JOURNALOF GEOPHYSICALRESEARCH, VOL. 98, NO. E9, PAGES17,183-17,205, SEPTEMBER 25, 1993

Galileo Imaging Observationsof Lunar Maria and Related Deposits

RONALDGREELEY, 1 STEVEN D. KADEL,1 DAVIDA. WILLIAMS,1 LISA R. GADDIS,2 JAMESW. HEAD,3 ALFREDS. MCEWEN,2 SCOTT L. MURCHIE,3 ]•NGELBERT NAGEL, 4 GERHARD NEUKUM, 4 CARLEM. PIETERS,3 JESSICA M. SUNSHINE,3 ROLAND WAGNER? AND MICHAEL J. S. BELTON5

The Galileo spacecraftimaged parts of the westernlimb and far side of the Moon in December 1990. Ratios of 0.41/0.56 gin filter images from the Solid State hnaging (SSI) experiment provided information on the titanium content of mare deposits' ratios of the 0.76/0.99 gm imagesindicated 1gin absorptions associated with Fe 2+ in maficminerals. Mare ages were derived froin crater statistics obtained from Lunar Orbiter images. Results on mare coinpositionsin western Oceanus Procellarum and the Humorurn basin are consistent with previous Earth-basedobservations, thus providing confidence in the use of Galileo data to extract compositional information. Mare units in the Grimaldi and Riccioli basins range in age froin 3.25 to 3.48 Ga and consistof medium- to medium-high titanium (<4 to 7% TiO2) content lavas. The Schiller-Zucchius basin shows a higher 0.76/0.99 gm ratio than the surroundinghighlands, indicating a potentially higher mafic mineral content consistentwith previous interpretationsthat the area includesmare depositsblanketed by highland ejecta and light plains materials. The oldest mare materials in the Orientale basin occur in south-central Mare Orientale and are 3.7 Ga old; youngestmare materials are in Lacus Autumni and are 2.85 Ga old; theseunits are medium- to medium-hightitanium (<4 to 7% TiO2) basalts. Thus, volcanism was active in Orientale for 0.85 Ca, but lavas were relatively constant in composition. Galileo data suggestthat Mendel-Rydberg mare is similar to Mare Orientale; cryptomareare presentas well. Thus, the mare lavas on the westernlimb and far side (to 178øE) are remarkably uniform in coinposition,being generally of medium- to medium-high titanium content and having relatively low 0.76/0.99 gin ratios. This region of the Moon is between two postulatedlarge impact structures,the Procellarum and the South Pole-Aitken basins, and may have a relatively thick crust. In areas underlain by an inferred thinner crust, i.e., zones within large basins (as at Apollo), titanium content is often higher. However, no mare deposits with titanium abundancesapproaching those of the high-titanium (9 to 14% TiO2) Apollo 11 and 17 basaltsnor of the high-titanium regions of central Oceanus Procellarum are seenon the westernlimb or easternfar side. Light plains depositsare generallyindistinct froin the surroundinghighlands in the SSI data and are inferred to be derived primarily from the same material that forins the highlands. Some of the light plains are too young to be related to basin-forming impacts, suggesting possible volcanic origin. Dark mantle deposit compositionsderived from SSI data are consistentwith Earth-basedobservations of similar near-sidedeposits and are interpretedto be pyroclasticmaterials. However,the modernitc albedo and 1 gin absorption of the dark mantle deposit on the southwest margin of the Orientale basin suggestit is a local pyroclasticdeposit contaminated with underlyinghighland materials from the Orientale impact.

1. INTRODUCTION 1Departmentof Geology,Arizona State University, Tempe. The Galileo flyby of the Moon in 1990 [Belton et al., 2U.S.Geological Survey, Flagstaff, Arizona. 1992]acquired the first new spacecraftlunar data in 15 years 3Departmentof Geological Sciences, Brown University, and includedobservations of far-side regionswith modem Providence, Rhode Island. instruments.Galileo missionobjectives for the Moon were 4GermanAerospace Research Establishment (DLR), outlinedby Fanale [1990] and preliminaryresults from the Institute for Planetary Exploration, Berlin/ Solid StateImaging (SSI) experimentwere givenby Belton Oberpfaffenhofen,Germany. et al. [1992]. 5NationalOptical Astronomy Observatories, Tucson, Arizona. Someof the key scienceobjectives focused on lunar maria andrelated deposits. Although mare materials represent <1% of the lunar crust [Head, 1976], they provideinsight into Copyright1993 by the AmericanGeophysical Union. crustalevolution, thermal history, and the Moon's interior. Consequently,information on the locationand composition Paper number93JE01000. of mare deposits,combined with agesderived from impact 0148-0227/93/93 JE01000505.00 crater statistics, enable better understandingof magma

17,183 17,184 GREELEYET AL.: GALILEOIMAGING OBSERVATIONS OF LUNARMARIA evolution and lunar volcanis•n. Although much has been only areascovered by •naturemare soilscan be classifiedfor learnedabout maria in thelast 25 years(reviewed by Headand TiO2, regions of young crater ejecta were not assessed. Wilson [1992]), compositionaldata for much of the surface Table 1 gives the classification used here for titanium are lacking. Multispecu'aldata obtainedfrom Earth provide contents. A comparisonof our classificationscheme with insight into surfacecompositions [Pieters, 1991], but are others,from both the spectraland petrologicperspective, is limited to the near side. Galileo provided the first given in Table 2. opportunity to collect multispectral data for parts of the Albedois broadlyindicative of lithologicdifferences among westernli•nb and far side. Our approachwas to obtain data mare, highland,and fresh cratermaterials [Pieters et al., this on near-sidemaria previouslyobserved and for which re,note issue]. Unlike normal albedo(reflectance at 0ø phaseangle) sensingdata were calibratedusing Alx)11o s,-unples [Pieters et usedby Pieters [1978], we determinedalbedo using the al., this issue]. This enabled calibration of SSI data to assess 0.56 gm filter (normalized to MH0 and 20ø phase angle mare compositionsfor the westernlimb and far side. [Pieters et al., this issue]). Aremsanalyzed (Figures la and lb m•dPlate 1) includemare Mafic minerals, such as pyroxene, produce strong deposits, light plains materials, and dark mantle deposits absorptionsnear 1 gm [e.g., Pieters, 1978]. The Galileo SSI (DMD). In each area we assessedthe local stratigraphy, 0.76/0.99 gm ratio, usedhere to estimate1 gm absorption obtainedcrater countsto determineages (except for DMD), strength,is influencedby soil maturity, grain size, viewing derived spectral characteristics, and interpreted the geometry,glass content, and the model abundances of Fe 2+ compositionsof the deposits. bearing minerals. However, by studying only areas of mature, uncont,'uninatedmare soils it is possibleto use the 2. DATA CHARACTERISTICSAND PROCESSING 0.76/0.99 grn ratio as an indicationof the relativeamount of maficminerals and Fe 2+ bearing glass present in mareunits. The Galileo imagingsequence [Belton et al., 1992] began at the easternterminator over the Apollo 12 and 14 landing 2.2. Data Correlation to Previous Studies sites, continued across the western near side, and extended to To assessSSI datafor analyzinglunar maria, we compared the far side to 178øE. The highestresolution images were SSI and Earth-based spectral information for parts of -3.5 Pan/pixel,centered at 7øS 25øW; the lowest resolution O. Procellarum and M. Humorurn [e.g., Whitaker, 1972; images analyzed were -7.6 Pan/pixelfor the Apollo basin. Pieters et al., 1975; McCord et al., 1976]. O. Procellarum These imageswere processedwith photometric,phase angle, mafia appearto be associatedwith eitherthe "mega-Imbriuln" and radiometric calibrations [McEwen et al., this issue], and basin [Spudis et al., 1988] or the Procellarum basin geometric control, and were reprojectedto a standardmap [Cadogan, 1974, 1981]. The formation of the postulated format with subpixelregistration applied. The result was a Procellarum basin would have had an enoHnous effect on the highly correlated data set of multispectral mosaicsin five Moon [Wilhelms,1987] and may haveresulted in thinningof wavelengths (0.41, 0.56, 0.66, 0.78, 0.99 [.tm). These the near-sidelithosphere, crustal deformation, mare extrusion, mosaicsserved as the primary data sourcefor thisstudy. and perhapsthickening of far-side crust [Cadogan, 1974, 1981]. 2.1. SpectralAnalysis Whitford-Starkand Head [1977, 1980] mappedthe lava Pieters et al. [this issue] discuss the calibration of Galileo flows in O. Procellarum,the youngestof which is the Sharp images to extract spectral data and the limitations of the Formation. Pieters [1978] and Pieters et al. [1980] found derived information. Imaging sequenceswere designedto this unit to be 3-11 wt % TiO2. The Hermann Formation, obtaindata over stand,'u'dsites for Earth-basedmultispectral the next youngestunit, coversnearly half of O. Procellarum analyses,including one in Mare Humoruln (designatedMH0). and is <3 wt % TiO2 [Pieters, 1978]. The Telemann Pieters[1978] outlineda four-par,'uneterclassification scheme Formationunderlies both the Sharpand HermannFormations for lunar basaltsbased on telescopicspectral reflectance data, and is <2wt % TiO2 [Pieters, 1978]. The Repsold includingthe 0.40/0.56 gm (UV/VIS) ratio, normalalbedo, Formation is the oldest unit, covers -1% of northwestern 1 gm absorption band strength, and 2 gm absorption O. Procellarum,and is 3-6 wt % TiO2 [Pieters, 1978]. The stxength. This scheme was adapted here, but without the boundariesof theseunits are clearly distinguishedon a color latterparmeter becauseGalileo lacksa 2 gm filter. map of the SSI 0.41/0.56 gm ratio (Plate 2) and the relative The Galileo SSI 0.41/0.56 gm ratio relative to a spectral titaniumcontents (following the schemein Table 1) inferred standard site in M. Serenitatis (MS-2) is used to evaluate for the Telemann (low-titanium basalt), Hermann (low- to titaniumabundance using an empiricalrelation developed by medium-titaniumbasalt), and Sharp (high-titanium basalt) Charetteet al. [1974]. This relationcorrelates weight percent Formationsare comparableto thosederived l¾om Earth-based TiO2 with the spectralreflectance ratio at 0.40/0.56 gm for assess•nents.However, SSI data indicate that the Repsold Apollo •nare soil s,-unples. The relation is restricted to Formation is co•nposedof low- to •nedium-titaniumbasalt mature(>70% agglutinates)mare soils and is thoughtto be which is less Ti-rich than previousesti•nates. Exposuresof accurate to -1% for samples >4% TiO2 with larger the Repsold Formation are on the western limb and are uncertainties for smaller TiO2 contents[Pieters, 1978]. difficult to observe fi'om Earth; because Galileo data were Althoughreasons for the uncertainties,'u'e poorly understood, obtained at low phase angles, they are consideredto be a factors may include differences in soil maturity and the better indicationof its composition. presence of glasses, agglutinates, and opaque •ninerals Pieters et al. [1975] studied mare units in the Hu•noruln [Johnsonet al., 1991], all of which influence spectral basinand defineda standardarea, MH0. They tbundthat lava reflectance. Despite thesedifficulties, Johnsonet al. [1991] flows on the westernside of the basin are -2 wt % TiO2, show that the Charetterelation is useful for mappingsoils whereasthose on the southeasternedge appear to be higher- derived from different Ti-content mare lava flows. Because albedo,low-titanimn basalts (similar to thoseof Apollo 12) GREELEYET AL.: GALILEOIMAGING OBSERVATIONS OF LUNAR MARIA I7,185 and have a weaker 1 [an absorptionthan the westernflows. 2.4. Crater Statistics Mare unitsin centralHumorum are -5 wt % TiO2 and show a strong absorptionnear 1 }am, attributed to clinopyroxene. In order to place the mare depositsand light plains into a Galileo SSI data are consistent with these values. stratigraphicsequence, hnpact craters were counted by sizeon key areas (Table 3) using Lunar Orbiter images. After 2.3. StatisticalAnalysis of SSI SpectralData analysisof selectedregions of interestand identificationof A statisticaluncertainty analysis was applied to 13 test units in which spectraland morphologicvariations suggested areasin the SSI westernlimb data to assessthe validity of compositionaldifferences, boundaries outlining these units 0.41/0.56 gm ratios for mapping purposes. Eleven value were markedon the photographs.Crater countingfollowed comparisonswere made using the modified t-test, or Smith- the procedureof Neukumet al. [1975] and Neukum [1983] in Satterthwaitetest, for parameterswhose standard deviations which film transparenciesare exmnined under a Zeiss PM2 are not equal [DeVore, 1982]. The test statistic,T, is given stereo comparator. Each crater di,'uneteris measured in the by left-right (generallyeast-west) direction on the image from the inner shadowmargin to the outermargin to obtaina rim- T= X-Y to-rim measurement. Crater dimneters are generally >200 mm underthe comparator,which hasan uncertaintyof s2+ s2 2 5 mm. Hence, uncertainty in diameter measurementsis <2.5% [Neukum et al., 1975]. in which X and Y = average DN values for two different spectralboxes, S1 and S2 _arestandard deviations for the Crater measurementsare recordedand partitionedinto 18 regionsindicated by X and Y, respectively,and m and n are bins of increasingcrater diameters D basedon the relation the number of pixels in each spectralbox, respectively. Two D = a 1ff•, (5) statisticallydistinct regions are presentwhen in which n = -3 ..... 3 and a are bin sizes of 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.7, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 6.0, 7.0, 8.0, T > orT_< (2) 9.0 km. For any measureddiameter D and bin diameter in which t is the critical value for a chosen confidence level, boundariesDa, DOwith Da < Dr,,D is put in the respective o: and v are the number of degreesof freedom, in which v = bin and set to the lower bin dimneterD a for cumulative m + n - 2. A rigorous statistical analysis requires the numbers.These binned data are usedto prtxlucea cumulative propagationof errors from the beginningof data processing crater size-frequencydistribution plot with corresponding to the final product. Unfortunately, statisticaluncertainties statistical errors, following the procedures outlined in for the intial processingsteps (i.e., flat-field correctionerrors, Arvidson et al. [1979]. Bin uncertaintiesare includedon the scatteredlight, and pixel misregistration)are unavailable. cumulativeplot as an error bar producedusing the following Hence, this analysis considersonly uncertaintiesrelated to formula: processingperformed on the multispectralmosaics by the authors,such as the standarddeviations in the MH0 region and the variationsresulting when the 0.41/0.56 ratio is made. +-{Jdiam= log Considerthe outputi•nage, Xa, producedwhen ,'m original Ndiam+A• Ndiam t (6) image, X•, is scaledor divided by the reflectancevalues of MH0 (XMH0) in which Ndiam is the cumulative number of craters for a givendiameter and A is the surfacearea counted. X A leastsquares curve fit is appliedto the cumulativedata to XMHO (3) normalizethe distributionto the standardcrater size-tYequency curve derived by Neukum [1983]. This stmidard curve The standard deviation of the output image is (Figure 2) was producedby usinga least squaresfit through determinedusing the followingformula normalized crater production data for 11 geologically homogeneousareas which are relatively free of secondary craters and that representa wide range of ages. ttence, an SOUT_Sr2 + S•qH0 1l th degreepolynomial equation, which representsthe curve X(•UT X?" XI•iH0 fit, is given by

11 •n which SOuT/XouT representsthe dev•aHond•v•ded• bE logN = Z ai (logD) i + F(to) averagereflectance in the outputMH0-scaled image, Sr/X•- i=0 (7) representsthe deviation divided b]( average reflectance in the original,unscaled i•nage, and SkH0/XhH0 representsthe in which N is the cumulative number of craters, D is crater deviation divided by averagereflectance values obtainedfor diameter, and a l =-3.6269, a2 = 0.4366, a3 = 0.7935, MH0. The resultingvalues for T showthat the differencesin a4 = 0.0865, a5 = -0.2649, a6 = -0.0664, a7 = 0.0379, the 0.41/0.56gm ratio for mostspectral boxes >8 pixelsin a8= 0.0106,a9 = -0.0022,a•0 = -5.180x 10-4, a• = 3.970 area are statisticallydistinct for mappingpurposes at a high x 10-5. F(to) is an additionalterm taking into account the confidence level. exposure time to the cratering flux. For the numerical Fig. la. Shadedairbrush relief map showingthe nmnesm•d locations of the axe&,;and featuresanalyzed with the Galileo SSI data.

Fig. lb. Mosaicof Galileoalbedo (0.56 •m) images(relative to MH0 at 20ø phaseangle) covering the areashown in Figurela. Closedcircles represent outer rings of basins[after Wilhelms, 1987]; dashed line marksapproximate rim positionof SouthPole-Aitken basin. Asterisksmark craterscontaining mare and/orlight plains deposits discussed in text. Whitedots mark dark mantle deposits. Plate 1. Color •napof 0.41/0.56 g•n ratio of Galileo SSI data for the m'eashown in Figure la. Red, low titaniumbasalt (<2 wt % Tie2)' yellow, •nediwn lilaniwn basalt(<4 wt % Tie2); green,•nedium-high titaniumbasalt (3-7 wt % Tie2); and blue, high titaniwnbasalt (>6 wt % Tie2).

N

Repsold Fm.

Repsold Fm.

Telemann Fm. Hermann Fm. 'Gerard.•, East Member Rumker Repsold Fro. Hills

Hermann Fm.

., .. Sharp .... Fm UlughBeigh •'Aristarchus SharpMember, Fro.r 'ß •Plateau'

Plate 2. Color map of 0.41/0.56 g•n ratio of Galileo SSI data, superimposedon a shadedairbrush relief map, showingpart of O. Procellm'mnincluding the Repsold,Telmnann, Herinram, and Sh,-u'pFormations. Unit.boundaries m'e shnilm' to Woseof Whitfi)rd-Starkand Head [1980]. 17,188 GREELEYET AL.: GALILEOIMAGING OBSERVATIONS OF LUNARMARIA

TABLE 1. E•npiricalRelation Between G,-dileo SSI 0.41/0.56 •n Ratio ,andTiO2 Contentof Matm'eM,-u'e Soils [after Pieters, 1978, and Pieters et al., this issue]

Designation SSI value Mare soils 0.41/0.56 •m* Estbnatedwt % TiO2

High Titanium > 1.08 >6 Med-HighTitaniuln 1.02 - 1.08 3 - 7 Medium Titanium 0.97 - 1.02 <4

Low Titanium <0.97 <2

*Relative to standardarea in Mare Serenitatis(MS-2).

approximationof the polynomialto the data,ao + F(to) is Uncertaintyvalues m'e determined for the craterretention age chosenas -2.534. The normalizationprocedure used on the of a given countusing the following formula: Neukum data resultsin a craterretention age N(1) from which a cratering model age can be determinedusing the lunar crateringchronology curve (Figure 3). +-ON=IN(1) ' 5A¾N(1) ] (9) The lunar cratering chronologycurve was producedby correlatingthe radiolnetricages of the Apollo andLuna sites in which N(1) is the craterretention age c•dculatedfor craters (e.g., compiled in Hartmann et al. [1981]) with crater at 1 Bn diameter and A is surfacearea of the countedregion. frequencymeasurements for the correspondingsites [e.g., The+o N valuesdefine the upper and lower limits of theerror Neukum, 1983; Neukum et al., 1975]. The procedureof bar, which are then used to determine the uncertainties in Neukum et al. [1975] h•,• an empirical dependenceof crater crateringmodel age fi'omthe lunarchronology curve (Figure frequencyon surfaceformation age. This dependencewas 3). Becauseof the changein slopeof the lunar chronology determinedusing a least squarescurve fit, resulting in the curve with time, the younger branch of the error bar in following equation crateringmodel ages tends to be larger. This effect is caused by a changein impactflux, whichbeco•nes nearly constant at N (D = 1 km)= 5.44x 10'14 (e6.931.1) + 8.38 x 10'41 (8) ages<3.2 Ga. The craterfrequency to agedependence is well in whicha crateringmodel age (t) in 109years is foundfrom definedfor the period3.2-4.3 Ga, allowingthe determination the crater retention age N(1). For a given unit, a cratering of modelages fl'o•n crater tYequency measurements of N with modelage t canbe determinedfrom thecrater retention age (y ell'Orsof +30% or +30 in.y. for ages>3.5 Ga and elTorsOf value of the size-frequencyplot) at D = 1 km (x value). ~ 100 tO 300 m.y. for agesbetween -3.2 and 3.0 Ga.

TABLE 2. Comparisonof TiO2 Ch•,;sificationSchelnes Used for DefiningLunar Mare BasaltTypes

Weight % TiO2 Specu'al Petrologic

Values Thiswork 1 2 3' 4 5 6

High >6 >7 8-14 >5 >6 9-14 9-14

Intermediate 5-9 5-9

Medium-high 3-7 3.5-5 Medium <4 2-3.5

Low <2 <4 1.5-5 <2 1-6 1.5-5 1-5

VLT <1.5 <1 <1.5 <1

1, Johnsonet al. [1991];2, Lofgrenet al. [1981]:3, Pieters[1978]; 4, Neal andTaylor [1992]: 5, Taylor[1982]; ,and6, Papikeanti Vaninmn[1978]. * Valuesinferred t¾om data presented. GREELEYET AL.: GALILEOIMAGING OBSERVATIONS OFLUNAR MARIA 17,189

TABLE 3. CrateringModel Agesof SelectedBasins, Light Plains,and Mare Deposits

Feature Name CraterRetention Age N (1) Model Age t (+ error;- error),Ga

Imbrium (Basin) 0.0371 3.92* (+0.03;-0.06)

Nectaris (Basin) 0.1000 4.07* (+0.04;-0.04)

Humorum (Basin) O.O595 3.99* (+0.04;-0.05)

OceanusProcellarum/Apollo 12 (1) 0.0036 3.18 (+0.1;-0.1)

NW Oceanus Procelh'u'um Voskresensky-Russell(2) 0.0075 3.63 (+0.03;-0.04) Gerard East (3) 0.0138 3.75 (+0.03;-0.04)

Grimaldi Mare East (1) 0.0021 2.49 (+0.51 ;-0.46) Mm'e West (2) 0.0031 3.25 (+0.11;-0.33) Sum of mare (largercraters) 0.0062 3.58 (+0.03;-0.06)

Riccioli Mare unit (3) 0.0045 3.48 (+0.05;-0.19)

Schiller-S chickard Schiller-Zucchiusplains (1) 0.0105 3.70 (+0.03;-0.04) Crater Schiller (2) 0.0121 3.73 (+0.04;-0.06) Schickardmaria (3) 0.0179 3.80 (+0.02;-0.04) Schickardlight plains(4) 0.0233 3.84 (+0.03;-0.04)

Orientale (B a•sin) 0.0224 3.84* (+0.04;-0.05) Mare Orient'de West (1) 0.0042 3.45 (+0.05;-0.11) Mare Orient,de Southeast(2) 0.0042 3.45 (+0.05;-0.12) Mare Odemale South-cenmd (3) 0.0106 3.70 (+0.04;-0.06) Lacus Amumni (4) 0.0024 2.85 (+0.37;-0.67) Lacus Veris (5) 0.0053 3.50 (+0.05;-0.08)

South Pole-Aitken Van de Graaff mare unit 0.0078 3.64 (+0.04;-0.07) Apollo maria (1) 0.0078 3.63 (+0.05;-0.06)

Korolev (Basin) 0.0812 4.04* (+0.04;-0.04) Light Plainswithin basin(1),(3) < 0.0038 < 3.4 Light Plainswithin basin (3) 0.0112 3.71 (+0.04;-0.07) Light Plainswithin basin (2) 0.0127 3.74 (+0.05;-0.05) Light Plainswithin basin(3) 0.0295 3.88 (+0.05;-0.07) Light Plainswithin basin(1) 0.0711 - 0.1010 4.02- 4.07 + 0.03

*Neukum [1983].

In many cases,the cratersize-frequency distributions do not 3. DESCRIPTIONAND ANALYSIS precisely follow the ideal productioncurve. Some curves have one or more "kinks", suggestingresurfacing events by 3.1. Western Oceanus Procellarum lava emplacement, ejecta deposition, or other processes [Neukumand Horn, 1976]. By fitting the productionsize- Many flows of western O. Procellarum embay adjacent frequencycurve to different segmentsof suchdistributions, highlandsas thin units [Sunshineet al., 1992], two of wlfich agesfor the "resurfacing"events in an ,'u'eacan be extracted were studied,the Gerard East ,andthe Vos-kresensky-Russell (e.g., Figure 4). areas(Figure 5). GerardEast, originally •napped as DMD by 17,190 GREELEYET AL.: GALILEOIMAGING OBSERVATIONS OFLUNAR MARIA

Scott et al. [1977], was later shown to be mare material [Coorobs and Hawke, 1992]. Cratering model ages E10ø \ LunarCratering Chronology (Figure6; Table 3) are 3.75 Ga and 3.63 Ga, respectively,or ,• Terrae 10'a late Imbrian. Thus, these mm'ia are among the oldest in the

area and may reflect em'ly filling of the ProcellaruInbasin. • - \ A14 Galileo spectra(Figure 7; Table 4) suggestthat Gerard East is vE • •5 10 1ø• composedof medium-titmfiumbasalt soils (0.41/0.56 gin = All x, A17 Z 0.99-1.00) whereasVos•'esensky-Russell includes ,nediron- • 10'•(L•-K • A 11(High-K B•s) high-titanium basalt soils (0.41/0.56 [t•n = 1.04). Both ß ]- Lma16'•1• 10's• regions have relatively low 0.76/0.99 g•n ratios when comp,'u'edto units in central O. Procellarum, suggesting • 10-3;- Luna2•A15 ••A 12 mixing of highlad material froIn iInpacts,-red cont,'unination > (•e & •n•, lg• of thin mare unitswith underlyinghighlands inaterials. • 10• - Tyro(A 17) + •E N•hRay (A16) • 10'7

105 10'5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0

STANDARD LUNAR CRATER Age(10 * years) 10 SIZE DISTRIBUTION CURVE Fig. 3. Lunarcratering chronology curve of Neukun•[1983].

3 lO 3.2. GrimaMi-RiccioliRegion The Grimaldi-Riccioli site (Figure 8) includesthe double- lO ringed Grimaldi basin and crater Riccioli, both of pre- Nect,'u'ianage [Wilhelms,1987]. Apollo 17 datasuggest that 1 Grimaldi has one of the h'u'gestpositive gravity anomalies (mascon)on the Moon and that mm'e filling may be very thick [Sjogren et al., 1974]; alternatively, Phillips and lO0 Dvorak [1981] concluded that the Grimaldi mascon was N~D-2.9 causedby mantle uplift. Stressmodels for the Grimaldi ,area 10'1 Shoemaker et al., suggesta lithospheric thicknessof-25 k,n during ,nare 1970 extrusion [Solomonanti Head, 1980]. It may have been easierfc)r dikes to propagateto the surfacein Grimaldi thanin 10'2 basins formed over thicker lithosphere, facilitating the emplacementof the unusually thick mare unit (-3.6 kin in the center)for sucha s•nallbasin [Solomonand Head, 1980]. 10-3 N~D'2'0Hartmann • \ A small m,'u'eunit is presentin northernRiccioli. Riccioli andWood, 1971 x \ is on the outerring of Grimaldi and the eruptionof •na,'elava 10-4 may have been facilitated by ring fractures. A chain of in'egulardepressions in northexstemRiccioli was inte,'preted N~D'I'8 Baldwin, 1971 by Schultz[1976] as a seriesof subsidedlava 1,•es, perhaps 10-5 indicatingthe locationof eruptivevents. Crater statisticsfor the Gri•naldi-Riccioli maria (Figure 9; Table 3) are compm'edto the Orientale basin (inodel age of lO-6 3.84 Ga; Neukum [1983]). Model agesof 3.58 Ga (all ,nare) m•d 3.25 Ga (west side) are obtainedti'o,n Grimaldi ,naria and 10'7 3.48 Ga for Riccioli maria, all of which are youngerthan the Orientale impact [Wilhelntsand McCauley, 1971; Scott et al., 1977]. Craterdata (Figure 9a) ,alsosuggest resurfacing in lO-s east-central Grimaldi at 2.49 Ga, which could represent obliteration of small (<1 lan) cratersby volcanic deposits, 10'9 mantlingfrom ejecta, or seis•nic"jostling" [e.g., Schultz anti 10'3 10'2 10'1 100 101 102 103 Gault, 1975] from an impactsuch as Gfimaldi B. Galileo SSI data were analyzed for six mare areas of Crater Diameter D (Kin) Grimaldi-Riccioli(Figure 10; Table 4). Grimaldi has two Fig. 2. Standardcrate,' size-f,'equencycurve of Neukun• mare units:a high-albedounit in the northeastwhich has a [1983], co,nparedto cu,'ve,sderived by otherinvestigators. spectralsignature consistent with cont,'uninationby ejecta GREELEYET AL.' GALILEOIMAGING OBSERVATIONS OFLUNAR MARIA 17,191

10 • pm'tof the unit. However, suchsmall vm'iationscannot be deter•ninedwith confidencebecause they m'enero' the limit of spatialresolution. 'E 10ø 3.3. Schiller-SchickardRegion 3.3.1. Schiller. Schiller (Figure 11) is a lm'ge,elongate z crater 180 -kmby 80 lan. Theoriesregm'ding its formation o>,10 'l include low angle impacts[Wilhelms, 1987] and volcano-

LL.. 10'2

10-3

1 O'4

10-s 10-2 10'1 100 101 102 Crater Diameter D(Km) Fig. 4. Crater size-tYequencydistribution showing inferred resurfacingof light.plains units in Korolev region (data froln LO I 38 H). The curve on the left •n,'u'ksthe age of the resurfacingevent.. The curve on the right mm'ksthe age of the underlyingsurface. from crater Grimaldi B m•d a western unit composed of medium-high-titaniumbasalt soils (0.41/0.56 !.tm = 1.03- 1.05) with a low 0.76/0.99 !.tm ratio. These values ,are consistentwith Earth-basedstudies by Hawke et al. [1991] who reported low titanium abundances using the 0.40/0.73 grn ratio (0.3252). SSI resultsm'e also consistent with titanium estimates(3.8 +1.3 wt % TiO2) derived from Apollo 16 T-rayspectroscopy [Adams et al., 1981]. The central and easternpm'ts of Grimaldi ,aremedium- titanium basaltssoils (0.41/0.56 !.tm= 1.01-1.02) with low 0.76/0.99 grn ratios. This m'eamay alsocontain a separate, younger unit of lower titanium content than in the west. Although the extreme eastern parts of Grimaldi are contmninatedby highland ejecta from hem'by craters, the compositionaldifference extendsbeyond the ejecta from Grimaldi B and other Copernicancraters. We suggest.that these relations indicate a volcanic origin lbr the 2.49 Ga msurfacingevent noted above. The Riccioli mm'e is a medium-high-titanimn basalt soil (0.41/0.56 !.tm= 1.06-1.07) simil,'u'to (but perhapssligl•tly Fig. 5. Lun,'u' Orbiter photographsshowing highlands more titanium-rich than) basalts in southwestern Grimaldi embayedby mm'ia in northwesternO. Procellm'uln. Areas and has a low 0.76/0.99 !.tmratio. Although the low ratio where crater counts (nmnbers) mid spectral inforlnation agreeswith Em'th-baseddata [Coorobset al., 1992], some (letters) were obtained m'e outlined and labeled: (top)the variation in compositionis indicatedby a lower titanium Gerard eastm'ea (LO IV 189 H); (bottmn) the Vos:la'esensky- signature(0.41/0.56 !.tm= 1.04-1.05) for the northeastern Russell ,area(LO IV 183 M). 17,192 GREELEYET AL.' GALILEOIMAGING OBSERVATIONS OF LUNARMARIA

101

10o 3)

(2) 10-1

10-2

10-3

Apollo 12 3.18 Ga (1 10-4

10-s [ I I I I Ill[ 10-2 10-1 100 10 1 10 2 Crater Diameter D(Km) Fig. 6. Crater size-fi'equencydistributions for l•orthwestel'nO. Procellarum units. Nmnbers in pm'enfl•esescorrespond to regionsin Figure 5.

1.70 1.70 tectoniccollapse [Schultz, 1976; Qffield, 1971]. An on-axis connection between Schiller and a smaller crater to the

--o--G•'ard E• A southeast(which also lies on the basin ring) suggeststhat ß-o-- G•'ard E• B 1.50 1.50 concentric faults associatedwith basin rings could act as ' Voekremd

1.30 1.30 Light plains fill low m'easand crater floors in the region. These plains may be ejecta, including material from the Orientale and hnbrium impacts [Wilhelmset al., 1979] or volcanicmaterial [e.g., Q[h'eld,1971]. Hartmannand Wood 1.10 1.10 [1971], Schultzand Spudis[1979], Ha•vkeanti Bell [1981], and Bell and Hawke [1984] suggestedthat the light plainsare patchesof ancient lava (i.e., "cryptomm'ia")which were subsequentlycovered by ejectadeposits. D,'u'k-halo craters on some light plains may representexcavation of underlying mare material [Schultzand Spudis,1979; Hawke and Bell, 0,70 .... I .... I .... I .... I .... I .... 0.70 0.3 0•, 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1981; Bell and Hawke, 1984]. Crater statistics(Figure 12; Table 3) yield a model age of Wavelength(•m) 3.73 Ga for Schiller light plains and 3.70 Ga for Schiller- Fig. 7. Spectraof Gerard East and Voslo'esensky-RussellZucchius plains. Becauseboth dates ,'u'eyounger than the mare deposits,shown relative to MH0 standardarea. Letters Orientale and hnbrium impacts, it is unlikely that the plains of areasmmlyzed correspond to Figure 5. can be atU'ibutedto ejecta from these basins. Rather, the GREELEYET AL.: GALILEOIMAGING OBSERVATIONS OF LUNAR MARIA 17,193

TABLE 4. Galileo SSI Specu'alBox Locationsfor LunarMaria and RelatedDeposits

Features Box Size, SpectralBox Upper Left SpectralBox Lower Right 0.41/0.56 0.76/0.99 pixels GeographicCoordinates GeographicCoordinates Ratio Ratio

Latitude Longitude Latitude Longitude

Gerard East A 4 x 4 43.13 77.38 42.63 76.88 0.991 0.946 Gerard East B 4 x 4 44.38 77.00 43.88 76.50 1.004 0.927 Voslcresensky-RussellC 6 x 5 29.25 81.63 28.63 80.88 1.038 0.938 Grimaldi A 6 x 6 -5.13 69.75 -5.88 69.00 1.034 0.963 Grimaldi B 6 x 6 -6.25 69.50 -7.00 68.75 1.045 0.940 Grimaldi C 6 x 6 -6.38 68.25 -7.13 67.50 1.020 0.960 Gfi•naldi D 6 x 6 -5.13 67.50 -5.88 66.75 1.011 0.953 Riccioli E 2 x 2 -2.63 74.25 -2.88 74.00 1.068 0.898 Riccioli F 2 x 2 -2.13 73.88 -2.38 73.63 1.046 0.911 Schiller A 9 x 8 -53.00 38.75 -54.00 37.60 0.958 0.964 Schiller B 4 x 4 -51.13 41.13 -51.63 40.63 0.967 0.969 Schiller-Zucchius Plains C 3 x 6 -53.50 45.38 -54.25 45.00 0.946 0.971 Schiller-Zucchius Plains D 7 x 8 -51.38 45.25 -52.38 44.38 0.958 0.961 Mare Schickard A 6 x 12 -42.13 57.00 -43.63 56.25 0.976 0.986 Mare Schickard B 3 x 5 -45.63 52.00 -46.25 51.63 0.972 0.951 SchickardLight Plains C 3 x 4 -44.75 56.38 -45.25 56.00 0.955 0.983 Hevelius Formation 20 x 20 -1.00 82.50 -3.50 80.00 N/a N/a Orientale A 3 x 2 -17.63 98.50 -17.88 98.13 1.059 0.982 Orientale B 4 x 3 -20.50 98.00 -20.88 97.50 1.021 0.954 Orientale C 5 x 5 -22.88 95.00 -23.50 94.38 1.061 0.934 Orientale D 8 x 10 -22.50 93.38 -23.75 92.38 1.065 0.924 Orientale E 5 x 3 - 19.25 92.38 - 19.63 91.75 1.022 0.951 Lacus Veris F 2 x 4 -13.00 87.75 -13.50 87.50 1.060 0.908 Lacus Veris G 2 x 3 -18.00 85.13 -18.38 84.88 1.051 0.935 Lacus Autumni H 2 x 2 - 14.00 81.50 - 14.25 81.25 1.023 0.904 Lacus Autumni I 3 x 2 - 10.88 84.25 - 11.13 83.88 1.067 0.892 Mendel-RydbergA 2 x 3 -51.13 93.38 -51.50 93.13 1.047 0.961 Mendel-RydbergB 2 x 1 -51.13 93.88 -52.25 93.63 1.081 0.962 Mendel-RydbergC 5 x 2 -51.88 94.00 -52.13 93.38 1.013 0.972 ApolloMare A 6 x 7 -36.12 153.00 -37.00 152.25 1.024 0.917 ApolloMare B 2 x 2 -36.25 152.25 -36.50 152.00 1.099 0.866 ApolloMare C 7 x 4 -41.38 153.25 -41.88 152.38 1.001 0.905 KorolevLight Plains A 3 x 3 -3.13 159.50 -3.50 159.13 0.963 0.994 KorolevM LightPlains B 2 x 2 -36.25 152.25 -8.38 157.13 0.964 0.984 Highlanck,;C 16 x 16 -1.25 144.25 -3.25 142.25 0.981 0.982 Afistarchus A 5 x 5 29.50 52.13 28.88 51.50 N/a N/a J. Herschel B 5 x 6 62.38 37.88 61.63 37.25 N/a N/a Hmnomm C 5 x 12 -27.88 43.88 -29.38 43.25 N/a N/a Western Sinus Aestuum D 3 x 4 6.25 15.13 5.75 14.75 N/a N/a NE Grimaldi E 2 x 4 - 1.88 64.75 -2.38 64.50 N/a N/a Orientale F 5 x 4 -27.75 97.88 -28.25 97.25 N/a N/a

plains may be ejecta depositsfrmn adjacent,young craters, 3.3.2. Schickard. Schickard, a 227 -km crater, contains such as Zucchius (340 •n to the southwest) or of other both light plainsand mare deposits(Figure 12). Furrows, origins. ridges,and secondary craters from Orientale are superposed on Galileo SSI spectra(Figure 13; Table 4) for Schiller light the crater,especially on the southwestrim and floor. Light plains are similar to the highlands. The Schiller-Zucchius plainsalso occur outside the crater which may represent ejecta plains have an enhancedmafic signature(higher 0.76/0.99 depositsfrom Orientale[Schultz, 1976; Wilhelms,1987]. gm ratio), which may reflect excavation and mixing of Schickardmm'e deposits,'u'e mapped as Eratosthenianand underlying mare basalt. This suggestsmm'e volcanism interpretedas relatively thin lava flows overlying thicker occurred in the Schiller-Zucchius basin prior to some lightplains [Karlstrom, 1974]. resurfacingevent(s) between 3.60 Ga and 3.76 Ga and is Hawke and Bell [1981] and Bell and Hmvke [1984] obtained consistentwith telescopicresults [Blewett et al., 1992]. telescopicreflectance spectra of Schickardmaria and dark-halo 17,194 GREELEYET AL.: GALILEOIMAGING OBSERVATIONS OF LUNAR MARIA

a. 101

. • ;::,,.•. :.'• ..,?' :...: '-•-::.:•.. (1) Odentaleevent :...::.....•i:,'*--•:.. :' ", D ':• 3.84 Ga ...... ::--"•,' ", .....:;' BI ;'E' •.•:...... ,...-•...... , • /, .... .-•;. 100 . ,••, - ....' ....•. •.,. • • . .,;...•, - (2)

.s::. ?:;.:'3,: ;•"','...... ß...... '" '-"'' 10-1 •;•,•:...... ?,:?,•; ß ...,..%.,:.....,..-,:..•,...... ,.. . ' •t•- .,.-.-'•q:. "..: .' ': ½:."".,'W':??:.;L });• .... :.•' :./...... ::.....½:•.:.:..•.-..• ß.•,.... ß ..• 10'2 ,'...k..;,':4.''•;;'•,;.} .... "-;.-'4' '".:? ' • ''

..

.. 10'3 __-- --

..

-- ß'½. ,.• :... --

.. 3.58C-,a -

Fig.8. Lunm'Orbiter photograph (LO IV 182M) showing 10-4 _-- fl•e Grimaldi-Riccioliregion and m'easwhere crater counts (numbers)and specu'al information (letters) were obtained.

10'$ I • •1 I I I I 1111[ I I I I Ill cratersin light.plains. They foundthat the reflectm•cecurves 10-2 10'1 100 10 4 10 2 of the din'k-halocraters were nearly identical to thoseof fresh Crater Diameter D(Km) cratersin the mm'eunits. Thus, it. is likely that the din'k- haloswere produced by impactsthat excavated dm'ker, underlying m,-u'ebasalt. Because dark-halo craters are abundant.throughout the region (except. within crater Schiller), pre-Orientalemare volcanismmay have been 100 widespread[Schultz antiSpudis, 1979] and the region may (3) haveappem'ed shnilar to present-dayMare Australe[Bell anti Orientale event Hawke,1984]. 3.84 Ga Crater statistics (Figure 14; Table 3) show that the 10-1 Schickm'dlight plains ,'u'ethe s,'uneage (3 ß84 Ga) as the Orientaleimpact and, thus, the plains m'e probably Orientale

eiecta.' The model age for the •nm'eis 3.80 Ga (slightly youngerthan Orientale)indicating an Imbrian, not 10-2 Eratosthenian,age. SSIspectra (Figure 15; Table 4) forSchickm'd maria suggestlow- to medium-titaniumbasalt. soils (0.41/0.56 gm = 0.97-0.98),with some ,'u'eas having slightly enhanced 10-3 0.76/0.99gm ratios relative to other western-rim mm'ia. The 0.76/0.99 !.tmratio is more indicativeof nero'-sidebasalts thanthat. of western limb/f,'u'-side units. The model age for Schickm'dmaria (Figure14) indicatesemplacement prior to 10'4 resurfacing-in the Schiller-Zucchius basin (Table 3). Consequently,Schick,'u'd mm'ia may be part.of an episodeof mare volcanismwhich includedthe materialsunderlying the Schiller-Zucchiusplains. Spectraand craterages for light. 10-s plainsin Schickardare similarto thosefor the surrrounding 10'2 10'1 100 10 • 102 Orientaleejecta deposits, but haveenhanced 0.76/0.99 !.tm Crater Diameter D(Km) ratiospossibly caused by mixing of highlandsejecta with Fig. 9. Crater size-frequencydistributions for units in the underlyingbasalt. Grbnaldi-Riccioliarea compm'ed to thecurve rapresenting the We conclude that the Schickard light plains consist. Orientaleimpact; numbers correspond to labeledregions in primm'ilyof ejectafrom Odemale and apparently m'e unrelated Figure 8: (a) Gri•naldi (data frown LO IV 168 H); to the light plainsin the Schiller-Zucchiusbasin. (b) Riccioli (data fi'omLO IV 173 H). GREELEYET AL.' GALILEOIMAGING OBSERVATIONS OFLUNAR MARIA 17,195

1.80 1.80 Maunder and Holunann Q. Although the mm'ia generally have low 0.76/0.99 gin ratios, slight heterogeneitiesm'e evident,including a higherratio in northwesternM. Orientale --o-- Grim•iB 1.60 ß--•-- G•aldi C 1.60 and a lower ratio in southeasternM. Orientale which may • GrimalcliD ; Rkx•E indicate different basalts. These results suggest either = P,•c•F multiple eruptions from a slightly heterogeneoussource 140 1.40 regionor multiplesources of slightlydifferent comlx, sitions. Cratercounts yield modelages of 3.50 Ga for L. Veris and 2.85 Ga for L. Autumni (Figure 19; Table 3). SSI data for

120 120 L. Veris suggest a medium-high-titanium basalt soil (0.41/0.56 gm ratio -- 1.05-1.06). L. Autumni shows medium-high-titanium(0.41/0.56 gm ratio = 1.06-1.07)soil in the north and a medium-titanium(0.41/0.56 gin ratio -- 1.01-1.02) soil in the south(Figure 20; Table 4). However, southern L. Autumni also contains several outlicrs of

0.80 .... t .... i .... I .... i .... t .... i .... • .... 0.80 highlandswhich may accountfor the slightlylower titanium 03 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1,1 signature. These results m'e consistentwith Earth-based Wavelength studies [Hawke et al., 1991] for L. Veris and Autumni and have similm'0.76/0.99 gm ratios which ,'u'eslightly weaker Fig. 10. Spectraof m,'u'edeposits in the Grilnaldi-Riccioli than those in Orientale. region, shownrelative to MH0 standm'darea. Locationsof ,'u'eascon'esponding to letteredspectra m'e shown in Figure8.

3.4. Orientale Maria

Orientaleis the youngestbasin on the Moon (Figure 16) and servesas a model for lm'gehnpact structures[Moore et al., 1974]. Mm'e units include M. Orientale, Lacus Veris, L. Autumni, and sinall isolated patches,as found in crater Schliiter.M. Orientalecovers ~47,000 kin 2 in thecenter of the basin and is at least 1 kin thick [Head, 1974]. It. has several lm'ge depressions interred to result frown lava subsidence.Lava "benches"suggest that M. Orientale was emplacedrapidly m•d ponded as a lava hake,perhaps frown flood eruptions [Greeley, 1976]. Mm'e ridges probably originatedfrom subsidenceof lava crestover the centerof the basin [Solomonand Head, 1980]. L. Veris and Autulnni form discontinuous patches basinward of the outer Rook and Cordillera mountains, respectively.Some mm'ia include small shieldvolcanoes and sinuousrilles, suggestinglow rates of effusion [Greeley, 1976]. Although L. Veris has an albedo similar to M. Orientale, L. Autumni has a higher albedo, perhaps indicating compositional differences [Schultz, 1976] or contmninationby highh-mds•naterials. Spectralreflectance data of Spudis et al. [1984] suggest that L. Veris m•d Autumni m'e contaminatedby inaterial derived locally froin highlands.Telescopic data of Hawke et al. [1991] suggesta titaniron content for L. Veris and Autulnni silnilm' to central Serenitatis. Crater statistics(Figure 17; Table 3) show that the oldest exposedmm'e was depositedin the south-centralbasin at. 100 km 3.70 Ga. Subsequentmm'ia were emplacedin westernmid southeasternM. Orientale at 3.45 Ga. SSI data (Figure 18: Fig. 11. Lung Orbiterphotograph (LO IV 155 M) showing Table 4) show that both units in M. Orientale m'e medium- Schickm'dand pm't of the Schiller-Zucchiusbasin; Schiller is high-titaniumbasalt soils (0.41/0.56 gm ratio = 1.06-1.07). the elongatecrater at lower-right;Schickm'd is in the upper The lower titanium signatures in northeasternand west- center. Areas m'e shown where crater counts(numbers) mid central M. Orientale m'e attributed to ejecta frowncraters spectralinformation (letters) were obtained. 17,196 GREELEYET AL.' GALILEO IMAGING OBSERVATIONSOF LUNAR MARIA

--0-- SchillerA 101 I I IIIlt:l ---o-- SchillerB -_-- I I I I IIII I -- -- Schller-Zu(•us Rains C -- 2.,3O 230 = Schller-7.u(•us Rains D -- --e-- HevellusFormalion lO o

-- -- 2.10 2.10 --

--

--

-- event _ 3.84 Ga 10-1 1.90

--

--

--

--

--

-- 1,70 1.70 10':'

1.50 1.50 (2) 10-3

1.30 .... I .... I .... • .... I .... I .... t .... I,,,, 1.30 03 0.4 0.5 0.6 0.7 0.8 0,9 1.0 1.1

10-4 Wavelength(• m) (1) Fig. 13. Spectraof Schiller and Schillcr-Zucchiuslight plains units, shown relative to MIt0 stm•dardarea. Locations 10-s of areas correspondingto lettered spectra are shown in 10-2 10-1 100 101 102 Figure 11. Spectrum obtained frownnearby highlands (HeveliusFormation) is includedfor co•npm'ison. Crater Diameter D(Km) Fig. 12. Crater size-frequencydistributions for light plains units in the Schiller-Zucchius basin m'ea (data from LO IV 160 H). Numbers correspondto labeled regions in Figure 11.

In summary, although •nare volcanis[n in the Orientale m, 100 _ basinspanned atleast 0.85 Ga, the lavas have a fah'lyn,m'ow • range of compositionsrelative to the diversityseen in nero'- v event side•nare deposics. z 3.84 Ga

10'• •._--

-- 3.5. Mendel-Rydberg --

-- This region is the site of a possibleNectarian or pre- -- Neetarianbasin between the cratersMendel and Rydberg u_ 10-2 _ [Wilhelms,1987]. The basin floor is sculpturedwith ejecta frownOrientale and containsa s•nall patch of Eratosthenian/hnbrian•nare (Figure 21) [Scottet al., 1977: O Wilhelmset al., 1979] Galileo spectraldata (Figure22; 10-3 ___ Table4) for the•nare indicate a •nedimn-titanimnbasalt soil (0.41/0.56 gm ratio = 1.01-1.02) with a low 0.76/0.99 ratio. '5

A secondsmall , sparsely cratered area is medimn-high-1:: tit,-miumbasalt soil (0.41/0.56 g•n ratio = 1.04-1.05) with a 10-4 -- low 0.76/0.99 ratio. Theseresults suggest •nultiple lava flows of differentcompositions. Fm'thennore, Mustard et al. [1992]proposed that cryptomaria in thisregion represent part 10-s of a larger area of pre-Orientale•nare volcanis•n,based on 10-2 10'1 10 0 101 102 spectral•nixing models using Galileo data. Crater Diameter D(Km) 3.6. South Pole-Aitken Basin Fig. 14. Crater size-fi'equencydistributions for Schickard The Soufl•Pole-Aitken basin is thelargest i•npact structure mare and light plains units (data fi'o•n LO IV 160 1t). on the lunar far side. Hartmann and Kuiper [1962] first Nmnberscon'eslxmd to labeledragions in Figure 11. GREELEYET AL.' GALILEOIMAGING OBSERVATIONS OF LUNARMARIA 17,197

2.20 2.20 Unlike most of the fro' side, the SP-Aitken basin contains many small mare patches,principally in impact structures, -.a--Mare SchickardB includingthe Apollo (Figure 23), Planck,and Ingenii basins, +--e-.- Schicka•Mare SchickardLighlARains and Leibnitz, Van de Graaff, and Von Kin'man craters. Although image resolution is insufficient to obtain crater statistics for most of these m,'u'ia, data obtained for Van de

1.$0 1.80 Graaff depositsyield a model age of 3.64 Ga (Figure 24; Table 3). The relative resU'iction of far-side maria to crater interiors

1.60 suggeststhat the basin-formingimpact locally thinned the lithosphereand enhancederuptions in a manner similar to that proposed for the near side by the Procellarum basin [Wilhelms, 1987]. Estimates of the thickness and 1 A0 1 A0 compositionof the fm'-sidecrust [Bills anti Ferrari, 1977; Haines and Metzget, 1980] also suggestthat it is thiimer beneath the SP-Aitken basin than in other far-side regions. 120 .... ; .... • .... • .... • .... • .... • .... • .... 120 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Wilhelms [1987] suggestedthat the sp,'u'sityof mm'iain SP- Aitken relative to Procellarum was caused pm'tly by the Wavelength(.u.m) absenceof large superposedbasins, its smallersize (2500 lon Fig. 15. Spectra of Schickm'dmm'e and light plains units, versus 3200 km), and the relatively thicker crust on the far shown relative to MI10 standm'd m'ea. Locations of m'eas side. correspondingto letteredspecu'a ,'u'e shown in Figure 11. The Apollo b•[sin,centered at 36øS 151øW, is 505 k•n in di,'uneterand is in the northeasternpm't of the SP-Aitken basin. Apollo containsseveral inm'e deposits with cratering suggestedits existence,based on observationof inountains model ages (Figure 24; Table 3) suggestingeinplacelnent at seen ne,'u'the South Pole; later Zend observations [Rodinov 3.63 Ga, approxiinately conteinporaneouswith inm'ia in et al., 1971, 1977] and Apollo data [Wollenhaupt anti crater Van de Graaff. Spectral signatures(Figure 25; Table Sjogren,1972; Bills anti Ferrari, 1975:Kinsler et al., 1975] d•x:umenteda depressionsome 5-7 km deep. SeveralWOl-kcrs noted that mountainscomprise a discontinuousring pm'tly surroundingthe depression[Howard et al., 1974; Schultz, 1976; Stuart-Alexander, 1978]. (3) 100 _

event 3.84 Ga (1),(2) 10-1 c

o' 10-2

10-3

10-4

250 km 10-s ...... • --.t•' '5: • :'.•'.:•:•:':-"•q•'::': 10-2 10-• 100 10 • 10 2

Fig. 16. Galileo SSI albedo (0.56 Hm) image of the Crater Diameter D(Km) Orientaleb•sin, superhnposedon shadedairbrush map. Areas Fig. 17. Crater size-frequency distributions for Mare where crater counts (numbers) and spectral information Orientale (data froln LO IV 195 H). Nulnbers correspondto (letters)were obtainedm'e labeled. labeledregions in Figure 16. 17,198 GREELEYET AL.: GALILEOIMAGING OBSERVATIONS OF LUNARMARIA

limb/fm'-sidemm'ia observed by Galileo and have slightly higher 0.76/0.99 !.tm ratios. }towever, the low spatial ---o--Idare Od• A resolution of images for the Apollo region gives less -' MareOd• B ---o--ldare Orle•tale½ 1.80 confidencein characterizingthe s•nallerunits. -" MareOrle•tale D The interior of the SP-Aitken basin has a strongmafic --A--Mare Od• E signature(high 0.76/0.99 !.unratio). This resultis attributed

1.60 to iron-richmaterial excavated frmn the lower crustor upper mantleor, alternatively,to thepresence of cryptomm'ia[Head et al., this issue;Pieters et al., this issue]. Although the

1,40 1.40 confinementof the mafic signaturewithin the basinsupports the presenceof cryptomaria,an m'eashowing the s,'unemafic signature extends outside the basin rim, suggesting excavationof deepmaterial by impact. 120 120

3.7. Korolev Basin

Korolevis a 440 )an multi ringedbasin of lower Nectm'i,'m 0,3 0.4 0.5 0.6 0.7 O• 0.9 1.0 1.1 age [Wilhelms, 1987], locatedat 4.5øS, 157øW(Figure 26). Wavdength• m) Stuart-Alexander[1978] mappeds•nooth plains in the basin, Fig. 18. Spectraof Mare Orientale,shown relative to MH0 with older light plains within the inner basin ring and stand,'u'd,area. Locations of ,'u'eascon'esponding to lettered youngerlight plainsfilling cratersand low ,areasbetween the spectram'e shown in Figure 16. inner and outer basin rings. The plains •nay be fluidized ejecta[Eggleton and Schaber,1972], ballisticejecta [Moore et al., 1974], mass-wastedand local •naterial reworked by 101 I ! I I IIIII I I iiiiii I I I iiiiii I I I IIIii:i: secondaryhnpacts [Oberbeck et al., 1975], volcanicmaterial [Neukum,1977], or mm'edeposits •nantled by Orientale[Bell antiHawke, 1984] m•d }Iertzspmng ejecta. 100 Crater statisticssuggest a pre-Nectarim•age (4.04 Ga) for Korolev (Table 3). The light plainswere subdividedfor crater countingbased on mappingby Stuart-Alexander[1978]. As Orientale event many as four ages are suggested(Figure 27; Table 3): >., 10'• 3.84 Ga (1) 4.02-4.07 Ga (plainsunits INp [Stuart-Alexander,1978] contemporaneouswith basinformation but possiblyrelated to the fmxnationof the slightlyyounger Hertzsprung basin), (2) 3.88 Ga (ph'-dnsunits lp [Stuart-Alexander,1978] nearly u_ 1 contemporaneouswith the formationof the Orientalebasin), (3) 3.70-3.75 Ga (plainsunits Ip [Stuart-Alexander,1978] similar to the age of light plains depositsin the Schiller- Zucchiusbasin), and (4) <3.4 Ga. CD 10-3 Galileo spectra(Figure 28; Table 4) ,areof low spatial resolutionbut show that the light plaitis are simil,'u'to the E surroundinghighlands, but of slightly lower albedo. The :3 0_4 plainshave a higheralbedo mid 0.76/0.99 !.tm ratio relative to (D 1 Orientale ejecta deposits. Korolev, Hertzsprung, and Orientaleejecta are the mostlikely sourcesfor the two older light plainsin Korolev. The younger(post-Orientale) light 10'5 plains(including xnaterial on the floor of craterKorolev M) 10'2 10'1 10ø 101 102 have no obvious impact source and may represent late- Imbrianmaria thinly blanketedby craterejecta. Crater Diameter D(Km) Fig. 19. Crater size-frequencydistributions for LacusVcris 3.8. Dark Mantle Deposits (data from LO IV 187 H) and Lacus Autumni (data from LO IV 181 H). Numbers correspond to labeled regions in Dm'k mantledeposits (DMD) occurin •nany,areas of the Figure 16. Moon. They include regional deposits,such as those at Taurus-Littrow, and local depositsassociated with features inferred to be volcanic vents that involved fire-fountaining 4) show coinpositionsdo]ninated by ]nedium- to mediuln- eruptions[e.g., Heiken et al., 1974;Head anti Wilson,1979]. high titanium basalt soils (0.41/0.56 lam = 1.01-1.03). Volcanicglass spherules coated with volatileswere sampled Small, sparsely-crateredpatches appem' to be richer in duringApollo 17 and are consideredto be derivedfrom the titanium(0.41/0.56 !.tm= 1.08-1.10) thanany of the western deep lunar interior [Delano, 1986]. Such glassesare GREELEYET AL.: GALILEOIMAGING OBSERVATIONS OFLUNAR MARIA 17,199

2.10 , 2.10

---e--t.acus Veis G --a--- I.•us AutumniH .•0 --O'--I-a•s-- I.aoJ•Va'is AutumniFi I

1.70 .70

1.50 1.5o

1.30 1.3o

1.10 1.10 03 0.4 0.5 0.6 0.7 0.8 0,9 1.0 .1 Wavelength(lam)

Fig. 20. Spectraof Lacus Veris and Lacus Autumni mare deposits,shown relative to MH0 standardarea. Locationsof areas corresponding to lettered spectra are shown in Figure 16.

importantcomponents of severalmajor DMD [Gaddiset al., 1985]. Spectralreflectm•ce studies [e.g., Adams et al., 1974; Pietet:•et al., 1974; Gaddiset al., 1985; Luceyet al., 1986] showthat iron-bearingpyroelastic glasses, olivine, m•dpartly crystallized,titmfium-rich beads m'e mnong the componentsof lunm'pyroelastic deposits. Ch'thopymxene associated with the Fig. 21. Lunar Orbiterphotograph showing •mu'e deposits in lunm'highlands and clinopyroxeneassociated with •nm'iam'e the Mendel-Rydbergbasin. Areaswhere spectr•d information (letters) was obtainedm'e labeled (LO IV 193 H). also infen'edto be associatedwith glassymaterials of several DMD [Luceyet al., 1984; Gaddiset al., 1984; Hawke et al., 1989].

Galileo dam were acquiredfor severalDMD (Figure 29), 2.30 2.30 including those at western Sinus Aestuuln, the Aristm'chus Plateau, southwest of M. Humorurn, and at a site on the southwesternmm'gin of the Orientalebasin for which spectra --o-- IdendeI-RydbergA 2.10 : = MeadeI-RydbergB 2.10 were previouslyunavailable. In addition,spectra for DMD --o-- MeadeI-RydbergC northeast. of Grimaldi and on the floor of J. tterschel crater wereobtained (Figure 30; Table4). Galileodata for ne•-side 1.90 DMD m'econsistent with previousresults from Earth-based spectra. All depositshave low albedoscompm'ed to MH0. We considerthe high 0.41/0.56 gm ratio for DMD at western 1.70 Sinus Aestuum to representtitanium-rich devitrified glass 1.70 beads simih-'u'to thoseobserved at Taurus-Litu'ow [Gaddis et al., 1985]. Basedon 0.76/0.99 gm ratios, spectrashow a strongmafic signaturefor DMD at.Aristm'chus, a possible marie signature in the spectra for Humorum, northeast Grimaldi, J. Herschel, and Orientale, m•d weak •nafic signaturesin DMD at. western Sinus Aestuum. The results 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 are consistentwith iron-bem'ingvolcanic glasses inferred for the soils of the Aristarchusmid (possibly)Humorum DMD. Wavelenglh(gin) Galileodata support. the int.e•I•retationof Luceyet al. [1986] Fig. 22. Spectraof Mendel-Rydberg]n,'u'e deposits, sh()wll that a strong,symmetrical 1 gm abso•I•t.ion band in spectra relative to MH0 standard area. Locations of areas for AristarchusDMD was producedby nem'ly pure, iron- correspondingto letteredspectra m'e shown in Figure21. bemingvolcanic glass. 17,200 GREELEYET AL.: GALILEO IMAGING OBSERVATIONSOF LUNAR MARIA

- I I I I IIiiJ i I Ililtt '::•:.:T:•'%.'"C'?-:?E,.:•:.•::....'' '• '•'...... •...... • •:'::-"• ...... •.-::•:-•...... : '-'::•-:-•:•..'_..•-'" '.•':'•:' ...... :./ •"•:•-•.•-:- •...... •..• ...... • ...... '- :.:•:•:•:.•.:• ß .':...... :•..,.,•:...... :. .. ß•:::.: ...... •..•. a---.... • '.':.:;L' ::.:•...... :.... ::..•:.•..:-- •::::•:•.•..•.....•:•:%• :•:-...... •:•:•::.•- ::...:- •:•:•:•-•...... :::•..•,::.-...•::•-:.. ß :.. •:.:.••:•..-:.•. :.: •:...... •:...... ::•:::• •:•. •:. •:-•.•.:::•.:.••:•..• ...... '•.::'•...... : ..:..•'• ...... :...?• •..•.....• ::--.•:;:• :...... •::•:•..•..:..::..•..:....::•:•::•...•:• ::•.::• •.•:•::::•..•: .... :':':':' ...... •:"•..:•: .. :•.'•'•.•.'•: ::•'•'•'•":•...... '"•:/•.•.... •: ::•:: I : :-•'•T•... :.. ' ::..• ....'.L. '"'7::. ';-.--•.•.•:.•:.•: "..•.•L.. % ':'•::::•::•.'.:.'.'.'•...-•...,•? '-:•..:•.:.:•::C•::•:':•:.$..•'•::•-:..•;:•.•::•.: • .'-.:.:-.- •...... • ...... :.:..:.:.:::....::•:...... % ...... -... •...... :. :...... •.:.. :& .•..• . • •..•;.•:•:•". -:. .:...... :•• • ...... • ...... •-• ...... 10o _ :.....-t•.... '•.•--'• -4::• ...."•"....•...... '-.:'•: ...... :: .....:...... •.:- .•i:- .:..•:::::..•:•::•-...... •:•.:.:. •:...... -•--: ...... • --...::.-:-:-.:.. === Ga 104 •__ ..::.. . --•:.:....-:•:-.::.:•.. •.. :•:.•'•' -.:-.. •:'( ?•?'..:.?;5%::•.•,•??•;•'"'•:,:•..•½,•'•...•: ".•..';•.. {- E•...... :'-;:•:•:•-•...... '.•. .•r•Z•;':• '" -.... '-...•. •':(:•':•:•:•'::•'•:'•'•::•:::•."•':....-:-:::.;::"%:½•;:::.:•:•'•'• ...... • •::•"? , . :•::•-"•L.?•::.'•...... •.*•.•'-•..•'::•':....:...... " - "•:: '::.?.-...... •½:':' :•:•-.:::•L '-':-....:.... ::;:: ....,..)• ...... 5::,::•' • •...... :.::•.-•:-:•..•;:??:..'•.•.(. "..:•:'•.•..:•:• •..-:.•.::..•-&d:•::•: :•... ':%::':•::.. . ' 10-2 ;•;•:E:..;...•½•½' .....'i...•:'.--•-.. j'....: •'-'-•---"::'.:-•-+..:..e.,.-..:-:-•:-.•:...... •;:.'•:;:•'-":;"'7::',• .....:';:.:•::t::: •P' ':::. ':'. ':':'½•:5: ':..'..... ':-:'•..•-•5:...:E:::.-.' '•::•:'.:, '.::-•::'--....-% ß'::J?•:•'::-'"-'; •.:--..:•:::.•.:•: -..•:.•..;:;•:•::::)•::•'"'.'...... i: ::¾:,-"•..... •'•:.•---..•:•::;;.;-:':•:.-. ::•%•.....: .• .:•:-:•::-•' -.:....• .: "•: .... .:.•:..:.•;•:;::...... :::.y;;" • "•:•:' '":t"•. •"';• :.- • •.':'•:. ' c) ...... E.• :.'..•:...:•.•.--•---,.•::•,. '•:•-:...... ;½•;....:...•::.....:..• .. ß....ß...... •,..•:•..:•:•:'•::•"----:....: --•-...::•:• ...... •:::::::• ...... ::.•>:•:•:•:•.•::•:.•-:•.: ..... • .....•.-. ß•:-•::•&<•:-• . •::•'.... )" .• :•-.q...... ::..-•'-'•:•-'".•::-'-•:--•::::::;:E-:v ...... 10 -a

,, .,,.,,,

. . .. ::.•: .• . ';•:•-•--..•:•::.:...... :..•..... •,..:-•..• ::*...... :.• • ::•: . :. •.- -+..•:';...:.....-- (1) ß:•. ..•-...... [•_• ....• ::: ..,:• •-• •-:•.:•:-::.,.:::.•-•..:.•-•...... ,...... '--.•.?.:...:.. ...:•::::..., •: ...... •.: ...... :.. .:.••:.•..•...... :.•:..-•:..".---•:...•:-•:.:...•:•: v...:.•...... •.-'. v:.':.:,. E " •...... --:•.•...... : -•::•,-•.•-:--•.:-:•'.-'•::•:: ...... •;::.-.:-•:.:...... •.•:•:•:•'•:•:::..•:•:•:::½:::.:'E•':::..:•:---:':-•:: ...... • -..• ..•:.'-•...'"':'...... ½--:'•:•:-. --"'•"'.;'--':':::'-•" .. ... •:::-•:;•. •.•::::•:.'. .:....:'" •:"•.::..• -...•:• ...... ".j. •::: ..•:. -----...••::: .... ß.... :.:..•. '-..•;•:• ":•'•:::•. •;..•::.;. . 10-4 ...... :.• ...... ,..:...:::::.•....:::•....•:•.:•;::::..•:.•::....?::",.•:•:..'"•:• c) ...... ß...... :5 -,...... •.:::'I':• ...... :'...... •½:•.. 10-5 , ß...... - ..- ...... 10-2 10-'• 10o 10 '• 102 .:•...... •.... -?•.•- :•...•:•...•:.•:.,..-:•.•• -.-•- ... :' '. .. "•.':':: . "•....•: .....,.• '.• :':,',' ....:•....•: ...•...... :•-'•••• .... Crater Diameter D(Km)

• • ...... ,, -- • -:::::::.-L..:• ...... : ..... Fig. 24. Crater size-frequencydisu'ibutions for Van de Graaff (data from AS 15 M-74, AS15 M-75) and Apollo (data fi'o•n .• .. •• ...... •..: .:•-::•:::.-.... • ...... •. •.... •-: ...... • ••..-'-•:.. LO V 30 H) mm'e units. Nu•nberscorrespond to labeled - ' .... ":• .--• ': :. '•-'•.::-:.•..•:'•:•'::•':"•:---•:L•. ,...•' regionsin Figure 23.

Fig. 23. Lunar Orbiter photographshowing the Apollo basin. Areas where a crater count (nu•nber) and spectral information(letters) were obtained m'e labeled (LO V 30 It). .80 1.80

1.60 Telescopic data for norlheast Gri•naldi DMD show a --e--Ap•b MareB ]noderate0.41/0.56 Iron ratio, •noderate ,albedo, mid a possible --a--Ap•b MareC 1 pm absorption.These resultssuggest the prescnceof a --O--Ai•bMareA! highlands co]nponent, possibly Ol'lhopyroxcnc, as a 1AO 1AO contmnin,'mtof the low-albedopymclastic materials [Lttct(y et al., 1984; Gaddis et al., 1984; Hawke et al., 1989]. No clear distinctionc,'m be madein theSSI spectrabetween absorption 1.20 120 bandsat 0.95 lain (as would be expectedfor orthopyroxenc) and thoseat 1 Iron. The possible1.0 Ironabsorplion infcrred fi'mn the SSI spectrmnfor J. Herschelsuggests the presence of olivine [e.g.,McCord et al., 1981]. The low albedo,high 0.41/0.56 pm ratio, and weak (or absen01 Ironabsorption

signaturesat westernSinus Aestuum m'e attributed to reduced 0.80 spectr,'dconu'ast and weaker absorptioncaused by ihnenitein 03 0.4 0.5 0.6 02 0.8 0.9 1.0 1.1 devitfifiedvolcmfic beads, shnilm' to flmsesampled at Apollo Wavelength(IJ.m) 17/Taums-Litu'ow[Gaddis et al., 1985]. The Orientale DMD were interpretedas seriesof vents Fig. 25. Spectraof Apollo •nm'edeposits, shown relative to MH0 standm'dm'ea. Locationsof ,areascorresponding to associatedwith a 175 -lanbasin [Schultzand Spudis,1978]. letteredspectra m'e shown in Figure23. They suggestedthat the ventswere heavily modified (but not. obliterated) by collapse of the Orientale basin transient GREELEYET AL.' GALILEO IMAGING OBSERVATIONSOF LUNAR MARIA 17,201

a. 101

,tale event 3.84 Ga 3.39 Ga (1) 100 Korolev event 4.04 Ga (1)

10'1 t--

10'2

10'3

10-4

Fig. 26. Lunar Orbiter photographshowing the Korolev basin. Areas wilere crater counts (]lumbers) and spectral inforlnation (letters) were obt,'finedare labeled (I_,O1 38 M). 10-5 10-2 10-1 100 10• 102 Crater Diameter D(Km) cavity. The relatively]noderate albedo mid the 0.76/0.99 B•n b. ratio in the SSI data for tile Orientale DMD suggestlocal pyroclastic deposits (possibly contaminatedby highland - •Orientale event ejecta), supporting the Sch.ttltz anti Spttdis [1978] - 3.84 Ga (3) interpretation. 3.28Ga (3) • 100 4. DISCUSSION 3.74 Ga (2) Shnple]nodels of mare bas,dtpetrogenesis were developed in the mid-1970s that involvedhigh- and low-titaniumlavas. 10-• es Ga (3) Low-titmliumlavas were proposedto representearly volc,-mic o eruptions from shallow sources, with high-titanium lavas o erupting later from deeper sources [e.g.,Do•vt),, 1975; cr N),quistet al., 1977].In laterstudies, maria were recognized t• 10-2 as having a wide rmlgeof titanium contentsmid neither their agesnor apparent depths of origincorrelated well with • titaniumcontent [Papike and Vaniman,1978]. Studiesof tO - pyroelasticglasses [Delano, 1980; Delano etal., 1980; Hess, o> 10'3 _=- 3.71 Ga (3) 1991;Ryder, 1991] suggested that source regions t•r mare '.•oa basalts are heterogeneousand that high- and low-titanium '• basaltsare produced at. a varietyof depthsand times. Tile $::: Galileo data confirm and add to this complexity. However, tO 10-4 -- the resultsalso showa remarkablespectral similarity of mare depositsin Grimaldi, Riccioli,Mare Orientale,,'red parts of westernO. Procellarum. All are interpretedto be of medium- 10-s I I I I•!!tl to medium-high titanium content (0.41/0.56 Bin = 0.97- 10-2 10 -1 100 101 102 1.08), yet sp,'ma wide rangeof ages(early Irabrian through Eratosthenian), suggesting a more constant, long-lived Crater Diameter D(Km) homogeneousmagma source region(s) for the westernregion Fig. 27. Crater size-frequencydistributions for light plains of the Moon th,'m on the central near side. This observation units (a) in the center of tile Korolev basin, and (b) in tile ]night.be attributedto tile partof tileMoon wherethese maria outer portionof the Korolev basin(data from LO I 38 H). occur, i.e., between tile two largest postulated impact. Numberscorrespond to labeledregions in Figure26. 17,202 GREELEYET AL.' GALILEO IMAOINO OBSERVATIONSOF LUNAR MARIA

270 270 structures, the Procellarmn basin and the SP-Aitken basin where the lunar crust may be relatively thick [Wilhehns, 1987]. As previously suggested,the titaniron content of

2.5O 2.5O mare lavas erupted in such areas tends to be low to intermediate, and high-titanium basalts are absent. These values are consistentwith the •nedimn- to mediu•n-high- titanium contentsinferred from this study. Moreover, SSI • 2.30 2.30 data suggestthat parts of the Apollo ]naria cont,'finedwithin the SP-Aitken basin (where the lithospheremay be thinner) ]nay have higher titanium contents. .• 2.10 - 2.10 5. SUMMARY AND CONCLUSI()NS

Galileo data show that maria of tile western limb are 1.90 --o-- KomlevUghl Plains A 1.90 ---o-- KorolevM LighlPlains B generallyhomogeneous with titanium contentsranging from • HighlandsC -- HeveliusForm• medium- to medium-high(0.41/0.56 gm= 0.97-1.08), with generallylow 0.76/0.99 [tm ratios. This low 0.76/0.99 ratio 1.70 .... • .... • .... • .... • .... • .... • .... • .... 1.70 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 is i]nportant in indicating tile unco]mnonlyweak 1 gin absorptions of limb/far-side basalts relative to near-side Wavelength(p.m) basalts. When comparedwith the other spectralpm'mneters, Fig. 28. Spectra of Korolev light plains units, shown the weak 1 gm absorptionfor thesebasalts is interpretedto relative to MtI0 standard area. Locations of areas suggest.a differentform of mafic ]nineralogynot seenbefore, correspondingto lettered spectra are shown in Figure 26. perhapscaused by different olivine/pyroxeneproportions or Speclzmnobtained from the Hevelius Formationis included exceptionally fine-grained textures [see Pieters et al., this for comparison. issue]. Light plains on the western li•nb and in the interior

HerOditusH q-.,

H-er:od'•tUSA. 0

Fig. 29. Galileo SSI albedo(0.56 gin) imagesof dark ]nantlcdeposits, showing areas (boxes) wilere spectral information was obtained: (a) Aristarchus, (b)J. Iterschel, (c) southwest of Hu]norum, (d) westernSinus Aestuum, (e) northeastGrim,-ddi, (/) southwestof Mare Orientale. GREELEYET AL.' GALILEOIMAGING OBSERVATIONS OF LUNARMARIA 17,203

t .--o-.k,,isia, d• A REFERENCES I ,' J.Ha'schelB I--o--Hu• C Adams, J.B., C.M. Pieters,and T.B. McCord, Orange glass: 2.00 J -- WesletoSinus Aesluum D 2.00 Evidence for regional depositsof pyroelasticorigin on the Moon, Proc. Lunar Phmet. Sci. Conf., 5th, 171-186, - '- Od•taleF 1974. 1.80 --J• Northeasl GdmaldiE 1.80 Adams, J.B., C.M. Pieters, A.E. Metzger, I. Adler, T.B. McCord, C.R. Chapman, T.V. Johnson, and M.J. Bielefield, Remote sensingof basalts in the solar 1.6o 1.60 system,in BasalticVolcanism on the TerrestrialPlanets, pp. 439-493, Pergmnon,New York, 1981. Arvidson, R., et al., Stand,'u'dtechniques for presentationand 1.4o 1.40 analysisof crater size-frequencydata, Icarus, 37, 467- 474, 1979. Baldwin, R.B., On the historyof lunar i•npactcratering: The 12O 120 absolute time scale and the origin of planetesimals, Icarus, 14, 36-52, 1971. Bell, J.F. and B.R. Hawke, Lun,'u'dark-haltyed hnpact craters: Origin and i•nplications for early •nare volcanis•n, J. Geophys.Res., 89, 6899-6910, 1984. Belton, M.J.S., et al., Lunm' impact basins and crustal 0.3 0,4 05 0.6 0.7 0,8 0,9 1.0 1.1 heterogeneity:New westernlimb and far side data from Galileo, Science, 255, 570-576, 1992. Wavaength(Fro) Bills, B.G. and A.J. Ferrm'i,Lunar topography,Proc. Lunar Fig. 30. Spectraof dark •nantledeposits, shown relative to Planet. Sci. Conf., Geochint. Cosmochint.Acta, 6th, MIt0 standard m'ea. Locations of areas corresponding to suppl.6, frontispiece,pl. 2, 1975. letteredspectra are shownin Figure29. Bills, B.G. and A.J. Ferrari, A lunm'density •nodel consislent with topographic,gravitational, librational, and seismic dam,J. Geophys.Res., 82, 1306-1314, 1977. of the South Pole-Aitken basin show enhanced inafic Blewett, D.T., B.R. Hawke, P.G. Lucey, J,'unesF. Bell III, signatures. Where these signaturescorrespond to zones of G.J. Taylor, C.A. Peterson,and P.D. Spudis,A near-IR dm'khalo craters,they are probablyindicative of ancientlava spectralinvestigation of the Schiller-Schickardregion of the Moon (abstract), Lunar Planet. Sci., 23, 123-124, flows mantled by ejecta and other deposits. If this 1992. intel•mtafionis COITeCt,•nare volcmfis•n was ]nore widespread Cadog•, P.H., Oldestand largestlunar basin?,Nattire, 250, •d longer-livedfl•m• previously considered. 315-316, 1974. Light plains deposits, as in Schiller and Korolev, are Cadogan, P.H., The Moon-Our Sister Planet, 391 pp., co•npositionallyindistinct frown the smxoundinghighlands, CronbridgeUniversity Press, New York, 1981. althoughslightly higher 0.76/0.99 g•n ratios are observedin Charette, M.P., T.B. McCord, C. Pieters, and J.B. Admns, someareas. Cratering•nodel ages sut,•e'• that so•nelight Applicationof remote specu'alrellectance measure•nents plainswere resin'faced<3.4 Ga agoby stoneprocess, •rhaps to lunar geology classification and determination of by volcanism. More likely, however, upper-hnbrian •nm'ia titanium content of lunm' soils, J. Geophys.Res., 79, me bl•keted by highlands•nateri•s whose sourcesare not 1605-1613, 1974. readilyidentifiable. Coo•nbs,C.R. and B.R. Hawke, Pyroelasticdeposit.,; on the Galileo •ta for DMD on the western li•nb and fro' side were western li•nb of the Moon, Proc. Lunar Planet. Sci. comp•ed with spectraof DMD at the Apollo 17 siles and Conf., 22nd, 303-312, 1992. other ne•-side sites. Resultssuggest that the westernSinus Cooinbs, C.R., B.R. Hawke, and M.S. Robinson, Aestumn DMD •e Ti-rich glasses, the Aristarchus and Pyroelastic deposits on the northwesternli•nb of the Moon (absu'act),Lunar Planet. Sci., 23, 249-250, 1992. Hmnoru•n DMD are Fe-bem'ing •las,.es,0 '• the J. ]Ierschcl Delano, J.W., Apollo 15 red glass:Chmnistry and liquidus DMD is olivine-rich glass,and that the northeastGri•n•di phaserelations (abstract), Ltmar Planet. Sci., 11, 210- DMD is con•minated by highland •naterials. Orientale 212, 1980. DMD •e consideredto be pyroelasticdeposits contminated Delano, J.W., Pristine lunar glasses: Criteria, data and by highl•ds materies. implications,Proc. Lunar Planet. Sci. Conf. 16th, Pm't 2, J. Geophys.Res., 91, suppl.,D201-D213, 1986. Acknowledgments. We thank the Galileo project for a Delano, J.W., S.R. Taylor, and A.E. Ringwood, successfulencounter with the Moon and in providingthe data Compositionand structureof the deep lunar interior for analysis. P. Spudisand M. Robinsonprovided reviews (abstract),Lunar Planet. Sci., 11, 225-226, 1980. which resulted in substantial improve•nents in the DeVore, J.L., Probabilityand Statisticsfi•r Engineeringanti manuscript. We also thank Dan Ball, Sue Selkirk, and the Sciences,640 pp., Brooks/Cole,Monterey, Calif., Shana B lixt for photographicsupport, preparation of the 1982. figures,and word processing.This work was supportedby Dowty, E., Shalloworigin of mm'ebasalts, in Coqferenceon the Galileo Project through the NASA Jet Propulsion Originsof Mare Basalts,pp. 35-39, Lunar and Planetary Laboratorycontract PVJ 6164. Institute, Houston, Tex., 1975. 17,204 GREELEYET AL.: GALILEOIMAGING OBSERVATIONS OF LUNARMARIA

Eggleton, R.E. and G.G. Schaber, Cayley Formation Hess, P.C., Diapirism and the origin of high TiO2 mare interpretedas basinejecta, Apollo 16 preliminaryscience glasses,Geophys. Res. Lett., 18, 2069-2072, 1991. report, NASA Spec.Pap., 315, 29-7 to 29-16, 1972. Howard, K.A., D.E. Wilhelms, and D.tt. Scott, Lunar basin Fanale, F.P., Galileo's Earth-Moon encounter set for formation and highland stratigraphy,Rev. Geophys.,12, December 8, Eos Trans. AGU, 71, 1803-1804, 1990. 304-327, 1974. Gaddis, L.R., P. Lucey, J. Bell, and B.R. Hawke, Remote Johnson, J.R., S.M. Larson, and R.B. Singer, Remote sensinganalyses of localizedlunar d,'u'kmantle deposits, sensing of potential lunar resources, 1, Nearside Reportsof the PlanetaryGeology GeophysicsProgrmn, compositionalproperties, J. Geophys.Res., 96, 18,861- NASA Tech. Mento., TM-87563, 399-401, 1984. 18,882, 1991. Gaddis, L.R., C.M. Pieters, and B.R. Hawke, Remote Karlstrom, K.N.V., Geologic map of the Schickard sensingof lunar pyroclasticmantling deposits,Icarus, quadrangleof the Moon, U.S. Geol. Surv. Map, 1-823, 61,461-489, 1985. 1974. Greeley, R., Modes of eraplacementof basalt terrainsmid an Kinsler, D.C., R.B. Merrill, and L.J. Smka, Apollo laser analysisof mare volc,'mismin the OrientaleBasin, Proc. altimetry: Proc. Lunar Planet. Sci. Conf., 6th, Lunar Planet. Sci. Conf., 7th, 2747-2759, 1976. Geochim. Cosmochim.Acta, suppl. 6, frontispiece, Grieve, R.A.F. and M. Dence, The terrestrialcratering record pl. 1, 1975. II. The craterproduction rate, Icarus, 38, 230-242, 1979. Lofgren, G.E., et al., Pctrology and chemistryof terrestrial, Haines, E.L. and A.E. Metzget, Lunar highland crustal lunar, and meteoritic basalts, in Basaltic Volcanism on modelsbased on iron concenu'ations:Isostasy ,'red center the Terrestrial Planets, pp. 236-265, Pergmnon,New of mass displacement,Proc. Lunar Planet. Sci. Conf., York, 1981. 11th, 689-718, 1980. Lucey, P.G., L.R. Gaddis, J.F. Bell, mid B.R. Hawkc, Nero'- Hm'Unann,W.K. and G.P. Kuiper, Concentric structures infrared spectral reflectance studies of localized dark surroundinglunar basins,Lunar Planet. Lab. Contnt.1, mantle deposits(abstract), Lunar Planet. Sci., 15, 495- pp. 51-66, Univ. of Ariz., Tucson, 1962. 496, 1984. Hartmann, W.K. and C.A. Wood, Moon: Origin and Lucey, P.G., B.R. Hawke, C.M. Pieters, J.W. }lead, and evolution of multi-ring basins,Moon, 3, 3-78, 1971. T.B. McCord, A compositionalstudy of the Aristarchus Hm'tmann,W.K., et ,'fl., Chronology of planetm'yvolcmfism region of the moon using near-infrared reflectance by compm'ativestudies of planetarycratering, in Basaltic spectroscopy,Proc. Lunar Planet. Sci. Coqœ16th, Part Volcanism on the Terrestrial Pictnets,pp. 1050-1130, 2, J. Geophys.Res., 91, suppl.,D344-D354, 1986. Pergamon,New York, 1981. McCord, T.B., C. Pieters, and M.A. Feierberg, Multi- Hawke, B.R. and J.F. Bell, Remote sensingstudies of lunar specu'almapping of the lunm'surface using •,rto ) und-based dark-halo impact craters: Preliminm'y results and telescopes,Icaru& 29, 1-34, 1976. implications for em'ly volcanism, Proc. Lunar Planet. McCord, T.B., R.N. Clark, B.R. Hawke, L.A. McFadden, Sci. Conf., 12th, 665-678, 1981. P.D. Owensby, C.M. Pieters, and J.B. Admns, Moon: Hawke, B.R., C.R. Coorobs,L.R. Gaddis, P.G. Lucey, and Near-infrared spectral reflectance, a first good look, P.D. Owensby, Remote sensingand geologic studiesof J. Geophys.Res., 86, 10,883-10,892, 1981. localizeddm'k mantle depositson file Moon, Ptvc. Lunar McEwen, A.S., L.R. Gaddis, G. Neukem, II. Itoffman, Planet. Sci. Conf, 19th, 255-268, 1989. C.M. Pieters, and J.W. Head, Galileo observations of Hawke, B.R., P.G. Lucey, G.J. Taylor, J.F. Bell, post-hnbriumlunar cratersduring the first Earth-Moon C.A. Peterson, D.T. Blewerr, I.C. Itorton, G.A. S•nilh, flyby, J. Geophys.Res., this issue. and P.D. Spudis, Remote sensing studies of the Moore, H.J., C.A. Ilodges, and D.H. Scott, Multiringed Orientale region of lhe Moon: A pre-Galileo view, basins- illusu'atedby Orientale and associatedfeatures, Geophys.Res. Lett., 18, 2141-2144, 1991. Proc. Lunar Planet. Sci. Conf., 5th, Geochim. Head, J.W., Orientale •nulti-ringed basin interior and Cosmoshim.Acta, suppl. 5, 71-100, 1974. implications for the petrogenesis of lunar highland Mustard, J.F., J.W. Head, S.M. Murchie, C.M. Pieters, s,'unples,Moon, 11, 327-356, 1974. M.S. Belton, and A.S. McEwen, Schickardcryptomare: Head, J.W., Lunar volcanism in space and time, Rev. Interactionbetween Orientale ejecta and pre-basinmare Geophys.,14, 265-300, 1976. from spectral mixture analysis of Galileo SSI data Head, J.W. mid L. Wilson, Alphonsus-typedin'k-halo craters: (abstract), Lunar Planet. Sci., 23, 957-958, 1992. Morphology, •nol-phometryand eruption conditions, Neal, C.R. and L.A. Taylor, Petrogenesisof mare basalts:A Proc. Lunar Planet. Sci. Conf, loth, 2861-2897, 1979. record of lunar volcanism, Geochim. Cosmochim. Acta, Head, J.W. and L. Wilson, Lunar mare volcanism: 56, 2177-2211, 1992. Stratigraphy,eruption conditions,and the evolution of Neukum, G., Different agesof lunar light plains, Moon, 17, secondary crusts, Geochim. Cosn•ochim. Acta, 56, 383-393, 1977. 2155-2175, 1992. Neukum, G., Meteoritenbombardement und datierung Head, J.W., S. Murchie, J.F. Mustard, C.M. Pieters, planetarer oberfi•ichen, 186 pp., Habilitationsschrifl, G. Neukum, A. McEwen, R. Greeley, E. Nagel, and Ludwig Maximilianis Universit•it,Mitachen, 1983. M.J.S. Belton, Lunar impact basins: New data for the Neukmn, G. and P. Horn, Effects of lava flows on lunar western limb and far side (Orientale and South Pole- craterpopulations, The Moon, 15, 205-222, 1976. Aitken basins)from the first Galileo flyby, J. Geophys. Neukum, G., G. K6nig, and J. Arkani-Hmned, A study of Res., this issue. lunar impact crater size-distributions,Moon, 12, 201- Heiken, G.H., D.S. McKay, and R.W. Brown, Lunar 229, 1975. deposits of possible pyroclastic origin, Geochim. Nyquist, L.E., B.M. Bansal, J.L. Wooden, and Cosmochin•. Acta, 38, 1703-1718, 1974. H. Wisemann, Sr-isotopic constraints on the GREELEYET AL.: GALILEO IMAGING OBSERVATIONSOF LUNAR MARIA 17,205

petrogenesisof Apollo 12 •nm'e basalts, Proc. Lunar of the west side of the Moon, U.S. Geol. Surv. Invest. Planet. Sci. Conf., 8th, 1383-1415, 1977. Ser. Map, 1-1034, 1977. Oberbeck,V.R., F. HOrz, R.H. Morrison, W.L. Quaide, and Shoemaker,E.M., et at., Preliminarygeologic investigation D.E. Gault, Oil the origin of the lunar smoothplains, of Apollo 12 landing site, Part A' Geology of the Moon, 12, 19-54, 1975. Apollo 12 landingsite, in Apollo 12 prelilninaryscience Offield, T.W., Geologic map of Schiller quadrangleof the report, NASA Spec. Pap., 235, 1-13 to 1-56, 1970. Moon, U.S. Geol. Surv. Map, 1-691, 1971. Sjogren, W.L., R.N. Wi•nberly, and W.R. Wollenhaupt, Papike, J.J. and D.T. Vaniman, Luna 24 fen'o-basaltsand tile Lunar gravity: Apollo 17, Moon, 11, 41-52, 1974. mare basaltsuite: Comparativechemistry, mineralogy, Sololnon, S.C. and J.W. ttead, Lunar lnascon basins: Lava and petrology,Mare Crisium: The view from Luna 24, filling, tectonicsand evolution of the lithosphere,Rev. Geochim. Cosmochim.Acta, suppl.9, 371-401, 1978. Geophys.,18, 107-141, 1980. Phillips, R.J. and D. J. Dvorak, The origin of lunar Spudis,P.D., B.R. ttawke, and P. Lucey, Colnpositionof mascons: Analysis of the Bonguer gravity associated Orientale basin depositsand ilnplicationstbr tile lunm' with Grimaldi, Multi-ring Basins, Proc. Lunar Planet basin-fonning process,Proc. Lunar Planet. Sci. Conf., Sci. Conf., 12A, 91-104, 1981. 15th, Part 1, J. Geophys.Res., 89, suppl.,C197-C210, Pieters, C.M., Mare basalt types on the front side of tile 1984. Moon: A summary of spectral reflectance data, Proc. Spudis,P.D., B.R. Hawke, and P.G. Lucey, Materials and Lunar Planet. Sci. Conf., 9th, 2825-2849, 1978. formation of the hnbrimn Basin, Proc. Lunar Planet. Pieters,C.M., Colnpositionaldiversity and stratigraphyof Sci. Con.L, 18th, 155-168, 1988. the lunar crust derived fi'o•n reflective spectroscopy,in Stuart-Alexander,D.E., Geologiclnap of the centralf,'u'side of Remote Geochentical Analysis: Elentental and the Moon, U.S. Geol. Sttrv.Map, 1-1047, 1978. Mineralogical Contposition,edited by C. Pietcrs and Sunshine, J.M., C.M. Pieters, J.W. Head, A.S. McEwen, P. Engler, Calnbridge University Press and Lun,-u' and R. Greeley, OceanusProcellm'uln as viewed by PlanetaryInstitute, New York, 1991. Galileo:Evidence tbr co•nlx,sitional diversity in the•nare Pieters, C.M., T.B. McCord, M.P. Charette, and depositsand at Marius Hills plateau (abstract), Lunar J.B. Adams, Lunar surface: Identification of the dm'k Planet. Sci., 23, 1387-1388, 1992. manfling •naterial in the Apollo 17 soil salnples, Taylor, S.R., Planetat3,Science: A Lunar Perspective,481 Science, 183, 1191-1194, 1974. pp., Lunar and PlanetaryInstitute, Itouston, Tex., 1982. Pieters, C.M., J.W. Head, T.B. McCord, J.B. Admns, and Whitaker,E.A., Lunarcolor lx, undaries and their relationship S. Zisk, Geochelnical and geological units of Mare to topographicfeatures: A preli•ninm'ysurvey, Moon, 4, Hmnorum: Definition using telnote sensingand lunar 348-355, 1972. smnpleinformnation, Proc. Lunar Planet. Sci. Conf., 6th, Whitlbrd-Stark, J.L. and J.W. Itead, The Procellaru•n 2689-2710, 1975. volcanic provinces: Contrasting slyles of volcanis•n, Pieters, C.M., J.W. Head, J.B. Admns, T.B. McCord, Proc. Lunar Planet. Sci. Con.f., 8th, 2705-2724, 1977. S. Zisk, and J.L. Whitford-Stm'k, Late high-titaniu•n Whirford-Stark,J.L. and J.W. Ilead, Stratigraphyof Oce,'mus basaltsof the western•naria: Geologyof tile Flmnsteed Procellarumbasalts: Sourcesand stylesof mnplace•nent, region of OceanusProcellaru•n, J. Geophys.Res., 85, J. Geophys.Res., 85, 6579-6609, 1980. 3913-3938, 1980. Wilhelms, D.E., The geologic history of the Moon, Pieters, C.M., et al., Crustal diversity of the Moon: U.S. Geol. Surv. Prof. Pap., 1348, 302 pp., 1987. Compositionalanalyses of Galileo solid statei•naging Wilhelms, D.E. and J.F. McCauley, Geologic Inap of the data, J. Geophys.Res., this issue. near side of the Moon, U.S. Geol. Surv. Map, 1-703, Rodinov, B.N., et al., New data on the Mooifs figure and 1971. relief based on results fi'o•n the reduction of Zond 6 Wilhehns, D.E., K.A. Howard, mid H.G. Wilshh'e,Geologic photographs,Cosmic Res., 9, 410-417, 1971. map of the south side of the Moon, U.S. Geol. Surv. Rodinov, B.N., A.A. Nefed'ev, M.I. Shpekin, S.G. Valeev, Map, 1-1162, 1979. and V.V. Kiselev, Relief of the Moon's reverse side Wollenhaupt, W.R. and W.L. Sjogren, Co•mnents on the accordingto Zond 8 photographs,Cosmic Res., 14, 548- figure of the Moon based oil preli•ninary results fi'o•n 552, 1977. laser altilnetry, Moon, 4, 337-347, 1972. Ryder, G., Lunar ferroan anorthosites and •nare basalt M.J.S. Belton, National Optical Astronomy sources:The •nixed connection,Geophys. Res. Lett., 18, Observatories,950 N. Cherry Avenue, Tucson, AZ 85726- 2065-2068, 1991. 6732. Schultz, P.H., Moon Morphology, 626 pp., University of L.R. Gaddisand A.S. McEwen, U.S. GeologicalSurvey, Texas Press, Austin, 1976. 2255 N. Gemini Drive, Flagstaff, AZ 86001. Schultz, P.H. and D.E. Gault, Seislnic-induced modification R. Greeley,S.D. Kadel, and D.A. Willirons,Department of Geology, Arizona State University, Tempe, AZ 85287- of lunar surfacefeatures, Proc. Lunar Sci. Conf., 6th, 1404. 2845-2862, 1975. J.W. Head, S.L. Murchie, C.M. Pieters, and Schultz, P.H. and P.D. Spudis,The dm'kring of Orientale: J.M. Sunshine,Department of Geological Sciences,Brown hnplicationsfor pre-basinlnare volcanis•nand a clue to University, Providence, RI 02912. the identificationof Ihe transientcavity ri•n (abstract), E. Nagel, G. Neukmn,and R. Wagner,German Aerospace Lunar Planet. Sci., 9, 1033-1035, 1978. Research Establishment (DLR), Institute for Planetary Schultz, P.II. and P.D. Spudis, Evidence for ancient •n,'u'e Exploration,Berlin/Oberpfaffenhofen, Germany. volcanisin,Proc. Lunar Planet. Sci. Con.f., loth, 2899- (Received June 2, 1992; 2918, 1979. revised January 21, 1993; Scott, D.H., J.F. McCauley, and M.N. West, Geologic•nap accepted April 13, 1993.)