spatial variation of radar derived basal conditions on kamb ice stream west antarctica radar profiles of bed echo intensity can survey conditions at the ice bed interface and test for the presence or absence of water however extracting information about basal conditions from bed echo intensities requires an estimate of the attenuation loss through the ice we used the relationship between bed echo intensities from constant offset radar data and ice thickness to estimate depth averaged attenuation rates at several locations on and near kamb ice stream kis west antarctica we found values varying from 29 db km 1 at siple dome to 15 db km 1 in the main trunk region of kis in agreement with a previous measurement and models using these attenuation rate values we calculated the relative bed reflectivity throughout our kis surveys and found that most of the bed in the trunk has high basal reflectivities similar to those obtained in the location of boreholes that found water at the bed areas of lower bed reflectivity are limited to the sticky spot where a borehole found a dry bed and along the margins of kis these results support previous hypotheses that the basal conditions at locations like the sticky spot on kis control its stagnation and possible reactivation

analysis of cassini cirs limb spectra of titan acquired during the nominal mission i hydrocarbons nitriles and co2 vertical mixing ratio profiles observations of the composite infrared spectrometer cirs during the entire nominal cassini mission 2004 2008 provide us with an accurate global view of composition and temperature in the middle atmosphere of titan between 100 and 500 km we investigated limb spectra acquired at 0 5 cm 1 resolution at nine different latitudes between 56 s and 80 n with a better sampling in the northern hemisphere where molecular abundances and temperature present strong latitudinal variations from this limb data acquired between february 2005 and may 2008 we retrieved the vertical mixing ratio profiles of c2h2 c2h4 c2h6 c3h8 ch3c2h c4h2 c6h6 hcn hc3n and co2 we present here for the first time the latitudinal variations of the c2h6 c3h8 co2 c2h4 and c6h6 vertical mixing ratios profiles some molecules such as c2h6 or c3h8 present little variations above their condensation level the other molecules except co2 show a significant enhancement of their mixing ratios poleward of 50 n c2h4 is the only molecule whose mixing ratio decreases with height at latitudes below 46 n regions depleted in c2h2 hcn and c4h2 are observed around 400 km 0 01 mbar and 55 n we also inferred a region enriched in co2 located between 30 and 40 n in the 2 0 7 mbar pressure range at 80 n almost all molecules studied here present a local minimum of their mixing ratio profiles near 300 km 0 07 mbar which is in contradiction with global circulation models that predict constant with height vertical profiles due to subsidence at the north pole 2009 elsevier inc all rights reserved

seasonal evolution of titan s stratosphere during the cassini mission titan s stratosphere exhibits significant seasonal changes including break up and formation of polar vortices here we present the first analysis of mid infrared mapping observations from cassini s composite infrared spectrometer cirs to cover the entire mission ls 293 93 2004 2017 mid northern winter to northern summer solstice the north polar winter vortex persisted well after equinox starting break up around ls 60 and fully dissipating by ls 90 absence of enriched polar air spreading to lower latitudes suggests large scale circulation changes and photochemistry control chemical evolution during vortex break up south polar vortex formation commenced soon after equinox and by ls 60 was more enriched in trace gases than the northern mid winter vortex and had temperatures 20 k colder this suggests early winter and mid winter vortices are dominated by different processes radiative cooling and subsidence induced adiabatic heating respectively by the end of the mission ls 93 south polar conditions were approaching those observed in the north at ls 293 implying seasonal symmetry in titan s vortices

tidewater glacier beds insights from iceberg debris in kongsfjorden svalbard the sediment in icebergs offers an opportunity to sample the otherwise inaccessible beds of modern tidewater glaciers this paper presents a discussion of the benefits and problems of using iceberg debris to make inferences about the nature of the beds of modern tidewater glaciers as an example we present data obtained from icebergs calved from fast flowing tidewater glaciers in kongsfjorden svalbard sediment samples obtained from 12 icebergs show that clast rich muddy diamicton is the dominant facies although muddy gravel and mud are also present in some debris layers sediment texture and clast shape analyses indicate that most debris in the icebergs is of basal origin micro morphological analyses of thin sections of the diamicton also indicate pervasive deformation of the sediment although individual icebergs only represent small samples of the bed of a tidewater glacier and problems remain in linking iceberg sediments to their pre calving transport history beneath the glacier this method provides a potentially fruitful way of linking sediments to glacier dynamics

automated mapping of local layer slope and tracing of internal layers in radio echograms an automated method is presented for tracing layers in radio echograms the method is designed to work with most radio echo sounding echograms and has been successfully tested with a 180 210 mhz multichannel coherent depth sounder to accurately trace layers first approximate layer positions are calculated by integrating the local layer slope which is inferred by the intensity response to a slanted filter then the positions are refined using an iterative optimization the layers are traced using an active contour model or snake which can be constrained to conserve both echogram features and smooth layers with this technique it is possible to trace internal layers over distances of several hundred kilometers the method was tested between two greenland deep ice cores where the age depth relation is known

temporal changes in the radiophysical properties of a polythermal glacier in spitsbergen in order to study the seasonal and inter seasonal variations in radio wave velocity rwv radiophysical investigations were made at hansbreen a polythermal glacier in spitsbergen in july august 2003 and april 2004 these investigations included repeated radar profiling 20 and 25mhz along a transverse profile repeated common midpoint measurements continuous radar measurements during 8 days at a fixed site meteorological observations and continuous ice surface velocity monitoring by differential gps seasonal and inter seasonal rwv changes in the temperate ice layer are attributed respectively to rapid water redistribution within it during the summer and to variations in water content from 2 1 in summer to 0 4 in spring the reflection properties of the temperate ice layer correlate well with the air temperature with a nearly semi diurnal time lag the temporal variability of the reflection properties of the internal horizon suggests enlargement of water inclusions or water drainage from the horizon repeated profiling shows a stable spatial pattern in bed reflection power interpreted as changes in water content controlled by bedrock topography the spatial variations of internal reflection energy along the repeated profile correlate with the thickness of the cold ice layer and the occurrence of drainage and crevasse systems

the morphology of ice streams a b and c west antarctica and their environs airborne radar soundings of the ice sheet surface made in 1984 1985 together with elevations measured by oversnow traverses between 1957 and 1964 have been used to produce a surface elevation map of ice streams a b and c and much of the region around them the flight positions and elevations were tied to many ground control stations whose positions in three dimensions were fixed by satellite tracking ice thicknesses were also measured along the flight lines but have not yet been mapped the surfaces of active ice streams a and b exhibit a longitudinal ridgetrough topography of uncertain origin prominent surface valleys are associated with most but not all of their marginal shear zones there is a deep subglacial trough beneath the grid northeastern side of ice stream a that connects to the subglacial trough beneath reedy glacier the boundary between ice streams bl and b2 the two tributaries of ice stream b persists as a suture zone for 250 km downstream from their merger ice stream bl overlies a relatively deep subglacial trough that continued under the corresponding part of ice stream b until it disappears close to the grounding line between ice streams bl and b2 there is a complex zone containing several regions of undisturbed ice separated by bands of disturbed ice which suggest to us that rafts of ice are being incorporated into the ice streams the previously reported negative mass balance for the drainage system of ice stream b may arise from the expansion of the ice stream into the interior of the west antarctic ice sheet crevasse bands at some places on ridge ab together with high glacial velocities reported elsewhere also suggest this expansion close to the ross ice shelf ice stream b widens into a nearly flat area where the mean surface slope is only 3 5 10 4 and the surface elevations are only a few tens of meters above hydrostatic equilibrium inactive ice stream c differs from active ice streams a and b in surficial and basal characteristics no elongated ridges and troughs are observed instead the ice stream surface exhibits several terraces including some maxima and minima in elevation radar sounding reveals areas where basal echoes are strong and steady indicating subglacial water alternating with areas of weaker echoes with short fading lengths calculations indicate that the strong reflections require a water layer at least several centimeters to several meters thick depending on the electrical conductivity of the water due to reversals in the surface slope the hydrologic potential gradient changes sign several times along ice stream c which implies that subglacial water should collect in some locations

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

an investigation into the mechanisms controlling seasonal speedup events at a high arctic glacier bingham r g hubbard a l nienow p w sharp m j 2008 an investigation into the mechanisms controlling seasonal speedup events at a high arctic glacier journal of geophysical research earth surface 113 f2 f02006 sponsorship nerc nserc the university of alberta circumpolar institute and northern science training programme the geological society of america

diagnostics of titan s stratospheric dynamics using cassini cirs data and the 2 dimensional ipsl circulation model abstract the dynamics of titan s stratosphere is discussed in this study based on a comparison between observations by the cirs instrument on board the cassini spacecraft and results of the 2 dimensional circulation model developed at the institute pierre simon laplace available at http www lmd jussieu fr titandbase rannou p lebonnois s hourdin f luz d 2005 adv space res 36 2194 2198 the comparison aims at both evaluating the model s capabilities and interpreting the observations concerning 1 dynamical and thermal structure using temperature retrievals from cassini cirs and the vertical profile of zonal wind at the huygens landing site obtained by huygens dwe and 2 vertical and latitudinal profiles of stratospheric gases deduced from cassini cirs data the modeled thermal structure is similar to that inferred from observations cassini cirs and earth based observations however the upper stratosphere above 0 05 mbar is systematically too hot in the 2d cm and therefore the stratopause region is not well represented this bias may be related to the haze structure and to misrepresented radiative effects in this region such as the cooling effect of hydrogen cyanide hcn the 2d cm produces a strong atmospheric superrotation with zonal winds reaching 200 m s 1 at high winter latitudes between 200 and 300 km altitude 0 1 1 mbar the modeled zonal winds are in good agreement with retrieved wind fields from occultation observations cassini cirs and huygens dwe changes to the thermal structure are coupled to changes in the meridional circulation and polar vortex extension and therefore affect chemical distributions especially in winter polar regions when a higher altitude haze production source is used the resulting modeled meridional circulation is weaker and the vertical and horizontal mixing due to the polar vortex is less extended in latitude there is an overall good agreement between modeled chemical distributions and observations in equatorial regions the difference in observed vertical gradients of c 2 h 2 and hcn may be an indicator of the relative strength of circulation and chemical loss of hcn the negative vertical gradient of ethylene in the low stratosphere at 15 s cannot be modeled with simple 1 dimensional models where a strong photochemical sink in the middle stratosphere would be necessary it is explained here by dynamical advection from the winter pole towards the equator in the low stratosphere and by the fact that ethylene does not condense near the winter pole 80 n some compounds c 4 h 2 c 3 h 4 exhibit an interior minimum in the observed abundance vertical profiles whereas 2d cm profiles are well mixed all along the atmospheric column this minimum can be a diagnostic of the strength of the meridional circulation and of the spatial extension of the winter polar vortex where strong descending motions are present in the summer hemisphere observed stratospheric abundances are uniform in latitude whereas the model maintains a residual enrichment over the summer pole from the spring cell due to a secondary meridional overturning between 1 and 50 mbar at latitudes south of 40 50 s the strength as well as spatial and temporal extensions of this structure are a difficulty that may be linked to possible misrepresentation of horizontally mixing processes due to the restricted 2 dimensional nature of the model this restriction should also be kept in mind as a possible source of other discrepancies

topographic hadley cells when a rotating fluid over sloping topography is heated from below and or cooled from above horizontal temperature gradients develop which drive convection cells aligned with isobaths we refer to these cells as topographic hadley cells laboratory experiments reveal that sinking occurs in small cyclonic vortices situated in relatively shallow regions this is balanced by slower upwelling in adjacent deeper regions the cross isobath motions which connect the upwelling and downwelling are accelerated by coriolis forces resulting in strong jets which follow isobathic contours for anticlockwise rotation the surface jets keep the shallows to their left when looking in the direction of flow which is opposite to both kelvin and rossby wave propagation

macro scale bed roughness of the siple coast ice streams in west antarctica subglacial bed conditions are key to understanding ice stream behaviour and evolution with bed roughness re ecting substrate composition and ow resistance here we present an analysis of bed roughness in the siple coast region of west antarctica from airborne radio echo sounding data the ice streams are associated generally with low bed roughness values which decrease downstream the bed of the slow owing ice stream c 10 000 km2 is also characterized by being smooth at all scales wavelengths ranging from 5 km to in excess of 40 km furthermore the bed is smooth either side of ice stream c this suggests the location of the ice stream is controlled by internal ice sheet dynamics rather than by bed morphology if the ice stream were encouraged to migrate laterally when active there would be little resistance offered by the subglacial morphology other inter ice stream regions are rough however indicating a subglacial topographic in uence on ice stream position bed roughness increases up ow of ice streams which unless the bed is modi ed may limit the inland migration of these systems copyright 2004 john wiley sons ltd

till beneath ice stream b 3 till deformation evidence and implications most of the velocity of ice stream b near the upstream b camp upb west antarctica appears to arise from deformation of a seismically detected subglacial till layer that averages 6 m thick available evidence indicates that the entire thickness of this till layer is deforming and is eroding subjacent bedrock into flutes parallel to ice flow and hundreds of meters across the resulting till flux beneath upb is equivalent to an average erosion rate of about 0 4 mm yr 1 in the catchment area and suggests that till deltas tens of kilometers long have been deposited at the grounding line during the holocene such deltas should be characterized by partial ice till decoupling across a water film and by a small ice air surface slope they may have been discovered by recent geophysical work

response of subglacial sediments to basal freeze on 1 theory and comparison to observations from beneath the west antarctic ice sheet 1 we have constructed a high resolution numerical model of heat water and solute flows in sub ice stream till subjected to basal freeze on the model builds on quantitative treatments of frost heave in permafrost soils the full version of the clapeyron equation is used hence ice water phase transition depends on water pressure osmotic pressure and surface tension the two latter effects can lead to supercooling of the ice base this supercooling in turn induces hydraulic gradients that drive upward flow of pore water which feeds the growth of segregation ice onto the freezing interface this interface may progress into the till and form ice lenses if supercooling is sufficiently strong hence the ice segregation process develops a stratified basal ice layer in our model a high basal temperature gradient 0 054 c m 1 triggers ice stream stoppage and the loss of basal shear heating leads to relatively high basal freeze on rates 3 5 mm a 1 in response the subglacial till experiences comparatively rapid consolidation till porosity can decrease from 40 to 120 kpa approximately within one century basal supercooling arising from redistribution of solutes and ice water interfacial effects amounts to ca 0 35 c below the pressure melting point fine grained till is in our model associated with widely spaced thick ice lenses coarse grained till yields thinner ice lenses that are more closely spaced our model results compare favorably although not in all details with available observational evidence from borehole studies of west antarctic ice streams

dynamic perennial firn aquifer on an arctic glacier ice penetrating radar and gps observations reveal a perennial firn aquifer pfa on a svalbard ice field similar to those recently discovered in southeastern greenland a bright widespread radar reflector separates relatively dry and water saturated firn this surface the phreatic firn water table is deeper beneath local surface elevation maxima shallower in surface lows and steeper where the surface is steep the reflector crosscuts snow stratigraphy we use the apparent deflection of accumulation layers due to the higher dielectric permittivity below the water table to infer that the firn pore space becomes progressively more saturated as depth increases our observations indicate that pfas respond rapidly subannually to surface forcing and are capable of providing significant input to the englacial hydrology system

temporal variations of titan s middle atmospheric temperatures from 2004 to 2009 observed by cassini cirs we use five and one half years of limb and nadir viewing temperature mapping observations by the composite infrared radiometer spectrometer cirs on the cassini saturn orbiter taken between july 2004 and december 2009 ls from 293 to 4 northern mid winter to just after northern spring equinox to monitor temperature changes in the upper stratosphere and lower mesosphere of titan the largest changes are in the northern winter polar stratopause which has declined in temperature by over 20 k between 2005 and 2009 throughout the rest of the mid to upper stratosphere and lower mesosphere temperature changes are less than 5 k in the southern hemisphere temperatures in the middle stratosphere near 1 mbar increased by 1 2 k from 2004 through early 2007 then declined by 2 4 k throughout 2008 and 2009 with the changes being larger at more polar latitudes middle stratospheric temperatures at mid northern latitudes show a small 1 2 k increase from 2005 through 2009 at north polar latitudes within the polar vortex temperatures in the middle stratosphere show a 4 k increase during 2007 followed by a comparable decrease in temperatures in 2008 and into early 2009 the observed temperature changes in the north polar region are consistent with a weakening of the subsidence within the descending branch of the middle atmosphere meridional circulation

glacial conditions that contribute to the regeneration of fountain glacier proglacial icing bylot island canada proglacial icings are one of the most common forms of extrusive ice found in the canadian arctic however the icing adjacent to fountain glacier bylot island is unique due to its annual cycle of growth and decay and perennial existence without involving freezing point depression of water due to chemical characteristics its regeneration depends on the availability of subglacial water and on the balance between ice accretion and hydro thermal erosion the storage and conduction of the glacial meltwater involved in the accretion of the icing were analyzed by conducting topographic and ground penetrating radar surveys in addition to the modelling of the subglacial drainage network and the thermal characteristics of the glacier base the reflection power analysis of the geophysical data shows that some areas of the lower ablation zone have a high accumulation of liquid water particularly beneath the centre part of the glacier along the main supraglacial stream a dielectric permittivity model of the glacier sediment interface suggests that a considerable portion of the glacier is warm based allowing water to flow through unfrozen subglacial sediments towards the proglacial outwash plain all these glacier related characteristics contribute to the annual regeneration of the proglacial icing and allow for portions of the icing to be perennial copyright 2013 john wiley sons ltd

evidence for subglacial ponding across taylor glacier dry valleys antarctica hubbard alun lawson w anderson b hubbard b p 2004 evidence for subglacial ponding across taylor glacier dry valleys antarctica annals of glaciology 39 1 79 84 rae2008

the evolution of titan s mid latitude clouds spectra from cassini s visual and infrared mapping spectrometer reveal that the horizontal structure height and optical depth of titan s clouds are highly dynamic vigorous cloud centers are seen to rise from the middle to the upper troposphere within 30 minutes and dissipate within the next hour their development indicates that titan s clouds evolve convectively dissipate through rain and over the next several hours waft downwind to achieve their great longitude extents these and other characteristics suggest that temperate clouds originate from circulation induced convergence in addition to a forcing at the surface associated with saturn s tides geology and or surface composition

small scale composition and haze layering in titan s polar vortex abstract fine scale layering of haze and composition in titan s stratosphere and mesosphere was investigated using visible uv images from cassini s imaging science sub system iss and ir spectra from cassini s composite infra red spectrometer cirs both iss and cirs independently show fine layered structures in haze and composition respectively in the 150 450 km altitude range with a preferred vertical wavelength of around 50 km layers are most pronounced around the north polar winter vortex although some weaker layers do exist at more southerly latitudes the amplitude of composition layers in each trace gas profile is proportional to the relative enrichment of that species in the winter polar vortex compared to equatorial latitudes as enrichment is caused by polar subsidence this suggests a dynamical origin we propose that the polar layers are caused by cross latitude advection across the vortex boundary this is analogous to processes that lead to ozone laminae formation around earth s polar vortices

the relationship between sticky spots and radar reflectivity beneath an active west antarctic ice stream isolated areas of high basal drag or sticky spots are important and poorly understood features in the force balance and dynamics of west antarctic ice streams characterizing sticky spots formed by thin or drying subglacial till using ice penetrating radar is theoretically possible as high radar bed returned power brp is commonly related to an abundance of free water at the ice bed interface provided losses from englacial attenuation can be estimated in this study we use airborne radar data collected over evans ice stream to extract brp profiles and test the sensitivity of brp to the adopted englacial attenuation correction we analyse 11 20 km profiles in four fast flow areas where sticky spots have been inferred to exist on the basis of model and surface data inversions in the majority of profiles we note that the increase in basal drag is accompanied by a decrease in brp and suggest that this is evidence both for the presence of a sticky spot in those locations and that local variations in subglacial hydrology are responsible for their existence a comparison is made between empirical and numerical modelling approaches for deriving englacial attenuation and our findings generally support previous studies that advocate a modelling approach

anisotropic radio wave scattering from englacial water regimes mýrdalsjokull iceland colinear polarized 5 mhz radar profiling data were obtained on m rdalsjokull a temperate glacier in iceland radar transects and therefore polarization planes were aligned approximately parallel transverse and oblique to the ice flow direction echoes from the shallower half to two thirds of the ice were 10 20db stronger on the oblique and longitudinal transects than those on the transverse transects anisotropy as a function of depth is clearly seen at the sites where the transects cross strong scattering on longitudinal transects apparently caused extinction of a radar reflecting layer that was continuously profiled on the transverse transects a radio wave scattering model shows that scattering from a longitudinal water filled conduit parallel to the glacier surface can explain the observed azimuthal variations of the echo we conclude that low frequency mhz radio waves can help to characterize englacial water regimes

bed reflectivity beneath inactive ice streams in west antarctica radio echo sounding res techniques are used to examine spatial changes inbed reflectivityacrossrelictice streams inwest antarctica measurementsfrom adjacent interstream ridgesare usedto correctthe measuredpower returned fromthebed for attenuation and losses due to geometric spreading scattering and absorption res measurements near boreholes drilled on ice stream c isc indicate high coefficients of bed reflectivity r40 1 in locations where the bed wasthawed and boreholes connected to thebasalwatersystem andlow reflectivitycoefficients r50 02 atlocationsthat were frozen and not connected intermediate values of bed reflectivity were measured at locations where the connection to the basal water systemwas weak measurements across four relictmargins show thatbedreflectivity usuallyjumpsfromlow tohighvaluesseveral kilometers inside the outermost buried crevasses we interpret this to be a transition from frozento thawedbasal conditions anddiscuss implications of these observations

englacial and subglacial water flow at skalafellsjokull iceland derived from ground penetrating radar in situ glacsweb probe and borehole water level measurements we reconstruct englacial and subglacial drainage at skalafellsjokull iceland using ground penetrating radar gpr common offset surveys borehole studies and glacsweb probe data we find that englacial water is not stored within the glacier water content 0 0 3 instead the glacier is mostly impermeable and meltwater is able to pass quickly through the main body of the glacier via crevasses and moulins once at the glacier bed water is stored within a thin 1 m layer of debris rich basal ice 2 water content and the till the hydraulic potential mapped across the survey area indicates that when water pressures are high most of the year water flows parallel to the margin and emerges 3 km down glacier at an outlet tongue gpr data indicates that these flow pathways may have formed a series of braided channels we show that this glacier has a very low water storage capacity but an efficient englacial drainage network for transferring water to the glacier bed and therefore it has the potential to respond rapidly to changes in melt water inputs

global and temporal variations in hydrocarbons and nitriles in titan s stratosphere for northern winter observed by cassini cirs mid infrared spectra measured by cassini s composite infrared spectrometer cirs between july 2004 and january 2007 ls 293 328 have been used to determine stratospheric temperature and abundances of c2h2 c 3h4 c 4h2 h cn and hc3n over 65 000 nadir spectra with spectral resolutions of 0 5 and 2 5 cm 1 were used to probe spatial and temporal composition variations in titan s stratosphere cassini s 180 orbital transfer in mid 2006 allowed low emission angle observations of the north polar region for the first time in the mission and allowed us to probe the full latitude range we present the first measurements of composition variations within the polar vortex which display increasing abundances right up to 90 n the lack of a homogeneous abundance latitude variation within the vortex indicates limited horizontal mixing and suggests that subsidence is greatest at the vortex core contrary to numerical model predictions and tropospheric cloud observations we do not see any evidence for a secondary circulation cell near the south pole which suggests a single hadley type circulation in the stratosphere at this epoch this difference can be reconciled if the secondary cell is restricted to altitudes below 100 km where there is no sensitivity in our data temporal variations in composition were observed in the south with volatile species becoming less abundant as the season progressed the observed variations are compared to numerical model predictions and observations from voyager

basal conditions beneath enhanced flow tributaries of slessor glacier east antarctica radio echo sounding data are used to investigate bed roughness beneath the three enhanced flow tributaries of slessor glacier east antarctica slow moving inter tributary areas are found to have rough beds while the bed of the northernmost tributary is relatively smooth a reconstruction of potential subglacial drainage routing indicates that water would be routed down this tributary and investigations of basal topography following isostatic recovery reveal that the bed would have been below sea level in preglacial times so marine sediments may have accumulated here together these factors are further support for the dominance of basal motion in this tributary reported elsewhere conversely although the other two slessor tributaries may have water routed beneath them they would not have been below sea level before the growth of the ice sheet so cannot be underlain by marine sediments they are also found to be rough and within the range of uncertainties it is likely that basal motion does not play a major role in the flow of these tributaries perhaps the most interesting area however is a deep trough where flow rates are currently low but the bed is as smooth as the northern slessor trough it is proposed that although ice deformation currently dominates in this trough basal motion may have occurred in the past when the ice was thicker

till beneath ice stream b 1 properties derived from seismic travel times seismic experiments conducted on ice stream b part of the marine ice sheet of west antarctica show a meters thick layer immediately beneath the 1000 m thick ice a seismic experiment consisting of wide angle reflection profiling along a line parallel to ice stream flow was conducted to determine the properties of this layer inversion of seismic travel times yields a compressional wave speed of less than 1700 m s 1 and a shear wave speed less than 160 m s 1 for the layer these very low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure based on wave speeds in other saturated unconsolidated sediments we believe that a porosity substantially greater than 0 32 probably around 0 4 and an excess of overburden pressure over pore pressure of only 50 kpa 0 5 bar characterize the layer at this location

bedrock platforms within the ross embayment west antarctica hypotheses for ice sheet history wave erosion cenozoic extension and thermal subsidence ice penetrating radar mostly airborne and marine seismic surveys have revealed plateaus and terraces about 100 350 m below sea level beneath parts of the ross embayment including the west antarctic ice sheet the ross ice shelf and the eastern ross sea these surfaces cover many thousands of square kilometers and are separated by bedrock troughs occupied by the west antarctic ice streams prominent plateaus are under edward vii peninsula siple dome and roosevelt island marine seismic data and gravity data over the buried plateaus support an interpretation that they were caused by erosion into basement the flat and level nature of the surfaces that were formed by erosion are near the same depth over large distances and fringe the buried rugged bedrock topography of marie byrd land supports an interpretation of marine rather than glacial erosion marine seismic reflection profiles over one of the plateau remnants show it as an angular unconformity cut into gently dipping sediments of late oligocene age draped with thin flat lying sediments implying that the plateau is early miocene or younger the younger limit for plateau erosion follows from the interpretation of the mechanism erosion must predate permanent ice along the coast associated with the growth of the west antarctic ice sheet probably prior to 10 ma and possibly prior to 14 ma the plateaus along the siple coast with depths around 350 meters do not rebound to close to sea level and wave base for models of removing past and present ice load this discrepancy can be explained if ross embayment lithosphere has been cooling and subsiding since significant extension in eocene to oligocene time our interpretation requires crustal subsidence in the ross sea which in turn implies higher bedrock elevations in the past within this region higher standing topography presented an opportunity for the accumulation of local ice sheets and caused sea level fluctuation prior to late cenozoic time

predicted age depth scales for siple dome and inland wais ice cores in west antarctica geophysical data are used with ice flow models and generalized accumulation histories to estimate age and annual layer thickness versus depth for two anticipated ice core sites in west antarctica siple dome 81 65 s 148 81 w and an inland site on the west antarctic ice sheet wais this modeling experiment predicts that 104 year old ice is at 50 depth and 105 year old ice is at 90 depth at both sites both of these cores could contain climate information through the last glacial cycle with annual resolution through the holocene the predicted similarity in resolution and record length between the two cores suggests that they could be compared in detail to obtain both spatial and temporal information about the paleoclimate and history of the west antarctic ice sheet

mapping the zonal structure of titan s northern polar vortex abstract saturn exhibits an obliquity of 26 7 such that the largest moon titan experiences seasonal variations including the formation of a polar vortex in the winter hemisphere titan s polar vortex is characterised by cold stratospheric temperatures due to the lack of insolation over the winter pole and an increase in trace gas abundance as a result of complex organic chemistry in the upper atmosphere combined with polar subsidence meridional variations in temperature and gas abundance across the vortex have previously been investigated but there has not yet been any in depth study of the zonal variations in the temperature or composition of the northern vortex here we present the first comprehensive two dimensional seasonal mapping of titan s northern winter vortex using 18 nadir mapping sequences observed by the composite infrared spectrometer cirs instrument on board cassini we investigate the evolution of the vortex over almost half a titan year from late winter through to mid summer ls 326 86 2007 2017 we find the stratospheric symmetry axis to be tilted from the solid body rotation axis by around 3 5 although our results for the azimuthal orientation of the tilt are inconclusive we find that the northern vortex appears to remain zonally uniform in both temperature and composition at all times a comparison with vortices observed on earth mars and venus shows that large scale wave mechanisms that are important on other terrestrial planets are not as significant in titan s atmosphere this allows the northern vortex to be more symmetrical and persist longer throughout the annual cycle compared to other terrestrial planets

simulating the one dimensional structure of titan s upper atmosphere 1 formulation of the titan global ionosphere thermosphere model and benchmark simulations 1 we employ a newly developed navier stokes model the titan global ionosphere thermosphere model t gitm to address the one dimensional 1 d coupled composition dynamics and energetics of titan s upper atmosphere our main goals are to delineate the details of this new theoretical tool and to present benchmark calibration simulations compared against the ion neutral mass spectrometer inms neutral density measurements first we outline the key physical routines contained in t gitm and their computational formulation then we compare a series of model simulations against recent 1 d work by cui et al 2008 strobel 2008 2009 and yelle et al 2008 in order to provide a fiducial for calibrating this new model in paper 2 and a future paper we explore the uncertainties in our knowledge of titan s atmosphere between 500 km and 1000 km in order to determine how the present measurements constrain our theoretical understanding of atmospheric structures and processes

radar reflections reveal a wet bed beneath stagnant ice stream c and a frozen bed beneath ridge bc west antarctica digital airborne radar data were collected during the 1987 88 antarctic field season in nine gridded blocks covering the downstream portions of ice stream b 6 km spacing and ice stream c 11 km spacing together with a portion of ridge bc between them an automated processing procedure was used for picking onset times of the reflected radar pulses converting travel times to distances interpolating missing data converting pressure transducer readings correcting navigational drift performing crossover analysis and zeroing remanent crossover errors interpolation between flight lines was carried out using the minimum curvature method maps of ice thickness estimated accuracy 20 m and basal reflection strength estimated accuracy 1 db were produced the ice thickness map confirms the characteristics of previous reconnaissance maps and reveals no new features the reflection strength map shows pronounced contrasts between the ice streams and ridge bc and between the two ice streams themselves we interpret the reflection strengths to mean that the bed of ice stream c as well as that of ice stream b is unfrozen that the bed of ridge bc is frozen and that the boundary between the frozen bed of ridge bc and the unfrozen bed of ice stream c lies precisely below the former shear margin of the ice stream

bed topography and lubrication inferred from surface measurements on fast flowing ice streams observations of surface elevation s and horizontal velocity components u and v are inverted to infer the topography b and lubrication c at the bed of an ice stream based on a linearized perturbation theory of the transmission of flow disturbances through the ice thickness synthetic data are used to illustrate non uniqueness in the inversion but also demonstrate that effects of b and c can be separated when s u and v are specified even with added noise to simulate measurement errors we have analyzed prominent short horizontal scale 2 km features in topography and velocity pattern in a local 64 km by 32 km area of the surface of ice stream e west antarctica our preferred interpretation of bed conditions beneath the most prominent features on the surface identifies a deep trough in the basal topography with low lubrication in the base of the trough

transient slip events from near field seismic and geodetic data on a glacier fault whillans ice plain west antarctica 1 bidaily tidally modulated stick slip speed ups of the whillans ice plain wip provide insight into glacier dynamics and failure at a naturally repeating fault asperity we installed a network of continuously operating gps receivers in 2007 and deployed on ice broadband seismometers during the austral summer of 2008 on whillans ice stream wis west antarctica and recorded 26 glacier speed up events previous work during the 2004 field season suggested that these speed ups initiate as failure of an asperity on or near ice raft a that triggers rupture across the entire wip our results for 2008 locate the slip initiation farther to the south of this feature closer to the grounding line and the southernmost extent of the ross ice shelf the initiation may be controlled by a discontinuity in basal boundary conditions at the suture between two ice streams a strong correlation between the amplitude of seismic waves generated at the rupture front and the total slip achieved over the duration of the slip event 30 min suggests slip predictable behavior i e the ability to forecast the eventual slip based on the first minute of seismic radiation successive slip events propagate with different rupture speeds 100 300 m s that strongly correlate r 2 0 73 with the interevent duration in addition the amount of slip achieved during each event appears to be correlated with the rupture speed we use these observations to constrain basal shear stress to be 4 kpa by calculating conditions for basal freezing our observations yield information regarding mechanics and dynamics of ice streambeds at the scale of tens to hundreds of kilometers subglacial processes are notoriously difficult to constrain on these large scales which are relevant to the understanding of regional and continental ice motion

vertical profiles of hcn hc3n and c2h2 in titan s atmosphere derived from cassini cirs data mid infrared limb spectra in the range 600 1400 cm 1 taken with the composite infrared spectrometer cirs on board the cassini spacecraft were used to determine vertical profiles of hcn hc3n c2h2 and temperature in titan s atmosphere both high 0 5 cm 1 and low 13 5 cm 1 spectral resolution data were used the 0 5 cm 1 data gave profiles at four latitudes and the 13 5 cm 1 data gave almost complete latitudinal coverage of the atmosphere both datasets were found to be consistent with each other high temperatures in the upper stratosphere and mesosphere were observed at titan s northern winter pole and were attributed to adiabatic heating in the subsiding branch of a meridional circulation cell on the other hand the lower stratosphere was much colder in the north than at the equator which can be explained by the lack of solar radiation and increased ir emission from volatile enriched air hc3n had a vertical profile consistent with previous ground based observations at southern and equatorial latitudes but was massively enriched near the north pole this can also be explained in terms of subsidence at the winter pole a boundary observed at 60 n between enriched and un enriched air is consistent with a confining polar vortex at 60 na nd hc 3n s short lifetime in the far north layers were observed in the hc3n profile that were reminiscent of haze layers observed by cassini s imaging cameras hcn was also enriched over the north pole which gives further evidence for subsidence however the atmospheric cross section obtained from 13 5 cm 1 data indicated a hcn enriched layer at 200 250 km extending into the southern hemisphere this could be interpreted as advection of polar enriched air towards the south by a meridional circulation cell this is observed for hcn but not for hc3n due to hcn s longer photochemical lifetime c2h2 appears to have a uniform abundance with altitude and is not significantly enriched in the north this is consistent with observations from previous cirs analysis that show increased abundances of nitriles and hydrocarbons but not c2h2 towards the north pole

changes in the configuration of ice stream flow from the west antarctic ice sheet surface based ice penetrating radar profiles on the northeast flank of siple dome support the hypothesis that a curvilinear scar first observed in advanced very high resolution radiometer satellite imagery represents the margin of a formerly active ice stream the scar defines the southwestern boundary of an ice stream flowing from ice stream c to ice stream d close to where it enters the ross ice shelf our studies show that the scar coincides with a trough and upward step in surface topography approximately 5 km across underlain by a zone of disturbed internal stratigraphy revealed by the radar burial depth of the disturbed zone enables us to calculate the time of shutdown as occurring prior to approximately 1 3 ka the configuration of the ice streams draining the west antarctic ice sheet into the ross ice shelf evidently changes with time and attempts to predict the evolution of the ice sheet must incorporate this observation

depth and time dependent vertical strain rates at siple dome antarctica as part of a project to investigate the flow of ice at low effective stress two independent strain gauge systems were used to measure vertical strain rate as a function of depth and time at siple dome antarctica the measurements were made from january 1998 until january 2002 at the ice divide and a site 7 km to the northeast on the flank the strain rate profiles place constraints on the rheology of ice at low stress show the expected differences between divide and flank flow with some structure due to firn compaction and probably ice stratigraphy and suggest that the flow of the ice sheet has not changed much in the last 8 6 kyr the strain rates show an unexpected time dependence on scales ranging from several months to hours including discrete summer events at the divide time dependence in strain rate water pressure seismicity velocity and possibly basal motion has been seen previously on the siple coast ice streams but it is especially surprising on siple dome because the bed is cold

the elevation history of ice streams and the spatial accumulation pattern along the siple coast of west antarctica inferred from ground based radar data from three inter ice stream ridges measurements of the surface and internal layer geometry from ice penetrating radar and global positioning system surveys on three inter ice stream ridges in west antarctica siple dome ridge de and ridge bc are examined with ice flow models to infer 1 the history of the divide position at each site and 2 the spatial pattern of accumulation across the ridges we find that the divide position is most steady at siple dome somewhat steady at ridge de and highly variable at ridge bc data from siple dome and ridge de show evidence for steady northward motion of the ice divide for the past few thousand years the layers beneath ridge bc suggest a 5 km northward shift of the divide position within the past several hundred years assuming the divide shifts are all due to changing elevation of the bounding ice streams we infer the relative elevation history for segments of ice streams b e the northward displacement of the divide for all ridges implies a progressive relative thinning of the ice streams from e to b with most dramatic recent thinning 100 m in 103 years of ice stream b relative to ice stream c analysis of the internal layer pattern across the ridges indicates a south north accumulation gradient with higher accumulation rates on the northern flanks of the ridges in all three cases the inferred accumulation distribution is nearly uniform on the northern flanks decreases sharply within a few ice thicknesses across the divides and then decreases gradually farther to the south the north south decrease is strongest for ridge de and weakest for ridge bc this spatial pattern and the reduction in gradient strength with distance from the amundsen sea is consistent with the hypothesis that storms from the amundsen sea carry moisture first south then west over west antarctica and deposit more snow on the windward side of ridges due to orographic lifting this pattern has been stable for at least the past several thousand years

antarctic ice streams a review an ice stream is a part of an inland ice sheet that flows rapidly through the surrounding ice the distinction between an ice sheet and an outlet glacier is clear in principle but muddy in practice many antarctic glaciers are intermediate in character the ross ice streams which flow through the west antarctic inland ice into the ross ice shelf are distinct in character differing even from other ice streams in the marine ice sheet of west antarctica their surface elevation profiles are low their bed slopes are low and smooth and their driving stresses diminish monotonically downglacier in transverse profile they are broader in relation to ice thickness and exhibit shallower subglacial troughs than other ice streams many models for the fast sliding of glaciers have been applied to the ross ice streams most have included in some form a reduction in basal drag resulting from a lesser effective than glaciostatic pressure at the bed most models are only semi physical at best so need to be tuned by fitting to assumed parameters at some place or places in the real ice sheet the recent discovery of a very small effective pressure beneath one ice stream consequently has led to some gross errors in the velocities predicted by the models the difficulty may be resolved if it is true as recent experiments suggest that ice stream b and by extrapolation other ross ice streams as well slide on a deforming bed that absorbs most or all of the differential motion between the ice and the bedrock