JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 105, NO. E2, PAGES 4217-4233, FEBRUARY 25, 2000
Geology of the Smythii and Marginis region of the Moon'
Using integrated remotely senseddata
Jeffrey J. Gillis
Departmentof Earth and PlanetarySciences,WashingtonUniversity,Saint Louis, Missouri
Paul D. Spudis
Lunar and Planetary Institute, Houston,Texas
Abstract. We characterized
the diverseand complexgeologyof the easternlimb regionof the
Moon usinga trio of remote-sensing
datasets:Clementine,LunarProspector,
andApollo. On the
basisof Clementine-derived
iron andtitaniummapswe classifythehighlandsinto low-iron(3-6
wt % FeO) andhigh-iron(6-9 wt % FeO) units. The association
of the latterwith basaltdeposits
westof Smythiibasinsuggests
thatthe highlandchemicalvariationis theresultof mixingbe-
tweenbasaltandhighlandlithologies.Mare SmythiiandMareMarginissoilsarecompositionally
similar,containingmoderateiron (15-18 wt % FeO) andtitanium(2.5-3.5 wt % TiO2). Smythii
basin,in additionto the basaltdeposits,containsan older,moderate-albedo
plainsunit. Our investigationrevealsthatthe darkbasinplainsunit hasa distinctalbedo,chemistry,andsurface
textureandformedasa resultof impact-mixingbetweenhighlandandmarelithologiesin approximatelyequalproportions.Clementineiron andmaturitymapsshowthatswirlsalongthe
northernmarginof Mare Marginishavethe sameironcomposition
asthe surrounding
nonswirl
materialand indicatethatthe swirl materialis brightbecauseof its low agglutinatecontent.
Gravity datafor the easternlimb showhigh,positiveBouguergravityanomaliesfor areasof thin
basaltcover(e.g., SmythiibasinandcomplexcratersJoliot,Lomonosov,
andNeper). We deduce
thattheuplift of densemantlematerialis theprimary(andmarebasalticfill the secondary)
source
for generatingthe concentrationof massbeneathlargecratersandbasins.
The geologyof the easternlimb regioncontainsvaluableinformationaboutprimary and secondarycrustalformationprocThe Smythii-Marginisregionis one of the mostgeologically esseson the Moon. Numerousmultiring basinsand largecraters
diverseareason the Moon (Figure 1) [Spudisand Hood, 1988]. on the easternlimb have sampledhighlandterra materialfrom
We havestudiedthe easternlimb regionof the Moon from 70ø to differentdepthsin the crust,the depthof excavationbeing pro100ø longitudeand from 10øSto 30øN utilizing Lunar Orbiter portionalto the diameterof the crater[e.g.,Spudis,1993]. Using
andApollophotographs,
Apolloy- andX-ray data[e.g.,La Jolla the compositionof the ejectablankets,centralpeaks,and basin
Consortium,1977], Clementinemultispectral,gravity, and to- rims, we have constructedthe three-dimensionalcompositional
pographydata [e.g.,Nozetteet al., 1994], and recentlyobtained structureof the highlandscruston the easternlimb of the Moon.
Lunar Prospectorgravity and y-ray data [Binder, 1998]. Three
Studyingthe durationandtiming of magmatismin this region
primarygeologicunitsarefoundin thisarea: crateredhighlands, will aid in understanding
the volcanicand thermalhistoryof the
characterized
by high albedoand low FeO (-3-10 wt % FeO); Moon. Volcanism in the area has occurred in at least three disbasinplainsmaterial,characterized
by moderatealbedoandFeO tinct episodes. Observationsof the craterdensitiesfor the volcontent(6-11 wt %); and mare material,characterized
by low canic flows in Smythii suggestthat theseflows are amongthe
albedoand high FeO content(15-18 wt %) (Figure2; Plate 1; youngeston the Moon [Schultzand Spudis, 1983; Spudisand
Table 1).
Hood, 1988]. The basalt depositsin Mare Marginis have a
The regionalgeologyis controlledby pre-Nectarianbasins, higherdensityof cratersand are thereforeolder. Finally, dark
from the oldestand mostdegradedbasins(e.g., Balmer, Margi- halo craterson the floors of the Lomonosov/Flemingand Alnis, Lomonosov/Fleming,
and Al-Khwarizmi/King)to the rela- Khwarizmi/King basinsare evidenceof buried basalt deposits
tively well-preservedSmythii basinwith its ruggedinner rim. [Schultzand Spudis,1979, 1983], indicatingthat volcanismocFeldspathichighlandsmaterial is the dominantunit exteriorto curredbeforethe end of heavybombardment,-3.85Gyr ago.
the basins. The mare and dark basin material are contained
Mare basaltsfill most of the nearsidebasins,thus erasingthe
withinthe SmythiiandMarginisbasinsandlargecraterssuchas early stagesof basin-fillingvolcanism.Smythiibasin,becauseit
Joliot(25øN, 94øE; 170 km diameter),Neper(2øS,85øE;140 km is only partly filled, affordsthe opportunityto studythe early
diameter),Lomonosov(27øN,94øE;95 km diameter),andHub- stagesof volcanic basinfilling. In additionto the mare unit in
ble(22øN,87øE;85 km diameter)(Figures1 and2).
Smythii,the basinfloor consistsof a moderatealbedoplainsunit
(Figures 1, 2, and 3). The origin of this unit is hypothesizedas
either a mixture of highlandsand volcanicmaterial [Conca and
Copyright2000 by theAmericanGeophysicalUnion.
Hubbard, 1979] or ejectafrom the Crisium basin[Stewartet al.,
Papernumber 1999JE001111.
1975]. We will demonstratethat a fifty-fifty mixture of high0148-0227/00/1999JE001111
$09.00
landsand mare material correctlymatchesthe compositionand
1. Introduction
4217
4218
GILLIS
AND
SPUDIS'
GEOLOGY
OF THE
MOON'S
EASTERN
LIMB
Figure 1. This Apollo imageshowsa regionalview of the easternlimb of the Moon (10øS-40øNlatitudeand
70ø-100ø longitude). Extendingacrossmuchof the regionarethe craterraysof GiordanoBruno(GB), one of the
youngestcraterson the Moon. Largecraterspartly filled with basaltare Lomonosov(L), Joliot(J), Hubble (H),
and Neper (N). Lunar swirls(LS) northof Marginisbasinare associated
with largesurfacemagneticanomalies.
Mare-filled basinsare Mare Marginis(MM) and Mare Smythii(MS). Note how the dark basinplains(dbp) material is almostas dark as the basaltin Smythii and maintainsan arcuateedgealong the southwestern
inner ring of
Smythii (AS14-75-10314).
albedo of the Smythii basin floor material. Furthermore,the
mafic compositionof the ejectafrom craterson the basinfloor
unit, and its relationshipwith surroundingunits,is evidenceof a
highland-maremixture.
agesfromthe threebandpasses
(415, 750, and950 nm) of the
ultraviolet-visible(UV-VIS) camerawere processedusing Inte-
gratedSoftwarefor ImagingSpectrometers
(ISIS), developed
by
the U.S. GeologicalSurvey(USGS),Flagstaff,Arizona[Gaddis,
1996;Eliason,1997]. All filterswerecoregistered
to the750 nm
filter to alignsurfacefeaturesin eachof thethreebandsandpre2. Clementine Data Processing
ventpixeloffsetwhenusingthe imagesfor quantitative
calculadeveloped
by the USGS,Flagstaff
Clementine multispectralimages from 21 orbits obtained tions. Calibrationparameters
fiat-field,frametransfer,
duringthe first and secondmonthsof data acquisitioncovering (e.g.,gain,offset,dark-fieldsubtraction,
corrections),were usedto radiometrically
the easternlimb region were used in this study (Figure 2). Im- and exposure-time
GILLIS AND SPUDIS'GEOLOGYOF THE MOON'S EASTERNLIMB
lO
Figure
2. Clementine
750nmimage
mosaic
showing
theeastern
limbregion
under
highSunillumination.
For
partoforbit
268,data
were
notobtained
atthe750nmwavelength'
thus
900nmdata
from
thatorbit
were
forced
to fit thereflectance
of the750nmdatain orderto increase
surface
coverage.
Images
fromthisorbitarenotin-
cluded
inanyofthecalculations.
Thisimage
isinsinusoidal
projection
andcovers
between
10¸ south
to30¸ north
and 75 ¸ to 95 ¸ east.
4219
4220
GILLIS
AND
SPUDIS'
GEOLOGY
80OE
OF THE
MOON'S
EASTERN
LIMB
90øE
FeO
Wt.øo
>18
16
14
12
lO
8
6
4
2
10 ø
eø
.10 ø
70øE
80øE
90øE
Plate 1. FeO mapof the Smythii-Marginis
region.The ironcontentfor the basaltmaterialwithinthetwo basinsis similar. The moderatealbedounit within Smythiibasinis muchlowerin iron thanthe basaltunit.. The
highlandterrato thewestof Smythiibasinis higherin ironrelativeto thehighlands
eastof thebasin
GILLIS
AND
SPUDIS:
GEOLOGY
OF THE MOON'S
EASTERN
LIMB
4221
Table 1. Units Within the EasternLimb RegionDefinedby Their
Chemical Signature
Unit
Highlands
East
FeO+l TiO2+l A1203
wt% MgOwt%
wt '70
wt %
Apollo
Apollo
3-7
<1
28-29
Th(ppm) 'Fh(ppm)
Apollo
Prospector
4-5
0.5
0.7
Highlands
West
6-10
<1
26-27
8.3-9.6
0.7
1.4
MareSmythii
16-18
2.5-3.5
16-22
9.9-12.4
2.4
1.4
Basinplains
6-11
0.75-1.8
22-25
9.6-10.3
1.2-1.6
1.5
MareMarginis
15-17
2.6-3.6
1.2
FeO and TiO2 concentrations
are derivedfrom the Clementinc
415,750 and950 nmdata
[Luceyet al., 1998].Apollox-raydatayieldA1203
andMgOabundances
[ConcaandHubbard,
1979],andTh is fromtheApollo)'-raydata[Davis,1980]andProspector
),-raydata[Lawrence
et al., 1998; Gillis et al., 1999].
correctthe data set (convert 8-bit digital numbersto 32-bit ab- were not obtainedduring a previousorbit, 111, as a result of a
solutereflectance(I/F)) [McEwene! al., 1998]. The photometric computerresetwhich preventedClementincfrom recordingdata.
functionby McEwen [1996] and revisionsby McEwenet al. The data described above were not used in the construction of the
[1998] were then appliedto normalizeimagesto a standard regional mosaic;their absenceaccountsfor the "checkerboard"
viewing geometry(phase=30
ø, emission=&,incidence=30
ø) to gaps evident in Figure 2 and Plates 1 and 2. Furthermore,we
allow directcomparisonof reflectancevalues. Final imagemo- have not processedthe 900 or 1000 nm data, as their inclusion
the sizeandnumberof thesedatagaps.
saicswere createdwith a resolutionof 250 m per pixel in sinu- into the mosaicincreases
soidalmapprojection.
The first month's data, from 5øSthrough10øN,were obtained 3. Smythii Basin
with phaseanglesof 5ø or less. As a result,reflectancevaluesfor
the first month'sdata are higher(_<4%)than the reflectanceval- 3.1. Ring Structure
ues for the overlappingsecondmonth's data after photometric
Smythii basin (2øS, 87øE) is pre-Nectarian [El-Baz and
normalization.
We assumed that the second month's data better
Wilhelms,1975]. There existssomedebateconcerningwherethe
represent
the true reflectanceof the lunarsurfacebecauseof their basinring structuresare located. Wilhelms[1987] mappedfour
greaterphaseangles(>20ø). This conclusionwas made on the ring structureswith diametersof 360, 660, 840, and 960 km.
basis that photometricmodels are most accurateat moderate Spudis[1993], however,suggestedfive basin rings: 260, 320,
phaseangles and that the photometricnormalizationmodifies 540, 740, and 1130 km in diameter. The topographicbasinrim
reflectancevalues less for an image with a phaseangle of 20ø diameter is reported as 740 km [Spudis, 1993] or 840 km
than for an image with a phaseangleof 5ø An empiricalfit was [Wilhelms,1987]. On the basisof Clementincaltimetry(Figure
performedon each filter in the first month's data between5øS 4), and Lunar Orbiter and Apollo images,we find evidencefor
and 10øN in order to matchthe reflectancedata in overlapping threeringswith diametersof 360, 660, and 840 km. The toporegionsfrom the secondmonth'sorbits.
graphicallyhighestof the threestructures
is the 840 km ring;
We useda three-channel,false-colorcompositemosaicto es- thereforewe considerit the main topographicrim.
timate qualitatively the regolith compositionand glass content.
Maps of estimatedFeO (Plate 1) andTiO 2 were createdusingthe
3.2. Highland Material
methoddevelopedby Lucey e! al. [1995, 1996, 1998] and with
refinementsby Blewerr et al. [1997]. Excellent agreementbeThe innermostring of Smythii stands4 km above the basin
tweenthe spectralFe and Ti parametersand the averageFeO and floor (Figure 4) and representsboth a physical and chemical
TiO2 contentsfrom the Apollo and Luna samplingsitesdemon- boundarybetweenthe inner and outer basin. Plagioclase-rich
stratesthat these techniquescan confidently be applied to the highland material is the dominant geologic unit outsidethe inClementincUVVIS coverageof the Moon globally [Blewerret nermostring of Smythii. This materialis mostlyejectafrom the
al., 1997].
Smythii basin, mixed with the ejecta of nearby youngerbasins
Image gaps occur in the Clementincmosaicsfor the eastern and large craters. The low-iron (3-7 wt % FeO; Plate 1) and
limb regionfor multiple reasons. In eight out of the 10 second high-aluminacontent(Table 1; 26-29 wt % A1203[Concaand
month's orbits (264-266, 268, 269, 271,273), imagesfrom one Hubbard, 1979]) of the soilsin this areasuggests
that the crustis
of the threefilters usedto constructeitherthe color ratio irnage thick andhighlyfeldspathicin composition
to the depthsampled
or iron or titanium abundancemaps (415, 750, or 950 nm) was by the Smythii basinimpact(-30 km). However,the highlands
droppedor the imagesobtainedwere >15ø off nadir (orbits264 west and north of the basin exhibit elevated iron concentrations
and 265). The previousdata errorresultedfrom a softwareglitch (6-10 wt % FeO) relative to the easternhighlands. Andre et al.
that preventedthe filter wheels from resettingafter an imaging [1977] and Concaand Hubbard [1979] report similar composisequence(T. C. Sorensen, personal communication,1997). tional differencesfor A1 and Mg betweenthe easternand western
Thus, when the next sequenceof images was taken, the filter highlandsboundingSmythii (Table 1). Terra materialseast of
wheel was not in the designatedpositionandwould substitutethe Smythii are higher in A1 and lower in Mg than the thoseto the
broadband"F" filter for one of the otherfive filters. Imageswere west,asnotedin the Clementinc-derived
iron maps.
taken off nadir during segmentsof orbits 264 and 265, because
The east-westhighlandcompositional
divisionwaspreviously
the spacecraftslued as a maneuverto recoverlost data. Data attributedto mafic debris from later impact events, such as
4222
GILLIS
70 ø E
20 ø N
AND
SPUDIS:
8.0ø
GEOLOGY
OF THE
MOON'S
EASTERN
LIMB
100 ø E
:20 ø N
90ø
NpNt
NpNt
l0 ø
Im •
lm
o
o
10 ø
20 ø S
70øE
8.0ø
90ø
100ø E
Figure 3. Geologicmapof the easternlimbregionmodifiedfrom Wilhehns
and El-Baz[1977] Solidlinesin'
dicatedfull basinrings,anddashedlinesindicatepartialbasinrings. Seelegendfor description
of mapunits.
GILLIS
AND
SPUDIS:
GEOLOGY
OF THE
MOON'S
EASTERN
LIMB
4223
Map Legend
Explanationof Map units:
Cc
- Copernicancrater;materialof sharp-rimmed,rayedcraters.
Ec
- EratosthenianCrater;material of sharp-rimmedcraters.
- Eratostheniandark mantle material; Eratosthenianage pyroclasticmaterial.
- Eratosthenianmare material;Eratosthenianage basalticmaterial.
Edm
Em
Ic
Im
Idhc
Ifc
Im 2
Idmt
Idbp
Ilp
Idp
Im•
Nhit
- Imbrian crater;materialof lesssharp-rimmedcraters.
- Imbrian mare materialundivided; Imbrian agebasaltmaterial,age relative to Oriental unknown.
- Imbrian dark halo crater;craterswith low-albedoejectadeposits.Thesecratershavepenetrated
throughoverlyinghighlandsmaterialto excavatedsubsurface
mafic materialin their ejecta.
- Imbrian floor-fracturedcrater;characterized
by a raised,fracturedcraterfloor whichmay contain
basaltand/orpyroclasticmaterial.
- Imbrian mare material;Imbrian agebasalticmaterial youngerthan the Orientalebasin.
- Imbrian dark mantledterra;topographicallyrough,low-albedopyroclasticmaterialmantledover
highlandsterrane.
- Imbrian dark basinplains;low-lying, pitted, moderate-albedo
material. A mixture of volcanic
material andbasin floor material of Smythii.
- Imbrian light plains;smoothlight coloredplainsmaterial.
- Imbrian dark plains,smoothlow-albedomaterial.
- Imbrian marematerial;Imbrian agebasalticmaterialolderthanthe Orientalebasin.
Nt
- Nectarianhigh-ironterra;highlandmaterialwith elevatediron concentrations.
- Nectarian terra;heavily crateredhighlandsmaterial.
Nc
- Nectarian crater; subdued-rimmed craters.
NpNt
pNc
pNcb
pNbr
pNbf
-
Nectarian,preNectarianundividedterra.
preNectariancratermaterial;very subdued-rimmed
crater.
preNectariancorrugated
basinmaterial;smooth,angularfracturedmaterial.
preNectarianbasinrim; basinmassifmaterial.
preNectarianbasinfloor; basinfloor material.
crater
rim crest
Crest of Basin Ring
Dashed
Figure 3. (continued)
where
inferred
4224
GILLIS AND SPUDIS: GEOLOGY
I00 ø
OF THE MOON'S
80 ø
EASTERN
LIMB
100 ø
.......?,.:.....
30 ø
90 ø
.?:•:!:
'"-::4" :il
.::.•. •.
::::.:•'J
.......
:...::.
:.,•
'--,'-
':•.t:::
....
.::•'"•;
•,,.......• ...
-•:•.
;:'
.......
:..
20 ø
10 ø
i.
10 ø
;:"•'
'<':..•-'-,
I•:.;•'
'
'•.
'ø'
'5,:--•
-.';.•'•½-'
..•'*'"11
0 o •..,½-.:½.
ß
.10 ø
80ø
90ø
.
-10ø
Figure 4. Topographyof the Smythii-Marginisregionas shownby Clementinealtimetry[Zuberet al., 1994J.
Eachcontourline represents
a differencein elevationof 1000m. The topographyis mergedwith the shadedrelief
mapto illustratemorphologicand topographiccorrelations.The innernm of the Smythiibasinis very apparent,
while a similarcirculartopographichigh is notdetectedfor the Marginisbasin.
CrisiumBasin,superposed
on the westernsideof the feldspathic
Smythii basin ejecta [Andre e! al., 1977]. The C!ementinederivediron map (Plate 1) indicates,however,thatthe compositional dichotomybetweenthe highlandsto the eastand west of
Smythiiis a manifestation
of maredepositspresentto the westof
Smythiiand absenteastof Smythii(Figures2 and 3). There are
multiplebasaltdepositsscatteredaboutthe westernedgeof the
Smythii basin. The iron map showshigh-ironbasaltdeposits
(10-14 wt % FeO) encompassed
by overlappingapronsof loweriron (6-10 wt % FeO) material. Thus gardeningof high-iron,
low-aluminumbasalt depositswith low-iron, high-aluminum
highlandsmaterialhaselevatedthe ironcontentat the expenseof
aluminain this region.
C!ementine data reveal anorthositic material in the innermost
ring structureof Smythii. The iron distributionmap showsthat
therangein FeOconcentration
alongmostof thebasinringis 26 wt %, compositionally
similarto the surrounding
highlands
(Plate1, Table 1). Thesedatasupportthefindingby Spudis'
and
Hood [1988] of highly feldspathicmaterial observedin the
northwestcornerof the 360 km diameterring and extendthe
known occurrences
of anorthoaticmaterialalong mostof this
ring (Figure 3). The age and relativelydegradedstateof the
innermostbasin ring suggestthat this anorthositicmaterial is
more likely to be comminutedsoil, low in iron, rather than
blocky outcropsof pure anorthositesuchas thoseseenat the
Orientale,Humorurn,and Nectarisbasins[Busseyand Spudis,
1997;HawIce,1993;Spudiset al., 1984, 1989].
In additionto the innerbasinringof Smythii,centralpeaksof
largecraterson the easternlimb alsosamplematerialrich in anorthosite.The centralpeaksof Joliot,Neper,andLomonosov
all
GILLIS
AND SPUDIS'
GEOLOGY
OF THE MOON'S
EASTERN
LIMB
4225
Figure 5. An Apollo photograph
showingthe geographyof SmythiiBasin.Avery (A); Cam6ens(C); Dumas
(D); Doyle (DO); Haldane(H); Helmeft(HE); Kiess(K); Kao (KA); K/istner(KS), K/istnerG (KSG); Mcadie(M);
Neper (N); Peek(P); Runge(R); Swasey(S); Widmannst/ittan
(W); Warner(WA); raceis placedabovecraters
with mafic craterejecta;Rilles andarrowdepictpromontorywith sinuousrilles;the white box outlinesthe location
of Plate2. The blackcrosses
arefiduciarymarkson the Apollo Hasselbladprints(AS 15-95-12991).
havecompositions
low in iron, 2-6 wt % FeO (Plate 1). Evi- basinfloor of Smythii affirms the protractedelapseof time bedenceof feldspathicmaterialwithin both the inner ring of tween basinformationand mareemplacement.
Smythiibasinanditsejectaandwithinthecentralpeaksof com-
plex craterssuggeststhat the bulk crustin this regionis anortho- 3.4. Basalt Composition
sitic betweendepthssampledby cratercentralpeaks(---10km
The volcanicdepositsin Smythiiare 16-18wt % FeO and2.5[Grieve and Garvin, 1984]) and the inner basin ring structure 3.5 wt % TiO2, as determinedfrom the C!ementinedata. These
(20-30 km [Grieveet al., 1981])[Gilliset al., 1997].
dataare consistentwith Mariner 10 data,4+1 wt % TiO 2 [Robinson et al., 1992] and the Apollo y-ray concentrations
for FeO,
3.3. Basalt Deposits
13+2.5 wt %, and TiO2, 4+1.8 wt %, reportedby Davis [1980].
Mare basaltdepositsfill the northeastinteriorof Smythiibasin The agreementin measuredFeO and TiO2 content between
C!ementineand Mariner 10 multispectraldata [Robinsonet al.,
and form isolated lenses within some of the modified floorfracturedcraters(Figures3 and5). The mainmaredepositoccu- 1992]andApollo gamma-raydata[Davis, 1980] offersassurance
[Luceyet
piesan areaof 32,000 km2 or--•25%of the basinfloor surface thatthe techniquesfor calculatingelementalabundaces
area. The isolatedbasaltdepositshave a combinedareal extent al., 1998] are valid and can be appliedwith confidenceto the
of 9000 km2. The basaltsarecharacterized
by low-albedo,
high- Moon globally. The Clementinedatasethasthe addedbenefitof
iron content, a smooth surface (with the exception of mare higher resolution(250 m/pixel) than the Mariner 10 (---10km
ridges),and low crater densities. The lack of visible domes, [Robinsonet al., 1992]) andthe Apollo y-ray data,approximately
sinuousrilles, and flow fronts in the main mare depositis evi- 100 to 200 km [Davis, 1980]. Resolutionof this quality allows
dencefor highly effusive,flood-typeeruptions[Greeley, 1976]. us to correlatechemicalfeaturesin the FeO and TiO 2 mapswith
featureson the surfaceof the Moon (e.g.,Plate2).
Craterdensitiesfor the entiremain mare unit in Smythii, relative geomorphic
to the lava flows at the dated Apollo sites, indicatethat these 3.5. Basalt Thickness
flowsareamongthe youngeston the Moon, 1-2 Gyr old [Schultz
and Spudis, 1983; Spudisand Hood, 1988]. Age estimatesby
Plate2 illustratesthat the highestiron concentrations
in Mare
Boyceand Johnson[1978] suggestthat the depositmight be as Smythii are shifted to the north and east from the centerof the
old as 2.5+0.5 Gyr. The accumulationof premarecraterson the maredeposit. If the observediron contentof Mare Smythiiwas
4226
GILLIS
AND
SPUDIS:
GEOLOGY
OF THE
MOON'S
EASTERN
LIMB
FeO
Wt. øo
>8
16
1,
1
0
8
6
Plate 2. The same Apollo image as in Figure 5 reprojectedand mergedwith the correspondingarea in the
C!ementineiron map (Plate 1). This imageillustratesthe relationof morphology(e.g., floor-fracturedcratersand
mafic craterejecta)with composition(e.g., low-iron craterrims and high- and low-iron ejectadeposits). White
arrowspoint to craterswith low-ironejecta,andthe yellow arrowpointsto the maximum-sizecraterwith high-iron
ejecta. Peek(13 km diameter)is a macroscale
exampleof what a craterwith low-ironejectalookslike.
the previous
causedby horizontalmixing, for example,producedby crater portionMare Smythiihasforcedus to supplement
ejecta,thenthe patternof FeO concentration
wouldmimicthe thicknessmeasurementsusing additional techniques. Accordboundariesof the mare unit. However,verticalmixing causedby ingly, we measuredthe diametersof two ghost craters(2øN,
meteoriteimpactswouldproducea greateramountof mixingand 88.5øE, diameter 8 km; IøN, 89.25øE, diameter 8.75 kin) and,
from Pike [1974, 1977],calculated
thatit would
contaminationof low-iron materialsin the thinner parts of the usingequations
basaltunit than for the thicker areas. The vertical mixing model require300 and 325 m of basalt,respectively,
to inundatethese
indicates
thatthe ironcomposition
of the basaltis directlyrelated craters. Thus we estimatethat the basalt fill of Smythii is on
measurements
occuroutside
to its thicknessand thereforeexplainswhy the area of highest average300 m thick. All thickness
within the basalt;thusthe
iron content does not coincide with the center of the deposit. the areaof highestiron concentration
Thuswe concludethatthebasaltdepositwithin Smythiiis thick- deepestportionsof thebasinareslightlythickerthan300 m. Our
measurements of basalt thickness are consistent with the Stewart
estin the northeastern
portionof the basin.
The thicknessof the basaltin Mare Smythii is quantifiedby et al. [I 975] calculationof an averagemarethicknessof 350 m
bracketing
the depthsat whichcratershaveexcavated
highlands and nqaxinqumthickness of 450-475 m. In addition, De Hon and
materialor not. The compositionof ejectablanketswas deter- Waskom[1976] suggestthatthe marebasaltsaverage230 m and
minedusingthe C!ementine
750/950 imageratioandironabun- reach a maximum thickness of 500-600 m in the northern floor of
dancemaps(Plate 2). For simplecraters,<20 km in diameter, the basin.
typicalexcavation
depthsareon theorderof onetenththeirap- 3.6. Surface features within the mare
parentdiameter[Croft, 1980]. Cratersthathaveexcavated
mateRemnantsinuousrilles locatedon a patchof high-standbasin
rial froma depthof-300 m or less(e.g.,an unnamedcrater3.25
km in diameter located at 0.7øN, 87.5øE) have not excavated floor material(89.25øE, 3.25øN) avoidedburial by mare flooding
low-ironmaterials(e.g.,theirejectablanketshavea high750/950 (Figure 5). These rilles emplacedsome basalt in the area, but
ratio and wt % FeO). Craterswith excavationdepths>350 m their length and width (10 km long, 500 m wide) suggestthat
(e.g., an unnamedcrater3.5 km in diameterlocatedat 0.6øN, they were not responsiblefor the majorityof the basaltdeposited
86.5øE and one 3.7 km in diameter located at 3øN, 89.8øE) have within the basin. Additional volcanic rilles are likely to have
excavatednonmarematerialfrom beneaththe basalt(e.g., at least existedon the basin floor of Smythii, but they were submerged
partof thecraterejectablankethasa low 750/950ratioandwt % by the continuederuptionof marebasalt.
Wrinkle ridgeswithin Mare Smythii trend in a northeasterly
FeO).
The absence of C!ementine data combined with the absence of
direction. The mare ridgescorrelatewith the thickestaccumulacraterslarge enoughto excavatenonmarematerialin central tion of basalt. Their nonarcuateshapeimplies that they do not
GILLIS
AND
SPUDIS:
GEOLOGY
OF THE
MOON'S
EASTERN
LIMB
4227
Table 2. Mixing Results
Unit
415 nm
750 nm
Avg. Highlands
0.107
0.175
Mare Smythii
0.070
0.112
Mixture 50:50
0.088
0.143
Dark basinplains
0.086
2.4
% Difference
950 nm
FeO
TiO2
A120•
MgO
Th
0.1991
5.0
0.1
0.1242
17.0
3.0
28.5
4.5
0.7
19.0
11.5
0.1617
11.0
1.6
23.75
2.4
8
1.55
0.142
0.1614
9.0
1.3
23.5
10
1.4
0.9
0.1
22
24
1.1
20
10.7
Usinga two-component
linearmixtureof thespectra
andcomposition
lbr thehighlands
surrounding
the
Smythiibasinandthemarebasalts
withinthebasin,a hypothetical
darkbasinplainslithology
(mixture50:50)
wascalculated.
Themodelresultiscompared
to thespectra
andcomposition
of theobserved
darkbasinplains
unitandreported
aspercentdifference
(%Difi: = (observ-calc)/observx100).
A fifty-fiftymixtureof thetwo
lithologies
closelymatches
thespectral
databest,buta sixty-lbrty
mixture
of highlands
andmarebetterapproximates
thecomposition
of theobserved
darkbasinplains.
reflect buriedpremaretopography. Thus we attributetheir formationto stressesdevelopedduring consolidation,local isostatic
adjustmentsof the basin, or saggingof the basin floor in responseto the overburdenof the basaltflows.
Greeley et al. [1977] proposedthat the mare lavas flowed
generally from the southeastand west toward the northeastand
accumulatedin the lowestpartsof the basin. However, we observethat the northeasternareaof the basinmaintainsthe highest
elevation(-4.25 km) and slopesdownwardto the west (-4.5 kin)
and southwest(-4.4 km) below the mean datum of 1738 km
(Figure 4). We infer from this observationthat eruptionsoccurred in the northeast and flowed
to the west and southwest.
This hypothesiswould explainhow the basaltthinsto the southwest while lacking any obviousvolcanic surfacefeatures. We
concludefrom the absenceof visible flow fronts, uniform spectral character,crater density, and iron and titanium concentrationsof the main basaltdepositthat the majorityof the volcanic
surfacewasdeposited
in a singleeruptiveeventandfroma single
sourceat depth.
Some areasin the westernand easternSmythii appearanalo-
gousto the corrugated
MaunderFormationof the Orientalebasin
(Figure 6) [Head, 1974; Scott et al., 1977; McCauley, 1977].
This unit consistsof smooth,angularblockswith concentricand
radial fractures, has an FeO concentrationof 6-8 wt %, and is
situatedbetweenthe innerring andmarebasalt. The arealextent
is limited to the northwesternand easternedgesof the basin.
This limited surfaceexposureis a consequence
of greaterdegradationby cratererosionand burial of $mythii relativeto Orien-
*.7g
(A)
(B)
Figure 6. Imagescomparing
(a) the Corrugated
Maunder(CM) andSrnoothMaunder(SM) Formationof the
Orientalebasin(LO lV-195-HI) and(b) thesmoothangularblocksfoundin Mare Smythii(LO-I-19M). The black
linein Figure6a separates
theOrientalemeltsheet(CM andSM) fromMareOrientale(MO). The smoothtexture
of theblocksin Smythiibasin(Sms)is similarin morphology
to theOrientaleMaunderFormationandby analogy
couldbe partof the meltsheetfromthe Smythiiimpact. An archetypal
darkhalocrater(dhc)is locatedin the
uppercenterof the image. The craterhasexcavated
darkerpossible
pyroclastic
materialfrombeneath
the mare
surface.A wrinkle ridge(wr) runsnorth-south
alongthe left-handsideof the image.
4228
GILLIS
AND
SPUDIS:
GEOLOGY
OF THE MOON'S
tale. If thissmoothcorrugated
materialis similarin originto the
MaunderFormation,then it consistsof the impactmelt from the
Smythii basin-formingevent. Likewise, the inner ring of the
Smythii basin,360 km, would then be analogousto the 320 km
innerRook ring of the Orientalebasin. An alternativeexplanation is that concentricand radial fracturesare fault grabensdevelopedin response
to tensionwithin the marebasalts.Sagging
of the basininterior,due to the overlyingweightof basalt,producedgrabenstructures
alongthe edgeof the basin.
4. Moderate
Albedo
Unit
Apollo and Clementineimages(Figures1) of the Smythiibasin showthat moderate-albedo
materialoccupiesthe southwestern andcentralfloor of Smythiibasin(dbp in Figure1 and Idbp
unit in Figure3). This geologicunit is hummockyin appearance
and is heavilycrateredby smallimpacts(Figure5). It is intermediatein albedo,A1/Si and Mg/Si ratios[Stewartet al., 1975;
Andre et al., 1977], and iron compositionbetweenmare and
adjacenthighlandterra(Plate2; Table 1). Craterdensitiesof the
moderatealbedobasinplains materialyield an age estimateof
-3.5 Gyr [Boyce and Johnson, 1978], older than the mare.
Topographicallyhigherthan the main mare deposit(Figure 4),
the moderatealbedo unit forms an elongatearch that stretches
from northwestto southeastand dividesthe main basaltdeposit
to the northeastfrom isolatedbasaltpatchesin the cratersDumas,
Kiess,Widmannst•itten,
Helmeft,andKao (Figures3 and5).
On the basisof albedoand craterdata,Stewartet al. [1975]
concludedthat the moderatealbedobasinmaterialwas emplaced
as a debris blanket of mixed origin and composition,derived
from the Crisiumbasin,otherlargeimpactbasins,and cratersin
the area. Concaand Hubbard[1979] statethat the high-albedo
of the mantled terra unit rules out the presenceof substantial
amountsof volcanicmaterial(< 25%) as eitherpyroclasticmaterial or lava flows.
There
are some inconsistencies
with these
Observations
EASTERN
LIMB
of craters within
the moderate-albedo
basin
plainsunit havingmaficejectablankets(Figure5; Plate2) supportthehypothesis
of buriedmare. The ejectaof multiplecraters
within the moderate-albedo
unit (e.g., Avery and threeunnamed
craterslabeledmce (Figure 5)) exhibita mafic signaturein the
Clementinethree-colorratio imageandin the iron map(Plate2).
Furthermore,the ejectafrom an unnamedcrater(83.4øE,3.5øS)
wasrevealedby Concaand Hubbard[1979] to havean elevated
Mg/Si ratio in the Apollo X-ray data. Finally, an unnameddark
halocrateris locatednorthof Widmannst•itten
(85.1øE,3.75øS);
itsirregularshapesuggests
thatit is possiblyof endogenic
origin
[Headand Wilson,1979]. Theseobservations
provideevidence
thatthe chemicalcomposition
of the dark plainssubsurface
unit
is moremafic thanthe exposedsurfacelayer. Additionalpossible eruptivesourceareasare degradedpit cratersand rilles locatedwithin the dark basinmaterialon the southeast
marginof
the basinbetweenthe cratersAvery andDumasandto the north
of KiessandWidmannst•itten
(Figure5).
We concludethat mare basalteruptedearly, conformedto the
innerring of Smythii,and subsequently
was overlainby highlandsmaterialejectedfrom nearbycraters.We proposethatthe
postbasin
cratersresponsible
for depositinghighlandsejectaover
the mare on the southwestern
side of Smythii are Doyle, Haldane,Kiess,Warner,Widmannst•itten,
Neper,La P6rouse(10øS,
76øE;70 kin), Ansgarius(13øS,80øE;90 kin), Sklodowska(19øS,
96øE; 115 km), and Humboldt (27øS,81øE;200 km). Over time,
gardeningof the surfaceby meteoritebombardmentmixed the
two lithologies[Quaide and Oberbeck,1975] to producethe
final basinplainsmaterial,intermediatein albedoand composition.
5. Floor-Fractured
Craters
Premare craters within the boundaries of the moderate albedo
plainshave beenmodifiedby endogenicprocesses
which include
models. First, the age of the dark basinmaterialis late Nectarian upliftedand fracturedfloors,pyroclasticdeposits,sinuousrilles,
or early lmbrian, too youngto be the resultof Crisium,which is andbasaltdeposits.The craterrims,centralpeaks,and uplifted
mid-Nectarianage [Wilhelms,1987]. Second,it is fortuitousthat annularringsof thesefloor-fractured
cratersare lowerin iron(4the moderatealbedounit was emplacedonly within the confines 8 wt % FeO) thanthe mareandbasinplainsmaterialsurrounding
of Smythiibasin. Other localtopographiclows,suchasthe cra- them (Plate 2). This indicatesthat thesecratersformed on the
ters K•istnerand K•istnerG, do not displaythe moderatealbedo surfaceof the ancientbasinfloor. Two modelshavebeenproseenin the dark plainsunit (Figures1, 2, and5). Bothcratersare posedto explain the processesthat have modified thesecrater
filled with light plainsmaterialwith iron concentrations
of only floors: volcanism and plutonism [Brennan, 1975; Schultz,
2-8 wt %, lower than the dark basin floor material (6-11 wt %
1976b]andthe viscousrelaxationof cratertopography
[Baldwin,
FeO, Plate 1). Finally, a fifty-fifty mixture of highlandsand 1968;Hall et al., 1981]. The first modelemploysdike-fed,bamarematerialwould producethe observedreflectancevaluesfor saltic intrusionsinto the fracturedmaterialbeneaththe impact
the dark basin material (Table 2). This samemixture of high- crater. As intrusionspersist,the crater floor elevatesand fraclandsand mare would also yield the observediron and titanium tures. Along the deepestfractures,volcaniceruptionsmay occur.
concentrations.
In contrast,the viscousrelaxation model incorporatescrustal
Similar values for averageA!/Si ratios [Andre et al., 1977] heatingto locally lower viscosity,allowing relaxationof crater
and FeO concentrations
for the highlandswest of Smythii (Plate topography. Mantle uplift is the mechanismfor the relatively
1), for portionsof A1-Khwarizmi/King
[ClarkandHawke, 1991], near-surface
crustalheatingthat providesthe thermalconditions
and for materialin the unfloodedportionof Smythii basinsug- for both viscous relaxation and volcanism.
gestthat thesedepositsall formedby similar processes.ConStudiesof the floor-fracturedcratersin Smythiibasinhaveled
ceivably,basalticmaterialmixed with highland-typelithologies previousworkers[Hall et al., 1981; Yingstand Head, 1998] to
to accountfor the intermediatealbedoand compositionbetween suggestthat crater floor modificationoccurredin responseto
mare and highlands. However, Conca and Hubbard [1979] viscous relaxation of the crater. However, our observational
notedthe difficulty with lateralmixing of two lithologies. Mix- data, as well as geophysicalmodeling [Dombard and Gillis,
ing would be more efficientif it were vertical[Rhodes,1977], 1999], reveal inconsistenciesin the viscous relaxation model.
eitheras a thin basaltdepositreworkedwith basinmaterialor as Floor-fractured craters and unmodified craters of similar size and
basaltcoveredby ejectafrom nearbycraters[Schultzand Spudis, age occurside by side; in other regions,relativelysmall craters
1979, 1983].
have undergonegreatermodificationand floor fracturingthan
GILLIS
,100 ø
AND
SPUDIS:
GEOLOGY
OF THE MOON'S
80 ø
90 ø
30
•'••:•11,'i!::•.•,•
ß
2-,.:
'..,•?.-:•
EASTERN
LIMB
4229
-I00 ø
?•...
•::•....i•i•-.
30 ø
20 ø
lO ø
,10 ø
80 ø
90 ø
.,!0 o
Figure 7. Bouguergravitymap [Zuberet al., 1994] mergedwith a shadedrelief imageof the easternlimb regionof the Moon. Eachcontourrepresents
100mGals. The greatestpositiveBouguersignaturedoesnot correlate
withthethickestareaof basaltwithintheSmythiibasin;thehighestgravityis offsetto thesouthwest.
The Marginis basindisplaysonly a moderatepositiveBougueranomaly. Gravity highsare alsoassociated
with the crater
Neperandan areanorthwestof MarginisBasin.
gravity anomaly (Figures 5 and 7), as required by the viscous
relaxationmodel [Brennan, 1975]. In addition, calculatedrates
for deformation[Yingst and Head, 1998] suggestthat craters
closestto the major mare deposit and largestgravity anomaly
have taken longer to modify than cratersof the samediameter
fartherfrom the area of greatestthermalactivity. For example,
Runge(38 km diameter)has taken almost5 times longerto deanomalies. However, crater floor modification by subsurface form than the crater Haldane (39 km diameter),3.5x106 versus
adjacentlargercraters[Schultz,1976b]. Moreover,comparing
a
mapof theglobaldistribution
of floor-fractured
craters[Schultz,
1976b]with marebasaltdistribution,
basinlocationandgravity
datashow[Lemoineet al., 1997]thata numberof floor-fractured
cratersoccurat significantdistances
from the nearestmareunit,
impactbasin,or gravityanomaly.The viscousrelaxation
model
requirescraterswith fracturedfloorsto be proximalto thermal
magmaemplacement
is lessdependent
than crustalheatingto
nearbythermal anomaliesbecauseisolatedmare units occur
1.6x10* years(a differenceof 12.5 million years)[Hall et al.,
1981]. Even Dumas(17 km diameter),locatedfarthestfrom the
within even the thickestcrustof the farside(e.g., Kohlsch'dtter mainmaredeposit,deformedmorequicklythanRunge(l.2x10*
and Lacus Solitudinis),far removedfrom any major body of years [Hall et al., 1981]). The viscousrelaxationmodel also
mare basalt.
requiresthat the viscosityof the lithospherevary significantly
Within the Smythiibasin,all of the floor-fractured
cratersoc- over shorthorizontaldistances.This is the casefor Ritter (28 km
cur adjacentto the mareanddo not coincidewith the highest in diameter) and Sabine (30 km diameter) craters located in
4230
GILLIS AND SPUDIS: GEOLOGY
OF THE MOON'S
EASTERN
LIMB
southwestern
Tranquillitatis.Thesecratersareequallydeformed. 3.6 wt % TiO2 (Plate 1; Table 1). Similar titaniumconcentrawere producedat approximatelythe sametime, and are located tions have been reportedby Robinsonet al. [1992] for Mare
within the samebasaltstrata,and their rims are separatedby only Marginis(3+1 wt % TiO2) and Mare Smythii (4+1 wt % TiO2).
4 kin, but it took 4.4x107yearslongerfor the modificationof
The extentof mare is 50,300 km2, slightlylargerthan Mare
Sabinecrater. A variationin viscosityof this magnitudeacross Smythii. Mare Marginis is olderthan Mare Smythii,as indicated
suchsmalldistancesseemsgeologicallyimplausible.
by highercraterdensities[Boyceand Johnson,1978]. There are
The fact that fractures do not extend outside the crater rims
two distinctepisodesof basalticvolcanismwithin the basin:Im•
suggests
that the mechanismfor formationis nearthe surfaceand and lm2 in Figure 3. The chemicalcompositions
of the basalt
localized beneaththe crater. Moreover, the processhas to be flows are similar (Plate 1), suggestingthat they originatedfrom a
able to accountfor deformationof large craters(e.g., Humboldt relatedsourceregion. The two units are distinguishedby their
209 km [Baldwin, 1968]) as well as small craters(e.g., unnamed different crater densities. The younger of the two units covers
cratersin Aitken, 6 km in diameter [EI-Baz, 1973; Bryan and mostof the surfaceof Mare Marginis. The older unit is present
Adams,1974]). It is counterintuitivethat the deformationcaused alongthe easternhighland-mareboundaryof the basinand along
by mantle upwellingscould be exclusivelycontainedwithin a the southcentralportionof the basin,northof the cratersNeper
crater with a radius of 6 kin. Furthermore,viscositiesused to and Jansky.
model viscous relaxation
are inferred to be on the order of the
Earth'smantle(1022poise[Hall et al., 1981]). In orderfor the
6.3.
Basalt Thickness
floor of the craters to fracture, they must be brittle; the low
theologiesof the near-surfacelithosphere,requiredby the viscousrelaxationmodel, are contradictoryto this fact.
The Smythii basincontainsat least 11 floor-fracturedcraters,
the densitiesof which suggestthat this regionhashad an intense
amountof subsurfaceigneousactivity, consistentwith our interpretationof the origin of the moderatealbedo unit discussedin
The absenceof craters larger than 2 km within the central
portion of Mare Marginis preventsus from using the method
describedin section3.5 for estimatingbasaltthickness;craters
with ejecta consistingof continuous,low-iron material are not
observed. Thus a secondmethodof calculatingmare thickness
from measurements
of the exposedrim height of partiallyburied
craters[Eggleton et al., 1974; De Hon and Waskom,1976; De
section 4. The collection of floor-fractured craters illustrates a
Hon, 1979] was usedto estimatethe maximumthicknessof the
continuum of modification (Figure 5); from craterswith wellbasalt. The mare thicknessaround three, unnamed,partially
developedannularringsandlittle mare(Haldane,Doyle, Mcadie, buried craters(Plate 1; 13øN, 87.8øE, 12.5 km diameter; 13øN,
and Warner)to thosethat are more envelopedby basalt(Runge, 88.7øE, 12.3 km diameter; and 13.2øN, 83.7øE, 11.25 km diCam6ens,Kiess, Widmannst•itten,
Helmert, Kao, and Swasey). ameter) range between 150 and 320 m. Thereforewe suggest
Pyroelasticmaterial is associatedwith irregularpits and rilles that the averagebasalt thicknessin Marginis basin is •250 m.
found within and adjacentto the floor-fracturedcraters. The Moreover, we conclude that the thickest portion of the basalt
irregular pits within the floor-fracturedcraters suggestthat a corresponds
with the region of highestiron content,the southnear-surfacevoid was present,founderingthe overlyingsurface west centralportionof Mare Marginis (Plate i), on the basisthat
and allowing lava to drain back into the subsurface. Floor- vertical mixing has been shown to dominate over horizontal
fracture craterssuch as Haldane, Runge, Swasey, Warner, and mixing [Rhodes,1977].
Widmannst•ittencontain pyroelasticmaterial within their crater
walls. The floor-fracturedcraters Doyle, Cam6ens,Tasso, and 6.4. Volcanic Surface Features
Keisshave pyroelasticmaterialalongtheir peripheries.The pyThe absenceof volcanic domes, flow fronts, and sinuousrilles
roclasticdepositsand sinuousrilles indicatethat highlyeffusive,
in
Mare Marginis suggeststhat thesebasaltswere the result of
mare flood-typevolcanismoccurredin the centralportionof the
basin, while fire-fountaining,plains-typeeruptionsoccurred highly effusive,flood-typeeruptionslike thosein Mare Smythii.
The only notedexceptionis a degradedri!le locatedatopa promadjacentto the main maredeposit.
ontory (86.3øE, 13.8øN) (Plate I). Mare Marginis also has no
apparentwrinkle ridges. This fact indicatesthatthe basin-fillhas
6. Marginis Basin
not beentectonicallycompensated
subsequent
to basaltflooding
like mostmare-filledimpactbasins.
6.1. Regional Geology
Mare Marginis (20øN, 84øE) may or may not occupyan impact basin. The Marginis basin has been mappedas a preNectarJanimpact basin, 580 km in diameter [El-Baz and
Wilhelms,1975; I4/ilhelms,1987]. However, Clementinealtimctry showsonly a shallow, irregulardepressionlackingany topographic rim (Figure 4). Moreover, there is no indicationof an
anorthositicring surroundingthe basin in Clementineimages
(Plate 1). Marginis also lacks a strong(>200 mGals), circular
positive gravity anomaly (Figure 7) displayedby many large
impactbasins. Theseattributesare characteristic
of a very old,
highlydegradedimpactbasin,a coalescence
of largecraters,or a
structural
trough
exterior
andconcentric
to Smythii
basiniGillis
et al., 1997].
6.2. Basalt Deposits
The iron and titanium abundances for the basalt in Mare Mar-
ginisaresimilarto thosein Mare Smythii: 15-17wt % FeO;2.6-
7. Lunar
Swirl
Material
Unusualswirlpatternsof alternating
brightanddarkmaterial
occuralong the highland-mareboundaryof northernMarginis
basin(Figures1, and 2)[EI-Baz, 1972]. Associated
with the
swirls are strongmagneticanomaliesdetectedby the Apollo
subsatellite
magnetometer
[Srnkaand Schultz,1979;1toodand
Williams,1989]. The swirlshavebeensuggested
to be eithera
chemicalalterationof surfacematerials[Schultz,1976a],deposition of sublimatesdegassedfrom the interior of the Moon,
scouringof the surfaceby recentcometaryimpacts[Srnkaand
Schultz, 1979; Schultzand Srnka, 1980], or agglutinate-freeregolith that has been shieldedfrom the ion bombardment
of the
solarwind by the associated
anomaliesof high surfacemagnetism [Hood and Williams, 1989].
Usingthe Clementinedata,we can supporta swirl generation
mechanismthat producesan agglutinate-poorsoil from in situ
GILLIS
AND
SPUDIS'
GEOLOGY
OF THE MOON'S
Comparison of reflectance data for swirl and non-swirl materials
0.325
O
0.3
HighlandsMaterial
•
0.275
•
0.25
Highland
Swirls
•
ß
[]-Mare
Swirls
Mare material
0.225
0.2
0.125
O.
1
••••-•-•-•-•
gravity readingsoccur to the south and east of the main mare
depositand do not correspondwith the thickestdepositof basalt
as reportedby Brennan[1975] andEI-Baz and Wilhelms[1975].
Furthermore,a masonof similar magnitudeshouldbe associated
with Mare Marginis becauseof its comparablethicknessto Mare
Smythii. Instead, there exists only a moderate(-125 mGals)
masconbeneaththe basalt flows of Mare Marginis. The basalt
flows in Smythii Basin are too thin (-300 m) to be the primary
causeof the mascon;it requiresa 18 km thick layer of material
tence of the submare mass is the result of the excavation of rela-
0.075
'
tively low densitycrustalmaterial(2.8 g/cm3) by the impact
t
500
'
I
'
I
600
'
700
t
800
'
I
900
'
1000
Wavelength (rim)
Figure 8. A comparisonbetweenreflectancevaluesof swirl
and nonswirlmaterial, on both highlandsand mare lithologies.
Data were collectedfrom the 415,750, and 950 nm wavelengths
from the Clementinedata set. Symbol size approximatesthe
error.
material. Both the scouringof the surfaceby a recentcometary
impact[Srnkaand Schultz,1979; Schultzand Srnka, 1980] and
ionic solar wind shielding by associatedmagnetic anomalies
[Hood and Williams, 1989] are consistent
with this modeof formation. Clementinc band ratios (415/750 and 750/950) yield
higher valuesfor the swirl materialthan the nonswirl material;
similarhigh valuesare recordedfor the brightCopernicancrater
raysof GiordanoBruno. Figure8 showsthat the regolithsuperposedby swirl material is -62% higher in reflectivity than the
same unit without
swirl material.
Mare basalt within
the swirl
materialhas a relativelyflatter or "bluet" continuumslopeand
deeper950 nm absorptionbandthan marebasaltoutsidethe swirl
material(Figure 8). Similarobservations
weremadeby Bell and
Hawke [1981] for the ReinerGammaFormationand were interpretedas signsof an immatureregolith,free of spaceweathering
effects. In addition,the iron map (Plate 1) showsthat the swirl
materialhas the same compositionas the backgroundmaterial
they lie upon(the methodfor calculatingiron abundance[Lucey
et al., 1998;Blewettet al., 1997] decouplesthe effectscausedby
albedoand soil maturitydifferences). These findingsare consistentwith data from Adams and McCord [1971], who compared Apollo 12 surfacesampleswith core tube samplestaken
from 20 cm below the surface.Material from 20 cm depthhada
40% higher reflectivity than the surface fines. Adams and
McCord [1973] also observeda negative correlationbetween
agglutinatecontentof Apollo 16 soilsandalbedo.
8. East Limb Gravity
In both the Clementinc[Zuberet al., 1994] and Lunar Prospector[Konoplivet al., 1998] gravity data, Smythii basinexhibits a strong(>500 mGals) Bouguergravity anomaly(Figure 7).
The
4231
Bouguergravity and thin basalticfill are notedfor the Nectaris
andOrientalebasins[Head, 1974, 1976]. Thereforewe conclude
that a submaremasscontributionis requiredto match the observedpositiveBouguergravity data over Smythii. The exis-
0.15
400
LIMB
with a densityof 3.3 g/cm3. Similarassociations
of largepositive
0.175
0.05
EASTERN
subsurface concentration
of mass or "mascon"
beneath
Smythii is asymmetricand displacedtowardthe easternpart of
the basin. If this masconwasthe soleconsequence
of the mare
basalt,thenthe greatestgravityreadingswould correlatewith the
thickestbasaltdepositsin Mare Smythii. Instead,the greatest
event, followed by a rising plug of greaterdensitymantle(3.3
g/cm3)to compensate
isostatically
for theexcavated
crustalmaterial [Wiseand Yates,1970;PhillipsandDvorak, 1981].
9. Geologic Evolution of the Eastern Limb
Region
Iron values for the centralpeaksof large cratersand for the
inner ring and ejecta of the Smythii basin show that the bulk
compositionof the easternlimb highlandsis uniform both laterally and vertically. Thereforeelevatedlevelsof iron and diminished levels of aluminum along the western boundaryof the
Smythii basincannotbe explainedby an impactinto the chemical heterogeneous
target. Instead,basaltdepositsto the west of
Smythii basin have been mixed with surroundinghighlandsby
meteoritechurningof the lunarregolith.
Mare Marginishasundergonemultipleepisodesof volcanism,
from early Imbrian to mid-Eratosthenian[Boyce and Johnson,
1978]. The differencein age betweenthe different units is detected by the greatercrater density of one unit over the other.
The chemicalcompositions
of the flows are similar,which suggeststhat they originatedfrom a similar sourcematerial. Also,
the lack of volcanicrilles, domes,and flow frontssuggests
that
botheruptiveeventsoccurredas flood-typevolcanism.
The marebasaltwithin the Smythiibasinis amongthe youngest on the Moon. There are two mechanismsfor the emplacement of basaltinto the Smythii basin. Plains-typevolcanismis
notedby the occurrenceof rilles and pyroclasticdepositsfound
among elevated topographyand within floor-fracturedcraters.
The smooth surface of the main mare deposit indicatesthese
units were depositedby highly effusive, flood-type volcanism.
The early stagesof basin filling were low-volume,plains-type
volcanism,andlatereruptiveemplacements
were floodbasalts.
Basalticmaterialwas not emplacedto an approximatehydrostaticlevel during mare volcanismin the easternlimb region. In
contrast,ToksOzand Solomon[1973] suggestthat thickeningof
the lunar lithospherewith time resultedin an increasein source
depth;thus basinsfilled to successively
higherlevels[Lucchitta
and Boyce, 1979]. In the Marginis-Smythiiregion, basaltshave
not been emplacedto a commontopographiclevel. El-Baz and
Wilhelms[1975] noted a depositat 73øE, 9øN, outsideof Mare
Smythii,that was 3 km higherin elevationthan the Smythiideposits. Even within Smythii alone,basaltsdo not reachan average hydrostaticlevel. The main maredepositlies between-4.6
and -4.2 km below the mean lunar radius(Figure 4). Basaltin
the cratersKiess,Widmannst•itten,
Kao, andHelmeftis only -2.8
4.232
GILLIS
AND
SPUDIS:
GEOLOGY
OF THE MOON'S
km below the mean datum. Thus we conclude that there is no
EASTERN
LIMB
quenceof an uplift of densermantle material in responseisostatic
correlation
betweenageof volcanicdeposits
andtheirelevation adjustmentsas a resultof craterexcavation.
for easternlimb of the Moon. There is a difference of 2 km be-
tweenthe surfacelevel of youngbasaltin Mare Smythii(-4.4
kin) andthe olderbasaltsin MareMarginis(-2.55 km). Thus
premare
topography
is the mainfactorcontrolling
the levelof
Acknowledgments. We would like to thank Ben Bussey, John
Guest, and internal reviewers Brad JollitY, Randy Korotev, and Ryan
Zeigler for their insightful comments and suggestionsthat helped to
improvethis manuscript. This paper is LPI contribution985. This researchwas supportedin part by the National Aeronauticsand Space
Administration Graduate Research Program and grant NAG5-6784
(BLJ).
marebasaltemplacement.
Basins with thin basalt flows and large positive gravity
anomalies(i.e., Smythii, Nectaris,and Orientale)are evidence
thatmasconsareprimarilythe resultof relief on the lunarmantle.
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J.J.Gillis,Departmentof EarthandPlanetarySciences,
Washington
University,CampusBox 1169. One BrookingsDr., St. Louis,MO
63130.(gillis•levee.wustl.edu)
P. D. Spudis.LunarandPlanetary.
Institute.Houston,TX 77058.
Scientific
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(ReceivedJune9, 1999; revisedOctober4. 1999;
acceptedOctober12, 1999.)