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Evolution of the Moon: The 1974 Model

Harrison H. SchmittAstronaut, Apollo 17*

The geology of the decade of Apollo and Luna probably will become one of the fundamentalturning points in the history of all science. For the first time, the scientists of the Earthhave been presented with the opportunity to interpret their home planet through the directinvestigations of another. Mankind can be proud and take heart in this fact.

The interpretive evolution of the Moon can be divided now into seven major stages be-ginning sometime near the end of the formation of the solar system. These stages andtheir approximate durations in time are as follows:

1. The Beginning: 4.6 billion years ago2. The Melted Shell: 4.6 to 4.4 billion years ago3. The Cratered Highlands: 4.4 to 4.1 billion years ago4. The Large Basins: 4.1 to 3.9 billion years ago5. The Light-colored Plains: 3.9 to 3.8 billion years ago6. The Basaltic Maria: 3.8 to 3.0 (?) billion years ago7. The Quiet Crust: 3.0 (?) billion years ago to the present

The Apollo and Luna explorations that permit us to study these stages of evolution eachhave contributed in progressive and significant ways. Through them we now can look withnew insight-into the early differentiation of the Earth, the nature of the Earth's protocrust,the influence of the formation of large impact basins in that crust, the effects of earlypartial melting of the protomantle, and possibly the earliest stages of the breakup ofthe protocrust into continents and ocean basins.

The probability is very great that thesynthesis of the geological discoveries ofApollo and Luna will become one of the fun-damental turning points in the history of allscience. For the first time, men have beenpresented with the opportunity to interprettheir own Earth through an understandingof a second planet. This second planet, theMoon, is now a pitted and dusty window intothe Earth's own origins and evolution.

The view through this window is new andat present incomplete. On the other hand, at

1 This paper is a revision and augmentation of apaper prepared for the Soviet-American Conferenceon Cosmochemistry of the Moon and Planets, Mos-cow, U.S.S.R., June 4-8, 1974. Adapted from "TheGeology of Apollo," William Smith Lecture, Geologi-cal Society of London, London, England, 19 Decem-ber 1973, Contribution No. 2480, Division ofGeological and Planetary Science, California Insti-tute of Technology, Pasadena, California.

the close of the decade of Apollo and Luna,we can speak with considerable confidenceabout the internal structure of the Moon, thecomposition of its crust, the past processesthat formed that crust, and the evolutionarysequence through which major portions ofthat small planet have passed. This paperwill review the new limits on our interpre-tive understanding of the Moon and thesequence of events by which we gained thisunderstanding. It also will suggest some ofthe new directions we can follow in searchfor understanding of the Earth as we attemptto apply our new vision of the Moon.

The summary of lunar science containedin this paper draws upon a broad spectrumof ideas and investigations performed bywhat has become known internationally as"The Lunar Science Team." One of the majordifficulties inherent in such a large joint

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64 COSMOCHEMISTRY OF THE MOON AND PLANETS

effort and in the intimate verbal contactamong those involved is the nearly impossibletask of properly acknowledging all contrib-utors to a given idea or area of discussion.This is manifestly more difficult in a reviewpaper such as this. For the present work,I hope that it will suffice to include somegeneral references and to say that what thereader finds he can agree with should becredited to the lunar science team as a whole;what he finds he cannot agree with shouldbe blamed on the author.

The Moon and Its Evolution

THE BEGINNING

Mysterious events were taking placearound our youthful Sun about 4.6 billionyears ago. The materials left over from thebirth of that Sun were combining in system-atic ways, but by largely unknown processes,to form the planets and their satellites. Thespecific beginning of the Moon remains un-clear, as is the case for all the planets andsatellites in the solar system; however, thereappears to be no reason to doubt that thebeginning occurred about 4.6 billion yearsago. On the other hand, the rate at whichnew information is being provided by thespace investigations of the Soviet Union andthe United States makes it almost certainthat an internally consistent model for theorigin of the solar system will appear in thenot too distant future. It is this very rate bywhich we are learning, plus the vast detailthat now constrains the possible models, thathas presently saturated our collective abilityto find a satisfactory model. A return to fun-damental principles, extrapolated under thenew constraints, has begun. However, thefirst new, generally accepted model for theevolution of the solar system has not ap-peared.

THE MELTED SHELL

Sunset on the farside of the Moon was notalways so starkly tranquil as it is now. About

4.6 billion years ago, when the growing Moonwas approximately its present size, the Sunprobably set on a glowing, splashing sea ofmolten rock. Storms of debris still sweptthis sea, mixing, quenching, outgassing, andremelting a primitive melted shell. Thisouter shell, and possibly the entire Moon,appear to have been melted by the greatthermal energy released by the last violentstages of the formation of terrestrial planets.The actual processes by which this energywas released and, in fact, the processes bywhich the seemingly closely related materi-als of the Moon and Earth came together inspace remain subjects of heated debate.

Inside the melted shell the crust and uppermantle of the Moon were gradually takingform through processes associated with thefractional separation of phases on a plane-tary scale. At the base of the melted shell orpossibly in the center of the completelymelted Moon, an immiscible, dense liquid ofiron and sulfur probably accumulated as themelting took place. The initial separation ofsilicate minerals in the outer melted shellthen produced a combined crust and uppermantle a few hundred kilometers thick; thecrust rich in calcium and aluminum (anor-thitic plagioclase), the upper mantle rich inmagnesium and iron (pyroxene and olivine).

Our lunar samples indicate that, in addi-tion to calcium-feldspar, about 30 percent ofthe volume of material of the outer 25 kilo-meters of the crust is made up of mineralsrich in magnesium and iron. Seismic infor-mation from beneath the area of our nearsidenet of seismometers indicates that the lowercrust, that is, a zone between depths of about25 and 65 kilometers, is similar in miner-alogical composition to the outer crust; how-ever, it appears to be of a much morecoherent and more uniform structure. Below65 kilometers and at least to about 200 kilo-meters beneath the Moon's surface, ourseismic evidence indicates that the upperlunar mantle is probably composed largelyof pyroxene and olivine. Both the uppermantle and crust, however, have beengreatly modified by later events and materi-als.

EVOLUTION OF THE MOON: THE 1974 MODEL 65

Most of the major chemical differentia-tion we have observed on the Moon may havebeen established with the formation andcooling of the outer melted shell. This dif-ferentiation included the fractionation ofsiderophile and chalcophile elements into theimmiscible iron-sulfur liquid; the fractiona-tion of many major, minor, and trace ele-ments between the crust and upper mantleduring the fractional crystallization of sili-cate minerals; and the loss of volatile ele-ments from the crust and upper mantle asthe continued rain of primordial debrismixed and splashed the outer melted shell inthe vacuum of space.

The distributions of rare earth elementspresently form one of the major tests of thevalidity of arguments for the existence ofan early melted shell as well as other eventsduring the evolution of the Moon. We canassume that differences and similarities inthe valence state and ionic radii of the rareearth elements, as compared with themselvesand other elements, would exercise the majorcontrols over the effect of various lunarprocesses on specific element distribution.The now obvious reduced oxidation stateof the Moon and the systematic decrease inionic radius with increasing atomic numberof the rare earth elements offer some simpli-fications to problems of interpretation. How-ever, the uncertainties in the nature of lunarevolutionary processes and in critical dis-tribution coefficients for these elements leavemuch room for alternative explanations ofthe observed distributions.

An additional uncertainty is the averageor beginning abundances of the rare earthelements in the Moon. In the absence of thesedata, the abundances in chondrites are usedas a standard for comparison; however, itis clear that the initial abundances andvariations in those initial abundances canaffect some interpretations.

The period of crystallization of the meltedshell appears to have been dominated by thesimultaneous formation of calcium-feldsparand one or more magnesium- and iron-richminerals. The dense magesium- and iron-richminerals would sink in response to gravity

to form a cumulate at the base of the meltedshell. It is probable that the less densecalcium-feldspar floated upward in the shell,for there has occurred a great enrichment ofthat mineral in the upper 60 km of the Moon.The dominant effects of these crystallizationprocesses on the rare earth elements of theMoon were the gradual enrichment of rareearths in the residual liquid and the relativedepletion of europium in that liquid as it wasextracted preferentially by the calcium-feldspar.

As our confidence grows in the "meltedshell" interpretation of the second phase oflunar evolution, we must emphasize the con-cept of early crustal melting and differenti-ation in our thinking about the early historyof the Earth. In addition to the creation ofthe protoforms ("proto" means "first") of acrust, mantle, and possibly a core at thistime, the earth probably also had accumulateda fluidsphere by virtue of a gravitationalfield strong enough to hold volatile com-ponents that would have been lost from theless massive Moon. The extreme depletionof the Moon's crust in components morevolatile than sodium relative to the Earthseems to reflect this difference in mass. OnEarth as on the Moon, it is probable that themajor radial controls on the distribution ofthe elements were established at the verystart of the planet's evolution.

It may be of interest to note at this pointthat in the oldest complexes on Earth thereare rocks called anorthosites, which are richin calcium and aluminum, as are the very oldcrustal rocks on the moon. The early differ-entiation of a plagioclase-rich crust on earthmay account for at least the initial concen-tration of elements composing these myste-rious rocks. Understanding their origin andevolution is not a trivial problem, as most ofour known titanium resources are found insuch rocks.

The early fluidsphere of the Earth is alsoof great interest. It can be assumed to havecontained nitrogen, water, and carbonbecause of the present great abundance ofthese components in the atmosphere and hy-drosphere. They are also abundant in meteor-


ites of the carbonaceous chondrite variety.The analysis of lunar volatile componentsthat are indigenous (in contrast to solar-wind-derived components) indicates that theearly fluidsphere of the earth probably alsocontained significant sulfur and chlorine.The exact chemical and physical nature ofthis fluidsphere would be greatly dependenton temperatures that are presently unknown.

The formation of a core is one of the mostimportant planetary phenomena that mayhave occurred at or soon after the time ofthe melted shell. There is geochemical, mag-netic, and seismic evidence suggesting thata core of a liquid solution of sulfur and ironaccumulated in the Moon at or prior to 3.9billion years ago. This would have occurredearly in lunar history if the entire Moon wasonce molten, or somewhat later if there wasa gradual gravitational migration of the im-miscible liquid from the melted shell periodthrough an always solid mantle.

Whenever the core formed, a remarkableand still little understood phenomenon ap-parently occurred: an electric dynamo prob-ably came into existence, began to perpetuateitself, and produced a magnetic field aboutl/25th the strength of that presently asso-ciated with the Earth. Other alternatives forthe creation of this field presently exist; how-ever, its previous presence is unquestioned.Although the field is not presently active,regional magnetic anomalies left in crustalrocks persist. The anomalies have dimensionson the order of 100 kilometers and thestrongest known is, appropriately, near thecrater Van de Graaff. The presence of hardremanent magnetism in soil breccias formedonly a few tens of millions of years ago sug-gests the possibility that the lunar magneticfield is only temporarily inactive.

If the creation of a lunar magnetic fieldwas dependent on the formation of a con-ducting core, as seems most likely, then sucha core was present at least 3.9 billion yearsago, the age of the oldest rock that has beenexamined for remanent magnetic evidenceof an ancient field. It also seems likely thata protective magnetic field existed aroundthe Earth at least as far into the past as that

of the Moon. The nature of the influence ofthis field on ancient climatic and biologicalprocesses on Earth is not yet known, butthere are many reasons to believe that thisinfluence would have been considerable. Fur-ther delineations of the history and originof the magnetic fields of the Moon and otherplanets will bear heavily on our understand-ing of these terrestrial processes and theirsignificance.

THE CRATERED HIGHLANDS

By about 4.4 billion years ago the surfaceof the Moon's outer crust was solid and musthave looked not unlike the cratered highlandareas we see today. As the debris stormscontinued their declining but still violentways, this cratered and broken outer crustwas saturated by craters 50 to 100 kilometersin diameter. It is now composed largely ofimpact-pulverized, shock-melted, and reag-gregated plagioclase feldspar, a silicate min-eral rich in calcium and aluminum. Theintensity and depth of the disturbance of theouter crust cannot be overemphasized. Byabout 4.1 billion years ago, a debris zoneformed that was at least 10 kilometers thick.The size of the craters with which the sur-face is saturated and the seismic data we haveaccumulated, indicate that the total distur-bance extended to about 25 kilometers belowthe surface. It is not yet certain that theinfall of debris declined in a continuous wayor that this decline may have occurred inpulses of varying intensity.

The four- to five-hundred million yearsduring which the cratered highlands werebeing profoundly modified by impactingdebris may have resulted in changes in thecharacteristics of the rare earth elementdistributions established during the meltedshell stage. Such changes would have beenmost extensive in the upper 10 kilometers ofthe cratered highlands. The first obviouspossibility is that new material with differentrare earth characteristics from those in theearly crust may have been added either fromdeep or extralunar sources. In addition,


selective volatilization and redeposition ofthe light rare earth elements may haveoccurred. Possibly most important, however,was the regional homogenization of the outercrust over areas on the scale of a fewhundred kilometers in diameter. Presentlyobserved geochemical heterogeneities onscales less than this must be the result oflate additions of new material. In particular,the most obvious geochemical anomaliesappear to be the result of the additions ofmaterial derived from depth through ex-cavation or magmatic activity.

It is highly probable that the protocrustof the Earth underwent disturbance compa-rable to that of the Moon. There is no clearevidence of this yet recognized on Earth;however, we should begin to consider the im-plications of the occurrence of such intensecratering. For example, the rates of mechan-ical and chemical weathering of the proto-crust in the environments of the fluidsphereprobably were greatly accelerated, with a re-sulting increase in the rates and degree ofgeochemical differentiation at the Earth'ssurface. The rates of early biological evolu-tion also may have been greatly enhanced bythe availability of nutrients and thermal en-ergy and by the continuous mixing causedby impacting debris. That debris also mayhave continuously supplied the early organicbuilding blocks of life which are presenteven now in some meteorites and in the in-terstellar medium.

THE LARGE BASINS

As the residue of creation was consumedby Earth and Moon alike, the debris stormsdecreased in frequency, although not with-out occasional unusually massive remindersof the past. Some time prior to about 4.1billion years ago, large basins began to beformed by major impact events at a timewhen they could not be obliterated by smallercollisions or by a subsequent period of largecollisions. Most of these basins are now par-tially filled by younger materials such asthe basaltic maria; however, certain gener-alizations can be made.

The relatively young large basins, such asSerenitatis, are circular in shape and havedeep original floors. Variations in gravita-tional accelerations measured from lunar or-bit show that these young basins overlie largeconcentrations of mass (mascons) and haveroughly concentric deficiencies of mass justinside their rims. The older large basins,such as Tranquillitatis, are irregular inshape, have shallow original floors, and con-tain no large concentrations or deficienciesof mass within them. Although all of thegreat basins appear to have been formed bymajor impact events, the general differencesbetween them suggest a major change in themechanical properties of the crust about 4.0billion years ago. The final upward migra-tion and crystallization of the highly frac-tionated residual liquids of the melted shellmay have occurred at this time.

The mechanics of the formation of thelarge basins and the detailed physical andchemical processes associated with their for-mation are poorly understood. Most of ourterrestrial experience with impact and explo-sion cratering has been on a scale where localshock effects, hemispherical excavation, bal-listic ejection, and limited movement ofshock-melted material have been dominant.We must now try to understand events thatoccur on scales ten to a hundred times moreenergetic, and which will interact with largeportions of an entire planet. Regional or glo-bal shock waves, large-scale lateral excava-tion, fluidized ejection and transport of pul-verized debris, and regional movement anddeep crustal injection of shock-melted ma-terial are phenomena that may need todominate our thinking about the formationand effects of the large basins.

Samples from the lunar surface and datafrom orbital sensors indicate that rocks pos-sibly formed from the residual liquids of themelted shell, and rich in alkalies, radioactiveisotopes, rare earth elements, and phospho-rus, are now present in varying amounts inthe debris on the Moon's surface. They havebeen referred to as the "KREEP" basalts.Their major known distribution limits arebetween longitudes 5° E and about 60° W.


They appear to be spatially associated withthe southern portion of the Imbrium Basinand its ejecta blanket to the south and south-west.

The separation of large volumes ofcalcium-feldspar and magnesium- and iron-rich minerals from the residual liquid of themelted shell appears to be the most likelyorigin for at least the parent materials of theKREEP basalts. In addition to the great en-richment of all the rare earth elements andthe greatly accented depletion of europiumin these materials, we find the expected rela-tive enrichment of the light over the heavyelements, imposed by differences in ionicradii.

With the completion of the formation ofthe large basins and prior to their partialfilling by younger materials, a major lunarsurface formation existed that is now largelycovered by light-colored plains materialsand basaltic maria. This formation is nowextensively exposed only inside the outermountain ring of the Orientale Basin whereit consists of a hummocky, cracked, and lo-cally draped layer that appears to have beenshock-melted lava that flowed within thebasin just after the impact event that cre-ated it. Although now largely covered else-where on the Moon, impacts into this surfacemay have contributed significant amounts ofits material to the debris that was formingon the surrounding highland regions.

There are many good reasons to believethat the early crust of the Earth suffered thesame violent indignities of large basin for-mation as did the Moon's crust. Although thesubsequent 4 billion years of dynamic Earthhistory have masked the effects of this vio-lence, there are now many new things to lookfor and many new lines of interpretation topursue. For example, the distribution of theearly ocean basins may have been deter-mined by the distribution of large impactbasins and groups of basins. Also, through-out the Earth's crust there have long beenrecognized regional provinces that are richin certain elements and which are the lociof ore deposits of those elements. For exam-ple, the southwestern United States is one

such geochemical province rich in copper.Our present understanding of the origin andstructure of these provinces is very limitedeven though much time, effort, and moneyhave been spent in endeavoring to under-stand. Locked in the mechanics of the forma-tion of the very large lunar basins, and theirpenetration into the crust, and in the dis-tribution of ejecta around such basins, maybe much of the understanding we seek.

THE LIGHT-COLORED PLAINS

The first event possibly generated by theMoon's interior processes about 3.9 billionyears ago was the surface deposition of light-colored, plains-forming materials. Visual andgeochemical studies conducted from lunarorbit indicate that these light-colored mate-rials are composed largely of the pulverized,but possibly annealed, remnants of the an-cient feldspar-rich crust. The surface featuresof the plains that fill large basins on the far-side of the Moon suggest that the materialsunderlying the plains are eruptive and havepartially filled all the great basins that thenexisted. These surface features include irreg-ular, nearly rimless maar-like craters and low,finely hummocky terrain overlain locally bysmooth, ponded material. The eruption ofsuch light-colored plains materials may havebeen driven by the first internal melting ofthe lunar mantle. These early melts probablywere rich in gaseous components. Throughadditional partial melting of the mantle, lessgaseous and more magnesium- and iron-richbasaltic melts would later concentrate inmuch larger volumes to form the maria.

There are other light-colored plains inold, irregular depressions on the Moon's sur-face. Many of these, particularly those thatare roughly circumferential to the large ba-sins near the limits of their ejecta blankets,may have formed from the ponding of finedebris ejected or remobilized by the impactsthat formed the basins. Still other plains asyet have no obvious origins. It is probablethat both eruptive and impact processes cre-ated light-colored plains during this periodof time.


THE BASALTIC MARIA

Just after the last of the large basins werecreated about 3.9 billion years ago, the finalmajor internally generated episode of evolu-tion took place. This chapter of lunar historyconsists of the flooding of all the great basinson the frontside of the Moon by vast, nowfrozen "oceans" of dark basalt. Only thevery deepest of basins on the farside, suchas Tsiolkovsky, were affected by the forma-tion of the maria. To some degree, however,all portions of the broken outer crust musthave been permeated up to a general mare"sea-level."

The tremendous upwellings and extrusionsof the basaltic maria were perpetuated byheat from radioisotopic decay, and, in thiscase, geochemical and petrogenetic argu-ments suggest that portions of the Moon'supper mantle or even deeper inner mantleprobably melted. The radioisotopic heat ac-cumulation was probably accentuated by atleast one of four factors: (1) the insulativeproperties of the outer crustal regolith pro-duced during the cratered highland period,(2) the additional insulative properties ofregional blankets of hot ejecta from large ba-sins, (3) the possible heat contribution fromthe less differentiated inner mantle, and (4)in some cases of the earliest maria eruptions,the local release of pressure caused by theformation of the large basins.

The extrusions of the products of the melt-ing appear to have been in pulses each ofwhich is now represented by the basaltic fill-ing of many of the large basins and of othertopographically low areas. The distributionof different chemical varieties appears to beroughly concentric to the Tranquillitatis re-gion. This suggests that successive surfaceeruptions of the maria may have been con-trolled by the interaction of the equipoten-tial surfaces of the lunar gravitational fieldwith an offset lunar figure and the bordersof impermeable crust beneath older mareeruptive areas. The limits of the large basinswould be the dominant lateral modifier ofthis idealized pattern of eruption.

The early extrusion of each major pulse

of mare basalt may have been very rapidthrough the intensely and deeply brokenouter crust of the preceding cratered high-land stage. If this was true, then each localbasalt-filled basin may be a single coolingunit and similar in internal structure to ourown planet's basaltic and ultramafic strati-form sheets. Most of the upper visible por-tions of these basins, however, are nowcomposed of extensive lava flows, 10 to 100meters thick, and, in some cases, several hun-dred kilometers long. These flows, along withsome pyroclastic debris, appear to haveerupted from late-stage, localized centers andthen spread over vast areas in and aroundthe large basins.

Variations in the internal composition ofthe Moon or in temperatures as a functionof depth caused the period of mare basinflooding to span at least the time from 3.8to 3.0 billion years ago. Such variations incomposition or depth of origin also causedmajor differences in the contents of titaniumand certain minor and trace elements in thebasalts that were produced as this timepassed. Finally, near the end of each of sev-eral periods of mare flooding, mantles ofchemically distinct, orange, black, red, andgreen basaltic material were deposited as py-roclastic debris over large areas. This debrisis unusually rich in magnesium, iron, somevolatiles, and primitive lead isotopes, andmay have been derived from the very deep,less differentiated interior of the Moon.

The details of the rare earth element dis-tribution after the crystallization of themelted shell may offer some clues to thedepths of origin of the mare basalts. Asthe magnesium- and iron-rich cumulateformed at the base of the shell, the lunarmantle was gradually formed. In an ex-tremely simplistic model, the cumulate as itformed would trap and isolate an intercumu-late liquid. Because of the progressive con-centration of the rare earth elements in theresidual liquids, the intercumulate liquidwould tend to have progressively higher con-centrations of these elements with decreasingdepth and age of accumulation in the mantle.In addition, the separation of calcium-feld-


spar from the residual liquid would cause theintercumulate liquid to have a progressivelygreater depletion of europium relative toother rare earths with decreasing depth inthe mantle.

A large number of the characteristics ofthe basaltic maria indicate that they origi-nated by the partial melting of selected por-tions of the lunar mantle. In particular, thenearly complete remelting of localized inter-cumulate material is suggested by the modelages (approximately 4.5 billion years) ofmany mare basalts. The characteristics ofthe rare earth abundances in the variousmare basalts suggest a depth-dependent se-quence of remelting of the mantle to producethe magmas from which they crystallized.This sequence is based on the expected in-creases with decreasing depth in (1) thetotal rare earth abundances, and (2) the rel-ative depletion of europium relative to otherrare earths. The probable sequence of mariasampled to date in order of decreasing depthof origin is Apollo 15 (3.2 b.y.), Apollo 12(3.1 b.y.), possibly Luna 16 (3.4 b.y.),Apollo 17 (3.8 b.y.), and Apollo 11 (3.7 b.y.).As is indicated by the apparent ages of themaria, the remelting of the mantle was gen-erally from the outside in, as has beensuggested by many workers based on geo-physical arguments.

There is a further suggestion in this se-quence that the titanium-rich portions ofthe original mantle, from which came theApollo 17 and 11 basalts, were formed rela-tively late (shallow mantle depths) in thecrystallization of the melted shell. The re-melting of titanium- and iron-rich oxidesin addition to the intercumulate materialmay account for the enrichment of the heavyrare earths relative to the light rare earthsin the Apollo 17 and Apollo 11 titanium-rich basalts.

With the completion of the eruptions ofthe maria, a relative quiet settled forever onthe surface of the Moon. When the wavesand currents in the surface flows of themaria had finally been arrested, the Moon'sappearance differed only slightly from thatof today.

We perhaps should note at this point thatthe oldest rock terrains on Earth alsocontain vast layered rock sheets which in ag-gregate are basaltic or ultramafic in compo-sition. Their resemblance to the lunar mariamay be more than coincidental. The possiblyhigher rate of radioisotopic heat accumula-tion in the Earth due to its larger volumeto surface area ratio would have acceleratedand prolonged any internal melting thatmight have produced mare-like materials.Again, as with the anorthosites, the questionof the existence of terrestrial maria is not oftrivial interest because large portions of thenickel, chromium, and platinum-group metalresources of our planet are located withinthese sheets of rock. In addition, the erup-tion and weathering of terrestrial basalticmaria may have produced a "geochemicalpulse" at the Earth's surface, of presentlyundefined significance.

THE QUIET CRUST

About 3 billion years ago, except for faintrumblings and occasional sharp ringings wehear now as seismic reminders of the past,the storied Moon apparently completed thevisible record of its tale. There are indica-tions of a brief period of later internalactivity, possibly a convective overturn ofthe mantle, but nothing like the continuingactivity of Earth and, apparently, of Mars.The stratigraphically young ridge and vol-canic system in Mare Procellarum, the greatregional graben systems across the southeastquadrant of the frontside of the Moon, andthe light-colored swirls of apparent altera-tion scattered around the whole Moon mayreflect the embryonic stresses of aborted evo-lution.

Thus, we bring ourselves to the Moon asit is at present. In many respects, the Moonis as chemically and structurally differenti-ated as the Earth, lacking only the continuedrefinements of mantle melting and convec-tion and crustal weathering and metamor-phism. In other respects, the Moon movesthrough space as an ancient text, related tothe history of the Earth only through the


interpretations of our minds, and as themodern archive of our Sun, recording in itssoils much of immediate importance toman's future well-being. Our only means ofreading the text and using the archive is tostudy what we now have and, most impor-tantly, to continue to go there.

The History of Apollo's andLuna's Science

TRANQUILLITY BASE—APOLLO 11

(July 20, 1969)

What was the history of events of Apolloand Luna that have led us to this first-orderinterpretation of the evolution of anotherplanet? The most important of those eventsoccurred in the southern region of MareTranquillitatis. That region holds a uniqueplace in the annals of science and of man-kind. The event which history will remem-ber as having changed forever the course ofthat same history was the landing of Apollo11 by Armstrong, Aldrin, and Collins.

Science for its part finally had real andfactual insight into the temporal dimensions,if not all of the actualities, of the evolutionof our sister planet. The ages of at least ma-jor portions of the rocks in the lunar crustwere found to be very old relative to knownterrestrial rocks and to most previous esti-mates of the probable ages of lunar rocks.Support for the arrested evolution of theMoon was also illustrated by the lack ofsignificant internal seismic activity. Thefact that the relatively young-appearingTranquillitatis mare was 3.7 billion years oldseemed to confirm that we would be studyingour own past along with that of the Moon.As a consequence of comparing the veryhighly cratered highlands of the Moon withthe relatively uncratered but nonethelessvery old maria, it was necessary to concludethat a major change in the frequency oflarge impact events occurred prior to 3.7 bil-lion years ago. This conclusion caused major

revisions in our stratigraphic interpreta-tions related to the time scale of lunar and,therefore, terrestrial evolution.

The rocks of the maria were found to bebasaltic, as predicted; however, they werenot only unusually rich in iron and titaniumand poor in sodium, carbon, and water byterrestrial standards, but were highly differ-entiated chemically, relative to solar and me-teoritic elemental abundances. Because ofthis and other new factors, the isotope andtrace element chemistry of the Moon was ob-viously not to be a straightforward applica-tion of fact and prejudice gained from stud-ies of the Earth and meteorites. Within thebasalts as a whole, we began to see that thecrust of the Moon was much richer in ura-nium and thorium than would be expectedfrom accepted solar abundances. The basaltsalso appeared to make up several flow unitsthat in turn appeared to be locally differen-tiated through fractional crystallization.Related to this crystallization is a still incom-pletely identified, immiscible sulphur-richgas phase that produced spherical holes andvugs in most of the basalt samples. The de-tailed nature of this gas phase in these andmany other rocks from all other landingsites remains a mystery.

Tranquillity Base gave us our first directexposure to the complexities and puzzles ofthe lunar soils, or what became known as the"lunar regolith." As had been supposed fromprevious investigations, including Ranger,Surveyor, and early Luna missions, most ofthe material in these soils appeared to havebeen derived by the pulverization, shockmetamorphism, shock melting, and local re-aggregation of the underlying basalts. Thereaggreation process produced dark matrixbreccias that are chemically and texturallynearly equivalent to the soils. Within thesoils and soil breccias there were found to besmall amounts of exotic materials includingnot only basaltic material from distant ornow covered maria, but also anorthositic andgranitic non-mare debris apparently de-rived from the cratered highlands to thesouth. Meteoritic debris, migrant volatilesfrom other regions, gases derived from the


solar wind, and the effects of galactic cosmicrays were also identified. The debris thatmay have come from the cratered highlandsseemed to confirm the Surveyor VII resultsat Tycho that the southern lunar highlands,and, therefore, the lunar crust, were rich incalcium and aluminum silicates.

Possibly most important to science, Apollo11 confirmed that much of our intellectualexperience in geoscience was applicable toour studies of the Moon; however, it alsoconfirmed that our intellectual insight wasin great need of expansion.

MARE COGNITUM—APOLLO 12

(November 19, 1969)

Conrad, Bean, and Gordon on Apollo 12landed within a few hundred meters of apreviously landed Surveyor III automatedspacecraft in Mare Cognitum southwest ofImbrium. Their mission returned obvious com-plexity to lunar science after the emotionalearly simplifications following the resultsfrom Apollo 11. The structure of the gar-dened upper few meters of the lunar surfacebecame a complex history book not only re-cording solar and cosmic events, but showingthat the relative mobility of volatile elementsin high vacuum would be of great signifi-cance in interpreting geochemical measure-ments. Representatives were uncovered ofheretofore unsuspected rocks rich in potas-sium, rare earth elements, and phosphorus(KREEP basalts) apparently recording afractionation event that occurred about 4.5billion years ago in the melted shell. Therange of ages of the basaltic maria was ex-tended downward to about 3.2 billion years;that is, the formation of the mare basaltscovered at least half a billion years. It alsowas found that the major chemical variabil-ity in basaltic maria extended to varietieswith relatively low quantities of titanium.The surface units of the maria were con-firmed to be differentiated flows on the orderof several tens of meters thick.

Of considerable importance to lunar stra-

tigraphy was the fact that the landing sitewas located on a ray of debris from the cra-ter Copernicus. Rays were thus seen to con-sist of large masses of material and not justdisruptions of the surface by relatively smallamounts of ejecta. Material from this raywas used to determine that the event thatformed Copernicus probably occurred 0.9 bil-lion years ago. This date now provides ananchor to much of the stratigraphic correla-tion of events that occurred after the forma-tion of the basaltic maria.

The geophysical data from Apollo 12,taken in concert with the strange findingsfrom Apollo 11, began to establish their ownspecial surprises. The seismometer showedus that the upper crust of the Moon ringslike a bell when hit. It has the unusual andunexpected combined properties of very lowattenuation (high Q) and very intense wavescattering. Such properties probably are theresult of a dry, pervasively fractured crustin which individual blocks have well-seatedcontact points against one another.

Magnetometers onboard automated or-bital spacecraft had previously shown thatthe Moon presently has essentially no globalmagnetic field (less than 1 gamma). In con-trast, the Apollo 12 surface magnetometershowed that local, low-intensity fields(around 100 gammas) were present. This,combined with the presence of hard rema-nent magnetism in the rocks, indicated thatthere had been a strong ancient global mag-netic field (2000-3000 gammas).

MARE FECUNDITATIS—LUNA 16

(September 20, 1970)

The sample return from the northwesternpart of Mare Fecunditatis by Luna 16 dem-onstrated an important new dimension tothe previous Luna and Surveyor automatedstudy of the Moon's surface. The data fromthe materials of the upper surface of thisgreat eastern plain permitted further gener-alizations to be made concerning the char-acter of the basaltic maria previously


sampled by Apollo 11 and Apollo 12in Mare Tranquillitatis and Mare Cognitum.Although each mission had sampled only avery small part of the vast region containedin these three, widely separated maria, the in-ternal consistency of the results of variousinvestigations on the samples increased theconfidence that many of the data from anindividual mission were representative ofbroad regions.

The crystallization age of a fragment ofthe local Fecunditatis basalt was determinedto be about 3.4 billion years, intermediateto the 3.7- and 3.2-billion-year crystal-lization ages measured for Tranquillitatisand Cognitum mare basalts, respectively.Some other characteristics of the Fecundi-tatis basaltic material also are intermediaterelative to Tranquillitatis and Cognitum, in-cluding the titanium and silicon contents. Onthe other hand, new ranges in the variabilityof basaltic compositions were established bythe Luna 16 analyses; rare earth elementconcentrations are lower than found for ear-lier missions, and the depletion of europiumrelative to chondrites is less.

The investigation of the regolith charac-teristics at the Luna 16 site indicates broadsimilarities with those of Tranquillity Base;however, the non-mare components of theregolith show many distinctive features rela-tive to both the Apollo 11 and 12 sites. Inparticular, the chemistry of the non-marecomponents indicates that the cratered high-lands (surrounding Fecunditatis) that havecontributed to the regolith, have retainedtheir own distinctive provincial character,as has been seen at all Apollo and Luna land-ing sites.

FRA MAURO—APOLLOS 13 AND 14

(February 5, 1971)

To the east of the Mare Cognitum landingsite of Apollo 12, and in the highlands southof the crater Copernicus, we planned thelanding of the Apollo 13 mission. We wereanticipating that through study of the Im-

brium Basin ejecta blanket, we would re-ceive insight into the intensity and timingof the event that formed the basin. Instead,we received new insight into ourselves. Thecourage of Lovell, Haise, and Swigert, as wellas the resourcefulness of the ground con-trollers of their mission following the ex-plosive destruction of the Service Module,provided one of history's most graphic ex-amples of man's potential in the face ofextreme adversity.

Apollo 14 and Shepard, Mitchell, andRoosa inherited Apollo 13's exploration planfor Fra Mauro. The mission told us that notonly did the Imbrium event occur barely 100million years before the oldest mare basaltextrusions, but that such massive collisionscause much more geologic disruption andtransfer much more heat energy into a plan-et's surface than we had ever before imag-ined. In fact, it now appears that much ofthe pulverized crustal material ejected fromthe large basins moved many hundreds ofkilometers across the Moon's surface andhad many of the mechanical, dynamic, andmetamorphic characteristics of volcanic ashflows.

The Apollo 14 mission also confirmed theextreme chemical differences between thehighlands and the maria detected on pre-vious missions. On the other hand, the abun-dance of rocks richer in alkalies, radioactiveisotopes, rare earth elements, and phospho-rus than were other known highland rockssuggested a well-defined provincial natureto the distribution of at least some lunar ma-terials other than the maria.

With Apollo 14 we finally established thebaseline of a net of seismometers. In con-junction with the Apollo 12 seismometer, itbecame possible to look at the structure andphysical properties of the lunar crustthrough the analysis of data from naturaland manmade seismic (impact) events. Mostimportantly, evidence appeared that at leastthe outer portions of the Moon are layered.Also, the first evidence of moonquakes beganto accumulate, indicating that, although veryquiet, the Moon was not yet completely deadinternally.


HADLEY-APENNINES—APOLLO 15

(July 30, 1971)

The Apollo 15 mission to Hadley Rille atthe foot of the lunar Apennine Mountainsintroduced a new scale to lunar exploration.First, Scott, Irwin, and Worden began tolook at the whole planet through the eyes ofprecision cameras and electronics as well asthe eyes of man. Then, on the Moon's surfacethey reached beyond our earlier hopes andwere the first to use a powered surface ve-hicle to rove and observe the wide variety offeatures available for investigation.

The varied samples and observations fromthe vicinity of Hadley Rille and the moun-tain ring of Imbrium pushed our knowledgeof lunar time and processes back past the3.9-billion-year barrier we had seemed tosee on previous missions. We discovered,however, that our interpretations of lunarhistory behind this barrier would have tocome through the mask of multiple cycles ofimpact brecciation. Nevertheless, throughthe clasts in the breccias we began to vaguelysee into the first half-billion years of lunarevolution and into some of the details of themelted shell period. In addition, we expandedour delineation of the complex volcanicprocesses that created the present surfacesof the maria. These processes now were seento include not only internally differentiatedlava flows but possible processes of volcanicerosion that could create the lunar sinuousrilles. We also saw once again how pervasiveare the effects of contamination of surfacematerials by the rays of distant impactevents.

With Apollo 15, we finally established ageophysical net, particularly a seismic net,by which we began to see into the inside ofa second planet; the structure of that planetas partly described earlier had begun to bedeciphered. This net and correlations of itsinformation with other facts have shownthat the general structure of at least majorportions of th'e Moon's interior is as follows:an upper, broken, calcium- and aluminum-rich silicate crust extending from 0 to 25 kilo-

meters; a lower, coherent, calcium- andaluminum-rich silicate crust from 25 to 65kilometers; an upper, magnesium- and iron-rich silicate mantle from 65 to about 200 or300 kilometers; an inner, probably chondriticand volatile-bearing silicate mantle fromabout 300 to about 600 kilometers; a lower,also probably chondritic and volatile-bearing,seismically active, locally melted mantle from

. about 600 to about 1100 kilometers; and anat least partially fluid, possibly iron-sulfurcore from about 1100 kilometers to theMoon's center at 1735 kilometers.

Our geophysical station at Hadley-Appen-nines also told us that the flow of heat fromthe Moon was possibly two times that ex-pected for a Moon of the approximate radio-isotopic composition of the Earth's mantle.If true, this tended to confirm earlier sugges-tions that much of the radioisotopic materialsin the Moon were concentrated in its crust.Otherwise, the interior of the Moon wouldbe more fluid and active than the record ofthe seismometers indicates.

We began to be able to correlate ourlanding areas around the whole Moon by vir-tue of geochemical X-ray and gamma-raymapping from orbit. These remote sensinginvestigations disclosed the provincial natureof lunar chemistry, particularly by high-lighting differences in the ratios of aluminumto silicon and of magnesium to silicon withinthe maria and the highlands. By outlininganomalies in the distribution of uranium,thorium, and potassium, the gamma-ray in-formation suggested that large basin-formingevents were capable of creating surface geo-chemical provinces through the ejection ofdeep-seated material.

From our orbital investigations, we alsogreatly expanded our knowledge of the dis-tribution and geological correlation of gravi-tational and magnetic anomalies in theMoon's crust. This was accomplished by useof a small satellite ejected by Apollo 15 priorto leaving lunar orbit for the return to Earth.

Possibly of equal importance with allthese discoveries by Apollo 15 was the reali-zation, by ourselves and through televisionby millions of people around the world, that


there yet existed beauty and majesty in viewsof nature previously outside human experi-ence.

APOLLONIUS REGION—LUNA 20

(February 21, 1972)

The second automated sample return,mission from the Moon, Luna 20, landed inthe Apollonius region south of the large basinCrisium and 120 kilometers north of the MareFecunditatis landing site of Luna 16. Aswith Luna 16's data on the basaltic maria,the most important aspect of the Luna 20sample was the increased global perspectiveit gave us with respect to the character oflunar highlands. When compared with theinvestigation of cratered highland materialsampled on Apollos 14 and 15 and later onApollos 16 and 17, the Luna 20 materialsemphasize the homogenization effects of thehalf-billion years of cratering that formedthe highland regions we now see. It is in theremaining traces of heterogeneity which re-flect ancient highland provinces that we seethe extent of the homogenization.

The materials of the Apollonius regionappear to be similar to the materials returnedslightly later by Apollo 16 from the Descartesregion. The major exceptions to this simi-larity are the significantly lower aluminumcontents of debris probably representative ofthe Apollonius region, and the abundance offragments representing a distinctive suiteof crystalline rocks known as the anorthosite-norite-troctolite suite. This suite first becamerecognized after Apollo 15 as possibly beingthe much reworked remnants of at least por-tions of the ancient lunar crust. Luna 20confirmed its importance. In addition tothese major distinctions, the Luna 20 materialshow differences in their trace element con-centrations relative to other highland areas.In particular, the rare earth elements arepresent in clearly lower abundances than inmaterials from Apollos 14, 15, and 16.

The last crystallization age of some of theLuna 20 rocks appears to be about 3.9 billion

years, and continued to point up this age asreflecting a major age limit in lunar history.The same general age for the cooling of high-land or highlandlike materials had beendetermined for the ejecta blanket of theImbrium Basin at Fra Mauro and for therocks of the Apennines, and soon would bedetermined for the highland rocks at Des-cartes. This age limit was now seen to repre-sent one of the following occurrences: (1) amajor thermal event associated with the for-mation of several of the large basins over arelatively short time period, (2) a majorthermal event associated with the formationof the light-colored plains, or (3) the rapidcessation of the period of major crateringthat had continually reworked the crateredhighlands until most vestiges of original ageshad disappeared and only the last local impactevent was recorded. As we attempt to explainthe absence of very old rocks on Earth, wealso should not forget these possibilities.

DESCARTES—APOLLO 16

(April 21, 1972)

Apollo 16 found that we were not yetready to understand the earliest chapters oflunar history exposed in the southern high-lands. In the samples returned by Young,Duke, and Mattingly from the Descartesarea, we seem to see that the major centralevents of that history were compressed intime far more than we had guessed. Thereare indications that the formation of theyoungest major lunar basins, the eruptionof light-colored plains materials, and theearliest extrusions of basaltic maria tookplace over about 100 million years of timearound 3.9 billion years ago. In addition,indications are present at Descartes that thelight-colored plains may be the loci of manyof the observed regional magnetic anomalies,suggesting that the plains formed as singlecooling units that were initially above theCurie point.

The extreme complexity of the problem ofinterpreting the lunar highland rocks and


processes became clearly evident even as theApollo 16 mission progressed. Rather thanrevealing materials of clearly volcanic originas had been expected, most informationsuggests that the samples from the Descartesregolith had been subjected to an interlockingsequence of igneous and impact processes.Some of these samples may have been de-rived from the distant, now covered surfacelayer of shock-melted lava in the large basins,as well as from bedrock beneath the Des-cartes highland region. A new chemical rockgroup known as "very high aluminum ba-salts" could be denned although their ances-try relative to other lunar materials has beenobscured by the final events that gave thecratered highlands their present form. Theresults of Apollo 16 have within them anintegrated look at almost all previously andsubsequently identified highland rock types.With this complexity comes a unique oppor-tunity to understand the formation andmodification of the Moon's, and potentiallythe Earth's, early crust.

Apollo 16 continued the broad-scale geolog-ical, geochemical, and geophysical mappingof the Moon's crust from orbit. This mappinggreatly expanded our knowledge of geochem-ical provinces and geophysical anomalies, andhas helped to lead to many of the generaliza-tions it is now possible to make about theevolution of the crust.

TAURUS-LITTROW—APOLLO 17

(December 11, 1972)

Near the coast of the great frozen sea ofSerenitatis, Apollo 17 carried Cernan, Evans,and me to visit the valley of Taurus-Littrow.The unique scientific character of this valleyhelps to mitigate the sadness that with ourvisit the Apollo explorations ended. If thisend had to be, it would have been difficult tofind a better locality to synthesize and ex-pand our ideas on the evolution of the Moon.

At Taurus-Littrow we have looked at andsampled the ancient lunar record rangingback from the extrusion of the oldest knownbasaltic maria, through the formation of the

breccias of the Serenitatis mountain ring,and thence back into clasts in these brecciasthat may reflect the very origins of the lunarcrust itself. Also, we have found and arestudying volcanic materials and debris-forming processes that range forward fromthe formation of the earliest basaltic mariasurface and through 3.8 billion years of modi-fication of that surface.

. The pre-mare events in the Taurus-Littrowregion that culminated in the formation ofthe Serenitatis Basin produced at least threemajor and distinctive units of multilithicbreccias. The oldest of these breccia unitscontains distinctive clasts of crystallinemafic and ultramafic rocks that appear to bethe remains of the fractional crystallizationof portions of the melted shell. This conclu-sion is supported by one of these distinctiveclasts, a crushed rock of magnesium olivine,which has an apparent crystallization age of4.6 billion years. The old breccia unit con-taining these clasts has been intruded andlocally metamorphosed by another brecciaunit which was partially molten at the time ofintrusion. This intrusive event appears tohave occurred about 3.9 billion years ago.Such intrusive breccias are probably the di-rect result of the massive impact event thatformed the nearby large basin of Serenitatis;however, an internal eruptive origin cannotyet be ruled out. The third unit appears topartially cap the tops of the mountains andit may be another old ejecta unit from one ofthe several large basins within range of thevalley. This breccia contains a wide varietyof clasts of mafic crystalline material plusother, previously unrecognized material thatincludes barium-rich granitic rock.

The valley of Taurus-Littrow and othernearby low areas appear to be a coincidentalstructural window that exposes some of theoldest, if not the oldest, basaltic maria ex-trusives on the Moon. With an age of 3.8billion years, they are 50 to 100 million yearsolder than the basalts at Tranquillity Base.Like the Tranquillity Base basalts, theTaurus-Littrow rocks are titanium-rich withup to 13 weight percent Ti02. Except fornear-surface, fine-grained varieties, the tex-


ture and composition of the Taurus-Littrowbasalt appears to be essentially uniform todepths of at least 120 meters. This suggeststhat the valley may be the top of a very thickcooling unit of basaltic material. Geophysicalevidence indicates that this unit of basalticmaterial may be as thick as two kilometersindicating also that, when taken with thepresent height of the surrounding massifs,the valley may have had an original depth ofover four kilometers.

One of the many major remaining puzzlesof potentially great significance is that of thetectonic history of the valley and the massifssurrounding it. Several facts suggest that thestructural boundary between the valley andthe massifs has been active throughout mostof its existence. A young scarp strongly re-sembling that of a fault suggests recentactivity. Also, there are no debris accumula-tions at the bases of the steep (20°-25°)slopes of the massifs; in fact, there tends tobe a continuous shallow moat instead. Inaddition, the soils and rocks that are presentat the bases of the massifs are all tens tohundreds of millions of years old rather thanhaving the imprint of the presumed 3.9-billion-year age of the massifs. All of theseconsiderations suggest that blocks of thelunar crust may be in continuous, but epi-sodic motion in spite of the obvious generalstrength and quiescence of that crust. Re-cently identified, but rare near-surface moon-quakes may also relate to this phenomenon.

Relatively recent modifications of the lunarsurface as a result of internal processes alsoare indicated by the visual and photographicdata on the mysterious light-colored swirlsthat were obtained by Apollo 17 from orbit.The swirls, of which Riner Gamma inOceanus Procellarum is the most well-knownexample, are much more widely distributedthan previously thought, particularly inbroad regions to the north and east of MareSmythii. In all known instances, the swirlshave no discernable topographic relief andare superimposed on all associated features.In addition, many swirls are zoned, withlight-colored zones bordering an inner dark-colored zone that is darker than the surround-

ing, unaffected surface. These characteristicssuggest that fluidized alteration processeshave formed the swirls. The fluids in turnmay have their origin in the continued,gradual degassing of the lunar interior.

The modifications of the surface of thevalley basalt included the addition of mantlesof beads of chemically distinctive orangeglass and black devitrified glass. These glassesmay have been formed as the result of pro-cesses once active within the deep interiorof the Moon. The titanium-rich, basaltic toultramafic glasses surprised us once again;their 10- to 30-million-year exposure age isyoung and was expected, for the dark man-tling deposits seen in photographs; but their3.5- to 3.7-billion-year cooling age was not ex-pected. The explanation for this differenceis complex and not yet completely under-stood. The glasses also have an unusual com-plement of trace elements, including lead,zinc, sulfur, chlorine, and others. Some of themore volatile trace components are presentas relatively low-temperature absorbed ma-terial. The volatile lead in the orange glassesis extremely enriched in primitive lead iso-topes and has other isotopic characteristicsthat indicate early isolation from the rocksystems that produced other lunar materialsexamined to date. Green glass beads found atHadley-Apennines on Apollo 15 have similarbut less definitive characteristics. The char-acteristics of the glass beads strongly suggesta volcanic source and a parent material in orbelow the mantle of the Moon and differentin major respects from the parent of themare basalts.

As a followup to the recognition of orangeand black pyroclastic materials on the lunarsurface at Taurus-Littrow, Apollo 17 alsoprovided visual and photographic data fromorbit that have permitted the identificationand interpretation of broad areas of similarmaterials around the southwestern, southern,and southeastern borders of the SerenitatisBasin. It is now clear that the previouslyidentified and mapped "dark-mantle mate-rials" in these areas (and most probably else-where on the Moon) are indeed volcanic de-posits made up of various combinations of


layers of orange, black, red, and green glassand devitrified glass.

THE FUTURE

For all of our Apollo missions we left theMoon before the lunar sunrise had progressedinto the vast regions of the lunar west: MareProcellarum, where the young mysteriousfeatures of that region's central ridge systemstill await the crew of a mission divertedafter Apollo 13; Mare Orientale, whose starkAlpine rings have been viewed closely byman only in the subdued blue light of theEarth. The promise of the story in theseregions has not diminished, but seeminglywatches for the progression of the sunrisesand the landing craft of another generationof explorers. When that time comes and wemerge the scientific revolution brought aboutby Apollo and Luna on the Moon with thesimultaneous revolution brought about bynew insight into the origins of ocean basinsand continents on the Earth, we may beginto understand the great stresses and strainswithin our planet's crust as ocean floors growand continents move. Within future under-standing of features like the basalts of MareProcellarum and its vast ridge and volcanicsystem may lie further inspiration for all ofus.

References

CONFERENCE PROCEEDINGS

Proc. Apollo 11 Lunar Science Conference, A. A.Levinson, ed., Pergamon Press, 1970; Geochimicaet Cosmochimica Acta, Supplement 1.

Proc. Second Lunar Science Conference, A. A. Levin-son, ed., MIT Press, 1971; Geochimica et Cosmo-chimica Acta, Supplement 2.

Proc. Third Lunar Science Conference, Elbert A.King, Jr., Dieter Heymann, and David R. Cris-well, eds., MIT Press, 1972; Geochimica et Cosmo-chimica Acta, Supplement 3.

Proc. Fourth Lunar Science Conference, Wulf A.Gose, ed., Pergamon Press, 1973; Geochimica etCosmochimica Acta, Supplement 4.

Proc. Fifth Lunar Science Conference, Wulf A.Gose, ed., Pergamon Press, 1974; Geochimica etCosmochimica Acta, Supplement 5.

PRELIMINARY SCIENCE REPORTS

Apollo 11 Preliminary Science Report, NASA SP-214, U.S. Government Printing Office, 1969.






REPORTS IN SCIENCE

Lunar Sample Preliminary Examination Team, Pre-liminary Examination of Lunar Samples PromApollo 11. Science, Vol. 165, September 19, 1969,pp. 1211-1227.

Lunar Sample Preliminary Examination Team, Pre-liminary Examination of Lunar Samples PromApollo 12. Science, Vol. 167, March 6, 1970, pp.1325-1339.

Lunar Sample Preliminary Examination Team, Pre-liminary Examination of Lunar Samples FromApollo 14. Science, Vol. 173, August 20, 1971, pp.681-693.

Apollo 15 Preliminary Examination Team, TheApollo 15 Lunar Samples: A Preliminary Descrip-tion. Science, Vol. 175, January 28, 1972, pp. 363-375.

Apollo 16 Preliminary Examination Team, TheApollo 16 Lunar Samples: Petrographic and Chem-ical Description. Science, Vol. 179, January 5, 1973,pp. 23-24.

Apollo 17 Preliminary Examination Team, Apollo 17Lunar Samples: Chemical and Petrographic De-scription. Science, Vol. 182, November 16, 1973,pp. 659-672.

Apollo Field Geology Investigation Team, GeologicExploration of Taurus-Littrow: Apollo 17 Land-ing Site. Science, Vol. 182, November 16, 1973, pp.672-680.

SCHMITT, H. H., Apollo 17 Report on the Valley ofTaurus-Littrow. Science, Vol. 182, November 16,1973, pp. 681-690.

COMPILATIONS OF ABSTRACTS

54th Annual Meeting, April 16-20, 1972, abstractsand program in EOS, Trans. Am. Geophys. Union,Vol. 54, April 1973, pp. 218-520.

Pall Annual Meeting, December 10-13, 1973, ab-stracts and program in EOS, Trans. Am. Geophys.Union, Vol. 54, November 1973, pp. 1060-1236.


GSA Meeting, Dallas, Texas, 1973, Symposium onGeology and Geochemistry of the Moon, W. R.Muehlberger and Harrison H. Schmitt, presiding,abstracts published in Geological Society ofAmerica, Vol. 5, October 1973; summary of sym-posium published in Geology, Vol. 2, March 1974,pp. 136-137.

OTHER ARTICLES

HINNEKS, N. W., The New Moon: A View. Rev.Geophys. and Space Phys., Vol. 9, August 1971, pp.447-522.

WASSEKBURG, G. J., The Moon and Sixpence of Sci-ence. Astronautics and Aeronautics, Vol. 10, April1972, pp. 16-21.

Luna 16 Issue, Earth. Planet. Sci. Letters, Vol. 13,January 1972, pp. 223-446.

Luna 20 Issue, Earth Planet. Sci. Letters, Vol. 17,December 1972, pp. 3-63.

Luna 20 Issue, E. Anders and A. L. Albee, eds., Geo-chimica et Cosmochimica Acta, Vol. 37, April 1973,pp. 719-1109.

U.S. GEOLOGICAL SURVEYAPOLLO GEOLOGY TEAM REPORTS

Apollo 11 geologic setting included in PET article,Science, Vol. 165, September 19, 1969, pp. 1221-1227.

Apollo 12 geologic setting included in PET article,Science, Vol. 167, March 6, 1970, pp. 1325-1339.

SWANN, G. A., N. J. TRASK, M. H. HAIR, AND R. L.SUTTON, Geologic Setting of the Apollo 14 Sam-ples. Science, Vol. 173, August 20, 1971, pp. 716-719.

Apollo Lunar Geology Investigation Team, GeologicSetting of the Apollo 15 Samples. Science, Vol. 175,January 28, 1972, pp. 407-417.

Apollo Field Geology Investigation Team, Apollo 16Exploration of Descartes: A Geologic Summary.Science, Vol. 179, January 5, 1973, pp. 62-69.

Apollo Field Geology Investigation Team, GeologicExploration of Taurus-Littrow: Apollo 17 Land-ing Site. Science, Vol. 182, November 16, 1972, pp.672-680.

APOLLO 11

U.S. Geological Survey, Interagency Report: Astro-geology 20. David Schleicher, ed., geologic tran-script from Apollo 11 mission, December 1969.

APOLLO 12

U.S. Geological Survey, Interagency Report: Astro-

geology 21. David Schleicher, ed., paraphrasedgeologic excerpts from Apollo 12 mission, June1970.

Information of the USGS sample location and orien-tation for Apollo 11 and 12 is also in R. L. Buttonand G. C. Schaber, Lunar Locations and Orienta-tions of Rock Samples from Apollo 11 and 12.Proc. Second Lunar Science Conference, Vol. 1,MIT Press, 1971, pp. 17-26.

APOLLO 14

U.S. Geological Survey, Interagency Report: Astro-geology 25. R. M. Batson, et al., preliminary logof 70mm pictures taken on the lunar surface dur-ing the Apollo 14 mission (magazines II, JJ, KK,LL, MM), March 1971.

U.S. Geological Survey, Interagency Report: Astro-geology 28. R. L. Sutton, et al., documentation ofthe Apollo 14 samples, May 1971 (supercedesAstrogeology 27).

U.S. Geological Survey, Interagency Report: Astro-geology 29. G. A. Swann, et al., preliminary geo-logic investigations of the Apollo 14 landing site,March 1971.

APOLLO 15

U.S. Geological Survey, Interagency Report: Astro-geology 32. Apollo Lunar Geology InvestigationTeam, preliminary report on the geology and fieldpetrology at the Apollo 15 landing site, August1971.

U.S. Geological Survey, Interagency Report: Astro-geology 34. R. L. Sutton, et al., preliminary docu-mentation of the Apollo 15 samples, August 1971.

U.S. Geological Survey, Interagency Report: Astro-geology 35. R. M. Batson, et al., preliminary cata-log of pictures taken on the lunar surface duringthe Apollo 15 mission, August 1971.

U.S. Geological Survey, Interagency Report: Astro-geology 36. G. A. Swann, et al., preliminary de-scription of Apollo 15 sample environments,September 1971.

U.S. Geological Survey, Interagency Report: Astro-geology 47. R. L. Sutton, et al., documentation ofApollo 15 samples, April 1972.

APOLLO 16

U.S. Geological Survey, Interagency Report: Astro-geology 48. Apollo Lunar Geology InvestigationTeam, preliminary report on the geology and fieldpetrology at the Apollo 16 landing site, April 1972.

U.S. Geological Survey, Interagency Report: Astro-geology 49. Apollo Lunar Geology Investigation


Team, progress report: Apollo 16 sample documen-tation, May 1972.

U.S. Geological Survey, Interagency Report: Astro-geology 50. R. M. Batson, et al., preliminary cata-logof pictures taken on the lunar surface duringthe Apollo 16 mission, May 1972.

U.S. Geological Survey, Interagency Report: Astro-geology 51. Apollo Lunar Geology InvestigationTeam, documentation and environment of theApollo 16 samples: a preliminary report, May1972.

APOLLO 17

U.S. Geological Survey, Interagency Report: Astro-geology 69. Apollo Lunar Geology Investigation

Team, preliminary report on the geology and fieldpetrology at the Apollo 17 landing site, December1972.

U.S. Geological Survey, Interagency Report: Astro-geology 70. K. B. Larson, et. al., preliminary cata-log of pictures taken on the lunar surface duringthe Apollo 17 mission, January 1972.

U.S. Geological Survey, Interagency Report: Astro-geology 71, Apollo Lunar Geology InvestigationTeam, documentation and environment of theApollo 17 samples: a preliminary report, January1973.

U.S. Geological Survey, Interagency Report: Astro-geology 72, Apollo Lunar Geology InvestigationTeam, preliminary geologic analysis of the Apollo17 site, March 1973.

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