post stagnation behavior in the upstream regions of ice stream c west antarctica the region where two active tributaries feed into the now stagnant ice stream c isc west antarctica is thickening in this region we observe a correlation between faster ice flow the tributaries and elevated topography we conclude that stagnation of isc resulted in compression and thickening along the tributaries eventually forming a bulge on the ice sheet surface modern hydraulic potential gradients would divert basal meltwater from isc to ice stream b isb these gradients are primarily controlled by the bulge topography and so likely formed subsequent to trunk stagnation as such we argue against water piracy as being the cause for isc s stagnation kinematic wave theory suggests that thickness perturbations propagate downstream over time but that kinematic wave speed decreases near the stagnant trunk this and modest diffusion rates combine to trap most of the tributary fed ice in the bulge region using interferometric synthetic aperture radar velocity measurements we observe that half of the ice within isc s southern tributary flows into isb that flow pattern and other observations of non steady flow in the region likely result from stagnation induced thickening along upper isc combined with a longer period of thinning on upper isb if current trends in thickness change continue more ice from upper isc will be diverted to isb

positive mass balance of the ross ice streams west antarctica we have used ice flow velocity measurements from synthetic aperture radar to reassess the mass balance of the ross ice streams west antarctica we find strong evidence for ice sheet growth 26 8 gigatons per year in contrast to earlier estimates indicating a mass deficit 20 9 gigatons per year average thickening is equal to 25 of the accumulation rate with most of this growth occurring on ice stream c whillans ice stream which was thought to have a significantly negative mass balance is close to balance reflecting its continuing slowdown the overall positive mass balance may signal an end to the holocene retreat of these ice streams

ross sea paleo ice sheet drainage and deglacial history during and since the lgm onshore and offshore studies show that an expanded grounded ice sheet occupied the ross sea embayment during the last glacial maximum lgm results from studies of till provenance and the orienta

glacial isostatic adjustment in response to changing late holocene behaviour of ice streams on the siple coast west antarctica the siple coast region of antarctica contains a number of fast flowing ice streams which control the dynamics and mass balance of the region these ice streams are known to undergo stagnation and reactivation cycles which lead to ice thickness changes that may be sufficient to excite a viscous solid earth response glacial isostatic adjustment gia this study aims to quantify siple coast ice thickness changes during the last 2000 yr in order to determine the degree to which they might contribute to gia and associated present day bedrock uplift rates this is important because accurate modelling of gia is necessary to determine the rate of present day ice mass change from satellite gravimetry recently published reconstructions of ice stream variability were used to create a suite of kinematic models for the stagnation related thickening of kamb ice stream since 1850 ad and a gia model was used to predict present day deformation rates in response to this thickening a number of longer term loading scenarios which include the stagnation and reactivation of ice streams across the siple coast over the past 2000 yr were also constructed and used to investigate the longer term gia signal in the region uplift rates for each of the ice loading histories based on a range of earth models were compared with regional gps observed uplift rates and an empirical gia estimate we estimate kamb ice stream to have thickened by 70 130 m since stagnation 165 years ago modelled present day vertical motion in response to this load increase peaks at 17 mm yr 1 i e 17 mm yr 1 subsidence for the weakest earth models tested here comparison of the solid earth response to ice load changes throughout the last glacial cycle including ice stream stagnation and reactivation across the siple coast during the last 2000 yr with an empirical gia estimate suggests that the upper mantle viscosity of the region is greater than 1 10 20 pa s when upper mantle viscosity values of 1 10 20 pa s or smaller are combined with our suite of ice load scenarios we predict uplift rates across siple coast that are at least 4 mm yr 1 smaller than those predicted by the empirical gia estimate gps data are unable to further constrain model parameters due to the distance of the gps sites from the study area our results demonstrate that late holocene ice load changes related to the stagnation and reactivation of ice streams on the siple coast may play a dominant role in defining the present day uplift signal however both the detailed earth structure and deglacial history of the region need to be better constrained in order to reduce uncertainties associated with the gia signal of this region

antarctic paleolake sediments and the search for extinct life on mars evidence of lakes in mars history is substantial the proposed similarities between the ancient martian environment and certain modern environments on earth have led exobiologists to study antarctic lakes as analogs to those purported to have existed on mars we have investigated modern sedimentation processes especially with respect to 13c of carbonate and organic matter in lakes in the mcmurdo dry valleys region of east antarctica and assessed various paleolake deposits with respect to their utility as martian analogs and targets for future mars exobiology missions three main types of paleolake deposit were identified and assessed strand lines perched deltas and lacustrine sand mounds deltas are usually identified as good targets but our research shows that authigenic carbonates are not readily identifiable in the sediments large deltas although most likely to attract attention through remote sensing generally are difficult sites for discovery of paleobiological matter and 13c signals follow no coherent pattern lacustrine sand mounds on the other hand contain abundant authigenic carbonate and freeze dried organic matter and appear to be excellent records of paleolimnological conditions the advantage of studying lake bottom deposits versus lake edge deposits is retrieval of a stable lake wide signal deltas are therefore most useful in that they are generally large scale features capable of drawing attention to a region of potential for the discovery of smaller lacustrine sand mounds

dynamics of ice stream temporal variability modes scales and hysteresis 1 understanding the mechanisms governing temporal variability of ice stream flow remains one of the major barriers to developing accurate models of ice sheet dynamics and ice climate interactions here we analyze a simple model of ice stream hydrology coupled to ice flow dynamics and including drainage and basal cooling processes analytic and numerical results from this model indicate that there are two major modes of ice stream behavior steady streaming and binge purge variability the steady streaming mode arises from friction stabilized subglacial meltwater production which may also activate and interact with subglacial drainage the binge purge mode arises from a sufficiently cold environment sustaining successive cycles of thinning induced basal cooling and stagnation low prescribed temperature at the ice surface and weak geothermal heating typically lead to binge purge behavior while warm ice surface temperature and strong geothermal heating will tend to produce steady streaming behavior model results indicate that modern siple coast ice streams reside in the binge purge parameter regime near a subcritical hopf bifurcation to the steady streaming mode numerical experiments exhibit hysteresis in ice stream variability as the surface temperature is varied by several degrees our simple model simulates heinrich event like variability in a hypothetical hudson strait ice stream including dynamically determined purge time scale till freezing and basal cooling during the binge phase these findings are an improvement on studies of both modern and paleo ice stream variability and provide a framework for interpreting complex ice flow models

duration of the ma adim vallis gusev crater hydrogeologic system mars abstract ma adim vallis and gusev crater aeolis subquadrangle of mars formed a complex hydrogeologic system through time crater statistics over five geologic units coupled with morphologic and geologic observations show that this system was active over a period ranging from 0 7 to about 2 billion years these results make the floor of gusev crater one of the most prominent martian depositional features the last aqueous sedimentary deposition could have occurred during early amazonian

on post glacial sea level i general theory summary modern analyses of sea level changes due to glacial isostatic adjustment gia are based on the classic sea level equation derived by farrell clark 1976 geophys j r astr soc 46 647 667 the connection between global sea level variations and changes to ocean height that is assumed within this equation breaks down in the presence of a time varying shoreline geometry we present a generalized sea level equation that overcomes this difficulty we also derive analytic expressions for and present schematic illustrations of the error in the ocean height change over finite time intervals introduced in published efforts to incorporate shoreline evolution into the theory of gia induced sea level change this comparison includes studies of shoreline migration due to either local sea level changes or the growth and ablation of marine based ice we conclude that the theories applied by johnston 1993 geophys j int 114 615 634 milne 1998 phd thesis university of toronto toronto and co workers are more accurate than the procedure advocated by peltier 1994 science 265 195 201 1998a geophys res lett 25 3955 3958 1998b rev geophys 114 615 634 although an improvement in the latter has recently been reported peltier drummond 2002 geophys res lett 29 10 1029 2001gl014273 our generalized theory is valid for any earth model in a companion paper we derive the equations necessary to treat the special case of a spherically symmetric linear viscoelastic and rotating earth and we quantify errors associated with previous work

basal mechanics of ice stream b west antarctica 2 undrained plastic bed model based on the results of our studies of the physical conditions beneath ice stream b we formulate a new analytical ice stream model the undrained plastic bed model henceforth the upb model mathematically the upb model is represented by a non linear system of four coupled equations which express the relationships among ice sliding velocity till strength water storage in till and basal melt rate we examine this system of equations for conditions of ice stream stability over short timescales that permit holding ice stream geometry constant less than hundreds of years temporal variability is introduced into the upb model only by the direct dependence of till void ratio changes e t on the basal melting rate m r since till strength b e and ice stream velocity u b b change as long as till void ratio varies the first condition for ice stream stability is that of constant till water storage 0 the second condition for ice stream stability arises from the feedback between ice stream velocity till strength and the basal melting rate which depends on shear heating m r u b b this is the weak till condition which requires that in a steady state till strength is a small fraction of the gravitational driving stress b n 1 1 d the salient feature of the upb model is its ability to produce two thermo mechanically controlled equilibrium states one with a strong bed and slow ice velocities ice sheet mode and one with a weak bed and fast ice velocities ice stream mode this bimodality of basal conditions is consistent with the available observations of subglacial conditions beneath slow and fast moving ice in west antarctica basal conditions that do not correspond to these two steady states may occur transiently during switches between the two stable modes the upb model demonstrates that ice streams may be prone to thermally triggered instabilities during which small perturbations in the basal thermal energy balance grow leading to generation or elimination of the basal conditions which cause ice streaming

an assessment of forward and inverse gia solutions for antarctica in this work we assess the most recent estimates of glacial isostatic adjustment gia for antarctica including those from both forward and inverse methods the assessment is based on a comparison of the estimated uplift rates with a set of elastic corrected gps vertical velocities these have been observed from an extensive gps network and computed using data over the period 2009 2014 we find systematic underestimations of the observed uplift rates in both inverse and forward methods over specific regions of antarctica characterized by low mantle viscosities and thin lithosphere such as the northern antarctic peninsula and the amundsen sea embayment where its recent ice discharge history is likely to be playing a role in current gia uplift estimates for regions where many gia models have traditionally placed their uplift maxima such as the margins of filchner ronne and ross ice shelves are found to be overestimated gia estimates show large variability over the interior of east antarctica which results in increased uncertainties on the ice sheet mass balance derived from gravimetry methods

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

capabilities and limitations of numerical ice sheet models a discussion for earth scientists and modelers the simulation of dynamically coupled ice sheet ice stream and ice shelf systems poses a challenge to most numerical ice sheet models here we review present ice sheet model limitations targeting

uplift rates from a new high density gps network in palmer land indicate significant late holocene ice loss in the southwestern weddell sea the measurement of ongoing ice mass loss and associated melt water contribution to sea level change from regions such as west antarctica is dependent on a combination of remote sensing methods a key method the measurement of changes in earth s gravity via the grace satellite mission requires a potentially large correction to account for the isostatic response of the solid earth to ice load changes since the last glacial maximum in this study we combine glacial isostatic adjustment modelling with a new gps dataset of solid earth deformation for the southern antarctic peninsula to test the current understanding of ice history in this region a sufficiently complete history of past ice load change is required for glacial isostatic adjustment models to accurately predict the spatial variation of ongoing solid earth deformation once the independently constrained effects of present day ice mass loss have been accounted for comparisons between the gps data and glacial isostatic adjustment model predictions reveal a substantial misfit the misfit is localized on the southwestern weddell sea where current ice models under predict uplift rates by approximately 2 mm yr 1 this under prediction suggests that either the retreat of the ice sheet grounding line in this region occurred significantly later in the holocene than currently assumed or that the region previously hosted more ice than currently assumed this finding demonstrates the need for further fieldwork to obtain direct constraints on the timing of holocene grounding line retreat in the southwestern weddell sea and that grace estimates of ice sheet mass balance will be unreliable in this region until this is resolved

the importance of the inelastic and elastic structures of the crust in constraining glacial density mass change and isostatic adjustment from geodetic observations in southeast alaska

evidence for late pleistocene thinning of siple dome west antarctica 1 we use a two dimensional full stress thermomechanical flow band model to investigate late pleistocene and holocene histories of ice thickness and ice flow at siple dome in west antarctica measurements obtained from an ice core and borehole near the ice divide and measurements of radar detected internal stratigraphy are used as targets for modeling models that account for the climate history accumulation rate and temperature alone yield a poor match to observations histories of ice dynamics ice thickness ice divide location and ice flow enhancement also need to be included our favored model includes 350 m of thinning at siple dome between 15 000 and 14 000 years ago onset of divide flow 3000 years ago ongoing divide migration starting 2500 years ago and evolution of ice fabric over the past 30 000 years the timing of thinning is earlier than that inferred from geologic evidence from the margins but is coincident with a 20 m rise in global sea level about 14 500 years ago meltwater pulse 1a this opens the possibility that thinning was a result of increased activity of the ice streams in the late pleistocene forced by meltwater pulse 1a the contribution to meltwater pulse 1a as a result of this thinning was likely small

impact crater lakes on mars the robotic search for life on mars centers on identifying accessible environments where the biological catalyst water has existed the formation of large impact craters on mars 65 km diameter may have resulted in the creation of ice covered impact crater lakes which would not freeze for thousands of years even under present climatic conditions water could be supplied from deep confined aquifers penetrated by the impact craters without the need for surface melt water freezing of the lakes is postponed owing to heat from impact generated melt bearing deposits from impact related uplift of hotter rocks from depth and from the latent heat of freezing of a deep crater lake abundant morphologic evidence for ancient crater lakes has not been found in viking images except for craters associated with outflow channels however ice covered crater lakes could have formed and further searches for evidence of these lakes are warranted the lake deposits from dissected impact craters may represent one of the best targets for future surface exobiology investigations or sample return missions from mars

fast flow signature in the stagnated kamb ice stream west antarctica among the major ice streams that drain west antarctica kamb ice stream formerly called ice stream c is unique in that it stagnated 150 yr ago but its former fast flow conditions are virtually unknown here we presentsurface based radar profiles of the ice stream s undulating internal stratigraphy which records these conditions our analysis of the profiles indicates that pre stagnation flow velocities averaged over a period 740 yr exceeded 350 m yr 1 in the trunk of the ice stream this velocity constraint would be lower if the ice had been thickening higher if thinning but suggests mass loss from the ice stream catchment that is of sufficient magnitude to reverse the gain estimated for today s siple coast region analysis of other ice streams would allow comparison of velocities over millennial time scales with observations of present day velocities useful for evaluating how west antarctic ice drainage has evolved

thermal effects on the location of ice stream margins west antarctic ice streams move rapidly over beds that are thawed by heat generated by the motion intervening zones of slow moving ice are probably freezing at their bases where heat generation is absent heat is also generated in the shear zone between the slow and fast moving ice through modeling of coupled heat and mass flow we show that the position of the boundary between freezing and melting at the bed is unstable for an otherwise morphologically uniform bed above a threshold speed in the ice stream heat generation in the shear zone allows melting beneath the slow ice and possible widening of the stream below this threshold heat generation at the edge of the thawed zone is insufficient to maintain melting leading to freezing and narrowing of the stream the threshold speed appears to be a function of the temperature at the base of slow ice relative to the melting point and is also highly sensitive to the inflow of colder ice from the slow zone for environmental conditions typical of west antarctica the threshold speed is about 100 m yr 1 in the absence of inflow and increases to typical ice stream speeds 500 m yr 1 for inflow speeds of a few meters per year thus the ice streams can be in a delicate thermal balance with the potential of melting outward or freezing inward rapidly moving ice streams could expand unless blocked by bed properties that would prevent fast motion even when there is melting at the bed

on postglacial sea level summary an exact method is presented for calculating the changes in sea level that occur when ice and water masses are rearranged on the surface of elastic and viscoelastic non rotating earth models the method is used to calculate the instantaneous elastic and delayed vi scoelastic sea level changes following the partial melting of late quaternary ice sheets we find that there can be large errors in the usual assumption that changes in sea level are uniform over the ocean basins if a quantity of ice equivalent to a uniform 100 m rise in sea level melts from the laurentide and fennoscandian ice sheets then in the south pacific the instantaneous rise in sea level can be as large as 120m in the north atlantic the instantaneous rise is always less than 100 m there is a zone in the north atlantic with almost no sea level change and near greenland and norway the sea level falls rather than rises by over 100 m one thousand years after the melting a forebulge migrating towards the ice loads causes water to flow from the south pacific into the north pacific suggesting that raised beaches should occur in the south pacific the gravitational attraction of an ice mass upon a nearby ocean tends to hold sea level high in the vicinity of the ice this extra load near the ice may have a significant influence on postglacial isostatic adjustment

porphyrins in upper jurassic source rocks and correlations with other source rock descriptors the porphyrin content and composition in the extractable organic matter has been analysed in 13 kimmeridge clay source rocks spread from the north to the south of the norwegian continental shelf the samples are predominantly immature early mature the distribution of nickel and vanadyl porphyrins within the samples is markedly different effects of the thermal maturity on the composition of porphyrins is evaluated and compared with biomarker parameters based on sterane and hopane ratios maturity is modelled using partial least squares pls multivariate analysis with t max as a maturity indicator porphyrin parameters based on the total carbon number distribution of dpep and etio series from the mass spectral data of the nickel and vanadyl porphyrin fractions effectively describe the t max of the samples with a better correlation than aliphatic biomarker ratios selected porphyrin and biomarker ratios were used for principal component analysis pca modelling and found to describe two types of variation that may be indicative of the prevailing depositional redox conditions

stagnation of ice stream c west antarctica by water piracy the dynamic behavior of the west antarctic ice sheet is of interest because of the possibility that it may change and cause rapid sea level rise attention is focused on the fast moving and rapidly responding ice streams that drain the ice sheet one of these ice stream c largely stopped about a century ago and some models for this shutdown postulate negative feedbacks that would tend to stabilize the ice sheet here new data are presented indicating that the slowdown of the ice stream is restricted to its lower part and occurred because of loss of lubrication on localized sticky spots at the bed of the ice stream the increased friction probably arises from a topographic accident of the glacier bed that has directed lubricating water to the neighboring ice stream b together with slow drawdown of the ice sheet rather than from any general stabilizing feedbacks

melting and freezing beneath the ross ice streams antarctica we have estimated temperature gradients and melt rates at the bottom of the ice streams in west antarctica measured velocities were used to include the effects of horizontal advection and strain heating in the temperature model and to determine shear heating at the bed our modeled temperatures agree well with measured temperatures from boreholes in regions of steady flow we find that ice stream tributaries and the inland ice account for about 87 of the total melt generated beneath the ross ice streams and their catchments our estimates indicate that the ice plains of whillans ice stream and ice stream c even when active have large areas subject to basal freezing confirming earlier estimates that import of water from upstream is necessary to sustain motion the relatively low melt rates on whillans ice stream are consistent with observations of deceleration over the last few decades and suggest a shutdown may take place in the future possibly within this century while there are pockets of basal freezing beneath ice streams d and e there are larger areas of basal melt that produce enough melt to more than offset the freezing which is consistent with inferences of relatively steady flow for these ice streams over the last millennium

distribution classification and ages of martian impact crater lakes paleolakes in impact craters on mars are characterized at global scale using the viking orbiter data we identified 179 paleolakes in impact structures formed by the influx of water and sediment derived from valley networks and channels that can be classified into three different fluviolacustrine systems closed open and lake chain systems we show the hydrogeologic implications for each of the three systems and their significance in terms of duration of fluviolacustrine activity this study provides a catalog of areographic physical and physiographic data for each of the studied impact crater lakes associated with the description of the sedimentary structures observed it also identifies environments where life and or precursors to life could have found favorable conditions to evolve

the impact of lateral variations in lithospheric thickness on glacial isostatic adjustment in west antarctica differences in predictions of glacial isostatic adjustment gia for antarctica persist due to uncertainties in deglacial history and earth rheology the earth models adopted in many gia studies are defined by parameters that vary in the radial direction only and represent a global average earth structure referred to as 1 d earth models oversimplifying the actual earth structure leads to bias in model predictions in regions where earth parameters differ significantly from the global average such as west antarctica we investigate the impact of lateral variations in lithospheric thickness on gia in antarctica by carrying out two experiments that use different rheological approaches to define 3 d earth models that include spatial variations in lithospheric thickness the first experiment defines an elastic lithosphere with spatial variations in thickness inferred from seismic studies we compare the results from this 3 d model with results derived from a 1 d earth model that has a uniform lithospheric thickness defined as the average of the 3 d lithospheric thickness irrespective of the deglacial history and sublithospheric mantle viscosity we find higher gradients of present day uplift rates i e higher amplitude and shorter wavelength in west antarctica when using the 3 d models due to the thinner than 1 d average lithosphere prevalent in this region the second experiment uses seismically inferred temperature as an input to a power law rheology thereby allowing the lithosphere to have a viscosity structure modelling the lithosphere with a powerlaw rheology results in a behaviour that is equivalent to a thinner lithospheremodel and it leads to higher amplitude and shorter wavelength deformation compared with the first experiment we conclude that neglecting spatial variations in lithospheric thickness in gia models will result in predictions of peak uplift and subsidence that are biased low in west antarctica this has important implications for ice sheet modelling studies as the steeper gradients of uplift predicted from the more realistic 3 d model may promote stability in marine grounded regions of west antarctica including lateral variations in lithospheric thickness at least to the level of considering west and east antarctica separately is important for capturing shortwavelength deformation and it has the potential to provide a better fit to global positioning system observations as well as an improved gia correction for the gravity recovery and climate experiment data

paleolakes and impact basins in southern arabia terra including meridiani planum implications for the formation of hematite deposits on mars 1 the hematite deposit in meridiani planum was selected for a mars exploration rover mer landing site because water could be involved in the formation of hematite and water is a key ingredient in the search for life our discovery of a chain of paleolake basins and channels along the southern margin of the hematite deposits in meridiani planum with the presence of the strongest hematite signature adjacent to a paleolake basin supports the possible role of water in the formation of the hematite and the deposition of other layered materials in the region the hematite may have formed by direct precipitation from lake water as coatings precipitated from groundwater or by oxidation of preexisting iron oxide minerals the paleolake basins were fed by an extensive channel system originating from an area larger than texas and located south of the schiaparelli impact basin on the basis of stratigraphic relationships the formation of channels in the region occurred over much of mars history from before the layered materials in meridiani planum were deposited until recently the location of the paleolake basins and channels is connected with the impact cratering history of the region the earliest structure identified in this study is an ancient circular multiringed basin 800 1600 km diameter that underlies the entire meridiani planum region the mer landing site is located on the buried northern rim of a later 150 km diameter crater this crater is partially filled with layered deposits that contained a paleolake in its southern portion

the rotational stability of an ice age earth summary predictions of glaciation induced changes in the earth s rotation vector exhibit sensitivities to earth structure that are unique within the suite of long wavelength observables associated with glacial isostatic adjustment henceforth gia and despite nearly a quarter of a century of research these sensitivities remain enigmatic previous predictions of present day true polar wander tpw speed driven by gia have indicated for example a strong sensitivity to variations in the thickness of the elastic lithosphere and the treatment phase or chemical of the density discontinuity at 670 km depth nakada recently presented results that suggest that the predictions are also sensitive to the adopted rheology of the lithosphere however his results have introduced an intriguing paradox in particular predictions generated using a model with an extremely high viscosity lithospheric lid do not converge to results for a purely elastic lithosphere of the same thickness mitrovica as cited by nakada has suggested that the paradox originates from an inaccuracy in the traditional rotation theory e g wu peltier associated with the treatment of the background equilibrium rotating form upon which any load and rotation induced perturbations are superimposed we revisit these issues using a new treatment of the linearized euler equations governing load induced rotation perturbations on viscoelastic earth models we demonstrate that our revised theory in which the background form of the planet combines a hydrostatic component and an observationally inferred excess ellipticity resolves the apparent paradox calculations using the revised theory indicate that earlier predictions based on earth models with purely elastic lithospheric lids are subject to large errors indeed previously noted sensitivities of tpw speed predictions to the thickness and rheology elastic versus viscous of the lithosphere largely disappear in the application of the new theory significant errors are also incurred by neglecting the stabilizing influence of the earth s excess ellipticity finally we demonstrate that the contribution from rotational feedback on predictions of present day rates of change of the geoid sea surface and crustal velocities are overestimated by the traditional rotation theory and this has implications for analyses of ongoing satellite e g grace missions and geodetic gps surveys

increased ice loading in the antarctic peninsula since the 1850s and its effect on glacial isostatic adjustment antarctic peninsula ap ice core records indicate significant accumulation increase since 1855 and any resultant ice mass increase has the potential to contribute substantially to present day glacial isostatic adjustment gia we derive empirical orthogonal functions from climate model output to infer typical spatial patterns of accumulation over the ap and by combining with ice core records estimate annual accumulation for the period 1855 2010 in response to this accumulation history high resolution ice sheet modeling predicts ice thickness increases of up to 45 m with the greatest thickening in the northern and western ap whilst this thickening is predicted to affect grace estimates by no more than 6 2 gt yr it may contribute up to 7 mm yr to the present day gia uplift rate depending on the chosen earth model with a strong east west gradient across the ap its consideration is therefore critical to the interpretation of observed gps velocities in the ap

increased ice losses from antarctica detected by cryosat 2 we use 3 years of cryosat 2 radar altimeter data to develop the first comprehensive assessment of antarctic ice sheet elevation change this new data set provides near continuous 96 coverage of the entire continent extending to within 215 km of the south pole and leading to a fivefold increase in the sampling of coastal regions where the vast majority of all ice losses occur between 2010 and 2013 west antarctica east antarctica and the antarctic peninsula changed in mass by 134 27 3 36 and 23 18 gt yr 1 respectively in west antarctica signals of imbalance are present in areas that were poorly surveyed by past missions contributing additional losses that bring altimeter observations closer to estimates based on other geodetic techniques however the average rate of ice thinning in west antarctica has also continued to rise and mass losses from this sector are now 31 greater than over the period 2005 2010

variable deceleration of whillans ice stream west antarctica the whillans ice stream ice plain wip has been slowing since at least 1963 prior constraints on this slowdown were consistent with a constant long term deceleration rate new observations of ice velocity from 11 continuous and 3 seasonal gps sites indicate the deceleration rate varies through time including on interannual time scales between 2009 and 2012 wip decelerated at a rate 6 1 to 10 9 2 m yr2 that was double the multidecadal average 3 0 to 5 6 2 m yr2 to identify the causes of slowdown we used new and prior velocity estimates to constrain longitudinal and transverse force budget models as well as a higher order inverse model all model results support the conclusion that the observed deceleration of wip is caused by an increase in basal resistance to motion at a rate of 10 to 40 pa yr subglacial processes that may be responsible for strengthening the ice stream bed include basal freeze on changes in subglacial hydrology or increases in the area of resistant basal substrate through differential erosion the observed variability in wip deceleration rate suggests that dynamics in subglacial hydrology plausibly driven by basal freeze on and or activity of subglacial lakes plays a key role in modulating basal resistance to ice motion in the region

lower satellite gravimetry estimates of antarctic sea level contribution a new model of glacial isostatic adjustment used in conjunction with grace satellite data suggests that ice loss from antarctica is contributing 0 19 millimetres per year to global mean sea level which is substantially less than previous grace based estimates estimates for recent mass loss from antarctica vary widely making it difficult to calculate any subsequent contributions to sea level rise one prominent approach uses the grace gravity recovery and climate experiment satellites to detect changes in gravity but the final estimates are dependent on the modelled response of the continent to changes in ice loading over time and to changes in lithospheric elastic rebound matt king and colleagues apply a new model of this glacial isostatic adjustment to the grace data and estimate antarctic mass loss between 2002 and 2010 to be about 69 gigatonnes per year much lower than previous estimates and equivalent to an annual sea level rise of about 0 2 millimetres recent estimates of antarctica s present day rate of ice mass contribution to changes in sea level range from 31 gigatonnes a year gt yr 1 ref 1 to 246 gt yr 1 ref 2 a range that cannot be reconciled within formal errors3 time varying rates of mass loss2 4 5 6 contribute to this but substantial technique specific systematic errors also exist3 in particular estimates of secular ice mass change derived from gravity recovery and climate experiment grace satellite data are dominated by significant uncertainty in the accuracy of models of mass change due to glacial isostatic adjustment7 8 gia here we adopt a new model of gia developed from geological constraints which produces gia rates systematically lower than those of previous models and an improved fit to independent uplift data9 after applying the model to 99 months from august 2002 to december 2010 of grace data we estimate a continent wide ice mass change of 69 18 gt yr 1 0 19 0 05 mm yr 1 sea level equivalent this is about a third to a half of the most recently published grace estimates2 5 which cover a similar time period but are based on older gia models plausible gia model uncertainties and errors relating to removing longitudinal grace artefacts destriping confine our estimate to the range 126 gt yr 1 to 29 gt yr 1 0 08 0 35 mm yr 1 sea level equivalent we resolve 26 independent drainage basins and find that antarctic mass loss and its acceleration is concentrated in basins along the amundsen sea coast outside this region we find that west antarctica is nearly in balance and that east antarctica is gaining substantial mass

possible ancient oceans on mars evidence from mars orbiter laser altimeter data high resolution altimetric data define the detailed topography of the northern lowlands of mars and a range of data is consistent with the hypothesis that a lowland encircling geologic contact represents the ancient shoreline of a large standing body of water present in middle mars history the contact altitude is close to an equipotential line the topography is smoother at all scales below the contact than above it the volume enclosed by this contact is within the range of estimates of available water on mars and a series of extensive terraces parallel the contact in many places

sea level as a stabilizing factor for marine ice sheet grounding lines climate change could potentially destabilize marine ice sheets which would affect projections of future sea level rise1 2 3 4 specifically an instability mechanism5 6 7 8 has been predicted for marine ice sheets such as the west antarctic ice sheet that rest on reversed bed slopes whereby ice sheet thinning or rising sea level leads to irreversible retreat of the grounding line however existing analyses of this instability mechanism have not accounted for deformational and gravitational effects that lead to a sea level fall at the margin of a rapidly shrinking ice sheet9 10 11 here we present a suite of predictions of gravitationally self consistent sea level change following grounding line migration our predictions vary the initial ice sheet size and also consider the contribution to sea level change from various subregions of the simulated ice sheet using these results we revisit a canonical analysis of marine ice sheet stability5 and demonstrate that gravity and deformation induced sea level changes local to the grounding line contribute a stabilizing influence on ice sheets grounded on reversed bed slopes we conclude that accurate treatments of sea level change should be incorporated into analyses of past and future marine ice sheet dynamics

switch of flow direction in an antarctic ice stream fast flowing ice streams transport ice from the interior of west antarctica to the ocean and fluctuations in their activity control the mass balance of the ice sheet the mass balance of the ross sea sector of the west antarctic ice sheet is now positive that is it is growing mainly because one of the ice streams ice stream c slowed down about 150 years ago1 here we present evidence from both surface measurements and remote sensing that demonstrates the highly dynamic nature of the ross drainage system we show that the flow in an area that once discharged into ice stream c has changed direction now draining into the whillans ice stream formerly ice stream b this switch in flow direction is a result of continuing thinning of the whillans ice stream and recent thickening of ice stream c further abrupt reorganization of the activity and configuration of the ice streams over short timescales is to be expected in the future as the surface topography of the ice sheet responds to the combined effects of internal dynamics and long term climate change we suggest that caution is needed when using observations of short term mass changes to draw conclusions about the large scale mass balance of the ice sheet

west antarctic ice stream discharge variability mechanism controls and pattern of grounding line retreat west antarctic ice streams show pronounced flow variability in their downstream reaches with changes stranding formerly fast flowing ice and redirecting discharge a simple model in which the temperature gradient in basal ice provides control of fast sliding in the downstream reach can explain this behavior downstream thinning steepens the temperature gradient near the bed increasing upward heat flow and the tendency toward basal freezing the basal temperature gradient is steepest and the tendency toward basal freezing the strongest in ice that has experienced the most rapid downstream thinning that is the fastest flowing ice the most successful rapid outflows are regions where basal water from elsewhere is likely to be consumed freezing here leads to episodic slow downs and redirections of flow the history of which appears in satellite imagery as ice rises distorted streaklines and margin jumps created when discharge migrates to areas with more favorable basal conditions one compelling consequence of this process is that it makes catastrophic collapse less likely if discharge currents are forced to slow when they become too fast thin then there may be an upper bound on the retreat rate and discharge flux of the west antarctic ice sheet wais ice stream system under the present climate

holocene accumulation and ice sheet dynamics in central west antarctica 1 we derive depth age relationships across the ice divide between the ross and amundsen seas by tracking radar detected layers from the byrd ice core and a dated 105 m core near the divide the depth age relationships and an ice flow model are used to establish histories of accumulation and ice sheet dynamics over the past 8000 years results show that accumulation was approximately 30 higher than today from 5000 to 3000 years ago antarctic climate variability today is dominated by periodic fluctuations in strength of the circumpolar vortex which raises the possibility that the vortex was systematically weaker during the period of high accumulation accumulation today decreases almost linearly across the divide it is unlikely that this pattern has changed through the holocene the radar detected stratigraphy shows no evidence of the arched layers that are expected beneath a stable divide that is frozen to its bed implying that the divide has also been migrating and or the basal ice has been sliding through the holocene we cannot rule out the possibility of sliding because the basal ice is near its pressure melting point other evidence indicates that divide migration is likely the ross sea sector is now near steady state but it had a strong negative imbalance 200 years ago when kamb ice stream was active in contrast recent speedups of pine island and thwaites glaciers have likely caused the mass balance of the amundsen sea sector to become negative the divide is likely migrating toward the ross sea today

climate variability in west antarctica derived from annual accumulation rate records from itase firn ice cores thirteen annually resolved accumulation rate records covering the last 200 years from the pine island thwaites and ross drainage systems and the south pole are used to examine climate variability over west antarctica accumulation is controlled spatially by the topography of the ice sheet and temporally by changes in moisture transport and cyclonic activity a comparison of mean accumulation since 1970 at each site to the long term mean indicates an increase in accumulation for sites located in the western sector of the pine island thwaites drainage system accumulation is negatively associated with the southern oscillation index soi for sites near the ice divide and periods of sustained negative soi 1940 42 1991 95 correspond to above mean accumulation at most sites correlations of the accumulation rate records with sea level pressure slp and the soi suggest that accumulation near the ice divide and in the ross drainage system may be associated with the mid latitudes the post 1970 increase in accumulation coupled with strong slp accumulation rate correlations near the coast suggests recent intensification of cyclonic activity in the pine island thwaites drainage system

experimental delta formation in crater lakes and implications for interpretation of martian deltas 1 the morphology of delta deposits in crater lakes on mars is indicative of upstream e g flow discharge and sediment properties and downstream e g basin characteristics parameters from which the hydrological conditions at the time of formation can be inferred to investigate the influences of these parameters on delta morphology we experimentally created deltas in crater shaped basins by feeding a range of constant flow discharges over a feeder channel of various sand textures we reproduced three categorically different types of deltas including most of the types that have been identified on mars our most striking observation is that water level behavior itself a unique function of flow discharge basin size and hypsometry and basin floor permeability can explain most delta morphologies observed on mars stepped retrograding deltas formed during water level rise prograding deltas formed during water level still stand and during water level fall deltas were partially destroyed by erosion on mars numerous retrograding and prograding deltas were preserved most of them without indications of channel incisions or other fluvial modification we conclude that the main difference between a single foreset prograding delta and a multiple foreset retrograding delta is the behavior of the water level in the basin these simple delta morphologies cannot be reconciled with long duration hydrological activity because that would imply crater lake fluctuations due to inherent complex water level histories along with complex sediment delivery histories our experiments and numerical verifications demonstrate that such deltas preferentially form during one aqueous event which parsimoniously argues for short duration hydrological activity

postglacial sea level change on a rotating earth summary we present a complete derivation of the equation governing long term sea level variations on a spherically symmetric self gravitating maxwell viscoelastic planet this new sea level equation extends earlier work by incorporating in a gravitationally self consistent manner both a time dependent ocean continent geometry and the in uence of contemporaneous perturbations to the rotation vector of the planet we also outline an e cient pseudo spectral numerical methodology for the solution of this equation and present a variety of predictions based on a suite of earth models of relative sea level rsl variations due to glacial isostatic adjustment gia these results show that the contribution to the predicted rsl signal from gia induced perturbations to the rotation vector can reach 7 8 m over the postglacial period in geographic regions where the rotationally induced signal is a maximum this result is sensitive to variations in the adopted lower mantle viscosity and is relatively insensitive to variations in the adopted lithospheric thickness we also show that the rotationally induced component of rsl change is su cient to in uence previous estimates of late holocene melting events and ongoing sea level change due to giawhich were based on a rsl theory for a non rotating earth in particular estimates of antarctic melting over the last 5 kyr based on the amplitude of sea level highstands from the australian region may require an adjustment downwards of the order of 0 5 m of equivalent sea level rise furthermore present day rates of sea level change are perturbed by as much as 0 2 mm yr 1 by the rotational component of sea level change and this has implications for gia corrections of the global tide gauge record over the period from the last glacial maximum to the present we predict a distinctly non monotonic variation in the rotation induced component of rsl this is in agreement with our previous preliminary study milne mitrovica 1996 but contrasts signi cantly with predictions presented by han wahr 1989 and bills james 1996 we demonstrate that the disagreement arises as a consequence of approximations adopted in the latter studies we furthermore refute an assertion by bills james 1996 that previously published constraints on mantle viscosity and ice sheet histories which did not incorporate a rotation induced rsl component are largely invalidated by this omission

surviving the limits to life at the surface of mars the harshness of the environment at its surface severely limits the survival of unprotected terrestrial organisms on mars however the martian geologic record is consistent with an earlier history with much different climatic conditions more conducive to life followed by an extended climate change which led to the currently inhospitable state such a transition is ideal for evolutionary adaptation to new conditions because it provides both the forcing function and the extensive time for progressive specializations it is shown that against each of the current limits to life organics deprivation restricted amounts and physical forms of h2o limited availability of other key nutrients ionizing radiation insult and strongly oxidizing chemical species there are plausible countermeasure solutions most but not all of these mechanisms already exist as known examples in terrestrial biota a combination of these adaptations could conceivably allow highly tailored organisms either active or dormant to survive at or very near the surface of mars even in carefully preserved regolith samples returned to earth microorganisms could be difficult to detect without specific biochemical probes and or purposeful biological amplification by selective enhancement of the supply of nutrients optimized incubation parameters and highly sensitive detection of metabolic activity

modeling the formation of bright slope deposits associated with gullies in hale crater mars implications for recent liquid water abstract our study investigates possible formation mechanisms of the very recent bright gully deposits bgds observed on mars in order to assess if liquid water was required we use two models in our assessment a one dimensional 1d kinematic model to model dry granular flows and a two dimensional 2d fluid dynamic model flo 2d o brien et al 1993 flo engineering to model water rich and wet sediment rich flows our modeling utilizes a high resolution topographic model generated from a pair of images acquired by the high resolution imaging science experiment hirise aboard the mars reconnaissance orbiter for the 1d kinematic modeling of dry granular flows we examine a range of particle sizes flow thicknesses initial velocities flow densities and upslope initiation points to examine how these parameters affect the flow run out distances of the center of mass of a flow our 1d modeling results show that multiple combinations of realistic parameters could produce dry granular flows that travel to within the observed deposits boundaries we run the 2d fluid dynamic model flo 2d to model both water rich and wet sediment rich flows we vary the inflow volume inflow location discharge rate water loss rate water rich models only and simulation time and examine the resulting maximum flow depths and velocities our 2d modeling results suggest that both wet sediment rich and water rich flows could produce the observed bright deposits our modeling shows that the bgds are not definitive evidence of recent liquid water on the surface of mars

history of the grounded ice sheet in the ross sea sector of antarctica during the last glacial maximum and the last termination knowledge of variations in the extent and thickness of the antarctic ice sheet is key for understanding the behaviour of southern hemisphere glaciers during the last ice age and for addressing issues involving global sea level ocean circulation and climate change insight into past ice sheet behaviour also will aid predictions of future ice sheet stability here we review ter restrial evidence for changes in ice geometry that occurred in the ross sea sector of antarctica at the last glacial maximum lgm and during subsequent deglaciation during the lgm a thick grounded ice sheet extended close to the continental shelf edge in the ross embayment this ice reached surface elevations of more than 1000 m along the coast of the central and southern trans antarctic mountains and marie byrd land the local lgm occurred by 18 ka on the coast but as late as 7 10 ka inland the first significant thinning took place at roughly 13 ka with most ice loss happening in the holocene this history makes it unlikely that the ross sea sector was a major contributor to meltwater pulse 1a mwp 1a resolution of a possible antarctic origin for mwp 1a awaits detailed reconstructions from all sectors of the ice sheet the antarctic ice sheet contains the world s larg est reservoir of fresh water and is a major control on southern hemisphere climate a history of changes in ice sheet extent and thickness is vital for answering questions involving the initiation of southern hemisphere glaciation and deglaciation

limnologic analysis of gusev crater paleolake mars abstract the survey of the hydrogeologic system formed by gusev crater and ma adim vallis aeolis subquadrangle of mars points out evidence for the existence of an ice covered lake in gusev crater a first lake was formed by the drainage of the aquifer in the region surrounding gusev before the entry of ma adim vallis in the crater the existence of a former lake in gusev is deduced from the morphology of the ma adim delta its comparison with terrestrial antarctic analogs argues for the presence of an ice covered lake in gusev at the time that the southern part of the crater s rampart was breached by ma adim first release and for a subice lacustrine construction of the valley s delta our survey shows that ma adim vallis may have entered gusev crater as late as late hesperian early amazonian as part of a second lake episode the relationship between the variation of the gusev lake water level the volume of the lake and the surface of the lake bed is established by our bathymetric model the elevation of the former lake is deduced from the elevation of the mesa like structures in the delta of ma adim vallis furthermore the correlation of the crater frequency of gusev rampart with mars stratigraphic age shows that lakes may have occupied gusev crater over a period of time covering 2 gyrs from the formation of the crater to the last episode of water release from ma adim vallis though it is most likely that the lake was episodical the recurrence of abundant water in gusev crater makes this site a high priority for missions either for martian resource exploration or for the search of life

past rates of accumulation in central west antarctica 1 the spatial variation in ice accumulation is critical to the form and stability of ice sheets in west antarctica knowledge of present ice accumulation rates is restricted to sparse measurements and interpolations across data free regions while information on former ice accumulation is restricted to a small number of ice cores and numerical modeling results here isochronous internal ice sheet layers recorded by airborne radio echo sounding and dated at their stratigraphic position at the byrd ice core site are used to reconstruct the spatial pattern of accumulation across the main west antarctic ice divide for the past 16 000 years mean accumulation calculated for the last 3100 years matches well to present day accumulation rates which is compatible with previous observations on siple dome accumulation between 6400 and 16 000 years ago is calculated to be around half that at present in accordance with recent ice sheet modeling results index terms 1223 geodesy and gravity ocean earth atmosphere interactions 3339 1827 hydrology glaciology 1863 9310 information related to geographic region antarctica 9604 information related to geologic time cenozoic 3349 meteorology and atmospheric dynamics polar meteorology citation siegert m j and a j payne 2004 past rates of accumulation in central west antarctica geophys res lett 31 l12403 doi 10 1029 2004gl020290

continued deceleration of whillans ice stream west antarctica 1 earlier observations indicated that whillans ice stream slowed from 1973 to 1997 we collected new gps observations of the ice stream s speed in 2003 and 2004 these data show that the ice stream is continuing to decelerate at rates of about 0 6 yr2 with faster rates near the grounding line our data also indicate that the deceleration extends over the full width of the ice plain extrapolation of the deceleration trend suggests the ice stream could stagnate sometime between the middle of the 21st and 22nd centuries

noble gases in iddingsite from the lafayette meteorite evidence for liquid water on mars in the last few hundred million years we analyzed noble gases from 18 samples of weathering products iddingsite from the lafayette meteorite potassium argon ages of 12 samples range from near zero to 670 91 ma these ages confirm the martian origin of the iddingsite but it is not clear whether any or all of the ages represent iddingsite formation as opposed to later alteration or incorporation of martian atmospheric 40ar in any case because iddingsite formation requires liquid water this data requires the presence of liquid water near the surface of mars at least as recently as 1300 ma ago and probably as recently as 650 ma ago krypton and xe analysis of a single 34 g sample indicates the presence of fractionated martian atmosphere within the iddingsite this also confirms the martian origin of the iddingsite the mechanism of incorporation could either be through interaction with liquid water during iddingsite formation or a result of shock implantation of adsorbed atmospheric gas our strongest conclusion is that the iddingsite in lafayette formed on mars in agreement with the microstratigraphic arguments of gooding et al 1991 and treiman et al 1993 a preterrestrial origin of the iddingsite is required both by the many non zero k ar ages and by the presence of xe that is isotopically distinct from any terrestrial xe the xe is accompanied by kr but the kr and xe have been fractionated if they are derived from the present martian atmosphere this is presumably the result of either incorporation via interaction with liquid water drake et al 1994 bogard and garrison 1998 or by adsorption from the martian atmosphere perhaps accompanied by shock see also gilmour et al 1998 1999 although the iddingsite is enriched in kr and xe compared to whole rock analyses it is not clear whether iddingsite is the dominant carrier of the atmospheric derived gas drake et al 1994 or merely a minor carrier gilmour et al 1999 our 40ar 39ar experiment was disappointing in that it mostly served to confirm that the iddingsite which contains fine grained clays is susceptible to recoil loss of 39ar during irradiation only one sample of five gave a clear signal of radiogenic or extraterrestrial 40ar and that was only by 3 potassium argon ages of the second set of samples were more successful ranging from near 0 to 670 91 ma it is not clear whether any or all of the ages represent iddingsite formation as opposed to later alteration the fact that a rb sr experiment shih et al 1998 gave an apparent age for iddingsite of 679 66 ma 2a suggests that perhaps formation of iddingsite occurred or began 650 ma ago and that some samples either formed or were thermally altered later the ages could be even younger than 650 ma if the samples have incorporated martian atmospheric 40ar this means that liquid water was certainly present on mars in the last 1300 ma the formation age of lafayette and probably within the last 650 ma

exploring for a record of ancient martian life the immediate task facing exopaleontology is to define a strategy to explore mars for a fossil record during the decade long exploration program that lies ahead consideration of the quality of paleontological information preserved under different geological conditions is important if we are to develop a strategy with broad applicability the preservation of microbial fossils is strongly influenced by the physical chemical and biological factors of the environment which acting together determine the types of information that will be captured and retained in the rock record in detrital sedimentary systems preservation is favored by rapid burial in fine grained clay rich sediments in chemical sedimentary systems preservation is enhanced by rapid entombment in fine grained chemical precipitates for long term preservation host rocks must be composed of stable minerals that are resistant to chemical weathering and that form an impermeable matrix and closed chemical system to protect biosignatures from alteration during subsequent diagenesis or metamorphism in this context host rocks composed of highly ordered chemically stable mineral phases like silica e g cherts or phosphate e g phosphorites are especially favored such lithologies tend to have very long crustal residence times and along with carbonates and shales are the most common host rocks for the precambrian microfossil record on earth although we make the defensible assumption that mars was more like the earth early in its history clearly the geological and historical differences between the two planets are many such differences must be carefully considered when adapting an earth based strategy to mars

a case for ancient evaporite basins on mars observations indicate that a martian analog to the earth s salt pans and saline lakes of arid regions may have existed in crater basins during mars early noachian epoch terraced and channelized crater basins point to ponding of surface water as well as possible prolonged and evolving base levels in addition supportive evaporite basin analogs are offered for three other morphologic features of martian crater basins an evaporite basin model for crater basins on mars has major implications for the mechanical chemical and even biological processes that potentially have operated in mars past and represent a spectrum of potential mineral resources

ma adim vallis evolution geometry and models of discharge rate abstract episodes of confined flows in ma adim vallis are reconstructed by surveying the terraces in the valley three major levels and their associated tributary systems are identified and mapped this study points out the evidence of the cessation of the valley activity between the formation of levels 1 and 2 characterized by degraded and large perched valleys which floor are not adjusted to the following flow episode level a strong asymmetry in the tributary distribution points toward a groundwater supplied water origin of the valley potential source areas are demonstrated to be in volcano tectonic regions implying a hydrothermal origin of the water that may include melted permafrost calculation of hydraulic radii valley sections and models of discharge rates associated with the morphologic and geologic survey shows that ma adim vallis was an episodic channel the lifetime of which spanned between the noachian and amazonian

earth analogs for martian life microbes in evaporites a new model system for life on mars abstract the prospect of life on mars today is daunting especially problematic for a potential life form is the lack of water particularly in a liquid state extremely cold temperatures ultraviolet and ionizing radiation and soil oxidants yet oases where life might persist have been suggested to occur in rocks in analogy with endolithic microorganisms described from deserts around the world in polar ice caps in analogy with snow and ice algae and in possible volcanic regions in analogy with chemoautotrophs living in deep sea hydrothermal vents all are critically examined microorganisms are known to be able to survive in salt crystals and recently it has been shown that organisms can metabolize while encrusted in evaporites because evaporites are thought to occur on mars and can attenuate light in the uv range while being far more transparent to radiation useful for photosynthesis 400 700 nm and because of the properties of these endoevaporitic organisms i propose that such communities provide a new model system for studying potential life on mars on the basis of this model i suggest possibilities for site selection for future exobiological experiments on mars

computations of the viscoelastic response of a 3 d compressible earth to surface loading an application to glacial isostatic adjustment in antarctica and canada s u m m a r y we develop a 3 d finite element model to study the viscoelastic response of a compressible earth to surface loads the effects of centre of mass motion polar wander feedback and selfconsistent ocean loading are implemented to assess the model s accuracy we benchmark the numerical results against a semi analytic solution for spherically symmetric structure we force our model with the ice 5g global ice loading history to study the effects of laterally varying viscosity structure on several glacial isostatic adjustment gia observables including relative sea level rsl measurements incanada and present day time variable gravity and uplift rates in antarctica canadian rsl observations have been used to determine the earth s globally averaged viscosity profile antarcticgps uplift rates have been used to constrainantarcticgia models and gia time variable gravity and uplift signals are error sources for grace and altimeter estimates of present day antarctic ice mass loss and must be modelled and removed from those estimates computinggia results for a 3 d viscosity profile derived from a realistic seismic tomography model and comparing with results computed for 1 d averages of that 3 d profile we conclude that 1 a gia viscosity model based on canadian relative sea level data is more likely to represent a canadian average than a true global average 2 the effects of 3 d viscosity structure on grace estimates of present day antarctic mass loss are probably smaller than the difference between gia models based on different antarctic deglaciation histories and 3 the effects of 3 d viscosity structure on antarctic gps observations of present day uplift rate can be significant and can complicate efforts to use gps observations to constrain 1 d gia models

the antarctica component of postglacial rebound model ice 6g_c vm5a based on gps positioning exposure age dating of ice thicknesses and relative sea level histories

ejection of rock fragments from planetary bodies the recent discovery of alha81005 a small meteorite that undoubtedly came from the moon has raised questions about how an intact rock fragment could survive acceleration to planetary escape velocities this acceleration could only have been produced by a large impact on the lunar surface a small amount of material 0 005 to 0 02 projectile volume may be ejected from an impact crater at speeds exceeding lunar escape velocity without suffering petrographically detectable shock damage the ejected material is protected by stress wave interference close to the free surface the existence and size of this zone depend upon parameters such as the rise time of the stress pulse produced by an impact the fragment size is a function of ejection velocity the lunar meteorite was ejected during an impact event that produced a crater at least 3 6 km in diameter other meteorites may have originated in a similar way

origin and evolution of valleys on martian volcanoes morphological analyses of six martian volcanoes ceraunius tholus hecates tholus alba patera hadriaca patera apollinaris patera and tyrrhena patera indicate that fluvial processes were the dominant influence in the initiation and subsequent development of many dissecting valleys lava processes and possibly volcanic density flows were also important as valley forming processes fluvial valleys are especially well developed on alba patera ceraunius tholus and hecates tholus these valleys are inset into the surrounding landscape they formed in regions of subdued lava flow morphology contain tributaries and tend to widen slightly in the downstream direction lava channels on alba patera are located on the crest of lava flows and have a discontinuous irregular surface morphology and distributary patterns these channels sometimes narrow toward their termini possible volcanic density flow channels are located on the northern flank of ceraunius tholus valleys dissecting apollinaris patera hadriaca patera and tyrrhena patera appear to have a complex evolution probably a mixed fluvial and lava origin they are inset into a subdued possibly mantled surface lack tributaries and either have fairly constant widths or widen slightly downvalley valleys surrounding the caldera of apollinaris appear to have formed by fluvial and possibly by volcanic density flow processes while those on the apollinaris fan structure may have a mixed lava and fluvial origin valleys on tyrrhena have broad flat floors and theater heads which have been extensively enlarged probably by sapping hadriaca valleys have a trough shaped form like those on the fan structure on apollinaris but are more enlarged and have theater heads fluvial valleys on the martian volcanoes generally followed a two stage evolutionary sequence similar to that of the hawaiian volcano valleys including 1 initial formation by surface runoff and 2 subsequent enlargement by groundwater sapping because basalt is highly permeable valleys on hawaii island formed only in regions where lava flows were mantled by less permeable volcanic ash similarly fluvial valleys on the martian volcanoes particularly those on alba patera formed in regions where lava flow morphology is subdued probably by ash mantling thus the presence or absence of this mantling may explain why fluvial valleys are present on some martian volcanoes but not on others of the same age the formation of fluvial or fluvial modified valleys on the older volcanoes ceraunius hecates tyrrhena apollinaris and hadriaca implies that surface and near surface water activity was widespread during late noachian to early hesperian period of heavy bombardment however the presence of anomalously young fluvial valleys on alba patera indicates that fluvial valley activity did not entirely cease near the end of late heavy bombardment but rather persisted in isolated regions until mid to late amazonian

separating geophysical signals using grace and high resolution data a case study in antarctica to fully exploit data from the gravity recovery and climate experiment grace we separate geophysical signals observed by grace in antarctica by deriving high spatial resolution maps for present day glacial isostatic adjustment gia and ice mass changes with the least possible noise level for this we simultaneously i improve the postprocessing of gravity data and ii consistently combine them with high resolution data from ice cloud and land elevation satellite altimeter icesat and regional atmospheric climate model 2 3 racmo we use gps observations to discriminate between various candidate spatial patterns of vertical motions caused by gia the icesat racmo combination determines the spatial resolution of estimated ice mass changes the results suggest the capability of the developed approach to retrieve the complex spatial pattern of present day gia such as a pronounced subsidence in the proximity of the kamb ice stream and pronounced uplift in the amundsen sea sector

air blast produced by the meteor crater impact event and a reconstruction of the affected environment using scaling relationships determined from nuclear explosions the radial extent of the air blast produced by the meteor crater impact event is estimated the wind velocity at a distance of 5 crater radii 3 km from the point of impact should have exceeded 2000 km h hurricane force winds would have existed as far away as 20 to 40 km depending on the exact explosive energy of the impact event to determine how this event may have affected the environment surrounding the crater the topography vegetation and animal life that existed at the time of the impact are reconstructed for example if the coniferous woodlands were 100 m lower than they are presently and they had moved farther out onto the plains then the air blast would have flattened trees within a 16 to 22 km radius of the point of impact and damaged them over an area of 4100 to 8500 km2 the distance over which the damage occurred may have been up to 2 larger in some directions around the crater because of additional effects produced by the ballistic shock wave unfortunately since the trajectory of the projectile is not well known the direction of the ballistic shock wave effects cannot yet be determined

experimental investigation of foundation on collapsible soils abstractgeotechnical engineers face serious problems when building on collapsible soils these types of soil show considerable strength when dry and experience excessive and sudden settlement when

a 3 d coupled ice sheet sea level model applied to antarctica through the last 40 ky abstract we present results from a three dimensional ice sheet shelf model of antarctica coupled to a gravitationally self consistent global sea level model that incorporates maxwell viscoelastic deformation of the solid earth the coupled model captures complex post glacial changes in sea level associated with the gravitational deformational and rotational effects of the evolving surface mass ice plus ocean load over the global ocean including at the grounding lines of marine based ice the simulations are initiated at 40 ka and we focus on ice distributions and sea levels from the last glacial maximum to present our results extend and confirm the key conclusions of earlier work using a simplified 1 d ice sheet model by demonstrating that the sea level coupling has a significant stabilizing influence on marine ice sheet grounding lines the feedback of sea level changes into the ice sheet model acts to slow down the retreat and advance of the grounding line relative to simulations in which the full coupling is not incorporated differences in ice thickness between these simulations can reach 1 km close to the grounding line finally we perform preliminary comparisons of our results to relative sea level rsl histories and gps derived present day uplift rates at sites near the margins of antarctica we find that the coupling improves fits to uplift rates in several regions and that the rsl predictions of the coupled model yield a fit to the observations that is comparable to recent uncoupled simulations in which the underlying earth model was varied to obtain a best fit to the rsl histories

century scale discharge stagnation and reactivation of the ross ice streams west antarctica 1 flow features on the surface of the ross ice shelf west antarctica record two episodes of ice stream stagnation and reactivation within the last 1000 years we document these events using maps of streaklines emerging from individual ice streams made using visible band imagery together with numerical models of ice shelf flow forward model experiments demonstrate that only a limited set of discharge scenarios could have produced the current streakline configuration according to our analysis whillans ice stream ceased rapid flow about 850 calendar years ago and restarted about 400 years later and macayeal ice stream either stopped or slowed significantly between 800 and 700 years ago restarting about 150 years later until now ice stream scenarios emphasized runaway retreat or stagnation on millennial timescales here we identify a new scenario century scale stagnation and reactivation cycles as well as lateral communication with adjacent ice streams through thickness changes on lightly grounded ice plains this introduces uncertainty into predictions for future sea level withdrawals by the west antarctic ice sheet which are based in part on recent slowing of whillans ice stream and the stagnant condition of kamb ice stream

variability in the mass flux of the ross ice streams west antarctica over the last millennium we synthesize previously published remote sensing observations radar data and model output to obtain a 1000 year ice flow history for the siple coast ice stream system in west antarctica to investigate the timing and magnitude of changes in mass flux the synthesized history shows significant short term variability in ice stream shear margin and grounding line position due to internal variability of the coupled system the chronology highlights the interplay between adjacent ice streams which implies that the behavior of any individual ice stream should not be examined in isolation furthermore individual events cannot be fully interpreted without an understanding of the broad scale long term variability in the ice sheet in the context of this millennium scale history we interpret the relatively recent stagnation of kamb ice stream kis as just one stage in the thermodynamic cycle of an ice stream in this region the changes in mass balance that result from the kis stagnation may thus be viewed as century scale noise relative to the longer term trend understanding and characterizing this noise is a necessary step before accurate model based predictions of ice sheet mass balance for the next century can be made

complex evolution of paleolacustrine systems on mars an example from the holden crater 1 the 150 km wide holden crater lies in an area characterized by high density of valley networks implying conditions conducive to forming of water related environments we undertook geological mapping and a stratigraphic survey in order to probe the evolution of water related landforms and their paleoenvironmental implications our investigations lead us to propose that the holden area was subjected to a wet lacustrine phase of hesperian age and an icy phase during the amazonian deltaic coastal and lacustrine environments occurred during the wet phase some displaying a cyclic depositional pattern presumably related to autogenic processes water was delivered to the basin by the uzboi vallis and by surface runoff channels from a series of drainage basins along the crater walls fan delta geometries and coastal onlap enabled estimation of major water levels two levels of major stand of the water have been recognized possibly reflecting allogenic controls geologic units related to this wet lacustrine phase were subsequently eroded by glacial abrasion and plucking and were disconformably overlain by glacial deposits of amazonian age defining an icy phase these features are consistent with a warm based glacier entering the holden crater through the wide uzboi vallis to form a proglacial lake in the central part of the crater changes in sedimentary units reflect changes of depositional environments probably connected with climatic variation

recent antarctic ice mass loss from radar interferometry and regional climate modelling observed estimates of ice losses in antarctica combined with regional modelling of ice accumulation in the interior suggest that east antarctica is close to a balanced mass budget but large losses of ice occur in the narrow outlet channels of west antarctic glaciers and at the northern tip of the antarctic peninsula

west antarctic ice streams draining into the ross ice shelf configuration and mass balance extensive airborne radar soundings closely tied to accurately located ground stations made over a portion of the west antarctic inland ice sheet and the ross ice shelf have enabled us to map the boundaries of the ice streams and their flow bands on the ross ice shelf the surfaces of the active ice streams a and b are heavily crevassed but there are no visible crevasses on ice stream c however the existence of numerous crevasses at a depth of about 35 m implies that ice stream c ceased to be active about 250 years ago there is a complex zone at the head of ice stream b that suggests that ice stream b is currently widening and advancing toward the interior of the ice sheet the grounding lines between the inland ice and the ice shelf were mapped from the surface elevations using the requirement of hydrostatic balance and the character of the radar echoes the inland ice is more extensive than previously mapped and includes a large weakly grounded area downstream of ice streams a and b where the surface elevations are barely above hydrostatic equilibrium crary ice rise extends 70 km upstream of the previously mapped position to relate surface elevation data from satellite observations to sea level three geoidal models were tested we conclude that the gem 10c model is the best of the three for this section of antarctica a new map of surface elevation shows deep marginal troughs and longitudinal ridges along ice streams a and b but not on ice stream c whose surface contours show signs of stagnation ice stream c shows a strongly positive and ice stream b a significantly negative net balance rate ice streams a and d also show a significantly negative net flux whereas ice streams e and f do not the overall net balance of this part of the west antarctic inland ice is suggestively negative 23 15 km3 yr 1 from the measured flux into the ross ice shelf and previous measurements we calculate an average basal melt rate from beneath the ice shelf of 0 12 0 03 m yr 1

thermophiles early biosphere evolution and the origin of life on earth implications for the exobiological exploration of mars mars holds great interest for exobiology and presently stands center stage in our plans to explore the inner solar system for signs of past or present life with the spectacular success of the pathfinder mission golombek 1997 and upcoming mapping efforts by the mars global surveyor orbiter albee et al 1998 it is an especially appropriate time to revisit some important issues in mars exobiology the papers making up this journal of geophysical research planets special issue on mars were the outgrowth of an american geophysical union special session entitled exploring for a martian biosphere which was presented at the fall meeting in san francisco in 1995 farmer 1995a together these papers provide a context for evaluating some fundamental issues in the exobiological exploration of mars topics covered fall into the following broad categories 1 environments and conditions required for the origin of life hydrothermal environments as sites for prebiotic chemical synthesis shock and schulte this issue 2 early martian environments as a context for the evolution and diversification of life on mars early impact hazards sleep and zahnle this issue early volatile and climate history of mars haberle this issue and ice covered lakes as habitats for early life doran et al this issue 3 exploring for extant martian life results of the viking missions klein this issue environmental extremes for martian life clark this issue and planetary protection and mars sample return devincenzi et al this issue and 4 exploration strategies phosphate minerals as targets for chemofossils mojzsis and arrhenius this issue and the role of robotic surface missions stoker this issue

does crater saturation equilibrium occur in the solar system abstract similarity is found in crater densities on the most heavily cratered surfaces throughout the solar system this is hypothesized to result from a steady state saturation equilibrium being approached or achieved by cratering processes this hypothesis conflicts with some recent interpretations however it accounts for 1 a similarity in maximum relative crater density below certain theoretically predicted values on all heavily cratered surfaces 2 a leveling off at this same relative density among 100 m scale secondary craters in populations on lunar maria and other sparsely cratered lunar surfaces 3 the approximate uniformity of maximum relative densities on saturn satellites in spite of dramatic variations predicted from nonsaturation models assuming heliocentric impactors the lunar frontside upland crater population sometimes described as a well preserved production function useful for interpreting other planetary surfaces is found not to be a production function it was modified by intercrater plains formed at least partly by early upland basaltic lava flooding recently confirmed spectrophotometrically consistent with this counts in pure uplands those lacking intercrater plains match the proposed saturation equilibrium density variations among large d 64 km crater populations are found but these may involve several hypothesized mechanisms that rapidly obliterate large craters especially on icy surfaces recent models in which different populations of interplanetary bodies hit different planets need further appraisal

the influence of subglacial hydrology on the flow of kamb ice stream west antarctica 1 ice streams on the siple coast west antarctica have a complex history of flow because their basal motion is governed by time varying basal conditions although the mechanical interaction between ice and till is well established very little is known about the potential effect of regionally scaled water transport in a basal water system which has only recently become apparent to investigate the combined effect of hydrological and mechanical processes we developed the hydrology ice and till model in which ice flow is coupled to a coulomb plastic till layer and a basal water system consisting of discrete conduits when the model is applied to kamb ice stream kis results confirm that it is capable of oscillating between fast and stagnant modes of flow we show that when subglacial conduits are disregarded or do not extend to the grounding line the oscillatory behavior of the ice stream is governed by the basal thermal regime when conduits extend to the grounding line the modelled ice stream oscillation period is increased peak speeds are reduced and oscillations may ultimately cease if the volume of water supplied is sufficiently high three different hydrological states characterize the behavioral patterns of ice flow and these states are distinguished by conditions at the grounding line modelled ice stream velocities were found to oscillate with fast and slow periods typically lasting a few hundred years although varying according to hydrological activity our results indicate that kis could reactivate this century given its hydrological setting and 170 years of stagnation

changes in west antarctic ice stream velocities observation and analysis 1 we have produced a map of velocity covering much of the siple coast ice streams the map confirms earlier estimates of deceleration on whillans ice stream comparison with bed elevation data indicates that subglacial topography and the location of consolidated sediment play a strong role in determining the location of the tributaries feeding the ice streams force balance estimates based on these data indicate that the tributaries have beds nearly an order of magnitude stronger than those beneath many of the ice streams we have used a theoretical analysis to examine the controls on fast flow this analysis suggests that ice plains very wide ice streams are inherently unstable this instability may be responsible for the current deceleration on the ice plain of whillans ice stream and the shutdown of ice stream c 150 years ago thinning induced reductions in driving stress may also explain some of the observed deceleration particularly in upstream areas the active portions of ice stream c coincide well with the areas where we estimate that melt should be taking place current topography and inferences of large thickening following a shutdown suggest the upstream migration of a stagnation front that initiated at the ice plain uncertainty remains about the basal conditions on ice stream d while the basal resistance on ice stream e is large enough to ensure basal melting

recent aqueous environments in martian impact craters an astrobiological perspective the discovery of gullies and debris aprons raises the question of the existence of aqueous environments on mars in recent geological times and its astrobiological implications three cases of such environments are surveyed at moc high resolution in the e gorgonum chaos and newton and hale craters the regional setting of these craters suggests that the mechanisms of aquifer destabilization flow discharge and gully formation in these three cases result from local geological triggers that can include impact cratering and tectonic processes rather than climate or insolation factor we take as a working hypothesis that microbial life appeared on mars in ancient geological times probably in a geothermal environment but potentially evolving via infrared detection systems to give photosynthetic communities under the selective pressure of energetic solar radiation we hypothesize that some microbial communities could have survived underground in either dormant or active state or that their biomolecules could be preserved either frozen or desiccated in the subsurface beneath the upper oxidized zone we assess the known environmental constraints for life and what type of potential habitats are provided in these three craters by aquifer discharges using comparison with terrestrial analogues and their associated microbial communities these environments include 1 the release of water on a dry crater floor in e gorgonum and the possibility for microorganisms and preserved biomolecules to be flushed out and mixed in with the sediment exposed at the surface 2 the evidence of a recent lacustrine episode in the newton crater with analogy to antarctic dry valley lakes 3 the exposure on the floor of hale crater of material from a regional subsurface that is likely to have retained traces of one of the oldest martian bodies of water recognized to date parker et al 2000 in the argyre basin we show how the water in argyre 3 8 billion years ago gya was likely to have been alterated by hydrothermal processes and how the subsequent formation of the 150 km hale crater on the northern ring of argyre generated hydrothermal pumping this accounts for the anomalously high location of the springs on the crater crests today with respect to the rest of the regional subsurface distribution finally we envision current impact cratering as a factor for destabilizing aquifers on mars today thus creating new environments we analyze the implications of impacts for two geological types of rock units that could harbor traces of life as a result we compare the potential of astrobiological exploration of crater floors rims and ejecta on future missions to mars

spatial and temporal variability in snow accumulation at the west antarctic ice sheet divide over recent centuries mean annual accumulation rates of 0 200 0 204 and 0 221 mweq a 1 respectively small scale spatial variability ssv was determined using an analysis of variance of accumulation in the ice core array thereby quantifying the uncertainty in individual accumulation records results indicate that the spatial variability was 0 030 mweq a 1 o r approximately 15 of the average annual accumulation an accumulation record representative of the wais divide local area over recent centuries was developed using a principal component analysis to identify the coherent accumulation signal the wais divide local record exhibited 14 interannual variability 1 standard deviation of the mean with the ssv reduced to 0 017 mweq a 1 correlations of the wais divide local accumulation record with atmospheric indices e g antarctic oscillation exhibited periods when the records oscillate in and out of phase thus reconstructing local and global atmospheric indices from wais divide accumulation records over recent centuries may prove problematic

a deglacial model for antarctica geological constraints and glaciological modelling as a basis for a new model of antarctic glacial isostatic adjustment abstract we present a new reconstruction of the antarctic ice sheets between 20 ka bp and the present day our reconstruction is derived using a numerical model to generate a physically consistent ice surface across the whole of the continent we define the extent of the ice sheet at five time slices 20 15 10 5 and 0 ka bp assuming an equilibrium state for the 20 ka bp time slice and a transient state for the deglacial time slices the evolution of the ice sheet within the numerical model is driven by variations in temperature accumulation rate and relative sea level in order to reconstruct the concave profile of the ice sheet in marine grounded regions such as the ross and weddell seas we force our model to develop channels of faster flow by defining greater basal sliding along the trajectory of former ice streams we find a strong dependence upon the basal sliding parameters and also the position of the grounding line we use an extensive data base of geological and glaciological data to constrain our ice sheet reconstruction grounding line extent is prescribed from marine geological data and we test ice sheet thickness against onshore geological data at 62 sites of the five time slices considered our 20 ka bp reconstruction is the best constrained by data and has an rms misfit of 147 6 m when compared to observations of ice thickness change between 20 ka bp and the present day across all time slices there are large regions of the ice sheet which are poorly constrained especially after 20 ka bp we estimate the spatial distribution of uncertainty in our ice sheet reconstruction and note that the solutions are least reliable in regions of complex topography we predict that the antarctic ice sheets contributed 9 1 5 m of eustatic sea level to the global ocean between 20 ka bp and the present and our reconstruction with minimum misfit contributes 8 m eustatic sea level during this period these values which we argue are an upper bound are lower than many previous estimates the reconstructed pattern of ice unloading can serve as a new input for glacial isostatic models

the thermal stability of near surface ground ice on mars the existence of subsurface water ice on mars has been predicted in several theoretical studies1 5 but there are no definitive observations of its present distribution geomorphic features on the surface of mars have been widely interpreted as evidence for the presence of ground ice6 12 but many of these features are found at near equatorial latitudes where thermal models have predicted that near surface water ice should not be stable under present climate conditions3 13 14 here i present the results of thermal calculations which show that observed geographic variations in the thermal and reflectance properties of martian soils significantly affect subsurface temperatures my results indicate that in certain regions ground ice deposits could exist much closer to the surface and much closer to the equator than previously thought in the future these deposits could be a valuable resource for human exploration15

sequential stagnation of kamb ice stream west antarctica 1 ice penetrating radar data confirm the presence of two relict northern margins of kamb ice stream kis west antarctica in an area called the duckfoot between the two relict margins deep layers are preserved and buried crevasses remain at a constant depth until roughly 1 5 km from the innermost youngest margin suggesting that this outer portion of the ice stream stagnated suddenly 10 years stagnation was likely accomplished through changes in basal conditions i e reduced lubrication beneath this outer portion of the ice stream and resulted in a narrowing of the ice stream trunk by 27 and a reduction in flow speed 200 years prior to complete stagnation flowstripe deformation as a result of duckfoot stagnation provides an estimate of kis paleo velocity of 210 m a after duckfoot stagnation

onset of deglacial warming in west antarctica driven by local orbital forcing an annually resolved ice core record from west antarctica indicates that warming driven by local insolation resulting from sea ice decline began in that region about 2 000 years before warming in east antarctica reconciling two alternative explanations for deglacial warming in the southern hemisphere

antarctic contribution to sea level rise observed by grace with improved gia correction antarctic volume changes during the past 21 thousand years are smaller than previously thought and here we construct an ice sheet history that drives a forward model prediction of the glacial isostatic adjustment gia gravity signal the new model in turn should give predictions that are constrained with recent uplift data the impact of the gia signal on a gravity recovery and climate experiment grace antarctic mass balance estimate depends on the specific grace analysis method used for the method described in this paper the gia contribution to the apparent surface mass change is re evaluated to be 55 13 gt yr by considering a revised ice history model and a parameter search for vertical motion predictions that best fit the gps observations at 18 high quality stations although the gia model spans a range of possible earth rheological structure values the data are not yet sufficient for solving for a preferred value of upper and lower mantle viscosity nor for a preferred lithospheric thickness grace monthly solutions from the center for space research release 04 csr rl04 release time series from january 2003 to the beginning of january 2012 uncorrected for gia yield an ice mass rate of 2 9 29 gt yr the new gia correction increases the solved for ice mass imbalance of antarctica to 57 34 gt yr the revised gia correction is smaller than past grace estimates by about 50 to 90 gt yr the new upper bound to the sea level rise from the antarctic ice sheet averaged over the time span 2003 0 2012 0 is about 0 16 0 09 mm yr

the evolution of lacustrine environments on mars is mars only hydrologically dormant abstract this study surveys the evolution of martian impact crater paleolakes through time by establishing their age comparing their locations and describing the formation of lacustrine sedimentary structures and features over time we explore their meaning in terms of global climate changes but also on local and regional scales we observe a co location between recent impact crater paleolakes and regions where gcm modeling shows that mars is currently experiencing days above triple point and below boiling point of water some source areas show morphologies that could be consistent with ancient glacier features in the argyre basin darwin crater region in locations that are also consistent with gcm models of past snow precipitation on mars we propose that at least for some paleolakes our results are best explained by recent and abundant aqueous activity on mars with some meteorological contribution these results are supported by other recent findings on martian meteorite ages by the analysis of new mgs data showing recent volcanic and fluvial activity and by climate modeling they point toward the possibility that mars currently is only hydrologically dormant and that there is a potential for hydrological activity to be generated again in the future following changes in the planet s orbital parameters

antarctic marine ice sheet retreat in the ross sea during the early holocene geological constraints on the timing of retreat of the last glacial maximum lgm antarctic ice sheets provide critical insights into the processes controlling marine based ice sheet retreat the over deepened landward sloping bathymetry of antarctica s continental shelves is an ideal configuration for marine ice sheet instability with the potential for past and future ice sheet collapse and accelerated sea level rise however the chronology of retreat of the lgm ice sheet in the ross sea is largely constrained by imprecise radiocarbon chronology of bulk marine sediments or by coastal records that offer more reliable dating techniques but which may be influenced by local piedmont glaciers derived from east antarctic outlet glaciers consequently these coastal records may be ambiguous in the broader context of retreat in the central regions of the ross sea here we present a sedimentary facies succession and foraminifera based radiocarbon chronology from within the ross sea embayment that indicates glacial retreat and open marine conditions to the east of ross island before 8 6 cal calibrated kyr b p at least 1 k y earlier than indicated by terrestrial records in mcmurdo sound comparing these data to new modeling experiments we hypothesize that marine based ice sheet retreat was triggered by oceanic forcings along most of the pacific ocean coastline of antarctica but continued holocene retreat into the inner shelf region of the ross sea occurred primarily as a consequence of bathymetric controls on marine ice sheet instability

crater size frequency distributions and a revised martian relative chronology a revised martian relative chronology is determined which dates geologic units with respect to the end of the period of heavy bombardment this analysis differs from previous studies by using viking 1 2m photomosaics to map all 25 826 craters 8km diameter which postdate the lithologic units making up the martian surface the relative plotting technique is used because it shows changes in the shape and density of the crater size frequency distribution curves more clearly than cumulative plots the changes in shapes of the distribution curves are interpreted to be due to two different impacting populations one dominating during the heavy bombardment period and the other emplaced during the post heavy bombardment period the 8 km diameter limit was chosen to exclude the majority of secondary craters and to minimize obliteration effects acting over the age of the planet the results obtained are average ages valid for units with typical thicknesses between about 1 and 3 km and extending over an area of about 105 km2 the shapes and densities of the crater distribution curves indicate that approximately 60 of the martian lithologic units in which these craters occur formed during the period of heavy bombardment many of the small volcanoes those generally less than 200 km diameter including most in the tharsis and elysium regions date from this period the hellas and argyre impacts bracket the average age of the intercrater plains in the highlands the ridged plains distribution curves indicate formation during the period of heavy bombardment but their lower crater density indicates that they are the last geologic unit to retain the scars from this population of impactors they therefore likely formed near the end of heavy bombardment the remaining 40 of martian terrain units show distribution curves indicative of emplacement since the end of heavy bombardment the wide range in crater densities among different regions of plains indicate a range in formation ages although the general view of martian geologic evolution does not differ significantly from previous studies the present analysis explicitly states the relationship of numerous geologic units to the period of heavy bombardment and has revised the relative ages of localized regions across mars

global glacial isostasy and the surface of the ice age earth the ice 5g vm2 model and grace abstract the 100 kyr quasiperiodic variation of continental ice cover which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of earth history has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n body effects in the solar system on the geometry of earth s orbit around the sun the impacts of the changing surface ice load upon both earth s shape and gravitational field as well as upon sea level history have come to be measurable using a variety of geological and geophysical techniques these observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid earth and on the detailed spatiotemporal characteristics of glaciation history this review focuses upon the most recent advances that have been achieved in each of these areas advances that have proven to be central to the construction of the refined model of the global process of glacial isos

a mossbauer investigation of iron rich terrestrial hydrothermal vent systems lessons for mars exploration hydrothermal spring systems may well have been present on early mars and could have served as a habitat for primitive life the integrated instrument suite of the athena rover has as a component on the robotic arm a mossbauer spectrometer in the context of future mars exploration we present results of mossbauer analysis of a suite of samples from an iron rich thermal spring in the chocolate pots area of yellowstone national park ynp and from obsidian pool ynp and manitou springs colorado we have found that mossbauer spectroscopy can discriminate among the iron bearing minerals in our samples those near the vent and on the surface are identified as ferrihydrite an amorphous ferric mineraloid subsurface samples collected from cores which are likely to have undergone inorganic and or biologically mediated alteration diagenesis exhibit spectral signatures that include nontronite a smectite clay hematite alpha fe2o3 small particle nanophase goethite alpha feooh and siderite feco3 we find for iron minerals that mossbauer spectroscopy is at least as efficient in identification as x ray diffraction this observation is important from an exploration standpoint as a planetary surface instrument mossbauer spectroscopy can yield high quality spectral data without sample preparation backscatter mode we have also used field emission scanning electron microscopy fesem in conjunction with energy dispersive x ray edx fluorescence spectroscopy to characterize the microbiological component of surface sinters and the relation between the microbiological and the mineralogical framework evidence is presented that the minerals found in these deposits can have multi billion year residence times and thus may have survived their possible production in a putative early martian hot spring up to the present day examples include the nanophase property and the mossbauer signature for siderite which has been identified in a 2 09 billion year old hematite rich chert stromatolite our research demonstrates that in situ mossbauer spectroscopy can help determine whether hydrothermal mineral deposits exist on mars which is significant for exobiology because of the issue of whether that world ever had conditions conductive to the origin of life as a useful tool for selection of samples suitable for transport to earth mossbauer spectroscopy will not only serve geological interests but will also have potential for exopaleontology