hirise observations of gas sublimation driven activity in mars southern polar regions iv fluid dynamics models of co2 jets abstract the high resolution imaging science experiment hirise onboard mars reconnaissance orbiter mro has been used to monitor the seasonal evolution of several regions at high southern latitudes on mars and in particular the jet like activity which may result from the process described by kieffer kieffer h h 2007 j geophys res planets 112 e08005 doi 10 1029 2006je002816 involving translucent co 2 ice in this work we concentrate on attempting to model the dusty co 2 gas jets using a computational fluid dynamics code models that included surface slopes of up to 20 as an analogy to the jet activity seen in inca city 81 s 296 e wind from 0 to 6 m s 1 variable vent cross section and length particles including a particle size distribution and mass loading with dust to gas ratios exceeding 1 were investigated the structure of the resulting gas jets the particle distribution within the jets the deposition patterns including their dependence on particle size and the appearance of jets when viewed from different orientations including from a nadir pointing camera have been investigated for a range of input parameters the results provide predictions for the size dependency of altitudes of particles within a plume and the distribution of particle sizes in the deposition fans where slopes are a dominant influence larger particles are expected to be seen furthest from the vent where wind is dominant smaller particles should travel to larger distances models producing deposition patterns consistent in length 80 m and form with fans observed by hirise on mro have been demonstrated the models also suggest that downward flow of gas produced by drag effects from particles falling from the jet under gravity could provide a mechanism for the production of bright haloes which are observed to surround dark fan deposits in moc hirise and crism

sub surface co2 gas flow in mars polar regions gas transport under constant production rate conditions 1 sublimation at the ice substrate interface with pressure build up is an accepted mechanism for the production of fan deposits on the southern polar co2 ice cap on mars fluid dynamics modeling has been used to investigate gas outflow through vents in a co2 slab ice small 5 25 m in length fan deposits seen on the annual southern co2 ice cap can be produced by a steady state in which open vents continuously outgas in response to sub surface sublimation generated by diurnal energy input through translucent impermeable to gas ice slabs this would produce diurnally controlled deposits which would change orientation with time in response to winds thereby explaining observations made by hirise on mro gas flow below the ice can reach up to 25 m s close to the vent dust flow in the sub ice cavity has also been computed but velocities are much lower with the main acceleration occurring 0 5 m from the vent this indicates that erosion is likely to be restricted to the immediate vicinity of the vent unless channels form in the cavity between the ice and the surface

acquisition geometry analysis in complex 3d media increasingly we must deal with complex subsurface structures in seismic exploration often resulting in poor illumination and therefore poor image quality consequently it is desirable to take into consideration the effects of wave propagation in the subsurface structure when designing an acquisition geometry we developed a new model based implementation of the previously introduced focal beam analysis method the method s objective is to provide quantitative insight into the combined influence of acquisition geometry overburden structure and migration operators on image resolution and angle dependent amplitude accuracy this is achieved by simulation of migrated grid point responses using focal beams note that the seismic response of any subsurface can be composed of a linear sum of grid point responses the focal beams have been chosen because any migration process represents double focusing in addition the focal source beam and focal detector beam relate migration quality to illumination properties of the source geometry and sensing properties of the detector geometry respectively wave equation modeling ensures that frequency dependent effects in the seismic frequency range are incorporated we tested our method by application to a 3d salt model in the gulf of mexico investigation of well sampled all azimuth long offset acquisition geometries revealed fundamental illumination and sensing limitations further results exposed the shortcomings of narrow azimuth data acquisition the method also demonstrates how acquisition related amplitude errors affect seismic inversion results

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

ice state evolution during spring in richardson crater mars abstract the martian climate is governed by an annual cycle that results in the condensation of co2 ice during winter up to a meter thick at the pole and thousands of kilometers in extension water and dust may be trapped during the condensation and freed during the sublimation in addition ice may be translucent or granular depending on the deposition process snow vs direct condensation annealing efficiency and dust sinking process the determination of ice translucency is of particular interest to confirm or reject the cold jet model also known as kieffer model this work is focused on the dune field of richardson crater 72 s 180 w in which strong interactions between the water dust and co2 cycles are observed we analyzed crism hyperspectral images in the near ir using radiative transfer model inversion we demonstrate that among the states of co2 ice the translucent state is observed most frequently the monitoring of surface characteristics shows a decrease in the thickness of the ice during the spring consistently with climate models simulations we estimate a very low dust content of a few ppmv into the co2 ice consistent with the formation scenario of cold jets the water impurities is around 0 1 v almost stable during the spring suggesting a water escape from the surface of subliming co2 ice layer the water ice grain size varies in a range 1 to 50 microns from these results we propose the following new mechanism of small water ice grain suspension as a cold jet occurs water ice grains of various sizes are lifted from the surface these jets happen during daytime when the general upward gas flux from the subliming co2 ice layer is strong enough to carry the smaller grains while the bigger fall back on the co2 ice layer the smaller water grains are carried away and integrated to the general atmospheric circulation

present day erosion of martian polar terrain by the seasonal co 2 jets abstract here we report on the detection of new dendritic troughs created by the seasonal co2 jet activity over several martian years according to kieffers hypothesis kieffer 2007 in martian polar areas seasonal co2 ice sublimation creates gas jets that deposit fans of mineral dust and sand on top of the co2 ice these jets and the related sub ice gas flows are believed to carve troughs in the underlying polar deposits which after multiple repetitions of this process throughout martian spring seasons create araneiform terrains the high resolution imaging science experiment hirise on the mars reconnaissance orbiter mro detected the new troughs during its campaign for seasonal monitoring of the polar areas the newly detected dendritic troughs are small shallow branching troughs 1 4 m wide similar to the seasonal furrows previously detected in the northern hemisphere bourke 2013 the essential difference between the new troughs and furrows lies in the fact that the troughs in the south are persistent while the northern furrows are erased each martian year by the sand movement due to summer winds from year to year the new southern troughs extend and develop new tributaries and their overall geometry turns from linear to dendritic a characteristic shared with araneiform terrains we believe that furrows have the same origin as the southern dendritic troughs but do not develop into dendritic shapes because of the high mobility of the dune material into which they are carved several locations where new dendritic troughs are observed lie in the vicinity of dunes this gives us an observational indication that presence of erosive sand material is an important factor in creating or at least starting erosive processes that lead to the formation of dendritic troughs by extrapolation the same mechanism should be acting to create the much larger araneiform terrains detection of the present day erosion working in polar areas and creating new topographical features is important for understanding of the processes that shape polar areas several years of hirise observations provide us with the information about the current rate of erosion and hence help estimate minimum ages of the araneiforms and the surface into which they are carved to be 1 3 103 martian years

salar de atacama basin a record of compressional tectonics in the central andes since the mid cretaceous the salar de atacama basin lies in the inner fore arc of northern chile topographically and structurally it is a first order feature of the central andes the sedimentary fill of the basin constrains the timing and extent of crustal deformation since the mid cretaceous we have studied good exposures along the western edge of the basin and have correlated them with seismic reflection sections and data from an exploration well throughout most of its history the basin developed in a foreland setting during periods of thin skinned and thick skinned thrusting growth strata provide evidence for coeval sedimentation and thrust motions during mid cretaceous paleogene and neogene times pre neogene deformation was significant in the basin and in surounding areas of the early central andes models that attempt to explain the current thickness of the central andes should consider late cretaceous and paleogene shortening as well as the more obvious neogene and quaternary shortening

single bubble air gun array for deep exploration large tuned air gun arrays operated in off shore petroleum exploration are also used for deep penetration marine seismic reflection surveys conducted to define structures in the earth s crust because of the attenuation of higher frequencies the useful upper frequency limit of these records is usually about 50 60 hz the aim of this paper is to report on a method of seismic pulse generation that preferentially concentrates the air gun s energy in the low range of the seismic frequency band by centering the output on the first bubble pulse instead of the initial primary pulse experimental results show that due to the increased low frequency energy content of this single bubble pulse air gun arrays considerably reduced both in size and volume can generate the necessary acoustic energy for deep seismic exploration

hirise observations of gas sublimation driven activity in mars southern polar regions iii models of processes involving translucent ice abstract enigmatic surface features known as spiders found at high southern martian latitudes are probably caused by sublimation driven erosion under the seasonal carbon dioxide ice cap the mars reconnaissance orbiter mro high resolution imaging science experiment hirise has imaged this terrain in unprecedented details throughout southern spring it has been postulated kieffer h h titus t n mullins k f christensen p r 2000 j geophys res 105 9653 9700 that translucent co 2 slab ice traps gas sublimating at the ice surface boundary wherever the pressure is released the escaping gas jet entrains loose surface material and carries it to the top of the ice where it is carried downslope and or downwind and deposited in a fan shape here we model two stages of this scenario first the cleaning of co 2 slab ice from dust and then the breaking of the slab ice plate under the pressure built below it by subliming ice our modeling results and analysis of hirise images support the gas jet hypothesis and show that outbursts happen very early in spring

february 2003 marine atmospheric conditions and the bora over the northern adriatic 1 a winter oceanographic field experiment provided an opportunity to examine the atmospheric marine conditions over the northern adriatic mean february winds are from a northeasterly direction over most of the adriatic and a more northerly direction along the western coast wind speeds are fastest in jets over the ne coast during bora events and weakest in the mid northwestern adriatic diurnal air temperature cycles are smallest on the ne coast and largest in the midwestern adriatic the maximum sea air difference is 10 c on the eastern coast and near zero on the midwestern adriatic boras are northeasterly from wind events that sweep off croatia and slovenia bringing slightly colder and drier air over the northern adriatic the main bora season is december to march winter 2002 2003 was normal for bora events synoptic scale temporal variations are correlated over the northern adriatic fastest bora winds and highest wind stress over the northern adriatic is concentrated in four topographically controlled jets the strongest is the senj jet while the trieste jet extends across the entire northern adriatic between each two jets is a weak wind zone the greatest mean net heat loss is in bora jets in the ne adriatic where it was 438 w m 2 and is weakest in the midwestern northern adriatic where it was near zero wind stress is concentrated over the ne half of adriatic in four bora jets while wind stress is weak in the nw adriatic there is significant variation in wind stress mean and standard deviation structure over the northern adriatic with each bora event

how martian araneiforms get their shapes morphological analysis and diffusion limited aggregation model for polar surface erosion abstract araneiforms are radially converging systems of branching troughs exhibiting fractal properties they are found exclusively in the southern polar regions of mars and believed to be result of multiple repetitions of cold co2 gas jets eruptions araneiform troughs get carved by the overpressurized gas rushing underneath a seasonal ice layer towards a newly created opening current work is an attempt to quantitatively analyze araneiforms patterns and model their formation mechanism araneiform shapes range from small and simple structures of one or two connected branches to intricate dendritic patterns several kilometers across the dendritic quality of most araneiforms are suggestive that they can be described in terms commonly applied to terrestrial rivers the main difference between rivers and araneiforms from the morphological point of view is that araneiform patterns are not directed or in any way influenced by the local gravitational gradient we have adapted and for the first time applied to martian araneiforms qualitative morphological analysis typically used for terrestrial rivers for several locations in the martian southern polar regions we have calculated the largest order of tributaries tributary densities and bifurcation ratios we have shown that the large and well developed araneiforms with tributary orders larger than 4 closely follow horton s law of tributary orders and have bifurcation ratio that falls well inside the range of terrestrial rivers we have implemented a two dimensional diffusion limited aggregation dla model that describes formation of dendrite shapes by mathematical probabilistic means we compared modeled dendrite shapes to the araneiform shapes observed in the martian polar regions and evaluated their similarity using the morphological analysis of araneiforms we showed that dla model can successfully recreate 2d shapes of different observed araneiforms to simulate the patterns that deviate from central symmetry it is required to modify the main governing parameter of dla model its two dimensional probability field an analog to the governing probability field in the case of araneiforms is the gas pressure in the cavity underneath the ice layer right before the gas starts moving towards the newly created escape vent thus modeling the creation of araneiform patterns with dla leads to better understanding of seasonal processes that create them

application of acoustic full waveform inversion to retrieve high resolution temperature and salinity profiles from synthetic seismic data 1 recent works show that multichannel seismic mcs systems are able to provide detailed information on the oceans fine structure the aim of this paper is to analyze whether 1 d full waveform inversion algorithms are suitable to recover the extremely weak acoustic impedance contrasts associated to the oceans fine structure as well as their potential to image meso scale objects such as meddies we limited our analysis to synthetic noise free data in order to identify some methodological issues related to this approach under idealistic conditions e g 1 d wave propagation noise free data known source wavelet we first discuss the influence of the starting model in the context of the multi scale strategy that we have implemented then we show that it is possible to retrieve not only sound speed but also salinity and temperature contrasts within reasonable bounds from the seismic data using neural network relationships trained with regional oceanographic data sets potentially the vertical resolution of the obtained models which depends on the maximum frequency inverted is of the order of 5 10 m whereas the root mean square error of the inverted properties is shown to be 0 5 m s for sound speed 0 1 c for temperature and 0 06 for salinity to conclude this study we have inverted synthetic data simulated along an oceanographic transect acquired during the eu funded geophysical oceanography go project the results demonstrate the applicability of the method for synthetic data as well as its potential to define oceanographic features along 2 d transects at full ocean depth with excellent lateral resolution

sublimation of mars s southern seasonal co2 ice cap and the formation of spiders 1 in this paper we define and describe morphological features that have colloquially been termed spiders and map their distribution in the south polar region of mars we show that these features go through a distinct seasonal evolution exhibiting dark plumes and associated fan shaped deposits during the local defrosting of the seasonal cap we have documented the seasonal evolution of the cryptic region and have found that spiders only occur within this terrain these observations are consistent with a geyser like model for spider formation association with the transparent cryptic portion of the seasonal cap is consistent with basal sublimation and the resulting venting of co2 gas also consistent with such venting is the observation of dark fan shaped deposits apparently emanating from spider centers spiders are additionally confined to the polar layered deposits presumably due to the poorly consolidated and easily eroded nature of their upper surface

cold jets in the martian polar caps 1 mars seasonal polar caps display dark ice local darker spots aligned elongate patches and radially dendritic forms that reverse albedo contrast the unexpected variety and sequence of these features are explained on the basis of processes involving co2 dust sand and h2o these processes are largely related to the atmosphere being near its saturation temperature and they have few terrestrial analogies in the simplest case the 1 m thick seasonal cap initially dusty cleans itself and becomes translucent after sunrise and is impermeable over extensive regions except for local vents the slab ice sublimates at the base and is levitated on high pressure gas causing humidity exchange with deeper layers subslab gas converging toward the vents erodes channels in the soil and ejects this material in high velocity jets recent spectral observations indicate great variety in the details

joint inversion of marine magnetotelluric and gravity data incorporating seismic constraints preliminary results of sub basalt imaging off the faroe shelf breakup of the north atlantic during the early tertiary was accompanied by widespread and massive magmatism resulting in the coverage of large areas of the north atlantic with flood basalts these flood basalts hamper seismic investigations of underlying sequences and thus the understanding of the rifting subsidence and evolution of the margin which in turn increases the risk for hydrocarbon exploration in this paper we present a methodology for the simultaneous joint inversion of diverse geophysical datasets i e free air gravity and magnetotelluric soundings mt using seismic a priori constraints the attraction of the joint inversion approach is that different geophysical measurements are sensitive to different properties of the sub surface so through joint inversion we significantly reduce the null space and produce a single model that fits all datasets within a predefined tolerance using sensitivity analysis of synthetic data we show how each data set contains complementary important information of the supra and sub basalt structure while separate inversions of individual datasets fail to image through the basalt layer our joint inversion approach leads to a much improved sub basalt structure application of the joint inversion algorithm to satellite gravity data and mt data acquired on the flare10 seismic line south west of faroe islands supports the existence of a 1 km to 2 km thick low velocity region that might be indicative of the existence of a sedimentary basin underneath the basalt layer though in this paper we demonstrate the use of joint inversion on a sub basalt target we believe it has wider applicability to other areas where conventional seismic imaging fails

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

equivalent medium representation of fractured rock a similarity may be found between various approaches for determining the effects of parallel fractures or aligned cracks on seismic wave propagation at wavelengths that are long compared with the scale length of the cracks fractures can be modeled using an empirical linear slip condition however natural fracture surfaces can also be simulated directly as planar distributions of small isolated areas of slip cracks model 1 or conversely as planar distributions of imperfect interfacial contacts model 2 an alternative is plane surfaces separated by thin continuous layers of viscous fluid or a soft material model 3 we present analytic expressions for the fracture compliances for these three models and using these analytic results compute the effective compliances and stiffnesses of the fractured material as a result it is possible to relate the measured compliances or stiffnesses directly to the statistics of the microstructural details of a fracture given appropriate a priori information on the fracture surfaces the results for model 1 are equivalent to a volume distribution of cracks as studied by hudson 1980 1981 for small crack density the results for model 2 are basically the same as those given by white 1983 for a packing of spheres and finally the results for model 3 are in agreement with those given by backus 1962 for combinations of two constituent layers these results can be extended to the case of nonaligned fractures and to allow for fluid flow between cracks and into a porous matrix rock finally it is shown that the ratio of the normal to shear fracture compliance is a good indicator of the properties of the fracture infill

long term observations of southern winters on mars estimations of column thickness mass and volume density of the seasonal co2 deposit from hend odyssey data 1 in this paper we have analyzed neutron spectroscopy data gathered by the high energy neutron detector hend instrument on board mars odyssey for two martian years of orbital mapping we focused on the observations of the martian southern seasonal cap for this period of time nuclear methods based on numerical modeling allow us to derive an estimation of the distribution of co2 frost column density and mass from the temporal variations of neutron flux between summer and winter seasons we have estimated the seasonal behavior of both the column density and mass of co2 frost comparison between two martian years revealed minor 10 interannual variations of co2 thickness at the south pole during the middle of southern winter it was found that maximal mass of the southern seasonal cap is achieved at ls 160 and is equal to 6 6 0 6 1015 kg we found that this result corresponds well with previous hend results nasa ames gcm predictions gamma ray spectrometer estimates and estimations from gravity models based on mgs and mars odyssey tracking data in our analysis we also compared to processed mola data linear thicknesses of snow depths to measure the value of the volume density of the southern seasonal cap and we found that it may vary in the range 400 1250 kg m3

north and south subice gas flow and venting of the seasonal caps of mars a major geomorphological agent 1 dark polygons associated with fans and spots appear during the spring on the southern seasonal cap the basal sublimation of the translucent cap and the venting of the co2 gas are responsible for their formation as previously proposed for the spots and fans dark polygons appear when dark material emerges from elongated vents whereas spots and fans form from point sources a class of erosive features etched polygons is associated with depressions a few meters to tens of meters in diameters connected to a network of radiating troughs spiders spiders are shaped by the scouring action of the confined gas converging toward point sources whereas the etched polygons result from the forced migration of the co2 gas over longer distances the minimum age of the spiders is 104 years they result from one of the most efficient erosive processes on mars displacing 2 orders of magnitude more dust per year than a typical dust storm or than all the dust devils during the same time period in the north parts of the seasonal cap are translucent between ls 355 and ls 60 and are associated with spots fans dark polygons and possibly spiders suggesting that the basal sublimation and venting of the cap triggers a subice gas and dust flow that is modifying the morphology of the surface layer however perennial features are extremely uncommon on the north regolith indicating that the conditions for their formation or conservation are not met the reduced basal energy budget of the north cap compared to the south and the shorter seasonal life time of the north translucent ice may explain the relative scarcity of features in the north the polar layered deposits contain the stratigraphic record of climatic changes and catastrophic events both polar deposits may have been locally disrupted by the seasonal subice gas flow and the stratigraphic record may have been partially lost

use of low frequencies for sub basalt imaging many prospective passive ocean margins are covered by large areas of basalts these basalts are often extremely heterogeneous and scatter the seismic energy of the conventional seismic reflection system so that it becomes difficult to obtain information on deeper reflectors since high frequencies are scattered more than low frequencies we argue that the acquisition system for sub basalt targets should be modified to emphasize the low frequencies using much larger airguns and towing the source and receivers at about 20 m depth in the summer of 2001 we obtained seismic reflection data over basalt in the northeast atlantic using a system modified to enhance the low frequency energy these new data show deep reflections that are not visible on lines shot in the same places with a conventional system

internal structure of the erlend tertiary volcanic complex north of shetland revealed by seismic reflection detailed seismic reflection mapping 100 km n of shetland has revealed a large subcrop of palaeocene basalts proved by exploration drilling the geometry of internal reflectors within the buried basalt suggests the existence of two partially eroded volcanoes a vent of about 2 km in diameter surrounded by radially outward dipping basalts is centred over the erlend igneous complex whose existence had been postulated previously on gravity and magnetic evidence alone a new volcano the more deeply eroded west erlend centre is postulated by interpretation of seismic data and by gravity modelling an approximately cylindrical mafic or ultramafic pluton is inferred from the gravity modelling to underlie each of the two centres the diameter of the erlend pluton is about 14 km and that of west erlend about 5 7 km the seismic stratigraphy of volcanic rocks can now be recognized as a valid new field of study with particular relevance to hydrocarbon exploration

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

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

observations of the northern seasonal polar cap on mars iii crism hirise observations of spring sublimation we analyze a series of targeted crism and hirise observations of seven regions of interest at high latitudes in the northern polar regions of mars these data allow us to investigate the temporal evolution of the composition of the seasonal ice cap during spring with a special emphasis on peculiar phenomena occurring in the dune fields and in the vicinity of the scarps of the north polar layered deposits nplds the strength of the spectral signature of co2 ice continuously decreases during spring whereas the one of h2o ice first shows a strong increase until ls 50 this evolution is consistent with a scenario previously established from analysis of omega data in which a thin layer of pure h2o ice progressively develops at the surface of the volatile layer during early spring ls 10 widespread jet activity is observed by hirise while strong spectral signatures of co2 ice are detected by crism later around ls 20 40 activity concentrates at the dune fields where crism also detects a spectral enrichment in co2 ice consistent with kieffer s model kieffer h h 2007 j geophys res 112 e08005 doi 10 1029 2006je002816 for jet activity effects of wind are prominent across the dune fields and seem to strongly influence the sublimation of the volatile layer strong winds blowing down the scarps could also be responsible for the significant spatial and temporal variability of the surface ice composition observed close to the npld

possible mechanism of boulder clustering on mars whatever causes small scale polygonal patterned ground diameter 20 m to form pervasively in the northern plains of mars appears to also cause boulders to cluster at polygon margins assuming that these polygons are caused by the seasonal thermal contraction and expansion of ice in the near surface to meters of depth we propose a mechanism that ratchets boulders towards the edges in seasonal cycles on present day mars during the contraction cooling phase a co2 frost layer embeds the boulders buffered by condensation and sublimation the solid co2 is isothermal and does not contract we propose that it locks the boulders in place while the h2o ice in the near surface continues cooling and contracts beneath them in spring summer seasons when the frost sublimates and heating of the surface begins the boulders freely move with the expanding near surface ice we show that over many seasons this process would lead to progressive boulder movement towards polygon edges with an estimated rate of boulder migration 0 1 mm yr under present mars conditions

evolution of south seasonal cap during martian spring insights from high resolution observations by hirise and crism on mars reconnaissance orbiter 1 we use data from the high resolution imaging science experiment hirise camera and the compact reconnaissance imaging spectrometer for mars crism imaging spectrometer onboard the mars reconnaissance orbiter to follow the evolution of the appearance and composition of 12 regions of the south polar layered deposits from spring to summer time we distinguish three steps in the evolution of the volatile layer a decrease of both co2 band strength and albedo until ls 190 210 a significant increase in both until ls 240 260 and finally a rapid decrease until the complete defrosting of the ground in contrast the water ice band displays a more monotonic decrease analysis of hirise color images acquired simultaneously with crism data allows a plausible interpretation of this evolution in early springtime ls 200 intense jet activity results in deposition of fans of large mineral grains and a wide spatial distribution of fine grains the small scale topography controls the presence and location of the jets by allowing more solar energy to be collected on slopes grains from the dust fans warm and sink through the co2 layer resulting in a bluish color at the locations of the fans around ls 190 210 as the atmosphere warms up the surface of the ice layer sublimes and releases dust and water resulting in its brightening the last phase of the process consists in a progressive defrosting resulting in a patchwork of frozen and unfrozen areas

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

how the martian residual south polar cap develops quasi circular and heart shaped pits troughs and moats the martian residual south polar cap rspc is a 1 10 m thick deposit of permanent co 2 ice perched on the much larger h 2o ice cap the co 2 ice is dissected into mesas by erosional landforms that can be broadly classified as i quasi circular pits ii heart shaped pits iii linear troughs and iv moats we use hirise 25 50 cm px images taken at a cadence of days to months to track meter scale changes in the rspc in order to investigate the mechanisms that lead to the development of these four distinct morphologies for the first time we report the development of dark fans on the sides of the co 2 mesas and the fracturing and deterioration of the initially smooth upper surface of co 2 mesas we interpret these features as indicating the sublimation and subsequent escape of co 2 from the interiors of mesas which undermines structural support of mesa tops causing them to collapse the collapse of mesa tops along with uneven deposition of co 2 ice creates steep scarps that erode during the summer due to preferential sunlight absorption during the winter co 2 deposition acts to smooth topography creating gently sloping ramps we propose that the interplay between the steep scarps and gentle slopes leads to either quasi circular pits heart shaped pits linear troughs or moats depending on local conditions

broadband receiver response from dual streamer data and applications in deep reflection seismology a method to process dual streamer data with under and over configuration is presented the method combines the results of dephase sum and dephase subtraction methods in the dephase methods the response of one streamer is time shifted so that the primary arrivals on both streamers are aligned and these responses are then summed or subtracted the method provides a broad spectral response from dual streamer data and increases the signal to noise ratio by a factor of 1 5 testing was done on synthetic data and then applied to a real data set collected by the british institutions reflection profiling syndicate birps its application to a deep seismic reflection data set from the british isles shows that the reflections from the lower crust contain frequencies up to 80 hz suggesting that some of the lower crustal reflectors may have sharp boundaries and could be 20 30 m thick

numerical modelling of seismic waves scattered by hydrofractures application of the indirect boundary element method summary we use a numerical method that can model the seismic waveforms scattered from an arbitrary number of fractures that are either empty or contain elastic or fluid material the indirect boundary element method bem is capable of generating the full elastic wavefield and is programmed in 2 d the governing equations and discrete implementation of the technique are described we explain in detail a new approach for evaluating the improper boundary integrals the method is shown to be highly accurate from a comparison with mode summation subsequently the bem is applied to modelling hydrofractures synthetic examples calculated for cross well and single well geometries demonstrate the effects of crack length opening and infill on recorded displacements it is shown that diffractions from the tips can in principle be used to locate and determine the hydrofracture size these arrivals depart from ray theoretical travel times due to defocusing over a fresnel zone s wave diffractions generally have a larger amplitude than the diffracted p waves and so may provide a better indication of fracture size energy which is converted to interface waves and subsequently diffracted from the crack tips is also observed the presence of water allows energy to pass through the fracture this is clearly evident on the cross well seismograms closure of the fracture causes a further increase in transmission amplitude as less energy is attenuated through internal multiples

the development of late stage continental breakup seismic reflection and borehole evidence from the danakil depression ethiopia during continental breakup the locus of strain shifts from a broad region of border faulting and ductile plate stretching to a narrow zone of magma intrusion in a young ocean basin recent studies of volcanic rifts and margins worldwide suggest this shift occurs subaerially before the onset of seafloor spreading we test this hypothesis using recently acquired seismic reflection and borehole data from the danakil depression ethiopia a unique region of transition between continental rifting and seafloor spreading our data located near dallol 30 km northwest of the erta ale volcanic segment reveal a remarkably thick 1 km sequence of young 100 ka evaporites in a basin bound by a major 400 m throw east dipping normal fault to generate such a large amount of subsidence in such a relatively short time we propose that upper crustal extension in danakil is currently dominated by faulting not magmatic intrusion given the region s markedly thinned crust 15 km thick relative to elsewhere in afar where magma assisted rifting dominates and maintains crustal thickness at 25 km mechanical extension in danakil is likely coupled with ductile extension of the lower crust and mantle lithosphere despite proximity to the voluminous lavas of the active erta ale volcanic segment evidence for igneous material in the upper 2 km of the 6 to 10 km wide basin is limited late stage stretching was likely aided by thermal strain induced lithospheric weakening following protracted magma assisted rifting basin formation immediately prior to the onset of seafloor spreading may also explain the accumulation of thick marine seepage fed evaporite sequences akin to those observed for example along the south atlantic rifted margins

the high resolution imaging science experiment hirise during mro s primary science phase psp the high resolution imaging science experiment hirise on the mars reconnaissance orbiter mro acquired 8 terapixels of data in 9137 images of mars between october 2006 and december 2008 covering 0 55 of the surface images are typically 5 6 km wide with 3 color coverage over the central 20 of the swath and their scales usually range from 25 to 60 cm pixel nine hundred and sixty stereo pairs were acquired and more than 50 digital terrain models dtms completed these data have led to some of the most significant science results new methods to measure and correct distortions due to pointing jitter facilitate topographic and change detection studies at sub meter scales recent results address noachian bedrock stratigraphy fluvially deposited fans in craters and in or near valles marineris groundwater flow in fractures and porous media quasi periodic layering in polar and non polar deposits tectonic history of west candor chasma geometry of clay rich deposits near and within mawrth vallis dynamics of flood lavas in the cerberus palus region evidence for pyroclastic deposits columnar jointing in lava flows recent collapse pits evidence for water in well preserved impact craters newly discovered large rayed craters and glacial and periglacial processes of particular interest are ongoing processes such as those driven by the wind impact cratering avalanches of dust and or frost relatively bright deposits on steep gullied slopes and the dynamic seasonal processes over polar regions hirise has acquired hundreds of large images of past present and potential future landing sites and has contributed to scientific and engineering studies of those sites warming the focal plane electronics prior to imaging has mitigated an instrument anomaly that produces bad data under cold operating conditions

efficient waveform tomography for lithospheric imaging implications for realistic two dimensional acquisition geometries and low frequency data summary we provide a series of numerical experiments designed to test waveform tomography under i a reduction in the number of input data frequency components efficient waveform tomography ii sparse spatial subsampling of the input data and iii an increase in the minimum data frequency used these results extend the waveform tomography results of a companion paper using the same third party 2 d wide angle synthetic viscoelastic seismic data computed in a crustal geology model 250 km long and 40 km deep with heterogeneous p velocity s velocity density and q factor structure accurate velocity models were obtained using efficient waveform tomography and only four carefully selected frequency components of the input data 0 8 1 7 3 6 and 7 0 hz this strategy avoids the spectral redundancy present in full waveform tomography and yields results that are comparable with those in the companion paper for an 88 per cent decrease in total computational cost because we use acoustic waveform tomography the results further justify the use of the acoustic wave equation in calculating p wave velocity models from viscoelastic data the effect of using sparse survey geometries with efficient waveform tomography were investigated for both increased receiver spacing and increased source spacing sampling theory formally requires spatial sampling at maximum interval of one half wavelength 2 5 km at 0 8 hz for data with receivers every 0 9 km conforming to this criterion artefacts in the tomographic images were still minimal when the source spacing was as large as 7 6 km three times the theoretical maximum larger source spacings led to an unacceptable degradation of the results when increasing the starting frequency image quality was progressively degraded acceptable image quality within the central portion of the model was nevertheless achieved using starting frequencies up to 3 0 hz at 3 0 hz the maximum theoretical sample interval is reduced to 0 67 km due to the decreased wavelengths the available sources were spaced every 5 0 km more than seven times the theoretical maximum and receivers were spaced every 0 9 km 1 3 times the theoretical maximum higher starting frequencies than 3 0 hz again led to unacceptable degradation of the results