Evolution of Mars Water-Ice Detection Research from 1990 to 2024

Abstract

As the most similar planet to Earth in the solar system, Mars’ surface and subsurface water ice provide important clues for studying extraterrestrial life and planetary evolution. Since the 1960s, the exploration of Martian water ice has gradually become a focus of scientific research. This article reviews the evolution of Mars water-ice detection technology from 1990 to 2024 through bibliometric analysis, with a focus on the application of key technologies such as radar detection, image analysis, in situ analysis, thermal infrared imaging, and neutron spectroscopy. The analysis results indicate that research in the field of Mars water-ice exploration has been increasing year by year, with major research institutions including National Aeronautics and Space Administration (NASA) and the California Institute of Technology (CIT), and key researchers such as Professor James W. Head making significant contributions. Keyword analysis shows that current research is focused on the distribution and status of water ice and its relationship with the Martian climate, and the application of modern exploration technology has also become a hot topic. However, despite continuous technological advancements, issues such as detection depth and data analysis accuracy remain challenges. The complex terrain and extreme climate conditions make water-ice detection difficult. This article also points out that future research should focus on integrating multiple high-precision detection techniques for consistent results and the application of new technologies such as time-varying gravity. Moreover, combined with the application of artificial intelligence, this will provide new directions for the precise detection and data-processing of Martian water ice.

1. Introduction

Mars is the planet in the solar system with the most similar environment to Earth and may hold the best record of the time of the origin of life in the solar system and the catastrophic changes in planetary evolution [1]. It is of great comparative significance for studying the origin and evolution of Earth and is one of the most valuable settings for exploring major scientific topics such as extraterrestrial life and the origin and evolution of life. Moreover, Mars is relatively close to Earth and is expected to be the first exoplanet to be explored by humans. Therefore, it has always been a key target of international planetary exploration and is the most explored extraterrestrial body by humans besides the Moon [2].

Since the 1960s, humans have embarked on a journey to explore Mars. Early Mars exploration missions mainly focused on “following the water” [3]. Water is fundamental to life. Therefore, the search for various forms of water on Mars remains a key scientific objective of its exploration [4]. At present, the main means of detecting Martian water ice rely on remote-sensing technology and landing systems. Remote sensing enables orbiters to capture high-resolution images and multispectral data, providing scientists with a comprehensive understanding of Martian terrain and water-ice distribution. Orbital probes such as NASA’s Mars Reconnaissance Orbiter (MRO) can obtain high-resolution images and detailed terrain data through sophisticated remote-sensing equipment, helping scientists identify water flow traces and geological features on the surface of Mars [5,6]. In addition, the landing system also plays a crucial role in detecting Martian water ice, by safely delivering the probe to the Martian surface for ground analysis and sample collection. Innovative landing technologies such as the “aerial crane” system are used to accurately deploy heavy probes such as Curiosity and Perseverance to designated locations, allowing them to directly contact Martian soil and rocks for deeper water-ice exploration [7].

Modern Mars probes not only employ advanced remote-sensing technology and landing systems but are also equipped with various specialized scientific instruments to obtain more detailed data. For example, ground-penetrating radar (GPR) is utilized by the Curiosity rover to probe Martian subsurface geological features and water-ice layers. This technology helps scientists better understand the geological structure and water resource distribution of Mars by emitting electromagnetic waves and analyzing their reflected signals. Additionally, neutron detectors play a crucial role in measuring the moisture content on the Martian surface and subsurface. NASA’s Mars Reconnaissance Orbiter (MRO) and Curiosity spacecraft both carry this instrument, which provides important data on the water distribution by monitoring changes in neutron flux, laying the foundation for evaluating Mars’ water resources and potential life-supporting conditions. Thermal analysis technology has also been applied to modern Mars probes, especially the Perseverance probe. The detector is equipped with thermal analysis instruments, and by analyzing the thermal radiation signals of soil and rock samples, scientists can infer the composition and physical state of these materials. This technology not only helps to gain a deeper understanding of the geological history of Mars but also provides important clues when searching for signs of life.

So far, scientists have confirmed there is evidence of water on the surface of Mars, including dry riverbeds, salt deposits, and polar ice caps. Due to significant geological changes throughout history, the planet has transitioned into its current cold state. The water detected so far primarily exists in the polar ice caps and beneath the surface [8]. In addition, the Mars probe has also discovered the presence of organic molecules and methane, which are potential indicators of the existence of life. In this context, multiple studies have reviewed the exploration of water ice on Mars. For example, Liu Zhenghao et al. provided a detailed introduction to the detection methods and technological progress of water ice on Mars, revealing the distribution of water ice in the polar and mid-latitude regions of Mars [9]. Zhao Jiannan et al. discussed the research progress on the detection, exploitation, and in situ utilization of water resources on Mars, reviewed methods such as imaging, spectroscopy, and radar detection, and analyzed the occurrence status of available water resources on Mars [10]. Xiao Yuan and others reviewed the current status of exploration and research on the subsurface of Mars, emphasizing the importance of using radar technologies such as MARSIS and SHARAD to detect Martian water ice and its role in studying the formation and evolution history of Mars [11]. These reviews provide valuable perspectives for understanding the current status of research on Martian water ice, but they often lack a quantitative analysis of research trends and dynamics. Therefore, using bibliometric analysis can provide us with a systematic framework to identify key research topics, collaborative networks of researchers, and the impact of research results in this field, thereby providing valuable guidance for future research directions.

Bibliometric analysis is a method of evaluating the development status of a research field through statistical analysis of literature data. By analyzing relevant literature from 1990 to 2024, the evolution trajectory, research hotspots, and future research directions of Mars subsurface water-ice exploration research can be revealed. This not only helps scientists understand the research progress in this field but also provides valuable references for novice researchers.

2. Data and Methods

The research on Mars surface water-ice exploration is divided into two main parts: data collection and analysis. Figure 1 shows the complete workflow of our research.

2.1. Data Acquisition

The data for this study were sourced from the Web of Science (WoS) database. We conducted a literature search using the keywords “Mars” and “water ice”, with a search time range from 1990 to 31 December 2024, to ensure that all relevant research in the field of subsurface water-ice exploration on Mars is covered. The final collected literature includes journal articles, conference papers, and technical reports, ensuring the diversity and comprehensiveness of the data.

2.2. Scientometric Analytical Methods

Scientific research databases such as Web of Science (WoS) provide rich metadata for publications, including titles, keywords, publication sources, and author affiliations [12]. By conducting quantitative analysis of these data, we can gain a deeper understanding of publication, citation, and usage patterns in the academic literature [13].

In our study, we employed the Visualizing the Ontology of Similarity (VOS) method to effectively visualize the relationships within the data. The core of the VOS method involves constructing a similarity matrix to represent the relationships between objects, and then mapping these objects to a low-dimensional space for visualization [14]. This approach includes constructing a similarity matrix S = ($s_{ij}$), which satisfies the following conditions:

$$\left\{\begin{array}{c}\begin{array}{cc}{s}_{ij}& \ge 0,\\ s_{ii}& =0,\\ s_{ij}& =s_{ji}\end{array}\\ i,j\in 1,\dots ,n\end{array}\right.$$

(1)

Here,$s_{ij}$ represents the similarity between objects (i) and (j).

A low-dimensional Euclidean space (X) is defined to represent the visualization coordinates of each object, typically in two dimensions, where $X\in R^{n\times 2}$. The goal of the VOS method is to minimize the total weighted squared distance between all pairs of objects, ensuring that the positions in the coordinates (X) reflect the similarities $s_{ij}$ as closely as possible. Therefore, the objective function is the following:

$$E\left(X;S\right)=\sum _{i<j}{s}_{ij}\parallel x_i-x_j{\parallel }^2$$

Minimizing this function yields the visualization coordinates. To maintain non-degeneracy, the following constraint is imposed:

$$\sum _{i<j}\parallel x_i-x_j\parallel =1$$

3. Results

3.1. Multi-Perspective Analysis of the Published Literature

3.1.1. Publication Characteristics

1.

Characteristics of Publications

From 1990 to 2024, the number of research articles on Mars water-ice detection has shown a general upward trend, with particularly significant peaks in 2003, 2010, 2019, and 2021 (see Figure 2). These fluctuations are closely linked to key Mars exploration missions and technological advancements. In 2003, Mars exploration entered a new phase with the launch of the European Space Agency’s Mars Express orbiter and NASA’s Opportunity and Spirit rovers. The successful landing of the latter sparked increased research on surface water indications and subsurface water ice on Mars. Meanwhile, the Mars Express carried the MARSIS radar instrument, which conducted the first in-depth study of the distribution of subsurface water ice on Mars, significantly advancing research in this area.

In 2010, the progress of NASA’s Mars Reconnaissance Orbiter (MRO) and Mars Science Laboratory (MSL) missions further accelerated research. The MRO released a wealth of high-resolution imaging and radar data, which provided key support for the detection of Martian glaciers, polar ice caps, and subsurface water ice. The open access to and widespread use of these data contributed to a notable increase in related research.

In 2019, with the preparations for the Mars 2020 mission and the MARSIS radar’s discovery of evidence for liquid water at the Martian south pole, research on Martian water resources reached a new peak, capturing global attention. Finally, in 2021, following the successful execution of the Tianwen-1 and Perseverance missions, the volume of research in these fields surged significantly.

In the field of Mars water-ice detection research, several journals have provided major platforms for the publication of relevant literature, including ICARUS, the Journal of Geophysical Research: Planets, Planetary and Space Science, Geophysical Research Letters, Astrobiology, and Earth and Planetary Science Letters (see Figure 3). These journals cover a wide range of research topics, from remote-sensing image analysis of Martian water ice to planetary geology and biology, offering a wealth of resources that contribute to scientific advancements in the field.

2.

Analysis of Country and Institution Contributions

As shown in Figure 4, the publication output of the top 10 countries in the field of Mars water-ice detection reveals that the United States leads the field, benefiting from the support of NASA and its collaborative institutions, particularly in the areas of planetary science and geology. France has made significant contributions in the fields of climate modeling and remote-sensing technologies, driving in-depth research into Martian water ice. The United Kingdom has focused on the study of planetary surfaces and climate change, thereby enhancing its influence in Mars science. Germany has excelled in technological research and development, as well as in supporting Mars missions, providing crucial theoretical and experimental data for Mars water-ice detection. In recent years, with the successful landing of the Tianwen-1 mission, China has gradually emerged as an influential player in the study of Mars water ice.

Over the past 43 years, numerous research institutions worldwide have played a crucial role in the exploration of Martian water ice, as shown in Figure 5. NASA, CIT, and the French National Center for Scientific Research (CNRS) rank as the top three institutions in terms of number of publications. With strong financial support, advanced technological infrastructure, and an extensive collaboration network, NASA has led numerous major Mars exploration projects, accumulating a vast amount of high-resolution imagery, radar data, and spectral analysis results, which have significantly advanced research on Martian water ice.

The California Institute of Technology plays a key role in Mars research, particularly through its affiliated NASA Jet Propulsion Laboratory (JPL). Researchers from CIT have contributed to multiple Mars missions by analyzing data and developing models, using remote-sensing techniques and geological simulations to study the distribution of Martian water ice. CNRS, the largest scientific research institution in France, also has strong expertise in planetary science. Additionally, the University of Arizona, Sorbonne University, and Paris-Saclay University have made substantial contributions to this field.

3.

Analysis of the Contributions of Authors

As illustrated in Figure 6, James W. Head dominates the field with 129 publications, pioneering groundbreaking discoveries including the identification of mid-latitude periglacial features (“brain terrain” morphology) and polar stratigraphic ice deposits. His formulation of the “Snowball Mars” hypothesis revolutionized our understanding of planetary glacial cycles, while his development of dual-polarization radar inversion techniques significantly enhanced SHARAD’s capability to resolve shallow subsurface ice layers (<1 km depth). Closely following, Michael D. Smith’s 72 publications establish critical frameworks for atmospheric ice dynamics through innovations in aerosol vertical profiling algorithms and global D/H isotope ratio mapping via CRISM/NOMAD spectrometers—methodologies now institutionalized in ExoMars and Tianwen-1 mission protocols. Other notable authors include Forget F (61 publications), Mellon MT (60 publications), Wolff MJ (60 publications), and Christensen PR (43 publications). These authors frequently collaborate, reflecting the international collaborative nature of this research field, as illustrated in Figure 7.

3.1.2. Hotspot Analysis

• The most frequently used keywords

Recent advancements in Mars water-ice detection have made significant progress and this has gradually become one of the central topics in Mars exploration. Figure 8 illustrates the distribution of research keywords in the field of subsurface water-ice detection on Mars over the past 34 years, revealing the evolution of research focus. Currently, the core research direction centers on Mars’ water ice and its associated characteristics. Keywords such as “water ice”, “liquid water”, “ground ice”, and “subsurface ice” appear frequently, indicating that the state and changes in water on and beneath the Martian surface are central to Martian scientific research. This research direction is not only closely linked to Mars’ climatic conditions but also plays a crucial role in the exploration of potential signs of life on the planet.

Based on long-term temperature monitoring data from the Mars Pathfinder, scientists have discovered that surface temperature variations on Mars exhibit significant diurnal cycles, with the lowest temperatures dropping to −78 °C and the highest temperatures around −8 °C in the afternoon. This observation further confirms that liquid water cannot exist on the Martian surface, as the surface temperature remains consistently below the freezing point of water. Additionally, the Pathfinder also detected the presence of fog on the Martian surface, suggesting the potential existence of a water vapor cycle. This discovery not only enhances our understanding of Mars’ atmosphere but also provides important clues for future water resource detection on the planet.

Moreover, as research on Mars’ surface geological features has deepened, keywords such as Gale Crater, Meridiani Planum, and Martian gullies have frequently appeared, indicating that scientists are increasingly focused on the geological processes occurring on Mars’ surface. For example, in 2012, NASA reported that the Curiosity rover had discovered evidence of past water flow in an ancient riverbed within Gale Crater [15,16,17,18,19]. This finding provided direct evidence of substantial water movement on Mars in the past. Through studies on the deuterium-to-hydrogen ratio, scientists further discovered that Mars underwent significant water loss processes prior to the formation of the lakebed in Gale Crater, which may have lasted for hundreds of millions of years. This discovery has greatly advanced our understanding of the history of water on Mars and revealed the potential distribution of water ice both on the surface and below ground.

In addition to research on the distribution of water ice, the impact of climate change on Mars has gradually become a key research direction. Keywords such as climate change, temperature, atmosphere, and the general circulation model indicate that the evolution of Mars’ climate system has a significant impact on the distribution of water ice and the interactions between water and rock. Existing studies not only focus on the historical changes in Mars’ climate system but also explore how climate change has shaped the planet’s water resources, particularly in the context of the formation and melting processes of water ice under extreme conditions.

Finally, with the advancement of aerospace technology, the application of remote-sensing techniques has become an indispensable tool in the study of Mars’ water ice. The appearance of technologies such as the spectroscope, thermal emission spectrometer, and orbiter camera highlights the critical role of remote sensing in acquiring data from Mars’ surface and atmosphere. These technologies enable in-depth analysis of the Martian surface, providing more precise physical and chemical information and further advancing our understanding of Mars’ water ice, climate, and geology.

2.

The top 10 most frequently cited articles

In the field of Mars research, the highly cited literature reveals the development of the discipline across several key directions. Figure 9 presents the top ten most cited Martian research papers, covering a broad range of topics from early foundational theories to recent technological applications. From the highly cited papers, it is evident that the research focus of Mars science has gradually diversified, encompassing a wide array of fields, including basic surface physical exploration, climate modeling, and modern remote-sensing technological applications. The three most cited papers are “Mars Surface Diversity as Revealed by the OMEGA/Mars Express Observations” [20], “Mars Global Surveyor Thermal Emission Spectrometer Experiment: Investigation Description and Surface Science Results” [21], and “Distribution of Hydrogen in the Near Surface of Mars: Evidence for Subsurface Ice Deposits” [22].

The publication years of these highly cited references span more than two decades from 1991 to 2014, reflecting the gradual deepening of Mars research from theoretical discussions to actual exploration. Early works such as Baker [23] and Clifford [24] focused on the ancient hydrological environment of Mars and the changes in its ice caps, providing theoretical support for subsequent studies on Mars’ water resources and climate models. After entering the 21st century, with the advancement of Mars exploration technologies, more publications began to concentrate on the specific scientific findings of Mars missions. For example, the Mars Global Surveyor Thermal Emission Spectrometer Experiment provided data on Mars’ surface temperature and mineral distribution. In Figure 9, the highest citation points for each reference are marked with asterisks, indicating the peak citation year for each paper. These peaks often correspond to the times when significant Mars missions or research breakthroughs occurred. For instance, citation peaks in 2005, 2006, and 2007 are closely linked to important scientific achievements of Mars exploration missions, such as those of MGS and OMEGA.

3.2. Analysis of Martian Water-Ice Exploration Technology

3.2.1. Mars Exploration Mission

The successful launch and landing of spacecraft are crucial prerequisites for studying Martian water ice. Mars exploration dates back to the 1960s, and since then, seven countries or organizations—namely the Soviet Union, the United States, Japan, the European Space Agency (ESA), India, the United Arab Emirates (UAE), and China—have successfully launched missions to Mars. The first attempt to send a spacecraft to Mars was made by the Soviet Union with the launch of Mars 1960A in 1960. In 1964, the United States followed suit with the Mariner 3 mission, its first Mars probe. Japan, ESA, and India each launched their inaugural Mars missions in 1998, 2003, and 2013, respectively, while the UAE and China launched their first missions in 2020. To date, a total of 48 Mars exploration missions have been undertaken (See Figure 10).

Table 1. Detailed information on the results of Mars exploration missions.

No.	Name	Status	Country	Launch Date	Key Achievements
1	Mariner 4	Successful	USA (Flyby)	28 November 1964	First Mars images and long-range atmospheric observations
2	Mariner 6	Successful	USA (Flyby)	24 February 1969	First atmospheric exploration and returned images
3	Mariner 7	Successful	USA (Flyby)	27 March 1969	93 long-range and 33 close-up Mars surface images
4	Mariner 9	Successful	USA	30 May 1971	First Mars orbiter; studied atmosphere and surface
5	Viking 1	Successful	USA	20 August 1975	High-res images, soil samples, and biological experiments
6	Viking 2	Successful	USA	9 September 1975	Analyzed soil and atmosphere, similar to Viking 1
7	Mars Global Surveyor	Successful	USA	7 November 1996	High-res surface images, topography and mineral mapping
8	Mars Pathfinder	Successful	USA	4 December 1996	Surface survey, imaging, and chemical analysis
9	Mars Odyssey	Successful	USA	7 April 2001	Mineral composition and temperature maps; Phoenix data support
10	Mars Express/Beagle 2	Successful/Failed	ESA	2 June 2003	Discovered water ice, methane, and evidence of plate tectonics
11	Spirit Rover	Successful	USA	4 January 2004	Revealed Mars’ basaltic surface and water-related rock formations
12	Opportunity Rover	Successful	USA	25 January 2004	Evidence of salt lakes and new insights into Mars’ history
13	Mars Reconnaissance Orbiter	Successful	USA	12 August 2005	High-res imaging and atmosphere and climate data
14	Phoenix Lander	Successful	USA	25 May 2008	First polar lander; discovered water and ice, analyzed soil
15	Curiosity Rover	Successful	USA	6 August 2012	Advanced rover; nuclear power; searched for signs of life
16	MAVEN	Successful	USA	18 November 2013	Studied atmosphere and solar wind interaction
17	Mangalyaan	Successful	India	5 November 2013	Studied Mars’ formation and potential for life
18	ExoMars/Beagle 2	Successful/Failed	ESA/Russia	14 March 2016	Monitored methane and other gases for biological/geological sources
19	InSight Lander	Successful	USA	5 May 2018	Studied Mars’ interior via seismic and geophysical surveys
20	Hope Orbiter	Successful	UAE	20 July 2020	Monitored Mars’ atmosphere and weather patterns
21	Tianwen-1	Successful	China	15 May 2021	First mission to orbit, land, and deploy a rover; studied surface and atmosphere
22	Perseverance Rover	Successful	USA	30 July 2020	Searched for ancient life, collected samples, and studied Mars’ geology

gives information on each of the Mars exploration missions. The United States has launched 23, the Soviet Union/Russia 19, the ESA 2, and Japan, India, the UAE, and China 1 each. Out of these missions, 27 have been successful or partially successful, resulting in an overall success rate of approximately 56%. While many early missions encountered failures due to technological and launch-related issues, subsequent missions have seen considerable success, thanks to advances in science and technology.

The early stage of Mars exploration was led by the Soviet Union and the United States. However, due to technological limitations and challenges in launch capabilities, the success rate of these missions was relatively low. In 1960, the Soviet Union launched Mars 1A and Mars 1B, but both missions failed due to rocket malfunctions. During this period, most Mars missions struggled to achieve their intended objectives, especially in terms of landing and data transmission, which posed significant obstacles. Nonetheless, in 1964, the United States’ Mariner 4 successfully flew by Mars, sending back the first images of the Martian surface and marking humanity’s first remote observations of the planet [30]. In 1971, the Mariner 9 spacecraft successfully entered Mars’ orbit, conducting a series of scientific investigations and providing critical data on the Martian surface and atmosphere, thus laying the foundation for future missions [31,32].

In the 1990s, Mars exploration entered a new era of advancement. In 1992, the United States’ Mars Global Surveyor successfully entered Martian orbit, capturing high-resolution images of the planet’s surface and becoming the first mission to conduct long-term observations of Mars’ surface and atmosphere [33,34]. Over the course of a decade, the mission provided extensive data on Mars’ geology and mineral distribution, significantly advancing our understanding of the planet’s history and environment. In 1996, the United States’ Mars Pathfinder successfully landed on Mars and carried the Sojourner rover, marking humanity’s first successful surface exploration of Mars [35,36,37]. In 2001, the Mars Odyssey spacecraft entered Martian orbit, discovering large amounts of water ice on the Martian surface and serving as a relay station for subsequent missions [34,38,39,40]. In 2003, the Spirit and Opportunity rovers successfully landed on Mars and conducted extensive geological surveys, revealing various types of Martian rocks and sedimentary layers and providing evidence that water had once existed on the planet [41,42,43,44,45]. In 2005, the Mars Reconnaissance Orbiter successfully captured more high-resolution images of the Martian surface and provided detailed information on Mars’ atmosphere [46,47,48,49]. In 2007, NASA’s Phoenix lander successfully landed at the Martian north pole, confirming the presence of water ice on Mars and collecting evidence of past liquid water [50,51].

Entering the 2010s, Mars exploration missions became more diversified and comprehensive. In 2011, NASA’s Curiosity rover successfully landed in Gale Crater, where it conducted detailed studies of the Martian surface, rocks, and climate, with a particular focus on whether Mars once had conditions suitable for life [52,53,54]. In 2013, India’s Mangalyaan (Mars Orbiter Mission) successfully entered Mars’ orbit, making India the first Asian country to reach Mars and marking a breakthrough in Indian space technology [55]. In 2018, NASA’s InSight lander, equipped with a seismometer, a temperature measurement device, and the Rotation and Interior Structure Experiment (RISE), successfully conducted the first-ever investigation of Mars’ internal structure, providing important data on the planet’s internal composition [56]. In 2020, the United Arab Emirates’ Hope orbiter successfully entered Martian orbit and began long-term monitoring of Mars’ atmosphere [57,58,59,60]. The same year, China’s Tianwen-1 mission successfully completed all three phases of its mission—orbiting, landing, and roving—providing key data on Mars’ surface morphology, soil properties, and atmospheric composition, marking a significant advancement in China’s Mars exploration efforts [61,62]. Also in 2020, NASA’s Perseverance rover began its exploration of ancient signs of life on Mars, collecting samples and conducting in-depth studies of crater geology [63,64,65]. It also carried the Ingenuity Mars helicopter, which opened up new pathways for aerial exploration, offering invaluable experience for future aerial investigations of Mars [66,67].

3.2.2. Water-Ice Detection Techniques

• Remote-sensing images and elevation data analysis

The advancement of Mars exploration technology has made remote-sensing imagery and elevation data crucial tools in the study of Martian geology and water ice. In the early research stages of the 1960s, Mars observations primarily relied on ground-based telescopes. While these telescopes were capable of identifying large-scale features such as the polar ice caps [68,69], their resolution was insufficient to reveal finer details of Mars’ surface structure. With the launch of the Mariner spacecraft series, particularly the successful orbital insertion of Mariner 9 in 1971, the resolution of Martian surface images significantly improved, providing new insights into the distribution of water ice and Martian geomorphology [70]. Although the image resolution of Mariner 9 ranged from 100 to 1000 m, it was a notable improvement over ground-based telescopes, offering clearer details and revealing, for the first time, features such as dried riverbeds, canyons, and possible traces of ancient flowing water, including the 4020 km long Ma’adim Vallis. These discoveries provided evidence of erosion caused by flowing water on Mars in the past [71,72].

By the late 1970s, Mars exploration technology had made further advancements. The Viking mission, through the deployment of orbiters and landers, provided higher-resolution imagery. In particular, global imagery with a resolution of 300 m per pixel and targeted observations at 8 m per pixel allowed for more in-depth studies of Martian water ice-related geomorphology. Researchers utilized Viking orbiter images to analyze three potential types of glacial landforms indicative of subsurface water ice: tongue-shaped debris flows, concentric impact crater fill, and surface softening. These features were found predominantly in the mid to high latitudes, above 30° north and south, suggesting the widespread presence of subsurface water ice in these regions. The Viking mission not only captured images of impact craters and geological structures suggestive of ice melting processes but also identified features in the Utopia and plains regions that were associated with the presence of liquid water, such as thermokarst landforms caused by the dissolution of subsurface water ice [73], further supporting the theory that liquid water once existed on Mars. As shown in Figure 11, this global color mosaic, produced from approximately 1000 Viking Orbiter images, provides a detailed and gap-free view of Mars’ surface. The mosaic offers valuable insights into the Martian topography and albedo, supporting the analysis of glacial landforms and surface processes potentially linked to water ice.

Entering the 21st century, high-resolution imaging technologies used in Mars exploration missions have significantly improved the accuracy of image analysis. For example, the Mars Orbiter Camera (MOC) on the Mars Global Surveyor (MGS) were able to capture image details with a resolution of 2 m per pixel, revealing polygonal terrain caused by ice melting and young gullies on steep slopes—geomorphological features closely related to the presence of water [74,75]. These data suggest that there was once long-term liquid water flow on the Martian surface, particularly in areas such as Nanedi Vallis and Nirgal Vallis. Additionally, images from the Mars Odyssey spacecraft, launched in 2001, further supported the evidence of water on Mars by revealing signs such as ancient lakebeds, branching valley patterns, and river delta-like features.

The European Space Agency’s Mars Express orbiter used its high-resolution stereo camera to capture images of the vast plains and the ice layers at the bottoms of impact craters in the Martian northern polar region. More importantly, the Mars Reconnaissance Orbiter (MRO), equipped with the High-Resolution Imaging Science Experiment (HiRISE) camera, offers an exceptional resolution of 0.25 m per pixel, allowing a detailed observation of Martian polar ice caps, mid-latitude glaciers, and shallow subsurface water ice. It has even directly detected exposed water ice [76]. For instance, Dundas et al. utilized HiRISE data to observe steep scarps in eight regions, where enhanced color images revealed blue material distributions in cross-sectional profiles, indicating the presence of water ice or hydrated minerals, with the exposed water ice being thicker than 100 m [76].

At the same time, MRO’s Context Camera (CTX) also provided global images with a resolution of 6 m per pixel, offering evidence of the existence and changes in water throughout Martian history. These data revealed signs of ancient hot springs, impact craters, and volcanic activity. Furthermore, in Mars’ mid-to-high latitudes, exposed water ice has been observed within impact craters. Through high-resolution remote-sensing image interpretation, researchers found that the ice in these craters can remain bright for several months to years before gradually darkening [77].

Figure 12 shows the global color image data obtained by China’s first Mars exploration mission, the Tianwen-1 orbiter. Through the medium-resolution camera of the Tianwen-1 orbiter, a segmented map product was formed after photometric correction, geometric correction, and global mapping processing, containing a total of 36 pieces of data with a resolution of 76 m. These images cover multiple regions of the Martian surface, revealing geological features, sand dunes, rocks, and other topographical details of Mars. By analyzing these images, researchers are able to observe the distribution and geological structure of Martian surface materials, revealing the complex geological evolution process of Mars.

Although remote-sensing imagery can reveal the geomorphological features of Mars, its coverage and temporal resolution are limited, making it difficult to capture subtle changes. Furthermore, the interpretation of imagery often relies on the experience of scientists, and similar geomorphological features may be subject to misinterpretation due to differing analytical approaches. To compensate for the limitations of remote-sensing imagery, elevation data have become an important supplement. Launched in September 1992, the Mars Observer spacecraft carried the world’s first Mars Laser Altimeter System (MOLA-1), which obtained high-precision surface elevation data through laser ranging, laying the foundation for subsequent studies of Martian topography. Although the mission was not completed successfully, the data it provided continue to offer valuable support for future Martian topographical research. Subsequently, the Mars Global Surveyor (MGS) carried the MOLA-2 system, which continued comprehensive laser altimetry measurements and produced a high-precision topographic map of Mars with a resolution of approximately 463 m per pixel [34]. These data have been crucial for Martian geophysical research and for supporting subsequent exploration missions.

To enhance the resolution and coverage of Martian topography, the blending of MOLA and HRSC data, shown in Figure 13, provides a high-precision topographic map with a resolution of 200 m per pixel. This blended DEM integrates MOLA’s laser altimetry measurements with HRSC’s stereo imagery, offering a more comprehensive and refined view of Mars’ surface, supporting studies related to thermal modeling and geomorphological analysis [79,80]. The combined dataset addresses the gaps in MOLA’s coverage, improving the accuracy and reliability of Mars topography for ongoing and future exploration missions.

2.

Spectral detection

Spectral detection technology provides strong support for the study of water ice on Mars by analyzing the absorption features at specific wavelengths in the reflected spectra. This method can distinguish water ice from other minerals and gases, such as dry ice, rocks, and soil. By observing spectral data across different wavelengths, scientists can reveal the distribution of water ice on the Martian surface. For example, the Visible and Infrared Mineralogical Mapping Spectrometer (OMEGA) aboard the Mars Express spacecraft confirmed the presence of water ice in the Martian south polar cap through near-infrared data, and for the first time, detected hydrated minerals on Mars, such as phyllosilicates and sulfates, whose crystal structures contain water [82]. The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard the Mars Reconnaissance Orbiter (MRO) has tracked the seasonal sublimation process of Martian water ice by comparing spectral data obtained at different times. For instance, Byrne et al. [83] used CRISM and HiRISE images along with spectral data to study exposed subsurface water ice in the mid-latitude regions of Mars, discovering that the water ice undergoes sublimation and migration as a result of seasonal changes.

Compared to imaging observations, spectral detection offers a range of unique advantages in the study of water ice on Mars. Firstly, spectral detection can directly provide information on the chemical composition and mineral characteristics of substances. By analyzing the reflected spectra at different wavelengths, spectrometric instruments can accurately identify and distinguish water ice, minerals, and other materials on the Martian surface. Additionally, spectral detection possesses strong quantitative capabilities. By obtaining reflectance data across various wavelengths, scientists can quantitatively analyze the content, distribution, and other physicochemical properties of water ice.

However, spectral detection also has certain limitations. One key issue is surface obstruction, which may affect the accuracy of the detection results. Environmental factors on Mars, such as dust, sand dunes, or cloud cover, may interfere with the reflection and absorption of spectral signals. Furthermore, despite the high-resolution data that spectral detection can provide, there are still spatial resolution limitations when detecting small or localized bodies of water ice. Some small-scale distributions of water ice may be difficult to detect effectively. Another challenge in spectral detection is the phenomenon of spectral mixing, where the diverse minerals and rock types on Mars’ surface may produce spectral features similar to those of water ice, making it more complex to differentiate water ice from other substances in the spectral data.

3.

Thermal analysis

Thermal analysis methods, by measuring the thermal energy radiated by objects, have become one of the key technologies for verifying the presence of water ice on Mars. Different materials emit thermal radiation at specific wavelengths depending on their temperature. Water ice has unique absorption and emission characteristics in the thermal infrared spectrum, distinct from other minerals. By analyzing these thermal radiation signals, scientists can infer the composition and state of materials, thereby confirming the presence of water ice. In 1996, the thermal emission spectrometer (TES) aboard the Mars Global Surveyor (MGS) conducted the first measurements of Mars’ surface in the thermal infrared and visible near-infrared spectra, providing critical data for studying the distribution and history of Martian water ice [21]. The TES not only helped scientists identify the mineral composition of Mars’ surface but also offered key insights into the planet’s water history. For example, the TES was used to study the mineral composition of the Nili Fossae region, which is primarily composed of olivine, with some surface crusts potentially formed by the cementation of mineral-rich liquids. Additionally, the TES helped scientists monitor the temporal changes in the surface temperature of Mars, accurately defining the boundaries of the planet’s polar ice caps [34].

In 2001, the Thermal Emission Imaging System (THEMIS) aboard the Mars Odyssey spacecraft significantly improved the resolution of temperature data [84]. Compared to the TES, the THEMIS provides more precise temperature measurements, enabling scientists to monitor changes in the Martian polar ice caps with greater accuracy. The THEMIS not only provides high-resolution surface temperature data but also supports the analysis of dynamic changes in water ice and its spatial distribution. Through the thermal–physical property measurements and simulations of the Martian south polar cap by the THEMIS and TES, scientists confirmed the presence of water ice in the southern polar cap. In Figure 14, the Mars Odyssey Thermal Emission Imaging System (THEMIS) daytime infrared (IR) global mosaic image (version 12) released by Arizona State University is presented, with a resolution of 100 m per pixel. This image shows the daytime temperature distribution on the surface of Mars, and the data are processed through stretching and blending to present a more continuous visual effect. This mosaic image was released in the summer of 2014 and provided by the Center for Astrogeological Sciences of the United States Geological Survey (USGS).

Thermal analysis techniques have provided important high-resolution temperature data for studying the distribution and dynamic changes in water ice on Mars. However, these methods also face certain challenges. The thermal radiation characteristics of different materials on the Martian surface (such as rocks and dust) may interfere with data interpretation, adding complexity to the analysis. Additionally, the limited coverage and field of view of the instruments may result in some regions being inadequately monitored, potentially hindering in-depth studies of water ice in specific areas.

4.

Neutron detection

Neutron detection technology is one of the key methods for studying the distribution of water ice on Mars. By monitoring the hydrogen abundance in the Martian surface, scientists can infer the distribution and dynamic changes in water ice. Since 2001, when NASA’s Mars Odyssey spacecraft equipped with a neutron detector was launched, this technology has played a crucial role in Mars water-ice research. The Neutron Spectrometer (NS) aboard Mars Odyssey is specifically designed to detect fluxes of thermal neutrons, epithermal neutrons, and fast neutrons. The basic principle of neutron detection is based on the interaction between cosmic rays and Martian surface materials. When high-energy cosmic rays strike the Martian surface, they generate different types of neutrons. Some of these neutrons, with high energy, escape into space and are termed fast neutrons; others collide with atoms in the Martian surface, forming epithermal and thermal neutrons. In particular, when neutrons collide with hydrogen atoms, their velocity significantly decreases, resulting in thermal neutrons. This characteristic enables the effective detection of hydrogen, making it possible to identify the presence of water ice. By analyzing the changes in neutron flux, the neutron detector can precisely assess the hydrogen content of the Martian surface and further infer the distribution of water ice.

The neutron spectrometer on Mars Odyssey has detected significant numbers of surface hydrogen signals globally, strongly indicating the presence of water ice on the Martian surface. Water-ice distribution varies significantly across different latitudes. Specifically, in regions south of 60° latitude (such as Terra Sabaea, around the Elysium volcano, and the northwest of Terra Sirenum), water ice is more widely distributed. In areas north of 60° latitude, the ice content is even higher, with the ice concentration reaching nearly 25% at 70° latitude. In polar regions, the concentration of water ice approaches 100%, and much of the ice is covered by dust or rock layers, which could affect its stability. Neutron detection technology not only reveals the widespread distribution of water ice but also provides important information for studying its dynamic changes. Mars Odyssey’s observations suggest that the concentration of water ice on the Martian surface is influenced by seasonal changes, particularly in the polar regions. Due to the instability of ice on the Martian surface, water ice may migrate with seasonal variations, further supporting the dynamic nature of Mars’ water resources. Additionally, data from the neutron detector have revealed the water ice content in the Martian surface soil. Near the equator, the water-ice content in the Martian soil is relatively low, ranging between 2% and 10%, while in high-latitude regions, particularly in the polar areas, the concentration of water ice in the soil increases significantly, with certain areas approaching 50%. These data provide scientists with spatial and temporal information on the distribution of Mars’ water resources, further confirming the presence of substantial amounts of water ice on the Martian surface.

The Dynamic Albedo of Neutrons (DAN) aboard the Curiosity rover, as one of the detection tools, estimates the distribution of underground water and water-bearing minerals by measuring the hydrogen content beneath the Martian surface. In 2013, the DAN instrument analyzed samples from the Bradbury landing site to a depth of 60 cm in the Yellowknife Bay area, successfully providing evidence of underground water. By detecting changes in the hydrogen content in the Martian surface layer, the DAN has revealed potential regions for subsurface water distribution [86,87,88,89]. Mars Odyssey also carries a Gamma Ray Spectrometer (GRS), which can analyze the Martian surface water-ice distribution through neutron detection data. According to data obtained in 2002, the Martian surface’s water is equivalent to a global water layer (GWL) at a depth of 0.5 to 1.5 km, offering valuable insights into the global distribution and seasonal variations in Martian surface water resources.

Neutron detection technology offers significant advantages, particularly for large-scale regional surveys. By monitoring variations in hydrogen abundance across different regions, scientists can uncover the distribution of water ice and its dynamic evolution. However, the technology also faces certain limitations. For example, the neutron detectors require complex calibration processes to accurately differentiate between different types of neutron signals. Furthermore, the composition of Martian surface materials, dust, or rock layers may interfere with the accuracy of neutron detection signals, necessitating the use of complementary detection methods for validation.

5.

Radar detection

On Earth, radar detection technology is one of the most valuable methods for detecting liquid water under glaciers. It mainly uses antennas to emit and receive electromagnetic waves to detect the characteristics and distribution of substances inside the medium. Radar emits electromagnetic waves to ground targets, and some of the energy is reflected by the surface, while more propagates in the medium below the surface and is reflected and transmitted again at different medium interfaces, enabling the radar to distinguish different medium layers [90,91,92,93,94]. Radar detection technology has been widely applied to the internal structure information of the Earth’s polar ice caps and the detection and research of subglacial lakes. Using ice radar observation data, previously undiscovered subglacial lakes under polar ice caps have been discovered [95,96]. Using a combination of qualitative and quantitative analysis methods, liquid water has been detected beneath ice sheets in Antarctica and Greenland [97,98]. Researchers have discovered subglacial lakes in eastern Antarctica and proposed specific standards for lake detection and classification [99], which were applied to subglacial lake detection in Greenland [100]. Recently, previously undiscovered subglacial lakes have been discovered in Greenland using this standard [101]. Given the powerful detection capability and advantages of radar remote-sensing technology, this method is particularly suitable for detecting the internal and underground structural information of subsurface sediments in extraterrestrial bodies such as the Moon and Mars, and it has become the main observation method for detecting water and ice in extraterrestrial bodies [102]. At present, radar detection of Martian water-ice information mainly includes three methods: ground-based radar, rover ground-penetrating radar, and orbiter detection radar.

(1)

Ground-based radar

Ground-based radar is the initial method of applying radar technology to Mars exploration. It is a high-power active radar observation that emits electromagnetic waves of a specific frequency from ground equipment to deep space exploration targets. When the electromagnetic waves encounter the surface and internal dielectric constant discontinuity interface of the target celestial body, they are reflected. By using the received signal, the surface structure and surface dielectric characteristics of deep space targets are studied. In 1963, humans first used ground-based radar to obtain echoes from the surface of Mars. Afterwards, this technology obtained information on physical properties such as the surface morphology, reflectivity, roughness, and dielectric constant of Mars during its opposition to the Sun [91,103]. Due to the limitations of the ground-based observation time and resolution, ground-based radar echo signals are weak, making it difficult to obtain effective subsurface information from strong electromagnetic background noise. But after Mars exploration enters the stage of orbiting and landing exploration, ground-based radar detection can play an auxiliary role in selecting landing areas [104].

(2)

Ground-Penetrating Radar

Ground-penetrating radar is mainly used on Mars rovers during Mars exploration missions to conduct in situ observations of the subsurface of Mars. In the early Mars exploration missions by Russia, France, and the European Space Agency, Mars rovers were equipped with ground-penetrating radar. However, due to the cancellation or failure of these missions, subsurface exploration using Mars rovers with ground-penetrating radar has yet to be successfully carried out [105,106,107]. The US Mars 2020 ‘Perseverance’ and China’s ‘Tianwen-1’ successfully arrived on Mars in 2021. Both rovers are equipped with vehicle-mounted ground-penetrating radar, which can provide in situ observation data for the exploration of Martian subsurface structures. Combined with remote-sensing observations from orbiter radar, satellite–ground mutual verification can be carried out to obtain fine subsurface structural information and deepen the understanding of Martian subsurface structures. However, due to the functional limitations of the activity range of the inspector, it can only obtain small-scale in situ observation information and cannot obtain subsurface structural information covering the whole world.

(3)

Surrounder detection radar

The orbiter subsurface detection radar is currently the only remote-sensing instrument capable of studying the subsurface of Mars from orbit. The radar signal can penetrate soil, ice, rocks, and other media to a certain extent, detect the subsurface structure of Mars from a few meters to several kilometers, obtain geological stratification information of Mars’ subsurface, and provide scientific evidence for detecting the distribution and abundance of water ice on Mars, as well as the origin and evolution history of Mars [108,109].

At present, typical subsurface exploration radars for Mars orbiters are mainly radar observation instruments carried by European and American Mars exploration missions, as well as China’s Tianwen-1 Mars exploration mission. The radar observation instruments carried in the European and American Mars exploration missions mainly include the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) launched by the European Space Agency in 2003, and the Shallow Subsurface Radar (SHARAD) carried by the Mars Reconnaissance Orbiter launched by NASA in 2005 [110]. MARSIS is a multi-band radar detector with a main frequency range of 1.3–5.5 MHz, a signal bandwidth of 1 MHz, and a wavelength of 55–230 m. It can detect the subsurface structure of the Martian crust within a range of about 3–5 km. It is mainly used to search for water ice in the Martian subsurface and study charged particles in the Martian ionosphere [111,112,113,114,115,116]. The center frequency of the SHARAD radar is 20 MHz, the bandwidth is 10 MHz, and the wavelength is 12–20 m. It is used to explore the surface structure of the Martian subsurface at a depth of about 1–3 km and search for global groundwater ice on Mars [110,117,118]. MARSIS has the characteristics of a low operating frequency, wide bandwidth, long wavelength, and deep detection depth, while SHARAD has the characteristics of a high operating frequency, wide bandwidth, short wavelength, and shallow penetration depth. Due to the influence of background noise, at the same depth, the echo received by SHARAD will be weaker than that of MARSIS. However, in terms of distance resolution, trajectory resolution, and intersection resolution, SHARAF’s detection accuracy is significantly better than that of MARSIS.

The European and American Mars exploration mission orbiter detection radar has been operating for nearly 20 years and has obtained rich information on the subsurface structure of Mars. The orbiter orbit radar detector mainly uses electromagnetic responses based on different media (polar water ice, dry ice, layered sediments, etc.) to invert scientific information such as the dielectric constant, attenuation coefficient, and depth of the subsurface interface of subsurface materials using subsurface echo information. It speculates on the material composition below the Martian subsurface and provides a scientific basis for the detection of water ice in the Martian subsurface.

In mid-latitude regions, according to MARSIS data, the reflectance of the global surface of Mars at a frequency of 3–5 MHz indicates that the dielectric constant is higher in mid-latitude regions (20–40°) and lower in equatorial and high-latitude regions. The reflectance decreases along the polar direction in the latitude range of 50–60°, corresponding to the appearance of water ice in the soil [119]. According to SHARAD exploration data, in the eastern part of the Hellas impact crater at mid-latitudes in the southern hemisphere of Mars, the main component of the tongue-shaped debris slope is water ice [120]. In the southern and Malea Planum regions, the underground ice layer is thinner or less pure than that in the northern plains [121,122]. In the Utopia Lowland of mid-latitudes in the northern hemisphere of Mars, the SHARAD exploration data combined with the digital elevation model of the Martian surface measured by the Mars Orbital Laser Altimeter (MOLA) estimated the water-ice content in the region to be between 50% and 85% [123].

In low-latitude regions, based on radar detection data from MARSIS, it was found that the Medusae Fossae Formation (MFF) in the equatorial region of Mars is rich in water ice, but the content of impurities and soil is higher than that in polar sedimentary layers [124]. The data from SHARAD also indicate that the upper surface layer of MFF at several hundred meters has higher porosity, and the radar echo is weaker than in general plain areas, which may be due to greater surface roughness and lower near-surface magnetic permeability [125]. Based on the layered information of the subsurface structure inverted from SHARAD data, combined with the High Resolution Imaging Science Experiment (HiRISE) camera and DTM data, recent studies have found underground passages filled with water-ice mixtures in the central region of the Elysion Plain [110].

In the Martian polar regions, the Martian Arctic Plateau is the first exploration area of MARSIS, and its exploration data reflect the presence of a large amount of high-purity water ice with a low dust content in the North Polar Layered Deposits (NPLDs) of the Martian Arctic [126]. The estimated water-ice content is 7.8 ± 1.2 × 10⁵ m³, and the water-ice content in the bedrock unit is estimated to be 4.5 ± 1.0 × 10⁵ m³. The total water-ice content in the Martian Arctic is estimated to be 1.3 ± 0.2 × 10⁶ m³ [127]. The dust content in the Gemina Lingula area is 5% or lower [128], generally not exceeding 6% [129]. The detection of SHARAD in the Arctic Plateau also showed layered interfaces of different compositions of ice, dust, and sandstone subsurface media [130]. For the Martian Antarctic Plateau, MARSIS exploration data show that the South Polar Layered Deposits (SPLDs) are composed of water ice with an impurity content of no more than 15% [131], with a total content of about 1.6 × 10⁶ m³ and an average thickness of about 11 m on the Martian surface [132]. Using MARSIS radar detection and processing methods based on the detection of subglacial lakes in the Earth’s polar ice caps, evidence of the first liquid lake under an Antarctic glacier was obtained [133]. Recently, new data from 2010 to 2019 once again obtained a highly reflective and very flat radar echo reflection interface under the ice sheet, confirming the previous conclusion that there may be a lake with high salt and high chloride salt brine under the ice sheet [112]. Using SHARAD data, carbon dioxide ice layers hundreds of meters thick were detected at the top of SPLDs [134], and the three-dimensional structure of the subsurface layer in the Antarctic Prometheus tongue region helped to explain the sedimentary and erosion evolution history of the area [135]. It is estimated that the total volume of ice detected on Mars is equivalent to 34 m of ice on the entire Martian surface, of which 22 m is located beneath the polar subsurface [132,136].

China’s Tianwen-1 mission has advanced Mars water-ice detection through its dual-platform radar system [61,137]. The Mars Orbiter Subsurface Investigation Radar (MOSIR) radar utilizes dual-polarization (HH/HV) and multi-band detection (27.5 MHz frequency separation) to analyze ice dielectric properties via Stokes parameters, achieving a meter-scale resolution at 1.8–3.5 km depth with enhanced sensitivity compared to single-polarization radars like MARSIS/SHARAD [138,139]. The Zhurong rover’s RoSPR (Rover mounted Subsurface Penetrating Radar) combines low-frequency (1 km depth/meter resolution) and high-frequency (30 m depth/cm resolution) modes to enable multi-scale 3D imaging of subsurface stratigraphy. The mission pioneered bistatic radar coordination, where MOSIR signals reflected from Martian subsurface interfaces are captured by RoSPR, significantly improving water/dry ice identification [11]. In situ data from Utopia Planitia’s southern basin revealed layered structures suggesting episodic aqueous sedimentation during the Late Hesperian–Amazonian period [140]. While pure water ice remains undetected to an 80 m depth, potential brine–ice mixtures offer new insights into Martian hydrologic evolution. With its balanced depth–resolution capabilities and polarization sensitivity, Tianwen-1’s radar system fills critical observational gaps, providing a high-precision framework for studying the Martian water-ice distribution and habitability.

In addition, the “Perseverance” Mars Subsurface Experiment Radar Imager (RIMMAX) has introduced new technological advancements, using a frequency range of 200 to 800 MHz, capable of detailed exploration of underground structures with a centimeter-level resolution. The real-time data-processing capability enables it to operate efficiently even in extreme environments on Mars, greatly enhancing its ability to detect water ice, rock formations, and potential saltwater layers.

In summary, radar observation technology has produced valuable research results in the exploration of water ice on Mars. Humans have obtained valuable exploration information and made research progress on whether there is water ice and whether there are other environmental conditions that life relies on for survival on Mars.

6.

Other detection technologies

In addition to common methods such as remote-sensing imaging, spectral analysis, thermal analysis, neutron detection, and radar sensing, Mars orbiters and landers have also conducted in-depth studies of Mars’ atmosphere, soil, environmental humidity, and subsurface water ice by carrying specialized scientific payloads and applying advanced technologies. These studies have not only provided new evidence for the presence of water resources on Mars but also deepened our understanding of Mars’ water cycle and climate processes.

For example, the Mars Atmospheric Water Detector (MAWD) onboard the Pioneer orbiter conducted a global survey of the distribution of water vapor in the Martian atmosphere, revealing seasonal variations and vertical distribution patterns [141,142]. The research found that, at Mars’ aphelion, water vapor sublimated from the polar ice caps mainly stays in the lower atmosphere, while at perihelion, water vapor from the southern polar ice cap can reach higher atmospheric layers. These findings help to understand the dynamic variations in water resources on Mars and provide new insights for the study of water-ice resources. Additionally, thermal inertia data provided by the Mars Climate Sounder (MCS) further confirmed the depth distribution of subsurface water ice layers, offering valuable information for the potential development of Martian water-ice resources.

Meanwhile, the Environmental Monitoring Station (REMS) onboard the Mars rover continuously monitors surface temperature, humidity, and other environmental conditions, revealing significant fluctuations in surface humidity and an overall dry environment [143]. This provides key data to support the study of Mars’ surface water cycle, climate change, and the assessment of the suitability of the Martian environment for human exploration. Based on long-term climate observations, the Trace Gas Orbiter (TGO) from the European Space Agency, with its NOMAD detector, further revealed the seasonal transport and vertical distribution characteristics of Martian water vapor, offering valuable data for a deeper understanding of Martian water vapor dynamics and climate change.

During the Curiosity rover’s mission, the Mars Sample Analysis Suite (SAM) provided crucial support for the study of Martian water resources. SAM analyzed the chemical composition of soil samples to explore the water content in Martian soil. In October 2012, SAM conducted the first X-ray diffraction analysis of Martian soil, identifying minerals such as feldspar, pyroxene, and olivine, suggesting that Martian soil is similar to certain weathered basaltic soils on Earth. SAM also detected sulfur, chlorine, and water molecules in the soil, further supporting the possibility that Mars once had water.

4. Discussion

4.1. Comparison of Different Detection Methods

Through a comparative analysis of various methods for detecting water ice on Mars, it becomes clear that each technology has its own distinct characteristics in terms of spatial resolution, detection depth, and data accuracy, and these methods often complement each other. For example, remote-sensing imagery and elevation data (such as those from HiRISE and CTX) provide high-resolution surface images of Mars, revealing the distribution of water ice in surface regions, especially in areas with significant geomorphological changes, such as shallow water ice exposed on steep cliffs [76]. However, while remote-sensing imagery can reveal surface information, it cannot directly detect subsurface water-ice resources. Therefore, combining neutron detection (such as the neutron spectrometer on Mars Odyssey) with radar detection (such as the MARSIS radar) allows for deeper probing beneath the Martian surface, providing detailed information about subsurface water ice and its distribution. Unlike these indirect detection methods, spectral detection (such as CRISM and OMEGA) offers chemical composition data on surface water ice, revealing phase changes in water ice and its interactions with other minerals [144]. This chemical analysis is crucial for studying the long-term stability and seasonal variations in water ice. By combining thermal analysis techniques (such as thermal infrared data from Mars landers), scientists can further track the sublimation process of water ice and verify its dynamic changes under different seasonal and environmental conditions.

Although neutron detection and radar technologies play a key role in subsurface water-ice detection, these methods still face some challenges. Radar wave reflections are heavily influenced by the Martian geological layers, soil density, and the distribution characteristics of water ice, leading to a certain degree of error in the detection process. Neutron detectors, which rely primarily on variations in hydrogen abundance to infer the presence of water ice, can effectively detect both surface and subsurface water ice, but they cannot directly identify the phase state of the water ice or its precise depth distribution. To overcome these technical limitations, combining the strengths of different detection methods, such as neutron detection with spectral data, will help provide more comprehensive and accurate subsurface water-ice data. Meanwhile, ground sampling and lander-based surveys also provide direct evidence for the study of Martian water ice. For example, the Sample Analysis at Mars (SAM) instrument aboard the Curiosity rover analyzed the chemical composition of Martian soil samples and confirmed the presence of water molecules and hydrated minerals [145]. These ground-based data not only validate the results from other remote-sensing methods but also provide valuable direct evidence for studying the composition and stability of water ice. However, the limitations of lander surveys and the localized nature of ground sampling mean that this method cannot cover the Martian surface as broadly as remote-sensing technologies, thus serving more as a verification tool rather than an independent detection method.

In summary, a single detection method is insufficient to fully understand Martian water ice. By combining multiple technologies, scientists can obtain more comprehensive and three-dimensional data, allowing for more accurate mapping of the water-ice distribution on Mars. For instance, combining neutron detection with spectral data can better study the depth distribution and physicochemical properties of water ice; integrating thermal analysis with spectral data helps track the dynamic changes in water ice and investigate its long-term stability. Additionally, the combination of remote-sensing imagery and elevation data can offer a macro-scale spatial distribution map for Martian water ice, helping to identify key areas for further investigation, while the integration of neutron detection and radar technology provides a more in-depth perspective for subsurface water-ice detection. With the development of technology, future Mars water-ice exploration missions will not be limited to existing methods but will adopt more diversified and precise technological combinations to ensure a deeper understanding and broader application of Martian water ice.

4.2. Prospects and Future Work

The future development of Mars water-ice detection technologies will focus on more precise detection methods, innovative data fusion techniques, and groundbreaking tools to address the unique geological environment of Mars. In this regard, the SWIM (Surface Water Ice Mapping) project offers new perspectives for future water-ice exploration. The SWIM project quantifies the consistency between multiple independent data sources—such as radar, thermal infrared, and optical imagery—and the presence of ice, in order to assess the potential distribution of water ice. By employing a consistency mapping approach, the SWIM project integrates topography, geological features, and other relevant data, allowing for more accurate predictions regarding the potential water-ice distribution, particularly in the polar regions and specific latitude bands.

This method is designed to overcome challenges associated with independently inferring the presence of shallow ice using a range of technologies, each of which has distinct detection depths, footprints, and sensitivity to ice presence. The datasets utilized in the SWIM project range from neutron probes and radar surface analysis to thermal and geological mapping. Each technique has unique strengths, such as neutron data’s sensitivity to even low concentrations of water ice, and radar data’s capacity to detect surface and subsurface features up to several meters deep. However, the challenge lies in combining these diverse datasets into a cohesive map of water-ice presence. To address this, the SWIM project uses a consistency value (C), calculated from the agreement between multiple technologies, with a scale that ranges from −1 (indicating no consistency with ice) to +1 (indicating complete consistency). This method does not rely on binary results (ice/no ice) but instead allows for nuanced interpretation, where C values above 0.2 suggest moderate confidence in ice presence, and values above 0.5 signify high confidence in ice consistency across multiple techniques [146].

In addition to its use of remote-sensing technologies, the SWIM project also takes into account dynamic features of the Martian surface. For instance, the study tracks areas with consistent ice signatures and compares them to areas of recent impact events, confirming that higher consistency values are more likely to correlate with exposed ice deposits. This method also allows for the quantification of ice layer thickness, with SHARAD radar being instrumental in detecting buried ice. By identifying low-dielectric-constant regions and converting radar reflections to depth estimates, the SWIM project is enhancing its ability to generate more accurate models of ice distribution, thickness, and accumulation. The result is a more reliable map of potential water-ice resources on Mars, offering insights into areas that could support future exploration and resource utilization.

At the same time, InSAR (Interferometric Synthetic Aperture Radar) technology will play a crucial role in future Mars water-ice detection. By detecting surface deformation, InSAR can help identify the transitions between ice and water. This technology offers precise monitoring of ice sheet thickness and the dynamics of Martian polar ice caps, providing a unique perspective on changes in the distribution of subsurface water ice. InSAR will be especially valuable for observing surface deformation, revealing interactions between ice and the Martian environment and supporting the interpretation of ice behavior in response to seasonal or long-term climate variations.

Moreover, Mars’ gravity data, especially time-varying gravity data, will play a key role in future water-ice detection [147]. Similar to gravity studies on Earth, time-varying gravity can reveal dynamic processes such as ice sheet mass balance, hydrological cycles, and climate change. These data will provide valuable insights into the long-term variations, sources, and accumulation mechanisms of Mars’ water ice. By tracking changes in gravity, scientists will be able to infer the movement and accumulation of ice at various depths, improving our understanding of how water ice interacts with Mars’ changing climate. Time-varying gravity monitoring will become a crucial tool for understanding these phenomena, revealing the complex relationship between Martian climate factors—such as dust cycles—and water-ice distribution.

The exploration of subsurface water on Mars is crucial for identifying potential habitats for life. While radar detection systems, such as SHARAD and MARSIS, offer deep penetration capabilities, they are currently the only tools available for global subsurface exploration. Ground-penetrating radar, carried on rovers such as NASA’s Perseverance, China’s Tianwen-1, and the European Space Agency’s Rosalind Franklin rover, can provide more detailed surface mapping but is limited in terms of penetration depth and coverage. However, radar’s relatively higher frequency provides a better resolution, although it cannot match the penetration depth of SHARAD. Furthermore, these ground-based systems lack the ability to provide the large-scale coverage needed for a global search. As a result, instrument limitations continue to hinder a comprehensive understanding of subsurface environmental parameters. Further investigations remain imperative to resolve the critical question of groundwater existence on Mars. In addition, manual interpretation of subsurface radar imagery presents substantial challenges [148,149]. With rapidly growing volumes of Martian exploration data, artificial intelligence (AI) techniques hold promise for automated information extraction, thereby enhancing computational efficiency and analytical precision [135,150,151].

Another key electromagnetic method for subsurface exploration is Time Domain Electromagnetic (TDEM) sounding [108]. TDEM sounding works by inducing eddy currents in the subsurface and measuring the magnetic fields generated by these currents. This technique can be used to determine subsurface conductivity, which increases significantly in the presence of saline water. While TDEM sounding can achieve greater depth penetration than GPR, it is not suitable for orbital platforms due to the large size of the induced current loops required for deep sensing. As a result, TDEM sounding is not feasible for Mars exploration from orbit.

Looking ahead, future technological advancements will also involve the application of new technologies, such as small satellites and superconducting gravity gradiometers. Small satellites, with their flexibility in deployment and distributed gravity measurement capabilities, can provide more precise detection results than traditional single satellites. These satellite systems can conduct more dense and comprehensive gravity measurements, offering more detailed spatial data on the distribution of water ice on Mars. The introduction of superconducting gravity gradiometers will significantly enhance the sensitivity of gravity measurements. In particular, when detecting the distribution of subsurface water ice, their accuracy and resolution will be greatly improved, providing clearer data to support the detection of water ice on Mars.

5. Conclusions

Our comprehensive review of Mars water-ice detection technologies reveals the current state of research and the ongoing challenges faced in this field. In recent years, the study of Martian water ice has garnered increasing attention, particularly in the context of supporting future Mars missions and resource utilization. This systematic review covers the key literature on Mars water-ice detection from recent years, focusing on the application of various technological methods, including remote-sensing imagery, spectral detection, thermal analysis, neutron detection, and radar detection. Through the analysis of selected relevant studies, we find that radar detection and remote-sensing imagery remain the mainstream choices, although neutron detection and thermal analysis have gradually emerged in recent years. The continuous development of radar detection technology has progressively improved the precision of detecting subsurface water ice on Mars, with breakthroughs particularly in detecting deeper water-ice layers. Meanwhile, neutron spectrometer technology has played a significant role in inferring the hydrogen distribution on the Martian surface, providing scientists with indirect evidence of the presence of water ice. Nonetheless, these technologies still face certain challenges, particularly in terms of improving detection depth, reducing errors, and enhancing data analysis capabilities.

Finally, our review highlights some key issues in the field of Mars water-ice detection. Technical limitations, especially in detection accuracy, depth, and data-processing speed, remain the primary challenges in the research. The complex terrain, dynamic changes in Mars’ climate, and extreme surface conditions make the detection of water ice even more difficult. This calls for the continued development of more refined and efficient detection methods, while also enhancing the adaptability and reliability of existing technologies.

Future Mars water-ice detection technologies will rely on higher-precision radar, neutron spectrometers, in situ analysis instruments, and high-resolution gravity measurement techniques, which will break through the current limitations in depth and resolution, providing more accurate data on water-ice distribution. The application of new sensors and data fusion technologies will further enhance the accuracy and comprehensiveness of detection, while autonomous detection and sample return technologies will support in-depth research on the composition and evolution of water ice. The development of these technologies will greatly advance our understanding of Mars’ water ice resources, providing crucial technological support for future scientific research and Mars exploration missions.

In addition, with the rapid development of artificial intelligence and machine learning technologies, future Mars water-ice detection missions will increasingly rely on autonomous detection and data analysis technologies. These technologies can improve mission efficiency, reduce communication delays, and enhance the adaptability of Mars exploration missions in complex environments. Next-generation geodesy could be achieved without relying on new technologies, but new technologies are expected to bring more transformative research advancements. Examples include small satellites and radio beacons for synchronized and distributed gravity measurements; superconducting gravity gradiometers; and airborne processing technologies for handling large-scale data, particularly in experiments such as InSAR.