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

meteorology of proposed mars exploration rover landing sites 1 a descriptive study of the near surface meteorology at three of the potential mars exploration rover mer landing sites terra meridiani gusev crater and melas chasma is presented using global and mesoscale models the mesoscale model provides a detailed picture of meteorology on scales down to a few kilometers but is not well constrained by observations away from the viking and pathfinder landing sites as such care must be taken in the interpretation of the results with there being high confidence that the types of circulations predicted will indeed occur and somewhat less in the quantitative precision of the predictions and the local time phasing of predicted circulations all three landing sites are in the tropics and are affected by hadley circulation by diurnal variations due to the global thermal tide and by planetary scale topography in these particular cases from tharsis elysium and the global topographic dichotomy boundary terra meridiani is least affected by large variations in local topography mean winds at terra meridiani during mer landing would be less than 10 m s with little vertical shear however these low wind speeds result from strong mixing in the early afternoon convective boundary layer which creates its own hazard in the horizontal variation of vertical winds of up to 8 m s both upward and downward in gusev crater the topography of ma adim vallis and the crater rim generates strong diurnally reversing channeling of wind in ma adim vallis and diurnally reversing radial flow in the crater associated with thermal slope winds on the crater rim the overturning circulation in gusev crater slightly suppresses the daytime convective boundary layer melas chasma in valles marineris provides an example of strong topographic forcing of near surface circulation of particular interest is the channeling of regional scale wind toward the center of the tharsis plateau during the evening this results in a surface level jet along the canyon of over 25 m s drainage of air from the plateau and into the canyon produces vertical winds down the canyon walls in the evening of over 5 m s in contrast during the early afternoon mer landing time horizontal winds at the proposed mer landing site are relatively calm with little mean shear with height this results from the proposed site being in a region of local divergence and the action of daytime convection the nature of flow in melas chasma results in an interesting dual maximum in boundary convection and depth with the usual daytime afternoon free convective maximum being joined by a mechanically forced nighttime boundary layer of almost 2 km depth

paleoethologic implications of zoophycos from late cretaceous and paleocene limestones of the venetian prealps northeastern italy large specimens of zoophycos are abundant in paleocene pelagic limestone successions in the vicinity of feltre and belluno in northeastern italy these burrow systems are familiar examples of the strongly ribbed downwardly spiralling version of the ichnogenus containing well developed primary and secondary lamellae marginal tunnels and poorly preserved axial structures some of the specimens contain fecal pellets rich in non carbonate mud and carbonized phytodetritus suggesting surface deposit feeding and organized subsurface storage of fecal material by the zoophycos producer other examples of the burrows record a departure from the normal behavioral pattern in the form of conspicuous radial tunnels that diverge from the axial region cut across structural elements and follow the laminae exactly to the outer margin this special feature of fully developed zoophycos appears to represent the last time the trace producers recycled the banks of fecal material possibly during long intervals of meager food supply at the deep ocean floor

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

identification of gully debris flow deposits in protonilus mensae mars characterization of a water bearing energetic gully forming process gullies are a class of geologically recent landform on mars that show evidence of downslope transport of sedimentary material from recessed alcoves through incised channels to distributary fans or aprons the mechanisms invoked to account for the formation of gullies on mars range from completely dry granular flows or landslides to debris flows that incorporate some component of liquid water to fluvial erosion and alluvial fan like deposition each of these processes requires different amounts of liquid water and produces different characteristic morphologies we report on the identification of unusual lobate structures present in proximity to gullies in protonilus mensae the lobes are up to 3 m thick and terminate in rounded snouts these lobate structures are present mostly downslope of gullies and can be traced upslope through channels to gully fan termini and in places onto gully fan surfaces crater dating indicates that the deposits formed recently potentially within the past 500 ka we use hrsc digital elevation models to constrain mechanical properties of the lobate deposits and to compare their formation environment to that of typical martian gullies the protonilus mensae lobate deposits are interpreted to indicate local dominance of wet debris flows in the formation of the observed gullies and lobes these observations are consistent with 1 top down melting of the ice component of the latitude dependent mantle terrain in which the gullies form and 2 initiation of debris flows by mobilization of the dusty lithic component of the mantle the suite of morphological observations diagnostic of wet debris flow processes suggests the identification of an unusual environment in which the wet debris flow formational end member is locally the dominant gully forming process elsewhere on mars gully morphology may be more consistent with a range of other water related sediment transport mechanisms including fluvial erosion hyperconcentrated flow and low strength mudflows that deposit as fans rather than lobes these results suggest that 1 gully forming processes involve liquid water 2 that the water source is associated with the martian latitude dependent mantle and 3 that a range of water related sediment transport processes is involved in gully formation

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

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

northern hemisphere martian gullies and mantled terrain implications for near surface water migration in mars recent past 1 we report on a systematic study of gullies and mantled terrain in the northern hemisphere of mars consistent with previous work the mantled terrain is generally located on poleward facing slopes and gullies are found at all orientations new results are as follows 1 the range of orientations of mantled terrain increases toward the pole 2 near polar gullies are commonly less morphologically developed compared to ones in the temperate latitudes 3 gullies toward the pole are better preserved than those closer to the equator and 4 these studies are conducted in the northern hemisphere whereas most previous work investigated the south no gully model in the literature cleanly fits all these observations nor any other gully observations reported in other papers however the likely effects of obliquity and precession variations on snow formation preservation and melting are consistent with many of the results

frosted granular flow a new hypothesis for mass wasting in martian gullies abstract recent gully deposits on mars have been attributed to both wet and dry mass wasting processes in this paper frosted granular flow fgf is presented as a new hypothesis for recent mass wasting activity in martian gullies fgf is a rare type of granular flow observed on a talus slope in the province of quebec canada hetu b van steijn h vandelac p 1994 geogr phys quat 48 3 22 frost reduces dynamic inter particle friction enabling flows to mobilize onto relatively low slope gradients 25 30 compared to those involving dry granular flow of the same material 35 41 resulting erosional and depositional features include straight to sinuous channels levees and digitate to branching arrangements of terminal deposits similar features are commonly found in association with geologically young gully systems on mars based on terrestrial observations of fgf processes the minimum criteria required for their occurrence on mars include i readily mobilized unconsolidated sediment at the surface ii an upper slope gradient at or near the angle of repose iii frost accumulation at the surface and iv triggering by rock fall all four conditions appear to be met in many areas on present day mars though triggering mechanisms may vary compared to terrestrial fgfs which are lubricated by thin liquid films at inter particle contacts those occurring on mars are more likely lubricated by vaporization of co 2 and small amounts of h 2 o frost that becomes incorporated in the translating mass some recent mass wasting activity in martian gullies therefore could be interpreted as the product of fgf

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

water ice at low to midlatitudes on mars 1 in this paper we analyze water ice occurrences at the surface of mars using near infrared observations and we study their distribution with a climate model latitudes between 45 s and 50 n are considered data from the observatoire pour la mineralogie l eau les glaces et l actitite and the compact reconnaissance imaging spectrometer for mars are used to assess the presence of surface water ice as a function of location and season a modeling approach combining the 1 d and 3 d versions of the general circulation model of the laboratoire de meteorologie dynamique de jussieu is developed and successfully compared to observations ice deposits 2 200 m thick are observed during the day on pole facing slopes in local fall winter and early spring ice extends down to 13 latitude in the southern hemisphere but is restricted to latitudes higher than 32 in the north on a given slope the pattern of ice observations at the surface is mainly controlled by the global variability of atmospheric water precipitation and vapor with local ground properties playing a lower role only seasonal surface ice is observed no exposed patches of perennial ground ice have been detected surface seasonal ice is however sensitive to subsurface properties the results presented in this study are consistent with the recent discovery of low latitude subsurface ice obtained through the analysis of co 2 frost

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

observations of martian gullies and constraints on potential formation mechanisms ii the northern hemisphere abstract the formation process es responsible for creating the observed geologically recent gully features on mars has remained the subject of intense debate since their discovery we present new data and analysis of northern hemisphere gullies from mars global surveyor data which is used to test the various proposed mechanisms of gully formation we located 137 mars orbiter camera moc images in the northern hemisphere that contain clear evidence of gully landforms 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 northern hemisphere gullies are clustered in arcadia planitia tempe terra acidalia planitia and utopia planitia these gullies form in craters 84 knobby terrain 4 valleys 3 other unknown terrains 9 and are found on all slope orientations although the majority of gullies are equator facing most gullies 63 are associated with competent rock strata 26 are not associated with strata and 11 are ambiguous assuming thermal conductivities derived from tes measurements as well as modeled surface temperatures we find that 95 of the gully alcove bases with adequate data coverage lie at depths where subsurface temperatures are greater than 273 k and 5 of the alcove bases lie within the solid water regime the average alcove length is 470 m and the average channel length is 690 m 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 these findings gleaned from the northern hemisphere data are in general agreement with analyses of gullies in the southern hemisphere heldmann j l mellon m t 2004 icarus 168 285 304

defrosting dark flow features and dune activity on mars example in russell crater 1 defrosting processes observed on mars are among the most unusual features described on high resolution images a defrosting sequence has been observed near the crest of the megadune located in the russell crater on new high resolution images obtained from the hirise instrument and on hyperspectral cubes obtained from crism instrument both on board mro hyperspectral images show that frost overlaps the entire megadune in middle winter this frost is composed mainly of co2 and of a small amount of water ice the deepest ice signatures are mainly located near the crest of the megadune the defrosting sequence monitored by crism reveals spatial heterogeneity and refrosting processes on the morphological counterpart the defrosting sequence starts with the development of dark spots similar to those described in the cryptic regions after few sols we observe the emplacement of dark linear flow features that start from the dark spots downward of the main slope of the dune these linear flow features are 1 to 2 m wide and 50 to 100 m long they settle on the small rills visible on the frost cover they are interpreted as avalanches of a mixing of sand dust and unstable co2 gas released under pressure the avalanches would be triggered by the eruption of the dark spots the flux of material transported by the flow features has been estimated to 0 25 to 0 5 m3 by meter width each year on the megadune this flux is larger than flux transported by wind these dark flow features are thus very efficient to transport material on slopes located in frosted areas

ancient melting of mid latitude snowpacks on mars as a water source for gullies abstract we hypothesize that during past epochs of high obliquity seasonal snowfields at mid latitudes melted to produce springtime sediment rich surface flows resulting in gully formation significant seasonal mid latitude snowfall does not occur on mars today general circulation model gcm results however suggest that under past climate conditions there may have been centimeters of seasonal mid latitude snowfall mischna m a richardson m i wilson r j mccleese d j 2003 j geophys res planets 108 doi 10 1029 2003je002051 5062 gully locations have been tabulated by several researchers e g heldmann j l mellon m t 2004 icarus 168 285 304 heldmann j l carlsson e johansson h mellon m t toon o b 2007 icarus 188 324 344 malin m c edgett k s 2000 science 288 2330 2335 and found to correspond to mid latitude bands a natural question is whether the latitudinal bands where the gullies are located correspond to areas where the ancient snowfalls may have melted producing runoff which may have incised gullies in this study we model thin snowpacks with thicknesses similar to those predicted by mischna m a richardson m i wilson r j mccleese d j 2003 j geophys res planets 108 doi 10 1029 2003je002051 5062 we model these snowpacks under past climate regimes in order to determine whether snowmelt runoff could have occurred and whether significant amounts of warm soil t 273 k existed on both poleward and equatorward slopes in the regions where gullies exist both warm soil and water amounts are modeled because soil and water may have mixed to form a sediment rich flow we begin by applying the snowpack model of williams et al williams k e toon o b heldmann j e mellon m 2008 icarus 196 565 577 to past climate regimes characterized by obliquities of 35 600 ka before present and 45 5 5 ma before present and to all latitudes between 70 n and 70 s we find that the regions containing significant snowmelt runoff correspond to the regions identified by heldmann and mellon heldmann j l mellon m t 2004 icarus 168 285 304 heldmann et al heldmann j l carlsson e johansson h mellon m t toon o b 2007 icarus 188 324 344 and malin and edgett malin m c edgett k s 2000 science 288 2330 2335 as containing large numbers of gullies we find that the snowmelt runoff 1 mm with equivalent rainfall rates of 0 25 mm h and warm soil 1 cm depth would have occurred on slopes within the gullied latitudinal bands the snowfall amounts modeled are predicted to be seasonal mischna m a richardson m i wilson r j mccleese d j 2003 j geophys res planets 108 doi 10 1029 2003je002051 5062 and our modeling finds that under the previous climate regimes there would have been meltwater present on the slopes in question for brief periods of time on the order of days each year our model provides a simple explanation for the latitudinal distribution of the gullies and also suggests that the gullies date to times when water migrated away from the present poles to the mid latitudes

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

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

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

modeling the formation of bright slope deposits associated with gullies in hale crater mars implications for recent liquid water abstract our study investigates possible formation mechanisms of the very recent bright gully deposits bgds observed on mars in order to assess if liquid water was required we use two models in our assessment a one dimensional 1d kinematic model to model dry granular flows and a two dimensional 2d fluid dynamic model flo 2d o brien et al 1993 flo engineering to model water rich and wet sediment rich flows our modeling utilizes a high resolution topographic model generated from a pair of images acquired by the high resolution imaging science experiment hirise aboard the mars reconnaissance orbiter for the 1d kinematic modeling of dry granular flows we examine a range of particle sizes flow thicknesses initial velocities flow densities and upslope initiation points to examine how these parameters affect the flow run out distances of the center of mass of a flow our 1d modeling results show that multiple combinations of realistic parameters could produce dry granular flows that travel to within the observed deposits boundaries we run the 2d fluid dynamic model flo 2d to model both water rich and wet sediment rich flows we vary the inflow volume inflow location discharge rate water loss rate water rich models only and simulation time and examine the resulting maximum flow depths and velocities our 2d modeling results suggest that both wet sediment rich and water rich flows could produce the observed bright deposits our modeling shows that the bgds are not definitive evidence of recent liquid water on the surface of mars

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

recent debris flows on mars seasonal observations of the russell crater dune field 1 debris flows occur on slopes in the russell crater dune field the morphology of the erosion features resembles terrestrial viscous slurry flows mudflows and suggests that a flow of fine grained material mixed with liquid water might have been responsible for their formation seasonal mgs tes and moc imagery based observations of the dune field show 1 an annual frosting and defrosting cycle and 2 that liquid h2o could be stable within a limited time period in the summer of the southern hemisphere these observations lead to the conclusion that debris flows in the russell crater dune field may form under current climatic conditions by episodic or seasonal melting of small amounts of autumn winter condensed water ice

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

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

the trace of the pacific cocos nazca triple junction in the central pacific and the formation of an overlapping spreading centre the plate tectonic mirror image to the region of the cocos and nazca plates which are currently being subducted beneath central america is preserved in the central pacific around 120 w just south of the equator cruise so 180 investigated this remote area during project central and acquired new magnetic and bathymetric data a plate tectonic model for the mirror image is presented based on the newly acquired as well as reprocessed existing data discordant magnetic anomaly patterns and bathymetric structures indicate at least two major reorganization events 19 5 and 14 7 myr which can be detected both in the cocos nazca spreading system and in the east pacific rise irregularities in the anomaly pattern and curvilinear structures on the sea floor of the survey area are interpreted in terms of a fossil overlapping spreading centre at the location where the farallon break up originated

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