When high resolution measurements of the phase variation of the lunar disk center brightness temperature revealed that in situ regolith electrical losses were larger than those measured on returned samples by a factor of 1.5 to 2.0 at centimeter wavelengths, the need for a refinement of the regolith model to include realistic treatment of scattering effects was identified. Two distinct scattering regimes are considered: vertial variations in dielectric constant and volume scattering due to subsurface rock fragments. Models of lunar regolith energy transport processes are now at the state for which a maximum scientific return could be realized from a lunar orbiter microwave mapping experiment. A detailed analysis, including the effects of scattering produced a set of nominal brightness temperature spectra for lunar equatorial regions, which can be used for mapping as a calibration reference for mapping variations in mineralogy and heat flow

Keihm, S. J.

English

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https://ntrs.nasa.gov/search.jsp?R=19840006944 2020-03-21T01:19:36+00:00Z
(NASD-CR-174577) ENALTSIS CF EFGOLITU
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NAS!1-3744
SAI 1-141-nO-0374-00
July 31, 19A3
Stephen J. Keihm
Principal Investigator
Planetary Science Institute
203n F. Soeedway, Suite ?nl
Tucson, Arizona 19714
W2) W-033?
3
4
V
Q^
ORIGINAL PAGE IS
TAILF OF MNTFNTS	 OF POOR QUALITY
1.	 Introduction ........................................................1
111.	 Vertical Structure Models ...........................................?_
IV. Volume Scatterinq Models ............................................4
V. Summary and Recommendations for Future ''lork .........................5
:J1 1 -! 1I .
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A proposal entitled "Analysis of Regolith Electromagnetic Scattering as
Ponstrained by Nigh Resolution Farth-'used 4easurements of Lunar Microwave
Emission" was submitted to NASA's Lunar and Planetary Program on August 1, 19711.
`-	 The planned research constituted a continuation of lunar regolith studies
t
originally prposed by the Principal Investigator in 1979 while at the
t
Lamont-Doherty r`,eological ()bservatory of Columbia liniversity. The work described
F	 herein began in the sprinn of 1979 and covers a four-year period ending July 31,
^-	 The primary objectives of the proposed research were the development of
theoretical models of planetary regolith scattering processes and an analysis of
the effects of scattering on the interpretation of passive microwave and radar
remote sensing data. netailed results are presented in the attachments and will
only be summarized within this report. In Section II, knowledgs of lunar rnqolith
properties prior to this study is reviewed, and the justification for scattering
studies is presented. In Section III, results of the vertical structure modeling
studies are presented and applications to the moon and Mars described. In Section
Iv, the volume scatterinq modals are described, and results relevant to the
intepretation of lunar remote sensing data are outlined. A summary and
recommendations for future studies are presented in Section V.
11. lackground
The Apollo Program provided a wealth of data relevant to the thermal and
electrical properties of the lunar regolith. in " experiments and
laboratory measurements of returned samples yielded critical constraints on the
depth and/or temperature dependence of regolith density, thermal conductivity,
heat capacity, and dielectric properties. In particular, at the Apollo 1 5 and 17
heat flow sites, surface and subsurface temperatures were measured directly over
rr
L,
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OF POOR QUALITY
many lunations, permitting the develo pment of a detailed model of regolith
thermo h sical Properties (Keihm 	 1973• Keihm and LanP Y
	 P	 St ^.L .
	
seth 1973).9
This model was remarkably successful in matching previous earth-based observations
of lunar disk center phase and eclipse variations at both infrared and microwave
wavelengths (Keihm and Lanqseth, 1975).
This result generated confidence that the regolith properties measured at a few
local sites were representative on a global scale, and that high resolution
microwave measurements from a lunar orbiter could be utilized to map local
variations in mineralogy and heat flow.
In the mid-1970's, however, confidence in the potential of remote microwave
lunar map p ing suffered a setback when a number of theoretical studies (e/q.,
Fnqland, 1975; Fisher and Staelin, 1977) suggested that scatterinq, assumed to be
negligible in previous regolith models, could be significant in masking the
microwave signatures of both mineralogy and heat flow. In addition, new
hinh-resolution measurements of the nhase variation of the lunar disk center
hrightness temperature indicated that in jilt regolith electrical losses were
larger than those measured on returned samples by a factor of 1.5 - ?.9 at
centimeter wavelengths (^ary and Koihm, 1975). The additional component of
electrical loss was postulated to be due to scatterinq and the need for a
refinement of the regolith model to include a realistic treatment of scattering
effects was clearly identified. For the ensuing research, two distinct scattering
regimes were considered: (1) vertical variations in dielectric constant, and (?)
volume scatterinq due to subsurface rock fragments.
III. Vertical Structure 'iodels
Prior to this study, previous theoretical models have examined the effects of
vertical dielectric structure for specific appl%cations with limiting
approximations (e.q., lurvich of 1 j., 1973; Tsang and Konq, 1975; Tsang at
L
0r, x ON
, f. g-PAGE IS
OF POOR QUALITY
s31., 1475; Fisher and Staelin, 1977). The method employed for our analysis of
lunar regolith vertical structure places no restrictions on the dielectric
profiles which could be treated. The analysis is based on the formulation of
Stogryn (147) who outlined a numerical approach for solution of the wave equation
and derivation of the reflectivity and emission weightinq function of a vertically
varying half-space.
Three types of vertical variations, appropriate to the lunar regolith
}	 structure, were considered: (1) A two-layer model, composed of solid rock
overlain by a mantle of fine soil; M A multi-layer structure composed of
centimeter-scale strata of coarse and fine-grained soils with moderate dielectric
contrasts between adjacent layers; (3) A continuous variation of dielectric
constant with depth. 11odels were analyzed in terms of their effects on the zerothP	
i	
and first harmonic ( p hase variation) of the lunar disk center brightness
tenoerature. retailed results are presented in Keihm and butts (19A1; Attachment
1 ) and can be summarized as follows:
*Lame impedance contrasts (bedrock overlain by a soil mantle) will
E	 cause no significant spectral variation over the wavelength range 3-30 cm
unless interfaces occur within 3 m of the surface. Rock substrates
occurring at depth <1 m could produce a negative spectral gradient
^.	 comparable in mannitude to the effect of a heat flow gradient. However,
such shallow bedrock occurrence is believed to be rare for the lunar
frontside.
•Lunation variations in microwave brightness temperatures are not
affected by a rock substrate that is deeper than 11 cm.
I_ *The occurrence of multiple soil-strata layers of moderate dielectric
contrast (as suggested by Apollo core tube studies) is not likely to
affect either the absolute level or lunation variation of centimeter-wave
-3-	 -
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Tha
1
ORIGINAL PAGE IS
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briqhtness temperatures due to lateral variations in layer thicknesses
and depths.
•"lodels of the ra p id increase in soil density, believed to be
characteristic of the lunar regolith's upper 10 cm, predict a brightness
temperature decrease of ?-1 goK over the wavelength range 3-3g cm. The
onset of the spectral decrease occurs at a wavelenqth of 3-15 -m,
dependent on the thickness of the porous regolith surface layer
cm). The mannitude and slope of the s pectral decrease depend,
-etpectively, on the contrast and thickness of the density transition
gayer.
*Plausible continuous density variations do not significantly effect
the amplitude of diurnal brightness temperature variations or subsequent
inferences of electrical loss prooerties.
The two-layer models have also been utilized to examine the feasibility of
ronorted (? i sk and "•louq i n i s-"4ark, 1 L111) seasonal variations in the radar
i ,-
reflectivity of the Sinai p lanum region of "oars. We have proposed an eolian
explanation for the reflectivity variation, postulatinq a seasonal stripping and
recoatinq of near surface rock strata by local transport of dust. r)eiails are
presented in butts and Keihm, M3 U ttachment 5).
I -
i
IV. Volume Scatterinn 'yodels
°rior to this study, England (1875) apolied radiative transfer theory, using
the Raylerqh approximation, to the problem of lunar regolith fragment scattering.
Assuminq a single effective particle size, he concluded that spectral brightness
temperature variations of tens of degroes could occur at centimeter wavelengths,
comp letely masking the spectral siqnatures of mineralogy and heat flow.
method applied for the present study also utilizes the radiative transfer
formulation, but employs "tie scattering phase functions and allows for a
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.,	 l
ORtONVAL PAGE {g
OF POr^ "II ITY
continuous distribution of particle sizes. 'lodel results and implications for
lunar remote sensing interpretation are presented in Keihm (119?; Attachment 3)
and can be summarized as follows:
*For subsurface fragment size populations representative of Surveyor
mare sites, emission darkening of 1-4 0K is predicted. For freshly
cratered regions, such as Tycho, brightness temperature decreases of up
to 70K, relative to the non-scattering case, are predicted.
*Spectral signature due to fragment scattering depends on the
relative distribut i on of particle sizes. For mare itites
.
, a small
increase in brightness tem perature, approximately ?n'' of that due to
heat flow, is predicted over the wavelength ranqe 3-30 cm. Regions
with a hiqh fraction of larger size Q10 cm) fragments, such as Tycho,
can produce nenative s pectral gradients, comparahle in magnitude to the
heat flow signature.
*The amplitude of lunarion variations in brightness temperature can
he affected by fragment po pulations in the u pper 10 cm surface layer.
If mare-ty pe populations exist in this layer, scattering losses wi;l be
comparable to absor ption losses; and purely absorptive properties, such
as the electrical loss tangent, can be seriously overestimated from
remote measurements if scattering is not taken into account.
!'omparison of laboratory and remote inferences of electrical loss
suggest +hat scattering effects may be particularly significant at
M_,	 wavelengths of 9-13 cm for the lunar disk center.
oThe featurelessness of microwave brightness temperature maps of the
lunar frontside indicates that the scattering properties of the upper
t g cm of regolith are remarkably uniform on a 79 g km scale.
i	 ^
ORIGINAL PAGE tS
OF POOR QUALITY
'	 N. gunmaary and Recommendations for Future Work
lur understandinq of the thermo physical properties which control the thermal
and radiative transport of energy in the lunar reqolith exceeds that of any other
extraterrestrial object. Fxtensive evidence does exist that regolith properties
are remarkably uniform on a ?9n km scale. Pxxnprehensive models now exist which
could he used to generate infrared and microwave maps of the lunar frontside at
all phases to be used as an absolute calibration standard. In this regard, the
primary questions remaining center on the variation of emissivity and surface
roughness effects with wavelength.
()ur models of lunar regolith energy transport processes are now at the state
for which a maximum scientific return could be realized fr<xn a lunar orbiter
microwave mapping experiment. A detailed analysis, includinq the effects of
scattering, has produced a set of nominal briqhtness temperature spectra for lunar
equatorial regions, which could be used as a calibration reference for mapping
km-scale variations in mineraloqy and heat flow (Keihm, I (V13; Attachment 4). A
coMplementary radar reflectivity experiment at comparable microwave wavelengths
would greatly enhance the scientific return of a passive orbital experiment.
The techniques developed for modeling the lunar egolilth have broad application
to the interpretation of remote sensinq data of other dry, atmospherp ;ess bodies.
It is antici pated that our progress in understanding the thermal and radiative
hchavior of the lunar regolith will lead to investigations of other planetary
surfaces which can be pursued with increased confidence and scientific output.
t^-6-
is
OF POOR QUALITY	 Peferences
f"utts, J.A. and Keihm, S.J. (193). Time-variable radar reflectivity of 'bars:
S Tian explanations, manuscript to be submitted to Icarus, ()ctober.
England, A. 14. (11175). Thermal microwave emission from a scattering layer,
Geoohvs. $ga. 0n_, 4484-441)5.
Fisher, A.I. and Staelin, O.H. (11177). Possible effect of subsurface
inhomoe)9neities on the lunar microwave spectrum, Icarus ;?, q8-1M.
gary, R.L. and Keihm, S.J. MM. Interpretation of ground-based microwave
measurements of the moon using a detailed rngolith properties model, Proc.
Lun. P(, net. Sci. Qgnf. nth, ?115-7nnn.
gurvich, A.S., Kalinin, V.I., and 11atveyev, q .T. (1083). Influence of the
internal structure of glaciers on their radio emission, Atmos. Qcean
DM. , 71?-717.
Koihm, S.J. (198?). Effects of subsurface volume scattering on the lunar
microwave brightness temperature spectrum, Icarus g?, 570-594.
Keihm, S.J. (IW ). Interpretation of the lunar microwave brightness temperature
spectrum: Feasibility of orbital heat flow mapping, submitted to Icarus in
April.
!<eihm, S.J. and r,utts, J.A. (11181). Vertical-structure effects on planetary
microwave brightness temperature measurements: Applications to the lunar
regolith, jays A_1, 701-??q.
Keihm, c.J. and gary, P.L. (1075). Comparison of theoretical and observed X3.55
cm wavelength brightness tem perature maps of the full moon, Pr12G. Lun. planet.
Sci. Conf. 10th, 2311-?31q.
Knihm, S.J. and Lanqseth, 14. 11. (1 1 73). Surface brightness temperature at the
Apollo 17 heat flow site: Thermal conductivity of the upper 15 cm of regolith,
proc. Lun. Sci,  ronf. 4th, ?511;-?513.
i
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-8-
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OF POOR OOALITY
Xeihm, S.J. and Lanqseth, M.r,. (075). Lunar microwave brightness temnerature
observations reevaluated in the light of Apollo program findings, Icarus
Z1. i'l1-13M.
Keihm, S.J., Peters, K., Lanqseth, 4.(1., and '',huts , J.L. Jr. (071). Apollo 11
measurements of lunar surface brightness temperatures: Thermal conductivity of
the upper 1 1/2 meters of reqolith, caCth O lanet. d,
337-151.
qtolryn, A. (1970). The brightness temperature of zi vertically structured medium,
Radio $	 S, 1397-14n,+.
Tsang, L. and Kong, J.A. (1975). The brightness temperature of half-space random
medium with non-uniform temperature profile, Radio e&j. 12, 19?5_1833.
Tsang, L., Njoku, F., and Kon-1, J.A. (1975). 'Aicrowave thermal emission from a
stratified medium with non-uniform temperature distribution, 1. _AQDI.
2byj. 41, 9177-5133.


Analysis of regolith electromagnetic scattering as constrained by high resolution Earth-based measurements of the lunar microwave emission

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