VOL. 13, NO. 3 REVIEWS OF GEOPHYSICS AND SPACE PHYSICS JULY 1975 Lunar Science:The Apollo Legacy D. S. Burnett Divisionof Geologicaland Planetary Sciences,California Institute of Technology,Pasadena, CaliJ•ornia 91125 A general review of lunar scienceis presented,utilizing two themes:a summary of fundamental prob- lems relating to the composition,structure, and history of the moon and a discussionof somesurprising, unanticipatedresults obtained from Apollo lunar science.(1) The moon has a crust of approximately60- km thickness,probably composedof feldspar-richrocks. Such rocks are exposedat the surfacein the light-coloredlunar highlands.Many highlandsrocks are complex impact breccias,perhaps produced by large basin-formingimpacts. Most highlandsrocks have agesof •3.9 X 109yr; the record of igneousac- tivity at older timesis obscuredby the intensebombardment. The impact rate decreasedsharply at 3.8-3.9 X 109yr ago. The impact basinswere filled by flows of Fe- and, locally, Ti-rich volcanicrocks creating the dark mare regions and providing the strong visual color contrast of the moon, as viewed from earth. Crustal formation hasproduced enrichments in many elements,e.g., Ba, Sr, rare earths, and U, analogous to terrestrialcrustal rocks. Compared with theseelements, relatively volatile elementslike Na, K, Rb, and Pb_are highly depleted in the source regions for lunar surface rocks. These source regions were also separatedfrom a metal phase,probably beforebeing incorporated into the moon. The physicalproperties of the lunar mantle are compatiblewith mixturesof olvine and pyroxene,although Ca- and Al-rich com- positionscannot be ruled out. Deeper regions,below • 1000km, are probably partially molten. (2) Lunar rockscooled in the presenceof a magneticfield very muchstronger than the onethat existstoday, owing either to dynamo action in an ancient molten core or to an external magnetization of the moon. Lunar soil propertiescannot be explained strictly by broken-up local rocks. Distant impacts throw in exotic material from other parts of the moon. About 1% of the soil appearsto be of meteoritic origin. Vertical mixing by impactsis important;essentially all material sampledfrom lunar coresshows evidence of sur- face residence.The surfacelayers of lunar material exposedto spacecontain a chemical record of im- planted solar material (rare gases,H) and constituentsof a lunar atmosphere(4øAr, Pb). Large isotopic fractionation effects for O, Si, S, and K are present. Physical properties of the surface layers are dominated by radiation damage effects. Lunar rocks have impact craters (<1 cm) produced by microgram-sizedinterplanetary particles. The contemporarymicrometeorite flux may be much higher than is indicatedby the microcraterdensities, indicating time variations in the flux. Particle track studies on the returnedSurveyor camera filter first showedthat the Fe nucleiwere preferentiallyenhanced in solar flares. A. INTRODUCTION materials in terrestrial laboratories,the list of the important questionsmight have read as follows: Any flashbackon the scientificaccomplishments of the past 1. What are the differences in highlands and mare 4-6 yr in the earth and planetary sciencesmust includea sum- materials? Even to the naked eye the lunar surface is not mary of Apollo lunar science.A priori it was probable that homogeneous(Figure 1). From telescopicobservation it was major advancesin the understandingof the natureand history known that the dark (mare) regions were lower and less of a secondplanet would be forthcomingonce lunar samples densely cratered than the lighter-coloredhighlands, which were availablefor laboratory study.This hasin fact happened. have a very high density of craters.The differencesin crater Because of the vast amount of information available it is im- density indicated that the mare surfaces were younger. possiblefor any one personto write a scientificsummary of' Telescopicobservations suggested that the mare were lava Apollo, even if he has lived through the excitementfrom the flows, but the basic distinction between the mare and the very beginning.Also, no two lunar scientistswill probably highlands rock types were unknown, except that the distinc- agree on the relative importance of various discoveries.So tion was likely to be chemical in nature. with all due respectto my colleaguesand without any pretense 2. What is the compositionof the moon as a planet?The for completenessI want to reminisceon someof the findingsof nonuniform appearanceof the moon suggestedthat planetary Apollo lunar sciencethat I have found particularlyintriguing. differentiationhad occurred.But was this a predominantor a This paper is meant for other earth scientistswho have not minor effect?Could inferencesabout the compositionof the tried to follow the pace of lunar science. moon as a whole be drawn from chemical studies of returned If in the pre-Surveyorera I had written a realisticlist of fun- samples? damental lunar problems that might be solved or at least 3. Why is the densityof the moon low, comparedwith that significantlyconstrained by the opportunity to study lunar of the earth or the other inner planets?The large densitycon- trast between the moon (3.34 g/cma) and the earth (5.52 g/cm3) can only partially be accounted for by the self- Copyright¸ 1975 by the AmericanGeophysical Union. compressionaleffects of a comparativelylarge planet like the 13 14 APOLLO 17 APOLLO 1,5 LUNA 16 LUNA 2:0 APOLLO 12 .APOLLO 11 APOLLO 14 APOLLO 16 Fig.1. Sketchof themoon as made by the Czech astronomer Andel in 1926.The Apollo and Luna landing sites are in- dicated.The dark, sparsely cratered areas (maria) are topographically lowin comparisonwith the light-colored, densely crateredhighlands. The Apollo 15 landing site is on the bottom right rim of the large impact basin, Mare lmbrium. Figure courtesy of G. P. Russ. earth [Kot)achand Anderson,1965]. Moreover, the extreme homogeneousplanet [Toksifzet al., 1974]. But constrained comparison is between the moon and Mercury, which onlyby themass, total density,and momentof inertia,a large although its diameter is only 1.4 times the diameter of the degreeof internal structureis still permitted. moon, has a densityof 5.5 g/cm3. It had beenrecognized for 5. What is the stateof evolutionof the moon?Thermally, many yearsthat thesedensity differences must reflectchemical the earthis an activeplanet. The greatmajority of the surface variations,even within the inner solar system, most reasonably and near-surfacerocks of the earthhave ages that are onlya reflectingthe relative abundanceof iron, the most abundant fractionof the ageof the planetitself. In contrast,meteorites high atomic weight element. are primarilydebris that has survivedintact from the begin- 4. What is the internal structure of the moon? The moment ning of the solar system.Where doeslunar material fit into this of inertia of the moon is consistent with that of a hierarchy? 15 Fundamental problems always turn out to be difficult; the above list is no exception. Furthermore, it is clear that the ROCKS l i above problems cannot be isolated totally and attacked in- •5I'•• HI GHLANOS -•• dependently;they are stronglyintertwined. It was not obvious, pre-Apollo, that any answersto thesequestions could be ob- tained, and completelysatisfactory solutions on all countsdo •l, ,6L•/,•//• BASALTS I not exist at present. But is is important to reflect on where things stand and to considerwhat are the prospectsfor the future with respectto thesemajor problems.This will be one theme of this paper. - HIGH Ti However, there is a secondtheme that I wish to developin MARE parallel. The above problems could have been formulated BASALTS without specificknowledge of lunar materials. In this sense they were 'anticipated' problemsto be studied, even though the resultsobtained were in many ways surprising.However, •// MARE •• • BASALTSLOWTi in any venture into an unexploredresearch area, one always also hopesto find exciting'unanticipated' results, and I believe fCHONDRITES that this was the casewith lunar science.Even thoughit is pos- .siblethat in some casesthe resultswere unanticipatedonly by I I I I I I O 2 4 6 8 IO 12 14 me, I found the following resultsnew and exciting:(1) a highly TiO2 wgt. 5/0 nonuniform cratering rate in the early history of the moon, (2) Fig. 2. Correlation diagram of A1203 versusTiO2, impact_breccias as the dominant highlands rock type, (3) a showingfields occupiedby mare basalts,highlands lunar paleomagneticfield, perhapsas large as that of the earth, rocks, and chondrites. No lunar soils have been (4) exotic componentsin the lunar soils (regolith), (5) fun- plotted. The insert compares the lunar fields damentally different properties for lunar surface layers, (6) with the one-standard-deviationspread Jn the dis- tribution of A12Oaand TiO• analysesfor all ter- microcraters on rocks, and (7) an enhanced solar flare iron restrial basalts, including both continental and flux. oceanicsamples, as compiledby Manson [ 1967].The For reasonsof simplicity, brevity, and personalpreference terrestrial range is approximate;any correlation of the discussionof the above two lists will focus on general in- A12Oaand TiO• hasbeen neglected. The high Ti mare terpretations and conclusionsthat are relatively 'model in- basaltsare from Apollo 11 and 17 and differ greatly from any terrestrialcounterpart. The low Ti mare dependent.' Many models have been formulated to address basaltsare from Apollo 12 and 15. Mare and high- someof thesetopics more fully and to provide