Scalloped Depressions Research Articles

Evidence for thermokarst depressions on comet 67P/Churyumov-Gerasimenko: An interplanetary comparison

Aims. The nucleus of comet 67P/Churyumov-Gerasimenko (67P) is ice rich and shows evidence of morphologies driven by sublimation processes. In particular, the fine-particle deposits (FPD) that cover its surface reveal depressions of many tens of meters. A detailed analysis of these morphologies and of the properties of the fine-particle deposits could help to constrain the amount of water ice in the subsurface. The aim of this paper is to characterize these depressions and constrain their formation and evolution by a detailed quantitative study of their morphometry and thermal environment, and through a comparison with similar morphologies on Earth and Mars. Methods. To study the cometary depressions, we constructed a database of morphometrical parameters for 131 depressions on 67P, 50 alases on Earth, and 200 scalloped depressions on Mars. We measured these parameters manually with the ArcGIS software. We used the images of the Narrow Angle Camera for 67P (down to a resolution of 1 m pixel−1), of the USGC-Digital Orthophoto Quadrangle for the Earth (5 m pixel−1), and of the High Resolution Imaging Science Experiment for Mars (25 cm pixel−1). We also used the digital elevation models associated with these images. In addition, we performed a study of the thermal environment of each depression on 67P to estimate the maximum temperature, the total erosion, and the total energy received from the Sun during one revolution. The morphometrical and thermal environment parameters were then used together for a comparative study of the depressions on 67P, the Earth, and Mars. Results. Our results show that depressions on 67P are geological analogs to alases on Earth and scalloped depressions on Mars; we call them cometary thermokarst depressions. They have been formed by a thermokarst process (i.e., permafrost thawing that causes a subsidence) triggered by water-ice sublimation. The cometary thermokarst depressions on 67P are distributed throughout the nucleus, without preferential distribution for a particular hemisphere or region. They are exclusively located in the fine-particle deposit layer, which is a few meters thick and is thought to have to contain a volume fraction of water ice of about 50% for the subsidence to occur. The formation and evolution of the depressions is driven by the Sun; the side facing the dominant insulation is the preferential direction of erosion. Our results contribute to a better understanding of the periglacial system on comet 67P and of the erosion processes on the nucleus.

Sparse subsurface radar reflectors in Hellas Planitia, Mars

Geomorphological features potentially related to subsurface ice, such as scalloped depressions, expanded craters, pedestal craters, and banded terrain, are present in and around Hellas Planitia, Mars. We present a radar survey of the region using the Shallow Radar (SHARAD) instrument on board the Mars Reconnaissance Orbiter (MRO) to identify candidate subsurface reflectors that may be due to the presence of potentially ice-rich deposits. We found that the majority of radar returns are likely from off-nadir surface topography (“clutter”), arising from the rough topography of the region. There is no widespread radar return from any subsurface interfaces. However, we identify a group of six reflectors adjacent to each other on a plateau in Malea Patera in which we have higher confidence. Landforms associated with a likely ice-rich mantle are associated with the plateau, but the thickness of this mantle does not correspond to the expected depth of the reflectors. However, layers beneath the mantle and marginal pitting at the edge of the plateau are similar to those associated with pedestal craters, which may be ice rich and are a similar thickness to the expected depth of the reflectors. Malea Patera has been interpreted to be a volcanic caldera, so the reflectors may be associated with a volcanic deposit within the plateau, although the evidence for this is inconclusive. Because this radar detection is localized and its origin ambiguous, we cannot use it to make conclusions about the thickness of subsurface deposits in the Hellas region as a whole. The lack of widespread radar reflectors in this region, as compared to the northern mid-latitudes where extensive radar reflections have been mapped, may be due in part to higher surface roughness, which creates radar clutter that may obscure subsurface reflectors. However on the southern rim of the basin and south of the basin, the lack of reflectors may indicate that the possible ice-rich deposits observed geomorphologically in this region are too thin to be resolved by SHARAD, are dielectrically similar to the underlying unit, or have a gradual vertical transition in ice content that is not reflective for the radar. This would imply that recent climate processes may have favored widespread, thick ice deposition or preservation in the northern hemisphere as compared to the southern hemisphere.

Appreciation of the Vascular Anatomy of Aesthetic Forehead Reflation.

Worldwide, the brow is the most common facial site to receive aesthetic treatment. However, the forehead above the brows has been comparatively less well studied with respect to both neuromodulators and fillers. Age-related remodeling of the forehead with loss of facial bone has been demonstrated on detailed radiographic studies. Concurrent loss of facial fat deposits also adds to the volume depletion. The resulting shallow scalloped depressions in the central 2/3 of the forehead give a tired and aged appearance as do the deep etched horizontal forehead lines which are often associated. Temporal hollowing may be an important associated feature. Combination treatment of the upper face with neuromodulators to elevate the brows and diluted hyaluronic acid (HA) fillers to smooth the medial glabellar complex and reflate the forehead and temple has recently become a desired and popular treatment. Several techniques have been described in the literature. All these techniques are designed to allow forehead reflation with reduction of the possibility of vascular compromise. Avoidance of the supratrochlear and supraorbital vasculature with cosmetic filler injections is possible by insertion of needle or cannula into the subgaleal space above their transition from preperiosteal to subcutaneous level. Using this technique we have so far not seen any vascular compromise and we present this technique in the interest of patient safety.

Grid-mapping Hellas Planitia, Mars – Insights into distribution, evolution and geomorphology of (Peri)-glacial, fluvial and lacustrine landforms in Mars' deepest basin

Traditional maps of Hellas Planitia, the most prominent impact basin on Mars, have focused on the delineation of continuous surface units. We applied the newly developed grid-mapping method in order to quantitatively analyze the distribution and geostatistics of selected (peri)-glacial, fluvial, and lacustrine landforms. The study area was subdivided in grid cells with a mesh size of 20 × 20 km, and more than 10,000 grids have been inspected manually in a GIS environment at a mapping scale of 1:30,000. Each grid has been checked for the presence or absence of a landform. Thus, we were able to statistically evaluate the geographical behavior of landforms with respect to elevation, slope inclination, aspect, and other parameters. We searched for 24 pre-selected landforms. However, only 15 of them had a sufficient abundance for scientific research. Whereas the latitude-dependent mantle is widespread in most of Hellas, it was found to be mostly missing in the northeastern part, likely a result of desiccating winds circulating clockwise within the basin. The location and morphologic expression of scalloped terrain also seems to be influenced by winds, as the local orientation of scalloped depressions appears to be aligned along the dominant wind direction, indicating that insolation is not the only factor controlling their formation. Hellas Planitia has been suggested as the site of a former sea. We also searched each grid for the presence of possible shorelines. Despite the small scale of our mapping, no clear evidence for coastal landforms has been detected. Our results reveal a distinctive asymmetry with respect to fluvial channels and Noachian light-toned sediments along the rim of the impact basin. While the northern rim shows a high density of both channels and sediments, the southern counterpart basically lacks channels and light-toned deposits. We suggest different climatic conditions for this imbalance, as the northern part of Hellas likely experienced higher temperatures throughout most of Mars' evolution, while the colder conditions at the southern rim may have prohibited aqueous processes, preventing the development of channels and related sediments. As Hellas contains the deepest areas of the planet's surface, and thus the highest air pressure, its climatic environment can exceed the triple point of water until today, making it a potential habitat. However, our results have shown that the basin floor displays only a very low density of landforms that may indicate liquid water and ice, and especially gullies and viscous-flow features are scarce. The high air pressure and relatively mild temperatures in Hellas decrease the relative atmospheric water content, resulting in a desiccated air and soil, and hence, may explain the lack of viscous-flow features and gullies. All these findings extended our knowledge not only of Hellas Planitia, but of the screened landforms themselves too. In conclusion, small-scale grid-mapping made it possible to recognize large-scale patterns and distributions in Hellas Planitia.

Ice-rich (periglacial) vs icy (glacial) depressions in the Argyre region, Mars: a proposed cold-climate dichotomy of landforms

On Mars, so-called “scalloped depressions” are widely observed in Utopia Planitia (UP) and Malea Planum (MP). Typically, they are rimless, metres- to decametres-deep, incised sharply, tiered inwardly, polygonised and sometimes pitted. The depressions seemingly incise terrain that is icy and possibly thermokarstic, i.e. produced by the thermal destabilisation of the icy terrain. Agewise, the depressions are thought to be relatively youthful, originating in the Late Amazonian Epoch. Here, we report the presence of similar depressions in the Argyre region (AR) (30–60° S; 290–355° E). More importantly, we separate and differentiate these landforms into two groups: (ice-rich) periglacial depressions (Type-1); and, (icy) glacial depressions (Type-2a-c). This differentiation is presented to the Mars community for the first time. Based on a suite of morphological and geological characteristics synonymous with ice-complexes in the Lena Peninsula (eastern Russia) and the Tuktoyaktuk Coastlands (Northwest Territories, Canada), we propose that the Type-1 depressions are ice-rich periglacial basins that have undergone volatile depletion largely by sublimation and as the result of thermal destabilisation. In keeping with the terms and associated definitions derived of terrestrial periglacial-geomorphology, ice-rich refers to permanently frozen-ground in which ice lenses or segregation ice (collectively referenced as excess ice) have formed. We suggest that the depressions are the product of a multi-step, cold-climate geochronology: Atmospheric precipitation and surface accumulation of an icy mantle during recent high obliquities. Regional or local triple-point conditions and thaw/evaporation of the mantle, either by exogenic forcing, i.e. obliquity-driven rises of aerial and sub-aerial temperatures, or endogenic forcing, i.e. along Argyre impact-related basement structures. Meltwater migration into the regolith, at least to the full depth of the depressions. Freeze-thaw cycling and the formation of excess ice. Sublimation of the excess ice and depression formation as high obliquity dissipates and near-surface ice becomes unstable. The Type-2 depressions exhibit characteristics suggestive of (supra-glacial) dead-ice basins and snow/ice suncups observed in high-alpine landscapes on Earth, e.g. the Swiss Alps and the Himalayas. Like the Type-1 depressions, the Type-2 depressions could be the work of sublimation; however, the latter differ from the former in that they seem to develop within a glacial-like icy mantle that blankets the surface rather than within an ice-rich and periglacially-revised regolith at/near the surface. Interestingly, the Type-2 depressions overlie the Type-1 depressions at some locations. If the periglacial/glacial morphological and stratigraphical dichotomy of depressions is valid, then this points to recent glaciation at some locations within the AR being precursed by at least one episode of periglaciation. This also suggests that periglaciation has a deeper history in the region than has been thought hitherto. Moreover, if the hypothesised differences amongst the Argyre-based depressions are mirrored in Utopia Planitia and Malea Planum, then perhaps this periglacial-glacial dichotomy and its associated geochronology are as relevant to understanding late period landscape-evolution in these two regions as it is in the AR?

SHARAD detection and characterization of subsurface water ice deposits in Utopia Planitia, Mars

AbstractMorphological analyses of Utopia Planitia, Mars, have led to the hypothesis that the region contains a substantial amount of near‐surface ice. This paper tests this hypothesis using ground‐penetrating radar techniques. We have identified an expansive radar reflective region spanning approximately 375,000 km2 in SHAllow RADar (SHARAD) data over western Utopia Planitia. The SHARAD reflective regions coincides with high densities of scalloped depressions and polygonal terrain. The reflectors are associated with layered mesas ∼80–170 m thick. We find a value of 2.8 ± 0.8 for the dielectric constant of the material overlying the reflectors. This work finds that the dielectric constant is consistent with a mixture of ice, air, and dust, containing a water ice volume up to 14,300 km3 in this unit.

Effects of varying obliquity on Martian sublimation thermokarst landforms

Scalloped depressions in the Martian mid-latitudes are likely formed by sublimation of ice-rich ground. The stability of subsurface ice changes with the planetary obliquity, generally becoming less stable at lower axial tilt. As a result, the relative rates of sublimation and creep change over time. A landscape evolution model shows that these variations produce internal structure in scalloped depressions, commonly in the form of arcuate ridges, which emerge as depressions resume growth after pausing or slowing. In other scenarios, the formation of internal structure is minimal. Significant uncertainties in past climate and model parameters permit a range of scenarios. Ridges observed in some Martian scalloped depressions could date from obliquity lows or periods of low ice stability occurring <5 Ma, suggesting that the pits are young features and may be actively evolving.

火星表面の永久凍土と周氷河作用

Following the development of high-resolution imaging, digital elevation models, thermal and hydrological data, and onsite ground information during the early period of the 21st century, the periglacial geomorphology of the Martian surface advanced rapidly. Images can even resolute meter-scale landforms, enabling identification of most periglacial features and analysis of their global distributions and detailed morphologies. This review focuses on progress in research covering permafrost distribution, patterned ground, possible heave and subsidence features, lobate debris forms, and slope-lineated features during the last decade. Most of the Martian high-latitude surface is underlain by ice-rich ground called the latitude-dependent mantle (LDM), which favors permafrost-related features possibly developed under warm-humid conditions during past high-obliquity periods or partially s,till active under the present cold-dry conditions. Thermal contraction cracking is likely to prevail in the LDM, resulting in high-centered, flat-top polygons, possibly underlain by sublimation-type wedges or sand wedges that prevail at high latitudes. The surface patterns change into subdued or peak-top polygons toward the mid-latitudes, probably reflecting long-term sublimation of the LDM. Some researchers attribute stone circles to sorting due to freeze-thaw, but the features are much larger than candidates on the Earth. Small isolated domes with concentric cracks or craters at the top may include pingos, which also prevail at high- to mid-latitudes. Asymmetrical scalloped depressions may result from sublimation or thawing of the LDM, but there is a debate between pole-ward and equator-ward slope retreats. Lobate debris aprons may originate from creep of ice-rock mixtures or debris-covered glaciers, but the distinction between the two origins is unclear, as in the long-lasting debate on terrestrial candidates. Some thin, smaller debris lobes at high latitudes resemble stone-banked solifluction lobes on the Earth, which may indicate the occurrence of seasonal freeze-thaw cycles in the recent past. Time-series images indicate active slope features, including gullies, slope streaks, and recurrent slope lineaes that develop below cliffs. These active features may originate from outflows of brine that thaws far below the melting point of water ice.

Modeling the development of martian sublimation thermokarst landforms

Sublimation-thermokarst landforms result from collapse of the surface when ice is lost from the subsurface. On Mars, scalloped landforms with scales of decameters to kilometers are observed in the mid-latitudes and considered likely thermokarst features. We describe a landscape evolution model that couples diffusive mass movement and subsurface ice loss due to sublimation. Over periods of tens of thousands of Mars years under conditions similar to the present, the model produces scallop-like features similar to those on the martian surface, starting from much smaller initial disturbances. The model also indicates crater expansion when impacts occur in surfaces underlain by excess ice to some depth, with morphologies similar to observed landforms on the martian northern plains. In order to produce these landforms by sublimation, substantial quantities of excess ice are required, at least comparable to the vertical extent of the landform, and such ice must remain in adjacent terrain to support the non-deflated surface. We suggest that martian thermokarst features are consistent with formation by sublimation, without melting, and that significant thicknesses of very clean excess ice (up to many tens of meters, the depth of some scalloped depressions) are locally present in the martian mid-latitudes. Climate conditions leading to melting at significant depth are not required.

Possible ice-wedge polygons and recent landscape modification by “wet” periglacial processes in and around the Argyre impact basin, Mars

The Argyre basin and associated rim-materials in the southern hemisphere of Mars are ancient, having been formed by the impact of a large body ∼4Gya. This notwithstanding, the regional landscape continues to be altered by a multiplicity of geological and geomorphological processes.Three landforms, whose close spatial-association is identified in a new geological map of the Argyre region (∼290–360°E; ∼30–72°S), feature prominently in the list of very Late Amazonian Epoch alterations:(a)Small-sized (⩽∼20m in diameter) and unsorted polygons that exhibit metre to sub-metre elevated margins or shoulders, giving them a low-centred appearance; in “wet” permafrost environments on Earth low-centred polygons (LCPs) often are underlain by ice-wedges.(b)Gullies, seemingly formed by “wet” flow(s) and incised by the LCPs.(c)A putative (possibly ice-rich) latitude-dependent mantle (LDM) that underlies the LCPs and spatially-convergent “wet” gullies in all of our observations.These landforms occur from the middle to the high (near-polar) latitudes of the region and overlie geological units of all ages.Hitherto, the presence of the LCPs has been reported only fleetingly in the literature and only in as much as they have been observed on the walls of a few scalloped depressions in mid-Utopia Planitia. By contrast, we report the ubiquitous occurrence of the LCPs in and around the Argyre impact-basin on gully-margins and adjacent slopes as well as on relatively-flat inter-crater terrain.On the basis of three separate but related lines of reasoning we hypothesise that ice wedges could be present beneath LCP margins in our study region. If we are correct then these LCP sites are geomorphological expressions of boundary conditions that were relatively warm and wet in the recent past.First, substantial ice-wedging in permafrost environments on Earth requires the availability of meltwater. If, as some researchers propose, the formation of some “wet” gullies at the martian mid-latitudes is induced by the localised (slope-side and crater-wall) thaw of the underlying LDM at the gully sites themselves, then meltwater also should be available for ice-wedging and the formation of LCPs at these sites. Interestingly, LCPs are observed on gully-channel walls as expected if meltwater is associated with gully formation; however, in some instances the LCPs are also observed on the slope-side terrain that extends for hundreds of metres beyond the channel walls and even above the gully alcoves. This suggests that the distribution of icy terrain affected by thaw could be much more substantial than has been suggested hitherto.Second, LCPs that are identical in shape and scale to the slope-side LCPs are observed on relatively flat inter-crater terrain (also underlain by the LDM) that is distal from the “wet” gullies. By contrast, here, their distribution extends for kilometres. This too could be indicative of meltwater being more extensive in the regional landscape than most workers have thought possible.Third, on Earth the “dryness” or the “wetness” of a permafrost environment determines whether LCP margins are underlain by sand or by ice. When the observed LCPs and major deposits of sand in the study region are plotted on our new map of the Argyre impact-basin, we see that these deposits are rarely proximal. On the other hand, the LDM underlies the LCPs in all instances and, if modified by thaw, seems to be a likelier source of margin fill for the LCPs than sand.

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.

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.

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.

Distribution and evolution of scalloped terrain in the southern hemisphere, Mars

Scalloped depressions are a unique martian surface morphology found in the northern and southern hemisphere latitude-dependent dust and ice-rich surface mantles. These features exhibit a distinct asymmetric north–south slope profile, characterized by steep pole-facing scarps, flat floors and gentle equator-facing slopes. We examined High Resolution Stereo Camera (HRSC) images of the southern hemisphere to determine their longitudinal distribution, which revealed that a majority of scalloped terrain is located in the region of the southern wall of the Hellas Basin and northern Malea Planum. A detailed map of this area was produced where scallops were found to contour the southern wall of the basin, and where the ice-rich mantle was seen to be thickest. Scalloped terrain is concentrated along the topographic highs near the Amphitrites and Peneus Paterae and areal extent and depth decreases with increasing depth into the basin. We also examined existing hypothesis for the formation and evolution of scalloped depressions using High Resolution Imaging Science Experiment (HiRISE) images and data from the Thermal Emission Imaging System–Infrared (THEMIS–IR) and the Thermal Emission Spectrometer (TES). Our approach provides regional context for the development of scalloped terrains within the southern hemisphere, and offers detailed evidence of scallop depressions forming around small cracks, presumably caused by thermal contraction. Morphometric measurements show that scalloped depressions can be as much as 40 m deep, with typical depths of between 10 and 20 m. Our observations of scallop formation and development in the southern hemisphere support a solar-insolation model proposed by previous researchers (e.g. [Morgenstern, A., Hauber, E., Reiss, D., van Gasselt, S., Grosse, G., Schirrmeister, L., 2007. J. Geophys. Res. 112, CiteID E06010; Lefort, A., Russell, P.S., Thomas, N., McEwen, A.S., Dundas, C.M., Kirk, R.L., 2009a. J. Geophys. Res. 114, E04005; Lefort, A., Russell, P.S., Thomas, N., 2009b. Icarus, in press]). Observations made using HiRISE images suggest that scalloped depressions most likely form from small cracks in the mantle, which become larger and deeper through sublimation of interstitial ice from within the mantle. Sublimation is likely enhanced on equator-facing slopes because of increased solar insolation, which accounts for the asymmetric slope profile and hemispherical orientation and is demonstrated by THEMIS-IR images. We suggest that sublimation lag deposits can possibly be removed by dust devils or strong slope winds related to the Hellas Basin, offering an explanation as to why scalloped terrain is so abundant only in this area of the southern hemisphere. Daytime maximum summer temperatures suggest that sublimation in the study area of Malea Planum is possible under current conditions if the sublimation lag is removed. While it cannot be ruled out that scalloped terrain in Malea Planum is presently evolving, we attribute the extensive distribution to geologically recent obliquity excursions when conditions were more conducive to mesoscale modification of the ice-rich mantle.

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.

Observations of periglacial landforms in Utopia Planitia with the High Resolution Imaging Science Experiment (HiRISE)

The region of western Utopia Planitia (80–105°E, 40–55°N) displays several types of landforms similar to Earth periglacial features, including scallop‐shaped depressions and networks of polygonal terrains. The scalloped depressions have been proposed to originate from thermokarstic processes such as sublimation and/or melting of near‐surface ground ice. Using HiRISE imagery, we characterize these depressions and several associated, distinct polygon networks in unprecedented morphologic and topographic detail and investigate support for an ice‐based degradation process. The scalloped depressions and interior polygons and ridges are found to evolve together, mainly influenced by sublimation, local proximity of ground ice to the surface, and obliquity variations.

Dendritic changes in the spinal dorsal horn following transection of a peripheral nerve

In an attempt to examine the morphologic changes which take place in the spinal dorsal horn as a consequence of peripheral nerve injury, the superficial radial nerve was cut and prevented from regenerating in adult cats. In laminae I–V of the 7th cervical segment of spinal dorsal horn at 2 weeks, 1 and 2 months following transection, small and medium-sized dendritic shafts developed membrane-bounded cavities. These cavities varied in number and size in each dendrite and were sometimes connected to the agranular reticulum. Large cavities often hollowed out a dendrite, leaving little remaining cytoplasm between the cavity membrane and the plasma membrane. Small and large cavities were frequently found open to the intercellular space. Synaptic glomeruli showed a loss of small dendrites with empty scalloped depressions in the central axonal endings of these structures left exposed to enlarged intercellular spaces. In addition, clusters of many enlarged oval or irregular intercellular spaces were found in the neuropil where many cavitated dendrites were observed. At least some of these intercellular spaces were thought to be derived from the loss of dendrites. From these observations we conclude that small and medium-sized dendrites involute through cavitation and eventually disappear from the spinal dorsal horn when primary input is disturbed by transection of peripheral sensory nerves.

An electron microscopic analysis of the trans-synaptic effects of peripheral nerve injury subsequent to tooth pulp extirpations on neurons in laminae I and II of the medullary dorsal horn

To examine the effects of peripheral nerve injury on second-order neurons in laminae I and II of the medullary dorsal horn, tooth pulps of all mandibular teeth in adult cats on one side were extirpated. This procedure severed and removed the receptors and terminal branches of the primary trigeminal neurons which innervate the tooth pulps of these teeth. The empty pulp chambers were then filled with dental cement to prevent regeneration. At 30 and 60 days postoperatively, membrane-lined cavities had formed inside many of the small-caliber dendrites of second-order neurons in laminae I and II. Cavity formation occurred mainly in dendritic shafts less than 2 micron in diameter and involved dendrites with synaptic vesicles as well as those without synaptic vesicles. The cavities extensively hollowed out these dendrites, often occupying more than half the cross-sectional diameter of the shafts and extending for appreciable distances in the long axis of the shaft. The process of cavitation ultimately resulted in the destruction of the affected dendrites. Many cavities became patent to the intercellular space with the cavity membrane establishing continuity with the dendritic membrane. Many cavities often formed in a single dendrite, and such severely cavitated dendrites became reduced to a trabeculated shell which ultimately fragmented into several small pieces. The presence of synaptic connections from a number of different kinds of axonal endings, including scalloped and dome-shaped endings, was not sufficient to prevent cavitation. The actual severing of synaptic connections on the cavitated dendrite appeared to be a relatively late event in the process since small pieces of dendritic debris could still be found clinging to their axodendritic synapses. Evidence that dendrites were being lost from the neuropil was most readily apparent in many of the disrupted glomeruli in lamina II in which many of the scalloped depressions in the central axonal endings that normally contained small dendrites were empty. Many central axonal endings remained in synaptic contact with only a single dendrite which often showed signs of cavitation. Such central endings showed only subtle remaining traces of their normal scalloped contours. This study demonstrates that injury to the distal branches of primary trigeminal neurons which innervate tooth pulps resulted in trans-synaptic degenerative changes in the dendritic arbors of second-order neurons which destroyed fine-caliber higher order dendrites.

Dendroaxonic synapses in the substantia gelatinosa glomeruli of the spinal trigeminal nucleus of the cat

The glomeruli in the substantia gelatinosa layer of the spinal trigeminal nucleus of the cat contain three kinds of dendritic processes. One of these, the type 2 dendrite, contains large synaptic vesicles in its spine heads and in its shafts. The type 2 dendrite receivers axodendritic synapses from primary trigeminal afferent (C) axons and an occasional axodendritic synapse from small axonal (P) endings with small synaptic vesicles. The type 2 dendrites in turn form dendroaxonic synapses on the C endings. The dendroaxonic synapse and the axodendritic synapse of the C ending typically occur in reciprocal pairs. The axodendritic synapse usually lies in the depths of scalloped depressions in the surface of the C ending while the dendroaxonic synapse is found on the rim of the depression. Type 1 spines, i.e., dendritic spines receiving axodendritic synapses from the primary ending and lacking synaptic vesicles, also receive dendrodentritic synapses from type 2 dendrites. The type 2 dendrite with its large, rounded synaptic vesicles is considered to be excitatory at its dendroaxonic and dendrodendritic synapses. The type 2 dendrites course from glomerulus to glomerulus receiving their excitatory input through the axodendritic synapses of C axons. A type 2 dendrite, in response to C axon excitation would activate type 1 spines directly through their dendrodendritic synapses (C leads to 2 leads to 1) and indirectly by increasing transmitter release at the axodendritic synapses of the C axonal endings through their dendroaxonic synapses (2 leads to C leads to 1). The type 2 dendrites could serve two functions. First, they may prolong transmitter release from the axodendritic synapses of C axonal endings beyond the time of arrival of incoming potentials because of the reciprocal pairing of dendroaxonic and axodendritic synapses (C in equilibrium 2). Second, they may extend the spatial range of the excitatory output of active primary afferent axons to type 1 spines of glomeruli whose primary afferent axons may be inactive (C leads to 2 leads to 1).