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

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

early plate tectonics versus single plate tectonics on mars evidence from magnetic field history and crust evolution 1 the consequences of an early epoch of plate tectonics on mars followed by single plate tectonics with stagnant lid mantle convection on both crust production and magnetic field generation have been studied with parameterized mantle convection models thermal history models with parameterized mantle convection not being dynamo models can provide necessary but not sufficient conditions for dynamo action it is difficult to find early plate tectonics models that can reasonably explain crust formation as is required by geological and geophysical observations and allow an early magnetic field that is widely accepted as the cause for the observed magnetic anomalies dating of crust provinces and topography and gravity data suggest a crust production rate monotonically declining through the noachian and hesperian and a present day crust thickness of more than 50 km plate tectonics cools the mantle and core efficiently and the core may easily generate an early magnetic field given a sufficiently weak mantle rheology plate tectonics can explain a field even if the core is not initially superheated with respect to the mantle because the crust production rate is proportional to temperature however an early efficient cooling will frustrate later crust production and therefore cannot explain for example the absence of prominent magnetic anomalies in the northern crustal province and the northern volcanic plains in the early hesperian voluminous crust formation following plate tectonics is possible if plate tectonics heat transfer is inefficient but then the crust growth rate has a late peak about 2 ga b p which is not observed these models also require a substantial initial superheating of the core to allow a dynamo if one accepts the initial superheating then as we will show a simple thermal evolution model with monotonic cooling of the planet due to stagnant lid mantle convection underneath a single plate throughout the evolution can better reconcile early crust formation and magnetic field generation

lithospheric loading by the northern polar cap on mars abstract new topography data for the northern polar region on mars returned by the the mars orbiter laser altimeter mola during the aerobraking hiatus and science phasing orbits allow characterization of the topography of the present northern polar cap and its environs models for loading of an elastic shell by an axisymmetric load approximating the present polar cap geometry indicate that the maximum deflection of the subice basement is in the range 1200 to 400 m for an elastic lithosphere of thickness 40 to 200 km overlain by a cap of pure h 2 o ice corresponding model cap volumes increase from 1 5 to 1 8 10 6 km 3 as elastic lithosphere thickness decreases from 200 to 40 km the presence of sediments in the polar cap increases the depth to basement and resulting cap volume for a given value of elastic lithosphere thickness one dimensional heat flow calculations indicate that the temperature at the base of the cap may approach the melting point of cap material if the lithosphere underlying the cap is thin the basal temperature is 170 k for a 200 km thick lithosphere overlain by pure ice but is as great as 234 k for a 40 km thick lithosphere overlain by a cap with a high sediment ice ratio constraints on elastic lithosphere thickness are weak but geologic mapping and mola data suggest that a flexurally derived circumpolar depression filled with sediments is consistent with elastic lithosphere thickness values in the range 60 120 km gravity and topography over the whole cap are poorly correlated possibly due to viscous relaxation of long wavelength topography but gravity and topography over the western portion of the main cap are consistent with an elastic lithosphere thickness of 120 km for a crustal thickness of 50 km both mola data and geological information suggest a formerly larger northern polar cap the relationship of time scales for changes in the polar cap volume and extent to time scales for viscous relaxation of topography has important implications for investigations of even present polar cap topography viscoelastic calculations show that the maxwell time for an earth like mantle viscosity for mars 10 21 pa s is 10 5 yr the maxwell time scales directly with the martian mantle viscosity so that values as high as 10 7 yr are possible time scales for changes in polar cap volume are poorly constrained but major changes in cap volume over periods of 10 6 10 8 yr are consistent with current understanding of polar cap processes

constraints on magma ascent emplacement and eruption geochemical and mineralogical data from drill core samples at obsidian dome inyo chain california systematic chemical and mineralogical variability occurs in samples from drill holes through obsidian dome the conduit to the dome and a nearby associated feeder dike the drill hole samples from the margins of the conduit and most of the lower part of the dome are high ba low silica rhyolites they contain two populations of phenocrysts and represent commingled magmas whereas samples from the dike and upper parts of the dome are low ba higher silica rhyolites that do not reflect commingled magmas samples from the center of the conduit are low ba higher silica rhyolites that are only slightly mixed a major part of the variability within the drill core samples of the dome and conduit reflects the juxtaposition and commingling of two distinct magmas during their passage through the conduit

constraints on the martian lithosphere from gravity and topography data localized spectral admittances of the large martian volcanoes are modeled by assuming that surface and subsurface loads are elastically supported by the lithosphere in order to model the case where the load density differs from that of the crust a new method for calculating gravity anomalies and lithospheric deflections is developed the modeled gravity anomalies depend upon the elastic thickness crustal thickness load density and crustal density and these parameters were exhaustively sampled in order to determine their effect on the misfit between the observed and modeled admittance function we find that the densities of the martian volcanoes are generally well constrained with values of 3200 100 kg m 3 which is considerably greater than those reported previously these higher densities are consistent with those of the martian basaltic meteorites which are believed to originate from the tharsis and elysium volcanic provinces the crustal density is constrained only beneath the elysium rise to be 3270 150 kg m 3 if this value is representative of the northern lowlands then pratt compensation is likely responsible for the approximately 6 km elevation difference between the northern and southern hemispheres the elastic thicknesses of the major martian volcanoes when subsurface loads are ignored are found to be the following elysium rise 56 20 km olympus mons 93 40 km alba patera 66 20 km and ascraeus mons 105 40 km we have also investigated the effects of subsurface loads allowing the bottom load to be located either in the crust as dense intrusive material or in the mantle as less dense material we found that all volcanoes except pavonis are better modeled with the presence of less dense material in the upper mantle which is indicative of either a mantle plume or a depleted mantle composition an active plume beneath the major volcanoes is consistent with recent analyses of cratering statistics on olympus mons and the elysium rise which indicate that some lava flows are as young as 10 30 myr as well as with the crystallization ages of the shergottites of which some are as young as 180 myr

thermal and compositional evolution of the martian mantle effects of water abstract we present numerical models of the thermochemical evolution of the mantle of mars with particular attention paid to the effects of water with a two dimensional anelastic compressible convection algorithm in combination with a parameterized model of composition and thermoelastic properties we consider how water in nominally anhydrous minerals may have influenced the melting and rheology of the martian mantle and how it may have been redistributed by melting melt extraction and convection over a period of 4 gy the strongest effect of water is found to be a marked increase in convective vigor due to the lower viscosity which leads to faster cooling of the mantle but nonetheless enhances melt production modeling results are compared with observations and independent inferences from spacecraft observations and martian meteorite analyses some models with radionuclide concentrations from wanke and dreibus 1994 but with initial mantle water concentrations between their estimate of 36 ppm by weight and 10 times this value combined with 50 degassing of crust forming material partly match observed crustal concentrations of k th and water in order to account for the observed chemical variation in crustal rocks however the formation and survival of ancient mantle heterogeneities appears to have been necessary water loss from the mantle since the end of the magma ocean stage has been minor elevated water contents notwithstanding the depth variation of viscosity in the sublithospheric mantle of a given model is likely minor depths to the curie temperature at the end of the noachian range from 40 to 60 km for magnetite and hematite in the models and present day total crustal thicknesses lie between 75 and 100 km the depth to the brittle ductile transition increases from values around 30 km in the early stages of the models to 170 240 km at present an estimate of total core entropy renders the existence of a basal perovskite ferropericlase layer unlikely the absence of stable superplumes in all of our preferred models suggests that the volcanic provinces of tharsis and elysium were formed by a succession of more short lived events such as transient plumes from specific regions at the core mantle boundary

toward mapping the effective elastic thickness of planetary lithospheres from a spherical wavelet analysis of gravity and topography abstract the effective elastic thickness t e of the lithosphere controls the flexural response to transverse loading and can be used in conjunction with rheological models to remotely estimate surface heat flux of terrestrial planets in the vast majority of studies t e estimation is carried out in a two step process 1 the joint spectra admittance and or coherency of gravity anomalies free air and or bouguer and topography are calculated within finite size windows and 2 the spectra are inverted using a model for the loading of an infinite elastic plate or shell in recent years research in the spatio spectral analysis of cartesian grids and improvements in lithospheric loading models have allowed the mapping of t e on the earth at unprecedented resolution nevertheless the limitations imposed by working with cartesian data and models are hampering further advances in terrestrial t e studies the planetary community on the other hand has traditionally used spatio spectral methods that work directly on the sphere thereby avoiding the undesirable distortions and biases inherent in cartesian studies however the models and methods developed for the earth have never been applied to planetary data therefore also limiting further advances in the mapping of planetary t e this paper reviews the latest developments in t e studies and proposes to combine advances in both terrestrial and planetary studies to allow the mapping of t e of terrestrial planets using a spherical wavelet analysis of gravity and topography data we begin with a review of the recent literature on the challenges and debates concerning t e estimation from joint gravity and topography spectra we then briefly review the implementation of the continuous planar and spherical wavelet transforms cpwt and cswt and demonstrate that both transforms possess exactly the same resolving power however the cswt does not suffer from effects associated with non periodic boundaries we further re derive in a consistent manner the expressions for the admittance and coherency from flexural equations for the loading of thin elastic plates and shells and show that they result in similar spectra except at the longest wavelengths where the shell membrane stresses dominate we then estimate global grids of earth s continental t e by inverting the planar and spherical admittance and coherency functions both separately and jointly using either free air or bouguer gravity anomalies the correspondence between the cartesian and spherical approaches and the similarity of t e results for the earth s continents indicate that the spherical methods are robust results obtained from the joint inversion of admittance and coherency show that simple lithospheric loading models fail to capture the complexity of the data with adverse effects on the estimated parameters finally we extend the analysis to the moon and other terrestrial planets and discuss limitations and future applications of the fully spherical techniques

very recent and wide spread basaltic volcanism on mars new spacecraft data provide increasing evidence for a dynamic environment on presenta day mars exogenic processes such as impact cratering mass wasting processes and active dune migration have all been observed to modify the surface no traces of current endogenic activity have been found yet but some studies point to very localized volcanism in the last few millions of years however no systematic study of young volcanic surfaces had been performed so far we present absolute model age determinations of plains volcanism on mars as derived from impact crater sizefrequency distributions extended areas in tharsis the largest volcanoa tectonic region on mars have been resurfaced by lava flows in the last few tens of millions of years we also present results on the rheologic properties of these lava flows inferred from morphometric measurements yield strengths are in the range of 100a 300 pa and viscosities reach values of 10 2 to 10 3 pa s indicating basaltic compositions the results imply that mars retained until recently and probably still retains enough internal heat to produce widea spread plaina style volcanism producing lowa viscosity lava flows throughout large parts of tharsis

magmatic volatiles in explosive rhyolitic eruptions obsidian clasts in rhyolitic tephra deposits preserve preeruption magmatic volatile contents providing a direct means for determining the volatile content of explosively erupted magmas small to moderate volume plinian eruptions 10 sup 3 to 10 sup 1 km sup 3 appear to be driven by 0 5 1 0 wt volatiles consisting dominantly of h sub 2 o with minor co sub 2 analysis of obsidian from eruptive sequences consisting of tephra and flows indicates that this hydrous magma abruptly overlies magma with only 0 1 0 2 wt h sub 2 o

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

dielectric properties of jovian satellite ice analogs for subsurface radar exploration a review the first european mission dedicated to the exploration of jupiter and its icy moons jupiter icy moons explorer juice will be launched in 2022 and will reach its final destination in 2030 the main goals of this mission are to understand the internal structure of the icy crusts of three galilean satellites europa ganymede and callisto and ultimately to detect europa s subsurface ocean which is believed to be the closest to the surface among those hypothesized to exist on these moons juice will be equipped with the 9 mhz subsurface penetrating radar rime radar for icy moon exploration which is designed to image the ice down to a depth of 9 km moreover a parallel mission to europa which will host onboard reason radar for europa assessment and sounding ocean to near surface equipped with 9mhz and 60mhz antennas has been recently approved by nasa the success of these experiments strongly relies on the accurate prediction of the radar performance and on the optimal processing and interpretation of radar echoes that in turn depend on the dielectric properties of the materials composing the icy satellite crusts in the present review we report a complete range of potential ice types that may occur on these icy satellites to understand how they may affect the results of the proposed missions first we discuss the experimental results on pure and doped water ice in the framework of the jaccard theory highlighting the critical aspects in terms of a lack of standard laboratory procedures and inconsistency in data interpretation we then describe the dielectric behavior of extraterrestrial ice analogs like hydrates and icy mixtures carbon dioxide ice and ammonia ice building on this review we have selected the most suitable data to compute dielectric attenuation velocity vertical resolution and reflection coefficients for such icy moon environments with the final goal being to estimate the potential capabilities of the radar missions as a function of the frequency and temperature ranges of interest for the subsurface sounders we present the different subsurface scenarios and associated radar signal attenuation models that have been proposed so far to simulate the structure of the crust of europa and discuss the physical and geological nature of various dielectric targets potentially detectable with rime finally we briefly highlight several unresolved issues that should be addressed in near future to improve our capability to produce realistic electromagnetic models of icy moon crusts the present review is of interest for the geophysical exploration of all solar system bodies including the earth where ice can be present at the surface or at relatively shallow depths

degassing of rhyolitic magma during ascent and emplacement the degassing history of a rhyolitic igneous system was documented from analyses of drill core samples through the extrusive and intrusive portions of obsidian dome and of surface samples of associated tephra the initial volatile composition of the inyo magma was estimated to be 4 0 wt h2o 500 ppm f 800 ppm cl and 80 ppm s retained volatile contents of glassy and crystalline samples reflect the effects of decompression and second boiling decompression is rapid and involves loss of water rich fluid until a close approach to lithostatic equilibrium is achieved second boiling is a slower process and produces a chlorine rich fluid some of which can be trapped during development of extremely fine crystallization textures nearly complete dewatering during decompression of surface extruded magma strongly undercools the system t 175 c suppressing crystallization and yielding glassy rhyolitic lava partial degassing of shallowly intruded magma permits pervasive crystallization even at high cooling rates the subvolcanic intrusive regime is the zone of maximum volatile release because second boiling is incomplete in extrusives and volatile bearing crystalline phases are stable in magma crystallized at greater depth

thermal behavior and ice table depth within the north polar erg of mars abstract we fully resolve a long standing thermal discrepancy concerning the north polar erg of mars several recent studies have shown that the erg s thermal properties are consistent with normal basaltic sand overlying shallow ground ice or ice cemented sand our findings bolster that conclusion by thoroughly characterizing the thermal behavior of the erg demonstrating that other likely forms of physical heterogeneity play only a minor role and obviating the need to invoke exotic materials thermal inertia as calculated from orbital temperature observations of the dunes has previously been found to be more consistent with dust sized materials than with sand since theory and laboratory data show that dunes will only form out of sand sized particles exotic sand sized agglomerations of dust have been invoked to explain the low values of thermal inertia however the polar dunes exhibit the same darker appearance and color as that of dunes found elsewhere on the planet that have thermal inertia consistent with normal sand sized basaltic grains whereas martian dust deposits are generally lighter and redder the alternative explanation for the discrepancy as a thermal effect of a shallow ice table is supported by our analysis of observations from the mars global surveyor thermal emission spectrometer and the mars odyssey thermal emission imaging system and by forward modeling of physical heterogeneity in addition our results exclude a uniform composition of dark dust sized materials and they show that the thermal effects of the dune slopes and bright interdune materials evident in high resolution images cannot account for the erg s thermal behavior

numerical models of martian mantle evolution induced by magmatism and solid state convection beneath stagnant lithosphere 1 numerical models of magmatism in a convecting mantle are presented to understand martian mantle evolution the viscosity is strongly temperature dependent and the lithosphere is stagnant magmatism is modeled as a permeable flow of basaltic magma generated by decompression melting when the initial mantle temperature is sufficiently high a magma ocean develops to generate a thick basaltic crust and create a compositionally layered mantle the upper layer of the mantle consists of compositionally buoyant residue of the basaltic crust while the lower layer consists of compositionally denser materials that are not as depleted in the basaltic component hot plumes grow from the lower layer making it thinner with time by erosion the hot plumes also induce magmatism which extracts heat producing elements hpes from the mantle the hpe extraction regulates plume magmatism activity and controls the duration of plume magmatism plume magmatism also keeps the mantle temperature below the solidus by efficiently extracting heat as soon as the mantle temperature exceeds the solidus the martian mantle is likely to have evolved as a relaxation from a compositionally layered state formed by the magma ocean and plume magmatism has probably played a crucial role in the relaxation process

textural constraints on effusive silicic volcanism beyond the permeable foam model transitions between explosive and effusive phases of silicic volcanic eruptions have been related either to stratification of volatiles in the source magma body or to the loss of volatiles through the permeable host rock of the conduit one way to distinguish between these two models is to map and analyze the vesicular and glassy textures found in silicic lava flows in this paper we present textural observations and isotopic evidence from active and recent silicic lava flows which show that at least some vesiculation occurs during surface advance of extrusions after magma has reached the earth s surface this view is in contrast to the widely promoted permeable foam model which states that all volatiles escape during ascent of the magma and that all dense glassy material in lava flows forms from the collapse of pumiceous lava i e that silicic lavas emerge as highly inflated foam flows such interpretations which claim that silicic lavas are completely degassed upon extrusion and that all degassing must take place on the time scale of the eruption neglect several important pieces of evidence including the presence of obsidian in extremely small domes and of vesicular zones in the interiors of silicic flows the copious loss of volatiles through eruption plumes between eruptive phases and direct observations of surface vesiculation during growth of the mount st helens lava dome the permeable foam model also implies the unlikely requirement that explosive to effusive transitions be associated with an increase in eruption rate we present a more comprehensive model for the emplacement of silicic extrusions that allows for early gas loss during ascent as well as late stage vesiculation we then discuss how the redistribution of volatiles during surface flow can increase explosive hazards from silicic lavas days weeks or months after the lava emerges from the vent

magmatic model for the mount st helens blast of may 18 1980 analytical and numerical solutions to the hydrodynamic equations of motion constrained by physical properties of juvenile ejecta in the mount st helens blast deposit were used to investigate magmatic conditions required to produce the initial devastating blast phase of the eruption of may 18 1980 evidence that the blast was magmatic includes equivalence in volume of juvenile blast ejecta to preemption inflation of the cone substantial vesicularity of this ejecta and continued vesiculation of large juvenile clasts after eruption observed or inferred ejecta velocities of 100 to 250 m s are shown to require 0 2 to 0 7 wt water vapor preexisting in magma unloaded by a landslide 200 to 900 m thick these conditions imply total magmatic water contents of 0 7 to 1 7 wt respectively such low required water content suggests that volcanic blasts may be regarded as a normal consequence of magma intrusion into an unstable edifice

orbital change experiments with a mars general circulation model abstract we use a mars general circulation model to examine the effect of orbital changes on the planet s general circulation and climate system experiments are performed for obliquities ranging from 0 to 60 for two different longitudes of perihelion each experiment simulates a full mars year assuming a fixed atmospheric dust distribution and fixed amount of co 2 in the atmosphere cap system we find that global mean surface temperatures and pressures decline with increasing obliquity due to the increasing extent of the winter polar caps the seasonal co 2 cycle and intensity of the solstice circulation amplify considerably with increasing obliquity such that global dust storms are likely at both solstices the most significant feature of the high obliquity solstice circulations is the development of an intense low level jet associated with the return branch of the hadley circulation model surface stresses are used to map regions of preferred dust lifting which are defined in terms of an annual deflation potential for the present obliquity the model predicted regions of high deflation potential are in good agreement with cantor et al s 2001 j geophys res 106 23653 23688 observations which gives us some confidence in the model s ability to predict where lifting might occur when mars orbit parameters are different than they are today in general we find that the dust lifting potential increases sharply with obliquity and is greatest at times of high obliquity when perihelion coincides with northern summer solstice over an obliquity cycle the model global annual deflation potential ranges from several tenths of a millimeter at 0 obliquity to almost 15 mm at 60 obliquity much higher values are possible when the atmosphere is very dusty we find a strong correlation between the deflation potential and surface thermal inertia regions of high deflation potential correspond to regions of high thermal inertia high rock abundance and regions of low deflation potential correspond to regions of low thermal inertia high dust sand abundance furthermore while the regions of preferred lifting high deflation potential expand somewhat with increasing obliquity and dust loading the central parts of tharsis arabia and elysium show no tendency for significant lifting at any obliquity or longitude of perihelion these regions may therefore be very old and represent net long term sinks for atmospheric dust it is the topography of the planet through its influence on surface pressure and wind systems which ultimately determines where dust accumulates finally as was found by fenton and richardson 2001 j geophys res 106 32885 32909 we find no tendency for the development of east southeasterly winds at the pathfinder site for any of our orbital change experiments this suggests that the ancient wind regime discussed by greeley et al 2000 j geophys res 105 1829 1840 was produced by other factors such as polar wander

first high resolution stratigraphic column of the martian north polar layered deposits 1 this study achieves the first high spatial resolution layer scale measured stratigraphic column of the martian north polar layered deposits using a 1m posting dem the marker beds found throughout the upper north polar layered deposits range in thickness from 1 6 m 16 0 m 1 4 m and 6 of 13 marker beds are separated by 25 35 m thin layer sets have average layer separations of 1 6 m these layer separations may account for the spectral power peaks found in previous brightness profile analyses marker bed layer thicknesses show a weak trend of decreasing thickness with depth that we interpret to potentially be the result of a decreased accumulation rate in the past for those layers however the stratigraphic column reveals that a simple rhythmic or bundled layer sequence is not immediately apparent throughout the column implying that the relationship between polar layer formation and cyclic climate forcing is quite complex

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

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

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

effects of compressibility on the flow of lava lava contains gas bubbles and hence is a compressible liquid whose density increases as a function of pressure after eruption at the earth s surface it spreads at a rate which is a function of its thickness and it is compressed under its own weight therefore both thickness and spreading rate are determined by a balance between viscous and compressible effects theoretical equations are derived for the shape and velocity of a compressible liquid spreading on a horizontal surface solutions are obtained for a fixed eruption rate q the radial extent of the flow increases proportional to t1 2 a dimensionless number c is defined which characterizes the importance of flow compression c rho 0 beta g left frac mu q rho 0 2 g right 1 mathord left vphantom 1 4 right kern nulldelimiterspace 4 where 0 is bubbly lava density at atmospheric pressure compressibility viscosity and g the acceleration of gravity c can be thought of as the ratio of two characteristic length scales one for compression effects and one for viscous effects the larger c is the more important compressibility effects are as c is increased the flow becomes thinner because the liquid is compressed more and more efficiently compressibility acts to smooth out variations of flow thickness which provide the driving force thus all else being equal a compressible liquid flows less rapidly than an incompressible one when trying to infer the effective viscosity of a flow from its spreading rate the neglect of compressibility leads to an overestimate the various factors which act to determine the distribution of gas bubbles in lava flows are reviewed and discussed quantitatively comparison with data from obsidian dome eastern california shows that disequilibrium effects are important and that bubble resorption during burial in a thick flow is not a pervasive phenomenon the analysis is applied to the 1979 dome of soufriere de saint vincent w i an effective value of compressibility for this 100 m thick dome is 1 5x10 6 pa 1 this implies that all else being equal the viscosity of this lava may be overestimated by a factor of 5 if no account is taken of the compressible nature of the flow

density of mars south polar layered deposits both poles of mars are hidden beneath caps of layered ice we calculated the density of the south polar layered deposits by combining the gravity field obtained from initial results of radio tracking of the mars reconnaissance orbiter with existing surface topography from the mars orbiter laser altimeter on the mars global surveyor spacecraft and basal topography from the mars advanced radar for subsurface and ionospheric sounding on the mars express spacecraft the results indicate a best fit density of 1220 kilograms per cubic meter which is consistent with water ice that has 15 admixed dust the results demonstrate that the deposits are probably composed of relatively clean water ice and also refine the martian surface water inventory

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

gas transport and bubble collapse in rhyolitic magma an experimental approach a series of experiments was conducted to test concepts of porous flow degassing of rhyolitic magma during ascent and of the subsequent collapse of vesicles in degassed magma to form obsidian dense synthetically hydrated natural glasses were pressurized under water saturated conditions and then decompressed to achieve a range of porosities in the presence of a tracer vapor d2o rapid isotopic exchange indicative of vapor transport rather than of simple diffusion occurred at a porosity 60 vol in accord with earlier gas permeability measurements on cold natural samples in another series of experiments natural and synthetic pumices vesiculated by degassing to atmospheric pressure rapidly collapsed to dense glass on repressurization to the modest pressures prevailing in lava flows no relict bubble textures remained these results support the hypothesis that effusive eruptions result from the syneruptive escape of gas from permeable magmatic foam and that a process analogous to welding yields dense lavas when such foams are extruded

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

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

subsurface damage from oblique impacts into low impedance layers 1 layered planetary surfaces occur ubiquitously in the solar system where sedimentary sequences or icy layers overlay crystalline bedrock previous experimental studies investigated how the presence of weak layer overlying a strong basement affects crater morphology subsurface damage and soft sediment compression numerical studies generally focus on the final morphology as a function of thicknesses and burial depths of weak layers in field studies of impact craters the shock state of minerals is a key metric here we evaluate the effect of a surficial low impedance layer on peak pressure magnitudes and consequent damage extent in the competent substrate laboratory experiments coupled with 3d cth models of oblique 30 from horizontal hypervelocity impacts at laboratory and planetary scales show that surface layers with a thickness on the order of the projectile diameter shield the underlying surface and absorb scatter 70 of the impact energy numerical simulations reveal that surficial layers reduce peak pressure magnitudes within the subsurface by 60 70 while damage in the substrate is due to shear failure sedimentary layers are more efficient shields than icy layers but both reduce the extent of subsurface damage and the resulting shock levels recorded by minerals these results indicate that a thin surficial low impedance layer mitigates the expression of shocked minerals in the substrate even when a structural response is still observed

radar probing of jovian icy moons understanding subsurface water and structure detectability in the juice and europa missions abstract radar probing of jovian icy satellites is fundamental for understanding the moons origin and their thermal evolution as potential habitable environments in our solar system using the current state of knowledge of the geological and geophysical properties of ganymede europa and callisto we perform a comprehensive radar detectability study to quantify the exploration depth and the lower limit for subsurface identification of water and key tectonic structural elements to achieve these objectives we establish parametric dielectric models that reflect different hypotheses on the formation and thermal evolution of each moon the models are then used for fdtd radar propagation simulations at the 9 mhz sounding frequency proposed for both esa juice and nasa europa missions we investigate the detectability above the galactic noise level of four predominant subsurface features brittle ductile interfaces shallow faults brine aquifers and the hypothesized global oceans for ganymede our results suggest that the brittle ductile interface could be within radar detectability range in the bright terrains but is more challenging for the dark terrains moreover understanding the slope variation of the brittle ductile interface is possible after clutter reduction and focusing for europa the detection of shallow subsurface structural elements few kilometers deep such as fractures faults and brine lenses is achievable and not compromised by surface clutter the objective of detecting the potential deep global ocean on europa is also doable under both the convective and conductive hypotheses finally for callisto radar waves can achieve an average penetration depth of 15 km although the current understanding of callisto s subsurface dielectric properties does not suggest sufficiently strong contrasts to produce unambiguous radar returns

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

multi channel deconvolution of p waves at seismic arrays the results of a new multi channel deconvolution method applied to norsar and eka array recordings of teleseismic p waves are presented and interpreted in terms of possible surface reflections and other arrivals from the nevada test site and kazakh nuclear explosions the deconvolution method utilizes the well known fact that p wave spectra can be decomposed into source and receiver spectral factors for most events the source time functions appear to contain a pp arrival but they also show later unexplained arrivals the site functions are also complex in many cases about half of the late coda in p appears to be due to each source and receiver the recordings for a presumed kazakh cratering event show a distinctly different source function when compared to deeper buried explosions at the same test site from the deconvolved source time functions the cratering to noncratering mb bias is estimated to be between 0 09 and 0 22 magnitude units

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

a rheological investigation of vesicular rhyolite abstract the rheology of vesiculating rhyolitic systems exerts a strong control on the transport of silicic magmas in the subvolcanic to volcanic environments we present here an investigation of vesiculating and vesiculated rhyolites using dilatometric methods this study examines the effect of vesicle content on the viscosity of a natural supercooled rhyolitic liquid with 0 70 vesicles the experimental samples of rhyolitic glass are derived from fusion of a natural obsidian from little glass butte oregon crystal free rhyolite glasses of varying porosity were prepared by fusing obsidian powder in a pt crucible differing porosities were obtained by varying the temperature 1300 1650 c and duration 0 5 6 h of the fusions cylindrical samples of the resulting vesiculated rhyolites were cored from the crucible using diamond tools and their ends were ground flat and parallel for dilatometry the porosity of each sample was determined from archimedean buoyancy density determinations and comparison with bubble free rhyolite 2 331 g cm 3 porosity 1 p p o the density of foamed samples was determined using their mass volume and regular geometry viscosities were determined in the parallel plate mode at stresses of 5 10 3 to 10 5 pa the viscosimeter was calibrated using nbs 711 glass the bubble contents were microscopically investigated using a video reflected light system and image analysis software distribution functions of the size orientation aspect ratio and surface porosity were obtained the viscosity of rhyolite decreases with increasing bubble content a general relationship of the form 0 1 c describes the effect of porosity in volume fraction on the viscosity where c is a dimensionless constant 22 4 2 9 and log 10 0 10 94 0 04 pa s at 850 c

radar sounding of temperate permafrost in alaska analogy to the martian midlatitude to high latitude ice rich terrains 1 radar detection of subsurface ice on mars has been widely debated in part because the dielectric signature of ice as deduced from the dielectric constant can be confused with dry silicate rich materials to identify the ice dielectric signature it is crucial to estimate the imaginary part of the dielectric permittivity inferred from the dielectric attenuation after removing the scattering loss unfortunately the latter remains poorly quantified at both mars advanced radar for subsurface and ionospheric sounding marsis and shallow subsurface radar sharad frequencies to address this ambiguity we conducted multiple frequency ground penetrating radar and resistivity investigations in well characterized temperate permafrost in fairbanks alaska the area shows several geomorphologic similarities to midlatitude and high latitude terrains on mars this approach allowed us to quantify the dielectric and scattering losses in temperate permafrost over the 10 to 1000 mhz frequency band at 20 mhz our results suggest an average dielectric loss rate of 0 25 0 03 db m whereas the corresponding average scattering loss rate is 0 94 0 37 db m the scattering loss was found to represent 69 of the total signal attenuation considering this result and the study by heggy et al 2006a in volcanic environments we revised the interpretation of the attenuation coefficient calculated from sharad data over the deuteronilus mensae region and amazonis planitia we then used the reevaluated dielectric loss to estimate the imaginary part of the dielectric permittivity our results suggest that even if deuteronilus mensae deposits and the vastitas borealis formation may have similar dielectric constants their imaginary parts are different this implies that the two regions have different bulk compositions with the former being ice rich sediments and the latter being nonconsolidated volcanic deposits

implications of large elastic thicknesses for the composition and current thermal state of mars abstract the martian elastic lithosphere thickness t e has recently been constrained by modeling the geodynamical response to loading at the martian polar caps and t e was found to exceed 300 km at the north pole today geological evidence suggests that mars has been volcanically active in the recent past and we have reinvestigated the martian thermal evolution identifying models which are consistent with t e 300 km and the observed recent magmatic activity we find that although models satisfying both constraints can be constructed special assumptions regarding the concentration and distribution of radioactive elements the style of mantle convection and or the mantle s volatile content need to be made if a dry mantle rheology is assumed strong plumes caused by e g a strongly pressure dependent mantle viscosity or endothermic phase transitions near the core mantle boundary are required to allow for decompression melting in the heads of mantle plumes for a wet mantle large mantle water contents of the order of 1000 ppm are required to allow for partial mantle melting also for a moderate crustal enrichment of heat producing elements the planet s bulk composition needs to be 25 and 50 sub chondritic for dry and wet mantle rheologies respectively even then models resulting in a globally averaged elastic thicknesses of t e 300 km are difficult to reconcile with most elastic thickness estimates available for the hesperian and amazonian periods it therefore seems likely that large elastic thicknesses in excess of 300 km are not representative for the bulk of the planet and that t e possibly shows a large degree of spatial heterogeneity

modeling and observational occurrences of near surface drainage in utopia planitia mars during the past 15 years evidence for an ice rich planet mars has rapidly mounted become increasingly varied in terms of types of deposits and types of observational data and has become more widespread across the surface the mid latitudes of mars especially utopia planitia show many types of interesting landforms similar to those in periglacial landscapes on earth that suggest the presence of ice rich permafrost these include thermal contraction polygonal networks scalloped terrains similar to thermokarst pits debris flows small mounds like pingos and rock glaciers here we address questions concerning the influence of meltwater in the utopia planitia up landscape using analogs of near surface melting and drainage along ice wedge troughs on bylot island northern canada in utopia planitia based on the identification of sinuous channel like pits within polygonal networks we suggest that episodic underground melting was possible under severe periglacial climate conditions in up the collapse pattern and morphology of unconnected sinuous elongated pits that follow the polygon crack are similar to underground melting in bylot island nunavut canada based on this terrestrial analogue we develop a thermal model that consists of a thick insulating dusty layer over ice saturated dust during a period of slight climatic warming relative to today s climate in the model the melting point is reached at depths down to 150 m we suggest that small scale melting could have occurred below ground within ground ice polygonal fractures and pooled in underground cavities then the water may have been released episodically causing mechanical erosion as well as undermining and collapse after melting the dry surface dusty layer might have been blown away thus exposing the degraded terrain of the substrate layer

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

tropical mountain glaciers on mars altitude dependence of ice accumulation accumulation conditions formation times glacier dynamics and implications for planetary spin axis orbital history abstract fan shaped deposits up to 166 000 km 2 in area are found on the northwest flanks of the huge tharsis montes volcanoes in the tropics of mars recent spacecraft data have confirmed earlier hypotheses that these lobate deposits are glacial in origin increased knowledge of polar latitude terrestrial glacial analogs in the antarctic dry valleys has been used to show that the lobate deposits are the remnants of cold based glaciers that formed in the extremely cold hyper arid climate of mars mars atmospheric general circulation models gcm show that these glaciers could form during periods of high obliquity when upwelling and adiabatic cooling of moist air favor deposition of snow on the northwest flanks of the tharsis montes we present a simulation of the tharsis montes ice sheets produced by a static accumulation pattern based on the gcm results and compare this with the nature and extent of the geologic deposits we use the fundamental differences between the atmospheric snow accumulation environments mass balance on earth and mars geological observations and ice sheet models to show that two equilibrium lines should characterize ice sheet mass balance on mars and that glacial accumulation should be favored on the flanks of large volcanoes not on their summits as seen on earth predicted accumulation rates from such a parameterization together with sample spin axis obliquity histories are used to show that obliquity in excess of 45 and multiple 120 000 year obliquity cycles are necessary to produce the observed deposits our results indicate that the formation of these deposits required multiple successive stages of advance and retreat before their full extent could be reached and thus imply that spin axis obliquity remained at these high values for millions of years during the late amazonian period of mars history spin axis obliquity is one of the main factors in the distribution and intensity of solar insolation and thus in determining the climate history of mars unfortunately reconstruction of past climate history is inhibited by the fact that the chaotic nature of the solution makes the calculation of orbital histories unreliable prior to about 20 ma ago we show however that the geological record combined with glacial modeling can be used to provide insight into the nature of the spin axis orbital history of mars in the late amazonian and to begin to establish data points for the geologically based reconstruction of the climate and orbital history of mars

support of long wavelength loads on venus and implications for internal structure a thin shell formulation for computing stress distributions on the surfaces of one plate planets is developed the lithospheric model includes laterally varying density anomalies at two depths corresponding to undulations on a crust mantle boundary and variations in upper mantle density for a given set of model parameters values for the vertical displacement crustal thickness anomaly and mantle density anomaly required to satisfy the topographic and gravitational boundary conditions are computed along with the resulting stress field the computed stresses can then be compared to stress directions inferred from observed tectonic features this formulation is used to study loads on venus s lithosphere and it is found that most highland areas appear to have been formed in a state of isostasy or by uplift with a significant portion of their support probably derived from dynamic processes in the mantle a possible exception is ishtar terra whose circumferential mountain belts are most consistent with a radially compressive stress regime generated by flexural loading reasonable lithospheric models predict low density mantle beneath atla beta eisila thetis and tethus regiones and thick crust accompanied by higher density mantle under ishtar terra and ovda regio thus western aphrodite terra appears to be more closely related to ishtar than to the rest of the equatorial highlands these results favor a hot spot model of venus tectonics in which beta eastern aphrodite eisila and tethus are expected to be relatively young and thermally active while ovda and ishtar should be older and less active

viscoelastic evolution of lunar multiring basins 1 we investigate the evolution of multiring basins on the moon the orbital geophysical and geochemical data collected by lunar prospector allow three distinct groups of basins to be identified group 1 basins show well preserved topography a bull s eye gravity signature and little mare basalt fill group 2 basins contain large amounts of mare basalt and show mare dominated gravity and topography signatures group 3 basins are characterized by low amplitude topographic relief and weak gravity anomalies while the crustal structures of group 1 and 2 basins appear to date from the collapse of the transient cavity group 3 basins are highly degraded and show very little crustal thinning using a viscoelastic model we explore the evolution of the lunar multiring basins with the goal of explaining the degraded condition of group 3 basins our results show that while viscous relaxation is a plausible explanation for the state of group 3 farside basins requiring heat flux 25 40 mw m 2 the thin nearside crust renders subsolidus ductile flow in the crust unlikely the existence of many degraded basins on the nearside points to a weakened and possibly partially molten lower crust that south pole aitken appears not to have relaxed suggests that similar conditions did not exist there and that the nearside was hotter than the farside early on this is likely a consequence of the nearside farside crustal thickness dichotomy which would have caused the farside to reach full crystallization first leaving the nearside with the remaining liquid and an excess of heat producing elements

ocean worlds in the outer solar system many outer solar system bodies are thought to harbor liquid water oceans beneath their ice shells this article first reviews how such oceans are detected we then discuss how they are maintained when they formed and what the oceans likely characteristics are we focus in particular on europa ganymede callisto titan and enceladus bodies for which there is direct evidence of subsurface oceans we also consider candidate ocean worlds such as pluto and triton

shallow radar sharad pedestal craters and the lost martian layers initial assessments 1 since their discovery martian pedestal craters have been interpreted as remnants of layers that were once regionally extensive but have since been mostly removed pedestals span from subkilometer to hundreds of kilometers but their thickness is less than 500 m except for a small equatorial concentration in the medusae fossae formation the nearly exclusive occurrence of pedestal craters in the middle and high latitudes of mars has led to the suspicion that the lost units bore a significant fraction of volatiles such as water ice recent morphological characterizations of pedestal deposits have further supported this view here we employ radar soundings obtained by the shallow radar sharad to investigate the volumes of a subset of the pedestal population in concert with high resolution imagery to assist our interpretations from the analysis of 97 pedestal craters we find that large pedestals diameter 30 km are relatively transparent to radar in their majority with sharad being able to detect the base of the pedestal deposits and possess an average dielectric permittivity of 4 0 5 in one of the cases of large pedestals in malea planum layering is detected both in sharad data and in high resolution imagery of the pedestal margins we find that clutter is a major issue in the analysis of radar soundings for smaller pedestals and tentative detection of the basal reflection occurs in only a few of the cases examined these detections yield a higher average permittivity of 6 the permittivity value derived for the larger pedestals for which a basal reflection is unambiguous is higher than that of pure water ice but lower than that of most silicate materials a mixture of ice and silicates or an ice free porous silicate matrix can explain a permittivity of 4 and radar alone cannot resolve this nonuniqueness data from the compact reconnaissance imaging spectrometer crism tentatively confirms a mafic component in at least one pedestal in malea planum interpretation of sharad results can support either a mixture of ice and silicates or a porous silicate the former is compatible with a model where nonpolar ice is periodically deposited in the midlatitudes as a result of obliquity variations the latter is compatible with ash deposits at least in where pedestals appear in volcanic centers such as malea planum

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

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

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

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

early mars climate near the noachian hesperian boundary independent evidence for cold conditions from basal melting of the south polar ice sheet dorsa argentea formation and implications for valley network formation currently and throughout much of the amazonian the mean annual surface temperatures of mars are so cold that basal melting does not occur in ice sheets and glaciers and they are cold based the documented evidence for extensive and well developed eskers sediment filled former sub glacial meltwater channels in the south circumpolar dorsa argentea formation is an indication that basal melting and wet based glaciation occurred at the south pole near the noachian hesperian boundary we employ glacial accumulation and ice flow models to distinguish between basal melting from bottom up heat sources elevated geothermal fluxes and top down induced basal melting elevated atmospheric temperatures warming the ice we show that under mean annual south polar atmospheric temperatures 100 c simulated in typical amazonian climate experiments and typical noachian hesperian geothermal heat fluxes 45 65mw m 2 south polar ice accumulations remain cold based in order to produce significant basal melting with these typical geothermal heat fluxes the mean annual south polar atmospheric temperatures must be raised from today s temperature at the surface 100 c to the range of 50 to 75 c this mean annual polar surface atmospheric temperature range implies lower latitude mean annual temperatures that are likely to be below the melting point of water and thus does not favor a warm and wet early mars seasonal temperatures at lower latitudes however could range above the melting point of water perhaps explaining the concurrent development of valley networks and open basin lakes in these areas this treatment provides an independent estimate of the polar and non polar surface temperatures near the noachian hesperian boundary of mars history and implies a cold and relatively dry mars climate similar to the antarctic dry valleys where seasonal melting forms transient streams and permanent ice covered lakes in an otherwise hyperarid hypothermal climate 2012 elsevier inc

the influence of crustal radioactivity on mantle convection and lithospheric thickness on mars orbital measurements of gravity and topography demonstrate that the crust of mars varies in thickness by about 100 km across the surface as a result the heat flux due to crustal radioactivity varies laterally by nearly 15 mw m 2 in an important new paper plesa et al 2016 doi 10 1002 2016je005126 have assessed how these variations in crustal thickness and heat flow affect the spatial pattern of convection in the martian mantle their results have important implications for the interpretation of the martian heat flux that will be measured by the upcoming insight mission to mars

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

lithospheric flexure and thermal anomalies plots of effective elastic thickness te of the oceanic lithosphere as a function of age when elastically deformed show a general increase in te with the square root of age although there is much scatter some of the scatter can be eliminated by correcting for the effect of finite yield strength in rocks which makes te for more sharply bent plates underestimate the true depth tm to the rheological boundary at the base of the high strength mechanical lithosphere after converting effective values te to true thicknesses tm it appears that the mechanical lithosphere is always thinner beneath islands and seamounts compared to lithosphere of similar age flexed at subduction zones from several other lines of evidence such as depth anomalies seamount subsidence and heat flow data it has been suggested that the thermal structure in plates subjected to hot spot type volcanism is reset to values appropriate for much younger lithosphere the reduction in mechanical plate thickness beneath hot spot volcanoes is also consistent with this concept of thermal rejuvenation since the strength of rocks decreases exponentially with rising temperature this dependence implies that variations in mechanical plate thickness can be used to map temperature anomalies in the lithosphere since the mechanical plate thickness depends principally upon the temperature at one depth specifically the depth at which the temperature reaches 550 600 c depending on strain rate flexural data are particularly sensitive indicators of the vertical structure of lithospheric reheating compared to quantities dependent on depth integrals of temperature such as swell height and subsidence the thermal rejuvenation hypothesis predicts that the flexural profile should mimic viscoelastic relaxation in the first 10 ma after loading if initially reheating is confined to depths close to and below the 550 c isotherm the flexural data combined with depth anomaly information for hawaii point to extremely rapid reheating localized in the lower lithosphere rejuvenation in the society islands on the other hand is compatible with slower methods of heat injection involving the entire lithospheric column

radar subsurface mapping of the polar layered deposits on mars 1 determining the three dimensional 3 d structure of the martian polar caps is fundamental to understanding their hydrologic history dynamic behavior past climatic changes and the underlying lithosphere and radar sounding is the most promising technique to do so we model the dielectric profile of the martian polar layered deposits pld apply it to a 1 d electromagnetic plane wave propagation model and calculate the relative strength of radar reflections produced by layering within these deposits variations in dust fraction with depth derive from albedo profiles of a northern polar trough and a radiative transfer model detection of the fine scale layering present in the polar deposits with sharad is likely under a wide range of possible dielectric properties of the ice and silicate inclusions as long as minimum inclusion volumetric fractions are larger than 10 3 our models indicate also that stratigraphic mapping of the pld is feasible with a vertical resolution of 20 m under ideal conditions penetration depths depend greatly on the dielectric properties of silicate inclusions reaching at least 250 m for ice rich in highly conductive altered basaltic dust to at least 2 km for weakly conducting silicate inclusions because of the wide parameter space and unconstrained composition interpretation of polar radar data will be best accomplished in conjunction with updated thermal models and other data sets such as the recently published results from orbital infrared remote sensing that impose limits on ice grain size and dust

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

time dependent rotational stability of dynamic planets with viscoelastic lithospheres we extend previous work to derive a non linear rotational stability theory governing true polar wander tpw of terrestrial planets with viscoelastic lithospheres we demonstrate analytically and using numerical examples that our expressions are consistent with previous results in the limiting cases of low and infinite i e purely elastic viscosity lithospheres to illustrate the stability theory we compute tpw on mars driven by a simple prescribed mass loading our calculations demonstrate that on short time scales relative to the relaxation time of the viscoelastic lithosphere the rotation axis follows a constrained path that reflects stabilization by remnant strength in the lithosphere but that on long times scales this stabilization disappears and the load ultimately reaches the equator earlier work based on the assumption of a permanent remnant bulge in the case of a purely elastic lithosphere has suggested that martian tpw would not persist for any significant time after a load is emplaced and thus an equilibrium stability theory is sufficient to model long term order 1 myr or longer polar motion of the planet our results suggest in contrast that tpw on mars can continue over time scales on the order of the relaxation time of the lithosphere after load emplacement for sufficiently high lithospheric viscosities this time scale may be comparable to the age of the planet

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

radar sounding evidence for buried glaciers in the southern mid latitudes of mars lobate features abutting massifs and escarpments in the middle latitudes of mars have been recognized in images for decades but their true nature has been controversial with hypotheses of origin such as ice lubricated debris flows or glaciers covered by a layer of surface debris these models imply an ice content ranging from minor and interstitial to massive and relatively pure soundings of these deposits in the eastern hellas region by the shallow radar on the mars reconnaissance orbiter reveal radar properties entirely consistent with massive water ice supporting the debris covered glacier hypothesis the results imply that these glaciers formed in a previous climate conducive to glaciation at middle latitudes such features may collectively represent the most extensive nonpolar ice yet recognized on mars

stability of foams in silicate melts bubble coalescence and the spontaneous disruption of high porosity foams in silicate melts are the result of physical expulsion of interpore melt syneresis leading to bubble coalescence and diffusive gas exchange between bubbles melt expulsion can be achieved either along films between pairs of bubbles or along plateau borders which represent the contacts between 3 or more bubbles theoretical evaluation of these mechanisms is confirmed by experimental results enabling us to quantify the relevant parameters and determine stable bubble size and critical film thickness in a foam as a function of melt viscosity surface tension and time foam stability is controlled primarily by melt viscosity and time melt transport leading to coalescence of bubbles proceeds along inter bubble films for smaller bubbles and along plateau borders for larger bubbles thus the average bubble size accelerates with time in silicate melts the diffusive gas expulsion out of a region of foam is effective only for water and even then only at small length scales as the diffusion of co2 is negligible the results of our analyses are applicable to studies of vesicularity of lavas melt degassing and eruption mechanisms

inverted fluvial features in the aeolis zephyria plana western medusae fossae formation mars evidence for post formation modification 1 the aeolis and zephyria plana contain the western most portion of the medusae fossae formation mff an enigmatic and extensive light toned deposit located in the martian equatorial region and dated from the hesperian to amazonian epochs this area hosts a large population of sinuous ridges srs interpreted as inverted fluvial features formed by precipitation indurated by chemical cementation buried by subsequent deposition and finally exhumed this interpretation of srs as uniformly fluvial represents a modification to an earlier hypothesis for one particular sr of possible glaciofluvial i e esker formation these srs provide a tool to investigate the degree and character of post fluvial modification processes in this region we combined digital terrain models made from context camera ctx and high resolution imaging science experiment hirise stereo image pairs with individual data points from the mars orbiter laser altimeter mola to estimate relief cross sectional profiles longitudinal profiles and slope directions of selected srs longitudinal profiles of several srs display undulations with amplitudes of up to order 100 m while some of the lower amplitude undulations may be due to differential erosion undulations having amplitudes in excess of sr relief require alternative explanations our combined morphologic and topographic analysis suggests that multiple post flow processes including compaction of the deposits and tectonic displacements have modified the original sr profiles specification of the type s and magnitudes of these modification processes will contribute to understanding both the potential of post flow modification of fluvial profiles elsewhere on mars as well as the nature and properties of the mff

radar soundings of the subsurface of mars the martian subsurface has been probed to kilometer depths by the mars advanced radar for subsurface and ionospheric sounding instrument aboard the mars express orbiter signals penetrate the polar layered deposits probably imaging the base of the deposits data from the northern lowlands of chryse planitia have revealed a shallowly buried quasi circular structure about 250 kilometers in diameter that is interpreted to be an impact basin in addition a planar reflector associated with the basin structure may indicate the presence of a low loss deposit that is more than 1 kilometer thick