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Advances in Space Research 42 (2008) 275–280

Exploring links between crater floor mineralogy and layered lunar crust

Deepak Dhingra *

Planetary Sciences & Exploration Programme (PLANEX), Physical Research Laboratory, Ahmedabad 380009, India

Received 3 January 2007; received in revised form 19 June 2007; accepted 20 October 2007

Abstract

Floors of similar sized craters, representing material from similar depth horizons, have been studied to explore their suitability asmineralogy indicators at various depths within the lunar crust. Clementine UV–vis multispectral data was used to generate mineral abun-dance maps of crater floors and surroundings using a modified version of algorithm given by Pieters et al. (2001) [Pieters, C.M., Head,J.W., Gaddis, L., Jolliff, B. and Duke, M. Rock types of the south pole aitken basin and extent of basaltic volcanism, JGR (106) E11,28001–28022, 2001.]. Substantial processing of the crater floor material due to variety of geological processes is evident in the generatedmaps, making straight forward interpretations difficult. However, systematic compositional trends in fresh craters on the floors of targetcraters seem to indicate the feasibility of such an effort.� 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Moon; Remote sensing; Magma ocean; Crater mineralogy; Geological evolution

1. Introduction

Global mineralogical layering is envisaged on the Moonas a result of lunar magma ocean. The occurrence of anor-thositic crust, mafic mantle and a possible core representthe broad sub-divisions of such a layered set-up. Further,numerous evidences have been put forward to show thatthe crust is stratified (Wieczorek and Zuber, 2001). Studieshave revealed that the lunar crust – ranging in thicknessfrom 60 to 120 km – shows enormous heterogeneity incomposition (Fig. 1; Pieters, 1982, 1991; Tompkins andPieters, 1999; Wieczorek and Zuber, 2001). The mineralog-ical variation within the crust occurs both spatially and ver-tically. The variation with depth can be understood partly,within the framework of crystallization sequence in thelunar magma ocean. The spatial variation resulting fromvarious geological processes active over the Moon in thepast is however, difficult to discern with certainty. Largescale events like impact cratering (including the late heavy

0273-1177/$34.00 � 2007 COSPAR. Published by Elsevier Ltd. All rights rese

doi:10.1016/j.asr.2007.10.024

* Tel.: +91 9879803160.E-mail addresses: deepakd@prl.res.in, deepdpes@yahoo.co.in.

bombardment), volcanism and space weathering have con-tributed to these variations.

Global mineralogical studies carried out on centralpeaks of impact craters (Tompkins and Pieters, 1999) havealready made available a valuable dataset for inferring thevariability of mineralogy within the lunar crust. Centralpeaks expose material from great depths and representone of the most immature surfaces on the Moon as well.Both these facts make them strong scientific probes forstudying compositional variation with depth. Some system-atic trends in mineralogy of central peaks have alreadybeen reported (Pieters and Tompkins, 1999).

2. Scope of the present study

The present study aims at exploring whether lunar sub-surface mineralogy can be characterized by studying ‘craterfloors’ which represent various excavation depths and toevaluate if a unifying mineralogical character for eachdepth can be defined on a lunar wide scale. Dependingon the degree to which global mineralogical trends withdepth are preserved and observed, various evolutionaryaspects of magma ocean crystallization can be explored.

rved.

Table 1Details of craters selected for the present study

Name Location Diameter (km) Age

Bredikhin 17.3�N 158.2�W 59 NectarianLebedinkskiy 8.3�N 164.3�W 62 ImbrianRaimond 14.6�N 159.3�W 70 ImbrianKonstantinov 19.8�N 158.4�E 66 NectarianSanford 32.6�N 138.9�E 55 NectarianPoynting 18.1�N 133.4�W 128 ImbrianFitzgerald 27.5�N 171.7�W 110 NectarianAnderson 15.8�N 171.0�E 109 NectarianVavilov 0.8�S 137.9�W 98 CopernicanChaplygin 6.2�S 150.9�E 137 NectarianFersman 18.7�N 126�W 151 ImbrianMendel 48.8�S 109.4�W 138 ImbrianKeeler 10.2�S 161.9�E 160 Imbrian

Fig. 1. Telescopic reflectance spectra of selected locations on Moon indicatewide variations in mineral composition of the crust (Pieters, 1989).

276 D. Dhingra / Advances in Space Research 42 (2008) 275–280

The occurrence of similar mineralogy at similar depth atlocal, regional or global extent, if established, would indi-cate the geographic scale at which uniform layering mighthave taken place. This may be extrapolated through mod-eling to infer mineralogy at depths that are not accessibledirectly. Such information would certainly be useful inthe context of determining the extent of magma ocean.The depth of magma ocean is not well constrained withpresent estimates ranging between 250 and 1000 km (Van-iman et al., 1991).

Study of crater floor mineralogy might offer certainadvantages over central peaks. The limited spatial extentof central peaks may make them less representative of sub-surface lithology. They occupy a very small part of thetotal excavated area in a crater. It is important from geo-logical perspective since an anomalous composition seenin central peak might simply be an exposure of localizedheterogeneity (Pieters and Wilhems, 1985; Pieters, 1991).Crater floors on the other hand, expose a much larger areathat can be taken as representative of the excavated depth,provided they are not altered significantly.

Further, the occurrence of floor in craters of all sizesincreases the number of sampling points representing vari-ous depths in the lunar crust. In contrast, central peaks arelimited to craters of certain diameter range only.

Crater floors have not been widely explored as a proxyfor subsurface composition though some work has beendone (Heather and Dunkin, 2003). Subsequent events likeoccurrence of impact melts, affect the original composi-tional setting. Depending on the cratering dynamics andtarget lithology, impact melt can cover portions of the cra-ter floor and the surroundings or may attain a sizable vol-ume, ruling out accessibility to the crater floor. Bothphotogeological and spectroscopic criteria (Smrekar andPieters, 1985; Howard and Wilshire, 1973; Hawke et al.,1979) have been used to identify impact melts. However,an unambiguous identification has been difficult at theavailable resolution in many cases. Another aspect is alter-ation of the original composition by ejecta from later cra-tering events. The present study aims at looking into suchproblems and exploring possible solutions.

3. Dataset

The dataset comprise of 5-band UV–vis spectra forselected craters acquired by the Clementine mission. Spec-tral data was obtained through USGS Astrogeology Map-A-Planet website (http://pdsmaps.wr.usgs.gov/PDS/pub-lic/explorer/html/moonpick.htm). Three crater diametersviz. �60 km, �110 km and �150 km were selected forour studies at spatial resolution of 500 m/pixel. Their floorssample depths of about 6, 11 and 15 km, respectively. Twobroad criteria were used to make the initial selection of cra-ters: relatively simple stratigraphy and spatially uniformdistribution of the dataset as far as possible. All theselected craters are from the lunar far side since mare-fillinghas complicated the stratigraphy on the near side. Thebroad criteria were aimed at generating a dataset that issuitable for regional/global studies. Coupled to this, thethree crater sizes were selected to see the general trendand accordingly modify the dataset depending on theobtained results. The selection of individual craters wascarried out by examining data from lunar orbiter images,virtual Moon atlas (http://astrosurf.com/avl/UK_in-dex.html) and clementine images. Initially, a set of 25 cra-ters was selected which gradually reduced to 13 due to oneof the following factors: substantial ejecta from surround-ing craters, poor data availability.

Based on these criteria, the selection was optimized. Theselected craters, their location, diameters and age are listedin Table 1. The geographic spread of craters is illustratedin Fig. 2

4. Data analysis

Mineralogical maps were generated for all the selectedcraters using various spectral ratios representative of par-ticular minerals (Fig. 3) following a modified version ofthe approach described by Pieters et al. (2001). Specifically,the band curvature parameter was redefined in terms of adifferent spectral ratio combination. As compared to themeasurement of angular variation in curvature defined by750–900–1000 nm spectral reflectance trend, an additivecombination of spectral ratios 750 nm/900 nm and

Fig. 2. Selected craters for the present study and their geographical spread.

Fig. 3. Spectrum of common lunar minerals as seen through clementineUV–vis camera (Lucey, 2004).

D. Dhingra / Advances in Space Research 42 (2008) 275–280 277

1000 nm/900 nm was used. It is much easier to calculate ascompared to measuring angle subtended by 750–900–1000 nm reflectance trend. At the same time, the modifiedparameter is able to capture the variation in similar wayas indicated by the angular measurements (Pieters, per-sonal communication). For band tilt, 900 nm/1000 nm

reflectance ratio was used as it gives better results than900–1000 nm reflectance. The author therefore adoptedthe modified ratio combination for his work.

The spectral ratios used and the classification of miner-alogy are given in Table 2 following (Pieters et al., 2001).The qualitative abundance maps representing anorthosite(and mature soils), low-Ca pyroxene and high-Ca pyrox-ene/olivine have been colour coded for better visualisation.

5. Discussion

The obtained mineral abundance maps provide differentperspectives for the selected dataset. The anorthosite andolivine/high-Ca pyroxene maps capture mineralogical dif-ferences across the terrain. Fresh ejecta from younger cra-ters could be identified and their extent could be mapped.Based on this information, it was noted that large scalematerial processing occurring on the crater floors makesit difficult to have straightforward interpretations. Attimes, the degree of modification is such that it is not pos-sible to even discern the crater boundary. Comparisons ofmineral abundance maps were made across floors of craters

Table 2Algorithm used for determining dominant lithologies on lunar surface

Parameter Band strength Band curvature Band tilt

Wavelength combination used 1000 nm/750 nm (750/900 + 1000/900) nm 900 nm/1000 nmOlivine Low Medium + low HighestLow-Ca pyroxene Mild High + highest LowestHigh-Ca pyroxene Low High + low MediumAnorthosite Highest Lowest + low Low

Modified from Pieters et al. (2001).

278 D. Dhingra / Advances in Space Research 42 (2008) 275–280

of different sizes. However, at the resolution of the dataset,any systematic variation in composition linked to changingdepth was not seen. Interestingly, abundance maps for low-Ca pyroxene did not show any specific trend among thecrater floors studied. This could possibly be due to twomajor reasons. Firstly, the extremely noisy nature of theratio maps obtained for low-Ca pyroxene. Secondly, thevariation in the abundance of low-Ca pyroxene acrossthe lunar surface is relatively small (0–30 wt%; Lucey,2004) and therefore may not show up on abundance mapsunless there are contrasting compositions like in case ofcentral peaks of crater Bhabha and its surroundings.Therefore, low-Ca pyroxene abundance maps were notused for making interpretations.

At the same time, ‘‘radial mineralogical trends’’ wereseen in some of the fresh craters on the floors of selectedcraters. The anorthosite content was seen to decrease radi-

Fig. 4. Radial trends seen in case of a fresh crater (inset) on the floor of cr

ally with depth in case of a fresh crater on the floor of cra-ter Keeler (Fig. 4). The same crater when analysed forolivine/high-Ca pyroxene abundance, showed radialincrease with depth (Fig. 5) possibly, indicating a trend inmineralogy with depth. Such trends have been seen in acouple of other craters as well e.g. Konstantinov. The freshcraters have a diameter of about 3 km in case of cratersKeeler and Konstantinov. They must have therefore exca-vated material from a depth of about 300 m. The system-atic nature of radial trends in terms of composition seemsmore likely to be the crater floor rather than any impactmelt component which should be more chaotic in nature.However, further work guided by the obtained results isrequired to understand these trends better. Some work inthis regard has recently been done (Dhingra, 2007).

The obtained results provide two important inputs forfurther studies. It indicates the possibility of retrieving ori-

ater Keeler. The anorthositic content shows a decrease towards center.

Fig. 5. Radial trend observed for the fresh crater (inset) on the floor of Keeler on olivine/high-Ca pyroxene map. Though noisy, it is still discernible thatabundance is increasing inwards.

D. Dhingra / Advances in Space Research 42 (2008) 275–280 279

ginal composition of crater floors through a study of themineralogy of later forming small fresh craters present onmain crater floor. Further, one needs to select craters withlarge number of small fresh craters on floors to obtain arealistic estimate of floor composition.

6. Conclusions

1. There are no specific mineralogical trends discernablefor crater floors representing various depths based onthe present dataset. A larger, more constrained datasetwith better spectral and spatial resolution might berequired to make conclusive interpretations.

2. Some of the fresh craters on the floors of selected cratersshow a radial compositional trend. For example, in caseof crater Keeler, a reduction in anorthosite compositionwith depth is seen. For the same crater, an increase inolivine/high-Ca pyroxene content with depth is seen,though it does not have the same degree of clarity. Thesequalitative estimates for isolated cases need to besubstantiated.

3. The above observations suggest that similar trends mightbe present for the crater floors as well. The subduedtopography of many of the selected craters indicate thepossibility that such features got obliterated with time.

7. Further work

It is planned to work with a larger dataset using otherapproaches for retrieving mineralogy from spectral reflec-tance. The criteria for selecting craters will include matu-rity index, impact melt detection, indigenous materialestimation as also number of fresh craters on the craterfloor. New set of high spatial and spectral resolutiondata with broad spectral coverage is expected fromupcoming lunar missions like Chandrayaan-1, Change-1, SELENE, etc. in near future. It is expected that quan-titative estimation of mineral abundance will be possiblewhich would greatly enhance the interpretability of theavailable data.

Acknowledgments

The author thanks the reviewers for their comments.Discussions with Prof. Carle Pieters and Dr. StephanieTompkins were very useful in improving the manu-script. Their help is gratefully acknowledged. I alsothank Prof. J.N. Goswami for encouragement to carryout this work. Acknowledgments are due to USGSAstrogeology for making data freely available on theweb.

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