antarctic snow accumulation mapped using polarization of 4 3 cm wavelength microwave emission different parts of antarctica receive different amounts of snowfall each year in this paper we map the variations of the mean annual snow accumulation across the ice sheet we also quantify the uncertainty in our estimates more objectively than has been possible for earlier maps the new map is produced using observations from satellites and ground based measurements after a logarithmic transformation these are combined using the geostatistical method of continuous part universal kriging to give an estimate of the snow accumulation within each cell of a rectangular grid covering antarctica we also derive spatial averages over the major drainage systems of the ice sheet along with their confidence intervals we obtain a value of 143 4 kg m 2 a 1 for the average rate of snow accumulation upon the grounded ice sheet of antarctica

geochemical diversity of shergottite basalts mixing and fractionation and their relation to mars surface basalts the chemical compositions of shergottite meteorites basaltic rocks from mars provide a broad view of the origins and differentiation of these martian magmas the shergottite basalts are subdivided based on their al contents high al basalts al 5 wt are distinct from low al basalts and olivine phyric basalts both with al 4 5 wt abundance ratios of highly incompatible elements e g th la are comparable in all the shergottites abundances of less incompatible elements e g ti lu hf in olivine phyric and low al basalts correlate well with each other but the element abundance ratios are not constant this suggests mixing between components both depleted and enriched high al shergottites deviate from these trends consistent with silicate mineral fractionation the depleted component is similar to the yamato 980459 magma approximately 67 crystal fractionation of this magma would yield a melt with trace element abundances like que 94201 the enriched component is like the parent magma for nwa 1068 approximately 30 crystal fractionation from it would yield a melt with trace element abundances like the los angeles shergottite this component mixing is consistent with radiogenic isotope and oxygen fugacity data these mixing relations are consistent with the compositions of many of the gusev crater basalts analyzed on mars by the spirit rover although with only a few elements to compare other mars basalts fall off the mixing relations e g wishstone at gusev gale crater rocks their compositions imply that basalt source areas in mars include significant complexities that are not present in the source areas for the shergottite basalts

effects of soil heterogeneity on martian ground ice stability and orbital estimates of ice table depth abstract data from the mars odyssey gamma ray spectrometer grs instrument suite and results from numerical simulations of subsurface ground ice stability have been used to estimate the depth of martian ground ice geographic correlation between these estimates is remarkable the relative ice table depth distributions also agree well however grs based estimates of ice table depth are generally deeper than predictions based on ground ice stability simulations this discrepancy may be related to heterogeneities in the martian surface such as rocks dust and albedo variations we develop a multi dimensional numerical model of ground ice stability in a heterogeneous martian subsurface and use it to place the first quantitative constraints on the response of the ice table to meter scale heterogeneities we find that heterogeneities produce significant undulations topography in the ice table at horizontal length scales of a few meters decimeter scale rocks create localized areas of deep ice producing a vertical depression of 10 60 cm in the ice table over a horizontal range of 1 2 rock radii decimeter scale dust lenses produce locally shallow ice however the magnitude of the vertical deflection of the ice table is small 1 4 cm the effects of decimeter scale albedo variations on the ice table are nearly negligible although albedo very weakly enhances the effects of dark rocks and bright dust on the ice table additionally we investigate the role played by rocks in estimates of ice table depth based on orbital data surface rocks can account for more than half of the discrepancy between ice table depths inferred from grs data and those predicted by theoretical ice stability simulations that utilize thermophysical observations our results have considerable relevance to the up coming mars scout mission phoenix because they indicate that the uncertainty in the ice table depth of a given region is greater than differences between current depth estimates likewise small scale depth variability due to heterogeneities at the eventual landing site is potentially greater than differences between current depth estimates

constraints on martian differentiation processes from rbsr and smnd isotopic analyses of the basaltic shergottite que 94201 isotopic analyses of mineral leachate and whole rock fractions from the martian shergottite meteorite que 94201 yield rb sr and sm nd crystallization ages of 327 12 and 327 19 ma respectively these ages are concordant although the isochrons are defined by different fractions within the meteorite comparison of isotope dilution sm and nd data for the various que 94201 fractions with in situ ion microprobe data for que 94201 minerals from the literature demonstrate the presence of a leachable crustal component in the meteorite this component is likely to have been added to que 94201 by secondary alteration processes on mars and can affect the isochrons by selectively altering the isotopic systematics of the leachates and some of the mineral fractions initial 87sr 86sr of 0 701298 14 nd143 of 47 6 1 7 and whole rock nd142 of 0 92 0 11 indicate that que 94201 was derived from a source that was strongly depleted in 87rb 86sr and enriched in 147sm 144nd early in its history modeling demonstrates that the sm nd isotopic compositions of que 94201 can be produced by either four episodes of melting at 327 ma of cumulates crystallized from a magma ocean at 4 525 ga or five episodes of melting of an initially solid mars at 4 525 ga and 327 ma the neodymium isotopic systematics of que 94201 are not consistent with significant melting between 4 525 ga and 327 ma the estimated timing of these events is based on initial neodymium isotopic ratios and is independent of differentiation of the que 94201 parental magma rb sr based partial melting models are unable to reproduce the composition of que 94201 using the same model parameters employed in the sm nd based models implying a decoupling of rb sr and sm nd isotopic systems the initial decoupling of the two isotopic systems can be attributed to either cumulate or crust formation processes which are able to more efficiently fractionate rb from sr compared to sm from nd the fact that all martian meteorites analyzed so far define a rb sr whole rock isochron age of 4 5 ga suggests that virtually all rb was partitioned out of their mantle source regions and into either fractionated residual liquids trapped in the cumulate pile or into the crust at that time thus the martian mantle cumulates and restites are not expected to evolve past 87sr86sr of 0 700 and could not have been significantly enriched in incompatible elements by crustal recycling processes all martian meteorites have initial 87sr86sr values that are higher than 0 700 and are therefore likely to be produced by mixing between evolved crustal like and depleted mantle reservoirs the absence of crustal recycling processes on mars may preserve the geochemical evidence for decoupling of the rb sr and sm nd isotopic systems underscoring one of the fundamental differences between geologic processes on mars and the earth

pb evolution in the martian mantle abstract the initial pb compositions of one enriched shergottite one intermediate shergottite two depleted shergottites and nakhla have been measured by secondary ion mass spectrometry sims these values in addition to data from previous studies using an identical analytical method performed on three enriched shergottites alh 84001 and chassigny are used to construct a unified and internally consistent model for the differentiation history of the martian mantle and crystallization ages for martian meteorites the differentiation history of the shergottites and nakhla chassigny are fundamentally different which is in agreement with short lived radiogenic isotope systematics the initial pb compositions of nakhla chassigny are best explained by the late addition of a pb enriched component with a primitive non radiogenic composition in contrast the pb isotopic compositions of the shergottite group indicate a relatively simple evolutionary history of the martian mantle that can be modeled based on recent results from the sm nd system the shergottites have been linked to a single mantle differentiation event at 4504 ma thus the shergottite pb isotopic model here reflects a two stage history 1 pre silicate differentiation 4504 ma and 2 post silicate differentiation to the age of eruption as determined by concordant radiogenic isochron ages the values 238u 204pb obtained for these two different stages of pb growth are 1 of 1 8 and a range of 2 from 1 4 4 7 respectively the 1 value of 1 8 is in broad agreement with enstatite and ordinary chondrites and that proposed for proto earth suggesting this is the initial value for inner solar system bodies when plotted against other source radiogenic isotopic variables sri 187 os e 143 nd and e 176 hf the second stage mantle evolution range in observed mantle values display excellent linear correlations r 2 0 85 and represent a spectrum of martian mantle mixing end members depleted intermediate enriched

lithium isotopes and light lithophile element abundances in shergottites evidence for both magmatic degassing and subsolidus diffusion degassed magmatic water was potentially the major source of surficial water on mars we measured li b and be abundances and li isotope profiles in pyroxenes olivines and maskelynite from four compositionally different shergottites shergotty que 94201 lar 06319 and tissint using secondary ion mass spectrometry sims all three light lithophile elements lle are incompatible li and b are soluble in h2o rich fluids whereas be is insoluble in the analyzed shergottites li concentration decreases and be concentration increases from cores to rims in pyroxenes however b concentrations do not vary consistently with li and be abundances except in que 94201 pyroxenes additionally abundances of these three elements in olivines show a normal igneous fractionation trend consistent with the crystallization of olivine before magma ascent and degassing we expect that kinetic effects would lead to fractionation of 6li in the vapor phase compared to 7li during degassing the li isotope profiles with increasing 7li from cores to rims as well as li and b profiles indicate possible degassing of hydrous fluids only for the depleted shergottite que 94201 as also supported by degassing models conversely shergotty lar 06319 and tissint appear to have been affected by postcrystallization diffusion based on their lle and li isotope profiles accompanied by diffusion models this process may represent an overlay on a degassing pattern the lle profiles and isotope profiles in que 94201 support the hypothesis that degassing of some basaltic shergottite magmas provided water to the martian surface although evidence may be obscured by subsolidus diffusion processes

fast numerical method for growth and retreat of subsurface ice on mars abstract subsurface water ice on mars evolves due to exchange of vapor with the atmosphere in the form of loss of ice to the atmosphere or in the form of the growth of interstitial ice described here is an accelerated numerical method for the long term evolution of subsurface ice this accelerated method is five orders of magnitude faster than explicit vapor transport calculations enabling fundamentally new types of climate models its speed matches that of purely thermal models the speedup is achieved primarily by solving time averaged equations for vapor transport and ice volume change processes incorporated are growth of interstitial pore ice retreat of pore ice retreat of an ice sheet and retreat of pore ice due to geothermal heating from below two example applications illustrate this numerical method s capabilities near the permafrost margin at 55 latitude ice is periodically depleted and slowly recharged leading to a pore ice layer estimated to be currently no more than a few meters thick at the phoenix landing site it shows the formation of a three layered structure whereby the layer of pore ice can be very thin

martian high latitude permafrost depth and surface cover thermal inertia distributions 1 martian high latitudes have thermal properties consistent with an extensive high thermal inertia permafrost layer near the surface surface cover thermal inertias and permafrost depths at martian high latitudes 50 80 n s are derived from thermal emission spectrometer tes data and compared with previously published water ice depths determined from the mars odyssey neutron spectrometer mons the depth to the permafrost layer is correlated with surface cover thermal inertia albedo and latitude in general agreement with predicted trends of water ice stability comparison of permafrost depths with water ice rich layer depths derived from mons data displays good qualitative agreement although a divergence is present at greater burial depths this disparity may be due to the presence of hydrated minerals at shallow depths or a lower than expected permafrost thermal inertia corresponding with low water ice concentrations at greater depths surface cover thermal inertias are greater in the northern high latitudes than in the south and differences between these and previous results will have significant effects on the predicted depth of martian water ice stability several regions in the northern hemisphere display high surface cover thermal inertia associated with possible receding water ice deposits that are shallow enough to influence diurnal surface temperatures significant lateral and vertical heterogeneity in water ice distributions are present and the martian regolith is likely more complicated than can be described by simple two layered models and a single mode of water ice emplacement

halogen and cl isotopic systematics in martian phosphates implications for the cl cycle and surface halogen reservoirs on mars abstract the cl isotopic compositions and halogen cl f br and i abundances in phosphates from eight martian meteorites spanning most rock types and ages currently available have been measured in situ by secondary ion mass spectrometry sims likewise the distribution of halogens has been documented by x ray mapping halogen concentrations range over several orders of magnitude up to some of the largest concentrations yet measured in martian samples or on the martian surface and the inter element ratios are highly variable similarly cl isotope compositions exhibit a larger range than all pristine terrestrial igneous rocks phosphates in ancient 4 ga meteorites orthopyroxenite alh 84001 and breccia nwa 7533 have positive 37 cl anomalies 1 1 to 2 5 these samples also exhibit explicit whole rock and grain scale evidence for hydrothermal or aqueous activity in contrast the phosphates in the younger basaltic shergottite meteorites 37 cl anomalies 0 2 to 5 6 phosphates with the largest negative 37 cl anomalies display zonation in which the rims of the grains are enriched in all halogens and have significantly more negative 37 cl anomalies suggestive of interaction with the surface of mars during the latest stages of basalt crystallization the phosphates with no textural major element or halogen enrichment evidence for mixing with this surface reservoir have an average 37 cl of 0 6 supporting a similar initial cl isotope composition for mars the earth and the moon oxidation and reduction of chlorine are the only processes known to strongly fractionate cl isotopes both positively and negatively and perchlorate has been detected in weight percent concentrations on the martian surface the age range and obvious mixing history of the phosphates studied here suggest perchlorate formation and halogen cycling via brines which have been documented on the martian surface has been active throughout martian history

chronology and petrogenesis of young achondrites shergotty zagami and alha77005 late magmatism on a geologically active planet a study was undertaken to determine the chronology petrogenesis and relationships among the shergottites shergotty and zagami and the unique achondrite alha77005 these meteorites are the product of a variety of complex processes petrogenesis chondrite normalized abundance patterns of shergotty and zagami are very similar and show pronounced depletions of both the light ree la nd and heavy ree dy lu relative to sm gd these characteristic depletions are even more pronounced for alha77005 the light ree depletion is qualitatively consistent with the presence of cumulus pyroxene and or olivine in these meteorites but trace element models show that the parental magmas of all three meteorites were probably also light ree depleted both trace element model calculations and combined rb sr and sm nd isotopic systematics show that the meteorites could not have been co magmatic nor can alha77005 be representative of the source material of the shergottites light ree depletion of the parental magmas also implies light ree depletion of the source material the sm nd systematics of the shergottites require a time averaged sub chondritic light ree enriched sm nd ratio since 4 6 ae ago the sm nd systematics of alha77005 permit a time averaged super chondritic light ree depleted sm nd ratio if its crystallization age is less than tice 0 72 ae chronology rb sr internal isochrons for all three meteorites and a sm nd internal isochron for zagami are concordant at 180 myr 39ar 40ar plateau ages of shergotty and zagami maskelynite are 250 260 myr these ages apparently reflect resetting of these isotopic systems by shock metamorphism which converted the feldspar to maskelynite the concordance of these ages suggests a single shock event during which the meteorites were in close physical proximity the time of this event is most precisely given by the rb sr age of 180 4 myr for zagami the crystallization ages of the meteorites were not precisely determined extreme upper limits are determined by sm nd model ages relative to an eucrite initial 143nd144nd 0 505835 at 4 6 ae ago these model ages for shergotty zagami and alha77005 are 3600 3500 and 2850 myr respectively the sm nd whole rock age of 1340 60 myr for the three meteorites gives the crystallization age if the sm nd ratios of the precursor materials were always the same we consider this 1340 myr age as a best estimate upper limit best estimate lower limits for shergotty and zagami are taken from the average 39ar 40ar ages of 1200 and 900 myr of pyroxene separates the average 39ar 40ar age of a whole rock sample of alha77005 was 1600 myr and can be partitioned between a low temperature feldspar phase and a high temperature olivine pyroxene inclusions phase the average apparent 39ar 40ar age of the low temperature phase is 1050 myr which is chosen as the best estimate lower limit to the age the crystallization ages of shergotty zagami and alha77005 probably lie within the ranges of 1200 1300 900 1300 and 1000 1300 myr respectively the rb sr whole rock age of 4400 400 myr and single stage babi model ages of 4800 5100 myr are interpreted as reflecting differentiation of the parent body at 4600 myr ago the complex geochemical and isotopic evolution recorded by these meteorites suggests a geologically active parent body capable of sustaining melting at two or more epochs in its history

water on mars insights from apatite in regolith breccia northwest africa 7034 abstract determining the source of planetary water from the hydrogen isotope compositions of crustal samples is complicated by the overprinting of isotopically diverse source material by geologic and atmospheric processes as mars has no plate tectonics crustal material which may have isotopically exchanged with the martian atmosphere is not recycled into the mantle keeping the water reservoirs in the mantle and atmosphere mostly isolated buffered by the crust as the only known martian samples that are regolith breccias with a composition representative of the average crust of mars northwest africa nwa 7034 and its paired stones provide an important opportunity to investigate the water content and hydrogen isotope composition of the martian crust in particular apatites in distinct clasts as well as the brecciated matrix of nwa 7034 record a complex history including magmatic and impact processes and exchange with crustal fluids

evidence in tissint for recent subsurface water on mars we report unambiguous chemical evidence for subsurface water activity in the martian crust at 600 ma based on the data from tissint a fresh martian meteorite fall with minimal terrestrial weathering the impact melt pockets in tissint contain abundant volatiles h 2o co 2 f and cl and their concentrations are positively correlated with each other higher h 2o concentrations also accompany higher deuterium contents these correlations suggest mixing between two volatile sources the first source is h 2o in the precursor basalt inherited from martian magma magmatic h 2o in the basalt had low deuterium concentration and was likely stored in the nominally anhydrous minerals this source contributed little co 2 or halogens to the impact melts the second source is inferred to be aqueous alteration products introduced to the basalt by water activity after the basalt erupted these alteration materials contributed more volatiles to the impact melts than the magmatic source and had high deuterium abundance reflecting isotope equilibrium with recent martian atmosphere the water activities occurred beneath the martian surface after 600 ma crystallization age but before 1 ma ejection age the chemical and isotopic signatures of the alteration products in tissint resemble previously known martian samples associated with old water activities on mars which can be traced back to 4 2 billion years ago e g the mudstone at gale crater this similarity in chemistry and the wide age span indicate that such water activities were common on mars throughout its history which had the potential to form habitable environment however the rarity of the volatile rich zone in tissint suggests that martian crustal aqueous processes where they have occurred are generally limited in their extent of water rock reaction

searching for the source regions of martian meteorites using mgs tes integrating martian meteorites into the global distribution of igneous materials on mars the objective of this study was to identify and map possible source regions for all 5 known martian meteorite lithologies basalt lherzolite clinopyroxenite orthopyroxenite and dunite using data from the mars global surveyor thermal emission spectrometer mgs tes we deconvolved the tes data set using laboratory spectra of 6 martian meteorites los angeles zagami alh a77005 nakhla alh 84001 and chassigny as end members along with atmospheric and surface spectra previously derived from tes data global maps 16 pixels degree of the distribution of each meteorite end member show that meteorite like compositions are not present at or above tes detectability limits over most of the planet s dust free regions however we have confidently identified local scale 100s1000s km2 concentrations of olivine and orthopyroxene bearing materials similar to alh a77005 chassigny and alh 84001 in nili fossae in and near ganges chasma in the argyre and hellas basin rims and in eos chasma nakhla like materials are identified near the detection limit throughout the eastern valles marineris region and portions of syrtis major basaltic shergottites were not detected in any spatially coherent areas at the scale of this study martian meteorite like lithologies represent only a minor portion of the dust free surface and thus are not representative of the bulk composition of the ancient crust meteorite like spectral signatures identified above tes detectability limits in more spatially restricted areas tens of km are targets of ongoing analysis

tharsis sourced relatively dust free lavas and their possible relationship to martian meteorites abstract the launch sites for martian snc meteorites remain unknown but they must derive from young volcanic provinces we present the first geologic and spectroscopic characteristics of a suite of relatively dust free lava flows that extend from underneath the regional dust cover on the south flank of the tharsis volcanic province on mars the purpose of this work is to 1 characterize these flows geologically by using thermal emission imaging system themis data supplemented by mars orbital laser altimeter mola mars orbital camera moc and high resolution imaging science experiment hirise datasets and 2 use thermal emission spectrometer tes data in order to test the hypothesis that tharsis represents a source for the sncs these flows are morphologically similar to flood lavas in amazonis planitia with the flow surfaces being a complex mixture of surficial materials with limited exposure the spectrally derived mineral composition and geochemistry for three flows reveals basaltic compositions that have an affinity with sncs especially the basaltic shergottites however discrepancies between spectra and mineral percentages for the flows versus the shergottites together with the observation of surface deposits on the flow surfaces make any connection between these flows and the sncs tenuous although not impossible further testing of a tharsis snc relationship will require measurements at finer spatial resolutions perhaps even in situ

seasonal defrosting of the phoenix landing site 1 the retreat of the seasonal polar cap at the phoenix landing site is analyzed through observations of high resolution images and thermal modeling of the co2 frost mass numerical simulations indicate that the onset of co2 frost formation occurs around ls 220 at 68 n however the polar hood prevented imaging of the surface during this time in the late winter early spring a continuous layer of frost coated the surface obscuring the underlying polygonal patterns however rocks are clearly evident indicating that the frost depth was relatively shallow rare dark fans originating from rocks are also observed in an image from early spring frost dissipated from the polygon centers first and lingered longer in polygon troughs polygon centers were first exposed on ls 13 9 and the polygon troughs were frost free by ls 57 frost distributions varied slightly between geologic units however in general global observations at similar latitudes are consistent with what is seen in the phoenix region a series of hypotheses is also explored to explain the preference for the co2 frost to linger in the troughs lower thermal inertia material in the troughs aeolian redistribution of frost burial of the ice table via aeolian redistribution of dust and shadowing each of these hypotheses has drawbacks to explaining this phenomenon as stand alone components shadows do play a role in keeping the co2 frost on the surface however this effect is small and cannot alone account for the observations

modeling the effects of martian surface frost on ice table depth abstract ground ice has been observed in small fresh craters in the vicinity of the viking 2 lander site 48 n 134 e to explain these observations current models for ground ice invoke levels of atmospheric water of 20 precipitable micrometers higher than observations however surface frost has been observed at the viking 2 site and surface water frost and snow have been shown to have a stabilizing effect on antarctic subsurface ice a snow or frost cover provides a source of humidity that should reduce the water vapor gradient and hence retard the sublimation loss from subsurface ice we have modeled this effect for the viking 2 landing site with combined ground ice and surface frost models our model is driven by atmospheric output fields from the nasa ames mars general circulation model mgcm our modeling results show that the inclusion of a thin seasonal frost layer present for a duration similar to that observed by the viking lander 2 produces ice table depths that are significantly shallower than a model that omits surface frost when a maximum frost albedo of 0 35 was permitted seasonal frost is present in our model from ls 182 to ls 16 resulting in an ice table depth of 64 cm which is 24 cm shallower than the frost free scenario the computed ice table depth is only slightly sensitive to the assumed maximum frost albedo or thickness in the model

petrology and geochemistry of olivine phyric shergottites lar 12095 and lar 12240 implications for their petrogenetic history on mars larkman nunatak lar 12095 and lar 12240 are recent olivine phyric shergottite lnds we report the results of petrographic and chemical analyses of these two samples to understand their petrogenesis on mars based on our analyses we suggest that these samples are likely paired and are most similar to other depleted olivine phyric shergottites particularly dar al gani dag 476 and sayh al uhaymir sau 005 and samples paired with those the olivine megacryst cores in lar 12095 and lar 12240 are not in equilibrium with the groundmass olivines we infer that these megacrysts are phenocrysts and their major element compositions have been homogenized by diffusion the cores of the olivine megacrysts have mg 70 whereas megacryst rims and groundmass olivines typically have mg 58 60 the rare earth element ree microdistributions in the various phases olivine low and high ca pyroxene maskelynite and merrillite in both samples are similar and support the likelihood that these two shergottites are indeed paired the calculated parent melt i e in equilibrium with the low ca pyroxene which is one of the earliest formed ree bearing minerals has an ree pattern parallel to that of melt in equilibrium with merrillite i e one of the last formed minerals this suggests that the lar 12095 12240 paired shergottites represent the product of closed system fractional crystallization following magma emplacement and crystal accumulation utilizing the europium oxybarometer we estimate that the magmatic oxygen fugacity early in the crystallization sequence was iw finally petrographic evidence indicates that lar 12095 12240 experienced extensive shock prior to being ejected from mars

chronological evidence that the moon is either young or did not have a global magma ocean a new age determination for a moon rock collected close to the site of the apollo 16 lander has yielded a surprisingly young age of around 4 360 million years this age would require either that the moon solidified some 200 million years after the formation of the solar system significantly later than is assumed by most lunar formation models or that the long held theory that the moon once had a primordial global magma ocean is flawed in the latter case much of the lunar crust could have been produced by non magma ocean processes such as serial magmatism chemical evolution of planetary bodies ranging from asteroids to the large rocky planets is thought to begin with differentiation through solidification of magma oceans many hundreds of kilometres in depth1 2 3 the earth s moon is the archetypical example of this type of differentiation evidence for a lunar magma ocean is derived largely from the widespread distribution compositional and mineralogical characteristics and ancient ages inferred for the ferroan anorthosite fan suite of lunar crustal rocks the fans are considered to be primary lunar flotation cumulate crust that crystallized in the latter stages of magma ocean solidification according to this theory fans represent the oldest lunar crustal rock type2 3 4 attempts to date this rock suite have yielded ambiguous results however because individual isochron measurements are typically incompatible with the geochemical make up of the samples and have not been confirmed by additional isotopic systems5 6 7 8 9 by making improvements to the standard isotopic techniques we report here the age of crystallization of fan 60025 using the 207pb 206pb 147sm 143nd and 146sm 142nd isotopic systems to be 4 360 3 million years this extraordinarily young age requires that either the moon solidified significantly later than most previous estimates or the long held assumption that fans are flotation cumulates of a primordial magma ocean is incorrect if the latter is correct then much of the lunar crust may have been produced by non magma ocean processes such as serial magmatism10

high resolution subsurface water ice distributions on mars theoretical models predict that the depth at which water ice is stable in the subsurface of mars will vary due to local surface features such as rocks and slopes and the thermal inertia of the ground above the water so far measurements have been limited to overviews of the several hundred kilometre footprint covered by the ray spectrometer on board the mars odyssey spacecraft but joshua bandfield has now used the seasonal temperature response of the martian surface from odyssey s themis package to map water ice distribution at the sub kilometre scale the observations reveal significant regional and local water ice depth variability at potential landing sites for the mars phoenix mission due for launch later this year the phoenix lander s robotic arm will be able to dig up to half a metre or so into the soil to search for water ice and these results suggest that it may encounter considerable variation in the depth to the ice layer the seasonal temperature response of the martian surface is used to map local surface heterogeneities such as rocks slopes and thermal inertia of ground cover at subkilometre scale these observations show significant regional and local water ice depth variability and indicate that the depth to the water ice table is highly variable in the potential phoenix spacecraft landing ellipses theoretical models indicate that water ice is stable in the shallow subsurface depths of 1 2 m of mars at high latitudes1 2 3 4 5 6 7 these models have been mainly supported by the observed presence of large concentrations of hydrogen detected by the gamma ray spectrometer suite of instruments on the mars odyssey spacecraft8 9 10 the models and measurements are consistent with a water ice table that steadily increases in depth with decreasing latitude more detailed modelling has predicted that the depth at which water ice is stable can be highly variable owing to local surface heterogeneities such as rocks and slopes and the thermal inertia of the ground cover11 12 13 measurements have however been limited to the footprint several hundred kilometres of the gamma ray spectrometer suite preventing the observations from documenting more detailed water ice distributions here i show that by observing the seasonal temperature response of the martian surface with the thermal emission imaging system on the mars odyssey spacecraft14 it is possible to observe such heterogeneities at subkilometre scale these observations show significant regional and local water ice depth variability and in some cases support distributions in the subsurface predicted by atmospheric exchange and vapour diffusion models the presence of water ice where it follows the depth of stability under current climatic conditions implies an active martian water cycle that responds to orbit driven climate cycles15 16 17 several regions also have apparent deviations from the theoretical stability level indicating that additional factors influence the ice table depth the high resolution measurements show that the depth to the water ice table is highly variable within the potential phoenix spacecraft landing ellipses and is likely to be variable at scales that may be sampled by the spacecraft

in situ analysis of ice table depth variations in the vicinity of small rocks at the phoenix landing site 1 prior to the phoenix mission ice stability simulations predicted that surface rocks at the landing site would produce depressions in the ice table the boundary between dry and ice cemented soil if ground ice at the site were in diffusive equilibrium with atmospheric water vapor we use stereo images and digital elevation models to investigate variations in the depth of the ice table near two decimeter scale rocks at the phoenix landing site and compare the observed depth variations to ice stability simulations we find that ice table depth variations observed near rocks at the landing site are consistent with our theoretical predictions based on our results we conclude that 1 water transport in the near surface regolith occurs primarily via vapor diffusion 2 environmental conditions in which water transport is dominated by diffusion have been persistent at the landing site for 100s to 1000s of years or longer that is ground ice is in diffusive equilibrium with atmospheric water vapor and 3 in diffusive equilibrium thermal stability dictates the depth of the ice table on horizontal length scales as small as centimeters

ice table depth variability near small rocks at the phoenix landing site mars a pre landing assessment abstract we combine thermal simulations of ground ice stability near small rocks with extrapolations of the abundance of rocks at the phoenix landing site based on hirise rock counts to estimate the degree of ice table depth variability within the 3 8 m 2 workspace that can be excavated during the mission detailed predictions of this kind are important both to test current ground ice theory and to optimize soil investigations after landing we find that phoenix will very likely have access to at least one rock in the diameter range 5 cm to 1 m our simulations which assume the ice to be in diffusive equilibrium with atmospheric water vapor indicate that all rocks in this size range are associated with an annulus of deep ice free soil ice table depth variability of 1 5 cm is very likely at the landing site due to the presence of small rocks further there are scenarios in which phoenix might exploit the presence of individual large rocks and or the arrangement of small rocks to sample soils at depths 10 cm below the average depth predicted from orbit 4 cm scale analysis to constrain uncertainties in simulation results indicates that estimates of maximum depths may be somewhat conservative and that ice table depressions associated with individual rocks could be deeper and laterally more extended than indicated by formal predictions by mm to cm

results from the mars phoenix lander robotic arm experiment 1 the mars phoenix lander was equipped with a 2 4 m robotic arm ra with an icy soil acquisition device capable of excavating trenches in soil deposits grooming hard icy soil surfaces with a scraper blade and acquiring icy soil samples using a rasp tool a camera capable of imaging the scoop interior and a thermal and electrical conductivity probe were also included on the ra a dozen trench complexes were excavated at the northern plains landing site and 31 samples including water ice bearing soils were acquired for delivery to instruments on the lander during the 152 sol mission deliveries included sprinkling material from several centimeters height to break up cloddy soils on impact with instrument portals excavations were done on the side of the humpty dumpty and the top of the wonderland polygons and in nearby troughs resistive forces encountered during backhoe operations show that soils above the 3 5 cm deep icy soil interfaces are stronger with increasing depth further soils are similar in appearance and properties to the weakly cohesive crusty and cloddy soils imaged and excavated by the viking lander 2 which also landed on the northern plains adsorbed h2o is inferred to be responsible for the variable nature and cohesive strength of the soils backhoe blade chatter marks on excavated icy soil surfaces combined with rasp motor currents are consistent with laboratory experiments using grain supported icy soil deposits as is the relatively rapid decrease in icy soil strength over time as the ice sublimated on mars

the early differentiation of mars inferred from hf w chronometry abstract mars probably accreted within the first 10 million years of solar system formation and likely underwent magma ocean crystallization and crust formation soon thereafter to assess the nature and timescales of these large scale mantle differentiation processes we applied the short lived 182 hf 182 w and 146 sm 142 nd chronometers to a comprehensive suite of martian meteorites including several shergottites augite basalt nwa 8159 orthopyroxenite alh 84001 and polymict breccia nwa 7034 compared to previous studies the 182 w data are significantly more precise and have been obtained for a more diverse suite of martian meteorites ranging from samples from highly depleted to highly enriched mantle and crustal sources our results show that martian meteorites exhibit widespread 182 w 184 w variations that are broadly correlated with 142 nd 144 nd implying that silicate differentiation and not core formation is the main cause of the observed 182 w 184 w differences the combined 182 w 142 nd systematics are best explained by magma ocean crystallization on mars within 20 25 million years after solar system formation followed by crust formation 15 million years later these ages are indistinguishable from the i pu xe age for the formation of mars atmosphere indicating that the major differentiation of mars into mantle crust and atmosphere occurred between 20 and 40 million years after solar system formation and hence earlier than previously inferred based on sm nd chronometry alone

wood taphonomy in a tropical marine carbonate environment experimental results from lee stocking island bahamas abstract both teredinid bivales and limnorid isopods attacked wood that was deployed experimentally for two years in a carbonate environment on average wood samples collected after one year on the sea floor lost 17 of their original surface area to limnoria and 50 of their original volume to teredinids after two years on the sea floor wood samples lost an average of 39 of their original surface and 74 of their original volume all samples of kiln dried lumber pinus sequoia araucaria magnolia and quercus experienced significantly more attack by teredinids and limnoria than samples of quercus stellata post oak branch wood that experienced terrestrial decomposition in a warm temperate environment for five years before deployment on the sea floor taphonomic loss rates for wood in marine environments based on kiln dried lumber may overestimate loss rates for natural wood entering marine environments after initial decomposition in terrestrial environments burial appears to play an important role in wood taphonomy burial to a depth of 3 cm inhibited destruction by limnoria but not teredinids possibly due to the greater tolerance of molluscs for low o 2 levels and the ability of teredinids to raise their siphons above the wood surface both limnorids and teredinids may contribute to the formation of teredolites disaggregated sediment filled teredinid borings limnorid isopods remove the surface of wood exposing the calcite lined borings of teredinids once their calcite lined borings are exposed teredinid are susceptible to predation and in the aftermath of predation teredinid borings may become sediment filled exposure also causes the calcite lined borings to break and disaggregate leading to the formation of teredolites the fossil record of teredinids reaches back to the jurassic limnoria traces in the surface of wood are distinctive and diagnostic at the species level however limnoria have a sparse fossil record

high resolution thermal inertia mapping from the mars global surveyor thermal emission spectrometer abstract high resolution thermal inertia mapping results are presented derived from mars global surveyor mgs thermal emission spectrometer tes observations of the surface temperature of mars obtained during the early portion of the mgs mapping mission thermal inertia is the key property controlling the diurnal surface temperature variations and is dependent on the physical character of the top few centimeters of the surface it represents a complex combination of particle size rock abundance exposures of bedrock and degree of induration in this work we describe the derivation of thermal inertia from tes data present global scale analysis and place these results into context with earlier work a global map of nighttime thermal bolometer based thermal inertia is presented at 1 4 per pixel resolution with approximately 63 coverage between 50 s and 70 n latitude global analysis shows a similar pattern of high and low thermal inertia as seen in previous viking low resolution mapping significantly more detail is present in the high resolution tes thermal inertia this detail represents horizontal small scale variability in the nature of the surface correlation with albedo indicates the presence of a previously undiscovered surface unit of moderate to high thermal inertia and intermediate albedo this new unit has a modal peak thermal inertia of 180 250 j m 2 k 1 s 1 2 and a narrow range of albedo near 0 24 the unit covering a significant fraction of the surface typically surrounds the low thermal inertia regions and may comprise a deposit of indurated fine material local 3 km resolution maps are also presented as examples of eolian fluvial and volcanic geology some impact crater rims and intracrater dunes show higher thermal inertias than the surrounding terrain thermal inertia of aeolian deposits such as intracrater dunes may be related to average particle size outflow channels and valleys consistently show higher thermal inertias than the surrounding terrain generally correlations between spatial variations in thermal inertia and geologic features suggest a relationship between the hundred meter scale morphology and the centimeter scale surface layer

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

phoenix soil physical properties investigation 1 the phoenix lander investigated the polygonal terrain and associated soil and icy soil deposits of a high northern latitude site on mars the soil physical properties component involved the analysis of force data determined from motor currents from the robotic arm ra s trenching activity using this information and images of the landing site soil cohesion and angle of internal friction were determined dump pile slopes were used to determine the angle of internal friction of the soil 38 5 additionally an excavation model that treated walls and edges of the scoop as retaining walls was used to calculate mean soil cohesions for several trenches in the phoenix landing site workspace these cohesions were found to be consistent with the stability of steep trench slopes cohesions varied from 0 2 0 4kpa to 1 2 1 8 kpa with the exception of a subsurface platy horizon unique to a shallow trough for which cohesion will have to be determined using other methods soil on polygon mounds had the greatest cohesion 1 2 1 8 kpa this was most likely due to the presence of adsorbed water or pore ice above the shallow icy soil surface further evidence for enhanced cohesion above the ice table includes lateral increase in excavation force by over 30 n as the ra approached ice

the age of the martian meteorite northwest africa 1195 and the differentiation history of the shergottites samarium neodymium isotopic analyses of unleached and acid leached mineral fractions from the recently identified olivine bearing shergottite northwest africa 1195 yield a crystallization age of 347 13 ma and an end143 value of 40 1 0 9 maskelynite fractions do not lie on the sm nd isochron and appear to contain a martian surface component with low 147sm 144nd and 143nd 144nd ratios that was added during shock the rb sr system is disturbed and does not yield an isochron terrestrial sr appears to have affected all of the mineral fractions although a maximum initial 87sr 86sr ratio of 0 7016 is estimated by passing a 347 ma reference line through the maskelynite fraction that is least affected by contamination the high initial end143 value and the low initial 87sr 86sr ratio combined with the geologically young crystallization age indicate that northwest africa 1195 is derived from a source region characterized by a long term incompatible element depletion the age and initial sr and nd isotopic compositions of northwest africa 1195 are very similar to those of queen alexandra range 94201 indicating these samples were derived from source regions with similar sr nd isotopic systematics these similarities suggest that these two meteorites share a close petrogenetic relationship and might have been erupted from a common volcano the meteorites yamato 980459 dar al gani 476 sayh al uhaymir 005 008 and dhofar 019 also have relatively old ages between 474 and 575 ma and trace element and or isotopic systematics that are indicative of derivation from incompatible element depleted sources this suggests that the oldest group of meteorites is more closely related to one another than they are to the younger meteorites that are derived from less incompatible element depleted sources closed system fractional crystallization of this suite of meteorites is modeled with the melts algorithm using the bulk composition of yamato 980459 as a parent these models reproduce many of the major element and mineralogical variations observed in the suite in addition the rare earth element systematics of these meteorites are reproduced by fractional crystallization using the proportions of phases and extents of crystallization that are calculated by melts other shergottites that demonstrate enrichments in incompatible elements and have evolved sr and nd isotopic systematics have some geochemical systematics that are similar to those observed in the depleted group most notably although they exhibit a very limited range of incompatible trace element and isotopic compositions they have highly variable major element compositions this is also consistent with evolution from a common mantle source region by variable amounts of fractional crystallization if this scenario is correct it suggests that the combined effects of source composition and fractional crystallization are likely to account for the major element trace element and isotopic diversity of all shergottites

preservation of late amazonian mars ice and water related deposits in a unique crater environment in noachis terra age relationships between lobate debris tongues and gullies abstract the amazonian period of mars has been described as static cold and dry recent analysis of high resolution imagery of equatorial and mid latitude regions has revealed an array of young landforms produced in association with ice and liquid water because near surface ice in these regions is currently unstable these ice and water related landforms suggest one or more episodes of martian climate change during the amazonian here we report on the origin and evolution of valley systems within a degraded crater in noachis terra asimov crater the valleys have produced a unique environment in which to study the geomorphic signals of amazonian climate change new high resolution images reveal hesperian aged layered basalt with distinctive columnar jointing capping interior crater fill and providing debris via mass wasting for the surrounding annular valleys the occurrence of steep slopes 20 relatively narrow sheltered valleys and a source of debris have provided favorable conditions for the preservation of shallow ice deposits detailed mapping reveals morphological evidence for viscous ice flow in the form of several lobate debris tongues ldt superimposed on ldt are a series of fresh appearing gullies with typical alcove channel and fan morphologies the shift from ice rich viscous flow formation to gully erosion is best explained as a shift in martian climate from one compatible with excess snowfall and flow of ice rich deposits to one consistent with minor snow and gully formation available dating suggests that the climate transition occurred 8 ma prior to the formation of other small scale ice rich flow features identified elsewhere on mars that have been interpreted to have formed during the most recent phases of high obliquity taken together these older deposits suggest that multiple climatic shifts have occurred over the last tens of millions of years of martian history

accretion timescale and impact history of mars deduced from the isotopic systematics of martian meteorites abstract high precision sm nd isotopic analyses have been completed on a suite of 11 martian basaltic meteorites in order to better constrain the age of silicate differentiation on mars associated with the formation of their mantle sources these data are used to evaluate the merits and disadvantages of various mathematical approaches that have been employed in previous work on this topic ages determined from the sm nd isotopic systematics of individual samples are strongly dependent on the assumed nd isotopic composition of the bulk planet this assumption is problematic given differences observed between the nd isotopic composition of earth and chondritic meteorites and the fact that these materials are both commonly used to represent bulk planetary nd isotopic compositions ages determined from the slope of 146sm 142nd whole rock isochrons are not dependent on the assumed 142nd 144nd ratio of the planet but require the sample suite to be derived from complementary contemporaneously formed reservoirs in this work we present a mathematical expression that defines the age of formation of the source regions of such a suite of samples that is based solely on the slope of a 143nd 142nd whole rock isochron and is also independent of any a priori assumptions regarding the bulk isotopic composition of the planet this expression is also applicable to mineral isochrons and has been used to successfully calculate 143nd 142nd model crystallization ages of early refractory solids as well as lunar samples this permits ages to be obtained using only nd isotopic measurements without the need for 147sm 144nd isotope dilution determinations when used in conjunction with high precision nd isotopic measurements completed on martian meteorites this expression yields an age of formation of the martian basaltic meteorite source regions of 4504 6 ma because the sm nd model ages for the formation of martian source regions are commonly interpreted to record the age at which large scale mantle reservoirs formed during planetary differentiation associated with magma ocean solidification the age determined here implies that magma ocean solidification occurred several tens of millions of years after the beginning of the solar system recent thermal models however suggest that mars sized bodies cool rapidly in less than 5 ma after accretion ceases even in the presence of a thick atmosphere assuming these models are correct an extended period of accretion is necessary to provide a mechanism to keep portions of the martian mantle partially molten until 4504 ma late accretional heating of mars could either be associated with protracted accretion occurring at a quasi steady state or alternatively be associated with a late giant impact if this scenario is correct then accretion of mars sized bodies takes up to 60 ma and is likely to be contemporaneous with the core formation and possibly the onset of silicate differentiation this further challenges the concept that isotopic equilibrium is attained during primordial evolution of planets and may help to account for geochemical evidence implying addition of material into planetary interiors after core formation was completed

the origin of alteration orangettes in dhofar 019 implications for the age and aqueous history of the shergottites the shergottites are the largest group of martian meteorites and the only group that has not been found to contain definitive evidence of martian aqueous alteration given recent reports of current liquid water at the surface of mars this study aimed to investigate in detail the possibility of martian phyllosilicate within shergottite dhofar 019 optical and scanning electron microscopy followed by transmission electron microscopy confirmed the presence of alteration orangettes with a layered structure consisting of poorly ordered mg phyllosilicate and calcite these investigations identified maskelynite dissolution followed by mg phyllosilicate and calcite deposition within the dissolution pits as the method of orangette production the presence of celestine within the orangette layers the absence of shock dislocation features within calcite and the mg rich nature of the phyllosilicate all indicate a terrestrial origin for these features on dhofar 019

explosive erosion during the phoenix landing exposes subsurface water on mars abstract while steady thruster jets caused only modest surface erosion during previous spacecraft landings on the moon and mars the pulsed jets from the phoenix spacecraft led to extensive alteration of its landing site on the martian arctic exposed a large fraction of the subsurface water ice under the lander and led to the discovery of evidence for liquid saline water on mars here we report the discovery of the explosive erosion process that led to this extensive erosion we show that the impingement of supersonic pulsed jets fluidizes porous soils and forms cyclic shock waves which propagate through the soil and produce erosion rates more than an order of magnitude larger than that of other jet induced processes the understanding of explosive erosion allows the calculation of bulk physical properties of the soils altered by it provides insight into a new behavior of granular flow at extreme conditions and explains the rapid alteration of the phoenix landing site s ground morphology at the northern arctic plains of mars

unique meteorite from early amazonian mars water rich basaltic breccia northwest africa 7034 we report data on the martian meteorite northwest africa nwa 7034 which shares some petrologic and geochemical characteristics with known martian meteorites of the snc i e shergottite nakhlite and chassignite group but also has some unique characteristics that would exclude it from that group nwa 7034 is a geochemically enriched crustal rock compositionally similar to basalts and average martian crust measured by recent rover and orbiter missions it formed 2 089 0 081 billion years ago during the early amazonian epoch in mars9 geologic history nwa 7034 has an order of magnitude more indigenous water than most snc meteorites with up to 6000 parts per million extraterrestrial h2o released during stepped heating it also has bulk oxygen isotope values of 17o 0 58 0 05 per mil and a heat released water oxygen isotope average value of 17o 0 330 0 011 per mil suggesting the existence of multiple oxygen reservoirs on mars

apparent thermal inertia and the surface heterogeneity of mars abstract thermal inertia derivation techniques generally assume that surface properties are uniform at horizontal scales below the footprint of the observing instrument and to depths of several decimeters consequently surfaces with horizontal or vertical heterogeneity may yield apparent thermal inertia which varies with time of day and season to investigate these temporal variations we processed three mars years of mars global surveyor thermal emission spectrometer observations and produced global nightside and dayside seasonal maps of apparent thermal inertia these maps show broad regions with diurnal and seasonal differences up to 200 j m 2 k 1 s 1 2 at mid latitudes 60 s to 60 n and 600 j m 2 k 1 s 1 2 or greater in the polar regions we compared the seasonal mapping results with modeled apparent thermal inertia and created new maps of surface heterogeneity at 5 resolution delineating regions that have thermal characteristics consistent with horizontal mixtures or layers of two materials the thermal behavior of most regions on mars appears to be dominated by layering with upper layers of higher thermal inertia e g duricrusts or desert pavements over fines prevailing in mid latitudes and upper layers of lower thermal inertia e g dust covered rock soils with an ice table at shallow depths prevailing in polar regions less common are regions dominated by horizontal mixtures such as those containing differing proportions of rocks sand dust and duricrust or surfaces with divergent local slopes other regions show thermal behavior that is more complex and not well represented by two component surface models these results have important implications for mars surface geology climate modeling landing site selection and other endeavors that employ thermal inertia as a tool for characterizing surface properties

subsurface ice on mars with rough topography high latitude ground ice on mars discovered by the gamma ray spectrometer suite is thought to be thermally stable owing to the presence of vapor in the martian atmosphere however local slopes can alter surface and subsurface temperatures substantially and hence allow ground ice to persist at locations where it would otherwise be unstable global statistics of the topography of mars are computed processed and extrapolated to derive a description of surface roughness on spatial scales to which ground ice should be sensitive this slope distribution is convolved with a new thermal model for the dependence of subsurface ice on slope to produce a prediction of the global ice distribution that includes the effect of topographic roughness in the highest latitudes slopes reduce the amount of buried ice while in lower latitudes the ice fraction increases widening the geographic boundary of the ice table at the high latitudes where ice is stable beneath horizontal ground the estimated reduction of ice is small compared to the existing ice volume areas in the midlatitudes with high surface roughness that have previously been predicted to be ice free are predicted to contain quantities of ice that may be detectable at present and accessible in the future slopes cause ground ice to be stable to latitudes of about 25 degrees in both hemispheres including for example areas within the northern olympus mons aureole deposits hecates tholus and hellas basin ice is unstable at equatorial latitudes even when accounting for surface slopes

martian cratering 8 isochron refinement and the chronology of mars abstract this paper reviews and refines the technique of dating martian surfaces by using impact crater isochrons defined as size distributions of impact craters on undisturbed martian surfaces of specified ages in the 1970s this system identified not only abundant ancient martian volcanic surfaces but also extensive lava plains with ages of a few 108 y old this dating was initially controversial but confirmed in the 1980s and 90s by martian meteorites the present update utilizes updated estimates of the mars moon cratering ratio the most important calibration factor improves treatment of gravity and impact velocity scaling effects combines aspects of the crater size distribution data from earlier work by both neukum and hartmann and for the first time applies a correction for loss of small meteoroids in the martian atmosphere from popova et al 2003 meteorit planet sci 38 905 925 the updated isochrons are not radically different from the previous 2002 iteration but fit observed data better and give somewhat older model ages for features dated from small craters diameter d 100 m crater counts from young lava flows in various areas give good fits to the new isochrons over as much as 3 orders of magnitude in d confirming the general isochron shape and giving crater retention ages in the range of some 106 to some 108 y interpreted as lava flow ages more complex older units are also discussed uncertainties are greatest if only small craters d 100 m are used suggestions by other workers of gross uncertainties due to local secondary craters and deposition exhumation are discussed they do not refute our conclusions of significant volcanic fluvial and other geologic activity in the last few percent of martian geologic time or the importance of cratering as a tool for studying processes such as exhumation indeed crater count data suggest certain very recent episodes of deposition exhumation and ice flow possibly associated with obliquity cycles of 10 7 y timescale evidence from ancient surfaces suggests higher rates of volcanism fluvial activity glaciation and other processes in noachian hesperian time than in amazonian time

effects of shock and martian alteration on tissint hydrogen isotope ratios and water content abstract the tissint meteorite a picritic shergottite fell to earth in morocco on the 18th of july 2011 and is only the fifth martian meteorite witnessed to fall hydrogen isotope ratios and water contents are variable within different minerals in tissint ringwoodite and shock melt pockets contain elevated d h ratios relative to terrestrial values d 761 4224 these high ratios in recrystallized phases indicate significant implantation of hydrogen from the d rich martian atmosphere during shock in contrast although olivine has detectable water abundances 230 485 ppm it exhibits much lower d h ratios d 88 to 150 suggesting this water was not implanted from the martian atmosphere the minimal terrestrial weathering experienced by tissint gives confidence that the olivine hosted water has a martian origin but its high concentration indicates direct inheritance from the parental melt is improbable especially given the low pressure of olivine crystallisation incorporation of a low d crustal fluid or deuteric alteration during crystallisation could explain the relatively high water contents and low d h ratios in tissint olivine

insights into the martian mantle the age and isotopics of the meteorite fall tissint the recent witnessed fall of the meteorite tissint represents the delivery of a pristine new sample from the surface of mars this meteorite provides an unprecedented opportunity to study a variety of aspects about the planet s evolution using the rb sr and sm nd isotopic systems we determined that tissint a depleted shergottite has a crystallization age of 574 20 ma an initial e 143 nd 42 2 0 5 and an initial 87 sr 86 sr 0 700760 11 these initial nd and sr isotopic compositions suggest that tissint originated from a mantle source on mars that is distinct from the source reservoirs of the other martian meteorites the known crystallization ages geochemical characteristics ejection ages and ejection dynamics of tissint and other similarly grouped martian meteorites suggest that they are likely derived from a source crater up to approximately 90 km in diameter with an age of approximately 1 ma that is located on terrain that is approximately 600 million years old

observational evidence of a suppressed planetary boundary layer in northern gale crater mars as seen by the navcam instrument onboard the mars science laboratory rover abstract the navigation cameras navcam of the mars science laboratory rover curiosity have been used to examine two aspects of the planetary boundary layer vertical dust distribution and dust devil frequency the vertical distribution of dust may be obtained by using observations of the distant crater rim to derive a line of sight optical depth within gale crater and comparing this optical depth to column optical depths obtained using mastcam observations of the solar disc the line of sight method consistently produces lower extinctions within the crater compared to the bulk atmosphere this suggests a relatively stable atmosphere in which dust may settle out leaving the air within the crater clearer than air above and explains the correlation in observed column opacity between the floor of gale crater and the higher elevation meridiani planum in the case of dust devils despite an extensive campaign only one optically thick vortex 1 5 0 5 10 3 was observed compared to 149 pressure events 0 5 pa observed in rems pressure data correcting for temporal coverage by rems and geographic coverage by navcam still suggests 104 vortices should have been viewable suggesting that most vortices are dustless additionally the most intense pressure excursions observed on other landing sites pressure drop 2 5 pa are lacking from the observations by the rems instrument taken together these observations are consistent with pre landing circulation modeling of the crater showing a suppressed shallow boundary layer they are further consistent with geological observations of dust that suggests the northern portion of the crater is a sink for dust in the current era

a prelanding assessment of the ice table depth and ground ice characteristics in martian permafrost at the phoenix landing site 1 we review multiple estimates of the ice table depth at potential phoenix landing sites and consider the possible state and distribution of subsurface ice a two layer model of ice rich material overlain by ice free material is consistent with both the observational and theoretical lines of evidence results indicate ground ice to be shallow and ubiquitous 2 6 cm below the surface undulations in the ice table depth are expected because of the thermodynamic effects of rocks slopes and soil variations on the scale of the phoenix lander and within the digging area which can be advantageous for analysis of both dry surficial soils and buried ice rich materials the ground ice at the ice table to be sampled by the phoenix lander is expected to be geologically young because of recent climate oscillations however estimates of the ratio of soil to ice in the ice rich subsurface layer suggest that that the ice content exceeds the available pore space which is difficult to reconcile with existing ground ice stability and dynamics models these high concentrations of ice may be the result of either the burial of surface snow during times of higher obliquity initially high porosity soils or the migration of water along thin films measurement of the d h ratio within the ice at the ice table and of the soil to ice ratio as well as imaging ice soil textures will help determine if the ice is indeed young and if the models of the effects of climate change on the ground ice are reasonable

the age of dar al gani 476 and the differentiation history of the martian meteorites inferred from their radiogenic isotopic systematics samarium neodymium isotopic analysis of the martian meteorite dar al gani 476 yields a crystallization age of 474 11 ma and an initial end143 value of 36 6 0 8 although the rb sr isotopic system has been disturbed by terrestrial weathering and therefore yields no age information an initial 87sr 86sr ratio of 0 701249 33 has been estimated using the rb sr isotopic composition of the maskelynite mineral fraction and the sm nd age the sr and nd isotopic systematics of dar al gani 476 like those of the basaltic shergottite que94201 are consistent with derivation from a source region that was strongly depleted in incompatible elements early in the history of the solar system nevertheless dar al gani 476 is derived from a source region that has a slightly greater incompatible enrichment than the que94201 source region this is not consistent with the fact that the parental magma of dar al gani 476 is significantly more mafic than the parental magma of que94201 and underscores a decoupling between the major element and trace element isotopic systematics observed in the martian meteorite suite combining the end142 end143 isotopic systematics of the martian meteorites yields a model age for planetary differentiation of 4 513 0 033 0 027 ga using this age the parent daughter ratios of martian mantle sources are calculated assuming a two stage evolutionary history the calculated sources have very large ranges of parent daughter ratios 87rb 86sr 0 037 0 374 147sm 144nd 0 182 0 285 176lu 177hf 0 028 0 048 these ranges exceed the ranges estimated for terrestrial basalt source regions but are very similar to those estimated for the sources of lunar mare basalts in fact the range of parent daughter ratios calculated for the martian meteorite sources can be produced by mixing between end members with compositions similar to lunar mare basalt sources two of the sources have compositions that are similar to olivine and pyroxene rich mafic cumulates with variable proportions of a rb enriched phase such as amphibole whereas the third source has the composition of liquid trapped in the cumulate pile i e similar to kreep after 99 crystallization correlation between the proportion of trapped liquid in the meteorite source regions and estimates of fo2 suggest that the kreep like component may be hydrous the success of these models in reproducing the martian meteorite source compositions suggests that the variations in trace element and isotopic compositions observed in the martian meteorites primarily reflect melting of the crystallization products of an ancient magma ocean and that assimilation of evolved crust by mantle derived magmas is not required furthermore the decoupling of major element and trace element isotopic systematics in the martian meteorite suite may reflect the fact that trace element and isotopic systematics are inherited from the magma source regions whereas the major element abundances are limited by eutectic melting processes at the time of magma formation differences in major element abundances of parental magma therefore result primarily from fractional crystallization after leaving their source regions

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

hydrogen isotopic composition of the martian mantle inferred from the newest martian meteorite fall tissint the hydrogen isotopic composition of planetary reservoirs can provide key constraints on the origin and history of water on planets the sources of water and the hydrological evolution of mars may be inferred from the hydrogen isotopic compositions of mineral phases in martian meteorites which are currently the only samples of mars available for earth based laboratory investigations previous studies have shown that d values in minerals in the martian meteorites span a large range of 250 to 6000 the highest hydrogen isotope ratios likely represent a martian atmospheric component either interaction with a reservoir in equilibrium with the martian atmosphere such as crustal water or direct incorporation of the martian atmosphere due to shock processes the lowest d values may represent those of the martian mantle but it has also been suggested that these values may represent terrestrial contamination in martian meteorites here we report the hydrogen isotopic compositions and water contents of a variety of phases merrillites maskelynites olivines and an olivine hosted melt inclusion in tissint the latest martian meteorite fall that was minimally exposed to the terrestrial environment we compared traditional sample preparation techniques with anhydrous sample preparation methods to evaluate their effects on hydrogen isotopes and find that for severely shocked meteorites like tissint the traditional sample preparation techniques increase water content and alter the d h ratios toward more terrestrial like values in the anhydrously prepared tissint sample we see a large range of d values most likely resulting from a combination of processes including magmatic degassing secondary alteration by crustal fluids shock related fractionation and implantation of martian atmosphere based on these data our best estimate of the d value for the martian depleted mantle is 116 94 which is the lowest value measured in a phase in the anhydrously prepared section of tissint this value is similar to that of the terrestrial upper mantle suggesting that water on mars and earth was derived from similar sources the water contents of phases in tissint are highly variable and have been affected by secondary processes considering the h2o abundances reported here in the driest phases most likely representing primary igneous compositions and appropriate partition coefficients we estimate the h2o content of the tissint parent magma to be 0 2 wt

petrology and trace element geochemistry of tissint the newest shergottite fall the fall and recovery of the tissint meteorite in 2011 created a rare opportunity to examine a martian sample with a known short residence time on earth tissint is an olivine phyric shergottite that accumulated olivine antecrysts within a single magmatic system coarse olivine grains with nearly homogeneous cores of mg 80 suggest slow re equilibration many macroscopic features of this sample resemble those of lar 06319 including the olivine crystal size distribution and the presence of evolved oxide and olivine compositions unlike lar 06319 however no magmatic hydrous phases were found in the analyzed samples of tissint minor and trace element compositions indicate that the meteorite is the product of closed system crystallization from a parent melt derived from a depleted source with no obvious addition of a lree rich crustal component prior to or during crystallization the whole rock ree pattern is similar to that of intermediate olivine phyric shergottite eeta 79001 lithology a and could also be approximated by a more olivine rich version of depleted basaltic shergottite que 94201 magmatic oxygen fugacities are at the low end of the shergottite range with log fo2 of qfm 3 5 to 4 0 estimated based on early crystallized minerals and qfm 2 4 estimated based on the eu in pyroxene oxybarometer these values are similarly comparable to other depleted shergottites including sau 005 and que 94201 tissint occupies a previously unsampled niche in shergottite chemistry containing olivines with mg 80 resembling the enriched olivine phyric shergottite lar 06319 in its crystallization path and comparable to intermediate olivine phyric shergottite eeta 79001a depleted olivine phyric shergottite dag 476 and depleted basaltic shergottite que 94201 in its trace element abundances and oxygen fugacity the apparent absence of evidence for terrestrial alteration in tissint particularly in trace element abundances in the whole rock and individual minerals confirms that exposure to the arid desert environment results in only minimal weathering of samples provided the exposure times are brief

water undersaturated mantle plume volcanism on present day mars based on meteorite evidence the present day martian mantle has a combined abundance of up to a few hundred ppm of h2o cl and f which lowers the solidus and enhances the magma production rate adiabatic decompression melting in upwelling mantle plumes is the best explanation for young last 200 myr volcanism on mars we explore water undersaturated mantle plume volcanism using a finite element mantle convection model coupled to a model of hydrous peridotite melting relative to a dry mantle the reduction in solidus temperature due to water increases the magma production rate by a factor of 1 3 1 7 at 50 ppm water and by a factor of 1 9 3 2 at 200 ppm water mantle water also decreases the viscosity and increases the vigor of convection which indirectly increases the magma production rate by thinning the thermal boundary layer and increasing the flow velocity at conditions relevant to mars these indirect effects can cause an order of magnitude increase in the magma production rate using geologic and geophysical observations of the late amazonian magma production rate and geochemical observations of melt fractions in shergottite meteorites present day mars is constrained to have a core mantle boundary temperature of 1750 to 1800 c and a volume averaged thermal rayleigh number of 2 106 to 107 indicating that moderately vigorous mantle convection has persisted to the present day melting occurs at depths of 2 5 6 gpa and is controlled by the rayleigh number at the low pressure end and by the mantle water concentration at high pressure

martian meteorite tissint records unique petrogenesis among the depleted shergottites tissint a new unaltered piece of martian volcanic materials is the most silica poor and mg fe rich igneous rock among the depleted olivine phyric shergottites fe mg zoning of olivine suggests equilibrium growth 0 1 c h 1 in the range of fo80 56 and olivine overgrowth fo55 18 through a process of rapid disequilibrium 1 0 5 0 c h 1 the spatially extended up to 600 m flat top fe mg profiles of olivine indicates that the early stage cooling rate of tissint was slower than the other shergottites the chemically metastable outer rim of olivine fo55 consists of oscillatory phosphorus zoning at the impact induced melt domains and grew rapidly compared to the early to intermediate stage crystallization of the tissint bulk high ca pyroxene to low ca pyroxene and high ca pyroxene to plagioclase ratios of tissint are more comparable to the enriched basaltic and enriched olivine phyric shergottites dominance of augite over plagioclase induced augite to control the ca buffer in the residual melt suppressing the plagioclase crystallization which also caused a profound effect on the al content in the late crystallized pyroxenes mineral chemical stability phase assemblage saturation and pressure temperature path of evolution indicates that the parent magma entered the solidus and left the liquidus field at a depth of 40 80 km in the upper mantle petrogenesis of tissint appears to be similar to lar 06319 an enriched olivine phyric shergottite during the early to intermediate stage of crystallization a severe shock induced deformation resulted in remelting 10 15 vol recrystallization most fe rich phases and exhumation of tissint in a time scale of 1 8 yr tissint possesses some distinct characteristics e g impact induced melting and deformation forming phosphorus rich recrystallization rims of olivine and shock induced melt domains without relative enrichment of lrees compared to the bulk and shared characteristics e g modal composition and magmatic evolution with the enriched basaltic shergottites evidently reflecting unique mantle source in comparison to the clan of the depleted members

uranium lead isotope systematics of mars inferred from the basaltic shergottite que 94201 abstract uranium lead ratios commonly represented as 238u 204pb calculated for the sources of martian basalts preserve a record of petrogenetic processes that were active during early planetary differentiation and formation of martian geochemical reservoirs to better define the range of values represented by the source regions of martian basalts we completed u pb elemental and isotopic analyses on whole rock mineral and leachate fractions from the martian meteorite queen alexandra range 94201 que 94201 the whole rock and silicate mineral fractions have unradiogenic pb isotopic compositions that define a narrow range 206pb 204pb 11 16 11 61 in contrast the pb isotopic compositions of weak hcl leachates are more variable and radiogenic the intersection of the que 94201 data array with terrestrial pb in 206pb 204pb 207pb 204pb 208pb 204pb compositional space is consistent with varying amounts of terrestrial contamination in these fractions we calculate that only 1 7 contamination is present in the purified silicate mineral and whole rock fractions whereas the hcl leachates contain up to 86 terrestrial pb this terrestrial pb contamination generated a 206pb 207pb array in the que fractions that appears to represent an ancient age which contrasts with a much younger crystallization age of 327 10 ma derived from rb sr and sm nd isochrons borg l e nyquist l e taylor l a wiesmann h and shih c y 1997 constraints on martian differentiation processes from rb sr and sm nd isotopic analyses of the basaltic shergottite que 94201 geochim cosmochim acta 61 4915 4931 despite the contamination and accepting 327 10 ma as the crystallization age we use the u pb data to determine the initial 206pb 204pb of que 94201 to be 11 086 0 008 and to calculate the value of its mantle source to be 1 82 0 01 the value calculated for the que 94201 source is the lowest determined for any martian basalt source and when compared to the highest values determined for martian basalt sources indicates that values in martian source reservoirs vary by at least a factor of two additionally the range of source values indicates that the value of bulk silicate mars is approximately three the amount of variation in the values of the mantle sources 2 4 is greater than can be explained by igneous processes involving silicate phases alone we suggest the possibility that a small amount of sulfide crystallization may generate greater extents of u pb fractionation during formation of the mantle sources of martian basalts

h layering in the top meter of mars we explore the capability of a method of mapping the depth distribution of a hydrogen rich layer in the top meter of mars from the neutron currents measured by the mars odyssey neutron spectrometer assuming the soil can be modeled by two layers of known composition having different hydrogen contents simulations allow an inversion of the neutron data into knowledge of depth and hydrogen content of the lower layer the determination of these variables is sensitive to the hypothesis of chemical composition of the soil we quantify this contribution to the uncertainty in the method first in terms of individual chemical elements and then in terms of macroscopic absorption cross sections to minimize this source of error an average composition was inferred from mars exploration rover data possible compositions having a wide range of macroscopic absorption cross sections were used to evaluate the uncertainty associated with our calculations we finally compare our results to ice table depth estimates predicted by two published theoretical models at locations where the composition is relatively well known the fit is excellent in the southern high latitudes but questionable in the northern high latitudes possible explanations of these differences include the high geographical variations of the neutron currents relative to the spatial width of the response function of the instrument and the overly simple model we of necessity used for surface layering

active boulder movement at high martian latitudes lobate stony landforms occur on steep slopes at high latitudes on mars we demonstrate active boulder movement at seven such sites submeter scale boulders frequently move distances of a few meters the movement is concentrated in the vicinity of the lobate landforms but also occurs on other slopes this provides evidence for a newly discovered common style of activity on mars which may play an important role in slope degradation it also opens the possibility that the lobate features are currently forming in the absence of significant volumes of liquid water

ground ice at the phoenix landing site stability state and origin 1 a primary objective of the phoenix mission was to examine the characteristics of high latitude ground ice on mars we report observations of ground ice its depth distribution and stability characteristics and examine its origins and history high latitude ground ice was explored through a dozen trench complexes and landing thruster pits over a range of polygon morphological provinces shallow ground ice was found to be abundant under a layer of relatively loose ice free soil with a mean depth of 4 6 cm which varied by more than 10x from trench to trench these variations can be attributed mainly to slope effects and thermal inertia variations in the overburden soil affecting ground temperatures the presence of ice at this depth is consistent with vapor diffusive equilibrium with respect to a mean atmospheric water content of 3 4 1019 m 3 consistent with the present day climate significant ice heterogeneity was observed with two major forms ice cemented soil and relatively pure light toned ice ice cemented soils which comprised about 90 of the icy material exposed by trenching are best explained as vapor deposited pore ice in a matrix supported porous soil light toned ice deposits represent a minority of the subsurface and are thought to consist of relatively thin near surface deposits the origin of these relatively pure ice deposits appears most consistent with the formation of excess ice by soil ice segregation such as would occur by thin film migration and the formation of ice lenses needle ice or similar ice structures

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

global distribution of neutrons from mars results from mars odyssey global distributions of thermal epithermal and fast neutron fluxes have been mapped during late southern summer northern winter using the mars odyssey neutron spectrometer these fluxes are selectively sensitive to the vertical and lateral spatial distributions of h and co 2 in the uppermost meter of the martian surface poleward of 60 latitude is terrain rich in hydrogen probably h 2 o ice buried beneath tens of centimeter thick hydrogen poor soil the central portion of the north polar cap is covered by a thick co 2 layer as is the residual south polar cap portions of the low to middle latitudes indicate subsurface deposits of chemically and or physically bound h 2 o and or oh