Mars: Wind streak production as related to obstacle type and size

The characteristics of wind streaks associated with Martian craters and hills in the size range of ～100 m to ～80 km (corresponding to obstacle heights of a few to several hundred meters) have been analyzed from Viking Orbiter images. Both dark erosional and bright depositional streaks form over the entire obstacle size range, but there are preferred obstacle sizes for producing streaks. Bright streaks form more readily in association with relatively smaller obstacles than do dark streaks. Small obstacles produce both types of streaks more effectively than do large ones. Hills produce streaks as effectively as do craters of comparable height. Alternative explanations of bright streak formation are evaluated in terms of their ability to account for these observations. The most satisfactory models invoke blocking of atmospheric flow downwind of an obstacle and consequent deposition of dust within the sheltered zone.     

Wind-blown streaks,splotches, and associated craters on Mars: Statistical analysis of Mariner 9 photographs

Crater morphology and size play a major role in determining whether wind-blown streaks emanating from craters or dark splotches within craters will form. Both bright and dark streaks emanate almost exclusively from bowl-shaped craters. Dark splotches are found mainly in flat-floored craters, especially those that are deep and have high rim relief. Trends of dark splotches in the northern to southern midlatitudes closely follow those of bright streaks, suggesting both were formed by similar winds. In the high southern latitudes, on the other hand, dark splotch trends closely follow those of dark streaks.Qualitative models of streak and splotch formation have been derived from these data and results of Sagan et al. (1972, 1973). Bright streaks probably form by trapping and simultaneous streaming of bright dust downwind. Dark splotched craters in regions with bright streaks usually have upwind bright patches, suggesting these features form by dumping of bright dust over crater rims with some minor redistribution of dark materials toward the downwind sides of craters. Data are consistent with dark streaks forming by erosion or nondeposition of bright material or by trapping of dark material. Dark splotches in these regions are probably mainly the result of trapping of dark sand in the downwind sides of crater floors. Craters with dark splotches and dark streaks are usually rimless and shallow. This is consistent with ponded dark sands easily washing over crater walls and extending downwind.Plots of streak length versus crater diameter suggest a complex history of streak formation for most regions.Bright streak trends and latitudinal distributions are consistent with return flow of dust to the southern hemisphere. Some dark streaks may be direct relics of passing sand and dust storms. Trends of dark streaks and splotches away from the south pole are consistent with the spreading of a debris mantle from the polar regions toward the equator.     

Global thermal inertia and surface properties of Mars from the MGS mapping mission

We present a new high-resolution map of thermal inertia derived from observations of planetary brightness temperature by the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) obtained during the entire MGS primary mapping mission. Complete seasonal coverage provides a nearly global view of Mars, including the polar regions, at a spatial resolution of approximately 3 km. Our map of nighttime thermal-bolometer-based thermal inertia covers approximately 60% of the surface between 80° S and 80° N latitudes. We confirm the global pattern of high and low thermal inertia seen in lower resolution mapping efforts and provide greater detail concerning a third surface unit with intermediate values of both thermal inertia and albedo first identified by Mellon et al. 2000, Icarus 148, 437-455. Several smaller regional units with distinct characteristics are observed. Most notably, a unit of low thermal inertia () and low-to-intermediate albedo (0.09-0.22) dominates the region polewards of 65° S. We consider possible causes for these characteristics and conclude that a low-density mantle formed by desiccation of a previously ice-rich near-surface layer is the most likely explanation for the observed thermophysical properties. Global comparison of thermal inertia and elevation shows that high and low thermal inertia values can be found over a broad range of elevation, with only low values (30-) occurring at the highest elevations and the highest values occurring only at lower elevations. However, the lowest values () are found only at lower elevations, implying that the distribution of low thermal inertia material is not solely controlled by atmospheric pressure and the trapping of fines at high elevations. A new estimate of thermal inertia for the Viking and Pathfinder landing sites helps establish an important link between surface characteristics observed in situ and those derived from remote-sensing data.     

Three decades of slope streak activity on Mars

Slope streaks are surficial mass movements that are abundant in the dust-covered regions of Mars. Targeting of slope streaks seen in Viking images with the Mars Orbiter Camera provides observations of slope streak dust activity over two to three decades. In all study areas, new and persisting dark slope streaks are observed. Slope streaks disappeared in one area, with persisting streaks nearby. New slope streaks are found to be systematically darker than persisting streaks, which indicates gradual fading. Far more slope streaks formed at the study sites than have faded from visibility. The rate of formation at the study sites was 0.03 new slope streaks per existing streak per Mars year. Bright slope streaks do not presently form in sudden events as dark slope streaks do. Instead, bright streaks might form from old dark slope streaks, perhaps transitioning through a partially faded stage.     

“Dust” streaks on Mars

Global mapping and photometry of selected areas on Mars are used to investigate the nature of bright and dark wind streaks that extend from topographic obstacles. Occurrence of both bright and dark streaks is strongly latitude dependent and is only weakly correlated with surface properties such as albedo and thermal inertia. Data on the colors, albedos, and phase behavior of streaks are consistent with models of bright streaks as mosaics of plains material and brighter, redder dust. Less than 20% of the ground need be covered by the optically thick dust in the brightest parts of the streaks; the amount of dust in optically thick layers could be as little as 10^?3 g/cm². Dark streaks can be interpreted as erosional windows in a patchy dust cover. Our model of dust deposition in optically thick patches is sedimentologically different from scenarios involving the deposition of ubiquitous, optically thin layers. It has the advantage that large amounts of dust can be deposited without affecting regional albedos.     

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.     

A statistical study of crater-associated wind streaks in the North Equatorial Zone of Mars

A statistical study of all crater-related wind streaks visible on Mariner 9 A-camera frames between latitudes 0 and 30°N has been completed. Of the 2325 streaks identified 1914 (82%) are light tone streaks, 189 (8%) are dark tone, and the remaining 222 (10%) are of mixed tone. Nine parameters characterizing each streak and its associated crater were measured and intercorrelated. Because of the large number of light streaks in our sample fir findings for this type of streak are most significant statistically: light tone streaks occur preferentially in Pc terrain (heavily cratered plains); they are preferentially associated with fresh craters; the surface density of light streaks is not a strong function of elevation; a significant latitude effect does emerge—the density of light tone streaks reaches a maximum between 10 and 15°N, and drops off appreciably both toward the equator and toward higher latitudes; the mean angular width of light streaks is about 25°—long light streaks are significantly narrower than short ones; about 50% of streaks have streak length/crater diameter ratios of ?4; light streak directions conform closely to the wind regime expected at the season of global dust storms (southern summer). Generally speaking, the results for dark and mixed tone streaks in the northern equatorial zone are similar, with the following possible exceptions: dark streaks may show a slight preference to form at higher elecations; dark streaks may be slightly wider on average than light or mixed tone streaks; mixed tone streaks do not share the preference for sharp craters exhibited by light and dark streaks; in general, the directions of dark streaks do not conform to the general circulation pattern expected at the season of global dust storms as well as do those of the light streaks.     

Mesoscale linear streaks on Mars: environments of dust entrainment

Variable surface albedo features on Mars are likely caused by the entrainment and deposition of dust by the wind. Most discrete markings are associated with topographic forms or with regional slopes that serve to alter the effective wind shear stress on the surface. Some of the largest variable features, here termed mesoscale linear streaks, are up to 400 km in length and repeatedly occur in one of the smoothest regions of Mars: Amazonis Planitia. Their orientations and apparent season of variability as observed by Viking and Mars Orbiter cameras indicate linear streak formation by enhanced surface wind stresses during regional or local dust storms and during the initial stages of global dust storms. They provide an example of the ability of large-scale winds, without significant local enhancement, to initiate dust motion on Mars. The sizes and spacing of the linear streaks may be controlled by boundary layer rolls. The repetitive formation of these streaks, over a span of more than 11 Mars years, gives one measure of the stability of Mars’ eolian processes.     

Meteorological assessment of the surface temperatures on Titan: constraints on the surface type

The latitudinal profile of near-surface air temperature on Titan retrieved by Voyager 1 has been difficult to understand and raised several speculations about possible exotic processes that might be occurring near Titan's surface, while the thermal properties of the surface itself are unknown. This study systematically investigates the seasonal and spatial variation of the surface temperature and air temperature in the lower troposphere by a 3-dimensional general circulation model for different putative surface types (porous icy regolith, rock-ice mixture, hydrocarbon lakes). For any viable surface type the surface temperature is unlikely to be constant through the year and should more or less vary seasonally and even diurnally, most likely by a few K. Recent observations of tropospheric clouds may be evidence of seasonal variation of the surface temperature and the model predicts in the case of solid surface the development of a convective layer with superadiabatic lapse rates near the surface exactly at those latitudes and seasons where clouds have been identified. The latitudinal profile of the surface temperature retrieved from Voyager 1 infrared spectra can be explained without invoking exotic effects, provided the thermal inertia of the surface is relatively small and/or the surface albedo is low. A dominance of water ice (high thermal inertia and high albedo) at the surface is unfavorable to reproduce the observation. The latitudinal gradient of the surface temperature is particularly large at the hydrocarbon lake surface due to low albedo and small surface drag. Local anomalies of the surface albedo or surface thermal inertia are likely to cause substantial inhomogeneities of the surface temperature. Quasi-permanent accumulation of stratospheric haze at both poles would create a perennial equator-to-pole contrast of the surface temperature, but also a substantially lower global-mean surface temperature due to an enhanced anti-greenhouse effect in summer. The air temperature in the lower troposphere exhibits a tiny latitudinal gradient and a pole-to-pole gradient due to the presence of a pole-to-pole Hadley circulation, indicating that the temperature within the planetary boundary layer may exhibit a vertical profile characteristic of season, location and scenario. There may be a shallow near-surface inversion layer in cold seasons and a shallow convective layer in warm seasons.     

Mineralogy of Terra Meridiani and western Arabia Terra from OMEGA/MEx and implications for their formation

Analyses of Mars Express OMEGA hyperspectral data (0.4-2.7 μm) for Terra Meridiani and western Arabia Terra show that the northern mantled cratered terrains are covered by dust that is spectrally dominated by nanophase ferric oxides. Dark aeolian dunes inside craters and dark streaks extending from the dunes into the intercrater areas in mantled cratered terrains in western Arabia Terra have similar pyroxene-rich signatures demonstrating that the dunes supply dark basaltic material to create dark streaks. The dissected cratered terrains to the south of the mantled terrains are dominated spectrally by both low-calcium and high-calcium pyroxenes with abundances of 20-30% each retrieved from nonlinear radiative transfer modeling. Spectra over the hematite-bearing plains in Meridiani Planum are characterized by very weak but unique spectral features attributed to a mixture of a dark and featureless component (possibly gray hematite) and minor olivine in some locations. Hydrated minerals (likely hydrous ferric sulfates and/or hydrous hydroxides) associated with poorly ferric crystalline phases are found in the etched terrains to the north and east of the hematite-bearing plains where erosion has exposed ∼1 km of section of layered outcrops with high thermal inertias. These materials are also found in numerous craters in the northern Terra Meridiani and may represent outliers of the etched terrain materials. A few localized spots within the etched terrain also exhibit the spectral signature of Fe-rich phyllosilicates. The ensemble of observations show that the evidence for aqueous processes detected by the Opportunity Rover in Meridiani Planum is widespread and confirms the extended presence of surface or near-surface water over this large region of Mars. The scenarios of formation of Terra Meridiani (“dirty” acidic evaporite, impact surge or weathering of volcanic ash) cannot satisfactorily explain the mineralogy derived from the OMEGA observations. The formation of the etched terrains is consistent with leaching of iron sulfides and formation of sulfates and hydrated iron oxides, either in-place or via transport and evaporation of aqueous fluids and under aqueous conditions less acidic than inferred from rocks examined by Opportunity.     

Apparent thermal inertia and the surface heterogeneity of Mars

Thermal inertia derivation techniques generally assume that surface properties are uniform at horizontal scales below the footprint of the observing instrument and to depths of several decimeters. Consequently, surfaces with horizontal or vertical heterogeneity may yield apparent thermal inertia which varies with time of day and season. To investigate these temporal variations, we processed three Mars years of Mars Global Surveyor Thermal Emission Spectrometer observations and produced global nightside and dayside seasonal maps of apparent thermal inertia. These maps show broad regions with diurnal and seasonal differences up to 200 J m⁻² K⁻¹s^−1/2 at mid-latitudes (60° S to 60° N) and 600 J m⁻² K⁻¹s^−1/2 or greater in the polar regions. We compared the seasonal mapping results with modeled apparent thermal inertia and created new maps of surface heterogeneity at 5° resolution, delineating regions that have thermal characteristics consistent with horizontal mixtures or layers of two materials. The thermal behavior of most regions on Mars appears to be dominated by layering, with upper layers of higher thermal inertia (e.g., duricrusts or desert pavements over fines) prevailing in mid-latitudes and upper layers of lower thermal inertia (e.g., dust-covered rock, soils with an ice table at shallow depths) prevailing in polar regions. Less common are regions dominated by horizontal mixtures, such as those containing differing proportions of rocks, sand, dust, and duricrust or surfaces with divergent local slopes. Other regions show thermal behavior that is more complex and not well-represented by two-component surface models. These results have important implications for Mars surface geology, climate modeling, landing-site selection, and other endeavors that employ thermal inertia as a tool for characterizing surface properties.     

The cool surfaces of binary near-Earth asteroids

Here we show results from thermal-infrared observations of km-sized binary near-Earth asteroids (NEAs). We combine previously published thermal properties for NEAs with newly derived values for three binary NEAs. The η value derived from the near-Earth asteroid thermal model (NEATM) for each object is then used to estimate an average thermal inertia for the population of binary NEAs and compared against similar estimates for the population of non-binaries. We find that these objects have, in general, surface temperatures cooler than the average values for non-binary NEAs as suggested by elevated η values. We discuss how this may be evidence of higher-than-average surface thermal inertia. This latter physical parameter is a sensitive indicator of the presence or absence of regolith: bodies covered with fine regolith, such as the Earth’s moon, have low thermal inertia, whereas a surface with little or no regolith displays high thermal inertia. Our results are suggestive of a binary formation mechanism capable of altering surface properties, possibly removing regolith: an obvious candidate is the YORP effect.We present also newly determined sizes and geometric visible albedos derived from thermal-infrared observations of three binary NEAs: (5381) Sekhmet, (153591) 2001 SN₂₆₃, and (164121) 2003 YT₁. The diameters of these asteroids are 1.41 ± 0.21 km, 1.56 ± 0.31 km, and 2.63 ± 0.40 km, respectively. Their albedos are 0.23 ± 0.13, 0.24 ± 0.16, and 0.048 ± 0.015, respectively.     

Revisiting the thermal inertia of Iapetus: Clues to the thickness of the dark material

The energy balance at the surface of an airless planetary body is strongly influenced by the bolometric Bond albedo and the surface thermal inertia. Both of these values may be calculated through the application of a thermal model to measured surface temperatures. The accuracy of either, though, increases if the value of the other is better constrained. In this study, we used the improved global bolometric Bond albedo map of Iapetus derived from Cassini VIMS and ISS and Voyager ISS data in conjunction with Cassini CIRS temperature data to reevaluate surface thermal inertia across Iapetus. Results showed the thermal inertia of the dark terrain varies between 11 and 14.8 J m⁻² K⁻¹ s^−1/2 while the light material varies between 15 and 25 J m⁻² K⁻¹ s^−1/2. Using an approximation to the thermal properties of the dark overburden derived from our thermal inertia results, we can implement our thermal model to provide estimates on the dark material thickness, which was found to lie between 7 cm and 16 cm. In order to develop an accurate global thermal model, a weighted function that approximates the surface thermal inertia across Iapetus was developed and verified via our measurements. The global bolometric Bond albedo map, surface thermal inertia map, and the thermal model are then used to synthesize global temperature maps that may be used to study the stability of volatiles.     

Sporadic formation of slope streaks on Mars

Slope streaks are a form of gravity-driven mass-movements that frequently occur on Mars today. The cause of slope streak formation remains unclear; both, dry and wet processes have been suggested. Here, we observationally constrain the time of the year during which slope streaks form. Imagery from four Mars-orbiting cameras is mined to identify locations that have been imaged repeatedly, and the overlapping images are surveyed for streak activity. A search algorithm automatically finds the locations on the surface that have been imaged most often based on a graph representation. Dark slope streaks are found to form sporadically throughout the Mars year. At one study site in the Olympus Mons Aureole, observations constrain slope streak formation to at least five distinct time intervals within a single Mars year. New slope streaks form spatially isolated or in small groups within a few kilometers of one another. The observations suggest that slope streak triggering is unrelated to season and not caused by any large regional events. Most slope streaks are caused by sporadic events of small spatial extent.     

Brownish inclusions and dark streaks in Libyan Desert Glass: Evidence for high‐temperature melting of the target rock

Abstract– Dark streaks and different types of inclusions in Libyan Desert Glass (LDG) collected from the LDG strewn field in Egypt were investigated. Rare transparent spherules enclosed in the glassy matrix are characterized by concentric cracks, irregular internal cracks, intense twinning, and considerable amounts of Ti and Al. Raman spectra show that the spherules are α‐cristobalite. Their occurrence together with lechatelierite indicates quick heating of the source rock to at least 1550 °C, followed by rapid quenching leading to crystallization of β‐cristobalite, which upon cooling inverted into α‐cristobalite. Brownish inclusions are irregularly shaped, elongated objects with smooth contacts to the surrounding glass. They contain small roundish to elliptical droplets, and a few larger angular grains, which compositionally and according to their Raman spectra most closely resemble low‐Ca, Al‐rich orthopyroxene. Composition and texture of the orthopyroxene suggest that the brownish inclusions formed by incomplete melting of an Al‐rich orthopyroxene bearing precursor, e.g., mafic phases present in desert surface sands or also of orthopyroxene‐bearing granulite dykes in the LDG target. Experimental data on Ca‐poor enstatite also support that the inclusions were heated to about 1550 °C. Analyses of dark streaks in LDG reveal high abundances of Al, Ti, Mn, Cr, Fe, and Ni and a pronounced correlation between the abundances of Cr, Mn, Fe, and Ni. As the Fe/Ni, Mn/Ni, and Cr/Ni ratios are all clearly nonchondritic, the source of this material is most likely terrestrial and the dark streaks studied here represent a different type of schlieren compared to those which contain a meteoritic component. These findings suggest LDG formation during a short high‐temperature event. Melting of Al‐rich orthopyroxene bearing target material seems to suggest an asteroid impact rather than a near‐surface airburst.     

Distribution and time-variation of spire streaks at Pavonis Mons on Mars

We documented the distribution and the time-variation of the specific dark wind streaks at Pavonis Mons. We focused on the streaks we named “Spire Streaks”, which are overlapping spindle shaped dark streaks at the upper boundary of the coalesced dark streaks on Tharsis volcanoes. We investigated both visible and infrared images obtained by Viking orbiter camera, Mars Orbiter Camera (MOC), THEMIS, CTX and HiRISE of the spire streaks at Pavonis Mons. We also used topographic data obtained by Mars Orbiter Laser Altimeter (MOLA) to see the relationship between the topography and the distribution of the spire streaks. The spire streaks at Pavonis Mons provide us high-resolution information about the direction of the nighttime slope wind, and could be indirect clues for the time-variation of the nighttime environment. We conclude that the spire streaks are erosional features. However, some features of the spire streaks reported in this paper are outside the scope of previous modeling for erosional process, and we need a new category of model for the formation.     

The scale size of chondrule formation regions: Constraints imposed by chondrule cooling rates

Abstract— Meteoritic data strongly suggest that most chondrules reached maximum temperatures in a range of 1650–2000 K and cooled at relatively slow rates of 100–1000 K/h, implying a persistence of external energy supply. The presence of fine‐grained rims around chondrules in most unequilibrated chondrites also indicates that a significant quantity of micron‐sized dust was present in chondrule formation regions. Here, we assume that the persistent external energy source needed to explain chondrule cooling rates consists primarily of radiation from surrounding heated chondrules, fine dust, and gas after the formation event. Using an approximate one‐dimensional numerical model for the outward diffusion of thermal radiation from such a system, the scale sizes of formation regions required to yield acceptable cooling rates are determined for a range of possible chondrule, dust, and gas parameters. Results show that the inferred scale sizes depend sensitively on the number densities of micron‐sized dust and on their adopted optical properties. In the absence of dust, scale sizes > 1000 km are required for plausible maximum chondrule number densities and heated gas parameters. In the presence of dust with mass densities comparable to those of the chondrules and with absorptivities and emissivities of ～0.01 calculated for Mie spheres with a pure mineral composition, scale sizes as small as ～100 km are possible. If dust absorptivities and emissivities approach unity (as may occur for particles with more realistic shapes and compositions), then scale sizes as small as ×10 km are possible. Considering all uncertainties in model parameters, it is concluded that small scale sizes (10–100 km) for chondrule formation regions are allowed by the experimentally inferred cooling rates.     

Spatial inhomogeneity of the martian subsurface water distribution: implication from a global water cycle model

In an effort to test and to understand the global hydrogen distribution in the shallow subsurface of Mars retrieved by the Mars Odyssey gamma-ray spectrometer, the present state and movement of water are investigated by a coupled global subsurface-atmosphere water cycle model. It was found that the observed global subsurface hydrogen distribution is largely consistent with the modeled global water cycle, so a large fraction of hydrogen is likely to exist as water, at low and mid latitudes in the form of adsorbed water. Under the present climate the water content in the shallow subsurface becomes higher in the northern hemisphere than in the southern hemisphere as a result of global water cycle, regardless of the initial water distribution in the soil or adsorptive capacity. The higher annual maximum soil temperature in the south, stronger net northward transport of atmospheric water vapor, and the emission of vapor from the northern residual polar cap in northern summer contribute to this hemispheric asymmetry. The generally higher adsorptive capacity of clay minerals in the northern plains may further increase this bias. The longitudinal inhomogeneity is caused by several factors, such as thermal inertia, adsorptive capacity, and atmospheric surface pressure. The water abundance is locally high in low thermal inertia regions (e.g., Arabia Terra) and at deep places where the surface pressure is high (e.g., Hellas); it is low in soil with a low adsorptive capacity (e.g., Tharsis) and high thermal inertia regions (e.g., Solis Planum). Most of the soil humidity near the surface at low and mid latitudes may originate from the atmosphere. The model implies that the upper soil layer should be largely ice-free because otherwise an excessive sublimation and vapor emission into the atmosphere in warm seasons would violate the observational constraints. Moreover, the more uniform latitudinal variation of the observed hydrogen abundance near the surface compared to that of deeper layers is indicative of the presence of adsorbed water instead of ground ice because the adsorbed water content does not as steeply depend on latitude as the ground ice stability. Concerning the regolith mineralogy, montmorillonite can much better account for the observed water cycle than palagonite. While the presence of permanent ground ice appears likely in the polar region below a thin layer, large seasonal cycle of phase change between pore ice and adsorbed water may be possible. Regolith adsorption/desorption is neither negligible nor crucial for the seasonal atmospheric water cycle, but the surface-atmosphere coupling is a major prerequisite for the long-term evolution of subsurface water distribution.     

Secondary chaotic terrain formation in the higher outflow channels of southern circum-Chryse, Mars

Higher outflow channel dissection in the martian region of southern circum-Chryse appears to have extended from the Late Hesperian to the Middle Amazonian Epoch. These outflow channels were excavated within the upper 1 km of the cryolithosphere, where no liquid water is expected to have existed during these geologic epochs. In accordance with previous work, our examination of outflow channel floor morphologies suggests the upper crust excavated by the studied outflow channels consisted of a thin (a few tens of meters) layer of dry geologic materials overlying an indurated zone that extends to the bases of the investigated outflow channels (1 km in depth). We find that the floors of these outflow channels contain widespread secondary chaotic terrains (i.e., chaotic terrains produced by the destruction of channel-floor materials). These chaotic terrains occur within the full range of outflow channel dissection and tend to form clusters. Our examination of the geology of these chaotic terrains suggests that their formation did not result in the generation of floods. Nevertheless, despite their much smaller dimensions, these chaotic terrains are comprised of the same basic morphologic elements (e.g., mesas, knobs, and smooth deposits within scarp-bound depressions) as those located in the initiation zones of the outflow channels, which suggests that their formation must have involved the release of ground volatiles. We propose that these chaotic terrains developed not catastrophically but gradually and during multiple episodes of nested surface collapse. In order to explain the formation of secondary chaotic terrains within zones of outflow channel dissection, we propose that the regional Martian cryolithosphere contained widespread lenses of volatiles in liquid form. In this model, channel floor collapse and secondary chaotic terrain formation would have taken place as a consequence of instabilities arising during their exhumation by outflow channel dissection. Within relatively warm upper crustal materials in volcanic settings, or within highly saline crustal materials where cryopegs developed, lenses of volatiles in liquid form within the cryolithosphere could have formed, and/or remained stable.In addition, our numerical simulations suggest that low thermal conductivity, dry fine-grained porous geologic materials just a few tens of meters in thickness (e.g., dunes, sand sheets, some types of regolith materials), could have produced high thermal anomalies resulting in subsurface melting. The existence of a global layer of dry geologic materials overlying the cryolithosphere would suggest that widespread lenses of fluids existed (and may still exist) at shallow depths wherever these materials are fine-grained and porous. The surface ages of the investigated outflow channels and chaotic terrains span a full 500 to 700 Myr. Chaotic terrains similar in dimensions and morphology to secondary chaotic terrains are not observed conspicuously throughout the surface of Mars, suggesting that intra-cryolithospheric fluid lenses may form relatively stable systems. The existence of widespread groundwater lenses at shallow depths of burial has tremendous implications for exobiological studies and future human exploration. We find that the clear geomorphologic anomaly that the chaotic terrains and outflow channels of southern Chryse form within the Martian landscape could have been a consequence of large-scale resurfacing resulting from anomalously extensive subsurface melt in this region of the planet produced by high concentrations of salts within the regional upper crust. Crater count statistics reveal that secondary chaotic terrains and the outflow channels within which they occur have overlapping ages, suggesting that the instabilities leading to their formation rapidly dissipated, perhaps as the thickness of the cryolithosphere was reset following the disruption of the upper crustal thermal structure produced during outflow channel excavation.     

The surface of asteroid 951 Gaspra

I present new maps, photomosaics and geological interpretations of asteroid 951 Gaspra. Facets and limb concavities suggest a long history of large impacts producing 5 to 7 km diameter craters. Craters 1 to 4 km in diameter date the last facet-forming impact, though it is not clear which facet this formed. These craters are more numerous than previously thought because much of the area seen at high resolution seems to be depleted in these larger craters. Craters in that area probably date the last body-jolting impact. Linear features, probably the surface expressions of deep fractures, form at least two groups with different trends and probably different ages. Previously noted fresh and spectrally distinct materials are concentrated on ridges. One or two dark markings occur on a steep slope seen at high sun. Smooth materials, probably consisting of thicker or more mobile regolith than elsewhere, occur on steep slopes, usually on rotational leading surfaces.