physical processes affecting circulation and hydrography in the sable gully of nova scotia abstract the sable gully is the largest submarine canyon along the shelf break off the east coast of north america the circulation and hydrography in the gully have significant temporal and spatial variability this paper presents a numerical study of the three dimensional circulation and hydrography in the gully using a multi nested ocean circulation model the model is forced by tides wind stress and surface heat freshwater fluxes model results are in fair agreement with the current and hydrographic observations made in the gully in 2006 and 2007 a process study is conducted to examine the main physical processes affecting the circulation and hydrography including tide topography interaction wind forcing and the shelf scale circulation over the eastern canadian shelf the model results demonstrate that the circulation and hydrography above the canyon rim are influenced significantly by wind particularly during storm events while the subsurface flow over the shelf slope is affected by the shelf scale circulation there is also significant tide topography interaction inside the gully

remote sensing of desert dune forms by polarimetric synthetic aperture radar sar abstract this report describes remote sensing experiments using highly capable polarimetric synthetic aperture radar sar in preparation for the launch of the sir c x sar mission nasa s jpl airborne polarimetric sar airsar was deployed over many locations worldwide the report describes sar observations of desert dune fields in north america bolivia and australia acquired by airsar and in namibia acquired by sir c x sar the use of polarimetric and multiwavelength sar is reviewed and comments on the advantages of the various combinations are noted sar data are very useful in mapping dunes because of the ability to control illumination parameters such as look angle wavelength and polarization furthermore the unique responses of sar to surface roughness are found very useful in mapping dune forms for most sites the longer wavelengths p and l band at 68 cm and 24 cm are advantageous over the shorter c band at 5 6 cm providing better contrast overall copolarized channels hh and vv are found best for mapping the dunes and dune types when trying to discriminate active from inactive dunes cross polarized channels provide essential information on vegetation

a tidal model for eastern juan de fuca strait and the southern strait of georgia a three dimensional barotropic finite element model is used to calculate the tidal flows in eastern juan de fuca strait and the southern strait of georgia the harmonics of eight constituents are computed and compared with those from previous finite difference models and those from historical tide gauge and current meter observations root mean square differences between observed and calculated sea level amplitudes are within 2 0 cm for all constituents and the phases are within 4 0 for all constituents except k2 horizontal currents from the model are found to reproduce the observed vertical variations in shear except in regions where stratification effects and internal tides exist in particular the model representation of currents at six stations in the canadian tide and current tables volume 5 is accurate the pattern of tidal residual currents has much more detail than previous coarser resolution models and numerous eddies are predicted computed energy flux fields reveal that over a 29 day period only 38 of the tidal power entering juan de fuca strait is transmitted into the southern strait of georgia and of the 39 entering haro strait 36 is dissipated within the strait itself

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

sand dune attributes estimated from sar images abstract dunes such as linear dunes oriented in a constant direction their size and interdune spacing are very regular in the deserts therefore they manifest a periodic pattern in terms of spatial distribution there are several modulation transfer functions between sar images and the actual periodic surface parameters and the image brightness is sensitive to the local incidence angle according to previous studies a linear mapping process can describe these modulation processes this study proposes an algorithm for estimating the linear dune features including linear dune direction the prevailing wind direction interdune spacing and rms slope and dune height from sar images to estimate rms slope two algorithms were developed a local incidence angle method and a modulation transfer function method the linear dune height was then calculated based on the rms slope of the surface finally the obtained results were compared with the truth data from field investigations the estimated physical quantities did not completely agree with the truth data and possible error sources are discussed

observations of the internal tide in monterey canyon abstract data from two shipboard experiments in 1994 designed to observe the semidiurnal internal tide in monterey canyon reveal semidiurnal currents of about 20 cm s 1 which is an order of magnitude larger than the estimated barotropic tidal currents the kinetic and potential energy evidenced by isopycnal displacements of about 50 m was greatest along paths following the characteristics calculated from linear theory these energy ray paths are oriented nearly parallel to the canyon floor and may originate from large bathymetric features beyond the mouth of monterey bay energy propagated shoreward during the april experiment itex1 whereas a standing wave that is an internal seiche was observed in october itex2 the difference is attributed to changes in stratification between the two experiments higher energy levels were present during itex1 which took place near the spring phase of the fortnightly 14 8 days cycle in sea level while itex2 occurred close to the neap phase further evide

linear dune pattern variability in the vicinity of dry valleys in the southwest kalahari linear sand dunes in the southwest kalahari dunefield have been widely observed to change pattern size and orientation in the vicinity of river valleys yet few explanations for this change have been put forward this paper identifies and describes four main associations between dunes and valleys which occur in the southwest kalahari i dunes which adjoin the valley on both sides with no discernible change in pattern or orientation ii dunes which adjoin the valley and change pattern iii dunes which adjoin the upwind valley flank but not the downwind side iv dune free zones on both sides of the valley the two main factors which influence the evolution of these associations are identified as the effect of the valley upon sediment supply and the effect of the valley upon wind regime our current state of knowledge about the potential geomorphological effects of these two variables is evaluated with reference to each association described

mean circulation and high frequency flow amplification in the sable gully abstract the sable gully a broad shelf break submarine canyon approximately 40 km east of sable island on the eastern scotian shelf separates banquereau and sable island banks unique among canyons on the eastern canadian continental shelf because of its depth steep slopes and extension far onto the shelf its ecological significance and increasing human pressures led to its designation in 2004 under canada s oceans act as the first marine protected area mpa in the atlantic region to improve the state of knowledge of the gully mpa a multi disciplinary field program was carried out in 2006 07 the physical oceanographic component consisted of the deployment april 2006 and recovery august 2007 of four current meter moorings and ctd surveys analysis of this 16 month mooring deployment demonstrates that the mean circulation above the canyon rim 200 m is characterized by a southwestward flow that appears unaffected by the canyon topography there is also some indication of the existence of an eddy at rim depth below 500 m the circulation is dominated by an upcanyon flow of order 0 02 m s 1 at the mooring array halfway between the canyon head and mouth the mean 200 m bottom transport towards the head of the gully was estimated as 35 500 m3 s 1 implying an upwelling velocity of 1 7 10 4 m s 1 14 m d 1 over the area results also show bottom intensified tidal flows and non linear constituents due to the interaction of k1 o1 m2 and s2 components along the thalweg of the canyon the strong overtides and compound tides observed in the gully make it unique among canyons further analyses provide evidence of enhanced mixing in the gully kv 180 10 4 m2 s 1 which is approximately 20 times that observed on the adjoining scotian shelf total variance of the currents in the gully is about 2 5 times greater than that observed on the nearby continental slope with an equivalent water depth

dynamic processes acting on a longitudinal seif sand dune abstract field measurements were made on a longitudinal dune in the sinai desert in order to understand its morphology and dynamics the field measurements contradicted the wind structure indicated by the helicoidal flow theory rather it was found that winds coming from two basically different directions at different times and striking the dune obliquely were responsible for sand transport and erosion or deposition along the lee flank the essence of this mechanism is the deflection of the wind airflow on the lee flank of the dune to a direction parallel to the crest line the occurrence of erosion or deposition depends upon the angle of incidence between the wind and the crest line when this angle is 40 the velocity of the deflected wind drops and deposition takes place on the lee flank the angle of incidence in each wind storm is changed intermittently between 30 and 100 along the dune because the dune meanders and because of the sinuous outline of the crest line in this manner sand transport and erosion or deposition occurs along the lee flank depending on the angle of incidence between the wind and the crest line as a result of the deflection of the wind the dune elongates at an average rate of more than 1 m per month peaks and saddles along the crest line advance at an average rate of 0 7 m per month the lack of uniformity in the effects of the wind on both sides of the dune creates a lack of uniformity in the rate of erosion and deposition this can explain the formation of peaks along the crest line of the dune

resonant amplification of subinertial tides in a submarine canyon 1 tidal oscillations dominate the flow field in many submarine canyons observations have shown that semidiurnal tidal energy in submarine canyons is significantly amplified with respect to adjacent shelves this amplification is thought to be caused by focusing of propagating internal tides incident from the open ocean or local in canyon generation on critical canyon floor slopes these mechanisms require freely propagating internal tides with superinertial frequencies we present results from a moored array in a canyon at 44 n where the observed velocities reached over 0 8 m s 1 the canyon flow field was highly unusual because it was dominated by the subinertial diurnal tide this occurred despite the fact that the barotropic tide was predominantly semidiurnal the diurnal tide was dramatically amplified in the canyon its velocities increasing toward the seafloor and canyon head the diurnal oscillations also exhibited marked modulation in time by the background barotropic forcing length scales suggest that the diurnal tide should be resonant in the canyon an analytical framework is used to explain the mechanisms behind the strong diurnal currents observed by the moored array in the model along shelf barotropic flow sets up a double kelvin wave response in the canyon generating along canyon velocities which are subsequently amplified by resonance the pattern of the model predictions is in excellent agreement with the observed velocity pattern

bottom trapped subinertial motions over midocean ridges in a stratified rotating fluid abstract linear analytical solutions for bottom trapped subinertial oscillatory flow over simple ridge topographies in a stratified two layer rotating fluid are presented results are compared to moored current meter observations of bottom intensified motions over the endeavour segment of juan de fuca ridge in the northeast pacific the solutions reproduce many of the observed features including preferential amplification of the clockwise rotary component of velocity over the ridge and increased velocity amplification with proximity to the ridge crest for a given internal deformation radius the degree of current amplification increases with increased bottom slope ridge height and oscillation frequency amplification decreases with increased width of the ridge relative to the deformation radius

upwelling flow dynamics in long canyons at low rossby number 1 submarine canyons topographic features incising the continental slope vary in both shape and size the dynamics of short canyons have been observed and described in the field in the laboratory and with numerical simulations flow within long canyons such as juan de fuca canyon located between vancouver island and washington state in the pacific northwest is less well understood physical models of both long and short canyons have been constructed to understand the upwelling dynamics in long canyons and how upwelling changes as compared with the dynamics of short canyons at low rossby number stratification and rotation both important parameters in determining the dynamics in canyons can be controlled and scaled accordingly for replication of oceanic conditions the physical model is spun up to an initial rotation rate and the flow is forced by increasing the rotation rate over the equivalent of several days flow visualization is used to determine the strength and location of upwelling the strength and mechanisms generating vorticity as well as the differences between the flow within the long and short canyons the pattern of upwelling between the two canyons is significantly different in the horizontal with upwelling occurring through the canyon head in the short canyon and upwelling occurring close to the mouth along the downstream rim in the long canyon at high rossby number upwelling is similar in both the long and short canyon and is driven by advection however as rossby number decreases the flow in the long canyon is more strongly affected by the strong convergence of the isobaths near the canyon than by advection alone

the effects of boundary turbulence on canyon flows forced by periodic along shelf currents previous laboratory experiments and associated numerical models of laminar flows forced by oscillatory along shelf background currents are extended to include some of the effects of boundary generated turbulence the experiments are conducted in the 13 m diameter rotating flow facility in grenoble france two pairs of case studies one at a large forcing velocity designated as fast for which the boundary layers are fully turbulent during part of the flow cycle and one at relatively smaller forcing slow for which transitional boundary layers are operative at the higher speeds of the background flow are conducted smooth and artificially roughened boundaries are considered respectively for each of these pairs phase averaged and time mean velocity vertical vorticity and horizontal divergence fields are found to be qualitatively similar to those of previous laminar experiments the similarities in the time mean fields are that i within the canyon they are dominated by cyclonic vorticity with maxima centered near the shelf break ii within and in the vicinity of the canyon the general circulation pattern includes a net transport into the canyon through its mouth a net upwelling in the canyon interior a transport away from the canyon over the shelf break along both sides of the canyon and by inference a return flow to the deep ocean and iii the interior time mean flow is characterized by a well defined coastal current whose axis follows the shelf in the vicinity of the shelf break with the coast on the right it is found that the measurements of the characteristic speed of the residual or time mean flow within the canyon for the transitional and fully turbulent experiments do not follow the scaling law derived earlier for laminar experiments an alternative scaling analysis for large reynolds number flows is thus derived although sufficient numbers of experiments are not available to test the hypothesis fully the measurements available for the fully turbulent flows are consistent with the theory advanced

observations and linear analysis of sill generated internal tides and estuarine flow in haro strait 1 current meter records from two sets of observations 20 years apart in haro strait british columbia canada are analyzed haro strait is a deep channel separated from larger bodies of water on either side by relatively shallow sills the estuarine flow in this region is in approximate geostrophic balance and is apparently unaffected by the tidally driven spring neap cycle in vertical stratification strong baroclinic variability with an amplitude 1 m s 1 close to the amplitude of the barotropic tide is present a linear theory for internal tides in silled basins is developed using no bottom flow boundary conditions to represent sill effects by comparison with observations it is concluded that an internal tide is generated at the seaward sill and propagates inshore although local damping appears to be weak no return signal is found from the inshore sill suggesting that either dissipation is strong in that area or that the internal tide is transmitted across that sill

solution convergence of flow over steep topography in a numerical model of canyon upwelling 1 the convergence as resolution is increased is examined for a numerical model simulation of upwelling over a continental shelf canyon a series of idealized one and two dimensional models are used to plot the dependence of bottom ekman layer velocity structure and transport on model resolution using these results as a guide a bottom ekman layer resolving three dimensional numerical model is constructed this model is used to simulate a laboratory model of canyon upwelling alongshore velocity volume of fluid upwelled through the canyon onto the shelf and change in depth of the canyon water are examined at three grid resolutions numerical model particle trajectories are compared with a laboratory model of canyon upwelling to validate the model the results suggest that current computational cluster power is sufficient to accurately simulate all aspects of upwelling in a steep canyon with the exception of flow separation from the canyon wall

the internal tide and resonance in indian arm british columbia the nature of the internal tide in indian arm is discussed in the winter of 1983 1984 this internal tide was observed to change from a predominantly m2 internal response to a predominantly k1 one this change in the response is explained as a partial resonance response of the system during the 1983 1984 winter the resonance period steadily increased from 14 hours at the start to 22 hours at the end it is suggested that the enhanced internal response at the k1 frequency late in the winter is due to resonance the fitting of normal modes was done to look at the energy flux in indian arm about 20 30 of the energy flux is found to return from the head of the inlet a finding supporting the resonance hypothesis which requires energy to be reflected from the head

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

physical modeling of tidal resonance in a submarine canyon current observations in submarine canyons poleward of 30 are usually dominated by the semidiurnal m2 tidal frequency which is superinertial at these latitudes observations from a submarine canyon at 44 n the gully nova scotia suggest that canyons can be dominated by the subinertial k1 tidal frequency if length scales are correct for resonance of the k1 frequency a model of the gully was constructed in a tank on a rotating table and tidal currents generated to determine factors that influence resonance resonance curves were fit to measurements from the laboratory canyon for a range of stratifications background rotation rates and forcing amplitudes dense water was observed upwelling onto the continental shelf on either side of the laboratory canyon and traveled at least one canyon width along the shelf friction values measured in the laboratory were much higher than expected probably due to upwelled water surging onto the shelf on each tidal cycle similar to a tidal bore by scaling observations from the laboratory to the ocean and assuming friction in the ocean is also affected by water traveling onto the shelf a resonance curve for the ocean was created because of the broad resonance curve the diurnal tide remains strong year round at the gully even as stratification at the shelf break changes dense water surging onto the shelf on tidal frequencies may affect friction and mixing at other nonresonant canyons

late neanderthals at jarama vi central iberia previous geochronological and archaeological studies on the rock shelter jarama vi suggested a late survival of neanderthals in central iberia and the presence of lithic assemblages of early upper paleolithic affinity new data on granulometry mineralogical composition geochemical fingerprints and micromorphology of the sequence corroborate the previous notion that the archaeological units jvi 2 1 to jvi 2 3 are slackwater deposits of superfloods which did not experience significant post depositional changes whereas the artifact rich units jvi 3 and jvi 1 mainly received sediment inputs by sheetwash and cave spall new ams radiocarbon measurements on three samples of cut marked bone using the ultrafiltration technique yielded ages close to or beyond the limit of radiocarbon dating at ca 50 14c ka bp and hence suggest much higher antiquity than assumed previously furthermore elevated temperature post ir irsl luminescence measurements on k feldspars yielded burial ages for subunits jvi 2 2 and jvi 2 3 between 50 and 60 ka finally our reappraisal of the stone industry strongly suggests that the whole sequence is of mousterian affinity in conclusion jarama vi most probably does not document a late survival of neanderthals nor an early upper paleolithic occupation in central iberia but rather indicates an occupation breakdown after the middle paleolithic

applications of spaceborne radar laboratory data to the study of aeolian processes aerodynamic roughness z0 is an important parameter in studies of sand and dust transport as well as atmospheric circulation models aerodynamic roughness is a function of the size and spacing of surface roughness elements and is typically determined at point locations in the field from wind velocity profiles because field measurements require complex logistics z0 values have been obtained for very few localities if radar can be used to map z0 estimates can be obtained for large areas in addition because aerodynamic roughness can change in response to surface processes e g flooding of alluvial surfaces radar remote sensing could obtain new measurements on short timescales both z0 and the radar backscatter coefficient 0 are dependent on topographic roughness at the submeter scale and correlation between these two parameters was developed based on radar data obtained from aircraft airsar the spaceborne radar laboratory srl afforded the opportunity to test the correlation for data obtained from orbit srl data for sites in death valley california lunar lake nevada and gobabeb namibia were correlated with wind data and compared with previous radar z0 relations correlations between 0 and z0 for l band 24 cm hv h vertically and v vertically polarized modes l band hh and c band 5 6 cm hv compare favorably with previous studies based on these results maps of z0 values were derived from srl data for each site demonstrating the potential to map z0 for large vegetation free areas from orbit using radar systems

tidal resonance in juan de fuca strait and the strait of georgia abstract the resonant period and quality factor q are determined for the semienclosed sea comprising juan de fuca strait puget sound and the strait of georgia the observed tidal elevation gain and phase change from the pacific ocean to this inland sea are fitted to the predictions of simple analytic models which give a resonant period of 17 21 h and a q of about 2 the low q value indicative of a highly dissipative system is consistent with the need for numerical models for the area to employ large bottom friction coefficients these include the effects of form drag

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

spin up and the effects of a submarine canyon applications to upwelling in astoria canyon 1 a parameterization for the on shelf mass flux induced by upwelling through a shelf break submarine canyon is estimated by laboratory spin up experiments we determine the effects of a submarine canyon on flow evolution implicitly by measuring the topographic drag force in the context of a heuristic model trials were performed across a range of values for the shelf break velocity coriolis frequency and buoyancy frequency assuming the drag force within the canyon is balanced locally by rotation we propose a parameterization for upwelling through a canyon provided that the coriolis frequency buoyancy frequency shelf break velocity and canyon dimensions at the shelf break depth are known we use our results to compare wind forced and canyon forced upwelling over astoria canyon off the coast of washington state the analysis suggests that canyon forced upwelling through astoria canyon is of equal importance to wind forced upwelling directly above it on seasonal scales

resonant modulation of the flow in a tidal channel 1 the coupling between a quarter wave resonance in a coastal bay and a helmholtz mode in an adjacent cove connected to the bay through a narrow channel is investigated by comparing field measurements to analytical and numerical model predictions pressure and velocity spectra from locations throughout the bay reveal an oscillation with a period of approximately 1 hour consistent with a quarter wave seiche mode the associated sea surface displacements throughout the bay are small less than 5 cm rms i e only 10 of the tidal elevation velocities within the channel are significantly modulated in the 1 hour band with amplitudes up to 40 of the peak tidal current the analytical model shows that the modulation of the channel flow results from the interaction between the quarter wave mode in the main basin and a helmholtz resonance in the cove also with a period near 1 hour the amplitude and phase of the 1 hour oscillation varies through the tidal cycle because of the change in helmholtz frequency with tidal elevation good quantitative agreement between the data and the model predictions is obtained if a drag coefficient approximately 3 4 times larger than the classical value of 3 x 10 3 is used in the channel and cove

internal hydraulic jumps and overturning generated by tidal flow over a tall steep ridge abstract recent observations from the hawaiian ridge indicate episodes of overturning and strong dissipation coupled with the tidal cycle near the top of the ridge simulations with realistic topography and stratification suggest that this overturning has its origins in transient internal hydraulic jumps that occur below the shelf break at maximum ebb tide and then propagate up the slope as internal bores when the flow reverses a series of numerical simulations explores the parameter space of topographic slope barotropic velocity stratification and forcing frequency to identify the parameter regime in which these internal jumps are possible theoretical analysis predicts that the tidally driven jumps may occur when the vertical tidal excursion is large which is shown to imply steep topographic slopes such that dh dxn 1 the vertical length scale of the jumps is predicted to depend on the flow speed such that the jump froude number is of order unity the numerical results agree with the theoreti

observation of internal tidal currents in the kaoping canyon off southwestern taiwan abstract internal tidal currents and associated water mass displacements were investigated during multiple cruises in the kaoping submarine canyon off southwestern taiwan observations from both moored and shipboard acoustic doppler current profilers and hydrographic casts were conducted along the canyon the velocity data showed that in the lower layer the major axis of the tidal currents aligned with the orientation of the canyon and currents moved up canyon during flood and down canyon during ebb the vertical phase shift and amplitude of the currents indicated that the semidiurnal internal tide dominated with intensity increasing with depth toward the canyon head tidal energy was channeled from the shelf landward with a beamlike internal wave guided by bottom topography the estimated phase velocity was 1 4 1 7 m s 1 based on normal mode analysis and the phase lag between sampling stations empirical orthogonal function analysis of hydrographic profiles confirmed that the first mode m 2 internal tide explained 70 of the total variance the strong convergence of internal tidal currents near the canyon head during flood may play an important role in the daily migration of cherry shrimps which burrow along the canyon wall

resonant diurnal internal tides in the north atlantic using a large acoustical array located midway between puerto rico and bermuda we have observed enhanced diurnal tidal signals associated with the lowest internal wave mode these signals result from a diurnal internal wave resonantly trapped between the shelf just north of puerto rico and the turning latitude a distance of about 1100 km the data obtained using the large acoustical array are consistent with the predicted airy function variation with latitude of diurnal internal waves near the turning latitude the existence of this wave is a striking demonstration of the long spatial and temporal coherence of oceanic internal tides in general the energy radiated by such waves is a loss of energy from the earth moon system and a source of energy for mixing in the deep ocean

titan s inventory of organic surface materials 1 cassini radar observations now permit an initial assessment of the inventory of two classes presumed to be organic of titan surface materials polar lake liquids and equatorial dune sands several hundred lakes or seas have been observed of which dozens are each estimated to contain more hydrocarbon liquid than the entire known oil and gas reserves on earth dark dunes cover some 20 of titan s surface and comprise a volume of material several hundred times larger than earth s coal reserves overall however the identified surface inventories 3 104 km3 of liquid and 2 105 km3 of dune sands are small compared with estimated photochemical production on titan over the age of the solar system the sand volume is too large to be accounted for simply by erosion in observed river channels or ejecta from observed impact craters the lakes are adequate in extent to buffer atmospheric methane against photolysis in the short term but do not contain enough methane to sustain the atmosphere over geologic time unless frequent resupply from the interior buffers this greenhouse gas at exactly the right rate dramatic climate change on titan is likely in its past present and future

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

cross shelf exchange driven by oscillatory barotropic currents at an idealized coastal canyon abstract numerical simulations are used to study on shelf transport of dense water by oscillatory barotropic currents incident upon an isolated coastal canyon the physical system is a laboratory scale annulus in which forcing is provided by an oscillatory modulation in the rotation rate and for which the external nondimensional parameters match those from an appropriate oceanic analog the numerical simulations were conducted for comparison with a companion set of laboratory experiments and these prior studies have shown good qualitative and quantitative agreement between the two here the numerical simulations are examined in further detail to determine the three dimensional structure of the time mean currents and density field on the shelf to quantify the resulting onshore transport of dense water and to expose the underlying nondimensional parameter dependencies the incident barotropic currents produce in equilibrium an extensive pool of anomalously dense fluid on the shelf surrounding the canyo

multifrequency and multipolarization radar scatterometry of sand dunes and comparison with spaceborne and airborne radar images understanding the unusual radar scattering characteristics of sand dunes is necessary in the analysis of radar images of aeolian landscapes of the earth and of other planets in this paper we report on airborne radar scatterometer data of sand dunes acquired at multiple frequencies and polarizations radar backscatter from sand dunes is very sensitive to the imaging geometry at small incidence angles the radar return is mainly due to quasi specular reflection from dune slopes favorably oriented toward the radar a peak return usually occurs at the incidence angle equal to the angle of respose for the dunes the peak angle is the same at all frequencies as computed from specular reflection theory at larger angles the return is significantly weaker the scatterometer measurements verified observations made with airborne and spaceborne radar images acquired over a number of dune fields in the united states central africa and the arabian peninsula the imaging geometry constraints indicate that possible dunes on other planets such as venus will probably not be detected in radar images unless the incidence angle is less than the angle of repose of such dunes and the radar look direction is approximately orthogonal to the dune trends