Much of the Martian surface is covered by a weathering layer (regolith or soil) produced by long-term surface processes such as impact gardening, eolian erosion, water weathering, and glacial modifications. China’s first Martian mission, Tianwen-1, employed the Mars Rover Penetrating Radar (RoPeR) to unveil the detailed structure of the regolith layer and assess its loss tangent. The RoPeR radargram revealed the local regolith layer to be highly heterogeneous and geologically complex and characterized by structures that resemble partial or complete crater walls and near-surface impact lenses at a very shallow depth. However, comparable radar data from the Lunar far side are rather uniform, despite the two surfaces being geologically contemporary. The close-to-surface crater presented in this study shows no detectable surface expression, which suggests an accelerated occultation rate for small craters on the surface of Mars as compared to the rate on the Moon. This is probably due to the relentless eolian processes on the Martian surface that led to the burial of the crater and thus shielded it from further erosion. The high loss tangent indicates that the regolith at the Tianwen-1 landing site is not dominated by water ice.

1. Geological Society of America | GEOLOGY | Volume 51 | Number 3 | www.gsapubs.org 315
Manuscript received 1 August 2022
Revised manuscript received 9 October 2022
Manuscript accepted 2 November 2022
https://doi.org/10.1130/G50632.1
© 2023 Geological Society of America. For permission to copy, contact editing@geosociety.org.
CITATION: Chen, R., et al., 2023, Martian soil as revealed by ground-penetrating radar at the Tianwen-1 landing site: Geology, v. 51, p. 315–319, https://doi​
.org/10.1130/G50632.1
Martian soil as revealed by ground-penetrating radar at the
Tianwen-1 landing site
Ruonan Chen1
*, Ling Zhang1,2
*, Yi Xu1†
, Renrui Liu1
, Roberto Bugiolacchi1,3
, Xiaoping Zhang1
, Lu Chen1
,
Zhaofa Zeng2
and Cai Liu2
1
State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, China
2
College of Geo-exploration Science and Technology, Jilin University, Changchun 130026, China
3
Earth Sciences, University College London, London WC1E 6BT, UK
ABSTRACT
Much of the Martian surface is covered by a weathering layer (regolith or soil) produced
by long-term surface processes such as impact gardening, eolian erosion, water weathering,
and glacial modifications. China’s first Martian mission, Tianwen-1, employed the Mars
Rover Penetrating Radar (RoPeR) to unveil the detailed structure of the regolith layer and
assess its loss tangent. The RoPeR radargram revealed the local regolith layer to be highly
heterogeneous and geologically complex and characterized by structures that resemble partial
or complete crater walls and near-surface impact lenses at a very shallow depth. However,
comparable radar data from the Lunar far side are rather uniform, despite the two surfaces
being geologically contemporary. The close-to-surface crater presented in this study shows
no detectable surface expression, which suggests an accelerated occultation rate for small
craters on the surface of Mars as compared to the rate on the Moon. This is probably due to
the relentless eolian processes on the Martian surface that led to the burial of the crater and
thus shielded it from further erosion. The high loss tangent indicates that the regolith at the
Tianwen-1 landing site is not dominated by water ice.
INTRODUCTION
Through the observation of valleys on Mars
and other geological considerations, scientists
have placed a high probability on the potential
habitability of the planet, at least in its remote
past. This has motivated a huge international
effort to send both orbital and landing missions
to explore this possibility. The characterization
of the physical, chemical, and mineralogical
properties of Mars’ weathering layer (regolith
or soil) represents a key target of this enterprise.
The regolith layer developed through various
types of geological processes, and unraveling
its evolution would address many of the out-
standing key geological questions, including
the potentiality of liquid water on the surface
or near-surface.
Compared to the regolith of airless planetary
bodies such as the Moon, the Martian regolith is
rather more complex, consisting of drifting fine
dust and sand overlying cemented soil and rock
fragments (e.g., McSween and Keil, 2000). The
layer was modified by the combined action of
impact gardening, erosion from eolian, liquid,
and igneous processes, and chemical weather-
ing by fluids (e.g., Cannon et al., 2019). As on
the Moon, on Mars, impact gardening is a major
process by which regolith is produced; however,
the 10-mbar-thick atmosphere has produced a
different size-frequency distribution of craters
(e.g., Hartmann and Neukum, 2001; Warner
et al., 2017) by both shielding the surface from
smaller bolides and affecting the craters’ evo-
lution following excavation. The accumulation
of air-transported dust and eolian modification
processes have also played a significant role in
the formation of the fine-grained regolith com-
ponent in some regions.
Decades of exploration have resulted in a
tremendous increase in the information available
about the physical, chemical, mineralogical, and
electrical properties of the Martian soil and dust
(see the Supplemental Material1
). Compared to
remote sensing approaches such as NASA’s
MarsAdvanced Radar for Subsurface and Iono-
spheric Sounding (MARSIS) (e.g., Picardi et al.,
2004) and the Shallow Radar (SHARAD) (e.g.,
Seu et al., 2007), in situ detection instead can
investigate the fine structure and properties of
subsurface materials. The Mars Exploration
Probe Tianwen-1 carrying the Zhurong rover
landed in Utopia Planitia on 15 May 2021 with
the task of investigating the surface composition,
regolith characteristics, water-ice distribution,
and environment at the surface (e.g., Liu et al.,
2022). The landing region was mapped in orbital
images as a late Hesperian lowland unit that is
mostly composed of Vastitas Borealis Forma-
tion (VBF) materials. Study of these materi-
als is fundamental to gaining a better under-
standing of the geologic history of the northern
plains and constraining their origin as due to
either fluvial, lacustrine, or marine processes.
Zhurong is equipped with six scientific instru-
ments (e.g., Tan et al., 2021), including the Mars
Rover Penetrating Radar (RoPeR) with dual-
frequency channels (CH1: 35–75 MHz; CH2:
0.45–2.15 GHz) (e.g., Zhou et al., 2020), and
the Navigation and Terrain Camera (NaTeCam).
RoPeR is the first multi-polarization radar effort
to sound the near subsurface through two dis-
tinct frequencies to reveal subsequent surface
processes that occurred in the largest Martian
unit and hopefully offer clues about the source
of the uppermost deposition materials. The
CH1 data expose an ∼80-m-thick, multi-lay-
ered structure that has been interpreted as the
occurrence of an episodic hydraulic flooding
sedimentation process (Li et al., 2022). Due to
the strong reflections between the rover and the
ground surface, the detailed structure of the first
10 m layer was not revealed, though the CH2
1
Supplemental Material. Supplemental text, methods of signal processing and loss tangent calculation, Figures S1–S11 and Table S1 and S2. Please visit
https://doi​.org​/10​.1130​/GEOL.S.21899427 to access the supplemental material, and contact editing@geosociety.org with any questions.
†
E-mail: yixu@must.edu.mo
*These authors contributed equally to this work.
Published online 9 February 2023
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2. 316 www.gsapubs.org | Volume 51 | Number 3 | GEOLOGY | Geological Society of America
data could complement the near-surface part of
the layer.
Our study presents the detailed subsurface
structure derived from CH2 data gained during
the first six months of the exploration mission
(Fig. 1). The heterogeneous distribution of scat-
tered energy inferred from the radargram when
compared with lunar regolith suggests a rather
more complex modification history than that of
the Moon.
MATERIALS AND METHODS
The NASA Mars Reconnaissance Orbiter
(MRO) High Resolution Imaging Science
Experiment (HiRISE) data (0.25 m/pixel) were
used to characterize the geomorphological fea-
tures of the Zhurong landing area. The NaTe-
Cams, binocular stereo cameras mounted on the
Zhurong rover, obtained field-of-view images of
the landing area. The detailed processing steps
for CH2 data and the process for deriving loss
tangent values are provided in the Supplemen-
tal Material.
RESULTS
The Tianwen-1 touched down in the southern
part of the Utopia Planitia and west of the Ely-
sium volcanic province (109.925°E, 25.066°N).
This is a region near the dichotomy boundary
that separates the southern Noachian highlands
and the younger northern lowlands, which are
dated to ∼3.1–3.36 b.y. ago (e.g., Ivanov et al.,
2017). However, recent studies have indicated
that the regional surface age of the landing
area obtained using statistics of smaller cra-
ters (1 km in diameter) is only between ca.
757 Ma and ca. 1.12 Ga (e.g., Zhao et al., 2021;
Wu et al., 2022). This could be interpreted as the
age of main local settlement/resurfacing events/
processes that occurred in the regional area.
The radargram generated from the CH2 HH
(horizontal transmitting and horizontal receiv-
ing) polarization mode data is ∼1200 m long
and 4.5 m deep (Fig. 2). The image derived does
not feature noticeable continuous reflections or
distinctive layered stratification, as seen in Fig-
ure 2. The surface elevation varied only ∼9 m
over the full traveling distance. We observe
some randomly distributed scattered signals
with strong reflectance. Two types of distinct
structures in the radargram were matched with
their corresponding surface images from the
NaTeCams (Figs. 3 and 4, respectively). The
first type is represented by two small, close-to-
surface craters named “A” and “B”. The second
relates to several sloping structures.
During its exploration journey, the rover has
driven over several small craters; accordingly,
we would expect to see some related under-
ground structures in the radargram. Crater A
is located ∼900 m away from the landing spot
(96–97 waypoints; Fig. 3A), with an apparent
diameter of ∼6.8 m, and features a shallow
depression on the surface as visible in the NaTe-
Cams image (Fig. 3B). Rock debris marks its
outer extent, and its interior is populated by fine-
grained, loose, and porous materials. The crater
is rimless and appears as a sandy depression
when observed in the terrain image in Figure 3B.
There is a distinct subsurface reflector (black
dashed line, Fig. 3D) that possibly originates
from a chord profile of the inner wall of crater
A. The length (note that the rover circled the
crater but did not cross its center) of crater A
indicated by the subsurface reflection echoes
(Fig. 3D) is 10 ± 1 m, larger than its diameter
as it is expressed on the surface. The energy of
the echoes inside crater A is chaotic and messy.
These chaotic signals may be related to the bur-
ied rocks, such as debris generated by crater
wall erosion. Modeling of crater degradation
has suggested that wall erosion could provide
most of the infill material (e.g., Forsberg-Taylor
et al., 2004). Considering the effects of the per-
mittivity, scanning path of RoPeR on the mea-
sured depth and actual diameter of the buried
crater, we determined its depth-to-diameter to be
0.06, which also fits in the range of 1/10–1/20,
the typical ratio for degraded craters.
A deeper bowl-shaped structure with smooth
boundaries located beneath sites 164–166 is
interpreted as the cross-section chord of a bur-
ied crater (crater B), as shown in Figure 3G.
Figure 1. The exploration
path (white line) of the
Zhurong rover (Chinese
Mars mission Tianwen-1)
in the first six months (File
ID: ESP_069665_2055_
RED). Yellow boxes
correspond to the local
regions in Figures 3 and 4.
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3. Geological Society of America | GEOLOGY | Volume 51 | Number 3 | www.gsapubs.org 317
Unlike crater A, the rim of crater B is invis-
ible in the surface image, except for a string
of slightly sloping exposed boulders (yellow
arrows in Fig. 3F). The absence of a rim can
be explained by the effects of strong erosion,
while the exposed boulders may originate from
the impact process.
In addition to shallow impact craters, slopes
are another type of commonly observed under-
ground structures, with their corresponding
surface terrain containing larger boulders com-
pared to those of highly degraded/buried craters
(Fig. 3B versus Figs. 4B and 4E). The first set
of strong scattering signals below sites 29–31
is shown in Figure 4C.A slope of ∼19° appears
within the depth of 2.5 m (black dashed line).
Another slope with the same tilt direction (∼24°)
extends from the surface to a depth of 4.5 m and
is marked with a set of red arrows; a reflection
in the opposite direction of these tilting struc-
tures with a larger slope of ∼35° is marked by
blue arrows. A strip of convex structures can be
seen in the orbital image in Figure 4B marked
with white arrows. They are probably associ-
ated with the subsurface oblique reflections. The
other two sloping reflections with strong reflec-
tance below sites 54–55 are shown in Figure 4F;
here, we observe a downslope reflector (∼23°)
followed by a rising (∼17°) one. However, the
corresponding surface terrain is very flat, with
only some randomly distributed rubble visible
(Fig. 4E). This might represent the surviving
part of the crater wall or subsurface polygonal
wedge casts produced by unstable relict deposits
that have flattened due to aggradation, although
only one such structure was observed within the
traveling path (∼1.2 km).
The loss tangent parameter is related to
the radar signal attenuation in the material.
Apollo samples show that the loss tangent is
mainly determined by the material composi-
tion, especially the iron and titanium content
(e.g., Carrier, 1991). The estimated loss tan-
gent of the Martian regolith at the Tianwen-1
site is 0.0124 ± 0.0006, 0.0179 ± 0.0006, and
0.0235 ± 0.0006 using average radar power
with geometric compensation of R2
, R3
, and
R4
, respectively. R3
correction is adopted here.
DISCUSSION
The potential buried craters are preserved in
radargram within the depths of less than ∼4.5 m
and show a distinctly heterogeneous distribution
of subsurface structures. In contrast, the lunar
regolith in the Von Kármán crater measured by
the ground-penetrating radar of theYutu-2 rover
of the Chinese Chang’e-4 mission is rather more
homogeneous except for a couple of continuous
thin ejecta layers caused by local small impact
events that have an average thickness of ∼11 m
(see Fig. S11A). No well-structured craters or
other strongly reflective buried structures have
been found within the layer of fine-grained lunar
regolith. The buried craters observed at the
Chang’e-4 landing site are located at a depth of
∼12–25 m. One example is given in Fig. S11C.
The buried structure on Mars is not as eroded
as the lunar example, which suggests a lower
cumulative effect of impact gardening. The pre-
served craters and missing shallow small-scale
ejecta layers also confirm prior studies asserting
that the Martian atmosphere has a weakening
effect on the impact of micrometeorites (e.g.,
Warner et al., 2017).
The absence of surface expressions of well-
preserved buried craters on Mars also implies a
faster obliteration speed of small craters on Mars
than on the Moon. The surface crater density is
significantly different between the Chang’e-4
landing area and Tianwen-1: many more small
craters can be clearly seen in the orbital image
of the Chang’e-4 landing area (Fig. S11B). Stud-
ies have shown that Martian craters are more
degraded than their lunar counterparts, partic-
ularly at smaller diameters (e.g., Irwin et al.,
2018). Crater Size Depth Frequency Distribu-
tion (CSDFD) surveys suggest that the northern
lowland has experienced an abnormally high
obliteration rate since 1.5 Ga ago (e.g., Breton
et al., 2022). Therefore, in addition to impacts,
other surface processes on Mars would play an
important role in the obliteration of crater edi-
fices. As observed during the rover’s traverse,
A
B
Figure 2. Surface elevation of the Zhurong rover (Chinese Mars missionTianwen-1) exploration path and a radargram in the first six months of
rover travel. (A) Surface elevation of the Zhurong exploration path. (B) Radargram of the first six months. Black line represents the maximum
elevation difference of the current exploration path (∼9 m), and the black dashed line represents the detection depth of the radargram (∼4.5 m).
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4. 318 www.gsapubs.org | Volume 51 | Number 3 | GEOLOGY | Geological Society of America
the existence of many active dunes suggests that
the eolian process may accelerate their oblitera-
tion. Over time, rims are weathered down and
the interiors are infilled with sand transported
by the wind. Subsequently, these small relief
craters are quickly buried by transported materi-
als and fade away.
Additional reworking processes, such as
micro-bombardment and space weathering, are
slowed down and do not have enough time to
destroy the impact melt at the crater floor or on
the wall. The radargram shows that the echoes
above the walls of cratersA and B are relatively
weak (Figs. 3D and 3G), although they are more
pronounced in crater B, which indicates that
these materials are somewhat porous, loose,
and fine-grained.
Measurements taken by the SHARAD show
the loss tangent value at the northern Utopia
Planitia (located at ∼139°E, ∼40°N) to be
0.009 ± 0.004, and at the Elysium Planitia
area next to the Tianwen-1 landing site, it is
0.022 ± 0.01144 (e.g., Campbell and Morgan,
2018). The loss tangent obtained at the Tian-
wen-1 landing site is 0.0179 ± 0.0006 (R3
cor-
rection), which is between the values of these
two areas. Due to the different detection fre-
quencies, the SHARAD data are representa-
tive of the material properties within a few tens
of meters, whereas the RoPeR measured the
uppermost surface layer. The surface layer in
the northern Utopia Planitia contains a high per-
centage of water ice; however, the increased loss
tangent indicates that the regolith at the Tian-
wen-1 landing site is not dominated by water
ice (e.g., Stuurman et al., 2016).
The RoPeR on board the Zhurong Martian
rover provides the first direct measurement of
the detailed subsurface structure of Martian
soil in southern Utopia Planitia within a depth
of 4.5 m. Radargram does not reveal a layered
structure within such depth. The reason could
be due to the thickness of the local regolith layer
being greater than 4.5 m or because the dielec-
tric contrast at the interface is not detectable by
the RoPeR.
Compared with the lunar radar results, the
Martian regolith appears to be more heteroge-
neous and contains parts of paleo-craters or the
entire crater walls of paleo-craters, in addition to
a sloping structure, whereas the lunar regolith at
the Chang’e-4 site contains a 10-m-thick layer
of homogenous and fine-grained particles with
no apparent structure.
The rover will continue to explore the sub-
surface structure of pitted cones and kilome-
ter-sized grabens, which should contribute to
our understanding of their origins and test the
hypothesis of the existence of a large standing
body of water in the northern lowland.
ACKNOWLEDGMENTS
We thank the Tianwen-1 payload team for mission
operations, and the China National Space Adminis-
tration for providing the Tianwen-1 data that made
this study possible. The data set was processed and
produced by the Ground Research and Application
System (GRAS) of China’s Lunar and Planetary Explo-
ration Program, provided by the China National Space
Administration (http://moon​.bao​.ac​.cn). This work was
supported by the CivilAerospace Pre-research Project
(grant D020101); the Science and Technology Devel-
opment Fund of Macau (grants 0089/2018/A3 and
0049/2020/A1); the Postdoctoral Innovation Talent
Support Program of China (grant BX20200148); the
China Postdoctoral Science Foundation Funded Project
(grant 2020M681041); and the National Natural Sci-
ence Foundation of China (grant 42104141).
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Printed in USA
A B
C
D E
F
Figure 4. Underground slope beneath the Zhurong rover (Chinese Mars mission Tianwen-1)
route path. (A,D) Zhurong track. Numbers are waypoints. (B) A terrain image around waypoints
29–31; white arrows indicate rocks. (C) A slope of ∼19° beneath waypoints 29–30 (black dashed
lines). Red (24°) and blue arrows (35°) point to tilting structures beneath waypoints 30–31. (E)
Terrain image around waypoints 54–55. White arrows indicate the route of the rover. Objects
in the background are the rover parachute and back shell. (F) The downslope and upslope
reflectors beneath waypoints 54–55: blue (23°) and red arrows (17°).
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