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The Global Search for Liquid Water on Mars from Orbit: Current and Future Perspectives


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Due to its significance in astrobiology, assessing the amount and state of liquid water present on Mars today has become one of the drivers of its exploration. Subglacial water was identified by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) aboard the European Space Agency spacecraft Mars Express through the analysis of echoes, coming from a depth of about 1.5 km, which were stronger than surface echoes. The cause of this anomalous characteristic is the high relative permittivity of water-bearing materials, resulting in a high reflection coefficient. A determining factor in the occurrence of such strong echoes is the low attenuation of the MARSIS radar pulse in cold water ice, the main constituent of the Martian polar caps. The present analysis clarifies that the conditions causing exceptionally strong subsurface echoes occur solely in the Martian polar caps, and that the detection of subsurface water under a predominantly rocky surface layer using radar sounding will require thorough electromagnetic modeling, complicated by the lack of knowledge of many subsurface physical parameters. Higher-frequency radar sounders such as SHARAD cannot penetrate deep enough to detect basal echoes over the thickest part of the polar caps. Alternative methods such as rover-borne Ground Penetrating Radar and time-domain electromagnetic sounding are not capable of providing global coverage. MARSIS observations over the Martian polar caps have been limited by the need to downlink data before on-board processing, but their number will increase in coming years. The Chinese mission to Mars that is to be launched in 2020, Tianwen-1, will carry a subsurface sounding radar operating at frequencies that are close to those of MARSIS, and the expected signal-to-noise ratio of subsurface detection will likely be sufficient for identifying anomalously bright subsurface reflectors. The search for subsurface water through radar sounding is thus far from being concluded.
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The Global Search for Liquid Water on Mars from
Orbit: Current and Future Perspectives
Roberto Orosei 1,* , Chunyu Ding 2,† , Wenzhe Fa 3,† , Antonios Giannopoulos 4,† ,
Alain Hérique 5,† , Wlodek Kofman 5,6,† , Sebastian E. Lauro 7,† , Chunlai Li 8,9,† ,
Elena Pettinelli 7,† , Yan Su 8,9,† , Shuguo Xing 10,† and Yi Xu 11,†
1Istituto di Radioastronomia, Istituto Nazionale di Astrofisica, Via Piero Gobetti 101, 40129 Bologna, Italy
2School of Atmosphere Sciences, Sun Yat-sen University, 2 Daxue Road, Xiangzhou District,
Zhuhai City 519000, China;
3Institute of Remote Sensing and Geographical Information System, School of Earth and Space Sciences,
Peking University, Beijing 100871, China;
School of Engineering, The University of Edinburgh, Alexander Graham Bell Building, Thomas Bayes Road,
Edinburgh EH9 3FG, UK;
5Université Grenoble Alpes, CNRS, CNES, IPAG, 38000 Grenoble, France; (A.H.); (W.K.)
6Centrum Badan Kosmicznych Polskiej Akademii Nauk (CBK PAN), Bartycka 18A, 00-716 Warsaw, Poland
7Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84,
00146 Roma, Italy; (S.E.L.); (E.P.)
8Key Laboratory of Lunar and Deep Space Exploration, National Astronomical Observatories,
Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100101, China; (C.L.); (Y.S.)
9University of Chinese Academy of Sciences, No.19(A) Yuquan Road, Shijingshan District,
Beijing 100049, China
10 Piesat Information Technology Co., Ltd, Beijing 100195, China;
11 State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology,
Avenida Wai Long, Taipa, Macau;
These authors contributed equally to this work.
Received: 22 June 2020; Accepted: 20 July 2020; Published: 24 July 2020
Due to its significance in astrobiology, assessing the amount and state of liquid water
present on Mars today has become one of the drivers of its exploration. Subglacial water was
identified by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) aboard
the European Space Agency spacecraft Mars Express through the analysis of echoes, coming from
a depth of about 1.5 km, which were stronger than surface echoes. The cause of this anomalous
characteristic is the high relative permittivity of water-bearing materials, resulting in a high reflection
coefficient. A determining factor in the occurrence of such strong echoes is the low attenuation of the
MARSIS radar pulse in cold water ice, the main constituent of the Martian polar caps. The present
analysis clarifies that the conditions causing exceptionally strong subsurface echoes occur solely in
the Martian polar caps, and that the detection of subsurface water under a predominantly rocky
surface layer using radar sounding will require thorough electromagnetic modeling, complicated by
the lack of knowledge of many subsurface physical parameters. Higher-frequency radar sounders
such as SHARAD cannot penetrate deep enough to detect basal echoes over the thickest part of the
polar caps. Alternative methods such as rover-borne Ground Penetrating Radar and time-domain
electromagnetic sounding are not capable of providing global coverage. MARSIS observations over
the Martian polar caps have been limited by the need to downlink data before on-board processing,
but their number will increase in coming years. The Chinese mission to Mars that is to be launched
in 2020, Tianwen-1, will carry a subsurface sounding radar operating at frequencies that are close
to those of MARSIS, and the expected signal-to-noise ratio of subsurface detection will likely be
Life 2020,10, 120; doi:10.3390/life10080120
Life 2020,10, 120 2 of 15
sufficient for identifying anomalously bright subsurface reflectors. The search for subsurface water
through radar sounding is thus far from being concluded.
Keywords: habitability; space missions; space technologies
1. Water Inventory on Mars
Most of the ice present on Mars today is located in the polar regions, which are covered by
ice sheets extending for millions of square kilometers and possessing a thickness of thousands of
meters. These deposits are constituted by several geologic units differing in origin, composition,
and age. The most recent and dynamic ones are the seasonal deposits of CO
ice, produced by the
condensation of the atmosphere and persisting through the Martian winter with a thickness below
one meter [
]. The residual ice caps cover only part of the polar ice sheets and consist of high-albedo
deposits of water ice with a thickness well below that of the ice sheets themselves [
]. Below them lie
the so-called Polar Layered Deposits (PLD), which consist of hundreds of layers of ice mixed with dust
in proportions that differ in every layer depending on climatic conditions at the time of deposition.
The overall dust content of the Southern PLD (SPLD) is estimated to be at
15% by volume [
while that of the North PLD (NPLD) is lower, at 5% or less over Gemina Lingula [
], and above 6%
overall [
]. The SHARAD radar sounder detected deposits of CO
ice hundreds of meters thick on
top of the SPLD [
]. There are older and dustier ice-bearing deposits beneath the PLD, known as the
Basal Unit in the North and the Dorsa Argentea Formation in the South. The Basal Unit (BU) is a
deposit consisting of water ice and lithic fines, lying stratigraphically beneath the North Polar Layered
Deposits. It consists of two geologic units, namely the Rupes Tenuis unit at the bottom, and the Boreum
Cavi unit on top, both of Amazonian age. The Boreum Cavi unit appears to consist predominantly of
sandy material [
]. The Dorsa Argentea Formation (DAF) is a vast Hesperian-aged unit surrounding
and partially underlying the South Polar Layered Deposits. Volcanic activity, debris flows, aeolian
deposition, and glacial activity have been proposed as formation mechanisms [2], but the hypothesis
that the DAF is the remnant of a large ice sheet [7] seems to be better supported by evidence.
Ice is also present in mid-latitudes landforms such as lineated valley fills and lobate debris aprons,
which are thought to be the remains of glaciers from a recent ice age [
]. In addition to that, neutron
spectroscopy revealed the widespread occurrence of ground ice outside the polar caps, even at low
latitudes [
]. The depth to which such ice extends is unknown, but it is thermodinamically limited by
the lower boundary of the cryosphere, which is the volume of the subsurface in which ground ice is
stable. The cryosphere extends from a few meters to several kilometers below the surface, depending
mainly on the geothermal heat flow from the interior of the planet and the thermal properties of the
crust [
]. Lastly, water molecules can be bound to minerals by processes such as alteration, hydration,
and serpentinization.
The total volume of ice currently present at Mars has been estimated to be equivalent to a layer of
34 m over the entire surface of the planet [
] (this quantity is usually referred to as Global Equivalent
Layer or GEL), 22 m of which are in the polar deposits [
]. The quantity of liquid water present in
early Mars has been estimated through various methods, for example the study of geological features
is presumed to have been carved by its action. Valley networks are systems of branching valleys found
in the most ancient terrains of Mars and resembling fluvial drainage basins, which appear to have
formed in the Noachian age approximately between 4.1 and 3.7 Gyr ago. A recent estimate of the total
volume of water needed to carve them led to a conservative lower limit of 640 m GEL of water present
at the time of their formation [13].
The ratio between normal and deuterated water on Mars has changed over time, because lighter
O molecules escape form the planet more easily than heavier HDO. This fact has been used to
extrapolate the total amount of water lost over the ages. The measured D/H ratio in the current
Life 2020,10, 120 3 of 15
Martian atmosphere is much higher than the one observed in ancient rocks such as the Martian
meteorites, confirming the loss of a large quantity of water inferred from observed atmospheric loss
rates [
]. It has been estimated [
] that the total water present on the surface of Mars 4.5 Gyr ago
must have been 6–7 times the quantity existing today.
2. The Search for Liquid Water
Liquid water can now be present at the surface of Mars only briefly and under uncommon
circumstances because of low temperature and atmospheric pressure. However, there is ample
geological [
] and mineralogical [
] evidence that water once flowed on the surface of the planet,
whose climate thus had to be very different from the current one, at least for part of its history. Due to
its significance in astrobiology, assessing the amount and state of liquid water present on Mars today
has become one of the drivers of its exploration.
Evidence of a geologically recent (i.e., less than a few million years) occurrence of liquid water at
the surface of Mars was first reported in [
], in which networks of narrow, incised channels called
gullies were interpreted as being carved by groundwater seepage and surface runoff. Gullies were
initially discovered on steep slopes, mostly on impact crater walls, but were later found also in
different settings such as sand dunes [
]. A Martian gully is characterized by an alcove at its head,
an incised channel, and a downslope depositional apron. The volume of the apron is lower than
the volume of the material that has been removed to form the alcove and channel, thus suggesting
that some volatile component was initially part of the material flowing through the gully and was
lost after its formation [
]. The mapping of gullies over the Martian surface has shown that they
occur in the 30–90
latitude band of both hemispheres, and that their presence is anti-correlated with
massive ice deposits. Gullies in the 30
latitude range are pole-facing, while those polewards
of 40
are predominantly oriented toward the equator. Such a distribution appears to be related to
the availability of near-surface ice deposits [
]. The formation of gullies has been explained through
different mechanisms, some of which do not require liquid water, such as dry granular flows in the
presence of CO
ice. Terrestrial formation mechanisms that have been considered potential analogs
for Martian gullies include pyroclastic flows and dry snow avalanches (as examples of natural dry
granular flows), and fluvial flows, debris flows, and slushflows as processes involving the presence of
liquid water. As discussed in [
], morphological evidence and laboratory experiments seem to point
to liquid-water debris flows resulting from surface melting as the most plausible formation mechanism
for gullies.
High-resolution imaging has recently revealed the presence of the so-called recurring slope
lineae, which are narrow (a few meters wide), dark streaks occurring on Sun-facing steep slopes close
to the equator. Appearing and gradually growing during warm seasons, they fade in cold seasons [
and have been interpreted as either water flows caused by the melting of ground ice or dry grain
flows [
]. Spectrographic analysis of recurring slope lineae has provided no evidence of water, but it
has revealed the presence of perchlorated salts [
], which would lower the freezing point of subsurface
water brines. Recent observations, however, point to a dry grain flow mechanism at the origin of
recurring slope lineae (e.g., [27]).
Evidence for surface liquid water in the current Martian climate is inconclusive, but water could be
present underground below the cryosphere. In the early warm Mars, water would naturally percolate
into the ground until it reached an impermeable layer, thus forming an aquifer similarly to what
happens on Earth. As the mean surface temperature decreased over the ages, a global cryosphere
would form, which would effectively seal groundwater in place [
]. There is widespread evidence for
groundwater upwelling in the Martian past, requiring the presence of a global groundwater system
(e.g., [
]). It has been suggested that such system could be replenished by surface water through
the basal melting of the polar caps [
], but estimates of lithospheric heat flow for the current epoch
are less than one fourth those of Earth, making basal melting unlikely [31].
Life 2020,10, 120 4 of 15
Recently, evidence for subglacial liquid water beneath the South polar cap has been obtained
through orbital radar sounding [
]. Quantitative analysis of radar echoes from an anomalously bright
reflector, about 20 km across at a depth of
1.5 km, yielding estimates of its relative permittivity
and matching that of water-bearing materials. Alternative mechanisms producing strong basal
echoes are the presence of a CO
ice layer at the top or the bottom of the SPLD, or a very low
temperature of the H
O ice throughout the SPLD, enhancing basal echo power compared to surface
reflections. However, such phenomena either require very specific physical conditions or they do
not cause sufficiently strong basal reflections. Thermophysical modeling of the conditions needed to
generate liquid water beneath the South polar cap yields estimates of the required lithospheric heat
flow exceeding accepted values for Mars. This result seems to imply the presence of a subsurface
thermal anomaly for liquid water to be present [
]. Modeling of the subglacial hydraulic potential
beneath the South polar cap, based on radar-derived basal topography, provided estimates of the
location of subglacial lakes that do not match the bright radar reflector. This finding is consistent
with a hydraulically isolated liquid body confined by cold-based ice, rather than with a subglacial
lake [
]. In spite of the theoretical difficulties in reconciling the presence of liquid water with
the known characteristics of the SPLD, recent observations acquired by MARSIS over the same
region, and analyzed using signal processing procedures commonly applied on Earth to discriminate
between wet and dry subglacial areas, are in agreement with the earlier detection of subglacial water,
and provide evidence for other wet areas in its surroundings, suggesting the presence of a complex
hydrologic system [35].
3. Radar Sounding and Subsurface Water Detection
Subglacial water was detected by the MARSIS [
] radar sounder aboard the European Space Agency
spacecraft Mars Express. Orbital radar sounding is based on the same principle as radioglaciology;
a well-established geophysical technique employed since the mid-20th century to probe the interior of
ice sheets and glaciers in Antarctica, Greenland, and the Arctic [
]. It is based on the transmission of
radar pulses at frequencies in the Medium Frequency (MF, 300 kHz – 3 MHz), High Frequency (HF,
3–30 MHz) and Very High Frequency (VHF, 30–300 MHz) range into the surface, to detect signals
reflected from dielectric discontinuities associated with compositional and/or structural changes in the
subsurface. Radar sounders have been successfully employed in planetary exploration since the times
of the Apollo program (e.g., [
]), and they still are the only remote sensing instruments allowing
the study of the subsurface of a planet from orbit. In particular, by transmitting a 1 MHz-bandwidth
pulse centered at 1.8, 3, 4, or 5 MHz, MARSIS has been able to reveal echoes coming from depths of
more than 3.5 km beneath the Martian polar cap [12].
An electromagnetic wave encountering a discontinuity in the medium through which it is
propagating is partially reflected, while the remainder is transmitted. In the ideal case of a plane
parallel geometry in which the wave is perpendicular to the discontinuity, the partition between
reflected and transmitted power is described by the reflectance (or reflectivity, or power reflection
, and the transmittance (or transmissivity, or power transmission coefficient)
at normal
incidence [42]:
T=1R (2)
is the complex relative permittivity in the medium from which the electromagnetic wave
propagates and
is the same parameter for the medium past the discontinuity. It can be seen from
Equation (1) that the greater the difference between
, the more energy is backscattered towards
the radar transmitting the electromagnetic wave.
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The equation describing the amount of power received by a radar illuminating some target is
known as the radar equation. A specialized form of this equation for an orbiting radar sounder over a
plane surface has been presented in [43]:
is the power of the surface echo received by the radar,
is the power of the transmitted pulse,
is the radar antenna gain,
the pulse wavelength,
the spacecraft altitude, and
the Fresnel
reflection coefficient at normal incidence for the surface. The geometric term
the geometric loss due to the spherical expansion of the wavefront (both in transmission and after
reflection) multiplied by the squared area of the Fresnel circle producing the specular reflection.
The term
in Equation (3) implies that part of the radar pulse energy propagates into the
subsurface and can be reflected back to the radar in the presence of a subsurface dielectric discontinuity.
In this case, the subsurface echo power
received by the radar can be computed through the
following expression [43]:
Pss =Pt×Gλ
s×Rss ×exp (2πftan δ τ)(4)
is the depth of the subsurface dielectric discontinuity,
the surface transmission coefficient,
the subsurface Fresnel reflection coefficient at normal incidence,
the radar frequency,
tan δ
loss tangent of the medium between the surface and the subsurface discontinuity, while
is the time
delay between the reception of the surface and subsurface echoes. The loss tangent is the ratio between
the imaginary and real parts of the complex relative permittivity, and the term
exp (2πftan δτ)
expresses the attenuation of the radar signal because of dielectric losses as it propagates through the
subsurface. Depth zand subsurface echo delay τare related through the following expression:
is the speed of light in vacuo and
is the real part of the relative permittivity of the medium
between the surface and subsurface interface. In the following discussion we will refer to such a
medium as the surface layer, while the material below the discontinuity causing the reflection will
be called the basal layer. Both layers are considered homogeneous unless stated otherwise. For ease
of reference, the real part of the relative permittivity will be called, although somewhat improperly,
dielectric constant.
The identification of liquid water in a radar signal is based on its different electromagnetic
response compared to ices and other geomaterials. The surface of Mars is constituted predominantly
by igneous rocks (e.g., [
]), while its polar caps consist mostly of water ice together with dry ice and
dust [
]. Although hydrated minerals have been identified on the planet, they cover only a small
fraction of its surface [
]. Table 1below lists the values of relative permittivity for these materials,
together with those of liquid water and brine.
Table 1.
Values of the complex relative permittivity of materials present on the Martian surface in the
MHz frequency range.
Material Dielectric Constant ε0Loss Tangent tan δSource
Volcanic rocks 4–9 103–102[45,46]
H2O ice 3.1 107–101[47]
CO2ice 2.2 4 ×103[48]
Water 80 103[46]
Brine 80–110 10–100 [46]
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Most materials listed in Table 1do not exhibit a strong dependence of their relative permittivity
on temperature in the range expected for the Martian surface. The loss tangent of water ice, however,
increases by orders of magnitude with temperature [
], drastically affecting the attenuation of the
radar wave (see Equation (4)). The loss tangent can be computed as a function of temperature using
formulas presented in [
] and is shown in Figure 1below. It can be seen that cold, pure water ice
has a loss tangent that is orders of magnitude below that of other substances, resulting in very little
attenuation, and it is thus extremely transparent to radar waves. As ice temperature approaches the
melting point, its
tan δ
becomes higher than that of rock, thus strongly limiting the penetration of the
radar signal.
Figure 1.
Loss tangent of pure water ice and of an ice/dust mixture with a volumetric fraction of
dust equal to 0.1 as a function of temperature, for the four Mars Orbiter Subsurface Investigation
Radar (MOSIR) operating frequencies. The loss tangent of water ice is computed according to formulas
presented in [
], while the permittivity of volcanic rock in Table 1has been used to represent that of
dust. The effective permittivity of the ice/dust mixture has been obtained through Equation (6).
Materials on the surface of Mars can consist of mixtures of different substances, as in the case
of the ground ice found outside the polar caps [
], or the Polar Layered Deposits. There are many
different models for the effective relative permittivity of a mixture of materials (see e.g., [
] for a
discussion), several of which are specialized for particular geometries within the medium. Due to a
lack of knowledge about the small-scale structure of Martian materials, past studies have often resorted
to the simple and yet widely used Polder-van Santen model. This model has the special property that
it treats the inclusions and hosting material symmetrically, i.e., it balances both mixing components
with respect to the unknown effective medium, using the volume fraction of each component as a
weight. Its formula, as given in [49], is:
=0 (6)
is the volume fraction of inclusions in the mixture,
is the relative permittivity of the host
that of the inclusions, and
the relative permittivity of the mixture. This formula requires
algebric manipulation to obtain an expression for the solution. The result is a quadratic equation with
Life 2020,10, 120 7 of 15
two roots: The correct solution must be greater than 1 and comprised between the relative permittivity
values of the host and of the inclusion.
According to Equation (6), materials such as porous rocks should increase their relative
permittivity if their pores are saturated with water and experimental evidence on Earth shows
that permittivity values greater than 15 are seldom associated with dry materials [
]. The high
relative permittivity of water bearing materials will result in a high reflection coefficient, according to
Equation (1), and indeed the detection of subglacial lakes by means of radar sounding (which in the
context of Earth polar studies is called Radio-Echo Sounding, RES) is chiefly based on the detection of
an increase in basal echo strength relative to the immediate surroundings (e.g., [
]). This was also the
main evidence in identifying subglacial water on Mars [
], but because of the low spatial resolution of
MARSIS, it was not possible to corroborate the identification through qualitative information such as
bedrock morphology in the radar image, which is an important criterion in terrestrial studies. For this
reason, a probabilistic inversion method based on Equations (3) and (4) had to be developed to estimate
the dielectric constant of the material below the South Polar Layered Deposits of Mars [
], obtaining
values above 20 that require the presence of liquid water.
The identification of liquid water on Mars through radar sounding is thus based on the detection
of areas of strong subsurface echoes. Indeed, in the case of [
] it was found that echoes coming from
below the polar ice sheet at a depth of
1.5 km were stronger than surface echoes by several dB,
as shown in Figure 2below.
Figure 2.
) Radargram for MARSIS orbit 10737. A radargram is a bi-dimensional color-coded section
made of a sequence of echoes in which the horizontal axis is the distance along the ground track of the
spacecraft, the vertical axis represents the two-way travel time of the echo (from a reference altitude
of 25 km above the reference datum), and brightness is a function of echo power. The continuous
bright line in the topmost part of the radargram is the echo from the surface interface, whereas the
bottom reflector at about 160
s corresponds to the interface between the Southern Polar Layered
Deposits (SPLD) and the bedrock. Strong basal reflections can be seen at some locations, where the
basal interface is also planar and parallel to the surface. (
) Plot of surface and basal echo power for
the radargram in (
). Red dots mark surface echo power values, while blue ones mark subsurface
echo power. The horizontal scale is along-track distance, as in (
), while the vertical scale reports
uncalibrated power in decibels (dB). The basal echo between 45 km and 65 km along track is stronger
than the surface echo even after attenuation within the SPLD (adapted from [32]).
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Subsurface echoes can be stronger than surface echoes because of the higher relative permittivity
of water-bearing materials. This can be verified computing the ratio between subsurface and surface
echo power by dividing Equation (4) by Equation (3):
s×exp (2πftan δ τ)(7)
where we neglect the difference between the term
in Equation (3) and the term
in Equation (4) because MARSIS operates between 250 and 800 km of altitude
probing the Martian subsurface down to a few kilometers, and thus
. As mentioned above,
this expression assumes that both the surface and subsurface interface are plane and parallel, and thus
the effects such as scattering due to surface roughness can be neglected. The Rayleigh roughness
criterion is used to determine if a surface can be considered specular:
8 cos θ(8)
is the maximum standard deviation of the topographic height for a surface to be considered
is the wavelength of the incident electromagnetic radiation, and
is the angle of incidence.
For normal incidence and a frequency of a few MHz as in the case of MARSIS,
is of the order of a
few tens of meters, whereas the topographic height variation in the area where water was detected
is of the order of a few meters over areas of the size of the MARSIS footprint [
]. Although the
standard deviation of the topography at the bottom of the polar cap cannot be determined from radar
measurements alone, we will assume that it is negligible at least in the area of strong subsurface
reflections in which water was identified.
The relative permittivities of surface and subsurface materials have to be defined to compute
values of
Pss /Ps
through Equation (7). The dielectric constant of the surface layer is varied between
the lowest value for dry materials reported in Table 1, corresponding to 2.2 for CO
ice, to a value of 9
matching dense volcanic rocks, to explore the effect of surface layer composition on the strength of
subsurface echoes. To simplify the analysis, and as a way to maximize
Pss /Ps
by neglecting dielectric
losses, the loss tangent of the surface layer is assumed to be negligible. To compare the most favorable
cases for the occurrence of strong subsurface reflections by dry and water-bearing materials, the relative
permittivity of the dry bedrock has been assumed to be the highest in the range of values for dry
volcanic rocks in Table 1, that is
9 and
tan δ=
, while the relative permittivity of the liquid
water body has been taken to be the highest reported for brines in Table 1, namely
110 and
tan δ=
100. Brines are considered to be more plausible than liquid water as the source of strong basal
echoes because temperatures at the base of the SPLD are expected to be well below the freezing point
of water (see discussion in [32]). Once the relative permittivities have been defined, the terms Rss , Rs,
and Tsin Equation (7) can be computed by means of Equations (1) and (2).
It can be seen in Figure 3that the occurrence of
Pss /Ps>
1 in the absence of water-bearing
materials is possible only for a dielectric constant below that of water ice, leaving CO
ice as the
main possible constituent of surface material (Table 1). However, CO
ice is considered to be a minor
component of the Southern polar cap of Mars [
], and it has not been detected outside the polar
regions. By contrast, in the absence of appreciable attenuation within the surface material, brine
would produce strong reflections even if the permittivity of surface material was that of basaltic rocks
(Table 1).
Life 2020,10, 120 9 of 15
Figure 3.
Values of
computed according to Equation (7) for a bedrock consisting of dry volcanic
rock and a body of subglacial brine, by varying the dielectric constant value of the surface layer between
that of CO
ice and that of dense volcanic rock, and by assuming that there is no signal attenuation due
to dielectric losses in the surface material (see text for details).
Attenuation, as measured by the loss tangent, is the other key factor determining the relative
power of surface and subsurface echoes. By setting
Pss /Ps=
1 as a limit condition, Equation (7)
can be inverted to determine the values of
tan δ
that are compatible with the occurrence of strong
subsurface echoes. Figure 4has been produced assuming a basal relative permittivity value at the
upper end of the range for brines (
110 and
tan δ=
100, as in Figure 3) and a time delay of the
subsurface echo τ= 160 µs, as in [32].
Figure 4.
Values of surface material loss tangent that result in
1 according to Equation (7),
for a basal relative permittivity value at the upper end of the range for brines and a time delay of the
subsurface echo of 160 µs, as in [32] (see text for details).
Life 2020,10, 120 10 of 15
A comparison of loss tangent values in Figure 4with material properties in Table 1and plots
in Figure 1reveals that strong subsurface echoes at depths of the order of a kilometer are possible if
attenuation in the surface material is similar to that of cold ice with or without a small fraction of dust,
and perhaps to that of porous rock, but not if it is that of dense basaltic rocks. Because the estimated
depth of the Martian water table is of the order of a few to several kilometers [
], identification of
subsurface liquid water outside the polar caps is made challenging by the likely weakening of radar
echoes, and in fact it has been predicted that the Martian water table could not be detected at all by
MARSIS if its depth is greater than a few kilometers [55].
Because the dielectric constant of materials listed in Table 1is constant or nearly constant in the
MHz to GHz frequency range [
], the values of
Pss /Ps
in Figure 3can be considered independent
from frequency. While the loss tangent of water ice is inversely proportional to frequency [
], the loss
tangents reported in Table 1are frequency-independent. As the term
exp (2πftan δ τ)
in Equation (4)
includes frequency
, then attenuation in pure water ice is frequency-independent, while it increases
with frequency in all other materials. A decrease of
Pss /Ps
with increasing MARSIS frequencies was
already noted in [
]; this phenomenon was interpreted as due to the presence of dust in the ice,
making the loss tangent of the Southern Polar Layered Deposits similar to that of low temperature
ice/dust mixture shown in Figure 1. This property of dust-contaminated ice was also invoked to
explain the absence of basal echoes in radar echoes collected by SHARAD, which is a radar sounder
similar to MARSIS aboard NASA’s Mars Reconnaissance Orbiter [
] operating at a central frequency
of 20 MHz and transmitting a 10 MHz-bandwidth pulse. Extrapolating the value of
Pss /Ps
frequencies based on the trend observed in MARSIS leads in fact to the prediction that the basal echo
will be near or below the detection threshold, as shown in Figure 5.
Figure 5.
Estimate of the ratio of subsurface to surface echo power over the bright reflector in [
] as a
function of frequency, extrapolated from MARSIS data. The light blue diagonal strip represents the
area of the best fit to the data extending to the 90% confidence level. The colored rectangles highlight
the operation bands of different radar instruments.
The discussion above is based on the implicit assumption that the surface layer is homogeneous
down to the depth of the interface producing subsurface echoes. This assumption is not verified even
Life 2020,10, 120 11 of 15
in the case shown in Figure 2, the starting point for this analysis, in which the layered structure within
the South Polar Layered deposits is clearly visible. Internal layering will result in a loss of energy of
the propagating pulse due to multiple reflections, and thus in the weakening of subsurface echoes.
Furthermore, if the subsurface interface is topographically rough, as determined through Equation (8),
then the pulse would be scattered in directions that differ from the specular one, thus weakening
subsurface echoes even further. Even in the ideal case of a plane parallel geometry and homogeneous
media, resonance effects may artificially enhance or depress both the surface and subsurface echoes,
leading to measured values of
Pss /Ps
that cannot be explained through the use of Equation (7)
(e.g., [56]).
In the absence of morphological evidence such as that available for terrestrial subglacial lakes,
the search for subsurface water through radar sounding is an inverse electromagnetic problem to
determine the relative permittivity of the material producing a measured radar echo. This is a complex
problem fraught by the lack of knowledge of many parameters such as attenuation within the surface
layer and roughness of subsurface interfaces. Several approaches have been proposed over the years
(e.g., [52,5759]), but all of them required some ad hoc assumptions that prevented generalization.
4. Conclusions and Perspectives
The investigation leading to the discovery of subsurface liquid water on Mars through radar
sounding was prompted by the detection of echoes, coming from a depth of about 1.5 km, which were
stronger than surface echoes [
]. The cause of this anomalous characteristic is the high relative
permittivity of water-bearing materials, resulting in a high reflection coefficient (Equation (1)).
A determining factor in the detectability of such strong echoes is the low attenuation of the MARSIS
radar pulse in cold water ice (Figure 1), the main constituent of the Martian polar caps.
The present analysis clarifies that the conditions causing exceptionally strong subsurface
echoes occur solely in the Martian polar caps, and that the detection of subsurface water under
a predominantly rocky surface layer will require thorough electromagnetic modeling, complicated
by the lack of knowledge on many subsurface physical parameters. As signal attenuation in rocks
increases with frequency, a future radar operating at frequencies below those of MARSIS could in
principle detect deeper and stronger water-related echoes. However, because the maximum plasma
frequency of the Martian ionosphere is several hundred kHz even in favorable conditions (i.e., on the
night side [
]), it is not possible to probe the subsurface at frequencies much lower than a MHz,
which would result in only a modest increase of Pss/Ps.
The search for strong basal echoes beneath the Martian polar caps is far from being complete,
however. As discussed in [
], the small size of strong subsurface reflectors compared to the MARSIS
footprint required the use of data that have not been processed on board before being downlinked to
Earth, because such processing drastically reduced the radar sampling rate along the ground track.
These raw data could be acquired only after a modification of the on-board software and constitute a
small fraction of the MARSIS dataset. Coverage of the polar caps in this mode is thus sparse, but it
is bound to increase in coming years, so that more bright subsurface reflectors could be potentially
discovered in the future.
The Mars Express spacecraft was launched in 2003 and it is thus expected that MARSIS will
continue collecting data on the Martian polar caps for no more than a few years. Fortunately,
the Chinese mission to Mars to be launched in 2020, Tianwen-1, will carry the Mars Orbiter Subsurface
Investigation Radar (MOSIR), which will operate in the 10–15 MHz, 15–20 MHz, and 30–50 MHz
frequency ranges. The lowest band is at frequencies that are intermediate between those of MARSIS
and those of SHARAD [
]. As shown in Figure 5, such a high frequency range will probably result
in weaker subsurface echoes even in the presence of liquid water, but the signal-to-noise ratio of
subsurface detection will likely be sufficient to identify anomalously bright subsurface reflectors in
comparison to their surroundings.
Life 2020,10, 120 12 of 15
In spite of its limitations, orbital radar sounding is currently the only technique that allows a
global search of subsurface water from orbit. Ground Penetrating Radars (GPR) will be carried by
NASA’s Perseverance [
] and China’s Tianwen-1 [
] rovers, to be launched in 2020, as well as by
ESA’s Rosalind Franklin rover [
], whose launch has been postponed to 2022. Although capable of a
much better resolution, these experiments operate at higher frequencies, and thus cannot penetrate
as deep as MARSIS. Furthermore, they lack the mobility needed to achieve large-scale coverage.
An alternative electromagnetic method for deep subsurface study is time-domain electromagnetic
(TDEM) sounding [
], which works by inducing eddy currents in the subsurface and by measuring
the magnetic fields produced by such currents. This technique allows the determination of subsurface
conductivity, which increases by orders of magnitude in the presence of saline water, and can achieve
deeper penetration than GPR at the cost of lower resolution. However, because the size of the loop
used to induce ground currents must be comparable to the depth of probing and because the loop
needs to be close to the medium in which currents are to be induced, this method is not suitable for
orbiting platforms.
The exploration of the Martian subsurface is critical in the search for life on Mars [
]. Stable bodies
of subsurface water are considered among the most promising potential habitats existing on today’s
Mars (although isolation from the surface could prevent the actual presence of life [
]), and detecting
them remains one of the prime goals of Martian exploration. In spite of a long-sought initial success,
much work is still to be done before the search for subsurface water can be considered concluded.
Author Contributions:
Conceptualization, R.O.; methodology, R.O.; software, R.O.; validation, R.O., C.D., W.F.,
A.G., A.H., W.K., S.E.L., C.L., E.P., Y.S., S.X. and Y.X.; formal analysis, R.O., C.D., W.F., A.G., A.H., W.K., S.E.L.,
C.L., E.P., Y.S., S.X. and Y.X.; investigation, R.O., C.D., W.F., A.G., A.H., W.K., S.E.L., C.L., E.P., Y.S., S.X. and Y.X.;
writing—original draft preparation, R.O.; writing—review and editing,R.O., C.D., W.F., A.G., A.H., W.K., S.E.L.,
C.L., E.P., Y.S., S.X. and Y.X.; visualization, R.O., C.D., W.F., A.G., A.H., W.K., S.E.L., C.L., E.P., Y.S., S.X. and Y.X.;
supervision, R.O.; project administration, R.O.; funding acquisition, R.O., W.K. and Y.X. All authors have read and
agreed to the published version of the manuscript.
This research was funded by the Italian Space Agency through contract ASI-INAF 2019-21-HH.0,
by the French Space Agency CNES, and by the Science and Technology Development Fund (FDCT) of Macau
(Grant 0089/2018/A3).
This work has been supported by the team “Searching for Subglacial Water on Mars with
Orbiting Ground Penetrating Radars” of the International Space Science Institute (ISSI).
Conflicts of Interest: The authors declare no conflict of interest.
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2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
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... The proposed approach that combines the imaging technique with the standard MARSIS data products for the detection of subglacial water bodies results in better and more extensive coverage than what is possible to achieve with the available raw echo data and potential discovery of unreported subglacial hydrated features. This is very important as the search for liquid water on Mars and across the solar system, in general, is compelling due to its astrobiological significance [24]. The results of this article are also useful for upcoming radar sounding missions planned to the Jupiter Icy moons namely Radar for Icy Moon Exploration (RIME) [25] and Radar for Europa Assessment and Sounding (REASON) [26]. ...
... where tan δ is the loss tangent, c 0 is the light speed in vacuum, and f 0 is the radar central frequency. In the MHz range, the value tan δ(z) for water ice is in the order of 10 −3 and the dielectric constant r is equal to 3.1 [24]. It is worth noting that the value of tan δ is mainly dictated by the probed material electrical conductivity and it generally has a strong dependence over ice temperature [34]. ...
... This could explain the increase in subsurface smoothness with α (i.e., reduction ofρ(z, α) as α increases). The surface roughness of the region under investigation is very low as also reported in [5] and [24]. At MOLA horizontal scale, the average slope within the study region is equal to 0.24 • ± 0.60 • . ...
Research based on Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) data detected unusual radar bright basal reflections located at about 1.5 km depth in a Mars region denoted as Ultimi Scopuli. These reflections were interpreted as a signature of subglacial liquid water even though this interpretation is still being debated in the literature. In this article, we propose a novel approach to the detection and imaging of candidate subglacial liquid water from radar sounding data. The approach combines the radar echo power traces with a suitable digital elevation model to provide a bidimensional representation of the surface. Even if the imaging method reconstructs a representation of the surface, we prove that it can be used to identify subsurface bright reflections in icy regions. Imaging is feasible even if the basal interface is not directly included in the processed data for image generation. To support this experimental evidence, we show that a relationship exists between the value of the reflected echo power originating from the englacial layers and the basal-to-surface-echo-power ratio. The observed relationship holds on both Ultimi Scopuli radar sounding data acquired on Mars and Lake Vostok data acquired on Earth. Our results show that the 2-D imaging provides an alternative way for locating candidate subglacial liquid water bodies on Mars over large areas also where the basal interface is not directly measured. The proposed approach complements previous research for further evaluation of the actual presence of liquid water on Mars.
... At the location where the bright radar return is, P ss /P s at 4 MHz is consistently measured higher than 0 dB with a median at about 2.5 dB. It exhibits excursions above 5 dB but rarely exceeds 10 dB (Lauro et al., 2020;Orosei et al., 2018Orosei et al., , 2020. Under the assumption of a 205 K SPLD ice with 10% dust content, Orosei et al. (2018) have discussed that a P ss /P s > 0 dB corresponds to a basal permittivity 15, indicative of the presence of liquid water below polar deposits on Earth (Oswald & Gogineni, 2008;Peters, 2005). ...
... Finally, the relative basal reflection coefficient P ss /P s for a global ice sheet is proportional to the ratio between R 23 and the surface reflection coefficient of the ice R 12 (Orosei et al., 2020) = 23 12 2 12 2 ...
... The corresponding loss tangent at 4 MHz is rather poorly constrained but is estimated to be 0.001-0.005 (Orosei et al., 2020;Plaut et al., 2007). We consider tan δ 2 = 0.001 as it also matches the theoretical derivation of the loss tangent for a 10%-impure ice (Orosei et al., 2020). ...
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Plain Language Summary The presence of stable liquid water on Mars might provide an environment appropriate for the sustainability of extraterrestrial life. However, the current temperature and pressure at the Red Planet makes stable liquid water unlikely at the surface. A debate has emerged over the recent years from the detection of an anomalously bright radar return from the base of the South polar cap, down to 1.4 km deep beneath the ice. Early interpretations have proposed this signal to originate from water‐bearing material, or even subglacial lakes. This is being disputed by laboratory measurements suggesting clay, metallic inclusions or saline ice as alternative materials. Here, we aim to determine if Martian terrains today could produce strong basal echoes if they were covered by a planet‐wide ice sheet. The strength of this approach is to rely on already measured radar reflection properties of the surface across the whole planet. We find that some existing volcanic‐related terrains could produce a very strong basal signal analog to what is observed at the South polar cap. Our analysis strengthens the case against a unique hypothesis based solely on liquid water for the nature of the polar basal material.
... Doing this for data in which surface echo power has a standard deviation below 1 dB (as noted in Table S1 in the Supplementary Materials of Orosei et al., 2018aOrosei et al., , 2018b, leaves only data at 4 and 5 MHz. The difference between normalized power at those two frequencies has a median value of 1.36 dB, which can be used for a back of the envelope computation based on equations found in Porcello et al. (1974) to obtain an estimate of the loss tangent (see also Orosei et al., 2020 for the effect of such loss tangent value on SHARAD measurements). The resulting value is approximately 0.03, which is about fifteen times greater than the one used by Lalich et al. (2021b). ...
The principal objective of the radar sounder MARSIS experiment is to look for ice and water in the Martian subsurface. One particular focus of investigations, since 2005, has been the search for basal liquid water in the south polar layered deposits (SPLD). Anomalously strong basal echoes detected from four distinct areas at the base of the deposits at Ultimi Scopuli have been interpreted to indicate the presence of bodies of liquid water in this location, beneath a 1.5 km thick cover of ice and dust. Other explanations for the bright basal reflections have been proposed, however, including the possibility of constructive interference in layered media. Here, we test this mechanism through simulations of MARSIS radar signals propagating in models of CO2-H2O ice sequences. We then compare the results to real MARSIS data acquired over Ultimi Scopuli, finding that no CO2-H2O ice model sequence reproduces the set of real data. The results of our work have implications in relation to the global CO2 inventory of Mars.
... One of the requirements for life as we know it is the presence of liquid water. There is now abundant evidence of periods of surface water on ancient Mars, and more controversial suggestions of liquid water on present-day Mars, including in the deep subsurface [1,[6][7][8]. ...
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High pressure deep subsurface environments of Mars may harbor high concentrations of dissolved salts, such as perchlorates, yet we know little about how these salts influence the conditions for life, particularly in combination with high hydrostatic pressure. We investigated the effects of high magnesium perchlorate concentrations compared to sodium and magnesium chloride salts and high pressure on the conformational dynamics and stability of double-stranded B-DNA and, as a representative of a non-canonical DNA structure, a DNA-hairpin (HP), whose structure is known to be rather pressure-sensitive. To this end, fluorescence spectroscopies including single-molecule FRET methodology were applied. Our results show that the stability both of the B-DNA as well as the DNA-HP is largely preserved at high pressures and high salt concentrations, including the presence of chaotropic perchlorates. The perchlorate anion has a small destabilizing effect compared to chloride, however. These results show that high pressures at the kbar level and perchlorate anions can modify the stability of nucleic acids, but that they do not represent a barrier to the gross stability of such molecules in conditions associated with the deep subsurface of Mars.
... The quest for possible ancient forms of life in the Universe and, in particular, for Martian life is largely a search for areas of former water abundance, e.g., [1][2][3][4]. Morphological indications for water include outflow channels between the southern highlands and the north plains, and valley networks carved on ancient and cratered areas of the planet [5][6][7][8][9][10]. In addition, numerous clues indicate that the northern plains were once an oceanic floor [6,11,12]. ...
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Mars has held large amounts of running and standing water throughout its history, as evidenced by numerous morphologies attributed to rivers, outflow channels, lakes, and possibly an ocean. This work examines the crater Antoniadi located in the Syrtis Major quadrangle. Some parts of the central area of the crater exhibit giant polygonal mud cracks, typical of endured lake bottom, on top of which a dark, tens of kilometers-long network of dendritic (i.e., arborescent) morphologies emerges, at first resembling the remnant of river networks. The network, which is composed of tabular sub-units, is in relief overlying hardened mud, a puzzling feature that, in principle, could be explained as landscape inversion resulting from stronger erosion of the lake bottom compared to the endured crust of the riverine sediments. However, the polygonal mud cracks have pristine boundaries, which indicate limited erosion. Furthermore, the orientation of part of the network is the opposite of what the flow of water would entail. Further analyses indicate the similarity of the dendrites with controlled diffusion processes rather than with the river network, and the presence of morphologies incompatible with river, alluvial, or underground sapping processes, such as overlapping of branches belonging to different dendrites or growth along fault lines. An alternative explanation worth exploring due to its potential astrobiological importance is that the network is the product of ancient reef-building microbialites on the shallow Antoniadi lake, which enjoyed the fortunate presence of a heat source supplied by the Syrtis Major volcano. The comparison with the terrestrial examples and the dating of the bottom of the crater (formed at 3.8 Ga and subjected to a resurfacing event at 3.6 Ga attributed to the lacustrine drape) contribute to reinforcing (but cannot definitely prove) the scenario of microbialitic origin for dendrites. Thus, the present analysis based on the images available from the orbiters cannot be considered proof of the presence of microbialites in ancient Mars. It is concluded that the Antoniadi crater could be an interesting target for the research of past Martian life in future landing missions.
... 182 Rhawn Joseph Copyright © 2021 beneath the surface [67,69,75]. These include dust covered glaciers and multiple subglacial lakes that may contain liquid water as well as water-ice [70,76,77,70]; some of which are heated by thermal anomalies [18,19]. ...
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In the ancient and recent past, various niches on Mars were habitable and possibly inhabited by organisms that have evolved and adapted to extreme surface and subsurface environments. Habitability is promoted by the high levels of iron that promotes melanization of various organisms that protects against radiation. Glacial and water-ice below the surface provides moisture to organisms at temperatures below freezing due to salts in these ices and heat generated from anomalous thermal sources. Impact craters formed over 3.7 bya appear to be highly magnetized thus providing additional protection against radiation; and if initially hosting a large body of water may have triggered the formation of hydrothermal vents. Tube worms, sulfur-reducing and other chemoautotrophs have thrived and likely still inhabit subsurface aquifers within Endurance Crater which was formed over 3.7 bya, has hosted large bodies of water, and also has the mineralogy of hydrothermal vents and surface holes surrounded by tubular specimens. Formations resembling fossil tube worms have also been observed in the ancient lake beds of Gale Crater which was formed over 3.7 bya. A comparative quantitative analysis of the Gale and Endurance Crater tubular specimens provides additional confirmation for the tube-worm hydrothermal vent hypothesis.
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Mars Orbiter Subsurface Investigation Radar (MOSIR) is carried by China's first Mars probe, Tianwen‐1 orbiter, investigating the Martian subsurface stratification. Surface clutter from topography off‐nadir will overlap with the subsurface echoes, which affects the recognition of Martian subsurface reflections. Surface clutter simulation can effectively distinguish the nadir and off‐nadir radar echoes. In this paper, we choose the facet method to model the Mars surface topography and combine the roughness parameter with the radar backscatter function. We also provide an analytic expression of the echo phase considering the distance variation in the whole facet. The Chinese first Mars landing site is on Utopia Planitia, which is also one of the key investigating regions of MOSIR. Therefore, we also carried on surface clutter simulation of this region and generated simulation radargram with the Chirp Scaling algorithm. Furthermore, we use the contrast method to compensate for ionospheric error introduced by the NeMars Mars ionosphere model. Our surface clutter simulation program will significantly support MOSIR subsurface investigation, and provide a chance to verify the related data processing.
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Vesicular rocks and thick clumps of green-colored matter photographed in Utopia Planitia and Chryse Planitia by NASA's Viking landers were subject to morphological and computerized quantitative pattern analysis. These vesicular rocks are not homogenous and include those similar to vesicular basalts, marine trace fossils, and "tafoni" which on Earth are fashioned via the interactional influences of moisture, powerful winds, the leaching of salts and lichen-chemical weathering. Upon magnification the green-colored vesicular substances closely resemble "vegetative matter" similar to green algae, lichens, mosses and vesicular mats. The green colors (based on false colors derived from spectra) may be indicative of chlorophyll and the capacity to produce oxygen via photosynthesis. These observations, when coupled with the continual replenishment of atmospheric oxygen and evidence of surface frost, subsurface water-ice, and past cycles of flooding and ponding of water, are supported by the positive results from the Viking Labeled Release and Gas Exchange experiments and should be viewed as confirming that beginning in 1976 the USA and NASA's Viking Landers 1 and 2 detected, photographed and discovered life and evidence of past life on Mars.
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Hundreds of tubular and spiral specimens resembling terrestrial tube worms and worm tubes were photographed in the soil and atop and protruding from “rocks” on Sols 177, 199 and 299 in the vicinity of Endurance Crater, Meridiani Planum. Dozens of these putative “worms” and tubes are up to 3 mm in size. These tubular specimens display twisting, bending, and curving typical of biology and are different from abiogenic structures. Morphological comparisons with living and fossilized tube worms and worm tubes also supports the hypothesis that the Martian tubular structures may be biological as they are similar and often nearly identical to their terrestrial counterparts. The literature concerning abiotic and biotic formation of mineralized tubular formations is reviewed and the Martian tubular structures meet the criteria for biology. In addition, larger “anomalous” oval-specimens ranging from 3 mm to 5 mm in diameter were photographed and observed to have web-like appendages reminiscent of crustacean pleopods. That marine organisms may have evolved and flourished in the vicinity of Endurance Crater, Meridiani Planum, was originally predicted by NASA’s rover Opportunity crew in 2004, 2005, and 2006. This area is believed to have hosted a briny body of water that was heated by hydrothermal vents; and these are favored habitats of tube worms. Further, all these specimens were photographed adjacent to vents in the surface and the mineralogy of Endurance Crater is similar to that produced by tube worms and their symbiotes. However, if any of these specimens are alive, fossilized, mineralized or dormant is unknown. Abiotic explanations cannot be ruled out and it cannot be stated with absolute certainty they are biological.
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This paper describes the scientific objectives and payloads of Tianwen-1, China’s first exploration mission to Mars. An orbiter, carrying a lander and a rover, lifted-off in July 2020 for a journey to Mars where it should arrive in February 2021. A suite of 13 scientific payloads, for in-situ and remote sensing, autonomously commanded by integrated payload controllers and mounted on the orbiter and the rover will study the magnetosphere and ionosphere of Mars and the relation with the solar wind, the atmosphere, surface and subsurface of the planet, looking at the topography, composition and structure and in particular for subsurface ice. The mission will also investigate Mars climate history. It is expected that Tianwen-1 will contribute significantly to advance our scientific knowledge of Mars.
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The detection of liquid water by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) at the base of the south polar layered deposits in Ultimi Scopuli has reinvigorated the debate about the origin and stability of liquid water under present-day Martian conditions. To establish the extent of subglacial water in this region, we acquired new data, achieving extended radar coverage over the study area. Here, we present and discuss the results obtained by a new method of analysis of the complete MARSIS dataset, based on signal processing procedures usually applied to terrestrial polar ice sheets. Our results strengthen the claim of the detection of a liquid water body at Ultimi Scopuli and indicate the presence of other wet areas nearby. We suggest that the waters are hypersaline perchlorate brines, known to form at Martian polar regions and thought to survive for an extended period of time on a geological scale at below-eutectic temperatures.
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Plain Language Summary The north polar cap of Mars is a tremendous reservoir of ices and dust of unknown concentration and composition. It is transparent to radar giving us a unique insight into its structure and composition. Here we use a novel technique that combines radar and elevation data along with a flexure model, to invert for the polar cap composition and the strength of the underlying lithosphere. Similar to previous studies, we find that the lithosphere below the north pole is extremely rigid and does not deform much under the load of the polar cap. This implies that the north polar region is currently colder than the rest of the planet, which has profound implications for our understanding of the structure and evolution of the Martian interior. Our inferred compositions suggest that for reasonable dust contents, about 10 vol% CO 2 ice is mixed within the polar deposits. This is the first time a large quantity of CO 2 ice is constrained to exist in the north polar cap. Like on Earth, where the composition of buried ices gives hints on the climatic evolution, having CO 2 ice at the north pole of Mars will help improve scenarios for the climate evolution of the planet.
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Liquid water was present on the surface of Mars in the distant past; part of that water is now in the ground in the form of permafrost and heat from the molten interior of the planet could cause it to melt at depth. MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) has surveyed the Martian subsurface for more than fifteen years in search for evidence of such water buried at depth. Radar detection of liquid water can be stated as an inverse electromagnetic scattering problem, starting from the echo intensity collected by the antenna. In principle, the electromagnetic problem can be modelled as a normal plane wave that propagates through a three-layered medium made of air, ice and basal material, with the final goal of determining the dielectric permittivity of the basal material. In practice, however, two fundamental aspects make the inversion procedure of this apparent simple model rather challenging: i) the impossibility to use the absolute value of the echo intensity in the inversion procedure; ii) the impossibility to use a deterministic approach to retrieve the basal permittivity. In this paper, these issues are faced by assuming a priori information on the ice electromagnetic properties and adopting an inversion probabilistic approach. All the aspects that can affect the estimation of the basal permittivity below the Martian South polar cap are discussed and how detection of the presence of basal liquid water was done is described.
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The discovery of an ~20‐km‐wide area of bright subsurface radar reflections, interpreted as liquid water, beneath the Martian south polar layered deposits (SPLD) in data from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument, and the discovery of two geologically recent potential eskers (landforms produced by subglacial melt) associated with viscous flow features in Martian midlatitudes, has suggested recent basal melting of Martian ice deposits may be feasible, possibly due to locally elevated geothermal heating. Locations of terrestrial subglacial lakes and major drainage axes have been successfully predicted from subglacial hydraulic potential surfaces calculated from surface topography and ice thickness. Here, we use surface topography from the Mars Orbiter Laser Altimeter and SPLD bed elevations derived from MARSIS data to calculate the subglacial hydraulic potential surface beneath the SPLD and determine whether the observed high reflectance area coincides with predicted subglacial lake locations. Given the sensitivity of terrestrial predictions of lake locations to basal topography, we derive over 1,000 perturbed topographies (using noise statistics from the MARSIS data) to infer the most likely locations of possible subglacial water bodies and drainage axes. Our results show that the high reflectance area does not coincide with any substantial predicted lake locations; three nearby lake locations are robustly predicted however. We interpret this result as suggesting that the high reflectance area (assuming the interpretation as liquid is correct) is most likely a hydraulically isolated patch of liquid confined by the surrounding cold‐based ice, rather than a topographically‐constrained subglacial lake.
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Recent analysis of radar data from the Mars Express spacecraft has interpreted bright subsurface radar reflections as indicators of local liquid water at the base of the south polar layered deposits (SPLD). However, the physical and geological conditions required to produce melting at this location were not quantified. Here we use thermophysical models to constrain parameters necessary to generate liquid water beneath the SPLD. We show that no concentration of salt is sufficient to melt ice at the base of the SPLD in the present day under typical Martian conditions. Instead, a local enhancement in the geothermal heat flux of >72 mW/m ² is required, even under the most favorable compositional considerations. This heat flow is most simply achieved via the presence of a subsurface magma chamber emplaced 100 s of kyr ago. Thus, if the liquid water interpretation of the observations is correct, magmatism on Mars may have been active extremely recently.
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The scale of groundwater upwelling on Mars, as well as its relation to sedimentary systems, remains an ongoing debate. Several deep craters (basins) in the northern equatorial regions show compelling signs that large amounts of water once existed on Mars at a planet‐wide scale. The presence of water‐formed features, including fluvial Gilbert and sapping deltas fed by sapping valleys, constitute strong evidence of groundwater upwelling resulting in long term standing bodies of water inside the basins. Terrestrial field evidence shows that sapping valleys can occur in basalt bedrock and not only in unconsolidated sediments. A hypothesis which considers the elevation differences between the observed morphologies and the assumed basal groundwater level is presented and described as the "dike confined water" model, already present on Earth and introduced for the first time in the Martian geological literature. Only the deepest basins considered in this study, those with bases deeper than ‐4000 m in elevation below the Mars datum, intercepted the water‐saturated zone and exhibit evidence of groundwater fluctuations. The discovery of these groundwater discharge sites on a planet‐wide scale strongly suggests a link between the putative Martian ocean and various configurations of sedimentary deposits that were formed as a result of groundwater fluctuations during the Hesperian period. This newly recognised evidence of water‐formed features significantly increases the chance that biosignatures could be buried in the sediment. These deep basins (groundwater‐fed lakes) will be of interest to future exploration missions as they might provide evidence of geological conditions suitable for life.
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The surface of Mars has been well mapped and characterized, yet the subsurface — the most likely place to find signs of extant or extinct life and a repository of useful resources for human exploration — remains unexplored. In the near future this is set to change.
Recurring Slope Lineae (RSL) are narrow, dark features that typically source from rocky outcrops, incrementally lengthen down Martian steep slopes in warm seasons, fade in cold seasons and recur annually. In this study we report the first observations of RSL at Hale crater, Mars, during late southern summer by the Color and Surface Science Imaging System (CaSSIS) on board ESA's ExoMars Trace Gas Orbiter (TGO). For the first time, we analyze images of RSL acquired during morning solar local times and compare them with High Resolution Imaging Science Experiment (HiRISE) observations taken in the afternoon. We find that RSL activity is correlated with the presence of steep slopes. Our thermal analysis establishes that local temperatures are high enough to allow either the melting of brines or deliquescence of salts during the observation period, but the slope and aspect distributions of RSL activity predicted by these processes are not consistent with our observations. We do not find any significant relative albedo difference between morning and afternoon RSL. Differences above 11% would have been detected by our methodology, if present. This instead suggests that RSL at Hale crater are not caused by seeping water that reaches the surface, but are best explained as dry flows of granular material.
The observation of low albedo, elongated and transient features lengthening downhill on Mars has attracted wide interest because of the possible role of aqueous fluids in their formation. These recurring gravity-driven processes, called Recurring Slope Lineae (RSL), remain mysterious in that, although the influence of local climate conditions has been established, their nature (dry versus wet) and the mechanisms that govern their growth and fading are debated. We present three different physical models for the growth of RSL, the first two are wet-based models with different aqueous fluid evaporation models and the last is based on dry granular theory. We discuss the prediction of each model with regards to the growth and fading of RSL and their morphology. We finally discuss the strengths and weaknesses of these models in light of what we currently observe on Mars. We find that both wet and dry RSL scenario face challenges, mostly regarding aspects of mass balance. However, water-based scenarios provide a consistent framework to reconcile the different sets of morphological observations made on RSL.