Utopia Planitia Research Articles

Sub-kilometre (intra-crater) mounds in Utopia Planitia, Mars: character, occurrence and possible formation hypotheses

At the middle latitudes of Utopia Planitia (∼35–45°N; ∼65–101°E) hundreds of small-sized mounds located in sub-kilometre impact craters dot the landscape. Their shape varies from circular to crescentic and their height ranges from ∼10 to 50m. Often, metre to decametre pitting is observed, as is metres-thick banding or stratification. Mound albedo is relatively high, i.e. ∼0.16. The plain’s terrain in the region, previously linked to the latitude-dependent mantle (LDM) of ice–dust, displays pitting and albedo similar to the small intra-crater mounds. Some workers have suggested that the mounds and the plain’s terrain share a common ice–dust origin. If so, then scrutinising the mounds could provide analogical insight on the key geological characteristics and spatial distribution of the LDM itself. Other workers have hypothesised that the mounds are eroded sedimentary landforms or periglacial mounds underlain by a perennial ice-core (closed-system pingos).In this article we develop and then discuss each of the three mound-hypotheses in a much more substantial manner than has been done hitherto. Towards this end we use high-resolution images, present a detailed regional-map of mound distribution and establish a regional platform of topographical analysis using MOLA data superposed on a large-scale CTX mosaic. Although the ice–dust hypothesis is consistent with some observations and measurements, we find that a (loess-based) sedimentary hypothesis shows greater plausibility. Of the three hypotheses evaluated, the pingo or periglacial one is the weakest.

Habitable periglacial landscapes in martian mid-latitudes

Subsurface permafrost environments on Mars are considered to be zones where extant life could have survived. For the identification of possible habitats it is important to understand periglacial landscape evolution and related subsurface and environmental conditions. Many landforms that are interpreted to be related to ground ice are located in the martian mid-latitudinal belts. This paper summarizes the insights gained from studies of terrestrial analogs to permafrost landforms on Mars. The potential habitability of martian mid-latitude periglacial landscapes is exemplarily deduced for one such landscape, that of Utopia Planitia, by a review and discussion of environmental conditions influencing periglacial landscape evolution. Based on recent calculations of the astronomical forcing of climate changes, specific climate periods are identified within the last 10Ma when thaw processes and liquid water were probably important for the development of permafrost geomorphology. No periods could be identified within the last 4Ma which met the suggested threshold criteria for liquid water and habitable conditions. Implications of past and present environmental conditions such as temperature variations, ground-ice conditions, and liquid water activity are discussed with respect to the potential survival of highly-specialized microorganisms known from terrestrial permafrost. We conclude that possible habitable subsurface niches might have been developed in close relation to specific permafrost landform morphology on Mars. These would have probably been dominated by lithoautotrophic microorganisms (i.e. methanogenic archaea).

Evidence of an eolian ice-rich and stratified permafrost in Utopia Planitia, Mars

Western Utopia Planitia (UP) is dotted with scalloped depressions, small-sized polygons and pingo-like mounds. Within the planetary science community, there seems to be a general agreement that these relatively recent landscape features are indicative of an ice-rich permafrost. However, questions about the concentration of ice-content and the origin of the permafrost remain unanswered. The scalloped depressions (∼100m to few km in diam.) are thought to be the product of degradation of ground-ice by thawing or sublimation. Indeed, most of the scalloped depressions display bright bands on their floors. These have been described as possible exposed sedimentary layers, markers of recessional ponded water or slumped material by previous works. As the depressions could represent probes of the permafrost, therefore the study of the inner bands could help to investigate the permafrost. Here, we evaluate the disparate hypotheses of band origin using several HiRISE images and a HiRISE DEM. We show that the depressions have an inner stepped-profile. This profile is reminiscent of exhumed and tilted sedimentary layers of different cohesion. Using ArcGIS, we estimate the dip of several layers (n=52). The stratification is complex comprising layers of ∼2–4m thick having different shallow dips with generally a north or south plunge sense. This geometry of tilted layers is typical on Earth of fluviatile or eolian sedimentation. In the last few years, several evidences on Mars, among them the subkilometer-scale smoothing of the topography and climatic simulations, suggested that the northern mid-latitudes have been influenced by eolian processes. The inferred complex stratification inside scalloped depressions may support an eolian origin of the permafrost in UP. In periglacial regions on Earth where thermokarst lakes are formed by extensive thawing of ground-ice, ice-rich permafrost are composed of fluvial or eolian sediments containing ∼15–80% of ice by volume. By analogy, the wide occurrence of kilometric scalloped depressions in UP could assume an ice-rich permafrost of possibly same ice-content. The presence of this ice-rich and stratified permafrost raises interesting questions about its relatively recent formation and climatic significance.

Basement controls on the scale of giant polygons in Utopia Planitia, Mars

We investigate the role of buried basement topography in controlling the scale of the giant polygons of Utopia Planitia (49°N, 233°W), Mars. Thermal Emission Imaging System (THEMIS) Infrared Daytime image mosaics reveal ∼6 km average spacing of polygon troughs along selected basin‐radial transects. If graben spacing correlates to thickness of the Utopia basin sediments, this measured trough spacing suggests ∼3 km of sediment, in contrast to inferences of 1–2 km thick sediments within this portion of the basin. Irregular basement topography can influence trough development within the contracting sediments by localizing stresses at the top of basement slopes so that the trough spacing correlates to topographic variations rather than layer thickness. Along the slopes, locally large strains responsible for graben development may develop with only modest levels of sediment contraction. Mars Orbiter Laser Altimeter (MOLA) data from the highlands region, which may represent the buried topography in Utopia basin, are used to create a stochastic suite of numerical simulations of the Utopia basin, where fracture seeds along the base of the model represent potential basement topography beneath the basin. At low strain, our models that simulate wet, fine sediment produce surface fractures with ∼4–7 km spacing, which matches the spacing of troughs in the polygonal terrain of the Utopia basin. This match supports the idea that buried topography below contracting wet sediments may exert primary control on the scale of polygonal troughs. Furthermore, volumetric contraction can produce locally high extensional strains with or without a contribution from tectonic uplift.

A re-interpretation of the recent stratigraphical history of Utopia Planitia, Mars: Implications for late-Amazonian periglacial and ice-rich terrain

Utopia Planitia, one of the great northern plains of the Mars, is a region where landscape modification by cold-climate processes, i.e. glacial and periglacial, is thought to be widespread. In the middle latitudes of this region a metres-thick mantle, possibly comprising an ice-dust admixture, has been reported; the occurrence of putative periglacial landforms such as flat-floored (thermokarst-like) depressions, small-sized (possibly thermal-contraction) polygons and polygon trough/junction pits also has been noted. Recently, some workers have suggested that the location of the putative periglacial landforms in mid Utopia Planitia is synonymous with that of the mantle and that the former evolve as the latter degrades. By contrast, preliminary work by others has proposed that this synonymy is misperceived, for two reasons: first, the putative periglacial landforms often are observed in areas of Utopia Planitia where the mantle is absent; second, in areas where the two landscape types are observed concurrently, the putative periglacial landforms either underlie the mantle and, stratigraphically, must predate the mantle, or they are adjacent to the mantle and at a lower datum of elevation. If the geological evolution of Utopia Planitia is to be constrained properly, then each of these hypotheses must be explored.Towards this end, we have mapped the location and distribution of the mantle and putative periglacial landforms across a broad latitudinal and longitudinal swath of the Utopia Planitia and its margins (∼55°–125°E and ∼30°–60°N). This map incorporates all the relevant images of these features and provides a regional scale of analysis. Previous discussions and/or maps of cold-climate landscapes in Utopia Planitia have been much narrower in latitudinal and longitudinal focus. An evaluation of high-resolution images containing the mantle material and putative periglacial landforms, underpinned by the MOLA-based topographic profiles, comprises a local scale of analysis. This too has not been developed fully in earlier work.Using the map, high-resolution photogeological evidence and the MOLA topographic profiles, we show three things. First, in mid Utopia Planitia the reach of the putative periglacial landforms extends well beyond the location of the possible dust-ice mantle. Second, the latter overprints the former in all observed instances and, consequently, the former cannot be a product of the latter. Third, perhaps the origin and evolution of the putative periglacial landscapes in mid Utopia Planitia is not as recent as some workers have proposed.

Polygon pattern geomorphometry on Svalbard (Norway) and western Utopia Planitia (Mars) using high-resolution stereo remote-sensing data

Polygonal systems formed by thermal contraction cracking are complex landscape features widespread in terrestrial periglacial regions. The manner in which cracking occurs is controlled by various environmental factors and determines dimension, shape, and orientation of polygons. Analogous small-scale features are ubiquitous in Martian mid- and high-latitudes, and they are also inferred to originate from thermal contraction cracking. We studied the geomorphometry of polygonally-patterned ground on Svalbard to draw a terrestrial analogy to small-scale polygonal structures in scalloped terrain in Martian mid-latitudes. We performed a comparative quantitative terrain analysis based on high-resolution stereo remote-sensing data (HRSC-AX and HiRISE) in combination with terrestrial field data and multivariate statistics to determine the relationship of polygon geomorphometry to local environmental conditions. Results show that polygonal structures on Svalbard and in Utopia Planitia on Mars are similar with respect to their size and shape. A comparable thermal contraction cracking genesis is likely. Polygon evolution, however, is strongly related to regional and local landscape dynamics. Individual polygon dimensions and orthogonality vary according to age, thermal contraction cracking activity, and local subsurface conditions. Based on these findings, the effects of specific past and current environmental conditions on polygon formation on Mars must be considered. On both Earth and Mars, the smallest polygons represent young, recently-active low-centered polygons that formed in fine-grained ice-rich material. Small, low-centered Martian polygons show the closest analogy to terrestrial low-centered ice-wedge polygons. The formation of composite wedges could have occurred as a result of local geomorphological conditions during past Martian orbital configurations. Larger polygons reflect past climate conditions on both Earth and Mars. The present degradation of these polygons depends on relief and topographical situation. On Svalbard the thawing of ice wedges degrades high-centered polygons; in contrast, the present appearance of polygons in Utopia Planitia is primarily the result of contemporary dry degradation processes.

Martian surface heat production and crustal heat flow from Mars Odyssey Gamma-Ray spectrometry

[1] Martian thermal state and evolution depend principally on the radiogenic heat-producing element (HPE) distributions in the planet's crust and mantle. The Gamma-Ray Spectrometer (GRS) on the 2001 Mars Odyssey spacecraft has mapped the surface abundances of HPEs across Mars. From these data, we produce the first models of global and regional surface heat production and crustal heat flow. As previous studies have suggested that the crust is a repository for approximately 50% of the radiogenic elements on Mars, these models provide important, directly measurable constraints on Martian heat generation. Our calculations show considerable geographic and temporal variations in crustal heat flow, and demonstrate the existence of anomalous heat flow provinces. We calculate a present day average surface heat production of 4.9 ± 0.3 × 10−11 W · kg−1. We also calculate the average crustal component of heat flow of 6.4 ± 0.4 mW · m−2. The crustal component of radiogenically produced heat flow ranges from <1 mW · m−2 in the Hellas Basin and Utopia Planitia regions to ∼13 mW · m−2 in the Sirenum Fossae region. These heat production and crustal heat flow values from geochemical measurements support previous heat flow estimates produced by different methodologies.

The Utopia/Isidis overlap: Possible conduit for mud volcanism on Mars

The largest areal concentration of pitted cones on Mars is located in the southwest section of Utopia basin. This particular area of pitted cones has been attributed to mud volcanism; several factors may have facilitated extensive mud volcanism at this location. The concentration of pitted cones is located where Utopia basin intersects Isidis basin; both features are multi-ring impact basins. On Earth, seismic investigations have shown that the outer rings of the Chicxulub multi-ring impact basin extend to the Mohorovičić discontinuity (Moho). If this is true on Mars as well, the fractures could act as conduits for water from Utopia Planitia, the site of a large, putative water body. It has been shown that methane can be generated at the mantle on Earth. On Mars this possible source of methane could combine with the infiltrated water to generate clathrates. While methane is not currently being released at the location of the pitted cones it could have been in the past. Three locations of methane release have been observed on Mars, two of which are located on the same outer ring of Isidis basin that intersects the pitted cone population. The area of Utopia basin that contains the large population of pitted cones is adjacent to the highland/lowland boundary where extensive deposition would have occurred. Extensive deposition combined with the potential for methane release may have contributed to the large population of pitted cones in this area of the Utopia basin.

Using complex independent component analysis to extract weak returns in MARSIS radar data and their possible relation to a subsurface reflector on Mars

Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) radar signals returned from the surface and subsurface of the northern lowlands of Mars are mostly characterized as a prominent surface pulse followed by a tail of weak signals. We applied the method of blind source separation (BSS) using complex independent component analysis (CICA) to examine if there is any subsurface information hidden in the apparent clutter and noise in the tails. In this paper we describe the application of the method with a case study in a selected region in the western part of Utopia Planitia (88°E–91°E, 45°N–51°N) where an unnamed large (∼160 km in diameter), semifilled crater is located. Applying CICA, we find a particularly significant signal from one of the blind sources. This signal is interpreted as representing a subsurface reflection interface, based on analysis of characteristics of the separated sources and their relation to surface roughness inside and outside the crater. A numerical simulation is conducted to examine the conditions under which the method tends to be effective. We conclude that CICA is helpful for recognizing subsurface signals above the noise level but obscured by clutter and thermal noise, provided the surface is relatively smooth.

Scalloped depressions and small-sized polygons in western Utopia Planitia, Mars: A new formation hypothesis

Flat-floored depressions with scalloped-shapes and spatially associated small-sized polygons (diameter <∼100 m) dot the landscape of western Utopia Planitia (centered at 45°N–95°E). The scalloped depressions are thought to be the result of ice-rich regolith undergoing degradation by sublimation or thaw. Current models suggest that the formation and development of the depressions occur in a poleward direction due to the enhanced sublimation of their equator-facing slopes. By contrast, we propose a conceptual model that shows the equatorward growth of depressions due to preferential degradation by sublimation of their pole-facing slopes. Our model is based on a geomorphological study of the depressions and small-sized polygons in western Utopia Planitia (80°–110°E, 35°–50°N), using images from the High Resolution Imaging Science Experiment (HiRISE) and topographical data from the Mars Orbiter Laser Altimeter (MOLA) and a HiRISE stereo Digital Elevation Model (DEM). Here we describe (i) a morphological evolution of small-sized polygons within the depressions, from low-centered to high-centered, that facilitates one's understanding of depression growth and development; and (ii) occurrence of v-shaped alcoves, failure cracks and semicircular hollows that point to a retrogressive degradation of the pole-facing slopes of depressions. We propose that the development of the depressions is due to heightened insolation of their pole-facing slopes, leading to enhanced sublimation of ground-ice. Based upon the inferred asymmetric insolation, we suggest that the equatorward expansion of depressions occurred during recent high-obliquity periods of Mars.

Intra-crater glacial processes in central Utopia Planitia, Mars

We describe and interpret a series of previously unidentified glacial-like lobes (34–43°N; 107–125°E) that were discovered as part of a survey of large (D>5km) impact craters in Utopia Planitia, one of the great northern plains of Mars. The lobes have characteristics that are consistent with a glacial origin. Evidence includes curvilinearity of form, lineations and ridges, and surface textures that are thought to form by the sublimation of near-surface volatiles. The lobes display morphologies that range from wedge-shaped to near-circular to elongate. The flow directions are towards the northern walls in the case of craters with large single lobes, and in all directions in the case of the largest (D>30km) craters. Concentric crater fill is also interspersed within craters of our study region, with such craters having much higher filling rates than those with flow lobes. We suggest that the impact crater population in south-west Utopia Planitia demonstrates a spectrum of glacial modifications, from low levels of filling in the case of craters with elongate lobes at one extreme, to concentric crater fill in highly-filled craters at the other.

Rift zone volcanism and associated cinder cone field in Utopia Planitia, Mars

We analyzed a small cone field in SW Utopia Planitia that shows striking similarities to volcanic rift zones on Earth. This is of particular interest as the study area lies off any of the volcanic centers of Mars in the northern hemisphere lowlands and is surrounded by material of the Amazonian aged Vastitas Borealis Formation (VBF). We believe that the study area could be the first direct evidence for volcanic activity in Utopia Planitia prior to the resurfacing by VBF material. The most striking characteristics that we observed are (1) a set of broad eruptive fissures showing signs of ongoing extension during cone formation, (2) parallel dike swarms, (3) magmatic intrusions, and (4) a number of lava flows that have erupted from the fissures and pitted cones that surround a larger cone in the center of the study area and that are often associated with, or aligned along, the fissures and dikes. Cratering model ages indicate that the study area is of Late Noachian to Early Hesperian age and is thus distinctly older than the surrounding VBF, falling into the time frame of the main phases of activity in the volcanic centers of Syrtis Major, Elysium Planitia, and the southern highlands.

Thermokarst in Siberian ice‐rich permafrost: Comparison to asymmetric scalloped depressions on Mars

On Earth, the thawing of permafrost deposits with high‐ground ice content results in massive surface subsidence and the formation of characteristic large thermokarst depressions. Slope asymmetries within thermokarst depressions suggest lateral growth, which occurs due to thermoerosion and gravimetric mass wasting along these slopes. It has been proposed that rimless, asymmetrically shaped depressions (called scalloped depressions) on Mars were formed by insolation‐driven ground ice sublimation. We investigated a large thermokarst depression in ice complex deposits in the Siberian Arctic as a terrestrial analogue for scalloped depressions in Martian volatile‐rich mantle deposits. Our results from field studies, insolation modeling, and geomorphometric analyses suggest lateral thermokarst development in a northern direction. This conclusion is obvious due to steeper slope angles of the south facing slopes. Insolation and surface temperatures are crucial factors directly influencing thermokarst slope stability and steepness. Comparative analyses of Martian scalloped depressions in Utopia Planitia were conducted using high‐resolution (High‐Resolution Imaging Science Experiment, Context Camera) and thermal infrared (Thermal Emission Imaging System) satellite data. By direct analogy, we propose that the lateral scalloped depression development on Mars was primarily forced on the steep pole‐facing slopes in the equator‐ward direction. Insolation modeling confirms that this must have happened in the last 10 Ma during an orbital configuration of higher obliquity than today. Development would have been maximized if the orbit was both highly oblique and highly eccentric, and/or the Martian summer coincided with perihelion. Relatively short events of increasing sublimation or even thawing of ground ice led to fast slumping processes on the steep pole‐facing slopes.

Examination of gully sites on Mars with the shallow radar

Martian gullies, found on steep slopes along broad mid‐latitudinal bands, have morphologies resembling those of water‐carved gullies on Earth and have been dated to &lt;10 Ma. As such, one of the leading hypotheses, though not unique, is that martian gullies formed by the flow of liquid water in the very recent geologic past. Since the permittivity of liquid water is about one order of magnitude higher than that of most silicates, it is plausible that subsurface geologic interfaces involving liquid water may be detected via ground penetrating radar. We have surveyed a substantive portion of the martian gully population with data from the Shallow Radar (SHARAD) instrument, on board the Mars Reconnaissance Orbiter (MRO), in search of strong subsurface radar reflections indicative of the presence of liquid water reservoirs, which would serve as sources to the flows occurring within gullies. No such reflections are found at most of the locations surveyed, suggesting that either liquid water is not likely present in detectable amounts or that the shallow martian subsurface is unusually electrically conductive (i.e., lossy) at all of the locations examined. Strong subsurface reflections occur in the vicinity of gullies at two locations in the northern lowlands: Arcadia and southeastern Utopia Planitiae. In both cases, the reflectors occur at a range in depth of 45 to 90 m, considering a range in permittivity of 3 to 10, and −20 to −30 dB weaker than the surface reflection. In the case or Arcadia, the reflector corresponds to the eastern edge of Plaut et al.'s (2009) extensive radar subsurface unit; in both Arcadia and Utopia we interpret the reflectors as ground ice. Though our results offer a general assessment of the gully population, SHARAD is continuing its survey of gully rich locations.

Evidence of widespread degraded Amazonian-aged ice-rich deposits in the transition between Elysium Rise and Utopia Planitia, Mars: Guidelines for the recognition of degraded ice-rich materials

Widespread deposits surrounding mesas, in craters and in valley systems are observed in the transition zone between the Elysium Rise and the Utopia Planitia Basin. They are characterized by their relatively high albedo, the presence of ring-mold crater (RMC) morphologies and their pitted surfaces, with textures ranging from lineations and fish-scale-patterns to widely distributed knobs. These deposits are interpreted to be modified ice-rich material in the form of degraded deposits of concentric crater fill (CCF), lineated valley fill (LVF) and lobate debris aprons (LDA). The degraded CCF deposits are observed from 31.2–40°N, 138–150°E over an elevation range of almost 9 km. This wide-ranging distribution demonstrates that degraded ice-rich deposits exist at every altitude and latitude in the study area, indicating that icy mantle materials were initially deposited over extensive areas and were stable over a long time period, allowing the deposits to coexist and interact with different processes under very different conditions. The degraded LDA deposits represent the largest unit of modified ice-rich material, with an area of ∼15,700 km 2, and are populated with a range of ring-mold crater morphologies that is interpreted to be related to a degradational sequence between previously described RMC and newly observed RMCs that appear to be more degraded. A distinctive frequency difference in the distribution of normal and degraded RMCs permits an evaluation of different degradation stages of the LDA deposits; we show how an RMC distribution can be used as a key tool for evaluation of altered LDA, LVF and CCF deposits. Taken together, these observations suggest that ice-rich material has played a major role in shaping the present-day landscape in the transition zone between the Elysium Rise and the Utopia Planitia Basin, and they provide a link for understanding Amazonian-aged degradation processes of ice-rich deposits in an area with no significant topographic relief.

Spectral evidence of volcanic cryptodomes on the northern plains of Mars

The composition and detailed morphology of dome-shaped features located in western Arcadia Planitia and just west of Utopia Planitia were examined in this study utilizing data from Mars Reconnaissance Orbiter and Mars Odyssey sensors. The domes have diameters averaging 1.5 km and heights averaging 160 m, and are generally dark-toned, although some are lighter toned or have split dark and light-toned surfaces. The domes are surrounded by annular deposits comprising, with increasing distance from the domes, dark-toned aprons, light-toned aureoles, and dark-toned aureoles. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data over several areas in the western Arcadia region show that spectra from the flanks of several domes have 1 and 2 μm absorption features consistent with the presence of olivine and a high-Ca pyroxene, nominally augite. Modified Gaussian Model (MGM) analysis of these spectra indicates Fe-rich olivine compositions. The tops of domes and the aprons surrounding many domes have negative sloping flat spectra in the near infrared, which is consistent with tachylite-rich, glassy compositions. High Resolution Imaging Science Experiment (HiRISE) images over several domes indicate that relatively high thermal inertia values associated with the tops of domes can be attributed to boulder strewn surfaces. HiRISE images also reveal that light-toned aureoles around domes consist of crenulated ground resembling “brain terrain” textures previously described for ice-rich concentric crater fill elsewhere on the northern plains. The plains surrounding the domes also display lineations that are interpreted to be lava channels or tubes. The combination of volcanic and ice-related features are consistent with the domes having formed as cryptodomes in the near sub-surface. We suggest that the domes could be basaltic in composition if the magmas were degassed and/or highly crystallized, and thus more viscous than typical basaltic magmas. The intrusion of these magmas into an ice-rich horizon would have produced a pervasively jointed chilled margin on the domes, which, once the domes were exposed, would have mechanically weathered to form the dark aprons. The domes could have served as local centers for ice accumulation during periods of high orbital obliquity, which ultimately would have led to the formation of the “brain terrain” surrounding the features. The domes represent late stage volcanic products on the northern plains of Mars and associated features provide more evidence for the role that ice accumulation and modification has played in recent martian history.

Comparison of ground- and aerial-based approaches for quantifying polygonal terrain network geometry on Earth and Mars via spatial point pattern analysis

Recently, a particular statistical method – spatial point pattern analysis (SPPA) – has been introduced as an effective means by which qualitative, observable variations in polygonal terrain network arrangements on Earth and Mars can be quantified. A number of ground- and aerial-based techniques are available from which to derive the required input data: the spatial ( x– y) coordinates of all polygon trough intersections within the site. However, each of the data collection methods may contain some level of error. Thus, the overarching question addressed by this research is: “how are the results of SPPA affected by the method by which the input data were generated?” At two polygonal terrain sites in the Canadian High Arctic, we performed ground-based surveys using differential and non-differential Global Positioning Systems (GPS) as well as photogrammetric analysis of aerial and satellite images of varying resolution to determine the trough intersection coordinates. It was found that the most robust statistical results were produced when using data from a combination of differential GPS surveys and high-resolution (∼0.25 m/pixel) aerial images. Images of pixel size ≥1 m were found to be unsuitable for this type of analysis. With respect to the investigation of similar Martian landforms, HiRISE and MOC images of polygonal terrain sites in southwestern Utopia Planitia were analyzed. Our results show that it is strongly preferable to perform SPPA using HiRISE images, though an empirical model is outlined that could be used to correct for errors arising from the reduced resolution inherent to MOC images.

Lineated valley fill at the Martian dichotomy boundary: Nature and history of degradation

The fretted terrain of the Martian dichotomy boundary is a key region for investigating landforms related to creep of ice and debris as it exhibits landforms comparable to morphologies of periglacial environments. Although features known as lobate debris aprons, lineated valley fills and concentric crater fills have been studied in great detail, basic questions concerned with the composition and the style of emplacement and degradation still remain unanswered. This study focuses on morphologies which are located in a near‐circular depression located at the dichotomy escarpment in Deuteronilus Mensae. Analysis of high‐resolution image data suggests an early formation of these features as the result of backward thermokarstic degradation of highland terrain. Geologically younger processes caused deposition and degradation of an ice‐rich mantling deposit, which ultimately led to formation of creep morphologies that might have even been active in the geologically recent past. Intermixing of both degradational landform units form complex patterns that cannot be explained by a late stage (glacial) process alone. Morphological comparisons of lineated valley fill units with concentric crater fill landforms in Utopia Planitia strongly suggest comparable emplacement and degradation styles of these features. The sequential development consisting of an initial probably widespread thermokarstic degradation followed by later cyclic deposition of volatile material and continual thermokarstic degradation suggests that the development of parts of the Martian fretted terrain is directly related to climatic variations in the planet's history.

Pedestal crater heights on Mars: A proxy for the thicknesses of past, ice-rich, Amazonian deposits

Mid-latitude pedestal craters on Mars offer crucial insights into the timing and extent of widespread ice-rich deposits during the Amazonian period. Our previous comprehensive analysis of pedestal craters strongly supports a climate-related formation mechanism, whereby pedestals result from impacts into ice-rich material at mid latitudes during periods of higher obliquity. The ice from this target deposit later sublimates due to obliquity changes, but is preserved beneath the protective cover of the armored pedestal. As such, the heights of pedestals act as a proxy for the thicknesses of the paleodeposits. In this analysis, our measurement of 2300 pedestal heights shows that although pedestals can reach up to ∼260 m in height, ∼82% are shorter than 60 m and only ∼2% are taller than 100 m. Mean pedestal heights are 48.0 m in the northern mid latitudes and 40.4 m in the southern mid latitudes, with the tallest pedestals located in Utopia Planitia, Acidalia Planitia and Malea Planum. We use these data in conjunction with prior climate model results to identify both regional and global trends regarding ice accumulation during obliquity excursions. Our data provide evidence for multiple episodes of emplacement and removal of the mid-latitude ice-rich deposit based on stratigraphic relationships between pedestal craters and the close proximity of pedestals with significantly different heights.

Scalloped terrains in the Peneus and Amphitrites Paterae region of Mars as observed by HiRISE

The Peneus and Amphitrites Paterae region of Mars displays large areas of smooth, geologically young terrains overlying a rougher and older topography. These terrains may be remnants of the mid-latitude mantle deposit, which is thought to be composed of ice-rich material originating from airfall deposition during a high-obliquity period less than 5 Ma ago. Within these terrains, there are several types of potentially periglacial features. In particular, there are networks of polygonal cracks and scalloped-shaped depressions, which are similar to features found in Utopia Planitia in the northern hemisphere. This area also displays knobby terrain similar to the so-called “basketball terrains” of the mid and high martian latitudes. We use recent high resolution images from the High Resolution Imaging Science Experiment (HiRISE) along with data from previous Mars missions to study the small-scale morphology of the scalloped terrains, and associated polygon network and knobby terrains. We compare these with the features observed in Utopia Planitia and attempt to determine their formation process. While the two sites share many general features, scallops in Peneus/Amphitrites Paterae lack the diverse polygon network (i.e. there is little variation in the polygon sizes and shapes) and large curvilinear ridges observed in Utopia Planitia. This points to a more homogeneous ice content within the substrate in the Peneus/Amphitrites Paterae region and implies that scallop formation is independent of polygon formation. This work shows that, as in Utopia Planitia, sublimation of interstitial ice is a likely process explaining the formation of the scalloped depressions in the region of Peneus/Amphitrites Paterae. Therefore, we provide a simplified scallop formation model based on sublimation of interstitial ice as proposed for Utopia Planitia. We also show that the differences in scallop morphologies between the two regions may be explained by differences in near-surface ice content, sublimation rates and age of formation of the scalloped terrains.