Content uploaded by Shashwat Shukla
Author content
All content in this area was uploaded by Shashwat Shukla on Jun 14, 2019
Content may be subject to copyright.
Content uploaded by Shashi Kumar Singh
Author content
All content in this area was uploaded by Shashi Kumar Singh on Apr 28, 2019
Content may be subject to copyright.
This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
IEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS AND REMOTE SENSING 1
Petrophysical Insights Into Lunar Mafic Extrusive
Basalts Over Reiner Gamma Formation
Shashwat Shukla , Shashi Kumar , and Valentyn A. Tolpekin
Abstract—The enigmatic swirls of Reiner Gamma Formation
(RGF) have been found to be associated with localized magnetic
surges and high albedo markings. The intriguing diversity within
deep-seated lithologies unravels the compositional evolution of
the RGF soil, thereby providing new insights into the geological
formation of lunar swirls. The present work contributes to the
petrophysical significance of possible mafic units by utilizing the
multisensor data from recent lunar missions. The compositional
characterization of mafic-rich regolith facilitates an improved high-
resolution eigenvector-based pyroxene spectroscopy. This provides
evidence of endogenic magmatic water in the spectra of high Mg
orthopyroxene with pronounced olivine content. Some regions in
the western part contribute to the presence of chromium with the
possible emplacement of mafic-rich basalts in the oxygen-deficient
environment. The sensitivity of the radar echo to the petrographic
units has been utilized to derive the physical characteristics of the
regolith grains. The regions with enhanced hydration attribute
to an increased dielectric permittivity with an associated surge in
bulk density and circular polarization ratio. This is in concordance
with the degree of maturity in the regolith due to space weathering.
Moreover, a relatively compact confinement of the soil pattern has
been observed in the immature mafic-rich patches, emphasizing the
mechanical stability of the regolith. The regional geomorphology
indicates the emanation of wrinkled ridges, wherein higher mafic
abundance zone attributes to the primitive source of extruded
magma. In essence, this study proposes the inclusion of local
charged dust variant in the global formation perspective of the
RGF.
Index Terms—Circular polarization, geology, magnetic fields,
moon, permittivity, radar, remote sensing, soil, spectroscopy.
I. INTRODUCTION
THE study of the early thermal evolution of the Moon has
provided new understanding in the bulk repository of the
lunar interior. This proves to be significant for extracting the
formation stages and constitution of the inner layers in conjunc-
tion with the Lunar Magma Ocean hypothesis [1]. According
Manuscript received September 29, 2018; revised March 6, 2019 and March
26, 2019; accepted April 1, 2019. This work was supported in part by the Faculty
of Geoinformation Science and Earth Observation (ITC), University of Twente,
Enschede, The Netherlands. (Corresponding author: Shashwat Shukla.)
S. Shukla is with the Geo-Informatics Department, Indian Institute of Re-
mote Sensing, ISRO, Dehradun 248001, India, and also with the Faculty
ITC, University of Twente, Enschede 7522 NB, The Netherlands (e-mail:,
sshashwat@ieee.org).
S. Kumar is with the Photogrammetry and Remote Sensing Department,
Indian Institute of Remote Sensing, ISRO, Dehradun 248001, India (e-mail:,
shashi@iirs.gov.in).
V. A. Tolpekin is with the Faculty ITC, University of Twente, Enschede 7522
NB, The Netherlands (e-mail:,v.a.tolpekin@utwente.nl).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JSTARS.2019.2909352
to the hypothesis, the initial cooling of the magma crystallizes
the denser magnesium-rich mafic minerals toward the interior.
Subsequently, due to the less dense nature of the plagioclase, it
forms the primary feldspathic lunar crust by floating over the
residual magma upon its crystallization. At this stage, the entire
bulk gets enriched with iron and a layer of incompatible ele-
ments, resulting from the late stage residual magma, emerges
in between. This produces a dense concentration of KREEP
(Potassium, Rare Earth Elements, Phosphorus) and Fe-abundant
cumulates like ilmenite. Eventually, this hypothesis ends up in
arranging a gravitationally unstable realization, thereby initiat-
ing secondary melting by forcing the heat producing elements
into the lunar interior [2]. Despite this overturn, the surficial
lithological composition of the lunar crust is anorthositic while
it evolves to gabbroic or noritic at depth [1]–[3].
One of the diagnostic attributes of the postintrusive lunar
magmatism is the presence of basalts. These are produced as
a consequence of fragmentary crystallization of silica-rich man-
tle along with melting of near-surface stratigraphic units. The
compositional characterization of different lithologies has un-
dergone a major alteration due to the early separation of arma-
colite, chromite, olivine and Fe-rich ilmenite from the extruding
basaltic magma [4]. Interestingly, low viscosity of the magma
has resulted in a diversified late-stage formation of basalts with
reduced SiO2amounts [5]. In order to capture different asso-
ciated cooling histories, the textures of the basalts have to be
critically analyzed. The returned lunar samples show a wide
range of sequential textures and crystallographic configurations
from vitrophyric to equigranular [4], [5]. In most of the basaltic
samples, the soil grains follow a fine crystalline structure with an
average size of 0.5 mm [6]. Some samples, however, contained
mafic minerals in the skeletal phenocrysts of around 1 cm (vit-
rophyric) and other samples contained interconnected vesicles
between the grains (equigranular) [7]. The physical properties of
the basaltic grains play a significant role in retrieving the mag-
matic origin of the separated near-surface mineralogical phases,
thereby understanding their cooling history.
Among all silicate and oxide minerals, the most abundant one
is pyroxene whereas the least abundant one is plagioclase. The
spectral properties of the pyroxene may be used to characterize
the evolution of the basaltic melts and provide new insights
into postintrusive/extrusive magmatic events [8], [9]. Several
studies have been performed in examining the spectroscopy
and thermometry of the pyroxenes using in situ lunar regolith
samples, consisting mainly of brecciated mixtures of clasts
[8]–[13]. However, due to the propinquity of the lunar surface,
1939-1404 © 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
2 IEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS AND REMOTE SENSING
the telescopic and orbital remote sensing observations have
gained extensive attention in characterizing the geochemical
composition of the broader lithology classes. In particular, the
diversity of the pyroxenes ranges from low calcium pigeonite
to orthopyroxene to high calcium augites [14]. The phenocryst
of a basaltic regolith grain enriched in pyroxene is complexly
zoned grading from a more magnesian pigeonite core to a
rim attributing to high calcium augite [4]. The geochemical
ferroan anorthosite (FAN) suite lithology of the lunar high-
lands has been correlated with a higher proportion of Fe-rich
orthopyroxene [6], [15]. The existence of orthopyroxenes
in FAN suite reflects their slower cooling compared to the
basalts. Essentially, this emphasizes the evolution of pigeonite
to the orthopyroxene upon cooling. On the contrary, the mafic
intrusive Mg suite rocks are composed of coarsely grained
orthopyroxenes crystalized directly from the basaltic magma.
In spectral measurements, pyroxene displays strong diagnos-
tic absorption dips at 1000 and 2000 nm. The Clementine mis-
sion has shown its potential in mapping the general pyroxene
abundance using reflectance spectroscopy [16]. Interestingly,
the immature regions with low exposure to the space environ-
ment and steeply sloped terrains representing the local plane-
tary bedrock influence the characteristic absorption depths. The
more mature regolith is characterized with finer crystalline grain
as compared to that of the coarsely grained immature soil. Sev-
eral maturity models have been developed for quantifying the
changes in the spectral properties of the regolith due to the in-
teraction of space weathering agents like solar wind, galactic
cosmic rays, and meteorites [17]–[20]. The maturity model pro-
posed by Lucey and coworkers utilizes the Clementine’s UVVIS
750 and 950 nm spectral reflectance bands, denoting the ferrous
electronic absorptions [17]. This model assumes that the ma-
terial with similar iron content varies radially with maturity.
However, less accurate maturity measurements have been found
due to the observed parallel trends in the mare regions [21]. In
order to overcome this effect, an improved algorithm has been
developed to detect the iron content and represent the vertical
stratigraphy in terms of maturity and iron composition [19].
Alternatively, a qualitative approach has been developed by
incorporating the spectral changes due to space weathering
agents and producing a high-resolution maturity color composite
through Chandrayaan-1 Moon Mineralogy Mapper (M3) obser-
vations [20]. Eventually, one of the effects of space weathering
is the increase of mafic absorption band depth with the decrease
in the size of regolith grain. This can be further extended toward
determining the physical association of pyroxene abundant de-
posits with the maturity of the surface. In addition, a number
of basalts also contain mafic intrusive iron-rich olivine, which
often integrates with pyroxene to form a three-phase assem-
blage (Ca, Fe pyroxene–Fe olivine–silica) [6]. These associated
lithologies attribute to their metastable crystallization from the
excess of Fe in the basalt melts during the post extrusive lunar
magmatism. The spectral signature of the olivine in such mafic
assemblages is hidden in a more abundant pyroxene mineral
[22]. Hence, the surficial exposures of olivine can be detected
by using the compositional aspects of high-resolution pyroxene
spectroscopy, which could provide petrogenetic origins of the
lunar mantle stratigraphy.
Traditionally, many approaches have been adopted in deriv-
ing the spatial distribution of a mineral in both terrestrial and
lunar environments [23]. Combining different bands can pro-
vide a qualitative measure of an abundant mineral in the form
of a false color composite. However, for minute exposures as-
sociated with a magma derived soil, such combinations can-
not provide a good discrimination even after adequate con-
trast stretching. Subsequently, band ratios and differences have
proved to be better techniques for characterizing specific litholo-
gies based on the selected bands having diagnostic absorption
dips [24].
As space weathering influences the chemical and physical
properties of the lunar soil, the inclusions of metallic iron in the
accreted rims of the grain attenuates the absorption depths of
the mafic minerals. Similarly, in tropical environments, lateri-
zation is a prolonged chemical weathering phenomena which
influence the grade, chemistry, and ore mineralogy of the soils
derived from iron-poor rocks [24]. For both cases, the band ratio
has limited use in recognizing the mafic stratigraphic units for
the former and iron oxide lithologies for the later. Moreover,
the contrast stretch degrades the quantitative characterization of
a particular mineral due to the nonlinear transformation of two
bands. A linear stretch to such result could remove the subtle
information at extreme reflectance measurements. Differencing
refers to the band contrast in a linear way, and hence, linear trans-
form does not cause information loss. However, the calculation
provides a biased solution in which the result is dominated by
the brighter image. The common spectral information between
the bands is lost. An alternative to this approach is the use of
balanced contrast enhancement, which provides a convenient
way for compensating the bias, thereby preserving the spectral
signature of the hidden lithologies [25].
Another popular technique which compensates the inherent
similarity between the bands is the principal component analysis
(PCA). This technique effectively reduces the dimensionality of
the data, whereby uncorrelated bands are generated and used
for subsequent processing [26]. The subtle differences associ-
ated with different minerals, however, are not included in the
highest order PCs. The measurements further make the spectral
features sensitive to noise and thereby, unsuitable for mineral
exploration. To overcome the limitations, decorrelation stretch-
ing has been used for providing a feature space-based stretching
and inverting the rotation, as in the case of PCA [27], [28]. This
generates a three band composite having all the color varia-
tions while retaining the same original color relations. The ma-
jor drawback of applying this variation is that it uses only three
bands while neglecting the information from others. Consider-
ing the theoretical spectral response of a mineral, the PCs can be
identified by investigating the association of weighted eigenvec-
tors and spectral signature of abundant mineral exposures [25].
This technique often involves selecting PCs based on the char-
acteristic absorption of a mineral, wherein eigenvector loadings
are utilized to retrieve the specific mineral-rich sites. The afore-
mentioned approach, feature-oriented principal component se-
lection (FPCS), introduces an enhanced scene variability along
with preserving the textural information, which offers an addi-
tional attempt to visualize the physical and geomorphological
entities of the abundant mineral zones [25].
This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
SHUKLA et al.: PETROPHYSICAL INSIGHTS INTO LUNAR MAFIC EXTRUSIVE BASALTS OVER REINER GAMMA FORMATION 3
Unlike the spectral observations for determining the miner-
alogical property of the surface, long wavelength microwave
radiations from the synthetic aperture radar (SAR) also play a
significant role in retrieving the physical properties of the re-
golith grains such as dielectric constant, loss tangent, bulk den-
sity, porosity, and void ratio [29]–[33]. These parameters are
strongly related to the compaction of the lunar soil, the response
of regolith grains toward the electromagnetic (EM) field, and the
presence of void spaces between the grains. Eventually, the phys-
ical measures have some degree of correlation with the regolith
exposure age [6]. This alignment can further be demonstrated
by quantifying the exposure rate in terms of maturity.
Immature regions often depict a higher range of complex di-
electric permittivity as compared to that of the mature soil. Due
to the variability in the particle size of the lunar grains, the poros-
ity and void ratio increases as the soil gets more exposed to the
space weathering agents. This, in turn, reduces the bulk density
and thereby, corresponding dielectric permittivity of the soil.
However, the redeposition of the liberated iron when transition
metal-bearing mafic minerals are vaporized causes the inclu-
sion of small blebs of metallic iron into the aggregates of glass
welded mineral fragments, called agglutinates [6]. This process
increases the particle size of the grain and, hence, opposes the
aforementioned theory. But, with the regolith being less influ-
enced by agglutination, the overall bulk density of the local sur-
face attenuates with maturation. This effect can be examined
more clearly over high albedo anomaly regions which exhibit
unclear explanations for maturity measures.
Lunar swirls are often interpreted as albedo anomalies due
to their extremely brighter sinuous textures and high immatu-
rity while the surrounding surface darkens with time [34]. Sev-
eral studies have shown the surge of magnetic fields in these
swirl regions with almost no relation to topographical deforma-
tions [35]–[38]. Three major hypotheses have been formulated
for understanding the evolution of these swirls on the surface:
1) deflection of solar wind ions by shielding of basin impact an-
tipode generated magnetic field; 2) cometary impact, scouring
the regolith with hypervelocity interparticle collisions result-
ing in associated magnetic field anomaly; and 3) levitation of
electrostatic lunar dust during terminator crossings [39]–[42].
These models have been tested for deriving the interpretations
based on the spectral data of the rims and proximal ejecta of
small craters situated over the entire swirl region. The presence
of magnetic shielding to deflect solar wind ions indicates the
higher density of fresh and immature craters on the brighter trail
of the swirls while the impact due to comet produces a set of
randomly distributed small immature craters throughout the re-
gion [39]–[41]. Furthermore, the hypothesis leading to the lofted
lunar dust attributes to the presence of bright feldspathic mate-
rial in the soil grains [42]. The petrophysical properties of the
soil are still uncertain and no conclusive remark can be made
on the most reliable hypothesis responsible for the geological
formation of the swirls.
The present study investigates the importance of the physical
characteristics of the lunar grains on the mafic extrusive basalts
over the swirl region. In this, a multisensor approach has been
adopted by using the data from recent lunar missions. The prime
Fig. 1. Color composite image of the RGF, red: 1000 nm, green: 1309 nm,
and blue: 2000 nm. The region is highlighted by red box in the image of the
Moon on the bottom right. The swirls are represented by high albedo region with
surrounding dark basalts.
focus of this work involves the characterization of mafic units
based on high-resolution pyroxene spectroscopy for understand-
ing the thermal evolution and composition of the basalts. More-
over, the petrophysical properties of the surface, attributing to
the regolith evolution processes, have been extracted by utiliz-
ing the long wavelength microwave emissions. Along with this,
geomorphological analysis has been performed to evaluate the
topographical abnormalities in the region and unravel the matu-
rity alterations between the on-swirl, inter-swirl, and surround-
ing off-swirl terrains. In essence, the multisource observations
have been considered to constrain the formation hypothesis for
the corresponding lunar swirl.
II. STUDY AREA
Reiner Gamma Formation (RGF) is one of the most prominent
lunar swirls due to its distinctive sinuous markings and relatively
higher albedo than the surrounding dark basalts. It is located near
the Reiner crater in the western mid-latitudinal mare region of
Oceanus Procellarum. The center of the RGF lies at 7.5° N,
59.0° W in the selenographic coordinate system. Between the
period of 3.9–1.2 Ga, the basaltic magma has completely cov-
ered the Oceanus Procellarum region by characterizing its plain
through the emplacement of two major mare stratigraphic units
i.e., Nectarian and Copernican [43]. The basalts have extended
up to 160–650 m in depth, making the region suitable to explore
out the magmatic evolution of the deep-seated lithologies [44].
In the past, the brightness of the RGF compared to the
rest of Oceanus Procellarum has been misinterpreted to be the
rays emanating from a fresh crater and, hence, regarded as a
highland region [36]. However, on further examining, the re-
gion has provided evidence of nonshadowed plains in the ab-
sence of topographical slopes, thereby emerging as an albedo
anomaly. Among other lunar swirl regions, this exhibits a rela-
tively stronger localized magnetic field and different lithology as
compared to that of the surrounding anorthositic basalts. Fig. 1
This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
4 IEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS AND REMOTE SENSING
shows the false color composite of the RGF with a brighter por-
tion of the swirl representing the mafic absorptions while the
surrounding basalts almost neglecting the effect of nanophase
ferrous iron (npFe2+) in the soil. Furthermore, the RGF is char-
acterized by a higher density of smaller craters following the
albedo trail.
III. DATA AND METHODS
A. Datasets From Recent Lunar Missions
In this study, the multiwavelength data have been utilized
for examining the mafic composition, petrophysical attributes,
and geomorphological insights of the RGF. The M3onboard
Chandrayaan-1 provides a high-resolution spectral data over
a wavelength range of 0.43–3.0 μm [45]. The thermally and
photometrically corrected level 2 reflectance data have been
employed for performing spectroscopic investigation of the
mafic-rich basalts. The M3data have been acquired between two
optical periods depending on the approvable viewing conditions
[45]. The present study uses the data from the part B of the first
optical period, which is operational at an altitude of 100 km with
a high solar zenith angle. The reason for choosing M3data over
well-established Clementine scientific data is its capability of
high-resolution mapping along with a wider spectral coverage.
In addition, the hybrid polarimetric data of lunar reconnais-
sance orbiter (LRO) miniature radio frequency (MiniRF) instru-
ment have been deployed in conjunction with M3spectroscopy.
This radar transmits circular polarized signal, while receives or-
thogonal linear polarized backscatter from the targets [46]. The
data are presented in the form of the classical Stokes vector. In
radar astronomy, the estimated four Stokes parameters are uti-
lized for generating child products which are then evaluated for
realizing different scattering mechanisms. This study examines
the performance of S-band (12.6 cm) MiniRF circular transmit
linear receive (CTLR) data for retrieving the physical properties
of the lunar regolith over the RGF.
Apart from this, the LRO narrow angle camera (NAC) pro-
vides a detailed geomorphological perspective of the lunar sur-
face with monochrome images at 0.5 m/pixel. The understanding
of the lunar regolith evolution for mafic enriched grains over the
RGF could be strengthened by using such a high-resolution data
along with other observations. In order to support the local topo-
graphical variations within the region, the Global Lunar Digital
Terrain Model 100 m (GLD100) has been employed. These mea-
surements are acquired from the stereo processing of LRO wide
angle camera data with a vertical accuracy of 10–30 m and a
pixel spacing of 100 m. The added benefit of this data over lunar
orbiter laser altimetry is the absence of data gores and 99.84%
cross-track coverage capabilities.
B. Methods Adopted
The proposed methodology of the research is shown in Fig. 2.
The first part of the methodology involves utilizing the M3data
strips for preparing a mosaic of the RGF. This analyzes the
high-resolution pyroxene spectroscopy for characterizing the
mafic rich units over a broader basalt lithology. As discussed, a
Fig. 2. Proposed methodological framework of the research.
TAB LE I
EIGENVECTOR LOADING OF PCS
First row (header) represents the selected spectral reflectance bands (in nm), while the PCs
link the original bands through individual eigenvector loadings.
more appropriate approach has been developed for mapping the
spectral enhancement of the pyroxene occurrences by FPCS.
This technique demonstrates the applicability of the principal
components (PCs) in contributing to the spectral information of
a particular surficial target. It evaluates the eigenvectors and tries
to associate PCs with the original bands of actual data. Based on
the eigenvector loadings of the original bands, certain PCs are
chosen and displayed as the desired signature of the target. The
selection criteria depend on the prior knowledge of the theoreti-
cal spectral response of pyroxene. For this, the bands exhibiting
around 1000 and 2000 nm along with reflection bands of 2896,
2936, and 2976 nm are selected and PCA is performed on them.
Table I represents the eigenvector loadings computed in con-
junction with covariance matrix, thereby contributing to the in-
dividual PCs. In this context, no significant spectral features
have been detected in PC1 due to the combination of positive
eigenvectors of all bands. This displays the spatial information
of topographical and albedo variations. Significantly, PC2 and
PC3 observe a spectral absorption region of 1000 and 2000 nm,
thereby making it applicable for pyroxene spectroscopy. The
information in PC4 corresponds to a stronger domination of
2000 nm absorption than 1000 nm with a negative loading of
higher wavelength bands. However, due to the lack of topo-
graphical information in the three dimensional (3-D) PC cube,
the interpretation of geomorphological component and accurate
locality of the pyroxene deposits provides an inappropriate spec-
tral mapping. This introduces the fourth dimension to increase
This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
SHUKLA et al.: PETROPHYSICAL INSIGHTS INTO LUNAR MAFIC EXTRUSIVE BASALTS OVER REINER GAMMA FORMATION 5
the spatial variability of the spectral mapping scheme. Hence,
PC1 is multiplied with PC2, PC3, and PC4 to produce a color
composite image of texturally distinguished pyroxenes over the
RGF basalts.
Compositionally, the spectra from the FPCA-based abundant
pyroxenes are further analyzed by utilizing the modified Gaus-
sian model (MGM). The purpose of MGM is to evaluate the
individual spectrum by deconvolving it into specific electronic
absorptions and isolate the pyroxene phases from the composite
mafic spectra [47]. This technique is utilized to model the band
centers of the diagnostic absorption dips and compare with the
in situ measured band positions of the synthetic and lunar pyrox-
enes [48]. For each fit, 5-6 bands are implemented by assuming
that only one pyroxene is present in one location. In this, two
bands attribute to the charge transfer of the metal oxygen, three
represents the crystal field bands of the pyroxenes with spin-
allowed configuration, and the last band denotes the hydroxyl
absorption feature near 2800 nm. Moreover, the linear wave-
length continuum slope has been chosen for the final fits as it
presents a relatively lower residual error around significant ab-
sorption bands.
The M3spectroscopy is further extended toward quantifying
the effects of space weathering on the spectral properties of the
lunar soil. The variations in the maturity within the study area
are investigated by using the Nettles model [20]. In this, a color
composite image is generated by considering the following spec-
tral parameters: Albedo (1548 nm), IBD1000 (Integrated Band
Depth at 1000 nm, for reduced absorption band depth effect)
and continuum ratio (1508/730 nm, representing the spectral
reddening). The significance of albedo parameter attributes to
the darkening of the surface, and hence, 1548 nm is chosen to ig-
nore the contribution of ferrous absorption in the spectral region
of mafic mineralogy. In this, IBD1000 is used as a spectral con-
trast measure to estimate the band strength of the entire 1000-nm
ferrous absorption relative to the continuum, as in (1). This is a
more robust technique than simple band depth or band ratio as
it enhances the spectral absorption variability. The third com-
ponent signifies the spectral reddening effect, represented by a
simple ratio on either side of the 1000-nm ferrous absorption:
IBD1000 =
38
n=0
1−R(730 + 20n)
Rc(730 + 20n).(1)
Based on the exposure age of the basalts, they can be divided
into two broad chemical groups: Older low-Ti and Younger high-
Ti. These compositions are highly significant in deriving the
petrogenetic origins of mafic lithologies and, hence, can be de-
picted as TiO2variations within the RGF. Significantly, higher
TiO2occurrences in the basalts are evident for producing lunar
liquid oxygen due to the unconsolidated distribution of ilmenite-
rich soil, thereby proving its potential toward lunar exploration
paradigm. For characterizing the regolith on the basis of TiO2
abundance, the following algorithm has been adopted by utiliz-
ing the M3spectral reflectance bands of 540 and 750 nm [18]:
TiO2=3.708 ×arctan(R540/R750 )−0.42
R750 5.979
(2)
where (R540,R
750)are the spectral reflectance at 540 and
750 nm, respectively. The reason of choosing 540 nm is due
to the absence of 415 nm spectral channel in case of M3.This
presents the quantitative high-resolution mapping of TiO2for
understanding the basaltic compositions of the RGF in conjunc-
tion with spectral and physical measures.
The associated petrophysical properties of the FPCA-derived
pyroxene sites in the RGF have been estimated by adopting an
integrated approach. This can be divided into two sections: elec-
trical properties (complex dielectric permittivity, and loss tan-
gent) and geotechnical properties (bulk density, porosity, and
void ratio). The LRO MiniRF S-Band data have been utilized in
deriving the physical characteristics of the regolith. In this study,
an attempt has been made to evaluate the potential of 2 ×2 Jones
matrix in estimating the real dielectric permittivity based on the
shape and orientation of the regolith grains. The model assumes
the regolith to be a random mixture of dielectric scatterers hav-
ing the size much smaller than that of the incident microwave
wavelength. Based on the returned Apollo samples, the regolith
particles have exhibited their average size to be less than 1 cm
[6]. Moreover, the previous literatures have shown that the re-
golith resembles the natural media with randomly distributed
dipoles [6], [30], [49]. The model also assumes the reflection
symmetry condition for all the surfaces. However, there may be
some rugged terrains with slopes greater than 10° for which the
condition cannot be met.
One of the basic products derived from hybrid polarimetric
architecture is the Stokes parameter. This describes the polar-
ization state of an EM wave by characterizing the orientation
and ellipticity of the polarization ellipse [50]. The capability of
Stokes parameter in representing the partially polarized wave
makes it useful for hybrid pol case. This can be expressed in the
form of
[S]= ⎡
⎢
⎢
⎢
⎣
S1
S2
S3
S4
⎤
⎥
⎥
⎥
⎦
=⎡
⎢
⎢
⎢
⎣
S1
S1cos (2ψ)cos(2χ)
S1sin (2ψ)cos(2χ)
S1sin (2χ)
⎤
⎥
⎥
⎥
⎦
.(3)
Here, S1represents the total intensity image, S2exhibits the
horizontal or vertical polarization state of the wave, S3and S4
describes the phase information, ψis the orientation angle of
the polarization ellipse, and χis the ellipticity parameter. The
scattering response of the target is influenced by the physical
properties of the surface upon interaction with the EM wave.
This signifies the mixed complex scattering scenario by most of
the scatterers, and hence, a more informative coherency matrix
is desired. In case of MiniRF, the hybrid polarimetric mode al-
lows the Stokes parameter to display partially polarized waves,
thereby constructing a 2 ×2 complex Hermitian Jones matrix
or wave coherency matrix [50], [51]
[J]= EHE∗
HEHE∗
V
EVE∗
HEVE∗
V=JHH JHV
JVH JVV (4)
where EHand Evare the horizontal and vertical polarized pow-
ers, ∗denotes the complex conjugate, and ...represents the
ensemble operation with an assumed stationary wave. This ma-
trix can further be utilized for deriving the eigenvectors and
This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
6 IEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS AND REMOTE SENSING
eigenvalues to physically interpret the modeled diagonal form
between a set of target vectors [50]. The matrix is represented
as
[J]=[U2][D][U2]∗T
[U2]=cos α−sin αe−iδ
sin αeiδ cos α,[D]=(λ1+λ2)P10
0P2
Pi=λi2
j=1
λj.
(5)
λ1and λ2are the eigenvalues of the matrix [J]with λ1≥
λ2≥0and [U2]is the 2 ×2 unitary matrix with two columns
of orthogonal eigenvectors, as shown in (6)
u
˜=cos αsin αeiδ T.(6)
The eigenvectors signify the polarization state of the wave.
However, the probability by which they can be denoted is given
by Pi. When the EM wave interacts with the lunar regolith,
then the polarization is broken into different degrees of polar-
ized state depending on the target. This can be demonstrated by
knowing the probability measures of the Jones matrix. Further-
more, the normalized eigenvalues depend on the degree of po-
larization (m) attributing to the statistical disorder of individual
targets [51]
λ1,2=(1±m)/2.(7)
Significantly, mis a sensitive indicator for determining the
polarity of the backscatter [50]. Lower values of mattribute to
the presence of anisotropic particles, thereby leading to a depo-
larized volumetric scattering mechanism. However, the higher
range identifies the smooth mare regions and naturally oriented
rocks associated with crater walls and basaltic floors. This can
be estimated by utilizing the ratio of polarized power to the total
power
m=S2
2+S2
3+S2
4/S1;0≥m≥1.(8)
The ratio of eigenvalues for interpreting the geometry and
orientation of the particle is regarded as Anisotropy (A). It can
also be realized in terms of polarizability [50]
A=λ1−λ2/λ1+λ2.(9)
When the eigenvalues are equal, anisotropy is zero. This
attributes to the case of random scattering mechanism from
the regolith. The shape of the particle can be governed by its
anisotropic properties. Moreover, the ability of a target to atten-
uate the incoming EM wave, due to the dielectric polarization,
also bounds the anisotropy parameter [52]. As the dielectric con-
stant (´ε) of a medium decreases, the characteristics of anisotropy,
being a shape indicator, diminishes
1/´ε<A<´ε+1/2.(10)
In order to perform inversion modeling of dielectric constant,
the lower and upper bounds of Ahave been utilized [53]. This
allows ´εto have a range between 1/A and 2A−1, as in (11).
The increase of Areduces the ´εestimates and follows a linear
increment beyond unity:
´ε=1/A +|1/A −(2A−1)|.(11)
The real part of complex dielectric permittivity can be used to
estimate the bulk density of the uppermost layers of the regolith.
In order to evaluate the compaction of soil for the underlying re-
golith layers, the penetration depth of the radar signal play an
important role [54]. Hence, in this study, the choice of S-band is
crucial as it offers an additional penetration as compared to that
of X-band. The bulk density (ρ) of the lunar regolith varies from
around 1.3 g/cm3to the higher range of 1.9 g/cm3in rugged
heterogeneous terrains including the steep sloped features, at-
tributing to the mineralogical diversity of the bedrock [6], [54],
[55]. The attenuation of the incident microwave radiation as it
propagates through the regolith medium is measured quantita-
tively by loss tangent. Several studies have shown the physical
dependence of the loss tangent with the bulk density and ilmenite
content (S) of the lunar regolith [54], [55]. The penetration of the
S-band radiation into the regolith is governed by the amount of
FeO+TiO2abundances and, hence, varies on a large scale from
0.5 m to greater than 5 m [55]. Furthermore, the loss tangent
(tanδ) can be interpreted as the ratio of the imaginary part (ε)
to the real part of the complex dielectric permittivity. It examines
the shift in the phase of dielectric polarization corresponding to
the transmitting electric field caused by the attenuation loss
ρ=3.53 log ´ε
tanδ=10
0.038S+0.312ρ−3.26
ε =´εtanδ.
(12)
The geotechnical properties of the basalts considered in this
study mainly constitute porosity and void ratio. The porosity is
a measure of void spaces between the regolith grains caused due
to the defects in the crystallographic structure of the constituent
grains [56]. It is defined as the ratio of volumetric estimates
of the void spaces to the total volume of the grain. Based on
the correlation analysis, the relationship between bulk density,
porosity, and specific gravity can be represented as
ρ=Gρw(1 −n)(13)
where Gis the specific gravity of the regolith material, ρwis the
density of water (taken as 1 g/cm3), and nis the porosity of the
soil. This can be further simplified in the form of
n=1−ρ/Gρw=1−ρ/3.1.(14)
The value of specific gravity ranges between different types
of material in the regolith like breccia, agglutinate, basalt, and
glass particle. The typical in situ measurement for basalts has
been estimated to be greater than 3.32. However, for scientific
purposes involving geotechnical analysis of lunar soils, a value
of 3.1 is recommended [56]. Another parameter can be consid-
ered for evaluating the damage made in the crystal structure of
the soils due to the continuous bombardment of space weather-
ing agents [56]. This is represented as a void ratio (v), attributing
to the volumetric quantification of void spaces between the solid
This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
SHUKLA et al.: PETROPHYSICAL INSIGHTS INTO LUNAR MAFIC EXTRUSIVE BASALTS OVER REINER GAMMA FORMATION 7
Fig. 3. Maturation rate of different lunar materials, as a function of space
weathering. (a) OMAT color composite of the RGF, red: continuum ratio, green:
1548 nm band, and blue: IBD1000. Green and blue bands are inverted to repre-
sent the least mature features in dark tones. (b) Spectral signatures of on-swirl
(blue), off-swirl (black-dotted), and inter-swirl (red-dotted) regions, depicting
pyroxene absorptions in the immature high albedo regolith.
particles, including the porosity of the soil
v=n/ (1 −n).(15)
These geotechnical parameters are responsible for govern-
ing the electrical resistivity, thermal conductivity, regolith slope
stability, and penetration of the incoming ionizing radiations
including those of solar winds and galactic cosmic rays [6].
The study further implements the topographical variations and
high-resolution geomorphological differences between the com-
positional phases of pyroxenes in the RGF. All the results are
integrated to form a basis on deriving the petrophysical insights
of the supported formation hypothesis of the lunar swirls.
IV. RESULTS AND DISCUSSIONS
A. Compositional Variability of the RGF Soil
The fragmented crystallization of early magma attributes to
the evolution of fine-grained anorthositic crustal regolith due
to the heavy bombardments. In order to study the effects of
space weathering on the RGF soil, the maturity has been com-
puted as suggested by Nettles et al. [20]. The spectral parameters
have proved its importance in quantifying the effects of darken-
ing, reddening, and mafic absorption attenuation. As stated in
previous approaches, the evaluation of optical maturity for the
Clementine mission has utilized two-dimensional spectral space
to show the variations in the trend [17]–[19]. However, the quan-
titative aspects of space weathering have been incorporated to
enhance the diverse variations in conjunction with the age of the
soil. In Fig. 3(a), the differences in the maturity of the RGF soil
have been visualized in 3-D space of albedo, continuum ratio,
and IBD. The RGF characterization attributes to the occurrences
of relatively coarser sized soil grains as compared to the mare
basalts. This can be depicted by the darker blue tone representing
the increased albedo of the swirl. In addition, several microim-
pacts over the central part of the swirl region provide the green
color information contributing to the equal proportion of band
depth and spectral reddening. The increased localized magnetic
field slower the rate of maturation for these craters due to the
shielding from the solar wind interaction. There is also an even
random distribution of small and immature craters surrounding
the swirl. This attributes to the sputtered remains of the possi-
ble scouring of the soil due to high impact cometary event. The
RGF has a controlling influence of immature feldspathic mate-
rial along with large quantitative estimates of basalts. Further-
more, the crystalline structure of the mature surrounding retains
a higher amount of solar wind gases due to the increased surface
area to volumetric ratio and, hence, remains one of the promi-
nent sites for future mining exploration. From the observations,
it can be established that determining a robust quantitative ma-
turity index can cause inaccurate measures as lunar regolith do
not mature to a common endpoint.
The spectral quantification between different features of the
basalts in the RGF can be represented in Fig. 3(b). The re-
flectance spectra of the mature crystalline material exhibit the
evidence of anorthosite with the wavelength dependences of the
compositional heterogeneity present in the soil. In essence, there
is an additional component of space weathering in order to pro-
vide the variations between different materials (spectra 1). The
spectra collected over the immature on-swirl region show two
dominant absorption dips at 1000 and 2000 nm, attributing to the
implications of mafic composition in the RGF basalts. As ob-
served in Fig. 3(b), the intensity of the reflectance from on-swirl
to off-swirl region happens to decrease as the darkening of the
soil increases, thereby flattening the spectra. This also attributes
to the inclusion of larger blebs of metallic iron with a diameter
ranging from 40 to 60 nm. However, the weathering of the ma-
terials along the albedo trail lowers the rate of maturation with a
much relatively smaller iron deposition and, thus, an increased
amount of spectral reddening. The inter-swirl region depicts the
similar effect of darkening with no change in the slope and pos-
sibly higher intensity than mature off-swirls due to the effect of
the increased localized magnetic field. Both the spectra under the
influence of darkening (spectra 2 and 3) represents a more dom-
inant yet smaller 1000-nm absorption, typically mimicking the
nature of basalts. Although the RGF appears to be fresher than
the surrounding, the basaltic composition of the soil varies from
the high albedo on-swirl site to the small craters formed due to
the microimpacts. Moreover, the nature of the RGF basalts can
be known by estimating Ti content. In principle, the basalts are
divided into three categories: 1) very low Ti (<1.5 wt% TiO2);
2) low Ti (1.5–9 wt% TiO2); and 3) high Ti (>9wt%TiO
2).
This leads to the retrieval of titanium content for the subsequent
characterization of the RGF basalts.
Normally, as the size of the regolith grain increases over the
Ti-rich soil, the relative Ti abundance compared to the underly-
ing soil is substantially enhanced. But this is not the case with the
RGF soil. In this, the retrieval of Ti content has been associated
with the surface phenomenon, thereby incrementing the occur-
rences in the inter-swirl region. From (2), the algorithm utilized
for evaluating the TiO2abundance for the RGF actually consid-
ers the darkening effect on the spectrum alone. However, the sub-
sequent darkening after the increase in the spectral slope should
have been assumed for the case of lunar swirls. From spectra
This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
8 IEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS AND REMOTE SENSING
Fig. 4. Classification of basalts on the basis of Ti content in the regolith.
(a) Variability of TiO2has been found to be aligned with the effects of space
weathering. The regolith has a modal abundance of 3.2 wt%, represented in
green color. Black arrows depict the higher Ti content in the basalts correlated
well with equal domination of 1000-nm absorption feature and spectral slope.
(b) More detailed view of the RGF swirls exhibiting extremely low abundanceof
Ti with mean value of 1.25. In this region, white arrows denote the microcraters
having green color. This attributes to the decreasing albedo of the features, as
seen in Fig. 1.
1, the mafic absorptions can be correlated with the RGF basalts
and, hence, the influence on the spectral slope has to be dealt
before comparing with the variations in the albedo. The change
in the intensity (spectra 2 and 3) attributes to the varying amount
of proportion of ilmenite and anorthosite in the soil. Spectra 2 of
the inter-swirl region is inclined more toward the contribution
of ilmenite in the mafic basalts, thereby showing an increased
Ti estimate in Fig. 4(a). The surrounding mare contributes to a
mixed composition of both ilmenite and anorthosite in the mafic
lithology. As albedo increases, the presence of nanophase iron
decreases and, hence, the ilmenite concentration reduces. This
correlates well with the result as shown in Fig. 4 for normal
space weathering regions. The inter-swirl lanes exhibit a faster
rate of maturation to the on-swirl albedo because it contributes
to only darkening.
The increased diameter of nanophase iron results due to the
welding of surrounding glasses and mineral fragments into an
aggregate during micrometeorite impact. This replicates the
spectral patterns of high-Ti regolith providing new insights into
early lunar volcanism. In essence, there is an increased propor-
tion of larger sized nanophase deposition in the nonmagnetically
influenced regions. Furthermore, the smaller craters present in
the central part of RGF exhibit higher amounts of Ti attributing
to the gradual darkening of the soil due to the decreased rate
of maturation, in Figs. 3(a) and 4(b). Hence, a more general-
ized and robust TiO2algorithm needs to be defined, in future,
for compensating the spectral effects of mafic absorption in the
subsequent albedo variation zones.
B. New Insights Into Pyroxene Spectroscopy Using FPCS:
Regional Exposures of Mafic Mineralogy in the RGF
Following the FPCS approach, the mafic spectra of subcrustal
affinity has been characterized for subsequent pyroxene abun-
dance. This involves the understanding of crystal field theory of
transition metal-bearing minerals. In the case of pyroxene spec-
troscopy, the inclusion of Fe2+/Ca2+in the ligand field exhibits
two prominent spectral absorption features, namely 1000 and
2000 nm. The crystal field spectrum of pyroxene attributes to
its formation as ferromagnesium silicates in the mafic extrusive
basalts with distinctive coordination sites. This provides new
insights into the association of mineral structure, exsolution,
zoning, chemical composition, and particle size for determining
the petrographic properties of the pyroxene-rich basalts. In this
study, a new spectral perspective has been visualized in terms
of implementing FPCS for the RGF regolith. In Fig. 5(a), the
central albedo region registers the majority of the pyroxene sig-
natures with less exposure in the tail portions. This is highlighted
by orange to red color in the image while sinuous features are de-
picted in greyish yellow hue. The surrounding high-Ti basalts in
the topmost right corner are indicated in light blue color with low
pyroxene enrichment due to the possible accumulation of oxide
phases (ilmenite and armacolite) within the extruded basaltic
liquid. An early-stage pyroxene crystal with high TiO2content
may also suggest that crystallization of the pyroxene has just
started in a liquidus containing ilmenite. Moreover, Ti is usu-
ally incompatible with Fe-rich/Mg-rich pyroxenes. The lower
amount of pyroxene may also be caused due to the inconsis-
tent crystal fractionation resulting from partial melting of the
pyroxene deficient source. This compositional diversity can be
identified in the regions that lack yellowish hue, thereby attribut-
ing to the late-stage lunar volcanism. The sequence of events in
the basalt crystallization also demonstrates some possible expla-
nation for this diversity. In this, due to the delayed nucleation of
plagioclase, the quantitative aspects of calcium have been altered
significantly resulting in fragmentary reaction of early formed
armacolite and olivine. Moreover, the influenced chemical tra-
jectories of other minerals have produced accessory phases in
the form of free silica from undersaturated SiO2contents. The
maximal enrichment of Fe in the residual liquid for the low-
Ti basalts has contributed toward the considerable amounts of
pyroxenes (in yellow and orange colors). However, during the
last phases of volcanism, Fe has been completely consumed by
early formed ilmenite attributing to the absence of pyroxene sig-
natures in the high-Ti regions.
Several spectra (1–12) have been characterized for different
regions within the pyroxene abundant RGF, as in Fig. 5(b). One
of the pixels exhibiting red color in the western part has provided
ample absorption in 1000 nm with an almost flattened spectrum
at 2000 nm (spectra 1). Although the region is dominated by py-
roxenes, the inclusion of this olivine spectral behavior attributes
This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
SHUKLA et al.: PETROPHYSICAL INSIGHTS INTO LUNAR MAFIC EXTRUSIVE BASALTS OVER REINER GAMMA FORMATION 9
Fig. 5. FPCA-based pyroxene spectroscopy for characterizing mafic units in
the RGF. (a) Color composite image, red: PC1 ×PC2, green: PC1 ×PC3, and
blue: PC1 ×PC4. Red color in the RGF shows the higher abundance of pyroxene,
while the bluish hue represents the high Ti content in the surrounding basalts.
Different colors signify the geological formation rate of the basalts depending
on the exposure to the space environment and degree of partial melting of the
magma. (b) Spectra are extracted from the regions indicated in (a) and analyzed
further. The red oval-shaped box depicts the presence of 2800-nm absorption
feature, attributing to the water/hydroxyl signatures. Moreover, the back box for
the spectra 10–12 includes 650-nm absorption feature, indicative of chromium
exposures.
to the presence of an Mg-rich intrusive igneous petrographic
feature in the lower crust. Since the topographical variations
are minimal, the excavation of lunar mantle upon cooling and
hardening of the crust can be ruled out. In the inter-swirl dark
lanes, three pixels have been sampled and their respective re-
flectance spectra are extracted (spectra 2–4). These spectral sig-
natures have replicated the similar effects as observed in spectra
1, thereby signifying the darkening of the spectra due to the influ-
ence of nanophase iron over the accreted rims of the agglutinated
product.
Moreover, the effects of space weathering play an important
role in vaporizing the mafic minerals, present in these lanes,
to produce the liberated form of metallic iron. The central part
of the on-swirl region has also been identified in conjunction
with the reflectance spectra of sampled pixels (spectra 5–9).
These spectra reveal the pyroxene response with stronger ab-
sorption dips at 1000 and 2000 nm. However, from the shape of
the spectrum, the continuum of pyroxene also incorporates the
Fig. 6. Spectra are characterized from different regions of the RGF and mod-
eled into specific corresponding absorption feature using MGM. The on-swirl
regolith provides high Mg orthopyroxene abundance with inclusion of felds-
pathic dust in purple fitted line over cyan asterisks. Cr-rich mafic regolith is
depicted by green asterisk dip at around 0.65-µm wavelength. Surrounding off-
swirl anorthosites are indicated by red-fitted line over yellow asterisks.
absorption features from feldspar. This is indicated by the grad-
ual flattening of the reflectance peak between 1000 and 1500 nm.
The higher albedo produced from the immature feldspathic dust
governs the spectral changes observed in the soil and, hence,
an important observation to consider. Interestingly, the pixels
are characterized by a prominent 2800-nm absorption in asso-
ciation with pyroxene bearing lithologies. This attributes to the
possible magmatic origin of hydration feature observed in the
spectra, provided the equatorial locality and mafic compositional
diversity of the RGF. The presence of mafic minerals, mainly py-
roxene, captures the magmatic information leading to inclusions
of lunar melts. Prior to volcanism, these inclusions undergo ex-
cessive degassing through a minimal posteruptive event, thereby
resulting in high amounts of magmatic volatiles including wa-
ter. Apart from this, a small patch in the western end displays
a strong 650-nm absorption feature (spectra 10–12). This as-
sociation with olivine dominated pyroxene spectra signifies the
existence of chromite-bearing diopside. The seldom and irregu-
lar formation of the symplectic lamellae enriched in chromium
results in two-phase diopside intergrowth. Moreover, such inner
worm-like textures attribute to the mesostasic inclusions aligned
with incompatible residual magma produced during the last ma-
jor phases of lunar volcanism.
Three pixels from on-swirl, off-swirl, and chromium-rich re-
gions have been sampled to provide compositional insights us-
ing MGM. The corresponding reflectance spectra of the pixels
are modeled has been performed by deconvolving the individ-
ual spectrum into its individual components, as shown in Fig. 6.
The MGM fits for the pyroxene deposits over the swirl region
indicate that the absorption band centers for 1000 and 2000 nm
lie around 910 and 1825 nm, respectively.
These results are in concordance with relatively high Mg-
enriched orthopyroxene. Toward the eastern end, some pixels
are characterized by high Ca content suggesting the occurrence
of 1000 nm at a relatively longer wavelength due to the pres-
ence of olivine feature in the orthopyroxene spectrum. This also
attributes to the higher proportion of iron content in the region.
This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
10 IEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS AND REMOTE SENSING
Fig. 7. Black and white arrows represent the region with higher abundance of
pyroxene, as derived from spectral parameter in [56] and proposed FPCA-based
approach, respectively. In the first image, the moderate abundance marked by
green color has been depicted by yellow hue in case of FPCA derived maps.
The modeling of off-swirl spectrum depicts the anorthositic ori-
gin with stronger absorption near 700 nm, implying the presence
of nanophase iron in a more mature surrounding. This is also an
indicator for charge transfer absorptions of weathered metallic
iron. In addition, there are comparatively weaker absorptions
at 1250 nm due to low FeO content-based plagioclase feldspar.
The inclusion of olivine and pyroxene multiple components in
the basalt demonstrates 1050-nm diagnostic absorption feature
associated with the off-swirl spectrum. Results from MGM fits
of the chromium-rich region provide a moderate range of Mg
value for olivine dominated spectrum attributing to KREEP-
related Mg suite intrusions. These sites are further found near
the wrinkled ridges that are typically formed due to the tectonic
faulting of the RGF.
The applicability of FPCS for deriving the qualitative abun-
dance of lunar pyroxenes has been validated by using the pro-
posed spectral parameter by [57]. The higher occurrences of
pyroxenes have been concentrated at the central part of the
RGF with a moderate quantification of parameter surrounding
it. There are some microcraters associated with high-Ti with a
slower maturation rate depicting exposures of low Ca pyroxenes.
The FPCS-based results are found to be consistent in relating the
abundant regions with that of the selected spectral parameter, as
shown in Fig. 7. However, the limitation lies in generating a
global pyroxene index for the entire regolith. This could be later
Fig. 8. CPR measurements of the mafic abundant regions in the RGF. Black
arrows indicate fresh immature microcraters exhibiting higher values of CPR,
due to roughness and possible sources of hydration in the lithology. The higher
penetration signifies the distribution of small craters in the swirl region.
utilized for quantifying the abundance based on a relative scale
of 0 to 1. As part of future work, an attempt could be made to
integrate the existing quantitative spectral parameter with FPCS
generated maps using geostatistical based regression and uncer-
tainty modeling, thereby proving its ability to differentiate lunar
materials based on compositional diversity of mafic lithologies.
C. Petrophysical Properties of the Mafic Abundant Regolith
Grains: Results From LRO MiniRF S-Band Data
An attempt has been made to deploy SAR data for retrieving
the physical characteristics of the grains enriched in mafic miner-
alogy. This facilitates the understanding of the nature of the soil
to gain new insights into the factors governing the possible for-
mation of the RGF. In one of the previous studies, spectral related
compaction regolith behavior has been investigated in conjunc-
tion with the maturation trends of the RGF to shed lights on the
swirl formation mechanism [58]. However, radar plays a pivotal
role in characterizing the regolith more accurately based on the
physical properties due to its higher penetration capabilities. In
this study, the applicability of S-band MiniRF monostatic CTLR
data has been examined to signify the scattering responses from
the mafic abundant grains.
The sensitivity of the radar echo to the hidden subsurface
anisotropic units and surface roughness can be interpreted in
terms of circular polarization ratio (CPR). This parameter has
been used as a primary indicator for retrieving the scattering
mechanisms from dielectric inhomogeneities [54]. In essence,
the mafic-enriched zones are mapped for understanding the mi-
croscale roughness based on the CPR estimations. The central
part of the RGF is characterized by low CPR values, as in Fig. 8.
However, intermediate-to-high values has been observed in the
region of smaller microimpact craters, attributing to the gradual
evolution of the fine-grained regolith structure devoid of surface
rocks. Significantly, the higher Mg content in the orthopyroxene
deposits exhibits enhanced values of CPR greater than 1 (indi-
cated by red pixels). This result is in concordance with the impli-
cations of magmatic water in the equatorial lunar melt inclusions
This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
SHUKLA et al.: PETROPHYSICAL INSIGHTS INTO LUNAR MAFIC EXTRUSIVE BASALTS OVER REINER GAMMA FORMATION 11
Fig. 9. Scattering power retrieval using m-chi decomposition. White arrows
denote the replication of albedo pattern to some extent by equal contribution of
even bounce and volumetric scattering (consistent with feldspar presence).
of the RGF. Furthermore, the olivine-dominated orthopyroxene-
rich soil with chromite exposures in the western albedo on-swirl
region indicates moderate values of CPR with plausible evidence
of buried basalt substrate formed in a low oxygen environment.
This can also be explained in terms of low-rate regolith garden-
ing due to the magnetic shielding from solar wind and galactic
cosmic rays. In addition, the inter-swirl region associated with
the agglutination process represents moderate to high range of
CPR. This attributes to the increased surface/subsurface rough-
ness due to the presence of welded materials. The inclusion of
hydrated olivine-rich mineral fragment in the surrounding glass
could also be responsible for the increased values of CPR. More-
over, the dark lanes are characterized by the nonmagnetic influ-
enced maturation leading to a higher proportion of nanophase
metallic iron deposition.
Especially, for equatorial regions, the higher values of CPR
can be interpreted for detecting the multiple scattering compo-
nents associated with m-chi decomposition of the MiniRF data
[59]. This facilitates the response of regolith to the incoming EM
wave in the form of single bounce, odd bounce, and volumetric
polarized signatures [59]. In order to understand the nature of
the scatterers, m-chi has been performed for characterizing the
RGF morphological units. The high albedo trail in the upper
portion of the RGF has been observed to be replicated, to some
extent, by the mixture of odd bounce and dipole scattering mech-
anism. This is clearly indicated by the yellow color draped over
more dominated blue color, as in Fig. 9. Also, the reason for this
combination lies in the topographical relief of the regolith. Such
regions are distinguished by the coarser nature of regolith grains
with reduced maturation rate, thereby acting as randomly dis-
tributed natural corner reflectors. However, this behavior has not
been depicted by CPR image due to the constructive enhance-
ment of the same sense polarization power. The opposite sense
backscatter measures the top surface scattering of the regolith
while m-chi decomposition models the scattering pattern along
with a subsurface component of the buried rocks and volatiles.
Moreover, the presence of fine feldspathic dust in the coarsely
grained on-swirl regolith governs the size of the particles. It
assumes that the particle is of similar size as compared to the
Fig. 10. Dielectric permittivity of the RGF with modal estimation of 3.32. The
ratio of the maximum real part to the minimum value is 1.66, while the maxi-
mum of the imaginary part is 2.38 times the minimum estimate. Black arrows
signify immature impacts with gradual darkening, probably shielded by mini-
magnetospheres. They exhibit higher dielectric values in yellowto red hues. Few
sites with high Mg and hydrated olivine orthopyroxene assemblages attribute to
elevated dielectric signature. Western part exhibits gradual topographic undula-
tions with plausible chromite exposures by white arrow.
surroundings, attributing to a much smoother surface of the high
albedo patch. This can be reflected in the high-resolution m-chi
decomposition image, wherein blue color shows its dominance.
However, a much-diversified terrain has been visualized with
an enhanced number of fresh microcraters shielded in the on-
swirl region. Some regions, in cyan color, depict the presence
of smaller fragmentary clasts in breccia rocks. This constitutes
the mixed contribution of single bounce and even bounce scat-
tering powers, suggesting the possible existence of quasi-planar
scatterers transparent to the radar wavelength. In essence, the
decomposed color composite demonstrates a powerful tool in
differentiating materials within RGF swirl petrographic units
based on the physical characteristics of the grains. Furthermore,
the penetration capability of the radar (∼2 m for S-band) at-
tempts to include the probable scattering from subsurface buried
geological inclusions.
In order to critically understand the response of different com-
positional mafic units to the EM wave, the dielectric behavior of
the RGF has been explored. The hybrid polarimetric data have
been converted into 2 ×2 Jones matrix under reflection sym-
metry condition. However, the idea of reconstructing pseudo
quad-pol data in the form of 3 ×3 coherency matrix has been
ruled out. The expansion of backscattering information of the
observed field into 3 ×3 coherency matrix proves to be un-
reliable as it cannot clearly elucidate the ideal responses of the
scatterers. Moreover, even if an assumption is made in regards to
compressing 3 ×3 matrix into 2 ×2 matrix by some argument,
the polarimetric scattering information may not be preserved.
This could further indicate the loss of information in the derived
pseudo quad-pol data [59]. Hence, the applicability of the Jones
matrix has been investigated to understand the dielectric signifi-
cance, thereby identifying the lithological evolution of the RGF
basalts.
The on-swirl region in the central part has been characterized
with low-to-moderate values of complex dielectric permittivity,
as in Fig. 10. The real part varies between 2.62 and 4.35 with
This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
12 IEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS AND REMOTE SENSING
a mean of 3.23, attributing to the presence of different strati-
graphic units in the regolith. The surrounding mature terrain
provides evidence of extensive gardening responsible for agglu-
tinated products in the debris. This can be demonstrated by lower
values of dielectric permittivity attributing to an increased rate
of space weathering and the ability of lunar materials to be elec-
trically charged for prolonged periods. Such behavior also indi-
cates the smoother appearance of the regolith toward EM wave.
Moreover, the enhanced powers of surface scattering contribute
to a specular surface with fine-grained regolith and increased
deposition of nanophase iron, as observed in maturity images.
This also has a geological implication of widespread distribution
of low viscous basaltic magma during the formation of Oceanus
Procellarum, thereby evolving into a relatively smoother and
regular crustal surface.
The high albedo region displays a unique perspective in
characterizing the gradual rate of on-swirl maturation for
micro-craters. The higher dielectric permittivity of such regions
introduces a factor of increased roughness due to the impact
deformations. Moreover, the prominent sites, exhibiting strong
orthopyroxene signatures in the central part, have been observed
to reveal an elevated trend of dielectric permittivity. This may be
due to the relatively higher Ti content in the microcrater floor,
indicative for the darkening of the RGF soil. Interestingly, the
possible association of magmatic water in the olivine-dominated
orthopyroxene assemblages attributes to a dielectric surge in
conjunction with an increased proportion of mixed volumetric
scattering and CPR enhancements. These regions have been
characterized to witness reduced amounts of EM wave propaga-
tion within the material, up to a designated penetration limit. In
essence, the dielectric properties correlate well with the observed
effects of space weathering, leading to maturation of the soil.
This has a deep significance with the geotechnical parameters re-
sponsible for classifying the mechanical stability of the regolith.
One of the fundamental properties affecting the dielectric
measures of the regolith is bulk density. This has been computed
for top 2-m regolith estimates by using the proposed method of
Olhoeft and Strangway [55]. The region corresponding to off-
swirl regolith attributes to a lower range of bulk densities as
compared to fresh immature soil. This may be due to the effect of
comminution with increasing maturation, thereby reducing the
mass of the grains within a cubic centimeter volume. The space
weathered regolith is also affected by agglutination, wherein
the crystalline materials are welded together to evolve into a
coarser grain. Such deposition drives an opposing constraint to
the theory, and hence, the region emerges out with higher values
of bulk density. Moreover, the characterization of these regions
could be further utilized as an effective tool for realizing the
repository of agglutinated glasses in the regolith. The immature
soil has been characterized with moderate-to-high bulk density
values, with the probable deposition of denser lithological units
(like pyroxenes and olivine). This also attributes to the ubiqui-
tous presence of fresh feldspathic dust in the bimodal mixture
of on-swirl grains, suggestive of increased particle size.
In addition, the attenuation losses of the ionizing radiation
can be examined by the ratio of imaginary and real part of di-
electric permittivity. This is also influenced by the amounts of
ilmenite due to its electroconductive nature. The lost energy in
the EM wave propagation within a medium proves to be signif-
icant in determining the electrical conductivity of the material.
In the case of RGF regolith, the mature surrounding region cor-
responds to a lower attenuation loss and, hence, resulting in an
increased strength of received radar echo. The noteworthy sites
of enhanced endogenic hydration in the western part exhibit
relatively higher values of loss tangent, attributing to possible
storage of lost energy in the polarization of the olivine-rich de-
posits. These deposits are also associated with the increased
power of volume-subsurface scattering, indicative of lower re-
ceived radar echo strength and higher CPR. In essence, such sites
draped with fine dust could act as polarized dipole moments in
the presence of solar wind-induced weak electric fields, thereby
resulting in charged dust levitation. During terminator cross-
ings, the lofted feldspathic dust could experience electrostatic
repulsions and, hence, contribute to tenuous dust storms with
maximal accumulation in the magnetically influenced swirls.
However, lower samples of the aforementioned sites provide a
localized perspective of the RGF, excluding the expanded cen-
tral and eastern part along with sinuous patches. The reason for
extreme reflectance in the western part could be further eluci-
dated by adopting charged dust as a local varying hypothesis
along with the proposed cometary gas disruption [41].
The compaction of the RGF soil is determined by implement-
ing the in situ physical model of porosity and void ratio. In this,
the porosity has been classified into three measures: 1) inter-
granular porosity (volume of the void spaces between individual
particles); 2) intragranular porosity (volume of the reentrant sur-
face); and 3) subgranular porosity (volume of the enclosed void
spaces) [6]. The measured specific gravity incorporates all three
variants of porosity in order to examine the overall estimate. The
subgranular voids occupy two-thirds of the total volume of the
regolith grain when subjected to a specific gravity of 1. As in
Fig. 11(a), the surrounding off-swirl region attributes an overall
higher in situ porosity with an average percentage of 55. The re-
duction of larger regolith grains into finer crystalline materials,
due to enhanced maturation rate, is responsible for such behav-
ior. Moreover, this combination of different materials includes
intergranular and intragranular porosity in their crystallographic
structure. The enclosed void within the interior of the grain tends
to get destroyed, following the evolution to the smaller structure.
This excludes the estimates of subgranular porosity while con-
sidering other parameters for in situ porosity measurements.
There are some regions with relatively moderate-to-low
porosity (indicated in cyan to blue color) over the mature terrain.
The slightly increased porosity may signify the possible inclu-
sion of glass-welded mineral fragments in the soil mixture. This
may also arise due to the existence of younger microimpacts,
randomly distributed, in the mature surroundings.
In the on-swirl region, several patches of immature deposits
correspond to a moderate range of porosity values. Apart from
this, orthopyroxene-rich soil exhibits a lower porosity of 38%,
attributing to the orthorhombic crystal structure of the mineral
fragment. The lower porous soil has also been observed in the
denser olivine-rich sites associated with coarser particle size.
This attributes to a tighter confinement of the regolith and, hence,
This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
SHUKLA et al.: PETROPHYSICAL INSIGHTS INTO LUNAR MAFIC EXTRUSIVE BASALTS OVER REINER GAMMA FORMATION 13
Fig. 11. Geotechnical characteristics of the RGF regolith. (a) In situ porosity
estimation. Black arrows represent the low porous soil, attributing to a tighter
arrangement of the grain within the soil. An astronomer inside the rover would
expect smooth exploration in such areas. Regions with yellow to red hues depict
loose to medium particle packing of the regolith. (b) Void ratio provides a sup-
plement to porosity for measuring the stability of the soil. Black arrows provide
regions corresponding to lower porosity and less capable of retaining solar wind
implanted volatiles. As compared to the off-swirl surrounding, these results are
aligned with slower rate of maturation.
could be utilized for future establishment of a lunar base for rover
operations.
The bulk density is strongly governed by intragranular poros-
ity, thereby increases with the decrease in the void space between
the grains. Significantly, this can be quantified by computing
the void ratio of the regolith. In Fig. 11(b), the average value of
1.01 has been observed for the off-swirl basalts, which tends to
decrease as the depth of the regolith increases [6]. The higher
volume of voids in the basaltic grains mainly comprises intrin-
sic vesicles and microcracks. The vesicles may have formed
due to the outgassing of volatile elements, like CO, CO2, and
sulfur-rich species, during the basaltic magma crystallization.
This results in the emergence of bubbles when the gas rise in the
magma, thereby escaping to the surface. Moreover, the crustal
trail of the lava flow transforms into an irregular and blocky ap-
pearance due to the aforementioned volatile outgassing. Apart
from this, intrinsic microcracks are produced across grains due
to the instantaneous cooling of magma flow in some regions.
This may also occur along the boundaries of the grain as a re-
sult of divergent thermal contraction for different lithological
fragments and agglutinated glasses.
In some regions, the average void ratio decreases with the
reduction of in situ porosity. The immature microcraters in the
on-swirl region enhance the mechanical stability of the regolith
by the fewer proportion of void spaces between the grains. This
may also be due to the abundance of orthopyroxene and olivine in
the soil. However, the porosity of the top few cm regolith evolves
with time as the exposure of deep-seated lithologies transforms
its crystal shape and structure under the influence of extreme
temperatures due to the interaction of solar wind. However, this
rate declines at magnetically influenced sites, thereby porosity
and void ratio measures in the on-swirl region remain accurate
with fewer uncertainties as compared to off-swirl surroundings.
In essence, the geotechnical variants prove to be an important
aspect for planning future investigations concerned with seismic
wave propagation and improved in situ experiments by employ-
ing planetary rovers over the swirls.
The physical properties of the RGF regolith fail to distin-
guish the high albedo markings, wherein the trend continues
to appear as that of the surroundings. Due to the penetration
of S-band (∼2 m), the dielectric behavior of high albedo im-
mature material may have been dominated by the underlying
coarsely grained regolith. This attributes to a relatively thinner
membrane-like surface impression of albedo trail with localized
smoothing, invisible to microwave wavelength. The dielectric
model used in this study assumes the reflection symmetry condi-
tion of the Jones matrix. Under this, the copolarized information
decorrelates with the cross-polarized channel, thereby satisfying
for most terrestrial terrains.
However, for the regions with steep topographical slopes, this
condition may not hold satisfactorily. In regards, the mean slope
of the RGF has been found to be less than 10°, and hence, the
estimations depict the robustness of the algorithm over the RGF.
The smoothness of the terrain can be further quantified by de-
riving the local incidence of the MiniRF radar wave. This is
calculated by taking into account the surface slopes in azimuth
and range directions along with absolute incidence angle of the
radar (∼49° for MiniRF), as proposed by Liu et al. [60]. The blue
color indicates the lower local incidence angle, thereby implying
to an increased smoothness of basaltic terrain with small-scale
blocky deformation due to the formation of intrinsic vesicles in
the magma (see Fig. 12). Such deformations are depicted in cyan
color, representing a gradual change of slope (∼1°). The imma-
ture microcraters exhibit yellow color with an average slope of
4°. These are formed due to the micrometeorite impact and most
of them in the on-swirl region are shielded from the subsequent
weathering through localized minimagnetospheres. Moreover,
the wrinkled ridges in the western end show slope enhance-
ments in yellow to orange color, attributing to possible surface
depressions due to the regional tectonic activities. In addition,
the blocky ejecta of the crater situated at the right side depicts
darker tone of yellow, attributing to an average slope of 5.5°.
The local incidence influenced by topographical slopes char-
acterizes the region with the degree of roughness varying from
a mature regolith to a much younger swirl.
A detailed geomorphological survey has been conducted
across the swirl and nonswirl region by utilizing very high res-
olution LRO NAC images. One of the interesting views of RGF
is depicted in Fig. 13, providing an oblique perspective from the
western direction over the swirls. This highlights the presence
of wrinkled ridges, the only physical deformities on the surface
besides microcraters. The observed pattern resembles that of a
This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
14 IEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS AND REMOTE SENSING
Fig. 12. Local incidence estimation, as an additional measurement for surface
roughness. The RGF region has been characterized with low incidence angle
and topographical slope variations. However, some regions with pronounced
microcrater density display a gradual increase in the incidence by ∼1.27° (black
arrows). From this measurement, the regional geomorphology can be visualized,
as the case for wrinkled ridges in the western part exhibiting higher incidence
angle (black arrows).
Fig. 13. LRO NAC oblique monochrome image, with resolution ∼0.5 m. This
perspective gives an idea about the geomorphology of the RGF. The topograph-
ical variations are minimal (also aligned with local incidence measures) along
with thrust faulting in the form of wrinkled ridges, as indicated by black arrows.
This can also be seen in 2-m resolution DEM as separating two regions with dif-
ferent elevation (from green-yellow-red-yellow-green). This image also shows
a relatively slower rate of weathering in the albedo region as compared to the
dark basalts.
parallel and uniform orientation of ridges with the gently sloped
terrain, which can be clearly seen in the local incidence image.
These wrinkled ridges have been found to be associated with
minor processes in the form of thrust faulting and flexing. This
could also be explained in conjunction with the geological evo-
lution of mare regions. Eventually, due to the possible melting of
the deep interior, the extrusion of silicate-rich magma onto the
surface results in the filling of large impact basins with succes-
sive layers of dark basalt. However, the relatively thin crust sinks
the heavier mare during the basin formation processes, thereby
separating mare from the underlying regolith. This results in a
sliding movement of mare toward the sinking center, wherein
imperfect decoupling characterizes the bunched-up areas as a
series of faults. Such deformities are originally transformed at
the base, which appears as wrinkled ridges on the surface. Some
regions within the ridges exhibit a higher abundance of high Mg
olivine content in the orthopyroxene signatures. This attributes
Fig. 14. LRO NAC acquisition reveals varying trends in maturity for the on-
swirl and inter-swirl regions. LOC 1 represents a fresh and less exposed re-
golith with crystalline feldspathic dust (indicated by arrows) over the underlying
coarser regolith soil. LOC 2 indicates a large population of small microcraters
darkening at a faster rate due to the absence of shielding. These observations are
in concordance with maturity image, as displayed in Fig. 2.
to the minimal chemical alterations in the extruded magma of
subcrustal affinity. In addition, the ridges in the western part
are observed to be optically mature than the swirls, thereby in-
dicative for the premature emergence of regional compressive
stresses. The result is also in concordance with a relatively tighter
compaction of the soil by reflecting higher dielectric permittivity
and lower porosity values. This further signifies the evolution of
high albedo swirls draped over the pre-existing wrinkled ridges.
In essence, the lower topographical relief proves to be a reli-
able criterion for exploring the enigmatic swirls and associated
tectonic activities with a rover.
An additional attempt has been made to unravel the effects of
space weathering on the spectral properties of the RGF regolith.
In Fig. 14, a closer perspective of the western part focuses the
inter-swirl lane composed of several small microcraters along
with brighter swirl region with variability in the degree of re-
flectance (LOC 2). As seen, the craters in the dark lane reveal
brighter crystalline materials underneath the regolith. This char-
acterizes the inter-swirl region as a prominent surface veneer.
However, similar behavior has not been observed in the case
of craters over the swirl regolith. The high albedo region exhibits
opposition to the weathering processes, thereby not initiating the
darkening of the soil (LOC 1). There may be a lower degree of
chemical alteration in the brighter regolith due to the possible
associated crustal magnetic field. This suggests the formation
of swirls occurs as a consequence of physical alteration process
affecting the top micrometer of the regolith. The lowering mat-
uration rate in the on-swirl region attributes to the formation of
low strength minimagnetospheres providing shielding from the
incoming charged particles.
This can be further demonstrated by analyzing temperature
profile of the RGF. For information regarding derivation of sur-
face temperature from M3data, refer to [61]. As seen in Fig. 15,
the swirl region has been marked with blue color suggestive of
relatively lower surface temperature than the surrounding soil.
This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
SHUKLA et al.: PETROPHYSICAL INSIGHTS INTO LUNAR MAFIC EXTRUSIVE BASALTS OVER REINER GAMMA FORMATION 15
Fig. 15. M3estimated surface temperature of the RGF. The on-swirl regions
are found to be relatively cooler due to the less interaction of space weather-
ing agents (blue color). Majority of the off-swirl basalts have a temperature of
approximately ∼335 K (green color).
The difference in the daytime temperature profile between on-
swirl and off-swirl regolith is ∼9 K. This provides possible im-
plications for magnetic shielding from excessive temperature
surges caused due to the solar wind plasma bombardment. The
sensitivity of daytime temperature depends on the albedo and
Ti content of the regolith. High Ti content in the basalts re-
flects yellow to the red color with a high average temperature of
∼345 K. The microcraters residing over on-swirl region observe
an increase in the temperature resulting from gradual darken-
ing, thereby affecting thermophysical properties of the regolith.
However, these estimated measures derived from M3instrument
assume lower limits and actual surface temperature may be a few
degrees higher. The reason for using the data is to showcase the
applicability of solar wind hypothesis in conjunction with rela-
tive temperature interpretations of the RGF.
Based on the observations, a new geological perspective for
the evolution of RGF swirls has been presented. The reflectance
spectra from high albedo regions resemble immature deposits
with strong signatures of Mg-rich orthopyroxenes and olivine
content. This mineralogical deposition may attribute to the em-
placement of subcrustal-derived mafic-enriched flood basalts
peripheral to the Imbirium base, followed by the formation of
regional wrinkled ridges. The MiniRF observations support the
lunar dust hypothesis responsible for the probable formation
of western brighter swirls, thereby proving its significance
on a local scale [42]. Furthermore, lower temperature and
immaturity of the swirls indicate that the minimagnetosphere
shield the surface from the effects of solar wind interaction
[39]. The creation of magnetosphere in such localized regions
hints toward the rich magnetic history of the Moon. This is
associated with immense heat produced during magma flow,
wherein metallic iron gets liberated in an oxygen-free environ-
ment. During the entire period of lunar magnetism, the iron is
supposedly aligned with the direction of magnetic surge [62].
The hypothesis can also be reformed into describing a different
source of derived iron when there is an interaction with the
space weathering agents. However, more work has to be done
in understanding the evolution of space weathering toward
solving the mystery of magnetic anomalies. In addition, the
enhanced hydration in some regions, as detected in the spectra,
may also be associated with the deposition of fresh material
through hypervelocity cometary gas event [41].
V. C ONCLUSION AND FUTURE SCOPE
In this study, the applicability of radar data has been ex-
plored to evaluate the electrical and geotechnical properties of
the mafic-rich regolith. The petrophysical characteristics of the
regolith have been observed to be strongly correlated by the
rate of maturation. The broad scale mineralogical distribution
indicates the abundance of pyroxene in the central part of the
RGF. The further examination of spectral analysis using MGM
signifies the possible consequence of mantle-derived primary
melts with negligible chemical variations during magma em-
placements. The hydration in the RGF regolith is found to be as-
sociated with endogenic origins. Moreover, the increased olivine
content in the olivine orthopyroxene assemblages attributes to
the plausible cooling of the pyroxene-rich liquidus. This study
hypothesizes the brighter swirl region as a thinner impression of
the surface less than the penetration depth. The geomorpholog-
ical analysis of wrinkled ridges suggests an early formation of
faults before the emergence of high albedo sinuous markings.
In addition, the radar observations propose a locally occurring
event of charged dust levitation in the western part during the
formation of the swirls. The present study contributes toward
exploring the mystery of swirl formation by means of petro-
physical characterization of regional mineralogy.
As part of future work, the local dust event along with
global formation hypothesis needs to be explored through in-
tensive numerical modeling approaches. The wider wavelength
spectrum of imaging IR spectrometer in the upcoming ISRO
Chandrayaan-2 mission will potentially provide deeper insights
into the significance of water/hydroxyl absorptions in the lunar
swirls. Moreover, the dual frequency L- and S-band SAR in-
strument onboard Chandrayaan-2 will contribute by means of
capturing reliable scattering mechanisms from the subsurface
due to longer penetration of L-band. Further, a more robust di-
electric estimation needs to be performed by utilizing physics-
based backscattering model. Thus, the understanding of the RGF
swirls can be improved by adopting multisensor approach rather
focusing on a single sensor data.
ACKNOWLEDGMENT
The authors are thankful to the Lunar PDS Geoscience Node,
Washington University at St. Louis for providing free access to
Chandrayaan-1 M3and LRO (S-band MiniRF, NAC) data prod-
ucts. They would also like to acknowledge the Indian Institute of
Remote Sensing, ISRO, India, and Faculty of Geo-information
Science and Earth Observation (ITC), University of Twente, The
Netherlands, for providing all the required resources which guar-
anteed the successful completion of the research work.
This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
16 IEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS AND REMOTE SENSING
REFERENCES
[1] P. H. Warren, “The magma ocean concept and lunar evolution,” Annu. Rev.
Earth Planet. Sci., vol. 13, pp. 201–240, 1985.
[2] C. K. Shearer and J. J. Papike, “Magmatic evolution of the Moon,” Amer.
Mineral., vol. 84, no. 10, pp. 1469–1494, 1999.
[3] P. C. Hess and E. M. Parmentier, “A model for the thermal and chemi-
cal evolution of the Moon’s interior: Implications for the onset of mare
volcanism,” Earth Planet. Sci. Lett., vol. 134, nos. 3/4, pp. 501–514, 1995.
[4] J. J. Papike, F. N. Hodges, A. E. Bence, M. Cameron, and J. M. Rhodes,
“Mare basalts: Crystal chemistry, mineralogy, and petrology,” Rev. Geo-
phys., vol. 14, no. 4, pp. 475–540, 1976.
[5] D. F. Weill, R. A. Grieve, I. S. McCallum, and Y. Bottinga, “Mineralogy-
petrology of lunar samples. Microprobe studies of samples 12021 and
12022; viscosity of melts of selected lunar compositions,” in Proc. 2nd
Lunar Sci. Conf., 1971, no. 2, pp. 413–430.
[6] F. Horz, R. A. Grieve, G. Heiken, P. D. Spudis, and A. Binder, “Lunar
surface processes,” in Lunar Sourcebook. Cambridge, U.K.: Cambridge
Univ. Press, 1991, pp. 61–120.
[7] B. Mason, E. Jarosewich, W. G. Melson, and G. Thompson, “Mineral-
ogy, petrology, and chemical composition of lunar samples 15085, 15256,
15271, 15471, 15475, 15476, 15535, 15555, and 15556,” in Proc. 3rd
Lunar Sci. Conf., vol. 1, 1972, pp. 785–796.
[8] R. L. Klima, C. M. Pieters, and M. D. Dyar, “Pyroxene spectroscopy:
Probing composition and thermal history of the lunar surface,” in Proc.
Lunar Planet. Sci. Conf., vol. 40, 2009.
[9] R.L. Klima, C. M. Pieters, and M. D. Dyar, “Characterization of the 1.2 µm
M1 pyroxene band: Extracting cooling history from near-IR spectra of
pyroxenes and pyroxene-dominated rocks,” Meteoritics Planet. Sci. Arch.,
vol. 43, no. 10, pp. 1591–1604, 2008.
[10] J. M. Sunshine and C. M. Pieters, “Estimating modal abundances from the
spectra of natural and laboratory pyroxene mixtures using the modified
Gaussian model,” J. Geophys. Res., vol. 98, no. 93, pp. 9075–9087, 1993.
[11] E.A. Cloutis and M. J. Gaffey, “Pyroxene spectroscopy revisited: Spectral-
compositional correlations and relationship to geothermometry,” J. Geo-
phys. Res., vol. 96, no. E5, pp. 22809–22826, 1991.
[12] J. B. Adams, “Visible and near-infrared diffuse reflectance spectra of py-
roxenes as applied to remote sensing of solid objects in the solar system,”
J. Geophys. Res., vol. 79, no. 32, pp. 4829–4836, 1974.
[13] E. A. Cloutis et al., “The 506 nm absorption feature in pyroxene spec-
tra: Nature and implications for spectroscopy-based studies of pyroxene-
bearing targets,” Icarus, vol. 207, no. 1, pp. 295–313, 2010.
[14] R. L. Klima, M. D. Dyar, and C. M. Pieters, “Near-infrared spectra of
clinopyroxenes: Effects of calcium content and crystal structure,” Mete-
oritics Planet. Sci., vol. 46, no. 3, pp. 379–395, 2011.
[15] J. T. Cahill and P. G. Lucey, “Radiative transfer modeling of lunar high-
lands spectral classes and relationship to lunar samples,” J. Geophys. Res.
E Planets, vol. 112, no. 10, E10007, 2007.
[16] S. Tompkins and C. M. Pieters, “Mineralogy of the lunar crust: Results
from Clementine,” Meteoritics Planet. Sci., vol. 34, pp. 25–41, 1999.
[17] P. G. Lucey, D. Blewett,J. Taylor, and R. Hawke, “Imaging of lunar surface
maturity,” J. Geophys. Res. Planets, vol. 105, no. E8, pp. 20377–20386,
Aug. 2000.
[18] P. G. Lucey, D. Blewett, and R. Hawke, “Mapping the FeO and TiO2
content of the lunar surface with multispectral imagery,” J. Geophys. Res.
Planets, vol. 103, no. E2, pp. 3679–3699, Feb. 1998.
[19] B. B. Wilcox, P. G. Lucey,and J. J. Gillis, “Mapping iron in the lunar mare:
An improved approach,” J. Geophys. Res. E Planets, vol. 110, no. 11,
pp. 1–10, 2005.
[20] J. W. Nettles et al., “Optical maturity variation in lunar spectra as measured
by moon mineralogy mapper data,”J. Geophys. Res., vol. 116, no. E00G17,
pp. 1–12, 2011.
[21] M. I. Staid and C. M. Pieters, “Integrated spectral analysis of mare soils
and craters: Applications to eastern nearside basalts,” Icarus, vol. 145,
no. 1, pp. 122–139, 2000.
[22] L. Moroz, U. Schade, and R. Wäsch, “Reflectance spectra of olivine-
orthopyroxene-bearing assemblages at decreased temperatures: Implica-
tions for remote sensing of asteroids,” Icarus, vol. 147, no. 1, pp. 79–93,
2000.
[23] F. F. Sabins, “Remote sensing for mineral exploration,” Ore Geol. Rev.,
vol. 14, nos. 3/4, pp. 157–183, 1999.
[24] E. A. Cloutis, “Hyperspectral geological remote sensing: Evaluation of
analytical techniques,” Int. J. Remote Sens., vol. 17, no. 12, pp. 2215–
2242, 1996.
[25] A. P. Crosta and J. M. Moore, “Enhancement of LANDSAT thematic
mapper imagery for residual soil mapping in SW Minas Gerais State,
Brazil: A pospecting case history in Greenstone Belt Terrain,” in Proc. 7th
Thematic Conf. Remote Sens. Exploration Geol., 1989, pp. 1173–1187.
[26] J. A. Richards and X. Jia, Remote Sensing Digital Image Analysis, 4th ed.
Berlin, Germany: Springer-Verlag, 2006.
[27] D. A. Rothery, “Improved discrimination of rock units using Landsat The-
matic Mapper imagery of the Oman ophiolite,” J. Geol. Soc. London,
vol. 144, no. 4, pp. 587–597, 1987.
[28] A. B. Kahle and L. C. Rowan, “Evaluation of multispectral middle infrared
aircraft images for lithologic mapping in the East Tintic Mountains, Utah,”
Geology, vol. 8, pp. 234–239, 1980.
[29] B.A. Campbell, B. R. Hawke, and T. W. Thompson, “Regolith composition
and structure in the lunar maria: Results of long-wavelength radar studies,”
J. Geophys. Res. E Planets, vol. 102, no. E8, pp. 19307–19320, 1997.
[30] T. Hagfors, “Remote probing of the moon by infrared and microwave
emissions and by radar,” Radio Sci., vol. 5, no. 2, pp. 189–227, 1970.
[31] Y. Shkuratov, “Regolith layer thickness mapping of the moon by radar and
optical data,” Icarus, vol. 149, no. 2, pp. 329–338, 2001.
[32] M. J. Campbell and J. Ulrichs, “Electrical properties of rocks and their
significance for lunar radar observations,” J. Geophys. Res., vol. 74, no. 25,
pp. 5867–5881, 1969.
[33] B. Campbell, J. Grant, and T. Maxwell, “RADAR penetration in mars
analog environments,” in Proc. Lunar Planet. Sci. Conf., vol. 33, 2002,
pp. 1–2.
[34] G. Kramer et al., “The lunar swirls a white paper to the NASA decadal
survey,” Appl. Phys., pp. 1–6, 2009.
[35] L. L. Hood and G. Schubert, “Lunar magnetic anomalies and surface op-
tical properties,” Science, vol. 208, no. 4439, pp. 49–61, 1980.
[36] N. C. Richmond et al., “Correlation of a strong lunar magnetic anomaly
with a high-albedo region of the Descartes mountains,” Geophys. Res.
Lett., vol. 30, no. 7, pp. 2–5, 2003.
[37] D. T. Blewett, E. I. Coman, B. R. Hawke, J. J. Gillis-Davis, M. E. Purucker,
and C. G. Hughes, “Lunar swirls: Examining crustal magnetic anomalies
and space weathering trends,” J. Geophys. Res. E Planets, vol. 116, no. 2,
pp. 1–30, 2011.
[38] G. Y. Kramer et al., “M3 spectral analysis of lunar swirls and the
link between optical maturation and surface hydroxyl formation at mag-
netic anomalies,” J. Geophys. Res. E Planets, vol. 116, no. 9, pp. 1–20,
2011.
[39] L.L. Hood and C. R. Williams, “The lunar swirls: Distribution and possible
origins,” in Proc. Lunar Planet. Sci. Conf., vol. 19, 1989, pp. 99–113.
[40] P. H. Schultz and L. J. Srnka, “Cometary collisions on the Moon and
Mercury,” Nature, vol. 284, no. 5751, pp. 22–26, 1980.
[41] P. C. Pinet, V. V. Shevchenko, S. D. Chevrel, Y. Daydou, and C.
Rosemberg, “Local and regional lunar regolith characteristics at Reiner
Gamma: Optical and spectroscopic properties from Clementine and Earth-
based data,” J. Geophys. Res., vol. 105, no. E4, pp. 9457–9475, 2000.
[42] I. Garrick-Bethell, J. W. Head, and C. M. Pieters, “Spectral properties,
magnetic fields, and dust transport at lunar swirls,” Icarus, vol. 212, no. 2,
pp. 480–492, 2011.
[43] H. Hiesinger, J. W. Head, U. Wolf, R. Jaumann, and G. Neukum, “Ages
and stratigraphy of mare basalts in Oceanus Procellarum, Mare Nubium,
Mare Cognitum, and Mare Insularum,” J. Geophys. Res., vol. 108, no. E7,
p. 5065, 2003.
[44] D. J. Heather and S. K. Dunkin, “A stratigraphic study of southern Oceanus
Procellarum using Clementine multispectral data,” Planet. Space Sci.,
vol. 50, nos. 14/15, pp. 1299–1309, 2002.
[45] C. M. Pieters et al., “The moon mineralogy mapper (M3) on
Chandrayaan-1,” Curr. Sci., vol. 96, no. 4, pp. 500–505, 2009.
[46] S. Nozette et al., “The lunar reconnaissance orbiter miniature radio fre-
quency (Mini-RF) technology demonstration,” Space Sci. Rev., vol. 150,
nos. 1–4, pp. 285–302, 2010.
[47] J. M. Sunshine, C. M. Pieters, and S. F. Pratt, “Deconvolution of min-
eral absorption bands: An improved approach,” J. Geophys. Res., vol. 95,
no. B5, p. 6955, 1990.
[48] R. L. Klima et al., “New insights into lunar petrology: Distribution and
composition of prominent low-Ca pyroxene exposures as observed by
the Moon Mineralogy Mapper (M3),” J. Geophys. Res., vol. 116, no. E6,
p. E00G06, Apr. 2011.
[49] A. Ishimaru, “Wave propagation and scattering in random media and rough
surfaces,” Proc. IEEE, vol. 79, no. 10, pp. 1359–1366, Oct. 1991.
[50] S. R. Cloude, Polarization: Application in Remote Sensing. Oxford, U.K.:
Oxford Univ. Press, 2009.
This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
SHUKLA et al.: PETROPHYSICAL INSIGHTS INTO LUNAR MAFIC EXTRUSIVE BASALTS OVER REINER GAMMA FORMATION 17
[51] S. R. Cloude, “The dual polarisation entropy/alpha decomposition,” in
Proc. Int. Workshop Sci. Appl. SAR Polarimetry Polarimetric Interferom-
etry, vol. 3, 2007, pp. 1–6.
[52] B. P. Ablitt, Characterisation of Particles and Their Scattering Effects on
Polarized Light. Nottingham, U.K.: Univ. Nottingham, 2000.
[53] A. Bhattacharya, A. Porwal, S. Dhingra, S. De, and G. Venkataraman,
“Remote estimation of dielectric permittivity of lunar surface regolith us-
ing compact polarimetric synthetic aperture radar data,” Adv. Space Res.,
vol. 56, no. 11, pp. 2439–2448, 2015.
[54] W. Fa, M. A. Wieczorek, and E. Heggy, “Modeling polarimetric radar
scattering from the lunar surface: Study on the effect of physical properties
of the regolith layer,” J. Geophys. Res., vol. 116, no. E3, p. E03005, 2011.
[55] G. R. Olhoeft and D. W. Strangway, “Dielectric Properties of the first
100 meters of the Moon,” Earth Planet. Sci. Lett., vol. 24, pp. 394–404,
1975.
[56] W. D. Carrier III, G. R. Olhoeft, and W. Mendell, “Physical Properties of
the Lunar Surface,” in Lunar Sourcebook, G. H. Heiken, D. T. Vaniman,
and B. M. French, Eds. Cambridge, U.K.: Cambridge Univ. Press, 1991.
[57] S. M. Pelkey et al., “CRISM multispectral summary products: Parame-
terizing mineral diversity on Mars from reflectance,” J. Geophys. Res.,
vol. 112, no. E08S14, pp. 1–18, 2007.
[58] M. Bhatt et al., “Regolith alteration processes at Reiner gamma shed light
on the formation of lunar swirls,” in Proc. Lunar Planet. Sci. Conf., vol. 49,
2018, pp. 1–2.
[59] R. K. Raney, T. S. Cahill, G. W. Patterson, and D. J. Bussey, “The m-chi
decomposition of hybrid dual-polarimetric radar data with application to
lunar craters,” J. Geophys. Res., vol. 117, no. E00H21, pp. 1–8, 2012.
[60] N. Liu, H. Ye, and Y. Q. Jin, “Dielectric inversion of lunar PSR media with
topographic mapping and comment on ‘Quantification of water ice in the
Hermite-A Crater of the lunar north pole,’” IEEE Geosci. Remote Sens.
Lett., vol. 14, no. 9, pp. 1444–1448, Sep. 2017.
[61] P. Isaacson, S. Besse, N. Petro, J. Nettles, and M3 Team, M3 Overview
and Working with M3 Data, 2011, p. 62.
[62] D. J. Hemingway and S. M. Tikoo, “Lunar swirl morphology constrains
the geometry, magnetization, and origins of lunar magnetic anomalies,”
J. Geophys. Res. Planets, vol. 123, no. 8, pp. 1–19, 2018.
Shashwat Shukla received the M.Sc. degree (Cum
Laude) in geoinformatics, in 2019 from the Faculty
of Geoinformation Science and Earth Observation
(ITC), University of Twente, Enschede, The Nether-
lands, under the Joint Education Program framework
with the Indian Institute of Remote Sensing, ISRO,
Dehradun, India.
His research interests include planetary remote
sensing with special emphasis on imaging radar, geo-
physical spectroscopy, multisensor data integration,
and solar wind interaction studies.
Shashi Kumar received the B.Sc. (Hons.) degree in
physics from Veer Kunwar Singh University, Arrah,
India, in 2002, and the M.Sc. degree in physics from
Patna University, Patna, India, in 2006. He completed
the M.Sc. Geoinformatics course, in 2009 under the
Joint Education Program of the Indian Institute of
Remote Sensing (IIRS), Dehradun, India, and the
Faculty of Geoinformation Science and Earth Obser-
vation (ITC), University of Twente, Enschede, The
Netherlands.
He is currently a Scientist with the IIRS, ISRO,
Dehradun, India. He has supervized several research works on SAR remote
sensing and its utilization. He is actively involved in the NASA ISRO Syn-
thetic Aperture Radar and ISRO Chandrayaan-2 missions. He was a member
of SAR Task Group to develop SAR protocols for carbon forest inventory of
Indian forests under the USAID Forest PLUS Technical Assistance Program.
His research interests include SAR remote sensing with special emphasis on
polarimetric SAR, polarimetric SAR interferometry, and SAR tomography for
structural and biophysical characterization of manmade and natural features.
Valentyn A. Tolpekin received the Ph.D. degree in
physics from the University of Twente, Enschede, The
Netherlands, in 2004.
He is currently an Assistant Professor with the De-
partment of Earth Observation Science, ITC. His re-
search interests focus on mathematical and statistical
tools for satellite image analysis, SAR image analysis,
contextual image analysis including Markov random
fields, super-resolution mapping, and issue of scale in
image analysis.
