seasonal activity and morphological changes in martian gullies abstract recent studies of martian dune and non dune gullies have suggested a seasonal control on present day gully activity the timing of current gully activity especially activity involving the formation or modification of channels which commonly have been taken as evidence of fluvial processes has important implications regarding likely gully formation processes and necessary environmental conditions in this study we describe the results of frequent meter scale monitoring of several active gully sites by the high resolution imaging science experiment hirise on the mars reconnaissance orbiter mro the aim is to better assess the scope and nature of current morphological changes and to provide improved constraints on timing of gully activity on both dune and non dune slopes our observations indicate that 1 gully formation on mars is ongoing today and 2 the most significant morphological changes are strongly associated with seasonal frost and defrosting activity observed changes include formation of all major components of typical gully landforms although we have not observed alcove formation in coherent bedrock these results reduce the need to invoke recent climate change or present day groundwater seepage to explain the many martian gullies with pristine appearance

transport processes induced by metastable boiling water under martian surface conditions liquid water on the martian surface is expected to be metastable owing to low atmospheric pressure experiments at martian conditions reveal that water and briny flows induce grain saltation and slope destabilization with geomorphic consequences

controls on the large scale spatial variations of dune field properties in the barchanoid portion of white sands dune field new mexico previous studies have shown that sediment fluxes and dune sizes are a maximum near the upwind margin of the white sands dune field and decrease to first order with increasing distance downwind these patterns have alternatively been attributed to a shear stress overshoot associated with a roughness transition localized at the upwind margin and to the influence of long wavelength topography on the hydrology and hence erodibility of dune field sediments i point out an issue that compromises the shear stress overshoot model and further test the hypothesis that long wavelength topographic variations acting in concert with feedbacks among aerodynamic granulometric and geomorphic variables control dune field properties at white sands building upon the existing literature i document that the mean and variability of grain sizes sand dryness aerodynamic roughness lengths bed shear stresses sediment fluxes and ripple and dune heights all achieve local maxima at the crests of the two most prominent scarps in the dune field one coincident with the upwind margin and the other located 6 7 km downwind computational fluid dynamics cfd modeling predicts that bed shear stresses erosion rates and the supply of relatively coarse poorly sorted sediments are localized at the two scarps due to flow line convergence hydrology and the spatially distributed adjustment of the boundary layer to variations in dune size as a result the crests of the scarps have larger ripples due to the granulometric control of ripple size larger grain sizes and or larger ripples lead to larger dunes and hence larger values of bed shear stress in a positive feedback

early views of the martian surface from the mars orbiter camera of mars global surveyor high resolution images of the martian surface at scales of a few meters show ubiquitous erosional and depositional eolian landforms dunes sandsheets and drifts are prevalent and exhibit a range of morphology composition inferred from albedo and age as seen in occurrences of different dune orientations at the same location steep walls of topographic depressions such as canyons valleys and impact craters show the martian crust to be stratified at scales of a few tens of meters the south polar layered terrain and superposed permanent ice cap display diverse surface textures that may reflect the complex interplay of volatile and non volatile components low resolution regional views of the planet provide synoptic observations of polar cap retreat condensate clouds and the lifecycle of local and regional dust storms

long term monitoring of martian gully formation and evolution with mro hirise abstract gully landforms are commonly taken as evidence for surface liquid water in the recent geological history of mars repeat observations with the high resolution imaging science experiment hirise instrument on the mars reconnaissance orbiter demonstrate widespread activity in gullies in the southern hemisphere particularly in those with the freshest morphologies this activity includes substantial channel incision and large scale mass movements and constitutes ongoing gully formation rather than degradation of older landforms new apron deposits that are bright dark and neutrally toned have all been observed the timing of gully activity is seasonally controlled and occurs during the period when seasonal frost is present and defrosting these observations support a model in which currently active gully formation is driven mainly by seasonal co 2 frost gullies in the northern hemisphere are far less active than those in the south this may be due to the current timing of perihelion near the northern winter solstice integrated over time activity like that observed within the past few years appears capable of forming all of the martian gully landforms on timescales of millions of years additionally the current style and rate of activity is able to erase meter to decameter scale surface features that might have been uniquely produced by other processes during the last obliquity high 0 4 ma although it is impossible to rule out a past role for water in the formation of martian gullies a model in which gullies form only through currently active processes with little or no liquid water is consistent with our observations

physical properties and rock physics models of sediment containing natural and laboratory formed methane gas hydrate this paper presents results of shear strength and acoustic velocity p wave measurements performed on 1 samples containing natural gas hydrate from the mallik 2l 38 well mackenzie delta northwest territories 2 reconstituted ottawa sand samples containing methane gas hydrate formed in the laboratory and 3 ice bearing sands these measurements show that hydrate increases shear strength and p wave velocity in natural and reconstituted samples the proportion of this increase depends on 1 the amount and distribution of hydrate present 2 differences in sediment properties and 3 differences in test conditions stress strain curves from the mallik samples suggest that natural gas hydrate does not cement sediment grains however stress strain curves from the ottawa sand containing laboratory formed gas hydrate do imply cementation is present acoustically rock physics modeling shows that gas hydrate does not cement grains of natural mackenzie delta sediment natural gas hydrates are best modeled as part of the sediment frame this finding is in contrast with direct observations and results of ottawa sand containing laboratory formed hydrate which was found to cement grains waite et al 2004 it therefore appears that the microscopic distribution of gas hydrates in sediment and hence the effect of gas hydrate on sediment physical properties differs between natural deposits and laboratory formed samples this difference may possibly be caused by the location of water molecules that are available to form hydrate models that use laboratory derived properties to predict behavior of natural gas hydrate must account for these differences

indications of brine related local seepage phenomena on the northern hemisphere of mars abstract springtime low albedo features called dark dune spots on the seasonal frost covered dunes on mars between 77 n and 84 n latitude have been analyzed two groups of these spots have been identified small and large ones where large spots have diameters above 4 m and complex internal structure from these large spots branching seepage like features emanate and grow on the steep slopes they show a characteristic sequence of changes first only wind blown features emanate from them while later a bright circular and elevated ring forms and dark seepage features start from the spots these streaks grow with a speed between 0 3 m day and 7 m day respectively first only from the spots later from all along the dune crest during this seepage period the temperature is between 150 k and 180 k at a 3 9 km spatial resolution scale indicating that co2 ice free parts must be present at the observed dark spots around the receding northern seasonal co2 cap an annulus of water ice lags behind which is probably present in the spots too where the co2 frost has sublimated our model estimates show in the present work and in kereszturi et al kereszturi a mohlmann d berczi sz ganti t kuti a sik a horvath a 2009b icarus 201 492 503 that the warming driven by solar insolation may produce not only interfacial water but also bulk brines around the dune grains the brine can support the movement of liquids and dune grains enhances the sublimation of co2 frost and produce the dark features as well as liquid modifies the optical properties of the surface signs of movement of dune material after the total defrosting of the terrain is also visible but it is uncertain because of the limit of resolution in our previous work kereszturi et al 2009b we showed that resembling seepage like streaks at the southern hemisphere might have been formed by ephemeral interfacial water as well as these northern features such wet environments may have astrobiological importance too

avalanche slope angles in low gravity environments from active martian sand dunes 1 the properties of granular material have an important effect on surface landforms and processes recently it has been suggested that material properties called dynamic and static angle of repose vary with gravitational acceleration which would have a significant effect on many planetary surface processes such as crater collapse and gully formation in order to test that hypothesis we measured lee slopes of active aeolian sand dunes on mars using the high resolution imaging experiment hirise dtms digital terrain model we examined dune fields in nili patera herschel crater and gale crater our measurements showed that the dynamic angles of repose for the sands in these areas are 33 34 in the first region and 30 31 in the other two these results fall within the 30 to 35 window for the dynamic angles of repose for terrestrial dunes with similar flow depths and grain properties and thus show that this angle does not significantly vary with decreasing gravity

laboratory simulation of debris flows over sand dunes insights into gully formation mars gully morphology often summarized as comprising an alcove channel and debris apron is one of the key elements used to support the argument for liquid water in the recent past on mars nevertheless the processes that create different gully morphologies on both mars and earth are not fully understood one of the puzzling morphologic attributes of martian dune gullies is their apparent lack of an apron or terminal deposit which has caused debate about their formation process several physical processes such as runoff debris flows granular flows and sliding blocks falling downslope could explain the formation of these gullies in this work we focus on the role of liquid in the substrate as well as in the flow and choose to experimentally test the plausibility of this hypothesis we performed a series of analogue experiments to investigate the formation of gullies on sand dune like substrates we used controlled flows of water over an inclined sand box to produce gully like forms ice rich sedimentary substrates were used including substrates that included a thin liquid water saturated thawed layer an active layer above the ice saturated zone to give an analogue for a periglacial environment we quantitatively demonstrate that debris flow processes in periglacial experiments are conducive to the formation of narrow and long channels with small terminal deposits with perched channels by re analysis of martian elevation data for dune gullies on mars we have found good evidence that such terminal deposits could exist our experiments revealed that increased water content in the thawed layer above the frozen bed increases flow length due to the subsequent reduction in infiltration capacity water is incorporated into the flow by erosion of the wet thawed layer sand plus water and by drainage of the thawed layer using a mars environment simulation chamber we found that atmospheric pressure conditions seem to have a limited influence on the morphology of the flows our experimental investigation allowed the reproduction of terrestrial debris flow and martian gully morphologies suggesting that a substrate that is resistant to infiltration could be present beneath the dune gullies on mars we suggest that like in our laboratory experiments the presence of ice at shallow depth is a possible explanation for the formation of these morphologies and that a wet thawed layer is a possible explanation for the long flow length

non uniform seasonal defrosting of subpolar dune field on mars abstract images of the sub polar regions of mars show existence of various seasonal depending features including mosaics of dark patches called dalmatian spots we interpret formation of dark patches on dune fields by means of the non uniform sublimation of winter co 2 ice deposit

present day seasonal gully activity in a south polar pit sisyphi cavi on mars abstract the large amount of multi temporal high resolution images acquired in the last few years offers the opportunity to identify morphological changes associated with recent geologic activity on the surface of mars in this study we focus on a single gully in sisyphi cavi located in the south polar region at 1 44 e and 68 54 s the gully incises the gullied equator facing slope of an isolated polar pit within an infilled impact crater it is important to notice that the following investigations describe the activity and modifications of an existing gully and not the formation of the gully itself high resolution image data analyses show new deposits at the terminus of the gully channel and on the gully apron within spring after solar longitudes of 236 of martian years my 29 and 31 our morphological investigations show that the identified new deposits were formed by dark flows through the entire gully deposited on top of the apron between solar longitudes l s 218 and 226 thermal data show a temperature increase between l s 218 and 226 near infrared spectral data show relatively constant band strengths of co 2 ice and h 2 o ice in this time range after the formation of the dark flows after l s 226 temperatures increase rapidly from 180 k to 270 k at l s 250 at this time spectral data indicate that all volatiles on the surface sublimated however an earlier beginning of sublimation when the dark flows were observed between l s 218 and 226 is likely due to the fact that the instruments can only show the last phase of sublimation decrease of volatile band strengths spectral modeling shows that from winter to mid spring the surface of the studied area is covered by translucent co 2 slab ice contaminated by minor amounts of h 2 o ice and dust furthermore our spectral modeling indicates that the dark material most likely flows on top of the co 2 slab ice cover three different scenarios were proposed to explain the identified dark flows including 1 flows supported by liquid h 2 o 2 flows supported by co 2 gas and 3 dry flows on the basis of our study we find that scenario 1 is unlikely because of the very low temperatures while scenario 2 is consistent with the observed beginning of co 2 ice sublimation in the study area it is unlikely because of the limitation of the activity to only one gully compared to surrounding gullies that share the same morphologies slope angles and volatile contents also with scenario 3 dry flows the activity of only one gully is difficult to explain thus we propose a mixture of scenario 2 and 3 dry flows supported by the ongoing sublimation of co 2 ice within the gully to be the most plausible scenario when the observed active gully comprises different source material than the surrounding gullies i e a higher content of probably sand sized material from outcrops located in the alcove

recent rheologic processes on dark polar dunes of mars driven by interfacial water abstract in springtime on hirise images of the southern polar terrain of mars flow like or rheologic features were observed their dark color is interpreted as partly defrosted surface where the temperature is too high for co2 but low enough for h2o ice to be present there these branching streaks grow in size and can move by an average velocity of up to about 1 m day and could terminate in pond like accumulation features the phenomenon may be the result of interfacial water driven rheologic processes liquid interfacial water can in the presence of water ice exist well below the melting point of bulk water by melting in course of interfacial attractive pressure by intermolecular forces van der waals forces e g curvature of water film surfaces and e g by macroscopic weight acting upon ice this melting phenomenon can be described in terms of premelting of ice it is a challenging consequence that liquid interfacial water unavoidably must in form of nanometric layers be present in water ice containing soil in the subsurface of mars it is the aim of this paper to study possible rheologic consequences in relation to observations which seem to happen at sites of dark polar dunes on mars at present the model in this work assumes that interfacial water accumulates at the bottom of a translucent water ice layer above a dark and insolated ground this is warmed up towards the melting point of water the evolving layer of liquid interfacial water between the covering ice sheet and the heated ground is assumed to drive downward directed flow like features on slopes and it can at least partially infiltrate seep into a porous ground there in at least temporarily cooler subsurface layers the infiltrated liquid water refreezes and forms ice the related stress built up is shown to be sufficient to cause destructive erosive processes the above mentioned processes may cause change in the structure and thickness of the covering ice and or may cause the movement of dune grains all these processes may explain the observed springtime growing and downward extension of the slope streaks analyzed here

surface runoff features on mars testing the carbon dioxide formation hypothesis 1 fresh gully like features on mars strongly suggest that fluid flowed on the surface in the recent past here we consider the possibility that co2 vapor supported flows formed the gullies we find that neither condensed co2 nor co2 clathrate hydrate are likely to accumulate in significant quantities in the martian crust in addition if condensed co2 were present under lithostatic pressures exposure to the atmosphere would produce features analogous to terrestrial pyroclastic flows not surface runoff finally the source volume of co2 required to support a flow excludes clustered or episodic gully formation therefore we conclude that co2 cannot have formed the gullies in light of these results liquid water flow remains the preferred formation mechanism for the recent surface runoff features

a new dry hypothesis for the formation of martian linear gullies long narrow grooves found on the slopes of martian sand dunes have been cited as evidence of liquid water via the hypothesis that melt water initiated debris flows eroded channels and deposited lateral levees however this theory has several short comings for explaining the observed morphology and activity of these linear gullies we present an alternative hypothesis that is consistent with the observed morphology location and current activity that blocks of co2 ice break from over steepened cornices as sublimation processes destabilize the surface in the spring and these blocks move downslope carving out leveed grooves of relatively uniform width and forming terminal pits to test this hypothesis we describe experiments involving water and co2 blocks on terrestrial dunes and then compare results with the martian features furthermore we present a theoretical model of the initiation of block motion due to sublimation and use this to quantitatively compare the expected behavior of blocks on the earth and mars the model demonstrates that co2 blocks can be expected to move via our proposed mechanism on the earth and mars and the experiments show that the motion of these blocks will naturally create the main morphological features of linear gullies seen on mars

sediment transport over the continental slope offshore northern israel an analysis by means of electron microscopy abstract seven subsamples from cores collected from the continental terrace offshore northern israel were examined by tem and sem these samples represent different sedimentary environments the most homogeneous sample was subjected to various stresses the highest corresponding to the overburden loading of a 1000 m thick sedimentary column the tem photomicrographs were quantitatively analyzed using computerized methods detrital clay domains mostly smectite constitute up to 70 of the sediment bulk highly compacted silt sized argillaceous clasts lumps consisting of very dense yet discernible clay domains and varying quantities of quartz grains and skeletal carbonate particles are randomly interspersed within the clay matrix the clayey platelets and the argillaceous clasts are of east and north african origin mostly transported by the nile river and in part by dust storms the lumps originate from stiff overconsolidated subbottom sediments detached and transported downslope by slumping current transported quartz grains and skeletal debris from the continental shelf are mixed with these sediments the analysis of the tem and sem photomicrographs supports the model of sediment dispersal patterns offshore northern israel which was evaluated from radiographic studies of the cores and other data

debris flows over sand dunes on mars evidence for liquid water 1 this study focuses on the formation and physical properties of the gullies observed over large martian dunes especially those of the russell crater 54 s 347 w geomorphic features like sinuosities and connections of the channels show that gullies over dunes involve flows with a significant proportion of liquid the occurrence of levees implies that these flows are debris flows with a yield strength characteristics of bingham plastic materials we apply terrestrial methods to estimate viscosity and velocity of these flows from levee size and sinuosities we obtain average velocities in the range of 1 to 7 m s 1 and apparent viscosities of 2 8 to 46 000 pa s with an average at 740 pa s compared with the 0 001 pa s of pure water these viscosities and velocities are in the range of terrestrial debris flows with a proportion of 10 to 40 of h2o these properties are typical of water holding debris flows but not of pure water surface runoff or co2 driven flows the debris flows over dunes are oriented on south facing slopes like other recent gullies meltwater from ground ice formed during a recent period of high obliquity is the more likely explanation for the formation of such flows over dunes

multitemporal observations of identical active dust devils on mars with the high resolution stereo camera hrsc and mars orbiter camera moc abstract active dust devils were observed in syria planum in mars observer camera wide angle moc wa and high resolution stereo camera hrsc imagery acquired on the same day with a time delay of 26 min the unique operating technique of the hrsc allowed the measurement of the traverse velocities and directions of motion large dust devils observed in the hrsc image could be retraced to their counterparts in the earlier acquired moc wa image minimum lifetimes of three large avg 700 m in diameter dust devils are 26 min as inferred from retracing for one of these large dust devil 820 m in diameter it was possible to calculate a minimum lifetime of 74 min based on the measured horizontal speed and the length of its associated dust devil track the comparison of our minimum lifetimes with previous published results of minimum and average lifetimes of small 19 m in diameter avg min lifetime of 2 83 min and medium 185 m in diameter avg min lifetime of 13 min dust devils imply that larger dust devils on mars are active for much longer periods of time than smaller ones as it is the case for terrestrial dust devils knowledge of martian dust devil lifetimes is an important parameter for the calculation of dust lifting rates estimates of the contribution of large dust devils 300 1000 m in diameter indicate that they may contribute at least regionally to 50 of dust entrainment by dust devils into the atmosphere compared to the dust devils

lee slope sediment processes leading to avalanche initiation on an aeolian dune 1 in order to detail the governing conditions through which a slipface matures to the point of failure dry sand avalanches were observed in the dune simulation wind tunnel on a 1 1 replica transverse dune with a crest height of approximately 1 2 m areal distributions of grainfall and reptation were measured using traps changes in the slipface elevation were observed using 3 d laser scanning with a vertical accuracy of 0 096 mm for approximately every 1 mm2 of surface area grainfall decayed exponentially from the brink with a constant rate across all wind velocities reptation removed sediment from areas close to the brink and deposited it downslope creating low amplitude cross slope ripples on the slipface a critical length scale separating grainscale and bulk sediment behavior is identified and it defines the lower limit to the validity of angle of repose measurements avalanche initiation occurred in an area of steep surface slope below a sediment bulge with distance from the brink independent of wind velocity the time between avalanches was found to be constant for constant wind velocity

evidence for present day gully activity on the russell crater dune field mars 1 on the basis of high resolution imaging science experiment hirise images of two successive years we observed changes of a gully on the russell crater dune field within the first year the length of a 2 m wide incised gully channel grew about 50 m downslope and 120 m within in the second year gully activity occurred in early spring between ls 198 and 218 in the first year and between ls 192 and 221 in the second year based on the channel morphology spectral data and modeled surface temperatures the changes can be best explained with transient melting of small amounts of h2o ice triggering slurry flows consisting of sand mixed with liquid water

hirise observations of gas sublimation driven activity in mars southern polar regions i erosion of the surface the high resolution imaging science experiment hirise on the mars reconnaissance orbiter mro has imaged the sublimation of mars seasonal co2 polar cap with unprecedented detail for one complete martian southern spring in some areas of the surface beneath the conformal coating of seasonal ice radiallyorganized channels are connected in spidery patterns the process of formation of this terrain erosion by gas from subliming seasonal ice has no earthly analog the new capabilities high resolution color and stereo images of hirise enable detailed study of this enigmatic terrain two sites are analyzed in detail one within an area expected to have translucent seasonal co2 ice and the other site outside that region stereo anaglyphs show that some channels grow larger as they go uphill implicating gas rather than liquid as the erosive agent dark fans of material from the substrate are observed draped over the seasonal ice and this material collects in thin to thick layers in the channels possibly choking off gas flow in subsequent years resulting in inactive crisscrossing shallow channels in some areas there are very dense networks of channels with similar width and depth and fewer fans emerging later in the season are observed subtle variations in topography affect the channel morphology a new terminology is proposed for the wide variety of erosional features observed

debris flows on the great kobuk sand dunes alaska implications for analogous processes on mars abstract we observed niveo aeolian deposits denivation features and small meltwater induced debris flows that had formed at the great kobuk sand dunes northwestern interior alaska in late march 2010 this high latitude cold climate dune field is being studied as a planetary analog to improve our understanding of factors that may trigger debris flows on the lee slopes of martian aeolian dunes debris flows consisted of a sand and liquid water mixture that cascaded down the lee slopes of two barchanoid dunes on days when measured ground surface temperatures were below freezing we hypothesize that relatively dark sand on snow caused local hot spots where solar radiation could be absorbed by the sand and conducted into the underlying snow enabling meltwater to form and sand to be mobilized this investigation provides insights into the interactions between niveo aeolian deposition slope aspect and insolation thawing and initiation of alluvial processes these debris flows are morphologically similar to those associated with seasonal gullies or erosion tracks visible on the slopes of mid to high latitude dune fields in both martian hemispheres localized heating and thawing at scales too small for orbital sensors to identify may yield martian debris flows at current climate conditions

intercomparison and validation of techniques for spectral unmixing of hyperspectral images a planetary case study as the volume of hyperspectral data for planetary exploration increases efficient yet accurate algorithms are decisive for their analysis in this paper the capability of spectral unmixing for analyzing hyperspectral images from mars is investigated for that purpose we consider the russell megadune observed by the compact reconnaissance imaging spectrometer for mars crism and the high resolution imaging science experiment hirise instruments in late winter this area of mars is appropriate for testing linear unmixing techniques because of the geographical coexistence of seasonal co2 ice and defrosting dusty features that is not resolved by crism linear unmixing is carried out on a selected crism image by seven state of the art approaches based on different principles three physically coherent sources with an increasing fingerprint of dust are recognized by the majority of the methods processing of hirise imagery allows the construction of a ground truth in the form of a reference abundance map related to the defrosting features validation of abundances estimated by spectral unmixing is carried out in an independent and quantitative manner by comparison with the ground truth the quality of the results is estimated through the correlation coefficient and average error between the reconstructed and reference abundance maps intercomparison of the selected linear unmixing approaches is performed global and local comparisons show that misregistration inaccuracies between the hirise and crism images represent the major source of error we also conclude that abundance maps provided by three methods out of seven are generally accurate i e sufficient for a planetary interpretation

sand water flows on cold climate eolian dunes environmental analogs for the eolian rock record and martian sand dunes abstract sand water flows were observed on the slopes of cold climate eolian dunes in southwestern saskatchewan canada during episodes of intense thawing in the winters of 2004 and 2005 meltwater produced flows with varying grain water contents on slopes of active blowout hollows and parabolic dunes the sand water flows emplaced small structureless lobate and tongue shaped deposits and alluvial fans the greatest concentration of deposits occurred on south facing slopes which frequently thaw in winter from intense insolation transport modes ranged from high density viscous slurry flows to lowdensity hyperconcentrated flows the triggering cause of these flows is high pore water pressure that develops in thawed nearsurface sand due to impeded infiltration by frozen sand with pore ice at depth these observations broaden the environmental context of sand water flows on dunes and contribute to interpretations of these deposits in the eolian rock record and to recent alluvial features observed on martian sand dunes

formation of recent martian debris flows by melting of near surface ground ice at high obliquity the observation of small gullies associated with recent surface runoff on mars has renewed the question of liquid water stability at the surface of mars the gullies could be formed by groundwater seepage from underground aquifers however observations of gullies originating from isolated peaks and dune crests question this scenario we show that these landforms may result from the melting of water ice in the top few meters of the martian subsurface at high obliquity our conclusions are based on the analogy between the martian gullies and terrestrial debris flows observed in greenland and numerical simulations that show that above freezing temperatures can occur at high obliquities in the near surface of mars and that such temperatures are only predicted at latitudes and for slope orientations corresponding to where the gullies have been observed on mars

magnitude and frequency of grain flows on a desert sand dune preliminary studies of the magnitude and frequency of lee side avalanches grain flows on a namib crescentic dune show that the frequency of grain flows for a given segment of the lee face is dependent on the wind speed and sand transport rate for the period preceding their initiation and the magnitude of the flows as described by their area is inversely proportional to the interval between flows and thus wind speed and sand transport rates these studies indicate the potential of using a simple digital video camera technique to document the magnitude frequency and geometry of grain flows on desert sand dunes

seasonal erosion and restoration of mars northern polar dunes despite radically different environmental conditions terrestrial and martian dunes bear a strong resemblance indicating that the basic processes of saltation and grainfall sand avalanching down the dune slipface operate on both worlds here we show that martian dunes are subject to an additional modification process not found on earth springtime sublimation of mars co2 seasonal polar caps numerous dunes in mars north polar region have experienced morphological changes within a mars year detected in images acquired by the high resolution imaging science experiment on the mars reconnaissance orbiter dunes show new alcoves gullies and dune apron extension this is followed by remobilization of the fresh deposits by the wind forming ripples and erasing gullies the widespread nature of these rapid changes and the pristine appearance of most dunes in the area implicates active sand transport in the vast polar erg in mars current climate

laboratory simulations of martian gullies on sand dunes 1 small gullies observed on mars could be formed by groundwater seepage from an underground aquifer or may result from the melting of near surface ground ice at high obliquity to test these different hypotheses a cold room based laboratory simulation has been performed the experimental slope was designed to simulate debris flows on sand dune slopes at a range of angles different granulometry and permafrost characteristics preliminary results suggest that the typical morphology of gullies observed on mars can best be reproduced by the formation of linear debris flows related to the melting of a near surface ground ice with silty materials this physical modelling highlights the role of the periglacial conditions especially the active layer thickness during debris flow formation

spectral evidence for hydrated salts in recurring slope lineae on mars transient streaks that appear seasonally on martian slopes are consistent with brine flows but evidence of water or salts has been lacking analysis of spectral data reveals hydrated salts associated with the streaks confirming a briny origin

hirise observations of gas sublimation driven activity in mars southern polar regions ii surficial deposits and their origins abstract the high resolution imaging science experiment hirise onboard mars reconnaissance orbiter mro has been used to monitor the seasonal evolution of several regions at high southern latitudes and in particular the jet like activity which may result from the process described by kieffer jgr 112 e08005 doi 10 1029 2006je002816 2007 involving translucent co 2 ice in this work we mostly concentrate on observations of the inca city 81 s 296 e and manhattan 86 s 99 e regions in the southern spring of 2007 two companion papers hansen et al this issue and portyankina et al this issue discuss the surface features in these regions and specific models of the behaviour of co 2 slab ice respectively the observations indicate rapid on set of activity in late winter initiating before hirise can obtain adequately illuminated images ls 2 titus et al 2007 agu abstract p41a 0188 these deposits originate from araneiform structures spiders boulders on ridges cracks on slopes and along linear cracks in the slab ice on flatter surfaces the type of activity observed can often be explained qualitatively by considering the local topography some dark fans are observed to shorten enormously in length on a timescale of 18 days we consider this to be strong evidence that outgassing was in progress at the time of hirise image acquisition and estimate a total particulate emission rate of 30 g s 1 from a single typical jet feature brighter deposits at inca city become increasingly hard to detect after ls 210 in the inca city region the orientations of surficial deposits are topographically controlled the deposition of dark material also appears to be influenced by local topography suggesting that the ejection from the vents is at low velocity 1 and that a ground hugging flow process a sort of cryo fumarole may be occurring the failure up to this point to obtain a clear detection of outgassing though stereo imaging is consistent with low level transport the downslope orientation of the deposits may result from the geometry of the vent or from catabatic winds at many sites more than one ejection event appears to have occurred suggesting re charging of the sources around ls 230 the brightness of the surface begins to drop rapidly on north facing slopes and the contrast between the dark deposits and the surrounding surface reduces this indicates that the co 2 ice slab is being lost completely in some areas at around this time by ls 280 at inca city the ice slab has effectively gone crism band ratios and themis brightness temperature measurements are consistent with this interpretation

observations of the northern seasonal polar cap on mars i spring sublimation activity and processes abstract spring sublimation of the seasonal co2 northern polar cap is a dynamic process in the current mars climate phenomena include dark fans of dune material propelled out onto the seasonal ice layer polygonal cracks in the seasonal ice sand flow down slipfaces and outbreaks of gas and sand around the dune margins these phenomena are concentrated on the north polar erg that encircles the northern residual polar cap the mars reconnaissance orbiter has been in orbit for three mars years allowing us to observe three northern spring seasons activity is consistent with and well described by the kieffer model of basal sublimation of the seasonal layer of ice applied originally in the southern hemisphere three typical weak spots have been identified on the dunes for escape of gas sublimed from the bottom of the seasonal ice layer the crest of the dune the interface of the dune with the interdune substrate and through polygonal cracks in the ice pressurized gas flows through these vents and carries out material entrained from the dune furrows in the dunes channel gas to outbreak points and may be the northern equivalent of southern radially organized channels araneiform terrain albeit not permanent properties of the seasonal co2 ice layer are derived from timing of seasonal events such as when final sublimation occurs modification of dune morphology shows that landscape evolution is occurring on mars today driven by seasonal activity associated with sublimation of the seasonal co2 polar cap

near tropical subsurface ice on mars near surface perennial water ice on mars has been previously inferred down to latitudes of about 45 and could result from either water vapor diffusion through the regolith under current conditions or previous ice ages precipitations in this paper we show that at latitudes as low as 25 in the southern hemisphere buried water ice in the shallow 1 m subsurface is required to explain the observed surface distribution of seasonal co2 frost on pole facing slopes this result shows that possible remnants of the last ice age as well as water that will be needed for the future exploration of mars are accessible significantly closer to the equator than previously thought where mild conditions for both robotic and human exploration lie copyright 2010 by the american geophysical union

present day formation and seasonal evolution of linear dune gullies on mars linear dune gullies are a sub type of martian gullies as their name suggests they only occur on sandy substrates and comprise very long compared to their width straight or sinuous channels with relatively small source areas and almost non existent visible deposits linear dune gullies have never been observed on terrestrial dunes and their formation process on mars is unclear here we present the results of the first systematic survey of these features in mars southern hemisphere and an in depth study of six dunefields where repeat imaging allows us to monitor the changes in these gullies over time this study was undertaken with hirise images at 25 30 cm pix and 1 m pix elevation data derived from hirise stereo images we find the latitudinal distribution and orientation of linear dune gullies broadly consistent with the general population of martian gullies they occur predominantly between 36 3 s and 54 3 s and occasionally between 64 6 s and 70 4 s they are generally oriented toward ssw at bearings between 150 and 260 we find that these gullies are extremely active over the most recent 5 martian years of images activity comprises 1 appearance of new channels 2 lengthening of existing channels 3 complete or partial reactivation and 4 disappearance of gullies we find that gully channels lengthen by 100 m per year the intense activity and the progressive disappearance of linear dune gullies argues against the hypothesis that these are remnant morphologies left over from previous periods of high obliquity millions of years ago the activity of linear dune gullies reoccurs every year between the end of winter and the beginning of spring ls 167 4 216 6 and coincides with the final stages of the sublimation of annual co ice deposit this activity often coincides spatially and temporally with the appearance of recurrent diffusing flows rdfs digitate shaped dark patches with low relative albedo up to 48 lower than the adjacent dune that encompass the active site south and ssw facing dune slopes are those which preferentially host co2 frost deposits however it is only those with angles of 20 just below the crest which possess linear dune gullies suggesting a slope limited formation process these observations provide a wealth of temporal and morphometric data that can be used to undertake numerical modeling to direct future image monitoring and guide laboratory experiments that can be used to better constrain the formation process of these features

seasonality of present day martian dune gully activity martian slope gullies are argued to be evidence for recent liquid water flow on the surface of mars to explain the source of water a wide range of environmental conditions and processes has been invoked however a lack of information about the environmental context or timing of gully activity makes it difficult to evaluate the theories here we present new observations of extensive gully modification over the past 6 mars years within dune gullies with slope gully morphology observed activity within 18 gullies in 7 dune fields constrains timing to winter which is consistent with observed slope gully activity these observations show that fluvial processes are unlikely to cause present day martian dune gully activity and imply that co 2 frost accumulation may play the dominant role

bright dust devil tracks on earth implications for their formation on mars we report on the first observations of bright dust devil tracks bddts on earth observed in the turpan depression desert in northwestern china where raindrop impacts on sand surfaces form aggregates of sand silt and clay resulting in rough surface textures which are destroyed by passages of dust devils leading to smooth surface textures within the tracks the differences in photometric properties between the track and outside the tracks cause the albedo differences leading to the formation of bddts and similar processes might lead to bddts on mars in areas with thick dust covers

flow and deposition of pyroclastic granular flows a type example from the 1975 ngauruhoe eruption new zealand small volume pyroclastic density currents pdcs are generated frequently during explosive eruptions with little warning assessing their hazard requires a physical understanding of their transport and sedimentation processes which is best achieved by the testing of experimental and numerical models of geophysical mass flows against natural flows and or deposits to this end we report on one of the most detailed sedimentological studies ever carried out on a series of pristine small volume pdc deposits from the 1975 eruption of ngauruhoe volcano whose emplacement were also witnessed during eruption using high resolution gps surveys a series of lateral excavations across the deposits and bulk sedimentological analysis we constrained the geomorphology internal structure and texture of the deposits with respect to laterally varying modes of deposition deposition from these pdcs began only on slopes at or around the material s angle of repose c 30 in unconfined settings the granular pdcs are interpreted to have been quasi steady forming sheets and lobes around the angle of repose where flows were confined sheet like proximal facies made up around 10 of the deposit volume at the angle of repose but 90 of the material was deposited from apparently unsteady inertial granular pdcs as a distal levee and channel facies on slopes well below the repose angle hence confined pdcs were able to travel up to 50 farther than unconfined flows in the distal facies the deposit width is inversely correlated to the local slope and the height of the levees above the deposit centreline is positively correlated with slope internally the deposits comprise three parts a coarse grained fines free sole layer that laterally connects to levees zone i an ashy matrix supported central body zone ii and an overlying coarse plaster of clasts zone ii trends in grain size data suggests these zones derive from a continuous un mixing of coarse particles from the initial bulk material by granular segregation that preferentially drives large particles to the upper free surface of the flow where they are concentrated at the front of flow before being deposited and overrun by comparison to analogue experiments we suggest a model of flow and deposition where the temporally and spatially varying mode of deposition is determined by the flow velocity the local slope the vertical velocity gradient the velocity gradient at the free surface and the vertical deposition rate using this model estimated vertical deposition rates of c 5 cm s 1 from the ngauruhoe pdcs agree with those determined in laboratory experiments on inertial granular flows

dust devils on mars columnar cone shaped and funnel shaped clouds rising 1 to 6 kilometers above the surface of mars have been identified in viking orbiter images they are interpreted as dust devils confirming predictions of their occurrence on mars and giving evidence of a specific form of dust entrainment

formation of gullies on mars by debris flows triggered by co2 sublimation martian gully landforms resemble terrestrial debris flows formed by the action of liquid water and have thus been interpreted as evidence for potential habitable environments on mars within the past few millennia however ongoing gully formation has been detected under surface conditions much too cold for liquid water but at times in the martian year when a thin layer of seasonal co 2 frost is present and defrosting above the regolith these observations suggest that the co 2 condensation sublimation cycle could play a role in gully formation here we use a thermo physical numerical model of the martian regolith underlying a co 2 ice layer and atmosphere to show that the pores beneath the ice layer can be filled with co 2 ice and subjected to extreme pressure variations during the defrosting season the subsequent gas fluxes can destabilize the regolith material and induce gas lubricated debris flows with geomorphic characteristics similar to martian gullies moreover we find that subsurface co 2 ice condensation sublimation and pressurization occurs at conditions found at latitudes and slope orientations where gullies are observed we conclude that martian gullies can result from geologic dry ice processes that have no terrestrial analogues and do not require liquid water such dry ice processes may have helped shape the evolution of landforms elsewhere on the martian surface

metastability of liquid water on mars abstract a simple model of local heat transport on mars demonstrates that transient melting of ice may occur in depressions and gullies nearly anywhere on the planet where thin ice is illuminated by normal incidence insolation an experiment has been performed to confirm the model of evaporation rate at low pressure reduction of radiative cooling due to gully geometry is shown to be important since appropriate meteorological topographic and optical conditions may occur on slopes nearly anywhere on the planet hydrological features such as gullies would likely form only where such ice might accumulate notably in sheltered locations at high latitudes it is suggested that cold trapping of winter condensation could concentrate a sufficient amount of ice to allow seasonal melting in gullies

the presence and stability of ground ice in the southern hemisphere of mars abstract we calculate new estimates of ground ice stability and the depth distribution of the ice table the depth boundary between ice free soil above and ice cemented soil below and compare these theoretical estimates of the distribution of ground ice with the observed distribution of leakage neutrons measured by the neutron spectrometer instrument of the mars odyssey spacecraft s gamma ray spectrometer instrument suite our calculated ground ice distribution contains improvements over previous work in that we include the effects of the high thermal conductivity of ice cemented soil at and below the ice table we include the surface elevation dependence of the near surface atmospheric humidity and we utilize new high resolution maps of thermal inertia albedo and elevation from mars global surveyor observations results indicate that ground ice should be about 5 times shallower than in previous predictions while results are dependent on the atmospheric humidity depths are generally between a few millimeters and a few meters with typical values of a few centimeters results are also geographically similar to previous predictions with differences due to the higher resolution of thermal inertia and the inclusion of elevation effects on humidity comparison with the measured epithermal neutron count rates in the southern hemisphere indicate that the geographic distribution of the count rate is best correlated with ground ice in equilibrium with 10 to 20 pr m precipitable micrometers column abundance of atmospheric water assuming a uniform distribution with co2 however given the uncertainties 5 to 30 pr m also may be viable this water abundance represents a longer term average over 100 to 1000 yr there is a high degree of correlation between the depth of the ice table and the epithermal count rate that agrees remarkably well with predicted count rates as a function of ice table depth these results indicate that ground ice in the upper meter of the martian soil is in diffusive equilibrium with the atmosphere since ground ice in this depth zone is expected to undergo saturation desiccation cycles with orbital variations this ice should be younger than about 500 kyr and was emplaced under similar cold and dry climate conditions of today remaining differences between the predicted depths of the ice table and those inferred from the neutron data are likely to be due to subpixel heterogeneity in the martian surface including the presence of rocks slopes and patches of soil with varying thermophysical properties