . F o r the " s t e a d y - s t a t e " distributions on level surfaces, the frequency o f craters is prop o r t i o n a l to the reciprocal o f the square o f the diameters, m o r p h o l o g i e s o f craters range from fresh to old, a n d the size o f craters to which the " s t e a d y - s t a t e " disEarth-Sei. Rev., 7 (1971) 5-33

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tribution applies increases with the ages of the surfaces. Thus, one concludes that erosion cycles consistent with theory and uniformity exist on the Moon. Lunar roek-stratigraphic units Mappable units. Lunar surfaces may be classified using the fundamental geologic concept of mappable units,(SHoEMAKERand HACKMAN, 1962, pp.289-300). Lunar

Fig.12. Geologic map of the crater Aristarchus. White bar is 40 km long. Map units are: central peak (cp), crater floor (cff), crater wall (cw), crater terrace (t), crater rim (cr), crater flank (cf), flow-like surfaces (f), and rimmed depressions (some of the rimmed depressions are indicated by letter S). Base is Lunar Orbiter IV high resolution photograph 150.

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map units are identified using surface characteristics such as topography, albedo (reflectivity during full-moon illumination), and, where available, radar reflectivity and infrared emission. Extensive mapping of the Moon's surface by the U.S. Geological Survey (see HARBOUR, 1969) has shown that such mappable units are extensive, that they often show simple geometrical relationships with one another, and that they are repeated from place to place on the Moon. For example, the crater Aristarchus, which measures about 40 km across, is characterized by a high albedo, 3,630 m of relief, and sharp crisp topography and structures around its flanks and within it. Bright rays are alined along radials from the crater. Eight mappable units can be recognized on Lunar Orbiter V photographs of Aristarchus (Fig. 12): (I) a bright blocky central peak, cp, which rises above the crater floor, (2) a bright generally level crater floor, cff, with irregular fissures, blocks and local small domical structures, (3) bright steep-terraced crater walls, cw, strewn with blocks and streaked by radial channels, (4) flat fissured terrace fills, t, (5) a bright crater rim, cr, with concentric ridges and valleys, radial shallow valleys, and radial rimmed channels, (6) generally bright crater flanks, cf, with radial, braided, and chevron ridges, (7) smooth flowlike surfaces extending over part of the rim and flanks and short ribbonlike to stubby ridges, f, and (8) alined, isolated to clustered, rimmed depressions beyond the flanks; the rims around the depressions vary, but typically form chevron ridges whose apices point to Aristarchus. With the exception of the flowlike surfaces, these mappable units are arranged concentrically around one another, and boundaries between them are gradational. Interpretation. Ideally, identification of map units with a given set of properties is independent of their origin. Interpretation follows identification. The initial interpretation made by geologists is that the surfaces with more or less uniform characteristics are underlain by relatively uniform rocks forming a distinctive geologic unit in three dimensions. Map units with great areal extent are inferred to represent the surface expression of underlying rock-stratigraphic units which are laterally continuous. Such an inference stems from terrestrial experience with volcanic and impact features where rock-stratigraphic units are demonstrably laterally continuous. Their concentric arrangement implies that the units mapped in and around Aristarchus are interrelated. With the exception of the depressions beyond the flanks of Aristarchus, continuity and areal extent of the map units suggest that the rock-stratigrapbic units are laterally continuous. Arguments that most of the map units are sheetlike rest on terrestrial experience with craters and their ejecta. The depressions beyond the flanks of Aristarchus are inferred to be related to Aristarchus because their frequency per unit area decreases away from the crater, they are partly alined along radials from the crater, and their chevron rims point toward the crater. Other lunar craters, such as Tycho and Copernicus, are very similar to Aristarchus. Thus the map units are repeated elsewhere on the Moon. Tycho and Earth-Sci. Rev., 7 (1971) 5-33

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Copernicus are not only similar in albedo, topography, relief, and distribution of map units, but also in their radar reflectivities (THOMPSON and D¥CE, 1966) and infrared emission properties (SAARI and SHORXnlLL, 1966). Although the visual-photographic and reflection-emission data cannot be unequivocally interpreted, close correlation between some interpretations of the data has been substantiated by orbital and landed spacecraft. Telescopic observations combined with geologic inference lead to the conclusion that materials around craters such as Copernicus (SHOEMAKER, 1962) and Aristarchus (MOORE, 1965) are composed of large blocks and crushed rock. SINTON (1962, pp.470-471 ) showed that the eclipse temperatures of Tycho were consistent with rock surfaces covered by a fraction of a millimeter of dust, while the environs of Tycho were covered by thick dust. MURRAYand WILDEY (1964) find, from an analysis of lunar night-time temperatures, that extensive local areas around Tycho and Copernicus contain materials more conductive than the average lunar material. PETTENGILL and HENRY (1962, pp.4884-4885) suggested that part of the 70-cm radar echo enhancement from Tycho might be the result of a material with a greater density than the materials of the surrounding landscape. Photographs of Tycho taken by Lunar Orbiter V and Surveyor VII show that the interpretations of data made prior to spacecraft exploration were remarkably good. Lack of perfect agreement and changes in mapping and interpretation should be expected. The first geologic maps of the Moon were made using earth-based telescopes with resolutions limited to about 1 kin, and without the benefit of infrared and radar data. Thus, as new techniques are developed and resolution of imagery improves, maps must be revised. A second problem besets the lunar mapper--it is the same problem encountered by terrestrial geologists. Terrestrial rocks are mostly covered by soils, alluvium, colluvium, and talus. Lunar rocks are also covered by a layer of loose material, the regolith. For the Moon, this fact becomes clear on the highest resolution photographs, which show that the topography of the fine-scale features is nearly the same in most areas. This is due to repeated bombardment of the surface by debris from space, which produces a "steady-state" surface and develops a regolith which varies in thickness from about a meter to more than tens of meters (QUAIDEand OBERBECK,1968, 1969; OBERBECK and QUAIDE, 1967, 1968). Superposition. Relative ages of some lunar features may be established by means of

the principle of superposition. This principle is well illustrated by the crater flank materials (cf) of Aristarchus (Fig.12), which partially fill rilles to the north, extend across both the rim and floor of Herodotus (the large crater to the southwest), and rest upon the smooth mare surfaces to the south and east. Rimmed depressions (s), which are interpreted as secondary impact craters produced by debris ejected from Aristarchus, are superposed on virtually all of the assorted topographies around Aristarchus. Thus, the rock units produced by the Earth-Sei. Rev., 7 (1971) 5-33

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formation of Aristarchus are younger than most, if not all, rock units in Fig.12 associated with features a few kilometers across and larger. REFLECTION--EMISSIONDATA

Studies of reflection and emission of electromagnetic radiation from the Moon produce valuable data on the nature of the lunar surface and near-surface materials. Interpretations of data on the reflection and polarization of sunlight, infrared and radio emission, and radar reflectivity of the lunar surface collected from terrestrial observatories, indicated that the lunar surface is very rough at optical wavelengths, but that it is smoother at longer radar wavelengths and that the surface materials are porous and probably fragmental. Early students (LYOT, 1929; BARABASHOV, 196l; DOLLFUS, 1961, 1962) found that the lunar polarization and reflection of sunlight and earthlight are similar to polarization and reflection of light from finely fragmented volcanic tuffand volcanic ash from Mount Vesuvius. Other workers stressed that the lunar surface must be porous and rough in texture at optical wavelengths (ORLOVA, 1956; M1NNAERT, 1961 ). Later and more extensive studies showed that fine darkened rock powders (10/~ average diameter) reflect and polarize light in much the same way as the lunar surface (HAPKE and VAN HORN, 1963; HAPKE, 1966, 1968). Comparison of infrared emissivity of the lunar surface with that of silica suggests that the lunar surface materials are very porous (PETTIT, 1961, pp.415-416), and thermal inertia constants derived from infrared observations during lunar eclipse and radio emission observations are close to those of pumice or dust in a v a c u u m (MAYER, 1961, p.455). Analysis of lunar night-time temperatures indicated that lunar surface materials are not composed solely of dust but that conductive materials are either commonly present on the surface or within a few centimeters of it (MURRAY and WILDEr, 1964). Radar echo studies indicate that the lunar surface materials are porous and broken up and that their average relative dielectric constant is near 2.7, somewhat like dry soils (EVANS,1962; PETTENGILLand HENRY, 1962). In addition, studies of radar ecbos indicate that the lunar surface is smoother at radar wavelengths than optical wavelengths (EVANS, 1962; HAGFORSand EVANS, 1968), but rougher than is indicated by slope measurements made by the photoclinometric (VAN DIGGELEN, 1960; ROWAN and MCCAULEY, 1966) and shadow techniques for base lengths of 3/4 km (McCAULEY, 1964). Although quantitative estimates of densities of the surface material varied significantly, as did estimates of root-mean-square slopes, pre-spacecraft data on reflection-emission properties captured the essence of the lunar surface and its materials. Continued effective use of radar echo data for lunar studies and classifying and mapping the lunar surface is illustrated by TYLER (1968) and THOMeSON and DYCE (1966). Using a 2.2-m wavelength transmitter in Explorer 35 and the receiver at Stanford University, California, Tyler measured a Brewster angle of Earth-Sci. Rev., 7 (1971) 5-33

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60 ° for lunar surface materials near the lunar crater Grimaldi and then calculated a relative dielectric constant near 3.0, which is consistent with a loosely compacted fragmental material composed of rock fragments with an effective dielectric constant of about 7. He also noted that certain areas reflect more strongly than others, and he correlated the strongly reflective areas with thin layers of fragmental material. That there are such areas with varying thicknesses of fragmental material is clearly shown by studies of the morphologies of small lunar craters and their ejecta (OBERBECK and QUAIDE, 1968). THOMPSONand DYCE (1966) postulated that relative ages of craters and other lunar features might be established by radar Doppler mapping at different wavelengths. The youngest craters would have high reflectivities at all wavelengths. Slightly older craters would have high reflectivities at intermediate and long wavelengths but not short wavelengths. Much older craters would have high reflectivities at only long wavelengths, whereas the oldest craters would have only average reflectivity. Additional information on relative ages of craters can be obtained from infrared data (SAARIand SHORTHILL, 1966). UNMANNED SOFT LANDINGS

Unmanned soft-landing spacecraft of the Surveyor program (ANONYMOUS, 1969b) and Luna series (Akademiya Nauk S.S.S.R., 1966) provided data on (1) the topography of the lunar surface at the fine scale, (2) the physical properties of the lunar near-surface materials, and (3) the chemical composition of the surface materials. The data will be briefly discussed below.

Topography As shown by all high-resolution photography, the fine-scale topography of the lunar surface is dominated by craters a few centimeters across to several hundred meters across (SHOEMAKER et al., 1969). Typically, the cumulative frequency distribution of the craters is close to the "steady-state" distribution for craters smaller than some limiting size, which is a function of the age of the surface. This limiting size is near 2 m for a young surface, such as the flank of the crater Tycho where Surveyor VII landed, and is about 100 m for the older surface of Sinus Medii where Surveyor VI landed (SHOEMAKERet al., 1969). Morphologies of the smaller craters range from fresh and highrimmed to subtle depressions; they meet the expectations for a "steady-state" surface, as discussed previously. Most, but not all, small lunar craters are consistent with the theory that they were produced by impact of meteors and debris from space (QuAIDE and OBERBECK, 1968). Notable exceptions include the dimple craters in the area where Surveyor V landed (SHOEMAKERet al., 1969), where there is a large population of such craters, and the dimple craters in Mare Cognitum (UREY, 1965). Rimless cratels about 100 m across, with large depth/diameter ratios, are found near Earth-Sci. Rev., 7 (1971) 5-33

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Surveyor III. These ratios, which are near 1:5, are too large for eroded impact craters this size. Such craters may have formed by some other process, such as collapse (KuIPER et al., 1966) or volcanism. Positive relief features such as blocks, clods, and fragments become increasingly abundant with decreasing size until, at the smallest sizes, they are predominant in the lunar landscape. Shapes of blocks vary. They may be equidimensional, tabular, or elongate; and they may be angular to rounded (SHOEMAKER et al., 1969). Abundant rectangular and angular blocks are found around fresh craters shown in Lunar Orbiter and Surveyor photographs. Such blocks were photographed on the flank of a 30-m crater by Surveyor I, and they are common features near Surveyor VII (SHOEMAKER et al., 1969). Rectangular blocks are familiar to geologists, and they probable represent separation of rocks along joints during ejection from craters. Rounded blocks and fragments found on the lunar surface may have been initially rounded, whereas others may have been rounded by some erosion process. Many round vesicular rocks probably represent bomblike ejecta from impact or volcanic craters. Other rocks have been eroded. For example, a partly buried fragment picked up by Surveyor VII was rounded where exposed, but angular where buried; thus, the process that rounded the fragment operated above the surface (ScoTT and ROBERSON, 1969). Other positive features may be clods. Clods of weakly cohesive surface materials were produced by the penetration of the Surveyor spacecraft footpads during landing and by the surface sampler during trenching (CHOATE et al., 1969; SCOTT and ROBERSON, 1969). Such clods became disaggregated when dropped on the upper surface of the footpads from small heights. Thus it seems reasonable that some positive features around small fresh craters are weakly cohesive clods. Physical properties The physical properties of lunar surface materials have been studied using the reaction of the surface to (1) various parts of the spacecraft during landing, (2) the surface sampler, (3) small stones set in motion by the spacecraft, (4) spacecraft thermal sensors, and (5) spacecraft landing radar. Such studies permitted determination of the bearing capacity of the lunar surface, the grain size of surface materials, thermal inertias, and dielectric constants. Bearing capacity of the lunar surface materials increases with depth (CHOATE et al., 1969). Loading by the alpha scattering sensor head places the static-bearing capacity near 2 • 1 0 4 dynes/cm z at depths near 1-2 mm. Rupture of aluminum sheets of the crushable blocks in the footpads, which occurs at 2.4 • l05 dynes/cm 2, indicates a static-bearing capacity of 1.8 • 105 dynes/cm 2 at depths near 1-2 cm. Surveyor I footpad analyses place the static-bearing capacity near 4.2-4.6 • 105 dynes/cm 2 at a depth of 4 cm. Cohesions estimated by studying the reaction of the soil to vernier engine exhausts during firing, ranged from 0.07 • 104 to 1.2 • 1 0 4 dynes/cm z (CHOATE et al., 1969, p.160). Interaction of the surface sampler with the Earth-Sci. Rev., 7 (1971) 5-33

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lunar surface materials indicated that the materials behave like terrestrial soils with angles of internal friction of 35 °-37 ° and cohesions near 0.35 • 10¢-0.7 • 104 dynes/cm 2 (ScoTT and ROBERSON, 1969, p.178). Such friction angles are in general agreement with maximum slopes of about 34 °, measured north of the Surveyor VII spacecraft. Bulging and doming of the lunar surface around the surface sampler suggested that the soil-like materials that it penetrated were incompressible and failed by general shear at depths of several centimeters (ScoTT and ROBERSON, 1969, pp.176-178). However, examination of the deformations caused by rolling fragments and the crushable block revealed no evidence of dilation of the uppermost few millimeters (CHoATE et al., 1969; SCOTT and ROBERSON, 1969); thus, this uppermost layer is compressible. The grain size of the bulk of the near-surface materials was estimated by two different methods to range from 10-60/~. One estimate was obtained by comparison of footpad imprints on the lunar surface with those in terrestrial soils (CHOATE et al., 1969, p.140). The other involved analyses of the response of surface materials to rocket motor and vernier engine exhausts (CHOATE et al., 1969, pp.158-159). This 10-60 /1 range was also predicted by extrapolation of the size-frequency distributions of particles counted on photographs returned by Surveyors 11I, V, and Vl (SHOEMAKERet al., 1969) where it is found that the entire surface or volume of lunar material is filled by grains larger than 10-30 # and that most of the grains are between 10 and 60/~. However, estimates based on extrapolation of frequency distributions from Surveyors 1 and VII do not yield satisfying results. Thermal inertias and dielectric constants were estimated from Surveyor data (LucAs et al., 1969; MUHLEMANet al., 1969). Estimates of thermal inertias ranged from about 500-800 cgs units. These were significantly lower than earth-based determinations from eclipse measurements, which were near 1300. Estimates of dielectric constants were consistent with earth-based determinations. It is also noteworthy that dielectric constants estimated for the Surveyor I and VII sites were higher than those for the Surveyor Ill, V, and VI sites. This is consistent with the low frequencies of blocks and fragments around Surveyors IIl, V, and VI. Chemical composition Lunar surface materials at the Surveyor V, VI, and VII sites were analyzed by means of alpha scattering instruments (TuRKEVlCH et al., 1969), and found to consist mostly of silicon and oxygen, with lesser amounts of magnesium, aluminum, calcium group elements, and iron group elements. The composition determined was consistent with that of basaltic rocks. These chemical data, when combined with geologic interpretation, suggested that common minerals such as plagioclase feldspars and pyroxenes should occur in the lunar rocks (SHOEMAKER et al., 1969, p.78), along with silicate glass formed by impact and volcanic processes.

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CONCLUSIONS Interpretation of observational data on the Moon, prior to manned landings, has been remarkably successful. Geologic interpretation using the principle of uniformity also yields ready explanations for the complex topography and morphology of large- and small-scale features on the lunar surface. Comparison of morphologies of lunar and terrestrial features leaves little doubt that tracks produced by boulders rolling and bouncing downslope, blocky craters produced by single disruptive events, craters produced by vulcanism, and other features are common to both the Moon and Earth. Experimental data on craters produced by projectile impacts and explosives in rock and sand targets, provide acceptable explanations for many of the crater morphologies seen on the Moon. Additionally, experimental simulation of meteor bombardment of the Moon produced a surface remarkably similar to the lunar surface at the fine scale and illustrated the importance of a lunar erosion cycle in shaping lunar topography at the fine scale. Mapping and interpretation of lunar rock-stratigraphic units have shown that the map units occur in more than one place on the Moon. Geologic interpretation of some map units has been very successful. In particular, superposition indicates that the materials of bright craters such as Tycho, Copernicus, and Aristarchus are younger than surrounding materials--a result that is in keeping with reflection-emission data. Interpretation of lunar reflection-emission data, prior to manned landings on the Moon, has been remarkably successful. Interpretations of data on light reflectivity, radar reflectivity, and infrared emissions are especially noteworthy because they formed the basis for prediction of the loose porous nature of the lunar near-surface materials and the progressive increase in roughness at progressively finer scales. The Surveyor program collected data on the fine-scale topography of the lunar surface, determined the physical properties of the lunar surface materials, and analyzed the chemical composition of the lunar surface materials. APPENDIX Results obtained from the Apollo 11 and 12 manned missions on the lunar surface have not only added a tremendous amount of data on the Moon but also demonstrated the success of the unmanned program of lunar exploration. Most significantly, the Apollo Lunar Modules landed with the knowledge of the mechanical properties of the surface materials gained using Surveyor spacecraft and with the knowledge of the lunar topography gained using Ranger, Lunar Orbiter, and Surveyor spacecraft. Many additional earlier interpretations were confirmed. For examples: (l) the surface materials are very fine grained, and contain scattered fragments, (2) the thickness of the very fine grained regolithic materials are in Earth-Sci. Rev., 7 (1971) 5-33

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substantial agreement with previous estimates, (3) shocked and fused grains in the regolith are consistent with the postulated model of topographic evolution by continual meteroid bombardment of the lunar surface, (4) rounded fragments with superposed small glass-lined impact craters are in keeping with the postulated model of erosion and in filling by continual meteoroid bombardment at the lunar surface, (5) unshocked rocks, exhumed from beneath the regolith by impact cratering events, are igneous rocks with the expected chemical composition, and (6) the igneous rocks are probably from volcanic flows at the Apollo 11 site. Thus Apollo results have generally confirmed many previous results and interpretations. Although our understanding and knowledge of the Moon and its rocks has increased phenomenally as a result of Project Apollo, we should keep in mind the substantial and necessary contributions of the unmanned program of lunar exploration which made Apollo possible.

REFERENCES AKADEMIYA NAUK S.S.S.R., 1966. The first panoramic views of the lunar surface. U.S. Natl. Aeronautics Space Admin., Tech. Transl. F-393, 125 pp. ANONYMOUS, 1964a. Ranger VII, Photographs of the Moon, I, Camera " A " series. U.S. Natl. Aeronautics Space Admin., Spec. Publ., SP-62, 200 pp. ANONYMOUS, 1964b. Ranger Vll, Photographs of the Moon, III, Camera "P" series. U.S. Natl. Aeronautics Space Admin., Spee. Publ., SP-63, 200 pp. ANONYMOUS, 1966. Ranger IX, Photographs of the Moon, Cameras "A", "B", and "P". U.S. Natl. Aeronautics Space Admin., Spec. Publ., SP-112, 170 pp. ANONYMOUS, 1969a. Lunar Orbiter Photographic Data. Data Users' Note NSSDC 69-05, Natl. Space Sci Data Center, Greenbelt, Md, 37 pp. ANONYMOUS, 1969b. Surveyor project final report, II--Science results. U.S. Natl. Aeronautics Space Admin., Spec. Publ., SP-184, 423 pp. APOLLO 11 EXPERIMENTERS, 1970. The Moon Issue. Science, 167(3918): 447-784. BARABASHOV, N. P., 1961. Rocks that may constitute the lunar surface. In: Z. KOPAL and Z. K. MIKHAILOV(Editors), The Moon--Proc. Intern. Astron. Union, Syrup. 14, Leningrad, 1960. Academic Press, New York, N.Y., pp.379-384. CHOATE,R., BATTERSON,S. A., CHRISTENSEN,E. M., HUTTON, R. E., JAFFE, L. D., JONES, R. H., KO, H. Y., SCOTT, R. F., SPENCER,R. L,, SPERLING,F. B. and SUTTON,G. H., 1969. Lunar surface mechanical properties. In: Surveyor Program Results--U.S, Natl. Aeronautics Space Admin., Spec. Publ., SP-184, pp.129-169. DAVIS, W. M., 1909. Geographic Essays. Ginn, Boston, Mass,, 777 pp. DOELFUS, A., 1961. Polarization studies of the planets. In: G. P. KUIPER and B. M. MIDDLEHURST (Editors), Planets andSatellites--The Solar System, 3. University of Chicago Press, Chicago, Ill., pp.343-399. DOLLFUS, A., 1962. Polarization of moonlight. In: Z. KOPAL (Editor), Physics and Astronomy of the Moon. Academic Press, New York, N.Y., pp.131-159. EGGLESTON,J. M., PATTERSON,A. W., THROOP,J. E., ARANT~W. H. and SPOONER,D. L., 1968. Lunar "rolling stones". Photogrammetric Eng., 84: 246-255. EVANS,J. V., 1962. Radio echo studies of the Moon. In: Z. KOPAL(Editor), Physics and Astronomy of the Moon. Academic Press, New York, N.Y., pp.429~179. FIELDER, G., 1961. Structure of the Moon's Surface. Pergamon, London, 266 pp. FILICE, A. L., 1967. Lunar surface strength estimate from Orbiter II photograph. Science, 156 (3781): 1486-1487.

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GAULT, D. E., 1970. Saturation and equilibrium conditions for impact cratering on the lunar surface: criteria and implications. Radio Science, 5(2): 273-291. GAULT, D. E., QUAIDE,W. L., OBERBECK,V. R. and MOORE, H. J., 1966. Luna 9--Evidence for a fragmental surface layer. Science, 153(3739): 985-988. HADLEY, J. B., 1964. Landslides and related phenomena accompanying the Hegben Lake earthquake of August 17, 1959. In: The Hegben Lake, Montana, Earthquake of Augttst 17, 1959. U.S. Geol. Surv. Profess. Paper, 435: 107-138. HAGFORS, T. and EVANS,J. V., 1968. Radar studies of the Moon. In: J. V. EVANSand T. HAGFORS (Editors), Radar Astronomy. McGraw-Hill, New York, N.Y., pp.219-273. HAPKE, B. W., 1966. Optical properties of the Moon's surface. In:W. N. HESS, D. H. MENZEL and J. A. O'KEEFE (Editors), The Nature of the Lunar Surface--Proc. IAU-NASA syrup., Greenbelt, 1965. Johns Hopkins, Baltimore, Md., pp.141-154. HAPKE, I . W., 1968. Lunar surface-composition inferred from optical properties. Science, 159(3810): 76-79. HAPKE, B. W. and VAN HORN, H., 1963. Photometric studies of complex surfaces, with applications to the Moon. J. Geophys. Res., 68(15): 4545--4570. HARBOUR, J., 1969. Geologic mapping of the Moon. Allgem. Vermessungs-Nachr., 5: 211-219. HUBBERT, M. K., 1967. Critique of the principle of uniformity. In: C. C. ALBRITTON,Jr. (Editor), Uniformity and Simplicity. Geol. Soc. Am., Spec. Papers, 89, pp. 3-33. JAEFE, L. D., 1965. Depth of the lunar dust. J. Geophys. Res., 70(24): 6129-6138. KUIPER, G. P., STROM, R. G. and LEPOOLE, R. S., 1966. Interpretation of the Ranger records. In: Ranger V1H and IX, pt. ll--Experimenters' Analyses and Interpretations. Calif. Inst. Technol., Jet Propulsion Lab., Tech. Rept., 32-800, pp.35-248. LUCAS, J. W., HAGEMEYER,W. A., SAARI,J. M., STIMPSON,L. D. and VICKERS,J. M. F., 1969. Lunar surface temperatures and thermal characteristics. In: Surveyor Program Results. U.S. Natl. Aeronautics Space Admin., Spec. Publ., SP-184, pp.181-202. LUNAR SAMPLEPRELIMINARYEXAMINATIONTEAM, 1970. Preliminary Examination of Lunar Samples from Apollo 12. Science, 167(3928): 1325-1339. LYOT, B., 1929. R6cherches sur la polarisation de la lumi6re des plan6tes et de quelques substances terrestres [Research on the polarization of light from planets and from some terrestrial substances]. Paris Obs. Ann., 8(1), 161 pp; translated, 1964: U.S. Natl. Aeronautics Space Admin., Tech. Transl., F-187, 144 pp. McCAULEY, J. F., 1964. A preliminary report on the terrain analysis of the lunar equatorial belt. U. S. Geol. Surv., open-file rept., 44 pp. McCAULEY, J. F., 1968. Geologic results from the lunar precursor probes. Am. Inst. Aeronautics Astronautics J., 6(10): 1991-1996. MAClVER, B. N., 1967. The formation and initial stability of slopes on cohesionless materials-Project Plowshare. U.S. Atomic Energy Comm., PNE-5009, 59 pp. MAYER, C. H., 1961. Radio emission studies of the Moon. In: G. P. KUIPER and B. M. MIDDLEHURST (Editors), Planets and Satellites. The Solar System, 3. University of Chicago Press, Chicago, Ill., pp.442-472. MERIFIELD, P. M., SAARI, J. M., SHORTHILL,R. W., WILDLY, R. L., WILHELMS,D. E. and WILLIAMS, R. S., JR., 1969. Interpretation of extraterrestrial imagery. Photogrammetric Eng., 34: 477492. MINNAERT, M., 1961. Photometry of the Moon. In: G. P. KUIPER and B. M. MIDDLEHURST(Editors), Planets and Satellites, The Solar System, 3. University of Chicago Press, Chicago, Ill., pp.213-245. MOORE, H. J., 1964. Density of small craters on the lunar surface. In: Astrogeologic studies, ann. prog. report, August 24, 1962-July 1, 1963, D. U.S. Geol. Surv., open-file rept., pp.34-51. MOORE, H. J., 1965. Geologic map of the Aristarchus region of the Moon. U.S. Geol. Surv., Misc. GeoL Invest., Map 1-465. MORRIS, E. C. and WILHELMS,O. E., 1967. Geologic map of the Julius Caesar quadrangle of the Moon. U.S. Geol. Surv., Misc. Geol. Invest., Map 1-510. MORRIS, E. C., BATSON,R. M., HOLT, H. E., RENNILSON,J. J., SHOEMAKER,E. M. and WHITAKER, E. A., 1968. Television observations from Surveyor VI. In: Surveyor 1I/, a Preliminary Report. U.S. Natl Aeronautics Space Admin., Spec. PubL, SP-166, pp.114-40.

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