ice core evidence for extensive melting of the greenland ice sheet in the last interglacial evidence from ice at the bottom of ice cores from the canadian arctic islands and camp century and dye 3 in greenland suggests that the greenland ice sheet melted extensively or completely during the last interglacial period more than 100 ka thousand years ago in contrast to earlier interpretations the presence of dirt particles in the basal ice has previously been thought to indicate that the base of the ice sheets had melted and that the evidence for the time of original growth of these ice masses had been destroyed however the particles most likely blew onto the ice when the dimensions of the ice caps and ice sheets were much smaller ice texture gas content and other evidence also suggest that the basal ice at each drill site is superimposed ice a type of ice typical of the early growth stages of an ice cap or ice sheet if the present day ice masses began their growth during the last interglacial the ice sheet from the earlier illinoian glacial period must have competely or largely melted during the early part of the same interglacial period if such melting did occur the 6 meter higher than present sea level during the sangamon cannot be attributed to disintegration of the west antarctic ice sheet as has been suggested

disturbed radiometric ages and their bearing on interregional correlations in the sw baltic shield abstract rb sr whole rock and zircon u pb isotopic work is reported from the northern part of the region between the mylonite and protogine zones in the southern baltic shield three age determinations on polymetamorphic gneissic granites were carried out two of these yielded u pb upper intercept ages of 1650 and 1675 ma respectively which is approximately one hundred ma less than expected from the combined field evidence and earlier isotopic age determinations although the new discordias appear well defined various criteria suggest that the u pb isotope system was disturbed thus the rb sr system had been opened but nevertheless the rb sr errorchrones are consistent with the u pb upper intercept ages and an abraded zircon fraction suggests an older age than the unabraded fractions the third u pb age determination resulted in a poorly constrained discordia its upper intercept ranges between 1575 and 1605 ma depending on the number of fractions considered this paper tests a three stage model where the intrusion age is set identical to the 1780 ma intrusion age of the undeformed assumed protolith the first two analysed rocks are consistent with a model suggesting intrusion at 1780 ma followed by partial opening of the isotopic systems early 1200 ma and late 950 ma during the sveconorwegian orogeny an abraded fraction together with the time of the suggested last opening of the system 950 ma ago defines a two point discordia with an upper intercept age of 1745 ma this age is in close agreement with the assumed intrusion age the isotope data from the third gneissic granite cannot be fitted to this simple model it is concluded that the analysed rocks could belong to the granitoids of the transscandinavian igneous belt but if they do they were reworked during the sveconorwegian orogeny as a corollary it follows that the regions separated by the northern part of the protogine zone largely had a common pre sveconorwegain geological history another important implication is that the intrusion ages in the area between the mylonite and protogine zones are older than those of the trans labrador batholith in canada

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

glaciohydraulic supercooling in iceland we present evidence of glaciohydraulic supercooling under jokulhlaup and ablation dominated conditions from two temperate icelandic glaciers observations show that freezing of sediment laden meltwater leads to intraglacial debris entrainment during normal and extreme hydrologic regimes intraglacial frazil ice propagation under normal ablation dominated conditions can trap copious volumes of sediment which forms anomalously thick sections of debris rich ice glaciohydraulic supercooling plays an important role in intraglacial debris entrainment and should be given more attention in models of basal ice development extreme jokulhlaup conditions can result in significant intraglacial sediment accretion by supercooling which may explain the concentration of englacial sediments deposited in heinrich layers in the north atlantic during the last glaciation

models of climate cycles recorded in martian polar layered deposits abstract layered strata observed in orbital images of polar sedimentary terrains on mars have been hypothesized to contain information about recent variations in the martian climate celestial mechanicians have determined that mars has experienced large amplitude quasi periodic variations in obliquity and eccentricity planetary meteorologists have inferred that the resulting changing patterns of solar heating must have had dramatic effects on martian climate accordingly it might be possible to link the known recent history of orbit axial variations with the physical characteristics of the most recently deposited strate determined from orbital imagery our paper explores that possibility quantitatively for various models of the formation of the polar layered deposits and the layering developed within them we modeled the formation of polar layered deposits during the past 10 my for two general classes of model uniform deposition rate udr models assume that the major constituent of the layered deposits is deposited at a uniform rate and that layering is established by climate induced variations in the concentration of a minor constituent climate modulated deposition rate cmdr models assume that the deposition rate of the major constituent is controlled by climate and that layer boundaries correspond to intervals of nondeposition in both models abrupt changes in deposition regime occur when a function of the orbit axial parameters crosses a threshold value detailed physical mechanisms for the control of sedimentation by orbit axial variations were developed for two specific models of the cmdr type in one model the deposits are assumed to be primarily composed of dust in the other they are assumed to be predominantly water ice we calculated the times at which abrupt changes in deposition regime occur and determined the relative thicknesses of successive layers which can be compared against layer thicknesses made from orbital imagery in all these models obliquity variations exert the dominant influence on the sequential variations in layer thickness although eccentricity and precession index longitude of perihelion may take recognizable contributions as well the layer patterns for the north and south polar deposits are identical for the dust model and only slightly different for the ice model but this is at least partly a consequence of simplifying assumptions the sequential variation in layer thickness is also very sensitive to those model parameters which control the fraction of time for which deposition occurs when the climate threshold for deposition is low deposition occurs a large fraction of the total time and characteristic patterns form with one thick layer and a number of much thinner ones such patterns would be easily recognized in photogeological measurements of layer thickness when the threshold for deposition lies in the mid range with respect to excursions of the climate function deposition occurs approximately half of the time and there is only slight modulation of layer thickness when the threshold for deposition is high deposition occurs only a small fraction of the total time and groups of layers form which are separated form other groups by long periods of nondeposition the stratigraphic unconformities that separate groupings of five to ten layers in the polar deposits may indicate that the martian polar region has experienced such a depositional history the feasibility of using these model results to verify the theory of climate control of polar stratigraphy on mars and to gain additional insights into the significant processes is considered the stratigraphy issues raised here are also explored in the context of recent work on the interpretation of stratigraphy of ocean margins from this comparison there emerge some valuable insights into the stratigraphic effects of periods of nondeposition and the uncertaintiesin interpreting images of exposed polar strata on mars

chasma boreale mars a sapping and outflow channel with a tectono thermal origin abstract a detailed geomorphological study of chasma boreale a widely known feature of mars north polar cap has been carried out for the first time along with a quantitative paleohydrological model it is concluded that the chasma was eroded by a flow whose discharge was on the order of magnitude of 10 9 m sec 1 this catastrophic flow is thought to have been preceded by a powerful sapping process caused by a tectonically focused thermal event several lines of evidence indicate tectonic control in the distribution and present aspect of most of the polar troughs this tectonic forcing probably played a major role in the inception of not only chasma boreale but also other polar reentrants as well

glaciohydraulic supercooling a freeze on mechanism to create stratified debris rich basal ice ii theory simple theory supports field observations lawson and others 1998 that subglacial water flow out of overdeepenings can cause accretion of layered debris bearing ice to the bases of glaciers the large meltwater flux into a temperate glacier at the onset of summer melting can cause rapid water flow through expanded basal cavities or other flow paths if that flow ascends a sufficiently steep slope out of an overdeepening the water will supercool as the pressure melting point rises and basal ice accretion will occur diurnal occasional or annual fluctuations in water discharge will cause variations in accretion rate debris content of accreted ice or subsequent diagenesis producing layers under appropriate conditions net accretion of debris bearing basal ice will allow debris fluxes that are significant in the glacier sediment budget

stratigraphic relationships within martian polar cap deposits abstract deposits of layered ice and dust accumulate on the smooth and banded terrains of the north and south polar cap deposits while erosion of equator facing slopes of layered terrain expose these layers banded terrain occurs at exposures of irregular layers resulting from slight erosion of the feather edges of layers deposited on pole facing trough walls these deposits unconformably overlap onto adjacent layered terrain stripping of layers occurs in areas of strong multidirectional wind forming striped terrain local wind scour has also produced steep scarps with crescentic planform often associated with dune fields positioned immediately downwind unconformities are common within layered deposits in areas of complex topography at the north pole these indicate that episodes of deposition of 5 10 layers alternate with slight erosion of pole facing trough walls troughs that strike north south alternate between erosional widening and deepening and depositional infilling creating multiple unconformities troughs near the equatorward limit of polar deposits increase in relief by deposition on adjacent smooth terrain and locally by erosional deepening troughs may migrate poleward replacing the relief increase by depositional infilling when they near the poles

the mars water cycle at other epochs recent history of the polar caps and layered terrain abstract the martian polar caps and layered terrain presumably evolve by the deposition and removal of small amounts of water and dust each year the current attributes of the cap therefore represent the incremental transport during a single year as integrated over long periods of time we have investigated the role of condensation and sublimation of water ice in this process by examining the seasonal water cycle during the past 10 7 years in our model axial obliquity eccentricity and l s of perihelion vary according to dynamic models at each epoch we calculate the seasonal variations in temperature at the two poles keeping track of the seasonal co 2 cap and the summertime sublimation of water ice into the atmosphere net exchange of water between the two caps is calculated based on the difference in the summertime sublimation between the two caps or on the sublimation from the other cap if one is covered with co 2 frost all year despite the simple nature of our model and the tremendous complexity of the martian climate system our results suggest two significant conclusions 1 only a relatively small amount of the available water in the polar deposits actually cycles between the poles on orbital evolution time scales such that it is to some extent the same water molecules moving back and forth between the two caps 2 the difference in elevation between the two caps results in different seasonal behavior such that there is a net transport of water from south to north averaged over long time scales this transport would cause the north cap to grow at the expense of the south cap until the product of water sublimation rate and cap area was equal for the caps minimizing any subsequent net transport these results can help explain 1 the apparent inconsistency between the timescales inferred for layer formation and the much older crater retention age of the cap and 2 the difference in sizes of the two residual caps with the south cap being smaller than the north

coastal geomorphology of the martian northern plains the paper considers the question of the formation of the outflow channels and valley networks discovered on the martian northern plains during the mariner 9 mission parker and saunders 1987 and parker et al 1987 1989 data are used to describe key features common both in the lower reaches of the outflow channels and within and along the margins of the entire northern plains it is suggested that of the geological processes capable of producing similar morphologies on earth lacustrine or marine deposition and subsequent periglacial modification offer the simplest and most consistent explanation for the suit of features found on mars

how glaciers entrain and transport basal sediment physical constraints abstract simple insights from the physics of ice water and sediment place constraints on the possible sediment transport behavior of glaciers and ice sheets because glaciers concentrate runoff streams generated by glaciers transport much sediment and may erode bedrock rapidly deforming glacier beds also can transport much sediment particularly in marginal regions rapid sediment entrainment producing thick debris rich basal zones may occur by regelation into subglacial materials and by freeze on from rising supercooled waters numerous other mechanisms may be important but primarily near ice margins especially those of advancing or fluctuating glaciers several sediment entrainment mechanisms may be active beneath a single glacier but one process is likely to be dominant at any place and time

thermophysical properties of alaskan loess an analog material for the martian polar layered terrain the martian surface has several regions where thermal inertia measurements indicate a porous ice free insulating surface yet are mechanically competent enough to sustain substantial slopes in support of the interpretation of those regions within the martian polar layered terrain we report measurements of thermal conductivity for loess from the field and in the usa crrel permafrost tunnel permafrost tunnel loess is a desiccated material that can form vertical walls but is of low density 800 1000 kg m modest shear strength 4 kpa and has a low thermal conductivity 0 1 w m k at 1 bar these properties are similar to the inferred properties of the martian polar layered terrain the birch hill field sample has a density of 1160 kg m and a conductivity of 0 15 w m k the chena spur road sample has a density of 1360 kg m and a conductivity of 0 7 w m k the relatively high conductivity for the chena spur road is due to the cementation of soil grain contacts its higher density coarser grain size and higher quartz grain content

assessing the martian surface distribution of aeolian sand using a mars general circulation model a sand transport model using white s 1979 sand flux equation and the mars general circulation model pollack et al 1990 was developed to understand the erosional sources transport pathways and depositional sinks of windblown sand on mars an initially uniform distribution of sand 4 mm over the entire surface is regionally transported based on wind stress saltation threshold and percentage of topographic trapping results are consistent with the observed polar and hellespontus dunes and christensen s 1986 modeled block size distribution but only for an extremely low saltation threshold 0 024 n m 2 low thresholds generally result in transport of sand sized particles originating in the northern mid latitudes to the north pole and transport from the northern lower latitudes to the southern hemisphere our results indicate that the polar dune fields could form in 50 000 years consistent with the active polar dunes and lack of longitudinal dunes observed on the surface of mars

mars landscape evolution influence of stratigraphy on geomorphology in the north polar region abstract lithology and physical properties of strata exposed at the earth s surface have direct influence on the erosion and geomorphic expression of landforms while this is well known on our planet examples on mars are just coming to light among the tens of thousands of airphoto quality images resolutions 1 5 12 m pixel acquired since 1997 by the mars global surveyor mgs mars orbiter camera moc specific examples occur among martian north polar layered materials which moc images reveal are divided into two distinct stratigraphic units a lower dark toned layered unit and a younger upper lighter toned layered unit the lower unit is less resistant to wind erosion than the upper unit the upper unit most likely consists of stratified dust and ice while the lower unit contains abundant poorly cemented sand sand is more easily mobilized by wind than dust the lower resistance to erosion of the lower unit results from the presence of sand where wind erosion in polar troughs has penetrated to the lower unit geomorphic change has proceeded more rapidly sand has been liberated from the lower unit and arcuate scarps have formed as the upper unit has been undermined wind erosion of the lower unit thus influences the geomorphology of the north polar region this result likely explains the genesis of the large polar trough chasma boreale and the relations between dunes and arcuate scarps that have puzzled investigators for nearly three decades the properties of the stratigraphic units suggest that the upper limit for the amount of water contained in the north polar layered materials may be 30 50 less than previously estimated

the effects of martian orbital variations upon the sublimation and relaxation of north polar troughs and scarps abstract the north polar layered deposits pld of mars are climatologically significant because they represent the largest actively exchanging reservoir of martian water the kilometer scale topography of the north pld is dominated by troughs and scarps which exhibit highly correlated surface slopes and total depths the most widespread theories of pld evolution presume that the asymmetrical nature of north pld troughs characterized by equatorward facing slopes that are generally steeper than poleward facing slopes is the result of preferential h 2 o sublimation from equatorward facing trough walls however our orbitally modulated simulations indicate that there is no long term sublimation advantage of equatorward facing trough walls because of the effects of obliquity upon the slope dependence of sublimation rate in contrast we propose that viscous relaxation of subsurface water is consistent with the slope and depth distributions of north pld troughs and scarps the results of our finite element simulations suggest that a mere 2 k difference in the subsurface temperatures of opposing trough walls is sufficient to produce the observed slope disparity due to the slower rate of uplift of colder poleward facing trough walls both our sublimation and relaxation simulations indicate that present day north pld troughs have formed since 5 ma and are not sites of long term deposition additionally our results imply that glacial flow may govern the large scale evolution of the north pld especially at high obliquity

impact melts and glasses on mars 1 dark mobile materials on mars are widely considered to be volcanically derived windblown particulates such a conclusion is consistent with the large volcanic edifices on mars and the assumption that a volatile rich crust would not favor significant impact melt generation several post viking discoveries however provide new perspectives for reconsidering alternative origins not only for some of the mobile dark veneers but also certain materials at the pathfinder and viking landing sites first certain dark materials on mars do not exhibit the expected mafic absorption features in their reflectance spectra second the pathfinder mission revealed unexpected feldspathic compositions of vesicular blocks that lead to a preliminary conclusion that they represent andesites or basaltic andesites rather than basalts third recent discoveries of holocene to miocene impacts in argentine loessoid deposits provide new insights for the nature of melts generated from eolian sediments fourth global dispersal of impact melt products are now recognized in the terrestrial record throughout the cenozoic consequently the generation and preservation of impact glass on mars should be reconsidered possible occurrences include possible tektite like strewnfields downrange from recent oblique impacts dark avalanches emerging from backwasting cliffs of easily eroded unconformable deposits global dispersal of glassy ejecta from hesperian and amazonian craters of chicxulub scale and dark mobile veneers emerging from noachian craters we propose that the mobile dark materials comprising many of the named features of mars as well as some blocks at the pathfinder viking landing sites may represent impact melt products

geographic variations in the thermal and diffusive stability of ground ice on mars to investigate the stability of ground ice within the top several meters of the martian regolith time dependent models of the thermal and diffusive behavior of the regolith have been developed the geographic distribution of thermal inertia and albedo as well as the latitudinal variation in insolation have been included in calculations of surface and subsurface temperatures between 60 latitude ground ice was found to be stable where the annual mean surface and subsurface temperatures were below the atmospheric frost point this generally occurs poleward of the mid latitudes the latitude poleward of which ground ice is stable varies by about 20 to 30 from one longitude to another geographic variations in thermal inertia and albedo are the primary factors controlling regional variations in ice stability calculations of temperatures at high and low obliquity suggest that ground ice would be stable globally at high obliquity and would not be stable between 60 latitude at low obliquity thermally driven diffusion of atmospheric water vapor within the regolith was modeled accounting for both ordinary molecular and knudsen transport and equilibrium between ice vapor and adsorbed phases atmospheric water vapor was found to be able to supply the top few meters of the regolith with ice in regions where the annual mean surface temperature was below the atmospheric frost point ice was found to begin condensing in as short as 1000 martian years rapid accumulation of ice in the pore space of the upper layers of the regolith acted to choke transport to lower layers and slow the diffusion process even so after 105 martian years in some cases as much as 30 to 40 of the available pore space accumulated ice the total amount of subsurface ice ranged from a few to more than 25 g cm2 within the top few meters in regions of stability ground ice was found to form below a depth where the annual average vapor pressure over ice was equal to the annual average atmospheric vapor pressure near the surface atmospheric vapor was not found to accumulate as ice below a depth where the seasonal temperature oscillations gave way to the geothermal gradient the time scales for condensation of ground ice were found to be comparable to that of orbital oscillations suggesting that the present geographic distribution of ground ice may depend on the orbital history of mars

episodic deposition and preservation of eolian sands a late paleozoic example from southeastern utah the upper pennsylvanian and lower permian stratigraphic sequence in canyonlands national park contains about 40 flat topped eolian sand bodies rhizoliths burrows and traces of former evaporites abound at the tops of these tabular genetic units upper surfaces of units were created by eolian deflation to the level of the groundwater table analysis of cross stratification reveals that sands accumulated via migration and climb of relatively small dunes brief episodes of sand accumulation were triggered by regression and were terminated when upwind sand supplies were depleted upper surfaces of genetic units are diastems and represent much more geologic time than the rocks themselves

evidence of ancient continental glaciation in the martian northern plains whorled ridges spaced about 2 6 km and forming lobate patterns with lobe widths of about 150 km occur at many locations in the northern plains of mars commonly in close association with sinuous troughs that contain medial ridges these landforms resemble moraines tunnel channels and eskers found in terrestrial glacial terrains such as the midcontinent of north america some martian landscapes may have formed by disintegration of continental glaciers that covered much of the northern plains into the early amazonian i e late in martian geologic history meltwater processes apparently were important in the collapse of these hypothesized ice sheets hence the glaciers apparently were wet based in part whereas striking similarities exist among areas of the northern plains and some glaciated pleistocene terrains on earth there are also important differences notably drumlin fields such as those in many glacial landscapes on earth are rare absent or not yet resolved in images of the martian northern plains another major difference is that postglacial fluvial and other water related modifications especially erosion of pleistocene terrains are substantial but similar modifications are not observed in the northern plains a virtually complete and sudden decline in the activity of liquid surface water following glaciation in the northern plains seems to be implied the climatic implications of the hypothesized martian glaciers and their decline are unclear we investigate two possibilities alternatively involving a relatively warm paleoclimate and the modern martian climate the hypothesized ice sheets in the basins within the northern plains generally at elevations lower than 1 km suggest a relationship of these frozen bodies of water with former regional lakes or seas which may have formed in response to huge discharges of water from martian outflow channels this possible relationship has been modeled glaciers may have evolved from seas by their progressive freezing and then grounding and sublimational redistribution of sea ice the transition to glaciation may have taken several million years if the climate was very cold comparable to today s or tens of thousands of years if the climate was as warm as modern antarctica a glacierized sea may have involved an extended period of glaciolacustrine and ice shelf processes

preliminary geological assessment of the northern edge of ultimi lobe mars south polar layered deposits we have examined the local base of the south polar layered deposits spld exposed in the bounding scarp near 72 74 s 215 230 w where there is a clear unconformable contact with older units sections of layering up to a kilometer thick were examined along the bounding scarp permitting an estimate of the thinnest individual layers yet reported in the spld rhythmic layering is also present locally suggesting a similarly rhythmic variation in environmental conditions and a recorded climate signal at least in some spld strata locally angular unconformities may be present as has been reported for the north polar layered deposits npld and may likewise imply intervals of subaerial erosion in the spld the outcropping layers display a broad range of weathering styles and may reflect more diverse conditions of depositions erosion and diagenesis than might have been expected from simple aeolian depostion modulated only by astronomically driven climatic fluctuations an unexpected finding of our study is the presence of locally abundant small pits close to the bounding scarp these quasi circular negative rimless features probably originated as impact craters and were modified to varying degrees by local endogenic processes as well as locally variable blanketing a nominal exposure age for the most heavily cratered region in our study area is about 2 million years and the crater statistics appear consistent with those for the overall spld although there are large uncertainties in the absolute ages implied by the crater size frequency statistics as in all martian crater ages another new finding is the presence of mass wasting features along the steepest portion of the retreating bounding scarp as well as a number of examples of brittle fracture consistent with large scale slumping along the bounding scarp and probably also ancient basal sliding both subhorizontal and high angle faults appear to be exposed in the bounding scarp but the dips of the faults are poorly constrained these fractures along with the relatively undeformed layers between them suggest to us that whatever horizontal motion may have taken place outward from the central cap region was accomplished by ancient basal sliding rather than large scale glacial like flow or ice migration by differential ablation as proposed recently for the northern permanent cap and underlying npld we have also obtained the first direct estimate of the regional dip of the spld around 2 3 outward northward in one area

high resolution topography and albedo of the south polar layered deposits on mars using a new photoclinometric technique with high resolution mariner 9 images maximum slopes of 10 20 are found to occur on an exposure of layered deposits within the south polar residual cap of mars stereophotogrammetry is used to constrain the photoclinometric solutions which resolve layer thicknesses of 100 300 m albedo variations are correlated with slope indicating that frost is present on level areas there is evidence for temporal changes in frost distribution in the 7 days 4 of l s between the two images used in this study the magnitude of the slopes derived here and consideration of the stability of water ice at the surface of the layered deposits strongly suggest the presence of a competent weathering rind the weathered surface may be composed of dark filamentary sublimation residue particles that protect the underlying ice from solar heating this hypothesis is consistent with previous studies of the regional color and albedo of the layered deposits which indicate that the deposits are slightly darker and less red than the bright dust that mantles much of the south polar region furthermore the proposed weathering mechanism provides a plausible source of dark saltating material for the martian polar dune fields

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

dark material in the polar layered deposits and dunes on mars viking infrared thermal mapping and bistatic radar data suggest that the bulk density of the north polar erg material is much lower than that of the average martian surface or of dark dunes at lower latitudes we have derived a thermal inertia of 245 280 j m 2 s 1 2 k 1 5 9 6 7 10 3 cal cm 2 s 1 2 k 1 for the proctor dune field and 25 150 j m 2 s 1 2 k 1 0 6 3 6 10 3 cal cm 2 s 1 2 k 1 for the north polar erg the uniqueness of the thermophysical properties of the north polar erg material may be due to a unique polar process that has created them the visible and near infrared spectral reflectance of the erg suggests that the dark material may be composed of basalt or ferrous clays these data are consistent with the dark material being composed of basaltic ash or filamentary sublimate residue fsr particles derived from erosion of the layered deposits dark dust may be preferentially concentrated at the surface of the layered deposits by the formation of fsr particles upon sublimation of water ice further weathering and erosion of these areas of exposed layered deposits may form the dark saltating material that is found in both polar regions dark fsr particles may saltate for great distances before eventually breaking down into dust grains re mixing with the global dust reservoir and being recycled into the polar layered deposits via atmospheric suspension

surface ages and resurfacing rates of the polar layered deposits on mars interpretation of the polar stratigraphy of mars in terms of global climate changes is complicated by the significant difference in surface ages between the north and south polar layered terrains inferred from crater statistics we have reassessed the cratering record in both polar regions using viking orbiter and mariner 9 images no craters have been found in the north polar layered terrain but the surface of most of the south polar layered deposits appears to have been stable for many of the orbital axial cycles that are thought to have induced global climate changes on mars the inferred surface age of the south polar layered deposits about 10 ma is two orders of magnitude greater than the surface age of the north polar layered deposits and residual cap at most 100 ka similarly modeled resurfacing rates are at least 20 times greater in the north than in the south these results are consistent with the hypotheses that polar layered deposit resurfacing rates are highest in areas covered by perennial ice and that the differences in polar resurfacing rates result from the 6 4 km difference in elevation between the polar regions deposition on the portion of the south polar layered deposits that is not covered by the perennial ice cap may have ceased about 5 million years ago when the obliquity of mars no longer exceeded 40

eolian deflation by ancient katabatic winds a late quaternary example from the north alaska range strong katabatic winds are frequently generated in glaciated mountain valleys of the alaska range by downglacier gravitational flow of chilled air stronger katabatic winds probably occurred during late quaternary time when glaciers were more extensive and such winds may have greatly influenced eolian sedimentation in nenana river valley and in most of adjacent teklanika river valley the entire 1 to 2 m thick eolian cap is postglacial in age and the beginning of loess deposition coincides almost exactly with rapid recession of late wisconsin ice previously deposited loess and bluff edge dunes were apparently deflated during the last glaciation between 25 000 and 12 000 yr b p loess deposition in these mountain valleys thus is exclusively an interglacial phenomenon glacial phases are characterized by deflation meteorological modeling of katabatic winds generated during the last glaciation predicts that paleowind velocities greatly exceeded the entrainment thresholds required for eolian deflation when both nenana river and teklanika river valleys are compared theoretical differences in the potential for generating katabatic winds and differences in valley physiography explain why pre holocene loess is preserved in downstream reaches of teklanika river valley and not nenana river valley the presence of strong katabatic winds in mountain valleys beyond former glacier limits has significant implications for late pleistocene paleoecology and archeology

sedimentary deposits in the northern lowland plains mars the lowland plains on mars have surfaces marked by large polygonal fracture patterns it was recently proposed that the fracture patterns were developed on sedimentary deposits from outflow channels we support this hypothesis because of the following observations 1 polygonal fracture patterns tend to occur in low areas on mars that apparently received influx of sediments the area of northern plains in which the fractures occur also forms a regional low 2 the fracture patterns are concentrated near reentrants of the plains projecting into the southern highlands not only are these reentrants associated with outflow channels but channels and polygonally fractured deposits have similar ages 3 sinuous ridges at the mouths of martian outflow channels resemble ridges in antarctic ice streams and ice shelves the similarity suggests that the martian material was transported from the southern highlands toward the northern plains as the antarctic material is transported from the continent toward the adjacent ocean overall it appears that a massive transfer of material took place midway in martian history either through the outflow channels or elsewhere along the northern highland scarp and that this material probably formed the deposits now characterized by polygonal fracture patterns

polar basal melting on mars if ice is present throughout the cryosphere anywhere on mars thermodynamic considerations suggest that it is most likely at the poles given this condition the deposition of dust and h2o at the polar surface will ultimately result in a situation where the equilibrium depth to the melting isotherm has been exceeded melting ice at the base of the cryosphere until thermal equilibrium is once again established should deposition persist the polar deposits will ultimately reach a thickness where melting will occur at their actual base at this point the cap may reach a state of equilibrium where the deposition of any additional ice is balanced by geothermal melting thermal calculations yield basal melting thicknesses that are consistent with the inferred 4 6 km thickness of the present north polar cap however in the south the deposits appear sufficiently thin 1 2 km that geothermal melting is likely to be relegated to a depth that lies well below the regolith polar cap interface similar conclusions are reached from consideration of the polar caps theoretical equilibrium profiles in the north a basal yield stress characteristic of ice at or near the melting point is indicated while in the south it appears the cap has yet to achieve the necessary height for significant deformation to occur at its base the potential importance of basal melting is illustrated through the discussion of four examples 1 the origin of the major polar reentrants 2 the removal and storage of an ancient martian ice sheet 3 the mass balance of the polar terrains and 4 the possibility of basal melting at temperate latitudes this analysis suggests that the process of basal melting may play a key role in understanding the evolution of the martian polar terrains and the long term climatic behavior of water on mars

glaciohydraulic supercooling a freeze on mechanism to create stratified debris rich basal ice i field evidence debris laden ice accretes to the base of matanuska glacier alaska u s a from water that supercools while flowing in a distributed drainage system up the adverse slope of an overdeepening frazil ice grows in the water column and forms aggregates while other ice grows on the glacier sole or on substrate materials sediment is trapped by this growing ice forming stratified debris laden basal ice growth rates of 0 1 m a 1 of debris rich basal ice are possible the large sediment fluxes that this mechanism allows may have implications for interpretation of the widespread deposits from ice that flowed through other overdeepenings including heinrich events and the till sheets south of the laurentian great lakes

the basal ice layer of glaciers and ice sheets abstract in many glaciers and ice sheets there is a basal ice layer bil in which the ice is conditioned primarily by processes operating at the bed the bil is chemically and physically distinctive and is characterised by a component of basally derived sediment the bil is a rheological control on ice sheet dynamics an indicator of subglacial conditions and processes an agent of subglacial geologic processes the source of a substantial proportion of glacial sediments a limit to the downward extension of the climate record from deep ice cores if debris characteristics of the bil are preserved in glacial sediments former glacier conditions can be inferred analysis of stable isotopes gas and cation composition ice crystallography debris composition and structural glaciology has elucidated many mechanisms for bil formation common elements can be identified from a range of locations key issues are entrainment of ice and debris from the bed formation of new ice metamorphism of existing ice in the bil and thickening of the bil by deformation a distinction can be made between basal ice formed by accretion of material from the bed stratified facies and basal ice formed by metamorphic processes within the ice close to the bed dispersed facies

extensive hesperian aged south polar ice sheet on mars evidence for massive melting and retreat and lateral flow and ponding of meltwater local and regional mars orbiter laser altimeter mola topographic data support the presence of an extensive hesperian aged volatile rich south polar deposit the dorsa argentea formation hd and related units underlying the present amazonian aged polar cap api residual ice and apl layered terrain and covering a surface area that could be as large as 2 94 106 km2 about 2 of the surface of mars over twice the area of the present amazonian aged south polar deposits the deposit characteristics indicate that it contained significant quantities of water ice in amounts comparable to present day polar deposits several lines of evidence for melting indicate that the ice sheet deposits underwent melt back and liquid water drainage into surrounding lows including a large valley near the crater schmidt and the argyre basin narrow sinuous ridges lie in a broad linear depression extending from a high near the present polar cap continuously downslope to near the distal portion of hd the new topographic data support the interpretation of these ridges as eskers representing meltwater distribution networks at the base of the receding deposit extensive development of large pits and depressions cavi have previously been interpreted as eolian etching or basal melting of ice rich deposits analysis of mola topography supports the interpretation that they represent basal melting of ice rich deposits and shows that they have links to the esker systems volumetric considerations and topographic lineations suggest that some of the basal melting occurred beneath regions presently occupied by apl and that some of the liquid water formed ponds and lakes in the distal parts of hd the presence of pedestal craters is further evidence of the removal of extensive volatile rich deposits and contributes to the quantitative measure of the former deposit thickness where did the melt products go inspection of the margins of the dorsa argentea formation reveals several large channels that begin there and drain downslope for distances between 900 and 1600 km onto the floor of the argyre basin some 3 5 4 0 km below their origin these channels do not exhibit tributaries their broad lateral distribution supports other evidence that deposit melting was areally very widespread and volumetrically significant and that a large part of the meltwater entered the surface distribution system and was deposited on the floor of the argyre basin over 1000 km away estimates of the present deposit thicknesses together with amounts of the deposit removed by meltback suggest that the original volume could have been as much as 5 9 106 km3 equivalent to a global layer of water 20 m deep if the deposit consisted of 50 volatiles a portion of these volatiles migrated across the surface to pond in adjacent valleys and basins and into the groundwater system a significant portion of the volatiles remain in the deposit representing a net removal from the atmosphere and from the active hydrologic system in early to middle mars history and forming an accessible record of aqueous conditions and possible biological environments dating from that time

chasma boreale mars topographic characterization from mars orbiter laser altimeter data and implications for mechanisms of formation 1 chasma boreale a large reentrant in the north polar cap of mars is distinct in scale detailed topography and orientation from the spiraling troughs that characterize the majority of the polar cap we use new high resolution mars orbiter laser altimeter mola data and mars orbiter camera moc and viking images of the chasma region to assess hypotheses of origin of chasma boreale these hypotheses include formation by glacial flow and ablation by eolian processes such as katabatic winds and sublimation by outflow of meltwater or by a combination of two or more of these processes we find that these new data support initial formation by outflow of meltwater with later modification by katabatic winds and sublimation on the basis of our analysis we suggest a possible scenario in which outflow has occurred in several stages the outflow appears to have begun near an enclosed depression at the head of the chasma meltwater migrated within and below the ice down the regional slope and eventually broke out to the surface removal of material by melting caused subsidence and erosion of the polar deposits forming the chasma which has subsequently undergone modification by eolian and sublimation processes the outflow deposited most of its sediment in the form of a lobe that stratigraphically overlies the polygonal plains mapped at the mouth of the chasma and scoured some of the circumpolar sediment mantle in the surrounding region the lowest portions of the north polar basin received the rest of the sediment as a deposit a few meters thick mola data also reveal evidence for similar events of smaller magnitude elsewhere on the margins of the polar cap

geologic history of the polar regions of mars based on mars global surveyor data ii amazonian period based on mars orbiter laser altimeter mola and mars orbiter camera moc observations of amazonian polar layered deposits pld morphology composition and thickness we conclude that the plds are composed of porous unconsolidated layers that have not experienced significant basal melting or other glacial type processes morphologic features and associations within the pld chasmata including preserved craters and sinuous ridges indicate that the dominant process of chasmata formation is wind scouring our detailed analysis of south polar spiral trough topography in conjunction with the identification of similar layered stratigraphy within north polar spiral troughs suggests that trough migration due to preferential ablation of sun facing slopes cannot be demonstrated within the layered sequences we have not identified widespread unconformities discontinuities or pinch outs that would indicate an accublation origin of the plds we therefore postulate that the well defined plds at both poles eventually reached and maintained their present form following deposition without extensive deformation or redeposition large cuspate ridges in the ultimi lobe of planum australe appear to be layered and may be unusual erosional remnants of a once thicker pld in this area beneath the north polar layered deposits in planum boreum we have identified a platform of older highly degraded polar deposits as much as a kilometer thick that may have once covered an area larger than planum boreum

topography and stratigraphy of martian polar layered deposits abstract topography and stratigraphy of young sediments in the martian north polar region are being studied from high resolution viking orbiter images to test and constrain theories of terrain evolution driven by cyclical climate change several steepened slopes upon which layers are exposed in section were found to have local relief of 200 to 800 m and slopes ranging from 1 to 8 first attempts to calculate average layer thicknesses from portions of sections yielded results ranging from 14 to 46 m linearly correlated with slope in some cases under the assumption of short range lateral continuity a greater number of layers was inferred to be present than was actually resolved on a particular section when apparent layer thickness are revised accordingly the correlation with slope is destroyed we conclude that images of the shallowest sloping sections of layered deposits should yield the most stratigraphic information and layers thinner than 14 m may occur indeed they may even be common or typical because of the sensitivity of summer frost distribution to local slope and other properties of exposed strata interseasonal comparisons of images promise to enhance the capability to detect layers and differentiate them by material properties

the distribution and behavior of martian ground ice during past and present epochs mars undergoes significant oscillations in its orbit which will have a pronounced effect on its climate and in particular on the behavior of subsurface water ice we explore and map the behavior of ice in the martian near surface regolith over the past 1 m y using a diffusion and condensation model presented in an earlier paper with two primary modifications to include orbitally induced variations in insolation and atmospheric water abundance we find that the past behavior of ground ice differs significantly from that at the present epoch primarily the result of high amplitude oscillations in obliquity presently 25 in midlatitude and equatorial regions ground ice will condense from atmospheric water during times of higher obliquity filling the top few meters of the regolith with significant amounts of ice at an obliquity of 32 ground ice becomes stable globally during times of lower obliquity ground ice will sublime and diffuse back into the atmosphere dessicating the regolith to a depth of about 1 to 2 m equatorward of 60 to 70 latitude in the high latitude regions these oscillations are considerably subdued below this depth of cyclic saturation and dessication a long term stability of ice exists in some geographic regions we present a map of the distribution of ice expected at the present epoch cyclic exchange of water between the global regolith and polar regions will have significant implications for surface geology and the polar layered deposits

resurfacing history of the northern plains of mars based on geologic mapping of mars global surveyor data 1 geologic mapping of the northern plains of mars based on mars orbiter laser altimeter topography and viking and mars orbiter camera images reveals new insights into geologic processes and events in this region during the hesperian and amazonian periods we propose four successive stages of lowland resurfacing likely related to the activity of near surface volatiles commencing at the highland lowland boundary hlb and progressing to lower topographic levels as follows highest elevations indicated stage 1 upper boundary plains early hesperian 2 0 to 2 9 km stage 2 lower boundary plains and outflow channel dissection late hesperian 2 7 to 4 0 km stage 3 vastitas borealis formation vbf surface late hesperian to early amazonian 3 1 to 4 1 km and stage 4 local chaos zones early amazonian 3 8 to 5 0 km at acidalia mensa stage 2 and 3 levels may be lower 4 4 and 4 8 km respectively contractional ridges form the dominant structure in the plains and developed from near the end of the early hesperian to the early amazonian geomorphic evidence for a northern plains filling ocean during stage 2 is absent because one did not form or its evidence was destroyed by stage 3 resurfacing remnants of possible amazonian dust mantles occur on top of the vbf the north polar layered deposits appear to be made up of an up to kilometer thick lower sequence of sandy layers early to middle amazonian in age overlain by late amazonian ice rich dust layers both units appear to have outliers suggesting that they once were more extensive

evidence for recent climate change on mars from the identification of youthful near surface ground ice ground ice in the crust and soil may be one of the largest reservoirs of water on mars1 2 3 near surface ground ice is predicted to be stable at latitudes higher than 40 ref 4 where a number of geomorphologic features indicative of viscous creep and hence ground ice have been observed5 mid latitude soils have also been implicated as a water ice reservoir6 the capacity of which is predicted to vary on a 100 000 year timescale owing to orbitally driven variations in climate7 it is uncertain however whether near surface ground ice currently exists at these latitudes and how it is changing with time here we report observational evidence for a mid latitude reservoir of near surface water ice occupying the pore space of soils the thickness of the ice occupied soil reservoir 1 10 m and its distribution in the 30 to 60 latitude bands indicate a reservoir of 1 5 6 0 104 km3 equivalent to a global layer of water 10 40 cm thick we infer that the reservoir was created during the last phase of high orbital obliquity less than 100 000 years ago and is now being diminished

north south geological differences between the residual polar caps on mars polar processes can be sensitive indicators of global climate and the geological features associated with polar ice caps can therefore indicate evolution of climate with time the polar regions on mars have distinctive morphologic and climatologic features thick layered deposits seasonal co 2 frost caps extending to mid latitudes and near polar residual frost deposits that survive the summer the relationship of the seasonal and residual frost caps to the layered deposits has been poorly constrained mainly by the limited spatial resolution of the available data in particular it has not been known if the residual caps represent simple thin frost cover or substantial geologic features here we show that the residual cap on the south pole is a distinct geologic unit with striking collapse and erosional topography this is very different from the residual cap on the north pole which grades into the underlying layered materials these findings indicate that the differences between the caps are substantial rather than reflecting short lived differences in frost cover and so support the idea of long term asymmetry in the polar climates of mars

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

the presence and stability of ground ice in the southern hemisphere of mars abstract we calculate new estimates of ground ice stability and the depth distribution of the ice table the depth boundary between ice free soil above and ice cemented soil below and compare these theoretical estimates of the distribution of ground ice with the observed distribution of leakage neutrons measured by the neutron spectrometer instrument of the mars odyssey spacecraft s gamma ray spectrometer instrument suite our calculated ground ice distribution contains improvements over previous work in that we include the effects of the high thermal conductivity of ice cemented soil at and below the ice table we include the surface elevation dependence of the near surface atmospheric humidity and we utilize new high resolution maps of thermal inertia albedo and elevation from mars global surveyor observations results indicate that ground ice should be about 5 times shallower than in previous predictions while results are dependent on the atmospheric humidity depths are generally between a few millimeters and a few meters with typical values of a few centimeters results are also geographically similar to previous predictions with differences due to the higher resolution of thermal inertia and the inclusion of elevation effects on humidity comparison with the measured epithermal neutron count rates in the southern hemisphere indicate that the geographic distribution of the count rate is best correlated with ground ice in equilibrium with 10 to 20 pr m precipitable micrometers column abundance of atmospheric water assuming a uniform distribution with co2 however given the uncertainties 5 to 30 pr m also may be viable this water abundance represents a longer term average over 100 to 1000 yr there is a high degree of correlation between the depth of the ice table and the epithermal count rate that agrees remarkably well with predicted count rates as a function of ice table depth these results indicate that ground ice in the upper meter of the martian soil is in diffusive equilibrium with the atmosphere since ground ice in this depth zone is expected to undergo saturation desiccation cycles with orbital variations this ice should be younger than about 500 kyr and was emplaced under similar cold and dry climate conditions of today remaining differences between the predicted depths of the ice table and those inferred from the neutron data are likely to be due to subpixel heterogeneity in the martian surface including the presence of rocks slopes and patches of soil with varying thermophysical properties

sand dune materials and polar layered deposits on mars abstract the relationship of dark sand dunes to the polar layered deposits of mars is investigated by mapping the colors and morphologic settings of the dunes in and near the polar deposits in the north polar region the dunes have restricted sources in the layered deposits that are associated with are arcuate scarps 10 50 km in length and

entrainment at cold glacier beds here we present measurements of the gas content and isotopic composition of debris rich basal layers of a polar glacier meserve glacier antarctica which has a basal temperature of 17 c these measurements show that debris entrainment has occurred without alteration of the glacial ice and provide the most direct evidence to date that active entrainment occurs at the beds of cold glaciers without bulk freezing of water entrainment at subfreezing temperatures may have formed the u shaped trough containing meserve glacier in addition to possibly allowing some cold based glaciers to be important geomorphic agents entrainment at subfreezing temperatures provides a general mechanism for formation of the dirty basal layers of polar glaciers and ice sheets which are rheologically distinct and can limit the time span of ice core analyses furthermore accumulating evidence suggests that geomorphologists should abandon the assumption that cold based glaciers do not slide and abrade their beds