Ding et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-020-00340-4(2020) 7:32

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open Access

Compositional variations along the route of
Chang’e-3 Yutu rover revealed by the lunar
penetrating radar
Chunyu Ding1,2, Zhiyong Xiao1,3,4* , Yan Su2,5, Jiannan Zhao6 and Jun Cui1,2,4Abstract
Using the high-frequency lunar penetrating radar data obtained by the Chang’e-3 mission, we apply the frequencyshift method to calculate the decay rate of the electromagnetic wave in the regolith-like ejecta deposits of theZiwei crater. The radar data are divided into segments according to the navigation points along the traverse routeof the Yutu rover. For each segment, we calculate the bulk loss tangent of materials within the top ~ 50 ns of theradar data based on the frequency decreasing rate of the electromagnetic wave. The loss tangent varies from ~0.011–0.017 along the route of Yutu, and it is within the range of the measured loss tangent of Apollo regolithsamples. Using the empirical relationship between loss tangent and TiO2 + FeO content derived from the Apollo
lunar samples, we estimate the TiO2 + FeO content for the bulk regolith along the route of Yutu, which is ~ 23–30
wt.%. This value is comparable with that estimated using both orbital reflectance spectral data and in situ
observation made by the Yutu rover. The loss tangent derived along the route of Yutu is larger than the averagevalue of returned lunar samples, which is mainly caused by the larger content of TiO2 + FeO at the landing site
compared to the global average. Variations of the TiO2 + FeO content along the route of Yutu are mainly due tothe excavation of the Ziwei crater. The TiO2 + FeO content map derived by the radar has a much higher spatial
resolution compared to orbital observation, testifying the feasibility of this technique for regional geology study.
Keywords: Moon, Regolith, Ground penetrating radar, Loss tangent, Chang’e-3

Introduction
Most of our knowledge about the composition of theMoon is derived from orbital observations, since thenumber of both returned lunar samples and collected
lunar meteorites is still rather limited. Orbital observations using spectrometers that work at various wavelengths have perfect spatial coverage, but they areusually restricted in the spatial resolution. On the other
hand, highly heterogeneous materials exist at any placesof the Moon due to intense material transportation, mixing, and metamorphism by impact cratering (Huang* Correspondence: xiaozhiyong@mail.sysu.edu.cn
1Planetary Environmental and Astrobiological Research Laboratory, School ofAtmospheric Sciences, Sun Yat-sen University, Zhuhai, China
3
State Key Laboratory of Lunar and Planetary Sciences, Space Science
Institute, Macau University of Science and Technology, Macau, China
Full list of author information is available at the end of the article

et al. 2017). Therefore, there is a gap in the spatial resolution of interpreted compositions using orbital and laboratory measurements, and caution should be used
when directly comparing the compositional data obtained from orbit and those measured for samples at laboratories. For example, reflectance spectra obtained
from orbit (normally > 10 m/pixel) are usually comparedwith those measured at laboratories for typical minerals
and lunar samples to determine the possible compositions (e.g., Pieters et al. 2000), but the two measurements have a scale difference of at least an order ofmagnitude. Recently, Wu and Hapke (2018) analyzed thevisible–near-infrared spectrometer data obtained on the
lunar surface by the Chang’e-3 mission (i.e., CE-3), andheterogeneous reflectance spectra were observed both at
the surface and in the near-subsurface materials.

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Ding et al. Progress in Earth and Planetary Science(2020) 7:32

Lunar penetrating radar (i.e., LPR) can also be used todeduce the composition of surface materials, especiallythe content of TiO2 + FeO (Schaber et al. 1975). The
rate of energy decay of electromagnetic waves in lunar
regolith is mainly controlled by the content of TiO2 +FeO (Strangway et al. 1977), so that the loss tangent oflunar regolith derived from the energy decay rate ofelectromagnetic waves can be used to estimate theamount of TiO2 + FeO (Campbell et al. 1997).
Pommerol et al. (2010) noticed that radar echoes
returned from high-Ti regions on the Moon were low,which were detected by the lunar radar sounder onboard
the Selenological and Engineering Explorer Kaguya mission. Applying this method, Campbell et al. (1997) estimated the bulk composition of regolith in the lunar
mare using Earth-based radar data. The LPR onboard
the Yutu rover of the CE-3 mission obtained the first in
situ radar profile on the surface of the Moon (Su et al.
2014). Compared to the radar data obtained by Earthbased telescopes (e.g., Campbell et al. 1997), the LPR
data obtained by CE-3 have a much shorter wavelength(“Data and method” section) and higher spatial resolution. This work utilizes the CE-3 LPR data to derive
the composition along the route of Yutu, which will fill
the observation gap in terms of the spatial resolution between the orbital (e.g., Zhao et al. 2014) and in situ measurements (e.g., Wu and Hapke, 2018). Furthermore,
instead of probing only the top-most materials, the LPRderived contents of TiO2 + FeO represent the averagevalue of materials within the radar detection ranges.
We introduce the data obtained by the CE-3 highfrequency LPR in the “Data” section, the frequency-shifttechnique used to estimate the loss tangent of lunar materials and the method used to derive the TiO2 + FeO
content are introduced in the “Estimate of loss and tangent”. The derived loss tangent values and the TiO2 +FeO contents are discussed in the “Estimate of loss andtangent” and “Bulk TiO2 + FeO content” sections, respectively, and the reliability of these results is verified
in the “Reliability of the loss tangent estimated” and “Reliability of the TiO2 + FeO contents estimated”. Indications of the results are discussed in “Indications toregional geology” section.

Data and methodData

We use the CE-3 high-frequency LPR data in this study.
The LPR system onboard the Yutu rover consists of twochannels that have center frequencies of 60 and 500
MHz, respectively (Fang et al. 2014; Su et al. 2014). The
transmitted pulse of the LPR was generated by a digital
integrated circuit, so that the transmitted waveform wasconstant (Fang et al. 2014), enabling the derivation ofloss tangent from the measured frequency drift rate

Page 2 of 11(Irving and Knight, 2003). The high-frequency LPR was
operated with different gain values during the mission(Feng et al. 2017), and a constant gain of 0 dB was usedfrom the navigation point N105 to N208 (Fig. 1). To
avoid uncertainties raised by the different gain values(Feng et al. 2017), we use the radar data from N105 to
N208 here, which contain ~ 1600 valid traces after removing the redundant data.
The radar data are processed following the routine
procedure (e.g., Su et al. 2014). The Yutu rover was
driven with different speeds from the navigation points
N105 to N208 (Feng et al. 2017). To remedy distortionsin the radargram that are caused by the different moving
speeds (e.g., data sections N202–203 and N204–206
shown in Fig. 1c), we perform a trace equivalent calculation for the data based on the average rover speed (i.e.,
0.055 m/s). Afterward, the standard processing procedure (e.g., remove direct current component and background, and band-pass filtering) is applied to obtain the
final LPR radargram (Fig. 2a).
We use the top ~ 50 ns of the radargram for thisstudy. There is a general consensus that this part of theradargram corresponds to the continuous ejecta depositsof the Ziwei crater (Fig. 1a, e.g., Xiao et al. 2015; Fa et al.
2015; Zhang et al. 2019), which is supported by both thegeological context (Qiao et al. 2016) and the signal transition of radar echoes (Xiao et al. 2015). Data at larger
depths are less clear in terms of the geological interpretation (Xiao et al. 2015; Zhang et al. 2015; Zhang et al.
2019), and the signal-to-noise ratio is lower (Xing et al.
2017). To avoid possible controversies regarding the reliability of both the data and geological interpretation atlarger depths, we focus on the ~ 50 ns of the radargram
here. Previous studies have found that materials within
this depth (i.e., the continuous ejecta deposits of Ziwei)
have similar properties with typical regolith materials(Lai et al. 2016; Xiao et al. 2015).
Estimate of loss tangent

We focus on the imaginary part of the relative permittivity (i.e., loss tangent) to invert the content of TiO2 +FeO. Radar permittivity consists of real and imaginaryparts. For the top ~ 50 ns LPR radargram, the real partof the relative permittivity has been estimated using
hyperbolic reflectors in the radargram, resulting invalues below 5 (Feng et al. 2017; Lai et al. 2016). However, the imagery part for the lunar surface at the CE-3
landing site was taken as a constant value of ~ 0.014,
and this value was estimated using the empirical relationship between loss tangent and the content of TiO2+FeO of the surface regolith (~ 27.8% on average), which
was obtained by the Active Particle-induced X-ray Spectrometer (APXS) onboard the Yutu rover (Ling et al.
2015). However, the APXS observations are only for 4

Ding et al. Progress in Earth and Planetary Science(2020) 7:32

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Fig. 1 The landing site of the Chang’e-3 mission and the traverse route of the Yutu rover. a Geological context of the landing site. The landingsite (white star) is located on the rim of the ~ 450 m diameter Ziwei crater. The base image is obtained by the Lunar Reconnaissance Orbit(M102285549LE+RE; 1.66 m/pixel). b Image acquired by the descent camera (image ID: CE3_BMYK_LCAM-3006) on the Chang’e-3 lander shows
numerous small craters within the landing site. The landing site is marked by the white star and the white line is the route of the Yutu rover. cTraverse route and speed of the Yutu rover from the navigation points N101 to N208 (black dots). The color bar is for the moving speed of therover. Navigation points that are started with ‘1’ represent the first lunar day, and ‘2’ represents the second lunar day. The white star represents
the position of landerDing et al. Progress in Earth and Planetary Science(2020) 7:32

points (Ling et al. 2015), and it is our target to resolve the
TiO2 + FeO content along the route of the Yutu rover.
Measurements for the permittivity of returned lunar
regolith revealed that at radar frequencies larger than 105
Hz, the loss tangent of lunar regolith is primarily affectedby the content of TiO2 + FeO, while the loss tangent isonly weakly related to the bulk density (Strangway et al.
1977). On the other hand, the Apollo 17 mission performed both radar and seismic detections for the subsurface, and a sudden increase was observed in both theimaginary and real parts at the possible boundary between
the regolith layer and the competent basalts, and this increase was interpreted to be caused by the larger bulk
density of competent basalts (Strangway et al. 1977). The
top ~ 50 ns of the CE-3 LPR radargram is restrictedwithin regolith-like materials (Xiao et al. 2015; Lai et al.
2016), so that the content of TiO2 + FeO can be estimatedusing the loss tangent derived from the LPR data.
Loss tangent can be estimated in both the time-domain(i.e., a decay function of signal amplitude with time; Brzostowski and McMechan, 1992) and frequency-domain (i.e.,a decrease function of centroid frequency with time;
Brzostowski and McMechan, 1992; Turner and Siggins,1994). Compared to the time-domain method, the advantage of the frequency-domain method is that the frequency shift of the reflected electromagnetic waves is not

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affected by reflection losses or far-field geometricalspreading (Liu et al. 1998). Difficulties raised by the amplitude decay of the reflected electromagnetic waves can beignored using frequency-domain method.
The relationship between the dielectric losses anddownshift of the centroid frequency (Quan and Harris,1997) can be applied to estimate the loss tangent of thelunar regolith (Irving and Knight, 2003; Lauro et al.
2017). This method is also widely used to estimate theloss tangent of media both in seismic detections (e.g.
Quan and Harris, 1997) and ground penetrating radar(Irving and Knight, 2003; Liu et al. 1998; Quan and Harris, 1997). When electromagnetic waves are propagatingin loss materials such as lunar regolith, the centroid frequency of the received electromagnetic wave (fr) is lower
than that transmitted (ft), a phenomena called wavelet
dispersion, which is caused by dielectric losses (Turnerand Siggins, 1994). To estimate the loss tangent using
the frequency shift method, the LPR data are transformed to the frequency domain, and the centroid frequency and its time decay for each trace of the LPR data
are calculated using the short-time Fourier transformation (STFT; Griffin and Lim, 1984). The decreasing rateof the centroid frequency of the received signal (i.e.,
downtrend slope) is computed using a linear least squarefit (Irving and Knight, 2003). Loss tangent is

Fig. 2 Radargram obtained by the high-frequency LPR from the navigation points N105 to N208. The two-way travel time of the electromagnetic
wave is shown in the right y-axis. a The final-processed radargram. The left y-axis is the depth corresponding to an assumed dielectric constant of
1. b Interpretations of subsurface structure of the radargram. The solid green lines are uneven reflectors that are interpreted as the boundary
between the ejecta deposits of Ziwei and deeper more competent materials (e.g., Xiao et al. 2015; Fa et al. 2015). The dashed pink lines mark theapproximate boundary where the signal transition is not obvious. The distance values shown in the lower panel are from ~ 33.2 to 107.4 m thatare the route lengths from the landing site. The left y-axis mark depths that correspond to an assumed dielectric constant of 3.2 (Fa et al. 2015)

Ding et al. Progress in Earth and Planetary Science(2020) 7:32

proportional to the downtrend slope (Quan and Harris,
1997; Irving and Knight, 2003).
The frequency drift of the received electromagnetic
waves is calculated based on the assumption that forelectromagnetic waves in radar wavelengths, the amplitudes of the transmitted signals (x), those loss throughmedium (r), and those of the received signals (y) follow
a linear system (Liu et al. 1998; Quan and Harris, 1997).
Therefore, at each time (τ), the amplitude of the transmitted and that of the received radar waves can be calculated via convolution based on the amplitude losses in
the medium as shown in Fig. 3. In a linear signal system,
the time-domain signal can be translated to a frequencydomain signal using the Fourier transformation (FFT;
Rabiner and Gold, 1975), which can simplify the calculation procedure (Cooley et al. 1969). Equation (1) shows
the linear system in the frequency domain (Fig. 3).
Y ð f Þ ¼ Xð f Þ  Rð f Þ

ð1Þ

where f is frequency, Y(f) is the frequency spectrum of
the received signal. X(f) is the frequency spectrum of thetransmitted radar pulse. R(f) is the propagation functionof the radar pulse, which includes both attenuation andphase terms. The transmitted radar pulse of the high frequency LPR channel can be approximated as a plane
wave (Irving and Knight, 2003), so that the frequency
spectrum of the received signal Y(f) at a given distance d
can be stated as Eq. (2).
Y ð f Þ ¼ Xð f Þ  e−αd  e− jβdð2Þwhere α is the attenuation term, and β is the phase termin the loss medium R(f). The loss tangent (tanδ) of thelossy medium is proportional to the attenuation function(α), and their relationship is expressed as Eq. (3). Constant tanδ not depending on frequency is assumed inthis study for simplicity.
pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ 1
ω ½ 1 þ tan2 δ−1 2
α¼
υp
2

f

g

ð3Þ

where ω = 2πf, f is the frequency of the radar pulse. vp isthe velocity of the radar pulse in the medium. The loss
tangent of typical lunar regolith is ~ 0.005 (Strangway
et al. 1977), so that the power of the loss tangent (tan2δ)is far less than one. Applying the binomial approximapﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
tion 1 þ χ 2 −1 ≈ χ 2 =2 to Eq. (3), we obtainαð f Þ ¼

π tanδ
fυp

ð4Þ

The electromagnetic wave transmitted by the highfrequency LPR follows a constant Ricker function (Fang

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et al. 2014), which can also be approximated by theGaussian function shown in Eq. (5) (Lauro et al. 2017):"#
2
4ð f − f 0 Þ2
ð5Þ
X ð f Þ ¼ pﬃﬃﬃ exp −
f0 π
f 20
where f0 is the dominant frequency of the transmitted
radar pulse, which is also recognized as the peak frequency (Zhang et al. 2002). Substituting Eq. (4) and (5)into Eq. (2), the frequency spectrum of the received signal can be expressed as the Eq. (6). More detailed derivation process can be found in the supplementary
information..
8 h 
i2 9
π tanδ f 20 d
>
>
<
=
4
f
−
f
−
0
8υp
2
Y ð f Þ ¼ pﬃﬃﬃ exp −
>
>
f0 π
f 20
:
;
"
2 #
π tanδ f 0 d
−2 −4
 exp
4υp


expð−jβd Þð6ÞThe first exponential expression of Eq. (6) is theGaussian format of the transmitted radar pulse. The second is the attenuation of the radar pulse, and the third is
the phase of the radar pulse.
The centroid frequency of the received signal (fr) is
shown in the Eq. (7) (Irving and Knight, 2003; Liu et al.
1998; Quan and Harris, 1997).
R∞
f  Y ð f Þdf
f r ¼ 0R ∞
ð7Þ
0 Y ð f Þdf
The centroid frequency of transmitted radar wave X(f)is ft, which is expressed as Eq. (8) (Quan and Harris,1997).
R∞f  X ð f Þdf¼ f0
f t ¼ 0R ∞
ð8Þ
0 X ð f Þdf
Combining Eqs. (6), (7), and (8), the centroid frequency of the received signal (fr) is related with that ofcentroid frequency of the transmitted signal (ft) and theloss tangent (Quan and Harris, 1997), and expressed asf r ¼ f t−

π tanδ f 2t
τ
8

ð9Þ

where τ = d/υp is propagation time. Fitting the centroid
frequency of the received signal (fr) and the propagation
time (τ), we can estimate the loss tangent (tanδ) of theelectromagnetic wave and the centroid frequency of thetransmitted signal (ft). Equation (10) shows the relationship between loss tangent (tanδ) and the frequency driftrate(Δfr/Δτ).

Ding et al. Progress in Earth and Planetary Science(2020) 7:32

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Fig. 3 The LPR data are treated as a linear signal system. Fourier transformation and Inverse Fourier transformation are used to transform thesignal system from time domain to frequency domain (Rabiner and Gold, 1975). In the time domain, the received signal y(τ) is derived based on
the convolution between the transmitted signal x(τ) and that of the loss medium r(τ). In the frequency domain, the amplitude spectrum of thereceived signal Y(f) equals the product of the amplitude spectra of the transmitted waveform X(f) and the loss medium R(f)

.
Δf r
Δτ
tanδ ¼ −8
π f 2tBulk TiO2+FeO content

ð10Þ

In the calculation, STFT is applied to the first ~ 50 ns ofeach A-scan to transform the received LPR signal fromtime-domain, y(τ), to frequency-domain, Y(f). The ~ 50 ns
boundary (i.e., solid green and dashed pink curves; Fig. 2b)corresponds to the approximated depth of surface regolithalong the rover route. The sampling frequency is set to 3.2
GHz, which corresponds to the time sampling interval(0.3125 ns) of the high frequency LPR (Su et al. 2014). Each
A-scan is divided into 8 equally sized segments (i.e., ~ 6 nseach), and the overlapped time width is set to 50% of thesegment width (i.e., 3 ns) to ensure reliable frequency resolution (Niethammer et al. 2000). A 3 ns Hamming window
is applied for each segment of the A-scan for the STFT.

Results
Estimate of loss tangent

For each segment of the LPR data that were obtained between adjacent navigation points (Fig. 1), the frequency
drift rate for the radar data is calculated. Figure 4 shows
the centroid frequency of the received signals versus timedelays, which are fitted using linear equations. It is notablethat fr here is derived from the data after reducing the system noise. Deriving fr from the raw data would cause substantial oscillation in the echo pattern. Table 1 shows theresults and the related errors. In general, the loss tangent
derived is ~ 0.011–0.017 (Table 1), and the average value
is 0.014. Referring to the relationship between loss tangent
and penetration depth of the high-frequency LPR (Xinget al. 2017; Fig. 5a), our estimated loss tangent corresponds to penetrating depths of ~ 12.5–15.7 m (ε = 2.9),
which is consistent with the actual penetrating depth of
the high-frequency LPR (Xiao et al. 2015; Fa et al. 2015).

The average density of materials within the top ~ 50 ns of
LPR data is estimated to be 1.8 g/cm3 (Fa et al. 2015),
which is consistent with the density of the typical lunar
regolith (Carrier et al. 1991). Therefore, applying the empirical relationship between the loss tangent and TiO2 +FeO content of lunar regolith samples (Eq. 11; Carrier
et al. 1991), the bulk TiO2 + FeO content of materials
within the ~ 50 ns of the radargram can be calculated.
tanδ ¼ 10ð0:030ð%TiO2 þ%FeOÞ−2:676Þ

ð11Þ

The TiO2 + FeO content is estimated as ~ 23–30 wt.%
(Table 1), and the average value is ~ 27 wt.%.
The estimated bulk concentration of TiO2 + FeO exhibits a decreasing trend from the navigation points
N105 to N202 (Fig. 5b). In general, navigation points
further away from the rim of Ziwei (Fig. 1) exhibit ahigher content of TiO2 + FeO (Fig. 5b). However, theTiO2 + FeO concentration at N107–N108 is distinctly
less than the surroundings, suggesting that the composition of materials in this region is patchy in distribution.
Assuming that the change of the bulk TiO2 + FeO contents along the traverse route of Yutu is continuous outward, the TiO2 + FeO contents along the route of Yutu
are interpolated to derive a regional trend (Fig. 5c) using
the natural neighbor method (Sibson, 1981).

Discussion
Reliability of the loss tangent estimatedIn this study, the loss tangent of subsurface materials
along the route of Yutu is estimated using the relationship between the centroid frequency of the received signals and the time delay. Table 1 shows that the centroid
frequency of the LPR transmitter is ~ 495.1–510.4 MHz.
The average value (501.3 MHz) is identical with that of

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Fig. 4 The relationship between the centroid frequency of the received signal (fr) and the time-delay for each segment of the LPR data. The data
segments (a–j) are divided according to the adjacent navigation points from N105 to N208. The red lines are the best-fit between fr and τ using
the least square method. The slopes of the best-fit lines are the drift rate of fr with τ

the designed centroid frequency (500 MHz) of the highfrequency LPR (Fang et al. 2014), suggesting that thebase data used to estimate the loss tangent is reliable.
The top ~ 50 ns of the CE-3 LPR data are restrictedwithin the continuous ejecta deposits of the Ziwei crater(Xiao et al. 2015; Fa et al. 2015). The ejecta deposits are
dominated by pre-impact regolith as evident by the geological context (Qiao et al. 2016). This is also consistent
with the relative permittivity value of ~ 2–5 estimatedusing the LPR data (Fa et al. 2015; Lai et al. 2016; Fenget al. 2017), which is substantially less than that of typical lunar rocks (Carrier et al. 1991). The derived loss
tangent values shown in Table 1 are within the range
and toward the upper end of the measured loss tangentof returned regolith samples by the Apollo missions(0.00057 to 0.0232; Carrier et al. 1991). Typical lunar
regolith samples exhibit loss tangent less than 0.01, andthis is also true for in situ measurements performed on
the Moon. The Surface Electrical Properties Experimentcarried out by the Apollo 17 mission has measured thepermittivity and loss tangent of materials at the landing
site, and the interpreted regolith layer (~ 7 m thick) has
a bulk loss tangent of 0.008 and a relative permittivity of
3.8 (Strangway et al. 1977). On the contrary, the loss
tangent of the interpreted bedrocks at the Apollo 17
landing site is 0.035 and the relative permittivity is 7.7(Strangway et al. 1977). It is notable that the value ofloss tangent cannot be used to determine whether or not

Table 1 Parameters estimation for each navigational points. The centroid frequency of the received radar signal, the drift slope ofthe centroid frequency, the derived loss tangent, and the derived TiO2 + FeO contents at different segments of the LPR data. The
uncertainties for all the estimated values are the 95% confidence bounds. The navigation points are shown in Figs. 1c and 5b
LPR
segments

Centroid frequency(ft, MHz)

Frequency drift rate
(Δf r ,MHz/ns)

Loss tangent(tanδ)

TiO2 + FeO (wt.%)

N105–N106
N106–N107
N107–N108
N108–N202
N202–N203
N203–N204
N204–N205
N205–N206
N206–N207
N207–N208

504.75 ± 1.22
502.10 ± 1.58
496.52 ± 1.70
510.37 ± 1.40
505.30 ± 1.31
501.44 ± 1.41
498.57 ± 1.38
495.11 ± 2.28
497.40 ± 1.30
501.24 ± 1.74

− 1.623 ± 0.043− 1.661 ± 0.059− 1.217 ± 0.066− 1.653 ± 0.047− 1.219 ± 0.040− 1.079 ± 0.044− 1.128 ± 0.042− 1.152 ± 0.071− 1.107 ± 0.043− 1.536 ± 0.070

0.0162 ± 0.0004
0.0168 ± 0.0005
0.0126 ± 0.0006
0.0162 ± 0.0004
0.0122 ± 0.0003
0.0109 ± 0.0004
0.0116 ± 0.0004
0.0120 ± 0.0006
0.0114 ± 0.0004
0.0156 ± 0.0006

29.54 ± 0.33
30.02 ± 0.44
22.58 ± 0.70
29.48 ± 0.35
25.36 ± 0.41
23.82 ± 0.52
24.63 ± 0.47
25.13 ± 0.77
24.42 ± 0.50
28.94 ± 0.58

Ding et al. Progress in Earth and Planetary Science(2020) 7:32

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Fig. 5 a The relationship between the different loss tangent and the penetration depth of the high frequency channel (Xing et al. 2017). The
pink, green and blue lines show the electromagnetic wave propagation in media that have a relative permittivity of 2.3, 2.9, and 3.5, respectively.
The red dashed line shows the position of our estimated average loss tangent, and the rest two dashed lines correspond to the lower and upper
limits of our estimated loss tangent, respectively. b The estimated content of TiO2 + FeO from the navigation points N105 to N208 along theroute of Yutu. c An interpolated trend map for the content of TiO2 + FeO in the traverse region of Yutu. This map is based on the measurement
shown in panel b using a natural neighbor interpolation method. The base map of the main frame is obtained by the Lunar Reconnaissance
Orbiter (M102285549LE + RE; 1.66 m/pixel), and that for the inset is obtained by the descent camera onboard the CE-3 lander (image
ID CE3_BMYK_LCAM-3006)

the medium is porous regolith or competent bedrock. The
Lunar Radar Sounder (operating frequency of 5 MHz) onboard the Kaguya orbiter (SELENE) estimated that theloss tangent for the ~ 200 m thick mare units at the Oceanus Procellarum is ~ 0.001–0.01, but the estimated relative permittivity is ~ 5.76–8.08 (Ono et al. 2009).
Reliability of the TiO2 + FeO contents estimated

The TiO2 + FeO contents derived from the loss tangent
are the average values for materials within the top ~ 50 nsof the LPR data. This value is consistent with various

orbital and in situ observations for this region. The
Gamma Ray Spectroscopy onboard the Lunar Prospector
find that the TiO2 + FeO content on the mare surface
where the CE-3 landed is ~ 25.2 wt.% (Prettyman et al.
2006). The ultraviolet-visible spectrometer onboard theClementine mission found that the TiO2 + FeO near theCE-3 landing site is 24–26 wt.% (with a FeO content of ~19 wt.% and a TiO2 content of ~ 5–7 wt.%; Ling et al.
2015). Likewise, the FeO and TiO2 content estimated by
the Multispectral Imager onboard the Kaguya mission(Fig. 6a, b) are also consistent with the measurements

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Fig. 6 The TiO2 (a), FeO (b), and their combined (c) contents at the CE-3 landing site (white stars). The data are derived from the Multispectral
Imager data, and the maps are revised from Zhao et al. (2014). The white dashed line is the boundary between the Imbrian- (north) and
Eratosthenian-aged mare units (down). North is up in all the panels

done here (Fig. 6c and Table 1), which are basically consistent those estimated by the Clementine mission (Zhaoet al. 2014). Furthermore, in situ measurements performedby the CE-3 APXS found that the TiO2 + FeO content is~ 27.8 wt.% (Ling et al. 2015). Therefore, the estimatedTiO2 + FeO content are in line with previous studies.
The higher content of TiO2 + FeO compared to the global average of mare surfaces (12%; Lucey et al. 1995) explains the larger than average loss tangent derived in Table
1. The measured loss tangent of returned lunar regolithsamples exhibit a large range (from less than 0.001 to ~0.03), indicating that the loss tangent of lunar regolith isweakly dependent on the density or relative permittivity
and the dominant factor is the content of TiO2 + FeO(Olhoeft et al. 1975; Strangway et al. 1977). Regolith samples that have > 0.01 loss tangent uniformly feature a highcontent of TiO2 + FeO (Table 1). Therefore, while the CE3 landing region has the highest content of TiO2 + FeO
among lunar mare surfaces (Zhao et al. 2014), the regolithlayer, which is the dominant materials within the ejecta deposits of Ziwei, also features the largest loss tangent.
Indications to regional geology

Along the route of Yutu, places further away from the rimof Ziwei generally exhibit a slightly larger TiO2 + FeO content of ~ 30 wt.%, e.g., navigation points N105–N107 (Fig.
5b). This content is comparable with that of the
Eratosthenian-aged mare surface around the landing site
(white star in Fig. 6c). However, places closer to the rim ofZiwei have a content of TiO2 + FeO ~ 24 wt.% (e.g., navigation points N203–N207; Fig. 5b), which is closer to the Late
Imbrian-aged mare basalts to the north (Fig. 6c). This comparison indicates that the generally lower content of TiO2 +FeO closer to the rim of Ziwei may be caused by the impact
excavation of deeper Imbrian-aged materials, as the
Eratosthenian-aged basalts have been penetrated throughby Ziwei. Also, the route between navigation points N107–

N108 features a less content of TiO2 + FeO compared tothe surrounding area, indicating that the excavated low
TiO2 + FeO materials follow a patchy distribution. Since
the relative permittivity of materials within the top ~ 50 nsof the radargram are close to that of lunar regolith (e.g., Faet al. 2015), the excavated Imbrian-aged materials are most
likely from the paleo-regolith that was developed between
the eruptions of the Erathothenian- and Imbrian-aged marebasalts. This interpretation is consistent with the geologicalinterpretation of the LPR data (Xiao et al. 2015). Furthermore, most of the continuous ejecta deposits of the Ziwei
crater exhibit a high content of TiO2 + FeO (Fig. 6c), suggesting that most of ejecta is from shallow part of Ziwei
crater site and Imbrium-agedpaleo-regolith with low TiO2
+ FeO low contents from deep part is minor and limited tocrater rims and restricted locations (e.g., N107–N108; Fig.
5c) because Ziwei crater is small (Qiao et al. 2016).
The TiO2+FeO content derived here bridges the resolution gap between in situ and orbital observations,attesting the advantage of ground penetration radar inestimating the bulk composition of regolith materials.
Compared to compositional data obtained by orbital observations (e.g., the maps shown in Fig. 6 represent the highest
resolution maps obtained from orbit), the large variationsof the TiO2 + FeO content along the route of Yutu (Fig. 5b,c) is not expected (Fig. 6c), appealing for more carefulinterpretations of compositional data obtained from orbit.

Conclusion
The high-frequency radar data obtained by the lunar penetration radar onboard the Yutu rover, Chang’e-3 is used toderive the content of TiO2 + FeO for the shallow materials
along the route of Yutu. The frequency-shift method isused for the top ~ 50 ns radar data to evaluate the drift
rates of the received radar signals, which are related to theloss tangent of the bulk materials. The estimated loss tangent is ~ 0.011–0.017, which is consistent with, but slightly

Ding et al. Progress in Earth and Planetary Science(2020) 7:32

larger than the measured results for returned lunar regolith. The larger than average loss tangent for the bulk material along the route of Yutu is mainly caused by of thehigh content of TiO2 + FeO (~ 23–30 wt.%), which is consistent with both orbital and in situ observations ( Prettyman et al. 2006; Ling et al. 2015; Zhang et al. 2015). The
resolved TiO2 + FeO contents exhibit a lower value towards the rim of Ziwei, which is caused by the excavationof the Late-Imbrian-aged paleo-regolith. This study showsthat ground penetration radar can be applied as a usefulsupplementary tool to investigate the bulk composition oflunar regolith, as both the spatial resolution and detection
depth are better than orbital observations.

Supplementary information
Supplementary information accompanies this paper at https://doi.org/10.
1186/s40645-020-00340-4.
Additional file 1. Supplementary material.
Abbreviations
LPR: Lunar penetrating radar; CE-3: Chang’e-3; SELENE: Selenological andEngineering Explorer; APXS: Active particle-induced X-ray spectrometer;
FFT: Fast Fourier transformation; STFT: Short time Fourier transformation
Acknowledgements
We thank Dr. Sebastian Emanuel Lauro and Dr. Federico Di Paolo for helpful
discussion. Comments and suggestions provided by two anonymos
reviewers significantly helped clarifying and improving the manuscript.
Authors’ informations
Dr. Chunyu Ding is an associate researcher at Sun Yat-sen University. He
received a Ph.D of Astronomy from the National Astronomical Observatories,
Chinese Academy of Sciences in 2017. Dr. Zhiyong Xiao is an associate
professor at the Planetary Environmental and Astrobiological ResearchLaboratory, Sun Yat-sen University. Dr Jiannan Zhao is a postdoc at China
University of Geosciences (Wuhan). Dr. Yan Su is the PI of data acquisition
subsystem at the Ground Application System of Lunar Exploration, National
Astronomical Observatories, Chinese Academy of Sciences. Dr. Jun Cui is the
director of Planetary Environmental and Astrobiological Research Laboratory,
Sun Yat-sen University.
Authors’ contributions
C.D. and Z.X. proposed the topic, conceived and designed the study, andanalyzed the data. Y.S, J.Z., and J.C. collaborated with the corresponding
author in the construction of manuscript. All authors read and approved thefinal manuscript.
Funding
The authors are supported by the B-type Strategic Priority Program of theChinese Academy of Sciences (XDB41000000), the National Natural Science
Foundation of China (41773063, 41525015, and 41830214), the Science andTechnology Development Fund of Macau (0042/2018/A2), the Pre-research
Project on Civil Aerospace Technologies (D020101, D020202) of CNSA, andthe Opening Fund of the Key Laboratory of Lunar and Deep Space Exploration, Chinese Academy of Sciences (ldse201702; ldse201908).
Availability of data and materials
Source codes and data used in this study open to public access (https://doi.
org/10.5281/zenodo.3884522). The LPR data are also available at the Ground
Application System of Lunar Exploration, National Astronomical
Observatories, Chinese Academy of Sciences (http://moon.bao.ac.cn).
Competing interests
The authors declare that they have no competing interests.

Page 10 of 11

Author details
1
Planetary Environmental and Astrobiological Research Laboratory, School ofAtmospheric Sciences, Sun Yat-sen University, Zhuhai, China. 2Key Laboratory
of Lunar and Deep Space Exploration, National Astronomical Observatories,
Chinese Academy of Sciences, Beijing, China. 3State Key Laboratory of Lunarand Planetary Sciences, Space Science Institute, Macau University of Scienceand Technology, Macau, China. 4Chinese Academy of Sciences, Center for
Excellence in Comparative Planetology, Hefei, China. 5University of Chinese
Academy of Sciences, Beijing, China. 6Planetary Science Institute, School ofEarth Sciences, China University of Geosciences (Wuhan), Wuhan, China.
Received: 28 October 2019 Accepted: 22 May 2020

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