Topographic-Compositional Units on the Moonand the Early Evolution of the Lunar Crust

Paul G. Lucey,* Paul D. Spudis, Maria Zuber, David Smith,Erick Malaret

The distribution of elevations on the moon determined by Clementine deviates stronglyfrom a normal distribution, suggesting that several geologic processes have influencedthe topography. The hypsograms for the near side and far side of the moon are distinctlydifferent, and these differences correlate with differences in composition as determinedby Apollo orbital geochemistry, Clementine global multispectral imaging, and ground-based spectroscopy. The hypsograms and compositional data indicate the presence ofat least five compositional-altimetric units. The lack of fill of the South Pole-Aitken Basinby mare basalts suggests poor production efficiency of mare basalt in the mantle of thisarea of the moon.

The marked difference between the nearand far side of the moon was evident fromthe first high-quality images obtained of thefar side. The intensely cratered far sidelargely lacks the vast basaltic plains thatcover a third of the nearside surface. Datacollected by Apollo underscore this differ-ence. The Apollo orbital geochemistry ex-periments showed that the farside crustoverflown by those instruments is deficientrelative to the near side in Th, Ti, and Feand has lower Mg/Al ratios. The Apollolaser altimeter demonstrated that this near-side-farside dichotomy extends to lunar to-pography as well, showing that the near sidestands an average of 3.2 km below the farside (1).

The Clementine mission provides datawhich strengthen the contrast betweennear and far sides. This mission collectednear-global topographic data with a laserranger (2). It also collected global multi-spectral imaging data in 12 wavelengthsfrom 415 nm to 8 Kum. The latter data setwill make its greatest contribution after alengthy calibration and data preparationprocess. However, we have produced a glo-bally co-registered visible and near-infrared(NIR) data set with approximately 50-kmresolution providing a global view of lunaralbedo and color (3) between the latitudesof +75° and -75°. Figure 1 shows the950-nm albedo, the 950/750-nm ratio, andlunar global topography, also between + 75°and -75° latitude.

The hypsogram (the distribution of ele-vation as a function of area) of a planetary

P. G. Lucey, Hawaii Institute of Geophysics and Planetol-ogy, 2525 Correa Road, University of Hawaii at Manoa,Honolulu, HI 96822, USA.P. D. Spudis, Lunar and Planetary Institute, 3600 BayArea Boulevard, Houston, TX 77058- 1113, USA.M. Zuber and D. Smith, Goddard Space Flight Center,Greenbelt, MD 20771, USA.E. Malaret, Applied Coherent Technology, 209 EldenStreet, Herndon, VA 22070, USA.

*To whom correspondence should be addressed.

surface provides insight into very large-scaleplanetary processes. For example, the terres-trial hypsogram is bimodal and reflects amarked difference between the oceanic andcontinental lithospheres. The lunar hypso-gram, although not distinctly bimodal,clearly shows more structure than the sim-ple normal distribution of altitudes thatmight be expected from a saturated crateredsurface (Fig. 2). Additionally, the nearsideand farside hypsograms are distinctly differ-ent, not only in mean elevation, but inwidth as well. The nearside hypsogramshows a narrow distribution of elevationswith a mean elevation of about 2 km belowdatum with a width at half height of about2.5 km. The nearside curve appears to haveat least two major modes. In contrast, thefarside hypsogram is much broader, having awidth at half height of almost 5 km andshowing both the highest and lowest eleva-tions on the planet. The farside hypsogramshows three major modes.

Our examination of the near- and far-side hypsograms suggests that there are fivemajor hypsographic units present on themoon, with mode peaks at -5.5, -3, -2, 0,and +4 km above datum. Although theseunits strongly overlap, placing unit bound-aries between the mode peaks at -4, -2.5,-1, and +3 km shows that these hypso-graphic units exhibit contiguous spatial oc-currences (Fig. 3). The unit at lowest ele-vations, between -7 and -4 km, is on thefar side and corresponds primarily to theSouth Pole-Aitken impact basin. The unitbetween -4 and -2.5 km corresponds pri-marily to the nearside mare basalts. Thehypsographic units are not always confinedto one hemisphere. The unit between - 2.5and -1 km contains much but not all of thenearside highlands and extends onto the farside in the north and east. The unit be-tween -1 and +3 km occurs chiefly on thefar side, but like the -2-km mode, it is notconfined to one hemisphere. Portions of

SCIENCE * VOL. 266 * 16 DECEMBER 1994

this hypsographic unit extend around to thenear side to the south, to the west nearOrientale and Humorum, and in a broad arcfrom the mid-latitude east, around south ofNectaris up through and terminating in theeastern Central Highlands. The unit above+3 km is a compact area to the north ofKorolev Basin.We used Apollo orbital geochemistry,

ground-based NIR spectroscopy, and Clem-entine global visible and NIR color data tocharacterize the compositional characteris-tics of these units. A correlation of topog-raphy and composition is evident fromApollo data (4). As our units are definedaltimetrically, we were not surprised to finda similar relation coupling Clementine to-pography data with the Apollo orbital geo-chemistry data which covers a narrow rangeof equatorial latitudes. We also found cor-relations between topography and composi-tion using ground-based and Clementinecompositional data for portions of the moonnot covered by the Apollo gamma ray andx-ray spectrometers.

Each unit defined above has distinct, butsomewhat overlapping, geochemical char-acteristics based on examination of Apolloorbital geochemistry and Clementine mul-tispectral data (Figs. 4 and 5). Excludingthe mare and South Pole-Aitken Basinunits, the -2-km unit, occurring principal-ly on the near side, shows higher values forFe, Th, and Ti and Mg/Al ratio, whereasthe +4-km unit shows lower values for Fe,Th, and Ti. No Mg or Al data exist for the+4-km unit. The 0-km unit is intermediatein Fe, Th, and Ti and lower in Mg/Al ratiothan the - 2-km unit. With petrologic mapsbased on the Apollo geochemical data (5)and the results of ternary mixing modelswith ferroan anorthosite, Mg-suite-KREEP(potassium, rare-earth element, and phos-phorus), and mare basalt as end-members(6), the +4-km unit is clearly dominated byanorthosite, whereas the other two unitscontain higher proportions of the moremafic components.

The Clementine multispectral datashow that the -2-km unit has a somewhatlower albedo and a slightly lower value inthe 950/750-nm ratio than the 0-km unit(Fig. 5). These multispectral parameters arecommonly associated with mafic content,though this association must be treatedwith caution (7). The +4-km unit, indicat-ed by mixing models to be highly anortho-sitic, is not anomalous in albedo or maficratio, being almost indistinguishable fromthe rest of the farside highlands. Higheralbedos and lower values of the mafic-cor-related ratio do occur, but in a broad area inthe northern part of the central far side.These data may indicate that the correla-tion between Th and mafic content ob-served in the samples returned from the

1855

'411*100amemeLwamoommsawom

on

May

9, 2

012

ww

w.s

cien

cem

ag.o

rgD

ownl

oade

d fr

om

near side main not hold for the tar site.Finally, Earth-based spectroscopy, hais

shoxvn that exposures of the 0-km unitxvhich extend onto the near side containanorthosite in the rings of Orientale, Hu-morumi, and Nectaris basins (8). Except forthe discovery of anorthosite in Aristarchuscrater (9) and the central peak of Alphon-sus crater (10), nearside ainorthosites iden-tified to date are confined to the 0-km unit.

The primary characteristics to be ad-dressed rega,.rding the lunar nearsile-far-side dichotomy are the large differences inelevations, the compositional asymnmetry,and the difference in the wvidths of thehypsographic curves. Additionally, the eo-chemical dichotomy between the near aindlfar side may extend to the deep interior(1 1 ). On the lutnar near side, all topograph-ic lows have been filled by mare basalts,which maly have flooded to a hydrostaticlevel ( 12). However, the lunar far side,althouth in gleneral standing higrher, exhib-its a largle topographic low in the SouthPole-Aitken Basin, tf-ar bclo)w' the level ofthe nearsicle mare. The South Pole-AitkenBasin does exhibit a smiall aimount of miaremiaterial in its floor, but the basin is notfilled to the near side mare level, thotuhthis ancient basin predates the mare (13).An analysis of the emnplacement of mnarebasalts on the moon ( 14) showed that near-side crustal thicknesses recltiredi magmaoverpressures in excess of 20 MPa to allowextensive eruptions to oCctir, and clearlythis overpressure was abundantly availableon the near side. With the methods detailedin (14) and the 20-km crustal thicknesscalculated for the South Pole-Aitken Basin(2), a imagma overpressure of only 6.4 MPawould allow eruptions to occur in this basin.Some process has inhibited eruption ofmare basalts in this portion of the moon,and a deficiency of heat-producing ecl-merits relative to the near side is a likelycandidate. Luna 10 and 12 gamma ray re-suilts for an area near the center of theSouth Pole-Aitken Basin show vcry low Uand Tb abundances ( 15).

Of the several models that attempt toaccount for the gyeochemical dichotomy,only iipact and primordial ci ifferentiatiOnprocesses can plausibly account for a hemi-spheric difference in mean elevation andsurface composition. A single large impactor large mnultiple impacts could havethinned the nearside crust exposing a miaficlow^xer crust (16) and depositing moreanorthositic material on the far side ( 13).One wouild expect that the rim of this largebasin wotild showe the 1hight1est elevatlonsrather than the antipodes, but one couldalso envision an impact of jutst the right sizeand velocity so that the em-placement ofejecta on the spherical moon wxxould yield anantipodal thickening. This fortuitously sized

1856

inpact could also explain the narrowness ofthe nearside lhy'pso0,gram, as updoming ofisothermns would have promoted extensivevolcanills in the basin floor, lexeling* thelong- x-avelength tc~poporaplnh and account-ingy for the -2-kmn hypsographic init. It hasbeen suggested, on the basis of remote sens-ing evidence, that the near side experienced

a volcanic resuirfacing event (17), which issupported bv the presence of nmieroustypes of ancient volcanic rocks of unknownvolumetric importance in the sample col-lection (18). Howxever, it is difficult to en-vision an impact tlat could have also11im-printed a compositional hemispheric di-clotOihorom on the lunar mantle as sug(ecstedl

Fig. 1. (A) A 950-nm relative albedo image of the moon derived from Clementine multispectral imaging.The projection is simple cylindrical. (B) A 950/750-nm ratio image. The vertical striping in the central rightportion of the image is due to images obtained at extreme photometric geometries for which adequatephotometric correction has not yet been made. (C) Image of lunar topography obtained by the Clemen-tine laser ranger. Highest elevations are white and lowest elevations are black. The data were constructedby (2) and gridded to 2° sampling. This image was then filtered with a Gaussian spatial passband to a

resolution of 4`.

SC IENCE * \OL. 266 * 16 DECEMBER 1994

on

May

9, 2

012

ww

w.s

cien

cem

ag.o

rgD

ownl

oade

d fr

om

Fig. 2. The lunar hypsogram (solidline), with the nearside (dashed line)and farside (dotted line) hypso-grams superimposed. The hypso-grams were constructed from anequal-area projection of the datapresented in Fig. 1C.

Fig. 3. Unit mapbased on the alti-metric data present-ed in Fig. 1C. Unitsare defined by eleva~tion contours at-4.0, -2.5, -1.25,and +3.0 km abovedatum.

c

T2C)0ca)

0

0 _''; - I' ,- "h

-10,000 -5,000 0 5,000

Altitude above datum (m)

by the absence of mare basalts in the SouthPole-Aitken Basin and by other evidence(10).

As giant impact cannot easily accountfor the apparent deep-interior hemisphericcompositional dichotomy, we propose analternative hypothesis that the largest scaletopographic features on the moon are theresult of primordial crustal differentiationprocesses. These processes would have in-cluded large-scale lateral transport of lunarmaterial to produce the hemispheric com-

10,000 positional asymmetry. We suggest that theanorthositic material making uLp the earliestlunar crust was not only concentrated nearthe surface by means of plagioclase flotationbut accumulated on the far side over alarge-scale convective downwelling andmay have even formed a small "continent,"now represented by the +4-km unit. Acomplementary convective upwelling underthe nearside crust would serve to thin thecrust through thermal erosion from belowand to emplace surface lava flows due to netextensional stresses coupled with availabil-ity of magma near the surface (19). Theseprocesses provide explanations for (i) thenarrowness of the nearside hypsogram asdue to volcanic infilling of topographic lowsover a convective downwelling, (ii) thepresence of the +4-km hypsographic unit as

Fig. 4 (left). Scatter diagrams of Clementine albedo and color data.Each x-axis is the scaled albedo at 950 nm, and each y-axis is the valueof the 950/750-nm ratio. Each of the hypsographic units are represent-ed as well as the moon as a whole. Fig. 5 (right). Scatter diagramsof Th versus Fe derived from Apollo orbital geochemistry for the threehighland units and the -5.5-km unit which largely represents SouthPole-Aitken Basin.

SCIENCE * VOL. 266 * 16 DECEMBER 1994 1 857

on

May

9, 2

012

ww

w.s

cien

cem

ag.o

rgD

ownl

oade

d fr

om

a result of net compressive forces operatingon the hot early crust over a convectivedownwelling, and (iii) the hemispheric sur-ficial and interior dichotomy as due to lat-eral transport of material during differenti-ation. The model is in conflict with thesimple geophysical argument that the highRayleigh number of a 500-km-thick magmaocean will promote small-scale turbulentconvection rather than global-scale con-vection, but the current degree of ignoranceof nature, let alone behavior of the actuallunar magma ocean, may not make this atightly binding constraint.

The distribution of elevations on themoon is distinctly irregular, and the nearside and far side differ both in mean eleva-tion and shape of the hypsographic curve.There is a correlation between topographyand composition, and five hypsographicunits have been defined with differing com-positional characteristics. Two of theseunits, the South Pole-Aitken Basin and thenearside mare basalts, are clearly the resultof impact and volcanism, respectively. Thelack of extensive mare basalt fill in theSouth Pole-Aitken Basin, despite the thincrust in this region, suggests hemisphericcompositional asymmetry in the deep lunarinterior. Early giant impact can explainmany of the observed characteristics, butnot the deep-interior compositional asym-metry. Primordial differentiation processescan explain all of the observed composi-tional and hypsographic characteristics butsuffer from the lack of an understood mech-anism for causing global- rather than small-scale lateral convective transport.

REFERENCES AND NOTES

1. W. M. Kaula et al., Proc. Lunar Planet. Sci Conf. 5,3049(1974).

2. M. T. Zuber and D. E. Smith, Science 266, 1839(1994).

3. This data set was constructed by averaging all pixelsin each charge-coupled device (CCD) camera imageto a single pixel. Each "image-pixel" was then cali-brated for camera gain and offset and integrationtime. The data were then projected into simple cylin-drical coordinates. Each image was photometricallycorrected with the Lommel-Seliger photometricfunction. Finally, an empirical phase function wasderived from a portion of the far side highlands be-tween the longitudes of the eastern South Pole-Ait-ken Basin and the western portion of the OrientaleBasin.

4. A. E. Metzger et al., Proc. Lunar Planet. Sci. Cont. 8,949 (1977).

5. P. D. Spudis and P. A. Davis, J. Geophys. Res. 91(suppi.), E85 (1986).

6. P. A. Davis and P. D. Spudis, ibid. 92 (suppl.), E387(1987).

7. The albedo and the ratio of a band near 1 Rm and aband on the continuum, usually around 560 or 750nm, are typically used to infer the relative amounts ofmafic material in mature lunar soils in multispectralimages. However, these parameters are not unam-biguously coupled to the mafic content of lunar soils.The albedo of a mature soil is due to the reflectanceof the minerals making up the source rock, the re-flectance of impact glasses produced by microme-teorite bombardment, and the amount of reducediron in the soil in the form of submicroscopic blebs

1858

which are produced by the reduction of Fe2+ inminerals by solar wind-implanted hydrogen. Maficminerals in source rocks vary widely in their reflec-tances, and each mineral is subject to the reductionprocess at different rates. Work by E. M. Fischerand C. M. Pieters (J. Geophys. Res., in prepara-tion) shows that there is no correlation between Aland visible albedo for highland areas overflown byApollo x-ray spectrometers. The ratio of a bandnear 1 pum to a continuum band is not directlycoupled to mafic content in that it decreases invalue with increasing abundance of mafic mineralsbut increases in value with maturity, but the specificmineralogy of the source dictates the relative im-portance of these properties. Thus, these parame-ters must be used with care when specifying thecomposition.

8. B. R. Hawke, P. D. Spudis, G. J. Taylor, P. G. Lucey,C. A. Peterson, Meteoritics 28, 361 (1993).

9. A. S. McEwen et al., Science 266, 1858 (1994).10. C. M. Pieters, Rev. Geophys. 24, 557 (1986).11. P. H. Warren and J. T. Wasson, Proc. Lunar Planet.

Sci. Conf. 11, 431 (1980).12. S. C. Solomon, ibid. 6,1021 (1975).13. D. E. Wilhelms, U.S. Geol. Surv. Prof. Pap. 1348

(1987); J. A. Wood, Moon 8, 73 (1973).

14. J. W. Head and L. Wilson, Geochim. Cosmochim.Acta 56, 2155 (1992).

15. Y. A. Surkov, Proc. Lunar Planet. Sci. Cont. 12B,1377 (1981).

16. G. Ryder and J. A. Wood, ibid. 8, 655 (1977); M. P.Charette et al., ibid., p. 1049; P. D. Spudis et al., J.Geophys. Res. 90 (suppl.), C197 (1985).

17. C. G. Andre and P. L. Strain, J. Geophys. Res. 88(suppl.), A544 (1983); P. H. Schultz and P. D. Spu-dis, Proc. Lunar Planet. Sci. Conf. 10, 2899 (1979).

18. G. Ryder and P. D. Spudis, Geochim. Cosmochim.Acta (suppl.) 12, 353 (1980).

19. R. Phillips, personal communication.20. We would like to thank the efforts of the Navy Re-

search Laboratory, the Lawrence Livermore NationalLaboratory, the Ballistic Missile Defense Organiza-tion, the U.S. Air Force, the National Aeronautics andSpace Administration, and the numerous privatecontractors who made Clementine happen. This isSchool for Ocean and Earth Sciences and Technol-ogy contribution number 3759, Hawaii Institute ofGeophysics and Planetology contribution number785, and Lunar Planetary Institute contribution num-ber 849.

10 August 1994; accepted 1 November 1994

Clementine Observations of the AristarchusRegion of the Moon

Alfred S. McEwen,* Mark S. Robinson, Eric M. Eliason,Paul G. Lucey, Tom C. Duxbury, Paul D. Spudis

Multispectral and topographic data acquired by the Clementine spacecraft provide in-formation on the composition and geologic history of the Aristarchus region of the moon.Altimetry profiles show the Aristarchus plateau dipping about 10 to the north-northwestand rising about 2 kilometers above the surrounding lavas of Oceanus Procellarum to thesouth. Dark, reddish pyroclastic glass covers the plateau to average depths of 10 to 30meters, as determined from the estimated excavation depths of 100- to 1000-meter-diameter craters that have exposed materials below the pyroclastics. These craters andthe walls of sinuous rifles also show that mare basalts underlie the pyroclastics across

much of the plateau. Near-infrared images of Aristarchus crater reveal olivine-rich ma-

terials and two kilometer-sized outcrops of anorthosite in the central peaks. The anortho-site could be either a derivative of local magnesium-suite magmatism or a remnant of theferroan anorthosite crust that formed over the primordial magma ocean.

The Clementine mission has providedglobal multispectral mapping of the entiremoon in 11 bandpasses (415, 750, 900, 950,1000, 1100, 1250, 1500, 2000, 2600, and2780 nm) and altimetry profiles from about-75° to +75° latitude (1). In this report,we present preliminary interpretations of asmall portion of these observations, fromthe Aristarchus region (latitude 18'N to320N, longitude 42YW to 570W).

The geologic and compositional diversi-ty of the Aristarchus region (2-7) and evi-dence for active emission of volatiles fromAristarchus crater (8-9) have led to pro-

A. S. McEwen, M. S. Robinson, E. M. Eliason, U.S. Geo-logical Survey, Flagstaff, AZ 86001, USA.P. G. Lucey, Planetary Geoscience Division, University ofHawaii, Honolulu, HI 96822, USA.T. C. Duxbury, Jet Propulsion Laboratory, Pasadena, CA91109, USA.P. D. Spudis, Lunar and Planetary Institute, Houston, TX77058, USA.* To whom correspondence should be addressed.

SCIENCE * VOL. 266 * 16 DECEMBER 1994

posals for surface exploration and resourceexploitation (10-12). The 170 km by 200km Aristarchus plateau, an elevated crustalblock just west of the outer ring of theImbrium Basin (Fig. 1), includes the densestconcentration of sinuous rilles known onthe moon. Most of these rilles begin atdistinctive "cobra-head" craters (or irregu-lar depressions), the probable source ventsfor a reddish dark mantling deposit (DMD)that blankets the entire elevated plateau;the DMD probably consists of volatile-richpyroclastic glass (4, 13, 14). The plateau isembayed on all sides by mare basalts ofOceanus Procellarum. The well-preservedCopernican impact crater Aristarchus, 42km in diameter, lies on the southeasternedge of the plateau. The Aristarchus pla-teau may have been the source region for asignificant portion of the mare basalts ofOceanus Procellarum (15). The movementand temporary storage of a large volume of

on

May

9, 2

012

ww

w.s

cien

cem

ag.o

rgD

ownl

oade

d fr

om