the closing of tethys and the tectonics of the himalaya recent geological and geophysical data from southern tibet allow refinement of models for the closing of southern neo tethys and formation of the himalaya shelf sediments of the indian passive continental margin which pass northward into deep sea tethyan sediments of the indus tsangpo suture zone were deposited in the late cretaceous an andean type margin with a 2 500 km long trans himalayan kohistan ladakh gangdese granitoid batholith formed parallel to the southern margin of the lhasa block together with extensive andesites rhyolites and ignimbrites lingzizong formation the southern part of the lhasa block was uplifted deformed and eroded between the cenomanian and the eocene in the western himalaya the kohistan island arc became accreted to the northern plate at this time the northern part of the lhasa block was affected by jurassic metamorphism and plutonism associated with the mid jurassic closure of the bangong nujiang suture zone to the north the timing of collision between the two continental plates ca 50 40 ma marking the closing of tethys is shown by 1 the change from marine flysch like to continental molasse like sedimentation in the indus tsangpo suture zone 2 the end of gangdese i type granitoid injection 3 eocene s type anatectic granites and migmatites in the lhasa block and 4 the start of compressional tectonics in the tibetan tethys and indus tsangpo suture zone south facing folds south directed thrusts after the eocene closure of tethys deformation spread southward across the tibetan tethys zone to the high himalaya deep crustal thrusting barrovian metamorphism migmatization and generation of oligocene miocene leucogranites were accompanied by south verging recumbent nappes inverting metamorphic isograds and by south directed intracontinental shear zones associated with the main central thrust continued convergence in the late tertiary resulted in large scale north directed backthrusting along the indus tsangpo suture zone more than 500 km shortening is recorded in the foreland thrust zones of the indian plate south of the suture and 150 km shortening is recorded across the indian shelf zanskar range and the indus suture in ladakh there was also large scale shortening of the karakoram and tibetan microplates north of the suture as much as 1 000 km shortening occurred in tibet the more recent deformation however involved the spreading of this thickened crust and the lateral motion of the tibetan block along major approximately east west trending strike slip fault zones

orbital forcing of the martian polar layered deposits since the first images of polar regions on mars revealed alternating bright and dark layers there has been speculation that their formation might be tied to the planet s orbital climate forcing1 2 3 4 but uncertainties in the deposition timescale exceed two orders of magnitude estimates based on assumptions of dust deposition ice formation and sublimation and their variations with orbital forcing suggest a deposition rate of 10 3 to 10 2 cm yr 1 refs 5 6 whereas estimates based on cratering rate result in values as high as 0 1 to 0 2 cm yr 1 ref 7 here we use a combination of high resolution images of the polar layered terrains8 high resolution topography9 and revised calculations of the orbital and rotational parameters of mars to show that a correlation exists between ice layer radiance as a function of depth obtained from photometric data of the images of the layered terrains and the insolation variations in summer at the martian north pole similar to what has been shown for palaeoclimate studies of the earth10 11 12 for the best fit between the radiance profile and the simulated insolation parameters we obtain an average deposition rate of 0 05 cm yr 1 for the top 250 m of deposits on the ice cap of the north pole of mars

evidence for ice flow prior to trough formation in the martian north polar layered deposits the relative importance of surface mass fluxes and ice flow in shaping the north polar layered deposits npld now or in the past remains a fundamental and open question motivated by observation of an apparent ice divide on gemina lingula also known as titania lobe we propose a two stage evolution leading to the present day topography on that lobe of the npld ice flow approximately balances surface mass fluxes in the first stage but in the second stage ice flow has minimal influence and topography is modified predominantly by the formation of troughs we focus here on evidence for the first stage by testing the fit of topography between troughs to an ice flow model we find that independent model fits on distinct flow paths closely match inter trough topography uniformly over a broad region on gemina lingula with mutually consistent and physically reasonable fitting parameters however our model requires ice to occupy and flow in spaces where troughs currently incise the ice we therefore infer that the troughs and the distribution of mass balance that caused them post date deposition of the inter trough material and its modification by flow because trough formation has apparently altered inter trough topography very little we infer that trough formation must have been rapid in comparison to the still unknown time scale of flow since troughs began to form we view the evidence for past flow as strong but we do not think that topographic evidence alone can be conclusive observations of englacial stratigraphy using orbital sounding radars will yield conclusive tests of our inferred mechanism for the formation of inter trough topography

north polar region of mars topography of circumpolar deposits from mars orbiter laser altimeter mola data and evidence for asymmetric retreat of the polar cap we have used high resolution mars orbiter laser altimeter mola data to analyze the topography morphology stratigraphy and geologic history of the martian north circumpolar deposits the present polar deposits are offset about toward 0 w from the rotational pole an arc of irregular topography concentric to olympia planitia and the cap consists of polar material remnants depressions which we interpret to be kettles frost covered and residual ice filled craters and frost patches olympia planitia originally thought to be a flat sand covered plain is characterized by a convex upward topography contiguous with the polar cap we interpret olympia planitia to represent a now dune covered extension of the polar materials together olympia planitia and the outlying deposits delineate a former extent of the polar cap topographic data have clarified relationships among the circumpolar deposits contributors to these deposits include local volcanics fluvial and aqueous sediments from outflow channels and a possible standing body of water pyroclastic ash sublimation lag from the olympia lobe and eolian reworked materials significant events in the history of the region include 1 formation of the northern lowlands 2 emplacement of volcanic plains fluvial and aqueous sedimentation and subsequent desiccation forming polygonal patterns which in part underlie the present polar layered deposits 3 formation of the polar cap composed primarily of layered deposits 4 asymmetric retreat of the olympia lobe resulting in sublimation lag deposits polar remnants and kettles and 5 continued collection and reworking of sediments by eolian processes the cause of the asymmetrical retreat of the olympia lobe is unknown

earliest accumulation history of the north polar layered deposits mars from sharad abstract the approximately 2 km thick north polar layered deposits npld are often considered to contain the most complete and detailed stratigraphic records of recent climate of mars exposures of the dense layering within troughs and scarps allowed detailed reconstructions of the latest accumulation history of these water ice deposits but we lack knowledge of their initial emplacement the shallow radar sharad onboard mars reconnaissance orbiter mro penetrates the npld to their base and detects their internal layering overcoming the limitation of scarce and scattered visible outcrops of the lowermost sequences in this study we map reflectors in sharad data that result from discrete stratigraphic horizons in order to delineate the three dimensional stratigraphy of the lowermost 500 m npld sequence and reconstruct their accumulation history we confirm the large scale lateral continuity and thickness uniformity of the deposits previously detected within the lowermost npld however stratigraphic complexity in the form of pinch outs and significant thickness variations arises when we examine single radar units we find evidence of an initially limited geographic stability of water ice within two deposits that are centered at the north pole and present day gemina lingula a period of lateral ice sheet growth followed interrupted only once by a retreat episode lower net accumulation is observed on pre existing slopes suggesting a reduction of water ice stability due to increased solar radiation incidence and or transport by katabatic winds lateral transport of water ice by wind is also suggested by thickness undulations toward the top of the sequence resembling cyclic steps water ice accumulation models based on orbital forcing predict a sequence of deposition and retreat events that is generally compatible with our reconstructed accumulation history therefore we interpret the stratigraphic complexity that we observe as regional and possibly global climate change induced by orbital forcing we also find that at least two units are completely buried within the npld and do not outcrop and that npld deposition in some places was contemporaneous with deposition of the stratigraphically underlying cavi unit in other places both of these findings show that radar reflector mapping is a necessary complement to any stratigraphic reconstruction based on visible exposures

chaotic obliquity and the nature of the martian climate recent calculations of the martian obliquity suggest that it varies chaotically on timescales longer than about 107 years and varies between about 0 and 60 we examine the seasonal water behavior at obliquities between 40 and 60 up to several tens of centimeters of water may sublime from the polar caps each year and possibly move to the equator where it is more stable co2 frost and co2 h2o clathrate hydrate are stable in the polar deposits below a few tens of meters depth so that the polar cap could contain a significant co2 reservoir if co2 is present it could be left over from the early history of mars also it could be released into the atmosphere during periods of high obliquity causing occasional periods of more clement climate

testing for flow in the north polar layered deposits of mars using radar stratigraphy and a simple 3d ice flow model 1 the water ice rich north polar layered deposits npld of mars play a key role in the martian climate through an active exchange of water vapor with the atmosphere conditions are not currently amenable for flow of the npld however gross morphological evidence for past flow suggests the possibility of a warmer climate in the past here we present the first comparison of internal stratigraphy predicted by a flow model with that observed by an orbital radar sounder we have analyzed radar data from gemina lingula the southernmost tongue of the npld acquired by the shallow radar on board nasa s mars reconnaissance orbiter the data shows extensive internal reflections and several radar reflectors were mapped to create gridded surfaces of this part of the npld all the mapped radar reflectors were smooth with no sudden dips towards the surface or the bedrock the internal radar reflectors were then compared with modeled isochrones in two different areas of gemina lingula under the assumption of flow occurring results indicate that flow of ice is unlikely to have occurred between the main dome and gemina lingula furthermore we found no evidence for the existence of a prior ablation zone in gemina lingula as predicted in another study

viscous flow rates of icy topography on the north polar layered deposits of mars we investigate the importance of viscous flow in shaping topography at the north polar layered deposits npld of mars by using finite element modeling to calculate the distribution of stresses and flow velocities present day impact craters on thenpldare too small and cold for viscous relaxation tohavebeen an important mechanism in controlling their current dimensions this effect may be ignored when analyzing crater size frequency distributions scarps at the npldmargins where avalanches of dust and carbon dioxide frost occur are sufficiently steep high andwarm to experience significant viscous flow we find flow velocities at the base of these steep scarps on the order of tens to hundreds of cm yr which are fast enough to significantly affect their slopeover kiloyear timescales alternatively the scarps could be close to steady state in which observed block falls provide a competing effect to viscous flow

discovery of middle jurassic trench deposits in the bangong nujiang suture zone implications for the timing of lhasa qiangtang initial collision abstract the mesozoic trench deposits in the bangong nujiang suture zone in central tibet can provide critical information for reconstructing the paleotectonic evolution of the bangong nujiang ocean and constraining the timing of the lhasa qiangtang initial collision in this paper the gamulong formation which is well exposed in the dong co area western bangong nujiang suture central tibet was studied the sedimentological analysis shows that the gamulong formation was deposited in a submarine fan environment the youngest detrital zircon age and fossils from limestone pebbles within the gamulong formation combine to determine a middle jurassic age the pebbles in the conglomerate are dominated by limestones and sandstones which is consistent with the sandstone petrology showing limestone and sandstone fragments as the main framework components detrital zircon u pb ages from the sandstones yield age populations of 164 178 ma 200 500 ma 700 1000 ma 1700 2100 ma and 2500 ma the corresponding zircon ehf t values are distributed between 23 5 and 11 1 these zircon features are similar to those from the qiangtang terrane accordingly the provenance results imply that the gamulong formation was recycled from the accretionary complex mugagangri group melange in the bangong nujiang suture zone and the forearc basin deposits lower middle jurassic sewa shaqiaomu jiebuqu formations in the southern qiangtang subterrane the gamulong formation is characterized by sedimentary recycling in the trench likely in response to a tectonic event possibly the continental collision together with the previous studies this work on the occurrence of the gamulong formation indicates that the lhasa qiangtang initial collision could have occurred in middle jurassic time 166 ma

a procedure for testing the significance of orbital tuning of the martian polar layered deposits layered deposits of dusty ice in the martian polar caps have been hypothesized to record climate changes driven by orbitally induced variations in the distribution of incoming solar radiation attempts to identify such an orbital signal by tuning a stratigraphic sequence of polar layered deposits plds to match an assumed forcing introduce a risk of identifying spurious matches between unrelated records we present an approach for evaluating the significance of matches obtained by orbital tuning and investigate the utility of this approach for identifying orbital signals in the mars plds using a set of simple models for ice and dust accumulation driven by insolation we generate synthetic pld stratigraphic sequences with nonlinear time depth relationships we then use a dynamic time warping algorithm to attempt to identify an orbital signal in the modeled sequences and apply a monte carlo procedure to determine whether this match is significantly better than a match to a random sequence that contains no orbital signal for simple deposition mechanisms in which dust deposition rate is constant and ice deposition rate varies linearly with insolation we find that an orbital signal can be confidently identified if at least 10 of the accumulation time interval is preserved as strata addition of noise to our models raises this minimum preservation requirement and we expect that more complex deposition functions would generally also make identification more difficult in light of these results we consider the prospects for identifying an orbital signal in the actual pld stratigraphy and conclude that this is feasible even with a strongly nonlinear relationship between stratigraphic depth and time provided that a sufficient fraction of time is preserved in the record and that ice and dust deposition rates vary predictably with insolation independent age constraints from other techniques may be necessary for example if an insufficient amount of time is preserved in the stratigraphy

meter scale morphology of the north polar region of mars mars north pole is covered by a dome of layered ice deposits detailed 30 centimeters per pixel images of this region were obtained with the high resolution imaging science experiment on board the mars reconnaissance orbiter mro planum boreum basal unit scarps reveal cross bedding and show evidence for recent mass wasting flow and debris accumulation the north polar layers themselves are as thin as 10 centimeters but appear to be covered by a dusty veneer in places which may obscure thinner layers repetition of particular layer types implies that quasi periodic climate changes influenced the stratigraphic sequence in the polar layered deposits informing models for recent climate variations on mars

an ice age recorded in the polar deposits of mars layered ice deposits at the poles of mars record a detailed history of accumulation and erosion related to climate processes radar investigations measure these layers and provide evidence for climate changes such as ice advance and retreat we present a detailed analysis of observational data showing that 87 000 cubic kilometers of ice have accumulated at the poles since the end of the last ice age 370 000 years ago this volume is equivalent to a global layer of 60 centimeters the majority of the material accumulated at the north pole these results provide both a means to understand the accumulation history of the polar deposits as related to orbital milankovitch cycles and constraints for better determination of mars past and future climates

north polar stratigraphy and the paleo erg of mars an accurate self consistent way of coregistering the imaging and topographic data sets of the mars global surveyor mission was developed and used to begin a stratigraphic analysis of the northern polar region a distinct change in the layering style exists at a definite stratigraphic horizon near the base of the north polar layered deposits occurrences of the contact between two distinct layered units can be mapped hundreds of kilometers apart at nearly the same mars orbiter laser altimeter mola elevation the lower layered unit has a consistent association with sand dunes leading to the conclusion that it is an eroding sand rich deposit that predates most of the overlying north polar layered deposits which exhibits the expected features of a dust ice mixture these results suggest that an areally extensive erg was in existence before the present ice cap and that the present circumpolar erg is likely composed of material reworked from this older deposit the volume of this lower unit is estimated to be on the order of 10 5 km 3 the presence of this deposit implies that there existed a period in mars history when there was no icy polar cap a dramatic climatic change leading to the deposition of the upper layered icy unit in the present day polar layered deposits represents a major event in mars history however owing to uncertainties in the mechanics of layered deposits formation such an event cannot be dated at this time

the polar deposits of mars the tantalizing prospect of a readable record of martian climatic variations has driven decades of work toward deciphering the stratigraphy of the martian polar layered deposits and understanding the role of the residual ice caps that cover them spacecraft over the past decade have provided a massive infusion of new data into mars science polar science has benefited immensely due to the near polar orbits of most of the orbiting missions and the successful landing of the phoenix spacecraft in the northern high latitudes topographic thermal radar hyperspectral and high resolution imaging data are among the datasets that have allowed characterization of the stratigraphy of the polar layered deposits in unprecedented detail additionally change within the residual ice caps has been monitored with spacecraft instruments for several years these new data have provided a golden opportunity to understand the interplay between the martian orbit climate and polar ice

origin and characteristics of the mars north polar basal unit and implications for polar geologic history building upon previous studies we have used mars orbiter camera and mars orbiter laser altimeter data to characterize in detail the newly discovered north polar basal unit lying stratigraphically between the polar layered deposits from which it is likely separated by an unconformity and the vastitas borealis formation this unit has introduced new complexity into north polar stratigraphy and has important implications for polar history exposures of the basal unit in olympia planitia and chasma boreale reveal relatively dark layers which exhibit differential erosion eroded primarily by wind the basal unit may be the major if not sole source for the north polar dunes and ergs and has contributed material to the lower polar cap layers we investigate four possible origins for the basal unit outflow channel oceanic deposits basal ice paleopolar deposits and eolian deposits the patchy layering within the unit its likely sandy grain size and presence only in the north polar basin suggest that it is primarily an eolian deposit supporting byrne and murray s 2002 earlier conclusion this implies that at some time during the early to late amazonian migrating sand was mixed with water ice forming a relatively dark sandy deposit during this time either no classic polar layered deposits were forming or smaller caps were growing and shrinking possibly adding material to the basal unit

comparison of the north and south polar caps of mars new observations from mola data and discussion of some outstanding questions new high resolution data from the mars orbiter laser altimeter mola have provided detailed topographic maps for the north and south polar regions these new data allow one to compare the overall topography and geologic histories of the two polar regions and to highlight some specific outstanding questions in mars polar studies following earlier comparisons using viking and mars global surveyor data the new data show that the centers of symmetry of the polar cap deposits in map view which include both the layered terrain and residual ice are offset from the current rotational pole in antipodal directions offset in the north appears to be due to retreat of the polar materials from predominantly one direction 180 w lines of evidence for movement and melting in different forms e g lobes in young craters kettle like depressions candidate residual mantles overlying polar layered terrain and possible eskers have been seen at both poles the exact timing and causes of movement and melting is yet unknown differences in underlying topography large low flat depression in the north broad cratered high and edge of a large impact basin in the south may influence the accumulation flow and movement of polar material and the storage and movement of meltwater the small number of superposed craters has been interpreted to indicate a late amazonian age for both caps with the southern cap being somewhat older 7 15 106 years than the northern cap 100 103 years the late amazonian aged caps are surrounded and underlain by hesperian aged material indicating an apparent hiatus almost 3 byr in duration this apparent hiatus in the geologic record from the late hesperian to late amazonian at both poles may be accounted for in one of three ways 1 polar caps are recent events in the history of mars this scenario requires that conditions in the late amazonian changed to produce environments favorable for cap formation late in the history of mars or that polar wander brought the caps to their present position very recently 2 the polar caps are oscillating in this scenario caps are periodically deposited and then retreat and disappear possibly due to extremes in the obliquity cycle in this model the present caps are the latest example of cap deposition and represent the type of deposit that has come and gone numerous times in the amazonian 3 the present caps are old but have been renewed in this scenario the present caps have been in their current position for much of the amazonian but some process e g melting flow is periodically destroying preexisting craters to produce cap surfaces that appear very young in terms of crater retention ages documenting the nature of the emplacement history of the present polar deposits and distinguishing among these disparate scenarios for their history are among major challenges facing mars polar studies in the coming years

observations of the northern seasonal polar cap on mars i spring sublimation activity and processes abstract spring sublimation of the seasonal co2 northern polar cap is a dynamic process in the current mars climate phenomena include dark fans of dune material propelled out onto the seasonal ice layer polygonal cracks in the seasonal ice sand flow down slipfaces and outbreaks of gas and sand around the dune margins these phenomena are concentrated on the north polar erg that encircles the northern residual polar cap the mars reconnaissance orbiter has been in orbit for three mars years allowing us to observe three northern spring seasons activity is consistent with and well described by the kieffer model of basal sublimation of the seasonal layer of ice applied originally in the southern hemisphere three typical weak spots have been identified on the dunes for escape of gas sublimed from the bottom of the seasonal ice layer the crest of the dune the interface of the dune with the interdune substrate and through polygonal cracks in the ice pressurized gas flows through these vents and carries out material entrained from the dune furrows in the dunes channel gas to outbreak points and may be the northern equivalent of southern radially organized channels araneiform terrain albeit not permanent properties of the seasonal co2 ice layer are derived from timing of seasonal events such as when final sublimation occurs modification of dune morphology shows that landscape evolution is occurring on mars today driven by seasonal activity associated with sublimation of the seasonal co2 polar cap

late cretaceous evolution of the coqen basin lhasa terrane and implications for early topographic growth on the tibetan plateau the tectonic evolution of the lhasa terrane southern tibetan plateau played a fundamental role in the formation of the tibetan plateau however many uncertainties remain with regard to the tectonic and paleogeographic evolution of the lhasa terrane prior to the india asia collision to determine the early tectonic processes that controlled the topographic evolution of the lhasa terrane we analyze the cretaceous strata exposed in the coqen basin northern lhasa subterrane which comprises the langshan and daxiong formations the langshan formation unconformably overlies the volcanic rocks of the lower cretaceous zelong group and consists of similar to 80 m of orbitolina bearing limestones which were deposited in a low energy shallow marine environment micropaleontological analysis indicates that the langshan formation in the coqen basin was deposited from late aptian to early cenomanian times ca 113 96 ma the overlying daxiong formation similar to 1700 m thick consists of conglomerate coarse sandstone and siltstone with interbedded mudstone and represents deposits of alluvial fans and braided rivers the daxiong formation was deposited after the early cenomanian ca 96 ma and accumulated until at least ca 91 ma indicating accumulation rates of greater than 0 3 km m y 1 by combining paleocurrent data sandstone petrology detrital zircon u pb ages and hf isotope analysis we demonstrate that the daxiong formation was derived from lower cretaceous volcanic rocks and pre cretaceous strata in the northern lhasa subterrane during late cretaceous time two thrust systems with opposite vergence were responsible for transforming the northern lhasa subterrane into an elevated mountain range this process resulted in the evolution from a shallow marine environment langshan formation into a terrestrial depositional environment daxiong formation on the southern margin of the northern lhasa subterrane given the regional paleogeographic context we conclude that the daxiong formation in the coqen basin records local crustal shortening and flexure resulting in foreland basin development on the southern margin of the northern lhasa subterrane which implies early topographic growth of the northern lhasa subterrane in southern tibet prior to the india asia collision

is there an orbital signal in the polar layered deposits on mars do the polar layered deposits on mars refl ect orbital control or stochastic variability it is fi useful to determine whether an orbital signal would be detected even if present an estimate of the uncertainty in the time depth relationship of the polar stratigraphy shows that nonlinearities in this relationship and noise in the signal will hamper or preclude detection of orbital forcing even if layer composition is directly proportional to insolation indeed stratigraphic sections of the north polar layered deposits reconstructed from spacecraft images yield no clear evidence of orbital control and are largely consistent with an autoregressive stochastic formation process there is however a broad rise in spectral power centered on a wavelength of roughly 1 6 m that appears in many of the stratigraphic sections this bedding may record a time scale associated with processes internal to mars climate system perhaps related to dust storms alternatively if formed in response to variations in mars obliquity or orbital precession the 1 6 m bedding implies that the 1 km thick upper north polar layered deposits formed over 30 70 myr

present day erosion rate of north polar scarps on mars due to active mass wasting abstract at the margins of the north polar deposits of dusty water ice on mars steep scarps reveal layered deposits that preserve a record of the planet s recent climate changes some of these scarps are currently the most active places on mars with annual avalanches and block falls we estimated the erosion rate of a steep north polar scarp by automatically identifying newly apparent blocks throughout a time series of hirise images the total depositional volume over three mars years corresponds to a minimum erosion rate of 0 3 m3 per mars year per meter along the scarp or a minimum average scarp retreat rate of 0 2 m kyr this rate cannot balance published 0 01 1 m yr viscous flow rates implying that either lower rates of the latter occur or that other processes contribute more than block falls to the slopes steepness

seasonally active frost dust avalanches on a north polar scarp of mars captured by hirise 1 north polar temporal monitoring by the high resolution imaging science experiment hirise orbiting mars has discovered new dramatic examples that mars co2 dominated seasonal volatile cycle is not limited to quiet deposition and sublimation of frost in early northern martian spring 2008 hirise captured several cases of co2 frost and dust cascading down a steep polar scarp in discrete clouds analysis of morphology and process reveals these events to be similar to terrestrial powder avalanches sluffs and falls of loose dry snow potential material sources and initiating mechanisms are discussed in the context of the martian polar spring environment and of additional active aeolian processes observed on the plateau above the scarp the scarp events are identified as a trigger for mass wasting of bright fractured layers within the basal unit and may indirectly influence the retreat rate of steep polar scarps in competing ways

thermal annealing of fission tracks in fluorapatite chlorapatite manganoanapatite and durango apatite experimental results it is well known that the optically measured lengths of fission tracks in apatite crystals are a function of etching conditions crystallographic orientation of the track composition of the crystal and the state of thermal annealing in this study we standardize etching conditions and optimize track length measurability by etching until etch pits formed at the surface of each apatite crystal reached widths of about 0 74 m etching times using 5m hno3 at 21 c were 31 s for otter lake quebec fluorapatite 47 s for durango mexico apatite 33 s for portland connecticut manganoanapatite and 11 s for bamle norway chlorapatite an etching experiment using two etchant strengths 5m and 1 6m hno3 revealed that despite significant differences in etch pit shape fission track length anisotropy with respect to crystallographic orientation of the tracks is not a chemical etching effect a series of 227 constant temperature annealing experiments were carried out on nuclear reactor induced tracks in orient

recent formation and evolution of northern martian polar layered deposits as inferred from a global climate model 1 we present a time marching model which simulates the exchange of water ice between the martian northern cap the tropics and a high latitude surface reservoir net annual exchange rates of water and their sensitivity to variations in orbital rotational parameters are examined using the martian water cycle modeled by the lmd three dimensional global climate model these rates are propagated over the last 10 myr to follow the thickness of the reservoirs the effect of a sublimation dust lag is taken account to test simple models of layer formation periods of high mean polar summer insolation 5 10 ma ago lead to a rapid exhaustion of a northern polar cap and a prolonged formation of tropical glaciers the formation of a northern cap and of a high latitude icy mantle may have started 4 ma ago with the average decrease of polar insolation tropical ice may have disappeared around 2 7 ma ago but small glaciers could have formed during the last peaks of polar summer insolation over the last 4 myr most of the present cap may have formed at the expense of tropical and high latitude reservoirs forming distinct layers at almost each 51 kyr 120 kyr insolation cycle layers thickness ranges from 10 to 80 m variations being produced by the modulation of the obliquity with 2 4 and 1 3 myr periods because only 30 insolation cycles have occurred since 4 ma ago we found an inconsistency between the recent astronomical forcing the observed number of layers and simple astronomically based scenarios of layers formation