The Moon after Apollo Harold Masursky

Material collected by unmanned landings and from the manned landings of the Apollo programme, together with observations from a large number of orbiting satellites, have been subjected to detailed examination over the past twenty years. This article reviews the picture of the Moon’s structure, composition, and geological history that has emerged as a result.

Man has wondered about the Moon since his earliest days, and myths from every culture record his attempt to answer his questions about it. In 1609, Galileo was able to discriminate, for the first time, the essential differences between the dark and bright areas on the Moon’s near side, and thus began the scientific study of Earth’s enigmatic satellite. The subsequent development of increasingly sophisticated telescopes allowed astronomers to identify, map, and name conspicuous features on the near side; orbital photographic missions and the unmanned U.S. and USSR lander missions in the last two decades have provided invaluable data from which theories could be formulated about the fundamental questions concerning the Moon’s origin and history. But it was not until Neil Armstrong made his ‘giant leap for mankind’ in July 1969 that the data necessary to test these theories have been available. We can now begin to answer both the old questions and the many new ones that have been raised from the study of the Apollo data.

The far side of the Moon was photographed for the first time by a Soviet spacecraft, in 1959, and was first seen by man from orbit by American astronauts in 1968. The view of the far side (figure 1) dramatically portrays the bimodality of the lunar crust. Here the surface consists almost entirely of bright cratered, highland terrain that occupies nearly 85 per cent of the total lunar surface. The appearance of the lunar front side is dominated by smooth, relatively dark mare areas (so named because they were thought to resemble seas), which make up about 15 per cent of the lunar surface (figure 2).

The differences between the mare and terra areas that were first recognized by visual observation and further clarified by orbital photography have been confirmed and elucidated by study of the data (figure 3) from various instruments on board orbiting spacecraft and by analysis of the 385 kg of returned lunar surface materials. Study of these data and samples have resulted in much more precise estimates of differences in chemical composition and relative thickness of the mare and terra materials, as well as a difference in radius between the two types of terrain.

Orbital laser altimetry, S-band radar tracking data,

Harold Masursky

Is a member of the Geologic Division, Branch of Astrogeologic Studies, of the Geological Survey, US Department of the Interior. He has lately bean closely involved with evaluating results obtained from the Voyager encounter with Saturn.

Endeavour, New Seriesl Volume 6, No. 2, 1982 (0 Pergamon Press. Prtnted in Great Britain) 016~9327/82/02004%11 503.00.

and surface seismic information have allowed us to estimate the thickness of the mare and terra materials. Twenty km of basalt are thought to overlie an additional 20 to 40 km of crustal material in eastern Oceanus Procellarum. In the circular mare areas, such as Maria Crisium, Serenitatis, and Vaporum, the crustal materials that underlie the mare materials appear to be much thinner-about 5 km. Near-side and far-side terra areas are also thought to vary in crustal thickness, 48 km being the estimated thickness in the near-side highlands and as much as 74 km in the far-side highlands. The reasons for this variation in crustal thickness have not been identified certainly at this time. Variations may have been caused by early chemical differentiation of the crustal materials from a priomordial liquid melt; the early differentiate may then have been concentrated in the areas where it is now thickest by convection currents within the liquid material. Other contributing factors may have been gravitational forces exerted by the Earth, or the concentration of heat due to inequalities of chemical composition within the melt.

One of the most important results of the Apollo programme was the radiometric dating of the returned samples. By this technique, the Moon was shown to be very old-probably as old as the Earth, meteorites, and the rest of the solar system. Dating of materials collected at each of the landing sites has calibrated the probable sequence of geologic events on the Moon so that its history can now be outlined.

The Moon is now thought to have been formed about 4.6 billion years ago. Melting of the primordial material, either by internal sources or as a result of energy generated by collision and accretion of the impacting meteoroid, resulted in differentiation of a light, anorthositic-gabbroic continental crust and a dense interior. The solidified lighter materials that became the present terrae continued to be impacted by a heavy rain of meteoroids for the first half billion years of lunar history. These impacts saturated the newly solidified crust with craters that are still recognizable wherever the ancient crust of the lunar highlands is observed. During this period, and for a short time following it, several huge bodies impacted to form large basins. Materials ejected from two basins blanketed large parts of the near-side highlands. Then, during a period of volcanism between about 3.9 and 3.2 billion years ago, basaltic magmas were extruded on to the surface, flooding the irregular areas where the crust was low and thin, and filling the large impact basins.

Most dynamic activity seems to have ceased about 3 billion years ago; modification of the surface from that

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Figure 1 Thefarside ofthe Moon pictured in this metriccamera photograph presents a strikingly different aspectfromthatofthefamiliarnearside.ltiscomprisedmostlyofdenselycratered,ancientterraeor uplands.ThetwomareareasthatappearinthelowerleftsectionofthisphotographareMareMarginis ~ab~~,e),andMareSmythii(below).ThesefeatureappearontheMoon’seasternlimb,asviewedfrom

time until the present has been limited mostly to its gradual degradation by impact cratering at a drastically reduced rate, and by the solar wind. These latter actions have formed a regolith (soil made up of fragmentary debris) that was formed in situ from the underlying rocks, and therefore reflects their composition.

Geophysical and geochemical instruments on the orbiting spacecraft or placed on the surface by the astronauts have contributed additional information concerning localized or regional gravity field variations over craters of differing sizes and within mare and terra units. Craters up to 100 km in diameter are known to be deficient in mass; gravity lows are associated with them. Conversely, mare-filled craters that are more than 150 km in diameter have positive gravity anomalies associated with them; the high gravity measurements probably result from the dense basaltic lavas that fill the craters, or from uplifting of the underlying denser mantle materials at the time of impact, or from a combination of these effects. Positive gravity anomalies are also associated with the large circular basins (Imbrium, Crisium, and Serenitatis, for example) but are absent over irregular basins such as Oceanus Procellarium and

other unfilled basins on the far side. Other structural, geochemical, and topographic differences between circular and irregularly shaped basins have also been deduced from various obital experiments. Magnetic measurements obtained from orbit and on the ground correlate in some areas with gamma ray spectrometer measurements (figure 3). Deflection of the solar wind that was recorded by the subsatellite magnetometer over certain limb areas (Mare Smythii, for eample) are now thought to be caused by regions of high magnetization.

Although data obtained from the orbital and ground-based geochemical and geophysical instruments have added much to our knowledge of the Moon’s composition, the history of its evolution could be developed only through the combined disciplines of geologic interpretation of the Orbiter and Apollo photographs and radiometric dating and intensive chemical, petrological, and geophysical study of the returned samples.

About 20 per cent of the lunar surface was photographed in detail by the Apollo cameras. Stereographic coverage was obtained by both the metric and panoramic cameras. The ability to control the

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Figure 2 West side of the Mare Serenitatis: the differing characteristics of mare and terra surfaces are dramaticallyportrayed.ThelowanglebetweentheSun’sraysandthelunarsurfaceenhances topographical relief. Ridgeson the maresurface,sometimescalled’wrinkle ridges’, have formed parallel to the edge of Serenitatis basin, thought to have formed when a large meteorite impacted the surface earlyintheMoon’shistory; itisbounded-herebytheApennineMountains(left)andtheCaucasus Mountains (farcentre horizon). Arcuate rilles (Rimae Sulpicius Gallus) cut the maresurface and adjoining terra. Northeast-trending straight rillescan be seen cutting the mare margin (near lower-central margin); directlyto their right is a chain of intersecting craters.

elevation of the spacecraft and pointing direction of the cameras in order to obtain the desired coverage at optimum sun angles enhanced the value of the photographic products. On-site descriptions of the scene being photographed, recorded by the astronauts, were also valuable. The picures, and the geologic interpretations derived from them, were essential in developing the history of the area photographed and, by extrapolation of returned sample data, to the rest of the Moon.

The terrae Photographs of the terrae regions show striking evidence of the early period of intense meteoritic bombardment of the lunar surface. The far side of the Moon shown in figure 1 is dominated by craters of different sizes and ages that are superposed on one another. In general, the largest

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craters are the oldest. Repetitive bombardment has also been the major cause of modification of the surface here. Countless impacts have resulted in widespread redistribution of materials over the surface, brecciation of the displaced materials, and metamorphism by shock of the minerals that compose the rocks.

The antiquity of the terra had been hypothesized from study of photographs like figure 1. Their great age was proved when samples of a near-side terra region visited by Apollo 16, the Descartes site, were dated by radiometric techniques. Samples of the Cayley Formation (a thick, crudely stratified blanket of debris), or brecciated rocks in the area, and of the highlands materials were all shown, upon analysis, to be composed of fragments of highly shocked plutonic anorthosites and feldspathic gabbros that are nearly 4 billion years old.

Essential differences in the chemical compositions of

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Figure 3 Thesecurvesshowcorrelationsbetweentopographicandgeologicfeatures,andphysicaland chemicalpropertiesrecorded byselectedremotesensinginstrumentsontheApollo15(le~)andApollo16 (rightispacecraft. Positive gravity anomalies (‘mascons’) correlate with circular mare basins. High gamma radiationwasrecordedattheborderbetweenOceanusProcellarumandMareImbrium.Aninverse relationisseen betweenratiosofaluminumtosilicaandmagnesiumtosilica;thesechangesystematically frommariatoterrae.Elevationcurvesshowthemareareastobelowandtheterraareastobehigh,witha systematicdecreaseinelevationformareareasacrossthelunarfrontsidefromthewestlimbatOceanus ProcellarumtotheeastlimbatMareSmythii

the mare and terra materials were well documented by enriched in refractory elements and depleted in volatiles the orbital x-ray fluorescence experiment. These data and alkali. Some of the breccias of Imbrium ejecta showed that the magnesium/silica ratio was relatively returned from the Apollo 14 landing site and materials high over the mare areas and low over the terra areas, from the Apollo 17 site also contain clasts (rocks made of whereas the aluminiumisilica ratio showed an opposite fragmented material) of basaltic materials. In contrast, correlation. being low over the maria and high over the breccias and debris returned from the Apollo 16 landing terrae (figure 3). Analysis of samples returned from the site in the Descartes area of the near-side highlands are Apollo 11 and 12 landing sites at Mare Tranquillitatis and composed of fragmented anorthosites and feldspathic Oceanus Procellarum proved these mare areas to be gabbros. The composition of these materials, in composed of rocks similar to terrestrial basal&, but collaboration with the geochemical data obtained by

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Figure 4 ThelargecraterisLambert.Lavaflowsofmorethanoneagearepresent.Asinuousbandof smooth, sparsely cratered, mare material extends northeastward through the centre of the picture, It is mostcertainlyayounglavaflowandcontrastsstronglywiththemuchmoredenselycrateredoldermareto thesoutheast.Thewestboundaryoftheyounglavaflowclearlylapsuponandembaystheblanketof ejectadepositssurroundingLambert.Manyradialridgesofejectaandradialgroovesorchainsof secondarycratersradiating outwardfrom Lambertarefaintlyvisible beneath theyoungflownearitswest boundary.Theserelationsprovethattheyoungflowpostdatestheformationofthecrater. Manyclusters ofsecondarycratersderivingfromcratersotherthanLambertarepresent.Theshape,orientation,and freshnessofsomeimplythattheywereprobablyformedbyejectafromCopernicus,360kmfarthersouth. Thecratersarepresentontheoldermare,ontheejectafrom Lambert,andelsewhereinthisarea butnone is present on the young flow. (North is toward the top of picture.)

orbital experiments,.suggestsithat the material at this site is made up ot reworked, terra material and that the high-standing primordial lunar crust is composed of differentiated, low-density, anorthositic-gabbroic rocks. The samples thus confirm the theory that the lunar maria are basaltic, and show that materials derived from the terra province are similar in composition to anorthosites and gabbros that are known to be the oldest terrestrial continental rocks.

The subdued appearance of the near-side and far-side highlands is thought to result from mantling of the surface by ejecta blankets derived from two relatively recent impacts. The Cayly Formation apparently is part of one such blanket. Another example of mantled terra is seen in the crater Albategnius and the surrounding area. Light plains deposits cover all flat areas there, including the floor of Albategnius. However, the light plains surface is not completely level; vague circular and elongated depressions on the floor of Albategnius reveal

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irregularities in the underlying materials. Circular depressions indicate the position of buried craters. The elongated depressions are parallel to other straight clefts that cut the crater rim and the surrounding terrain; these linear features are known as ‘Imbrium sculpture’ and are thought to indicate structural deformation and ejecta scouring of the near-side highlands and chains of secondary craters that were associated with formation of the Imbrium Basin. They are proof that structural forces have also played a part in modifying the near-side terrae. Another example of such modification is the Sulpicius Gallus Rimae shown in figure 2; these faults (breaks in the crust) cut the margins of the Apennine Mountains. Other faults that trend northeast cut the terrae farther north.

The terrae have also been modified by early volcanic activity and mass wasting. In areas along the margins of the highland terrain (figure 2), fluid mare basal& have flooded ancient craters and other lower sections of the terra, giving the margin a serrated appearance. Terrae

Figure 5 The western edge of Mare lmbrium is marked bythe Harbinger Mountains (lower right) and the Aristarchus Plateau (uppercentre). Basaltic materialsembayand nearlycoverthese highlandterrains. Manytypical marefeaturesareseen in this photograph. The old crater Prinz has been flooded by mare basalts so that only part of its rim is still visible. The Prinz rilles, straight and arcuate,wander across the mare surface; several of the rilles originate in rimlessvolcaniccraters. Parallel alignment of partsof the rilles suggeststhattheyfollowed, in part, pre-existing faults later modified by lava flows from the source craters. The source crater for another extensive sinuous rille,Vallis Schroteri, is seen in the uppercentre of the photograph. Colourvariations in the mare materialsarevisible here; temporarychangesincolouror brightness had been recognized in this area previously by Earth-based telescopic observations. The large (43-km diameter) youngcraterAristarchusisseen intheupperleftpartofthe photograph.

Figure 6 AlthoughmareareasappearflatandmonotonouswhenviewedfromEarth,irregularitieson maresurfaceswereplainlyseen byApolloastronauts.lnthisnear-terminatorphotograph,thecrenulate profilesofmareridges(someashighas200m)areenhancedbythelowsunangle.Asinuousrillecutsthe mareridgenearthecentreofthepicture.OriginoftheAristarchusPlateau,thehighlandareainthelower leftpartofthephotograph,isnotknown,thoughitsrelativelystraightedgessuggestfaultcontrol.

deposits around the margins of several crater floors are evidence of modification by mass-wasting.

The terrae have been known for many years to be both higher and lighter in colour than the mare areas. Apollo photographs, such as figure 2, show these essential dif- ferences very clearly. Measurements made by orbiting geochemical and geophysical instruments have shown that the far-side terrae (figure 1) are as much as 5 km higher than the mean lunar radius; terra areas on the near-side are 2 to 3 km higher than the mean (figures 2,3). The lighter colour is caused, in part, by the greater roughness of the terrain, which results in higher albedo. The higher silica content of these ancient rocks is, however, the main reason for their lighter colour.

The maria Areas of dark, relatively smooth mare material occupy about 15 per cent of the Moon’s total surface; most of these areas occur on the lunar near side. Some of the mare materials fill irregular areas; others occupy multiringed circular basins that are thought to have been formed by the impact of large bodies during the early period of heavy cratering. All of the mare areas are low. As delineated by the Apollo altimeter (figure 3), these areas become successively lower from west to east across the near side, with Oceanus Procellarium (on the west) being 2 km lower than mean lunar radius and Mare Smythii (on the east limb) being nearly 5 km below the mean. This tilting of the mare-filled surface probably represents a tectonic distortion of the near side that is like a great tilted fault block.

Although all the mare materials returned from the Apollo landing sites are basaltic, variations in chemical composition were noted that correlate with colour differences recorded by spectral reflectance (figure 3). Figure 5 shows an example of colour variation in the boundary area between Oceanus Procellarium and Mare Imbrium; the mare surface to the west (upper part of photograph) displays a blue cast, whereas the eastern (lower) mare appears brownish in colour. Temporary changes in colour and brightness, called ‘transient lunar phenomena’ have also been recognized in this area.

It has been possible to determine the probable ages and compositions of mare materials located far distant from the Apollo landing sites by extrapolating colour differences and compositions noted in orbital photographs and by orbital instruments. However, geological interpretations of the photographs indicate that some mare materials may be much younger than the materials sampled at the Apollo sites. Figure 4 shows one area in a multiringed basin, Mare Imbrium, where geological units and morphologic features can be assigned relative ages by superposition relations. The absence of secondary craters derived from the nearby youthful (<l billion years B.P) crater Copernicus on the young lava flow indicates that the flow postdates Copernicus, and is therefore much younger than any of the mare materials returned from the Apollo landing sites; it may be less than 1 billion years old. If so, volcanism has occurred much later in the Moon’s history than was previously thought.

Low sun elevation photographs obtained by the orbiting Apollo spacecraft cameras enhance mare features and prove that these areas are not as smooth and bland as was originally thought. Mare ridges and arcuate

(bow-shaped), straight, and sinuous rilles (valleys) on the surface of Oceanus Procellarum are dramatically shown in figure 6. Here the mare flows embay the older highland materials of the Aristarchus Plateau; faulting at the plateau margin has created a graben (down-dropped block) that is filled with mare material.

Arcuate, sinuous, and straight rilles are ubiquitous features on mare surfaces; in a few places, rilles are also seen in highland terrain. Arcuate rilles are commonly found near the edges of mare basins (figure 2) or associated with other mare structure (figure 5). They were probably formed by tensional stresses in the lunar crust related to the basin-forming events. Straight rilles are also thought to be structurally controlled: sections of the Prinz rilles, seen in figure 5, are aligned parallel to one another, whereas other sections are aligned normal to this trend. Other straight rilles are commonly seen cutting the floors of large, mare-filled craters. The dark halos around some craters situated along and near the rilles suggest that volcanism also played a part in the formation of some rilles. These craters are interpreted as volcanic in origin because they lack rims; the dark halos are interpreted as fine-grained volcanic material that erupted from the crater vents. These deposits differ markedly from the ejecta patterns around bright, young impact craters.

Most sinuous rilles are considered to be volcanic; lavas either formed them or modified a previously existing fault trough. Hadley Rille (figure 7) is a striking example of this type of rille. The Apollo 15 astronauts landed near Hadley Rille, and collected data indicating that layered basaltic lava flows are exposed in the valley walls. Hadley Rille originated in a cleft in the Apennine Mountains, and flowed northward across the flood basalts of Palus Putredinus, becoming shallower and less distinct as it flowed downslope. The valley is v-shaped; irregularities on the floor suggest that lava erupting along fissures on the floor may have added material to the flows that emanated from the source crater. Other materials on the floor appear to be joint blocks that have fallen from the valley walls. At its distal end, the channel appears to be roofed, suggesting that the rille may have been formed, at least in part, as a lava tube that has collapsed along most of its present course. Other sinuous valleys emanate from source craters; the source crater for Vallis Schroteri can be seen on the upper edge of figure 5. Figure 6 also shows several sinuous rilles that emanate from source craters or depressions. The relative youthfulness of the rilles, compared to other mare features, is well documented in this figure.

The craters Impact cratering is the single significant process that has affected the entire lunar surface. Cratering effects are most evident on the Moon’s far side (figure 1) where the surface is saturated with the scars of impact. Crater sizes range from the mare-filled multiringed basins on the near side where diameters measure more than 1000 kilometres, to microcraters recognized on individual grains of material in the sample returned to Earth. The regolith (lunar soil), ejecta blankets, and other types of mantling materials have all been formed by this process.

The density and relative sizes of the impact craters on a surface have been used to determine the relative age of thesurface, asisshowninfigure4. Studiesof theshapesof

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Figure 7 TheApollo spacecraftobtainedthissynopticviewofitslandingsitenearHadleyRille.Therille -consideredoneoftheyoungestontheMoon-originatesinacleftneartheedgeoftheApennine Mountains, which trend northeast across the photo. Its course across the mare materials has been deflectedbyacrater(centre)andbySouthMassif,apartoftheApenninechain.Therilleis1.5kmwideand over 300 m deep nearthe landing site. The astronauts discovered that layered basalt crops out in the upper walloftherille,provingthat,atleastinthisarea,themareconsistsofseveraldiscrete,superposedbasalt flows. The rille is thought to be a giant channel along which lava erupted across the mare surface northwardandthenwestward. Nearitsnorthendtherilleappearstobeinterrupted; higherresolution photographsshowthattherilleisroofedoverinthisandothersmallerstretchesofitscourse,suggesting thatitis,inpart,alavatube.StraightrillescutthemareandterrasurfaceswestofHadleyandarealigned paralleltoit,suggestingthatthelavasthatformedHadleyRillemayhavefollowedandmodifieda pre-existing straight rille formed by tectonism. (North is at the top of picture.)

lunar craters and of the material ejected at the time of impact have been compared with man-made experimental craters on Earth. These studies have provided information on various types of projectile, their energies, and directions of impact; from this information, theories about the nature of an impacting body can be formulated. We have learned that when a high-velocity projectile enters a target, a compressional shock wave. followed by a rarefaction wave, spreadsoutward from the entrance point. Most of the material fractured by impact is moved out of the crater, either in rays of ballistically projected material that extends outward for 10 to 30 crater diameters, or as a continous blanket of material around the crater that extends outward for 1 to 2 crater diameters. Fluidized movement of the ejected material is called the ‘base surge’; this flowing material may form ridges and dunes that vary with the velocity of flow and the

shape of the impacted surface. Figure 4 shows the type of ejecta blanket that forms around a young crater in a mare surface. Although much of the ejecta material has been covered by a younger lava flow, the ridges and dunes of ejected material are clearly shown, and grooves or chains of secondary craters that radiate outward from Lambert are still visible beneath the flow. Following ejection of the fragmented or fluidized material, the floor of the crater rebounds; a central peak may be formed by rebound, depending on conditions of crater size and type of material impacted by the crater.

The central peak in crater King (figure 8a) is a striking example of rebound. Parallelism of the two arms of the peak, nicknamed ‘the lobster claw’, may indicate a pre-existing tabular body that was excavated by impact and brought to its present position by rebound of the crater floor. Comparative roughness of the central peak is

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Figure 8a CraterKing,onthelunarfarside, has beenexhaustivelystudied bygeologistsattemptingto understandthe mechanicsofcratering. Terracesthat have slumped inward from its regular,crisplydefined rim attest to the youth of this crater. The unusual shape of its central peak, the blocky appearance of its floor materials, and the fluid-like aspect of its ejecta blanket have been subjects of intensive study.

Figure 8b The panoramiccamera obtained this high-resolution pictureof part of a pool of dark material on King’s north flank. Features displayed herestronglysuggestthatthe material that makes upthepoolwasfluidatthetimeofformation. Pool material partially fills other nearby depressions and masks the underlying ejecta in other areas. Levied channels point downslope towards the large pool. The channels and festoon-like wrinkles on the surface resemble features associated with terrestrial lava flows and have led some investigators to theorize that the pool is filled with basaltic lava; othersthinkthefluidized material inthe pool is impact melt.

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Figure 9a Thecomplexcentralpeak,lava-floodedfloor,terracedwalls,andridgedejectablanket associatedwiththefar-sidecraterTsiolkovskyhavemadeitanotherfavouritesubjectforstudy.

Figure 9b Thisapron-likeexpanseofstriatedmaterialonthenorth asagiantlandslide,similartolandslidestriggeredbythe’GoodFrid

westflankofTsiolkovskyisinterpreted ay’earthquakeinAlaska.

one indication of the youth of thiscrater. Blocks that litter the crater floor and the top of the central peak, and terraces that have slumped inward from its sharply defined rim are other indications of youth.

Several possible volcanic features associated with crater King were delineated by the superior resolution of the Apollo cameras. Some dome-shaped structures with summit peaks can be seen on the floor of King (figure Sa); these resemble small terrestrial volcanic cones, but their origin is unclear. A large ‘pool’ of dark material on the crater’s north flank that was first recognized in Apollo 10 photographs is another feature (figure 8b) at much higher resolution. Flow channels, cracks, and wrinkles on the floor of the pool indicate that the material was originally fluid; whether the material is volcanic or consists or rock that was fluidized at the time of impact cannot be determined conclusively. A very fine-textured ejecta blanket mantles the surface around King for a distance of two diameters (see figure 8a). Peculiar flow lobes formed along the outer edge of the King ejecta blanket in area where the surging ejecta encountered an opposing slope; the appearance of these lobes testifies to the highly fluid nature of King’s ejecta.

Tsiolkovsky (figure 9), a larger and somewhat older far-side crater, resembles King in some ways; however, important differences are seen that exemplify differences in the processes that formed the two craters. A moderately dense population of small craters is preserved in the north-facing slopes of Tsiolkovsky’s central peak. Terrestrial cratering experiments have shown that central peaks are composed of bedrock that has been displaced upward by a distance equal to about one-tenth the crater diameter. Based on this calculation, the central peak material was once 20 kilometres below the pre-impact surface.

The floor of Tsiolkovsky is almost entirely covered by a dark, presumably basaltic lava flow. A comparison of crater populations and sizes on the floor with those on the central peak indicated that the lavas on the floor were emplaced much later than the peak. Small areas on the northwest and southwest edges of the floor wer?: not flooded by lava. These areas are lighter in tone and appear cracked, furrowed, and hummocky , resembling some areas on King’s floor. The floors of both craters probably consist of crustal material that melted at the time of impact and then solidified. Collection and study of samples from the central peak and floor of Tsiolkovsky are high on the list of goals for future lunar exploration.

The Tsiolkovsky ejecta blanket is dominated by a coarse pattern of ridges that radiate outward from the crater. Superposed on this pattern are small pools of light-coloured smooth material that filled depressions on the underlying surface. These pools appear to be composed of crustal rock that was melted by impact heat and pressure, which filled depressions before solidifying. Figure 9b shows an unusual feature on the northwest

flank of Tsiolkovsky. It has been interpreted to be a massive landslide because the relative smoothness of the material, the longitudinal striations, and the fan-like

shape resemble terrestrial landslides like the Sherman landslide near Anchorage, Alaska, which was triggered by the Good Friday earthquake in 1964. The striations apparently outline individual filaments or jets of debris like those that have been observed in experiments involving the flow of particulate matter travelling at high velocity. The slide originated from a scarp high on the rim of the crater, and gushed 50 km down the outer rim of the crater, coming to rest 3000 m below the source scarp. No similar features have been identified to date on the Moon, although some are seen in the canyonland area of Mars. The triggering mechanism for this landslide and those on Mars is uncertain; seismic events either from other impacts or from internal activity may have caused them. Many small, fresh craters dot the surface of the landslide in figure 9b and also Tsiolkovsky’s ejecta blanket shown in figure 9a; their random distribution, crisp appearance, and small size indicate that they are primary craters formed by impact on the surface at a later date. Other degraded craters can be seen outside Tsiolkovsky’s rim; these craters must predate Tsioikovsky because they are mantled by ejecta material from the larger crater.

Examples of fresh young primary craters that have resulted from impact on a mare surface are seen in figure 2. Their random distribution, raised circular rim crests, and smooth, bowl-shaped interiors attest to their youth. Ridges and dunes in the ejecta blanket of the crater are typical features associated with young craters.

Summary Photographs, samples, and geophysical data from the Apollo missions have been and continue to be examined in detail. These studies have proved conclusively that the lunarrocks, though similar to terrestrial anorthosites and basalts, are older and show minor but important chemical differences. The composition and form of the lunar rocks have provided a clue to the probable composition of terrestrial crust materials before they were subjected to dynamic changes - changes wrought by interaction with Earth’s atmosphere and reconstitution of the materials by tectonism and deformation deep within its interior. Interpretation of the Apollo photographs has allowed us to extrapolate the compositions and processes that are now known for the landing sites to other areas on the Moon’s surface. Those interpretations have provided a much clearer understanding of global processes and compositions on the Moon.

However, the most important results of the Apollo mission are still being formulated. The major gain from the Apollo missions will be the new understanding we will have of Earth, the beautiful, dynamic ball, floating in space, which was so strikingly photographed by the Apollo 15 astronauts (seecover). A better understanding of our own planet, its composition and history, and how Earth relates to the rest of the solar system, is the eventual goal of space exploration, which was so successfully inaugurated by the Apollo missions.

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