investigation of upper mantle discontinuity structure beneath the central pacific using ss precursors 1 using a high quality broadband seismic data set of precursors to the phase ss we investigate the structure of upper mantle discontinuities beneath the central pacific including the hawaiian hot spot we image structure by stacking over 4000 records into geographic bins retaining periods down to 5 s we consider the effects of correcting for four separate tomographic models of mantle heterogeneity excluding data at distances containing phases that potentially interfere with precursors we find evidence for peak to peak topography of 15 20 km on the 670 km discontinuity and 7 28 km of topography on the 400 km discontinuity weak reflections are detected from discontinuities near 220 and 520 km depth the average transition zone thickness beneath our region is approximately 242 3 km very similar to previous estimates of the global average lateral transition zone thinning and thickening weakly correlate with reduced and increased transition zone shear velocity respectively consistent with a thermal origin to topographical variations on the discontinuities within our study region the transition zone beneath hawaii and to the east of the line island chain is thinned by up to 20 km in a province spanning nearly 1000 km suggesting an excess mantle temperature of 200 k in the oldest crustal regions of our study area 100 ma the 400 km discontinuity is relatively shallow and the transition zone is relatively thick 250 255 km a possible explanation for this pattern includes small scale convection in the upper mantle bringing colder material into the transition zone

constraints on seismic anisotropy in the mantle transition zone from long period ss precursors the mantle transition zone mtz of earth is demarcated by solid to solid phase changes of the mineral olivine that produce seismic discontinuities at 410 and 660 km depths mineral physics experiments predict that wadsleyite can have strong single crystal anisotropy at the pressure and temperature conditions of the mtz thus significant seismic anisotropy is possible in the upper mtz where lattice preferred orientation of wadsleyite is produced by mantle flow here we use a body wave method ss precursors to study the topography change and seismic anisotropy near the mtz discontinuities we stack the data to explore the azimuthal dependence of travel times and amplitudes of ss precursors and constrain the azimuthal anisotropy in the mtz beneath the central pacific we find evidence for 4 anisotropy with a se fast direction in the upper mantle and no significant anisotropy in the mtz in subduction zones we observe 4 anisotropy with a trench parallel fast direction in the upper mantle and 3 anisotropy with a trench perpendicular fast direction in the mtz the transition of fast directions indicates that the lattice preferred orientation of wadsleyite induced by mtz flow is organized separately from the flow in the upper mantle global azimuthal stacking reveals 1 azimuthal anisotropy in the upper mantle but negligible anisotropy 1 in the mtz finally we correct for the upper mantle and mtz anisotropy structures to obtain a new mtz topography model the anisotropy correction produces 3 km difference and therefore has minor overall effects on global mtz topography

martian north polar cap summer water cycle abstract a key outstanding question in martian science is are the polar caps gaining or losing mass and what are the implications for past current and future climate to address this question we use observations from the compact reconnaissance imaging spectrometer for mars crism of the north polar cap during late summer for multiple martian years to monitor the summertime water cycle in order to place quantitative limits on the amount of water ice deposited and sublimed in late summer we establish here for the first time the summer cycle of water ice absorption band signatures on the north polar cap we show that in a key region in the interior of the north polar cap the absorption band depths grow until l s 120 when they begin to shrink until they are obscured at the end of summer by the north polar hood this behavior is transferable over the entire north polar cap where in late summer regions flip from being net sublimating into net condensation mode this transition or mode flip happens earlier for regions closer to the pole and later for regions close to the periphery of the cap the observations and calculations presented herein estimate that on average a water ice layer 70 microns thick is deposited during the l s 135 164 period this is far larger than the results of deposition on the south pole during summer where an average layer 0 6 6 microns deep has been estimated by brown et al 2014 earth planet sci lett 406 102 109

interannual observations and quantification of summertime h2o ice deposition on the martian co2 ice south polar cap abstract the spectral signature of water ice was observed on martian south polar cap in 2004 by the observatoire pour l mineralogie l eau les glaces et l activite omega bibring et al 2004 three years later the omega instrument was used to discover water ice deposited during southern summer on the polar cap langevin et al 2007 however temporal and spatial variations of these water ice signatures have remained unexplored and the origins of these water deposits remains an important scientific question to investigate this question we have used observations from the compact reconnaissance imaging spectrometer for mars crism instrument on the mars reconnaissance orbiter mro spacecraft of the southern cap during austral summer over four martian years to search for variations in the amount of water ice we report below that for each year we have observed the cap the magnitude of the h2o ice signature on the southern cap has risen steadily throughout summer particularly on the west end of the cap the spatial extent of deposition is in disagreement with the current best simulations of deposition of water ice on the south polar cap montmessin et al 2007 this increase in water ice signatures is most likely caused by deposition of atmospheric h2o ice and a set of unusual conditions makes the quantification of this transport flux using crism close to ideal we calculate a minimum apparent amount of deposition corresponding to a thin h2o ice layer of 0 2 mm with 70 porosity this amount of h2o ice deposition is 0 6 6 of the total martian atmospheric water budget we compare our minimum apparent quantification with previous estimates this deposition process may also have implications for the formation and stability of the southern co2 ice cap and therefore play a significant role in the climate budget of modern day mars

sublimation and transport of water from the north residual polar cap on mars several features of the viking mars atmospheric water detection observations have suggested that the residual north polar cap on mars is a major source of atmospheric water during northern summer and that the water it supplies to the atmosphere is transported equatorward to low northern latitudes by summer s end we have explored the role of the north residual cap in the current martian water cycle by constructing models to assess the ability of the cap to supply water to the atmosphere and the ability of the atmosphere to transport it out of the polar regions from our sublimation model we conclude that the residual cap can sublime the 7 1014 g of water that appears in the atmosphere after ls 80 provided surface winds are high 10 m s the cap emissivity is low 0 9 or the exposed soil in polar regions contains ice that is accessible to the atmosphere in our view these requirements seem unlikely and we are drawn to the conclusion that other sources of water exist this conclusion is further supported by the results of two dimensional transport simulations which show that most of the water sublimed from the cap remains at high latitudes the polar circulation simply lacks the intensity and scale to move water very far from the cap a weak circulation places additional constraints on cap sublimation rates since abundant clouds and significant ice deposits would form just equatorward of the cap edge according to the model if the cap is allowed to sublime 7 1014 g of water such predictions are inconsistent with available data we suggest that the nature of the additional source is water desorbing from the nonpolar regolith

experiments in migration processing of ss precursor data to image upper mantle discontinuity structure long period ss precursors result from underside reflections off upper mantle discontinuities by grouping and stacking global seismic data by ss bounce point location it is possible to map lateral variations in depths to the 410 and 660 km discontinuities a process analogous to common midpoint cmp stacking in reflection seismology because this method assumes horizontal reflectors energy arriving from dipping or intermittent reflectors may not be correctly imaged to address this possibility we experiment with techniques based on migration processing of shallow seismic reflection data the problem is complicated by the uneven distribution of sources and receivers for the ss precursor observations but the data are sufficiently dense beneath the northwest pacific ocean that reasonably good coverage can be obtained for this region we parameterize the model as a grid of point scatterers in latitude longitude and depth from the surface to 1000 km depth and compute travel times from each grid point to the source and receiver locations these times are used to construct a matrix equation that yields predicted ss precursor waveforms from the assumed scatterers to recover the model we experiment with both simple back projection and full inversions using a conjugate gradient method tests on noise free synthetic data generated using the same source receiver distribution as the actual data suggest that detailed resolution of discontinuity structure is possible at horizontal scales much smaller than the fresnel zone however the real data do not produce coherent results unless some degree of horizontal smoothing is imposed at least partially defeating the purpose of this approach results for the northwest pacific find structure on the 410 and 660 km discontinuities and hints of intermittent reflectors at other depths random resampling tests however suggest that most of these features are not reliably resolved with the exception of a depression on the 660 km discontinuity seen in the northwest pacific our experiments show that it is unlikely that small scale structure on the 660 km discontinuity near subducting slabs causes significant bias in maps of the large scale 660 km topography derived from long period ss precursor observations

evidence for a sharp 670 km discontinuity as inferred from p to s converted waves the fine structure of the upper mantle discontinuities is investigated using observations of converted waves on short period and broadband seismograms using a stacking technique to analyze the p wave coda of teleseismic records evidence is found for coherent near receiver p to s converted phases generated by the 400 and 670 km discontinuity beneath a number of the stations used in this study variations in travel time slowness and amplitude of these phases as observed among the stacks for stations of the regional seismic test network in the united states and of the network of autonomously recording seismographs in western europe are very likely the expression of upper mantle heterogeneity observations of coherent converted phases from the 400 km discontinuity are fewer in number than of phases converted at the 670 km discontinuity suggesting that the latter is more pronounced some of the seismograms especially of station rscp show extremely high amplitude p to s converted phases from the 670 km discontinuity these seismograms allow a detailed waveform comparison of the converted phase with the direct p phase and present evidence for a sharp 670 km discontinuity

mars south polar hood as observed by the mars climate sounder 1 we have used observations from the mars climate sounder to investigate south polar hood water ice clouds at 12 m including the first systematic examination of the vertical 5 km resolution and nighttime structure we find that the structure and evolution of the polar hood is controlled more strongly by atmospheric temperature variations than by intrinsic fluctuations in water vapor abundance the clouds form as a belt during ls 10 70 phase 1 and ls 100 200 phase 2 during phase 1 the cloud belt extends over a wide latitude range between 30 s and 75 s with a visible column optical depth between 0 075 and 0 15 the cloud belt then evaporates as temperatures warm during phase 2 the cloud belt reappears due to an increase in water vapor as a partial band of low opacity clouds south of the tharsis region and eventually becomes continuous in longitude with a visible column opacity between 0 125 and 0 25 as the southern spring equinox approaches the cloud belt shifts southward following the seasonal cap edge from ls 140 to ls 200 the daytime belt lies about 15 farther south than the nighttime belt due to tidally driven diurnal temperature differences the vertical structure of the cloud belt is consistent within and between the two seasonal phases and is characterized by a thick lower cloud deck and an upper layer whose altitude shifts between the nighttime and daytime because of thermal tidal control of the condensation altitudes overall the southern polar hood is observed to rapidly form and dissipate as the temperature crosses the saturation point of water vapor

global mapping of topography on the 660 km discontinuity long period precursors to the ss seismic phase are used to produce global maps of topography for the discontinuity at 660 km depth in the upper mantle these maps indicate discontinuity depth variations of up to 30 km and suggest a correlation between regional depressions in the 660 km discontinuity and subduction zones a result more consistent with models in which the subducting slabs are deflected horizontally at the discontinuity than with models of unhindered slab penetration into the lower mantle

observations of the south seasonal cap of mars during recession in 2004 2006 by the omega visible near infrared imaging spectrometer on board mars express 1 the omega visible near infrared imaging spectrometer on board mars express has observed the southern seasonal cap in late 2004 and 2005 and then in the summer of 2006 these observations extended from the period of maximum extension close to the southern winter solstice to the end of the recession at ls 325 the spectral range and spectral resolution of omega make it possible to monitor the extent and effective grain size of co2 ice and h2o ice on the ground the level of contamination of co2 ice and h2o ice by dust and the column density of m sized ice grains in the atmosphere the co2 seasonal cap is very clean and clear in early southern winter contamination by h2o ice spreads eastward from the hellas basin until the southern spring equinox during southern spring and summer there is a very complex evolution in terms of effective grain size of co2 ice and contamination by dust or h2o ice h2o ice does not play a significant role close to the southern summer solstice contamination of co2 ice by h2o ice is only observed close to the end of the recession as well as the few h2o ice patches already reported by bibring et al 2004a these observations have been compared to the results of a general circulation model with good qualitative agreement on the distribution of h2o ice on the surface and in the atmosphere resolving the remaining discrepancies will improve our understanding of the water cycle on mars

anisotropic shear wave velocity structure of the earth s mantle a global model 1 we combine a new large data set of surface wave phase anomalies long period waveforms and body wave travel times to construct a three dimensional model of the anisotropic shear wave velocity in the earth s mantle our modeling approach is improved and more comprehensive compared to our earlier studies and involves the development and implementation of a new spherically symmetric reference model simultaneous inversion for velocity and anisotropy as well as discontinuity topographies and implementation of nonlinear crustal corrections for waveforms a comparison of our new three dimensional model s362ani with two other models derived from comparable data sets but using different techniques reveals persistent features 1 strong 200 km thick high velocity anomalies beneath cratons likely representing the continental lithosphere underlain by weaker fast anomalies extending below 250 km which may represent continental roots 2 weak velocity heterogeneity between 250 and 400 km depths 3 fast anomalies extending horizontally up to 2000 3000 km in the mantle transition zone beneath subduction zones 4 lack of strong long wavelength heterogeneity below 650 km suggesting inhibiting character of the upper mantle lower mantle boundary and 5 slow velocity superplumes beneath the pacific and africa the shear wave radial anisotropy is strongest at 120 km depth in particular beneath the central pacific lateral anisotropic variations appreciably improve the fit to data that are predominantly sensitive to the uppermost and lowermost mantle but not to the waveforms that control the transition zone and midmantle depths tradeoffs between lateral variations in velocity and anisotropy are negligible in the uppermost mantle but noticeable at the bottom of the mantle

simultaneous inversion for mantle shear velocity and topography of transition zone discontinuities summary until now modelling of three dimensional 3 d velocity variations in the mantle and topography of the transition zone discontinuities have been considered separately velocity models were obtained assuming that the radii of the discontinuities are constant then the travel time data sensitive to the topography such as the ss precursors were corrected for the effect of 3 d seismic structure and inverted for depth variations of a discontinuity such a procedure is unsatisfactory as it may introduce artefacts that could significantly affect the topographic results the opposite trade off is less likely to introduce conceptually important changes in the velocity distribution but should also be considered in this study we bring together the same set of s velocity sensitive data as used by gu et al and combine it with a large set of differential travel times of ss s400s s s670s and direct measurements of s400s s670s we formulate the inverse problem in terms of the volumetric 3 d and topographic 2 d perturbations for both the 400 and 670 km discontinuities the best fitting model of the joint inversion significantly improves the variance reduction of ss s400s and ss s670s residuals the velocity distribution in the resulting model topos362d1 is very similar to that in model s362d1 with correlation coefficients 0 9 throughout the mantle which indicates that lateral variations of discontinuity depths have only minor influence on global modelling of velocity important changes however have been made to the topography of the 400 and 670 km discontinuities with respect to those obtained earlier assuming an existing velocity model the overall undulation of the 400 km discontinuity is considerably less than that reported by earlier global studies in topos362d1 its maximum variation does not exceed 12 km the strong degree 1 component before the joint inversion has decreased such that the correlation between the velocities above the discontinuity and the shape of the discontinuity itself has substantially diminished spatially this result means a significant diminution in the strength of the earlier reported depression of the 400 km discontinuity under the pacific the power spectrum of the topography of the 670 km discontinuity has been enriched in long wavelength component especially in degree 2 the range of depth variations is 18 km and its shape correlates well with the radially averaged velocity perturbations in the transition zone at wavelengths greater than 1000 km there is little correlation between the depth perturbations of the 400 and 670 km discontinuities the topography of the 400 km discontinuity does not appear to be strongly influenced by thermal structures potentially associated with subduction processes and plumes this implies that thermal influence on the olivine to phase transformation may not fully account for the observed depth variations dynamical effects and potential variations in composition may be important near the top of the transition zone

variability of mars north polar water ice cap i analysis of mariner 9 and viking orbiter imaging data abstract previous studies interpreted differences in ice coverage between mariner 9 and viking orbiter observations of mars north residual polar cap as evidence of interannual variability of ice deposition on the cap however these investigators did not consider the possibility that there could be significant changes in the ice coverage within the northern residual cap over the course of the summer season our more comprehensive analysis of mariner 9 and viking orbiter imaging data shows that the appearance of the residual cap does not show large scale variance on an interannual basis rather we find evidence that regions that were dark at the beginning of summer look bright by the end of summer and that this seasonal variation of the cap repeats from year to year our results suggest that this brightening was due to the deposition of newly formed water ice on the surface we find that newly formed ice deposits in the summer season have the same red to violet band image ratios as permanently bright deposits within the residual cap we believe the newly formed ice accumulates in a continuous layer to constrain the minimum amount of deposited ice we used observed albedo data in conjunction with calculations using mie theory for single scattering and a delta eddington approximation of radiative transfer for multiple scattering the brightening could have been produced by a minimum of 1 a 35 m thick layer of 50 m sized ice particles with 10 dust or 2 a 14 m thick layer of 10 m sized ice particles with 50 dust

determination and analysis of long wavelength transition zone structure using ss precursors summary global mapping of 410 and 660 km discontinuity topography and transition zone thickness has proven to be a powerful tool for constraining mantle chemistry dynamics and mineralogy numerous seismic and mineral physics studies suggest that the 410 km discontinuity results from the phase change of olivine to wadsleyite and the 660 km discontinuity results from the phase change of ringwoodite to perovskite and magnesiowustite underside reflections of the 410 and 660 km discontinuities arrive as precursors to ss with the recent development of a semi automated method of determining ss arrivals we have more than tripled the flanagan and shearer 1998a data set of handpicked ss waveforms we are able to increase resolution by stacking waveforms in 5 rather than 10 radius bins as well as increasing data coverage significantly in the southern hemisphere the resulting ss s410s and ss s660s times are heavily influenced by upper mantle velocity structure we perform a joint inversion for discontinuity topography and velocity heterogeneity as well as performing a simple velocity correction to the precursor differential times and find little difference between the two methods the 660 km discontinuity topography and transition zone thickness are correlated with velocities in the transition zone whereas the 410 km discontinuity topography is not in addition the 410 km discontinuity topography is not correlated with the 660 km discontinuity topography rather anticorrelated as expected due to the opposite signs of the clapeyron slopes of their respective phase changes these results suggest that whereas the topography of 660 km discontinuity could be dominated by thermal effects the topography of the 410 km discontinuity is likely dominated by compositional effects in addition unlike previous studies which find less topography on the 410 km discontinuity than on the 660 km discontinuity our 410 and 660 km topography have similar amplitudes

sensitivity kernels of pp precursor traveltimes and their limitations for imaging topography of discontinuities we analyze the sensitivity of pp precursor traveltimes that are often used to infer lateral variation in the depths of the 410 and 660 km discontinuities in the mantle previous results were inconclusive due to complex wave phenomena such as multiple energy conversions and focusing defocusing that hamper their interpretation using spectral element synthetics and frechet derivatives calculated with adjoint methods we compute sensitivity kernels for volumetric and boundary parameters in a 1 d model for representative epicentral distances of past studies and a dominant period of 11 25 s our results indicate that the boundary sensitivity of pp precursors is low and that these phases are not coherently seen in exact synthetics our most important finding is the strong sensitivity to both shear and compressional wave speeds indicating that wave interference and wave conversions are dominant the pp precursor traveltimes appear more sensitive to structural parameters that is compressional and shear wave speed than to the boundaries therefore they are unlikely sources for valuable insight into discontinuity topography

the nature of the 660 kilometer upper mantle seismic discontinuity from precursors to the pp phase global seismic network data were used to image upper mantle seismic discontinuities stacks of phases that precede the pp phase thought to be underside reflections from the upper mantle discontinuities at depths of 410 and 660 kilometers show that the reflection from 410 kilometers is present but the reflection from 660 kilometers is not observed a continuous lame s constant lambda and seismic parameter at the 660 kilometer discontinuity explain the missing underside p reflections and lead to a p wave velocity jump of only 2 percent whereas the s wave velocity and density remain unchanged with respect to previous global models the model deemphasizes the role of lame s constant lambda with regard to the shear modulus and constrains the mineralogical composition across the discontinuity

ss wave sensitivity to upper mantle structure implications for the mapping of transition zone discontinuity topographies 1 studies of the depths of the upper mantle discontinuities have relied on the differential travel times between the ss wave and its precursors we examine the effect of velocity heterogeneities on these differential travel time data ray theory which is used to correct these data for the effect of velocity heterogeneities is shown to be inaccurate if the characteristic heterogeneity length scales are similar to those seen in regional models or if the station is near the ss wave radiation nodal plane

perennial water ice identified in the south polar cap of mars the inventory of water and carbon dioxide reservoirs on mars are important clues for understanding the geological climatic and potentially exobiological evolution of the planet1 from the early mapping observation of the permanent ice caps on the martian poles2 3 the northern cap was believed to be mainly composed of water ice whereas the southern cap was thought to be constituted of carbon dioxide ice however recent missions nasa missions mars global surveyor and odyssey have revealed surface structures4 altimetry profiles5 underlying buried hydrogen6 and temperatures of the south polar regions that are thermodynamically consistent with a mixture of surface water ice and carbon dioxide7 here we present the first direct identification and mapping of both carbon dioxide and water ice in the martian high southern latitudes at a resolution of 2 km during the local summer when the extent of the polar ice is at its minimum we observe that this south polar cap contains perennial water ice in extended areas as a small admixture to carbon dioxide in the bright regions associated with dust without carbon dioxide at the edges of this bright cap and unexpectedly in large areas tens of kilometres away from the bright cap

an analysis of ss precursors using spectral element method seismograms summary the traveltimes of underside shear wave reflections i e ss precursors are widely used data for mapping the topography of mantle discontinuities in the upper mantle here we examine the accuracy of ray theory in estimating the contribution of shear velocity heterogeneity in the mantle to ss s400s ss s670s and s400s s670s traveltimes we analyse stacks of spectral element method waveforms computed for 34 shallow earthquakes and 16 020 globally distributed virtual seismometers the waveforms are computed for three versions of shear velocity model s20rts with variable strengths of the volumetric shear velocity perturbations within the layered structure of the preliminary reference earth model we find that ray theoretical corrections account for only 50 per cent of the traveltime variation due to large scale velocity heterogeneity for current tomographic models as represented by s20rts this translates into unpredictable long and short wavelength errors in maps of mantle discontinuity depths of about 5 km this amounts to roughly 10 per cent of published variations in mantle discontinuity depths however relative errors will be significantly larger if ray theoretical traveltime corrections are based on forthcoming models of shear velocity with much stronger heterogeneity at the smallest scales

multiresolution imaging of mantle reflectivity structure using ss and p p precursors summary knowledge of the mantle reflectivity structure is highly dependent on our ability to efficiently extract and properly interpret small seismic arrivals among the various data types and techniques long period ss pp precursors and high frequency receiver functions are routinely utilized to increase the confidence of the recovered mantle stratifications at distinct spatial scales however low resolution and a complex fresnel zone are glaring weaknesses of ss precursors while over reliance on receiver distribution is a formidable challenge for the analysis of converted waves from oceanic regions a promising high frequency alternative to receiver functions is ppprecursors which are capable of resolving mantle structures at verticalandlateralresolutionof 5and 200km respectively owingtotheirspectralcontent shallow angle of incidence and near symmetric fresnel zones this study presents a novel processing method for both ss or pp and ppprecursors based on deconvolution stacking radon transform and depth migration a suite of synthetic tests is performed to quantify the fidelity and stability of this method under different data conditions our multiresolution survey of the mantle at targeted areas near nazca south america subduction zone reveal both olivine and garnet related transitions at depths below 400km we attribute a depressed 660 to thermal variations whereas compositional variations atop the upper mantle transition zone are needed to explain the diminished or highly complex reflected scattered signals from the 410km discontinuity we also observe prominent ppreflections within the transition zone and the anomalous amplitudes near the plate boundary zone indicate a sharp 10km thick transition that likely resonates with the frequency content of ppprecursors the migration of ss precursors in this study shows no evidence of split 660 reflections but potential majorite ilmenite 590 640km and ilmenite perovskite transitions 740 750km areidentifiedbasedonsimilarlyprocessedhigh frequencyppprecursors additionalfindings of severely scattered energy in the lithosphere and distinct lower mantle reflections at 800km could be potentially important but require further verifications overall our improved imaging methods and the strong sensitivity of ppprecursors to the existence depth sharpness and strengthofreflective structuresoffersignificant futurepromisefortheunderstanding ofmantle mineralogy and dynamics

seismic constraints on mantle flow and topography of the 660 km discontinuity evidence for whole mantle convection recent seismic models of three dimensional mantle structure and topography on the transition zone seismic discontinuities permit the direct evaluation of the buoyancy forces that drive large scale mantle flow this buoyancy distribution coupled with radial viscosity models that are consistent with the observed geoid predicts flow patterns with significant vertical flow through the 660 km phase boundary the observed topography on the 660 km discontinuity acts to inhibit convection across the boundary but is two to four times too small to prevent whole mantle convection

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

the effect of topography of upper mantle discontinuities on ss precursors using the spectral element method we explored the effect of topography of upper mantle discontinuities on the traveltimes of ss precursors recorded on transverse component seismograms the latter are routinely used to infer the topography of mantle transition zone discontinuities the step from precursory traveltimes to topographic changes is mainly done using linearised ray theory or sometimes using finite frequency kernels we simulated exact seismograms in 1 d and 3 d elastic models of the mantle in a second simulation we added topography to the discontinuities we compared the waveforms obtained with and without topography by cross correlation of the ss precursors since we did not add noise the precursors are visible in individual seismograms without the need of stacking the resulting time anomalies were then converted into topographic variations and compared to the original topographic models based on the correlation between initial and inferred models and provided that ray coverage is good we found that linearised ray theory gives a relatively good idea on the location of the uplifts and depressions of the discontinuities it seriously underestimates the amplitude of the topographic variations by a factor ranging between 2 and 7 real data depend on the 3 d elastic structure and the topography all studies to date correct for the 3 d elastic effects assuming that the traveltimes can be linearly decomposed into a structure and a discontinuity part we found a strong non linearity in this decomposition which cannot be modelled without a fully non linear inversion for elastic structure and discontinuities simultaneously

constraints on upper mantle discontinuities from observations of long period reflected and converted phases stacked images combining over 5 years of long period global digital seismograph network data reveal many phases associated with reflections and conversions from upper mantle discontinuities these images show travel time and amplitude relative to a reference phase which is aligned and normalized on all seismograms prior to stacking results obtained for p s ss and pp reference arrivals resolve numerous phases from discontinuities near 410 and 660 km while some of the stacks also show evidence for a weaker discontinuity near 520 km phases of particular interest include p and sh multiples resulting from topside reflections precursors to ss from underside reflections and p to sv converted phases these phases can be clearly seen both in the waveform stacks and in cross correlation analysis of individual seismograms travel times for these arrivals are converted to discontinuity depths relative to velocities in the preliminary reference earth model and empirical corrections are applied for the effects of lateral velocity variations in the upper mantle average apparent depths to the discontinuities for the different phases agree to within 3 km for the 410 km discontinuity to within 4 km for the 520 km discontinuity and to within 8 km for the 660 km discontinuity the best global averages are obtained from the ss precursor data which indicate discontinuities at 415 519 and 659 km discontinuity depths obtained from the p to sv converted phases at over 35 individual seismic stations exhibit variations of less than 20 km apparent depths to the 660 km discontinuity consistently show greater variability than depths to the 410 km discontinuity supporting recent laboratory results which indicate that the clapeyron slope for the 660 km discontinuity is significantly larger in magnitude than the slope for the 410 km discontinuity precursors to ss seen between 110 and 180 are particularly useful for mapping possible lateral variations in discontinuity depths since each arrival can be associated with a single underside reflection point apparent discontinuity depths computed from ss precursors for different tectonic regions agree to within about 5 km the ss precursors have especially good coverage near the subducting slabs in the northwest pacific analysis of apparent discontinuity depths in this area suggests the presence of a broad 1000 to 1500 km wide region near the slab in which the 660 km discontinuity is depressed by about 20 km measuring absolute amplitudes for these phases is difficult due to the large corrections required to compensate for the effects of incoherent stacking relative amplitude analysis suggests that the p and s wave impedance changes at 410 km are about 0 8 1 1 times the size of the changes at 660 km and that the contrasts at 520 km are between 0 3 and 0 6 of the changes at 410 km

historical damage pattern and differential seismic effects in a town with ground cavities a case study from southern italy abstract this paper presents the results of a study focused on the analysis of differential seismic effects on a town characterized by the presence of ground cavities the event analysed is the 1930 irpinia earthquake io x mcs me 6 7 whereas the site investigated is rionero in vulture basilicata southern italy this town is built as several historical centres over the world on man made ground cavities going back to the past centuries due to the high quality of available historical information regarding the 1930 seismic event the analysed town represents an important case study both to improve the reconstruction methodology of the damage scenario and to test the role that ground cavities play on seismic damage in a site with high seismicity to this purpose we examined about 1400 unpublished technical reports the information drawn from written sources was integrated with in situ surveys furthermore we made a comparison with seismic effects which were recorded in some italian localities having ground cavities the results of this research can provide useful information for microzonation analysis and decision making for localities where seismic territorial and anthropic features are similar to rionero in vulture the paper is divided into four parts the first one analyses the geological features of the region and the town the second part describes the historical seismicity of the site the third part deals with the methodological approach in order to reconstruct the building damage finally the fourth part is focused on the analysis of the damage pattern with appraisals of the causes of differential effects

topography of the 410 km discontinuity from pp and ss precursors a number of previous studies have confirmed the global existence of a transition zone discontinuity at 410 km depth by aligning large numbers of long period seismograms on a surface reflection phase before stacking discussion of ss and pp precursors from the 410 km discontinuity termed p410p and s410s has largely focused on the long wavelength topography of this discontinuity recent models suggest that the precursor derived topography of the 410 km discontinuity is dominated by low degree spherical harmonics however there remain some discrepancies as to the details of the topography and its relation to mantle processes here we present a new model for 410 topography which is constrained using both p410p and s410s measurements and examine the relationship between our observations of 410 topography and seismic velocity anomalies in the transition zone we find a moderate negative correlation between discontinuity topography and transition zone seismic velocity anomalies at very low spherical harmonic degrees this is consistent with the expected relationships between the position of the olivine phase boundary seismic velocity anomalies and mantle temperatures however when shorter wavelengths are included the relationship becomes more complex in smaller regions beneath northeast asia and the western pacific we observe positive correlations between seismic velocities and discontinuity topography which we interpret as evidence for the effects of chemical heterogeneities on seismic velocities and the 410 km discontinuity our results suggest there is a long wavelength temperature pattern in the mantle however at shorter wavelengths the influence of chemical heterogeneities becomes visible in seismic velocity anomalies and mantle discontinuity topography this separation of the spectra for thermal and chemical heterogeneities suggests that short wavelength chemical heterogeneities can survive in convecting mantle

multiple transition zone seismic discontinuities and low velocity layers below western united states with p to s converted waves recorded at seismic stations of the us transportable array we image the fine structure of upper mantle and transition zone tz beneath the western us we map the topographies of seismic discontinuities by stacking data by common conversion points along profiles systematic depth and amplitude measurements are performed not only for the well known 410 and 660 interfaces but also for minor seismic discontinuities identified around 350 590 and 630 km depths the amplitude of conversion suggests shear wave velocity vs increase by 4 at the 410 and the 660 the observed 660 velocity contrast is smaller than expected from the 6 in iasp91 but consistent with a pyrolitic model of mantle composition the gorda plate subducted under northern california is tracked to the tz where it seems to flatten and induce uplift of the 410 under northern nevada maps of 410 660 amplitude topography reveal that the tz is anomalous beneath the geographical borders of washington oregon and idaho with i a thickened tz ii a sharp change in depth of the 660 iii a reduced 410 conversion amplitude in the north and iv a positive 630 discontinuity such anomalous structure might be inherited from the past history of plate subduction accretion a thinned tz under the yellowstone suggests higher than average temperatures perhaps due to a deep thermal plume both the 350 and the 590 negative discontinuities extend over very large areas they might be related either to an increased water content in the tz a significant amount of oceanic material accumulated through the past 100 my or both

marci and moc observations of the atmosphere and surface cap in the north polar region of mars abstract we used mgs moc and mro marci daily mapping images of the north polar region of mars from 16 august 2005 ls 270 to 21 may 2009 ls 270 covering portions of three consecutive martian years my 27 my 29 to observe the seasonal behavior of the polar ice cap and atmospheric phenomena the rate of cap regression was similar in my 28 and my 29 but was advanced by 3 5 of ls 7 8 sols in my 29 the spatial and temporal behaviors of dust and condensate clouds were similar in the two years and generally in accord with prior years dust storms 100 km2 were observed in all seasons with peak activity occurring at ls 10 20 from 50 n to 70 n and at ls 135 140 from 70 n to 90 n the most active quadrant was 0 90 w in my 28 shifting to 180 270 w in my 29 the majority of regional storms in both years developed in longitudes from 10 w to 60 w during late summer the larger storms obscure the north polar region in a cloud of dust that transitions to north polar hood condensate clouds around autumnal equinox changes in the distribution of perennial ice deposits especially in olympia planum were observed between the 2 years with the my 29 ice distribution being the most extensive observed to date modeling suggests that the small bright ice patches on the residual cap are not the result of slope or elevation effects rather we suggest that they are the result of local meteorological effects on ice deposition the annual darkening and brightening of peripheral areas of the residual cap around summer solstice can be explained by the sublimation of a brighter frost layer revealing an underlying darker ice rich layer that itself either sublimes to reveal brighter material below or acts as a cold trap attracting condensation of water vapor that brightens the surface an alternative explanation invokes transport and deposition of dust on the surface from the cap interior and later removal of that dust the decrease in cap albedo and accompanying increase in near surface atmospheric stability may be related to the annual minimum of polar storm activity near northern summer solstice

upper mantle discontinuity topography from thermal and chemical heterogeneity using high resolution stacks of precursors to the seismic phase ss we investigated seismic discontinuities associated with mineralogical phase changes approximately 410 and 660 kilometers km deep within earth beneath south america and the surrounding oceans detailed maps of phase boundary topography revealed deep 410 and 660 km discontinuities in the down dip direction of subduction inconsistent with purely isochemical olivine phase transformation in response to lowered temperatures mechanisms invoking chemical heterogeneity within the mantle transition zone were explored to explain this feature in some regions multiple reflections from the discontinuities were detected consistent with partial melt near 410 km depth and or additional phase changes near 660 km depth thus the origin of upper mantle heterogeneity has both chemical and thermal contributions and is associated with deeply rooted tectonic processes

mantle transition zone structure along a profile in the sw pacific thermal and compositional variations summary events from the south pacific recorded in the tien shan region are studied with a migration method to measure discontinuity depth along a profile between the mariana and molucca sea subduction zones deflections of the upper mantle discontinuities within and between the subduction zones are investigated using pp precursors filtered to periods of 3 to 10 s we find p wave reflections near the 410 520 and 660 km discontinuities the 410 km discontinuity is elevated in the subduction region and near the predicted depth 410 km in the average velocity region in some cases we find negative polarities of the reflection from the 410 km discontinuity which may indicate water induced melting above the olivine to wadsleyite transition the 520 km discontinuity is shallow between the two subduction zones and deepens towards the mariana backarc region this discontinuity topography may be due to detection of both the wadsleyite to ringwoodite transition in a depleted downwelling elevated region and the formation of calcium perovskite in a more fertile upwelling depressed region the 660 km discontinuity is split in the halmahera slab with one reflection each near 600 and 700 km depth consistent with a cold sinking slab and detection of phase transitions for both garnet and olivine to perovskite respectively in the region between the subduction areas we find a downward deflection of the 660 km discontinuity with short length scale 100 300 km undulations that may reflect compositional variation

vertical distribution of dust and water ice aerosols from crism limb geometry observations 1 near infrared spectra taken in a limb viewing geometry by the compact reconnaissance imaging spectrometer for mars crism on board the mars reconnaissance orbiter provide a useful tool for probing atmospheric structure specifically the observed radiance as a function of wavelength and height above the limb enables the vertical distribution of both dust and water ice aerosols to be retrieved more than a dozen sets of crism limb observations have been taken so far providing pole to pole cross sections spanning more than a full martian year radiative transfer modeling is used to model the observations taking into account multiple scattering from aerosols and the spherical geometry of the limb observations both dust and water ice vertical profiles often show a significant vertical structure for nearly all seasons and latitudes that is not consistent with the well mixed or conrath v assumptions that have often been used in the past for describing aerosol vertical profiles for retrieval and modeling purposes significant variations are seen in the retrieved vertical profiles of dust and water ice aerosol as a function of season dust typically extends to higher altitudes 40 50 km during the perihelion season than during the aphelion season 20 km and the hellas region consistently shows more dust mixed to higher altitudes than other locations detached water ice clouds are common and water ice aerosols are observed to cap the dust layer in all seasons

sensitivity analysis of seismic waveforms to upper mantle discontinuities using the adjoint method using spectral element simulations of wave propagation we investigated the sensitivity of seismic waveforms recorded on transverse components to upper mantle discontinuities in 1 d and 3 d background models these sensitivity kernels or frechet derivatives illustrate the spatial sensitivity to model parameters of which those for shear wave speed and the surface topography of internal boundaries are discussed in this paper we focus on the boundaries at 400 and 670 km depth of the mantle transition zone ss precursors have frequently been used to infer the topography of upper mantle discontinuities these seismic phases are underside reflections off these boundaries and are usually analysed in the distance range of 110 160 this distance range is chosen to minimize the interference from other waves we show sensitivity kernels for consecutive time windows at three characteristic epicentral distances within the 110 160 range the sensitivity kernels are computed with the adjoint method using synthetic data from our simulations we can draw three main conclusions i the exact frechet derivatives show that in all time windows and also in those centred on the ss precursors there is interference from other waves this explains the difficulty reported in the literature to correct for 3 d shear wave speed perturbations even if the 3 d structure is perfectly known ii all studies attempting to map the topography of the 400 and 670 km discontinuities to date assume that the traveltimes of ss precursors can be linearly decomposed into a 3 d elastic structure and a topography part we recently showed that such a linear decomposition is not possible for ss precursors and the sensitivity kernels presented in this paper explain why iii in agreement with previous work we show that other parts of the seismograms have greater sensitivity to upper mantle discontinuities than ss precursors especially multiply bouncing s waves exploiting the s wave triplications due to the mantle transition zone these phases can potentially improve the inference of global topographic variations of the upper mantle discontinuities in the context of full waveform inversion in a joint inversion for an elastic parameters and topography

martian clouds observed by mars global surveyor mars orbiter camera we have made daily global maps that cover both polar and equatorial regions of mars for ls 135 360 and 0 111 using the mars global surveyor mgs mars orbiter camera moc red and blue wide angle swaths taken from may 1999 to january 2001 we study the seasonal distribution of condensate clouds and dust clouds during roughly 1 martian year using these daily global maps we present the development and decay of the tropical cloud belt and the polar hoods the spatial and temporal distributions of lee waves and spiral clouds and an unusual aster cloud above the volcanoes consisting of rays around a central disk like the flower the tropical cloud belt contains mostly fibrous clouds during northern spring early summer and convective clouds during middle late northern summer the detailed development and decay of the tropical cloud belt is nonuniform in longitude two distinct stormy periods in late summer precede the formation of the north and south circumpolar hoods the north polar storms in late summer resemble baroclinic frontal systems on earth but contain both dust and condensate clouds spiral clouds occur only in the northern high latitudes and only during northern spring and summer the north polar hood displays a stationary wave number two structure during the fall and winter the south polar hood has fewer streak clouds and lee wave clouds than the north polar hood during this particular year the lee wave cloud abundance in the south had two peaks in early fall and late winter and the lee wave cloud abundance in the north had one peak in early fall

imaging mantle transition zone thickness with sds ss finite frequency sensitivity kernels summary we invert differential sds ss traveltime residuals measured from stacked waveforms and finitefrequency sensitivity kernels for topography on the 410 and 660 km discontinuities this approach yields higher resolution images of transition zone thickness than previous stacking methods which simply average smooth over topographic features apparent structure measured using simple stacking is highly dependent upon the bin size of each stack by inverting for discontinuity topography with a variety of bin sizes we can more accurately calculate the true structure the inverted transition zone model is similar to simple stack models with an average thickness of 242 km but the lateral variations in thickness are larger in amplitude and smaller in scale fast seismic velocities in 3 d mantle models such as sb4l18 correlate with areas of thicker transition zone the elongated curvilinear regions of thickened transition zone that occur near subduction zones are narrow and high amplitude which suggests relatively little lateral spreading and warming of subducted lithosphere within the transition zone the anomalously thin transition zone regions are laterally narrow and not broadly continuous if these variations in transition zone thickness are interpreted as thermal in nature then this model suggests significant temperature variations on small lateral scales

a mesoscale model study of summertime atmospheric circulations in the north polar region of mars 1 the oregon state university mars mm5 was used in a comprehensive high resolution study of northern polar summertime circulations three simulations ls 120 ls 135 and ls 150 characterize the changing circulation the atmosphere is dry and model dynamics are hydrostatic a modified tes thermal inertia map provides a realistic simulation of the polar thermal environment the highest resolution nest 18 km resolves complex flows near the cap zonal mean easterlies 10 m s and zonal mean katabatic winds 5 m s near the surface are relatively steady during this study katabatic flows are shallow 300 m the easterlies are deeper 1 5 km transient eddies are very important within the first scale height they are excited by mechanisms and at locations that change dramatically during this short study period at ls 120 they form along the residual cap edge with a zonal wave number one structure producing strong excursion winds 10 15 m s that blow consistently across the cap by ls 135 strong eddies are seen to form on the northern slopes of alba patera and tharsis these eddies are quite suggestive of the large annular cloud structures seen in hubble space telescope and mars orbital camera imagery at this location and season and can traverse the high latitudes to reach the residual cap before dissipating eddies in the earlier two simulations appear to be primarily excited by energetic flows near the surface by ls 150 an early fall polar jet causes strong winter like baroclinic eddies to develop the transient eddies found in this study are probably important in the water cycle of the northern residual cap

lateral variations in mantle velocity structure and discontinuities determined from p pp s ss and ss sds travel time residuals on the basis of p pp s ss arrival times and ss s410s ss s660s differential times we construct models of mantle p and s velocity structure and boundary topography of the 410 km and 660 km discontinuities events from the catalog of the international seismological centre isc are relocated relative to the international association of seismology and physics of the earth s interior 1991 iasp91 velocity model using both p and s arrival times the arrival times are corrected for ellipticity and the pp and ss residuals are corrected for the topography at the bounce point the cap averaged pp p and ss s differential time residuals plotted at the pp and ss surface reflection points form broad coherent patterns the geographic distribution of the cap averaged residuals agrees quite well with pp p and ss s differential time residuals derived from long period global digital seismograph network gdsn data a robust lp inversion scheme is used to infer global mantle structure synthetic tests indicate that for regions well sampled by ss s410s and ss s660s differential times the velocity estimates are not seriously contaminated by the topography of the 410 and 660 km discontinuities however estimates of boundary deflections may be influenced by extensive p and s velocity variations of 3 or greater we find the 410 km discontinuity to be depressed by as much as 24 km beneath north america conversely the discontinuity is deflected upward underneath eurasia in some regions the topography of the 660 km discontinuity is quite distinct from that of the 410 km discontinuity but the two appear to be positively correlated a series of depressions are found at several intersections of the 660 km discontinuity with known subduction zones the elevated topography in the 410 km discontinuity beneath europe is underlain by a trough in the 660 km discontinuity a number of subduction zones are characterized by a thinning of the transition zone negative p and s velocity anomalies underlying back arc basins and tectonically active continental regions encircle the pacific where they are resolved the stable continental cratons are systematically positive velocity features that extend below 200 km with the inclusion of pp and ss travel time residuals we are better able to constrain midmantle structure most notably in the depth range 35 660 km beneath the northwest pacific we observe high p velocity where they are resolved mid ocean ridges are most clearly imaged as low velocity features in the s model the northern portion of the mid atlantic ridge is underlain by negative s velocity anomalies in the pacific the east pacific rise is an extensive low s velocity anomaly

densification of water ice deposits on the residual north polar cap of mars abstract understanding the density evolution of any ice deposits on the martian north polar residual cap is key to understanding whether the cap contains a climate record in the form of occluded samples of past atmospheres we present a physical model for densification of water ice deposits on the cap due to gravity driven sintering and temperature gradient driven vapor transport the model predicts the steady state profile of porosity versus depth in the ice cap for specified time independent mass accumulation rate and surface deposition density and for surface temperatures with specified diurnal and seasonal variations that comprise an invariant annual cycle we find that in contrast to the analogous situation on earth vapor transport dominates densification in the current martian climate this mechanism can produce very low porosity near surface ice even when accumulation rates are small but positive comparable to those presently thought to occur thus recent inferences of high thermal inertia on the cap may not necessarily imply that the cap is ablating the model predicts a sharp transition between temperature and accumulation conditions under which a deep firn column results and those characterized by very low surface porosity finally we derive a temporal frequency dependent thermal inertia for a vertically inhomogeneous medium observations of this quantity are at least in principle useful for remote estimation of the ice cap density variations within about 10 m of the surface

summer evolution of the north polar cap of mars as observed by omega mars express the observatoire pour la mineralogie l eau les glaces et l activite omega visible infrared imaging spectrometer extensively observed regions of mars with latitudes above 70 n in late 2004 heliocentric longitude from ls 93 to ls 127 the extent of water ice at the surface and the size of ice grains were monitored as a function of time bright small grained frost which initially covered a large fraction of the polar cap waned in favor of large grained ice in outlying regions dominated by large grained ice the albedo increased over the period evaluating the dust content was model dependent however contamination of ice by dust was low

a radiative transfer model for sea ice with vertical structure variations a radiative transfer model designed for application to snow covered sea ice with vertical layering is presented which removes several of the computational problems associated with media whose optical properties are strong functions of wavelength and depth with this model it is possible to treat the effects of biological activity in the ice as well as vertical variations in brine volume and bubble density transmission spectra can also be calculated as a function of depth in the water column beneath the ice accurate results are achieved for both direct and diffuse incident irradiance for single scattering albedos ranging from 0 0 to 0 99999999 calculated albedos and extinction coefficients for snow and sea ice are consistent with both observations and previous models and the model results resolve apparent inconsistencies among certain observational data sets refraction is included for all layer interfaces and is found to reduce the albedo while the transmissivity responds to changes in the optical depth in addition to redirection of the radiation for bare sea ice ignoring refraction can introduce albedo errors as great as 24 and transmissivity errors of more than 70 absorption by algae in the ice is shown to reduce the transmitted radiation by more than a factor of 10 at certain visible wavelengths the predicted azimuthal dependence of the emergent radiances is very weak even for large solar incidence angles

global variability of transition zone thickness 1 we analyze recordings from more than 3000 shallow earthquakes and measure s410s s660s differential travel times on a global scale these differential times are obtained using a novel cross correlation technique that eliminates effects associated with a priori assumptions of crustal thickness and velocity variations above the upper mantle transition zone our measurements show absolute perturbations up to 8 s from the global average 242 km which imply transition zone thickness variations of 20 km on a length scale of 2000 km the transition zone under major subduction zones e g the western pacific and south america is significantly thicker than the global average this could be associated with large scale depressions of the 660 km discontinuity due to the accumulation of cold oceanic lithosphere at the base of the upper mantle a narrow transition zone is observed under the central pacific and northern atlantic oceans we also observe a modest anticorrelation between s410s s660s times and the delay times of shear waves predicted using s12 wm13 su et al 1994 in the transition zone which suggests considerable thermal influence on the large scale topography of the 410 and 660 km discontinuities the lack of a stronger anticorrelation suggests that 1 compositional heterogeneities may be present and 2 a significant fraction of thermal anomalies near the transition zone do not extend through the transition zone and influence both the 410 and the 660 km discontinuities a regional analysis shows that the transition zone is on average 6 8 km thicker under continents than under oceans this implies a temperature difference of 50 70 c however we do not observe a clear correlation between thickness and the age of the crust

subsurface structure of planum boreum from mars reconnaissance orbiter shallow radar soundings abstract we map the subsurface structure of planum boreum using sounding data from the shallow radar sharad instrument onboard the mars reconnaissance orbiter radar coverage throughout the 1 000 000 km2 area reveals widespread reflections from basal and internal interfaces of the north polar layered deposits npld a dome shaped zone of diffuse reflectivity up to 12 s 1 km thick underlies two thirds of the npld predominantly in the main lobe but also extending into the gemina lingula lobe across chasma boreale we equate this zone with a basal unit identified in image data as amazonian sand rich layered deposits byrne s murray b c 2002 j geophys res 107 5044 12 pp doi 10 1029 2001je001615 fishbaugh k e head j w 2005 icarus 174 444 474 tanaka k l rodriguez j a p skinner j a bourke m c fortezzo c m herkenhoff k e kolb e j okubo c h 2008 icarus 196 318 358 elsewhere the npld base is remarkably flat lying and co planar with the exposed surface of the surrounding vastitas borealis materials within the npld we delineate and map four units based on the radar layer packets of phillips et al phillips r j and 26 colleagues 2008 science 320 1182 1185 that extend throughout the deposits and a fifth unit confined to eastern gemina lingula we estimate the volume of each internal unit and of the entire npld stack 821 000 km3 exclusive of the basal unit correlation of these units to models of insolation cycles and polar deposition laskar j levrard b mustard j f 2002 nature 419 375 377 levrard b forget f montmessin f laskar j 2007 j geophys res 112 e06012 18 pp doi 10 1029 2006je002772 is consistent with the 4 2 ma age of the oldest preserved npld obtained by levrard et al levrard b forget f montmessin f laskar j 2007 j geophys res 112 e06012 18 pp doi 10 1029 2006je002772 we suggest a dominant layering mechanism of dust content variation during accumulation rather than one of lag production during periods of sublimation

h2o grain size and the amount of dust in mars residual north polar cap in mars north polar cap the probable composition of material residual from the annual condensation cycle is a mixture of fine dust and h2o grains of comparable size and abundance however metamorphism of such material will gradually lower its albedo by increasing the size of the h2o grains only if the cap is undergoing net annual sublimation as inferred from water vapor observations late summer observations should be of old ice with h2o grain sizes of 100 microns or more ice of this granularity containing 30 percent fine dust has a reflectivity similar to that of dust alone the observed albedo and computed ice grain size imply dust concentrations of 1 part per 1000 or less the brightness of the icy areas conflicts with what would be expected for a residual cap deposited by an annual cycle similar to that observed by viking and aged for thousands of years the residual cap surface cannot be old dirty ice it could be old coarse and clean or it could be young fine and dirty this brings into question both the source of the late summer water vapor and the formation rate of laminated terrain

receiver function imaging of upper mantle complexity beneath the pacific northwest united states abstract small scale topographic variations on the upper mantle seismic discontinuities provide important constraints on the thermal influences of upwellings and downwellings in geodynamically complex regions subduction of the juan de fuca plate and other tectonic processes dominating the pacific northwest united states in the cenozoic involve massive thermal flux that likely result in an upper mantle that has strong 3 d temperature variations we address the interaction of such processes in the region using receiver functions to image the upper mantle seismic discontinuities at 410 and 660 km we utilized over 15 000 high quality receiver functions gathered from 294 teleseismic earthquakes recorded at 277 regional broadband seismic stations primarily those of the earthscope usarray transportable array we find the average depths of the discontinuities to be 412 km and 658 km respectively with no obvious 520 km discontinuity detected the peak to peak range is greater on the 410 than the 660 suggesting the possibility of more significant regional dynamic processes at upper mantle depths our results are not consistent with a mantle plume below central oregon in the high lava plains region our observation of a thinner transition zone beneath the western snake river plain region however is consistent with a regional increase in mantle temperatures perhaps due to either asthenospheric flow from beneath and around the southern edge of the juan de fuca plate or to vertical flow in the form of regional mantle upwelling related to the snake river plain yellowstone hotspot track further our results are not consistent with a simple subducting juan de fuca slab morphology but rather suggest similar levels of significant complexity in slab structure found by recent regional tomographic studies we find evidence for a thickened and therefore cooler mantle transition zone beneath the wallowa idaho batholith region consistent with tomographic models which suggest broad scale increased seismic velocities to transition zone depths we speculate that this region may be the location of mantle downwelling at larger scales than previously proposed

gross errors in upper mantle discontinuity topography from underside reflection data summary currnet models of upper mantle discontinuity topography derived from traveltime variations of long period underside reflections pdp or sds waves may contain gross errors due to the use of geometrical optics in relating observed traveltimes to depth variations of the discontinuity near the reflection point the use of geometrical optics is not valid if variations in the depth of the discontinuities exist on a lateral scale smaller than the size of the fresnel zone in the data geometrical optics does not take into account the large size and the complex structure of the traveltime surface of underside reflections and may introduce spurious structure when used in inversions examples of synthetic long period pdp waveforms for reflections near subducting lithosphere show that scattering from expected small scale depth variations of the discontinuities inside slabs causes complex waveform variations for a 15 20s pdp wave these small scale depth variations with a scale length of 4 are smaller than the fresnel zone roughly 20 but larger than the pdp wavelength at 420 km depth about 150 200 km the synthetic waveforms are processed to obtain pdp traveltimes which are in turn converted to apparent discontinuity depth at the reflection point using geometrical optics the apparent discontinuity depth variations are not only of the same order of magnitude as those obtained from real data but also exhibit artefacts that closely resemble features observed in current models it is shown that large scale wavelengths of the order of 1000 km downwarping of the 670 km discontinuity near subduction zones derived from underside reflection data can be explained as the long wavelength manifestation of a single small scale wavelengths of several hundreds of kilometres deflection of the discontinuity inside and near subducting slabs

global variations of temperature and water content in the mantle transition zone from higher mode surface waves abstract we performed a fully nonlinear joint inversion of surface wave overtone phase velocities for velocity structure down to a depth of 1500 km and topography of transition zone discontinuities using neural networks the resulting models are compared with existing mantle models from linearized inversions techniques and the agreement between vertically averaged s wave velocity models and various 660 km topography models is very good whereas the agreement between all the 400 km topography models is very poor the latter might be due to a broader than previously reported 400 km discontinuity where the minerals co exist with water induced melt from our models of transition zone thickness and s wave velocity we estimated lateral variations of temperature and water content in earth s mantle transition zone linearly relating variations in s wave velocity and transition zone thickness to variations in temperature and water content within the transition zone we observe relatively cold subduction zones as well as relatively warm regions beneath continents and hot spots we find that the transition zone is wettest away from subduction zones