Meteoritics & Planetary Science 36, 3 1-42 (2001) Available online at http://www.uark.edu/rneteor

The optical properties of the finest fraction of lunar soil: Implications for space weathering

SARAH K. NOBLE*I, CAKE M. PIETERSI, LAWRENCE A. TAYLOR2, RICHARD V. MORRIS3, CARLTON C. ALLEN4, DAVID S. McKAY3 AND LINDSAY P. KELLER5

'Brown University, Providence, Rhode Island 02912, USA Wniversity of Tennessee, Knoxville, Tennessee 37996, USA 3NASA Johnson Space Center, Houston, Texas 77058, USA

4Lockheed Martin, 2400 NASA Road 1, Houston, Texas 77058, USA 5MVA Inc., 5500/200 Oakbrook Parkway, Norcross, Georgia 30093, USA *Correspondence author's e-mail address: noble@porter.geo.brown.edu

(Received 2000 February 22; accepted in revised form 2000 August 30)

Abstract-The fine fraction of lunar soils (<45 ,urn> dominates the optical properties of the bulk soil. Definite trends can be seen in optical properties of size separates with decreasing particle size: diminished spectral contrast and a steeper continuum slope. These trends are related to space weathering processes and their affects on different size fractions. The finest fraction (defined here as the <1 Opm fraction) appears to be enriched in weathering products relative to the larger size fractions, as would be expected for surface correlated processes. This 4 0 ,um fraction tends to exhibit very little spectral contrast, often with no distinguishable ferrous iron absorption bands. Additionally, the finest fractions of highland soils are observed to have very different spectral properties than the equivalent fraction of mare soils when compared with larger size fractions. The spectra of the finest fraction of feldspathic soils flatten at longer wavelengths, whereas those of the finest fraction of basaltic soils continue to increase in a steep, almost linear fashion. This compositional distinction is due to differences in the total amount of nanophase iron that accumulates in space weathering products. Such ground-truth information derived from the <I0 p m fraction of lunar soils provides valuable insight into optical properties to be expected in other space weathering environments such as the asteroids and Mercury.

INTRODUCTION

The finest fraction of lunar soils is geochemically and petrologically different than the larger size fractions. The less than 10 pm fraction tends to be more feldspathic in composition than the bulk soil from which it formed (Papike et al., 1981; Taylor et al., 1999a). Cratering experiments for regolith development have demonstrated that feldspar is more easily broken down, or comminuted, than pyroxene and other lunar minerals and thus is enriched in the finer fractions (Horz et al., 1984; Horz and Cintala, 1997). Modal analyses of lunar soils performed using point-count methods originally suggested that mineral fragments increase but agglutinates (glass-welded aggregates) decrease with decreasing particle size (e.g., Labotke et al., 1980; Houck, 1982). Recent technological advances have allowed more accurate modal analyses to be performed using x-ray digital imaging which, unlike point-count methods, is able to assess the percentage of minerals locked in multi- phase lithic fragments and fused soil particles (Taylor et al., 1996). This new technique provides a far more accurate

assessment of modal mineralogy and has demonstrated that, contrary to the results of point-count studies, agglutinitic glass content actually increases consistently with decreasing particle size all the way down to the finest fraction (Taylor et al., 1999a). Also, the mineral abundances are species-dependent and vary with size fraction.

Several studies of the optical properties of lunar materials have been carried out using particle size separates of lunar soils. Pieters et al. (1993) examined the optical properties of the <25 pm fraction and larger size fractions, comparing them to the bulk soil. Fischer studied size separates of a suite of highland soils of similar composition but different exposure histories (Fischer, 1995; Fischer and Pieters, 1995; Fischer et al., 1994). These studies demonstrate consistent and systematic changes in the spectra of lunar soils as a function of particle size. A "reddening" of the continuum slope ( i e . , reflectance increases at longer wavelengths) occurs with decreasing particle size in the smaller size fractions. There is a gradual loss of spectral contrast and the ferrous iron absorption band near 1 pm almost disappears as we approach the finest fraction. These

31 0 Meteoritical Society, 2001. Printed in USA.

32 Noble et al.

a 0 S m w 0 a

500 1000 1500 2000 2500 Wavelength (nm)

FIG. I . Bi-directional reflectance spectra of several size fractions of mature mare soil 10084 from Pieters et al. (1993). Note the similarity of the bulk to the fine fractions of the soil.

studies also demonstrate that the spectral bulk properties of any natural lunar soil are dominated by the finer fractions and tend to be most similar to the fractions in the 10-45 p m range (e.g., Pieters et al., 1993; Fischer, 1995). An example ofparticle size separates for a natural lunar soil is illustrated in Fig. 1.

It is well known that spectra of transparent material normally get brighter as particle size decreases (e.g., Adams and Filice, 1967; Pieters, 1983). This, however, does not appear to be the case universally with naturally formed lunar soils. While some increase in the albedo with decreasing particle size is observed in the near infrared along with the reddening of the continuum, reflectance in the visible range (near 0.4pm) often has very little, if any, change as a fimction of particle size. Pieters et al. ( I 993) demonstrated that synthetic particle size separates of lunar fractions produced by crushing larger fractions of lunar soils follow the normal pattern of brightening with decreasing particle size throughout and do not duplicate the optical effects of natural particle separates of lunar soils. This difference suggested that the structure of the natural surface is destroyed in the grinding process as fresh surfaces are exposed. Thus, they concluded that the optical properties of the finest fractions must be due largely to surface correlated weathering products rather than simply to particle size effects.

"Space weathering" is the term given to a collection of processes that result from exposure to the space environment due

to the continuous bombardment by energetic cosmic rays, solar wind particles, and micrometeoroids. These small-scale processes (nm to mm) occur on any body that is not protected by an atmosphere. The constant flux of high-energy particles and micrometeoroids, along with larger meteoroids, act to comminute, melt, sputter, and vaporize components of the soil, as well as to garden (mix) it. The products of these weathering processes include agglutinates, as well as surface-correlated products on individual soil grains, such as glass splashes, implanted rare gases, and solar flare tracks. In addition, it was noted in the 1970s that solar wind sputtering and impact vaporization create submicroscopic metallic iron particles (nanophase iron, npFe0) on grain surfaces (Housley and Grant, 1975, 1977; Baron et al., 1977; Hapke, 1973; Hapke et al., 1975).

The combined results of these weathering processes have systematic effects on the spectral properties of lunar soil. Space weathering of lunar materials results in a loss of spectral contrast, a reddened continuum slope, and an overall decrease in the albedo. As suggested earlier (Cassidy and Hapke, 1975), the red slope is believed to be linked to tiny inclusions of nanophase Feo on the order of 40-330 8, in diameter (Morris, 1980). Larger spheres of Feo are thought to result in darkening of the soil (Britt and Pieters, 1994). These blebs of Feo are found in a wide range of sizes both as a surface-correlated

The optical properties of the finest fraction of lunar soil 33

feature in the rims of grains, as well as components incorporated throughout agglutinates. Space weathering products, along with their spectral effects, are highly dependent on soil maturity, the average length of time particles in a given soil have been exposed to the space environment.

Recent detailed microanalytical studies (Keller et al., 1998; Wentworth et al., 1999) describe thin (60-200 nm) patinas, or rims, developed on lunar soil grains. The rims appear to be created by both subtractive and additive processes that are the combined result of radiation damage and vapor deposition (and or sputtering), respectively. It is within these rims that much of the npFe0 resides.

The <I0 p m fraction of lunar soils, while composing only about 10 to 20% of the soil by weight, constitutes as much as two-thirds of the surface area (Housley, 1980). As grain rims are inherently surface correlated, it is expected that several weathering products would be concentrated in the finest fraction, since it constitutes such a large percentage of the exposed surface. Additionally, these fine particles tend to surround and cling to larger particles electrostatically, leaving them more exposed while "protecting" the larger particles. It is no surprise then, that weathering products affect the optical properties of the fine fraction more strongly than larger particles.

The Lunar Soil Characterization Consortium is currently studying in detail a suite of mare soils in order to make direct links between soil chemistry and spectral properties (Taylor et al., 1999b, 2000a,b,c; Pieters et a[., 2000). Soil samples selected for the consortium study were chosen to represent a wide variety of compositions and maturities. The consortium is concentrating on the 10-20 and 20-45 p m size fractions because the optical properties of these sizes bear the greatest resemblance to those of the bulk soil. The <10pm fraction is also included because of its importance in weathering processes. Characterizing the finest fraction is essential to understand both weathering effects and the optical properties of the soil as a whole. Multiple processes interact to create the unique properties of the finest fraction and need to be investigated in great detail. Reported here are the constraints set by the optical properties of the 4 O p m fraction of lunar soil.

EXPERIMENTAL APPROACH

Several techniques for sieving lunar soils into different size fractions have been used:

Wet Sieving with Freon

The suite of highland samples (61 22 1, 6480 1, 6770 1, and 6746 1 ) studied by Fischer et al. (1 994) were originally prepared at Johnson Space Center (JSC) by wet sieving with large quantities of freon into eight size fractions: 500-1000, 250- 500, 150-250, 90-150, 45-90, 20-45, 10-20, and 4 0 pm. The finest fraction (<10pm) was collected after allowing the

fieon to evaporate. Samples of a lunar soil simulant, Minnesota Lunar Simulant I (Weiblen et al., 1990, and references therein) and a bytownite (feldspar) were prepared using the same technique (Fischer, 1995), in order to determine whether processing by freon alone has any effect on samples. In addition to Fischer's samples, we tested general interaction with freon by exposing <10 p m samples of lunar mare soil to various amounts of fieon. The soil selected for this test was the 4 0 p m fraction of soil 70181, which had been prepared using dry sieving techniques (see below). One <IOpm sample split was exposed for several days to large amounts of freon and the freon allowed to evaporate. The other <10 pm sample was exposed for several hours to only small amounts of freon. In this case, much of the freon was removed after the sample had settled before the remaining freon was evaporated.

Dry Sieving

Phase I of the Taylor et al. lunar soil characterization consortium suite of soils was sieved at JSC using a sonic sifter without the aid of any liquid. This suite consisted of nine mare soils sieved into 20-45 and 10-20 p m size fractions. Four of these soils (10084, 12030,70 18 1,7922 1) were also sieved into <1 Opm fractions. For comparison, a highland sample ofmature soil, 6850 1, was processed independently at Brown University by gently dry sieving by hand (i.e., no sonic sifter) into 45-125, 25-45, 10-25, and <10 p m size fractions. Fischer also processed samples of the simulant and bytownite by dry sieving for comparison with his wet sieved samples.

Wet Sieving with Water

Phase I1 of the Taylor et al. lunar soil characterization suite of mare soils were processed at JSC using ultra pure de-ionized water. The samples in this suite have been sieved into 20-45, 10-20, and <10 p m size fractions. Our study includes the following samples from this suite: 10084, 1200 1, 12030, 15041, 15071, 71061, 71501, 70181, and 79221.

Spectroscopic Measurements

All the visible to near-infrared spectra in this study were obtained using the RELAB bidirectional spectrometer at Brown University at a standard viewing geometry of 30" incidence and 0" emission (except where otherwise stated). Although not presented here, mid-infrared Fourier Transform-Infrared (FT-IR) biconical reflectance spectra (2-25 pm) were also obtained at the RELAB for many samples.

RESULTS

Although not all samples have been prepared in the same manner, sufficient data exist to recognize that the very finest fraction (<lo pm) exhibits distinct optical properties.

34 Noble et al.

0.4

0.35

0.3 a, 0 0.25 c (d 0 0.2 a, a, 0.15 [I:

c

- c

0.1

0.05

0

i Highland Soil 64801

- c1op.m - - 10-201rrn

500 1000 1500 2000 2500 Wavelength (nm)

FIG. 2. Bi-directional reflectance spectra of size fractions of mature highland soil 64801 from Fischer( 1995). The continuum ofthe <lO,um fraction displays a curvature that is not present in the larger fractions.

Several size fractions of a representative highland soil are shown in Fig. 2 and several size fractions of a representative mare soil are shown in Fig. 3. The highland samples were originally prepared for Fischer (1999, with the exception of sample #68501, which was prepared for this study (see Experimental Approach). The Fischer samples were chosen to represent similar compositions, but different degrees of maturity. The mare samples were originally measured for the lunar soil characterization studies (Taylor et al., 1998,1999a,b). Soil samples in that study were chosen to represent a wide variety of both compositions and maturities.

Seventeen <10pm size fraction samples of lunar soil have been analyzed, including thirteen mare samples from Apollos 1 1, 12, 15, and 17, and four highland samples from Apollo 16.

o,35 Mare Soil 70181

0.3

0.25 a,

a c $ 0.2

[I: 0.15

- c a,

0.1

0.05

0 500 1000 1500 2000 2500

Wavelength (nm) FIG. 3. Bi-directional reflectance spectra of several size fractions of submature mare soil 701 8 1.

Table 1 lists the <10 pm soil samples available for this study and the method of processing used to prepare the samples. Their bulk soil (<250pm) maturity indices, I,/FeO, as well as the <10 pm IJFeO, are provided where known. I,/FeO is a measure of the amount of npFe0 normalized to the total amount of iron in the sample and is directly related to the surface exposure time for that sample (Morris, 1977). A maturity index of 0 to 29 is considered immature, 30-59 is submature and over 60 is considered mature. The spectral properties of these samples are summarized in Fig. 4.

The <10pm fraction of mare soils exhibits a much steeper continuum slope than the larger size fractions and, with the exceptions of immature soils 12030 and 71061, very little ferrous iron absorption bands (Fig. 3). Soil 7 106 1 is particularly unusual in that it does not display the 1 and 2pm bands typical

TABLE 1. Less than 1 Opm samples of lunar soils.

Sample ## I,/FeO Dry sieved* Freon sievedt Water sieved! (<250 p n ) (<10pm I,/FeO) (<10,um I,/FeO) ( < l O p m I,/FeO)

10084 78 119 - 145 - 115 1200 1 56 -

12030 14 37.5 - 32 1507 1 52 - - 159

- 161 1504 I 94 -

36 61221 9 64801 71 - 145 -

81 6770 1 39 -

70181 47 X - 104 - 88 71501 35 - - 28 71061 14 -

7922 1 81 158.6 - I69

*Taylor et al. (1998). TFischer (1995). $This study. $Taylor ei al. (1999b, 2000a,b,c).

- -

- 68501 85 X (by hand)S - -

The optical properties of the finest fraction of lunar soil 35

12030 14 + 12001 56 + 10084 78 _t_ 71061 14 -ci- 15071 52 + 15041 94

500 1000 1500 2000 2500 Wavelength (nm)

1.3

1.2

1.1

1

0.9

0.8

Wavelength (nm) FIG. 4. (a) Bi-directional reflectance spectra of all <I0 ,um samples used in this study. Solid lines are mare, dotted are highlands. Line symbols represent immature soils, open symbols are submature, and closed symbols are the mature samples. The bulk soil I, values for each soil are given in the legend. (b) Reflectance spectra of all <lOpm samples used in this study scaled to 1 at 750 nm. Key is the same as (a).

36 Noble et al.

of most lunar soils, but rather its spectrum appears to be controlled by the large fraction of black beads in the sample (for a more detailed discussion, see Pieters et al., 2000). Spectra of the less than 10 p m fraction of mare soils typically display a distinctly red continuum with reflectance increasing towards longer wavelengths in a nearly linear fashion. The < l o p m fraction of highland soils, like their mare counterparts, exhibit little, if any, ferrous bands (Fig. 2). However, their continua exhibit a distinct curvature in the visible and a definite flattening towards longer wavelengths that is quite different from the spectral behavior of either the larger fractions of highland soils, or the finest fractions of mare soils.

IMPLICATIONS FOR SAMPLE PREPARATION METHODS

The various methods used to prepare the <IOpm samples have affected the results in subtle ways that provide additional information about the finest fraction.

Freon Processing

The highland suite prepared by Fischer (1995) were all processed with large amounts of freon, causing concerns that perhaps the freon itself or some type of contamination in the freon was affecting the spectra and possibly causing the flattening observed in the <10 p m fraction at longer wavelengths. The finest fraction, the so-called pan fraction because it sits in the collecting pan with the liquid, is exposed to the freon for much longer than the other fractions and the freon is evaporated off leaving any dissolved material behind. This time factor and the large amount of liquid used, in combination with the greater surface area of these tiny particles, would make the finest fraction more susceptible to such problems than larger fractions.

Fischer tested for possible effects of freon by processing a bytownite and soil sirnulant (Minnesota Lunar Simulant I) with and without freon. As shown in Fig. 5, the freon-exposed spectra exhibit an overall darkening, possibly due to minor contaminants. There was no apparent change in continuum shape however. Additionally, our separate experiments exposing soil 70 18 I to freon detected virtually no change in the sample exposed for brief periods, but the extended exposure sample displayed an overall darkening, which again, may be related to some type of contamination in the freon. These results are shown in Fig. 6 . Nevertheless, no flattening of the continuum was observed with either of the freon processed mare <10 p m samples. Our findings compliment those of Fischer: the use of freon does not appear to change the shape or character of the spectra, though under certain conditions it may result in a reduction of the overall albedo.

Dry Sieving

Early in the analysis of Phase I soil separates (dry sieved) it was noted that finer separates often have a systematically

0.5 bytownite - freon pro-

m

lunar sirnulant - no freon

0.2

0.1 k lunar sirnulant - freon processed

o ~ " ' ' ' " ' ' ~ ' ' " ~ ' ' " ' 500 1000 1500 2000 2500

Wavelength (nm) FIG. 5. Comparison ofreflectance spectra for samples prepared with and without using freon in the sieving process (from Fischer, 1995). The freon appears to have resulted in darkening of the soil, but has not substantially changed the character of the spectra.

lower value of I, than previously measured splits of the same sample (R. Morris, pers. comm.). This raised great concern about the sample preparation method. If space weathering effects are indeed surface correlated, then they may be affected by the process of mechanical dry sieving. It was hypothesized that the harsh interaction between particles may cause the weathered material in the outer rims of larger grains to be removed and concentrated in the finest fraction. Figure 7 compares the wet and dry sieved samples of several mare soils. It is clear that in all cases the dry sieved sample displays a noticeably steeper and less curved continuum slope, suggesting that the dry sieved samples are systematically different from the wet sieved samples.

The dry-sieved-by-hand <10 p m sample of 68501 still exhibited some flattening, though its continuum is much redder than the other highland samples. Soil 6850 1 is the most mature soil studied (I,/FeO = 85), so it is expected to be somewhat redder than the less mature highland samples. Separating the effects of weathering and dry sieving is difficult, but it is possible that even hand sieving may be too harsh, resulting in some concentration of weathering products.

Water Sieving

Due to the concerns raised by dry sieving and freon processing methods, Phase I1 of the lunar soil characterization study utilized wet sieving with ultrapure water at JSC. Water sieving appears to be acceptable, though fluid interaction should be kept to a minimum.

DISCUSSION OF THE OPTICAL PROPERTIES OF THE 4 0 p m SIZE FRACTION

Beyond the minor effects of various processing methods, there are several competing factors involved in creating the optical properties of the <10 p m fraction of lunar soils. Separating a soil into size fractions has significant

The optical properties of the finest fraction of lunar soil 37

0.3

0.25

a, QL 0.1

0.05

0

70181 - limited

500 1000 1500 2000 2500 Wavelength (nm)

FIG. 6. Reflectance spectra of the original dry sieved < l O p m fraction of submature lunar soil 71081 compared to the results after exposure to freon for several hours (limited) and several days (extended). As in Fig. 5, there is a darkening, but no real change in the character of the continuum.

1.25

1.2 - 1.15

% 1.05

- - 0.95 a> II:

0.9

0.85

+ 70181 dry - - 0 - - 70181 wet --s- 10084 dry . - a - - 10084 wet -e- 12030 dry - - + - - 12030 wet -79221 dry - - A - - 79221 wet li

1 I I I I I I I I I 1 1 I I I , I I I I I I I , I1 Wavelength (nm)

500 1000 1500 2000 2500

FIG. 7. A comparison of the < 1 0 p m fraction of lunar soils processed by dry and wet sieving techniques. The reflectance is scaled to one at 750 nm. In each case, the continuum of the dry sieved sample is redder and less curved than the water sieved sample.

38 Noble et al.

consequences for its optical properties. Once the size fractions are separated, standard mixture modeling, either linear or non- linear, cannot easily recombine the spectra in order to synthesize the spectral properties of the bulk soil (Fischer, 1995). The finest fraction is very important as part of the bulk, where the small particles tend to cling to and surround the larger ones. Radiation interacts with both before being scattered back to the viewer. In separating size fractions, this intimate interaction is removed, along with its spectral consequences. When light interacts with only one well defined (but artificial) size of particle, the spectral properties no longer mimic the natural environment.

Despite the inherent problems involved in isolating the finest fraction, much can be learned of the processes acting on this fraction from observed systematic trends in composition and maturity.

Compositional Trends

The finest fractions of the measured highland soils have very different spectral properties than those for mare soils. The continuum of the finest fraction of the feldspathic soils increases in the visible, then flattens at longer wavelengths (Fig. 2). The continuum of the finest fraction of basaltic soils continues to increase in a steep, almost linear fashion throughout the visible- near-infrared (Fig. 3). Measurements of the <10 y m fraction of the highland samples were also taken with several different viewing geometries to determine whether the observed flattening was due to photometric effects (Fischer, 1995). As shown in Fig. 8, no significant difference in spectral slope was observed over phase angles from 30" to 95", including specular geometry. Thus, viewing geometry is not the cause of the flattening seen in the highland samples. While preparation methods do appear to have minor effects on the spectral properties of this finest fraction (see Figs. 6 and 7), they also

0 . 4 5 , . , I I , I , I . , , , . I I , , I , . I , , 0.4

0.35

8 0.3 C m 5 0.25

a, [r 0.2

- c

500 1000 1500 2000 2500 Wavelength (nm)

FIG. 8. Bi-directional reflectance spectra of the <10,um fraction of lunar soil 64801 taken with various geometries relative to halon at the same geometry, corrected according to NIST standards. The incidence angle (i) and the emission angle (e) is indicated on the figure.

cannot account for this dramatic difference. Thus, we believe there is ample evidence that the difference between the nearly linear continuum of the mare and the flattened highland continuum is real, and directly related to the compositional differences between the iron-rich mare and the feldspathic highlands.

Maturity Trends

Space weathering causes systematic changes in the optical properties of lunar soil over time. As mentioned previously, with increasing maturity, it has been found that lunar soil spectra typically become darker, redder, and lose their spectral contrast. The finest fraction is no exception. Preliminary results show systematic variations between the <10 p m finest fraction of soils of varying exposure ages (maturity) as measured by I,/FeO. This is to be expected, as more mature soils have had time to accumulate a larger quantity of weathering products. The mature samples of both mare and highlands are darker than their immature counterparts. Also, more mature soils exhibit a greater loss of spectral contrast such that in the finest fraction, absorption bands have all but disappeared. Among the mare soils, the only ones with identifiable absorption bands are the immature soils (1 2030,70161). Among the highland samples, the immature and submature soils (61 22 1,6770 1, and 6746 1 ) show a slight 1 p m band, which has completely disappeared in the mature samples.

Additionally, from Fig. 4 it is clear that the immature and submature highland samples (6 122 1,67701,6746 1) show the most dramatic turn-over and flattening at longer wavelengths. In fact, the immature and submature samples are nearly indistinguishable. The mature highland samples, both the wet and dry sieved (64801, 68501), show significantly steeper slopes than their less mature relatives. It appears that as the highlands become more weathered, their continuum slopes become increasingly linear and redder although this effect is only seen for the most mature highland soils.

In contrast, it is the two immaturemare soils (12030,701 61) that stand out from their submature and mature counterparts. This is not only due to their brightness and distinct absorption bands, but also because the other mare soils all tend to converge in the visible while the immature soils are significantly brighter even down to those wavelengths. Additionally, if the strong absorption bands are ignored, the continua of 12030 and 70 16 1 are quite similar in character to the mature highland samples, displaying a slight, but noticeable curvature. In contrast to the highland soils, both the submature and the mature mare soils exhibit flatter and more linear continua than the immature samples.

Possible Explanation for Compositional Differences

Mare soils have an average of at least three times the total FeO content of highland soils. The more Fe that is available in the soil, the better the opportunity of creating nanophase

The optical properties of the finest fraction of lunar soil 39

"a 0.3 - LL

8 z 0.2 1

0.1 -

I&,$' 0 ' " J " " " " ~ " ' ~ ' ' ' ~ '

0 200 400 600 800 1000 1200 1400

's

FIG. 9. The 79 mare and highland soils plotted here (from Morris, 1977) fall along a best-fit line represented by the equation above. There is quite a strong linear relationship between I, values and wt% metallic Fe in lunar soils.

iron through weathering processes, so we expect the I, values of mare soils to be significantly higher than those of the highland soils. This is why I, values, which measure the amount of nanophase iron present in the sample, are normalized to FeO content in order to compare maturities from one soil to another. In Fig. 9, data from Morris (1980) are plotted to show the relationship between I, and Feom, the wt% metallic iron created by the exposure-induced reduction of ferrous iron. As expected, this is a linear relationship. A linear best-fit through the data gives a line with an R2 of 0.9537. From Morris' data, mare bulk (<250 pm) soils have average wt% F e o m of 0.10 (immature) to 0.27 (mature), while highland bulk soils average only 0.02 (immature) to 0.16 (mature) wt% FeOm. The finest fraction of both soil types should have higher values due to the greater concentration of weathering products. The data from Morris (1980) document that mare soils contain significantly more npFeo than highland soils.

In order to evaluate the optical effects of varying amounts of npFe0, Allen et al. (1 996) have performed experiments using silica gel powders (35-74 pm) as optical analogues for lunar soils. The gels contain pores 6 nm in diameter into which varying amounts of Fe203 were implaced. The powders were then heated and exposed to flowing hydrogen, resulting in reduction of the Fe2O3 and the creation of npFe0. Their results show that samples with very small concentrations of Feo (<2 wt%) display significant red slopes. Greater amounts of Fe darken the spectra so that anything more than about 2 wt% Feo results in a continuum that is essentially flat with extremely low reflectivity. The results of Allen's experiments with small amounts of Fe are shown in Fig. 10. The most relevant of Allen's analogues is the 0.2 wt% Fe spectrum. The continuum of this sample is only slightly red, but exhibits a steep curvature in the visible and flattening at longer wavelengths that is very similar in shape to the 4 0 pm highland samples. The 0.47 wt%

1.0,

FIG. 10. Results of Allen's lunar soil analog (Allen etal., 1996; pers. comm.): Spectra of silica gel particles (35-74pm in diameter) with various amount of npFeo filling pores 6 nm in diameter.

Fe sample of Allen's analog is much redder than any of our lunar soil samples, but the continuum is similar in shape to our "reddest" samples, the immature mare and mature highland. Subsequent analog samples with larger wt% Fe begin to lose redness as their whole spectrum becomes darker, eventually resulting in a completely black continuum.

Shown in Fig. 11 are the I, values of our samples plotted on the linear best-fit generated for data in Fig. 9. Good estimates of wt% F e o m for our samples are shown to range from 0.15 (immature) to 0.6 (mature) for the mare and 0.05 (immature) to 0.2 (mature) for the highlands. Although the data are limited, the soils generally fall into three groups. The immature and submature highland soils, with their curvature in the visible and flattening at longer wavelengths, form a group in the lower left with <0.1 wt% FeOm. The slightly curved and very red mature highland and immature mare form an intermediate group with slightly more FeORM. Finally, the submature and mature mare with their slightly less red and linear continuum form a third group with much larger amounts ofFeom (0.3 to 0.6 wt%). These concentrations are somewhat small in comparison to the absolute values for the experimental analogs with similar continuum shapes, but the qualitative relationship between the amount of FeoR, and spectral shape is the same.

The curvature of the spectrum is sensitive to small amounts of npFe0, an effect that is seen only in the finest fractions of soils and in Allen's experimental analog (which used 35-74 p m size particles). The physical effects of npFe0 are a function of overall transparency, or the amount of absorption as light is transmitted through a grain. It appears that complex lunar soil particles need to be <10 p m before they are transparent enough to see the additional effects of npFe0 on the continuum. The experimental analog used exceptionally transparent silica gels, and therefore, the effect was detectable even in much larger particles.

The amount of npFe0 is very important in determining the shape of the continuum in transparent or semi-transparent materials. In the finest fraction of lunar soils, the process can be broken into four stages: (1) Small amounts of npFe0 result in a large curvature in visible wavelengths, while leaving the

40

, I , 8 l " " I " ' I 1 I I I

- 79221 (81)- Highlands

Noble et al.

0.7

0.6

t 0.5 1 15071 (

I- 12001 (5t looyV

0.4 1 71081 (47 71501 (35)/

0.3

0.2

0.1

n 500 1000 1500

S I

FIG. 1 I . The best-fit line determined from Fig. 9a is plotted. The arrows show the <10pm I, values which can then be extrapolated to the line in order to obtain estimates for the weight percent of npFeo present in each sample. Solid arrows indicate mare samples, open arrows represent highlands. The soils are labeled with their sample number followed by their bulk soil I,/FeO value.

longer wavelengths largely unaffected. (2) As npFeO accumulates, the continuum becomes less curved and significantly redder, reaching a peak redness somewhere between 0.15 and 0.35 wt% Feo. (3) Additional npFe0 results in an increasingly linear continuum that starts to lose redness in the visible. (4) Following the trend of Allen's analog, if significantly more npFe0 could be added to the soil, we would expect that the continuum eventually would become dark and featureless. However, even our most mature Fe-rich mare sample ( 1 504 1 ) still has a significantly red slope. The natural soils seem to find an equilibrium state where there is a balance between the creation and destruction of weathered rims and the influx of new material. Mature mare soils end at stage 3. Mature highland soils, having less iron available to them, reach steady-state at stage 2 .

CONCLUSIONS

It is strongly recommended that the most gentle procedures be used in the preparation of size separates of natural lunar soils (e.g., wet sieving with water). Rims of optically active weathering products do exist on soil grains and may be disrupted by the mechanical action of severe dry sieving. Gently dry sieving by hand may reduce this problem, though caution is still warranted, particularly for mature samples.

Beyond the minor effects of processing, we find that the optical properties of the finest fraction of lunar soils are particularly sensitive to a combination of effects due to maturity and composition. In the decades since Apollo, it has been difficult to isolate these effects for study. The concentration of weathering products in the finest fraction allows us to explore the optical effects of space weathering in greater detail. The finest fractions of lunar soils show trends in their continuum shapes due to interrelated effects of composition and maturity. The Fe-poor, immature highland samples display a strong curvature in the visible with a flattening towards longer wavelengths. As the highland samples mature, their continua become redder and less curved, similar to immature (but Fe-rich) mare soils. With increasing maturity, mare soils display continua that become more linear, darker, and begin to lose their red slope.

Experimental analogs for lunar soils with varying amounts of npFeo (Allen et al., 1996) show trends in their continuum shapes similar to those seen in the finest fraction of lunar soils. It is the amount of npFe0 in surface-correlated rims on lunar soil grains that appears to control the overall continuum shape of the finest fraction. Thus npFe0 can explain the trends we see in both maturity and composition. Obviously, the more mature a soil, the more time it has had to build up weathering products (ie. , npFe0). It also follows naturally that more Fe-rich

The optical properties of the finest fraction o f lunar soil 41

( ie . , mare) soils can create npFe0 at a higher rate than Fe-poor soils (ic, highlands).

The finest fractions of lunar soils are an indispensable tool for understanding the effects of the space weathering process. However, they are only one component of soils and do not represent everything concerning space weathering. Still, much insight has been gained from the systematic trends seen in comparing and contrasting the finest fractions with larger size fractions and comparing samples of the finest fractions of different composition and maturity. The invaluable information provided by returned lunar soils is the only ground truth currently available for understanding other airless bodies. Factors controlling the development and abundance of npFe0, as well as the overall transparency of soil particles, are key properties affecting the products of space weathering observed for regoliths in other environments, such as asteroids and Mercury.

Acknowledgments-Spectra were acquired using RELAB, a multi-user facility under NASA grant NAGS-3871. The skilled assistance by Takahiro Hiroi in obtaining spectra is greatly appreciated. This analysis builds on several aspects of the earlier work on highland soils by E. M. Fischer (1995). Helpful reviews by B. Hapke and M. Cintala were greatly appreciated. This research has received support from the NASA grants of each of the Lunar Soil Characterization Consortium members, and for this we are collectively grateful.

Editorial handling: M. J. Gaffey

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