reading the climate record of the martian polar layered deposits abstract the martian polar regions have layered deposits of ice and dust the stratigraphy of these deposits is exposed within scarps and trough walls and is thought to have formed due to climate variations in the past insolation has varied significantly over time and caused dramatic changes in climate but it has remained unclear whether insolation variations could be linked to the stratigraphic record we present a model of layer formation based on physical processes that expresses polar deposition rates of ice and dust in terms of insolation in this model layer formation is controlled by the insolation record and dust rich layers form by two mechanisms 1 increased summer sublimation during high obliquity and 2 variations in the polar deposition of dust modulated by obliquity variations the model is simple yet physically plausible and allows for investigations of the climate control of the polar layered deposits pld we compare the model to a stratigraphic column obtained from the north polar layered deposits npld fishbaugh k e hvidberg c s byrne s russel p s herkenhoff k e winstrup m kirk r 2010a geophys res lett 37 l07201 and show that the model can be tuned to reproduce complex layer sequences the comparison with observations cannot uniquely constrain the pld chronology and it is limited by our interpretation of the observed stratigraphic column as a proxy for npld composition we identified however a set of parameters that provides a chronology of the npld tied to the insolation record and consistently explains layer formation in accordance with observations of npld stratigraphy this model dates the top 500 m of the npld back to 1 million years with an average net deposition rate of ice and dust of 0 55 mm a 1 the model stratigraphy contains a quasi periodic 30 m cycle similar to a previously suggested cycle in brightness profiles from the npld laskar j levrard b mustard f 2002 nature 419 375 377 milkovich s head j w 2005 j geophys res 110 but here related to half of the obliquity cycles of 120 and 99 kyr and resulting from a combination of the two layer formation mechanisms further investigations of the non linear insolation control of pld formation should consider data from other geographical locations and include radar data and other stratigraphic datasets that can constrain the composition and stratigraphy of the npld layers

non linear flow modelling of a martian lobate debris apron the martian mid latitudes contain numerous small water ice deposits collectively termed viscous flow features vffs the shape and topography of the deposits contain information on their past flow history and formation process in order to access this information it is imperative to get information on their deformational properties here we use a high resolution digital topography map and ice penetrating radar data in combination with an inverse method to constrain the deformational properties of a lobate debris apron a class of vff in the southern hemisphere of mars we find that while the creep parameter and accumulation rates are not well constrained in absolute values their ratio is robust we also find that the creep exponent is most likely n 3

new martian climate constraints from radar reflectivity within the north polar layered deposits the north polar layered deposits npld of mars represent a global climate record reaching back millions of years potentially recorded in visible layers and radar reflectors however little is known of the specific link between those layers reflectors and the global climate to test the hypothesis that reflectors are caused by thick and indurated layers known as marker beds the reflectivity of three reflectors was measured mapped and compared to a reflectivity model the measured reflectivities match the model and show a strong sensitivity to layer thickness implying that radar reflectivity may be used as a proxy for short term accumulation patterns and that regional climate plays a strong role in layer thickness variations comparisons to an orbitally forced npld accumulation model show a strong correlation with predicted marker bed formation but dust content is higher than expected implying a stronger role for dust in mars polar climate than previously thought

geological and hydrogeological controls on the accumulation of coalbed methane in the weibei field southeastern ordos basin abstract commercial exploration and production of coalbed methane cbm in the weibei field ordos basin china has rapidly increased since 2010 the weibei field has become one of the most productive cbm areas in china however relatively few studies have investigated the migration of gas and water in the coal reservoir and their controls on the gas accumulation this study conducts stable isotope analyses and quality tests for groundwater samples discusses the relationships between the fluid flow pathways and tectonics and concludes by discussing the geological and hydrological controls on potential gas accumulation in the weibei field the coalbed groundwaters contain primarily sodium and bicarbonate and are effectively devoid of sulfate calcium and magnesium the groundwaters are typically freshwater with total dissolved solids tds values ranging from 814 to 2657 mg l differences in hydrogeology and structural geology divide the study area into four gas domains in the northern hancheng area the predominant northwest flow of groundwater has resulted in higher gas content in the west 12 m 3 t than in the east 8 12 m 3 t even though the coals in the east have high thermal maturity 2 1 2 3 r o the area with the highest gas content 16 m 3 t is in the region near the downthrown side of the xuefeng nan thrust fault in the northern hancheng area and the fault forms a barrier to the northwestward flow of groundwater the area with the lowest gas content 3 t is in the southwest heyang area where there is no entrapment mechanism and the gas has been flushed out of the coals due to a reduction of hydrostatic pressure and active groundwater flow from the east structural and hydrodynamic mechanisms especially the intensity of the hydrodynamic activity and the groundwater flow pathways are important for gas accumulation in the weibei field

a probabilistic case for a large missing carbon sink on mars after 3 5 billion years ago abstract mars has a thin 6 mbar co2 atmosphere currently there is strong evidence for paleolakes and rivers formed by warm climates on mars including after 3 5 billion years ga ago which indicates that a co2 atmosphere thick enough to permit a warm climate was present at these times since mars no longer has a thick co2 atmosphere it must have been lost one possibility is that martian co2 was lost to space oxygen escape rates from mars are high enough to account for loss of a thick co2 atmosphere if co2 was the main source of escaping o but here using h isotope ratios o escape calculations and quantification of the surface o sinks on mars we show for the first time that o escape from mars after 3 5 ga must have been predominantly associated with the loss of h2o not co2 and therefore it is unlikely that 250 mbar martian co2 has been lost to space in the last 3 5 ga because such results require highly unfavored o loss scenarios it is possible that the presence of young rivers and lakes on mars could be reconciled with limited co2 loss to space if crater chronologies on mars are sufficiently incorrect that all apparently young rivers and lakes are actually older than 3 5 ga or if climate solutions exist for sustained runoff on mars with atmospheric co2 pressure

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

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

effects of dispersed particulates on the rheology of water ice at planetary conditions we have investigated the effects of initial grain size and hard particulate impurities on the transient and steady state flow of water ice i at laboratory conditions selected to provide more quantitative constraints on the thermomechanical evolution of the giant icy moons of the outer solar system our samples were molded with particulate volume fractions of 0 001 to 0 56 and particle sizes of 1 to 150 m deformation experiments were conducted at constant shortening rates of 4 4 10 7 to 4 9 10 4 s 1 at pressures of 50 and 100 mpa and temperatures 77 to 223 k for the pure ice samples initial grain sizes were 0 2 0 6 mm 0 75 1 75 mm and 1 25 2 5 mm stress strain curves of pure ice i under these conditions display a strength maximum u at plastic strains e 0 01 after initial yield followed by strain softening and achievement of steady state levels of stress ss at e 0 1 to 0 2 finer starting grain size in pure ice generally raises the level of u petrography indicates that the initial transient flow behavior is associated with the nucleation and growth of recrystallized ice grains and the approach to ss evidently corresponds to the development of a steady state grain texture effects of particulate concentrations 0 1 are slight at these concentrations a small but significant reduction in u with respect to that for pure water ice occurs mixed phase ice with 0 1 is significantly stronger than pure ice the strength of samples with 0 56 approaches that of dry confined sand the magnitude of the strengthening effect is far greater than expected from homogeneous strain rate enhancement in the ice fraction or from pinning of dislocations orowan hardening this result suggests viscous drag occurs in the ice as it flows around the hard particulates mixed phase ice is also tougher than pure ice extending the range of bulk plastic deformation versus faulting to lower temperatures and higher strain rates the high pressure phase ice ii formed in 0 56 mixed phase ice during deformation at high stresses bulk planetary compositions of ice rock 0 4 0 5 are roughly 2 orders of magnitude more viscous than pure ice promoting the likelihood of thermal instability inside giant icy moons and possibly explaining the retention of crater topography on icy planetary surfaces

the transitional depositional environment and sequence stratigraphy of chasma boreale abstract the depositional system within chasma boreale is unique in that it contains active aeolian environments expressed as dune fields and active cryosphere environments present as layered ice deposits as well as environments that transition between these this work presents a new analysis of the chasma boreale sediment system that creates an interpretative framework addressing a controls on the balance between aeolian and cryospheric processes in the modern depositional system b the stratigraphic architecture of related sedimentary deposits and c processes of sediment accumulation and preservation images from context camera ctx 6 m pixel are used to classify and map sedimentary environments surfaces and deposits on the reentrant floor to refine the established geologic map of the reentrant and to infer the stratigraphic record of the accumulation from chasma boreale s depositional system a spectrum of sedimentary environments occurring between those dominated by aeolian and by cryospheric processes are identified through time the boundaries of these sedimentary environments have shifted resulting in complex lateral changes in the configuration of sedimentary environments on the reentrant s floor vertically the stratigraphic record is characterized by the punctuation of sandy aeolian deposits by icy surfaces that indicate episodes of ice growth that preserve underlying deposits resulting in accumulation stabilized icy surfaces occur at multiple vertical temporal scales and lateral extents suggesting the influence of both regional climate change due to allogenic forcing as well as autogenic dynamics within the transitional system these observations demonstrate that the chasma boreale accumulation can be interpreted in an aeolian sequence stratigraphic framework this work contributes the first detailed description of the processes forming polar aeolian sequences with an emphasis on the competing and complementary dynamics between aeolian and cryospheric systems

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

a revised surface age for the north polar layered deposits of mars this work was funded by nasa grant nnx13ag72g m e l was supported by the national science foundation graduate research fellowship program grant dge 1143953 hirise images referenced are available on the instrument s public website https hirise lpl arizona edu the crater catalog used in this work is included with this paper as supporting information the authors thank s sutton for help with socet set software m m sori for useful discussion on viscous relaxation and m e banks for useful discussion on the impact population the authors additionally thank j a skinner p becerra d laikko m sori n barlow and an anonymous reviewer for helpful comments on the manuscript

radar sounding using the cassini altimeter waveform modeling and monte carlo approach for data inversion of observations of titan s seas recently the cassini radar has been used as a sounder to probe the depth and constrain the composition of hydrocarbon seas on saturn s largest moon titan altimetry waveforms from observations over the seas are generally composed of two main reflections the first from the surface of the liquid and the second from the seafloor the time interval between these two peaks is a measure of sea depth and the attenuation from the propagation through the liquid is a measure of the dielectric properties which is a sensitive property of liquid composition radar measurements are affected by uncertainties that can include saturation effects possible receiver distortion and processing artifacts in addition to thermal noise and speckle to rigorously treat these problems we simulate the ku band altimetry echo received from titan s seas using a two layer model where the surface is represented by a specular reflection and the seafloor is modeled using a facet based synthetic surface the simulation accounts for the thermal noise speckle analog to digital conversion and block adaptive quantization and allows for possible receiver saturation we use a monte carlo method to compare simulated and observed waveforms and retrieve the probability distributions of depth surface subsurface intensity ratio and subsurface roughness for the individual double peaked waveform of ligeia mare acquired by the cassini spacecraft in may 2013 this new analysis provides an update to the ku band attenuation and results in a new estimate for its loss tangent and composition we also demonstrate the ability to retrieve bathymetric information from saturated altimetry echoes acquired over ontario lacus in december 2008

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

3d imaging of mars polar ice caps using orbital radar data since its arrival in early 2006 various instruments aboard nasa s mars reconnaissance orbiter mro have been collecting a variety of scientific and engineering data from orbit around mars among these is the shallow radar sharad instrument supplied by agenzia spaziale italiana asi and designed for subsurface sounding in the 15 25 mhz frequency band as of this writing mro has completed more than 46 000 nearly polar orbits of mars 30 of which have included active sharad data collection by 2009 a sufficient density of sharad coverage had been obtained over the polar regions to support 3d processing and analysis of the data using tools and techniques commonly employed in terrestrial seismic data processing we have processed subsets of the resulting collection of sharad observations covering the north and south polar regions as sharad 3d volumes imaging the interiors of the north and south polar ice caps known respectively as planum boreum and planum australe after overcoming a series of cha

wide distribution and glacial origin of polar gypsum on mars the north polar cap of mars is associated with different kinds of superficial sediments including the circumpolar dune field interior dune fields and sedimentary veneers scattered over the ice cap in order to resolve the mineralogical composition and the regional distribution of these sediments we processed omega and crism hyperspectral data with an original method based on spectral derivation we find that gypsum is present in all areas where undefined hydrated minerals had been previously detected including superficial sedimentary veneers covering the north polar cap interior dune fields and the whole circumpolar dune field morphological and structural analyses reveal that these gypsum crystals derive directly from the interior of the ice cap the source of superficial sedimentary veneers is the dust that was previously contained in the upper part of the ice cap the ice rich north polar layered deposits npld this gypsum bearing dust was released on south facing slopes of spiral troughs and arcuate scarps by ice ablation controlled by katabatic winds by the analysis of all associations of erosional scarps and dune fields over the north polar cap we also demonstrate that the polar dunes are composed of sand sized particles that were previously contained in the sediment rich basal unit bu corresponding to the lower part of the ice cap these particles contain gypsum and were released from the bu by regressive ablation of ice at marginal scarps that border the north polar cap and by vertical ablation of ice on olympia planum from a reconstruction of wind streamlines over and around the ice cap we infer that katabatic winds descending from the polar high and rotating around the north polar cap control the release of these gypsum bearing particles by ice ablation and the redistribution of these particles in the circumpolar dune field 2011 elsevier b v

north polar cap of mars polar layered deposit characterization and identification of a fundamental climate signal of the cap 6 10 5 km 2 in the 13 images analyzed to date 3 assessment of geometric relationships shows that layers are not horizontal but rather have an apparent dip of approximately 0 5 degrees we interpret these results as follows 1 the fundamental 30 m signal is interpreted as a climate signal that may correspond to a 51 kyr insolation cycle 2 the lateral correlation and broad distribution of these layer sequences strongly imply that layer accumulation processes are widespread across the cap rather than confined within a single trough or region 3 local to regional variability in individual layer thicknesses and thus accumulation and sublimation rates is typically less than a factor of 2 5 providing the ability to study regional trends but often making it difficult to correlate visually the vertical sequences in individual cores finally initial examination of layers located deeper in the stratigraphic sequence within the north polar cap than the 300 m thick zone 1 provides evidence for a unit less than 100 m thick zone 2 in which the fundamental 30 m sequence is not detected we interpret this as a deposit having formed during a recent high obliquity phase of mars during which time polar volatiles underwent mobilization and were transport equatorward leaving a polar lag of dust rich material the most recent ice age 0 5 2 ma offers a plausible candidate for this period of ice cap removal and lag deposit formation an underlying zone 3 200 m contains a dominant 35 m signal and a lowermost zone 4 200 m contains multiple signals but no dominant one together these four zones represent 800 m of vertical stratigraphic section about one fourth of the total thickness of the cap these findings support earlier interpretations that orbital parameter variations could cause significant erosion and possibly complete removal of the polar caps the interpreted crater retention ages of the layered terrain are consistent with the correlations and vertical sequences described here suggesting that the polar caps wax and wane throughout geological history depending on the evolution of orbital parameters definition of the 30 m unit signal holds promise for determining 1 the detailed origin of individual layer types 2 the nature of deposition and sublimation processes and their relation to insolation geometry across the polar cap and 3 correlation with and comparison to the south polar layered terrain record

three dimensional structure and origin of a 1 8 km thick ice dome within korolev crater mars korolev is an 80 km diameter impact crater located at 72 7 n 164 5 e containing a large domed deposit perennial water ice is thought to be currently unstable there however using a 3 d analysis of the dome s internal radar stratigraphy we estimate that korolev s central mound contains between 1400 and 3500 km3 of water ice that is up to 1 8 km thick furthermore the stratigraphic structure of this ice dome is strikingly similar to the north polar layered deposits npld on planum boreum approximately 600 km to the north additionally our stratigraphic analysis suggests that korolev ice was not previously part of a once larger polar ice sheet but rather it was deposited locally we conclude that korolev crater s ice likely deposited during the same climate regime as planum boreum s npld but independently this implies that the incorporation of circumpolar ice deposits such as korolev may prove useful in reconstructing a unique mars polar climate history

onset and migration of spiral troughs on mars revealed by orbital radar the northern polar cap of mars containing enough water to cover the entire planet to a depth of several metres features two major landforms that stand above all others these are the enormous canyon chasma boreale and a series of spiral troughs the processes leading to their formation have remained unclear now two papers in this issue present detailed histories of both systems john holt and colleagues use penetrating radar imagery from mars reconnaissance orbiter s radar sounder to show that depositional processes rather than a catastrophic event formed the chasma boreale isaac smith and john holt use mars reconnaissance orbiter data to rule out erosional cutting of polar ice as the cause of the central troughs and instead conclude that they too are largely depositional having migrated polewards and upwards in elevation in the past two million years a pinwheel array of deep troughs has been one of the most perplexing features of the north polar layered deposits on mars many ideas have been put forward about how it formed but there is as yet no consensus here penetrating radar has been used to rule out erosional cutting as a mechanism for the formation of the array instead it is concluded that the troughs are largely depositional in origin and have migrated to the poles and upwards in elevation over the past two million years or so the landscape of the north polar layered deposits of mars npld is dominated by a pinwheel array of enigmatic spiral troughs1 the troughs have intrigued planetary scientists since the mariner 9 spacecraft returned the first close up image in 1972 but conclusive evidence of their origin has remained elusive debate continues regarding all aspects of the troughs including the possibility that they have migrated2 3 4 5 their age in relation to the current npld surface6 and whether they are fundamentally erosional6 7 or constructional2 4 features the troughs are probably related to climatic processes2 8 yet the nature of this relationship has remained a mystery previous data characterizing only the exposed nlpd surface were insufficient to test these hypotheses here we show that the central spiral troughs initiated after deposition of three quarters of the npld quickly reached a stable morphology and migrated approximately 65 kilometres poleward and 600 metres in altitude over the past two million years or so our radar stratigraphy rules out hypotheses of erosional incision post dating deposition6 7 9 10 and instead largely validates an early hypothesis for constructional trough migration2 3 4 5 with wind transport and atmospheric deposition as dominant processes these results provide hard constraints for palaeo climate models and a new context for evaluating imagery spectral data and now radar sounding data the better to understand the link between orbital parameters and climate the role of climate in shaping the polar ice of mars and eventually the age of the polar deposits themselves8 11 12 13

radar detectability of a subsurface ocean on europa abstract a spacecraft in orbit around jupiter s moon europa can use ice penetrating radar to probe for a possible liquid water ocean beneath europa s surface ice and to characterize other important subsurface structure consideration of available constraints on the properties of europa s ice possible subsurface temperature gradients and possible impurities in the ice places an upper limit of about 10 km on the depth to which an ocean might be detectable with an orbiting radar

radar sounder evidence of thick porous sediments in meridiani planum and implications for ice filled deposits on mars meridiani planum is one of the most intensely studied regions on mars yet little is known about the physical properties of the deposits below those examined by the opportunity rover we report the detection of subsurface echoes within the meridiani planum deposits from data obtained by the mars advanced radar for subsurface and ionospheric sounding marsis instrument the delay time between the surface and subsurface returns is indicative of materials with a real dielectric constant of 3 6 0 6 the real dielectric constant is strongly modulated by bulk density newly derived compaction relationships for mars indicate that the relatively low dielectric constant of the meridiani planum deposits is consistent with a thick layer of ice free porous basaltic sand the unique physiographic and hydrologic setting of meridiani planum may have provided an ideal sediment trap for eolian sands the relatively low gravity and the cold dry climate that has dominated mars for billions of years may have allowed thick eolian sand deposits to remain porous and only weakly indurated minimally compacted sedimentary deposits may offer a possible explanation for other nonpolar region units with low apparent bulk dielectric constants

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

dielectric constant estimation of the uppermost basal unit layer in the martian boreales scopuli region abstract an electromagnetic inversion model has been applied to echoes from the subsurface sounding shallow radar sharad to retrieve the dielectric properties of the uppermost basal unit bu beneath the north polar layered deposits of mars sharad data have been carefully selected to satisfy the assumption of the inversion model which requires a stratigraphy consisting of mostly plane parallel layers the resulting values of the dielectric constant have been interpreted in terms of a variable percentage of dust in an ice dust mixture through the use of a mixing model for dielectric properties the resulting dust content exceeds 65 reaching perhaps 95 depending on the permittivity values assumed for the dust such a concentration is higher than that obtained by selvans et al selvans m m plaut j j aharonson o 2010 j geophys res 115 e09003 this discrepancy could be justified considering that our observations refer to the uppermost bu layer whereas selvans et al selvans m m plaut j j aharonson o 2010 j geophys res 115 e09003 probed the bu full thickness moreover if the bu is considered spatially inhomogeneous with very different dust content and thickness tanaka k l skinner j a fortezzo c m herkenhoff k e rodriguez j a p bourke m c kolb e j okubo c h 2008 icarus 196 318 358 the discrepancy could be furtherly reconciled

the state and future of mars polar science and exploration as the planet s principal cold traps the martian polar regions have accumulated extensive mantles of ice and dust that cover individual areas of 10 6 km 2 and total as much as 3 4 km thick from the scarcity of superposed craters on their surface these layered deposits are thought to be comparatively young preserving a record of the seasonal and climatic cycling of atmospheric co 2 h 2o and dust over the past 10 5 10 8 years for this reason the martian polar deposits may serve as a rosetta stone for understanding the geologic and climatic history of the planet documenting variations in insolation due to quasiperiodic oscillations in the planet s obliquity and orbital elements volatile mass balance atmospheric composition dust storm activity volcanic eruptions large impacts catastrophic floods solar luminosity supernovae and perhaps even a record of microbial life beyond their scientific value the polar regions may soon prove important for another reason providing a valuable and accessible reservoir of water to support the long term human exploration of mars in this paper we assess the current state of mars polar research identify the key questions that motivate the exploration of the polar regions discuss the extent to which current missions will address these questions and speculate about what additional capabilities and investigations may be required to address the issues that remain outstanding

mars north polar deposits stratigraphy age and geodynamical response the shallow radar sharad on the mars reconnaissance orbiter has imaged the internal stratigraphy of the north polar layered deposits of mars radar reflections within the deposits reveal a laterally continuous deposition of layers which typically consist of four packets of finely spaced reflectors separated by homogeneous interpacket regions of nearly pure ice the packet interpacket structure can be explained by approximately million year periodicities in mars obliquity or orbital eccentricity the observed 100 meter maximum deflection of the underlying substrate in response to the ice load implies that the present day thickness of an equilibrium elastic lithosphere is greater than 300 kilometers alternatively the response to the load may be in a transient state controlled by mantle viscosity both scenarios probably require that mars has a subchondritic abundance of heat producing elements

high ice purity of martian lobate debris aprons at the regional scale evidence from an orbital radar sounding survey in deuteronilus and protonilus mensae

quantitative analysis of mars surface radar reflectivity at 20 mhz abstract the radar surface echoes taken from sharad observations are extracted to obtain a reflectivity map covering almost half of the martian surface and then compared to available roughness maps then we used a 2 step method based on a stochastic description of reflectivity in order to i separate the coherent incoherent components of the signal by means of a probability density function fitting of amplitude distributions and ii express these components with respect to roughness permittivity values by adapting common backscattering models to the nadir case scattering is found to be the most important process dominating reflectivity over the martian terrains reflectivity is best correlated with roughness maps derived from the slope parameter the stochastic behavior of reflectivity is confirmed by the excellent agreement of our models and demonstrates the low probability of retrieving permittivity from scattering surfaces the non stationary nature of most martian terrains is consolidated by a very good fit to k distributions justifying the use of the fractal theory for describing surface roughness on mars a few slightly rough regions exhibit both a coherent and incoherent component when surface correlation length is non significant within the resolution cell we show that the rms roughness at a decameters baseline can be derived with sub decimeter sensitivity without prior signal calibration improving the capabilities of sharad in determining surface roughness with a view to landing site selection sets of derived dielectric constants are obtained and analyzed with regard to the reference signal used for calibration given the very different implications of each set to the composition of martian terrains we emphasize the importance of good signal calibration as a major issue for sharad and the next interplanetary radar missions

permittivity estimation of layers beneath the northern polar layered deposits mars 1 martian polar layered deposits most likely dusty ice are transparent to radar waves in the north pole bedrock has been detected by marsis up to a depth about 2 km whereas in the south pole such interface has been located at about 3 7 km the reflected signal is generally above the noise and can be used to estimate the dielectric properties of the bedrock assuming the permittivity of the first layer is known we use a simplified inversion method to extract the bedrock relative dielectric permittivity as 4 9 below the north pld s these values are coherent with the basalt nature and so the approach could be used to identify the lithology beneath the martian poles

geology and insolation driven climatic history of amazonian north polar materials on mars a wealth of image and topographic data from the mars global surveyor and mars odyssey missions has been combined in a detailed geological map of the north polar region of mars one intriguing finding is a cycle of ice deposition similar in duration to a 1 470 year climate cycle of unknown origin on earth if both are externally driven a mechanism that affects both planets such as variation in solar luminosity must be involved mariner 9 and viking spacecraft images revealed that the polar regions of mars like those of earth record the planet s climate history however fundamental uncertainties regarding the materials features ages and processes constituting the geologic record remained1 2 3 4 5 6 recently acquired mars orbiter laser altimeter data7 and mars orbiter camera high resolution images8 from the mars global surveyor spacecraft and moderately high resolution thermal emission imaging system visible images9 from the mars odyssey spacecraft permit more comprehensive geologic and climatic analyses10 11 12 13 14 15 16 17 here i map and show the history of geologic materials and features in the north polar region that span the amazonian period 3 0 gyr ago to present 18 19 erosion and redeposition of putative circumpolar mud volcano deposits15 formed by eruption of liquefied fine grained material led to the formation of an early amazonian polar plateau consisting of dark layered materials crater ejecta superposed on pedestals indicate that a thin mantle was present during most of the amazonian suggesting generally higher obliquity and insolation conditions at the poles than at present brighter polar layered deposits rest unconformably on the dark layers and formed mainly during lower obliquity over the past 4 5 myr ref 20 finally the uppermost layers post date the latest downtrend in obliquity 20 000 years ago20

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

three dimensional radar imaging of structures and craters in the martian polar caps abstract over the last decade observations acquired by the shallow radar sharad sounder on individual passes of the mars reconnaissance orbiter have revealed the internal structure of the martian polar caps and provided new insights into the formation of the icy layers within and their relationship to climate however a complete picture of the cap interiors has been hampered by interfering reflections from off nadir surface features and signal losses associated with sloping structures and scattering foss et al the leading edge 36 43 57 2017 https doi org 10 1190 tle36010043 1 addressed these limitations by assembling three dimensional data volumes of sharad observations from thousands of orbital passes over each polar region and applying geometric corrections simultaneously the radar volumes provide unprecedented views of subsurface features readily imaging structures previously inferred from time intensive manual analysis of single orbit data e g trough bounding surfaces a buried chasma and a basal unit in the north massive carbon dioxide ice deposits and discontinuous layered sequences in the south our new mapping of the carbon dioxide deposits yields a volume of 16 500 km 3 11 larger than the prior estimate in addition the radar volumes newly reveal other structures including what appear to be buried impact craters with no surface expression our first assessment of 21 apparent craters at the base of the north polar layered deposits suggests a hesperian age for the substrate consistent with that of the surrounding plains as determined from statistics of surface cratering rates planned mapping of similar features throughout both polar volumes may provide new constraints on the age of the icy layered deposits the radar volumes also provide new topographic data between the highest latitudes observed by the mars orbiter laser altimeter and those observed by sharad in general mapping of features in these radar volumes is placing new constraints on the nature and evolution of the polar deposits and associated climate changes

a strategy for an accurate estimation of the basal permittivity in the martian north polar layered deposits this paper deals with the investigation of the mars subsurface by means of data collected by the mars advanced radar for subsurface and ionosphere sounding working at few megahertz frequencies a data processing strategy which combines a simple inversion model and an accurate procedure for data selection is presented this strategy permits to mitigate the theoretical and practical difficulties of the inverse problem arising because of the inaccurate knowledge of the parameters regarding both the scenario under investigation and the radiated electromagnetic field impinging on the mars surface the results presented in this paper show that it is possible to reliably retrieve the electromagnetic properties of deeper structures if such strategy is accurately applied an example is given here where the analysis of the data collected on gemina lingula a region of the north polar layer deposits allowed us to retrieve permittivity values for the basal unit in agreement with those usually associated to the earth basaltic rocks

holocene paleoclimate data from the arctic testing models of global climate change to evaluate the spatial variability of arctic climate change during the present interglacial cape project members compiled well dated terrestrial marine and ice core paleoenvironmental records spanning the past 10 12 thousand years ka six tundra biomes of increasing summer temperature requirements were defined based on regionally coherent pollen assemblages using a rule based approach pollen spectra were converted to tundra forest tundra or forest biomes ranked by their average growing season requirements marine sea surface reconstructions were based on proxy data following a similar rule based approach from these data based reconstructions departures in summer temperatures from modern normals were calculated in 1 ka time slices through the holocene to test predictive models data based summer temperature reconstructions were compared with general circulation model gcm simulations for 10 ka and 6 ka ago paleodata and model results both show that warming occurred earlier across beringia and asia relative to lands adjacent to the north atlantic and that late holocene cooling was most apparent in the north atlantic region however the gcm over predicts the magnitude of mid holocene warming over northern asia and underestimates the intensification of the north atlantic drift in the early holocene strong spatial variability in environmental response during the holocene despite symmetric insolation forcing suggests that any future changes whether caused by anthropogenic or natural factors are unlikely to result in a uniform change across the arctic adding additional complexity to forecasts of global impacts

potential role of vegetation feedback in the climate sensitivity of high latitude regions a case study at 6000 years b p previous climate model simulations have shown that the configuration of the earth s orbit during the early to mid holocene approximately 10 5 kyr can account for the generally warmer than present conditions experienced by the high latitudes of the northern hemisphere new simulations for 6 kyr with two atmospheric mixed layer ocean models community climate model version 1 ccm1 and global environmental and ecological simulation of interactive systems version 2 genesis 2 are presented here and compared with results from two previous simulations with genesis 1 that were obtained with and without the albedo feedback due to climate induced poleward expansion of the boreal forest the climate model results are summarized in the form of potential vegetation maps obtained with the global biome model which facilitates visual comparisons both among models and with pollen and plant macrofossil data recording shifts of the forest tundra boundary a preliminary synthesis shows that the forest limit was shifted 100 200 km north in most sectors both ccmi and genesis 2 produced a shift of this magnitude genesis 1 however produced too small a shift except when the boreal forest albedo feedback was included the feedback in this case was estimated to have amplified forest expansion by approximately 50 the forest limit changes also show meridional patterns greatest expansion in central siberia and little or none in alaska and labrador which have yet to be reproduced by models further progress in understanding of the processes involved in the response of climate and vegetation to orbital forcing will require both the deployment of coupled atmosphere biosphere ocean models and the development of more comprehensive observational data sets

north polar deposits of mars extreme purity of the water ice 1 the polar layered deposits are the largest reservoir of water on the surface of mars the physical properties of the ice and their spatial distribution are largely unknown 140 000 data points from the sounding radar sharad on the mars reconnaissance orbiter were analyzed over the gemina lingula region one fourth of the north polar layered deposits area maps of the dielectric properties of the bulk ice were drawn up there is no basal melting signature a drop of the dielectric constant in north west of gemina lingula could be explained by an abrupt 250 meter uplift of the base the bulk ice of the studied region has an average dielectric constant of 3 10 0 12 and a loss tangent 0 0026 0 0005 analytic interpretations shown the volume of ice is pure at 95 the impurities have a radial distribution with higher concentrations at margins

global permittivity mapping of the martian surface from sharad abstract sharad is a subsurface sounding radar aboard nasa s mars reconnaissance orbiter capable of detecting dielectric discontinuities in the subsurface caused by compositional and or structural changes echoes coming from the surface contain information on geometric properties at metre scale and on the permittivity of the upper layers of the martian crust a model has been developed to estimate the effect of surface roughness on echo power depending on statistical parameters such as rms height and topothesy such model is based on the assumption that topography can be characterized as a self affine fractal and its use allows the estimation of the dielectric properties of the first few metres of the martian soil a permittivity map of the surface of mars is obtained covering several large regions across the planet surface the most significant correspondence with geology is observed at the dichotomy boundary with high dielectric constant on the highlands side 7 to over 10 and lower on the lowlands side 3 to 7 other geological correlations are discussed

constraints on martian lobate debris apron evolution and rheology from numerical modeling of ice flow radar observations in the deuteronilus mensae region by mars reconnaissance orbiter have constrained the thickness and dust concentration found within mid latitude ice deposits providing an opportunity to more accurately estimate the rheology of ice responsible for the formation of lobate debris aprons based on their apparent age of 100 myr we developed a numerical model simulating ice flow under martian conditions using results from ice deformation experiments theory of ice grain growth based on terrestrial ice cores and observational constraints from radar profiles and laser altimetry by varying the ice grain size the ice temperature the subsurface slope and the initial ice volume we determine the combination of parameters that best reproduce the observed lda lengths and thicknesses over a period of time comparable to the apparent ages of lda surfaces 90 300 myr we find that an ice temperature of 205 k an ice grain size of 5 mm and a flat subsurface slope give reasonable ages for many ldas in the northern mid latitudes of mars assuming that the ice grain size is limited by the grain boundary pinning effect of incorporated dust these results limit the dust volume concentration to less than 4 however assuming all ldas were emplaced by a single event we find that there is no single combination of grain size temperature and subsurface slope which can give realistic ages for all ldas suggesting that some or all of these variables are spatially heterogeneous based on our model we conclude that the majority of northern mid latitude ldas are composed of clean 64 vol coarse p1 mm grained ice but regional differences in either the amount of dust mixed in with the ice or in the presence of a basal slope below the lda ice must be invoked alternatively the ice temperature and or timing of ice deposition may vary significantly between different mid latitude regions either eventuality can be tested with future observations

flexure of the lithosphere beneath the north polar cap of mars implications for ice composition and heat flow the geodynamical response of the lithosphere under stresses imposed by the geologically young north polar cap is one of the few clues we have to constrain both its composition and the present day t

integrating radar stratigraphy with high resolution visible stratigraphy of the north polar layered deposits mars abstract shallow radar sharad on board nasa s mars reconnaissance orbiter has successfully detected tens of reflectors in the subsurface of the north polar layered deposits npld of mars radar reflections are hypothesized to originate from the same material interfaces that result in visible layering as a first step towards verifying this assumption this study uses signal analyses and geometric comparisons to quantitatively examine the relationship between reflectors and visible layers exposed in an npld outcrop to understand subsurface structures and reflector geometry reflector surfaces have been gridded in three dimensions taking into account the influence of surface slopes to obtain accurate subsurface geometries these geometries reveal reflector dips that are consistent with optical layer slopes distance elevation profiling of subsurface reflectors and visible layer boundaries reveals that reflectors and layers demonstrate similar topography verifying that reflectors represent paleosurfaces of the deposit statistical and frequency domain analyses of the separation distances between successive layers and successive reflectors confirms the agreement of radar reflector spacing with characteristic spacing of certain visible layers direct elevation comparisons between individual reflectors and discrete optical layers while necessary for a one to one correlation are complicated by variations in subsurface structure that exist between the outcrop and the sharad observations as inferred from subsurface mapping although these complications have prevented a unique correlation a genetic link between radar reflectors and visible layers has been confirmed validating the assumption that radar reflectors can be used as geometric proxies for visible stratigraphy furthermore the techniques for conducting a stratigraphic integration have been generalized and improved so that the integration can be undertaken at additional locations

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

buried ice and sand caps at the north pole of mars revealing a record of climate change in the cavi unit with sharad nasa mars data analysis program nnx15am52g landmark software and services a halliburton company

method for anisotropic crystal orientation fabrics detection using radio wave depolarization in radar sounding of mars polar layered deposits the polar layered deposits plds provide a wealth of information about the past climate evolution of mars surface mass fluxes and ice flow mainly governed topography and layering of the pld china s mars probe including an orbiter and a landing rover will be launched by 2020 a new type satellite borne mars penetrating radar instrument has been selected to be a part of the payloads on the orbiter its main scientific objectives are to map the distribution of water water ice and to detect the soil characteristics at global scale on the martian crust compared with mars advanced radar for subsurface and ionospheric sounding and shallow radar the biggest difference is that the antenna system of this mars penetrating radar consists of two dipole antennas mutually perpendicular this special configuration enables the investigation of the ice flow of pld by detecting and analyzing the features of anisotropic crystal orientation fabric cof thus relying on the fact that the radio waves are depolarized while passing through an anisotropic cof layer in this paper a method for anisotropic cof detection based on this radar system is proposed the radar echo formulation of anisotropic cof is derived and the ratio of the signals measured by the two perpendicular antennas is used to analyze the anisotropy of cof we demonstrate that the ratio is an ideal criterion for the detection and analysis of cof since it contains all parameters about the anisotropy feature of cof and it is independent of the attenuation in the propagation path and the reflection coefficient in order to verify the validity of the derived analytical expression of the ratio for the detection of cof finite different time domain simulations are carried out based on a simple model of the subsurface of pld which contains an anisotropic cof layer the advantages of this method the potential application scenarios and the effects of the martian environment are also discussed

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

the 3 5 mhz global reflectivity map of mars by marsis mars express implications for the current inventory of subsurface h2o abstract we extracted the surface echo power from 2 years of marsis measurements the retrieved values are calibrated to compensate for changes in the distance of the spacecraft to the surface and for the attenuation of the signal by the ionosphere the results are used to build the first global map of surface echo power at 3 5 mhz the surface echo power variations are primarily caused by kilometer scale surface roughness then we derive the values of dielectric constant of the shallow subsurface materials by normalizing the surface echo power map using a simulation of marsis signal from the mola topography as a result we obtain a map that characterizes the dielectric properties of the materials down to a few decameters below the surface dielectric properties vary with latitude with high values in mid latitudes belts 20 40 and lower values at both equatorial and high latitudes from the comparison of marsis reflectivity map to grs observations we conclude that the reflectivity decrease observed poleward of 50 60 corresponds to the onset of water ice occurrence within the regolith assuming homogenous ground composition and texture at the scale of the marsis resolution cell our inferred volume of ground water ice is of 106 km3 equivalent to a polar cap low reflectivity areas are also observed in equatorial regions from radar studies alone equatorial low dielectric constant values could have different interpretations but the correlation with grs hydrogen distribution rather points toward a water related explanation

simulated climate and biomes of africa during the late quaternary comparison with pollen and lake status data new compilations of african pollen and lake data are compared with climate ccm1 ncar boulder and vegetation biome 1 2 gsg lund simulations for the last glacial maximum lgm and early to mid holocene emh the simulated lgm climate was ca 4 c colder and drier than present with maximum reduction in precipitation in semi arid regions biome simulations show lowering of montane vegetation belts and expansion of southern xerophytic associations but no change in the distribution of deserts and tropical rain forests the lakes show lgm conditions similar or drier than present throughout northern and tropical africa pollen data indicate lowering of montane vegetation belts the stability of the sahara and a reduction of rain forest the paleoenvironmental data are consistent with the simulated changes in temperature and moisture budgets although they suggest the climate model underestimates equatorial aridity emh simulations show temperatures slightly less than present and increased monsoonal precipitation in the eastern sahara and east africa biome simulations show an upward shift of montane vegetation belts fragmentation of xerophytic vegetation in southern africa and a major northward shift of the southern margin of the eastern sahara the lakes indicate conditions wetter than present across northern africa pollen data show an upward shift of the montane forests the northward shift of the southern margin of the sahara and a major extension of tropical rain forest the lake and pollen data confirm monsoon expansion in eastern africa but the climate model fails to simulate the wet conditions in western africa

introduction to the 4th mars polar science and exploration conference special issue five top questions in mars polar science abstract as an introduction to this icarus special issue for the 4th mars polar science and exploration conference we discuss five key questions in mars polar science gleaned from plenary discussions and presentations held at the conference these questions highlight major unknowns in the field 1 what are the physical characteristics of the polar layered deposits pld and how are the different geologic units within beneath and surrounding the pld related 2 how old are the pld and what are their glacial fluvial depositional and erosional histories 3 what are the mass and energy budgets of the pld and what processes control these budgets on seasonal and longer timescales 4 what chronology compositional variability and record of climatic change is expressed in the stratigraphy of the pld 5 how have volatiles and dust been exchanged between polar and non polar reservoirs and how has this exchange affected the past and present distribution of surface and subsurface ice

density variations within the south polar layered deposits of mars 1 the south polar layered deposits spld constitute the largest known reservoir of water on mars previous studies solved for the best fit uniform density of the deposits using a forward approach here we invert for the lateral density variations in the layered deposit using gravity data from radio tracking of mars reconnaissance orbiter topography from mola on board mars global surveyor and radar sounding data from marsis on board mars express we use the gravity anomalies outside the spld to construct a wiener filter which is applied to the gravitational signature of the spld to remove the short wavelength anomalies over the spld that are spectrally consistent with an origin in the crust or mantle we then use a constrained inversion for the vertically averaged density within the spld as a function of position the results suggest significant density variations within the spld an inverse relationship between the density and thickness of the spld suggests that thicker portions of the cap contain less dust alternatively the dorsa argentea formation may extend beneath the spld and result in the observed high gravity anomaly in the marginal area of the spld we find these conclusions to be robust against the choice of inversion constraint and perturbations to the applied filter a synthetic test is also performed to verify the recoverability of the density variation in our approach

environmental change in eastern greenland during the last 1300 years evidence from foraminifera and lithofacies in nansen fjord 68 n lithofacies and benthic foraminiferal analyses of two sediment cores bs1191 k13b and k14 from nansen fjord eastern greenland show evidence of changing oceanographic and sea ice conditions from

dielectric measurements and radar attenuation estimation of ice basalt sand mixtures as martian polar caps analogues the nature of the materials underlying the superficial deposits of mars can be inferred applying an inversion algorithm from the data acquired by the orbiting hf radars marsis and sharad this approach requires the knowledge of the electromagnetic properties of the shallow deposits and an accurate evaluation of the signal attenuation the present work is focused on the determination of the dielectric parameters of several geo materials we performed the measurements of the complex permittivity in a wide range of temperature 150 250 k and frequency 20 hz 1 mhz on pure water ice dry basalt sand and ice basalt mixtures with different sand volume fractions the data are presented in terms of attenuation as a function of basalt volume fraction frequency and temperature and discussed in terms of extrapolation to marsis and sharad frequency bands the results show that besides the expected dependence of the attenuation from temperature the presence of the solid inclusions in the ice strongly affects the behaviour of the attenuation versus frequency

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

former extent of glacier like forms on mars abstract mars mid latitude glacier like forms glfs have undergone substantial mass loss and recession since a hypothesised last martian glacial maximum lmgm stand to date there is a lack of knowledge of the nature and timing of the lmgm the subsequent mass loss and whether this mass loss has been spatially variable here we present the results of a population scale inventory of recessional glfs derived from analysis of 1293 glfs 3 identified within context camera ctx imagery to assess the distribution and controls on glf recession a total of 436 glfs were identified showing strong evidence of recession 197 in the northern hemisphere and 239 in the southern hemisphere relative to their parent populations recessional glfs are over represented in the low latitude belts between 25 and 40 and in areas of high relief suggesting that these zones exert some control over glf sensitivity and response to forcing this analysis is complemented by the reconstruction of the maximum extent and morphology of a specific glf for which high resolution imaging science experiment hirise derived digital elevation data are available using nye s nye j f 1951 proc roy soc lond ser a mat phys sci 207 554 572 perfect plastic approximation of ice flow applied to multiple flow lines under an optimum yield strength of 22 kpa we calculate that the reconstructed glf has lost an area of 6 86 km 2 with a corresponding volume loss of 0 31 km 3 since the lmgm assuming the loss reconstructed at this glf occurred at all mid latitude glfs yields a total planetary ice loss from mars glfs of 135 km 3 similar to the current ice volume in the european alps on earth

spiral trough diversity on the north pole of mars as seen by shallow radar sharad we combine observations from ground penetrating radar and optical instruments to conduct a comprehensive survey of spiral troughs on the north polar layered deposits of mars our survey reveals that all north polar troughs have common characteristics but there is strong regional diversity we divide the north polar layered deposits npld into eight regions based on observations of morphology and stratigraphy in order to classify the spiral troughs morphologic features that separate the regions include central promontories topographic undulations bedrock floors and trough asymmetry among others we demonstrate here that the troughs formed during two periods the first generation of troughs formed on the main lobe and in part of gemina lingula during accumulation of 600 m of ice and dust the second generation of troughs formed stratigraphically higher and immediately after large scale erosion in gemini scopuli which we conclude created the slopes necessary for troughs to form we test scenarios of trough evolution and find that only three concurrent processes are required for their formation wind transport insolation induced sublimation and atmospheric deposition our observations are inconsistent with other hypothesized processes to produce spiral troughs including incision after the formation of the npld fracturing due to flowing ice and sublimation driven patterns from insolation

the construction of chasma boreale on mars the northern polar cap of mars containing enough water to cover the entire planet to a depth of several metres features two major landforms that stand above all others these are the enormous canyon chasma boreale and a series of spiral troughs the processes leading to their formation have remained unclear now two papers in this issue present detailed histories of both systems john holt and colleagues use penetrating radar imagery from mars reconnaissance orbiter s radar sounder to show that depositional processes rather than a catastrophic event formed the chasma boreale isaac smith and john holt use mars reconnaissance orbiter data to rule out erosional cutting of polar ice as the cause of the central troughs and instead conclude that they too are largely depositional having migrated polewards and upwards in elevation in the past two million years the chasma boreale is a large canyon 500 km long up to 100 km wide and nearly 2 km deep that cuts into the north polar layered deposits on mars quite how it formed has been unclear however new penetrating radar imagery has now been used to show that depositional processes rather than catastrophic events were responsible the polar layered deposits of mars contain the planet s largest known reservoir of water ice1 2 and the prospect of revealing a detailed martian palaeoclimate record3 4 but the mechanisms responsible for the formation of the dominant features of the north polar layered deposits npld are unclear despite decades of debate stratigraphic analyses of the exposed portions of chasma boreale a large canyon 500 km long up to 100 km wide and nearly 2 km deep have led most researchers to favour an erosional process for its formation following initial npld accumulation candidate mechanisms include the catastrophic outburst of water5 protracted basal melting6 erosional undercutting7 aeolian downcutting7 8 9 and a combination of these processes10 here we use new data from the mars reconnaissance orbiter to show that chasma boreale is instead a long lived complex feature resulting primarily from non uniform accumulation of the npld the initial valley that later became chasma boreale was matched by a second equally large valley that was completely filled in by subsequent deposition leaving no evidence on the surface to indicate its former presence we further demonstrate that topography existing before the npld began accumulating influenced successive episodes of deposition and erosion resulting in most of the present day topography long term and large scale patterns of mass balance achieved through sedimentary processes rather than catastrophic events ice flow or highly focused erosion have produced the largest geomorphic anomaly in the north polar ice of mars

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

sedimentation waves on the martian north polar cap analogy with megadunes in antarctica complex interactions between katabatic winds and the cryosphere may lead to the formation of sedimentation waves at the surface of ice sheets these have been first described and named snow megadunes in antarctica here we use topographic data optical images subsurface radar soundings and spectroscopic data acquired by mars orbiters to show that the surface of the martian north polar cap displays two superimposed sets of sedimentation waves with differing wavelengths these sedimentation waves have similarities with antarctic snow megadunes regarding their surface morphology texture grain size asymmetry and internal stratigraphic architecture both sets of martian sedimentation waves present young ice and occasional sastrugi fields indicative of net accumulation on their shallow dipping upwind sides their tops and the intervening troughs old layers of dusty ice indicative of net ablation are exhumed on the steep dipping downwind sides of the larger waves smooth surfaces of coarse grained ice indicative of reduced accumulation associated with sublimation metamorphism cover the steep dipping downwind sides of the smaller waves these surface characteristics and the internal stratigraphy revealed by radar soundings are consistent with the interpretation that both sets of martian sedimentation waves grow and migrate upwind in response to the development of periodic accumulation ablation patterns controlled by katabatic winds the recognition of these sedimentation waves provides the basis for the development of a common model of ice wind interaction at the surface of martian and terrestrial glaciers martian smaller waves characterized by reduced net accumulation on their downwind sides are analogous to antarctic snow megadunes that have been described so far a terrestrial equivalent remains to be discovered for the larger martian waves characterized by net ablation on their downwind sides 2014 elsevier b v

planum boreum basal unit topography mars irregularities and insights from sharad shallow radar investigations of planum boreum mars basal unit bu deposit have revealed multiple reentrants morphologic irregularities and thickness trends that differ from those of the overlying north polar layered deposits we present detailed subsurface maps for these features and offer explanation for genesis of the deposit s morphologic asymmetry expressed in different erosional characteristics between 0 e 180 e and 180 e 360 e additionally this work revealed a depression in the basal unit that may have provided a site for spiral trough initiation interpretations of the findings suggest that antecedent bu topography has a marked impact on modern morphology and that aeolian forces have been the dominant driver of polar deposit accumulation since at least the end of rupes unit emplacement we find no results requiring explanation beyond common martian surface processes including aeolian erosion and impact armoring to add to the detailed morphologic study of the bu we mapped the variability of the bu radar reflection character combining generalized katabatic wind flow with the radar mapping results suggests that rupes unit material sourced the younger cavi we present clear evidence that while compositionally distinct from the overlying layered deposits the bu and its morphology are intimately linked to the morphology of the north polar layered deposits combining geologic evidence with paleoclimate modeling the deposits contain evidence for a long history of aeolian emplacement and modification

sequence architecture of the cavi unit chasma boreale mars abstract the principles of terrestrial sequence stratigraphy provide a proven approach for evaluating the cavi unit where it transitions into the north polar layered deposits npld three cavi unit outcrops at the head of chasma boreale were examined using high resolution imagery and topography and interpreted in a sequence stratigraphic framework lithofacies present in the cavi unit were identified and consist of 1 dark lithic material 2 dark material with icy foresets 3 icy cavi 4 isolated dunes 5 dirty ice and 6 layered ice these lithofacies represent a progression in the dominance of aeolian to cryosphere processes and have analogs in the modern chasma boreale depositional environment parasequences were defined as aeolian units bounded by outcrop scale unconformities permafrost layers and layered ice deposits the northern outcrop is dominated by sandy lithofacies with large bodies of sand also dominating the northern parts of the central and southern outcrops in the central and southern outcrops sandy lithofacies yield southward to icier accumulations concurrently vertical trends in lithofacies distribution indicate an increasing prevalence of ice deposits through time considered across all three outcrops the parasequences form a set that expresses the northward retreat of the cavi aeolian system coincident with transgression of an ice sheet originating in gemini lingula this retrogradational stacking pattern of cavi aeolian parasequences with gradual transgression of the npld environment implies that the transitional cavi unit and npld form a single sequence that may correspond to allogenic forcing by orbital parameters the dynamics between aeolian and cryosphere processes revealed at the parasequence scale indicate that preservation of the stratigraphic record is driven by the ice transgression suggesting that during episodes of ice sheet collapse the north polar sedimentary environment reverts to one dominated by aeolian erosion and reworking as represented by the basal cavi unit

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

paleoceanographic reconstructions of surface ocean conditions in the greenland iceland and norwegian seas through the last 14 ka based on diatoms prior reconstructions of the last glacial maximum and the holocene did not include detailed paleoclimate data from the greenland iceland and norwegian gin seas which cover sensitive areas for oceanographic boundaries sea ice distribution and deep water formation in order to fill this gap we studied nine well dated sediment cores from this area and reconstructed the paleoceanographic conditions of the surface gin seas at 2000 year intervals since 13 400 bp based on the diatom record results show that a n s extending sea ice free corridor had opened along norway already at 13 400 bp indicating a northward flow of a branch of the north atlantic drift possibly caused by a change in the jet stream flow due to the decreased height of the north american ice sheet a major change of climatic conditions occurred over the gin seas as the insolation anomaly reached its maximum around 9000 bp when the sea ice cover and the oceanic fronts retreated to a northwesterly position along greenland and the ocean temperatures rose the first half of the holocene is recorded as the warmest period during the last 13 400 bp in the gin seas the duration of this climatic optimum decreased both toward the north and toward the margins of the area the second half of the holocene is characterized by a cooling trend in step with the decreasing insolation these results support the proposal of imbrie et al 1992 that the gin seas region acts as an initial responder to insolation changes

orbital radar imagery and atmospheric modeling reveal an aeolian origin for abalos mensa mars 1 icy deposits surrounding planum boreum mars contain crucial information for deciphering paleoclimate and past geologic processes at the martian north pole one such deposit abalos mensa is an enigmatic wedge of material located near the 1 km high rupes tenuis its unique location and lobate morphology have fostered formation hypotheses that assume either fluvial or aeolian erosion of a once larger ice deposit the aeolian scenario posed previously requires impact shielding of ancient basal unit material to provide an erosional remnant which seeds later deposition while the fluvial hypotheses invoke cryovolcanism beneath the younger north polar layered deposits npld and associated outflow to erode the adjacent chasmata here we combine newly available radar sounding data high resolution imagery digital elevation models and atmospheric modeling to examine internal structure and infer both the mechanisms for and timing of abalos mensa formation from this integrative approach we conclude that abalos mensa formed as a distinct feature via atmospheric deposition following erosion of rupes tenuis and grew concurrently with the rest of planum boreum as the npld accumulated the required processes are consistent with those observed today no exotic phenomena cryovolcanism fluvial activity or impact shielding appear necessary to explain the formation of abalos mensa

volume of martian midlatitude glaciers from radar observations and ice flow modeling numerous glacier like forms have been identified in the midlatitudes of mars and within recent years the acquisition of radar sounding data has revealed that the features are chiefly composed of water ice here we use radar observations in combination with ice flow models and inverse methods to calculate the volume of ice present at the midlatitudes of mars in order to obtain ice thicknesses we infer the yield stress of the ice deposits and we find that they are consistently lower than those of most terrestrial glaciers we estimate the present ice volume of lobate debris aprons identified by levy et al 2014 on mars to correspond to 1 55 105 km3 with an uncertainty of 25 this corresponds to a global ice cover of 1 1m thus the water ice found at midlatitudes is an important water reservoir and an important part of the global surface ice budget

a subsurface depocenter in the south polar layered deposits of mars the south polar layered deposits spld are one of the largest water ice reservoirs on mars and their accumulation is driven by variations in the climate primarily controlled by orbital forcings patterns of subsurface layering in the spld provide important information about past atmospheric dust content periods of substantial erosion and variations in local or regional deposition here we analyze the spld using shallow radar sharad sounder data to gain a unique perspective on the interior structure of the deposits and to determine what subsurface layers indicate about the preserved climate history sharad data reveal a major deviation from the gently domical layering typical of the spld a subsurface elongate dome the dome most likely formed due to variations in the accumulation of ice and snow across the cap with a higher rate occurring in this region over a prolonged period this spld depositional center provides an important marker of south polar climate patterns

arecibo radar imagery of mars ii chryse xanthe polar caps and other regions abstract we conclude our radar imaging survey of mars which maps spatial variations in depolarized radar reflectivity using arecibo s band 12 6 cm observations from 2005 2012 whereas our earlier paper harmon et al 2012 arecibo radar imagery of mars the major volcanic provinces icarus 220 990 1030 covered the volcanic regions of tharsis elysium and amazonis this paper includes non volcanic regions where hydrologic and impact processes can be the dominant resurfacing agents affecting radar backscatter many of the more prominent and interesting radar bright features outside the major volcanic provinces are located in and around chryse planitia and xanthe terra these features are identified with a basin in northeast lunae planum containing the combined deposits from maja vallis and ganges catena outflows channel outwash plains in western and southern chryse basin plateaus bordering chasma chaos zones where surface modification may have resulted from hydrologic action associated with incipient chaos formation and some bright ejecta craters in chryse basin of a type otherwise rare on mars dark halo craters have also been identified in chryse and elsewhere that are similar to those seen in the volcanic provinces although the cratered highlands are relatively radar bland they do exhibit some bright depolarized features these include eroded crater rims several unusual ejecta flows and impact melts and terrain softened plains the rims of large impact basins hellas argyre isidis show a variety of radar bright features provisionally identified with massif slopes erosion sediments eroded pyroclastics impact melts and glacial deposits the interiors of these basins are largely radar dark which is consistent with coverage by rock free sediments tempe terra and acheron fossae show bright features possibly associated with rift volcanism or eroded tectonic structures and northwest tempe terra shows one very bright feature associated with glacial or other ice processes in the dichotomy boundary region the first delay doppler images of the radar bright features from the north and south polar icecaps are presented both poles show the circular polarization inversion and high reflectivity characteristic of coherent volume backscatter from relatively clean ice the south polar feature is primarily backscatter from the residual co 2 icecap with a lesser contribution from the polar layered deposits whose finite optical depth probably accounts for the feature s strong s x band wavelength dependence conversely the north polar radar feature appears to be mostly backscatter from the h 2 o ice rich polar layered deposits rather than from the thin residual h 2 o cap the north polar region shows additional radar bright features from korolev crater and a few other outlying circumpolar ice deposits

aeolian processes as drivers of landform evolution at the south pole of mars abstract we combine observations of surface morphology topography subsurface stratigraphy and near surface clouds with mesoscale simulations of south polar winds and temperature to investigate processes governing the evolution of spiral troughs on the south pole of mars in general we find that the south polar troughs are cyclic steps that all formed during an erosional period contrary to the troughs at the north pole which are constructional features the shallow radar instrument sharad onboard mars reconnaissance orbiter detects subsurface stratigraphy indicating relatively recent accumulation that occurred post trough formation in many locations using optical instruments especially the thermal emission imaging system themis we find low altitude trough clouds in over 500 images spanning 6 mars years the locations of detected clouds correspond to where recent accumulation is detected by sharad and offers clues about surface evolution the clouds migrate by season moving poleward from 71 s at l s 200 until l s 318 when the last cloud is detected our atmospheric simulations find that the fastest winds on the pole are found roughly near the external boundary of the seasonal co 2 ice cap thus we find that the migration of clouds and katabatic jumps corresponds spatially to the retreat of the co 2 seasonal ice as detected by titus 2005 and that trough morphology through recent accumulation is integrally related to this seasonal retreat