Optical measurements on Apollo 12 soil samples from lunar mare surface

Briggs, F.

Oleary, B. T.

English

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CRSR 420
OPTICAI, PROPERTIES OF
APOLLO 12 1,100H SAMPLES
Frank Briggs and Brj an O'Leary
Subml.t,tc d to Journal of Geophysical Research.
ABSTRACT
The photometric phase function, color, normal albodo,
POIC-IrImetric pliuse, functJ on, and spectrophotoi,,Ietry of the
Apollo 12 soil are presented. In general-, the optical prop-
erties of the Apollo 12 soil are very similar to those of the
'	 gApollo .1.3. soil and of lunar mare .surfaces in eneral. Sig-
nificant differences are that the Apol.lo 12 soil. Is 20 per
cent brighter and considerably redder than the Apollo 11. soil.
Thesc may be explained by the presence of material comprisin6
a ray of' the. crater Copernicus.
YJR
.
1	
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i
4
INTRODUCTION
During the past year, several papers on the optical
proport.i.e s of Apollo 1.1 lunar samples have appeared in the
literature. 5.1here has been considerable overlap in coverage,
with emphasis placed on the spec trophotome-t,ry of rocl^a and
	 ,
fines and on the photometric and polarimetric Bahr-,use functions
of the fines. The agreement among various experimenters ha s
been generally good, with the result that the optical proper-
ties of the Apollo 11 fines are similar to those for a several
kn- area of Mare Tranauillitatis surrounding the landing site
[Adams wid McCord, 1.910; O'Leary and Briggs, 19701,
Nevertheless, there remains some significant discrepan-
cies between various experiments, particularly in their in-
terpretation. it is not our purpose here to present a dis-
course on conflicting interpretations; only after much further
Y
study, maybe never, wAll. a coherent story arise regarding the
optical properties of the lunar surface,
In this paper, we present in detail the optical properties
of the Apollo 12 samples  and compare them to Apollo 11 samples
and to the Moon as a whole. This work is an extention of
similar studies performed on Apollo 11 samples [O'Leary and 	 j5
Briggs, 1970]. We have attempted to vary as many parameters	 t
as is reasonably possible in order to simulate the undisturbed
lunar surface. Only then is it possible to make inferences
regarding (1) the correlation between samples with large lunar
-1-
.
x6
At
ANK NOT FII.11a '
V'AVrMING I'AG 3— 3—
reflectivitles at 0.56µ wavelength of the Apollo 12 and Apollo
11 powders with a "mean moon" curve taken from Hapke [1.968 ]
and normalized to the normal albedo of the Apollo 11 si to
(Wildey and Pohn, 3.969). Both samples were prepared by grad-
ually dropping the fines from a height of about 2 centimeters
onto a sample tray. While the photometric curves of the two
samples have similar shapes, the Apollo 12 soil is noticeably
the brighter. For a given phase angle, the Apollo 12 soil is
about 20 per cent brighter than the Apollo 11. soil, suggesting
that an appreciable quantity of ray material is mixed in with
the Apollo 3-2 mare material.
For e = 600, the photometric functions of the soils are
flatter at large phase angles than is the moon. This effect
can probably be attributed to large scale roughness of the
lunar surface as will be discussed later in this paper.
Figure 2 gives reflectivity measurements for Apollo 12
soil in three colors. Extrapolation to zero phase angle re-
sults in the normal albedoes given in Table 1. Laboratory
determinations of the color index, B-V, as a function of phase
angle are plotted in Figure 1, along with the earthbased
observations of Gehrels, Coffeen and Owings [1964) corrected
for a solar B-V of 0.63. The Apollo 12 soil is redder than
both the Apollo 11 soil and the mean value of the moon, and
shows greater reddening with phase angle than Apollo 11 soil.
Both samples show a minimum in B-V at — 5 0 phase angle, implying
I
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a steeper opposition effect at longer wavelengths than at
shorter wavelengths. Unfortunately, information about the
normal a:l bedo and phase function of the Apollo 12 site from
remote sensing are not available, so a detailed quantitative
comparison between soil, and site is not available. However,
Mitchell and Pellieori [1970] have observed a 80 km-diameter
region in Oceanus Procellarum about 225 I m from the Apol-lo 1.2
landing site. Corrected for the solar B-V, their B-V value for
the region Is, 0.14 at 90 0
 please angle, which is considerably
bluer than the Apollo 12 soil and bluer than any of 17 other
lunar regions sampled. Again, this suggests that ray material
from Copernicus, which is much redder than mare regions [Coyne,
1965), is mixed in with the Apollo 12 'soil.
The polarization dependence on phase angle for the Apollo
12 soil is very similar to that of the moon as a whole [Hapke,
1968] as shown in Figure 3. However, both samples have their
peaks in polarization occurring at somewhat greater phase angles
than does the moon [Pellieori, 19693. The maximum polarization
t
of the Apollo 12 sample is in good agreement with earthbased
l	
observations while that of Apollo 11 is anomalously high.
Figure 4 gives phase dependence of polarization for the Apollo
12 soil in three colors, again in good agreement with the earth-
based studies of mare regions by Pellieori [1969].
A study of the optical behavior for three degrees of com-
paction resulted in the curves.of Figures 5 and 6. A process
0
.1	 I
s
—5—
of roughening, the soil surface by raking with the point of a
needle gave the "fluffed" state C.Hapke, 1968; Hap%e et a] ..
19'(0a and b ] . Lightly tamping the: sample with a flat siirfac.e
of stainless steel. approximately 1. cm in diameter produced a
smooth, level state of compaction labeled "packed". The
"dropped" stage was obtained by gradually dropping; the powder
as were the oampl,es for the previously presented curves.
The "Packed" curves are significantly different-, from both
states of Lesser compaction and from earthbased observations
of the moon, in that they show a broad speeula,r peak in reflec-
tivity and excessive polarization at large phase angles. The
range in normal albedoes of the various states was :Found to be
similar to that reported for the Apollo 11 soil. by Hapke et
[197 0a and b]. The "packed" and "fluffed" curves of reflec-
tivity and polarization bound the "dropped" curves, with the
"fluffed" sample showing slightly lower reflectivites and lower
polarizations than the "dropped" sample. Comparison of these
samples with lunar surface photographs immediately rule out the
""packed" state as a natural state of the lunar material. On
the other hand, the "fluffed" state appears to have an unnatural
roughness which is unlikely (though not impossible) to occur
on the lunar surface. From examination of photographs, the
true answer seems to lie between the "dropped" and "fluffed"
states, with a tendency toward the dropped state. It is for-
tunate that the optical properties of both dropped and fluffed
4
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samplc;s are alini1 ar, and that different fl.uffirigs and droppings
produce results which repeat quite faithfully.
,r	 It is now possible to examine certain discrepancies be-
tween Apollo 11 results with the benefit of our study of the
Apollo .12 soil.. First, Hapke et al. (3.97.0b) have sx)ggested
that the relative flatness of the ;phase function of the lunar
soil. with respect t,o the Moon as a whole (e.g., see Fig. 1
C.
 
= 600 ) would tend to disappear if the sample is raked and
fluffed, However, in Figure 5 we see that fluffing the sail
affects -the slope of the phase function only slightly, and it
appears unlikely that varying the compaction is sufficient to
produce the required match. It is more probably that large-
scale roughness, e.g., mountain and rock shadows, account for
the steeper phase curve for the moon as a whole.
The anomalously high polarization of our Apollo 11 soil
(Fig. 3) is difficult to explain, because similar measurements
by Geake et al [1970 do not show the anomaly. It is possible
that our sample may have compacted too much from its dropping
or that there was an inadvertent selective sifting in particle
size. We did not observe a similar anomaly with the Apollo 12
soil, so it is unlikely that our previous measurements suf-
fered from systematic instrumental errors. Moreover, the Apollo
11 polarization anomaly repeated for successive droppings sug-
Besting, that the effect is real. We are presently investigat-
ing possible sources of error attributable to sample preparation,
.1
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0
but are meanwhi3c forced to conclude that our Apollo 11 soil
exhibitod anomalotisly high polarizatio?vis,
Flina3ly.. tho normal albedo of our Aj)ollo 11 soil stimple
was ounsiderably 3.cA,:er than that measured on the Apollo 3.1
soil by Hapke et 
.
al (1970a and b), but in good agreement with
•	 that obtained from Apollo photography of a several. km ? area
surronnaingr Irraii(juillity Base (Wi1doy and Pohn ., 1969). It is
0
unlikely that the state of compaction or particle size selec-
t1on Ure sufficient to explaii. the discrepancy. Hapke et 8.1
[19701)] suggested that the Apollo . 11 soil was not typical, of
the Tranquillity site ., an explanation which is not required
for our results, In fact, the albedo of our Apollo 11 sample
integrated over a hemisphere was somewhat lower than that of
the lunar surface surrounding Tranqui.1lity Base [O'Leary and
Briggs, 1970]. At that time we suggested that the subsurface,
soil of the sample was slightly darker than the surface soil,
in agreement with Surveyor results (Jaffee ems, 1968; Shoe- ­
maker et al l, 1969) and consistent with the concept of ultra.
violet bleaching of the lunar surface by the sun (Cohen and
Hapke, 1968).
Figure 7 shows the diffuse reflectances of Apollo 12 soil
and rocks from 0.44 to 1.8µ wavelength as measured by a Cary
14 spectrophotometer with an integrating sphere. Spectro-
photometry of the rock required that two chIps (12022,88 and
12022,89) from the same rock be measured together because of
rk
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their smal-I size relative to the slit length. Included in
r
Figure 7 are the curves determined previously [ O l Leary and
Briggs, lg(0 ] for the Apollo 11 samplcns. The Apollo 12 soil.
shows a curve. of steadily increasing albedo toward longer wave
lengths, a behavior exhibited by the Apollo 11 material. and by
the moon as a whole. The albedo of the Apollo 1.2 soil, is
higher than that of Apollo 11, as in the case of the phase
function observations. The particular rock samples investicra
ted had a broad absorption band centered at about	 the
soil also sh(ms a weak band near this wavelength. Observations
of the Apollo 12 landing site coupled with the calibration work
of McCord and Johnson [1970] yield a. spectr6photometric curve
from 0.4. to 1.111 which is similar in slope to that of the
Apollo 12 soil. Similar conclusions may be drawn for the spec»
trophotometric results of Apollo 12 as those for Apollo 11
(Adams and Jones, 1970; Hapke ems, 1970a and b; O'Leary  and
Brigbs, 1970; Adwiis and McCord, 1970).
•
	 CONCLUSION
There appear to be no major differences between the opti-
cal properties of lunar soil returned from the Apollo 11 and
12 missions and those of the lunar mare surfaces.' Some dif-
ferences do arise in albedo, color and polarization ,  but they
can probably be attributed to differences in properties of the
.
6W9-
surface and subsurface soil ., and in an unkncAin blend of mare
and upland material. The proper degree of compaction of the
lunar sxirface can be reasonably reconstructed in tlie laboratory
without introducing major changes in optical, properties,
A few discreponeJ,as still exist Vetween various experi-
menters. More experiments on the Apollo 11 ana 12 samples, as
well as experiments to be performed on samples returned from
future sites, will most likely resolve these conflicts,
ACKMLEDGEMENT
We thank J. Smith for his valuable assistance. This work
was performed under NASA Grant NAS-98018,
A
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Tab 1e 1.
Normal Albedoes of the Lunar powder.	 Samples.
Blue Green Red
Filt(iii, Wavelength (}t) 0, 4.4 0.56 0,65
Normal
Apollo
Albedo'
3.2	 (•^-	 003). 0. Og^^ 0. ^2^ 0.13E
Normal
oApoll
A:lbedo '
(-1•	 . 11	 003) 0.0$3 ' 0.102 0.3-15
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REFERENCES
6
Adams ,,	 J.B.	 and R.L. Jones, Spectral refleQUvIty of lunar
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^a,^—
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,
rI+ 'JGiIHk; CAi^^1'ioNa
1+'i^jure :t .	 TYle rc^f:l.ect:tvity of AYa°la.o ll t^.nd 1.2 ^.ian^,r so1.l
'^	 sa2ti^^^,c:•N vc^rsiis pkis,se an^;1.e at U. S^{.^ wavc^.ength and for vlearr-
i27^; s,n^;7 cis e == 0° and GU°.	 (To;^) Cod or index l3-ST of tY^e
powac;^; ,aa,irip:l.r•:s v^;:^^sus phs.se ^b2"}^';:2.C: :['or r =^- 0°,	 Also pl.otf,ed
arc ^^,Y^e redcYen^.zag i'^u2^,cti,or^ oi' the Ent^.re mcaon and. scvera].
va:l.uc>:; for a re^^ion of Ms,r^: ^'ranquill^.ta^t,is ab determ3.ned Uy
Gc^^a:^c1.s e=: ^ a1 [ ^ yG 11• a and rorrec t;c^^d fora ;p olar. B-V of 0. ^3..^	 .
F1.gu:^^e ? . ^'hc .roflec^t;iv^_ty ofi' t17e Apollo soil versus pYz^.se
arlt;lr, s,t three wave^.cngths and for viewing angles c w Cho
a.nd GO° .
T^':i^;urc 3, The pols.rl2a^L^.on of Apollo 1], ana 1.2 ^.uns.r soils
versus Yailase angle a.t 0.5^u. wavelength L^27d view^.ng angles
E ^ 0° and 60°,
Figure ^^. The polari^atiori of the Apollo '12 soil. versus phase
an^;^_e at trsree wavelengths and viewing; angles E = 0° and
c = ^0°.
F9.gure a. The reflectivity of the Apollo l2_ soil versus phase
angle at O. 56µ wavelength and viewing angles E = 0° and 60°
for three degrees of cornpactian.
,^ Figure G. The polarization of the Apollo 12 soil versus phase
angle at O.56µ wavelength and v^.ewing angles ^ = 0° and 600
for three degrees of compaction,
I{'igurE 7, Diffuse reflectance versus wavelength of the Apollo
<i.l and l2 lunar sa2nples .
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Optical properties of Apollo 12 moon samples

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