generation of continental rifts basins and swells by lithosphere instabilities 1 continents may be affected simultaneously by rifting uplift volcanic activity and basin formation in several different locations suggesting a common driving mechanism that is intrinsic to continents we describe a new type of convective instability at the base of the lithosphere that leads to a remarkable spatial pattern at the scale of an entire continent we carried out fluid mechanics laboratory experiments on buoyant blocks of finite size that became unstable due to cooling from above dynamical behavior depends on three dimensionless numbers a rayleigh number for the unstable block a buoyancy number that scales the intrinsic density contrast to the thermal one and the aspect ratio of the block within the block instability develops in two different ways in an outer annulus and in an interior region in the outer annulus upwellings and downwellings take the form of periodically spaced radial spokes the interior region hosts the more familiar convective pattern of polygonal cells in geological conditions such instabilities should manifest themselves as linear rifts striking at a right angle to the continent ocean boundary and an array of domal uplifts volcanic swells and basins in the continental interior simple scaling laws for the dimensions and spacings of the convective structures are derived for the subcontinental lithospheric mantle these dimensions take values in the 500 1000 km range close to geological examples the large intrinsic buoyancy of archean lithospheric roots prevents this type of instability which explains why the widespread volcanic activity that currently affects western africa is confined to post archean domains

the influence of thermochemical convection on the fixity of mantle plumes abstract a general feature of both isochemical and thermochemical studies of mantle convection is that horizontal plume velocities tend to be smaller than typical convective velocities however it is not clear which system leads to a greater fixity of mantle plumes we perform two and three dimensional numerical calculations and compare both thermochemical and isochemical cases with similar convective vigor to determine whether presence of a dense component in the mantle can lead to smaller ratios of horizontal plume velocity to surface velocity we investigate different viscosity and density contrasts between chemical components in the thermochemical calculations and we perform isochemical calculations with both free slip and no slip bottom boundary conditions we then compare both visually and quantitatively the results of the thermochemical and isochemical calculations to determine which leads to greater plume fixity we find that horizontal plume velocities for thermochemical calculations are similar to those from isochemical calculations with no slip bottom boundary conditions in addition we find that plumes tend to be more fixed for isochemical cases with free slip bottom boundary conditions for two dimensional calculations however in three dimensions we find that plume fixity is similar to that observed in thermochemical calculations

possible liquid water origin for atacama desert mudflow and recent gully deposits on mars evidence of recent gully activity on mars has been reported based on the formation of new light toned deposits within the past decade the origin of which remains controversial analogous recent light toned gully features have formed by liquid water activity in the atacama desert on earth these terrestrial deposits leave no mineralogical trace of water activity but rather show an albedo difference due to particle size sorting within a fine grained mudflow therefore spectral differences indicating varying mineralogy between a recent gully deposit and the surrounding terrain may not be the most relevant criteria for detecting water flow in arid environments instead variation in particle size between the deposit and surrounding terrain is a possible discriminator to identify a water based flow we show that the atacama deposit is similar to the observed mars gully deposits and both are consistent with liquid water activity the light toned mars gully deposits could have formed from dry debris flows but a liquid water origin cannot be ruled out because not all liquid water flows leave hydrated minerals behind on the surface therefore the mars deposits could be remnant mudflows that formed on mars within the last decade

geomorphic signatures of glacial activity in the alba patera volcanic province implications for recent frost accumulation on mars 1 glacial periglacial landforms lying within impact craters on mars have led to the identification of two mechanisms for their formation 1 intermittent deposition of atmospherically emplaced snow ice during past spin axis orbital conditions and 2 flow of debris covered ice rich deposits the maximum presence of the young ice snow rich features thermal contraction crack polygons gullies arcuate ridges and lobate debris tongues was observed on the pole facing slope indicating that this slope was the preferred site for ice snow accumulation during the last 10 ma in this study we investigated 30 craters lying in the alba patera volcanic province in the latitudinal bands between 45 n and 32 4 n morphological comparison of the younger ice snow rich features in these craters led us to conclude that glacial periglacial features in alba patera are mainly present within pole facing slopes of craters lying within 45 n 39 n the craters lying within 40 2 n 40 n did not show any glacial periglacial features we suggest that the formation of these young ice snow rich features follows the same orientation trends as those of other older 10 ma glacial features debris covered ice snow rich large deposits at the base of the crater wall in the region the present work has revealed that the onset of physical processes that result in the formation of glacial periglacial landforms is also dependent on the changes in elevation ranges of the investigated craters in alba patera our results confirm past inferences for accumulation of ice snow on mars and suggest that the period of ice snow accumulation activity in alba patera occurred throughout the amazonian and lasted until the recent past i e 2 1 0 4 ma

numerical modeling of martian gully sediment transport testing the fluvial hypothesis 1 using a stereo pair of hirise images of a pole facing crater slope at 38 s 218 e we measure topographic profiles along nine gullies typical slopes of the interior channel region above the depositional apron are 20 we test the hypothesis that sediment transport on gully slopes occurs via fluvial transport processes by developing a numerical sediment transport model based on steep flume experiments performed by smart 1984 at 20 slopes channels 1 m deep by 8 m wide and 0 1 m deep by 3 m wide transport a sediment volume equal to the alcove volume of 6 105 m3 in 10 h and 40 days respectively under constant flow conditions snowpack melting cannot produce the water discharge rates necessary for fluvial sediment transport unless long term kyr storage of the resulting meltwater occurs if these volumes of water are discharged as groundwater the required aquifer thicknesses and aquifer drawdown lengths would be unrealistically large for a single discharge event more plausibly the water volume required by the fluvial transport model could be discharged in 10 episodes for an aquifer 30 m thick with a recurrence interval similar to that of martian obliquity cycles 0 1 myr

crustal structure of mainland china from deep seismic sounding data abstract since 1958 about ninety seismic refraction wide angle reflection profiles with a cumulative length of more than sixty thousand kilometers have been completed in mainland china we summarize the results in the form of 1 a new contour map of crustal thickness 2 fourteen representative crustal seismic velocity depth columns for various tectonic units and 3 a pn velocity map we found a north south trending belt with a strong lateral gradient in crustal thickness in central china this belt divides china into an eastern region with a crustal thickness of 30 45 km and a western region with a thickness of 45 75 km the crust in these two regions has experienced different evolutionary processes and currently lies within distinct tectonic stress fields our compilation finds that there is a high velocity 7 1 7 4 km s layer in the lower crust of the stable tarim basin and ordos plateau however in young orogenic belts including parts of eastern china the tianshan and the tibetan plateau this layer is often absent one exception is southern tibet where the presence of a high velocity layer is related to the northward injection of the cold indian plate this high velocity layer is absent in northern tibet in orogenic belts there usually is a low velocity layer lvl in the crust but in stable regions this layer seldom exists the pn velocities in eastern china generally range from 7 9 to 8 1 km s and tend to be isotropic pn velocities in western china are more variable ranging from 7 7 to 8 2 km s and may display azimuthal anisotropy

martian gullies in the southern mid latitudes of mars evidence for climate controlled formation of young fluvial features based upon local and global topography a new survey of mars orbiter camera moc narrow angle images of gullies in the 30 45 s latitude band includes their distribution morphology local topographic setting orientation elevation and slopes these new data show that gully formation is favored over a specific range of conditions elevation 5000 to 3000 m slope 10 and orientation 83 8 on pole facing slopes these data and the frequent occurrence of gullies on isolated topographic highs lead us to support the conclusion that climatic related processes of volatile accumulation and melting driven by orbital variations are the most likely candidate for processes responsible for the geologically recent formation of martian gullies

comprehensive analysis of glaciated martian crater greg abstract the 66 km diameter martian crater greg east of hellas hosts various distinctive features including dendritic valleys filled with chevron textured masses south wall and lobate tongues a few kilometers long north wall we analyze these features by various quantitative techniques to illuminate martian geologic and climatic history crater retention model ages indicate that greg is at least 1 3 gy old but surface layers of mantles and glacial features are orders of magnitude younger properties of the dendritic valleys combined with climate models suggest that fluvial activity began under a thicker warmer atmosphere soon after the crater s formation the oldest exposed fluvial systems have surface crater retention ages of a few hundred my indicating runoff in recent geologic time much of greg is covered by ice rich mantle deposits for which we infer gradual accumulation and depths of order 30 85 m they mask pre existing landforms the lobate tongues are interpreted as glaciers with mean slope of 10 2 2 3 and average thickness of 33 19 m our calculations and data suggest that these glaciers were originally ice rich and that their surface layers have been depleted by volatile loss the glaciers probably formed when ice rich mantle deposits reached critical thickness and flowed downhill the top 5 10 m of the mantle and glaciers show crater survival times of order a few my to 15 my which remarkably is the time since the last 1 4 episodes of obliquity 45 global climate models affirm that greg lies in one of two non polar areas with extremes of ice deposition during high obliquity epochs this match with observations supports the use of such models in studies of planetary climate change

distinct variations of crustal shear wave velocity structure and radial anisotropy beneath the north china craton and tectonic implications article i nfo this study presents the crustal shear wave velocity structure and radial anisotropy along two linear seismic arrays across the north china craton ncc from ambient noise tomography about a half to one year long ambient noise data from 87 stations were used for obtaining the inter station surface wave empirical green s functions egfs from cross correlation rayleigh and love dispersion curves within the period band 5 30 s were measured from the egfs of the vertical and transverse components respectively these dispersion data were then used to determine the crustal shear wave velocity structure vsv and vsh and radial anisotropy 2 vsh vsv vsh vsv from point wise linear inversion with constraints from receiver function analysis our results reveal substantial structural variations among different parts of the ncc the bohai bay basin in the eastern ncc is underlain by a thin crust 30 km with relatively low velocities particularly vsv and large positive radial anisotropy in the middle to lower crust such a crustal structure is no longer of a cratonic type and may have resulted from the widespread tectonic extension and intensive magmatism in this region since late mesozoic beneath the ordos basin in the western ncc the crust is relatively thicker 40 km and well stratified and presents a large scale low velocity zone in the middle to lower crust and overall weak radial anisotropy except for a localized lower crust anomaly the overall structural features of this region resemble those of typical precambrian shields in agreement with the long term stability of the region the crustal structure under the trans north china orogen tnco central ncc is more complicated and characterized by smaller scale velocity variations strong positive radial anisotropy in the middle crust and rapid change to weak to negative anisotropy in the lower crust these features may reflect complex deformations and crust mantle interactions probably associated with tectonic extension and magmaticunderplating during the mesozoic to cenozoic evolution of the region our structural images in combination with previous seismic geological and geochemical observations suggest that the phanerozoic lithospheric reactivation and destruction processes may have affected the crust especially the middle and lower crust of the eastern ncc and the effect probably extend ed to the tnco but may have minor influence on the crust of the western part of the craton

gully formation on mars two recent phases of formation suggested by links between morphology slope orientation and insolation history the unusual 80 km diameter noachian aged asimov crater in noachis terra 46 s 5 e is characterized by extensive noachian hesperian crater fill and a younger superposed annulus of valleys encircling the margins of the crater floor these valleys provide an opportunity to study the relationships of gully geomorphology as a function of changing slope orientation relative to solar insolation we found that the level of development of gullies was highly correlated with slope orientation and solar insolation the largest and most complex gully systems with the most well developed fluvial landforms are restricted to pole facing slopes in contrast gullies on equator facing slopes are smaller more poorly developed and integrated more highly degraded and contain more impact craters we used a 1d version of the laboratoire de meteorologie dynamique gcm and slope geometries orientation and angle driven by predicted spin axis orbital parameter history to assess the distribution and history of surface temperatures in these valleys during recent geological history surface temperatures on pole facing slopes preferential for water ice accumulation and subsequent melting are predicted to occur as recently as 0 5 2 1 ma which is consistent with age estimates of gully activity elsewhere on mars in contrast the 1d model predicts that water ice cannot accumulate on equator facing slopes until obliquities exceed 45 suggesting they are unlikely to have been active over the last 5 ma the correlation of the temperature predictions and the geological evidence for age differences suggests that there were two phases of gully formation in the last few million years an older phase in which top down melting occurred on equator facing slopes and a younger more robust phase on pole facing slopes the similarities of small scale fluvial erosion features seen in the gullies on mars and those observed in gullies cut by seasonal and perennial snowmelt in the antarctic dry valleys supports a top down melting origin for these gullies on mars 2010 elsevier inc

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

testing the water hypothesis quantitative morphological analysis of terrestrial and martian mid latitude gullies abstract although martian gullies resemble terrestrial counterparts two conflicting hypotheses exist for their formation still invoke fluvial processes on the one hand or lubricated co 2 flows on the other in this work we compared the quantitative morphology of terrestrial gullies known to have formed by liquid water and mid latitude martian gullies in the martian southern hemisphere we also compared these results with measurements of martian dry ravines adjacent to the gullies our results show a similarity between martian and terrestrial gully formation supporting the hypothesis that liquid water was involved in their erosion our results show dry ravines differ morphologically from gullies further suggesting fluidised flows as a likely origin of the latter variations in the relationships across various terrestrial and martian gullies indicate the significance of local environmental and geological conditions our work supports the idea that martian gullies may not have been formed by just one single process but may have evolved through a more complex interaction of processes and environment

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

degradation of mid latitude craters on mars abstract recent geomorphic remote sensing and atmospheric modeling studies have shown evidence for abundant ground ice deposits in the martian mid latitudes numerous potential water ice rich flow features have been identified in craters in these regions including arcuate ridges gullies and small flow lobes previous studies such as in newton basin have shown that arcuate ridges and gullies are mainly found in small craters 2 30 km in diameter these features are located on both pole facing and equator facing crater walls and their orientations have been found to be dependent on latitude we have conducted surveys of craters 20 km in diameter in two mid latitude regions one in the northern hemisphere in arabia terra and one in the southern hemisphere east of hellas basin in these regions prominent lobes potentially ice rich are commonly found on the walls of craters with diameters between 20 100 km additional water ice rich features such as channels valleys alcoves and debris aprons have also been found in association with crater walls in the eastern hellas study region channels were found to be located primarily on pole facing walls whereas valleys and alcoves were found primarily on equator facing walls in the arabia terra study region these preferences are less distinct in both study regions lobate flows gullies and arcuate ridges were found to have pole facing orientation preferences at latitudes below 45 and equator facing orientation preferences above 45 similar to preferences previously found for gullies and arcuate ridges in smaller craters interrelations between the features suggest they all formed from the mobilization of accumulated ice rich materials the dependencies of orientations on latitude suggest a relationship to differences in total solar insolation along the crater walls differences in slope of the crater wall differences in total solar insolation with respect to wall orientation and variations in topography along the crater rim can explain the variability in morphology of the features studied the formation and evolution of these landforms may best be explained by multiple cycles of deposition of ice rich material during periods of high obliquity and subsequent modification and transport of these materials down crater walls

testing the tracer ratio method for modeling active compositional fields in mantle convection simulations 1 tracer methods are attractive for modeling compositional fields because they offer the potential of zero numerical diffusion composition is typically taken to be proportional to the absolute local concentration of tracers but an increasingly popular method is to have dense and regular tracers with composition being equal to the local fraction of dense tracers this paper tests this ratio method using established benchmarks and by comparing the performance of the two tracer methods and grid based methods for simulating the long term evolution of a convecting mantle with a thick dense stable layer for this scenario the ratio method is found to have several advantages giving sharp stable long term layering with no tracer settling minimal statistical noise and low entrainment even with only 5 tracers per cell the method is equally applicable to finite volume and finite element treatments of the underlying flow entrainment in grid based advection methods is heavily dependent on resolution and numerical details and is reduced 1 order of magnitude by the filter proposed by a lenardic numerical determination of physically correct entrainment rates remains a challenging problem comparing tracer and grid based methods the spatial pattern of the thermal and chemical fields appear to be converging on the finest grids however the estimated entrainment differs significantly

timing of destruction of the north china craton abstract the north china craton ncc is the typical example of destruction of an ancient craton however the destruction mechanism and geodynamic controlling factors still remain enigmatic due to controversy on the timing of destruction which is the key to understanding the destruction processes based on temporal and spatial distributions of the igneous rocks and their sources and tectonic settings it is recognized that six stages of tectono magmatism occurred in the ncc during the phanerozoic i e carboniferous to early permian late permian to middle triassic late triassic jurassic early cretaceous and cenozoic among them cenozoic magmatism mainly consists of alkali basalts and is found only occasionally in the eastern ncc the first four stages of magmatism and tectonism related to the southward subduction of the paleo asian plate and the assembly of sino korean and yangtze cratons are locally distributed in limited parts of the ncc reflecting a multiple stage modification of the ncc from the late carboniferous to jurassic however the intensive development of early cretaceous magmatism extensional deformation and associated gold mineralization with significant continental crustal growth indicate that the eastern ncc was destroyed during this time period this destruction was the result of paleo pacific subduction beneath the eastern asian continent with lithospheric removal and or replacement of an ancient cratonic lithosphere by a juvenile oceanic lithosphere

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

martian gullies a comprehensive review of observations mechanisms and insights from earth analogues upon their discovery in 2000 martian gullies were hailed as the first proof of recent i e less than a few million years flowing liquid water on the surface of a dry desert planet many processes have been proposed to have formed martian gullies ranging from liquid water seepage from aquifers melting of snow ice and frost to dry granular flows potentially lubricated by co2 terrestrial analogues have played a pivotal role in the conception and validation of gully formation mechanisms comparison with the terrestrial landscape argues for gully formation by liquid water debris flows originating from surface melting however limited knowledge of sediment transport by sublimation is a critical factor in impeding progress on the co2 sublimation hypothesis we propose avenues towards resolving the debate a laboratory simulations targeting variables that can be measured from orbit b applications of landscape evolution models c incorporation of the concept of sediment connectivity d using 3d fluid dynamic models to link deposit morphology and flow rheology and e a more intense exchange of techniques between terrestrial and planetary geomorphology including quantitative and temporal approaches finally we emphasize that the present may not accurately represent the past and that martian gullies likely formed by a combination of processes

crustal thickness map of the chinese mainland from teleseismic receiver functions abstract we present a new crustal thickness map of the chinese mainland and adjacent areas based on a compilation of previously published data more than 1900 estimates of crustal thickness from receiver function studies were collected and the weighted averages were used to develop a contour map of crustal thickness beneath the chinese mainland the combined dataset provided good coverage especially over eastern and central china and our analysis revealed crustal thickness variations of the chinese mainland with considerably more detail than in previous models our crustal thickness map shows a large variation from a thickness of less than 30 km in the mesozoic basin of eastern china to over 80 km in the tibetan plateau the thickest crust was found in central tibet while the thinnest continental crust was observed beneath eastern china normal to slightly thick crust 38 51 km was found for the stable precambrian basins and orogenic belts comparison between our model and other models reveals that the models share similar large scale and smaller scale features over large portions of eastern china where the data coverage is good however large differences up to 25 km in crustal thickness occur in areas with complex crustal structure or poor data coverage we also evaluated the quality of previous models by comparing them with our newly compiled point observations of crustal thickness

elysium planitia lava flows crater count chronology and geological implications mars global surveyor mgs and viking images allow analysis of chronological and geological relations among lava flows in southern elysium planitia based on crater populations mgs and viking images clearly show morphological features of lava flows with some extremely sparsely cratered young flow units atop somewhat older surfaces there is no evidence of substantial dust mantling on the young flows and hence we infer that the crater populations date not sediment accumulation but the lava flows themselves the youngest flows have some of the lowest crater densities that we have seen on mars some 1 of lunar mare values at crater diameters below about 180 to 500 m we infer that thin scattered volcanic flows some tens of meters thick have been emplaced atop stratigraphic units within the last 100 myr and possibly within the last 10 myr volcanism is thus a continuing process in the recent geologic history of mars and this must constrain geophysical models of the planet

context camera investigation on board the mars reconnaissance orbiter 1 the context camera ctx on the mars reconnaissance orbiter mro is a facility instrument i e government furnished equipment operated by a science team not responsible for design and fabrication designed built and operated by malin space science systems and the mro mars color imager team marci ctx will 1 provide context images for data acquired by other mro instruments 2 observe features of interest to nasa s mars exploration program e g candidate landing sites and 3 conduct a scientific investigation led by the marci team of geologic geomorphic and meteorological processes on mars ctx consists of a digital electronics assembly a 350 mm f 3 25 schmidt type telescope of catadioptric optical design with a 5 7 field of view providing a 30 km wide swath from 290 km altitude and a 5000 element ccd with a band pass of 500 700 nm and 7 m pixels giving 6 m pixel spatial resolution from mro s nearly circular nearly polar mapping orbit raw data are transferred to the mro spacecraft flight computer for processing e g data compression before transmission to earth the ground data system and operations are based on 9 years of mars global surveyor mars orbiter camera on orbit experience ctx has been allocated 12 of the total mro data return or about 3 terabits for the nominal mission this data volume would cover 9 of mars at 6 m pixel but overlapping images for stereo mosaics and observation of changes and meteorological events will reduce this area ctx acquired its first instrument checkout images of mars on 24 march 2006

dynamics of unusual debris flows on martian sand dunes 1 gullies that dissect sand dunes in russell impact crater often display debris flow like deposits in their distal reaches the possible range of both the rheological properties and the flow rates are estimated using a numerical simulation code of a bingham plastic flow to help explain the formation of these features our simulated results are best explained by a rapid debris flow for example a debris flow with the viscosity of 102 pa s and the yield strength of 102 pa can form the observed deposits with a flow rate of 0 5 m3 s sustained over several minutes and total discharged water volume on the order of hundreds of cubic meters which may be produced by melting a surface layer of interstitial ice within the dune deposits to several centimeters depth

evidence of gully formation by regional groundwater flow in the gorgonum newton region mars abstract the discovery of gullies on mars suggests liquid water activity near the surface of the planet in recent times since liquid water is unstable under the present day p t martian conditions the formation mechanisms of the gullies and the source of the putative water have been a matter of debate for the last years to provide new insights into these matters we have approached the problem studying the gullies in relation to their regional setting a major point in our study relates to the geographic orientation of gullies an aspect that has been previously regarded as a crucial matter in different models and has profound implications regarding their origin we present a comprehensive and detailed survey of the gorgonum newton region and a study of the dao and nirgal vallis regions the survey was carried out with the aid of 965 high resolution moc images 752 for gorgonum newton 102 for nirgal vallis and 111 for dao vallis regions and mola derived dems we found that gullies display a clear regional pattern geographically and topographically consistent with a decreasing regional slope we interpret the results in terms of the existence of several groundwater flow systems operating at different scales which ultimately may have led to gully formation by seepage at the slopes of craters and canyons we suggest that aquifers discharging at gully systems may have recharged from the surface in response to the melting of young partially eroded ice rich deposits

mantle and gully associations along the walls of dao and harmakhis valles mars 1 evaluation of high resolution images allows a systematic characterization of the walls of dao and harmakhis valles mars variations in wall morphology are classified into three archetypes exposed mantled and incised gullies are exposed by removal of semi competent mantling deposits suggesting progressive development of feature types in an idealized sequence from i mantled walls to ii incised mantle and eventually iii walls that display exposed gullies analysis of latitude elevation and wall orientation of 420 representative sample sites reveals spatial patterns that display preferential clustering of the two end member morphologies exposed gullies are favored at lower latitude higher elevation and on east facing walls mantled walls are more prevalent at higher latitude lower elevation and on south and southwest facing walls these morphologies and their patterns are consistent with the hypothesis that gullies emerge from beneath an ice rich mantle that is degrading in response to local variations in total solar insolation

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

models of isostatic and dynamic topography geoid anomalies and their uncertainties knowledge of the dynamic topography at earth s surface caused by sublithospheric density contrasts would provide crucial constraints on models of mantle dynamics we calculate global models of this dynamic topography by subtracting from the observed topography estimates of the topography caused by assumed isostatically compensated density variations in the crust the oceanic lithosphere and the continental tectosphere we also calculate the isostatic geoid anomaly that would result from these compensated near surface density anomalies because there is controversy about the thermal structure of old oceanic lithosphere we investigate two models of the cooling of the oceanic lithosphere half space and plate and calculate the effect of each on the inferred dynamic topography peak amplitudes of dynamic topography from sublithospheric sources expanded to spherical harmonic degree 6 are approximately 1 km we estimate the uncertainties in the calculated dynamic topography and geoid by combining the errors in the data and systematic errors to obtain the spectral coefficients of the fields we use weighted least squares inversions the calculated dynamic topography arising from sublithospheric sources residual geoid and errors associated with these fields can be used in studies of mantle dynamics

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

sublithospheric small scale convection and its implications for the residual topography at old ocean basins and the plate model 1 the seafloor topography and heat flux differ significantly from the predictions of the half space cooling hsc model at old ocean basins understanding the deviations has important implications for thermal evolution of oceanic lithosphere and large scale mantle dynamics a widely used model that explains significant fraction of the deviations is the plate model but the dynamical feasibility of the plate model has never been demonstrated in this study we investigated the effects of sublithospheric small scale convection ssc and of internal heating on seafloor heat flux and topography and mantle thermal structure and we examined the dynamic feasibility of the plate model by formulating high resolution two dimensional numerical models of mantle convection with strongly temperature and depth dependent rheology we found that mantle convection with tectonic plates often leads to formation of a broad thermal anomaly below old lithosphere where the mantle is not cooled by subducted slabs and heat transfer is less efficient because of thick lithosphere especially when significant internal heating is present this trapped heat may exist in the middle mantle when the ssc is absent and it may also be redistributed by the ssc to shallow depths to reheat the lithosphere and to homogenize mantle temperature when internal heating accounts for 60 of the total heat output the trapped heat may provide sufficient heat supply to preferentially reheat old lithosphere via ssc while maintaining uniform mantle temperature we suggest that the trapped heat and the ssc are responsible for the residual heat flux and topography at old ocean basins relative to the hsc model predictions our models also show that for the plate model to be dynamically viable both the ssc and significant internal heating 60 are necessary this is because only the ssc in a mantle with significant internal heating can erode and reheat the lithosphere while maintaining a nearly constant mantle temperature below lithosphere which is the basic assumption of the plate model with the viscosity structure and internal heating rate for the present day mantle we think that the plate model is dynamically viable

a comparison of semiarid and subhumid terrestrial gullies with gullies on mars implications for martian gully erosion abstract comparison of martian hillside gullies with terrestrial analogues is a productive method for gaining an understanding of how surface processes operate on mars and on earth as well as their similarities and differences here we compare gullies within a semiarid region of island lagoon near woomera australia and periglacial gullies located at pasture hill new zealand with the martian gullies in noachis terra we discovered that fluvial processes dominate erosion at the terrestrial sites though in different forms concentration of overland flow and the presence of a resistant cap rock have played a key role in the location and shape of the island lagoon gullies the presence of steeper slopes and a greater abundance of volatiles such as water frost and snowmelt at pasture hill have helped carve channels equivalent in size to the martian gullies and the interaction of frost dry flows snowmelt and debris flows has led to a complex regime of gully erosion and deposition the observation of similar features on mars suggests that martian gully formation is not necessarily restricted to a single process in addition gully slopes are closely related to those of their host escarpment as well as the thickness of erodible material and presence of bedrock traditional indicators of water related activity such as slope and sinuosity need to be placed into the context of the environment of the study site

downslope sediment transport by boiling liquid water under mars like conditions experiments and potential implications for martian gullies gullies are widespread morphological features on mars for which current changes have been observed liquid water has been one of the potential mechanisms to explain their formation and activity however under present day martian conditions liquid water is unstable and should only be transiently present in small amounts at the surface yet little attention has been paid to the mechanisms by which unstable water transports sediment under low atmospheric pressure here we present the results of laboratory experiments studying the interaction between liquid water owing over a sand bed under mars like atmospheric pressure c 9 mbar the experiments were performed in a mars simulation chamber at the open university uk in which we placed a test bed of ne sand at a 25 slope we chose to investigate the in uence of two parameters the temperature of the water and the temperature of the sand we performed 27 experiments with nine different combinations of water and sand temperatures ranging from 278 to 297 k under all experimental conditions the water was boiling we investigated and compared the types and timing of sediment transport events and the shapes characteristics and volumes of the resulting morphologies in agreement with previous laboratory studies we found that more intense boiling increased the volume of sediment transported for a given volume of water we found four main types of sediment transport entrainment by overland ow grain ejection grain avalanches and levitation of saturated sand pellets our results showed that increasing sand temperature was the main driving parameter in increasing the sand transport and in modifying the dominant sediment transport mechanism the temperature of the water played a negligible or minor role apart from the duration of sand ejection and avalanches which lasted longer at low water temperature at low sand temperature the majority of the sand was transported by overland ow of the liquid water at higher sand temperatures the transport was dominated by processes triggered by the boiling behaviour of the water at the highest temperatures sediment transport was dominated by the formation of levitating pellets dry avalanches and ejection of the sand grains this resulted in a transport volume about nine times greater at a sand temperature of 297 k compared with 278 k our heat transfer scaling shows that the boiling behaviour will be enhanced under martian low gravity resulting in more ef cient transport of sediment by levitating sand pellets even at temperatures close to the triple point our results showed that the boiling intensity played an important role in the physics of sediment transport by liquid water this implied that the amount of water required to produce morphological changes at the surface of mars could be lower than previously estimated by assuming stable liquid water boiling is a critical process to be considered when assessing gully formation and modi cation mechanisms mobilized by liquid water our work could have similar implications for any water formed landform on mars which could include recurring slope lineae dark dune ows and slope streaks

intra crater glacial processes in central utopia planitia mars abstract we describe and interpret a series of previously unidentified glacial like lobes 34 43 n 107 125 e that were discovered as part of a survey of large d 5 km impact craters in utopia planitia one of the great northern plains of mars the lobes have characteristics that are consistent with a glacial origin evidence includes curvilinearity of form lineations and ridges and surface textures that are thought to form by the sublimation of near surface volatiles the lobes display morphologies that range from wedge shaped to near circular to elongate the flow directions are towards the northern walls in the case of craters with large single lobes and in all directions in the case of the largest d 30 km craters concentric crater fill is also interspersed within craters of our study region with such craters having much higher filling rates than those with flow lobes we suggest that the impact crater population in south west utopia planitia demonstrates a spectrum of glacial modifications from low levels of filling in the case of craters with elongate lobes at one extreme to concentric crater fill in highly filled craters at the other

a hydrological analysis of terrestrial and martian gullies implications for liquid water on mars abstract understanding the role and amounts of liquid water involved in martian gully formation is critical in studies of the martian hydrosphere and climate we performed hydrological analyses using the manning and darcy weisbach equations in order to infer flow velocities and discharge rates from channels at two gully sites in noachis terra mars the results of these analyses were compared with analogous hillside gullies in australia we found the velocities and discharge rates for the terrestrial gullies to be comparable to velocities and discharge rates of some small martian gully channels in contrast velocity and discharge in some larger martian gullies were almost an order of magnitude higher equating with catastrophic flows on earth we postulate that the larger gully channels were more likely formed by a number of smaller flows in a similar manner observed in some terrestrial gullies a scenario that does not require vast amounts of liquid water to be stable under martian conditions in addition we found that post fluvial channel widening may have acted on the martian gullies probably by dry mass wasting leading to larger channels than were originally carved by liquid water future hydrological analyses of martian gullies will lead to a greater understanding of the relative importance of dry mass wasting compared to liquid water erosion

the western qaidam basin as a potential martian environmental analogue an overview the early martian environment is interpreted as warmer and wetter before a significant change in its global climatic conditions irreversibly led to the current hyper arid environments this transition is one of the most intriguing processes of martian history the extreme climatic change is preserved in the salt deposits desiccated landscapes and geomorphological structures that were shaped by the evaporation of water however until a manned journey to mars is feasible many martian materials morphological structures and much of its evolutionary history will continue to be poorly understood in this regard searching and investigating martian analogues is still meaningful to find an earth environment with a whole set of martian structures distributed at a scale comparable to mars is even more important to test landing crafts and provide optimized working parameters for rovers the western qaidam basin in north tibetan plateau is such a martian analogue the area harbors one of the most extreme hyper arid environments on earth and contains a series of ancient lakes that evaporated at different evolutionary stages during the rise of the tibetan plateau large quantities of salts and geomorphological features formed during the transition of warmer and wet to colder and dry conditions provide unique references to study the modern martian surface and interpret the orbital data we present numerous similarities and results of investigations that suggest the qaidam basin as a potential analogue to study modern geomorphic processes on mars and suggest that this is an essential site to test future mars sample return missions

global documentation of overlapping lobate deposits in martian gullies abstract martian gullies are landforms comprising an alcove channel and debris apron in some gullies lobate deposits have been observed implying a debris flow like process but uncertainty remains as to the importance of this process in the gully population and whether lobate deposits are related to a specific context we document evidence of overlapping lobate deposits on gully fan surfaces within craters emplaced between 30 75 in both hemispheres we have identified 26 craters in which lobate deposits occur in gullies of which 6 were previously reported this corresponds to 3 39 of the 765 gullied craters studied using 1004 hirise images we show that gullies with lobate deposits 1 do not show any location preference 2 are poleward facing 3 are found in craters with and without latitude dependent mantle ldm and or glacier like forms 4 are emplaced at average slopes of 22 and 15 at the foot of fans in the northern and southern hemispheres respectively and 5 form in craters of all ages we infer that preservation of lobate deposits in gullies is the main factor leading to their relative paucity within the gully population additionally we find that the size of the clasts embedded in the lobes does not seem to vary with host crater age and the presence of ldm and or glacial landforms further we observe morphological features associated with terrestrial wet debris flows including overlapping tongue shaped terminal lobes levees channel backfilling plug formation and avulsion we have not observed any significant present day changes in the morphology and topography of gullies and or lobes we conclude that a debris flow like process is likely responsible for the majority of sediment transport in gully landforms but whether the fluidising agent is or was liquid water or co2 ice sublimation remains unknown

geomorphological characterisation and interpretation of a mid latitude glacier like form hellas planitia mars we describe and interpret the surface terrain types associated with a widely reported 4 km long midlatitude martian viscous flow feature vff the feature is located in the southern hemisphere on the poleward facing rim of a 60 km diameter crater in eastern hellas planitia high resolution imaging science experiment hirise images analysed in both 2d and 3d reveal that the upper margin of the feature is bounded by steep 30 headwalls typically some tens of metres high that are formed from unconsolidated material and characterised by a series of slope parallel linear incisions below these incised headwalls the feature flows at a general angle of 10 from a broad upper basin to a confined lower tongue that is bounded by a nested sequence of elongate raised ridges these characteristics are typical of several vffs in the region and are strikingly similar to moraine bounded valley glaciers on earth and we sub classify this feature as a glacier like form glf 1 the glf comprises five distinctive surface terrain types that contrast sharply with surface characteristics outside its bounding moraines four of these terrains scaly terrain polygonized terrain linear terrain and mound and tail terrain are located within the glf s innermost bounding moraine while the fifth rectilinear ridge terrain is located between its frontal moraines these terrains are mapped characterised and associated with possible mechanisms of formation to draw inferences about the glf s glaciology and glacial history this analysis suggests that the glf reached its maximal extent in the geologically recent past and that it may have been partially wetbased at that time subsequent to this phase the glf experienced an extended period of general recession that has been punctuated by several episodes of still stand or advance currently the glf s basin appears to be composed of a lower zone that is dominated by an exposed former glacier bed and an upper zone that may still contain a now degraded and dust mantled viscous mass similar to many partially glacierized basins on earth

multi agent gully processes evidence from the monaro volcanic province australia and in terra cimmeria mars abstract comparison of the similarities and differences between terrestrial and martian hillside gullies promotes understanding of how surface processes operate on both planets here we tested the viability of subsurface flow of water as a process affecting gully evolution we compared gullies within the monaro volcanic province near cooma new south wales australia to gullies possessing strong structural control near gasa crater terra cimmeria mars although cursory examination of the monaro gullies initially suggested strong evidence for aquifer erosion detailed field surveys showed the evidence to be ambiguous instead a complex regime of erosion dependent on multiple conditions and processes such as local geology surface runoff dry mass wasting and animal activity emerged we found the morphology of gullies near gasa crater to be consistent with erosion caused by liquid water while also being heavily influenced by the local environment including slope and geology additionally erosion at the martian site was not consistent with evidence of subsequent smaller scale erosion and channel modification by dry mass wasting local conditions thus play an important role in gully evolution further highlighting that processes forming martian gullies may be more diverse than initially thought

global distribution and migration of subsurface ice on mars a thermal diffusive model of h2o kinetics and equilibrium was developed to investigate the long term evolution and depth distribution of subsurface ice on mars the model quantitatively takes into account 1 obliquity variations 2 eccentricity variations 3 long term changes in the solar luminosity 4 variations in the argument of subsolar meridian in planetocentric equatorial coordinates 5 albedo changes at higher latitudes due to seasonal phase changes of co2 and the varying extent of co2 ice cover 6 planetary internal heat flow 7 temperature variations in the regolith as a function of depth time and latitude due to the above factors 8 atmospheric pressure variations over a 104 year time scale 9 the effects of factors 1 through 5 on seasonal polar cap temperatures and 10 knudsen and molecular diffusion of h2o through the regolith the migration of h2o into or out of the regolith is determined by two boundary conditions the h2o vapor pressure at the subsurface ice boundary and the annual average h2o concentration at the base of the atmosphere these are controlled respectively by the annual average regolith temperature at the given depth and seasonal temperatures at the polar cap starting from an arbitrary initial uniform depth distribution of subsurface ice h2o fluxes into or out of the regolith are calculated for 100 selected obliquity cycles each representing a different epoch in mars history the h2o fluxes are translated into ice thicknesses and extrapolated over time to give the subsurface ice depth as a function of latitude and time the results show that obliquity variations influence annual average regolith temperatures in varying degrees depending on latitude with the greatest effect at the poles and almost no effect at 40 lat insolation changes at the pole due to obliquity argument of subsolar meridian and eccentricity variations can produce enormous atmospheric h2o concentration variations of 6 orders of magnitude over an obliquity cycle superimposed on these cyclic variations is a slow monotonic change due to the increasing solar luminosity albedo changes at the polar cap due to seasonal phase changes of co2 and the varying thickness of the co2 ice cover are critically important in determining annual average atmospheric h2o concentrations despite the strongly oscillating character of the boundary conditions only small amounts of h2o are exchanged between the regolith and the atmosphere per obliquity cycle 10 g cm2 the net result of h2o migration is that the regolith below 30 40 lat is depleted of subsurface ice while the regolith above 30 40 lat contains permanent ice due to the depth of penetration of the annual thermal wave this result is supported by recent morphological studies the rate of migration of h2o is strongly dependent on average pore capillary radius for which we have assumed values of 1 and 10 m we estimate that the h2o ice removed from the regolith would produce a permanent ice cap with a volume between 2 106 and 6 106 km3 this generally agrees with estimates deduced from deflationary features at lower latitudes depositional features at higher latitudes and the mass of the polar caps

mantle convection as a boundary layer phenomenon summary the boundary layer nature of vigorous thermal convection is explored using high resolution numerical solutions to the governing hydrodynamic equations these solutions are obtained for a series of idealized models of the earth s mantle in which the viscosity is assumed to be constant a detailed analysis of the local energy balance within the horizontal and vertical thermal boundary layers is presented in terms of which a test of the fundamental assumptions of boundary layer theory is provided the results of this test have important geophysical consequences since the asymptotic predictions of boundary layer theory have been employed extensively in the context of thermal history modelling although boundary layer theory closely predicts the correct power law behaviour of various quantities it does not determine their absolute values accurately vertical advection is shown to play an important role in the energy balance within horizontal boundary layers at all rayleigh numbers horizontal and vertical advection dominate the energy balance within vertical plumes while horizontal diffusion plays a very minor role when heating is partially from within the fluid vertical advection into the upper thermal boundary layer can produce significant departures in the thermal structure from that found when heating is entirely from below for a free upper boundary this results in a relative flattening of the variations of surface topography and heat flow across the convection cells for a constant velocity upper boundary similar to plate motion the bathymetry flattens but the heat flow does not this result agrees with marine observations rigidity of the thermal boundary layer below the upper surface is not included explicitly in the model and it is not known whether the inclusion of this feature in future models would significantly alter the topographic expression if not the observed departure of the oceanic bathymetry from a 6 dependence at old ocean floor ages could be attributed to a small amount of internal heating in a mantle wide convective circulation

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

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

seismic evidence for significant lateral variations in lithospheric thickness beneath the central and western north china craton abstract we have migrated teleseismic s receiver functions to construct detailed lithospheric structure images that cover the three constituent parts of the north china craton ncc our images show that in contrast to the eastern ncc where significantly thinned lithosphere 60 100 km is widespread the central and western ncc are characterized by the coexistence of both preserved thick and dramatically thinned lithosphere the thick lithosphere 200 km is present beneath the stable ordos plateau and the thinned lithosphere up to 80 km is found in the late cenozoic yinchuan hetao and shaanxi shanxi rift areas with sharp changes occurring over a lateral distance of

gasa impact crater mars very young gullies formed from impact into latitude dependent mantle and debris covered glacier deposits abstract the mode of formation of gullies on mars very young erosional depositional landforms consisting of an alcove channel and fan is one of the most enigmatic problems in martian geomorphology major questions center on their ages geographic and stratigraphic associations relation to recent ice ages and if formed by flowing water the sources of the water to cause the observed erosion deposition gasa 35 72 s 129 45 e a very fresh 7 km diameter impact crater and its environment offer a unique opportunity to explore these questions we show that gasa crater formed during the most recent glacial epoch 2 1 0 4 ma producing secondary crater clusters on top of the latitude dependent mantle ldm interpreted to be a layered ice dust rich deposit emplaced during this glacial epoch high resolution images of a pre gasa impact crater 100 km northeast of gasa reveal that portions of the secondary crater covered ldm have been removed from pole facing slopes in crater interiors near gasa gullies are preferentially located in these areas and channels feeding alcoves and fans can be seen to emerge from the eroding ldm layers to produce multiple generations of channel incision and fan lobes we interpret these data to mean that these gullies formed extremely recently in the post gasa impact time period by melting of the ice rich ldm stratigraphic and topographic relationships are interpreted to mean that under favorable illumination geometry steep pole facing slopes and insolation conditions melting of the debris covered ice rich mantle took place in multiple stages most likely related to variations in spin axis orbital conditions closer to gasa in the interior of the 18 km diameter ldm covered host crater in which gasa formed the pole facing slopes display two generations of gullies early somewhat degraded gullies have been modified by proximity to gasa ejecta emplacement and later fresh appearing gullies are clearly superposed cross cut the earlier phase and show multiple channels and fans interpreted to be derived from continued melting of the ldm on steep pole facing slopes thus we conclude that melting of the ice rich ldm is a major source of gully activity both pre gasa crater and post gasa crater formation the lack of obscuration of gasa secondary clusters formed on top of the ldm is interpreted to mean that the gasa impact occurred following emplacement of the last significant ldm layers at these low latitudes and thus near the end of the ice ages this interpretation is corroborated by the lack of ldm within gasa however gasa crater contains a robustly developed set of gullies on its steep pole facing slopes unlike other very young post ldm craters in the region how can the gullies inside gasa form in the absence of an ice rich ldm that is interpreted to be the source of water for the other adjacent and partly contemporaneous gullies analysis of the interior floor and walls of the host crater suggest that prior to the gasa impact the pole facing walls and floor were occupied by remnant debris covered glaciers formed earlier in the amazonian which are relatively common in crater interiors in this latitude band we suggest that the gasa impact cratering event penetrated into the southern portion of this debris covered glacier emplaced ejecta on top of the debris layer covering the ice and caused extensive melting of the buried ice and flow of water and debris slurries on the host crater floor inside gasa the impact crater rim crest and wall intersected the debris covered glacier deposits around the northern pole facing part of the gasa interior we interpret this exposure of ice rich debris covered glacial material in the crater wall to be the source of meltwater that formed the very well developed gullies along the northern pole facing slopes of gasa crater

the dynamic topography of eastern china since the latest jurassic period x c and s l were supported by the national key research and development program of china 2017yfc0601401 and 2016yfc0601002 national science and technology major project 2016zx05004001 003 qingdao national laboratory for marine science and techonlogy 2016askj 3 and national programme on global change and air sea interaction soa gasi geoge 01 x c was also supported by the china scholarship council for 1 year of study at the university of sydney n f was supported by arc de160101020 and r d m was supported by arc ih130200012 and dp130101946 figures were prepared using the generic mapping tools and matplotlib we thank two anonymous reviewers for constructive comments that improved the manuscript dynamic topography results are available from http portal gplates org dynamic topography list

former extent of glacier like forms on mars abstract mars mid latitude glacier like forms glfs have undergone substantial mass loss and recession since a hypothesised last martian glacial maximum lmgm stand to date there is a lack of knowledge of the nature and timing of the lmgm the subsequent mass loss and whether this mass loss has been spatially variable here we present the results of a population scale inventory of recessional glfs derived from analysis of 1293 glfs 3 identified within context camera ctx imagery to assess the distribution and controls on glf recession a total of 436 glfs were identified showing strong evidence of recession 197 in the northern hemisphere and 239 in the southern hemisphere relative to their parent populations recessional glfs are over represented in the low latitude belts between 25 and 40 and in areas of high relief suggesting that these zones exert some control over glf sensitivity and response to forcing this analysis is complemented by the reconstruction of the maximum extent and morphology of a specific glf for which high resolution imaging science experiment hirise derived digital elevation data are available using nye s nye j f 1951 proc roy soc lond ser a mat phys sci 207 554 572 perfect plastic approximation of ice flow applied to multiple flow lines under an optimum yield strength of 22 kpa we calculate that the reconstructed glf has lost an area of 6 86 km 2 with a corresponding volume loss of 0 31 km 3 since the lmgm assuming the loss reconstructed at this glf occurred at all mid latitude glfs yields a total planetary ice loss from mars glfs of 135 km 3 similar to the current ice volume in the european alps on earth

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

the formation and evolution of youthful gullies on mars gullies as the late stage phase of mars most recent ice age gullies are extremely young erosional depositional systems on mars that have been carved by an agent that was likely to have been comprised in part by liquid water malin m c edgett k s 2000 evidence for recent groundwater seepage and surface runoff on mars science 288 2330 2335 mcewen a s et al 2007 a closer look at water related geologic activity on mars science 317 1706 1709 the strong latitude and orientation dependencies that have been documented for gullies require 1 a volatile near the surface and 2 that insolation is an important factor for forming gullies these constraints have led to two categories of interpretations for the source of the volatiles 1 liquid water at depth beneath the melting isotherm that erupts suddenly groundwater and 2 ice at the surface or within the uppermost layer of soil that melts during optimal insolation conditions surface near surface melting in this contribution we synthesize global hemispheric regional and local studies of gullies across mars and outline the criteria that must be met by any successful explanation for the formation of gullies we further document trends in both hemispheres that emphasize the importance of top down melting of recent ice rich deposits and the cold trapping of atmospherically derived h 2o frost snow as important components in the formation of gullies this provides context for the incorporation of high resolution multi spectral and hyper spectral data from the mars reconnaissance orbiter that show that 1 cold trapping of seasonal h 2o frost occurs at the alcove channel level on contemporary mars 2 gullies are episodically active systems 3 gullies preferentially form in the presence of deposits plausibly interpreted as remnants of the late amazonian emplacement of ice rich material and 4 gully channels frequently emanate from the crest of alcoves instead of the base showing that alcove generation is not necessarily a product of undermining and collapse at these locations a prediction of the groundwater model we interpret these various lines of evidence to mean that the majority of gullies on mars are explained by the episodic melting of atmospherically emplaced snow ice under spin axis orbital conditions characteristic of the last several myr

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

martian cratering 8 isochron refinement and the chronology of mars abstract this paper reviews and refines the technique of dating martian surfaces by using impact crater isochrons defined as size distributions of impact craters on undisturbed martian surfaces of specified ages in the 1970s this system identified not only abundant ancient martian volcanic surfaces but also extensive lava plains with ages of a few 108 y old this dating was initially controversial but confirmed in the 1980s and 90s by martian meteorites the present update utilizes updated estimates of the mars moon cratering ratio the most important calibration factor improves treatment of gravity and impact velocity scaling effects combines aspects of the crater size distribution data from earlier work by both neukum and hartmann and for the first time applies a correction for loss of small meteoroids in the martian atmosphere from popova et al 2003 meteorit planet sci 38 905 925 the updated isochrons are not radically different from the previous 2002 iteration but fit observed data better and give somewhat older model ages for features dated from small craters diameter d 100 m crater counts from young lava flows in various areas give good fits to the new isochrons over as much as 3 orders of magnitude in d confirming the general isochron shape and giving crater retention ages in the range of some 106 to some 108 y interpreted as lava flow ages more complex older units are also discussed uncertainties are greatest if only small craters d 100 m are used suggestions by other workers of gross uncertainties due to local secondary craters and deposition exhumation are discussed they do not refute our conclusions of significant volcanic fluvial and other geologic activity in the last few percent of martian geologic time or the importance of cratering as a tool for studying processes such as exhumation indeed crater count data suggest certain very recent episodes of deposition exhumation and ice flow possibly associated with obliquity cycles of 10 7 y timescale evidence from ancient surfaces suggests higher rates of volcanism fluvial activity glaciation and other processes in noachian hesperian time than in amazonian time

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

aeolian contributions to the development of hillslopes and scree sediments in graenagil torfajokull iceland abstract scree cones and slopes are common sedimentary landforms created by rock fall and rock particle fall in mountainous environments these formative processes are attributed to various weathering and particle detachment mechanisms however the aeolian contributions to the weathering of rock faces and formation of scree sediments are poorly understood and often underestimated a case study in the southern highlands of iceland provided a geological setting in a subarctic environment where the contribution of various erosion and deposition mechanisms to the development of scree deposits could be studied here moderately cohesive subglacial volcanic eruption products are continuously undercut by local streams creating exposed and steep sided canyon walls where scree cones and slopes are formed by dominant influences of freeze thaw cycles and the wind on rock particle fall the stratigraphy and the morphometry of these sedimentary landforms indicate that wet formative processes can contribute but are not as dominant avalanching of accumulated material occurs in dry conditions and creates distinct stratified sediments the aeolian contribution to the scree development was studied by determining the physical requirements of particle detachment using wind tunnel simulations simulated threshold wind speeds of the removal of fines show that these wind and gust conditions are common in these areas in iceland a detailed particle analysis of the sediment from an isolated scree cone and the contributing rock face showed that sediments were depleted in the silt fractions field observations confirmed the influence of deflation where fine material is removed by the wind from exposed sediments whereas larger particles are excavated by the deflation of the surrounding matrix on the overlying rock face the outcomes highlight the role that aeolian processes can fulfil in subarctic environments as similar glassy materials are also found on other planets the studied processes can for example offer insights in hillslope processes at the surface of planet mars

evidence for very recent melt water and debris flow activity in gullies in a young mid latitude crater on mars abstract terrestrial debris flows and their deposits are mainly studied and monitored because of their hazardous nature on mars they may serve as geomorphologic indicators of transient liquid water we compared the morphology of debris flow like deposits within a young 0 2 ma mid latitude crater on mars with debris flow fans on svalbard as possible terrestrial analogues it was our objective to constrain whether dry granular flow or processes related to water saturation at or close to the surface were responsible for the formation of the deposits within the crater we found that the morphological attributes of the deposits on mars are very similar to debris flows in svalbard and include overlapping terminal lobes debris tongues and snouts debris flow fans scoured channels with medial deposits debris plugs and clearly defined lateral deposits levees furthermore the interior crater walls display a range of landforms indicating aspect dependent degradation ranging from debris flow dominated pole facing slopes to east and west facing single channel gullies and north facing talus cones granular flow our findings suggest that the debris flows are not related to impact induced heating and release of meltwater we further suggest that degradation of a latitude dependent dust ice mantling unit may only have played a minor role in this youthful terrain instead we propose that the debris flows are mainly formed by melting of very recent snow deposits after the termination of the last martian ice age as such they may represent some of the most recent geomorphological indicators of transient liquid water in the martian mid latitudes the distinct north south asymmetry in degradation further demonstrates that insolation controlled slope processes are surprisingly efficient on mars during the last

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

mass movement on vesta at steep scarps and crater rims the quadrangles av 11 and av 12 on vesta are located at the northern rim of the giant rheasilvia south polar impact basin the primary geologic units in av 11 and av 12 include material from the rheasilvia impact basin formation smooth material and different types of impact crater structures such as bimodal craters dark and bright crater ray material and dark ejecta material av 11 and av 12 exhibit almost the full range of mass wasting features observed on vesta such as slump blocks spur and gully morphologies and landslides within craters processes of collapse slope instability and seismically triggered events force material to slump down crater walls or scarps and produce landslides or rotational slump blocks the spur and gully morphology that is known to form on mars is also observed on vesta however on vesta this morphology formed under dry conditions

initiation and flow conditions of contemporary flows in martian gullies understanding the initial and flow conditions of contemporary flows in martian gullies generally believed to be triggered and fluidized by co2 sublimation is crucial for deciphering climate conditions needed to trigger and sustain them we employ the ramms rapid mass movement simulation debris flow and avalanche model to back calculate initial and flow conditions of recent flows in three gullies in hale crater we infer minimum release depths of 1 0 1 5 m and initial release volumes of 100 200 m3 entrainment leads to final flow volumes that are 2 5 5 5 times larger than initially released and entrainment is found necessary to match the observed flow deposits simulated mean cross channel flow velocities decrease from 3 4 m s 1 to 1 m s 1 from release area to flow terminus while flow depths generally decrease from 0 5 1 m to 0 1 0 2 m the mean cross channel erosion depth and deposition thicknesses are 0 1 0 3 m back calculated dry coulomb friction ranges from 0 1 to 0 25 and viscous turbulent friction between 100 200 m s 2 which are values similar to those of granular debris flows on earth these results suggest that recent flows in gullies are fluidized to a similar degree as are granular debris flows on earth using a novel model for mass flow fluidization by co2 sublimation we are able to show that under martian atmospheric conditions very small volumetric fractions of co2 of 1 within mass flows may indeed yield sufficiently large gas fluxes to cause fluidization and enhance flow mobility

distinct lateral variation of lithospheric thickness in the northeastern north china craton abstract a detailed knowledge of the thickness of the lithosphere in the northeastern north china craton ncc is important for understanding the significant tectonic reactivation of the craton in the mesozoic and cenozoic time we achieve this goal by applying the newly proposed wave equation based migration technique to the s receiver functions recently collected in the region distinct negative signals are identified below the moho in all the s receiver function migrated images and stacks which we interpret as representing the s to p conversions from the lithosphere asthenosphere boundary lab the imaged lab is as shallow as 60 70 km in the southeast basin and coastal areas and deepens to no more than 140 km in the northwest mountain ranges and continental interior these observations indicate widespread lithospheric thinning in the study region in comparison with the 180 km lithospheric thicknesses typical of most cratonic regions the revealed topography of the lab generally agrees with the lateral variation in upper mantle seismic anisotropy previously measured through sks splitting analysis in particular a sharp lab step of 40 km is detected at the triple junction of the basin and mountains at almost the same place where an abrupt change from nw se to ne sw in fast polarization direction of shear waves was found these findings suggest a close correlation between the seismic anisotropy and hence deformation of the upper mantle the lithospheric thickness and the surface tectonics of the northeastern ncc while the thinned lithosphere and the nw se fast shear wave polarizations in the east areas probably are related to the dominant nw se tectonic extension in the late mesozoic cenozoic time the thicker lithosphere and the ne sw fast polarization direction in the west mountain ranges may reflect earlier contractional deformations of the region synthetic tests indicate that the lab beneath the northeastern ncc is a well defined zone 10 20 km thick combined with seismic tomography results and geochemical and petrological data this suggests that complex modification of the lithosphere probably accompanied significant lithospheric thinning during the tectonic reactivation of the old craton

crustal thickness and vp vs ratio of the central and western north china craton and its tectonic implications summary we obtained the crustal thickness h and average vp vs ratio for the central cncc and western north china craton wncc by h stacking of receiver functions our results show that h and varies significantly but similarly in the northern and southern cncc reflecting possible effects of the phanerozoic cratonotic reactivation on the region of the wncc the south is featured by values typical for precambrian shields 1 77 and variations of h mirroring surface topography the north presents large fluctuations of 1 68 1 93 and a crustal thickening up to 50 km uncorrelated with topography such n s differences were thought to be associated with a 1 95 ga suturing event occurred in the north with little effects on the south our observations suggest that distinct structural heterogeneities of the ncc not only reflect diverse regional tectonics in phanerozoic but also provide evidence for long term survival of crustal fabrics in stable continental interior

crust and upper mantle structure of the north china craton and the ne tibetan plateau and its tectonic implications abstract understanding the mesozoic cenozoic tectonic evolution of the north china craton ncc and the ne tibetan plateau tp requires detailed knowledge of the lithospheric structure using dense regional networks and temporary deployments as well as updated reference models we obtain the crust and upper mantle structure to 120 km depth our tomographic results show several major features which have particular implications for the weihe shanxi rift system wsrs deformation of the ne tp and lithospheric evolution of the ncc beneath the wsrs the crust gradually thickens from south to north the lithospheric mantle gradually becomes slower and the mid lower crustal velocities are lower in the weihe rift where rifting of the wsrs initiated we suggest that along strike variations of the lithospheric structures of the wsrs have played an important role in its multistage evolution a low velocity zone lvz in the mid crust beneath the qilian orogen is characterized by relatively higher velocities compared to lvzs in other parts of the tp thus coherent lithospheric deformation may occur due to the high viscosity of the lvz during early plateau growth causing strong anisotropy to develop the western ncc including the ordos block and part of the alashan block shows a high velocity cratonic root extending to the base of our model in contrast the lithosphere of the eastern ncc appears to have been completely modified during the mesozoic through cenozoic and presents a thin lithosphere of relatively low velocities underlain by hot asthenosphere we observed significant upper mantle heterogeneities in the ncc which may reflect its diachronous lithospheric modification

moho depth variations from receiver function imaging in the northeastern north china craton and its tectonic implications a detailed knowledge of the crustal thickness in the northeastern north china craton ncc is important for understanding the unusual phanerozoic destruction of the craton we achieve this goal by employing a 2 d wave equation based migration method to p receiver functions from 198 broadband seismic stations using ps conversions and surface reflected multiples by combining receiver function images along 19 profiles we constructed a high resolution moho depth model for the northeastern ncc the results present dominant e w moho depth variations similar to previous observations and new regional n s variations beneath both sides of the north south gravity lineament to the west while a deeper moho approximate to 42km appears in the interior of the trans north china orogen a relatively shallow moho approximate to 38km exists in the northern margin of the trans north china orogen to western ncc to the east the crust beneath the yan mountains in the marginal area is thicker approximate to 32 40km than that approximate to 26 32km beneath the bohai bay basin in the craton interior and the moho further shallows from ne approximate to 32km to sw approximate to 26km within the basin along with other observations we conclude that the dominant e w difference may have been associated with the paleo pacific plate subduction under eastern asia since the mesozoic the newly observed complex n s variations may have reflected the structural heterogeneity of the cratonic lithosphere inherited since the formation of the ncc in the paleoproterozoic or spatially uneven effects on the cratonic lithosphere of subsequent thermotectonic events during the long term evolution of the craton or both

arctic and antarctic submarine gullies a comparison of high latitude continental margins abstract submarine gullies are common features of high latitude continental slopes and over the last decade have been shown to play a key role in continental margin evolution submarine erosion downslope sediment transport slope deposits and the architecture of petroleum reservoirs however the processes that form these gullies the timescales over which they develop and the environmental controls influencing their morphology remain poorly constrained we present the first systematic and comparative analysis between arctic and antarctic gullies with the aim of identifying differences in slope character from which we infer differences in processes operating in these environments quantitative analysis of multibeam echosounder data along 2441 km of the continental shelf and upper slope and morphometric signatures of over 1450 gullies show that six geomorphically distinct gully types exist on high latitude continental margins we identify distinct differences between arctic and antarctic gully morphologies in the arctic data sets deep relief 30 m gully incision depth at 50 m below the shelf edge and shelf incising gullies are lacking these differences have implications for the timescales over which the gullies were formed and for the magnitude of the flows that formed them we consider two hypotheses for these differences 1 some antarctic gullies developed through several glacial cycles and 2 larger antarctic gullies were formed since the last glacial maximum as a result of erosive flows i e sediment laden subglacial meltwater being more abundant on parts of the antarctic margin over longer timescales a second difference is that unique gully signatures are observed on arctic and on antarctic margins environmental controls such as the oceanographic regime and geotechnical differences may lead to particular styles of gully erosion observed on arctic and antarctic margins

lithospheric structure variations between the eastern and central north china craton from s and p receiver function migration abstract i imaged the lithospheric structure beneath the bohai bay basin bbb in the eastern north china craton ncc and the taihang mountains tm and areas further to the west in the central ncc by applying the wave equation migration technique to both the s and p receiver functions rfs collected in this region the data images and synthetic modeling results indicate that s rf imaging is more appropriate than p rf imaging for mapping lithospheric discontinuities especially the lithosphere asthenosphere boundary lab in various types of regions even directly applicable to basin areas that are covered by thick sediments the constructed s rf images show that the lithospheric thickness of the study region is highly variable the imaged lab is on average 80 km beneath the bbb and displays obvious undulations from 120 km under the tm to 40 km under the mountain areas in the central ncc consistent with previous p rf imaging results the observed structural differences between the eastern and central ncc are likely associated with different lithospheric tectonics in these two regions while widespread lithosphere reactivation and extension in the eastern ncc during the late mesozoic time was plausibly responsible for both the thinned crust and lithosphere in the bbb lithospheric thinning in the central ncc may be equally heterogeneous but less significant and probably occurred mainly in cenozoic such a process may have given rise to the highly variable present day lithospheric thickness in this region however it probably did not noticeably affect the overlying crust that was reported to have thickened in response to the early magmatic underplating in the mesozoic time the different tectonic processes in the eastern and central ncc may have induced the formation of the nsgl which likely serves as a major tectonic boundary separating the two regions of contrasting lithospheric structures at present

physical weathering and modification of a rhyolitic hyaloclastite in iceland fragmental volcanic glass or hyaloclastite is a common glaciovolcanic eruption product that is formed in large abundance during basaltic andesitic and rhyolitic subglacial eruptions the physical weathering of rhyolitic hyaloclastites differs notably from basaltic hyaloclastites due to differences in cementation and edifice consolidation as rhyolitic glasses are also much rarer comparatively little is known about their physical weathering and fracturing characteristics in the presented study we provide a process oriented analysis of the physical modification of subglacially erupted rhyolitic hyaloclastites from the blahnukur edifice in torfajokull iceland frost weathering experiments were performed to determine how vesicular glass particles fragment to finer particle sizes the surficial porosity of the glass drives such frost weathering through the process of pore pressurisation and was quantified using high pressure mercury intrusion uniaxial compression experiments were carried out to understand how the glass structure responds to the application of external stress the observed fracturing in both experimental treatments was found to adhere to fractal statistics which allowed the compression experiments to be used in conjunction with the frost weathering experiments for inferring the fracturing characteristics of rhyolitic volcanic glasses transport processes by wind and gravity were simulated by long duration abrasion experiments in rock tumblers through granular avalanching but these low energy particle interactions were not found to significantly abrade particles a notable result from our fragmentation experiments was the production of 10 m particles this size range is considered respirable and illustrates how physical weathering can continuously create potentially harmful ash textures a process which is often overlooked in health hazard assessments after volcanic eruptions fragmentation by post eruptive weathering can lead to overestimations of the fine ash fraction produced by syneruptive fragmentation and granulometric studies therefore need to be appreciative of the effects of such secondary fracturing processes

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

thermokarst processes and the origin of crater rim gullies in utopia and western elysium planitia abstract we have identified a number of gullies that could be aqueous in origin near or at the rim of several impact craters in utopia planitia and western elysium planitia 30 0 59 0 n 241 0 291 0 w based on the sharpness of their incisions and the general absence of superposed craters we ascribe a relatively recent origin to the gullies scalloped depressions are commonplace throughout the region as well as on the crater walls rims and floors near the areas of gully issuance occasionally the depressions cross cut the gully debris aprons suggesting that the formation of some depressions is even more recent than that of the gullies previous research has proposed that the depressions are collapse basins formed by thermokarst processes on earth thermokarst landforms occur in areas of low gradient topography where the permanently frozen ground permafrost is ice rich and has undergone a change in thermal equilibrium this change can be triggered by long term or episodic cyclic climate change and accompanying rises in mean temperatures towards 0 c as well as by rises in seasonally sustained summer temperatures well above 0 c in order to explain the origin of the rim or near rim gullies we invoke high obliquity and the possibility that this region of mars experienced obliquity driven rises in temperature atmospheric pressure and humidity sufficient to keep surface water and near surface ground ice stable for extended periods of time we propose that gully formation is closely related to local freeze thaw processes that in turn generate a thermokarst landscape of which the gullies are a part this geological and climatological scenario comprises the following steps 1 an inundation of meltwater at high obliquity due to the thawing of an atmospherically deposited snowpack or ice sheet and the subsequent saturation of the underlying regolith to tens of metres of depth 2 loss of water on the surface perhaps as obliquity decreases slightly followed by the progressive freezing of the saturated regolith this creates an aggrading mass of ice rich regolith 3 obliquity induced temperature rises that engender the thaw drainage and partial evaporation of the near surface ice rich regolith 4 localised formation of thermokarst collapse basins alases as water is evacuated from these basins 5 formation of gullies near or at some impact crater rims as the result of meltwater migration from nearby alases through the thawed regolith to the areas of gully issuance although the plains materials in this region are in part very old possibly hesperian or even noachian the mantling deposits and their deformation by thermokarst processes appears to be relatively young this suggests that recent climatic conditions could have been episodically warmer and wetter than had been previously thought

martian flow features moraine like ridges and gullies terrestrial analogs and interrelationships ridges that resemble terrestrial moraines are commonly visible at the foot of many mid latitude crater walls in mars global surveyor mars orbiter camera images these moraine like ridges are often associated with hillside gullies mantling material and glacier like flows and are usually in contact with crater fill suggesting possible interrelationships we consider terrestrial glacier systems that may be analogs of martian moraine like ridges and glacier like flows and suggest that the formation of some gullies and crater fill is intimately tied to ice deposition ice flow and rock glacier processes upper limits on age suggest the possibility that many of these features formed during the last or last few high obliquity cycles

possible open system hydraulic pingos in and around the argyre impact region of mars abstract we report the observation of possible hydraulic open system pingos osps at the mid latitudes 37 s in and around the argyre impact basin osps are perennial water ice cored mounds they originate and evolve in periglacial and pro glacial landscapes on earth where intra or sub permafrost water under hydraulic artesian pressure uplifts localised sections of surface or near surface permafrost that then freezes in situ we invoke three lines of evidence in support of our analogue based interpretation 1 similarities of shape size and summit traits between terrestrial osps and the martian mounds 2 clustered distribution and the slope side location of the mounds consistent with terrestrial permafrost environments where osps are found and 3 spatially associated landforms putatively indicative of periglacial and glacial processes on mars that characterise osp landscapes on earth this article presents five osp candidate locations and nests these mound locations within a new geological map of the argyre impact basin and margins it also presents three periglacial hypotheses about the possible origin of the water required to develop the mounds alternative non periglacial formation hypotheses also are considered however we show that their robustness is not equal to that of the periglacial ones