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.     

Periglacial mass-wasting landforms on Mars suggestive of transient liquid water in the recent past: Insights from solifluction lobes on Svalbard

On Earth, periglacial solifluction is a slow mass-wasting process related to freeze–thaw activity. We compare the morphology of small-scale lobate features on Mars to solifluction lobes in Svalbard to constrain their processes of formation. The analysis is based on high-resolution satellite imagery of Mars (HiRISE, ～25 cm/pxl), aerial images of Svalbard with a similar spatial resolution (HRSC–AX, ～20 cm/pxl) acquired through an air campaign in summer 2008, and ground truth obtained during two summer expeditions in 2009 and 2011 on Svalbard. We present a detailed study of two crater environments on Mars displaying two types of lobate forms, characterized as sorted (clast-banked) and non-sorted lobes. On both Svalbard and Mars such lobes typically occur as clusters of overlapping risers (lobe fronts), pointing to differential velocities in the soil. The martian small-scale lobes have well-defined arcuate risers and lobe treads (surface). Lobe widths range between 14 and 127 m and tread lengths between 13 and 105 m. Riser height is estimated to be approximately 1–5 m. The lobes on Mars share the plan view morphology of solifluction lobes on Svalbard and their morphometry is within the range of values of terrestrial solifluction lobes. The lobes are distinct from permafrost-creep landforms such as rock glaciers. We show the results of a survey of 53 HiRISE images covering latitudes between 59°N and 81°N. Similar to Svalbard, the studied lobate features on Mars occur in close spatial proximity to gullies and thermal contraction polygons. The widespread distribution of the lobate forms in the northern hemisphere and their close association to ground-ice and gullies are best explained by mass-wasting processes related to frost creep, gelifluction and/or plug-like flow. This suggests a protracted process (thousand to several thousands of years) of freeze–thaw activity at the northern high latitudes on Mars. Age constraints on lobe deposits and superposition relationships with gullies and polygons imply a process involving liquid water within the last few million years.     

The Circum-Hellas Volcanic Province,Mars: Overview

Building on previous studies of volcanoes around the Hellas basin with new studies of imaging (High-Resolution Stereo Camera (HRSC), Thermal Emission Imaging System (THEMIS), Mars Orbiter Camera (MOC), High-Resolution Imaging Science Experiment (HiRISE), Context Imager (CTX)), multispectral (HRSC, Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA)), topographic (Mars Orbiter Laser Altimeter (MOLA)) and gravity data, we define a new Martian volcanic province as the Circum-Hellas Volcanic Province (CHVP). With an area of >2.1 million km², it contains the six oldest central vent volcanoes on Mars, which formed after the Hellas impact basin, between 4.0 and 3.6 Ga. These volcanoes mark a transition from the flood volcanism that formed Malea Planum ～3.8 Ga, to localized edifice-building eruptions. The CHVP volcanoes have two general morphologies: (1) shield-like edifices (Tyrrhena, Hadriaca, and Amphitrites Paterae), and (2) caldera-like depressions surrounded by ridged plains (Peneus, Malea, and Pityusa Paterae). Positive gravity anomalies are found at Tyrrhena, Hadriaca, and Amphitrites, perhaps indicative of dense magma bodies below the surface. The lack of positive-relief edifices and weak gravity anomalies at Peneus, Malea, and Pityusa suggest a fundamental difference in their formation, styles of eruption, and/or compositions. The northernmost volcanoes, the ～3.7–3.9 Ga Tyrrhena and Hadriaca Paterae, have low slopes, well-channeled flanks, and smooth caldera floors (at tens of meters/pixel scale), indicative of volcanoes formed from poorly consolidated pyroclastic deposits that have been modified by fluvial and aeolian erosion and deposition. The ～3.6 Ga Amphitrites Patera also has a well-channeled flank, but it and the ～3.8 Ga Peneus Patera are dominated by scalloped and pitted terrain, pedestal and ejecta flow craters, and a general ‘softened’ appearance. This morphology is indicative not only of surface materials subjected to periglacial processes involving water ice, but also of a surface composed of easily eroded materials such as ash and dust. The southernmost volcanoes, the ～3.8 Ga Malea and Pityusa Paterae, have no channeled flanks, no scalloped and pitted terrain, and lack the ‘softened’ appearance of their surfaces, but they do contain pedestal and ejecta flow craters and large, smooth, bright plateaus in their central depressions. This morphology is indicative of a surface with not only a high water ice content, but also a more consolidated material that is less susceptible to degradation (relative to the other four volcanoes). We suggest that Malea and Pityusa (and possibly Peneus) Paterae are Martian equivalents to Earth's giant calderas (e.g., Yellowstone, Long Valley) that erupted large volumes of volcanic materials, and that Malea and Pityusa are probably composed of either lava flows or ignimbrites. HRSC and OMEGA spectral data indicate that dark gray to slightly red materials (often represented as blue or black pixels in HRSC color images), found in the patera floors and topographic lows throughout the CHVP, have a basaltic composition. A key issue is whether this dark material represents concentrations of underlying basaltic material eroded by various processes and exposed by aeolian winnowing, or if the material was transported from elsewhere on Mars by regional winds. Understanding the provenance of these dark materials may be the key to understanding the volcanic diversity of the Circum-Hellas Volcanic Province.     

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.     

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.     

Winds and tides of Ligeia Mare,with application to the drift of the proposed time TiME (Titan Mare Explorer) capsule

We use two independent General Circulation Models (GCMs) to estimate surface winds at Titan’s Ligeia Mare (78° N, 250° W), motivated by a proposed mission to land a floating capsule in this ～500 km hydrocarbon sea. The models agree on the overall magnitude (～0.5–1 m/s) and seasonal variation (strongest in summer) of windspeeds, but details of seasonal and diurnal variation of windspeed and direction differ somewhat, with the role of surface exchanges being more significant than that of gravitational tides in the atmosphere. We also investigate the tidal dynamics in the sea using a numerical ocean dynamics model: assuming a rigid lithosphere, the tidal amplitude is up to ～0.8 m. Tidal currents are overall proportional to the reciprocal of depth—with an assumed central depth of 300 m, the characteristic tidal currents are ～1 cm/s, with notable motions being a slosh between Ligeia’s eastern and western lobes, and a clockwise flow pattern.We find that a capsule will drift at approximately one tenth of the windspeed, unless measures are adopted to augment the drag areas above or below the waterline. Thus motion of a floating capsule is dominated by the wind, and is likely to be several km per Earth day, a rate that will be readily measured from Earth by radio navigation methods. In some instances, the wind vector rotates diurnally such that the drift trajectory is epicyclic.     

Major episodes of geologic history of Isidis Planitia on Mars

We have mapped the area of Isidis Planitia (1–27°N, 75–103°E) in order to assess the geologic history of this region using modern data sets such as MOLA topography and the high-resolution images provided by the HRSC, CTX, and HiRISE cameras. Results of our mapping show that the geologic history of Isidis Planitia consists of three principal episodes. (1) Impact dominated episode (Noachian, until ～3.8 Ga): During this time, the oldest materials in the study area were formed mostly by impact reworking and mass-wasting. Other processes (e.g., volcanism and fluvial/glacial activity) likely operated at this time but played a subordinate role. (2) An episode related to volcanic and fluvial/glacial activities (late Noachian–early Amazonian, ～3.8–2.8 Ga): Volcanism appears as the most important process at the beginning of this episode (～3.8–3.5 Ga) and was responsible for the formation of a large circum-Isidis volcanic province by the early Hesperian epoch. Volcanic materials covered large portions of the Isidis rim, almost completely buried the previous crater record on the floor of the Isidis basin, and probably were the major contributors to the filling of the basin. Fluvial/glacial processes prevailed closer to the end of the episode (early Hesperian–early Amazonian, ～3.5–2.8 Ga) and were responsible for widespread resurfacing in the Isidis Planitia region, mostly at ～3.1–3.4 Ga. Glaciers and/or ice sheets probably resulted in a massive glaciation of the rim and the floor of the Isidis basin. The total volume of material eroded from the Isidis rim by glacial and fluvial activity is estimated to be about 35,000–50,000 km³, which is equivalent to a composite layer about 40–60 m thick on entire floor of the basin. More important, however, is that the eroded materials were likely saturated with ice/water and could form wet deposits on the floor. (3) Wind-dominated episode (since early Amazonian, ～2.8 Ga): Wind activity dominated the later geologic history of Isidis Planitia but resulted only in minor modification of the surface.     

Gasa impact crater,Mars: Very young gullies formed from impact into latitude-dependent mantle and debris-covered glacier deposits?

The mode of formation of gullies on Mars, very young erosional–depositional landforms consisting of an alcove, channel, and fan, is one of the most enigmatic problems in martian geomorphology. Major questions center on their ages, geographic and stratigraphic associations, relation to recent ice ages, and, if formed by flowing water, the sources of the water to cause the observed erosion/deposition. Gasa (35.72°S, 129.45°E), a very fresh 7-km diameter impact crater and its environment, offer a unique opportunity to explore these questions. We show that Gasa crater formed during the most recent glacial epoch (2.1–0.4 Ma), producing secondary crater clusters on top of the latitude-dependent mantle (LDM), interpreted to be a layered ice-dust-rich deposit emplaced during this glacial epoch. High-resolution images of a pre-Gasa impact crater ～100 km northeast of Gasa reveal that portions of the secondary-crater-covered LDM have been removed from pole-facing slopes in crater interiors near Gasa; gullies are preferentially located in these areas and channels feeding alcoves and fans can be seen to emerge from the eroding LDM layers to produce multiple generations of channel incision and fan lobes. We interpret these data to mean that these gullies formed extremely recently in the post-Gasa-impact time-period by melting of the ice-rich LDM. Stratigraphic and topographic relationships are interpreted to mean that under favorable illumination geometry (steep pole-facing slopes) and insolation conditions, melting of the debris-covered ice-rich mantle took place in multiple stages, most likely related to variations in spin-axis/orbital conditions. Closer to Gasa, in the interior of the ～18 km diameter LDM-covered host crater in which Gasa formed, the pole-facing slopes display two generations of gullies. Early, somewhat degraded gullies, have been modified by proximity to Gasa ejecta emplacement, and later, fresh appearing gullies are clearly superposed, cross-cut the earlier phase, and show multiple channels and fans, interpreted to be derived from continued melting of the LDM on steep pole-facing slopes. Thus, we conclude that melting of the ice-rich LDM is a major source of gully activity both pre-Gasa crater and post-Gasa crater formation. The lack of obscuration of Gasa secondary clusters formed on top of the LDM is interpreted to mean that the Gasa impact occurred following emplacement of the last significant LDM layers at these low latitudes, and thus near the end of the ice ages. This interpretation is corroborated by the lack of LDM within Gasa. However, Gasa crater contains a robustly developed set of gullies on its steep, pole-facing slopes, unlike other very young post-LDM craters in the region. How can the gullies inside Gasa form in the absence of an ice-rich LDM that is interpreted to be the source of water for the other adjacent and partly contemporaneous gullies? Analysis of the interior (floor and walls) of the host crater suggest that prior to the Gasa impact, the pole-facing walls and floor were occupied by remnant debris-covered glaciers formed earlier in the Amazonian, which are relatively common in crater interiors in this latitude band. We suggest that the Gasa impact cratering event penetrated into the southern portion of this debris-covered glacier, emplaced ejecta on top of the debris layer covering the ice, and caused extensive melting of the buried ice and flow of water and debris slurries on the host crater floor. Inside Gasa, the impact crater rim crest and wall intersected the debris-covered glacier deposits around the northern, pole-facing part of the Gasa interior. We interpret this exposure of ice-rich debris-covered glacial material in the crater wall to be the source of meltwater that formed the very well-developed gullies along the northern, pole-facing slopes of Gasa crater.     

Methane emissions from Earth’s degassing: Implications for Mars

The presence of methane on Mars is of great interest, since one possibility for its origin is that it derives from living microbes. However, CH₄ in the martian atmosphere also could be attributable to geologic emissions released through pathways similar to those occurring on Earth. Using recent data on methane degassing of the Earth, we have estimated the relative terrestrial contributions of fossil geologic methane vs. modern methane from living methanogens, and have examined the significance that various geologic sources might have for Mars.Geologic degassing includes microbial methane (produced by ancient methanogens), thermogenic methane (from maturation of sedimentary organic matter), and subordinately geothermal and volcanic methane (mainly produced abiogenically). Our analysis suggests that ～80% of the “natural” emission to the terrestrial atmosphere originates from modern microbial activity and ～20% originates from geologic degassing, for a total CH₄ emission of ～28.0×10⁷ tonnes year^?1.Estimates of methane emission on Mars range from 12.6×10¹ to 57.0×10⁴ tonnes year^?1 and are 3–6 orders of magnitude lower than that estimated for Earth. Nevertheless, the recently detected martian, Northern-Summer-2003 CH₄ plume could be compared with methane expulsion from large mud volcanoes or from the integrated emission of a few hundred gas seeps, such as many of those located in Europe, USA, Mid-East or Asia. Methane could also be released by diffuse microseepage from martian soil, even if macro-seeps or mud volcanoes were lacking or inactive. We calculated that a weak microseepage spread over a few tens of km², as frequently occurs on Earth, may be sufficient to generate the lower estimate of methane emission in the martian atmosphere.At least 65% of Earth’s degassing is provided by kerogen thermogenesis. A similar process may exist on Mars, where kerogen might include abiogenic organics (delivered by meteorites and comets) and remnants of possible, past martian life. The remainder of terrestrial degassed methane is attributed to fossil microbial gas (～25%) and geothermal-volcanic emissions (～10%). Global abiogenic emissions from serpentinization are negligible on Earth, but, on Mars, individual seeps from serpentinization could be significant. Gas discharge from clathrate-permafrost destabilization should also be considered.Finally, we have shown examples of potential degassing pathways on Mars, including mud volcano-like structures, fault and fracture systems, and major volcanic edifices. All these types of structures could provide avenues for extensive gas expulsion, as on Earth. Future investigations of martian methane should be focused on such potential pathways.     

Surface age of the ice–dust mantle deposit in Malea Planum,Mars

The mid- and high-latitudes of Mars are covered by a smooth young mantle that is interpreted as an atmospherically derived air-fall deposit of ice and dust related to recent climate changes. In order to determine relative and absolute ages of this surface unit within the southern hemisphere, a systematic survey of all available HiRISE and CTX images in the Malea Planum region from 55–60°S latitude and 50–70°E longitude was performed and the distribution and the morphology of small impact craters on the mantle deposit were investigated. Using crater size-frequency measurements, we derived absolute model ages of ～3–5 Ma for the surface of the mantle, immediately south of the Hellas basin rim. Morphologic observations of the mantle, its fresh appearance, very low number of craters, and superposition on older units support this very young Amazonian age. Nearly all observed craters on the smooth mantle in Malea Planum are small and show signs of erosion, evidence for the ongoing modification of the ice–dust mantle. However, this modification has not been strong enough to reset the surface age. Compared to the ice–dust mantle at higher latitudes in the northern and southern hemisphere, the surface of the mantle in Malea Planum is older and thus has been relatively stable during obliquity changes in the last ～3–5 Ma. This is consistent with the hypothesis that the ice–dust mantle is a complex surface deposit of different layers, that shows a strong latitude dependence in morphology and has been deposited and degraded at different times in martian history.     

Titan’s global crater population: A new assessment

We report a revised crater population for Titan using Cassini RADAR data through January 2010 (flyby T65), and make a size-dependent correction for the incomplete coverage (～33%) using a Monte-Carlo model. Qualitatively, Titan’s landscape is more heavily cratered than Earth, but much less than Mars or Ganymede: the area fraction covered by craters is in fact comparable with that of Venus. Quantitative efforts to interpret crater densities for Titan as surface age have been confounded by widely divergent crater production rates proposed in the literature. We elucidate the specific model assumptions that lead to these differences (assumed projectile density, scaling function for simple crater diameter, and complex crater size exponent) and suggest these are reasonable bounding models, with the Korycansky and Zahnle (2005) model representing a crater retention age of ～1 Ga, and the Artemieva and Lunine (2005) model representing a crater retention age of ～200 Ma. These estimates are consistent with models of Titan’s evolution that predict a thickening of its crust 0.3–1.2 Gyr ago.     

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.     

A multi-wavelength study of the 2009 impact on Jupiter: Comparison of high resolution images from Gemini,Keck and HST

Within several days of A. Wesley’s announcement that Jupiter was hit by an object on UT 19 July 2009, we observed the impact site with (1) the Hubble Space Telescope (HST) at UV through visible (225–924 nm) wavelengths, (2) the 10-m W.M. Keck II telescope in the near-infrared (1–5 μm), and (3) the 8-m Gemini-North telescope in the mid-infrared (7.7–18 μm). All observations reported here were obtained between 22 and 25 July 2009. Observations at visible and near-infrared wavelengths show that large (～0.75-μm radius) dark (imaginary index of refraction m_i ～ 0.01–0.1) particulates were deposited at atmospheric pressures between 10 and 200–300 mbar; analysis of HST-UV data reveals that in addition smaller-sized (～0.1 μm radius) material must have been deposited at the highest altitudes (～10 mbar). Differences in morphology between the UV and visible/near-IR images suggest three-dimensional variations in particle size and density across the impact site, which probably were induced during the explosion and associated events. At mid-infrared wavelengths the brightness temperature increased due to both an enhancement in the stratospheric NH₃ gas abundance and the physical temperature of the atmosphere. This high brightness temperature coincides with the center part of the impact site as seen with HST. This observation, combined with (published) numerical simulations of the Shoemaker-Levy 9 impacts on Jupiter and the Tunguska airburst on Earth, suggests that the downward jet from the terminal explosion probably penetrated down to the ～700-mbar level.     

Reduced sulfur–carbon–water systems on Mars may yield shallow methane hydrate reservoirs

Methane clathrate hydrate reservoirs capped by overlying permafrost have been proposed as potential sources of atmospheric methane plumes on Mars. However, the surface flux of methane from hydrate dissociation is limited by the diffusion rate of methane through the overlying ice. Assuming hydrates underlay the entire plume footprint, the maximum diffusion path length is expected to be less than 15 m, depths too shallow to stabilize pure methane hydrates under Mars geothermal and lithostatic conditions at low to mid latitudes. Therefore, pure methane hydrates confined within permafrost could not produce methane surface fluxes of the magnitude observed near the equator. However, the addition of 10% H₂S, a secondary gas commonly associated with methane production on Earth, expands the hydrate stability field, with clathrates expected within 10 m of the surface at the equator and at depths less than 1 m at higher latitudes. This indicates that H₂S would also be expected to be released as well as methane if the plumes have a confined hydrate reservoir source.     

The growth of wind-waves in Titan’s hydrocarbon seas

Ocean wave growth on Titan is considered. The classic Sverdrup–Munk theory for terrestrial wave growth is applied to Titan, and is compared with a simple energy balance model that exposes the effect of Titan’s environmental parameters (air density, gravity, and fluid density). These approaches are compared with the only previously-published (semi-empirical) model (Ghafoor, N.A.-L., Zarnecki, J.C., Challenor, P., Srokosz, M.A. [2000] J. Geophys. Res. 105, 12,077–12,091, hereafter G2k), and allow the impact of various parameters such as atmospheric density to be transparently explored.Our model, like G2k, suggests fully-developed significant wave heights on Titan H_s = 0.2 U², where U is the windspeed (SI units): in dimensionless terms this is rather close to H_s = 0.2 U²/g, a rule of thumb previously noted for terrestrial waves (we find various datasets where the prefactor varies by ～2). It is noted that liquid and air densities affect the growth rate of waves, but not their fully-developed height: for 1 m/s winds wave amplitude reaches 0.15 m (75% of fully-developed) with a fetch of only 1 km, rather faster than predicted by G2k. Liquid viscosity has no major effect on gravity wave growth, but does influence the threshold windspeed at which gravity–capillary waves form in the first place.The model is used to develop predicted ranges for wave height to guide the design of the Titan Mare Explorer (TiME), a proposed Discovery-class mission to float a capsule on Ligeia Mare in 2023. For the expected maximum 1 m/s winds, a significant wave height of 0.2 m and wavelength of ～4 m can be expected. Assuming that wave heights follow Rayleigh statistics as they do on Earth, then given the wave period of ～4 s, individual waves of ～0.6 m might be encountered over a 3 month period.For predicted Titan winds at Kraken Mare, significant wave heights may reach ～0.6 m in the peak of summer but do not exceed the tidal amplitude at its northern end, consistent with the area around Mayda Insula being a tidal flat, while elsewhere on Kraken and Ligeia and at Ontario Lacus, shorelines may be wave- or tidally-dominated, depending on the specific location.     

Rheologies and ages of lava flows on Elysium Mons,Mars

We present results of our study of the rheologies and ages of lava flows in the Elysium Mons region of Mars. Previous studies have shown that the geometric dimensions of lava flows reflect rheological properties such as yield strength, effusion rate and viscosity. In this study the rheological properties of lava flows in the Elysium Mons region were determined and compared to the rheologies of the Ascraeus Mons lava flows. We also derived new crater size-frequency distribution measurements (CSFDs) for the Elysium lava flows to identify possible changes in the rheological properties with time. In addition, possible changes in the rheological properties with the distance from the caldera of Elysium Mons were analyzed.In total, 35 lava flows on and around Elysium Mons were mapped, and divided into three groups, lava flows on the flanks of Elysium Mons, in the plains between the three volcanoes Elysium Mons, Hecates and Albor Tholus and lava flows south of Albor Tholus. The rheological properties of 32 of these flows could be determined. Based on our morphometric measurements of each individual lava flow, estimates for the yield strengths, effusion rates, viscosities, and eruption duration of the studied lava flows were made. The yield strengths of the investigated lava flows range from ～3.8 × 10² Pa to ～1.5 × 10⁴ Pa, with an average of ～3.0 × 10³ Pa. These yield strengths are in good agreement with estimates for terrestrial basaltic lava flows. The effusion rates are on average ～747 m³ s^?1, ranging from ～99 to 4450 m³ s^?1. The viscosities are on average ～4.1 × 10⁶ Pa s, with a range of 1.2 × 10⁵ Pa s to 3.1 × 10⁷ Pa s. The eruption durations of the flows were calculated to be between 6 and 183 days, with an average of ～51 days. The determined rheological properties are generally very similar to those of other volcanic regions on Mars, such as on Ascraeus Mons in the Tharsis region. Calculated yield strengths and viscosities point to a basaltic/andesitic composition of the lava flows, similar to basaltic or andesitic a’a lava flows on Earth.Absolute model ages of all 35 lava flows on Elysium Mons were derived from crater size-frequency distribution measurements (CSFD). The derived model ages show a wide variation from about 632 Ma to 3460 Ma. Crater size-frequency distribution measurements of the Elysium Mons caldera show an age of ～1640 Ma, which is consistent with the resurfacing age of Werner (2009). Significant changes of the rheologies with time could not be observed. Similarly, we did not observe systematic changes in ages with increasing distances of lava flows from the Elysium Mons caldera.     

Volatile transport on inhomogeneous surfaces: I – Analytic expressions,with application to Pluto’s day

The two orders of magnitude drop between the measured atmospheric abundances of non-radiogenic argon, krypton and xenon in Earth versus Mars is striking. Here, in order to account for this difference, we explore the hypothesis that clathrate deposits incorporated into the current martian cryosphere have sequestered significant amounts of these noble gases assuming they were initially present in the paleoatmosphere in quantities similar to those measured on Earth (in mass of noble gas per unit mass of the planet). To do so, we use a statistical-thermodynamic model that predicts the clathrate composition formed from a carbon dioxide-dominated paleoatmosphere whose surface pressure ranges up to 3 bars. The influence of the presence of atmospheric sulfur dioxide on clathrate composition is investigated and we find that it does not alter the trapping efficiencies of other minor species. Assuming nominal structural parameters for the clathrate cages, we find that a carbon dioxide equivalent pressure of 0.03 and 0.9 bar is sufficient to trap masses of xenon and krypton, respectively, equivalent to those found on Earth in the clathrate deposits of the cryosphere. In this case, the amount of trapped argon is not sufficient to explain the measured Earth/Mars argon abundance ratio in the considered pressure range. In contrast, with a 2% contraction of the clathrate cages, masses of xenon, krypton and argon at least equivalent to those found on Earth can be incorporated into clathrates if one assumes the trapping of carbon dioxide at equivalent atmospheric pressures of ～2.3 bar. The proposed clathrate trapping mechanism could have then played an important role in the shaping of the current martian atmosphere.     

Composition and structures of the subsurface in the vicinity of Valles Marineris as revealed by central uplifts of impact craters

Despite recent efforts from space exploration to sound the martian subsurface with RADAR, the structure of the martian subsurface is still unknown. Major geologic contacts or discontinuities inside the martian crust have not been revealed. Another way to analyze the subsurface is to study rocks that have been exhumed from depth by impact processes. The last martian mission, MRO (Mars Reconnaissance Orbiter), put forth a great deal of effort in targeting the central peaks of impact craters with both of its high resolution instruments: CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) and HiRISE (High Resolution Science Experiment). We analyzed the composition with CRISM and the physical characteristics on HiRISE of the rocks exhumed from depth from 31 impact craters in the vicinity of Valles Marineris. Our analyses revealed the presence at depth of two kinds of material: massive light-toned rocks and intact layers. Exhumed light-toned massive rocks are enriched in low calcium pyroxenes and olivine. Hydrated phases such as smectites and putative serpentine are present and may provide evidence of hydrothermal processes. Some of the rocks may represent portions of the volatile-rich, pre-Noachian martian primitive crust. In the second class of central peaks, exhumed layers are deformed, folded, and fractured. Visible-near infrared (VNIR) spectra suggest that they are composed of a mixture of olivine and high calcium pyroxene associated with hydrated phases. These layers may represent a Noachian volcanic accumulation of up to 18 km due to Tharsis activity. The spatial distribution, as well as the in-depth distribution between the two groups of rocks exhumed, are not random and reveal a major geologic discontinuity below the Tharsis lava plateau. The contact may be vertical over several kilometers depth suggesting the pre-existence of a steep basin (early giant impact or subsidence basin) or sagduction processes.     

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.     

On liquid phases in cometary nuclei

In this paper we review the relevant literature and investigate conditions likely to lead to melting of H₂O ice, methanol (CH₃OH) ice, ethane (C₂H₆) ice and other volatile ices in cometary nuclei. On the basis of a heat balance model which takes account of volatiles loss, we predict the formation of occasional aqueous and hydrocarbon liquid phases in subsurface regions at heliocentric distances, r_h of 1–3 AU, and 5–12 AU, respectively. Low triple-point temperatures and low vapour pressures of C₂H₆, C₃H₈, and some higher-order alkanes and alkenes, favour liquid phase formation in cometary bodies at high r_h. Microporosity and the formation of a stabilization crust occluding the escape of volatiles facilitate liquid-phase formation. Characteristics of the near-surface which favour subsurface melting include; low effective surface emissivity (at low r_h), high amorphous carbon content, average pore sizes of ～10 μm or less, presence of solutes (e.g. CH₃OH), mixtures of C₂–C₆ hydrocarbons (for melting at high r_h), diurnal thermal cycling, and slow rotation rate. Applying the principles of soil mechanics, capillary forces are shown to initiate pre-melting phenomena and subsequent melting, which is expected to impart considerable strength of ～10⁴ Pa in partially saturated layers, reducing porosity and permeability, enhancing thermal conductivity and heat transfer. Diurnal thermal cycling is expected to have a marked effect on the composition and distribution of H₂O ice in the near-surface leading to frost heave-type phenomena even where little if any true melting occurs. Where melting does take place, capillary suction in the wetted zone has the potential to enhance heat transfer via capillary wetting in a low-gravity environment, and to modify surface topography creating relatively smooth flat-bottomed features, which have a tendency to be located within small depressions. An important aspect of the “wetted layer” model is the prediction that diurnal melt–freeze cycles alter the mixing ratio vs. depth of solutes present, or of other miscible components, largely through a process of fractional crystallization, but also potentially involving frost heave. Wetted layers are potentially durable and can involve significant mass transport of volatile materials in the near-surface, increasing in extent over many rotations of the nucleus prior to and just after perihelion passage, and causing stratification and trapping of the lowest-melting mixtures at depths of several metres. A possible mechanism for cometary outbursts is proposed involving a heat pulse reaching the liquid phase in the deepest wetted zone, leading to supersaturation and triggering the sudden release under pressure of dissolved gases, in particular CO₂, CO, CH₄ or N₂, contained beneath a consolidated near-surface layer. This study indicates that liquid water can persist for long periods of time in the near-surface of some intermediate-sized bodies (10²–10³ km radius) within protoplanetary discs.