Characteristics and Climatic Indications of Ice-Related Landforms at Low Latitudes (0°–±30°) on Mars

Abstract

The deposition and evolution of ice-rich materials on Martian surfaces offer valuable insights into climatic evolution and the potential driving forces behind global climate change. Substantial evidence indicates that the mid-latitudes of Mars played a crucial role in the formation and development of glacial and periglacial landforms during the Amazonian period. However, few studies have comprehensively examined ice-related landforms in the low-latitude region of Mars. Whether extensive glacial activity has occurred in the equatorial region of Mars and whether there are any potential geological records of such activities remain unclear. In this study, we analyzed remote sensing data from the Martian equatorial region (0°–±30°) and identified existing glacial/periglacial features, as well as remnant landforms of past glaciation. Our findings reveal that glaciation at low latitudes is more widespread than previously thought, with ice-related remnants extending as far equatorward as 13°N in the northern hemisphere and 19°S in the southern hemisphere, highlighting a broader latitudinal range for ice-related landforms. These landforms span multiple episodes of Martian geological history, supporting the hypothesis on the occurrence of repeated glaciation and various high-obliquity events. Evidence of dynamic interactions between ice deposition and sublimation in low-latitude regions demonstrates substantial ice loss over time, leaving ice-related remnants that provide valuable insights into Mars’ climatic evolution. Based on volumetric estimates of the concentric crater fill (CCF), the low-latitude regions of Mars may contain up to 1.05 × 10³ km³ of ice. This corresponds to a global equivalent ice layer thickness ranging from 21.7 mm (assuming a pore ice with 30% ice content) to 65.1 mm (assuming glacial ice with 90% ice content), suggesting a potentially greater low-latitude ice reservoir than previously recognized.

1. Introduction

Numerous studies have emphasized the critical role of the mid-latitudes (±30°–60°) of Mars in the formation and evolution of glacial and periglacial landforms during the Amazonian period (~3.1 Ga–present) [1,2,3,4,5]. These regions host a variety of glacial landforms (Figure 1), including lineated valley fills (LVFs), lobate debris aprons (LDAs), concentric crater fills (CCFs), and glacier-like forms (GLFs) [6,7,8,9]. Recent studies suggest that many of these landforms were formed from debris-covered glaciers, with ice contents of up to 90% [10,11]. Radar interpretation results indicate that substantial quantities of ice are preserved in the subsurface [2,12].

Ring-mold craters (RMCs) and “brain-terrain” textures are frequently observed on the surfaces of these glacial landforms. RMCs are interpreted as the result of impacts on ice-rich substrates [7,13,14], causing plastic deformation of buried ice that uplifted the crater rim into the characteristic ringed morphology. While they currently lack direct terrestrial analogs, partially collapsed pingos in permafrost regions may bear a superficial resemblance to RMCs, although their origins differ significantly. “Brain-terrain” textures appear as irregular surface patterns on features such as LDAs, LVFs, and CCFs. Although these textures resemble polygonal patterned ground in terrestrial permafrost settings, such as Alaska and Siberia, they likely form due to different mechanisms. On Earth, patterned ground typically results from seasonal freeze–thaw processes, while Martian “brain-terrain” textures are interpreted to develop via the sublimation of ice-rich materials, thermal contraction cracking, and glacio-tectonic deformation [15]. Therefore, despite morphological similarities, “brain-terrain” textures on Mars represent a cold desert, sublimation-dominated analog rather than a freeze–thaw feature. In addition, small-scale polygonal patterns are widely distributed in the mid- to high-latitude regions of Mars and are considered to be closely related to thermal contraction and differential sublimation processes [16]. In contrast, large-scale polygons may indicate the potential presence of freeze–thaw cycles [16].

Global circulation models (GCMs) suggest that Mars’ fluctuating orbital obliquity drives the periodic sublimation of polar ice, with ice being transported to equatorial regions during periods of high obliquity [17,18]. At an average obliquity of 35°, ice is primarily stabilized at mid-latitudes, whereas at higher obliquities, such as 45°, ice can extend into equatorial regions [19]. The current interglacial period has led to unstable ice-rich materials near the equator [20,21]. Mars may have experienced multiple ice ages, with glaciers at mid-latitudes forming during periods of high obliquity, particularly in the middle to late Amazonian period [22,23]. Recent studies suggest that glacier-like forms (GLFs) represent remnants of the most recent obliquity shift, providing insights into regional glaciation and revealing stratigraphic relationships [24,25]. The “brain-terrain” textures observed in mid- to high-latitude mantle polygons reflects the sequential interaction between earlier glacial-like deposition events and subsequent modification by ice-rich mantling and thermal contraction cracking, during more recent cold-climate periods [15]. These findings support the idea of repeated glaciation during high-obliquity events.

Despite extensive research on Martian ice-related features at mid-latitudes, there is a notable gap in terms of studies focusing on low-latitude regions (0°–±30°). While some evidence suggests that the equatorial regions may have experienced glacial/periglacial activity during periods of high obliquity [26,27,28], comprehensive studies documenting the ice-associated features in these areas are lacking. Previous studies have largely concentrated on mid- and high-latitude regions, where glacial features are more prominent [4,29,30,31,32]. For example, large tropical glaciers in regions such as Tharsis Montes and Olympus Mons [33], as well as candidate ice-rich materials in Sinus Sabaeus [34], may have formed during early high-obliquity periods, but the overall extent of ice-related landforms at low latitudes remains unclear. Consequently, it remains unclear whether the absence of documented ice-related features at low latitudes indicates a lack of glacial activity or if these features have been overlooked due to insufficient coverage. Recent evidence revealed elevated hydrogen levels in central Valles Marineris, suggesting that this equatorial region may be a promising site for investigating subsurface water ice [35].

The primary goal of this study is to extend previous investigations by mapping glacier-related landforms in the low-latitude region of Mars (0°–±30°) and analyzing these features in greater detail via high-resolution remote sensing images. We focus on the spatial distribution and key parameters of ice-related landforms, including crater diameter, fill elevation, slope, aspect, and volume, and compare them to similar features found at mid- and high latitudes. By integrating datasets from different latitudes and sources, we seek to identify global patterns in the distributions of ice-related features. Additionally, chronological crater counts were conducted in representative areas to estimate the formation ages of these ice-related deposits, offering new insights into the geological and climatic evolution of Mars during the late Hesperian to Amazonian period.

2. Materials and Methods

2.1. Image Datasets and Surface Property Data

Image mosaics from the Mars Reconnaissance Orbiter (MRO) context camera (CTX; ~5–6 m/pixel) [36] and high-resolution image data from the high-resolution imaging science experiment (HiRISE; ~25 cm/pixel) [37] were used for visual interpretation and detailed geomorphological mapping. Topographic parameters, including elevation, slope, and aspect, were extracted and analyzed using the Mars MGS MOLA-MEX HRSC blended DEM (global, 200 m resolution) [38], which integrates data from the Mars Orbiter Laser Altimeter (MOLA) [39], aboard the NASA Mars Global Surveyor (MGS), and the High-Resolution Stereo Camera (HRSC; 2 m/pixel) [31] on the ESA Mars Express (MEX) spacecraft. The image databases were sourced from the Planetary Data System (PDS) Cartography and Imaging Science Discipline (CISD) Node and the Mars Orbital Data Explorer (ODE). All the CTX images covering the Martian low latitudes (0°–±30°) were systematically examined through detailed visual interpretation. This thorough, frame-by-frame assessment ensured complete coverage and consistency across the entire study area.

2.2. Mapping of a Global Database of Published Characteristics of Typical Glaciers and the Distribution of Buried Water Ice Resources

The identification and mapping of typical glacial surface features on Mars follow the frameworks summarized in previous studies [15,24,40]. Potential ice-related landforms, similar to those shown in the work of Diot et al. [41], were identified primarily as depressions and cracks within craters [42].

To distinguish glacial and periglacial landforms from morphologically similar features formed by other geological processes, we apply multiple diagnostic criteria, including morphology, context, and associated surface textures. For example, several crater floor depressions in the study area exhibit concentric talus scarp rings, linear depressions composed of pits, and crack networks. These features are interpreted as remnants of glacial ice-rich fill that underwent sublimation. Their circular symmetry, continuous talus scarp distribution, and a lack of evidence of flood discharge/fluvial erosion or slope collapse suggest that these depressions are not simply the result of gravitational mass wasting, but rather indicate progressive ice loss from crater interiors. Furthermore, their frequent occurrence in association with other glacial morphologies, such as lineated valley fills and lobate debris aprons, reinforces this interpretation. Outside of craters, the identified possible sublimation depressions (PSDNs) exhibit linear depressions and/or steep scarps, as well as depressions with concentric patterns and circular ridges, and lack fracture control or roof collapse structures typical of lava tubes or karstic subsidence features. Their geomorphic appearance, combined with their spatial clustering near known ice-related landforms (e.g., ring-mold craters), supports an interpretation that they are of an ice-related origin, most likely as surface expressions of ground ice sublimation and associated surface collapse or compaction. This integrated geomorphic analysis allows us to differentiate ice-related features from other surface processes and supports the interpretation of widespread glacial/periglacial activity at low latitudes (0°–±30°) on Mars.

To quantify crater fill in the lower latitudes of Mars, we extracted key parameters from the Mars Global ≥1 km Impact Crater Database [43] for craters intersecting our mapped regions. These parameters, including latitude, longitude, and diameter, were extracted to facilitate the calculation of the crater depth and fill volume.

Using the distribution of buried water ice resources mapped by the Mars Subsurface Water Ice Mapping (SWIM) team, we incorporated data from the geomorphology database and composite ice-consistency maps for depths of 1–5 m and >5 m [44]. These datasets are analyzed together in our discussion. We attempted to examine several glacial/periglacial features using publicly available SHARAD data; however, no clear subsurface reflections were detected. As a result, no new SHARAD radar data were processed in this study. The radar-related observations presented are based on previously published datasets and are used here for contextual interpretation.

2.3. Volume Calculation of Concentric Crater Fill (CCF)

To estimate the volume of concentric crater fill (CCF) with the most pronounced surface features at low latitudes, we employed the volume calculation method outlined by Levy et al. [22]. First, the depth of the unfilled impact crater (d) was calculated via the crater diameter (D), and this depth–diameter relationship was established by Garvin et al. [45]. Next, the depth of the fill within the crater (d_f) was determined by subtracting the fill thickness (d_m) from the depth of the unfilled crater (d). The fill thickness (d_m) was calculated as the difference between the maximum rim elevation of the crater and the average elevation of the fill within the crater. Finally, the final filled volume of the CCF was calculated by applying the cavity shape function from Garvin et al. [45] and integrating it over the crater diameter range from 0 to d_f.

2.4. Crater Size–Frequency Distribution (CSFD)

CSFD analysis [46,47] was used to estimate the absolute modeled ages of the deposits within craters in different typical regions. Small craters within these regions were identified through the use of CTX and HiRISE images. Georeferencing and measurements were performed via ArcGIS 10.8.1 software (https://www.esri.com; accessed on 1 September 2022), crater mapping was conducted via CraterTools (v2.1) [48], and the data analysis was conducted via Craterstats 2.0 software [49]. Crater identification was automated via an automatic crater detection (ACD) tool [50], supplemented by basic visual interpretation and manual corrections. To improve the reliability of the age estimates, we excluded from the statistics both craters superposed on crater wall deposits and those with diameters smaller than 30 m (d < 30 m). After excluding these factors, we applied the production function by Hartmann and Daubar [51] and the chronology function by Hartmann [47] to assess the absolute modeled ages of the deposits.

3. Results

This study surveyed entire low-latitude regions of Mars (0° to ±30°) and identified a total of 4307 ice-related landforms. These features were classified into five major morphological types, listed here in descending order by abundance: crater fills (CFs), lobate debris aprons (LDAs), possible sublimation depressions (PSDNs; non-crater forms), glacier-like forms (GLFs), and lineated valley fills (LVFs) [22].

3.1. Characteristics and Global Distribution of Ice-Related Landforms at Low Latitudes

3.1.1. Crater Fill (CF)

Crater fill (CF) features are the most abundant ice-related landforms in the lower latitudes of Mars, accounting for approximately 85% of the 4307 mapped features across the surveyed low-latitude regions of Mars. We classify CFs into four types on the basis of their morphology: concentric crater fill (CCF) (Figure 2), crater irregular fill (Figure 3), crater directional fill (Figure 4), and crater depression (Figure 5).

CCFs at low latitudes resemble those at mid-latitudes, with the crater floors displaying concentric rings or linear structures (Figure 2). The surface features commonly include “brain-terrain” textures [15] and/or RMC morphologies [13] (Figure 2b,d). Crater irregular fills exhibit ice-related textures (Figure 3), often characterized by linear, ice flow-like patterns and pits, which are likely formed by the sublimation of near-surface ice (Figure 3b). In addition, depressions and scarps near the crater floor edges resemble talus deposits, indicative of glacial retreat [6]. Unique features on the crater floors include circular mesas, degraded craters, and RMCs, although linear or “brain-terrain” textures are less common (Figure 3d). Circular mesas are generally considered erosional remnants, potentially shaped by wind or water-related processes, and, in some cases, may also be linked to sublimation-induced collapse in ice-rich terrains. On Earth, these landforms are part of a morphological continuum, akin to terrestrial plateaus, mesas, and buttes, distinguished by their size. RMCs are present on the surface of the deposited material, with cracks observed on the central mesa (Figure 3e), and a network of polygonal cracks is visible on smooth deposited surfaces (Figure 3c,e). Crater directional fills display the occurrence of deposits on the walls of polar-facing craters (Figure 4). For example, Figure 4a shows the directional filling within the interior of a double-layer ejecta rampart crater in Sabaea Terra. Crack networks are distributed across the depositional surface on the crater floor (Figure 4b). Crater depressions are widespread at low latitudes and exhibit a ring of talus, steep scarps parallel to the crater rim, linear depressions composed of pits, and linear cracks (Figure 5, Figure A1 and Figure A2). These materials are thicker near the crater center and thinner toward the rim (Figure A3). Newly observed ice-rich materials are located near 47°E, 15°N (Figure A1e).

CF features are mostly concentrated in Arabia Terra and northern Syrtis Major Planum in the northern hemisphere, as well as around northern Hellas in the southern hemisphere (Figure 6). In the northern hemisphere (15°–30° latitude), the number of CFs increases significantly with the latitude, and the types become more diverse. The CFs are also distributed west of Elysium, northwest of Amazonis Planitia, near Tartarus Colles, and around the northern edge of Hellas, with additional scattered occurrences in the southern highlands near the 30° latitude across longitudes −90° to 90°. Owing to the dichotomy of Martian north–south topography, the mean elevation of the CFs in the northern hemisphere (−1165 m) is significantly lower than that in the southern hemisphere (552 m). In the northern hemisphere, most CFs occur at negative elevations, referring specifically to the height of the deposit surface measured below the Martian datum (0 m), except in areas such as Ceraunius Fossae, parts of Syrtis Major Planum, and Elysium, where the elevations are positive. In the southern hemisphere, CFs near Uzboi Vallis and north of Hellas are generally below 0 m, whereas the CFs in other areas at similar latitudes tend to occur above 0 m, particularly north of Thaumasia, where elevations exceed 1700 m.

The majority of CCFs are concentrated in central Arabia Terra and Tartarus Colles, near the 30°N latitude in the northern hemisphere, and are also locally observed in northern Hellas in the southern hemisphere. Crater irregular fill and crater directional fill are distributed across similar latitudes, with additional occurrences between Uranius Fossae and Labeatis Fossae in the northern hemisphere. In the southern hemisphere, these types of crater fill are found south of Coprates, including eastern Solis Planum, Thaumasia Planum, and parts of Noachis Terra. In contrast, crater depressions are more abundant and geographically widespread than other crater fill types. They typically occur closer to the Martian equator, near 13°N and 19°S. In the northern hemisphere, prominent occurrences are found around Uranius Mons, localized areas of Ceraunius Fossae, the North Kasei Channel, and the Elysium Mons. In the southern hemisphere, crater depressions are most densely clustered between longitudes −95° and 110°, notably within the Sinus Sabaeus region [34]. A distinct spatial transition in crater fill morphology is observed from equatorial to mid-latitude regions, progressing from crater depressions to directional fills, then to irregular fills, and culminating in concentric crater fills at higher latitudes. This pattern may reflect latitudinal variations in climate-related ice deposition and degradation processes.

3.1.2. Lobate Debris Aprons (LDAs)

LDAs, the second largest category, account for approximately 6.6% of the total identified features and are almost exclusively distributed in the northern hemisphere (Figure 7). The morphology of LDAs at low latitudes differs somewhat from that at mid-latitudes, as their formation is constrained by specific geographical environments, possibly indicating distinct formation mechanisms. LDAs are located primarily in northwestern Amazonis Planitia, Kasei Valles, Arabia Terra, and northeastern Elysium Planitia.

In the northeastern Elysium Planitia and northwestern Amazonis Planitia, particularly around Tartarus Colles and Phlegra Dorsa, the surface morphology of LDAs becomes increasingly similar to that of mid-latitude LDAs as they approach the 30°N latitude. These LDAs exhibit an apron-like form, uniformly scattered around mesas or knolls (Figure 7a,b,d). Closer to the equator, the volume and number of LDAs decrease significantly, as shown in Figure 7c. On the western side of Orcus Patera, the apron-like morphology nearly disappears, leaving only the outermost ridge, as indicated by the black arrow in Figure 7e,f.

In Kasei Valles, LDAs exhibit topographic depressions, also known as moats, which encircle mesas or run parallel to hillocks (Figure 7g,h). Hauber et al. [52] interpreted these features as remnants of former LDAs, possibly eroded flood materials, lava flows, or debris flows analogous to volcanic mudflows, which solidified at the front edge of the LDAs. Owing to climate change, these features retreated and formed depressions over time (Figure 7g,h). In Arabia Terra, we identified outwardly parallel ridges on the inner walls of certain craters, extending from the crater rim to the base of the cliffs, and categorized them as LDAs (indicated by black arrows in Figure 7i,j), which is consistent with the classification used in the GIS datasets [11,22].

In the southern hemisphere, LDA-like features are sparse and sporadic, including moat-like structures around hills and ridges at their outermost edges (Figure A4). Possible sublimation lag deposits (Figure A4a,b) occur in the Mangala Valles [53]. In the Peta Crater and nearby craters (Figure A4c), former ice flow or sliding is suggested to have formed ridges [54,55]. This limited distribution may be due to the generally higher elevations of the southern highlands, which reduce the potential for ice accumulation and preservation compared with similar latitudes in the north.

3.1.3. Possible Sublimation Depressions (Non-Crater) (PSDNs)

In addition to crater-associated depressions, we identified depressions outside craters that may have formed due to the sublimation of near-surface ice. The surface of the area is characterized by linear depressions, which are termed “possible sublimation depressions (non-crater)” (PSDNs), based on their morphological features and spatial distribution.

The PSDNs include two subtypes: (1) linear depressions and/or steep scarps, which are typically located within valleys (Figure 8b,d,e,h), and (2) depressions, which exhibit concentric patterns and circular ridges (Figure 8f,g). The latter, as noted by Levy et al. [56], may have resulted from interactions between volcanic activity and subsurface ice. Additionally, staggered and continuous depressions are observed in Lycus Sulci, which are located near hills (Figure 8a). In Ceraunius Fossae, viscous flow material is present within graben floors, with surfaces covered by ice-rich material and ring-mold craters (Figure 8b,c). Valley-associated depressions represent the most common type of PSDNs (Figure 8b,d,e,h). These features are predominantly concentrated in the northern hemisphere, particularly near Lycus Sulci, Ceraunius Fossae, Kasei Valles, and Elysium Mons. In contrast, distinctive concentric depressions with circular ridges are rare, sparsely distributed, and found only in a few isolated locations (Figure 8f,g). No additional morphological subtypes indicative of ice-related processes were identified within the PSDN category, based on the available imagery and mapping resolution.

3.1.4. Glacier-like Form (GLF)

The surface morphology of GLFs at low latitudes is similar to that at mid-latitudes, with textures that flow along topographic gradients (Figure 9). For instance, the boundaries of GLFs are well-defined and terminate in distinct moraine-like ridges (Figure 9a,c,e,i; indicated by the black arrow). In contrast, Figure 9g,h shows less clearly defined boundaries, although the surface textures still exhibit flow patterns, aligned with the topographic gradient.

In the northern hemisphere, GLFs are predominantly distributed at an average latitude of 28.12°. They generally have small volumes and are confined within alcoves strongly influenced by the topography, primarily occurring on north-facing slopes. In the southern hemisphere, GLFs are distributed at an average latitude of 25.12°, tend to have large volumes, and are found mainly on southeast-facing slopes, along the inner or outer walls of craters (Figure 9e,f,I,j). While GLFs are slightly more abundant than LVFs, they remain relatively rare overall.

GLFs are more common in the northern hemisphere than in the southern hemisphere and are sprinkled near Olympus Mons (Figure 9a), where the broad glacial deposits have not yet been fully mapped, although individual GLFs have been identified. They are also observed in central Arabia Terra (Figure 9b) and near the north–south dichotomy of Syrtis Major, possibly indicating multiple glacial events northwest of Nili Fossae (Figure 9c,d). In the southern hemisphere, GLFs are predominantly found around northern Hellas (Figure 9e–j). Examples include moraine-like ridges in the Isil Crater [57,58] (Figure 9e), steep scarps and trim lines formed by glacial sublimation retreat and contraction in the Nako Crater [59] (Figure 9g), and craters on the northwest edge of the Terby Crater [60] (Figure 9h).

3.1.5. Lineated Valley Fills (LVFs)

At low latitudes, the morphology of LVFs resembles that observed in mid-latitude regions, with flow characteristics aligned parallel to the valley walls (Figure 10). However, the linear features on the surface of the LVFs in low-latitude regions are less distinct than those in mid-latitude areas. The occurrence of LVFs at low latitudes is extremely limited, with an average distribution near 28.3° north and south latitudes. LVFs are slightly more prevalent in the northern hemisphere than in the southern hemisphere. In the southern hemisphere, LVFs more clearly exhibit tightly spaced or folded linear features on their surfaces (Figure 10e,g,h). In the northern hemisphere, LVFs are primarily concentrated in Arabia Terra and some valleys to the east, such as the lower reaches of Auqakuh Vallis [61] (Figure 10f). In the southern hemisphere, LVFs are distributed mainly in Thaumasia Planum (Figure 10e,g,h) and the northern part of the Hellas Basin (Figure 10i,j).

3.1.6. Elevation Distribution of Ice-Related Landforms at Low Latitudes

The global mapping results are shown in Figure 11. A total of 4307 features were identified: 3653 crater fills (CFs; 84.82%), 284 LDAs (6.59%), 139 PSDNs (3.23%), 137 other types (3.18%), 79 GLFs (1.83%), and 15 LVFs (0.35%).

We analyzed the elevation distributions of these landforms via MOLA and HRSC blended DEM data, calculated the mean elevation for each 5° longitude interval (Figure A5a) and generated elevation maps for the low-latitude regions (Figure A5b). From 150°E to 225°E (−135°E), the ice-related landforms are mostly confined to the northern hemisphere near −2000 m, with elevations ranging from −4246.81 m to 1529.24 m (average −3061.71 m). From −135°E to −55°E, the average elevation of the ice-related landforms is 1726.58 m, ranging from −3213.14 m to 14,911.33 m. From −55°E to 95°E, the elevation ranges from −6530.90 m to 3082.36 m, with a mean of −824.96 m. From 95°E to 150°W, the elevation ranges from −4631.43 m to 13,624.03 m, with an average of 876.37 m.

Most ice-related landforms occur in regions with negative elevations, such as the eastern Elysium Planitia, western Amazonis Planitia, parts of Kasei Valles, Arabia Terra, and northern Hellas Basin. Ice-related landforms in other areas occur primarily at positive elevations.

3.2. Crater Fills (CFs) in Low-Latitude Regions

3.2.1. Statistical Analysis of the CFs

The slope and aspect measurements are based on a blend of DEM data. The number of CFs is notably greater in the northern hemisphere than in the southern hemisphere (Figure A6a, Figure A7 and Figure A8), with CFs mainly distributed in regions characterized by gentle slopes, based on MOLA–HRSC blended DEM data at a 200 m/pixel resolution (Figure A6b). Furthermore, the majority of fill materials are concentrated in craters less than 20 km in diameter (Figure A6c). The orientation of the CFs is determined by the directionality of the three fill types, other than CCFs. At low latitudes, the orientation of CCFs is less distinct, possibly because of the smaller number of CCFs. Crater irregular fill and crater directional fill are symmetrically distributed across the different hemispheres: the northern hemisphere predominantly features north- and northeast-facing slopes, whereas the southern hemisphere has south- and southeast-facing slopes. On the other hand, crater depressions tend to exhibit fill movement toward the east and southeast, with a surface morphology characterized by depression-dominated crater floors.

The morphological characteristics and slope variations also differ among the fill types. CCFs are typically found in craters 2–15 km in diameter, with slopes not exceeding 4°. Crater irregular fill is observed in craters 2–10 km in diameter, with fill slopes up to 6°. Crater directional fill occurs in craters of all sizes (1–30 km diameter), with slopes ranging from 1° to 13°. Crater depressions are found in relatively flat regions within craters less than 20 km in diameter and with slopes of 12° or less (Figure A7 and Figure A8).

3.2.2. Total Volume of Identified CCFs at Low Latitudes

The volume of the CCF was estimated based on the described calculation method. The total volume of CCF deposits in low-latitude regions was calculated to be approximately 1048.98 km³. To estimate the ice content in these deposits, we considered two hypotheses regarding their origin [22]: glacial ice with 90% ice content [11] and pore ice with 30% ice content [62]. Based on these assumptions, the total ice inventory of low-latitude CCFs was estimated to be 944.08 km³ for glacial ice and 314.69 km³ for pore ice, corresponding to globally equivalent ice thicknesses of 65.1 mm and 21.7 mm, respectively. No error estimations are provided for the estimated volume and ice content in this study, due to inherent limitations in the resolution of the DEM data and uncertainties associated with interpolating the full dataset [22]. As a result, the volume estimate of 30% pore space is a reasonable minimum value for these landforms, following previous studies [22]. This estimate includes only concentric fills within craters, excluding irregular and directional fills. Consequently, the actual ice content in the low-latitude region of Mars is likely even greater than these calculated values.

3.2.3. Age Estimates for the CFs

We analyzed the CSFDs of different crater fill types to estimate their model ages (

Table A1. Summary of crater size–frequency measurements for crater fill landforms.

Types	Specific Images	Area Size	Number of Craters	Estimated Age with Errors
Centric crater fill	Figure 12a	6.47 × 101 km2	132	220 ± 20 Ma
Crater irregular fill 1	Figure 12b	3.63 × 101 km2	6	140 ± 50 Ma
Crater irregular fill 2	Figure 12c	3.81 × 100 km2	14	2.8 + 0.4 Ga/2.8 − 0.7 Ga
Crater irregular fill 3	Figure 12d	2.62 × 101 km2	113	3.4 + 0.05 Ga/3.4 − 0.07 Ga
Crater directional fill	Figure A9	8.98 × 100 km2	26	4.2 ± 0.8 Ma

). The surveyed area covers approximately 140 km², within which 291 small craters were identified on the crater fill surface for chronological analysis. The model ages of the CCFs (220 ± 20 Ma, 140 ± 50 Ma) (Figure 12a,b) are broadly consistent with the depositional ages (100 Ma−700 Ma) reported for mid-latitude viscous flow glacial features in previous studies [63,64]. However, the CSFDs of irregularly filled craters reveal variations in the model ages depending on their surface morphologies. Deposits with only RMCs on the surface, lacking any flow patterns (Figure 12c,d), exhibit older model ages in the late Hesperian (~3.4 Ga) and early Amazonian (~2.8 Ga) periods. For an impact crater within the Arabia Terra region (52.49°E, 23.98°N), we analyzed the model ages for the crater directional fill (Figure A9). Although accurately measuring the age of the deposits on the polar-facing interior wall was difficult, only the region on the crater floor was selected for counting, and its modal age (4.2 ± 0.8 Ma) suggests a relatively young depositional period.

However, owing to the limited number and small size of the craters superposed on the deposit surface, the modeled ages should be regarded as approximate estimates. Furthermore, dating results may also be subject to uncertainty due to many other factors. For example, candidate craters that are too small in diameter may lead to biased dating. In addition, the accuracy of diameter identification for some RMCs with heavily degraded boundaries may also affect the results.

4. Discussion

At the global low-latitude scale, ice-related features are primarily found up to 13°N in the northern hemisphere and 19°S in the southern hemisphere. The previously observed restriction of ice indicating features restricted to latitudes above 32°N in the northern hemisphere is no longer consistent, aligning with predicted ice accumulation in the low-latitude Thaumasia region (110°W–60°W) of the southern hemisphere [66]. Levrard et al. [67] predicted that surface water ice accumulation at 40° obliquity would primarily occur near the Tharsis Volcanic Province and the Schiaparelli Basin. In the Tharsis region, water ice is likely deposited on the windward sides of volcanoes under monsoon-like atmospheric circulation [33,68]. In addition, remnant deposition in Sinus Sabaeus most likely occurred due to past obliquity-driven climate change [34].

At low latitudes, ice-related landforms exhibit less pronounced surface textures and are fewer in number and distribution than those at mid-latitudes. This is primarily because current Martian climatic conditions are unfavorable for stable accumulation and long-term preservation of ice-rich materials in low-latitude regions. In addition, intense surface degradation processes, such as aeolian activity or erosion, are more active at lower latitudes and may obscure or erode ice-related features. In many cases, any near-surface ice that may exist is likely buried beneath the regolith layer, thereby limiting its geomorphic expression on the surface. Like the GLFs and LVFs, these features are sporadically distributed in Arabia Terra and Syrtis Major near the dichotomy boundary in the northern hemisphere and in Thaumasia Planum and northern Hellas in the southern hemisphere. LDAs, predominantly located in Tartarus Colles and Kasei Valles, with some occurrences in Arabia Terra, are interpreted as glacial remnants at low latitudes [52]. In contrast, the CFs and PSDNs extend closer to the equator and exhibit broader distributions. Ice-related landforms in the northern hemisphere generally occur at significantly lower elevations than those in the southern hemisphere and are likely influenced by the Mars global dichotomy and atmospheric circulation patterns, which suggests a link between the local topography and favorable conditions for glacial ice accumulation and preservation [69]. A detailed analysis of the CFs revealed that northeast-facing (northern hemisphere) and southeast-facing (southern hemisphere) orientations dominated at low latitudes. This orientation may reflect local climatic and insolation conditions, allowing ice to persist in colder crater regions [5,31,70].

Figure 13 integrates previously published datasets with our results, illustrating the distribution of ice-related landforms on Mars from mid-latitudes to the equator. The two columns on the right of the legend represent prior datasets [8,22,71], which focus primarily on glacial features between 30° and 60° north and south latitudes. Our study extends this mapping into low-latitude regions, addressing a significant data gap. The comparative analysis reveals that our low-latitude results align well with the previous mid-latitude data near 30°N and 30°S, indicating spatial continuity in the landform distribution. Importantly, we introduce newly defined subcategories of ice-related landforms in low-latitude regions. Within the crater fill (CF) category, three subtypes, excluding concentric crater fill (CCF), are newly proposed in this study, based on distinct morphological patterns not previously classified as glacial or periglacial features. In addition, the total number of ice-related landforms mapped at low latitudes in our dataset exceeds that reported in earlier works. These additions provide a more comprehensive and refined characterization of Martian glaciation at low latitudes, enhancing our understanding of the spatial and morphological diversity of ice-related processes beyond the traditionally studied mid-latitude zones. Our results also show uniform coverage across both hemispheres as the dataset extends toward the equator, particularly in regions such as Arabia Terra in the northern hemisphere and northern Hellas in the southern hemisphere, where crater fill features are dominant.

We compared our dataset with the ice consistency maps generated by the SWIM team, which represent the inferred burial depths of subsurface ice, categorized as 1–5 m and >5 m [44]. To facilitate the comparison, we mapped the geomorphological features (a), alongside SWIM-derived data indicating estimated depths to subsurface ice in the range of 1–5 m (b) and >5 m (c) (Figure 14). Good agreement between our dataset and the SWIM results was observed in several key regions. In the northern hemisphere, the key regions include Tartarus Colles (160°–185°E), Uranius Fossae to Tempe Mensa (270°–292°E), and Arabia Terra to Nili Fossae (10°–80°E). In the southern hemisphere, the key regions include the northern Argyre margins (298°–350°E) and northern Hellas margins (39°–100°E). In addition, our findings in the Sinus Sabaeus (3°–15°E) region align well with observations by Shean [34]. Several other blue regions near the equator, some of which are associated with the Medusa Fossae Formation, contain pedestal craters, a landform type not covered by the data in this study. In Figure 14b, the dust layer in certain areas may influence the ground radar signals [40]. We examined several landforms with distinct glacial/periglacial features, using publicly available SHARAD radar data; however, no subsurface reflections were detected. This may be attributed to factors such as steep crater walls or the small, discontinuous nature of the ice-related deposits, which can generate surface clutter that overwhelms the radar signal. Additionally, the absence of reflections may result from the loss of near-surface volatiles or the presence of ice-rich material that is too thin to be detected by SHARAD.

This study suggests that extensive glacial activity occurred in the low-latitude regions of Mars during past periods of high Martian obliquity, potentially preserving substantial subsurface ice within large crater deposits. The crater retention ages of concentric crater fills (CCFs) and other ice-related landforms range from as early as the late Hesperian (~3.4 Ga) to as recent as ~3 Ma, indicating multiple episodes of ice accumulation and reworking. Sinha and Vijayan [5] summarized and compared the geomorphic and topographic characteristics of four typical late Amazonian glacial/periglacial flow features (LVF/LDA, CCF, and VFF), along with their age estimates and obliquity considerations. The flow feature is interpreted to have formed during a period between ~10 and 100 Ma, under obliquity conditions (~30°–45°), similar to those associated with the formation of LDA and LVF landforms [5]. These settings are analogous to the crater directional fill observed in this study, particularly those preserved on pole-facing slopes, suggesting shared climatic control. The wide age range of low-latitude ice-related features may reflect episodic glacial activity linked to successive high-obliquity intervals during the Amazonian period, which is consistent with orbital simulations and stratigraphic correlations across Martian mid- and low-latitudes. However, further constraints, such as regional thermal models and crater degradation analyses, are needed to better resolve the timing, duration, and climatic thresholds of each glaciation episode.

Hauber et al. [52] suggested that LDAs at 25°−30°N formed at approximately 1 Ga, with the current landscapes representing glacial remnants. This aligns with models that predict Martian climatic evolution under relatively high obliquity [20]. Similarly, crater irregular fill features only RMCs on their sediment surfaces, without significant flow patterns, and likely formed during high-obliquity periods, hundreds of millions of years ago. The formation of CCF viscous flow features at low latitudes appears to have aligned temporally with mid-latitude glaciation events, which are estimated to have occurred between 100 Ma and 700 Ma [63,64]. The observed geomorphological features reflect a complex interplay between ice deposition and ice loss through both sublimation and glacial recession. For example, impact crater floor cracks and talus accumulations along crater edges [6] suggest extensive sublimation of near-surface ice. Moreover, the presence of ice-rich debris on crater sidewalls may indicate former glacial activity and downslope transport, indicating transitions between periods of ice accumulation and recession, likely driven by variations in Martian obliquity.

The modal age of polar-facing crater wall deposits (4.2 ± 0.8 Ma) corresponds to relatively recent glaciation during the late Amazonian period, comparable to the CSFDs of small VFFs in Nereidum Montes (2.7 ± 0.8 Ma) [71]. These sediments likely represent the outcomes of recent glacial activity and depositional processes. Our findings are consistent with those of previous analyses, supporting the hypothesis that multiple debris-covered glacial events occurred during the late Amazonian period on Mars. These observations emphasize the dynamic and episodic nature of Martian glaciation, which was driven by climatic changes over geologic timescales. The preservation of young, debris-covered deposits on pole-facing slopes suggests that Martian glaciers may share key similarities with terrestrial rock glaciers. It is likely that past glaciers on Mars contained more extensive clean ice, which progressively transitioned to debris-covered forms through sublimation and surface armoring. While the internal ice content of these flow-like landforms cannot yet be fully explained, their morphology and spatial context strongly indicate past ice activity during their formation and emplacement.

A 3D calculation was performed in regard to the residual glacial landforms (i.e., CCFs in this study) on Mars, with an estimated total volume of 1048.98 km³. Assuming an ice content of 90%, this corresponds to a global ice layer thickness of 65.1 mm; with an ice content of 30%, it corresponds to a global ice layer thickness of 21.7 mm. These estimates are provided as end-member scenarios, assuming ice contents of 90% and 30%, respectively. While the higher end of this range would support an interpretation involving glacial processes, the lower end may indicate debris-dominated rock glaciers or other forms of periglacial activity. However, due to the current lack of direct subsurface ice measurements at these locations, we cannot draw definitive conclusions regarding the dominant process type. The volume of glaciers at low latitudes is much smaller than that of glaciers in mid- to high-latitude areas. Note that we did not estimate the ice volumes of subcategories other than CCFs for several reasons. First, the distributions of GLFs and LVFs are sparse, and their boundaries are often indistinct, making precise definitions challenging. Second, some LDAs are remnants of glacier ice or features resulting from glacial retreat, exhibiting complex characteristics that complicate measurements. Additionally, the sublimation of ice can create depressions and cracks inside or outside craters, further complicating accurate mapping of terrain changes. As only the volume of CCFs was measured, other subcategories were excluded, suggesting that the actual ice content at low latitudes is likely underestimated.

5. Conclusions

In this study, we conducted a detailed visual survey of the entire Martian low-latitude (0°–±30°) region and identified a total of 4307 ice-related landforms in these regions. The identified features include 3653 crater fills (CFs; 84.82%), 284 lobate debris aprons (LDAs; 6.59%), 139 possible sublimation depressions (non-crater) (PSDNs; 3.23%), 79 glacier-like forms (GLFs; 1.83%), and 15 lineated valley fills (LVFs; 0.35%). The remaining 137 features (3.18%) were categorized as “others”, referring to potential ice deposits that may have occurred on plains. The crater fill features were further classified into four subtypes, namely concentric crater fill (CCF), crater irregular fill, crater directional fill, and crater depression. By mapping and analyzing these features, we have expanded the understanding of Martian glacial/periglacial processes, particularly in regions closer to the equator, that were previously underexplored. The key findings are summarized as follows:

• Ice-related landforms in the lower latitudes of Mars are more extensive than previously thought. The latitudinal extent of ice-related remnants has extended equatorially to 13°N in the northern hemisphere and 19°S in the southern hemisphere, highlighting a broader range for ice-related landforms;
• The identified ice-related landforms were formed during multiple episodes of Martian geologic history, indicating that Mars has experienced repeated glacial/periglacial processes. These findings support the hypothesis that surface ice extends closer to the equator during periods of high obliquity;
• Evidence of a dynamic interplay between ice accumulation and ice loss (sublimation) has been observed at low latitudes. These regions had significantly higher ice contents in the past and have since undergone extensive sublimation and removal. Many ice-related remnants have been preserved;
• Crater fills (CFs) are the most abundant type of ice-related landform at low latitudes, comprising approximately 85% of all the observed features. Among these, irregular fill is the most prevalent subtype. CFs in the northern hemisphere are at lower average elevations (−1165 m) than those in the southern hemisphere (552 m), but their numbers are significantly greater. The elevation contrast between the northern and southern CFs reflects both Martian hemispheric topography and possible differences in climate-driven ice stability. Directional fills are primarily found on the polar-facing walls of craters;
• The total volume of CCFs at low latitudes on Mars was approximately 1.05 × 10³ km³. Assuming an ice content of 90%, this corresponds to a global equivalent ice layer thickness of 65.1 mm; based on a more conservative assumption of 30% ice content, the equivalent thickness is 21.7 mm. The ice content at low latitudes is likely higher than that suggested by previous studies, necessitating a revised estimate.