The Moon after Apollo

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  • 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 Moons 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 Moons near side, and thus began the scientific study of Earths 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 Moons 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).ThesefeatureappearontheMoonseasternlimb,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 Moons 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.ThelowanglebetweentheSunsraysandthelunarsurfaceenhances topographical relief. Ridgeson the maresurface,sometimescalledwrinkle ridges, have formed parallel to the edge of Serenitatis basin, thought to have formed when a large meteorite impacted the surface earlyintheMoonshistory; 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 Alba...