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

mars south polar spring and summer behavior observed by tes seasonal cap evolution controlled by frost grain size thermal emission spectrometer tes observations of the recession phase of mars south polar cap are used to quantitatively map this recession in both thermal and visual appearance geographically nonuniform behavior interior to the cap is characterized by defining several small regions which exemplify the range of behavior for most of the cap while temperatures remain near the co2 frost point albedos slowly increase with the seasonal rise of the sun then drop rapidly as frost patches disappear over a period of 20 days a cryptic region remains dark and mottled throughout its cold period tes observations are compared with first order theoretical spectra of solid co2 frost with admixtures of dust and h2o the tes spectra indicate that the cryptic region has much larger grained solid co2 than the rest of the cap and that the solid co2 here may be in the form of a slab the mountains of mitchel remain cold and bright well after other areas at comparable latitude apparently as a result of unusually small size of the co2 frost grains we found little evidence for a significant presence of h2o although co2 grain size may be the major difference between these regions incorporated dust is also required to match the observations a self cleaning process carries away the smaller dust grains comparisons with viking observations indicate little difference in the seasonal cycle 12 martian years later the observed radiation balance indicates co2 sublimation budgets of up to 1250 kg m 2 regional atmospheric dust is common localized dust clouds are seen near the edge of the cap prior to the onset of a regional dust storm and interior to the cap during the storm

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

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

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

mass wasting and ground collapse in terrains of volatile rich deposits as a solar system wide geological process the pre galileo view the polar terrains of mars are covered in many places with irregular pits and retreating scarps as are some of the surfaces of the outer planet satellites these features are interpreted by us as diagnostic of exogenic degradation due to the loss of a volatile rock forming matrix or cement in this study we propose that sublimation degradation is a plausible solar system wide geological process candidate examples have been identified on mars io and triton and possibly europa and ganymede we envision this process as having two end member expressions pits and scarps for which we hypothesize two end member mechanisms massive localized lenses and areally extensive basal layers in this study we focus on the role this process may play on the surfaces of the galilean satellites our principle modeling results are that for these satellites h2s co2 and nh3are the only viable candidate volatiles for sublimation degradation of landforms in light of galilean satellite cosmochemistry for io s polar regions only h2s and then only from slopes that face the sun and have thin lags is volatile enough to cause the observed sublimation induced erosion at those latitudes so2is not a viable candidate as an agent of erosion especially for these polar landforms in the case of europa only co2and h2s are viable candidates given surface age constraints both species could be efficient eroders in nonpolar regions h2s could generate erosion within the polar regions if the deposition and erosion conditions were essentially identical as those we invoked for io s polar regions for ganymede and callisto nh3might be an agent of erosion in equatorial terrains of great age the sublimation of co2and h2s is much more robust than nh3 the much slower rate of sublimation degradation from nh3might be detectable by galileo and used as a compositional indicator

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

on the unusual characteristics of the diamonds from letseng la terae kimberlites lesotho abstract the letseng la terae kimberlites are situated 3100 m above sea level in the maloti mountains of lesotho southern africa the principal economic bodies are two late cretaceous low grade 1 3 5 carats hundred ton cpht kimberlite pipes that host high value diamonds realising us 2000 2500 carat ct in 2008 terms locally the larger kimberlite body is referred to as the main pipe 17 2 ha and the smaller one is called the satellite pipe 5 2 ha these pipes and their associated eluvial and proximal alluvial deposits are renowned for yielding large d colour gem quality diamonds including 100 carat ct stones earlier artisanal effort 1959 1977 and formal mining 1977 1982 produced 335 000 carats cts including the 601 ct lesotho brown in 1968 in 2003 letseng diamonds limited re commenced mining operations and had produced 265 000 cts by the end of july 2008 including 24 100 ct diamonds the largest of which was the 603 ct lesotho promise we report here on the unusual characteristics of the letseng diamond population that include i 75 gem quality that is more commonly associated with alluvial diamond deposits ii large average stone size of ca 1 carat stone ct stn that is also more typical of certain alluvial diamond placers iii high yielding rounded to flattened irregular resorbed dodecahedral shapes main pipe 67 and satellite pipe 87 with subordinate dodecahedral macle main pipe 32 and satellite pipe 12 and broken ca 1 forms in both pipes the octahedral component is virtually absent iv economically favourable colour mix ca 33 white colour diamonds in both pipes v abundance of nitrogen free d colour type iia diamonds that dominate the internationally recognised special stone size fraction which covers all diamonds larger than 10 8 cts main pipe 32 and satellite pipe 51 during 2008 these larger special diamonds commanded prices in excess of us 15 000 ct contributing ca 75 of the revenue generated by the letseng mine furthermore of the 24 100 ct diamonds recovered between november 2003 and july 2008 18 75 were type iia d colour diamonds that also fetched prices mostly in excess of us 25 000 ct therefore the type iia diamonds boost significantly the revenue per unit measure in this case the us ton of the letseng la terae pipes making these low grade kimberlites economic to mine

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

initiation of martian outflow channels related to the dissociation of gas hydrate we propose that the disruption of subpermafrost aquifers on mars by the thermal or pressure induced dissociation of methane hydrate may have been a frequent trigger for initiating outflow channel activity this possibility is raised by recent work that suggests that significant amounts of methane and gas hydrate may have been produced within and beneath the planet s cryosphere on earth the build up of overpressured water and gas by the decomposition of hydrate deposits has been implicated in the formation of large blowout features on the ocean floor these features display a remarkable resemblance in both morphology and scale to the chaotic terrain found at the source of many martian channels the destabilization of hydrate can generate pressures sufficient to disrupt aquifers confined by up to 5 kilometers of frozen ground while smaller discharges may result from the water produced by the decomposition of near surface hydrate alone

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

slope morphology of hills at the mars pathfinder landing site abstract the study of hillslopes is a primary element of geomorphology and has successfully been used in many terrestrial arenas in this study we take advantage of high resolution imaging science experiment hirise imagery as well as mars orbiter camera moc derived dems of the pathfinder landing site to study regional hillslopes at resolutions many times greater than previously available and compare them with mars pathfinder lander images this site was thought to be modified by massive flooding 1 8 3 5 byr ago and although evidence of flood activity was not obvious at the finer scale of this study possible lee deposits and terracing were seen in some of the features evidence of post flood processes of ice related creep aeolian and dry mass wasting were observed at the site and have likely obscured flood related morphology present in these features regional slopes were found to vary with aspect and suggest processes intensities operating at different orientations possibly related to the prevailing wind direction as well as the origin of the ancient flood event

possible ancient oceans on mars evidence from mars orbiter laser altimeter data high resolution altimetric data define the detailed topography of the northern lowlands of mars and a range of data is consistent with the hypothesis that a lowland encircling geologic contact represents the ancient shoreline of a large standing body of water present in middle mars history the contact altitude is close to an equipotential line the topography is smoother at all scales below the contact than above it the volume enclosed by this contact is within the range of estimates of available water on mars and a series of extensive terraces parallel the contact in many places

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

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

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

episodic flood inundations of the northern plains of mars throughout the recorded history of mars liquid water has distinctly shaped its landscape including the prominent circum chryse and the northwestern slope valleys outflow channel systems and the extremely flat northern plains topography at the distal reaches of these outflow channel systems paleotopographic reconstructions of the tharsis magmatic complex reveal the existence of an europe sized noachian drainage basin and subsequent aquifer system in eastern tharsis this basin is proposed to have sourced outburst floodwaters that sculpted the outflow channels and ponded to form various hypothesized oceans seas and lakes episodically through time these floodwaters decreased in volume with time due to inadequate groundwater recharge of the tharsis aquifer system martian topography as observed from the mars orbiter laser altimeter corresponds well to these ancient flood inundations including the approximated shorelines that have been proposed for the northern plains stratigraphy geomorphology and topography record at least one great noachian early hesperian northern plains ocean a late hesperian sea inset within the margin of the high water marks of the previous ocean and a number of widely distributed minor lakes that may represent a reduced late hesperian sea or ponded waters in the deepest reaches of the northern plains related to minor tharsis and elysium induced amazonian flooding

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

high outflow channels on mars indicate hesperian recharge at low latitudes and the presence of canyon lakes abstract a number of martian outflow channels were carved by discharges from large dilational fault zones these channels were sourced by groundwater not surface water and when observed on high standing plateaus they provide indicators of elevated paleo groundwater levels we identify three outflow channels of hesperian age that issued from a 750 km long fault zone extending from candor chasma to ganges chasma two of these channels allegheny vallis and walla walla vallis have sources 2500 m above the topographic datum too high to be explained by discharge from a global aquifer that was recharged solely in the south polar region the indicated groundwater levels likely required regional sources of recharge at low latitudes the floodwaters that erupted from ophir cavus to form allegheny vallis encountered two ridges that restricted the flow forming temporary lakes the flow probably breached or overtopped these obstructions quickly catastrophically draining the lakes and carving several scablands after the last obstacle had been breached a single main channel formed that captured all subsequent flow we performed hydrologic analyses of this intermediate phase of the flooding prior to incision of the channel to its present depth using floodwater depths of 30 60 m we calculated flow velocities of 6 15 m s 1 and discharges in the range of 0 7 3 10 6 m 3 s 1 locally higher flow velocities and discharges likely occurred when the transient lakes were drained variable erosion at the channel and scabland crossing of mola pass 10644 suggests that the upper 25 30 m may consist of poorly consolidated surface materials underlain by more cohesive bedrock we infer that an ice covered lake with a surface elevation 2500 m probably existed in eastern candor chasma because this canyon is intersected by the ophir catenae fault system from which allegheny vallis and walla walla vallis originated we introduce a new hydrology concept for mars in which the groundwater system was augmented by recharge from canyon lakes that were formed when water was released by catastrophic melting of former ice sheets in tharsis by effusions of flood basalts this model could help to reconcile the expected presence of a thick cryosphere during the hesperian with the abundant evidence for groundwater as a source for some of the circum chryse outflow channels

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

present day impact cratering rate and contemporary gully activity on mars the mars global surveyor mars orbiter camera has acquired data that establish the present day impact cratering rate and document new deposits formed by downslope movement of material in mid latitude gullies on mars twenty impacts created craters 2 to 150 meters in diameter within an area of 21 5 106 square kilometers between may 1999 and march 2006 the values predicted by models that scale the lunar cratering rate to mars are close to the observed rate implying that surfaces devoid of craters are truly young and that as yet unrecognized processes of denudation must be operating the new gully deposits formed since august 1999 are light toned and exhibit attributes expected from emplacement aided by a fluid with the properties of liquid water relatively long extended digitate distal and marginal branches diversion around obstacles and low relief the observations suggest that liquid water flowed on the surface of mars during the past decade

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

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

runout and fine sediment deposits of axisymmetric turbidity currents we develop a model that describes the runout behavior and resulting deposit of a radially spreading suspension driven gravity current on a surface of negligible slope our analysis considers the separate cases of constant volume and constant flux sources it incorporates expressions for the conservation of volume a froude number condition at the current front and the evolution of the driving suspension due to settling of particles to the underlying bed the model captures the key features of a range of experimental observations the analysis also provides important scaling relationships between the geometry of a deposit and the source conditions for the deposit forming flow as well as explicit expressions for flow speed and deposit thickness as functions of radial distance from the source among the results of our study we find that in the absence of information regarding flow history the geometries of relatively well sorted deposits generated by flows with source conditions of constant volume or constant flux are virtually indistinguishable the results of our analysis can be used by geologists in the interpretation of some geologically important gravity surge deposits using our analytical results we consider three previously studied radially symmetric turbidites of the hispaniola caicos basin in the western atlantic ocean from gross geometry and grain size of the turbidites alone we estimate for the respective deposit forming events that upon entry into the basin the initial sediment concentrations were approximately 3 by volume and the total volumes were roughly between 30 km 3 and 100 kn1 3 each of the suspension driven flows is inferred to have spread into the basin with a characteristic speed of 3 5 m s 1 and reached its ultimate runout length of about 60 75 km while laying down a deposit over a period of about 10 12 hours

present day gully activity in sisyphi cavi mars flow like features and block movements abstract the continuously increasing number of multi temporal high resolution images from the surface of mars offers the possibility for detailed studies of present day surface activity in this study we investigated all gullies in the sisyphi cavi region 355 e 71 s of the south polar region of mars this region is influenced by the seasonal deposition of a decimeters thick translucent slab ice in late autumn winter and its subsequent sublimation in spring we mapped all gullies n 17 760 and measured their orientations we also identified gullies with contemporary activity n 35 using multi temporal hirise images for martian years my 28 to 34 we observed two different kinds of activity 1 dark flow like features and 2 movement of blocks for both sediment was transported from the source region alcove and or flanks of channels down the gully using image data from hrsc ctx and hirise we monitored the general defrosting of the study region we also analyzed the maximum daytime surface temperatures of the complete study region based on tes data from mys 24 to 26 to identify the origin and triggering mechanism of the observed activity we used 1 detailed topographic investigations e g slope angles of two extensively gullied slopes based on two hirise dtms 2 identification of small scale displacements with digital image correlation dic and 3 orientation measurements of active gullies and comparison to non active gullies we found that for the active gullies studied activity happens at the end of spring between ls 225 and 250 this is consistent with the timing of final stages of defrosting in the region at this time some surfaces are already defrosted while others still host the seasonal slab ice cover for the surfaces with slab ice dark defrosting spots and flows if the surface is inclined are observed on dark dunes as well as on gully aprons and in gully channels these spots form when triggered by basal sublimation generated overpressure sediment entrained in co2 gas is transported through cracks within the ice and redeposited onto the frosted surface we compared and linked both morphologic features dark dune spots and dark flow like features and concluded that these features have comparable or even the same triggering mechanisms based on this extensive study the most plausible mechanism for ongoing gully activity can be divided into two steps 1 accumulation of material within gully channels via small dry flows on top of the slab ice comparable to dark dune spots flows 2 when a critical mass is reached the sediment flows down the still frosted gully on top of the sublimating ice or as a mixture of dry material and ice in a catastrophic flow the triggering factor for the movement of blocks remains unclear as their timing could not be constrained with the available data we identified headwall erosion in one gully in the study region whereas a discrete source could not be identified for the other sites suggesting multiple failure mechanisms could be active in such gullies finally through volume balance calculations we show that active gullies in sisyphi cavi could have been formed within decades to several tens of thousands of my but gully morphology indicates a much longer period for formation of the entire gully landform

the instability of a south polar cap on mars composed of carbon dioxide abstract if the martian south polar cap were composed of solid carbon dioxide would it be sufficiently stable against collapse under its own weight this question is examined in the light of new experiments on the creep of solid carbon dioxide and new high resolution topographic mapping of the south polar cap the conclusion from a simple model is that with the strongest of three flow laws compatible with the measurements co 2 is strong enough to support a cap of this material with thickness about 1800 m for 10 7 years or more which is the age suggested by the cratering record however none of the three possible flow laws will allow a 3000 m cap the thickness suggested by stereogrammetry to survive for 10 7 years indicating that the south polar ice cap is probably not composed of pure co 2 ice

dynamic processes controlling evolution of rifted basins abstract the extension of the lithosphere controlling the development of rifted basins is driven by a combination of plate boundary forces frictional forces exerted on the base of the lithosphere by the convecting asthenosphere and deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system although mantle plumes are not a primary driving force of rifting they play an important secondary role by weakening the lithosphere and by controlling the level of rift related volcanic activity a distinction between active and passive rifting is only conditionally justified the extension of the lithosphere depending on its rate and magnitude and the potential temperature of the asthenosphere can cause by adiabatic decompression partial melting of the lower lithosphere and upper asthenosphere in rift systems the level and timing of volcanic activity is highly variable the lack of volcanic activity implies passive rifting an initial passive rifting stage can be followed by a more active one during which magmatism plays an increasingly important role magmatic destabilization of the moho may account for the frequently observed discrepancy between upper and lower crustal extension factors combined with evidence for thermal thinning of the mantle lithosphere this suggest that the volume of the lithosphere is not necessarily preserved during rifting as advocated by conventional stretching models the structural style of rifts is controlled by the rheological structure of the lithosphere the availability of crustal discontinuities that can be tensionally reactivated the mode orthogonal or oblique and amount of extension and the lithological composition of pre and syn rift sediments simple shear extension prevails in rifts that subparallel the structural grain of the basement pure shear extension is typical for rifts cross cutting the basement grain pre existing crustal and mantle lithospheric discontinuities contribute to the localization of rift systems the duration of the rifting stage of extensional basins is highly variable stress field changes can cause abrupt termination of rifting in major rift systems progressive strain concentration on the zone of future crustal separation entails abandonment of lateral rifts depending on constraints on lateral block movements crustal separation can be achieved after as little as 9 my and as much as 280 my of rifting activity syn rift basin subsidence is controlled by isostatic adjustment of the crust to mechanical stretching of the lithosphere its magmatic inflation and thermal attenuation of the mantle lithosphere post rift basin subsidence is governed by thermal reequilibration of the lithosphere asthenosphere system deep seated thermal anomalies related to syn rift pull up of the asthenosphere lithosphere boundary have decayed after 60 my by about 65 and after 180 my by about 95 the magnitude of post rift subsidence is a function of the rift induced thermal anomaly and crustal density changes the potential temperature of the asthenosphere and initial water depths intraplate stresses can have an overprinting effect on post rift subsidence stretching factors derived from post rift subsidence analyses must be corrected for such effects

martian cratering vi crater count isochrons and evidence for recent volcanism from mars global surveyor this paper develops a methodology to establish absolute martian ages by deriving isochrons on a plot of martian impact crater density vs crater diameter calibrated by lunar crater age data the isochrons illustrated here are based on a mars moon cratering ratio of 1 6 at constant size but there is a factor of 2 to 4 uncertainty in this ratio and the consequent model ages martian crater diameter distributions are determined in several areas down to diameter d 16 45 m the shapes of the curves in young areas are found to be close to that of the predicted isochrons and close to the standard production function found by neukum the youngest areas studied here display the lunar like production function down to d 30 m where saturation equilibrium sets in model crater retention ages of several volcanic units are found to be a few hundred million years or less with estimated uncertainties ranging from a factor of 2 lower to a factor of 4 higher the results are consistent with martian meteorite ages volcanism on mars has probably persisted into the last 10 to 15 of the planet s history and is likely ongoing because surfaces as young as a few hundred million years have reached crater saturation equilibrium at d 60 to 100 m mars is likely to have widespread impact produced regoliths at least a few meters deep and this may contribute to the widespread mobile dust and boulder fields of mars

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

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