lunar mare soils: space weathering and the major effects of

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. Ell, PAGES 27,985-27,999, NOVEMBER 25, 2001

Lunar Mare Soils: Space weathering and the major effects of surface-correlated nanophase Fe

Lawrence A. Taylor Planetary Geosciences Institute, University of Tennessee, Knoxville, Tennessee

Carl6 M. Pieters

Geological Sciences, Brown University, Providence, Rhode Island

Lindsay P. Keller, Richard V. Morris, and David S. McKay Earth Science and Solar System Exploration Division, NASA Johnson Space Center, Houston, Texas

Abstract. Lunar soils form the "ground truth" for calibration and modeling of reflectance spectra for quantitative remote sensing. The Lunar Soil Characterization Consortium, a group of lunar sample and remote sensing scientists, has undertaken the extensive task of characterization of lunar soils, with respect to their mineralogical and chemical makeup. This endeavor is aimed at deciphering the effects of space weathering of soils from the Moon, and these results should apply to other airless bodies. Modal abundances and chemistries of minerals and glasses in the <45 gm size fractions of nine selected mare soils have been determined, along with the bulk chemistry of each size fraction, and their Is/FeO values. These data can be addressed at http:/web. utk. edu/-pgi/data.html. As grain size decreases, the bulk composition of each size fraction continuously changes and approaches the composition of the agglutinitic glasses. Past dogma had it that the majority of the nanophase Fe ø resides in the agglutinitic glasses. However, as grain size of a soil decreases, the percentage of the total iron present as nanophase-sizod Fe ø increases dramatically, while the agglutinitic glass content rises only slightly. This is evidence for a large contribution to the Is/FeO values from surface-correlated nanophase Fe ø, particularly 'm the <!0 gm size fraction. This surficial nanophase Fe ø is present largely as vapor-deposited patinas on the surfaces of almost every particle of the mature soils. It is proposed that these vapor-deposited, nanophase Feø-bearing patinas may have far greater effects upon reflectance spectra of mare soils than the agglutinitic Fe ø.

1. Introduction

Remote sensing is a basic tool of planetary exploration, and reflectance spectroscopy has proven to be the major method for remote compositional and mineralogical analyses of planetary surfaces. The foundation for remote mineralogical analysis lies in linking spectral properties of materials measured in the laboratory to well-understood mineral species and their mixtures. The pioneering works by Burns [1970, 1993], Hapke et al. [1970], McCord and Adams [ 1973], McCord et al. [1981 ], and others have demonstrated the potential of spectral reflectance measurements for lunar materials. Pieters [1986, 1993] summa- rized the application of near-infrared spectroscopy for identifica- tion and effective mapping of a variety of lunar rock types on relatively unweathered lunar surfaces.

Unfortunately, accurate estimation of rock and mineral compositions is complicated by the ubiquitous blanketing lunar s0il (<1 cm fraction of the regolith), which comains the cumulative effects of "space weathering," that optically mask mineralogi- cally diagnostic spectral features. As noted by McCord and

Copyright 2001 by the American Geophysical Union.

Paper •umber 2000JE001402. 0148-0227/01/2000JE001402509.00

Adams [1973] and described by Fischer and Pieters [1994], three common optical manifestations of space weathering on lunar materials are the following: (1) overall reduction of reflectance, (2) general attenuation of diagnostic absorption bands, and (3) development of a red-sloped continuum. These effects generally increase with soil maturity (i.e., duration of surface exposure). A principal cause for these variations is believed to be accumulation of nanophase-sized Fe ø particles (native Fe of single-domain size, -3-33 nm) [Housley et al., 1973], which are concentrated in the finest fractions of the lunar soils. Indeed, it is the <45 lam fractions of the lunar soils that are most similar to and appear to dominate the spectral signature of bulk soil [Piet- ers et al., 1993].

The relative concentration of nanophase Fe ø, expressed as Is, normalized to the total iron content of a soil, expressed as FeO, results in the value Is/FeO. It is this value for the <250 lam portion of the soils that is used as the maturity index for lunar soils in general [Morris, 1976, 1978]. The normalization to FeO is necessary because the concentration of nanophase Fe ø is pro- portional to both the duration of surface exposure and the amount of FeO available for autoreduction to Fe ø. By implica- tion the lunar-surface regions mentioned above where reflectance spectra have been most successful in determining mineralogical compositions are those where the effects of space weathering are minimal [Pieter& 1993].

27,985

27,986 TAYLOR ET AL.: LUNAR MARE SOILS

The weathering processes on the Moon, although markedly different from those on Earth, are also representative, to a large extent, of those on relatively small, airless bodies in the Solar System, such as asteroids (e.g., Vesta, Eros) and moons such as Phobos and Deimos [Pieters et al., 2000]. Although the processes may be similar, the intensity and style of this weathering will vary with distance from the Sun, whereby bodies in the Asteroid Belt will not have near the velocities of impacting mi- crometeorites relative to the Earth's Moon at 1 AU. However, understanding the effects of space weathering upon lunar reflectance spectra provides the basis for application of this important remote sensing technique to all airless bodies, in general.

1.1. Rationale for the Present Study

Although bulk soil properties are greatly altered by the effects of space weathering, even mature lunar soils retain weak, yet distinct spectral signatures, commonly of pyroxenes, that are due to the inherent mineralogy of the dominant local lithology. However, it is the <45 gm particle-size domains of the lunar soil that are the most similar to and appear to dominate the spectral signature of the bulk soil [Pieters et al., 1993]. This is partly because fine particles coat larger particles, and photons that

enter large particles are unlikely to escape. Optically, the 10-20 [tm and 20-45 [tm size fractions are the most similar to the bulk soil [Fischer, 1995]. Larger size fractions are not representative of bulk soil properties [Pieters et al., 1993], and the <10 fractions are most sensitive to surface-correlated processes [Fischer, 1995; Noble et al., 2001; Pieters et al., 2000].

The detailed petrographic properties of lunar soils, particularly the finer fractions, are poorly known. Therefore modern techniques are required to characterize soil compositions with the accuracy necessary for integration with spectroscopic analyses. To make a more direct and quantitative link between soil mineralogy and chemistry and soil spectral properties requires acquisition of several data sets. It is imperative to accurately measure and characterize the petrography of lunar soils in terms relevant to remote analyses. This should be coupled with meas- urement of precise reflectance spectra and testing and use of appropriate analytical tools that identify and characterize the spectral effects of individual mineral and glass components (through spectral deconvolution of a mixture). Examples of the preliminary integration of the modai and chemical data, from this present study, with the reflectance spectra are given by No- ble et al. [2000] and Pieters et al. [2001 ].

I Morris I

I Pieters I

L.S. Curator

GMs

Lunar Sample Collection (Pristine Lab)

r <lmm McKay/We.•vo•h I

McKay

Sivin !•m 10•'-20 !•m 2045 !• rn Lunar Sample Curator Each Size Split 5 Ways

50*mg 10 mg 1-2 mg Taylor

MiniPet + Bulk Chem Pieters

Spec. Reft. Morris

FMR Keller

SEM/TEM

Modellinll Pieters & Co.

Figure 1. Flow sheet for lunar sample distribution by the Lunar Sample Curator, at Johnson Space Center, to the Lunar Soil Characterization Consortium.

TAYLOR ET AL.: LUNAR MARE SOILS 27,987

1.2. Lunar Soil Characterization Consortium

The Lunar Soil Characterization Consortium [Taylor et al., 1999] was recently established and consists of lunar-soil and remote-sensing scientists who address the integration of soil mineralogy and chemistry, particularly of the finest size fractions (i.e., <45 I•m), with the reflectance spectral characterization of these same fractions. This paper reports the significant new discoveries and understandings of lunar mare soils that have come from these integrated efforts. All mare soil chemical and physical characterization data presented in this paper and elsewhere can be retrieved on the Web site: http'.//web.utk. edu/ -pgi/data.html.

2. Methodology

The processing and distribution of the soil fractions for use by the several LSCC members is shown in Figure 1. Only "pristine" lunar soils were used for this major endeavor. The logis- tics of the many handlings of the soil splits, the immense amount of documentation, and the enormous quantities of pa- perwork, as well as the preparation of polished grain mounts of the soils, were conducted in good fashion by the Lunar Sample Curatorial personnel at Johnson Space Center (JSC).

2.1. Lunar Samples

Nine lunar mare soils were selected from the Apollo 11, 12, 15, and 17 sample collections based on similarities in chemical compositions and diversity in maturity, as reflected by their Is/FeO values [Morris, 1976]. There are four soils from Apollo 17, two each from Apollo 12 and 15, and the only soil sampled by Apollo 11. Pristine samples of these soils were received from the Planetary Materials Curation facility at Johnson Space Center. The Apollo 17 soils were the subject of a previous paper [Taylor et al., 2000a, 2001], but some of the data are in- cluded here for completeness and comparison.

2.2. Sieving

The nine Apollo mare soils were sieved with triply-distilled water to obtain a <45 •m size fraction. Al'ter removing material for analyses (Figure 1), the <45 •m fraction was further sieved into three finer size ranges: 20-45 •m, 10-20 •m, and <10 •m. Portions of each size separate were distributed for analyses to each laboratory of the members of the Lunar Soil Characteriza- tion Consortium.

2.3. Chemical Analyses

The bulk chemistry of the separated size fractions was determined by electron microprobe analyses of fused-glass beads. This technique is commonly used for determination of bulk compositions of lunar samples, which are particularly suited to this procedure because of their general lack of volatiles [Schuraytz and Ryder, 1990]. While being bathed in dry nitro- gen, a 5-mg portion of each size fraction was fused on a Mo- strip heater. The samples are easily melted with annealing times of 20-30 seconds, and our experience has shown that <0.5 wt% Mo has been determined to dissolve in this melt. Polished

mounts of the fused-glass beads were prepared, and at least 15 EMP analyses with a 20-•tm spot size were performed on each bead.

2.4. Modal and Digital-Imaging Analyses

Since the return of the first lunar samples, standard operating procedure for soil petrography has been to characterize a lunar soil by "particle counting" [e.g., Heiken and McKay, 1974; Simons et al., 1981]. Such analyses provide detailed information about the abundances of mineral and rock fragments, volcanic glasses, impact-produced glasses, and glass-bonded aggregates, called agglutinates. Particle counting simply involves classifying a soil fragment with a title (e.g., pyroxene, basalt, breccia, and agglutinate). These particle count data, however, do not provide information on the real percentages of minerals (modes) locked in rock fragments and fused-soil particles (e.g., agglutinates).

The actual amounts of the various minerals and glasses in the soil that interact with solar radiation are the important input data for the remote compositional analysis and space weathering study. Modal analysis, sensu stricto, is defined as the volme percentage (or calculated wt %) of the mineral constituents, not the particle type. It is essential that accurate quantitative modal analyses of the components of lunar soils be obtained. This is accomplished using the techniques described and illustrated by Taylor et al. [1993, 1996], Chambers et al. [1994], and Higgins et al. [1995]. Accurate modal analyses were performed with an Oxford Instrument Energy Dispersive Spectrometer Unit (EDS) with a Cameca SX-50 electron microprobe. This type of X-ray digital-imaging analyses on grain mounts of lunar soils, using the FeatureScan program of Oxford Instruments, is detailed by Taylor et al. [1996]. In addition to the modes, another program allows us to determine the average chemical composition of each phase (e.g., different types of pyroxene, plagioclase, high-Ti volcanic glass, high-A1 agglutinitic glass).

As part of the initiation of these analyses, the compositional limits of all minerals and glasses, especially of the agglutinitic glasses, are determined by direct analyses of-1000-2000 phases that previously have been optically identified. Each new soil must undergo this type of initial characterization because even subtle differences in chemistry can change the "chemical win- dow" for a mineral or glass. The agglutinitic glasses are all alumina-rich and are distinguished from plagioclase by their >0.3% FeO, from olivine by Al203, etc. Basically, all impact- produced glasses are reported as agglutinitic glass, since the compositions are identical and because essentially all of the impact-produced glasses contain nanophase Fe ø. In the modal values for agglutinitic glass, the other non-agglutinitic, impact glasses actually make up only <<10% of the amount measured. The compositions and modes of several different volcanic glasses are measured as well and collectively reported here as simply "volcanic glass." In addition, ilmenite, Cr-ulv6spinel, and Ti-chromite are also measured as separate mineral species. However, the spinel modes are mostly <2% and are reported here as "others," along with FeNi metal, troilite, K-rich glass (mesostasis), and other minor accessory phases. The details of the actual modes are on our Web site.

Previous studies on these mare soils have involved the 90-

150 •tm size fractions [Taylor et al., 1996]. However, at the <45 •tm grain sizes, many of the agglutinates have lost much of their diagnostic vesicular texture and have been largely broken into their individual mineral and glass components. The amount of this impact-produced glass, with its nanophase Fe ø, is important for evaluation of space weathering. In this study, the modal percentages and average chemical compositions of the different

27,988 TAYLOR ET AL.: LUN• MARE SOILS

Table 1. Modal Abundances of Minerals and Glasses in the Finest Size Fractions of Lunar Mare Soils a Studied by the Lunar Soil Characterization Consortium (LSCC)

10084-78 12030-14 12001-56

20- 10- <10pm 20- 10- <10pm 20- 10- <10pm 45pm 20pm 45pm 20pm 45pm 20pm

Ilmenite 6.4 5.2 5.0 2.6 3.2 3.0 2.6 1.8 1.6 ?lagioclase 16.8 17.1 17.4 15.3 14.0 18.0 13.4 13.9 15.6 Pyroxene 16.0 12.2 8.4 33.8 21.4 15.3 19.9 17.9 13.5 Olivine 1.4 1.1 0.9 4.3 3.7 2.5 3.4 4.2 2.2 Agglutinitic glass b 53.9 57.0 62.6 39.4 49.8 55.0 56.2 56.8 61.9 Volcanic glass 3.4 2.9 3.7 1.2 1.5 1.6 1.5 1.3 1.9 Others 2.1 4.5 2.0 3.4 6.4 4.6 3.0 3.8 3.3

100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

15071-52 15041-94 71061-14


Ilmenite 1.9 1.8 1.2 1.2 0.8 0.7 10.4 9.7 7.6 Plagioclase 18.1 19.4 19.8 15.5 16.2 18.0 13.9 15.2 18.1 Pyroxene 22.1 16.7 10.9 22.5 17.0 5.3 20.8 12.5 8.3 Olivine 3.9 2.8 1.9 3.3 2.4 0.6 3.9 4.5 3.8 Agglutinitic glass 47.6 49.2 59.7 51.3 56.7 70.4 31.4 37.9 45.4 Volcanic glass 4.0 4.1 3.6 2.3 2.6 1.9 18.9 18.8 15.7 Others 2.4 6.0 2.9 3.9 4.3 3.2 0.7 1.3 1.1

100.0 100.0 100.0 100.0 100.0 100.0 100•0 100.0 100.0

71501-35 70181-47 79221-81


Ilmenite 12.3 9.7 7.6 8.9 6.7 3.4 7.3 6.0 5.2 Plagioclase 16.5 19.8 20.0 16.9 18.3 18.5 16.9 16.0 18.6 Pyroxene 21.3 13.7 8.8 15.7 8.5 4.6 13.5 9.7 3.6 Olivine 3.6 3.4 3.2 3.6 3.8 3.2 4.8 3.4 2.2 Agglutinitic glass 38.3 44.8 53.1 43.4 51.7 58.3 46.5 54.3 61.5 Volcanic glass 6.7 7.5 5.9 10.1 9.2 10.3 10.9 9.2 8.0 Others 1.3 1.1 1.5 1.3 1.8 1.7 0.1 1.4 0.8

100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

aMaturity as ls/FeO of the <250 pm fraction [Morris, 1978] is given directly after the soil number, a value commonly used as the reference maturity for an entire soil.

•rhis designation effectively includes all impact-produced glass, the majority (>90 %)of which is agglutinitic glass; these are combined because they have similar compositions and both contain nanophase Fe ø.

mineral and glass phases (e.g., pyroxene, agglutinitic glass) were determined. The phase compositional data are accessible at http://web. utk.edu/-pgi/data. html.

2.5. Ferromagnetic Resonance (FMR) Analyses

The abundances of single-domain, nanophase Fe ø were determined by Ferromagnetic Resonance (FMR) measurements. The FMR values for nanophase Fe ø (designated as Is) in the lunar soil fractions were measured in the Magnetics Lab of R. V. Morris, at Johnson Space Center. This laboratory has exclu- sively performed Is measurements on lunar samples since 1972.

2.6. Spectral Reflectance Measurements

Spectra of all size separates were measured as horizontal, uncovered samples using the RELAB facility at Brown Univer- sity [Pieters, 1983]. A description of this facility can be found at http://www.planetary. brown.edu/relab/index.html. Visible to near-infrared spectra (0.3 to 2.6 gm) were measured at 5 nm sampling resolution using the RELAB bidirectional spectrometer. All measurements were obtained at i = 30 ø and e = 0 ø.

3. Results

3.1. Modal Abundances

The modal percentages of mineral and glass components for all the size fractions of the nine mare soils are given in Table 1 and shown graphically in Figure 2, along with the Is/FeO values for each fraction. The bulk soil maturities, expressed as Is/FeO values for the <250 lam portion of each soil, are given in Morris [1976; 1978]. The bulk soil chemistry for FeO and TiO2 are also given by Figure 2 for additional comparisons.

With studies of lunar-soil formational processes [e.g., Simons et al., 1981; Fischer, 1995], particle abundances are commonly reported, and each agglutinate contains 30-80% glass that binds these soil aggregates together. However, it is the absolute abundance of individual mineral phases and the agglutinitic glass that is important for chemical considerations [Hu and Tay- lor, 1977], as well as spectral reflectance modeling, and these data do not exist in the literature. Modal analyses of the phases in the soils also permit us to address the abundances of nanophase Feø-bearing agglutinitic glass, as a function of grain size.


Low-Ti Mare Soils

15071 -sz* 15041 •* 12030-•4- 12001-se

FeO = 15.4 16.3 17.2 14.3 TiO•= 3.6 1.6 2.8 1.7

'h-Ti Mare Soils

71501-ar 10084-7r 71061-44* 70181-4•* 79221-84.

16.2 17.4 16.4 16.2 15.4

7.3 9.5 8.1 7.3 6.5

8O

6O

4O

2O IslFeO of 250

I Others • VoI.GI.

• Olivine

•-• IIm

ß '"':. • Plag

• Aggl GI

Figure 2. Modal analyses of the finest size fractions of Mare soils. The soils are divided into high- and low-Ti soils for this presentation and further arranged left to right within these two groups from least to most mature. The small number after the sample number (e.g., 12030-14) is the Is/FeO value of the <250 •m fraction [Morris, 1978], commonly referenced as the maturity value for the soil in general. The FeO and TiO2 contents of the soils are shown for comparison. These soils were selected to have similar compositions, as well as contrasting maturities. Note the general increase in agglutinitic- glass and decrease in pyroxene contents as maturity increases between soils, as well as the same trends within a given soil with decrease in grain size.

In the nine mare soils examined, there is a large increase (i.e., >100%) in the agglutinitic glass contents between the 90-150 p.m and the 10-20 p.m size fractions [Taylor et al., 1996, 1999]. As can be readily seen in Figure 2, the concentration of nanophase Fe ø (expressed as IJFeO) increases with overall soil maturity, as does the modal percentage of agglutinitic glass.

Examination of the properties of the size fractions for individual soils reveals interesting and highly significant systematic changes. There is an increase in impact-produced agglutinitic- glass content with decreasing grain size. The modal abundances of crystalline plagioclase are relatively constant to slightly increasing, with decreasing grain size of the fractions. However, the abundances of all other components (pyroxene, oxides, volcanic glass, and olivine) decrease with particle size. This decrease for pyroxene is pronounced and very significant since pyroxene is probably the most optically active of the lunar minerals. An important conclusion is that the abundance of plagioclase increases relative to the mafic minerals with decreasing particle size. These effects would seem to be a function of the agg!utination process and that of selective comminution, ex- perimentally verified by Cintala and HOrz [1992].

Pyroxene is the most spectrally active mineral in lunar soils• and different types of pyroxene each have diagnostic spectral features. In addition to the fact that the reflectance spectra for orthopyroxene and clinopyroxene are distinctly different, the wavelength positions of peaks of absorption bands in pyroxene spectra are a function of chemistry [e.g., Burns, 1993]. There-

fore the modal abundances and chemistry of the pyroxenes were dissected into four chemical groups: Opx, Pig, Mg-Cpx, and Fe- Cpx (Table 2). This was considered essential for integration by modeling with spectral data and to quantitatively assess the character of pyroxene that coexists with the nanophase Feø [e.g., Noble et al., 2000].

3.2. Soil Chemistry

The nine lunar soils studied, representing both high-Ti and low-Ti mare soils, have systematic differences among one another, as well as among the different grain-size fractions of a given soil (Figure 3, Table 3). These permit comparisons of their chemistries, both as individuals and collectively, as a function of different maturities. With decreasing grain size, FeO, MgO, and TiO2 concentrations decrease, and CaO, Na20, and A1203 (plagioclase components) increase for all soils (Figure 4). By comparison with the modal values in Figure 2, these chemical variations parallel increases in agglutinitic glass and decreases in the oxide minerals (mainly ilmenite), pyroxene, oil- vine, and volcanic glass.

3.3. Mineral and Glass Chemistry

The compositions of the individual mineral and glass phases within a soil are relatively constant and independent of grain size. Agglutinitic glass compositions are reported in Figure 4 (see data for all phases at our web site). These analyses repre-


Table 2. Modal Percentages of Four Subsets of Pyroxenes in the Finest Size Fractions of Lunar Mare Soils a Studied by the LSCC a

10084-84 12030-14 12001-56

20- 10- <10pm 20- 10- <10•tm 20- 10- <10pm 45pm 20pm 45•m 20pm 45pm 20pm

Orthopyroxene 0.34 0.61 0.42 3.85 2.86 2.20 0.82 2.24 2.07 Pigeonite 3.94 3.23 1.81 15.20 10.18 7.07 8.77 7.36 6.06 Mg-Clinopyroxene 10.35 7.81 5.96 12.14 6.59 5.18 7.52 7.64 4.79 Fe-Clinopyroxene 1.38 0.57 0.25 2.59 1.75 0.89 1.77 0.70 0.58 Total Pyroxene 16.01 12.22 8.44 33.78 21.38 15.34 19.88 17.94 13.50

15071-52 15041-94 71061-14

20- 10- <10pm 20- 10- <10pm 20- 10- <10pm 45•m 20pm 45•m 20pm 45pm 20[tm


71051-35 70181-47 79221-81

20- 10- <10pm 20- 10- <10pm 20- 10- <10pm 45pm 20pm 45•m 20pm 45pm 20pm


aMamrity as Is/FeO of the <250 gm fraction [Morris, 1978] is given directly after the soil number, a value commonly used as the reference maturity for an entire soil.

•This designation effectively includes all impact-produced glass, the majority (>90 %) of which is agglutinitic glass; these are combined because they have similar compositions and both contain nanophase Fe ø.

ER

Di Hd

1oo84 ! Fe-Cpx 1.4-0.6-0.3

3.9-3.2-1.8

0.3.0.6.0.4 0 igs •x

Fs

Figure 3. Abundances of four pyroxene modes in the finest size fractions of mare soil 10084 (e.g., Opx, Pigs, Mg-Cpx, and Fe-Cpx). Pyroxene is the most spectrally active mineral in lunar soils; differ- em types of pyroxene each have distinctive spectral features. Therefore, it is essential that the modes and chemistry of differera pyroxenes be determined for integration by modeling with spectral data. The three numbers above each envelope represent the modal percentages of this pyroxene, from left to right, of the 20-45 pro, 10-20 pm, and <10 pm size fractions. The envelopes represent an - I sigma range in compositions.


Table 3. Bulk Chemistry and IdFeO Values of the Finest Size Fractions of Lunar Mare Soils Studied by the LSCC a

Sample 10084-78 12030-14 12001-56 Size <45 20- 10- <10 <45 20- 10- <10 <45 20- 10-

pm 45pm 20gm gm gm 45gm 20gm pm gm 45gm 20gm <1o

SiO2 41.7 41.3 41.2 42.1 46.4 46.1 46.3 46.2 45.3 45.3 45.0 46.0 TiO2 7.54 8.30 7.94 7.25 3.23 3.74 3.32 3.01 2.96 3.20 2.96 2.78 A1203 13.1 12.0 13.2 15.9 11.7 10.5 10.7 13.9 12.2 11.0 12.3 14.9 Cr203 0.29 0.30 0.30 0.27 0.42 0.40 0.50 0.43 0.44 0.41 0.46 0.42 MgO 8.12 8.46 7.98 7.20 9.51 9.94 9.86 8.37 10.3 10.6 10.0 8.79 CaO 11.8 11.6 11.8 12.3 10.0 9.09 9.64 10.4 10.1 9.83 10.2 11.2 MnO 0.21 0.21 0.21 0.19 0.21 0.23 0.20 0.19 0.22 0.21 0.23 0.19 FeO 14.8 15.5 14.7 12.0 16.0 17.6 17.2 14.3 16.0 16.9 15.9 12.5

Na20 0.42 0.39 0.43 0.46 0.47 0.41 0.44 0.53 0.38 0.39 0.44 0.51 K:O 0.13 0.12 0.13 0.15 0.29 0.26 0.26 0.35 0.21 0.21 0.23 0.30 P:O5 0.10 0.12 0.11 0.20 0.26 0.19 0.23 0.32 0.22 0.23 0.28 0.24 SO2 0.17 0.17 0.13 0.14 0.12 0.12 0.12 0.15 0.15 0.11 0.18 0.29

Total 98.38 98.50 98.13 98.16 98.61 98.58 98.27 98.14 98.48 98.39 98.18 98.12

Is/FeO 88 67 87 145 20 12 17 32 62 51 67 115

Sample 15071-52 15041-94 71061-14

Size <45 20- 10- <10 <45 20- 10- <10 <45 20- 10-

!.tm 45 !.tm 20 !.tm !.tm !.tm 45 !.tm 20 !.tm !.tm !.tm 45 gm 20 gm <1o

pm

SiO2 45.9 45.8 45.7 46.9 46.4 46.1 46.2 46.6 39.8 39.2 39.5 40.2 TiO2 1.81 2.33 1.88 1.57 1.83 2.03 1.88 1.79 8.76 9.48 8.94 7.89 A1203 13.1 12.4 12.9 17.1 13.5 12.5 13.5 16.4 10.5 9.33 10.8 13.8 Cr203 0.41 0.43 0.53 0.40 0.41 0.39 0.41 0.37 0.48 0.48 0.48 0.44 MgO 11.3 11.4 11.0 9.85 10.8 11.2 10.8 9.37 10.5 10.8 10.4 9.18 CaO 10.3 9.81 10.2 11.8 10.3 9.91 10.2 11.6 9.90 9.58 9.79 10.7 MnO 0.19 0.21 0.22 0.15 0.20 0.20 0.21 0.17 0.24 0.23 0.23 0.20

FeO 14.9 15.6 15.4 9.59 14.2 15.2 14.4 11.0 17.5 18.5 17.5 14.8

Na20 0.37 0.36 0.39 0.48 0.41 0.36 0.41 0.49 0.41 0.34 0.40 0.46 K20 0.13 0.14 0.18 0.22 0.19 0.16 0.18 0.23 0.09 0.07 0.09 0.11 P205 0.18) 0.15 0.19 0.09 0.21 0.19 0.24 0.20 0.06 0.04 0.06 0.05 SO2 0.12 0.10 0.10 0.14 0.13 0.11 0.12 0.11 0.15 0.17 0.20 0.25

Total 98.71 98.73 98.69 98.30 98.58 98.35 98.55 98.35 98.39 98.31 98.25 98.19

Is/FeO 71 49 80 159 93 66 92 161 16 9 14 28

Sample 71501-35 70181-47 79221-81

Size <45 20- 10- <10 <45 20- 10- <10 <45 20- 10-

pm 45 pm 20 pm pm pm 45 pm 20 pm prn pm 45 gm 20 pm <1o

gm

SiO2 39.7 38.4 39.0 40.4 40.8 40.7 40.4 41.5 41.7 40.5 40.9 42.3 TiO2 9.31 10.7 9.83 8.27 7.57 8.11 7.88 6.54 6.39 7.38 7.21 5.83 Al:O3 11.3 9.94 11.6 14.5 12.4 11.5 12.7 15.4 13.5 11.6 12.9 15.9 Cr:O3 0.42 0.46 0.45 0.40 0.42 0.43 0.42 0.39 0.37 0.40 0.40 0.35 MgO 9.73 9.97 9.52 8.76 10.1 10.1 9.97 9.12 10.3 10.9 10.4 9.59 CaO 10.2 9.94 10.1 11.2 10.6 10.3 10.4 11.5 10.8 10.3 10.4 11.7 MnO 0.22 0.24 0.23 0.19 0.21 0.22 0.23 0.18 0.21 0.22 0.20 0.17 FeO 16.5 17.8 16.4 13.5 15.3 16.0 15.5 12.7 14.0 15.8 15.0 11.3

Na20 0.38 0.35 0.39 0.42 0.39 0.35 0.34 0.46 0.41 0.38 0.39 0.49 K:O 0.09 0.07 0.09 0.11 0.08 0.08 0.08 0.13 0.09 0.09 0.10 0.15 P205 0.07 0.07 0.06 0.06 0.07 0.06 0.05 0.10 0.07 0.06 0.07 0.07 SO2 0.16 0.17 0.19 0.25 0.17 0.16 0.15 0.20 0.19 0.17 0.19 0.17

Total 98.09 98.21 97.93 98.11 98.06 98.07 98.15 98.16 98.05 97.85 98.23 98.10

Is/FeO 44 28 50 88 61 53 63 104 91 57 78 169

aThe chemistry was determined by EMP analyses of fused beads of the soil. Values of IJFeO are from FMR Aaalyses. Maturity as IJFeO of the <250 I•m fraction [Morris, 1978] is given directly at•er the soil number, a value commonly used as the reference maturity for an entire soil.


F•

18

16

14

12

10

12

10

8

6

4

2

0.5

0.4

150

IO0

50

Low-Ti Mare Soils Hih-Ti Mare Soils

12030- 4' 15071-52' 71061-14' 70181-47' 79221-8' 12001s* 15041-94' 71501-3* 10084-?8*

,•0 15.4 17.2 16.3 14.3 16.2 17.4 16.4 16.2 15.4 02 3.6 2.8 1.6 1.7 7.3 9.5 8.1 7.3 6.5

_

' CaO 3---ø"'ø . - -

_

ils ' "eO e. • j' .e,,,,e.• j ,• I I I I I I I I I I •B•••I I I'1 I I I I I I I

20-.45 <10 20-.45 <10 20-45 <10 20-.45 <10 20-.45 <10 10-20 •m 10-20 p,m 10-20 p,m 10-20 p,m 10-20

Figure 4. Chemistry of the finest size fractions of lunar mare soils. The soils are divided into high- and low-Ti soils for this presentation and further arranged in left to right order from least to most mature. The small number after the sample number (e.g., 12030-14) is the Is/FeO value of the <250 •m fraction [Morris, 1978], commonly used as the reference maturity value for the entire soil. The data for a given soil are presented from left to right for decreasing grain size of the soil fractions. With decreasing grain size, note the systematic increase in plagioclase components (CaO, A1203, Na20) and decrease in ferro- magnesian-mineral components (FeO, MgO, TiO2). Also, notice the large increases in Is/FeO with decreasing grain size for the individual fractions.

sent >10,000 individual analyses performed during our EMP characterizations of the soil fractions. In contrast to the systematic chemical variations observed in bulk chemistry for different size fractions (Figure 4, Table 3), the agglutinitic glass compositions remain relatively constant with changes in grain size and soil maturity. In fact, changes in the chemical compositions of the soil size fractions are caused mainly by the abundances of the phases (e.g., pyroxene, agglutinitic glass), not by distinct changes in the chemistry of the individual minerals and/or glasses.

3.4. Agglutinitic Glass

It was originally suggested by Hu and Taylor [1977] that the composition of agglutinitic glass in a soil mimics, to large extent, the chemistry of the bulk soil. However, the compositions of the agglutinitic glasses analyzed here, at least for the grain sizes <45 •tm, are more feldspathic than the overall bulk chemistry of the soil. Thus the composition of the agglutinitic glass is a

fair representation of the bulk chemistry plus a feldspathic component

In Figure 5, a comparison is made of the bulk chemistry of each size fraction and the composition of the corresponding agglutinitic glass, for the high-Ti mare soils. The systematic changes in soil composition with decreasing grain size reveal that the bulk chemistry of each size fraction becomes more feldspathic with increasing maturity, with the effect being most pronounced in the finest fractions (Figure 5). Notice that the composition of the agglutinitic glass is relatively invariant and even more feldspathic (i.e., rich in A1203) than even the <10 gm fraction, largely a function of the "Fusion of the Finest Fraction" (F 3) model [Papike et al., 1981 ]. However, the selective impact- produced vaporization of FeO may also play a role, as discussed below.

Thus it would appear that the bulk chemical compositions of the soil fractions, with decreasing grain size, approach the composition of the agglutinitic glasses. This relation not only


o

18

17

16

All Bulk = open symbol All Aggl = filled symbol

15 D 10084-78

14 • 71061-•4 /• 71501-3s

13 O 70181-47 12 •j• 79221

II [' -78,-14,-35,-47,-81: 10 L Is/FeO for <250 ••m ./ , , ,. .,. .•, • ']

9 10 II 12 13 14 15 16 17 18

AI203

Figure 5. Comparisons of changes in the bulk chemistry of the three finest size fractions for the high- Ti soils versus the compositions of their agglutinitic glasses. The small number after the sample number (e.g., 10084-78) is the Is/FeO value of the <250 !am fraction [Morris, 1978]. Note that the bulk compositions of decreasing-size fractions within a given soil converge on the composition of the agglutinitic glass by decreasing FeO and increasing A1203 contents.

strengthens the "fusion of the finest fraction" (F 3) hypothesis [Papike et al., 1981; Walker and Papike, 1981 ], but also high- lights the important role ofplagioclase in the formation of agglutinitic glass. The F 3 model suggests that the selective crushing of basalts enriches the finest fractions of the soil in plagioclase feldspar relative to the olivine/pyroxene components, and that this finest fraction is most readily melted by micrometeorite impact [Cintala and HOrz, 1992].

3.5. Ilmenite

The compositions of the several mineral and glass phases present in the soil fractions are identical for each size fraction of a given soil (Table 4), even for the composition of the agglutinitic glass (Figure 5). However, for all the various fine-size fractions of the five high-Ti mare soils, the compositions of the agglutinitic glass are significantly lower in TiO2 relative to that of the bulk-soil chemistry (compare Tables 3 and 5). This can be seen in Figure 6, which compares the chemistries of the different size fractions to the compositions of the agglutinitic glass, by normalizing all to the bulk chemistry of the total <45 gm fraction. In Figure 6, the relative enrichment of Al203 and depletion of FeO for agglutinitic glass is easily apparent.

This apparently anomalous depletion of TiO2 in the agglutinitic glass would seem t•o indicate that either (1) ilmenite does not readily comminute to the finest grain sizes involved in the formation of the agglutinitic melt or (2) ilmenite that is present in the finest fractions does not undergo the complete melting that the silicate minerals do. The abundance of ilmenite appears to mirror the abundance of mafic minerals (Figure 2, Ti) and is not severely depleted in the finest fraction. This anomalously low TiO2 content in the agglutinitic glass probably relates to differences between the melting properties of the ilmenite and those of the silicate minerals, such that ilmenite is retarded from entering the melt.

3.6. Is/FeO Values

The relative abundance of nanophase Fe ø, expressed as Is, was determined by ferromagnetic resonance (FMR). The value Is/FeO, where FeO is the total iron content of a soil size fraction, is the relative proportion of the total iron in a sample that is present as nanophase Fe ø. In general when comparing soils (Figure 2), the abundance of agglutinitic glass increases as the maturity Is/FeO values increase [Morris, 1978]. However, within a given soil, values of Is/FeO increase with decreasing grain size, even though the bulk FeO contents decrease (Figure 4). That is, the percentage of the total iron that is present as nanophase Fe ø increases substantially in the smaller size fractions. Note that the values for nanophase Fe ø in smaller size fractions is significantly higher than the increase in agglutinitic- glass content would seem to predict. This will be discussed in more detail below.

3.7. Spectral Properties

We have concentrated our analyses on the <45.[tm fraction since the optical properties of the bulk soil are dominated by the finest fraction [Pieters et al., 1993]. Spectra for the size separates for each soil are shown in Plate 1. Several detailed analyses are underway using these spectra to test various techniques for linking measured compositional properties with spectral features [e.g, Noble et al., 2000; Pieters et al., 2001]. Summa- rized here are the principal distinctions that are readily observed between soils.

This group of mare soils encompasses a range of maturity (Is/FeO: 14- 94). However, only the two most immature soils, 12030 and 71061 (both with Is/FeO = 14), diverge from the general trends seen in the group as a whole. There are no major elemental compositional distinctions between these immature soils and other soils from the same site. For mare soils, the opti-


ß o


::L It3

V

2.0

1.5

1.0

0.5

I ' •,gglut. I ? 10611 Bulk Chemistry Glass

20-45 •m / •::•O - _ 10-20 •m• •

SiO; TiO; AI203 Cr203 MgO CaO FeO Na;O K;O

Figure 6. Comparisons of the bulk chemistry of the finest size fractions of the high-Ti mare soil, 71061, to the compositions of the agglutinitic glasses in each size. The bulk-chemistry and agglutinitic- glass oxide compositions are ratioed to the <45 Ltm bulk oxide chemistry to obtain a common means of comparison. Notice the general depletion of the agglutinitic glass in TiO2, as discussed in the text.

cal effects of exposure (space weathering) are clearly very non linear with respect to common measures of maturity. The spectral effects of nanophase metallic iron are discussed by Pieters et al. [2000] and Noble et al. [2001 ].

All these mare soils exhibit features near 1 and 2 gm due to ferrous iron in pyroxene. The two immature soils (12030 and 71061) are the only soils with spectral features also observed at shorter wavelengths. Spectra for all size fractions of 12030 are linear from 750 to -400 nm where a sharp inflection toward the ultraviolet occurs. This inflection is associated with charge transfer transitions in the crystalline phases present, in this case, most likely pyroxene. The <10 gm spectrum for 71061 exhibits a different, broad but weak absorption cemered near 600-700 nm due to the unusual ilmenite contents of the abundant black

volcanic beads present in that immature soil. Apollo 17 sample 71061 provides insight into the optical

properties of dark mantle deposits. This soil contains abundant pyroxene, which is clearly evident in the prominent pyroxene absorption bands near 1 and 2 gm in the 20-45 gm size fraction of Plate 1. The bulk <45 gm spectrum exhibits one of the weakest Fe 2+ features near 1 gm of all the soils. This apparently contradictory combination is due to the presence of abundant unweathered "black" pyroelastic beads. These beads contain extremely fine-grained "feathery" ilmenite (-lgm) in an oil- vine/glass matrix [Weitz et al, 1999]. This unusual microphase ilmenite provides the strong broad absorption near 700 nm, the wings of which distort the olivine feature to ---1.2 gm, leaving a peak near lgm, easily seen in the <10 gm sample (see Plate 1). This ilmenite peak effectively cancels the pyroxene absorption in the <45 gm soils. The shorter path length in the <10 gm separate allows the black beads to dominate the spectrum.

It is noteworthy that for all but the most immature samples, the reflectance of all size fractions for mare soils from these four

sites (Apollo 11, 12, 15, 17) converge in the ultraviolet. Al- though there are albedo distinctions in the visible, this UV con- vergenee occurs for both high- and low-Ti basalt soils. Such an effect is not observed for particle size separates from artificially pulverized samples [Pieters, 1993]. In other words, in the UV region, natural lunar soils exhibit no optical variations with particle size.

4. Another Source of Nanophase Fe ø (npFe ø) Figure 7 is a comparison of the ldFeO values, for each/•ize

fraction of the nine mare soils, with the agglutinitic glass abundances, for these same soil splits. The numbers above each data point represent the percentage increase in the value with reference to the next coarsest size. For example for 79221, the I•/FeO value for the < 10 •tm fraction is 117% greater than that for the 10-20 grn size, while the agglutinitic glass content has only increased by 15%. Notice the large increases in the I•tFeO values with decreasing grain size contrasting with the small increases in the agglutinitic glass abundances for the same grain-size change.

Basu et al. [1996] measured I•tFeO values for individual agglutinate particles within a single soil and concluded that the large variation in l•tFeO between grains was in part a conse- quence of natural mixing processes. Therefore it is possible that the I•/FeO values and compositions of the agglutinitic glass in a lunar soil are variable grain to grain, but on average, they are more uniform and approximately unchanging with grain size. In the present study, we have determined that the average chemical compositions of the agglutinitic glass for each grain size of a given soil (as well as other phases) are similar (Table 4). Therefore there is no reason to suspect that the IJFeO contribution from this glass changes with grain size. The disproportion-


0 150

I&. 100

io 50

80

• 60

20

!11 II I I I I I I I I II I I .I [

,, i 65 . 72 . s / Fe .

' • ,"•36% ' 88 .10 -

." 4.•• ß .,56• '•79% I I I 11111 IIIIII I I I I I ii iii iiiiiiii iii i1111 ii i iillr

12030 12001 15071 15041 71061 71501 70181 10084 79221

27% Agqlutinitic GlaSS 9% 9% 8% 2 19% 1913% 6

20% 17

21% 13%

Figure 7. Comparisons of the ldFeO values for the finest fractions of mare soils with changes in agglutinitic-glass abundances between grain-size fractions. The numbers above each data point represent the stepwise percentage increase in the value on going from each coarser to finer grain size.

ately large increase in IdFeO with decreasing grain size cannot be accounted for by the nominal increase in agglutinitic glass.

4.1. Surface-Correlated Nanophase Fe ø

Recent discoveries have been made of another major source of npFe ø in lunar soils [Keller and McKay, 1993, 1997]. This Fe ø is present in thin patinas (0•0.1 pm) on the surfaces of most soil particles [Wentworth et al., 1999]. The major portion of this npFe ø formed by deposition of vapor produced by abundant micrometeorite impacts, as documented by the presence of multiple and overlapping patinas. A smaller portion seems to have formed by radiation sputtering [Bernatowicz et al., 1994]. The exact mechanism of formation of this surface-correlated Fe ø is not well understood [Keller and McKay, 1997], but its presence was predicted by Hapke et al. [1975]. It is this surface- correlated npFe ø that can account for the additional contribution to the IdFeO values of the finest fractions of lunar soils.

Several high-resolution TEM and EMP studies have described and documented the significant contributions from vapor-deposited, npFe ø present in the patinas of most lunar soil grains [e.g., Keller and McKay, 1997; Keller et al., 1999, 2000; Pieters et al., 2000; Taylor et al., 2000 a, 2000 b, 2001]. The BSE and X-ray maps produced show that even plagioclase grains contain thin rims with appreciable Fe contents, and silica- rich patinas are developed on ilmenite and olivine. These last two minerals also contain Fe-rich rims, as confirmed by HRTEM examination. It would appear that the presence of npFe ø in the vapor-deposited patinas (rims) on virtually all grains of a mature soil provides an additional and abundant source for the greatly increased I•/FeO values observed.

4.2. Magnitude of the Contribution of the Surface-Correlated Nanophase Fe ø

As a general rule, for the same masses of two soil size fractions, the surface area of the size fraction increases by a factor of 4, as the average grain size decreases by only 50%. If the increase in I•cFeO that is attributable to the increase in agglutinitic glass is accounted for in each change in grain size, the "resid- ual" is the possible surface-correlated I•/FeO contribution. On average, there is about a 100% increase in I•/FeO value between the finest two size fractions (Plate l). Therefore to a first ap- proximation, the increase of 2X in l•/FeO correlates well with the predicted 4X increase in particle surface area._ The contribution to the I•/FeO from surface-correlated npFe ø in the <10 pm fraction may be even greater than that made by the npFe ø in the agglutinitic glass.

4.3. Significance of Surface-Correlated Nanophase Fe ø to Reflectance Spectra

The presence of the rimming npFe ø adds further to our knowledge of the complex lunar soil, but what is its contribution to the space weathering effects upon reflectance spectra? Our Lunar Soil Characterization Consortium has also addressed this

query. Keller et al. [2000] and Keller and Clemett [2001] have recently shown that the average grain size of the vapor-deposited npFe ø is significantly smaller than that of the agglutinitic Fe ø. Importantly, this suggests that the effects of the finer vapor- deposited Feø may be accountable for the major space weathering effects on reflectance spectra. The agglutinitic npFe ø has a general darkening effect, but it is the vapor-deposited Fe ø that ap-

TAYLOR ET AL.' LUNAR MARE SOILS 27,997

0.3

0.2

0.1

0.3

0.2

0.1

I I

10084-78

•m,-

mm m

.min.

,• 0.:2 _

0.1

0 500 1000 1500 2000 2500

! ! I I " I'm 0.3

- 0.2

- ?.._• • o.• ; 15041-94

I ! i I ,, o 500 1000 1500 2000 2500

< 10gm 10-20 gm 20-45 gm < 45 pm

I I Im 0.3

12001-5•

500 1000 1500 2000 2500

i I ! I I

,,.,

m m

• 15071-5• - ! I I i !

500 1000 1500 2000 2500

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t' 0.1-

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0

,.,...

12030-;4.

i I, !,

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500 1000 1500 2000 2500

.,

70181-47 • I I i,, ml

500 1000 1500 2000 2500

0.3

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,

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500 1000 1500 2000 2500

", ' ' , ,•.,.• 0.3 / m , , • ,'

' -14 " 0.2 -

.

0.1 -

71501-35 - i i I 01 I I I I l

500 1000 1500 2000 2500

0.3

0.2

0.1

0

I ! I

79221-8•

i i i

500 1000 1500 2000 2500

Wavelength (nm) Plate 1. Bidirectional reflectance spectra of the size separates for nine mare soils in the Lunar Soil Characterization Consortium. For ease of comparison, the scale is the same on all figures. (for 12030, data are offset 0.05). The 2-digit number directly following the sample number (e.g., the 78 of 10084- 78) refers the Is/FeO value for the <250 pm bulk soil [Morris, 1978].


pears to contribute major spectroscopic effects [Keller et al. 2000]. By combining experimental [Allen et al., 1996], theoreti- cal [Hapke, 2000], and spectroscopic [Noble et al., 2001] data for npFe ø, Pieters et al. [2000] demonstrated that the optical effects of npFe ø abundance are nonlinear, but very distinctive and fully consistent with the wide range of data now available for these lunar soils. In brief, small amounts of npFe ø redden a spectrum in the visible region without affecting the near- infrared; larger amounts of npFe ø darken overall and produce a linear red-sloped continuum [Pieters et al., 2000; Noble et al., 2001].

The major assistance and expertise of Allan Patehen, our EMP "magician" at UT, with the numerous FeatureScan analyses is gratefully acknowledged. Sue Wentworth performed most of the time-consuming, detailed sieving, which is well appreciated. The constructive review of Barbara Cohen has added significantly to the quality of the above presentation. We are particularly indebted to Dave Vaniman and Pam Clark for their detailed reviews, especially the thorough editing and numerous constructive comments. The revisions fi•om their reviews have resulted in significant improvements to the paper. Paul Lucey is thanked for his handling of the JGR editing. RELAB at Brown University is a multiuser facility supported under NAGS-3871. The extensive studies reported in this paper have received support fi•om NASA grants to each of the Lunar Soil Characterization Consortium members, and for this we are collectively grateful.

5. Summary

The modal abundances and chemistry of the minerals and glasses have been determined in a selection of lunar mare soils, in addition to the bulk chemistry and Is/FeO values for the finest grain-sizes of these soils. These studies have important and highly significant ramifications for understanding the processes of lunar-soil formation and remotely obtained reflectance spectra.

The agglutinitic-glass compositions are relatively constant for all size fractions, being more feldspathic that any of the bulk compositions. However, TiO2 is significantly depleted in agglutinitic glass, possibly bemuse of its reluctance to readily enter into the silicate impact melt. As grain size decreases, the amount of agglutinitic glass increases, as does the relative abundance of plagioclase, whereas the amounts of other minerals decrease. With decreasing grain size, the bulk chemistry of each size fraction continuously changes, becoming more Al-rich (i.e., plagioclase- component rich) and Fe-poor (i.e., pyroxene- and olivine-component poor). The bulk composition of the finer fractions approaches the composition of the agglutinitic glasses, systematics never before documented. Much of these data support the F 3 model of agglutinitic glass formation [Papike et al., 1981].

The Is/FeO values of soil size fractions increase by greater than 100% (>2X) between the smallest grain sizes (10-20 !,tin and <10 !,tin), whereas the abundance of agglutinitic glass increases by only 10-15%. There is evidence for a large contribution to the Is/FeO values from surface-correlated nanophase Fe ø, particularly in the <10 !,tm size fraction of the mare soils. The surface-bonded nanophase Fe ø is present mainly as vapor- deposited patinas on the surfaces of almost every particle of mature soils, and to a lesser degree for the immature soils. The vapor-deposited patinas possibly have far greater effects upon reflectance spectra of mare soils than the agglutinitic nanophase Fe ø.

The scientific potential from having finely tuned, highly accurate, compositional and mineralogiml data for lunar soils is immense. Requisite toward improving and expanding the interpretations of reflectance spectra of lunar Maria is coupling of the data from this study with the highly diagnostic mineral absorption features. Now that the mare soil physical, chemical, and spectroscopic characterizations have been completed and detailed in this paper, their integration for predictive purposes with remote sensing data have been initiated by Noble et al [2000] and Pieters et al. [2001 ].

Acknowledgments. We would like to thank the Curation and Planning Team for Extra-Terrestrial Materials (CAPTEM) for their support of work such as that presented in this paper. The patience and assistance of Dawn Taylor in the preparation of the tables and figures is greatly appreciated.

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Pieters, C. M., L. A. Taylor, S. K. Noble, P. Keller, B. Hapke, R. V. Morris, C. C. Allen, and S. Wentworth, Space weathering on airless bodies: Re- solving a mystery with lunar samples, Meteor. Planet. Sci., 35, 1101- 1107, 2000.

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Taylor, L. A., C. M. Pieters, L. P. Keller, R. V. Morris, D. S. McKay, A. Patehen, and S. J. Wentworth, The effects of space weathering on Apollo 17 mare soils: Petrographic and chemical characterization, Me- teor. Planet. Sci., 288-299, 2001.

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L. P. Keller, D. S. McKay, and R. V. Morris, Earth Science and Solar System Exploration Division, NASA Johnson Space Center, Houston, TX 77058. (lindsay.p.kellerl •jsc.nasa.gov; david.s.mckay 1 •jsc.nasa.gov; richard.v.morrisl •jsc.nasa.gov)

C. Pieters, Geological Sciences, Brown University, Providence, RI 02912. (pieters•porter. geo.brown.edu )

L. A. Taylor, Planetary Geosciences Institute, University of Tennessee, Knoxviile, TN 37996. (lataylor•utk.edu)

(Received September 27, 2000; revised February 23,2001; accep[ed March 13,2001.)

lunar mare soils: space weathering and the major effects of

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