dune bedforms produced by dilute pyroclastic density currents from the august 2006 eruption of tungurahua volcano ecuador a series of pyroclastic density currents were generated at tungurahua volcano ecuador during a period of heightened activity in august 2006 dense pyroclastic flows were confined to valleys of the drainage network while dilute pyroclastic density currents overflowed on interfluves where they deposited isolated bodies comprising dune bedforms of cross stratified ash exposed on the surface here the description measurement and classification of more than 300 dune bedforms are presented four types of dune bedforms are identified with respect to their shape internal structure and geometry length width thickness stoss and lee face angles and stoss face length 1 elongate dune bedforms have smooth shapes and are longer in the flow direction than wide or thick internal stratification consists of stoss constructional thick lensoidal layers of massive and coarse grained material alternating with bedsets of fine laminae that deposit continuously on both stoss and lee sides forming aggrading structures with upstream migration of the crests 2 transverse dune bedforms show linear crests perpendicular to the flow direction with equivalent lengths and widths internally these bedforms exhibit finely stratified bedsets of aggrading ash laminae with upstream crest migration steep truncations of the bedsets are visible on the stoss side only 3 lunate dune bedforms display a barchanoidal shape and have stratification patterns similar to those of the transverse ones finally 4 two dimensional dune bedforms are much wider than long exhibit linear crests and are organized into trains elongate dune bedforms are found exclusively in proximal deposition zones transverse lunate and two dimensional dune bedforms are found in distal ash bodies the type of dune bedform developed varies spatially within an ash body transverse dune bedforms occurring primarily at the onset of deposition zones transitioning to lunate dune bedforms in intermediate zones and two dimensional dune bedforms exclusively on the lateral and distal edges of the deposits the latter are also found where flows moved upslope elongate dune bedforms were deposited from flows with both granular based and tractional flow boundaries that possessed high capacity and competence they may have formed in a subcritical context by the blocking of material on the stoss side we do not interpret them as antidune or chute and pool structures the dimensions and cross stratification patterns of transverse dune bedforms are interpreted as resulting from low competence currents with a significant deposition rate but we rule out their interpretation as antidunes a similar conclusion holds for lunate dune bedforms whose curved shape results from a sedimentation rate dependent on the thickness of the bedform finally two dimensional dune bedforms were formed where lateral transport exceeds longitudinal transport i e in areas where currents were able to spread laterally in low velocity zones we suggest that the aggrading ash bedsets with upstream crest migration were formed under subcritical flow conditions where the tractional bedload transport was less important than the simultaneous fallout from suspension this produced differential draping with no further reworking we propose the name regressive climbing dunes for structures produced by this process a rapid decrease in current velocity possibly triggered by hydraulic jumps affecting the entire parent flows is inferred to explain their deposition this process can in principle hold for any kind of particulate density current

titan s planetary boundary layer structure at the huygens landing site 1 huygens atmospheric structure instrument hasi for the first time performed an in situ measurement of the thermal structure in titan s atmosphere with a vertical resolution sufficient to analyze the planetary boundary layer pbl the vertical potential temperature profile reveals the presence of a weakly convective pbl with a surface layer thickness of 10 m and an outer layer with a depth of 300 m with a mean eddy diffusivity of only 7 4 10 3 m2 s 1 the turbulence in the pbl is weak the turbulent heat flux in the surface layer was upward but tiny 0 02 w m 2 indicating that the ground surface was marginally warmer than the air the surface heat flux is too small to cause a diurnal variation of the pbl except in the lowest few meters so the observed profile may be a nearly steady state feature within this season at the landing site in the surface layer the mean wind speed is likely to be less than 0 1 m s 1 given the tiny surface heat flux the buoyant production of turbulence is very weak the pbl structure reveals that the weather condition at the time and place of the huygens landing resembles a calm overcast day on earth but is clearly different from the pbl typical for the polar night

prediction of aeolian features on planets application to titan paleoclimatology we examine the conditions necessary to form aeolian features on planets such as ripples and dunes specifically we compute saltation and suspension speeds for the terrestrial planets triton titan and two possible primordial titan atmospheres and compare these with theoretical estimates of friction speeds we examine the availability of sediments and the mass transport rates needed to form aeolian features on titan and compare likely feature dimensions with the resolution of instruments on the cassini mission to evaluate the detectability of such features a novel element is the consideration of a weibull wind speed distribution adding a probabilistic element to the formation and destruction of dunes we find dunes unlikely to form in any titan atmospheres suggesting that dunelike bedforms are only likely to be found in areas that were once submerged by liquids

cassini sar radiometry scatterometry and altimetry observations of titan s dune fields large expanses of linear dunes cover titan s equatorial regions as the cassini mission continues more dune fields are becoming unveiled and examined by the microwave radar in all its modes of operation sar radiometry scatterometry altimetry and with an increasing variety of observational geometries in this paper we report on cassini s radar instrument observations of the dune fields mapped through may 2009 and present our key findings in terms of titan s geology and climate we estimate that dune fields cover 12 5 of titan s surface which corresponds to an area of 10 million km 2 roughly the area of the united states if dune sand sized particles are mainly composed of solid organics as suggested by vims observations cassini visual and infrared mapping spectrometer and atmospheric modeling and supported by radiometry data dune fields are the largest known organic reservoir on titan dune regions are with the exception of the polar lakes and seas the least reflective and most emissive features on this moon interestingly we also find a latitudinal dependence in the dune field microwave properties up to a latitude of 11 dune fields tend to become less emissive and brighter as one moves northward above 11 this trend is reversed the microwave signatures of the dune regions are thought to be primarily controlled by the interdune proportion relative to that of the dune roughness and degree of sand cover in agreement with radiometry and scatterometry observations sar images suggest that the fraction of interdunes increases northward up to a latitude of 14 in general scattering from the subsurface volume scattering and surface scattering from buried interfaces makes interdunal regions brighter than the dunes the observed latitudinal trend may therefore also be partially caused by a gradual thinning of the interdunal sand cover or surrounding sand sheets to the north thus allowing wave penetration in the underlying substrate altimetry measurements over dunes have highlighted a region located in the fensal dune field 5 latitude where the icy bedrock of titan is likely exposed within smooth interdune areas the hemispherical assymetry of dune field properties may point to a general reduction in the availability of sediments and or an increase in the ground humidity toward the north which could be related to titan s asymmetric seasonal polar insolation alternatively it may indicate that either the wind pattern or the topography is less favorable for dune formation in titan s northern tropics

two boundary layers in titan s lower troposphere inferred from a climate model saturn s moon titan has a dense atmosphere but its thermal structure is poorly understood simulations with a three dimensional general circulation model suggest that titan has a lower atmospheric structure with two boundary layers a seasonal deep layer and a shallower one that develops during the course of each day

electromagnetic constraints on a melt region beneath the central mariana back arc spreading ridge 1 an electrical resistivity profile across the central mariana subduction system shows high resistivity in the upper mantle beneath the back arc spreading ridge where melt might be expected to exist although seismic data are equivocal on the extent of a possible melt region the question arises as to why a 2 d magnetotelluric mt survey apparently failed to image any melt we have run forward models and inversions that test possible 3 d melt geometries that are consistent with the mt data and results of other studies from the region and that we use to place upper bounds on the possible extent of 3 d melt region beneath the spreading center our study suggests that the largest melt region that was not directly imaged by the 2 d mt data but that is compatible with the observations as well as the likely effects of melt focusing has a 3 d shape on a ridge segment scale focused toward the spreading center and a resistivity of 100 m that corresponds to 0 1 1 interconnected silicate melt embedded in a background resistivity of 500 m in contrast to the superfast spreading southern east pacific rise the 3 d melt region suggests that buoyant mantle upwelling on a ridge segment scale is the dominant process beneath the slow spreading central mariana back arc a final test considers whether the inability to image a 3 d melt region was a result of the 2 d survey geometry the result reveals that the 2 d transect completed is useful to elucidate a broad range of 3 d melt bodies

the latitudinal distribution of clouds on titan clouds have been observed recently on titan through the thick haze using near infrared spectroscopy and images near the south pole and in temperate regions near 40 s recent telescope and cassini orbiter observations are now providing an insight into cloud climatology to study clouds we have developed a general circulation model of titan that includes cloud microphysics we identify and explain the formation of several types of ethane and methane clouds including south polar clouds and sporadic clouds in temperate regions and especially at 40 in the summer hemisphere the locations frequencies and composition of these cloud types are essentially explained by the large scale circulation

variations in titan s dune orientations as a result of orbital forcing abstract wind blown dunes are a record of the climatic history in titan s equatorial region through modeling of the climatic conditions associated with titan s historical orbital configurations arising from apsidal precessions of saturn s orbit we present evidence that the orientations of the dunes are influenced by orbital forcing analysis of 3 titan general circulation models gcms in conjunction with a sediment transport model provides the first direct intercomparison of results from different titan gcms we report variability in the dune orientations predicted for different orbital epochs of up to 70 although the response of the gcms to orbital forcing varies the orbital influence on the dune orientations is found to be significant across all models furthermore there is near agreement among the two models run with surface topography with 3 out of the 5 dune fields matching observation for the most recent orbital cycle through comparison with observations by cassini we find situations in which the observed dune orientations are in best agreement with those modeled for previous orbital configurations or combinations thereof representing a larger portion of the cycle we conclude that orbital forcing could be an important factor in governing the present day dune orientations observed on titan and should be considered when modeling dune evolution

implications of dune pattern analysis for titan s surface history abstract analysis of large scale morphological parameters can reveal the reaction of dunes to changes in atmospheric and sedimentary conditions over 7000 dune width and 7000 dune spacing measurements were obtained for linear dunes in regions across saturn s moon titan from images t21 t23 t28 t44 and t48 collected by the synthetic aperture radar sar aboard the cassini spacecraft in order to reconstruct the aeolian surface history of titan dunes in the five study areas are all linear in form with a mean width of 1 3 km and mean crest spacing of 2 7 km similar to dunes in the african saharan and namib deserts on earth at the resolution of cassini sar the dunes have the morphology of large linear dunes and they lack evidence for features of compound or complex dunes the large size spacing and uniform morphology are all indicators that titan s dunes are mature features in that they have grown toward a steady state for a long period of time dune width decreases to the north perhaps from increased sediment stabilization caused by a net transport of moisture from south to north or from increased maturity in dunes to the south cumulative probability plots of dune parameters measured at different locations across titan indicate there is a single population of intermediate to large sized dunes on titan this suggests that unlike analogous dunes in the namib and agneitir sand seas dune forming conditions that generated the current set of dunes were stable and active long enough to erase any evidence of past conditions

dunes on titan observed by cassini radar thousands of longitudinal dunes have recently been discovered by the titan radar mapper on the surface of titan these are found mainly within 30 of the equator in optically near infrared and radar dark regions indicating a strong proportion of organics and cover well over 5 of titan s surface their longitudinal duneform interactions with topography and correlation with other aeolian forms indicate a single dominant wind direction aligned with the dune axis plus lesser off axis or seasonally alternating winds global compilations of dune orientations reveal the mean wind direction is dominantly eastwards with regional and local variations where winds are diverted around topographically high features such as mountain blocks or broad landforms global winds may carry sediments from high latitude regions to equatorial regions where relatively drier conditions prevail and the particles are reworked into dunes perhaps on timescales of thousands to tens of thousands of years on titan adequate sediment supply sufficient wind and the absence of sediment carriage and trapping by fluids are the dominant factors in the presence of dunes

compositional and spatial variations in titan dune and interdune regions from cassini vims and radar dunes cover about 15 of titan s visible surface and represent one of the largest reservoirs of hydrocarbon solids on titan and herein we use data from the cassini spacecraft to derive constraints on the compositional and regional variability of titan s dune and interdune regions by combining spectral information from the visual and infrared mapping spectrometer vims and spatial information from synthetic aperture radar sar data using the combined datasets we extract pure infrared spectra of dune and interdune regions by extrapolating linear correlations between vims reflectance and dune area fraction calculated in each vims footprint from sar images we applied the same method using the cassini radar radiometer dataset to extract the microwave surface emissivity of the dune and interdune regions globally the dune spectra show little variation but we find that the interdune spectra exhibit several different behaviors similarly we extract from passive radiometry a mean dune emissivity of 0 98 0 01 while interdune emissivity varies from 0 86 to 0 98 we find that the interdune regions are often spectrally similar to other titan terrain units namely caladan planitia the adiri mountains and sinlap crater while the dunes are spectrally distinct from all terrain units around sinlap crater the interdune regions correspond to the dark blue vims unit the dunes could be forming on top of the ejecta or the material corresponding to the blue unit could be depositing preferentially in the interdunes areas there was one region in the belet sand sea where we were unable to extract the dune and interdune spectra and emissivities in spite of high quality data which we interpret to result from a thick sand cover in the interdune regions implying inactive or saturated dune fields however the fact that we were able to extract distinct dune and interdune spectra and emissivities in most of titan s dune fields makes a strong case for sand free interdune areas of varying composition in these regions on depths from micrometers to decimeters this would imply that the sand dunes have been active recently on geologic timescales

a 3 km atmospheric boundary layer on titan indicated by dune spacing and huygens data abstract some 20 of titan s surface is covered in large linear dunes that resemble in morphology size and spacing 1 3 km those seen on earth although gravity atmospheric density and sand composition are very different on these two worlds this coincident size scale suggests that the controlling parameter limiting the growth of giant dunes namely the boundary layer thickness andreotti et al 2009 nature 457 1120 1123 is similar we show that a 3 km boundary layer thickness is supported by huygens descent data and is consistent with results from global circulation models taking the distinctive thermal inertia and albedo of the dune sands into account while the boundary layer thickness on earth controlling dunes can vary by an order of magnitude depending on the proximity of oceans which have very different thermal properties from dry land the relative invariance of dune spacing on titan is consistent with relatively uniform thermal properties near the dunes and no prominent variation with latitude is seen

global pattern of titan s dunes radar survey from the cassini prime mission 1 we present a map of the orientation and extent of titan s sand dunes derived from the complete radar imaging dataset from the cassini prime mission the 16 000 dune segments we have mapped cover 8 of the titan s surface suggesting a total coverage of 20 are confined within 30 of the equator and show local and regional deviations of dune orientation of up to about 40 from due eastwards there is no obvious global longitudinal pattern although some divergence with latitude is apparent the most striking pattern is one of collimation by and divergence around bright and or high terrain obstacles 100 300 m high obstruct dunes when the local slope is 1 50 or steeper while slopes 1 200 or shallower cause dunes to thin out as they climb or to deviate around the obstacles

linear dunes on titan and earth initial remote sensing comparisons thousands of dunes found in cassini radar images of the equatorial regions of titan a moon around saturn are similar in size and morphology to linear dunes on earth we present remote sensing images of terrestrial analogues to the dunes on titan obtained by landsat and radar both at considerably higher resolution than are available at titan that provide information about dune landforms and processes dunes are generally dark to radar indicating smooth surfaces and signal absorbing materials but at certain incidence angles dune surfaces can reflect the radar signal and lead to a bright return linear dunes on titan and earth diverge around topographic obstacles creating teardrop patterns that indicate mean direction of wind flow and sand transport when sand supply or wind conditions change in linear dune fields such as behind an obstacle or near the margin of the dune field dunes disappear change size and spacing or change dune type these comparisons of features on titan and earth provide a better understanding of the global sand transporting wind directions sand properties and supply and the nature of the underlying substrate on titan

venus atmospheric structure and dynamics from the vega lander and balloons new results and pds archive abstract the longest lived in situ measurement platforms at venus have been the soviet vega balloons in 1985 and the only high quality pressure temperature profile in the lowest 10 km of the atmosphere is that from the vega 2 lander here we review the mission and the resultant literature and report the archival of numerical data from these investigations on the nasa planetary data system atmospheres node to facilitate their access to the community we additionally report some new results including the striking absence of a signature of the planetary boundary layer in the near surface potential temperature profile from the vega 2 lander in contrast to the well defined boundaries seen in a comparable profile at titan

thermal structure of the low velocity zone derived from laboratory and seismic investigations laboratory measurements of seismic velocities in upper mantle peridotite can be used to constrain the thermal state in the low velocity zone the measurements carried out at high temperature and high confining pressure can be extrapolated to upper mantle conditions we compare laboratory velocities with seismic observations and estimate temperatures and partial melt fractions in the low velocity zone the comparison gives 1310 c and 7 vol partial melt at 45 km depth in 0 5 my asthenosphere beneath the iceland plateau and 1450 c and 3 vol partial melt at 85 km depth in 0 20 my asthenosphere beneath the pacific ocean the laboratory results show that up to a 6 velocity drop in the low velocity zone relative to the lithospheric velocity does not necessarily require the existence of partial melt this implies that partial melt may not exist in the low velocity zone older than 5 my and 50 my in the iceland plateau and the pacific ocean respectively

seismic anisotropy in the mantle beneath an oceanic spreading centre beneath an active mid ocean ridge the mantle upwells in response to the divergence of the newly formed plates leading to high temperatures and pressure release melting below the ridge axis the width of the upwelling region and the amount of melting depend on mantle rheology1 5 but all models predict a maximum decrease in seismic velocity at the ridge axis it has also been suggested however that the alignment of anisotropic minerals by shear in the upwelling mantle will increase seismic velocity for rays travelling subvertically through the upwelling zone6 7 here we report the observation of a consistent pattern of anomalously early p wave arrival times at an array of ocean bottom seismographs deployed across the axis of the southern mid atlantic ridge p waves from distant earthquakes arrive earlier at stations near the axis than at those further away our results are consistent with a model of anisotropy in which the degree of mineral alignment is greatest directly beneath the ridge axis and significant anisotropy extends tens of kilometres from the axis

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

texture and composition of titan s equatorial sand seas inferred from cassini sar data implications for aeolian transport and dune morphodynamics the texture composition and morphology of dunes observed in the equatorial regions of titan may reflect present and or past climatic conditions determining the physio chemical properties and the morphodynamics of titan s dunes is therefore essential to understanding of the climatic and geological history of the largest moon of saturn we quantitatively derived average surface properties of dune and interdune areas texture composition from modeling of the microwave backscattered signal and monte carlo inversion of the despeckled cassini sar data over titan s three largest sand seas belet shangri la and fensal we present the first analysis of the backscatter functions extracted from despeckled sar images that cover such a large range in incidence angles including data from the beginning of the cassini mission up to its grand finale we show that dunes and interdunes have significantly different physical properties dunes are found to be more microwave absorbent than interdunes additionally potential secondary bedforms such as ripples and avalanches may have been detected providing potential evidence for currently active dunes and sediment transport our modelling shows that the interdunes have multi scale roughnesses with higher dielectric constants than the dunes which have a low dielectric constant consistent with organic sand the radar brightness of the interdunes can be explained by the presence of a shallow layer of significantly larger organic grains possibly non mobilized by the winds together our findings suggest that titan s sand seas evolve under the current multi directional wind regimes with dunes that elongate with their crests aligned in the residual drift direction

search for extractable fullerenes in clays from the cretaceous tertiary boundary of the woodside creek and flaxbourne river sites new zealand abstract when fullerenes were first discovered to form spontaneously in condensing carbon vapors kroto et al 1985 it was suggested that they might be widely distributed in the universe searches for fullerenes in meteorites see devries et al 1993 were unsuccessful but c 60 and c 70 were reported to occur on earth in samples of shungite a meta anthracite from a deposit near shunga russia buseck et al 1992 and in fulgurite a substance formed when lightning strikes certain soils or rocks daly et al 1993 the occurrence of fullerenes in shungite is particularly surprising since fullerene synthesis in the laboratory has always involved gas phase chemistry at temperatures over 1000 c such conditions may be attained during lightning strikes but shungite is believed to have formed from carbonaceous material creeping into fissures of a precambrian rock which metamorphosed under extreme pressures if the original carbonaceous material did not already contain fullerenes perhaps from wildfires they must have formed during the metamorphism by as yet unknown solid or liquid phase mechanisms

spectroscopy morphometry and photoclinometry of titan s dunefields from cassini vims fine resolution 500 m pixel cassini visual and infrared mapping spectrometer vims t20 observations of titan resolve that moon s sand dunes the spectral variability in some dune regions shows that there are sand free interdune areas wherein vims spectra reveal the exposed dune substrate the interdunes from t20 are variously materials that correspond to the equatorial bright 5 m bright and dark blue spectral units our observations show that an enigmatic dark red spectral unit seen in t5 in fact represents a macroscopic mixture with 5 m bright material and dunes as its spectral endmembers looking more broadly similar mixtures of varying amounts of dune and interdune units of varying composition can explain the spectral and albedo variability within the dark brown dune global spectral unit that is associated with dunes the presence of interdunes indicates that titan s dunefields are both mature and recently active the spectrum of the dune endmember reveals the sand to be composed of less water ice than the rest of titan various organics are consistent with the dunes measured reflectivity we measure a mean dune spacing of 2 1 km and find that the dunes are oriented on the average in an east west direction but angling up to 10 from parallel to the equator in specific cases where no interdunes are present we determine the height of one set of dunes photoclinometrically to be between 30 and 70 m these results pave the way for future exploration and interpretation of titan s sand dunes

giant aeolian dune size determined by the average depth of the atmospheric boundary layer sand dunes common on earth and other planetary bodies still hold some mysteries the ways that small dunes form those with lengths of tens of metres are well understood but much less clear is how giant kilometre scale dunes come into existence what dictates their shapes and what limits their size andreotti et al use a combination of field measurements and aerodynamic calculations to show that the growth of giant dunes is limited by the confinement of the sand transport flow over the average depth of the atmospheric boundary layer the new findings can explain the mean spacing of wind generated giant dunes up to 3 5 km in length this paper shows that the growth of aeolian giant dunes ascribed to the non linear interaction between small scale superimposed dunes is limited by the confinement of the flow over the average depth of the atmospheric boundary layer the findings explain the mean spacing of aeolian giant dunes ranging from 300 metres in coastal deserts to 3 5 kilometres depending on the wind regime1 2 sand dunes exhibit linear3 4 crescent shaped or star like forms5 resulting from the interaction between dune morphology and sand transport6 7 8 small scale dunes form by destabilization of the sand bed9 10 11 with a wavelength a few tens of metres determined by the sand transport saturation length11 12 13 the mechanisms controlling the formation of giant dunes and in particular accounting for their typical time and length scales have remained unknown using a combination of field measurements and aerodynamic calculations we show here that the growth of aeolian giant dunes ascribed to the nonlinear interaction between small scale superimposed dunes4 10 14 15 is limited by the confinement of the flow within the atmospheric boundary layer16 17 aeolian giant dunes and river dunes form by similar processes with the thermal inversion layer that caps the convective boundary layer in the atmosphere18 acting analogously to the water surface in rivers in both cases the bed topography excites surface waves on the interface that in turn modify the near bed flow velocity this mechanism is a stabilizing process that prevents the scale of the pattern from coarsening beyond the resonant condition our results can explain the mean spacing of aeolian giant dunes ranging from 300 m in coastal terrestrial deserts to 3 5 km we propose that our findings could serve as a starting point for the modelling of long term evolution of desert landscapes under specific wind regimes

seismic attenuation tomography of the mariana subduction system implications for thermal structure volatile distribution and slow spreading dynamics 1 the anelastic structure of a subduction zone can place first order constraints on variations in temperature and volatile content we investigate seismic attenuation across the western pacific mariana subduction system using data from the 2003 2004 mariana subduction factory imaging experiment this 11 month experiment consisted of 20 broadband stations deployed on the arc islands and 58 semibroadband ocean bottom seismographs deployed across the fore arc island arc and back arc spreading center we compute amplitude spectra for p and s arrivals from local earthquakes and invert for the path averaged attenuation for each waveform along with the seismic moment and corner frequency for each earthquake additionally we investigate earthquake source parameter assumptions and frequency dependent exponents ranging from 0 to 0 6 tomographic inversion of nearly 3000 t estimates at 0 27 for 2 d qp 1 and qp qs structure shows a 75 km wide columnar shaped high attenuation anomaly with qp 43 60 beneath the spreading center that extends from the uppermost mantle to 100 km depth a weaker high attenuation region qp 56 70 occurs at depths of 50 100 km beneath the volcanic arc and the high attenuation regions are connected at depths of 75 125 km the subducting pacific plate is characterized by low attenuation at depths greater than 100 km but high attenuation is found in the plate between 50 and 100 km depth the fore arc shows high attenuation near the volcanic arc and beneath the serpentinite seamounts in the outer fore arc qs structure is less well resolved than qp because of a smaller data set but qp qs ratios are significantly less than 2 throughout the study region as temperatures estimated from qs 1 are unusually high we interpret the arc and wedge core anomalies as regions of high temperature with enhanced q 1 due to hydration and or melt the slab and fore arc anomalies as indicative of slab derived fluids and or large scale serpentinization and the columnar shaped high qp 1 anomaly directly beneath the back arc spreading center as indicative of a narrow region of dynamic upwelling and melt production beneath the slow spreading ridge axis

gravity anomalies and crustal thickness variations along the mid atlantic ridge between 33 n and 40 n multibeam bathymetry and gravity data have been obtained along an 800 km long section of the mid atlantic ridge from just south of the hayes fracture zone at 33 n to the northern edge of the azores platform near 40 n a three dimensional analysis of these gravity and topography data combined with results from earlier seismic refraction studies in this area reveal two different scales of crustal heterogeneity a systematic along axis segment scale 8 9 km near the middle of spreading segments and the thinnest crust 3 4 km near segment offsets the magnitude of this along axis variation in crustal thickness is proportional to the length of the spreading segment and the size of the adjacent ridge offset there also is a distinct asymmetry in crustal structure across the rift valley near large segment offsets with gravity highs inferred to be thinner crust beneath the inside corner highs adjacent to these offsets this segment scale crustal heterogeneity is similar to that reported from the kane to atlantis section of the mid atlantic ridge and from parts of the intermediate spreading southern mid atlantic ridge it is superimposed on a second longer wavelength variation in gravity and crustal thickness associated with the azores hot spot the most pronounced effect of the azores hot spot on the mid atlantic ridge occurs between 38 n and 40 n where the ridge axis rapidly shoals by more than 1000 m the crust thickens by over 2 km and the rift valley largely disappears the absence of a deep axial rift valley on the azores platform and near the midpoints of some ridge segments along the mid atlantic ridge to the south agrees well with the predictions of recent ridge crest thermal and rheological models that suggest a dependence of axial morphology on both crustal thickness and spreading rate the transition from a rift valley to an axial high morphology at these spreading rates 11 mm yr half rate occurs at a crustal thickness of about 9 1km

partial melting beneath a mid atlantic ridge segment detected by teleseismic pkp delays an array of six ocean bottom seismometers deployed across a segment of the mid atlantic ridge recorded pkp phases from four teleseismic earthquakes after correcting for normal moveout and elevation differences there is a systematic variation of anomalous travel time with distance from the ridge stations at the ridge axis are slowest the average anomaly at a station 16 km from the axis is 0 36 s relative to the axial stations most of this anomaly is probably caused by the presence of partial melt beneath the ridge axis perhaps concentrated primarily in the upper 30 km of the mantle the rapid variation across axis suggests that the melt is either associated with a narrow zone of buoyant upwelling or a region of veins and dikes forming a narrow transport zone connecting a deep melt production region to the crust

insights into titan s geology and hydrology based on enhanced image processing of cassini radar data the cassini synthetic aperture radar has been acquiring images of titan s surface since october 2004 to date 59 of titan s surface has been imaged by radar with significant regions imaged more than once radar data suffer from speckle noise hindering interpretation of small scale features and comparison of reimaged regions for change detection we present here a new image analysis technique that combines a denoising algorithm with mapping and quantitative measurements that greatly enhance the utility of the data and offers previously unattainable insights after validating the technique we demonstrate the potential improvement in understanding of surface processes on titan and defining global mapping units focusing on specific landforms including lakes dunes mountains and fluvial features lake shorelines are delineated with greater accuracy previously unrecognized dissection by fluvial channels emerges beneath shallow methane cover dune wavelengths and interdune extents are more precisely measured a significant refinement in producing digital elevation models is shown interactions of fluvial and aeolian processes with topographic relief is more precisely observed and understood than previously benches in bathymetry are observed in northern sea ligeia mare submerged valleys show similar depth suggesting that they are equilibrated with marine benches these new observations suggest a liquid level increase in the northern sea which may be due to changes on seasonal or longer timescales

a scaling law for aeolian dunes on mars venus earth and for subaqueous ripples abstract the linear stability analysis of the equations governing the evolution of a flat sand bed submitted to a turbulent shear flow predicts that the wavelength at which the bed destabilises to form dunes should scale with the drag length l drag s f d this scaling law is tested using existing and new measurements performed in water subaqueous ripples in air aeolian dunes and fresh snow dunes in a high pressure co2 wind tunnel reproducing conditions close to venus atmosphere and in the low pressure co2 martian atmosphere martian dunes the difficulty is to determine the diameter of saltating grains on mars a first estimate comes from photographs of aeolian ripples taken by the rovers opportunity and spirit showing grains whose diameters are smaller than on earth dunes in addition we calculate the effect of cohesion and viscosity on the dynamic and static transport thresholds it confirms that the small grains visualised by the rovers should be grains experiencing saltation finally we show that within error bars the scaling of with ldrag holds over almost five decades we conclude with a discussion on the time scales and velocities at which these bed instabilities develop and propagate on mars

field evidence for surface wave induced instability of sand dunes barchans are crescent shaped dunes that move faster over desert surfaces than most other dune types a three year field study shows how they do it collisions and changes in wind direction destabilize the dunes generating surface waves that form new smaller barchans this prevents the formation of a single giant dune so plays a fundamental role in the development of dune patterns field studies of barchans crescent shaped dunes that propagate over solid ground under conditions of unidirectional wind1 have long focused on the investigation of an equilibrium between sand transport by wind and the control of air flow by dune topography2 3 4 which are thought to control dune morphology and kinematics5 6 7 because of the long timescale involved however the underlying dynamic processes responsible for the evolution of dune fields remain poorly understood8 here we combine data from a three year field study in the moroccan sahara with a model study to show that barchans are fundamentally unstable and do not necessarily behave like stable solitary waves as suggested previously9 10 11 12 we find that dune collisions and changes in wind direction destabilize the dunes and generate surface waves on the barchans because the resulting surface waves propagate at a higher speed than the dunes themselves they can produce a series of new barchans of elementary size by breaking the horns of large dunes the creation of these new dunes provides a mechanism for sand loss that prevents dune fields from merging into a single giant dune and therefore plays a fundamental role in the control of size selection and the development of dune patterns

methane storms as a driver of titan s dune orientation titan s equatorial dunes propagate eastwards whereas titan s surface winds blow towards the west atmospheric simulations suggest that tropical methane storms generate strong eastward gusts that may dominate sand transport on titan s surface

crustal thickness on the mid atlantic ridge bull s eye gravity anomalies and focused accretion spreading segments of the mid atlantic ridge show negative bull9s eye anomalies in the mantle bouguer gravity field seismic refraction results from 33 s indicate that these anomalies can be accounted for by variations in crustal thickness along a segment the crust is thicker in the center and thinner at the end of the spreading segment and these changes are attributable to variations in the thickness of layer 3 the results show that accretion is focused at a slow spreading ridge that axial valley depth reflects the thickness of the underlying crust and that along axis density variations should be considered in the interpretation of gravity data