Radar Observations of Liquid Water in the South Polar Region of Mars: Indications from Astrobiology Perspectives

Abstract

In recent decades, extensive research has led to the understanding that Mars once hosted substantial liquid-water reserves. While the current Martian landscape boasts significant water-ice deposits at its North and South poles, the elusive presence of liquid-water bodies has remained undetected. A breakthrough occurred with the identification of radar-echo reflections at the base of the Martian South Pole, using MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding) in 2018. These radar echoes strongly suggest the presence of a highly concentrated liquid-water body. However, a counter-narrative has emerged, contending that the subterranean conditions beneath the ice cap, encompassing factors like temperature and pressure, may be inhospitable to liquid water. Consequently, alternative hypotheses posit that the observed bright echoes could be attributed to conductive minerals or water-absorbing clay-like materials. The ongoing discourse regarding the presence of liquid water beneath the southern polar ice cap is a hot topic in the realm of Martian exploration. The primary focus of this paper is to provide a comprehensive overview of Martian radar detection, the recent controversies regarding liquid water’s existence in the Martian South Pole, and the implications regarding the potential existence of Martian life forms in the water on Mars. The revelation of liquid water on Mars fundamentally suggests an environment conducive to the viability of Martian life, consequently furnishing invaluable insights for future exploratory endeavors in the pursuit of Martian biospheres. In addition, this paper anticipates the forthcoming research dedicated to Martian liquid water and potential life forms, while also underscoring the profound significance of identifying liquid water on Mars in propelling the field of astrobiology forward.

1. Introduction

The question of whether extraterrestrial life exists in the universe and whether other planets are suitable for human colonization has long been a primary focus of our space exploration [1,2]. The planet Mars, which is in the habitable zone of the solar system and is one of the closest planets to Earth, stands in contrast to Venus; therefore, it has attracted much attention and exploration as the most likely planet for future human colonization [3,4,5]. Its nature has sparked interest in the possibility of life’s existence on Mars [6,7,8]. Human exploration of Mars through spacecraft missions started in the 1960s, with the hope of discovering extraterrestrial life on the planet [6,9,10,11]. Over the subsequent decades of research, it has been observed that the current surface conditions on Mars do not support the long-term existence of liquid water [12]. However, the Martian morphology shows that the presence of extensive networks of channels, outflow channels, dried-up deltas, and alluvial fan landforms suggests a history of abundant liquid water in Mars’ past [12,13,14]. In the early 21st century, both Europe and the United States launched missions, such as the Mars Express and Mars Reconnaissance Orbiter, to explore the possibilities of life on Mars [15,16]. These missions carried radar instruments, such as MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding) and SHARAD (SHAllow RADar) that have extensively probed the Martian subsurface in the past decade, as shown in Figure 1a,b [17,18]. Those radars led to the discovery of significant water-ice deposits at the North and South Poles of Mars [19,20,21,22]. In 2020, China launched the Tianwen-1 mission to Mars, which also carried a radar payload on its orbiter, as shown in Figure 1c [23,24]. The name of the Chinese orbiter-mounted radar is MOSIR (Mars Orbiter Subsurface Investigation Radar). MOSIR is primarily aimed at probing the subsurface structure of Mars and finding water resources. Its detection frequency expands the frequency range of the other two radars [23,25].

In recent years, MARSIS has identified bright reflections at the bottom of the Subsurface Polar Layered Deposits (SPLDs) on the southern polar of Mars, and the detection principle is as shown in Figure 2. Orosei et al. [26] reasoned that these bright reflections are indicative of liquid water through the inversion of dielectric constants. They proposed that a high concentration of brines has lowered the freezing point of the liquid water, enabling its presence beneath the south polar ice cap [26]. Subsequently, Lauro et al. [27] discovered multiple bright reflections, using the latest MARSIS data, and suggested the existence of multiple liquid-water bodies at the bottom of the SPLDs. The presence of liquid water implies the possibility of life on Mars and provides potential directions for human colonization. As a result, discussions regarding the existence of liquid water at the South Pole of Mars have ensued. Some researchers argue that even with high-concentration brines, the current temperatures and heat flux on Mars would prevent the existence of liquid water [28]. They suggest that the cause of the bright reflections might be differences in the dielectric properties of minerals at the bottom of the SPLDs [29] or clay [30]. Additionally, layered interference within the SPLDs has been proposed as one of the causes of the bright reflections [31]. In response to these doubts, researchers have conducted laboratory experiments to measure the dielectric properties of relevant materials under corresponding conditions. They argue that materials such as clay cannot cause such bright reflections and they continue to support the view that the cause of the bright reflections is brines with high chloride content [32,33]. Currently, the cause of these bright reflections remains inconclusive. The study of this area continues to be one of the hot topics in Martian exploration.

In this paper, we primarily provide an overview of the radar-based discovery of liquid water at the South Pole of Mars and its controversies. Meanwhile, we catch a glimpse of the significance of discovering water on Mars for the possibility of life on Mars. The main framework of the paper is as follows: First, we briefly introduce the operational principles of the Martian radar instruments, e.g., MARSIS, SHARAD, and MOSIR (see Section 2.2). Second, we discuss the permittivity inversion methods employed to inverse the liquid water on Mars, using MARSIS radar (see Section 3). Third, the recent debates concerning the existence of liquid water beneath the SPLDs ice layers are outlined in Section 4. Finally, we offer insights into the future exploration of liquid-water bodies on Mars and the potential search for Martian life (see Section 5).

2. Detection Principles of Mars Orbital Radars

2.1. Introduction of Mars Orbital Radars

MARSIS is carried by the Mars Express spacecraft launched by the European Space Agency (ESA) (Figure 1a). It was initiated in August 2005 and is designed to explore the subsurface structures and ionosphere of Mars [34,35]. MARSIS consists of several components: (1) a 40 m-long dipole antenna oriented parallel to the Mars Express movement direction; (2) a 7 m-long monopole antenna perpendicular to the dipole antenna; (3) a dual-channel data processor; and (4) control units for power supply and electronic digital signals [36]. The primary antenna, the dipole antenna, is used to observe subsurface echoes from Mars. The secondary antenna, the monopole antenna, is employed to receive non-nadir reflections, reducing interference with the echoes received by the primary antenna. When probing subsurface structures, MARSIS operates at four different frequencies: 1.8 MHz, 3 MHz, 4 MHz, and 5 MHz, each with a bandwidth of 1 MHz [37] (

Table 1. Basic parameters comparison of MARSIS, SHARAD, and MOSIR radars [23,37,38].

Parameter	MARSIS	SHARAD	MOSIR
Operational frequency (MHz)	1.8, 3, 4, and 5	15–25	LF: 10–15/15–20 HF: 15–30
Depth resolution in free space (m)	150	15	30/7.5
Designed penetrating depth (km)	0.5–5	0.1–1	0.1–1

). The low-frequency pulses emitted by the radar enable the detection of structures several kilometers beneath the Martian surface [37].

SHARAD is carried by NASA’s Mars Reconnaissance Orbiter, launched in August 2005 [39]. It primarily operates during the Martian night-time, to observe the subsurface structure and water ice on Mars [15]. SHARAD features a pair of 10 m-long dipole antennas and it operates at frequencies between 15–25 Hz [38,40]. Its high-frequency pulses provide a higher resolution than MARSIS, but its penetration depth is less than that of MARSIS. Table 1 provides a comparison of parameters between the two radars. Unlike MARSIS, SHARAD has a weaker surface-penetrating capability, due to its higher-frequency radar, which limits its ability to probe the subsurface hundreds-to-thousands of kilometers below the Martian surface. However, its high-resolution characteristics allow the radar to achieve more precise detection of the subsurface at depths ranging from tens to hundreds of meters on Mars. Consequently, SHARAD focuses on studying surface-related terrain structures and material composition [15]. MARSIS and SHARAD complement each other, in terms of exploration depth and radar resolution [41]. Over the past two decades, both radars have been utilized to survey Mars’ polar regions [42,43,44] and other areas [45,46,47,48], mapping terrain [43,49,50] and investigating the distribution of water ice on Mars [16,47,51,52]. In recent years, these radars have also hinted at the possible presence of liquid water on Mars [26], and they are further discussed in detail in Section 4.

In addition to the MARSIS and SHARAD radar systems, Mars also features the MOSIR radar onboard China’s Tianwen-1 mission, launched in 2020 (Figure 1c). MOSIR operates at three different frequencies: 10–15 MHz, 15–20 MHz, and 30–50 MHz, enabling penetration depths of several hundred meters into the Martian surface [23]. MOSIR emits either low- or high-frequency pulses, with vertical resolutions of 30 m and 7 m, and horizontal resolutions of several hundred meters (along-track) by several kilometers (cross-track) (see Table 1). Unlike the previous two radar systems mentioned, MOSIR radar employs two pairs of orthogonal dipole antennas (9 m in the cross-track direction and 10 m in the along-track direction). The antennas in the along-track transmit radar pulse and the whole cross-dipole antennas are utilized for echo reception. This configuration enables the reception of cross-track echoes, providing additional information for material-property analysis [23]. Apart from investigating Martian water ice, MOSIR is also designed to provide insights into aspects such as searching for potential life, surface terrain, and the internal structures of Mars [23,53,54].

2.2. Detection Principles of Mars Orbital Radars

The radars exploring Mars operate in different modes, including MARSIS’ SS1, SS2, SS3, SS4, SS5, and AIS (Atmospheric and Ionospheric Sounding mode). MARSIS uses the SS1–SS5 modes (subsurface sounding mode) while detecting the subsurface of Mars (such as liquid water under the Martian ice caps) and selects different modes according to the diversity of the topography [17]. Similarly, MOSIR has three operational modes: HFSS, LFSS plus ISS, and ADM [23]. The radars select the appropriate operational modes based on the specific exploration requirements. The spacecraft carry multiple operational modes, and the radars generate substantial raw data, due to varying penetration depths and resolutions. The downlink of the data can potentially influence the radar operations. Therefore, the radars need to address issues related to onboard preprocessing, mode coordination, utilization of polarization information, compression ratios, etc. [55,56,57,58,59]. Furthermore, each operational mode requires tailored data types and adjustable parameters, enhancing the radars’ adaptability and flexibility in exploration [60]. When attempting to detect faint echoes from beneath the surface of Mars, radar systems that operate at lower frequency bands with wider bandwidths can experience waveform distortions. These distortions are caused by the transmission module, antenna, and reception module, and can lead to significant differences between received signals and ideal waveforms [40,61].

To address this, measurements and simulations of the radar’s transmission and reception modules, as well as antenna patterns, are required. These are then compared to the ideal signal spectrum, to compensate for waveform distortions [61,62]. During daytime surface exploration on Mars, the ionosphere can cause signal defocusing [55,63]. To mitigate this effect, the radar systems measure the total electron content of the ionosphere, to obtain compensation coefficients. Doppler analysis is also employed to achieve imaging results [64]. Additionally, the motion of the spacecraft itself can introduce signal variations. Doppler-beam sharpening techniques are utilized to compensate for motion in directions that are different from the vertical direction of the spacecraft [65,66]. Once the radar data are obtained, corresponding analysis is performed, based on the research objectives. This analysis aims to reverse the relevant electromagnetic parameters, such as dielectric permittivity and loss tangent. In Section 3, the method of reversing a subsurface material’s dielectric properties using orbiter-mounted radar data will be discussed in detail.

3. The Permittivity Inversion

3.1. The Permittivity

While using spacecraft radar to detect the subsurface of Mars, the permittivity is a physical parameter we commonly apply, to determine the composition of a substance. Permittivity, as a complex number, comprises both a real part and an imaginary part [67]. The real part, known as the relative permittivity, typically influences the propagation speed of electromagnetic waves in a medium, whereas the imaginary part primarily results in energy loss during the propagation of electromagnetic waves in the medium. In this paper, when discussing the permittivity, we generally refer to its real part. Through the inversion of radar data on the permittivity of subsurface materials beneath the Martian surface, we can ascertain the nature of the subsurface materials. This method allows us to infer the properties of these subsurface materials, consequently elucidating the nature of the substances detected by radar. Through this approach, we gain a deeper understanding of the structure and composition beneath the surface of Mars, providing a foundation for further research. As we cannot directly obtain samples of subsurface materials on Mars currently, inversely deducing the permittivity can help us reasonably infer the property of subsurface materials on Mars.

3.2. The Intensity Values Collected by the Radar

Due to incomplete radar data collection at other frequencies, using MARSIS to probe the SPLDs is hindered, especially by the severely affected radar data at 3 MHz due to the ionosphere. Lauro et al. [68] adopted the more complete radar data at 4 MHz. To mitigate power variance resulting from variable roughness, Lauro et al. [68] employed the method developed by Oswald and Gogineni [69]. They derived an equation to determine the along-track waveform-averaging window from the diameter of the pulse-limited footprint, which can be expressed as [68]

$$W=2\sqrt{c\frac{P}{2}\left(H+\frac{z}{n}\right)}\cong 2\sqrt{c\frac{P}{2}H},$$

(1)

where c is the speed of light in a vacuum, p is the transmitted pulse length (in this case, p = 1 $\mathsf{\mu }$s), and H represents the spacecraft altitude. As MARSIS radar operates on Mars, the value of H varies between 250 km and 900 km; z indicates the depth of the reflector, and n is the refractive index of water ice [68]. Utilizing the along-track waveform-averaging window, the aggregated power and the average echo power are presented in the following equation. Lauro et al. [68] calculated the intensity variability ${\sigma }_1\left(x_i\right)$, as indicated by

$${\sigma }_1\left(x_i\right)=\frac{10}{ln\left(10\right)}\frac{\sqrt{\frac{1}{N}{\sum }_{x=x_i-W/2}^{x_i+W/2}{\left[P_{\mathrm{ag}}\left(x\right)-〈P_{\mathrm{ag}}\left(x_i\right)〉\right]}^2}}{\langle P_{\mathrm{ag}}\left(x_i\right)\rangle },$$

(2)

where ${\mathrm{x}}_i$ is along each orbit at the position, which here could be the surface topography or the SPLDs thickness. $P_{\mathrm{ag}}\left(x\right)$ is the aggregated power, and the average echo power ($<\mathrm{Pag}\left({\mathrm{x}}_i\right)>$) can be exhibited as

$$\langle P_{\mathrm{ag}}\left(x_i\right)\rangle =\frac{1}{N}\sum _{x=x_i=W/2}^{x_i+W/2}{P}_{\mathrm{ag}}\left(x\right).$$

(3)

Based on the intensity variability, Lauro et al. [27] obtained the intensity values gathered along the radar track. These values were then utilized to calculate the basal permittivity, using the probabilistic-inversion approach introduced by Lauro et al. [68].

3.3. Permittivity Estimated by Probabilistic-Inversion Approach

3.3.1. Ice Layer Dielectric Properties

The probabilistic-inversion approach to determining the dielectric permittivity beneath the SPLDs on Mars was employed by Lauro et al. [68]. It is necessary to understand the characteristics of the SPLDs ice layers before proceeding. Previous studies have indicated that the SPLDs are composed of a mixture of water ice and silicate powder [70], and it is generally assumed that water ice and silicate powder are distributed uniformly on the SPLDs. To explore the various compositional scenarios of the SPLDs, prior knowledge of the dielectric properties of both pure and dirty ice is required. Although the node characteristics of pure water ice can be obtained through laboratory measurements, the dielectric properties measurements of pure water ice are inconsistent, due to factors such as the cooling process, aging, bubbles, and cracks within the ice [71,72]. Therefore, Lauro et al. [68] used data from Kawada [73] to reverse the dielectric properties of the bottom of the SPLDs. For dirty ice, being a two-phase mixture, it is assumed that the content of silicate dust is 5%, 10%, 15%, or 20%. The Maxwell–Garnett mixing rule is used to determine the effective dielectric properties of the mixture [74]. Figure 3 illustrates the variation curve of the attenuation $\alpha$ of the 4 MHz radar wave under different volume fractions (water ice–silicate) and at different temperatures. The real-part values of the dielectric constant for ice with different volume fractions are approximately 3. This is the dielectric permittivity of typical water ice on Mars.

3.3.2. Deriving the Permittivity of the SPLDs

According to the principle that radar electromagnetic waves propagate in a subsurface layered medium, we find the ratio of the base energy to the surface energy (${\mathrm{P}}_b$/${\mathrm{P}}_s$) in the radar echo signal. The following equation can be obtained for the inversion of the dielectric permittivity, which is shown as follows [68]:

$$\frac{P_b}{P_s}={\left[\frac{(1-{{\rho }_s}^2){\rho }_b}{{\rho }_s}\right]}^2{e}^{-4{\alpha }_{ie}{\tau }_{ie}{v}_{ie}},$$

(4)

where ${\rho }_s$, ${\rho }_b$ are the Fresnel reflection coefficients at the interfaces air/surface and ice-layer/basal material, and

$${\rho }_s=\frac{1-\sqrt{{\epsilon }_{ie}}}{1+\sqrt{{\epsilon }_{ie}}},$$

(5)

$${\rho }_b=\frac{\sqrt{{\epsilon }_{ie}}-\sqrt{{\epsilon }_b}}{\sqrt{{\epsilon }_{ie}+\sqrt{{\epsilon }_b}}}$$

(6)

where ${\epsilon }_b$ is the permittivity of the basal material underlying the SPLDs. Lauro et al. [68] employed the probability density function (PDF) of the measured data and the model parameters as inputs to solve the inversion of the equation. However, this approach requires making the following assumptions about the ice layer [68]:

• The ice layer consists of a uniform mixture of water ice and silicate, with the volume fraction of silicate (${\mathrm{f}}_v$) within the range of [0.05, 0.2].
• The temperature $T\left(z\right)$ of the ice layer linearly increases with depth, with the surface temperature Ts at 160 K and the basal temperature Tb within the range of 170 K to 270 K.
• Different values of ${\mathrm{f}}_v$ and $T\left(z\right)$ will influence the value of ${\epsilon }_{ice}$.

A detailed derivation of the procedure for calculating the permittivity of the basal material underlying the SPLDs can be found in Lauro et al. [68].

4. Current Existence of Liquid Water on Mars

4.1. Discovery of Liquid Water on Mars

Since the activation of MARSIS and SHARAD, numerous pieces of evidence related to the presence of water ice on Mars have been detected over the past few decades [51,75,76]. Water ice on Mars is predominantly found in layered deposits beneath the polar ice caps [19]. In recent years, the MARSIS radar identified a region of bright radar reflections at the base of Ultimi Scopuli (${193}^{\circ }$ E, ${81}^{\circ }$ S) in the Martian South Pole region [26]. In Figure 4a, two distinct interfaces are visible: the continuous upper interface corresponds to the surface echo, while the lower interface at approximately 160 $\mathsf{\mu }$s corresponds to the basal echo of the SPLDs. Additionally, within this interface, partial bright radar reflections can be observed [26]. By constructing a model that assumes the proportion of water ice and dust in the SPLDs, as well as the vertical temperature variation within the SPLDs, the calculated results show a region with a basal temperature of approximately 205 K and a permittivity greater than 15 (Figure 4e). This leads to the inference that the region probably corresponds to highly chlorinated brine [26]. The presence of highly chlorinated brine, which significantly depresses the freezing point of water, suggests the potential existence of liquid water on Mars. Applying signal-processing techniques commonly used for Earth’s polar ice sheets, Lauro et al. [27] employed the mesh-refinement technique to obtain a permittivity map beneath the SPLDs. Lauro et al. [27] identified multiple bright regions, and regions with permittivity greater than 15 were interpreted as potential areas of liquid water presence. This led to the hypothesis that multiple liquid-water bodies could exist beneath the SPLDs [27].

4.2. Dispute on the Existence of Liquid Water on the SPLDs

Radar observations have indicated the presence of multiple buried bodies of liquid water under the Martian south polar ice cap [26,27]. However, there are also numerous opposing viewpoints regarding the presence of liquid water in this region. Some argue that the bright radar reflections could be caused by factors other than a liquid-water body. Debates among researchers have ensued, regarding the composition of the underlying material in this area. The question of whether liquid water exists in this region undoubtedly remains a focal point in current Mars research. We summarize the latest findings on the debate on liquid water under the Martian south polar ice cap. Figure 5 illustrates the relevant debate.

4.2.1. Local Environmental Factors May Not Support the Existence of Liquid Water

Some viewpoints suggest that under the typical current conditions on Mars, the pressure and temperature at the SPLDs base do not support the formation of liquid water [28]. Sori and Bramson [77] constructed a heat-flux model for the SPLDs, based on the estimated basal permittivity by Orosei et al. [26], shown in Figure 6. They calculated that for the existence of liquid water, at least 72 mW/m${}^2$ of heat flux would be required [77]. However, the current global average heat flux on Mars is around 19 mW/m${}^2$, with a range of 14–25 mW/m${}^2$ [78,79,80]. Additionally, there has been no recent geological activity near the South Pole to provide the necessary heat, making it challenging for liquid water to exist at the base [77].

Figure 5. The latest research debate on the existence of liquid water on the SPLDs. The black boxes are negative views on the presence of water at the South Pole of Mars, and the blue boxes are the response to the negative views [26,28,30,31,32,33,81,82].

Figure 5. The latest research debate on the existence of liquid water on the SPLDs. The black boxes are negative views on the presence of water at the South Pole of Mars, and the blue boxes are the response to the negative views [26,28,30,31,32,33,81,82].

To address this issue, Lauro et al. [81] optimized the estimates for the basal temperature and permittivity by analyzing signal attenuation using MARSIS frequencies of 3 MHz, 4 MHz, and 5 MHz, respectively. They estimated the surface temperature and heat flux of the SPLDs and they utilized the Maxwell–Garnett mixing formula. As a result, they re-calculated the basal permittivity to be approximately 40, with a loss-tangent value of 0.0017 and a maximum temperature potential of 240 K, as shown in Figure 7. Based on the deduced permittivity values and temperature, they suggested that the material in this area of liquid water is reasonable and could be rich in brine [81].

4.2.2. CO${}_2$-Ice and Water-Ice Interference with Echoes

Lalich et al. [31] conducted simulations of three different subsurface structures of SPLDs (Figure 8a). The first scenario involved a water-ice layer without a layer of CO${}_2$ ice, serving as a reference. The second scenario included a layer of CO${}_2$ ice with a thickness ranging from 1 to 100 m between water ice and bedrock. The third scenario featured two layers of CO${}_2$ ice between water-ice layers. They simulated the waveforms generated by each scenario and compared these simulations with observations detected by MARSIS. They found that the simulation produced in the third scenario exhibited strong similarity to the observation detected by MARSIS (Figure 8b). Therefore, they concluded that the bright radar echoes detected by MARSIS are likely a result of interference between the CO${}_2$ ice and water ice, shown in Figure 8 [31]. However, Orosei et al. [82] evaluated the echoes generated by the simulated subsurface structures, using the theoretical framework of layered media propagation. Orosei et al. [82] argued that in such terrain structures involving coherent interference between layers, the interference produced by radar waves at one frequency cannot have the same effect at another frequency. Therefore, they countered the possibility that the bright echoes were caused by the interference of CO${}_2$-ice and water-ice layers [82].

In addition, Lauro et al. [83] focused on three key aspects—namely, the electromagnetic model, dielectric properties, and materials and geology—to address the simulation:

• Electromagnetic Model: In response to the findings by Lalich et al. [31], suggesting the third scenario as a potential cause for bright echoes, Lauro et al. [83] conducted experiments employing this scenario as a model, with the exclusion of the non-existent CO${}_2$-ice layer above. Deviating from the original model, they observed that the Normalized Basal Echo Power (NBEP) values at the radar’s three frequencies (3, 4, 5 MHz) did not simultaneously reach their maximum at the same thickness of the dry-ice layer. Instead, comparable NBEP values (approximately 4.55 dB) were attained only when the radar frequencies corresponded to thicknesses of 15.4 m, 11.4 m, and 9 m, respectively.
• Dielectric Properties: Lalich et al. [31] utilized dry-ice permittivity values from Pettinelli et al. [84] in their simulation. However, Lauro et al. [83] contested the accuracy of the data interpretation from that particular article, asserting a misinterpretation leading to the citation of incorrect permittivity values.
• Materials and Geology: Contrary to Lalich et al. [31]’s simulation, which suggested that Ultimi Scopuli’s surface is covered by the residual-ice-cap unit (RIC) rather than the seasonal-ice-cap unit (SIC), Lauro et al. [83] argued that, in reality, the significant distance of approximately 400 km between the center of the Antarctic RIC and Ultimi Scopuli makes it implausible for the RIC to cover the surface of Ultimi Scopuli. Accordingly, they proposed that the surface of Ultimi Scopuli should be influenced by the SIC, with its CO${}_2$-ice layer exhibiting periodic changes corresponding to the seasons.

4.2.3. Other Possibilities Causing the Bright Reflection

Smith et al. [30] propose that hydrated montmorillonite could also lead to bright echo reflections. They selected the common Ca-montmorillonite found on Mars as a sample, fully hydrating it to create a plastic paste-like substance. They performed dielectric measurements on the hydrated sample and discovered that at 230 K and a central frequency range of 3–5 MHz, the dielectric constant ranged from 15 to 25, accompanied by a loss tangent of 1.2 to 1.4, shown in Figure 9a,b. As montmorillonite is prevalent on Mars, they suggested that the bright echo reflection could be more reasonably attributed to hydrated montmorillonite than to a liquid-water body [30]. Additionally, some viewpoints propose that the radar-detected regions of liquid water might be conductive or semiconductive minerals, such as hematite or hydrated minerals like jarosite, which could also explain the bright reflection at the base [29].

Mattei et al. [32] conducted dielectric measurements on different levels of water content, clay content, and various types of minerals. They found that the apparent permittivity of clay deposits did not exceed 7 at 203 K and that significant effects on clay’s dielectric properties only occurred at temperatures above 250 K (Figure 9c). Additionally, they measured the apparent permittivity of Mg${\left(\mathrm{Cl}{\mathrm{O}}_4\right)}_2$ and CaC${\mathrm{l}}_2$ brines with different volume fractions and found that certain volume fractions of these brines could achieve the apparent permittivity values required for MARSIS inversion (Figure 9d). Therefore, they argued that, compared to other substances like minerals or clay, high-concentration brines are better suited to explaining the bright reflection at the SPLDs base [32]. On the other hand, Cosciotti et al. [33] replicated experiments measuring the dielectric properties of clay samples. They found that even in clay samples with high moisture content, the apparent permittivity exhibited was only 8.4 at 4 MHz and 230 K, and it was only 4.1 at 200 K. These values were significantly lower than the actual inversed values. This research has conclusively demonstrated that materials such as clays cannot produce the strong reflections observed by MARSIS [33].

In the various studies mentioned above, the researchers supporting the presence of liquid water at the base of the SPLDs have responded to every argument against the existence of liquid water. Currently, researchers who do not support the presence of liquid water at the base of the SPLDs have not yet responded to these latest arguments. Therefore, we maintain an optimistic outlook regarding the existence of liquid water at the base of the SPLDs. The future radar payloads on Mars could provide additional observations of the region, to validate the presence of liquid water and enhance the robustness of these findings.

5. The Discovery of Liquid Water Provides Insights for the Exploration of Martian Life

5.1. Life in Extreme-Cryogenic-Brine Ecosystems

The discovery of liquid water beneath the Martian south polar ice cap provides crucial clues regarding the current potential for life on Mars and points the way for future astrobiological research [85]. Orosei et al. [26] suggests the presence of highly concentrated perchlorate brine beneath the SPLDs base. The SPLDs basal temperature could reach a maximum of 240 K [81]. Therefore, the question arises: what kind of life could exist in the extreme environment beneath the ice layer, characterized by anaerobic, sunlight-deprived, cold conditions with high concentrations of perchlorates?

Micro-organisms on Earth can survive in low-temperature, oxygen-deprived environments, as has already been discovered. The micro-organisms in the saltwater Lake Vida, located in the Antarctic region, are completely enclosed by a permanent ice cover, as shown in Figure 10 [86,87]. The lake’s temperature is minus 13 degrees Celsius and, due to its ice-covered state, it also lacks sunlight and oxygen [88]. In addition to low temperatures, the high concentration of perchlorates in the environment restricts most potential microbial species. Most micro-organisms cannot adapt to such high perchlorate concentrations. Although the high perchlorate concentrations in Earth’s natural environment cannot reach the levels found in the SPLDs liquid-water bodies on Mars [89], environments on Earth with high concentrations of salt can still provide insights into the tolerance of micro-organisms to salt. Prior to the discovery of possible liquid water beneath the SPLDs ice layer, halophiles were considered the most likely candidates for microbial life on Mars, due to their ability to survive in extreme high-salinity environments [26,90,91]. Some halophiles can even survive in concentrated brines for short periods [92,93,94], and certain halophiles can exist in sub-zero cold environments [90,95], such as halobacterium lacusprofundi (H. lacusprofundi), isolated from a deep lake in Antarctica [96]. The Atacama Desert, one of the harshest environment on Earth, which includes different salt flats, is a suitable environment to research. Aerts et al. [97] used sophisticated extraction procedures to detect and analyze the amino acids from the salt flats. They found that the majority of samples contained diverse and complex microbial communities, with a large portion being halophilic bacteria. In addition, they found that the growth of the micro-organism was not hindered by salt, as they could adapt to higher concentrations of saline environment. Therefore, halophiles are one of the most likely candidates for microbial life beneath the SPLDs ice layer. As samples from beneath the SPLDs ice layer cannot currently be obtained, we need to determine whether micro-organisms can survive, thereby simulating the corresponding environment in Earth’s laboratory. DasSarma et al. [91] cultured H. lacusprofundi and Halobacterium sp. NRC-1 in a 40 mM (here nM is the molarity, 1 nM = 1 nmol/L) perchlorate environment under dark and oxygen-deprived conditions. They recorded the bacterial counts of both strains at zero and eight days and found that H. lacusprofundi was capable of growth in a high-perchlorate environment compared to other haloarchaea.

Additionally, DasSarma et al. [91] placed H. lacusprofundi and Halobacterium sp. NRC-1 in culture media containing different ions and observed the growth rates of the two strains. They discovered that [91]:

• H. lacusprofundi can adapt to higher concentrations of perchlorates, with less inhibition of growth, by increasing perchlorate concentrations within the 0–2 M range, compared to other haloarchaea.
• Divalent cations have a greater impact on microbial growth rates than monovalent cations.
• Perchlorate ions have a more significant impact on microbial growth rates than cations.

DasSarma et al. [91] concluded that although perchlorates can inhibit the growth of H. lacusprofundi, the currently assumed high-perchlorate concentrations on Mars would not be sufficient to completely inhibit its growth. Heinz et al. [89] discovered Debaryomyces hansenii and Purpureocillium lilacinum, the highest perchlorate-tolerant micro-organisms reported in all studies to date. To study eukaryotic organisms’ tolerance to perchlorates, they cultured Debaryomyces hansenii and Purpureocillium lilacinum separately in liquid growth media with varying concentrations of sodium perchlorate. They found that Debaryomyces hansenii can grow in a 2.4 mol/L NaCl${\mathrm{O}}_4$ environment, while all cells die after 10 days in a 2.6 mol/L NaCl${\mathrm{O}}_4$ environment (Figure 10b).

5.2. Exploring the Potential Existence of Life on Mars

The suitable way to determine if micro-organisms exist in the SPLDs region is to analyze samples collected from this area. However, the SPLDs region is located approximately 1.5 km below the Martian surface [26]. Therefore, it is currently extremely challenging to directly obtain samples from this area. As of now, obtaining surface samples from Mars is most likely through robotic missions or automated spacecraft equipped with sample collection tools. CNSA, NASA, and ESA all have corresponding Mars sampling programs. For example, ESA has proposed a staged plan to collect samples from Mars and return them to Earth, shown in Figure 11. China’s Tianwen-3 mission plans to collect surface samples from Mars and gather data from the surrounding environment [98]. The Tianwen-3 mission includes two major components [99]: one for landing and collecting Mars samples (lander) and one that will work on the satellite orbit (orbiter). The landing spacecraft will collect samples itself and send the collected samples back to the orbiter with its ascent vehicle. After docking with the ascent vehicle, the orbiter will carry the samples back to Earth. Two Long March 5 rockets will be used to launch the Tianwen-3 spacecraft. The orbiter will launch in 2028, arrive in August or September 2029, and return in October 2030 [99]. There are two planned missions regarding the timing of the lander launching and touching down. In plan A, the lander will launch in December 2028 and arrive on Mars in July 2029, and it will take about 6 months to collect samples. To avoid winter in the northern hemisphere, plan B proposes launching in May 2028 and landing in August 2030, then spending about three months collecting samples [99]. While collecting samples, a mini-helicopter and a four-legged robot could be used to detect the surrounding environment and collect the samples away from the lander [99]. But where to collect and what to collect is still uncertain, due to the diversity of Mars’ geological environment. Owing to the extreme environment (long-term cosmic ray radiation) on the surface of Mars, we may need to apply methods such as drilling and sampling, to obtain samples of the subsurface of Mars [100]. In the future, more advanced methods may become available for collecting samples from this region. Although obtaining samples from this area in the short term is not feasible, studying surface samples from Mars may provide valuable insights. If signs of organic compounds or indications of the presence of micro-organisms are found in samples from the Martian surface, it would make our speculation about the possible existence of micro-organisms at the base of the SPLDs more reasonable.

Before obtaining samples from the surface of Mars, analyzing Mars-surface samples and conducting field investigations in arid environments on Earth that resemble the Martian surface, such as the Atacama region in Chile [97], could help us understand the geological and mineral evolution of the Martian surface [101,102,103,104]. Consequently, this could contribute to an enhanced understanding of the characteristics of the Martian surface and further strengthen our ability to assess the resilience of the micro-organisms in the Martian environment.

Furthermore, research on Martian meteorites is of significant assistance in our quest for life on Mars. Certain carbon-containing meteorites may undergo reactions under the influence of water and heat, potentially generating various organic compounds, offering the possibility of the emergence of life [105,106]. The abundant water that once existed on the surface of Mars could have provided conditions conducive to the synthesis of certain organic compounds [105,107]. In 1984, a meteorite from Mars dating back approximately 15 million years was discovered [108], having landed on Earth about 13,000 years ago [109]. Although no organic compounds were found in the study of this Martian meteorite, the minerals were discovered to have been corroded by liquid water, which confirmed the past water erosion on the surface of Mars. Water-eroded active minerals may have catalyzed the formation of complex organic compounds during chemical evolution and may have some impact on the existence of life [110,111]. Numerous meteorites have reached the surface of Mars, and over years of exposure to water and heat, life may have potentially originated [106]. This study provides direction for the selection of Mars sample-collection sites. Samples collected from impact fractures in the Martian crust may provide us with answers [112].

Figure 11. Scenario of ESA’s prospective strategy for a Mars-sample return mission [113,114].

Figure 11. Scenario of ESA’s prospective strategy for a Mars-sample return mission [113,114].

6. Conclusions and Perspectives

Detecting biological features on Mars is prominent research in the current field of astrobiology. And water is the source of life—nearly all living organisms on Earth are intimately connected to it. Water provides the fundamental conditions for the survival of organisms, acting as both a reactant in various biological processes and a solvent for these reactions within organisms. Over the past few decades of research, evidence has suggested the past existence of liquid water on Mars and, in recent years, water ice has been discovered in regions such as the Martian poles. Since the discovery of liquid water beneath the south polar ice cap of Mars by Orosei et al. [26] in 2018 the presence of liquid water on Mars has become one of the hottest topics in Martian exploration. The discovery of liquid water at the base of the SPLDs on Mars and the summary of implications for Mars life studies are as follows:

• MARSIS detected bright radar echoes at the base of the SPLDs, and analysis indicates that these echoes were caused by a water body containing a high concentration of perchlorates. While there have been doubts suggesting that the bright radar echoes could be due to substances such as clay, simulations of the dielectric properties of relevant materials suggest that clay cannot produce such bright reflections. These studies suggest the likely presence of liquid water at the base of the SPLDs on Mars, albeit containing a significant amount of perchlorates.
• The presence of liquid water implies the possibility of microbial life. The high salinity, low temperature, and anaerobic conditions at the base of the Mars SPLDs seem unfavorable for the long-term survival of life. Research on similar extreme environments on Earth and laboratory experiments demonstrating the survival of halophilic bacteria in simulated extreme conditions suggest that if microbial life exists at the base of the Mars SPLDs, it is most likely to be halophilic bacteria.
• Additionally, studies of carbon-containing meteorites on Earth have shown that these meteorites may produce organic compounds under the influence of water and heat, which could potentially support microbial life. Mars has also been extensively impacted by meteorites, and similar reactions might have occurred on Mars. This provides insights for the selection of sampling sites and specimens in Mars sampling missions.

In future studies, the deployment of advanced radar systems to enable more precise observations of this region, along with the collection of additional data to improve the accuracy of dielectric properties inversion beneath the ice layers, could be considered valuable avenues for exploration. In addition, exploration of the Martian-North-Pole region, which also features substantial ice deposits, could provide insight into whether similar bright reflections exist there. Furthermore, laboratory investigations of future Martian soil samples could enhance our understanding of Martian soil composition, aiding in better predicting the constituents of the SPLDs base. The presence of liquid water on the subsurface of Mars implies the potential for the existence of micro-organisms. By combining this discovery with the surrounding Martian environment, we can more reasonably speculate about the possibility of and potential forms of life on Mars. The identification of liquid water will impact our Mars-exploration strategies, resulting in a heightened emphasis on observing liquid water and a more precise collection of soil samples. Moreover, studying how extremophiles survive in Earth’s extreme environments might offer valuable insights for studying potential Martian life, providing us with additional strategies for searching for life on Mars.

This paper provides a comprehensive summary of recent research on the liquid water at the base of the SPLDs on Mars and studies related to Martian micro-organisms. Including orbital radar exploration of the Martian subsurface and investigations into the survival of micro-organisms in simulated Martian environments, the paper covers various aspects of recent advancements in the study of Martian water bodies and micro-organisms. We believe that this paper will serve as a valuable reference for future orbital-radar exploration of Martian-subsurface water bodies, while also offering insights for the selection of sampling sites in China’s Tianwen-3 mission.