Lunar pyroclastic deposits represent one of the primary anticipated sources of raw materials for future human settlements. These deposits are fine-grained volcanic debris layers produced by explosive volcanism contemporaneous with the early stage of mare infilling. There are several large regional pyroclastic units on the Moon (for example, the Aristarchus Plateau, Rima Bode, and Sulpicius Gallus formations), and numerous localized examples, which often occur as dark-halo deposits around endogenic craters (such as in the floor of Alphonsus Crater). Several regional pyroclastic deposits were studied with spectral reflectance techniques: the Aristarchus Plateau materials were found to be a relatively homogeneous blanket of iron-rich glasses. One such deposit was sampled at the Apollo 17 landing site, and was found to have ferrous oxide and titanium dioxide contents of 12 percent and 5 percent, respectively. While the areal extent of these deposits is relatively well defined from orbital photographs, their depths have been constrained only by a few studies of partially filled impact craters and by imaging radar data. A model for radar backscatter from mantled units applicable to both 70-cm and 12.6-cm wavelength radar data is presented. Depth estimates from such radar observations may be useful in planning future utilization of lunar pyroclastic deposits

Campbell, B. A.

Campbell, D. B.

Hawke, B. R.

Stacy, N. J.

Thompson, T. W.

Zisk, S. H.

English

NASA Technical Reports Server

16 New Technologies /or Lunar Resource Assessment
concentrations from orbit under the assumption of an infinitely
deep homogeneous chemistry, when the major-element compo-
sition is simultaneously determined using a gamma ray detector
[7,81. Newer calculations reported here show that interpretation
of these data can be in error if the hydrogen content of the near-
surface regolith is not homogeneous.
Hydrogen is notable for its high efficiency in moderating cosmic-
ray-induced fast neutrons to thermal energies. The ratio of thermal
to epithermal neutrons (those in the process of slowing) is a very
strong function of the hydrogen content. In order to get high
precision at low H concentration, however, the abundances of
elements that strongly absorb thermal neutrons such as, e.g., Ti,
Fe, Sm, and Gd, also need to be determined.
Recent calculations [7,8] showed that a determination of the
hydrogen content down to levels of 10 ppm from orbit requires
only a 1% statistical precision. However, interpretation of these
data is subject to systematic uncertainties that may be larger than
the 1% level, depending on the nature and magnitude of H
abundance variations as a function of depth. Basically, the reduced
mean-scattering path of thermal neutrons caused by the presence
of H tends to concentrate neutrons from surrounding material
into the high-hydrogen-content side of abundance-changing inter-
faces. For example, comparison of a lunar meteoriffc chemistry
loaded uniformly with 100 ppm of hydrogen down to 200 g/cm2
below the surface, and then with 10 ppm H to the 900 g/cm
level, with the same chemistry loaded with 100 or 10 ppm H
uniformly down to the 900 g/cm2 level, shows about 2% variation
in subsurface gamma ray production rates (and hence in thermal
neutron number density).
Instrumentation: The neutron detection is made with two
identical 3He gas proportional counters, one being bare and the
other being wrapped in Cd. The bare detector is sensitive to both
thermal and epithermal neutrons, whereas the one wrapped in
Cd is sensitive only to epithermal neutrons because Cd is a strong
absorber of thermal neutrons. The abundances of the strong
neutron absorbers in the soil will be made with a simple scintillator
gamma fay spectrometer. Although this type of detector does not
offer the high sensitivity of cooled semiconductor detectors, they
are more than adequate for determination of the abundances of
Fe and Ti, the principal neutron absorbers on the Moon. Samarium
and Gd are not easily detected with this approach, but because
of our extensive database of lunar rock types, these elements can
be estimated easily from the abundances of other elements that
will be determined with the GRS, especially K, U, and Th.
Delivery and Support System: The instruments will be del-
ivered to the lunar surface via a penetrator with a separable
afterbody. Using the soil penetration equations of [9] and assuming
a penetrability comparable to dry silt or clay, a forebody mass of
30 kg, a diameter of 7.5 cm, and an impact velocity of 100 m/s,
the forebody will penetrate to a depth of about 10 m while the
afterbody will remain partially exposed on the surface to maintain
communication with Earth. Reduction in this maximum depth
can be readily achieved by reducing the impact speed. Both parts
will contain neutron and gamma detectors in order to compare
the differences between the surface and deep abundances. The
penetrator will be battery operated to provide a life of about one
week on the lunar surface.
Conclusions: Calculations of the depth dependence of ther-
mal, epithermal, and fast neutron fluxes and consequent captute
gamma ray production rates have shown their utility in deter-
mining the depth profile of H at the 10 to 100 ppm sensitivity
level. Orbital surveys of these same data are not capable of such
a determination. The two techniques therefore complement one
another. Whereas orbital surveys can provide comprehensive maps
of suspected hydrogen concentrations due to solar wind implan-
tation, a penetrator can provide ground truth for a small selection
of sites to allow estimates of the depth dependence of such deposits
and consequently an evaluation of the utility of such deposits as
a source of resources to support human habitation on the Moon.
References: [l)Wittenburgetal. ( 1986) Fusion Teen. , 10,167.
[2] Haskin (1989) LPSC XX, 387. [31 Arnold (1975) PTOC. LSC
6th, 2375. [4] Swindle et al. (1990) Mining Lunar Soils for }He.
UA/NASA SERC, Tucson. [5] Morris (1976) Proc. LSC 7th, 315.
[61 Jordan (1990) In Space Mining and Manu/acturing, VII-38, UA/
NASA SERC, Tucson. [7] Metzger A. E. and Drake D. M. (1991)
JGR, 96, 449-460. [8] Feldman W. C. et al., GRL, 18, 2157-2160.
[91 Young C. W. (1972)Sandia Lab. Re/>. SC-DR-72-0523.
724 •??
ESTIMATING LUNAR PYROCLASTIC DEPOSIT DEPTH
FROM IMAGING RADAR DATA: APPLICATIONS TO
LUNAR RESOURCE ASSESSMENT. B. A. Campbell1, N. J.
Stacy2, D. B. Campbell2, S. H. Zisk1, T. W. Thompson3, and
B. R. Hawke1, 'Planetary Geosciences, SOESTT University of
Hawaii, Honolulu HI, USA, 2NAIC, Cornell University, Ithaca
NY, USA, 3Jet Propulsion Laboratory, California Institute of
Technology, Pasadena CA, USA.
Introduction: Lunar pyroclastic deposits represent one of the
primary anticipated sources of raw materials for future human
settlements [1]. These deposits are fine-grained volcanic debris
layers produced by explosive volcanism contemporaneous with the
early stages of mare infilling [2,3). There are several large regional
pyroclastic units on the Moon (for example, the Aristarchus
Plateau, Rima Bode, and Sulpicius Callus formations), and numer-
ous localized examples, which often occur as dark-halo deposits
around endogenic craters (such as in the -floor of Alphonsus
Crater). Several regional pyroclastic deposits have been studied
with spectral reflectance techniques: The Aristarchus Plateau
materials were found to be a relatively homogeneous blanket of
iron-rich glasses [4,5). One such deposit was sampled at the Apollo
17 landing site, and was found to have ferrous oxide and ti tanium
dioxide contents of 12% and 5% respectively [6). While the areal
extent of these deposits is relatively well defined from orbital
photographs, their depths have been constrained only by a few
studies of partially filled impact craters and by imaging radar data
[7,H- In this work, we present a model for radar backscatter from
mantled units applicable to both 70-cm and 12.6-cm wavelength
radar data. Depth estimates from such radar observations may be
useful in planning future utilization of lunar pyroclastic deposits.
Radar Scattering Model: The depth of a pyroclastic layer is
estimated based upon the ratio of backscattered power between
a mantled region and an area of unmantled terrain assumed to
represent the buried substrate. Several conditions are required:
(1) Only the depolarized (same-sense circular) echo is modeled,
to avoid consideration of large single-scattering facets; (2) thete
is no volume scattering Within the pyroclastic (i.e., there are no
inclusions latge with respect to the radar wavelength); and
(3) the depolarized backscatter originates largely within the re-
golith substrate, and not at its upper surface. Conditions (2) and
(3) are supported by previous studies of the eclipse temperatures
https://ntrs.nasa.gov/search.jsp?R=19930008051 2020-03-24T07:24:47+00:00Z
LPJ Technical Report 92-06 17
incidence Angle-55
. Const-3.0
Ce/isiry-1.5 gfcn"3
Fig. l.
of pyroclastic deposits and analysis of tO-cm backscattering from
the lunar regolith [8,9].
Input parameters to the model are the radar wavelength, the
radar incidence angle, and the dielectric parameters of the pyro-
clastic layer and the regolith substrate. The model used here
permits analysis of a pyroclastic layer with varying density as a
function of depth. The electrical properties of an Apollo 17 glass
sample (6) as a function of density p are used here:
e, = 2.1" tan 8 = 0.0037 p
where e, is the real dielectric constant and tan 6 is the radar loss
tangent. The real dielectric constant of the lunar regolith has been
measured to be ~3.0, but may vary from mare to highland regions
(10). The incident circularly polarized wave is decomposed into
horizontal and vertical polarized components, and the Fresnel
transmission coefficients for each interface are calculated. Atten-
uation within the pyroclastic is assumed to be exp(-2aL), where
L is the path length and a is the attenuation coefficient [ -111
a = k (0.5*|(tanS2 + l)a5 - I])05
where k is the radar wavenumber in the pyroclastic deposit. This
expression is solved numerically in our model for the varying
density within the lossy medium.
All the scattered radiation within the regolith layer is assumed
to be randomly polarized, with some total backscattering efficiency
S. In our model, we thus combine the H- and V-polarized com-
ponents within the regolith, multiply by S, and split the remaining
power evenly between the two polarizations for the return trip
to the surface. The energy that exits the pyroclastic is assumed
to have random phase, but may have an elliptical polarization
due to the differing H and V transmission coefficients. The scatter
from an unmantled regolith is also calculated, and the two results
are combined into a ratio of total mantled/unmantledbackscatter.
This cancels the S term in both expressions. The ellipticity of
the final scattered wave is assumed to be close to unity. The results
are plotted as a function of power ratio vs. depth for a given set
of electrical and density parameters.
In mathematical terms, the H and V backscattered powers from
a mantled region are
Ph = 0.25 * S *A2 * (Ths/f * V + TVS'P * T/r)' Thr/p * Thr'5
Pv = 0.25 * S * A2' (Th5'11' V + TV5/P ' T/") * Tv"p * T/>
where A is the total attenuation factor, and the T terms refer
to H- and V-polarized Fresnel transmission coefficients for the
layer interfaces noted in their superscripts; s, p, and r refer to
space, pyroclastic, and regolith respectively. The backscattered
powers from an unmantled area are
Ph = 0.25 * S * (Ths/r + T/') * Vs
Pv = 0.25 • * S ' (Ths'r + T/') * Tvr/>
Results: Figure 1 shows an example of this model for an
incidence angle of 55°, corresponding to the Earth-based viewing
geometry for the Aristarchus Plateau. The pyroclastic density was
assumed to be a uniform 1.5 g/cm3. The regolith real dielectric
constant is assumed to be 3.0. Both 70-cm and 12.6-cm radar
wavelengths are tested, and it is seen that the shorter wavelength
is attenuated much more rapidly in the pyroclastic, as expected.
The ratio between average 70-cm returns from the plateau and
the mean lunar echo is —0.30, implying a depth of 6-7 m based
on these electrical parameters. This estimate is a lower bound;
the actual buried highland terrain is likely much brighter than
the Moon-wide average, implying a greater mantle depth.
Tests of the above model show that (1) it is relatively insensitive
to variations in incidence angle between 20° and 60°, (2) it is
insensitive to regolith dielettric variations from 2.5 to 4.0,
(3) the scattered wave ellipticity is >85%, and (4) density vari-
ations with depth in the pyroclastic, within the narrow range
permitted by realistic values of 1.5-2.0 g/cm3, are not important
unless the changes are very rapid. The value of the density at
the upper surface of the layer is probably a satisfactory estimate
for the overall density of such shallow deposits.
Conclusions: This technique may permit remote estimates
of pyroclastic mantle depths from either Earth-based or lunar-
orbital radar systems. A 3-km resolution map of the lunar nearside
is available for 70-cm radar wavelength [12], and ongoing work
at Arecibo Observatory provides additional data at 1 2.6-cm wave-
length. We anticipate that such studies will be required in the
planning stages of a lunar base project for resource assessment.
References: 1 1 1 Hawke B. R. et al. ( 1 990) Proc. LPS, Vol. 2 I ,
377-389. [2] Head J. W. (1974) Proc. LSCSth, 207-222. [3] Wilson
L. and Head J. W. (1981) JGR, 86, 2971-3001. (.4) Caddis L. R.
et al. (1985) /carus, 61. 461-489. |5) Lucey P. G. et al. (J986)
Proc. LPSC 1 6th, in JGR, 91, D344-D354. [61 Bussey H. E. (1979)
Proc. LPSC /Oth, 2175-2182. [7] Zisk S. H. et al. (1977) Moon.
17, 59-99. [8] Moore H. J. et al. (1980) USGS Pro/. Pa/>er 1046-
B. [9] Thompson T. W. and Zisk S. H. (1972) In Prog, in Aero.
and Astro., 83-117. |10) Hagfors T. and Evans J. V. (1968) In
Radar Astronomy, 219-273. [ I l l Ulaby F. T. et al. (1986) In
Microwave Remote Sensing, 67. [ 1 2] Thompson T. W. (1987) Earth
Moon PlflnetsJZ, 59-70. H *) S OO
A GROUND-BASED SEARCH FOR LUNAR RESOURCES
USING HIGH-RESOLUTION IMAGING IN THE INFRA-
RED. C. R. Coombs and T. S. McKechnie, POD Associates,
Inc., 2309 Renard Place SE, Suite 201, Albuquerque NM 87109,
USA.
Introduction: When humans return to the Moon (". . . this
time to stay ..." [ I ] ) lunar resources will play an important role
in the successful deployment and maintenance of the lunar base.
Previous studies have illustrated the abundance of resource mate-
rials available on the surface of the Moon, as well as their ready
accessibility [e.g., 2-5]. Particularly worth considering are the lunar
regional (2000-30,000 km2) pyroclastic deposits scattered about
the lunar nearside. These 30-50-m-thick deposits are composed


Estimating Lunar Pyroclastic Deposit Depth from Imaging Radar Data: Applications to Lunar Resource Assessment

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server