viscous flow features on the surface of mars observations from high resolution mars orbiter camera moc images 10 5 10 7 years of the material our shear stress estimates of 10 1 5 10 2 5 mpa yield strain rates on the order of 10 11 10 16 s 1 which are within the superplastic flow regime of ice mean annual surface temperatures age constraints and strain estimates show that it is possible for a meters thick ice dust mixture to undergo viscous deformation under past or present surface conditions for ice grain sizes 10 mm the meters thick layer in which the viscous flow features formed is morphologically similar to a degraded meters thick ice dust surface deposit dissected mantle terrain locations of the viscous flow features dissected mantle terrain and recent gullies are concentrated in the midlatitude regions and all three show identical distributions as a function of latitude with the maximum frequency of occurrence at 40 n and s the strong association between these small scale flow features and the dissected mantle terrain large scale viscous flow features and recent gullies imply that deposition deformation and removal of ice rich materials has played an important role in the modification of the surface in the midlatitudes of mars during the amazonian and possibly longer index terms 5470 planetology solid surface planets surface materials and properties 5416 planetology solid surface planets glaciation 1863 hydrology snow and ice 1827 1823 hydrology frozen ground 8168 tectonophysics stresses general keywords viscous flow viscous creep dissected mantle terrain ice rich deposits mars climate recent gullies

evaluating the reliability of the stream tracer approach to characterize stream subsurface water exchange stream water was locally recharged into shallow groundwater flow paths that returned to the stream hyporheic exchange in st kevin gulch a rocky mountain stream in colorado contaminated by acid mine drainage two approaches were used to characterize hyporheic exchange sub reach scale measurement of hydraulic heads and hydraulic conductivity to compute streambed fluxes hydrometric approach and reachscale modeling of in stream solute tracer injections to determine characteristic length and timescales of exchange with storage zones stream tracer approach subsurface data were the standard of comparison used to evaluate the reliability of the stream tracer approach to characterize hyporheic exchange the reach averaged hyporheic exchange flux 1 5 ml s 1 m 1 determined by hydrometric methods was largest when stream base flow was low 10 l s 1 hyporheic exchange persisted when base flow was 10 fold higher decreasing by approximately 30 reliability of the stream tracer approach to detect hyporheic exchange was assessed using first order uncertainty analysis that considered model parameter sensitivity the stream tracer approach did not reliably characterize hyporheic exchange at high base flow the model was apparently more sensitive to exchange with surface water storage zones than with the hyporheic zone at low base flow the stream tracer approach reliably characterized exchange between the stream and gravel streambed timescale of hours but was relatively insensitive to slower exchange with deeper alluvium timescale of tens of hours that was detected by subsurface measurements the stream tracer approach was therefore not equally sensitive to all timescales of hyporheic exchange we conclude that while the stream tracer approach is an efficient means to characterize surface subsurface exchange future studies will need to more routinely consider decreasing sensitivities of tracer methods at higher base flow and a potential bias toward characterizing only a fast component of hyporheic exchange stream tracer models with multiple rate constants to consider both fast exchange with streambed gravel and slower exchange with deeper alluvium appear to be warranted

mars orbiter camera observations of the martian south polar cap in 1999 2000 the spring summer recession of the south polar cap of mars in 1999 2000 has been investigated using the wide angle cameras of the mars orbiter camera experiment on mars global surveyor the 1999 2000 regression closely resembles the recession observed by viking orbiters in 1977 although the rates of recession are quite similar the current recession was slightly ahead of that in 1977 throughout spring the rapid development of dark terrain dubbed cryptic terrain by hugh kieffer in one sector of the south cap between ls 198 and 223 is documented the emergence and separation of the mountains of mitchel has also been recorded in detail culminating with its complete disappearance at solstice the 2000 residual cap is almost identical to that in 1977 this observation does not resolve the mystery of the very different appearance of the residual cap in 1972 the albedos in the residual cap region and in bright seasonal frost increase rapidly to high values at around solstice and then level out until the seasonal frost disappears observations of more martian years will be needed to constrain the amount of interannual variability and its relation to other phenomena such as dust storms

potential sand sources for the dune fields in noachis terra mars 1 no sand transport pathways are visible in a study performed in noachis terra a 60 35 region in the southern highlands of mars known for its many intracrater dune fields detailed studies were performed of five areas in noachis terra using mars orbiter camera moc wide angle mosaics thermal emission imaging system themis daytime and nighttime infrared mosaics mola digital elevation and shaded relief maps and moc narrow angle images the lack of observable sand transport pathways suggests that such pathways are very short ruling out a distant source of sand consistent dune morphology and dune slipface orientations across noachis terra suggest formative winds are regional rather than local e g crater slope winds a sequence of sedimentary units was found in a pit eroded into the floor of rabe crater some of which appear to be shedding dark sand that feeds into the rabe crater dune field the visible and thermal characteristics of these units are similar to other units found across noachis terra leading to the hypothesis that a series of region wide depositional events occurred at some point in the martian past and that these deposits are currently exposed by erosion in pits on crater floors and possibly on the intercrater plains thus the dune sand sources may be both regional and local sand may be eroding from a widespread source that only outcrops locally sand bearing layers that extend across part or all of the intercrater plains of noachis terra are not likely to be dominated by loess or lacustrine deposits glacial and or volcanic origins are considered more plausible

ideas about the surface runoff features on mars in their report evidence of recent groundwater seepage and surface runoff on mars 30 june p 2330 1 malin and edgett propose that the seepage and flow of water may have occurred on mars in the recent geologic past on the basis of the observation of a large number of channel features on

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

co2 gas fluidization in the initiation and formation of martian polar gullies 1 martian gully landforms indicative of rapid flow of sediment down steep slopes have been cited as evidence of the action of near surface liquid water in mars recent past gullies in polar regions cannot involve pure liquid water because ambient temperatures are too low here we show that polar gullies could be initiated by fluidization of sediment over a subliming seasonal deposit of co2 frost as has been proposed previously for gullies in sand the gas speed and co2 sublimation rate required for fluidization is calculated with the semi empirical ergun relation as validated in industrial applications for gullies in dust the gas speed and sublimation rate required for fluidization are estimated from laboratory experiments with comparable materials to determine if those rates can be achieved we compute diurnal and seasonal surface and subsurface temperatures through a martian year including co2 condensation and sublimation using the marstherm one dimensional finite difference thermal model models were run without sediment and with layers of sand or dust 10 4 10 1 m thick deposited over a seasonal layer of co2 frost the simulations show that in the spring sufficient heat reaches the co2 frost underlying the surface sediment layer to fluidize them this result confirms that martian gullies may have diverse origins and that polar and mid latitude gullies can be initiated on steep slopes by the fluidization of loose sediment on a sublimating seasonal deposit of co2 gas

formation of martian gullies by the action of liquid water flowing under current martian environmental conditions 1 geomorphic evidence suggests that recent gullies on mars were formed by fluvial activity the martian gully features are significant because their existence implies the presence of liquid water near the surface on mars in geologically recent times irrespective of the ultimate source of the fluid carving the gullies we seek to understand the behavior of this fluid after it reaches the martian surface we find that contrary to popular belief the fluvially carved martian gullies are consistent with formation conditions such as now occur on mars outside of the temperature pressure stability regime of liquid water our model of the action of flowing pure liquid water produces the observed gully length distribution only at surface pressures and temperatures below the triple point where liquid water simultaneously boils and freezes and thus suggests that gullies were formed under conditions similar to present day mars our results suggest a typical flow rate of 30 m 3 s to carve the gully channels at least 0 15 km 3 has flowed across the surface of mars to carvethegullysystemsobservedtoday andthiswouldrequireanaquifer recharge rate of 10 13 10 12 m yr the absence of gullies on mars that are long enough to have been created above the triple point pressure argues that the atmospheric pressure has not been significantly larger than it is now since the origin of the gullies this result may imply that mars does not possess a significant reservoir of condensed co2

distribution and orientation of northern hemisphere gullies on mars from the evaluation of hrsc and moc na data abstract martian gullies show morphologic characteristics that indicate the presence of a flowing liquid during their formation process however the current climate in the northern hemisphere of mars does not allow the existence of substantial amounts of liquid water and therefore an assessment of the regional distribution and characteristics of gullies is required in order to understand the most recent climatic development of mars we examined high resolution stereo camera hrsc and mars orbiter camera narrow angle moc na data covering the northern hemisphere in order to determine the distribution geological setting and orientation of gullied slopes we used these results to test the consistency of formation models proposed in earlier works and to define the climate dependence gully orientations were analyzed only at impact crater walls because craters have an azimuthally uniform distribution of slopes this is ideal when investigating the influence of insolation on the distribution and development of landforms we examined 230 hrsc and 21 000 moc na images and identified more than 2200 gullies on hrsc and more than 3100 gullies in moc na data both surveys show the highest densities of gullies between 35 n and 55 n and a shift of gully orientation at 40 n from pole facing slopes to slopes facing the equator orientation trends as well as the latitudinal distribution of identified gullies strongly support the view of gully formation caused by insolation climatic conditions this result suggests that formation mechanisms based on the assumption of atmospheric water ice deposition proposed in earlier work are more likely than processes related to groundwater flow further support for a theory related to deposition is gained through observation of the high occurrence of gullies at isolated knobs and hills which cannot be associated with formation models based upon groundwater flow

martian hillside gullies and icelandic analogs abstract we report observations of icelandic hillside gully systems that are near duplicates of gullies observed on high latitude martian hillsides the best icelandic analogs involve basaltic talus slopes at the angle of repose with gully formation by debris flows initiated by ground water saturation and or by drainage of water from upslope cliffs we report not only the existence of mars analog gullies but also an erosional sequence of morphologic forms found both on mars and in iceland the observations support hypotheses calling for creation of martian gullies by aqueous processes issues remain whether the water in each case comes only from surficial sources such as melting of ground ice or snow or from underground sources such as aquifers that gain surface access in hillsides iceland has many examples of the former but the latter mechanism is not ruled out our observations are consistent with the martian debris flow mechanism of f costard et al 2001c science 295 110 113 except that classic debris flows begin at midslope more frequently than on mars from morphologic observations we suggest that some martian hillside gully systems not only involve significant evolution by extended erosive activity but gully formation may occur in episodes and the time interval since the last episode is considerably less than the time interval needed to erase the gully through normal martian obliteration processes

formation of recent martian gullies through melting of extensive water rich snow deposits the observation of gullies on mars indicates the presence of liquid water near the surface in recent times1 2 which is difficult to reconcile with the current cold climate gullies have been proposed to form through surface runoff from subsurface aquifers1 3 or through melting of near surface ice under warmer conditions4 but these gullies are observed to occur preferentially in cold mid latitudes2 where the presence of liquid water is less likely and on isolated surfaces where groundwater seepage would not be expected making both potential explanations unsatisfactory here i show that gullies can form by the melting of water rich snow that has been transported from the poles to mid latitudes during periods of high obliquity within the past 105 to 106 years refs 5 6 melting within this snow7 can generate sufficient water to erode gullies in about 5 000 years my proposed model for gully formation is consistent with the age and location of the gullies and it explains the occurrence of liquid water in the cold mid latitudes as well as on isolated surfaces remnants of the snowpacks are still present on mid latitude pole facing slopes and the recent or current occurrence of liquid water within them provides a potential abode for life

investigating gully flow emplacement mechanisms using apex slopes abstract the origin of the martian gullies has been much debated since their discovery by the mars orbiter camera moc malin m c edgett k s 2000 science 288 2330 2335 several previous studies have looked at slope gradients in and around gullies but none have used digital elevation models dems from the high resolution imaging science experiment hirise mcewen a s and 14 colleagues 2007 j geophys res 112 e05 e0505s02 which has a pixel scale down to 25 cm pixel we use five 1 m post hirise dems to measure gully apex slopes the local channel gradient at the upslope extent of the gully debris apron which marks a shift from erosion to deposition the apex slope provides information about whether a flow was likely a typical dry granular flow begins depositing on slopes 21 or fluidized by some extra mechanism depositing on shallower slopes we find that 72 of the 75 gully fans studied were likely emplaced by fluidized flows relatively old gullies appear more likely to have hosted fluidized flows than relatively fresh gullies this suggests a time and location dependent fluidizing agent possibly liquid water produced in a different climate as previously proposed our results do not provide evidence for water rich flows in gullies today

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

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

yearly comparisons of the martian polar caps 1999 2003 mars orbiter camera observations abstract the mars global surveyor mars orbiter camera wide angle cameras were used to obtain images of the north and south seasonal and residual polar caps between 1999 and 2003 wide angle red camera images were used in assembling mosaics of the north and south polar recessions and regression rates were measured and compared there are small variations in the north polar recession between 2000 and 2002 especially between l s 7 and l s 50 however there is no evidence for the plateau in the recession curves that has been observed in some prior years the south polar recession changes very little from year to year and the 2001 dust storm had little if any effect on the average cap recession that year albedo values of the geographic north pole were measured using wide angle red and blue camera images and the residual south polar cap configuration was compared between the three years observed by moc the albedo of the geographic north pole generally varies between 0 5 and 0 6 as measured from moc wide angle red camera images there were only minor variations near the edges of the residual south polar cap between the three years examined

eutectic brines on mars origin and possible relation to young seepage features abstract if the recently observed young gullies on mars represent seeps or outflows of aqueous fluid that fluid must be highly concentrated brine to be even moderately stable under current martian surface conditions such brines are to be expected in the megaregolith if an initial saline hydrosphere underwent evapoconcentration via water escape from the atmosphere evolved into subsurface cacl 2 enriched brines by chemical interaction with mafic rocks and then froze until eutectic compositions were reached although most such brines would tend to sink deep into the megaregolith they could be preserved locally at relatively high elevations as aquifers or frozen paleoaquifers perched between ground ice and crystalline salts

evidence for paleolakes in erythraea fossa mars implications for an ancient hydrological cycle there is now widespread agreement that the surface of mars underwent some degree of fluvial modification but there is not yet full understanding of its surface hydrological cycle and the nature of standing bodies of water rivers and precipitation that affected its surface in this paper we explore erythraea fossa 31 5 w 27 3 s a graben adjacent to holden crater which exhibits strong evidence that it once housed a chain of three lakes had overland water flow and was subject to precipitation the inlet valley outlet valley and fan morphologies in the paleolakes are used to qualitatively discern the hydrologic history of the paleolakes based on topography constraints the three basins combined once held 56 km 3 of water depositional features within the basins that change with drainage area and nearby valleys that start near drainage divides indicate that the paleolakes may have been fed by precipitation driven runoff this suggests the presence of an atmosphere at least locally that was capable of supporting a hydrological cycle

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

geologically recent gully polygon relationships on mars insights from the antarctic dry valleys on the roles of permafrost microclimates and water sources for surface flow we describe the morphology and spatial relationships between composite wedge polygons and mars like gullies consisting of alcoves channels and fans in the hyper arid antarctic dry valleys adv as a basis for understanding possible origins for martian gullies that also occur in association with polygonally patterned ground gullies in the adv arise in part from the melting of atmospherically derived wind blown snow trapped in polygon troughs snowmelt that yields surface flow can occur during peak southern hemisphere summer daytime insolation conditions ice cemented permafrost provides an impermeable substrate over which meltwater flows but does not significantly contribute to meltwater generation relationships between contraction crack polygons and sedimentary fans at the distal ends of gullies show deposition of fan material in polygon troughs and dissection of fans by expanding polygon troughs these observations suggest the continuous presence of meters thick ice cemented permafrost beneath adv gullies we document strong morphological similarities between gullies and polygons on mars and those observed in the adv inland mixed microclimate zone on the basis of this morphological comparison we propose an analogous top down melting model for the initiation and evolution of martian gullies that occur on polygonally patterned mantled surfaces

the role of arcuate ridges and gullies in the degradation of craters in the newton basin region of mars abstract a survey of craters in the vicinity of newton basin using high resolution images from mars global surveyor and mars odyssey was conducted to find and analyze examples of gullies and arcuate ridges and assess their implications for impact crater degradation processes in the phaethontis quadrangle mc 24 we identified 225 craters that contain these features of these 188 had gullies on some portion of their walls 118 had arcuate ridges at the bases of the crater walls and 104 contained both features typically on the same crater wall a major result is that the pole facing or equator facing orientation of these features is latitude dependent at latitudes 44 s equator facing orientations for both ridges and gullies are prevalent but at latitudes

role of aquicludes in formation of martian gullies liquid water is not currently stable on the surface of mars but images provided by the mars orbiter camera aboard the mars global surveyor spacecraft reveal erosional landforms previously interpreted to be geologically young gullies formed by groundwater seepage we test the basic hypothesis that as on earth the location of these gullies is controlled in part by the presence of an impermeable rock layer aquiclude and that the depths of the gully heads below the surface should thus be correlated to subsurface geology we show that 1 gullies emanate from a specific cliff forming layer even if the layer is faulted and 2 the depth of gullies below the local surface ranges from 70 to 800 m and 3 is positively correlated to mapped geologic units gully formation is therefore dependent upon both favorable climatic conditions to produce and sustain liquid water and the presence of impermeable subsurface layers to collect the groundwater gullies may mark the distribution of subsurface impermeable layers globally and are prime targets for the search for present water and life on mars

mars nature and evolution of young latitude dependent water ice rich mantle 1 high resolution altimetry and imaging have revealed the presence of a meters thick sedimentary layer at middle to high northern and southern latitudes presently covering at least 23 of the planet the layer is interpreted to be water ice rich and to undergone degradation recently its activity very likely coincided with the last major obliquity excursion a few hundred thousand years ago the majority of the layer at higher latitudes however persisted for a much longer time in the late amazonian stratigraphic analysis suggests a complex history of successive episodes of deposition and removal repeated deposition and removal of the mantles are interpreted to be responsible for the unusual statistical properties of kilometer scale topography in the transitional mid latitude zones

absolute dune ages and implications for the time of formation of gullies in nirgal vallis mars 1 transverse dunes cover the valley floor of nirgal vallis a 670 km long valley network at 318 e and 29 s the dunes are superposed by small undeformed impact craters which implies that the dunes are inactive under present atmospheric conditions the last active phase of dune movement absolute age can be determined by crater size frequency distributions the derived absolute ages from high resolution mars global surveyor mars orbiter camera imagery range from approximately 300 000 years to 1 4 myr b p error of factor of 2 the dunes are also superposed by gully deposits aprons which implies a younger age of the gullies these superposition relationships show that the gullies in this midlatitude region are at the most 3 myr old but might be even younger than 300 000 years

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

southern high latitude dune fields on mars morphology aeolian inactivity and climate change in a study area spanning the martian surface poleward of 50 s 1190 dune fields have been identified mapped and categorized based on dune field morphology dune fields in the study area span 116 400 km2 leading to a global dune field coverage estimate of 904 000 km2 far less than that found on earth based on distinct morphological features the dune fields were grouped into six different classes that vary in interpreted aeolian activity level from potentially active to relatively inactive and eroding the six dune field classes occur in specific latitude zones with a sequence of reduced activity and degradation progressing poleward in particular the first signs of stabilization appear at 60 s which broadly corresponds to the edge of high concentrations of water equivalent hydrogen content observed by the neutron spectrometer that have been interpreted as ground ice this near surface ground ice likely acts to reduce sand availability in the present climate state on mars stabilizing high latitude dunes and allowing erosional processes to change their morphology as a result climatic changes in the content of near surface ground ice are likely to influence the level of dune activity spatial variation of dune field classes with longitude is significant suggesting that local conditions play a major role in determining dune field activity level dune fields on the south polar layered terrain for example appear either potentially active or inactive indicating that at least two generations of dune building have occurred on this surface many dune fields show signs of degradation mixed with crisp brinked dunes also suggesting that more than one generation of dune building has occurred since they originally formed dune fields superposed on early and late amazonian surfaces provide potential upper age limits of 100 my on the south polar layered deposits and 3 ga elsewhere at high latitudes no craters are present on any identifiable dune fields which can provide a lower age limit through crater counting assuming all relatively stabilized dune fields represent a single noncontiguous surface of uniform age their estimated crater retention age is 10 000 years an average sized uncratered dune field 94 km2 has a crater retention age 8 my this apparent youth suggests that present day climate conditions are responsible for the observed degradation and reduced level of aeolian activity a lack of observed transport pathways and the absence of large dune fields in the largest basins hellas and argyre planitiae are consistent with the previously proposed idea that dune sands are not typically transported far from their source regions on mars

liquid co2 breakout and the formation of recent small gullies on mars we show that the action of a co2 suspended flow could have produced the recent small gullies on mars and hence that liquid water is not required the model involves the build up of a liquid co2 aquifer behind and below a dry ice barrier dam cap rock in the pore spaces a few meters into the rock from the cliff face and on order of a hundred meters below the top of the cliff brink surface seasonal or obliquity cycle seasonal heating causes pinching out of the dry ice barrier and rapid release of the liquid co2 erosion of the gullies occurs as the rapid vaporization of the liquid co2 with entrainment of rock and clathrate hydrate ice produces a density flow analogous to a terrestrial nue ardente

comparison of small lunar landslides and martian gullies some lunar crater wall landslides strongly resemble martian gullies despite the lack of geologically active water on the moon today or in the past the lunar features indicate that alcove channel apron morphology attributed on mars to seepage of liquid water can also form via a dry landslide mechanism therefore a more stringent test than just an alcove channel apron morphology is necessary to differentiate dry landslides from water carved gullies

orientation and distribution of recent gullies in the southern hemisphere of mars observations from high resolution stereo camera mars express hrsc mex and mars orbiter camera mars global surveyor moc mgs data 1 geologically recent small gullies on mars display morphologies consistent with erosion by water or by debris flows suggested formation models are divided into two main categories 1 groundwater or 2 melting of near surface ice snow sourced from the atmosphere we have measured location and orientation and recorded the local contexts of gullies to constrain the likely models of gully formation more than 22 000 mars orbiter camera narrow angle moc na and 120 mars express high resolution stereo camera hrsc images in the southern hemisphere were searched for gullies discrete gullied slope sections with consistent orientation were recorded rather than individual gullies slope setting impact crater valley wall etc location and orientation were recorded for each slope section more than 750 moc images with gullies 900 distinct gullied slope sections and more than 40 hrsc images 380 distinct gullied slope sections were identified from both moc and hrsc gullies were found to be most common between 30 and 50 degrees latitude and to have an overall pole facing preference the preferred gully orientation for hrsc is southeast rather than south in moc owing to illumination effects that make gullies difficult to detect on south to southwest facing slopes in hrsc in both moc and hrsc surveys higher latitude gullies show less preference for pole facing than those at mid latitudes both data sets produced similar results demonstrating that our data are reliable we suggest that the observed latitudinal and orientation distributions of gullies show that insolation and atmospheric conditions play a key role in gully formation

accumulation of sedimentary debris in the south polar region of mars and implications for climate history abstract stratigraphic units of the south polar region of mars were mapped relative chronology determined and detailed modeling of the observed crater populations was used to set absolute constraints on the age of emplacement of materials significant secular variation in the net debris accumulation rate over history is evident an episode of enhanced crater obliteration at about 3 7 ga ago lasting a few hundred ma is inferred for south polar cratered terrains a similar peak in low latitude obliteration rates suggests that the event may have been global in scale whether the debris is volcanic or aeolian in origin the event suggests a possible enhancement in atmospheric density at the time modeling results imply that cratered terrains poleward of 65 south latitude have subsequently experienced steady state net accumulation of material at a rate of about 0 1 km ga while equatorial cratered terrains have been retained in relatively pristine form fifteen craters of impact origin were discovered on the south polar layered terrain formerly thought to be almost devoid of craters their presence implies that the surface is at least a few 100 ma old and that the net accumulation rate is no more than 10 km ga if layer formation is modulated by periodic climatic effects either the period of oscillation is a few ma or longer or the layer deposition mechanism ceased operating at least several 100 ma ago

kilometer scale roughness of mars results from mola data analysis the mars orbiter laser altimeter mola data are used to characterize the kilometer scale surface roughness of mars the median absolute value of the differential slope at a given baseline is proposed as a data derived measure of the surface roughness at this scale study of the scale dependence of roughness for the smoothest terrains gives an independent estimate of 20 cm for mola ranging accuracy the baseline lengths from 0 6 to 20 km are used and kilometer scale roughness is mapped for the entire surface the maps show that different geological units have distinctive roughness characteristics scale dependence of roughness for a number of geological units is presented and discussed the southern polar cap is rougher than the northern at kilometer and subkilometer scale which suggests differences in the sublimation condensation balance the vastitas borealis formation has a distinctive 3 km scale background surface topography which suggests a nonvolcanic origin for its upper layer young volcanic plains in amazonis planitia and the eastern part of elysium planitia are very similar to each other in their roughness characteristics and differ from other volcanic plains on mars which suggests a distinctive eruption style there are systematic latitudinal variations of roughness in both the southern highlands and the northern lowlands terrains at high latitude are smoother at short baselines the characteristic vertical scale related to this difference is several meters processes that could be responsible for formation of this trend include creep of ice rich near surface material at high latitudes treatment of the surface with repetitive deposition and sublimation of seasonal frost climate controlled deposition and or cementation of dust at high latitudes and repetitive sublimation and accumulation of subsurface ice at low latitudes with climate variations all mechanisms of origin could operate more effectively under different climate conditions relevant morphological observations favor mechanisms involving deposition of smooth blankets at high latitudes

an assessment of evidence for pingos on mars using hirise pingos are small hills with cores of ice formed by injection and freezing of pressurized water the possibility of pingos on mars is of particular interest because of the associated implications for liquid water we have systematically searched for candidate pingos using images from the high resolution imaging science experiment hirise camera since pingos are expected to develop surface fractures due to extension of the frozen ground over the ice core we have searched for fractured features and identified a variety of mounds these features are confined to the martian mid latitudes in the bands where gullies are also most common the observed fractured mounds have a variety of morphologies and are likely of multiple origins isolated fractured mounds found on the floors of gullied craters in the southern hemisphere match the general morphologic characteristics of terrestrial pingos and are the best candidates for martian pingos but there is currently no direct evidence for presence of ice cores and it is difficult to produce the necessary water volumes so these features should still be interpreted with caution other fractured mounds appear more likely to be erosional remnants of an unusual mantling layer or possibly thermokarst structures flat topped mounds in utopia have some characteristics fracture pattern and latitudinal distribution consistent with pingos but differ in other aspects such as shape and setting while we do not rule out a pingo origin we prefer an erosional model for these enigmatic features

constraints on mechanisms for the growth of gully alcoves in gasa crater mars from two dimensional stability assessments of rock slopes abstract the value of slope stability analyses for gaining insight into the geologic conditions that would facilitate the growth of gully alcoves on mars is demonstrated in gasa crater two dimensional limit equilibrium methods are used in conjunction with high resolution topography derived from stereo high resolution imaging science experiment hirise imagery these analyses reveal three conditions that may produce observed alcove morphologies through slope failure 1 a ca 10 m thick surface layer that is either saturated with h 2 o ground ice or contains no groundwater ice at all above a zone of melting h 2 o ice or groundwater and under dynamic loading i e seismicity 2 a 1 10 m thick surface layer that is saturated with either melting h 2 o ice or groundwater and under dynamic loading or 3 a 100 m thick surface layer that is saturated with either melting h 2 o ice or groundwater and under static loading this finding of three plausible scenarios for slope failure demonstrates how the triggering mechanisms and characteristics of future alcove growth would be affected by prevailing environmental conditions hirise images also reveal normal faults and other fractures tangential to the crowns of some gully alcoves that are interpreted to be the result of slope instability which may facilitate future slope movement stability analyses show that the most failure prone slopes in this area are found in alcoves that are adjacent to crown fractures accordingly crown fractures appear to be a useful indicator of those alcoves that should be monitored for future landslide activity

seasonal and diurnal variations of polardarn f region echo occurrence in the polar cap and their causes f region echo occurrence rates for the polar dual auroral radar network polardarn hf radars at inuvik inv rankin inlet rkn and clyde river cly are compared for observations in 2013 the cly radar shows somewhat smaller echo occurrence rates consistent with its more poleward geographic and geomagnetic location for all three radars the winter occurrence rates are roughly twice that of the corresponding summer rates for observations in the midnight sector strong equinoctial maxima are evident in terms of season and local time echo occurrence patterns are found to be roughly the same for all radars seasonally clear maxima are found near noon during both winter and summer while diurnally enhancements are found during equinoctial dusk a comparison of data from roughly the same scattering area shows that having strong electron density in the scattering volume is not sufficient for getting an hf echo propagation conditions along the propagation path are also important diurnal variations in the f region electron density and electric field both measured by canadian advanced digital ionosonde are compared to those of rkn echo occurrence rates for observations over resolute bay rb located at a geomagnetic latitude of 83 n these results show a reasonable correlation between occurrence and electron density for both winter and summer periods and correlation between occurrence and electric field during summer periods

evidence for recent climate change on mars from the identification of youthful near surface ground ice ground ice in the crust and soil may be one of the largest reservoirs of water on mars1 2 3 near surface ground ice is predicted to be stable at latitudes higher than 40 ref 4 where a number of geomorphologic features indicative of viscous creep and hence ground ice have been observed5 mid latitude soils have also been implicated as a water ice reservoir6 the capacity of which is predicted to vary on a 100 000 year timescale owing to orbitally driven variations in climate7 it is uncertain however whether near surface ground ice currently exists at these latitudes and how it is changing with time here we report observational evidence for a mid latitude reservoir of near surface water ice occupying the pore space of soils the thickness of the ice occupied soil reservoir 1 10 m and its distribution in the 30 to 60 latitude bands indicate a reservoir of 1 5 6 0 104 km3 equivalent to a global layer of water 10 40 cm thick we infer that the reservoir was created during the last phase of high orbital obliquity less than 100 000 years ago and is now being diminished

north south topographic slope asymmetry on mars evidence for insolation related erosion at high obliquity 1 a map of north south subkilometer scale slope asymmetry on mars obtained from statistical analysis of along track mola topographic profiles reveals well defined zonal belts of north south slope asymmetry at 40 50 latitude in both hemispheres in these narrow anomalous belts the pole facing slopes are systematically gentler than equator facing slopes this asymmetry is especially pronounced for the steepest 20 slopes in which pole facing slopes are three times less frequent than 20 slopes facing the equator we interpret these belts to be related to insolation asymmetry specifically we suggest that summertime melting of ground ice on pole facing slopes occurred during periods of very high obliquity 45 in the past and favored downslope movement of material and reduction of steep pole facing slopes

new and recent gully activity on mars as seen by hirise 1 fresh appearing gully deposits are found at tens of sites in the southern hemisphere of mars these deposits have latitudinal and azimuthal dependences similar to the overall preferences of southern hemisphere gullies suggesting that most gullies can undergo such events definite changes are seen at ten sites including two previously reported these include visible modification of gully channels and aprons those formation intervals constrained to better than one mars year tend to include winter and exclude summer suggesting seasonal activity this seasonal activity is consistent with proposed models for gully formation driven by co2 frost although at least some of the new deposits are probably due to dry granular flow with no volatile involvement as these deposits are capable of effecting distinct topographic changes to gully aprons and channels they represent a significant component of recent gully evolution

observations of martian gullies and constraints on potential formation mechanisms abstract the discovery of presumably geologically recent gully features on mars malin and edgett 2000 science 288 2330 2335 has spawned a wide variety of proposed theories of their origin including hypotheses of the type of erosive material to test the validity of gully formation mechanisms data from the mars global surveyor spacecraft has been analyzed to uncover trends in the dimensional and physical properties of the gullies and their surrounding terrain we located 106 mars orbiter camera moc images that contain clear evidence of gully landforms distributed in the southern mid and high latitudes and analyzed these images in combination with mars orbiter laser altimeter mola and thermal emission spectrometer tes data to provide quantitative measurements of numerous gully characteristics parameters we measured include apparent source depth and distribution vertical and horizontal dimensions slopes orientations and present day characteristics that affect local ground temperatures we find that the number of gully systems normalized to the number of moc images steadily declines as one moves poleward of 30 s reaches a minimum value between 60 63 s and then again rises poleward of 63 s all gully alcove heads occur within the upper one third of the slope encompassing the gully and the alcove bases occur within the upper two thirds of the slope also the gully alcove heads occur typically within the first 200 meters of the overlying ridge with the exception of gullies equatorward of 40 s where some alcove heads reach a maximum depth of 1000 meters while gullies exhibit complex slope orientation trends gullies are found on all slope orientations at all the latitudes studied assuming thermal conductivities derived from tes measurements as well as modeled surface temperatures we find that 79 of the gully alcove bases lie at depths where subsurface temperatures are greater than 273 k and 21 of the alcove bases lie within the solid water regime most of the gully alcoves lie outside the temperature pressure phase stability of liquid co2 based on a comparison of measured gully features with predictions from the various models of gully formation we find that models involving carbon dioxide melting ground ice in the upper few meters of the soil dry landslide and surface snowmelt are the least likely to describe the formation of the martian gullies although some discrepancies still exist between prediction and observation the shallow and deep aquifer models remain as the most plausible theories interior processes involving subsurface fluid sources are generally favored over exogenic processes such as wind and snowfall for explaining the origin of the martian gullies

recent bright gully deposits on mars wet or dry flow bright gully sediments attributed to liquid water flow have been deposited on mars within the past several years to test the liquid water flow hypothesis we constructed a high resolution 1 m pixel photogrammetric digital elevation model of a crater in the centauri montes region where a bright gully deposit formed between 2001 and 2005 we conducted one dimensional 1 d and 2 d numerical flow modeling to test whether the deposit morphology is most consistent with liquid water or dry granular flow liquid water flow models that incorporate freezing can match the runout distance of the flow for certain freezing rates but fail to reconstruct the distributary lobe morphology of the distal end of the deposit dry granular flow models can match both the observed runout distance and the distal morphology wet debris flows with high sediment concentrations are also consistent with the observed morphology because their rheologies are often similar to that of dry granular flows as such the presence of liquid water in this flow event cannot be ruled out but the available evidence is consistent with dry landsliding

patterns of accumulation and flow of ice in the mid latitudes of mars during the amazonian evidence has accumulated that non polar portions of mars have undergone significant periods of glaciation during the amazonian period this evidence includes tropical mountain glacial deposits lobate debris aprons lineated valley fill concentric crater fill pedestal craters and related landforms some of which suggest that ice thicknesses exceeded a kilometer in many places in some places several lines of evidence suggest that ice is still preserved today in the form of relict debris coved glaciers the vast majority of deposit morphologies are analogous to those seen in cold based glacial deposits on earth suggesting that little melting has taken place although these features have been broadly recognized and their modes of ice accumulation and flow analyzed at several scales they have not been analyzed and well characterized globally despite their significance for understanding the evolution of the martian climate a major outstanding question is the global extent of accumulation and flow of ice during periods of non polar glaciation as a mechanism to address this question we outline two end member scenarios to provide a framework for further discussion and analysis 1 ice accumulation was mainly focused within individual craters and valleys and flow was largely local to regional in scale and 2 ice accumulation was dominated by global latitudinal scale cold based ice sheets similar in scale to the laurentide continental ice sheets on earth in order to assess these end members we conducted a survey of ice related features seen in context camera ctx images in each hemisphere and mapped evidence for flow directions within well preserved craters in an effort to decipher orientation preferences that could help distinguish between these two hypotheses regional hemispheric glaciation or local accumulation and flow these new crater data reveal a latitudinal dependence on flow direction at low latitudes in each hemisphere 40 45 slopes of all orientations show signs of ice accumulation and ice related flow this latitudinal onset of concentric flow of ice within craters in each hemisphere correlates directly with the lowest latitudes at which typical pedestal craters have been mapped taken together these observations demarcate an important latitudinal boundary that partitions each hemisphere into two zones 1 poleward of 45 where net accumulation of ice is interpreted to have occurred on all surfaces and 2 equatorward of 45 where net accumulation of ice occurred predominantly on pole facing slopes these results provide important constraints for deciphering the climatic conditions that characterized mars during periods of extensive amazonian non polar glaciation

geologic settings of martian gullies implications for their origins 1 martian gullies are found on steep slopes of all origins on all sorts of terrains of all ages scattered across nearly all of mars gullies are observed on all manner of substrates layered massive shattered rubble with or without nearby mantling deposits gullies are most common in the southern midlatitudes but also occur in the northern hemisphere in near polar terrain on equatorial volcanoes and on northern plains most gullies in the southern hemisphere are on south facing slopes but they occur on slopes of all orientations gullies are among the youngest features on mars but locally are overlain by eolian deposits and cut by faults old or eroded gullies are rare and those found have been partially stripped from slopes leaving no rock debris behind most gully deposits contain no detectable rocks these data are inconsistent with published hypotheses of gully formation including seeps and breakouts of water or brine hydrothermal activity cryovolcanism and breakouts from liquid carbon dioxide the data are consistent with gullies being dry flows of eolian material dust and silt comparable to climax snow avalanches on earth eolian sedimentation should be correlated little with underlying geology cause of slope age of terrain type of terrain or the nature of the rocks eolian sedimentation should be correlated with wind deceleration which will cause suspended sediment to drop and areas with common gullies are those with strong wind deceleration predicted by global circulation model in such areas sediment will be deposited preferentially in the lee of obstacles for the gully rich areas of the southern midlatitudes winds blow from the nnw so that sediment is deposited on sse facing slopes i e poleward these predictions are in accord with observations