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Advances in Space Research 42 (2008) 281–284

Titanium estimates of the central peaks of lunar craters: Implicationsfor sub-surface lithology of moon

Neeraj Srivastava *

Planetary Sciences and Exploration Program, Physical Research Laboratory, Ahmedabad 380 009, India

Received 7 January 2007; received in revised form 30 October 2007; accepted 1 November 2007

Abstract

Central peaks of 24 lunar craters, having mafic rocks, were studied to estimate their average titanium content and infer the nature ofthe subsurface lithologies. Titanium contents were derived from Clementine UV–Vis data (415, 750 nm) following the approach of Luceyet al. [Lucey, P.G., Blewett, D.T. and Jolliff, B.L., Lunar iron and titanium abundance algorithms based on final processing of Clem-entine ultraviolet–visible images, J. Geophys. Res.106 (E8), 20297–20,305, 2000]. TiO2 content exceeding 1 wt% suggests presence ofmantle derived mafic sub-surface rock types (plutonic/volcanic) within the central peaks. Even though, the algorithm used for derivingtitanium content is susceptible to variation in topography and sun angle, especially at higher latitudes, careful selection and analyses ofdata for regions within the central peaks revealed compositional heterogeneities. The results indicate a preponderance of mafic lithologieswith low TiO2 content (<1 wt%) in the central peaks of lunar craters populating the equatorial region. Average titanium content of cen-tral peaks can serve as a useful tracer for distinguishing mantle derived mafic subsurface lithologies from those formed during globalmagma ocean episode.� 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Moon; Central peaks; Craters; Mafic rocks; Titanium concentration; Clementine UV–Vis data

1. Introduction

Geochemical and mineralogical study of lunar rocks is aprime objective of the lunar missions as it provides usefulinformation to further our understanding of the originand evolution of the moon both in space and time. A studyof events, such as, the occurrence of global magma oceanduring the early history of the moon or the partial meltingof the lunar mantle, requires knowledge of the composi-tional makeup of the lunar surface as well as its interior.A systematic study of possible heterogeneities within thelunar crust and, the character, scale, and abundance ofembedded plutons on a global scale is very important inthis regard (Pieters, 1991).

In the absence of sufficient and reliable geophysical mea-surements for the whole moon, the nature of the internal

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* Tel.: +91 7926314407; fax: +91 7926314900.E-mail address: sneeraj@prl.res.in

structure of the Moon is still being debated. Models ofcrustal evolution were proposed based on compositionaldata of lunar samples, and limited geophysical and remotesensing data from Apollo and Luna missions (Warren,1985; Wood et al., 1970). Analyses of lunar meteorites fur-ther enlarged the database (Warren, 1994) and led torefinement of the existing models (Korotev et al., 2003a).The remote sensing missions, Clementine and Lunar pros-pector, have produced global compositional maps of thelunar surface and provided significant new insights intothe internal structure of the moon (Tompkins and Pieters,1999; Wieczorek and Philips, 2000).

Impact craters on the lunar surface are characterized bysystematic morphological features. They provide an oppor-tunity to scan the litho-stratigraphic sequence at a particu-lar location through remote sensing techniques. Centralpeaks and the crater walls are units of an impact craterwhere subsurface lithology is exposed. These have beenused for assessing the variation of rock types on moon

rved.

282 N. Srivastava / Advances in Space Research 42 (2008) 281–284

(Tompkins and Pieters, 1999; Wieczorek and Zuber, 2001)both across spatial and depth scales. Compositional map-ping of these geomorphologic features is essential to under-stand the processes involved in the genesis of the associatedrock types. Mg, Fe and Ti are useful tracers in this contextas their concentrations allow us to discriminate variouslithologies present in these units. Among these, the tita-nium content has been used to classify mare basalt intovery low (<1 wt% TiO2), low (1–4 wt% TiO2), intermediate(4–8 wt% TiO2) and high (>8 wt% TiO2) titanium basalt(Wieczorek et al., 2006).

Lunar rock types enriched in titanium, such as some ofthe basalts and gabbros, are products of volcanic and plu-tonic activity, respectively. They are manifestation of par-tial melting in the mantle source regions and upwardascent of the resulting magma through faults and fissures.Ilmenite ðFeTiO3Þ, the principle carrier of titanium in theserock types is expected to have formed during the last stageof crystallization in the envisaged global magma oceanalong with other incompatible elements (Warren, 1990).Thus, enrichment of titanium in any rock type suggeststhe source rocks to be of mantle origin. Spectral effects oftitanium on lunar reflectance spectra in the UV–Vis regionhave been modeled to derive titanium abundance in vari-ous lunar rock types (Lucey et al., 2000).

Table 1Inferred TiO2 content and standard deviation of central peaks of 24 lunar cra

Crater Location Diameter (km) Peak lithologi

List of craters with (average TiO2 + r)cp < 1 wt%

King 0.6�N, 120.5� 77 AGN, GNTAJackson 22�N, 197� 71 GNTA1, GNVavilov 01.5�S, 221� 99 A, GNTA1, GCrookes 10.5�S, 195� 50 GNTA1, GNCopernicus 10�N, 340� 98 GNTA1, GNTsiolkovsky 20�S, 129� 185 A, GNTA1, GTheophilus 11.5�S, 26� 100 A, GNTA1, GBurg 45.5�N, 28.5� 40 GNTA2, ANFizeau 58.5�S, 224.5� 110 GNTA2, ANAristoteles 50�N, 17� 87 A, GNTA2, ABullialdus 20.7�S, 337.8� 61 GNTA1, GNTMaunder 14.5�S, 266� 55 GNTA1, GNTOhm 18�N, 246� 64 GNTA1, GNTLowel 13�S, 257� 66 GNTA1, GNTStevinus 33�S, 54.5� 75 GNTA2, AG,Langmuir 36.5�S, 231� 85 A, GNTA1, GBhabha 55.5�S, 195� 78 AGN, AN, G

List of craters with (average TiO2 + r)cp > 1 wt%

Eratosthenes 14.5�N, 348.5� 58 A, GNTA1, GScaliger 27�S, 109� 78 A, GNTA1, GBirkeland 30�S, 174� 85 GNTA2, AN,Finsen 42.5�S, 181.5� 87 GNTA2, GNWhite 44.6�S, 158.3� 39 GNTA2, AG,Tycho 43�S, 349� 85 GNTA2, AG,Zucchius 61�S, 310� 64 GNTA1, GN

a Rock type abbreviations: A, anorthosite; GNTA1, gabbroic–noritic–troctroctolitic anorthosite with 80–85% plagioclase; AN, anorthositic norite; AGtroctolite; N, norite; G, gabbro; T, troctolite; GN, gabbroic–norite.

b Uncertainty in TiO2 content due to topography is �50% for craters withincontent are lower limits due to exclusion of dark anti-sun slopes and shadows

In this study, an attempt has been made to classify themafic sub-surface lithologies exposed in the central peaksstudied earlier (Tompkins and Pieters, 1999) on the basisof their average titanium content (Table 1). The maficrocks include Gabbro (G), Anorthositic Gabbro (AG),Anorthositic Gabbro Norite (AGN), Gabbro Norite(GN), Norite (N), Anorthositic Norite (AN) and Anortho-sitic Troctolite (AT) (Tompkins and Pieters, 1999).

2. Analytical approach

We have used processed UV–Vis multi-spectral Clemen-tine data obtained at a spatial resolution of �500 m(source: USGS website PDS Map a Planet) to derive aver-age titanium content of the central peaks. Estimate of TiO2

(wt%) is obtained by using standard relations proposed byLucey et al. (2000) based on comparison of Clementinereflectance data for the Apollo and Luna landing sites withmeasured titanium concentration of corresponding lunarsamples. These relations, that also take into account theeffect of sample maturity, are given by:

hTi ¼ arctanf½ðR415=R750Þ � y0TiÞ�=½R750 � x0Ti�gwt%TiO2 ¼ 3:708� ½hTi�5:979

ters

esa after Tompkins and Pieters (1999) TiO2 wt% of Central peaksb

Average value(r)

2 0.29 (0.09)TA2,AGN 0.18 (0.11)

NTA2, AGN 0.27 (0.08)TA2, AT 0.35 (0.09)TA2, AT 0.22 (0.13)

NTA2, AT 0.46 (0.19)NTA2, AT 0.30 (0.10)

0.44 (0.16)0.19 (0.12)

N 0.47 (0.23)A2, AN, N 0.47 (0.24)A2, AG 0.60 (0.22)A2, AG 0.43 (0.11)A2, AG 0.44 (0.14)AGN, AN 0.36 (0.17)NTA2, AGN 0.47 (0.21)

N, G 0.43 (0.23)

NTA2, AGN 0.93 (0.67)NTA2,AG 0.70 (0.35)GN 1.22 (0.30)

, N,AGN, AN 0.71 (0.35)AN, GN 1.14 (0.40)AGN, G 0.56 (0.71)

TA2, AG,G 0.43 (1.05)

tolitic anorthosite with 85–90% plagioclase; GNTA2, gabbroic–noritic–N, anorthositic gabbronorite; AG, anorthositic gabbro; AT, anothositic

±20� latitude (Jolliff, 1999). For craters at higher latitudes inferred TiO2

in the analyses.

N. Srivastava / Advances in Space Research 42 (2008) 281–284 283

Here, hTi is titanium sensitive spectral parameter andR415;R750 correspond to reflectance values at wavelengths415 nm and 750 nm. The x0Ti and y0Ti are the co-ordinatesof the optimized origin on a plot of R415=R750 vs R750 withvalues of 0.0 and 0.42, respectively.

The reflectance values depend upon sun angle, localtopography (slope) and sun facing and anti-sun geometry.Determination of absolute TiO2 content at high resolution(�200 m) for areas with significant local topographic vari-ation within the central peaks is thus constrained by lack ofaccurate reflectance measurements and the absence of highresolution topographic data (Lucey et al., 1998; Jolliff,1999; Robinson and Jolliff, 2002). The extent of topogra-phy related errors are also sensitive to latitudinal variations(Lucey et al., 1998) as this leads to change in sun angle. Anattempt has been made in this study to take into accountthese aspects while inferring compositional informationof the central peaks based on their average titaniumconcentration.

Since the exact scale of topography variation is notknown for the central peaks and the high resolution(�200 m) data can have significant topography relatedeffects (Lucey et al., 1998; Jolliff, 1999; Robinson and Jol-liff, 2002), a spatial resolution of �500 m was chosen in thisstudy. Twelve out of 24 central peaks studied in this workare located within ±22� latitude, and the Clementineimages in this region were acquired at a phase angles(<30�), were the effect of topography on reflectance valuesis not severe. Thus, the estimates of average titanium con-tent derived for central peaks, particularly in the equatorialregion is expected to be least affected by topography. Jolliff(1999) reported that an underestimation of 57% is expectedfor a steep sun facing slope of 30� at �20� latitude (Apollo17 landing site). If we consider all the slopes within the cen-tral peak area to be 30� and sun facing, we expect an under-estimation of titanium content by �50% in the equatorialregion.

At higher latitudes (>±22�), the phase angles are high.This results in deep shadows and enhances the effect oftopography on reflectance values. The low sun angle athigher latitudes makes topography of the central peaksprominent and helped in visually discarding dark anti-sunslopes and areas under deep shadow. The average concen-tration thus obtained for central peaks at higher latitudesmay be underestimated due to rejection of dark anti-sunslopes and an apparent increase of sun facing slopes inthe analysis. Therefore, for central peaks away from theequatorial region we primarily considered those caseswhere TiO2 content is >1 wt%.

Average TiO2 concentration much lower than 1 wt%may imply low relative abundance of titanium enrichedlithologies within the central peaks (cp) or underestimationof titanium content. We have based our analyses on (aver-age TiO2 + r)cp to reduce this ambiguity. Care was alsotaken to avoid contamination from proximity to marebasalt regions, especially, for craters within mare basinssuch as Eratosthenes, Campanus, Bullialdus, etc. and for

those having their floor filled with mare basalt (e.g.,Tsiolskovsky).

3. Results and discussion

Average TiO2 content obtained for central peaks withmafic lithologies in twenty four lunar craters, located bothin basins and highland are given in Table 1. Out of these,seventeen show (average TiO2 + r)cp significantly <1wt%. Low titanium content expected in the central peakswith Anorthositic Troctolite as the dominant mafic lithol-ogy is consistent with the inferred values. The higher con-centration of titanium found in the central peaks of sevencraters (see Table 1), even allowing for all the uncertaintiesdiscussed in Section 2, indicate sampling of significant pro-portions of mantle derived subsurface mafic lithologieswith >1 wt% TiO2. In the Scaliger crater, high concentra-tion of titanium is reflected in the data for the ejecta, craterwall as well as central peak. This suggests the possibility ofan impact with a vertically extensive gabbroic/basaltic sub-surface lithology. We note that an earlier study (Tompkinset al., 1999) has suggested a similar formation environmentfor the Tycho crater.

Higher concentration of titanium is also found in thecentral peaks of Birkeland, Finsen and White, locatedwithin South Pole Aitken Basin. This may be due to thesampling of significant proportions of mantle or mantlederived plutonic/volcanic rocks by these central peaks.The low concentration of titanium observed in the centralpeaks of crater Bullialdus favors, though not conclusively,sampling of primary mafic lower lunar crustal rocks. Ear-lier studies (Tompkins and Pieters, 1999; Wieczorek andZuber, 2001) have suggested a similar possibility on thebasis of occurrence of this crater within a basin and geo-physically based dual layered crustal thickness model,respectively.

Our studies of latitudinal dependence on distribution oftitanium content in the subsurface lithologies have revealedthat out of the 12 craters studied between 22�N and 22�S,all of them except Eratosthenes, show titanium concentra-tion (average TiO2 + r)cp <1 wt%. On the other hand,central peaks of craters at higher latitudes (up to 61�Sand 50�N) are characterized by varying titaniumconcentrations.

The estimates of titanium concentration in the presentstudy using the algorithm of Lucey et al. (2000), haveyielded significant and consistent results and suggest thepossibility that the subsurface lithologies exposed in thecentral peaks in the equatorial (±22�) belt, is characterizedby low titanium content. However, the possible presence ofimpact melt glasses and grain size variations that can affectthe reflectance data need to be explored before making afirm conclusion. Further, the present study is constrainedby lack of exact topography information and possiblephase angle effects on the reflectance values. Combinedhyperspectral and topography data coupled with labora-tory reflectance spectroscopy are required to resolve these

284 N. Srivastava / Advances in Space Research 42 (2008) 281–284

issues. The Terrain Mapping Camera (TMC) onboardChandrayaan-1 will be providing high resolution (�10 m)topography data for the lunar surface. This coupled withhyperspectral datasets from Hyperspectral Imager (HySI),Moon Mineral Mapper (MMM) and Near-IR Spectrome-ter (SIR-2) onboard this mission will significantly enhanceour understanding of lunar subsurface lithology.

Acknowledgements

I thank Prof. J.N. Goswami, Prof. C.M. Pieters andPeter Isaacson for their comments and valuable suggestions.

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