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
SARAHK. NOBLE*I, CAKE M. PIETERSI, LAWRENCEA. TAYLOR2, RICHARD V. MORRIS3,
CARLTON
C. ALLEN4, DAVIDS. McKAY3 AND LINDSAYP. 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: [email protected]
(Received 2000 February 22; accepted in revisedform 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 10pm fraction tends to be more feldspathicin 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 multiphase 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 demonstrateconsistent 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
0Meteoritical Society, 2001. Printed in USA.
Noble et al.
32
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 fractionsmust 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-scaleprocesses
(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
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. (1994) were originally prepared
at Johnson Space Center (JSC) by wet sieving with large
quantities of freon into eight size fractions: 500-1000, 250500, 150-250, 90-150, 45-90, 20-45, 10-20, and 4 0 pm.
The finest fraction (<10pm) was collected after allowing the
33
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, 12030,70 181,7922 1) were also sieved into
<1Opm 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 p m ) 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 p m ) exhibits distinct optical properties.
Noble et al.
34
0.4
0.35
i Highland Soil 64801
o,35
0.3
0.3
a,
0
c
a,
0.25
a
$
-
c
0
0.25
c
(d
a,
c
Mare Soil 70181
0.2
0.2
c
a,
[I: 0.15
a, 0.15
[I:
-c1op.m
0.1
- - 10-201rrn
0.05
0.1
0.05
0
0
500
1000
1500
2000
Wavelength (nm)
2500
500
1000
1500
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.
10084
12001
12030
15071
1504I
61221
64801
6770 1
68501
70181
71501
71061
7922 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 p m 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 1061 is particularly
unusual in that it does not display the 1 and 2pm bands typical
I,/FeO
(<250 p n )
Dry sieved*
(<10pm I,/FeO)
Freon sievedt
(<10,um I,/FeO)
Water sieved!
( < l O p m I,/FeO)
78
56
14
52
94
9
71
39
85
47
35
14
81
119
-
-
-
145
115
32
159
161
*Taylor et al. (1998).
TFischer (1995).
$This study.
$Taylorei al. (1999b, 2000a,b,c).
2500
FIG. 3. Bi-directional reflectance spectra of several size fractions of
submature mare soil 7018 1.
TABLE1. Less than 1Opm samples of lunar soils.
Sample ##
2000
Wavelength (nm)
37.5
-
-
-
-
-
-
36
145
81
-
X (by hand)S
X
-
-
-
158.6
-
-
-
-
104
88
28
I69
The optical properties of the finest fraction of lunar soil
12030 14
+12001 56 +10084 78
_t_ 71061 14 -ci- 15071 52
500
35
1000
+15041 94
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).
Noble et al.
36
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
bytownite
0.5
-
freon pro-
m
lunar sirnulant - no freon
0.2
k lunar
0.1
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
70181
a,
-
limited
0.1
QL
0.05
0
500
2000
1500
1000
Wavelength (nm)
2500
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
%
a>
II:
1.2
1.15
1.05
+70181 dry
- - 0 - - 70181 wet
--s- 10084 dry
. - a - - 10084 wet
-e12030 dry
- - + - - 12030 wet
-79221
dry
- - A - - 79221 wet
0.95
0.9
0.85
1
I
I
500
I
I
I
I
I
I
1000
I
1
1
I
I
1500
I
, I I
I
2000
Wavelength (nm)
I
I
I
I
li
,I1
2500
FIG. 7. A comparison ofthe < 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 nonlinear, 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 visiblenear-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
0.45,.
,
I
I
,
I
,
I
.
,
,
,
.
I
I
,
,
I
,
.
I
,
,
0.4
0.35
8
C
0.3
5
-
0.25
m
c
a,
[r
0.2
500
1000
1500
2000
2500
Wavelength (nm)
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.
FIG.
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 (12030,70161). 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 (12030,70161)
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
1.0,
"LL
a 0.3-
8
z
0.2
1
0.1
-
0
I&,$'
0
'
"
J
"
"
200
"
"
~
"
'
400
~
'
'
'
~
600
'
800
1000
1200
1400
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.
'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 concentrationsof 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 p m highland samples. The 0.47 wt%
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
o f F e o m (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
Noble et al.
40
0.7
,
,
l
8
"
"
I
"
'
1
I
I
I
79221 (81)-
t
looyV
1
15071 (
12001 (5t
71081 (47
71501 (35)/
I0.4
I
Highlands
0.6 -
0.5
I
1
0.3
0.2
0.1
n
500
1000
1500
IS
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
( i e . ,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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