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 (～100 m 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 ～2–4 m 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.     

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 km², 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.     

Sorted stone circles in Elysium Planitia, Mars: Implications for recent martian climate

We have found sorted stone circles and polygons near the equator of Mars, using new 25 cm/pixel NASA HiRISE (High Resolution Imaging Science Experiment) images. The sorted circles occur in geologically recent catastrophic flood deposits in the equatorial Elysium Planitia region, and are diagnostic of periglacial processes: sorted polygons do not form from volcanic activity, as has been suggested for non-sorted polygons in this region. These landforms indicate that (i) a long-lived, geologically recent, active cryoturbation layer of ground ice was present in the regolith, (ii) there was some degree of freeze-thaw, and thus (iii) there were sustained period(s), likely within the last 10 Ma, in which the martian climate was 40 to 60 K warmer than current models predict.     

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.     

Stratigraphic analysis of the northern polar layered deposits of Mars: Implications for recent climate history

We examine the stratigraphy of the polar layered deposits (PLD) within the north polar cap of Mars to assess its layer continuity, correlations, cyclicity and structure and implications for the recent climate record. PLD sequences characterized using Fourier analysis and curve shape matching algorithms show that layers correlate throughout the upper part of the PLD. We tested for cyclicity and found that the uppermost ∼300 m contain a dominant wavelength layer packet of ∼30 m, interpreted to be a climate signal related to the 51 kyr precession cycle. Directly below this region we document a section of polar layered deposits ∼100 m thick without a dominant periodic signal; this is interpreted to represent a phase of low net accumulation and lag deposits formed during the last ice age, about 0.5-2 Ma ago. We further analyzed layer structure by combining these results with three-dimensional determinations of layer orientation (strike and dip) to assess the internal stratigraphy of the PLD and its implications for polar history. We show that individual layers within the PLD stratigraphy are not horizontal (no dip) but rather show broad variation in elevation with distance. Correlations suggest that the layer strikes and dips broadly follow present cap surface topography. Local variations in layer orientations in the vicinity of the troughs suggest that (1) trough structures were present at the time of layer accumulation and (2) dips may have been influenced by ice flow and/or static ice accumulation in the presence of preexisting troughs. This new information favors models in which the troughs are long-term structures of the PLD rather than (1) recently eroded into the PLD, or (2) very active and laterally migrating around the PLD. Our results strongly support the hypothesis that significant volumes of polar volatiles are mobilized and transported equator-ward during periods of increased obliquity. Our results predict that the upper ∼300 m of the north polar PLD accumulated in the last 500 ka, yielding net accumulation rates of ∼0.06 cm/yr. The presence and albedo of the no periodic signal zone suggest that polar net accumulation rates are very low and that dust rich lag deposits form during periods of sustained high obliquity. Layer sequences in the south polar and equatorial regions are examined and compared to those in the north; rhythmic sequences are observed in both regions but no direct correlations to the dominant signals of the north polar deposits have yet been found. These new techniques and observations provide a paradigm for further analysis of recent polar history (the upper kilometer of the record) and a basis for extending assessments to the lower part of the polar deposits and to other cyclic deposits in the geological record of Mars.     

The volcanic history of central Elysium Planitia: Implications for martian magmatism

Central Elysium Planitia (CEP) is located south of Elysium Mons. Back to the era of the Viking orbiters, clues accumulated in favor of recent volcanism in relation with ground water release and the formation of long sub-parallel fissures. Four aqueous flood channel systems emanate from linear fissures. Recent eruptions of low viscosity lavas originate from these fissures and from low shield volcanoes. The objective of this paper is to constrain the volcanic history of this region, and to determine the chronological relationships with fluvial/erosional processes. New observations (e.g., new shield volcanoes and one new fluvial event) are summarized on a context map. Thirty-five surfaces have been dated from the count of about 15,000 impact craters. Ages have been cross-checked with relative stratigraphy when possible. A probabilistic approach has been introduced to compare similar ages and define periods of volcanic activity. Our results confirm that some volcanic features are extremely recent (∼2 My). Active periods are found at 2.5-3 My, 4.3 My, 13.5-16.2 My, 19 My, 21-32 My, 58 My, 71 My, 85-95 My, 134 My, 173 My and 234 My, not excluding the possibility that some of the gaps would be filled with additional crater counts. The volcanic activity thus extended for at least the last 250 My. The lava volumes have been estimated from the topographic modeling of the floor of depressions filled up by volcanic products, including the volumes of several large crater cavities buried under lavas (>20% of the total volume). Our new estimation of the total lava volume is 1.5 ± 0.2 × 10⁵ km³. This value corresponds to an average thickness of one hundred meters of lavas for the young volcanic plain. As a consequence, the total eruption rate at CEP, defined as the total volume of lava divided by the time of emplacement 1.4 × 10⁻²-1.8 × 10⁻² m³/s is lower than values typically estimated for terrestrial hot spots or large igneous provinces, suggesting longer inactive periods. The concept of mantle plumes responsible for terrestrial flood volcanism may not be applicable to the case of CEP and the mechanism proposed in Schumacher and Dreuer (2007) offers a plausible alternative to explain our observations.     

Concentric crater fill in Utopia Planitia: History and interaction between glacial “brain terrain” and periglacial mantle processes

At martian mid-to-high latitudes, the surfaces of potentially ice-rich features, including concentric crater fill, lobate debris aprons, and lineated valley fill, typically display a complex texture known as “brain terrain,” due to its resemblance to the complex patterns on brain surfaces. In order to determine the structure and developmental history of concentric crater fill and overlying latitude-dependent mantle (LDM) material, “brain terrain” and polygonally-patterned LDM surfaces are analyzed using HiRISE images from four craters in Utopia Planitia containing concentric crater fill. “Brain terrain” and mantle surface textures are classified based on morphological characteristics: (1) closed-cell “brain terrain,” (2) open-cell “brain terrain,” (3) high-center mantle polygons, and (4) low-center mantle polygons. A combined glacial and thermal-contraction cracking model is proposed for the formation and modification of the “brain terrain” texture of concentric crater fill. A similar model, related to thermal contraction cracking and differential sublimation of underlying ice, is proposed for the formation and development of polygonally patterned mantle material. Both models require atmospheric deposition of ice, likely during periods of high obliquity, but do not require wet active layer processes. Crater dating of “brain terrain” and mantled surfaces suggests a transition at martian mid-latitudes from peak “glacial” conditions occurring within the past ∼10-100 My to a quiescent period followed by a cold-desert “periglacial” period during the past ∼1-2 My.     

Electromagnetic characterization of polar ice-wedge polygons: Implications for periglacial studies on Mars and Earth

Polygonal terrain is found in a variety of polar environments on Earth and Mars. As a result, many areas of northern Canada may represent ideal terrestrial analogues for specific regions of Mars - in particular the northern plains. In the Canadian Arctic, polygon troughs are commonly underlain by wedges of massive ice, with rare examples of other wedge types. If the same is true for Mars, this raises interesting implications for the processes that concentrate H₂O at the Martian poles. This study uses an electromagnetic induction sensor to investigate the electromagnetic characteristics of terrestrial polar ice-wedge polygons. Surveys were conducted in two regions of the Canadian Arctic using a DUALEM-1S dual-geometry electromagnetic induction sensor, which measures electrical conductivity in the first 1.5-2 m of the subsurface. At locations where strong geomorphological evidence of ice was found, polygon troughs corresponded to local conductive anomalies. Trenching confirmed the presence of ice wedges at one site and allowed ground-truthing and calibration of the geophysical data. Previously unknown bodies of massive ice were also identified through the use of this geophysical technique. This study shows that an electromagnetic induction sounder is a useful instrument for detecting and mapping out the presence of subsurface ice in the Canadian Arctic. Taking together with its small size, portability and ruggedness, we suggest that this would also be a useful instrument for any future missions to Mars’ polar regions.     

The morphologies of volcanic landforms at Central Elysium Planitia: Evidence for recent and fluid lavas on Mars

This paper focuses on physical parameters (flow rates and rheological properties) of lava flows observed in the Central Elysium Planitia (CEP) region of Mars. The flows are modeled as Newtonian fluids, using the Jeffrey's equation and the concept of Graetz number, or alternatively as Bingham fluids. In addition to these approaches, a theoretical model of the shape of shield volcanoes based on the solution for the porous flow of an unconfined aquifer is applied to 5 shields, providing independent quantifications of rheological variations between the shields. This analysis indicates that of the five volcanoes studied, two are partially buried by lava postdating their formation, a result which has been confirmed independently in one case by high resolution images. Our observations reveal that two types of lava flows may be found in the CEP region. The first group is composed of large lava flows with viscosities around ∼2.5×¹⁰⁵ Pa s or yield strengths ranging from 100 to 500 Pa. The second group includes small lava flows of the shield volcanoes and large leveed lava channels on the plains with viscosities below 10³ Pa s, or yield strengths less than 200 Pa. When compared with other volcanic regions on Mars investigated with similar approaches, these latter values are, at present, the lowest inferred for martian lava flows. Several hypotheses for the formation of these lavas are discussed in the context of CEP given that low viscosity can be the result of (1) high temperature, (2) low crystal content, (3) low Si abundance of the liquid phase, and/or (4) the presence of dissolved volatiles. Two scenarios are considered. In the first one, it is demonstrated that low viscosity lavas (of low silica content) can be produced in the context proposed by Schumacher and Breuer [Schumacher, S., Breuer, D., 2007. Geophys. Res. Lett. 34. L12202] for recent volcanism. However, geochemical maps derived from GRS measurements do not provide support for anomalously low silica concentrations in this region. In the second scenario, a water-rich magma is proposed, although arguments in favor of a water-rich mantle source below the CEP are not available at the present time.     

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.     

Characterization of hydrated silicate-bearing outcrops in Tyrrhena Terra,Mars: Implications to the alteration history of Mars

The Tyrrhena Terra region of Mars is studied with the imaging spectrometers OMEGA (Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité) onboard Mars Express and CRISM (Compact Reconnaissance Infrared Spectrometer for Mars) onboard Mars Reconnaissance Orbiter, through the observation of tens of craters that impacted into this part of the martian highlands. The 175 detections of hydrated silicates are reported, mainly associated with ejecta blankets, crater walls and rims, and central up-lifts. Sizes of craters where hydrated silicates are detected are highly variable, diameters range from less than 1 km to 42 km. We report the presence of zeolites and phyllosilicates like prehnite, Mg-chlorite, Mg-rich smectites and mixed-layer chlorites–smectites and chlorite–vermiculite from comparison of hyperspectral infrared observations with laboratory spectra. These minerals are associated with fresh craters post-dating any aqueous activity. They likely represent ancient hydrated terrains excavated by the crater-forming impacts, and hence reveal the composition of the altered Noachian crust, although crater-related hydrothermal activity may have played a minor role for the largest craters (>20 km in diameter). Most detected minerals formed over relatively high temperatures (100–300 °C), likely due to aqueous alteration of the Noachian crust by regional low grade metamorphism from the Noachian thermal gradient and/or by extended hydrothermal systems associated with Noachian volcanism and ancient large impact craters. This is in contrast with some other phyllosilicate-bearing regions like Mawrth Vallis where smectites, kaolinites and hydrated silica were mainly identified, pointing to a predominance of surface/shallow sub-surface alteration; and where excavation by impacts played only a minor role. Smooth plains containing hydrated silicates are observed at the boundary between the Noachian altered crust, dissected by fluvial valleys, and the Hesperian unaltered volcanic plains. These plains may correspond to alluvial deposition of eroded material. The highlands of Tyrrhena Terra are therefore particularly well suited for investigating the diversity of hydrated minerals in ancient martian terrains.     

Formation and disruption of aquifers in southwestern Chryse Planitia, Mars

We present geologic evidence suggesting that after the development of Mars' cryolithosphere, the formation of aquifers in southwestern Chryse Planitia and their subsequent disruption led to extensive regional resurfacing during the Late Hesperian, and perhaps even during the Amazonian. In our model, these aquifers formed preferentially along thrust faults associated with wrinkle ridges, as well as along fault systems peripheral to impact craters. The characteristics of degraded wrinkle ridges and impact craters in southwestern Chryse Planitia indicate a profound role of subsurface volatiles and especially liquid water in the upper crust (the upper one hundred to a few thousands of meters). Like lunar wrinkle ridges, the martian ones are presumed to mark the surface extensions of thrust faults, but in our study area the wrinkle ridges are heavily modified. Wrinkle ridges and nearby plains have locally undergone collapse, and in other areas they are associated with domical intrusions we interpret as mud volcanoes and mud diapirs. In at least one instance, a sinuous valley emanates from a modified wrinkle ridge, further indicating hydrological influences on these thrust-fault-controlled features. A key must be the formation of volatile-rich crust. Primary crustal formation and differentiation incorporated juvenile volatiles into the global crust, but the crustal record here was then strongly modified by the giant Chryse impact. The decipherable rock record here begins with the Chryse impact and continues with the resulting basin's erosion and infilling, which includes outflow channel activity. We propose that in Simud Vallis surface flow dissection into the base of the cryolithosphere-produced zones where water infiltrated and migrated along SW-dipping strata deformed by the Chryse impact, thereby forming an extensive aquifer in southwestern Chryse Planitia. In this region, compressive stresses produced by the rise of Tharsis led to the formation of wrinkle ridges. Zones of high fracture density within the highly strained planes of the thrust faults underlying the wrinkle ridges formed regions of high permeability; thus, groundwater likely flowed and gathered along these tectonic structures to form zones of elevated permeability. Volatile depletion and migration within the upper crustal materials, predominantly along fault systems, led to structurally controlled episodic resurfacing in southwestern Chryse Planitia. The erosional modification of impact craters in this region is linked to these processes. This erosion is scale independent over a range of crater diameters from a few hundred meters to tens of kilometers. According to our model, pressurized water and sediment intruded and locally extruded and caused crustal subsidence and other degradational activity across this region. The modification of craters across this wide range of sizes, according to our model, implies that there was intensive mobilization of liquid water in the upper crust ranging from about one hundred to several thousand meters deep.     

Contribution of Mars Odyssey GRS and Mars Reconnaissance Orbiter CRISM at Elysium Planitia: A case of mistaken identity

Two recent Icarus papers on the geology of Mars' Elysium plain (Diez et al. 2009 [Icarus 200, 19-29] and Jaeger et al. 2010 [Icarus 205, 230-243]) raise the issue of what gamma ray, neutron, and visible-infrared spectral observations (GRS-NS, CRISM¹ ) mean geologically. This question is important because spectrometric data are readily accepted as supporting evidence of a particular geology (e.g., flood-volcanism on Mars). However, chemical composition is not a proxy for lithology.     

Geomorphological characterisation and interpretation of a mid-latitude glacier-like form: Hellas Planitia, Mars

We describe and interpret the surface terrain types associated with a widely-reported ∼4 km long, mid-latitude martian viscous flow feature (VFF). The feature is located in the southern hemisphere, on the poleward-facing rim of a ∼60 km-diameter crater in eastern Hellas Planitia. High Resolution Imaging Science Experiment (HiRISE) images, analysed in both 2D and 3D, reveal that the upper margin of the feature is bounded by steep (∼30°) headwalls, typically some tens of metres high, that are formed from unconsolidated material and characterised by a series of slope-parallel linear incisions. Below these incised headwalls, the feature flows at a general angle of ∼10° from a broad upper basin to a confined lower tongue that is bounded by a nested sequence of elongate raised ridges. These characteristics are typical of several VFFs in the region and are strikingly similar to moraine-bounded valley glaciers on Earth, and we sub-classify this feature as a ‘glacier-like form’ (GLF)¹. The GLF comprises five distinctive surface terrain types that contrast sharply with surface characteristics outside its bounding moraines. Four of these terrains (scaly terrain, polygonized terrain, linear terrain and mound-and-tail terrain) are located within the GLF’s innermost bounding moraine, while the fifth (rectilinear-ridge terrain) is located between its frontal moraines. These terrains are mapped, characterised and associated with possible mechanisms of formation to draw inferences about the GLF’s glaciology and glacial history. This analysis suggests that the GLF reached its maximal extent in the geologically-recent past, and that it may have been partially wet-based at that time. Subsequent to this phase, the GLF experienced an extended period of general recession that has been punctuated by several episodes of still-stand or advance. Currently, the GLF’s basin appears to be composed of a lower zone that is dominated by an exposed former glacier bed and an upper zone that may still contain a now-degraded and dust-mantled viscous mass, similar to many partially-glacierized basins on Earth.     

Residual south polar cap of Mars: Stratigraphy, history, and implications of recent changes

The residual south polar cap (RSPC) of Mars includes a group of different depositional units of CO₂ ice undergoing a variety of erosional processes. Complete summer coverage of the RSPC by ∼6-m/pixel data of the Context Imager (CTX) on Mars Reconnaissance Orbiter (MRO) has allowed mapping and inventory of the units in the RSPC. Unit maps and estimated thicknesses indicate the total volume of the RSPC is currently <380 km³, and represents less than 3% of the total mass of the current Mars atmosphere. Scarp retreat rates in the CO₂ ice derived from comparison of High Resolution Imaging Science Experiment (HiRISE) data with earlier images are comparable to those obtained for periods up to 3 Mars years earlier. These rates, combined with sizes of depressions suggest that the oldest materials were deposited more than 125 Mars years ago. Most current erosion is by backwasting of scarps 1-12 m in height. This backwasting is initiated by a series of scarp-parallel fractures. In the older, thicker unit these fractures form about every Mars year; in thinner, younger materials they form less frequently. Some areas of the older, thicker unit are lost by downwasting rather than by the scarp retreat. A surprising finding from the HiRISE data is the scarcity of visible layering of RSPC materials, a result quite distinct from previous interpretations of layers in lower resolution images. Layers ∼0.1 m thick are exposed on the upper surfaces of some areas, but their timescale of deposition is not known. Late summer albedo changes mapped by the CTX images indicate local recycling of ice, although the amounts may be morphologically insignificant. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data show that the primary material of all the different forms of the RSPC is CO₂ ice with only small admixtures of water ice and dust.     

A geomorphic analysis of Hale crater, Mars: The effects of impact into ice-rich crust

Hale crater, a 125 × 150 km impact crater located near the intersection of Uzboi Vallis and the northern rim of Argyre basin at 35.7°S, 323.6°E, is surrounded by channels that radiate from, incise, and transport material within Hale’s ejecta. The spatial and temporal relationship between the channels and Hale’s ejecta strongly suggests the impact event created or modified the channels and emplaced fluidized debris flow lobes over an extensive area (>200,000 km²). We estimate ∼10¹⁰ m³ of liquid water was required to form some of Hale’s smaller channels, a volume we propose was supplied by subsurface ice melted and mobilized by the Hale-forming impact. If 10% of the subsurface volume was ice, based on a conservative porosity estimate for the upper martian crust, 10¹² m³ of liquid water could have been present in the ejecta. We determine a crater-retention age of 1 Ga inside the primary cavity, providing a minimum age for Hale and a time at which we propose the subsurface was volatile-rich. Hale crater demonstrates the important role impacts may play in supplying liquid water to the martian surface: they are capable of producing fluvially-modified terrains that may be analogous to some landforms of Noachian Mars.     

Structural evolution of Lavinia Planitia, Venus: Implications for the tectonics of the lowland plains

This work shows the results of a detailed structural analysis of the deformation belts of Lavinia Planitia. Ridge belts and graben and groove belts can be observed at the studied area, while wrinkle ridges and large individual grooves predominate in the smooth plains. Transcurrent components of displacement are commonly observed, and transpression and transtension zones are the rule rather than the exception at most of the studied belts. Along-strike azimuth changes of deformation belts are accommodated by internal variations in the predominance of contractional, transcurrent or extensional structures. The material of the surrounding plains embays most of these deformation belts. The kinematic analysis of this complex network of tectonic structures suggests a broadly synchronous activity of contractional, transcurrent and extensional structures. The maximum horizontal shortening axis determined in this work describes a steady, semi-circular pattern centered at Alpha Regio. This deformation continued, although with subdued activity, after embayment of the deformation belts by the material of the plains. Future study of the tectonic evolution of the lowland plains should take into account the importance of the coeval history of neighboring uplands and lowlands.     

Thermokarst processes and the origin of crater-rim gullies in Utopia and western Elysium Planitia

We have identified a number of gullies that could be aqueous in origin near or at the rim of several impact craters in Utopia Planitia and western Elysium Planitia (30.0°-59.0° N; 241.0°-291.0° W). Based on the sharpness of their incisions and the general absence of superposed craters, we ascribe a relatively recent origin to the gullies. Scalloped depressions are commonplace throughout the region, as well as on the crater walls, rims and floors near the areas of gully issuance. Occasionally, the depressions cross-cut the gully debris-aprons, suggesting that the formation of some depressions is even more recent than that of the gullies. Previous research has proposed that the depressions are collapse basins formed by thermokarst processes. On Earth, thermokarst landforms occur in areas of low gradient topography where the permanently frozen ground (permafrost) is ice rich and has undergone a change in thermal equilibrium. This change can be triggered by long-term or episodic/cyclic climate change and accompanying rises in mean temperatures towards ∼0 °C as well as by rises in seasonally sustained summer temperatures well above ∼0 °C. In order to explain the origin of the rim or near-rim gullies we invoke high obliquity and the possibility that this region of Mars experienced obliquity-driven rises in temperature, atmospheric pressure and humidity sufficient to keep surface water and near-surface ground-ice stable for extended periods of time. We propose that gully formation is closely related to local freeze-thaw processes that, in turn, generate a thermokarst landscape (of which the gullies are a part). This geological and climatological scenario comprises the following steps:

1.

An inundation of meltwater at high obliquity (due to the thawing of an atmospherically-deposited snowpack or ice-sheet) and the subsequent saturation of the underlying regolith to tens of metres of depth.

2.

Loss of water on the surface, perhaps as obliquity decreases slightly, followed by the progressive freezing of the saturated regolith; this creates an aggrading mass of ice-rich regolith.

3.

Obliquity-induced temperature rises that engender the thaw, drainage and partial evaporation of the near-surface, ice-rich regolith.

4.

Localised formation of thermokarst collapse-basins (alases), as water is evacuated from these basins.

5.

Formation of gullies near, or at, some impact-crater rims as the result of meltwater migration from nearby alases through the thawed regolith to the areas of gully issuance.

Although the plains' materials in this region are in part very old (possibly Hesperian or even Noachian), the mantling deposits and their deformation by thermokarst processes appears to be relatively young. This suggests that recent climatic conditions could have been episodically warmer and wetter than had been previously thought.     

Crater streaks in the Chryse Planitia region of Mars: Early Viking results

High-resolution images of Chryse Planitia and eastern Lunae Planum from the early revolutions of Viking Orbiter I permit detailed analyses of crater-associated streaks and interpretation of related eolian processes. A total of 614 light and dark streaks were studied and treated statistically in relation to: (1) morphology, morphometry, and orientation, (2) “parent” crater size and morphology, (3) terrain type in which they occured, (4) topographic elevation, and (5) meteorological data currently being acquired by Viking Lander I. Three factors are apparent: (1) light streaks predominate, (2) most streaks form in association with fresh bowl-shaped craters, and (3) most light streaks are of the “parallel” type, whereas dark streaks are approximately evenly divided between convergent and parallel forms; moreover, very few light or dark streaks are divergent or fan-shaped. Light streaks have an average azimuth of 218° (corresponding to winds from the northeast), which approximates the orientation of 197 ± 14° for eolian “drifts” observed by the Viking Lander imaging team (Binder et al., 1977). This lends support to the hypothesis that light streaks are deposits of windblown sediments. Dark streaks are oriented at an azimuth of 42° (approximately opposite that of light streaks) and are nearly in line with the dominant wind direction currently recorded by the Viking meteorology instruments (Hess et al., 1977). Although the size of the sample area is not uniform among the various terrain types, the highest frequency of streaks per unit area occurs in the knobby terrain. This is partly explained by the probable production of fine-grained material (weathered from the knobs) to form streaks and other eolian features, and the higher wind turbulence generated around the knobs. The lowest frequency of streaks occurs on the elevated plateaus. The light streaks in Chryse Planitia appear to be relatively stable and to result from deposition of windblown material during times of relatively high velocity northeasterly winds. Dark streaks are more variable and probably result from erosion by southwesterly winds. Both types will be monitored during the extended Viking mission and the results compared with lander data.     

Possible pingo fields in the Utopia basin, Mars: Geological and climatical implications

The presence of pingos on Mars has been hypothesized since the period of the Viking mission. In fact, a diverse range of pingo-like features has been found at various martian sites including Elysium, Chryse and Utopia Planitiae in the northern lowlands. Due to the morphology and the geological setting, some of those features were interpreted in different ways, creating some controversies, as happened in Athabasca Valles. This reflects the complexity of interpreting these features by remote sensing and multiple plausible interpretations of the same feature. With the objective of identifying new possible pingos or rootless cones on Mars, we selected a study area in Utopia Planitia (10-55° N, 210-260° W) where the presence of both features is possible due to its geological history (volcanic and hydrological). We analyzed more than 2100 Mars Orbiter Camera (MOC)-narrow angle images in addition to Viking, Thermal Emission Imaging System (THEMIS), and High Resolution Stereo Camera (HRSC) images, together with Mars Orbiter Laser Altimeter (MOLA)-derived Digital Elevation Models (DEMs) with a Geographic Information System (GIS). We found in 94 MOC-narrow angle images dome, cone, and ring-shaped features. We analyzed them from morphological and morphometrical points of view in order to compare them with relevant features on Mars and Earth. We tested different possible origins for those features following the approach of multiple working hypotheses. We conclude that the dome, cone, and ring-shaped features could be pingos, which is in agreement with their geological settings. Regarding the driving heat source for the formation of the purported pingos, we propose the existence of a heat source, possibly a magma chamber, underneath the surface of the Utopia basin. Together with possible climatic shifts, the past activities of the heat source may have caused melting of ground ice. The pingo growth due to freezing of the water would have occurred during the following cold climatic conditions.