geomorphic and geologic settings of the phoenix lander mission landing site 1 the phoenix lander touched down on the northern distal flank of the shield volcano alba patera in a 150 km wide valley underlain by the scandia region unit the geomorphology and geology of the landing site is dominated by the 0 6 ga 11 5 km wide bowl shaped impact crater heimdal and its areally extensive ejecta deposits the lander is located 20 km to the west of the crater and is sitting on a plains surface underlain by partially eroded heimdal ejecta deposits heimdal was produced by a hypervelocity impact into fine grained ice rich material and is inferred to have produced high velocity winds and a ground hugging ejecta emplacement mode that destroyed or buried preexisting surfaces and rock fields out to 10 crater radii patterned ground is ubiquitous with complex polygon patterns and rock rubble piles located on older plains 3 3 ga to the west of the ejecta deposits crater size frequency distributions are complex and represent equilibria between crater production and destruction processes e g aeolian infill cryoturbation relaxation of icy substrate rock abundances increase near craters for the older plains and rocks with their dark shadows explain the reason for the few percent lower albedo for these plains as opposed to the heimdal ejecta deposits many rocks at the landing site have been reworked by cryoturbation and moved to polygon troughs the evidence for cryoturbation and the lack of aeolian features imply that the soils sampled by phoenix are locally derived and mixed with a subordinate amount of windblown dust

mars odyssey neutron data 1 data processing and models of water equivalent hydrogen distribution 1 for more than 7 years the los alamos built mars odyssey neutron spectrometer mons has measured the neutron albedo from mars in three consecutive energy bands thermal epithermal and fast neutron ranges this paper synthesizes the teamwork on the optimization of the signal extraction the corrections for observational biases and instrument specific characteristics results are presented for neutron time series with an emphasis on seasonal variations at the poles frost free data are mapped on to the surface and the apparent random nature of the counting rate distribution per pixel is analyzed for epithermal neutrons the relative standard deviation is less than 0 5 equatorward of 45 and up to 2 5 above this latitude limit for thermal neutrons it is 1 and 2 5 respectively and for fast neutrons it is 3 and 5 5 respectively new science results are obtained with regards to the distribution of water equivalent hydrogen weh on mars under the assumption of a single uniform distribution of hydrogen with depth weh abundances range from 2 near the equator to 80 at the poles with 2 to 4 5 relative error bars a best approximation to a two layered global distribution of a lower level hydrogen rich substrate beneath an upper layer of varying thicknesses is generated using an average hydration level of an upper layer of 2 wt derived in the paper by feldman et al 2011 such results are discussed and compared with regard to previous publications on the mons instrument

recurrent spring fed rivers in a middle to late pleistocene semi arid grassland implications for environments of early humans in the lake victoria basin kenya the effect of changing palaeoclimate and palaeoenvironment on human evolution during the pleistocene is debated but hampered by few east african records directly associated with archaeological sites prior to the last glacial maximum middle to late pleistocene deposits on the shoreline of eastern lake victoria preserve abundant vertebrate fossils and middle stone age arte facts associated with riverine tufas at the base of the deposits which are ideal for palaeoenvironmental reconstructions new data from tufas identified on rusinga island and on the mainland near karungu kenya are provided from outcrop thin sections mineralogical stable isotopic and u series dating analyses tufa is identified in four sites nyamita 94 0 3 3 and 111 4 4 2 ka kisaaka aringo 455 45 ka and obware the age ranges of these tufa deposits demonstrate that spring fed rivers were a recurrent variably preserved feature on the pleistocene landscape for ca 360 kyr poor sorting of clastic facies from all sites indicates flashy ephemeral discharge but these facies are commonly associated with barrage tufas paludal environments with 13c values of ca 10 indicative of c3 plants and fossil hippopotamus all of which indicate a perennial water source other tufa deposits from nyamita obware and aringo have a mixed c3 c4 signature consistent with a semi arid c4 grassland surrounding these spring fed rivers the 18o values of tufa from nyamita are on average ca 1 more negative than calcite precipitated from modern rainfall in the region suggesting greater contribution of depleted monsoonal input similar to the last glacial maximum microdebitage and surface collected artefacts indicate that early modern humans were utilizing these spring fed rivers the presence of spring fed rivers would have afforded animals a reliable water source sustaining a diverse plant and animal community in an otherwise arid environment

boulder movement at high northern latitudes of mars 1 we examine a narrow latitudinal band 60 n 70 n on mars to place constraints on the seasonally averaged velocity q of boulder movements over patterned ground these latitudes comprise a region of the northern lowlands where patterned ground covers nearly every surface here boulders meters in diameter are consistently found to be concentrated at or near the cracks that define the polygonal networks indicating a mobilization process because impact craters are the source for many boulders we can use craters and their degradation to estimate the time scales for boulder movement we study and catalog 1018 degraded impact craters 100 m d 1 km in 55 high resolution imaging science experiment hirise images we find that crater degradation occurs on a time scale 1 ma which is too recent for melting related mechanisms in a past warmer epoch clustering of boulders occurs at a time scale of a few ma or shorter which means that boulders on 5 20 m diameter polygons move at seasonal velocities q 1 10 m yr or faster

the global stratigraphy of the cretaceous tertiary boundary impact ejecta the chicxulub crater ejecta stratigraphy is reviewed in the context of the stratigraphy of underlying and overlying rock sequences the ejecta sequence is regionally grouped in a thick polymict and monomict breccia sequences inside the crater and within 300 km from the rim of the crater known from drill holes in and close to the breater and exposures near the border of yucatan and belize b gulf of mexico region 2500 m from the crater with up to 9 m thick complex tsunami wave influenced tektite bearing sequences in shallow marine 500 m deep environments and tektite bearing decimeter thick gravity flow deposits in deep water sites c an intermediate region between 2500 and 4000 km from the crater where centimeter thick tektite bearing layers occur and d a global distal region with a millimeter thin ejecta layer the distal ejecta layer is characterized by sub millimeter sized microkrystites often rich in ni rich spinels and altered clinopyroxene wherever present the ejecta layers mark exactly the sudden mass mortality horizon of the k t boundary what exactly caused the mass mortality is still uncertain but it appears the main event leading to the k t mass extinctions

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

crustal fracturing and chert dike formation triggered by large meteorite impacts ca 3 260 ga barberton greenstone belt south africa the ca 3260 ma contact between the largely volcanic onverwacht group and overlying largely sedimentary fig tree group in the barberton greenstone belt south africa is widely marked by chert dikes that extend downward for up to 100 m into underlying sedimentary and volcanic rocks of the mendon formation onverwacht group in the barite valley area these dikes formed as open fractures that were filled by both precipitative fill and the downward flowage of liquefied carbonaceous sediments and ash at the top of the mendon formation spherules that formed during a large meteorite or asteroid impact event occur in a wave and or current deposited unit spherule bed s2 which widely marks the onverwacht fig tree contact and as loose grains and masses within some chert dikes up to 50 m below the contact four main types of chert dikes and veins are recognized type 1 irregular dikes up to 8 m wide that extend downward across as much as 100 m of stratigraphy type 2 small vertical dikes most 1 m wide which are restricted to the lower half of the mendon chert section type 3 small crosscutting veins most 50 cm across filled with precipitative silica and type 4 small irregular to bedding parallel to irregular veins mostly 10 cm wide filled with translucent precipitative silica type 2 dikes formed first and reflect a short lived seismic event that locally decoupled the sedimentary section at the top of the mendon formation from underlying volcanic rocks and opened narrow vertical tension fractures in the lower lithified part of the sedimentary section later seismic events triggered formation of the larger type 1 fractures throughout the sedimentary and upper volcanic section widespread liquefaction of soft uppermost mendon sediments and flowage of the liquefied sediments and loose impact generated spherules into the open fractures late stage tsunamis everywhere eroded and reworked the spherule layer the coincidence of crustal disruption dike formation spherule deposition and tsunami activity suggests that all were related to the s2 impact or impact cluster crustal disruption at this time also formed local relief that provided clastic sediment to the postimpact fig tree group including a small conglomeratic fan delta in the barite valley area remobilization and further movement of debris in the subsurface continued for some time locally the deposition of dense baritic sediments over soft dike materials induced remobilization of material in the dike causing foundering of s2 and 1 2 m of overlying baritic sediments into the dike spherule beds occur at the base of the fig tree group over wide areas of the barberton belt marking the abrupt change from 300 m y of predominantly anorogenic mafic and komatiitic volcanism of the onverwacht group to orogenic clastic sedimentation and associated felsic volcanism of the fig tree group this area never again returned to onverwacht style mafic and ultramafic volcanism but evolved 100 m y later into the kaapvaal craton these results indicate that this major transition in crustal evolution coincided with and was perhaps triggered by major impact events ca 3260 3240 ma

spinel from archean impact spherules abstract large meteorite impacts are recorded in distal fall deposits preserved in two early archean greenstone belt sequences 3 2 3 5 ga mapping stratigraphie and geochemistry studies have been used to identify at least four impact layers in the barberton greenstone belt of southern africa and one in the pilbara greenstone belt of western australia spherules that represent impact melt droplets distinguish these impact layers from associated tuffs spinel with unusual compositional traits occurs within spherules in two of these impact layers spinel is the only preserved primary phase and is found within up to 10 of the spherules in the best preserved layers some spherules contain up to 40 spinel which is often the only primary phase that crystallized within the spherules spinel morphologies include dendrites and skeletal octahedra typical of those found in high temperature melts spinel compositions are similar to those of cenozoic impacts including very high ni and ferric iron contents but the archean spinel is distinctive in being composed dominantly of the chalcophile elements fe ni cr and v this spinel is not metamorphic or metasomatic in origin closely associated komatiites in the underlying onverwacht group both fresh and highly altered have unaltered spinel with igneous compositions and morphologies quite distinct from the impact spinel characteristics of spinel preserved in these archean impact spherules suggest a rather complex series of steps following a large bolide impact initial high temperature reduction produced melt droplets some of which were immiscible silicate metal sulfide density differences allowed separation of some of these immiscible melts which subsequently evolved independently and formed spherules with highly variable chalcophile element contents oxidation during atmospheric reentry increased ni fe and fe 3 fe 2 in the chalcophile siderophile melts archean impact spinel has higher ni fe than cenozoic impact spinel though distinctly lower fe 3 fe 2 which probably reflects a substantially less oxidizing atmosphere during the archean the archean atmospheric oxidation may in fact be a transient phenomenon associated with a large impact into the terrestrial ocean resulting in very high atmospheric h 2 o h 2 bolide size is estimated to be 24 km in diameter based on the average diameter of 0 85 mm for the spinel bearing spherules this estimate is consistent with the observed ir fluence in spinel bearing impact layers of approximately 5 3 mg cm2 which would be produced by a 30 km diameter chondritic bolide

microfossils and possible microfossils from the early archean onverwacht group barberton mountain land south africa there is widespread textural evidence for microbial activity in the cherts of the early archean onverwacht group layers with fine carbonaceous laminations resembling fossil microbial mats are abundant in the cherty metasediments of the predominantly basaltic hooggenoeg and kromberg formations in rare cases filamentous microfossils are associated with the laminae the morphologies of the fossils as well as the texture of the encompassing laminae suggest an affinity to modern mat dwelling cyanobacteria or bacteria a variety of spheroidal and ellipsoidal structures present in cherts of the hooggenoeg and kromberg formations resemble modern coccoidal bacteria and bacterial structures including spores the development of spores may have enabled early microorganisms to survive the relatively harsh surficial conditions including the effects of very large meteorite impacts on the young earth

fate of outflow channel effluents in the northern lowlands of mars the vastitas borealis formation as a sublimation residue from frozen ponded bodies of water 1 we analyze the fate of the hesperian aged outflow channel effluents emplaced into the northern lowlands of mars we have modeled the evolution of these effluents under the assumption that they were emplaced under a range of atmospheric conditions comparable to those of today and thought to have prevailed in the hesperian under these conditions we find that the evolution of the sediment loaded water after it left the channels includes three phases phase 1 emplacement and intensive cooling violent emplacement of water followed by a short period of intensive evaporation from the surface and near surface layer and intensive convection during this phase the water maintained and redistributed its large suspended sediment load water vapor strongly influenced the climate at least for a geologically short time when the temperature of the water reached the temperature of the maximum density or the freezing point boiling and intensive convection ceased and the water deposited the sediments phase 2 freezing solid geologically rapid freezing of the water body accompanied by weak convective water movement occurred over a period of the order of 104 years phase 3 sublimation and loss this period involved sublimation of the ice and lasted longer than the freezing phase the rate and latitudinal dependence of the sublimation as well as the location of water vapor condensation crucially depend on the planetary obliquity climate and sedimentary veneering of the ice phase 3 would have been very short geologically 105 106 years if an insulating sedimentary layer did not build up rapidly if such an insulating layer did form rapidly sublimation could cease and residual ice hundreds of meters thick could remain below the surface today the northern lowlands of mars are largely covered by the late hesperian aged vastitas borealis formation vbf which has sharp boundaries clearly seen in the map of kilometer scale roughness and a distinctive kilometer scale roughness signature we examine detrended topography data and find evidence that the vbf is underlain at very shallow depths by an early hesperian ridged volcanic plains substrate rather than frozen water deposits remaining from the outflow channel events analysis of the vbf roughness characteristics suggests at least 100 m thickness of sediments on top of the underlying volcanic ridged plains the total inferred volume of the vbf material approximately corresponds to the volume of material removed from the outflow channels these results support a model in which the vastitas borealis formation predominantly represents the sedimentary residue remaining after the sediment laden water effluents of the outflow channels ponded in the northern lowlands and rapidly froze solid and sublimed the distinctive facies of the vbf are interpreted to be related to the freezing sublimation and residue of the outflow channel effluents ridged facies are dominantly marginal and are interpreted to be related to ice sheet lateral retreat knobby facies occur throughout and are interpreted to be due to a variety of causes including dewatering de icing and brine related processes and kame like ice residues grooved facies are interpreted to represent postsublimation uplift and fracturing due to load removal the mottled facies may represent bright ejecta whose albedo is related to buried sedimentary or evaporitic residues presently the most likely sites to find frozen remnants of the hesperian ocean are below the floors of stealth craters underlying the vbf on the basis of these findings and interpretations we make predictions for the fate of the outflow channel water vapor and the nature of the martian hydrologic cycle during the hesperian if noachian aged oceans existed conditions at that time would be similar to those described for a hesperian ocean as soon as a global cryosphere was formed if any residual deposits remain in the northern lowlands from a proposed noachian ocean they would lie below both the vbf and the underlying sequence of hesperian ridged plains

nonlinear spectral mixture modeling to estimate water ice abundance of martian regolith abstract we present a new technique to estimate the abundance of water ice in the martian permafrost using phoenix lander surface stereo imager ssi multispectral data past work estimated this abundance by employing radiative transfer methods to model the spectra of ice and regolith mixtures using the optical constants of water ice and a martian analogue soil our technique removed the uncertainty of using an analogue or of estimating a regolith composition by deriving the optical constants directly from observations of icy regolith acquired before and after sublimation of the ice at similar viewing geometries laboratory spectral measurements of known mixtures of water ice and dry soil at controlled viewing geometries allowed us to constrain the uncertainty in our technique we found that model fits across the observational uncertainty will contain the actual water ice fraction we then applied the technique to phoenix ssi observations of snow white trench and dodo goldilocks trench for the snow white trench we estimated water ice abundances consistent with pore fill in the near surface ice consistent with atmospheric diffusion for the dodo goldilocks trench we estimate water ice abundances larger than pore fill would allow these results have implications to our understanding of the history of water ice on mars and the role of the regolith at high latitudes as a reservoir of atmospheric h2o

the origin of carbonaceous matter in pre 3 0 ga greenstone terrains a review and new evidence from the 3 42 ga buck reef chert abstract the geological record of carbonaceous matter from at least 3 5 ga to the end of the precambrian is fundamentally continuous in terms of carbonaceous matter structure composition environments of deposition preservation and abundance in host rocks no abiotic processes are currently known to be capable of producing continuity in all four of these properties although this broad view of the geological record does not prove that life had arisen by 3 5 ga the end of the early archean it suggests a working hypothesis most if not all carbonaceous matter present in rocks older than 3 0 ga was produced by living organisms this hypothesis must be tested by studies of specific early geological units designed to explore the form distribution and origin of enclosed carbonaceous matter the carbonaceous environmentally diverse 3416 ma buck reef chert brc of the barberton greenstone belt south africa provides an opportunity for such a study upward facies progressions in the brc reflect deposition in environments ranging from shallow marine evaporitic brine ponds to a storm and wave active shelf to a deep low energy basinal setting below storm wave base abundances and ratios of al2o3 zr tio2 and cr track inputs of various types of volcaniclastic and terrigenous clastic materials in particular zr al2o3 and zr serve as proxies for concentration of windblown dust and indirectly as proxies for sedimentation rate cu zn ni and feo were concentrated in the most slowly deposited transitional and basinal sediments inconsistent with a hydrothermal setting but consistent with a normal marine setting the distribution of microfacies defined by associations and layering of clastic ferruginous and carbonaceous grains correlates with facies transitions fine carbonaceous laminations which occur only in shallow platform settings represent photosynthetic microbial mats these were ripped up and the debris widely redistributed in shallow and deep water by waves and storms the isotopic composition of carbonaceous matter ranges from 35 to 30 in shallow water settings and to 20 in deep water units the heavier 13c in deep water carbonaceous matter is thought to reflect microbial processing possibly by fermentation and methanogenesis of organic matter originally produced in shallow water hydrothermal origins for brc carbonaceous matter are clearly excluded by the inferred depositional setting of the rocks as a whole an inference supported by field petrographic and geochemical analysis we suggest that the biological model proposed here for brc carbonaceous matter is the best currently available the hypothesis that at least some carbonaceous matter present in rocks older than 3 0 ga was produced by living organisms should be regarded as likely until extraordinary contradictory evidence is presented

size frequency distributions of rocks on the northern plains of mars with special reference to phoenix landing surfaces 1 the size frequency distributions of rocks 1 5 m diameter fully resolvable in high resolution imaging science experiment hirise images of the northern plains follow exponential models developed from lander measurements of smaller rocks and are continuous with rock distributions measured at the landing sites dark pixels at the resolution limit of mars orbiter camera thought to be boulders are shown to be mostly dark shadows of clustered smaller rocks in hirise images an automated rock detector algorithm that fits ellipses to shadows and cylinders to the rocks accurately measured within 1 2 pixels rock diameter and height by comparison to spacecraft of known size of 10 million rocks over 1500 km2 of the northern plains rock distributions in these counts parallel models for cumulative fractional area covered by 30 90 rocks in dense rock fields around craters 10 30 rock coverage in less dense rock fields and 0 10 rock coverage in background terrain away from craters above 1 5 m diameter hirise resolves the same population of rocks seen in lander images and thus size frequency distributions can be extrapolated along model curves to estimate the number of rocks at smaller diameters extrapolating sparse rock distributions in the phoenix landing ellipse indicate 1 chance of encountering a potentially hazardous rock during landing or that could impede the opening of the solar arrays extrapolations further suggest rocks large enough to depress the ground ice table and small enough to be picked up or pushed by the robotic arm should be present within reach for study after landing

prolonged magmatism and time constraints for sediment deposition in the early archean barberton greenstone belt evidence from the upper onverwacht and fig tree groups the single zircon evaporation shrimp ion microprobe and conventional dissolution techniques were used to determine 207pb 206pb and u pb ages on samples from the upper onverwacht and fig tree groups of the early archean barberton greenstone belt south africa zircons from dacitic rocks of the upper hooggenoeg formation yield ages of 3445 3452 ma a tuff in the basal kromberg formation has a mean age of 3416 5 ma a tuffaceous band 5 cm thick in the uppermost kromberg formation contains igneous zircons with a mean age of 3334 3 ma the 1700 m section of kromberg formation between these two samples is composed of basaltic lavas minor komatiites and cherty metasediments the overlying mendon formation is composed of interbedded komatiitic lavas and metasediments with a minimum thickness of 600 m a cherty stromatolitic metasediment 300 m above the base contains several thin ash layers with a mean zircon age of 3298 3 ma the basal fig tree group has units as old as 3259 3 ma and upper units in the fig tree are as young as 3225 3 ma xenocrystic zircons in the upper onverwacht and overlying fig tree group samples suggest that successive igneous units inherited zircons from underlying units and that over several hundred million years episodes of intermediate to felsic igneous activity took place at 20 40 ma intervals structural repetition by isoclinal folding and thrust faulting are important components of late greenstone belt evolution but should not obscure the importance of the prolonged interval of magmatic evolution represented by the thick pile of volcanic rocks observed in the barberton greenstone belt

driven by excess climatic implications of new global mapping of near surface water equivalent hydrogen on mars abstract we present improved mars odyssey neutron spectrometer mons maps of near surface water equivalent hydrogen weh on mars that have intriguing implications for the global distribution of excess ice which occurs when the mass fraction of water ice exceeds the threshold amount needed to saturate the pore volume in normal soils we have refined the crossover technique of feldman et al 2011 by using spatial deconvolution and gaussian weighting to create the first globally self consistent map of weh at low latitudes our new maps indicate that weh exceeds 15 in several near equatorial regions such as arabia terra which has important implications for the types of hydrated minerals present at low latitudes at high latitudes we demonstrate that the disparate mons and phoenix robotic arm ra observations of near surface weh can be reconciled by a three layer model incorporating dry soil over fully saturated pore ice over pure excess ice such a three layer model can also potentially explain the strong anticorrelation of subsurface ice content and ice table depth observed at high latitudes at moderate latitudes we show that the distribution of recently formed impact craters is also consistent with our latest mons results as both the shallowest ice exposing crater and deepest non ice exposing crater at each impact site are in good agreement with our predictions of near surface weh overall we find that our new mapping is consistent with the widespread presence at mid to high martian latitudes of recently deposited shallow excess ice reservoirs that are not yet in equilibrium with the atmosphere

identification of sublimation type thermal contraction crack polygons at the proposed nasa phoenix landing site implications for substrate properties and climate driven morphological evolution 1 we identify a surface within the nasa phoenix landing site area d characterized by boulder topped polygonally patterned mounds comparable in radius and cross sectional morphology to terrestrial sublimation polygons found in antarctica both martian and antarctic polygons display topographic asymmetry with shallow equator facing slopes and steep pole facing slopes interpreted to indicate insolation dependent differential sublimation of buried ice on the basis of morphological similarities we classify the phoenix box 1 polygons as sublimation polygons terrestrial sublimation polygons form where ice volumes exceeding available pore space occur in the shallow subsurface and where near surface conditions are too cold and dry to permit the development of saturated active layers conditions comparable to those recently modeled for mars in the phoenix landing site area d box 1 the identification of sublimation polygons on mars would provide direct evidence for shallow ground ice exceeding pore ice volumes and shed light on the emplacement mechanism of this ice

a high continental weathering flux into paleoarchean seawater revealed by strontium isotope analysis of 3 26 ga barite controls on archean seawater chemistry remain controversial many studies have suggested that it was largely controlled by oceanic hydrothermal fluid circulation recent work however from clastic sequences hf o isotope data from detrital zircons and models for the rb sr evolution of the continental crust suggest that intense continental weathering and low temperature surface alteration were more important than previously thought during the early archean this is consistent with biogeochemical studies that suggest the archean had a diverse microbial ecology which would in part need to be sustained by nutrients e g phosphorus that were derived from continental weathering to further quantify continental weathering during the early archean we analyzed 3 26 ga barite from the fig tree group south africa for strontium oxygen and sulfur isotope compositions we propose that the seawater component of the barite is characterized by 87sr 86sr ratios 0 701 which is significantly more radiogenic than contemporaneous mantle 0 7007 0 7008 the radiogenic nature of seawater at this time suggests that the continental weathering flux at 3 26 ga had a large impact on ocean chemistry 400 million years earlier than previously suggested

mars exploration program 2007 phoenix landing site selection and characteristics 1 to ensure a successful touchdown and subsequent surface operations the mars exploration program 2007 phoenix lander must land within 65 to 72 north latitude at an elevation less than 3 5 km the landing site must have relatively low wind velocities and rock and slope distributions similar to or more benign than those found at the viking lander 2 site also the site must have a soil cover of at least several centimeters over ice or icy soil to meet science objectives of evaluating the environmental and habitability implications of past and current near polar environments the most challenging aspects of site selection were the extensive rock fields associated with crater rims and ejecta deposits and the centers of polygons associated with patterned ground an extensive acquisition campaign of odyssey thermal emission imaging spectrometer predawn thermal ir images together with 0 31 m pixel mars reconnaissance orbiter high resolution imaging science experiment images was implemented to find regions with acceptable rock populations and to support monte carlo landing simulations the chosen site is located at 68 16 north latitude 233 35 east longitude areocentric within a 50 km wide n s by 300 km long e w valley of relatively rock free plains surfaces within the eastern portion of the valley are differentially eroded ejecta deposits from the relatively recent 10 km wide heimdall crater and have fewer rocks than plains on the western portion of the valley all surfaces exhibit polygonal ground which is associated with fracture of icy soils and are predicted to have only several centimeters of poorly sorted basaltic sand and dust over icy soil deposits

recent high latitude icy mantle in the northern plains of mars characteristics and ages of emplacement 1 martian high latitude zones are covered with a smooth layered ice rich mantle containing characteristic polygonal patterns the mantle is especially uniform and homogeneous in the northern plains we performed a survey of decameter scale mantle textures and circular features including impact craters in the northern lowlands down to 50 n latitude strongly altered and mantled craters of impact and non impact origin form a population of subtle circular features below the mantle pits of non impact origin are numerous in some regions at lower latitudes impact craters superposed on the mantle are small and very sparse the inferred mean crater retention age of the mantle is 0 1 ma the spatial distribution of young craters suggests an age difference between the highest latitudes younger and some lower latitude regions older latitudinal trends in polygon textures agree with impact crater evidence for a latitudinal age progression of mantle properties

the current martian cratering rate abstract the discovery of 248 dated impact sites known to have formed within the last few decades allows us to refine the current cratering rate and slope of the production function at mars we use a subset of 44 of these new craters that were imaged before and after impact by mars reconnaissance orbiter s context camera a thoroughly searched data set that minimizes biases from variable image resolutions we find the current impact rate is 1 65 10 6 craters with an effective diameter 3 9 m km2 yr with a differential slope power law exponent of 2 45 0 36 this results in model ages that are factors of three to five below the hartmann hartmann w k 2005 icarus 174 294 320 and neukum et al neukum g ivanov b a hartmann w k 2001 space sci rev 96 55 86 ivanov ivanov b a 2001 space sci rev 96 87 104 model production functions where they overlap in diameter the best fit production function we measure has a shallower slope than model functions at these sizes but model function slopes are within the statistical errors more than half of the impacts in this size range form clusters which is another reason to use caution when estimating surface ages using craters smaller than 50 m in diameter

periglacial landforms at the phoenix landing site and the northern plains of mars 1 we examine potentially periglacial landforms in mars orbiter camera moc and high resolution imaging science experiment hirise images at the phoenix landing site and compare them with numerical models of permafrost processes to better understand the origin nature and history of the permafrost and the surface of the northern plains of mars small scale 3 6 m polygonal patterned ground is ubiquitous throughout the phoenix landing site and northern plains larger scale 20 25 m polygonal patterns and regularly spaced 20 35 m rubble piles localized collections of rocks and boulders are also common rubble piles were previously identified as basketball terrain in moc images the small polygon networks exhibit well developed and relatively undegraded morphology and they overlay all other landforms comparison of the small polygons with a numerical model shows that their size is consistent with a thermal contraction origin on current day mars and are likely active in addition the observed polygon size is consistent with a subsurface rheology of ice cemented soil on depth scales of about 10 m the size and morphology of the larger polygonal patterns and rubble piles indicate a past episode of polygon formation and rock sorting in thermal contraction polygons while the ice table was about twice as deep as it is presently the pervasive nature of small and large polygons and the extensive sorting of surface rocks indicates that widespread overturning of the surface layer to depths of many meters has occurred in the recent geologic past this periglacial reworking has had a significant influence on the landscape at the phoenix landing site and over the martian northern plains

noble metal abundances in an early archean impact deposit we report detailed analyses on the concentrations of the noble metals pd os ir pt and au in an early archean spherule bed s4 of probable impact origin from the lower fig tree group barberton greenstone belt south africa compared to other sedimentary deposits of known or suspected impact origin some noble metals are present in exceptionally high concentrations noble metal abundances are fractionated relative to abundances in chondrites with ratios of osir ptir pdir and auir at only 80 80 41 and 2 of these values in ci chondrites although an extraterrestrial source is favored for the noble metal enrichment the most plausible cause of the fractionation is by regional hydrothermal metasomatic alteration

modeling the development of martian sublimation thermokarst landforms abstract sublimation thermokarst landforms result from collapse of the surface when ice is lost from the subsurface on mars scalloped landforms with scales of decameters to kilometers are observed in the mid latitudes and considered likely thermokarst features we describe a landscape evolution model that couples diffusive mass movement and subsurface ice loss due to sublimation over periods of tens of thousands of mars years under conditions similar to the present the model produces scallop like features similar to those on the martian surface starting from much smaller initial disturbances the model also indicates crater expansion when impacts occur in surfaces underlain by excess ice to some depth with morphologies similar to observed landforms on the martian northern plains in order to produce these landforms by sublimation substantial quantities of excess ice are required at least comparable to the vertical extent of the landform and such ice must remain in adjacent terrain to support the non deflated surface we suggest that martian thermokarst features are consistent with formation by sublimation without melting and that significant thicknesses of very clean excess ice up to many tens of meters the depth of some scalloped depressions are locally present in the martian mid latitudes climate conditions leading to melting at significant depth are not required

geological and geochemical record of 3400 million year old terrestrial meteorite impacts beds of sand sized spherules in the 3400 million year old fig tree group barberton greenstone belt south africa formed by the fall of quenched liquid silicate droplets into a range of shallow to deep water depositional environments the regional extent of the layers their compositional complexity and lack of included volcanic debris suggest that they are not products of volcanic activity the layers are greatly enriched in iridium and other platinum group elements in roughly chondritic proportions geochemical modeling based on immobile element abundances suggests that the original average spherule composition can be approximated by a mixture of fractionated tholeiitic basalt komatiite and ci carbonaceous chondrite the spherules are thought to be the products of large meteorite impacts on the archean earth

cold and dry processes in the martian arctic geomorphic observations at the phoenix landing site and comparisons with terrestrial cold desert landforms 1 we analyze surface stereo imager observations of rocks sediments and permafrost related landforms in the vicinity of the phoenix lander comparing the imaged features to analogous examples of physical weathering and periglacial processes observed in the antarctic dry valleys observations at the phoenix landing site of pitted rocks puzzle rocks undergoing in situ breakdown perched clasts and thermal contraction crack polygon morphologies strikingly similar to terrestrial sublimation polygons all strongly suggest that stable non churning permafrost processes dominate the phoenix landing site morphological evidence suggests that cold desert processes in the absence of wet active layer cryoturbation and largely driven by sublimation of buried ice either pore ice excess ice or both are shaping the landscape

initiation and growth of martian ice lenses abstract water ice in the upper meters of the martian regolith is a major volatile reservoir although the geographic extent burial depth and thermal stability of this shallow ice are well understood its origin history and stratigraphy are not over the past decade a growing body of observational evidence has indicated that shallow ground ice exceeds the pore volume of its host soil over large regions of both martian hemispheres this is confounding given that 1 the physical theory that accurately predicts the location of ground ice also assumes that ice should be pore filling in the upper meter of regolith and 2 the phoenix spacecraft uncovered far more pore filling ice than excess ice at its landing site in the northern hemisphere the development of ice lenses by low temperature in situ segregation analogous to the processes that generate frost heave on earth has been hypothesized to explain shallow excess ice on mars we have developed a numerical model of ice lens initiation and growth in the martian environment and used it to test this hypothesis for the first time we carried out a large suite of numerical simulations in order to place quantitative constraints on the timing and location of ice lens initiation and on the magnitude of ice lens growth in a variety of host soils we find that ice lens initiation is a ubiquitous process in the martian high latitudes but the ultimate magnitude of lens growth or frost heave is sensitive to the properties of the host soil depending on the specific properties of martian soils in situ segregation may be a very slow process sufficient to explain the excess ice observed in the dodo goldilocks trench at the phoenix landing site but without regionally significant effects alternatively if clay sized particles or perchlorate salts are present in situ segregation may be a vigorous process that has significantly affected the stratigraphy of ground ice in the upper meter of regolith throughout the high latitudes

early archean silicate spherules of probable impact origin south africa and western australia layers of sand sized silicate spherules in 3 2 to 3 5 ga archean greenstone belts in south africa and western australia formed by the accumulation of quenched liquid silicate droplets having primary compositions resembling those of immediately underlying rock sequences the layers and particles appear to be unrelated to volcanic activity and we suggest that both occurrences may represent melt droplets formed during meteorite or comet impacts if so these spherules are the oldest known terrestrial impact products if these objects are representative of a larger population of early archean impact features it may be possible to directly estimate the meteorite flux on the early archean earth

subsurface volatile content of martian double layer ejecta dle craters abstract excess ice is widespread throughout the martian mid latitudes particularly in arcadia planitia where double layer ejecta dle craters also tend to be abundant in this region we observe the presence of thermokarstically expanded secondary craters that likely form from impacts that destabilize a subsurface layer of excess ice which subsequently sublimates the presence of these expanded craters shows that excess ice is still preserved within the adjacent terrain here we focus on a 15 km dle crater that contains abundant superposed expanded craters in order to study the distribution of subsurface volatiles both at the time when the secondary craters formed and by extension remaining today to do this we measure the size distribution of the superposed expanded craters and use topographic data to calculate crater volumes as a proxy for the volumes of ice lost to sublimation during the expansion process the inner ejecta layer contains craters that appear to have undergone more expansion suggesting that excess ice was most abundant in that region however both of the ejecta layers had more expanded craters than the surrounding terrain we extrapolate that the total volume of ice remaining within the entire ejecta deposit is as much as 74 km 3 or more the variation in ice content between the ejecta layers could be the result of 1 volatile preservation from the formation of the dle crater 2 post impact deposition in the form of ice lenses or 3 preferential accumulation or preservation of subsequent snowfall we have ruled out 2 as the primary mode for ice deposition in this location based on inconsistencies with our observations though it may operate in concert with other processes although none of the existing dle formation hypotheses are completely consistent with our observations which may merit a new or modified mechanism we can conclude that dle craters contain a significant quantity of excess ice today

geomorphic evidence for the distribution of ground ice on mars high resolution viking orbiter images show evidence for quasi viscous relaxation of topography the relaxation is believed to be due to creep deformation of ice in near surface materials the global distribution of the inferred ground ice shows a pronounced latitudinal dependence the equatorial regions of mars appear to be ice poor while the heavily cratered terrain poleward of 30 latitude appears to be ice rich the style of creep poleward of 30 varies with latitude possibly due to variations in ice rheology with temperature the distribution suggests that ice at low latitudes which is not in equilibrium with the present atmosphere has been lost via sublimation and diffusion through the regolith thereby causing a net poleward transport of ice over martian history

mineralogy and diagenesis of 3 24ga meteorite impact spherules abstract spherules in the s3 bed of the barberton greenstone belt south africa are distal fallout formed within an impact plume after a large impact event at 3 24 ga since that time diagenesis and lower greenschist grade metamorphism of the spherules has changed the mineralogy though shape and texture are largely preserved alteration of the s3 bed has resulted in spherules composed of quartz phyllosilicates ti and fe oxides and some ni rich chromites initially glassy spherules were palagonitized and silica cementation of the spherules began during low temperature interaction with seawater further alteration by si and k rich fluids resulted in a mineralogical assemblage of quartz feldspar and clays crystalline minerals were replaced by dissolution precipitation processes preserving relict textures further silica cementation resulted in complete lithification of the bed most of this alteration occurred at the seafloor and during shallow burial with continued burial amorphous silica recrystallized to microcrystalline quartz later recrystallization of clays to micas occurred during regional metamorphism at peak temperatures of 300 320 c late stage shearing and mineralization preferentially affected the northern region of the belt samples from different sections record highly variable local conditions water depth the amount of pre depositional transport location within the belt and proximity to igneous dikes all affect the diagenesis of the s3 spherules silica and barite concentrations are lower and carbonate concentrations are higher in the deep water depositional environments element mobility during diagenetic and lower greenschist grade metamorphism can be inferred based on studies of multiple sections throughout the bgb the most immobile elements are al zr ti sc and the high field strength elements and present element ratios can be used to infer original composition of the spherules the large ion lithophile elements are highly mobile as are the light rare earth elements rees which are particularly susceptible to mobilization during carbonate diagenesis and phosphate authigenesis of the rees ce and eu show the largest variability suggesting significant mobilization during diagenesis and low grade metamorphism consistent cr ir ratios particularly in the high concentrations suggest limited mobility and segregation of platinum group elements into ni rich chromite bearing spherules sulfide mineralization has not affected the ir concentration in the s3 layer

hirise observations of new impact craters exposing martian ground ice twenty small new impact craters or clusters have been observed to excavate bright material inferred to be ice at mid latitudes and high latitudes on mars in the northern hemisphere the craters are widely distributed geographically and occur at latitudes as low as 39 n stability modeling suggests that this ice distribution requires a long term average atmospheric water vapor content around 25 precipitable micrometers more than double the present value which is consistent with the expected effect of recent orbital variations alternatively near surface humidity could be higher than expected for current column abundances if water vapor is not well mixed with atmospheric co2 or the vapor pressure at the ice table could be lower due to salts ice in and around the craters remains visibly bright for months to years indicating that it is clean ice rather than ice cemented regolith although some clean ice may be produced by the impact process it is likely that the original ground ice was excess ice exceeding dry soil pore space in many cases observations of the craters suggest small scale heterogeneities in this excess ice the origin of such ice is uncertain ice lens formation by migration of thin films of liquid is most consistent with local heterogeneity in ice content and common surface boulders but in some cases nearby thermokarst landforms suggest large amounts of excess ice that may be best explained by a degraded ice sheet

patterned ground in martian high northern latitudes morphology and age constraints we performed a systematic survey of typical surface patterns and small 5 50 m impact craters in a zone between 50 and 70 latitude in the northern lowlands of mars we found that small scale 10 m polygonal pattern formed by narrow fractures is typical in the northern half of the studied zone we interpret this pattern to be formed by active thermal contraction polygons associated with the present day shallow ground ice these fresh small polygons are superposed over a larger scale 15 30 m polygonal network of smooth gentle troughs and mounds the latter pattern extends southward from the shallow ground ice boundary to the southern edge of the study zone we interpret this pattern to be a relic from different climate conditions in the geologically recent past possibly to the most recent periods of high obliquity of mars spin axis the population of small craters is sparse and indicates intensive resurfacing in the recent geological past the retention age of 5 10 m diameter craters in the northern part of our study zone is surprisingly young on the order of hundreds to thousands of years which means that quick obliteration of such craters occurs under the present day spin orbit configuration of mars the crater obliteration time scale increases for larger craters and toward the southern edge of the study area

martian cratering 11 utilizing decameter scale crater populations to study martian history new information has been obtained in recent years regarding formation rates and the production size frequency distribution psfd of decameter scale primary martian craters formed during recent orbiter missions here we compare the psfd of the currently forming small primaries p with new data on the psfd of the total small crater population that includes primaries and field secondaries p fs which represents an average over longer time periods the two data sets if used in a combined manner have extraordinary potential for clarifying not only the evolutionary history and resurfacing episodes of small martian geological formations as small as one or few km2 but also possible episodes of recent climatic change in response to recent discussions of statistical methodologies we point out that crater counts do not produce idealized statistics and that inherent uncertainties limit improvements that can be made by more sophisticated statistical analyses we propose three mutually supportive procedures for interpreting crater counts of small craters in this context applications of these procedures support suggestions that topographic features in upper meters of mid latitude ice rich areas date only from the last few periods of extreme martian obliquity and associated predicted climate excursions

long term wind erosion on mars abstract wind erosion and deposition are powerful agents of surface change on mars erosion is sensitive to the atmospheric density so feedback between orbit variations and atmospheric density can enhance the sensitivity of erosion rates to orbital parameters we use statistics derived from a 1 gyr integration of the spin axis of mars coupled with runs of the nasa ames 3 d general circulation model gcm at a variety of orbital conditions and pressures to explore this feedback we find that wind erosion in the gcm is associated with two factors baroclinic zone winds and strong cross equator solstice flows both of these factors are influenced by topography producing an asymmetry in the erosion pattern between the north and the south the erosion model averaged over 1 gyr produces potential erosion rates of 1 7 10 5 m yr 1 in the north and 1 0 10 5 m yr 1 in the south which increase by an order of magnitude in an early 28 mbar atmosphere the stability of these potential erosion patterns over geological time indicates that the lowland regions of mars are continuously eroded and that wind is capable of revealing ancient sedimentary deposits or eroding substantial deposits that may have otherwise been preserved

martian perched craters and large ejecta volume evidence for episodes of deflation in the northern lowlands the northern lowland plains such as those found in acidalia and utopia planitia have high percentages of impact craters with fluidized ejecta in both regions the analysis of crater geometry from mars orbiter laser altimeter mola data has revealed large ejecta volumes some exceeding the volume of excavation moreover some of the crater cavities and fluidized ejecta blankets of these craters are topographically perched above the surrounding plains these perched craters are concentrated between 40 and 70 n in the northern plains the atypical high volumes of the ejecta and the perched craters suggest that the northern lowlands have experienced one or more episodes of resurfacing that involved deposition and erosion the removal of material most likely caused by the sublimation of ice in the materials and their subsequent erosion and transport by the wind is more rapid on the plains than on the ejecta blankets the thermal inertia difference between the ejecta and the surrounding plains suggests that ejecta characterized by a lower thermal inertia protect the underneath terrain from sublimation this results in a decreased elevation of the plains relative to the ejecta blankets sublimation and eolian erosion can be particularly high during periods of high obliquity

dielectric map of the martian northern hemisphere and the nature of plain filling materials 1 a number of observations suggest that an extended ocean once covered a significant part of the martian northern hemisphere by probing the physical properties of the subsurface to unprecedented depth the marsis mars express provides new geophysical evidences for the former existence of a late hesperian ocean the vastitas borealis formation located inside a putative shoreline of the ancient ocean has a low dielectric constant compared with that of typical volcanic materials we show that the measured value is only consistent with low density sedimentary deposits massive deposits of ground ice or a combination of the two in contrast radar observations indicate a distribution of shallow ground ice in equilibrium with the atmosphere in the south polar region we conclude that the northern plains are filled with remnants of a late hesperian ocean fed by water and sediments from the outflow channels about 3 gy ago

the periglacial landscape at the phoenix landing site 1 the mars scout mission phoenix landed in the northern high latitudes of mars in an area thought to be dominated by subsurface ground ice and has provided a unique detailed examination of periglacial morphology and topography that is not visible from orbit the landing site and surrounding terrain is dominated by polygonal patterned ground typical of the martian northern plains the polygons exhibit a gentle u shaped trough and rounded mound topography with an average trough depth of 17 cm and an average surface slopes of only 6 7 polygons an average 4 3 m across form a pattern superimposed on all other geologic landforms including larger 20 25 m polygons and kilometer long linear ridges the polygon morphology and the predominance of subsurface ice cemented soil are consistent with the formation of subsurface sand wedges in a permanently frozen arid climate small scale soil furrows and cracks into the ice cemented soil indicate that the thermal contraction process responsible for the formation of the polygons is active today rocks appear to be sorted within the polygon troughs indicating a process of cryoturbation within the permanently frozen polygons the complexity of the patterns crosscutting relations and rock distributions indicate advanced maturity of the polygon networks and shifting crack patterns possibly facilitated by crack and wedge annealing during times of low obliquity

present day impact cratering rate and contemporary gully activity on mars the mars global surveyor mars orbiter camera has acquired data that establish the present day impact cratering rate and document new deposits formed by downslope movement of material in mid latitude gullies on mars twenty impacts created craters 2 to 150 meters in diameter within an area of 21 5 106 square kilometers between may 1999 and march 2006 the values predicted by models that scale the lunar cratering rate to mars are close to the observed rate implying that surfaces devoid of craters are truly young and that as yet unrecognized processes of denudation must be operating the new gully deposits formed since august 1999 are light toned and exhibit attributes expected from emplacement aided by a fluid with the properties of liquid water relatively long extended digitate distal and marginal branches diversion around obstacles and low relief the observations suggest that liquid water flowed on the surface of mars during the past decade

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

latitude dependence of martian pedestal craters evidence for a sublimation driven formation mechanism we report on the results of a survey to document and characterize pedestal craters on mars equatorward of similar to 60 degrees n and 65 degrees s latitude the identification of 2696 pedestal craters reveals a strong latitude dependence with the vast majority found poleward of 33 degrees n and 40 degrees s this latitudinal extent is correlated with many climate indicators consistent with the presence of an ice rich substrate and with climate model predictions of where ice is deposited during periods of higher obliquity in the amazonian we have measured key physical attributes of pedestal craters including the farthest radial extents of the pedestals pedestal heights and the circularity of the pedestal margins in conjunction with the geographic distribution our measurements strongly support a sublimation related formation mechanism this is in contrast to previous hypotheses which have relied on eolian deflation to produce the elevated plateaus the identification of marginal pits on the scarps of some pedestal craters interpreted to be sublimation pits provide direct evidence for the presence of ice rich material underlying the armored surface of pedestal craters on the basis of our findings we propose a formation mechanism whereby projectiles impact into a volatile rich dust snow ice substrate tens to hundreds of meters thick overlying a dominantly fragmental silicate regolith the area surrounding the resulting crater becomes armored pedestals extend to a distance of multiple crater radii farther than typical ejecta deposits necessitating an armoring mechanism that is capable of indurating the surface to a distance greater than the reach of the ejecta return to low obliquity causes sublimation of the volatile rich layer from the intercrater plains lowering the elevation of the regional terrain this yields generally circular pedestal craters elevated above the surrounding plains as a result the armored surfaces of pedestal craters have preserved a significant record of amazonian climate history in the form of ice rich deposits

possible mechanism of boulder clustering on mars whatever causes small scale polygonal patterned ground diameter 20 m to form pervasively in the northern plains of mars appears to also cause boulders to cluster at polygon margins assuming that these polygons are caused by the seasonal thermal contraction and expansion of ice in the near surface to meters of depth we propose a mechanism that ratchets boulders towards the edges in seasonal cycles on present day mars during the contraction cooling phase a co2 frost layer embeds the boulders buffered by condensation and sublimation the solid co2 is isothermal and does not contract we propose that it locks the boulders in place while the h2o ice in the near surface continues cooling and contracts beneath them in spring summer seasons when the frost sublimates and heating of the surface begins the boulders freely move with the expanding near surface ice we show that over many seasons this process would lead to progressive boulder movement towards polygon edges with an estimated rate of boulder migration 0 1 mm yr under present mars conditions

mars in ice ages for 25 of post noachian geologic history abstract mars is currently a hyperarid hypothermal desert planet whose surface inventory of water is primarily confined to the north and south polar ice caps previous studies have shown that mars has undergone massive shifts in its spin axis obliquity present day is 25 2 due to secular spin orbit resonances during periods of higher obliquity water ice from the polar caps is mobilized to the mid latitudes 35 obliquity and even the equator 45 obliquity where it is deposited as snow and accumulates over time to form thick regional surface ice sheets abundant evidence exists today for the remnants of these ice ages in the form of debris covered glaciers and ice deposits but due to the chaotic nature of orbital simulations beyond 20 ma it has remained unclear to what temporal extent mars has experienced such ice ages recent developments have suggested that impact events which formed in martian surface ice deposits exhibit a distinctive double layered ejecta morphology in tandem with cratering statistics this observation offers the potential to better our understanding of the history of ice ages on mars this work explores the timing of ice age events by evaluating the size frequency distribution of craters forming in surface ice using hartmann isochron model ages this work shows that mars has experienced mid latitude ice ages for up to a cumulative 680 myr out of the past 3 6 ga and experienced equatorial ice ages for up to a cumulative 250 myr within the same time period the results of this study indicate that mars has experienced mid latitude equatorial ice age states for up to approximately 25 of its post noachian geologic history emphasizing that much of the geologic history of mars is dominated by the presence of widespread non polar surface ice sheets

physics of crustal fracturing and chert dike formation triggered by asteroid impact 3 26 ga barberton greenstone belt south africa archean asteroid impacts reflected in the presence of spherule beds in the 3 2 3 5 ga barberton greenstone belt bgb south africa generated extreme seismic waves spherule bed s2 provides a field example it locally lies at the contact between the onverwacht and fig tree groups in the bgb which formed as a result of the impact of asteroid possibly 50 km diameter scaling calculations indicate that very strong seismic waves traveled several crater diameters from the impact site where they widely damaged onverwacht rocks over much of the bgb lithified sediments near the top of the onverwacht group failed with opening mode fractures the underlying volcanic sequence then failed with normal faults and opening mode fractures surficial unlithified sediments liquefied and behaved as a fluid these liquefied sediments and some impact produced spherules filled near surface fractures today represented by swarms of chert dikes strong impact related tsunamis then swept the seafloor p waves and rayleigh waves from the impact greatly exceeded the amplitudes of typical earthquake waves the duration of extreme shaking was also far longer probably hundreds of seconds than that from strong earthquakes dynamic strains of 10 3 occurred from the surface and downward throughout the lithosphere shaking weakened the onverwacht volcanic edifice and the surface layers locally moved downhill from gravity accommodated by faults and open mode fractures coast parallel opening mode fractures on the fore arc coast of chile formed as a result of megathrust events are the closest modern analogs it is even conceivable that dynamic stresses throughout the lithosphere initiated subduction beneath the onverwacht rocks

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

crater morphology and evolution at the phoenix landing site insights into timing of modern geological processes and subsurface ice abstract we report results of an in depth study of the morphology of small km scale and smaller craters over a 4000 km2 region on mars that includes the phoenix landing ellipse focused on the nature and timing of geological processes that have been active in the region the study seeks to constrain regolith depositional rates thickness of ground ice layer abundance of ice in the subsurface and the rate of crater degradation measurements of the size of ejecta blocks around craters show that the region has not been a net depositional site over the last billion years most craters smaller than 200 350 m lack ejecta blocks regardless of age indicating that there is a layer approximately 20 35 m deep that either does not form or does not retain ejecta blocks this layer is expressed both within and outside the extended ejecta blanket of heimdal crater implying that it is not associated with the ejecta blanket we propose that this unconsolidated layer consists dominantly of ice cemented soil which raises questions about the emplacement of ice at depth we also note clear progression in crater degradation morphology which allows us to place constraints on the rate and style of crater degradation small craters 20 to 200 m in diameter appear to lose their relief due to slow infilling of the bowl over time scales of 5 100 kyr the infilling of small craters perhaps by a combination of ice and dust presents a mechanism by which ice is buried down to a few tens of meters on the other hand larger craters 200 m to 1 km do not exhibit evidence for infilling but do exhibit concentric sets of fractures within and outside the rim we propose that loss of relief of the larger craters is due to viscous creep over time scales of a few to tens of myr this latter interpretation would require the presence of water ice in abundances greater than pore filling down to depths of at least tens of meters

viscous relaxation of craters within the martian south polar layered deposits abstract the south polar layered deposits pld are of fundamental importance to martian climatology as they may comprise the largest reservoir of near surface water on mars the south pld exhibit relatively young crater retention surface ages which are widely attributed to recent resurfacing however we show that both constructional and destructional resurfacing mechanisms such as dust deposition and water ice sublimation respectively are inconsistent with the size depth and spatial distributions of south pld craters we demonstrate that another process viscous creep relaxation of dusty water ice is more compatible with the observed cratering of south pld surfaces the results of our finite element relaxation simulations suggest that despite their apparent youthfulness the pld have been stable for at least several hundred million and perhaps even over a billion years consequently our modeling implies that the time scales for the formation and preservation of the layers characteristic of the south and possibly north pld are much longer than generally assumed

minimum effective area for high resolution crater counting of martian terrains the acquisition of high resolution imagery for the surface of mars has enabled mapping of spatially limited order of 200 m on four type terrains using mars reconnaissance orbiter mro context camera ctx imagery that span the noachian hesperian and amazonian epochs the counts from each location include a region covering 10 000 km2 ten 1000 km2 subsets of that larger area and approximately one hundred 100 km2 samples the data demonstrate significant variation in the crater size frequency and derived model ages across a single terrain type for the 100 km2 samples the crater size frequency at this area scale varies across a single uniform geologic unit by up to a factor of 2 3 on the four different terrains at 1000 km2 the local pattern variations that are relevant at the 100 km2 scale become less important and the age variations are tighter in all four terrain cases the 10 000 km2 and 1000 km2 samples capture distinct crater populations km sized craters that formed before and after resurfacing event s however due to the relatively high mean distance between km sized craters the 100 km2 size area samples more commonly than not exclude a statistically significant sample at the kilometer size range masking important information about the pre resurfacing history of the terrain we therefore suggest that due to the effect of pattern variability in cratering over 100 km2 and the susceptibility of smaller craters to resurfacing crater counts derived from small area samples are suspect to major uncertainties

compositions of subsurface ices at the mars phoenix landing site 1 nasa s phoenix lander uncovered two types of ice at its 2008 landing site on the northern plains of mars a brighter slab like ice that broke during robotic arm operations and a darker icy deposit spectra from the phoenix surface stereo imager ssi are used to demonstrate that the brighter material consists of nearly pure water ice which probably formed by migration and freezing of liquid water the darker icy material consists of 30 20 wt ice with the remainder composed of fine grained soil indicating that it probably formed as pore ice these two types of ice represent two different emplacement mechanisms and periods of deposition

chronology of early archaean granite greenstone evolution in the barberton mountain land south africa based on precise dating by single zircon evaporation abstract we report precise 207 pb 206 pb single zircon evaporation ages for low grade felsic metavolcanic rocks within the onverwacht and fig tree groups of the barberton greenstone belt bgb south africa and from granitoid plutons bordering the belt dacitic tuffs of the hooggenoeg formation in the upper part of the onverwacht group yield ages between 3445 3 and 3416 5 ma and contain older crustal components represented by a 3504 4 ma old zircon xenocryst fig tree dacitic tuffs and agglomerates have euhedral zircons between 3259 5 and 3225 3 ma in age which we interpret to reflect the time of crystallization a surprisingly complex xenocryst population in one sample documents ages from 3323 4 to 3522 4 ma we suspect that these xenocrysts were inherited during the passage of the felsic melts to the surface from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite trondhjemite plutons along the southern and western margins of the bgb and units predating any of the exposed greenstone or intrusive rocks several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 ma coeval with or slightly older than onverwacht felsic volcanism while the kaap valley pluton along the northwestern margin of the belt is coeval with fig tree dacitic volcanism these results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites some of the volcanic and plutonic units contain zircon xenocrysts older than any exposed rocks these indicate the existence of still older units possibly stratigraphically lower and older portions of the greenstone sequence itself older granitoid intrusive rocks or bodies of older unrelated crustal material our data show that the onverwacht and fig tree felsic units have distinctly different ages and therefore do not represent a single tectonically repeated unit as proposed by others unlike the late archaean abitibi greenstone belt in canada which formed over about 30 ma exposed rocks in the bgb formed over a period of at least 220 ma the complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones a surprising similarity in rock types tectonic evolution and ages of the bgb in the kaapvaal craton of southern africa and greenstones in the pilbara block of western australia suggests that these two terrains may have been part of a larger crustal unit in early archaean times

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

distribution of mid latitude ground ice on mars from new impact craters new impact craters at five sites in the martian mid latitudes excavated material from depths of decimeters that has a brightness and color indicative of water ice near infrared spectra of the largest example confirm this composition and repeated imaging showed fading over several months as expected for sublimating ice thermal models of one site show that millimeters of sublimation occurred during this fading period indicating clean ice rather than ice in soil pores our derived ice table depths are consistent with models using higher long term average atmospheric water vapor content than present values craters at most of these sites may have excavated completely through this clean ice probing the ice table to previously unsampled depths of meters and revealing substantial heterogeneity in the vertical distribution of the ice itself

sharad soundings and surface roughness at past present and proposed landing sites on mars reflections at phoenix may be attributable to deep ground ice we use the shallow radar sharad on the mars reconnaissance orbiter to search for subsurface interfaces and characterize surface roughness at the landing sites of viking landers 1 and 2 mars pathfinder the mars exploration rovers spirit and opportunity the phoenix mars lander the mars science laboratory curiosity rover and three other sites proposed for curiosity only at the phoenix site do we find clear evidence of subsurface radar returns mapping out an interface that may be the base of ground ice at depths of 15 66 m across 2900 km2 in the depression where the lander resides at the opportunity spirit and candidate curiosity sites images and altimetry show layered materials tens to hundreds of meters thick extending tens to hundreds of kilometers laterally these scales are well within sharad s resolution limits so the lack of detections is attributable either to low density contrasts in layers of similar composition and internal structure or to signal attenuation within the shallowest layers at each site we use the radar return power to estimate surface roughness at scales of 10 100 m a measure that is important for assessing physical properties landing safety and site trafficability the strongest returns are found at the opportunity site indicating that meridiani planum is exceptionally smooth returns of moderate strength at the spirit site reflect roughness more typical of mars gale crater curiosity s ultimate destination is the smoothest of the four proposed sites we examined with holden crater eberswalde crater and mawrth vallis exhibiting progressively greater roughness

deposition and degradation of a volatile rich layer in utopia planitia and implications for climate history on mars 1 we investigate the surface morphology of a study area in western utopia planitia mars which is characterized by a variety of landforms that resemble those of terrestrial periglacial landscapes polygonally fractured ground and thermokarst like depressions are found to be located in a young mantling deposit with a thickness of several tens of meters we observe a latitudinal dependence of the degradation of this mantling deposit larger depressions whose floors reveal the underlying basement rocks cover a higher fraction of the total terrain in the southern part of the study area than in the northern part indicating a more intense degradation of the mantle in the south all depressions have an asymmetric cross section in north south direction interpreted to be the result of the different solar radiation on differently oriented slopes on the basis of our morphological observations we develop a conceptual model for landscape evolution in western utopia planitia it involves subaerial deposition of a layered ice rich mantle and its subsequent degradation by polygon formation and sublimation a terrestrial analog to the polygonally fractured mantling deposit and its thermokarst like depressions is the siberian ice complex or yedoma which consists of subaerial ice rich deposits and is connected to nonglaciated landscapes with highly continental cold climatic environmental conditions our comparison suggests that no unusual or exotic processes need to be invoked to explain the current morphology of western utopia however the young age of the deposition and degradation implies climatic excursions in the very recent past on mars

early archean spherule beds chromium isotopes confirm origin through multiple impacts of projectiles of carbonaceous chondrite type three early archean spherule beds from barberton south africa have anomalous cr isotope compositions in addition to large ir anomalies confirming the presence of meteoritic material with a composition similar to that in carbonaceous chondrites the extra terrestrial components in beds s2 s3 and s4 are estimated to be approx l 50 60 and 15 30 respectively these beds are probably the distal and possibly global ejecta from major large body impacts these impacts were probably much larger than the cretaceous tertiary event and all occurred over an interval of approx 20 m y implying an impactor flux at 3 2 ga that was more than an order of magnitude greater than the present flux

the stratigraphy of the 3 5 3 2 ga barberton greenstone belt revisited a single zircon ion microprobe study abstract recent field and geochemical studies indicate a need to test the stratigraphy of the ca 3 5 ga barberton greenstone belt as it is presently adopted 1 2 this work uses the ion microprobe shrimp to attempt such a test results show that 1 volcaniclastic sediments of the theespruit formation

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