pore scale evidence of ion exclusion during methane hydrate growth and evolution of hydrate pore habit in sandy sediments abstract hydrate bearing sediments constitute complex hydrological systems within which the pore habit of hydrate significantly affects all physical properties however there is scarce pore scale evidence of the evolution of hydrate growth in porous media and the influence of water salinity in the micro morphology of hydrate in this paper we used x ray computed micro tomography ct to monitor three experiments of methane hydrate growth in sand partially saturated with either nabr or ki brine under excess gas conditions gas brine and hydrate are observed to co exist at the pore scale during the evolution of hydrate pore habit towards three phase equilibrium the experiments and data analysis reveal that 1 hydrate forms as a porous medium mixed with inclusions of brine and gas where hydrate and brine evolve gradually into separate phases as hydrate cages exclude salt ions 2 hydrate growth mobilizes water over fairly long distances resulting in heterogeneous hydrate distribution and 3 hydrate can exhibit interconnecting pore habit at local hydrate saturations higher than 50 even if grown under excess gas conditions the results imply that hydrate micro morphology and pore habit evolve during hydrate formation and we suggest so do the physical properties of hydrate bearing sediments our findings provide new insights into the micro morphology of hydrate and evolution of the properties of hydrate bearing sediments synthesized in the laboratory and in natural systems

the landscape of titan as witness to its climate evolution we investigated the range of titan climate evolution hypotheses regulated by the role sources and availability of methane we analyzed all available image data principally synthetic aperture radar sar of titan s landscape through the t 86 encounter starting with focused examinations of terrains that carry the markers of climate evolution traditional geologic and geomorphic landscape analysis was used to perform morphometric characterization establish time stratigraphic relationships and interpret local and regional geologic process oriented evolutionary histories we then assayed the distribution of terrains we identified with respect to both their latitudinal and altimetric occurrence our analysis of the terrain types and distributions was used to evaluate and rank the various climate evolution scenarios we favor progressive hypotheses which include a relatively brief period in which precipitation was able to affect geomorphic change in low latitudes at scales perceivable in sar data with subsequent gradual decline of precipitation intensity coupled with an increasing poleward restriction

mechanical properties of hydrate bearing turbidite reservoir in the first gas production test site of the eastern nankai trough abstract the study of mechanical properties of marine sediments is essential for the prediction of the occurrence of geohazards e g subsea landslides and seafloor subsidence and the design of submarine structures for offshore industry in this study triaxial compression tests of gas hydrate bearing sandy sediments and clayey silty sediments were conducted the sediments were recovered by pressure coring in the eastern nankai trough the area of the first japanese offshore production test soil index properties were measured and revealed porosity of 40 50 with porosity decreasing gradually with greater depth below the seafloor the mean particle size was less than 10 m for clayey silty sediments and approximately 100 m for sandy sediments permeability estimated by a consolidation process of triaxial testing and with x ray diffraction analysis depended on the content of fines which consisted chiefly of mica kaolinite and smectite the results of undrained compression tests for clayey silty sediments showed positive excess pore pressure under all test conditions this mechanical behavior indicates that the core samples are normally consolidated sediments drained compression tests showed that the strength and stiffness of sandy sediments increase with hydrate saturation furthermore the volumetric strain of hydrate bearing sediments changed from compression to dilative this result was obtained for hydrate saturation values sh of more than 70 the shear strength of hydrate bearing turbidite sediments of the eastern nankai trough is shown to be a function of the confining pressure

an asymmetric distribution of lakes on titan as a possible consequence of orbital forcing a set of lakes filled or partially filled with liquid hydrocarbon and empty lake basins have been discovered in the high latitudes of saturn s moon titan these features were mapped by the radar instrument on the cassini orbiter here we quantify the distribution of the lakes and basins and show a pronounced hemispheric asymmetry in their occurrence whereas significant fractions of the northern high latitudes are covered by filled and empty lakes the same latitudes in the southern hemisphere are largely devoid of such features we propose that in addition to known seasonal changes the observed difference in lake distribution may be caused by an asymmetry in the seasons on titan that results from the eccentricity of saturn s orbit around the sun we suggest that the consequent hemispheric difference in the balance between evaporation and precipitation could lead to an accumulation of lakes in one of titan s hemispheres this effect would be modulated by and reverse with dynamical variations in the orbit we propose that much like in the earth s glacial cycles the resulting vigorous hydrologic cycle has a period of tens of thousands of years and leads to active geologic surface modification in the polar latitudes

a radar map of titan seas tidal dissipation and ocean mixing through the throat of kraken abstract we present a radar map of the titan s seas with bathymetry estimated as proportional to distance from the nearest shore this naive analytic bathymetry scaled to a recent radar sounding of ligeia mare suggests a total liquid volume of 32 000 km 3 at the low end of estimates made in 2008 when mapping coverage was incomplete we note that kraken mare has two principal basins separated by a narrow 17 km wide 40 km long strait we refer to as the throat tidal currents in this strait may be dramatic 0 5 m s generating observable effects such as dynamic topography whirlpools and acoustic noise much like tidal races on earth such as the corryvreckan off scotland if tidal flow through this strait is the dominant mixing process the two basins take 20 earth years to exchange their liquid inventory thus compositional differences over seasonal timescales may exist but the composition of solutes and thus evaporites over croll milankovich timescales should be homogenized

rivers lakes dunes and rain crustal processes in titan s methane cycle titan exhibits ample surface and crustal processes including lakes and seas fluvial erosive features possibly subsurface reservoirs of liquid and rainfall together these constitute strong evidence for a multicomposition hydrological system composed mostly of methane and ethane as well as trace amounts of other alkanes estimates of the volume of liquid methane required in streams and rainfall to produce erosional features suggest that these could be relatively recent phenomena perhaps periodically renewed as the overall climate cycles between dry and wet periods the end state of the longer term chemical processing of methane in the upper atmosphere is expressed on the surface in the form of deposits of solid organics organized into dunes and lighter hydrocarbons such as ethane in the lakes acetylene and other hydrocarbons and nitriles the long term evolution of the methane cycle may have involved episodic resupply of methane to the surface or gradual depletion of a larger surface reservoir of

evidence of titan s climate history from evaporite distribution abstract water ice poor 5 m bright material on saturn s moon titan has previously been geomorphologically identified as evaporitic here we present a global distribution of the occurrences of the 5 m bright spectral unit identified with cassini s visual infrared mapping spectrometer vims and examined with radar when possible we explore the possibility that each of these occurrences are evaporite deposits the 5 m bright material covers 1 of titan s surface and is not limited to the poles the only regions with extensive long lived surface liquid we find the greatest areal concentration to be in the equatorial basins tui regio and hotei regio our interpretations based on the correlation between 5 m bright material and lakebeds imply that there was enough liquid present at some time to create the observed 5 m bright material we address the climate implications surrounding a lack of evaporitic material at the south polar basins if the south pole basins were filled at some point in the past then where is the evaporite

the lakes of titan the surface of saturn s haze shrouded moon titan has long been proposed to have oceans or lakes on the basis of the stability of liquid methane at the surface1 2 initial visible3 and radar4 5 imaging failed to find any evidence of an ocean although abundant evidence was found that flowing liquids have existed on the surface5 6 here we provide definitive evidence for the presence of lakes on the surface of titan obtained during the cassini radar flyby of titan on 22 july 2006 t16 the radar imaging polewards of 70 north shows more than 75 circular to irregular radar dark patches in a region where liquid methane and ethane are expected to be abundant and stable on the surface2 7 the radar dark patches are interpreted as lakes on the basis of their very low radar reflectivity and morphological similarities to lakes including associated channels and location in topographic depressions some of the lakes do not completely fill the depressions in which they lie and apparently dry depressions are present we interpret this to indicate that lakes are present in a number of states including partly dry and liquid filled these northern hemisphere lakes constitute the strongest evidence yet that a condensable liquid hydrological cycle is active in titan s surface and atmosphere in which the lakes are filled through rainfall and or intersection with the subsurface liquid methane table

investigating the effects of different landslide positioning techniques landslide partitioning approaches and presence absence balances on landslide susceptibility mapping the ziarat watershed located in the south of the golestan province iran has witnessed several destructive landslide episodes prompting a number of researchers to aspire to improve landslide susceptibility modeling lsm techniques we constructed three scenarios focusing on landslide positioning techniques pixel based centroid crown and toe training test sampling strategies mahalanobis distance md and random sampling rs with alternative landslide non landslide data balances 1 1 1 2 and 1 3 the data mining boosted regression trees brt model was used for the landslide susceptibility modeling using landslide data and 13 landslide controlling factors for the ziarat watershed the performance of the scenarios was assessed using the areas under the success and prediction rate curves ausrc and auprc a combination of pixel based md 1 2 showed the highest learning capability and goodness of fit with an ausrc value of 0 87 and the highest predictive power and generalization capacity with an auprc value of 0 79 conversely centroid based 1 3 rs crown based 1 3 rs and toe based 1 3 rs performed less well comparatively the pixel based md and 1 2 data balance scenarios surpassed their counterparts and outperformed the other models the results indicated a high spatial differentiation with a significant chi square value of 4549 46 at 95 confidence level moreover 15 21 of the study area containing almost 50 of the landslides was found to have a high susceptibility to landslides according to the premier scenario pixel based md 1 2 lithological formation distance from roads and ndvi with respective contributions of 31 4 12 9 and 12 are the main spatial controlling factors leading to landslide occurrences in the study area

storms in the tropics of titan methane clouds lakes and most fluvial features on saturn s moon titan have been observed in the moist high latitudes while the tropics have been nearly devoid of convective clouds and have shown an abundance of wind carved surface features like dunes the presence of small scale channels and dry riverbeds near the equator observed by the huygens probe at latitudes thought incapable of supporting convection and thus strong rain has been suggested to be due to geological seepage or other mechanisms not related to precipitation here we report the presence of bright transient tropospheric clouds in tropical latitudes we find that the initial pulse of cloud activity generated planetary waves that instigated cloud activity at other latitudes across titan that had been cloud free for at least several years these observations show that convective pulses at one latitude can trigger short term convection at other latitudes even those not generally considered capable of supporting convection and may also explain the presence of methane carved rivers and channels near the huygens landing site

an international code comparison study on coupled thermal hydrologic and geomechanical processes of natural gas hydrate bearing sediments abstract geologic reservoirs containing gas hydrate occur beneath permafrost environments and within marine continental slope sediments representing a potentially vast natural gas source numerical simulators provide scientists and engineers with tools for understanding how production efficiency depends on the numerous interdependent coupled processes associated with potential production strategies for these gas hydrate reservoirs confidence in the modeling and forecasting abilities of these gas hydrate reservoir simulators ghrss grows with successful comparisons against laboratory and field test results but such results are rare particularly in natural settings the hydrate community recognized another approach to building confidence in the ghrs comparing simulation results between independently developed and executed computer codes on structured problems specifically tailored to the interdependent processes relevant for gas hydrate bearing systems the united states department of energy national energy technology laboratory doe netl sponsored the first international gas hydrate code comparison study ighccs1 in the early 2000s ighccs1 focused on coupled thermal and hydrologic processes associated with producing gas hydrates from geologic reservoirs via depressurization and thermal stimulation subsequently ghrss have advanced to model more complex production technologies and incorporate geomechanical processes into the existing framework of coupled thermal and hydrologic modeling this paper contributes to the validation of these recent ghrs developments by providing results from a second ghrs code comparison study ighccs2 also sponsored by doe netl ighccs2 includes participants from an international collection of universities research institutes industry national laboratories and national geologic surveys study participants developed a series of five benchmark problems principally involving gas hydrate processes with geomechanical components the five problems range from simple geometries with analytical solutions to a representation of the world s first offshore production test of methane hydrates which was conducted with the depressurization method off the coast of japan to identify strengths and limitations in the various ghrss study participants submitted solutions for the benchmark problems and discussed differing results via teleconferences the ghrss evolved over the course of ighccs2 as researchers modified their simulators to reflect new insights lessons learned and suggested performance enhancements the five benchmark problems final sample solutions and lessons learned that are presented here document the study outcomes and serve as a reference guide for developing and testing gas hydrate reservoir simulators

pore scale investigation of permeability evolution during hydrate formation using a pore network model based on x ray ct abstract permeability in hydrate bearing sediment critically governs fluid flow and determines hydrate nucleation growth and distribution making it important to characterize the evolution of permeability with respect to water and gas during hydrate formation this study uses ct scanning of krypton hydrate formation in silica sand using the excess gas method together with a pore network model to investigate variations in hydrate morphology and associated permeation the results show that during hydrate formation the growth habit mainly varies from grain coating to patchy with an increase in hydrate saturation however at relatively low saturation hydrate preferentially grows as a grain cementing habit theoretical models of a capillary tube and of a kozeny grain both predict that permeability in grain coating hydrates will be higher than in patchy hydrates we thus recognise a correlation of permeability and hydrate saturation for multiple growth habits that is of interest for gas production from hydrate reservoirs under lower water saturation there is a decrease in relative permeability to water but an increase in relative permeability to gas due to the reduction in pore shape factors conversely an upward shift in relative permeability to water and a downward shift in relative permeability to gas is found with hydrate formation under higher water saturation owing to the jamin effect steeper curves of relative permeability to gas and water with increasing hydrate saturation resulted in a reduction of the co cementation zone for water and gas even if or though no gas migrates the results suggest that due to the low relative permeability of gas higher water saturation sediments result in an excess water yield accompanied by low gas production that is not desirable for natural gas hydrate production therefore improving the permeability and weakening the jamin effect are critical for gas production in marine hydrate reservoirs especially in low permeability sediments

microstructural evolution of gas hydrates in sedimentary matrices observed with synchrotron x ray computed tomographic microscopy the formation process of gas hydrates in sedimentary matrices is of crucial importance for the physical and transport properties of the resulting aggregates this process has never been observed in situ at submicron resolution here we report on synchrotron based microtomographic studies by which the nucleation and growth processes of gas hydrate were observed at 276 k in various sedimentary matrices such as natural quartz with and without admixtures of montmorillonite type clay or glass beads with different surface properties at varying water saturation both juvenile water and metastably gas enriched water obtained from gas hydrate decomposition was used xenon gas was employed to enhance the density contrast between gas hydrate and the fluid phases involved the nucleation sites can be easily identified and the various growth patterns are clearly established in sediments under saturated with juvenile water nucleation starts at the water gas interface resulting in an initially several micrometer thick gas hydrate film further growth proceeds to form isometric single crystals of 10 20 m size the growth of gas hydrate from gas enriched water follows a different pattern via the nucleation in the bulk of liquid producing polyhedral single crystals a striking feature in both cases is the systematic appearance of a fluid phase film of up to several micron thickness between gas hydrates and the surface of the quartz grains these microstructural findings are relevant for future efforts of quantitative rock physics modeling of gas hydrates in sedimentary matrices and explain the anomalous attenuation of seismic sonic waves

observations of pore scale growth patterns of carbon dioxide hydrate using x ray computed microtomography natural and artificial gas hydrates with internal pores of nano to centimeters and weak grain cementation have been widely reported while the detailed formation process of grain cementing hydrates remains poorly identified pore scale morphology of carbon dioxide co2 hydrate formed in a partially brine saturated porous medium was investigated via x ray computed microtomography x ray cmt emphasis is placed on the pore scale growth patterns of gas hydrate including the growth of dendritic hydrate crystals on preformed hydrate and water wetted grains porous nature of the hydrate phase volume expansion of more than 200 during the water to hydrate phase transformation preference of unfrozen water wetting hydrophilic minerals and the relevance to a weak cementation effect on macroscale physical properties the presented pore scale morphology and growth patterns of gas hydrate are expected in natural sediment settings where free gas is available for hydrate formation such as active gas vents gas seeps mud volcanoes permafrost gas hydrate provinces and co2 injected formation for the sake of geologic carbon storage and in laboratory hydrate samples synthesized from partially brine saturated sediments or formed from water gas interfaces

synchrotron x ray computed microtomography study on gas hydrate decomposition in a sedimentary matrix in situ synchrotron x ray computed microtomography with sub micrometer voxel size was used to study the decomposition of gas hydrates in a sedimentary matrix xenon hydrate was used instead of methane hydrate to enhance the absorption contrast the microstructural features of the decomposition process were elucidated indicating that the decomposition starts at the hydrate gas interface it does not proceed at the contacts with quartz grains melt water accumulates at retreating hydrate surface the decomposition is not homogeneous and the decomposition rates depend on the distance of the hydrate surface to the gas phase indicating a diffusion limitation of the gas transport through the water phase gas is found to be metastably enriched in the water phase with a concentration decreasing away from the hydrate water interface the initial decomposition process facilitates redistribution of fluid phases in the pore space and local re formation of gas hydrates the observations allow also rationalizing earlier conjectures from experiments with low spatial resolutions and suggest that the hydrate sediment assemblies remain intact until the hydrate spacers between sediment grains finally collapse possible effects on mechanical stability and permeability are discussed the resulting time resolved characteristics of gas hydrate decomposition and the influence of melt water on the reaction rate are of importance for a suggested gas recovery from marine sediments by depressurization this article is protected by copyright all rights reserved

x ray computed tomography imaging of gas migration in water saturated sediments from capillary invasion to conduit opening 1 the strong coupling between multiphase flow and sediment mechanics determines the spatial distribution and migration dynamics of gas percolating through liquid filled soft granular media here we investigate by means of controlled experiments and computed tomography ct imaging the preferential mode of gas migration in three dimensional samples of water saturated silica sand and silica silt sediments our experimental system allowed us to independently control radial and axial confining stresses and pore pressure while performing continuous x ray ct scanning the ct image analysis of the three dimensional gas migration provides the first experimental confirmation that capillary invasion preferentially occurs in coarse grained sediments whereas grain displacement and conduit openings are dominant in fine grained sediments our findings allow us to rationalize prior field observations and pore scale modeling results and provide critical experimental evidence to explain the means by which conduits for the transit of methane gas may be established through the gas hydrate stability zone in oceanic sediments and cause large episodic releases of carbon into the deep ocean

are the basins of titan s hotei regio and tui regio sites of former low latitude seas 1 features observed in the low latitude basins of hotei regio and tui regio on titan have attracted the attention of the cassini era investigators at both locations visual infrared mapping spectrometer vims observed isolated 5 m bright 500 km wide features described as lobate in shape several studies have proposed that these materials are cryo volcanic flows we propose an alternative explanation recently published topographic profiles across hotei regio and tui regio indicate these features appear to occur in large regional basins at least along the direction of the profiles cassini synthetic aperture radar sar images show that the terrains surrounding both topographically low lying 5 m bright features exhibit fluvial networks that appear to converge into the probable basins the 5 m bright features themselves correspond to fields of discrete radar bright depressions whose bounding edges are commonly rounded and cumulate in planform in sar images these fields of discrete radar bright depressions strongly resemble fields of features seen at titan s high latitudes identified as dry lakes thus the combination of 1 the resemblance to high latitude dry lakes 2 location in the centers of regional depressions and 3 convergence of fluvial networks are inferred by us to best explain the features of hotei regio and tui regio as sites of paleolake clusters and perhaps former now dry seas these low latitude paleolake clusters or former seas if real may be evidence of substantially larger inventories of liquid alkanes in titan s past

pore habit of methane hydrate and its evolution in sediment matrix laboratory visualization with phase contrast micro ct abstract describing the pore habit of methane hydrate in sediment matrices is essential for understanding natural distribution of methane hydrate methane trace transport and solidification in the hydrate stability zone physical properties of hydrate bearing sediments and the associated influence on potential gas production pore habit visualization in natural media at pore scale even with laboratory synthesized cores has been challenging due to the similar densities of methane hydrate from pore liquid in this work we used phase contrast assisted micro ct with potassium iodine doped brine to visualize four phases sand particles pore fluid methane hydrate and methane gas this study visualizes the pore habit of methane hydrate at various stages including during hydrate formation in excess gas systems its evolution after brine injection to replace pore fluid and hydrate formation in excess water systems hydrate tends to adopt round and smooth surfaces when in contact with water while exhibits relatively angular interfaces when in contact with methane gas hydrate formation in excess gas systems results in a partial cementing and partial mineral coating pore habit while hydrate in excess water systems develops mainly as pore filling and locally cementing or mineral coating where big gas pockets exist at the initial state pore liquid replacement from methane gas to brine triggers a shift of hydrate pore habit towards pore filling methane hydrate evolution over time produces bigger hydrate particles but with less contact area with sand particles the effects of hydrate pore habit become less important as hydrate particle size exceeds the pore size additionally hydrate formation could trap residual methane gas and brine as inclusions

the sand seas of titan cassini radar observations of longitudinal dunes the most recent cassini radar images of titan show widespread regions up to 1500 kilometers by 200 kilometers of near parallel radar dark linear features that appear to be seas of longitudinal dunes similar to those seen in the namib desert on earth the ku band 2 17 centimeter wavelength images show 100 meter ridges consistent with duneforms and reveal flow interactions with underlying hills the distribution and orientation of the dunes support a model of fluctuating surface winds of 0 5 meter per second resulting from the combination of an eastward flow with a variable tidal wind the existence of dunes also requires geological processes that create sand sized 100 to 300 micrometer particulates and a lack of persistent equatorial surface liquids to act as sand traps

load bearing characteristic of methane hydrate within coarse grained sediments insights from isotropic consolidation abstract gas hydrate as a solid pore constituent can potentially share external load with sediment matrix and critically impact geological evolution of its hosting sediments well integration and sediment settlement during gas production depending on its contribution to support external loading we aim to address the question of whether hydrate shares the load with the soil skeleton through isotropic consolidation experiments our results highlight the formation of methane hydrate does not affect the integrity of sediment skeletons if the effective stress previously applied on the skeleton remains constant which indicates that methane hydrate would not actively support the load subsequent external load applied to the sediments after hydrate formation can be supported by hydrate secondly hydrate as well as hydrate bearing sediments exhibit creep behavior as hydrate creeps the load transfers to the sediment skeleton with time and hydrate gradually retreats from the status of load bearing when hydrate saturation is low and the coexisting pore fluid can percolate through sediments the local deformation of hydrate due to sediment consolidation induces excess pore pressure which dissipates by pushing out the pore fluid when the coexisting pore fluid is enclosed in hydrate mass in high hydrate saturation environments the dissipation of excess pore pressure relies on the viscous flow of hydrate this pressure dissipation could last for millions of years according to analytical analysis based on terzaghi s consolidation theory during which hydrate cannot completely escape the status of load bearing these observations conclude that load bearing is a dynamic status rather than a stagnant pore habit countering conventional thinking that load bearing joins grain coating cementing and pore filling as a pore habit of gas hydrate

an experimental investigation on methane hydrate morphologies and pore habits in sandy sediment using synchrotron x ray computed tomography abstract as pore scale morphologies and spatial distribution pore habits of natural gas hydrates in marine sediments considerably affect their physical mechanical properties they have extensively been investigated by x ray computed tomography xrct and especially synchrotron x ray computed tomography sxrct while both image spatial and scan temporal resolutions are being improved over time it is still challenging to distinguish water from methane hydrate in an image due to their low absorption contrast in this study methane hydrate formation and growth in wet sand were observed at submicron micron scale using sxrct saline water potassium iodide ki was used in order to improve the image contrast evolutions of methane hydrate morphologies and distribution at both the pore and sample scales were observed results are discussed based on various mechanisms related to material behaviors and experimental conditions e g suction variation during methane hydrate formation local temperature gradient in the sample and particularly the interaction of x rays with the sample methane hydrate formation is interpreted as a dynamic process favoring the ostwald ripening furthermore morphologies and pore habits of methane hydrates under excess gas and excess water conditions are discussed some recommendations are finally given for further studies on methane hydrates bearing sediments via xrct or sxrct

strain rate dependent hardening softening characteristics of gas hydrate bearing sediments the presence of gas hydrates ghs increases the stiffness and strength of marine sediments in elasto plastic constitutive models it is common to consider gh saturation sh as key internal variable for defining the contribution of ghs to composite soil mechanical behavior however the stress strain behavior of gh bearing sediments ghbs also depends on the microscale distribution of gh and on gh sediment fabrics a thorough analysis of ghbs is difficult because there is no unique relation between sh and gh morphology to improve the understanding of stress strain behavior of ghbs in terms of established soil models this study summarizes results from triaxial compression tests with different sh pore fluids effective confining stresses and strain histories our data indicate that the mechanical behavior of ghbs strongly depends on sh and gh morphology and also on the strain induced alteration of gh sediment fabrics hardening softening characteristics of ghbs are strain rate dependent which suggests that gh sediment fabrics dynamically rearrange during plastic yielding events we hypothesize that rearrangement of gh sediment fabrics through viscous deformation or transient dissociation and reformation of ghs results in kinematic hardening suppressed softening and secondary strength recovery which could potentially mitigate or counteract large strain failure events for constitutive modeling approaches we suggest that strain rate dependent micromechanical effects from alterations of the gh sediment fabrics can be lumped into a nonconstant residual friction parameter we propose simple empirical evolution functions for the mechanical properties and calibrate the model parameters against the experimental data plain language summary gas hydrates ghs are crystalline like solids which are formed from natural gas molecules and water at high pressure and low temperature ghs and particularly methane hydrates are naturally abundant in marine sediments it is known that the presence of gh increases the mechanical stiffness and strength of sediments and there is strong effort in analyzing and quantifying these effects in order to understand potential risks of sediment destabilization or slope failure based on our experimental results from high pressure geotechnical studies we show that not only the initial amount and distribution of gh are important for the increased strength of gh bearing sediments but also the dynamic rearrangement of gh sediment fabrics during deformation characterizes the stress strain response and enables strength recovery after failure we propose that different microstructural mechanisms contribute to this rearrangement and strength recovery of gh sediment however we consider these complicated processes in a simplified manner in an improved numerical model which can be applied for geotechnical risk assessment on larger scales

study of hydrate occupancy morphology and microstructure evolution with hydrate dissociation in sediment matrices using x ray micro ct abstract hydrate occupancy morphology and spatial distribution in sediment pore can significantly affect the physical properties of hydrate bearing sediment but their evolutions with hydrate dissociation under various brine saturations are not well understood x ray micro ct is employed to image the pore scale dynamic process of methane hydrate dissociation in both sediments with different brine saturations on the basis of processing and statistical analysis of x ray ct images several topological parameters including volume shape factor fractal dimension and euler characteristic of individual hydrate cluster are selected to quantify hydrate occupancy morphology and microstructure evolution with hydrate dissociation in the host sediments results show that the degree of brine saturation within the sediments can exert a significant impact on hydrate pore occupancy morphology and microstructure connectedness during hydrate dissociation in contrary to those of fully brine saturated sediment the topological parameters of individual hydrate cluster usually experience more dramatic changes with hydrate dissociation in partially brine saturated sediment it is inferred that in partially brine saturated sediment the isolated hydrate clusters occupying in pore bodies are dissociated earlier in the initial experimental stage and the dissociation of interpore hydrate frameworks interconnected through pore throats get dominated when hydrate saturation declines below a critical threshold value on the other hand in fully brine saturated sediment the kinetics of pore scale hydrate dissociation exhibit a completely opposite trend

rapid and extensive surface changes near titan s equator evidence of april showers although there is evidence that liquids have flowed on the surface at titan s equator in the past to date liquids have only been confirmed on the surface at polar latitudes and the vast expanses of dunes that dominate titan s equatorial regions require a predominantly arid climate we report the detection by cassini s imaging science subsystem of a large low latitude cloud system early in titan s northern spring and extensive surface changes spanning more than 500 000 square kilometers in the wake of this storm the changes are most consistent with widespread methane rainfall reaching the surface which suggests that the dry channels observed at titan s low latitudes are carved by seasonal precipitation

large catchment area recharges titan s ontario lacus abstract we seek to address the question of what processes are at work to fill ontario lacus while other deeper south polar basins remain empty our hydrological analysis indicates that ontario lacus has a catchment area spanning 5 5 of titan s surface and a large catchment area to lake surface area ratio this large catchment area translates into large volumes of liquid making their way to ontario lacus after rainfall the areal extent of the catchment extends to at least southern mid latitudes 40 s mass conservation calculations indicate that runoff alone might completely fill ontario lacus within less than half a titan year 1 titan year 29 5 earth years assuming no infiltration cassini visual and infrared mapping spectrometer vims observations of clouds over the southern mid and high latitudes are consistent with precipitation feeding ontario s large catchment area this far flung rain may be keeping ontario lacus filled making it a liquid hydrocarbon oasis in the relatively dry south polar region

regional geomorphology and history of titan s xanadu province titan s enigmatic xanadu province has been seen in some detail with instruments from the cassini spacecraft the region contains some of the most rugged mountainous terrain on titan with relief over 2000 m xanadu contains evolved and integrated river channels impact craters and dry basins filled with smooth radar dark material perhaps sediments from past lake beds arcuate and aligned mountain chains give evidence of compressional tectonism yet the overall elevation of xanadu is puzzlingly low compared to surrounding sand seas lineations associated with mountain fronts and valley floors give evidence of extension that probably contributed to this regional lowering several locations on xanadu s western and southern margins contain flow like features that may be cryovolcanic in origin perhaps ascended from lithospheric faults related to regional downdropping late in its history radiometry and scatterometry observations are consistent with a water ice or water ammonia ice composition to its exposed eroded fractured bedrock both microwave and visible to near infrared v nir data indicate a thin overcoating of organics likely derived from the atmosphere we suggest xanadu is one of the oldest terrains on titan and that its origin and evolution have been controlled and shaped by compressional and then extensional tectonism in the icy crust and ongoing erosion by methane rainfall

titan s topography and shape at the end of the cassini mission with the conclusion of the cassini mission we present an updated topographic map of titan including all the available altimetry sartopo and stereo photogrammetry topographic datasets available from the mission we use radial basis functions to interpolate the sparse dataset which covers only 9 of titan s global area the most notable updates to the topography include higher coverage of the poles of titan improved fits to the global shape and a finer resolution of the global interpolation we also present a statistical analysis of the error in the derived products and perform a global minimization on a profile by profile basis to account for observed biases in the input dataset we find a greater flattening of titan than measured additional topographic rises in titan s southern hemisphere and better constrain the possible locations of past and present liquids on titan s surface

instrumented pressure testing chamber for characterizing sediment cores recovered at in situ hydrostatic pressure marine sediments retrieved with pressure coring systems such as those used by the integrated ocean drilling program maintain samples at in situ hydrostatic pressure throughout recovery such pressure cores are particularly important for the study of gas hydrate bearing sediments which must be maintained within the gas hydrate stability field during shipboard core characterization until now there has been no device capable of directly measuring a suite of physical properties on sediments contained in pressure cores without first depressurizing them this study describes the design construction and deployment of the instrumented pressure testing chamber iptc which was first used to measure the physical properties of pressure cores recovered 1530 m below the sea surface during 2005 drilling in the gulf of mexico the iptc permits drilling through the plastic liner while the core is under pressure and the sequential measurement of p and s wave velocities undrained strength and electrical conductivity preliminary results indicate that the seismic velocities measured on a pressure core with the iptc are more representative of in situ seismic velocities at comparable depths than are velocities measured on conventional cores from these same depths the flexibility of the iptc design allows future modifications to account for restoration of effective not just hydrostatic stress the addition of new types of measurements sampling of pore fluids and sediments under pressure and use of the device as a laboratory reactor for gas hydrates studies

fluvial features on titan insights from morphology and modeling fluvial features on titan have been identified in synthetic aperture radar sar data taken during spacecraft flybys by the cassini titan radar mapper radar and in descent imager spectral radiometer disr images taken during descent of the huygens probe to the surface interpretations using terrestrial analogs and process mechanics extend our perspective on fluvial geomorphology to another world and offer insight into their formative processes at the landscape scale the varied morphologies of titan s fluvial networks imply a variety of mechanical controls including structural influence on channelized flows at the reach scale the various morphologies of individual fluvial features implying a broad range of fluvial processes suggest that paleo flows did not occupy the entire observed width of the features disr images provide a spatially limited view of uplands dissected by valley networks also likely formed by overland flows which are not visible in lower resolution sar data this high resolution snapshot suggests that some fluvial features observed in sar data may be river valleys rather than channels and that uplands elsewhere on titan may also have fine scale fluvial dissection that is not resolved in sar data radar bright terrain with crenulated bright and dark bands is hypothesized here to be a signature of fine scale fluvial dissection fluvial deposition is inferred to occur in braided channels in paleo lake basins and on sar dark plains and disr images at the surface indicate the presence of fluvial sediment flow sufficient to move sediment is inferred from observations and modeling of atmospheric processes which support the inference from surface morphology of precipitation fed fluvial processes with material properties appropriate for titan terrestrial hydraulic equations are applicable to flow on titan for fully turbulent flow and rough boundaries for low reynolds number flow over smooth boundaries however knowledge of fluid kinematic viscosity is necessary sediment movement and bed form development should occur at lower bed shear stress on titan than on earth scaling bedrock erosion however is hampered by uncertainties regarding titan material properties overall observations of titan point to a world pervasively influenced by fluvial processes for which appropriate terrestrial analogs and formulations may provide insight