Sep 25, 1993 - measured as a function of wavelength can often be used to identify compositional properties of material present. For lunar samples, the type ...
JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 98, NO. E9, PAGES 17,127-17,148, SEPTEMBER 25, 1993
CrustalDiversity of the Moon' CompositionalAnalysesof Galileo Solid StateImaging Data C. M. PIETERS 1,J. W. HEAD1,J. M. SUNSHINE 1,E. M. FISCHER 1,S. L. MURCHIE1,2, M. BELTON 3, A. MCEWEN4, L. GADDIS 4,R. GREELEY 5,G. NEUKUM 6, R. JAUMANN 6, ANDH. HOFFMANN 6 The multispectral imagesof the lunarlimb andfarsideobtainedby the solidstateimaging(SSI) system on board the Galileo spacecraftprovide the first new pulse of compositionaldata of the Moon by a spacecraft in well overa decade.The wavelength rangecoveredby SSI filters(0.4-1.0 }•m) is particularly sensitiveto the composition of marebasalts,the abundance of mafic (ferrous)minerals,andthe maturityof the regolith. To a first order,the limb and farsidematerialis consistentwith previouscharacterization of nearsidelunar spectraltypesfor mare and highlandsoilsand craters. Most basaltsare of an intermediate TiO 2 composition and most of the highlandcrustis feldspathicwith local variationsin mafic content identifiedprincipallyat impactcraters. Dark manflingmaterialon the farsidecanbe interpretedin termsof knownpropertiesof lunarpyroclasticglass. Regionsof cryptornate are shownto have spectralproperties intermediatebetweenthoseof highlandand mare soils,as would be expectedfrom a mixture of the two. There are severalimportantexceptionsand surprises, however. Unlike the basalttypesidentifiedon the nearside,limb and farsidebasaltsexhibitan exceptionallyweak 1 [xmferrousabsorption band. This may indicatea compositionally distinctlunarbasaltgroupthat,for example,is moreMg-rich thanmostbasalts of the nearside. Someof the mostnotablecompositional anomaliesare associated with SouthPole-Aitken Basin. This large regionhas a muchlower albedothan surroundinghighlands. The inner, darkest,portion of the basin exhibitsoptical propertiesindistinguishable from low-Ti basalts. Depositsto the south exhibituniquepropertieswith a strongandbroadferrous1 [xrnabsorption, mostconsistent with abundant olivine. The unusualcompositions associated with SouthPole-Aitkenand their spatiale3tentsuggests the impactcreatingthishugelunarbasinexcavatedmafic-richlower crustor perhapsmantlematerial.
1. INTRO OUCrI'ION
primordialcrustand a mafic mantlethat later was to becomethe sourceregion of basalts [Wood et al., 1970; Wood, 1986]. The extraterrestrialsamplesreturned by the U.S. Apollo The variety of processesproposedfor early evolution of the program and the Soviet automated Luna missions changed lunar crust are summarizedin schematicform in Figure 1 [see forever our perspective of the Earth-Moon system and our Taylor, 1982; Heiken et al., 1991]. Extensiveearly melting is understanding of how planetarybodiesevolved during the first hypothesizedas a "magma ocean" which, with reasonable billion years of solar systemhistory. Since the Apollo-Luna assumptionsof starting composition, is believed to account era, lunar science has progressed significantly through for the enrichmentof plagioclasefeldspar (anorthosite)into increasingly sophisticated analyses of lunar samples and the crust and the accumulation of mafic mineral cumulates throughthe use of an evolving suite of instrumentson Earth- forming the lunar mantle. It was also recognizedfrom detailed based telescopes. Two decades after Apollo, the Galileo analysis of "pristine" lunar samples (fragments which have spacecraftpassedthroughthe Earth-Moonsystemon its way to escapedmuch of the subsequentreworking) that early crustal Jupiterandprovidedour first multispectral imagingview of the evolution also required smaller, more localized differentiation westernlunar limb andneighboringfarside. Discussedhere are events, or plutons, to account for additional, but distinct, the compositionalimplications of this relatively small, but compositionalcomponentsof crustal material [e.g., Warren, highly significant,pulseof new data for the Moon as acquired 1985]. by the solid state imaging (SSI) system. An overview of the In addition to the development of a primordial crust and SSI systemcan be foundin Beltonet al. [1992a]. manfie,physicalprocessesreworkedthe surfaceto an unknown Even with the first samples from Apollo 11 it was depth to form a variety of rock types. A few hundredmillion recognizedthat a global differentiationevent occurredearly in years after the formation of the Moon when the crust was lunar evolution which resulted in an anorthosite-enriched sufficientlydevelopedand rigid, major impact eventsproduced basinshundredsof kilometers in diameter, excavatingmaterial to a depth proportionalto their size. Some may have been 1Department of Geological Sciences, BrownUniversity,large enoughto excavateto the mantle, but this has not been Providence, Rhode Island. confirmed. These high energy events altered, mixed, and 2Now atLunar andPlanetary Institute, Houston, Texas. redistributed material across the Moon. As space debris 3KittPeakNational Observatory, National Optical Astronomy Observatories,Tucson, Arizona. continuedto bombardthe early Moon, the upper kilometer or 4U.S. Geological Survey, Flagstaff, Arizona. two of crust was heavily brecciatedand locally well mixed by 5Arizona State University, Tempe, Arizona. impact craters. This zone is referred to as the "megaregolith" 6DLR,Institute for Planetary Exploration, Oberpfaffenhofen, or brokenand brecciatedzone of the lunar crust[e.g., Heiken et Germany. al., 1991]. On a smallerscale, the well developedfine-grained regolith which we see today with remote sensorsis only a few Copyright1993 by the AmericanGeophysicalUnion. meters thick and representsthe uppermost zone which has Papernumber93JE01221. interacted with small scale processes (solar wind, 0148-0227/93/93JE-01221
$05.00
micrometeorite bombardment, etc.). 17,127
17,128
•RS
ETAL.:CRUSTALDIVERSITYOFTI-IEMOON
•/ {"•/ /.Heavy Bombardment •Y Mega-regolith Crust
Mantle
ß ..
;, ;.:
. , '.".
o. ß
'. '.' -/•!
' .{ Basaltic Volcanism ß
.
MagmaOcean Convection & Mixing Plutons.., ', '.,
.Igneous Activity 'r'" r,•
Basalt Pyroclastics
:• Plutons
{ Melting
Composition ;' Mafics . •, Anorthosite
•n• KREEP
•
Layered Intrusions
Fig. 1. Schematic summary of processes involvedin lunarcrustalevolution (seetext).
The structure and compositional diversity of the crust, however, is not well documented. Apollo seismic
crystalline noritic anorthosite, anorthosites, gabbroic anorthosite, and troctolite (trending to dunire), highly measurements estimated the nearside anorthositic crust to be on suggestiveof plutons[Luceyet al., 1986; Pieters, 1986, 1991; the order of 60 km thick [Toksoz et al., 1974]. The 10-20% of Luceyand Hawke, 1988]. the surface measuredby the Apollo x-ray and gamma-ray The Galileo encounterwith the Earth-Moon system in experimentsaboard the orbiting Apollo 15 and 16 Command Decemberof 1990 provided the first new compositionaldata for the lunar limb and farside in well over a decade. Science and ServiceModulesshowedthe highlandsto be generally rich and the maria to be Fe- and Mg-rich on a regional scale. highlightsof the Galileo SSI lunar encounterare discussedin This suggests that the mare and highland compositions Belton et al. [1992b]. Compositionalanalysesof SSI data are sampledat the nine siteson the easternequatorialnearsideare presentedin more detail in the following three sections. The representativeof principal lunar lithologieswith the highlands first summarizespreviousspectroscopic analysesapplicableto being feldspathicand the maria being basaltic. The limited this extended visible part of the spectru m. The second Apollo orbital experiments, however, also identified several describes the spectral calibration procedures using lunar anomaliesbeyondthe sampledsites,someof the mostnotable samples. The third, most extensive section presents and being the distinct concentrations of radiogenic elements discussesthe compositional diversity observed by SSI for associatedwith localized regions such as the Aristarchus different terrains and how they do or do not fit within our plateau [Metzget et al., 1983] and a modest, but regional currentperspectiveof lunar crustalevolution. increasein Fe- and radiogenicelementswhen the Apollo 15 module passedover the southcentral far side [Metzget et al., 2. BACKGROUND: COMPOSITIONAL INFORMATION !974; Arnold eta/., 1977;Davis, 1980]. DERIVED FROM THE EXTENDED VISIBLE The scaleand natureof crustaldiversityhasbeen addressed in more detail with measurements of the nearside lunar surface At visible and near-infrared wavelengths, measured using spectroscopicinstruments on Earth-based telescopes radiation from the Moon is entirely reflected solar radiation. [e.g., see reviews by Pieters, 1978, 1986, 1993]. Mare Since severalhighly diagnosticmineral absorptionfeatures basaltson the nearsideexhibit a great rangeof compositions occurin this part of the spectrum[Burns, 1970;Adams, 1974, and age, two thirds of which are distinctly different from the 1975; Gaffey et al., 1993], the reflectanceof the surface compositionssampled. The bulk of the nearsidefeldspathic measuredas a function of wavelengthcan often be used to highlandcrustis notablynoritic (plagioclase-rich, containing identify compositionalpropertiesof material present. For low-Ca pyroxenes)in characteralthoughgabbroiccomponents lunar samples, the type and abundance of mafic minerals (containinghigh-Ca pyroxene) appearto increasetoward the (olivine and pyroxene of various compositions) are west. On a local and regional scale spectroscopic studiesof particularlyimportantfor identifyingthe variety of distinct basin massifs and craters large enough to excavate material rock typescomprisingthe lunar crust (recentreview of the frombeneaththe megaregolith (---upper 2 km) haveshownthere compositionalimplicationsof lunar spectralpropertiescan be to be a great diversity of highland rock types including found in Pieters [1993]). The SSI camera containedfilters
PIETERSET AL.: CRUSTALDIVERSITY OF TtIE MOON
covering the spectral range from 0.4 to 1.0 [xm and the discussionbelow will focus on the compositionalinformation that can be derived using multispectral imaging in this extendedvisible part of the spectrum.
2.1. Propertiesof Lunar Materials: GroundTruth Shown in Figure 2 are visible to near-infrared laboratory reflectance spectra of representative lunar rock and soil samples. Sample 67455 is a noritic breccia from Apollo 16 and 12063 is a basalt from Apollo 12. These two particulate samples exhibit typical spectral properties of unaltered highland and mare rocks, respectively. The prominent
absorption bandsnear1 and2 gm aredueto Fe+2 andare indicative of the mafic mineral pyroxene. The shorter wavelengthsof band centersfor 67455 identify the pyroxene compositionas a low-Ca pyroxene[Adams,1974;Hazen et al, 1978], whereasthe longer wavelengthsof the bandsfor 12063 indicatea high-Ca pyroxene,as would be expectedfor basaltic material. The higher reflectanceof 67455 and proportionately weaker pyroxene bands result from the lower abundanceof pyroxene and higher abundanceof feldspar, respectively,
17,129
exposed feldspathic (highland) material such as 67455 generally has a much flatter continuum than compositionally comparablemature soil from the samesite (e.g., 62231). Shown in Figure 3 superimposed on extendedvisible lunar spectraare effective band passesfor five of the filters for the Galileo SSI system(two additional SSI narrow methanefilters were not included in this analysis due to calibration difficulties). Images of the Moon obtained in all filters were individually co-registered, allowing five point spectra for surface elements to be evaluated and compared. These are discussedin section4. Several diagnosticpropertiesof the 1 gm ferrous absorption band, for instance the width and wavelengthof band center, cannotbe measuredwith five filters in this part of the spectrum.Nevertheless,the SSI filters can provide a measureof the strengthof the overall mafic mineral absorption features near 1 [xm and the smooth spectral variations acrossthe visible region. Using the "ground truth" properties of the returned lunar samples as a basis for comparisonand confident interpretation,even theserelatively simple spectral properties provide compositionalinformation of immensevalue for unexploredlunar regions.
which axecharacteristics typical of highlandnoritic breccias. 2.2. Propertiesof Lunar Materials: Lunar SpectralTypesFrom Another common lunar marie mineral, olivine, exhibits a TelescopicMeasurements diagnosticabsorptionband at somewhatlonger wavelengths, The spectralpropertiesof variousterrainson the lunar near near 1.05 [xm. This absorption,not evident in these samples, exhibits a ferrous band that is much broader than that observed side were measuredfrom 0.32 to 1.1 gm with photoelectric detectorsin the late 1960's and 1970's. During that period, for pyroxene. The remaining spectra in Figure 2, shown with dashed spectra were obtained relative to a standard area in Mare lines, are for representativewell developedsoils from Apollos Serenitatis (MS2, located at 18ø 40' N, 21ø 25' E). Such 12 and 16 landingsites. The characteristic continuumof lunar "relativereflectance"spectranot only allowed instrumentaland soilshas been approximatedas a tangentto the spectrum(near atmosphericeffects to be removed, but they also emphasized 0.73 and 1.6 gm) and removedto allow the characterof subtle the small variations in color between lunar regions. Several absorptionsto be discerned. Mature soils are darker than hundredspectraof the lunar near sideobtainedwith Earth-based particulaterock samplesfrom the samesite and exhibit similar telescopesshow that mare, highland, and fresh impact crater but very subduedabsorptionbands. Both characteristics are due lithologiescan be readily distinguished. Lunar spectraltypes to the accumulationof weatheringproductsthat result from were first summarizedby McCord et al. [1972a, b] and later exposureto the spaceenvironment(cosmicray and solarwind expandedupon by Pieters and McCord [1976] and Pieters irradiation, micrometeorite bombardment, etc.). All mature [1977]. The McCord et al. summaryof lunar spectral types lunar soils exhibit an overall "red" continuum that increases in basedon relative reflectancespectraare shown in Figure 4. reflectance towards longer wavelengths. Bright, freshly These early surveysof the near side are importantto the Galileo
Lunar Soils and Rocks 1.1
0l ' I ' I ' I ' i ' I
67455
5 Lunar Soils
0.60
0.40
o
0.20
o.•o o.E
0.00
0.30
0.60
0.90
1.20
1.50
1.80
WavelengthIn Microns
2.1 0
2.40
.
i
/ .
I
,
I
,
I
50.600.80 1.001.201.401.601.80 WavelengthIn Microns
Fig. 2. Visible to near-infraredreflectancespectraobtainedin the laboratoryfor representative lunar rocksand soils. From Apollo 16, 67455 is a feldspathicbrecciapowder,and 62231 is a maturesoil. From Apollo 12, 12063 is a mare basalt powderand 12070is a maturemare soil. The spectraon the right showthe subtleabsorptions of the lunar soilsin expanded scaleaftera continuumhasbeenremoved.Verticallinesare drawnat 0.9 and 1.0 Ixmto allow comparison of bandcenters.
17,130
PIEIZRS ET AL.: CRUSTAL DIVERSITY OF THE MOON
0.70
degreeof shock alterationand length of exposureto the lunar environment. Feldspathicmaterialsexposedat fresh craters tend to have continua that are less steep ("Hatter") than surroundingmaturesoils (similar to the differencein continuum between Apollo 16 feldspathicbreccia 67455 and Apollo 16 maturesoil in Figure2). This is the principalcauseof the bluesloped trend in the reflectance spectra of highland craters (decreasetoward longer wavelengths) shown in Figure 4 relative to MS2. Mafic mineralspresentat a highlandcrater producean absorptionnear 1 Ltmsuperimposed on the distinct immaturefeldspathic"blue" continuum. Most of the common
0.60
0.50
0.40
0.30
2063
Ap 16
0.20
Ap 14 Ap 12
0.10
Ap 11 0.00
0.30
0.50
0.70
0.90
1.1 0
lunar lithologiesidentifiedspectroscopically are distinguished by variationsin the abundances of plagioclaseand pyroxene (the principal mafic mineral). As seen in Figures2 and 3, ferrous pyroxene absorptionbands are centeredbetween 0.9 and 1.0 I.tm and the spectrapeak near 0.75 I.tm. The subtle inflectionsthat occur near 0.76 I.tm in the relative reflectance spectra are thus an important indicator of compositional variations
associated with abundance of this common mafic
WavelengthIn Microns
mineral. As can be seenfrom the examplesin Figure 4, the strength of mafic mineral absorptionsvary from crater to Fig. 3. EffectiveSSI filter bandpassessuperimposed on representative lunarspectraThe rockpowdersareidentifiedby number(seeFigure2); crater. Feldspathiematerial also exhibits a distinctinflection maturesoilsare 62231, 142:59,12070,and 10084representing Apollo near 0.40 !.tm (see 67455), which ereares a peak at that 16, 14, 12, and 11, respectively. wavelengthin reflectancespectrarelative to mare soil [Adams and McCord, 1971b]. The wavelengthof the turndowntoward the bluecan be affectedby othercomponents presentas well as SSI analysesbecausethey coverthe samespectralrangeas SSI the shockhistory [e.g., Pieters, 1977; Pieters and Taylor, andprovidethe basicframeworkfor comparison with the new 1989]. SSI limb and far side data.
As can be seenin Figure4, spectraof freshmareimpact cratersarequitedifferentfrom thoseof maturemaresoils. (The spectralpropertiesof mare soils are discussed extensivelyin the next section.) The most prominentdistinctionbetween soilsandcratersis that the pyroxeneabsorption near 1 Ltmis muchstrongerin spectrafor freshmarecraters.Althoughnot evidentin Figure 4 (becausethe spectrahave been scaledto unity at 0.56 Ltm),freshcratersare alsoalwaysbrighterthan surrounding mare. This distinctionin albedoand strengthof
combined Fe+2absorptions in maficmineral is comparable to the differences betweenrockpowdersandmaturesoilsobserved in the laboratory(Figures2 and 3). Highland craters and mature soils exhibit a similar
distinctionin albedo; freshlyexposedmaterialat highland craters is the brightest material on the surface of the Moon
[Pohnand Widley,1970]. Unlike the diversityseenfor mature mare soils, however,most mature highlandsoils are observed to have somewhatsimilar spectralcharacteristics.Although minorvariationsoccurbetween0.5 and0.7 I.tm,the feldspathic highlandsoils tend to have a somewhatsteepercontinuumthan many mare regions (including MS2) and thus are relatively
"red" in the visible part of the spectrumwith increasing reflectancetowardlongerwavelengths.Reflectance spectraof highland soils relative to a mare soil, such as thoserelative to
MS2 in Figure4, exhibita positivebumpin the spectrum near 1 Ltm becausethe pyroxene absorptionbands are weaker in highland soils (mirroring the low abundance of mafic minerals). As will be discussed below, this normallyweak 1 I.tm absorption is an important mappable property of feldspathichighland soils. As a group,highlandimpactcratersexhibitmorediversity in the extendedvisiblepartof thespectrumthando marecraters [seePieters, 1977]. The causeof this diversityis a complex couplingof feldsparand mafic mineral abundance,as well as
2.3. Compositionof Mare SoilsFrom UV/VIS Measurements Perhapsthe most frequentlymeasuredspectralpropertyof the Moon is the color variation among mature mare soils betweenthe ultraviolet (UV) and the visible (VIS), typically measured as a UV/VIS ratio. The attention given to this spectralparametermerits specialdiscussion.The strengthsand limits to using this spectral parameter in mapping mare composition is discussedin detail below. The spectral variability from the ultraviolet to about 0.73 gm for different mare soils is apparentin the examplesof spectrain Figure 4. Furthermore,differencesamong these spectralpropertiesare spatiallycoherenton the scaleof the remotemeasurements (12 kin) and define extensive and relatively uniform flows or depositswithin the maria. The basaltic samplesfrom different lunar sites exhibit a range of compositions[Papike et al., 1976] that could account for observedspectral variations of surface units. Returned basalt samples also exhibit a range of textures, grain sizes, glass content, etc., and these petrographic properties are equally important for interpretation of spectroscopic measurements.Although the bulk propertiesof a basalt unit may be uniform, localized variability exists, particularly on the scale of individual samples. Soil formation processes (repeated cycles of cornmunition forming fine grained particulates, exposure to solar wind, bombardment by micrometeorites, formation of glass and recrystallized components, accumulation of singledomainreducedFe) tendto homogenizesurfacematerialon a regionalscale. Over billions of years a regolith several meters thick is developed,but the time scale for the uppermost soils to reach steady state maturation is short relative to the age of mare basalts [e.g., Adams and McCord, 1971b, 1973; Pieters et al., 1985].
PIETERSET AL.: CRUSTAL DIVERSITY OF TIlE MOON
17,131
17,132
PIETERS ETAL.:CRUSTAL DIVERSITY OFTI-• MOON
Because the process, however complex, is the same everywhere, regional spectral variations thus reflect inherent differencesof the basaltlithology and allow mare basalttypes to be mapped in a regional extent based on the spectral propertiesof mature soils. Excludingareasnear freshcraters,regionalmareunitshave been successfullymappedwith multispectralimagesthat span the ultraviolet to visible part of the spectrum[Whitaker, 1972;
mature mare soil spectra were used in the analysis. Rock powdersand immaturesoils, soils of mixed heritage,or soils that have not otherwise reached equilibrium with lunar alteration processes, did not follow this relationship. Similarly, in a comparisonof 0.40/0.56 I.tm measurements of telescopicspectrafor the landingsiteswith the TiO 2 of returned soils, the bluet soils were shownto be Ti-rich. Later analyses usingthe entire data baseof lunar soil spectraobtainedby J. B. McCord et al., 1976, 1979; Johnson et al., 1977a, b; 1991a, Adams in the early 1970's showed there to be much scatter b]. The distinct variation between mare in a UV/VIS ratio has when severalsoils from the samesite were used [Johnsonet al., been empirically linked to the TiO 2 content of the soils 1991a, b; H. Hoffmann et al., personalcommunication,1991]. [Charette et al., 1974; Pieters et al., 1973]. The quantitative This scatter is likely due to a combination of laboratory use of this relationship, however, has been substantially measurement variations and the small number of truly modified as additionallaboratoryand telescopicdatahave been representativeand mature soils in the collection. The preferredmethod of establishingan empiricalrelation analyzed [Head et al., 1978a; Pieters eta!., 1979; Pieters, 1978, 1993]. The relationbetweenthe UV/VIS ratio andTiO2 betweenUV/VIS color andTiO2 contentfor maresurfacesis to for mare soils is calibratedprincipally from empiricalevidence use a measurementof the UV/VIS obtainedremotely for each and, for reasonsoutlined below, is valid only within certain sampledsite in comparison with the TiO 2 of maturesoilsfrom constraints.
The notable variations in the UV/VIS
that ratio for mare soils
have no single physical explanation. Several compositional properties are known to affect this part of the spectrum: absorptions of Fe- and Ti-bearing glasses, charge transfer
site.
As
discussed
above,
the
scale of
remote
measurementsprovides a good measure of the regional bulk properties and is relatively insensitive to variations of a smaller
scale.
After additional telescopic and laboratory spectra were mineralabsorptions, Ti+3 electronictransitionabsorptions,obtained,a more completeTiO 2 versus UV/VIS relationship scattering and absorptionby submicroscopicmetal particles, was presentedin Pieters [1978]. This analysisincorporateda shock alteration,recrystallizedilmenite and other opaques,etc. rangeof measured TiO2 (sincetherewasno goodreasonto favor The overall spectral contrast of the bulk sample, which is one mature soil sampleover anotherto representbulk regional controlled by many of the above factors, also affects the properties)and a range of measuredUV/VIS for each landing measured color. Spectra of dark soils (with low spectral site (due to observationalvariations). UV/VIS for one site is contrast) tend to be relatively flat, which for lunar materials measured relative to that for the standard lunar mare area, MS2. results in a relatively blue spectrum. All of these The resultingTiO2 versusUV/VIS relationshipstill showsthat characteristics affect the measuredUV/VIS of soils,but for any dark,relativelyblue mare surfacesare TiO2-rich,but includesa given soil the relative importance of these competing rangeof uncertaintyfor TiO2 values estimatedfrom UV/VIS processesthat create the spectralvariationsobservedremotely measurements, with increasinguncertaintyfor lower valuesof is poorly known. The causeof the relationbetweenTiO2 and TiO2. UV/VIS for mature mare surfacesis most certainlycoupledto When usingsoil TiO2 estimates,recall that althoughmature the overall opacity of the soil particlesand the effect of these mare soils directly reflect the composition of underlying upon spectralcontrast,and is thusdependenton the abundance basalts,they are not identical to it. CrystallineTi-rich basalt of Fe and Ti in the soil. Since all basaltshave a high FeO samplesfrom Apollo 11 and 17, for example, are typically content which varies only slightly, variations in color are severalpercenthigher in TiO2 than local soil. We have used thoughtto be principallycorrelatedto largervariationsin TiO 2 the calibrationand estimatesof TiO 2 from Pieters [1978] for the current SSI analysesof mare soils, but have expandedthe (see discussionin Pieters [1978, 1993]). Isolating various optical effects is not easily achieved UV/VIS categories somewhat to accommodate variations usingreturnedsoils, due to the complexityof soil particlesand observedwithin SSI data (dominatedby large spectralcontrasts the 105 difference in scale between the remote measurements in Oceanus Procellarum). The nomenclature and estimated and laboratory samples. Because lunar soils are multirangeof UV/VIS usedhereare summarizedin Table 1 andFigure 5. component and complex, one frequently used procedure for quantifying compositionalinformation extractedfrom spectra For high-TiO2 soilsthe empiricalevidencesuggests a direct is to derive an empirical relation between composition and relationshipbetweenUV/VIS ratio measuredremotely and % spectral properties [e.g., Charette et al., 1974; Jaumann, TiO2, with thebluestbasalts(highUV/VIS) beingthemostTi1991]. Using Earth-basedcolor difference photography(two rich. Inversely, low-Ti soils have low UV/VIS values filters with matchedpositive and negative film characteristics) (relatively red). It is the intermediate range of measured it was recognizedearly in the Apollo programthat the regions UWVIS values that has the most uncertainty associatedwith aroundlanding sites from which Ti-rich basaltswere returned TiO 2 compositions. Mare soils with intermediatevalues of appeared relatively blue compared to other mare sites UV/VIS (medium 0.40/0.56 I.tm ratio: 0.97-1.02) have [Whitaker, 1972]. It was the lunar samples,coupled to the nonuniquepredictionsfor TiO 2 (see Figure 5). Despitethese telescopic measurementsthat substantiatedthis relationship. uncertaintiesin actual TiO 2 percentages,basalt soils in the In the early Charetteet al. [1974] analysis,the TiO2 contentof medium and medium-high categories can be readily soils measuredin the laboratorywas comparedto the slopeof distinguished by variationsin the UV/VIS ratio and therefore the reflectancespectrumbetween0.40 and 0.56 I.tm (measured mappedas separateunits with multispectralimages. In this as the ratio of 0.40/0.56 I.tm). Ti-rich soils of Apollo 11 and case, distinguishingand mapping distinct basalt units using 17 did indeedexhibit a "bluet" (higher 0.40/0.56 I.tm) spectrum UV/VIS measurements is more accurate than an estimation of % than lower Ti soils, provided that only accuratelymeasured TiO 2 for thesebasaltsoils.
PIETERS ETAL.:CRUSTAL DIVERSITY OFTHEMOON TABLE1. Empirical Relation Between UV/VISRatioandTiO2 Content of Mature Mare Soils [After Pieters, 1978]
'Terminology Used
The geometry of the Galileo Earth-Moon encounter
provided a favorable opportunity for spectral calibration. Half
SSI Measured
Mare Soils
0.40/0.56gin*
Est.wt%TiO2
> 1.07 1.02-1.07 0.97-1.02 < 0.97
High-Ti (HT) Med. High (MH) Medit•n (M)
17,133
>6 3-7 --- 54 SPA areas 28, 29, 30, and 32 are local soils or intercrater plains, .... ,,,, .... , .... , .... whereasareas73 and74 are local freshcraters(seeFigure6). 0.8 The intercratersoils are darker than most highlandsoils, as 300 400 500 600 700 800 900 1000 1100 is evidentboth from the spectraas well as the colored albedo Wavelength slicesof Plate 1. Comparedto typicalhighlandsoils (Figures Fig. 11. Representative reflectancespectrafor Galileo SSI areas(see 4 and 10), Schiller-Schickard soils also have a slightly Figure6 and Table 3 for locations). All reflectancespectra/Sun values stronger1 gm mafic absorption,most easily seencomparing maintainthe relativealbedovaluesmeasuredby Galileo SSI (seetext). the relative reflectancespectraof Figure lib and Figure 9, All relative reflectancespectra/MS2are scaledto unity at 0.56 gm to centerright. This lower albedoand strongermafic absorption allow comparison of principalspectralfeatures. (a) Dark Mantle. (b) Schiller-Schickard. (c) South Pole-Aitken. for the soils at Schiller-Schickard strongly suggests the '=
0.9
_
PIETERSET AL.: CRUSTALDIVERSITY OFTHE MOON
17,143
types of material are shown. First, the lowest albedo areas (areas 33, 34, 35, 36 shown with solid symbols and dashed lines) that are in the central part of the basin are redder than most highland soils but exhibit a mafic absorption band as strong as many mare. Second, the strongestmarie mineral absorptionsare not associatedwith the darkest material, but with areasthat are somewhatbrighter and occurin the southern part of the region (areas 39, 40, and 41, shown with solid lines). The spectraof theseregionsrelative to MS2 peak in the green(0.56 gin): they arerelativelyred (low UV/VIS) andhave a relatively broad and strong 1 I,tm absorptionband. (These areas are near the limb, but the relative absorptions would actuallybe strongerthan what is observedif the measurements contain residual scatteredlight since the data obtained in the greenfilter exhibit the least scatteredlight [seeMcEwen et al., this issue].) Third, the areas shown with open symbols and dotted lines (53, 54, 84) lie just exterior to the northeastern existence of an extensive ancient mare deposit whose optical basin massif and could be ejecta from Apollo Basin (see propertieshave been subsequently alteredby the additionof an locationsin Figure 6). These latter regionsexhibit properties more comparable to common mature and immature highland extensivehighland feldspathiccomponent. soils (less red than South Pole-Aitken interior and lower 0.76/0.99 gm with a positive inflection at 0.76 I,tm) and imply 4.7. South Pole-Aitken a sharpboundaryexistsbetweentheseand the principal South Pole-Aitken lithologies. The optical propertiesof the South Pole-Aitken basin are Although a high-Fe, low-A1 description of the interior unique. Small patchesof low albedo material, presumably South Pole-Aitken is accurate in a general sense, it does not basalt, occur throughoutthe basin [e.g., see Wilhelms, 1987], characterize the mineralogy and the important distinctions but as seen in Plate 1 this far side low albedo anomalyis betweenthe darker inner and the brighter outer portionsof the expansiveand appearsto encompass much of the entire basin. basin. The inner basin is as dark as many mare basalts and In addition,an unusuallystrongmafic absorptionassociated these soils exhibit a marie mineral band equal in strengthand with this region was an importantdiscoveryduringinitial SSI character to low-Ti basalt soils on the nearside. Because the analyses[Belton et al., 1992b]. SSI data are of low spatial and spectralresolution,however, It is well knownthat the Apollo 15 gamma-rayexperiment they cannot distinguish between mare and mare-like optical detectedan increasein iron contentand in radioactivityas the properties. Viable hypothesesdescribingthe low albedo inner spacecraftpassedacrossthe northernboundaryof the basin portion of South Pole-Aitken basin include (1) the presenceof [Arnold et al., 1977; Davis, 1980]. An increasein bulk Fe is numerous small patches of postbasin formation mare basalt, normallythoughtto createa lower albedosoil [e.g.,Adamsand (2) a cryptornate extensively reworked into the regolith by McCord, 1971a], whichin turn wouldbe coupledto a decrease impact bombardment,(3) soil formed on lower crustmafic-rich in A1. Since most of the basin can be delineated on the basis of lithology (compositionally similar to mare basalt) excavated its low albedo(Plate 1) a generalinterpretation of high Fe, low by basin formation, (4) soil developedon Fe-rich impact melt A1 soils across the basin appears consistent with the that dominatesthe optical propertieswhen mixed with crustal observationsand can be quantifiedby extrapolatingfrom the feldspathi½ lithologies. The opticalpropertiesof inner South correlationof x-ray A1/Si measurementsand albedo [Fischer et Pole-Aitken basin are unusualand indicate a mafic lineage, but
presenceof a low albedomafic component,such as a basalt. The two craters evaluated in the area have quite different spectra,eachof whichcanbe readilyinterpreted.SSI spectrum 73 in Figure 1lb has all the propertiesof a classichighland feldspathic crater, including the inflection upward towards longer wavelengthsat 0.76 [xm. SSI spectrum74, however, has a muchstrongerferrousabsorptionat 1 !xm. Althoughit is brighter than many mare craters, its spectral properties, and strengthof the 1 [xmband in particular,appearto be associated with freshly exposedbasalt (this associationshould be tested with higherspectralresolutionnear-infrareddata). Analysisof the spatialdistributionof marie soils in the Schiller-Schickard region and the mixing relationships between highland and mare lithologies presented by Head et al. [this issue] and Mustard et al. [1992] provide convincing evidence that this region is indeed an ancientpre-Orientale"cryptomare." Thus, both the geologic and compositional evidence support the
al., 1992].
there is insufficient
information
to determine
which
of the
Becauseof its low albedoand the strong1 gm absorption, above(or combination)are more likely. thereis little doubtthat the hugelunar regionrecognizedas The unusual but consistent properties of intermediate SouthPole-AitkenBasin is not typical feldspathichighland albedo areas39, 40, 41 (solid lines in Figure 11) presentan crust;it is clearly more mafic. The principalcompositional enigma. These areas all peak in the green (near 0.56 I,tm) issueis the type and abundance of marie lithologiespresent. relative to MS2, implying the presence of a broader marie Furthermore, the unusual properties of South Pole-Aitken absorptionthan that which occursfor pyroxene alone. Based material occur over an extensive region and have survived on spectroscopicanalysis of lunar and terrestrial materials in continuous bombardment since the formation of this ancient the laboratory, several types of materials can be identified basinvery early in lunar history. Within the resolutionof SSI which exhibit these unique spectralpropertiesassociatedwith images,it is also importantto note that no bright cratersare portions of the South Pole-Aitken basin region: (1) Marie observedin the basin implying that these optical, and hence minerals such as pyroxene with a ferric component often compositional, characteristicsare pervasive and not just exhibit overlapping near-infrared absorptions with a surficial. Sincethe currentsurfacehascertainlybeensubjected reflectance peak near 0.6 I,tm [e.g., Adams, 1975]. Such to the same weatheringenvironmentas the rest of the Moon, featuresare commonin terrestrialmaterials. (2) High iron, low the compositionalinformationmust be extractedlargely from titanium homogeneousglass (quenched, without crystalline well developed soils that represent the underlying regolith components)has a broad 1 I,tm glass band and a reflectance lithologies. peak in the green [Adams et al., 1974; Bell et al., 1976]. The RepresentativeSSI spectrafor several areas in and around "green glass"of Apollo 15 is the most common example with the South Pole-Aitkenregion are shownin Figure 11c. Three this property. For comparison,two examples of Apollo 15
17,144
PIE'rERSET AL.: CRUSTALDIVERSITY OF •
"green"glassare shownwith Apollo 17 orangeand black glass in Figure 12. (3) Olivine of some compositions(usually Mgrich) exhibit overlapping ferrous bands that extend toward shorter wavelengthscausing a reflectance peak near 0.6 I.tm. Severalolivine spectra[from Sunshineand Pieters, 1990] are shown in Figure 13. (4) Alternatively, because the natural systemis often extraordinarilycomplex, an unknownprocess or unsuspectedlithology could be responsiblefor the unusual spectralpropertiesat South Pole-Aitken. For the lunar case, option 1 is unlikely and 4 is undeterminable,leaving options2 and 3 as reasonable working hypotheses. Thus, either an abundanceof olivine and/orof Fe-rich,Ti-poor quenchedglass could account for the unusual properties observed in the southernSouth Pole-Aitken basin. Both imply an enhanced marie concentration.
MOON
1.001 ' ' • 'Spectra ' •' Oilvine
,FO I97. . i , , , . I
0.80
FO 92, 93, 90,
0.60
89, 86, 83 0.40
FO 50, 43, 40, 01 0.20
15 km. Given the size of the Mare and important implications of the South Pole-Aitken mafic Orientale deposits(-200 km in diameter), there shouldbe no anomaly is that the extensive exposuresof olivine or mafic difficulty obtaining accurateelementaldata with the proposed glass observedin the southernpart of the basin most likely gamma-rayand x-ray spectrometers[Moss et al., 1993; Clark The second critical piece of compositional representdeep-seatedmaterial excavatedby the basin forming et al., 1993]. event. Although the precisedimensionsof South Pole-Aitken information is characterizationof the basalt mafic mineralogy are not well determined,potentialdepthsof excavation[Spudis using high spectralresolutionmeasurements acrossdiagnostic and Davis, 1985] are estimatedto rangefrom 88-167 (assuming mineral absorptionbands in the near-infrared. The spatial a 2500 km diameter). Thesemafic materialsalongthe southern (-200 m) and spectral (-11 nm) resolution and continuous portionof the basincouldthusvery well be derivedfrom lower spectralcoveragefrom 0.35 to 2.4 [xm of the proposedorbital crust depths. They could also represent materials with a imaging spectrometer[Pieters et al., 1993] is well-suited to significant mantle component. such mineral characterizationobjectives. Mg-rich pyroxenes and/or olivine can be readily identified and distinguishedfrom 6. SCIENCE AND EXPLORATION IMPLICATIONS their Fe-rich counterpartsbased on the energy and shape of observed absorptions [Adams, 1974; Burns, 1970]. Even with the small pulse of new data over this limited Identification of mineral combinations is determined through spectral range, the Galileo SSI multispectral images of the deconvolutionof multiple absorptionbandssuperimposedon a lunar limb and far side have provided a wealth of information lunar continuum [Sunshine and Pieters, 1990, 1993a]. The aboutthe compositionalcharacterof unexploredlunar regions. high spectralresolutionof the imagingspectrometer will allow As frequently occursfor explorationmissions,the SSI results the subtle variations between mafia to be evaluated in terms of raise more questionsthan they answer. What, for example, is compositional variations in geologic context. The high the significanceof the unusual limb and far side basalts in spatial resolutioncompositionalinformation allows genetic terms of volcanic evolution of the Moon? If the South Polerelationsbetweenbasalt compositionsto be ascertained. Aitken impacttappedlower crustor mantle material,what is its The unusual maric depositsassociatedwith South Polecompositionand how does that affect estimatesof the bulk Aitken Basin provide perhapsthe most unique opportunityto compositionof the Moon? Hypothesesare testablewith more documentthe compositionof the lunar crust with depth. The completedata and global coverage. Additional multispectral SSI results have highlighted the compositionalanomaly at images,suchas thoseto -beacquiredby the Clementincmission SPA, but only hint at its extentandcharacter.A morecomplete (DSPSE) [Shoemakerand Nozette, 1993;Lucey, 1993], may determinationof the elemental and mineral compositionof providea valuablebasemap of generalsurfaceunitswhich will SPA deposits,in their geologic context and relation to each allow their distribution to be analyzed in geologic context. other, can provide the first direct compositionalmeasurement Fundamental lunarsciencequestions, however,awaitresolution of crustalstratigraphyfor any planetto suchdepths(> 50 kin).
3.
The huge marie-rich anomaly associatedwith South
Pole-Aitken
Basin dominates the southern far side crust.
Soils
17,146
PIETERS ETAL.:CRUSTAL DIVERSrrYOFTltEMOON
How is the compositionof the lower crust distinguishedfrom the upper manfie? Do stratigraphiczonesrich in incompatible elements (such as KREEP) exist within the crust as a magma ocean residual?
What is the relation of the low albedo interior
to the exterior depositsof SPA? Again, informationon both the elemental
and mineral
abundance
is fundamental
to such
science issues. The low albedo interior of SPA and the southem
mineralogy: Implications from visible and near infrared reflectivity of Apollo 11 samples,Geochim. Cosmochim. Acta, 1, suppl.,1937-1945, 1970. Adams, J.B., and T.B. McCord, Optical propertiesof mineral separates,glass, and anorthositicfragmentsfrom Apollo mare samples,Proc. Lunar Sci. Conf., 2nd, 2183-2195, 1971a
Adams, LB., and T.B. McCord, Alteration of lunar optical properties'Age and compositionaleffects, Science, 171,
SPA depositsof possibly olivine-rich material identified by GalileoSSI dataarespatiallylargeenoughto allow gamma-ray 567-571, 1971b. measurements of Fe, Ti, and radiogenicelements(essentialto Adams, J.B., and T.B. McCord, Vitrification darkeningin the assessthe KREEP component).Mineralogywill be assessed at lunar highlandsand identificationof Descartesmaterial at the Apollo 16 sites,Proc. Lunar Sci. Conf., 4th, 163-177, higher spatial resolutionwith the visible to near-infrared 1973. imaging spectrometer, allowing sharp boundaries and gradationalmixing of lithologies to be characterizedand Adams, J.B., C. Pieters, and T.B. McCord, Orange glass: Evidencefor regionaldepositsof pyroclasticorigin on the mappedthroughout the deposits.Confh-mation of thepresence
Moon, Proc. Lunar Sci. Conf., 5th, 171-187, 1974. of olivine and its distributionrequireshigh spectralresolution Adams, J.B., F. Horz, and R.V. Gibbons, Effects of shocknear-infraredspectraand deconvolutionof diagnosticolivine loadingon the reflectancespectraof plagioclase,pyroxene absorptionsfrom those of other mafic minerals such as and glass,Lunar Planet. Sci., 10, 1-3, 1979. pyroxene. With such data it is anticipated that the Adler, I. et al., The Apollo 15 x-ray fluorescenceexperiment, composition of the olivine, a principalmarkerfor magmatic Proc. Lunar Sci. Conf., 3rd, 2157-2178, 1972. evolution, can be discerned for those regions of high
Adler, I., J. Trombka, R. Schmadebeck, P. Lowman, N.
Blodget,L. Yin, and E. Eller, Resultsof the Apollo 15 and concentration[e.g., Sunshineand Pieters, 1993b]. Equally 16 x-ray experiment,Proc. Lunar Sci. Conf., 4th, 2783important for reconstructingcrustal stratigraphywill be 2791, 1973. characterization of the pyroxene composition of various lithologies and the mixing relations of olivine/pyroxene Arnold, J.R., A.E. Metzget, and R.C. Reedy, Computergeneratedmapsof lunar compositionfrom gamma-raydata, throughoutthe deposits,challengesreadily addressedwith Proc. Lunar Sci. Conf., 8th, 945-948, 1977. available advancedcomputationalcapabilities. Bell, A.F., and B.R. Hawke, Lunar dark haloed impact crater; One very importantlessonfrom the Galileo first EarthOrigin and implications for early mare volcanism, J. Moon encounteris that the Moon continuesto hold surprises. Geophys.Res., 89, 6899-6910, 1984. Before the encounter,it was expectedthat the depositsaround Bell P.M., H.K. Mao, and R.A. Weeks, Optical spectra and the Orientale Basin would exhibit notable variations, and many electron paramagnetic resonance of lunar and synthetic glasses:A study of the effects of controlledatmosphere, investigatorswere somewhat disappointedwhen the data compositionand temperature. Proc. Lunar Sci. Conf. 7th, indicateda predominantlyfeldspathiccomposition. On the 2543-2559, 1976. other hand, few (perhapsno one) expectedSouth Pole-Aitken to be so prominentand dramatic. Having 382 kg of lunar Belton, M. J. S. et al., The Galileo solid-state imaging experiment,SpaceSci. Rev., 60, 413-455, 1992a. samplesfor studyin Earth-boundlaboratoriesfor the last 20 Belton M. J. et al., Lunar impact basins and crustal years has laid the foundationfor further exploration,but has heterogeneity:New western limb and far side data from also limited our expectations.We did not expectto see such Galileo, Science, 255, 570-576, 1992b. obvious evidence for extensive deposits of mafic material Burns, R.G., Mineralogical Application of Crystal Field derived from the lower crust or mantle. Until the global Theory, 224 pp., Cambridge University Press, London, 1970. assessmentof the mineral and elemental compositionof the Moon is completed,we will alwaysbe limitedin whatwe think Charette, M.P., McCord, T.B., Pieters, C., and Adams, J.B., Applicationof remotespectralreflectancemeasurements to we know about the character and evolution of Earth's nearest lunar geology classification and determination of titanium neighbor. contentof lunar soils, J. Geophys.Res., 79, 1605-1613, 1974.
Acknowledgments.We gratefullyacknowledgethe GalileoTeam at JPL for their effortsleadingto a successful Earth-Moonencounter.Part of this researchwassupported by NASA grantsNAGW-28 andNAG9184 (C.M.P.). We are most thankful for the assistanceof Joel Plutchak in computerprogramming and analyses.
Clark, P.E., C.G. Andre, I. Adler, M. Podwysocki, and J. Weidner, Palus Somni: Anomalies in the correlation of
AI/Si x-ray flourescenceintensityratios and broad-spectrum visible albedos,Geophys.Res. Lett., 3 (8), 421-424, 1976. Clark, P.E., E. Eliason, C.G. Andre, and I. Adler, A new color
correlationmethod applied to XRF A1/Si ratios and other lunar remote sensingdata, Proc. Lunar Planet. Sci. Conf. REFERENCES
Adams, J.B., Visible and near-infrared diffuse reflectance
spectraof pyroxenesas appliedto remotesensingof solid objects in the solar system,J. Geophys.Res., 79, 4829-
9th, 3015-3027,
1978.
Clark, P.E., L.G. Evans, and J.I. Trombka, Remote sensingxray fluorescencespectrometryfor future lunar exploration missions, Lunar Planet. Sci., XXIV, 305-306, 1993.
Coombs, C.R., B.R. Hawke, and M.S. Robinson, Pyroclastic
depositson the northwesternlimb of the Moon, Lunar 4836, 1974. Planet. Sci., XXIII, 249-250, 1992. Adams,J. B., Interpretationof visible andnear-infrareddiffuse reflectancespectraof pyroxenesand other rock forming Davis, P.A., Jr., Iron and titanium distribution on the Moon from orbitalgamma-rayspectrometry with implicationsfor minerals,in Infrared and RamanSpectroscopy of Lunar and crustalevolutionarymodels,J. Geophys.Res., 85, 3209Terrestrial Materials, edited by C. Karr, pp. 91-116, Academic, New York, 1975.
3224,
1980.
Adams, LB., and T.B. McCord, Remote sensingof lunar surface Fart T.G., B.A. Bates,R.L. Ralph, and J.B. Adams,Effectsof
PIETERSET AL.: CRUSTAL DIVERSrrY OF THE MOON
overlapping optical absorption bands of pyroxene and glasson the reflectancespectraof lunar soils, Proc. Lunar Planet. $ci. Conf., 11th, 719-729, 1980. Fischer, E. M., C.M. Pieters, and J.W. Head, Lunar resources
17,147
TiO2 abundance map for the northernmaria,Proc. Lunar Sci. Conf., 8th, 1029-1036, 1977b. Lucey, P.G., The Clementine instrumentcomplement,Lunar Planet. $ci., XXIV, 907-908, 1993.
using moderatespectralresolutionvisible and near-infrared Lucey, P.G., and B.R. Hawke, A remote mineralogic spectroscopy:AL/SI and soil maturity, Workshop on New perspectiveon gabbroicunits in the lunar highlands,Proc. Technologiesfor Lunar ResourceAssessment,LPI Tech. Lunar Planet. Sci. Conf. 18th, 355-363, 1988. Rep. 92-06, Lunar and Planet. Inst., Houston,Tex., 1992. Lucey, P.G., B.R. Hawke, C.M. Pieters, J.W. Head, and T.B. Gaddis,L., C.M. Pieters,and B.R. Hawke, Remotesensingof McCord, A compositionalstudyof the Aristarchusregion of lunar pyroclasticmantling deposits,Icarus, 61, 461-489, the moon using near- infrared reflectance spectroscopy, 1985. Proc. Lunar Planet. $ci. Conf. 16th, Part 2, J. Geophys. Gaffey, S., L. McFadden, D. Nash, and C. Pieters, Ultraviolet, Res., 91, suppl., D344-D354, 1986. visible, and near infrared reflectance spectroscopy' Lunar Exploration Science Working Group (LExSWG), A Laboratory spectra of geologic materials,in Remote Planetary Science Strategy for the Moon, pp. 26, NASA Geochemical Analysis: Elemental and Mineralogical Spec. Pub. JSC-25920, 1992. Composition, edited by C. Pieters and P. Englert, 43-77, McCord, T.B., and J.B. Adams, Progressin remote optical CambridgeUniversity Press,New York, 1993. analysis of lunar surface composition,Moon, 7, 453-474, Greeley R., et al., Galileo imaging observationsof lunar maria 1973. and related deposits,J. Geophys.Res., this issue. McCord, T.B., M. Charette, T.V. Johnson, L.A. Lebovsky, Hawke, B.R., and J.F. Bell, Remote sensingstudiesof lunar C.M. Pieters, and J.B. Adams, Lunar spectral types, J. dark halo craters:Preliminary results and implicationsfor Geophys.Res., 77, 1349-1359, 1972a. early volcansim,Proc. Lunar Planet. Sci. Conf., 12th, 655- McCord, T.B., M.Charette, T.V. Johnson,L.A. Lebovsky, and 678, 1981. C.M. Pieters, Spectrophotometry(0.3 to 1.1 microns) of Hawke B. R., P.G. Lucey, G.J. Taylor, J.F. Bell, C.A. Peterson, visited and proposedApollo lunar landing sites, Moon, 5, D.T. Blewerr, K. Horton, G.A. Smith, and P.D. Spudis, 52-89, 1972b. Remote sensing studies of the Orientale region of the McCord, T.B., C.M. Pieters, and M.A. Feierberg, MultiMoon: A pre-Galileo view. Geophys.Res. Lett., 18, (11), spectral mapping of the lunar surface using groundbased 2141-2144, 1991. telescopes,Icarus, 29, 1-34, 1976. Hazen, R.M., P.M. Bell, and H.K. Mao, Effects of McCord, T.B., M. Grabow, M.A. Feieberg,D. MacLaskey, and compositional variation on absorption spectra of lunar C.M. Pieters,Lunar multispectralmaps:Part II of the lunar pyroxenes,Proc. Lunar Planet. Sci. Conf., 9th, 2919-2934, nearside, Icarus, 37, 1-28, 1979. 1978.
Head, J.W., J.B. Adams, T.B. McCord, C.M. Pieters, and S.H.
Zisk, Regional stratigraphyand geologic history of Mare Crisium, Mare Crisium: The Viewfrom Luna 24, pp. 43-74, Pergamon,New York, 43-74, 1978a. Head J.W., C.M. Pieters, T.B. McCord, J. Adams, and S.H. Zisk, Definition and detailed characterization of lunar
surface units using remote observations,Icarus, 33, 145172, 1978b.
Head, J.W., M. Belton, R. Greeley, C. Pieters,A. McEwen, G. Neukum, and T. McCord, Lunar Scout Missions: Galileo
encounterresults and applicationto scientific problemsand exploration requirements,Lunar Planet. Sci., XXIV, 625626, 1993. Head J.W., S. Murchie, J.F. Mustard, C.M. Pieters, G. Neukum,
A. McEwen, R. Greeley, and M.J.S. Belton, Lunar impact
McEwen, A.S., L.R. Gaddis, G. Neukum, H. Hoffman, C.M.
Pieters, and J.W. Head, Lunar craters and soils: Ages, colors, and regolith thicknesses,J. Geophys. Res., this issue.
Metzger, A.E., J.I. Trombka, R.C. Reedy, and J.R. Arnold, Element concentrations from lunar orbital gamma-ray measurements,Geochim. Cosmochim.Acta, 5, (2), suppl., 1067-1078, Frontispiece,pl. 2, 1974. Metzger, A., M.I. Etchegaray-Ramirez,E.L. Haines, and B.R. Hawke, Thorium concentrations in the lunar surface, IV, Deconvolution of the Mare Imbrium, Aristarchus, and
adjacentregions,Proc. Lunar Planet. Sci. Conf. 13th, Part 2, J. Geophys.Res., 88, suppl., A529-A543, 1983. Morris, R.V., Surface exposure indices of lunar soils' A comparativeFMR study, Proc. Lunar Sci. Conf., 7th, 315335, 1976.
basins:New data for the westernlimb and far side (Orientale and South Pole-Aitken basins)from the first Galileo flyby, J. Geophys.Res., this issue. Heiken, G. H., D.S. McKay, andR.W. Brown, Lunar depositsof
Morris, R.V., The surfaceexposure(maturity) of lunar soils: Some conceptsand Is/FeO compilation, Proc. Lunar Sci. Conf., 9th, 2287-2297, 1978.
possiblepyroclastic origin, Geochim. Cosmochim.Acta,
Nakano,and R.C. Reedy,Gamma-rayspectrometer for Lunar
38, 1703-1718, 1974. Heiken, G., D. Vaniman, and B.M. French, Editors, Lunar
Sourcebook, 736 pp., Cambridge University Press, New York, 1991.
Moss, C.E., W.W.
Burt, B.C. Edwards, R.A. Martin, G.H.
Scout II, Lunar Planet. Sci., XX/V, 1019-1020, 1993. Murchie S.L., J.W. Head, A.S. McEwen, J.F. Mustard, C.M.
Pieters, and M.S. Belton, Spectral propertiesof Orientale Basin materials from Galileo images, Lunar Planet. Sci.,
Jaumann, R., Spectral-chemical analysis of lunar surface XXIII, 947-948, 1992. materials,J. Geophys.Res., 96, 22,793-22,807, 1991. Mustard J. F., J.W. Head, S.M. Murchie, C.M. Pieters, M.S. JohnsonJ. R., S.M. Larson, and R.B. Singer, Remote sensing Belton, and A.S. McEwen, Schickard cryptomare: of potential lunar resources, 1, Near-side compositional Interaction between Orientale ejecta and pre-basin mare properties, J. Geo_phys.Res., 96 (E3), 18,861-18,882, from spectralmixture analysisof Galileo SSI data. Lunar 1991a.
Planet. Sci., XXIII,
957-958, 1992.
Johnson,J.R., S.M. Larson, and R.B. Singer, A teevaluation Papike, J.J., F.N. Hodges,A.E. Bence, M. Cameron,and J.M. of spectralratios for lunar mare TiO 2 mapping,Geophys. Rhodes, Mare basalts:Crystal chemistry, mineralogy, and Res. Lett., 18, 2153-2156, 1991b. petrology, Rev. Geophys.,14(4), 475-540, 1976. Johnson,T.V., J.A. Mosher, and D.L. Matson, Lunar spectral Pieters, C.M., Characterization and distribution of lunar mare basalttypes,lI, Spectralclassificationof fresh mare craters, units: A northern hemisphericmosaic, Proc. Lunar Sci. Conf., 8th, 1013-1028, 1977a. Proc. Lunar Sci. Conf., 8th, 1037-1048, 1977. Johnson, T.V., R.S. Saunders, D.L. Matson, and J.A. Mosher, Pieters,C.M., Mare basalttypeson the front side of the Moon:
17,148
PIEFERSET AL.: CRUSTAL DIVERSITY OF TIIE MOON
A summary of spectral reflectance data, Proc. Lunar Sci. lunar crust?:In,plicationsfor lunar crustal origin, Lunar Planet. Sci. XVI, 807-808, 1985. Conf., 9th, 2825-2849, 1978. Pieters,C.M., CopernicusCrater centralpeak: Lunar mountain Spudis, P.D., B.R. Hawke, and P. Lucey, Composition of of unique compositon,Science, 215, 59-61, 1982. Orientale Basin deposits and implications for the lunar Pieters, C.M., Compositionof the lunar highland crust from basin-formingprocess,Proc. Lunar Planet. Sci. Conf. 15th, near-infrared reflectance spectroscopy,Rev.of Geophys., Part I, J. Geophys.Res., 89, suppl., C197-C210, 1984. 24 (3), 557-578, 1986. Spudis, P.D., B.R. Hawke, and P. Lucey, Materials and formation of the Imbrium Basin, Proc. Lunar Planet. Sci. Pieters, C.M., Bullialdus: Strengtheningthe case for lunar plutons, Geophys.Res. Lett., 18, 2129-2132, 1991. Conf., 18th, 155-168, 1988. Pieters,C.M., Compositionaldiversity and stratigraphyof the Spudis,P.D., B.R. Hawke, and P. Lucey Geology and deposits lunar crustderivedfrom reflectancespectroscopy, in Remote of the Lunar NectarisBasin,Proc. Lunar Planet. Sci. Conf., 19th, 51-59, 1989. Geochemical Analysis: Elemental and Mineralogical Composition, edited by C.M. Pietersand P. Englert, pp. Sunshine,J.M., and C.M. Pieters,Extractionof compositional 309-339, CambridgeUniversityPress,New York, 1993. information from olivine reflectance spectra: A new Pieters, C.M., and T.B. McCord, Characterization of lunar mare capability for lunar exploration, Lunar Planet.Sci., XXI, 1223-1224, 1990. basalt types: A remote sensing study using reflection spectroscopy, Proc. Lunar Sci. Conf., 7th, 2677-2690, Sunshine, J. M., and C.M. Pieters, Estimating modal 1976. abundancesfrom the spectra of natural and laboratory Pieters, C.M., and G.J. Taylor, Millimeter petrology and pyroxene mixtures using the Modified GaussianModel. J. kilometer mineralogical exploration of the Moon, Proc. Geophys.Res., 98, 9075-9087, 1993a. Lunar Planet. Sci. Conf., 19th, 115-125, 1989. Sunshine, J. M., and C.M. Pieters, Determining the Pieters, C.M., T.B. McCord, S.H. Zisk, and J.B. Adams, Lunar composition of olivine on asteroidal surfaces, Lunar Planet.Sci., XXIV, 1379-1380, 1993b. black spotsand the natureof the Apollo 17 landingarea,J. Sunshine, J.M., C.M. Pieters, J.W. Head, A.S. McEwen, and R. Geophys.Res., 78, 5867-5875, 1973. Pieters, C.M., T.B. McCord, M.P. Charrette, and J.B. Adams, Greeley, Oceanus Procellarum as viewed by Galileo: Lunarsurface:Identificationof the dark manflingmaterialin Evidence for compositionaldiversity in the mare deposits and at the Marius Hills Plateau, Lunar Planet. Sci., XXIII, the Apollo 17 soil samples, Science, 183, 1191-1194, 1974.
1387-1388,
1992.
Pieters, C.M., J.W. Head, T.B. McCord, J.B. Adams, and S.
Taylor, S.R., Planetary Science:A Lunar Perspective,481 pp., Zisk, Geologicaland geochemicalunitsof Mare Humorurn: Lunar and PlanetaryInstitute,Houston,Tex., 1982. Furtherdefinitionusingremotesensingandlunar sampling Toksoz, M.N., A.M. Dainty, S.C. Solomon, and K.R. information, Proc. Lunar Sci. Conf., 6th, 2689-2710, Anderson, Structureof the Moon, Rev. Geophys.12 (4), 1975.
539-567,
1974.
Pieters, C.M., T.B. McCord, J.W. Head, J.B. Adams, and S.
Warren, P. H., The magmaoceanconceptand lunar evolution,
Zisk, Mare Crisium geologic units' Implications of additional remote sensingdata, Proc. Lunar Planet. Sci. Conf., loth, 2967-2973, 1979.
Whitaker, E.A., Lunar color boundariesand their relationship to topographicfeatures: A preliminary survey, Moon, 4,
Pieters, C.M., J.W. Head, J.B. Adams, T.B. McCord, S. Zisk,
Annu. Rev. Earth Planet. Sci., 13, 201-240, 1985.
348-355,
1972.
and J. Whitford-Stark, Late high titanium basaltsof the Wilhelms, D.E., The GeologicHistory of the Moon, U.S. Geol. westernmaria:Geologyof the Flaresteedregionof Oceanus Surv. Prof. Pap., 1348, 302 pp., 1987. Procellarum,J. Geophys.Res., 85, 3913-3938, 1980. Wood, J.A., J.S. Dickey, U.B. Marvin, and B.N. Powell, Lunar Pieters,C.M., J.B. Adams,P. Mouginis-Mark,S.H. Zisk, J.W. anorthosites and a geophysical model of the Moon, Head,T.B. McCord,andM. Smith,The natureof craterrays: Geochim.Cosmochim.Acta, !, suppl., 965-988, 1970. the Copernicus example, J. Geophys. Res., 90(B14), Wood, J.W., Moon over Mauna Loa: A review of hypotheses of 12,393-12,413, 1985. formation of Earth's Moon, in Origin of the Moon, edited Pieters, C.M., S. Pratt, H. Hoffmann, P. Helfenstein, and J. by W.K. Hartmann, R.J. Phillips, and G.J. Taylor, pp. 17Mustard,Bidirectionalspectroscopy of returnedlunar soils: 55, Lunar and Planetary Institute, Houston,Tex., 17-55, 1986. Detailed "groundtruth" for planetaryremotesensors, Lunar Planet. Sci., XXII,
1069:1070, 1991.
Pieters,C.M., J.W. Head,T.B. McCord,andtheMinMapTeam,
M. Belton, Kitt Peak National Observatory, National Optical
MinMap: An imaging spectrometerfor high resolution AstronomyObservatories,Tucson, AZ 85726-6732. E.M. Fischer, J.W. Head, C.M. Pieters, and J.M. Sunshine, compositionalmapping of the Moon, Lunar Planet. $ci., XXIV,
1145-1146, 1993.
Departmentof GeologicalSciences,BrownUniversity,Providence,RI 02912.
Pohn, H., and R.L. Widley, A photoelectric-photographic L. Gaddisand A. McEwen, U.S. GeologicalSurvey, Flagstaff,AZ study of the normal albedo of the Moon, U SGS 86001. Prof.Pap.Map, 559-E, 1970. R. Greeley,ArizonaStateUniversity,Tempe,AZ 85287-1404. Schultz, P.H., and P.D. Spudis, Beginning and end of lunar H. Hoffmann, R. Jaumann, and G. Neukum, DLR, Institute for mare volcanism, Nature, 302, 233-236, 1983.
PlanetaryExploration, 12484 Berlin and 82230 Oberpfaffenhofen,
Shoemaker,E.M., and S. Norette, Clementine:An inexpensive Germany. S.L. Murchie,Lunar and PlanetaryInstitute,Houston,TX 77058. mission to the Moon and Geographos,Lunar Planet. Sci., XXIV,
1299-1300,
1993.
Smrekar S., and C.M. Pieters, Near-infrared spectroscopyof probable impact melt from three large highland Lunar craters, Icarus, 63, 442-452, 1985.
Spudis, P.D., and P.A. Davis, How much anorthositein the
(ReceivedSeptember 9, 1992; revisedMay 6, 1993; acceptedMay 6, 1993.)
