environmental perturbations caused by the impacts of asteroids and comets we review the major impact associated mechanisms proposed to cause extinctions at the cretaceous tertiary geological boundary we then discuss how the proposed extinction mechanisms may relate to the environmental consequences of asteroid and comet impacts in general our chief goal is to provide relatively simple prescriptions for evaluating the importance of impacting objects over a range of energies and compositions but we also stress that there are many uncertainties we conclude that impacts with energies less than about 10 mt are a negligible hazard for impacts with energies above 10 mt and below about 104 mt i e impact frequencies less than one in 6 104 years corresponding to comets and asteroids with diameters smaller than about 400 m and 650 m respectively blast damage earthquakes and fires should be important on a scale of 104 or 105 km which corresponds to the area damaged in many natural disasters of recent history however tsunami excited by marine impacts could be more damaging flooding a kilometer of coastal plain over entire ocean basins in the energy range of 104 105 mt intervals up to 3 105 years corresponding to comets and asteroids with diameters up to 850 m and 1 4 km respectively water vapor injections and ozone loss become significant on the global scale in our nominal model such an impact does not inject enough submicrometer dust into the stratosphere to produce major adverse effects but if a higher fraction of pulverized rock than we think likely reaches the stratosphere stratospheric dust causing global cooling would also be important in this energy range thus 105 mt is a lower limit where damage might occur beyond the experience of human history the energy range from 105 to 106 mt intervals up to 2 106 years corresponding to comets and asteroids up to 1 8 and 3 km diameter is transitional between regional and global effects stratospheric dust sulfates released from within impacting asteroids and soot from extensive wild fires sparked by thermal radiation from the impact can produce climatologically significant global optical depths of the order of 10 moreover the ejecta plumes of these impacts may produce enough no from shock heated air to destroy the ozone shield between 106 and 107 mt intervals up to 1 5 107 years corresponding to comets and asteroids up to 4 and 6 5 km diameter dust and sulfate levels would be high enough to reduce light levels below those necessary for photosynthesis ballistic ejecta reentering the atmosphere as shooting stars would set fires over regions exceeding 107 km and the resulting smoke would reduce light levels even further at energies above 107 mt blast and earthquake damage reach the regional scale 106 km tsunami cresting to 100 m and flooding 20 km inland could sweep the coastal zones of one of the world s ocean basins fires would be set globally light levels may drop so low from the smoke dust and sulfate as to make vision impossible at energies approaching 109 mt 108 years the ocean surface waters may be acidified globally by sulfur from the interiors of comets and asteroids the cretaceous tertiary impact in particular struck evaporate substrates that very likely generated a dense widespread sulfate aerosol layer with consequent climatic effects the combination of all of these physical effects would surely represent a devastating stress on the global biosphere

compositions of large metal nodules in mesosiderites links to iron meteorite group iiiab and the origin of mesosiderite subgroups large mesosiderite metal nodules were studied petrographically and by neutron activation their large sizes minimized the effects of silicate contamination and dilution by fe reduced from feo the remarkable mixture of subequal amounts of metal and basaltic and pyroxenitic silicates in mesosiderites can be explained by the accretion of a core like metal mass from one asteroid onto the surface of another asteroid our data show that the accreted mass had a composition roughly similar to that of the iiiab iron meteorite core consistent with other evidence of close genetic links between these and other related groups the siderophile compositional pattern the very limited compositional range the inferred large projectile target ratio and the electrical interconnectedness of mesosiderite metal indicate that the bulk of the metal was molten when accreted mesosiderites have been divided into plagioclase and tridymiterich subgroup a and orthopyroxene rich subgroup b on the basis of modal silicate mineralogy roughly the same subgroups are defined by metal compositions au as ni and cu are generally higher and w lower in subgroup a than in subgroup b in our sample set a widmanstatten pattern is only observed in nodules from subgroup a a plausible model is that subgroup a best preserves the compositions of the target regolithic silicates and accreted core subgroup b is comprised of these materials together with a minor amount of coremantle materials consisting of olivine and refractory ni poor metal the independent covariation of ga re and ir may in part reflect fractionations during impact heating events the anomalous mesosiderite reckling peak a79015 is strongly depleted in refractory siderophiles and slightly enriched in volatile siderophiles such evolved metal requires a formation distinct from common mesosiderite metal

definitive evidence of hesperian basalt in acidalia and chryse planitiae 1 the surface materials of acidalia and chryse planitiae mars have been variably interpreted morphologically as basalt and compositionally as andesite or altered basalt a thorough survey using the compact reconnaissance imaging spectrometer for mars crism near infrared spectrometer reveals olivine and high calcium pyroxene signatures in the shallow subsurface exposed by impact craters these signatures indicate that the near surface of these regions are primarily basaltic in nature and are similar to other regions of hesperian volcanism across mars e g syrtis major previous spectroscopic studies using coarser resolution data sets have failed to identify these mafic signatures due to obscuration of the underlying material in addition to the relatively small scale exposures in impact craters the nature of the surficial obscuration material is likely multifaceted and includes the deposition of latitude dependent mantling material and the presence of alteration rinds this observation shows that the northern plains of mars are basaltic and must be factored into the thermal and surficial evolution of mars

high noble metal concentrations in a late pliocene sediment a 2 3 myr old layer in a sediment from the antarctic ocean contains ir and au at levels comparable with those at the cretaceous tertiary boundary a sizable fraction of the noble metals is contained in vesicular millimetre sized poly mineralic grains that closely resemble ablation debris from chondritic meteorites and there is little doubt that the noble metals resulted from the accretion of a large extraterrestrial object no massive extinctions or other evidence of environmental stress seem to be associated with this accretionary event

composition of impact melt debris from the eltanin impact strewn field bellingshausen sea abstract the impact of the km sized eltanin asteroid into the bellingshausen sea produced mm to cm sized vesicular impact melt rock particles found in sediment cores across a large area of the ocean floor these particles are composed mainly of olivine and glass with minor chromite and traces of nife sulfides some particles have inclusions of unmelted mineral and rock fragments from the precursor asteroid although all samples of melt rock examined have experienced significant alteration since their deposition in the late pliocene a significant portion of these particles have interiors that remain pristine and can be used to estimate the bulk composition of the impact melt the bulk composition of the melt rock particles is similar to the composition of basaltic meteorites such as howardites or mesosiderite silicates with a contribution from seawater salts and a siderophile rich component there is no evidence that the eltanin impact melt contains a significant terrestrial silicate component that might have been incorporated by mixing of the projectile with oceanic crust if terrestrial silicates were incorporated into the melt then their contribution must be much

geochronological evidence supporting antarctic deglaciation three million years ago the response of the antarctic ice sheets to increased global temperatures is an important unresolved issue in the assessment of future climate change in particular considerable controversy exists as to whether the east antarctic ice sheet suffered extensive deglaciation during the mid pliocene epoch 3 myr ago when temperatures were only slightly warmer than today although the ice sheet is widely assumed to have existed in something like its present form for the past 14 myr ref 1 marine diatoms eroded from the antarctic interior have been found in glacial till deposits high in the transantarctic mountains2 3 and have been bio stratigraphically dated at 3 myr before present this age has been disputed4 because it implies marine deposition in the antarctic interior and hence substantial deglaciation at a time when other evidence has been marshalled for the persistence of cold polar conditions4 here we report k ar and 40ar 39 ar ages for a volcanic ash bed in diatom bearing glaciomarine strata cored in ferrar fiord east antarctica by the ciros 2 drill hole5 which confirm the age of the diatoms at 3 myr and hence also confirm the mid pliocene deglaciation

dating of sirius group tillites in the antarctic dry valleys with cosmogenic3he and21ne we measured in situ produced cosmogenic3he and21ne in boulder and bedrock surface samples in and near sirius group tillites from two localities in southern victoria land antarctica pyroxene quantitatively retains cosmogenic3he for millions of years the deposit at mt fleming has a minimum exposure age of 6 5 ma while 6 ma is a probable minimum age for the table mountain tillite these lower limits not taking into account erosion of the sampled surfaces are based on the samples with the highest concentrations of cosmogenic noble gases and currently accepted production rates since the plagioclase bearing mt fleming samples almost certainly lost part of their cosmogenic ne the minimum exposure age at this location is presumably even 50 higher than the stated value the concentrations of cosmogenic ne in our samples constrain uplift rates of the transantarctic mountains to 170 m ma these data contradict the hypothesis of a collapse of the east antarctic ice sheet in the pliocene coupled with high uplift rates and support the model of a stable ice sheet since the middle miocene

the lomonosov crater impact event a possible mega tsunami source on mars recent research suggests that major meteorite impact events into a late hesperian early amazonian ocean likely produced a mega tsunami that would have resurfaced coastal areas in northwestern arabia terra the orientations of the associated lobate deposits a conspicuous type of landforms called thumbprint terrain suggests that if an impact event triggered the mega tsunami the most likely location of the source crater is within the northern plains regions situated north of arabia terra this study focuses on the identification of impact craters that impacted into the ocean and are likely to have produced the tsunami we selected 10 complex impact craters based on their diameters location and geomorphic characteristics of those the late hesperian 120 km diameter lomonosov crater exhibits a unique topographic plan view asymmetry compared to other similar sized and similar aged craters in the northern plains such as micoud korolev and milankovic we attribute its broad and shallow rim in part to an impact into a shallow ocean as well as its subsequent erosion from the collapsing transient water cavity the likely marine formation of the lomonosov crater and the apparent agreement in its age with that of the thumbprint terrain unit 3 ga strongly suggests that it was the source crater of the tsunami these results have implications for the stability of a late northern ocean on mars

the acidalia mensa region on mars a key element to test the mars ocean hypothesis abstract we present the results of a detailed photogeological study of a region surrounding acidalia mensa 43 50 n 15 35 w based on analysis of a mosaic of the ctx images nominal resolution 5 m px the main result of the study is the unfolding of the geologic history of the study region and determination of absolute model ages of its major episodes the documented history begins with the formation of a volcanic plateau of acidalia mensa 4 ga ago the surface of the acidalia plateau was strongly modified since its formation the crater size frequency distribution csfd on the plateau indicates that the resurfacing was likely consisted of multiple episodes the latest such episode recognizable by the crater statistics occurred 3 65 ga ago the vast majority of the surface around acidalia mensa is covered by materials of the background plains that correspond to the interior unit of the vastitas borealis formation vbf the csfd on the background plains corresponds to an absolute model age ama of 3 58 ga which is completely within the range of amas determined for the vbf along its boundary with the southern uplands small scale features such as bright cones mounds polygonal troughs and flow like mantling materials postdate the emplacement of the background plains and manifest the post formational evolution of the vbf the majority of crater ejecta in the study area have higher albedo in the ctx and hirise images but appear as dark spots in the themis ir nighttime images these wavelength dependent properties of the ejecta indicate the presence of light toned and fine grained material beneath the surface of the background plains this material was the source of numerous bright cones mounds we interpret the formation of this reservoir as the result of sorting of large amounts of water mud deposited in the study area and by implication within the northern lowlands the final episodes of the ancient geologic history of the study area were related to the emplacement of materials of the thick flows at topographically high levels within the acidalia plateau 3 47 ga ago the morphology of the flows is inconsistent with lava and we interpret the occurrences of the thick flows as the result of displacement of water saturated regolith in the form of either soli or gelifluction flows or ice cored rock glaciers

a depositional record of deglaciation in a paleofjord late carboniferous pennsylvanian of san juan province argentina the role of catastrophic sedimentation the combination of high sediment supply rates and ample accommodation within fjords provides high resolution records of deglaciation ancient fjord fills provide the potential for three dimensional views of the evolution of depositional environments in response to changing sediment supply and base level through the deglacial process we describe the fill jejenes formation of a well exposed late carboniferous pennsylvanian glacial valley and its tributaries these deposits record the progressive marine flooding and infilling of topography by glacially derived sediments the geometry of the valley and its tributaries is controlled by the underlying lower paleozoic lithologies a deep valley with steep sides exists where the bedrock is massive limestones and a broader shallower valley exists where the bedrock is generally a fine grained olistostrome the valleys are locally floored with diamictites including both in situ tillites and remobilized diamictites in the trunk valley these are locally overlain by a small shallow water delta the major part of the valley fill consists of a 150 m thick mudstone dominated succession probably generated by plumes of glacial outwash containing numerous dropstones that decrease in abundance down the fjord the mudstones contain numerous thin sandstone and conglomerate turbidites that were supplied laterally via subaqueous gravel fans feeding in from the tributary valleys each of which has a distinctive clast suite related to the local subcrop the entire succession is overlain by 300 m of sandy turbidites of which the upper part includes large mass transport complexes intercalated within the succession in the trunk valley are structureless graded silty mudstones lacking dropstones but with abundant large wood fragments close to the steep western margin of the trunk valley each of these massive mudstones is underlain by a slump or debris flow locally containing meter scale blocks of ordovician limestone from the valley side we interpret these as a consequence of rockfalls from the steeper valley sides triggering debris flows on the subaqueous fjord slopes we suggest that large solitary waves were generated as the rockfalls entered the water traveling along the fjord and stripping vegetation from the shoreline large amounts of mud and silt were thrown into suspension during these events and subsequently settled from suspension to form the structureless graded beds these deposits and other mass flow deposits within the succession emphasize the potential importance of catastrophic sedimentation within deglacial successions

the early cenozoic tectonic history of the southeast pacific abstract new marine geophysical data collected in the southeast pacific enable us to define better the early cenozoic tectonic history of the area and to interpret previously unexplained bathymetric features in terms of plate tectonic processes we have identified a major re organization in plate boundaries at anomaly 21 time when a large fragment was broken off the pacific plate and joined to the antarctic plate this interpretation is based on three pieces of evidence 1 a major ese trending feature the humboldt fracture zone marks the southern boundary of the farallon antarctic anomalies identified by weissel et al 1 2 a north south striking trough the hudson trough which represents the scar left by the pacific antarctic ridge when it rifted across old pacific crust 3 a sequence of north south striking magnetic lineations south of the humboldt fracture zone and east of the hudson trough is correlated to anomalies 27 to 22 and identified with pacific aluk spreading

on marine microfossil transport and pathways in antarctica during the late neogene evidence from the sirius group at mount fleming there are two extreme views of the evolution of the pliocene antarctic ice sheet dynamicists argue for ice sheet reduction and reexpansion on the basis of pliocene marine diatoms in a glacial deposit the sirius group that is widespread in the transantarctic mountains stabilists argue from other evidence that the antarctic cryosphere remained essentially constant in area and volume they propose marine diatom transport by eolian processes and emplacement into terrestrial glacial strata hence the inferred source area and transport mechanism of marine diatoms are of critical importance we tested the reduction hypothesis on an important outcrop of the sirius group at mount fleming south victoria land we observed very few unidentifiable marine diatom fragments in sirius group strata in contrast marine diatoms enclosed in a surface diamicton covering the sirius group were more abundant and identifiable our study further indicates that the sirius group at mount fleming was not deposited by the east antarctic ice sheet but rather by alpine ice originating on the transantarctic mountains on the basis of both data sets we infer that marine diatoms postdate sirius group deposition at mount fleming and that transport was by wind and we advance alternative scenarios for their source and transport pathways

preservation of miocene glacier ice in east antarctica antarctic climate during the pliocene has been the subject of considerable debate one view holds that during part of the pliocene east antarctica was largely free of glacier ice and that vegetation survived on the coastal mountains1a 4 an alternative viewpoint argues for the development of a stable polar ice sheet by the middle miocene which has persisted since then5a 10 here we report the discovery of buried glacier ice in beacon valley east antarctica which appears to have survived for at least 8 1 million years we have dated the ice by 40ar 39ar analysis of volcanic ash in the thin overlying glacial till which we argue has undergone little if any reworking isotope and crystal fabric analyses of the ice show that it was derived from an ice sheet we suggest that stable polar conditions must have persisted in this region for at least 8 1 million years for this ice to have avoided sublimation

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

calcareous plankton stratigraphy around the pliocene eltanin asteroid impact area se pacific documentation and application for geological and paleoceanographic reconstruction abstract conventional qualitative and semi quantitative analyses of calcareous nannofossils and planktic foraminifers from four cores ps2704 1 ps2708 1 ps2709 1 e13 4 documenting the area of the eltanin impact into the bellingshausen sea were done to a identify sediment disturbances and reworking b constrain the stratigraphic framework of the impact related sediment sequences and c evaluate probable environmental consequences of the impact the calcareous microfossil stratigraphy from the autochthonous sediments above and below the impact related sediments places the impact event into the late pliocene at around 2 15 ma and indicates that sediments as old as middle eocene were disturbed by the impact the qualitative and quantitative determination of displaced calcareous microfossils allows the reconstruction of sediment disturbances displacement and resettlement related to the impact event no signs for significant changes in the assemblages produced and sedimented after the impact could be observed indicating that the impact had no major influence on regional or global climate conditions that can be detected in the geological record

cenozoic marine sedimentation and ice volume variation on the east antarctic craton recycled cretaceous and cenozoic marine microfossils have been recovered from samples of the pliocene sinus formation samples were collected from outcrops in the reedy beardmore and ferrar glacier areas of the transantarctic mountains between lat 77 and 86 s the glaciogene sediments contained diatoms foraminifera calcareous nannoplankton silicoflagellates radiolarians sponge spicules palynomorphs and ostracodes of late cretaceous paleocene eocene late oligocene late miocene and pliocene age this suggests the presence of open marine basins on the east antarctic craton during late mesozoic and cenozoic time the apparent absence of early oligocene and early through middle and earliest late miocene assemblages suggests either that marine regression exposed the basin floors or that ice filled the basins during these times the high elevation setting of sirius formation outcrops suggests one of two hypotheses for their origin 1 they are in situ pliocene glaciomarine deposits that were uplifted 1 750 2 500 m with the transantarctic mountains to their present elevation 2 the sirius formation deposits are a mixture of derived sediments stripped from sub ice intracratonic basins and subsequently redeposited by ice flowing up the inland slope of the transantarctic mountains we favor the second hypothesis with transport to sites sometime within the past 3 m y

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

aznalcollar dam failure part 1 field observations and material properties the paper describes the failure of aznalcollar dam in southern spain in april 1998 the rockfill dam slid forward and released a flow of acid saturated tailings the geology and geomorphology of the site are described the results of a detailed laboratory testing programme on the tailings material and on the overconsolidated high plasticity foundation clay are given special attention is paid to the identification of cementation of the tailings and to the strength and consolidation properties of the foundation clay failure features are interpreted on the basis of the field evidence taking into account the measured geotechnical properties of the materials involved

pliocene pleistocene diatoms in paleozoic and mesozoic sedimentary and igneous rocks from antarctica a sirius problem solved there are two competing scenarios on the behavior of the east antarctic ice sheet during the late tertiary in one scenario the ice sheet was very dynamic and underwent major drawdown and renewal as late as the pliocene in the other the ice sheet was relatively stable during the late neogene the presence of marine diatoms in sirius group sedimentary rocks in east antarctica is at the center of the disagreement one side regards the diatoms as the major piece of evidence to support the drawdown and renewal hypothesis and infers that they were introduced into the sirius during renewed glaciation of east antarctica others suggest that these diatoms were likely introduced into the sirius by atmospheric largely eolian processes we propose a simple test of the eolian hypothesis if diatoms were introduced into the sirius by eolian processes then they should also be present in older paleozoic and mesozoic sedimentary and igneous rocks samples from two units of the beacon supergroup devonian to jurassic from beacon valley east antarctica were analyzed the beacon heights orthoquartzite devonian and the feather conglomerate permian triassic also examined was sediment found in cracks of paleozoic and mesozoic devonian to cretaceous igneous rocks from marie byrd land west antarctica largely pliocene pleistocene planktonic marine diatoms were found in all sample sets because neither beacon supergroup sedimentary rocks nor igneous rocks from marie byrd land are pliocene pleistocene in age such findings strongly suggest that diatoms were introduced into them by eolian processes this same scenario can be applied to sirius group sedimentary rocks

iridium concentrations and abundances of meteoritic ejecta from the eltanin impact in sediment cores from polarstern expedition ant xii 4 abstract the abundances of meteoritic ejecta from the eltanin asteroid impact have been examined in seven sediment cores recovered by the fs polarstern during expedition ant xii 4 using elemental concentrations of iridium and weights of coarse 500 m ejecta debris three cores with well preserved impact deposits ps2704 1 ps2708 1 and ps2709 1 each contain ir and ejecta fluences similar to those found previously in usns eltanin core e13 4 small ir anomalies and traces of ejecta were found in cores ps2706 1 and ps2710 1 but since these cores lack well defined deposits these are considered to be reworked and not representative of the fallout no evidence of ejecta was found in cores ps2702 1 and ps2705 1 these results confirm earlier speculation that the eltanin impact resulted in deposits of ejecta with up to 1 g cm2 of debris over a wide area of the ocean floor however there are still large uncertainties over the actual regional or global extent of this unique sediment deposit

the dispersal of pyroclasts from ancient explosive volcanoes on mars implications for the friable layered deposits a number of voluminous fine grained friable deposits have been mapped on mars the modes of origin for these deposits are debated the feasibility for an origin by volcanic airfall for the friable deposits is tested using a global circulation model to simulate the dispersal of pyroclasts from candidate source volcanoes near each deposit it is concluded that the medusae fossae formation and electris deposits are easily formed through volcanic processes and that the hellas deposits and south polar pitted deposits could have some contribution from volcanic sources in specific atmospheric regimes the arabia and argyre deposits are not well replicated by modeled pyroclast dispersal suggesting that these deposits were most likely emplaced by other means

characteristics of the london clay from the terminal 5 site at heathrow airport the innovative engineering approach adopted for the new terminal 5 at heathrow airport called for an advanced investigation of the london clay strata including detailed in situ profiling and stress path laboratory testing on high quality rotary cored samples although the scope of the investigations exceeded that normally specified for conventional design questions relating to the structure and anisotropy of stiffness and strength of the clay remained that could not be answered further research was required and the deep excavations at the site provided the opportunity for a team from imperial college to take multiple block samples from three depths supplemented by two additional dedicated rotary cored boreholes intensive research was performed at imperial college on these samples as described in three companion papers by gasparre et al and nishimura et al this overview paper integrates the findings from the recent research with those from the commercial investigation and with earlier studies to e

olivine and pyroxene diversity in the crust of mars datafromthe observatoire pour la mineralogie l9eau les glaces et l9activite omega on the mars express spacecraft identify the distinct mafic rock forming minerals olivine low calcium pyroxene lcp and high calcium pyroxene hcp on the surface of mars olivine and hcp rich regions are found in deposits that span the age range of geologic units however lcp rich regions are found only in the ancient noachian aged units which suggests that melts for these deposits were derived from a mantle depleted in aluminum and calcium extended dark regions in the northern plains exhibit no evidence of strong mafic absorptions or absorptions due to hydrated materials

late cenozoic glacial history of the ross embayment antarctica abstract knowledge of the cenozoic terrestrial and marine glacial history of antarctica has been developed through studies of on land surface outcrops and subsurface drillcores and through examination of drillcores and piston cores obtained from a variety of offshore marine geological settings these field and laboratory endeavors have been a major target of the antarctic earth science community for more than thirty years the ross sea sedimentary basins including the byrd basin west antarctica wilkes and pensacola basins east antarctica and the transantarctic mountains provide significant data on the cenozoic glacial record a record that has been traced back to at least 38 ma close to the eocene oligocene boundary this paper reviews the late cenozoic history of the transantarctic mountains and flanking sedimentary basins with emphasis on the debate between the stabilist view of antarctic ice sheet history see clapperton and sugden 1990 and our favored multi glacial dynamic ice sheet model

evidence for widespread aqueous sedimentation in the northern plains of mars high resolution imaging data sets were used to identify and investigate a discrete stratigraphic unit exposed in the walls of impact craters throughout chryse and acidalia planitiae mars the unique morphology and spectral signatures observed in this unit which include fine scale layering lensing pinching and elevated hydration signatures all indicate that this material was deposited in a suite of aqueous environments furthermore this unit s widespread distribution and relationship to regional topography suggests an origin from the circum chryse outflow channels while earlier studies have suggested that large quantities of outflow channel effluents are located throughout this region this analysis is the first to identify and characterize the outcrop scale morphology and spatial distribution of this extensive unit these observations imply that the northern plains of mars served as a global depocenter for immense volumes of water and sediment early in martian history

origin of giant martian polygons extensive areas of the martian northern plains in utopia and acidalia planitiae are characterized by polygonal terrane polygonal terrane consists of material cut by complex troughs defining a pattern resembling mudcracks columnar joints or frost wedge polygons on earth however the martian polygons are orders of magnitude larger than these potential earth analogues leading to severe mechanical difficulties for genetic models based on simple analogy arguments plate bending and finite element models indicate that shrinkage of desiccating sediment or cooling volcanics accompanied by differential compaction over buried topography can account for the stresses responsible for polygon troughs as well as the large size of the polygons although trough widths and depths relate primarily to shrinkage the large scale of the polygonal pattern relates to the spacing between topographic elevations on the surface buried beneath polygonal terrane material geological relationships favor a sedimentary origin for polygonal terrane material but our model is not dependent on the specific genesis our analysis also suggests that the polygons must have formed at a geologically rapid rate

avulsion deposits in lower eocene alluvial rocks bighorn basin wyoming abstract eocene alluvial rocks in the northern bighorn basin can be subdivided into three major kinds fine grained deposits on which cumulative paleosols developed lithologically heterogeneous packages associated with immature paleosols and major sheet sandstone bodies the heterogeneous packages which alternate vertically with the cumulative paleosols are dominated by fine grained deposits and also contain ribbon sandstone bodies and thin 1 m sheet sandstone bodies most ribbon sandstones are less than 3 m thick and are more common in the middle and upper parts of the packages cut and fill deposits with fine grained fills are locally important because the lithologic packages are readily apparent in the field because of the immature or simple paleosols developed on the fine grain d facies they are referred to as simple paleosol packages the major sheet sandstones were deposited by meandering trunk rivers and locally overlie and truncate the other deposits contrary to classical models of meandering streams only a portion of the fine grained rocks in the study area were deposited by overbank flooding of a trunk channel on the basis of their stratigraphic position and few ribbon sandstones the upper parts of some simple paleosol packages are interpreted as ancient levee deposits associated with a trunk channel farther from the trunk channel overbank floods deposited alluvium on which the cumulative paleosols formed the greater parts of the simple paleosol packages including their fine grained deposits were deposited rapidly during channel avulsion ribbon sandstone bodies represent ancient splay channels on the avulsion belt simple paleosol packages in the willwood formation support the contention of smith et al 1989 that n many meandering river systems a significant portion of the fine grained deposits were deposited as the trunk channel avulsed deposits that resemble the simple paleosol packages may provide a means of recognizing avulsion episodes in other alluvial sequences

the influence of structure on the behaviour of london clay an intensive investigation is described into the london clay units from heathrow terminal 5 intrinsic properties and composition were established and relating the behaviour of intact and reconsti

tectonic shear zones in the london clay formation subhorizontal shear zones observed in the london clay formation are described the physical properties of the shear zones are examined and it is demonstrated both that they are of tectonic origin the result of bedding plane slip during minor folding and that they were probably formed before there had been any significant erosion of the london clay or later overlying formations the occurrence of the shear zones is reviewed and it is shown that similar features have been recorded in other areas to the west of london and it seems likely that they will be encountered elsewhere in the london basin wherever minor folding occurs the shape of the london clay formation reading formation interface appears to give an indication of the degree of folding and hence of the possible occurrence of shear zones the displacements of the shear zones examined are believed to be small perhaps only a few centimetres and consequently the strength along the shear zones is probably slightly above the true large displace

the geologic history of margaritifer basin mars in this study we investigate the fluvial sedimentary and volcanic history of margaritifer basin and the uzboi ladon morava outflow channel system this network of valleys and basins spans more than 8000 km in length linking the fluvially dissected southern highlands and argyre basin with the northern lowlands via ares vallis compositionally thermophysically and morphologically distinct geologic units are identified and are used to place critical relative stratigraphic constraints on the timing of geologic processes in margaritifer basin our analyses show that fluvial activity was separated in time by significant episodes of geologic activity including the widespread volcanic resurfacing of margaritifer basin and the formation of chaos terrain the most recent fluvial activity within margaritifer basin appears to terminate at a region of chaos terrain suggesting possible communication between surface and subsurface water reservoirs we conclude with a discussion of the implications of these observations on our current knowledge of martian hydrologic evolution in this important region

a biostratigraphy for the london clay in london biostratigraphy enables specific layers of strata to be given a unique stratigraphic address that can be used to identify the horizons in which in situ tests are conducted and from which samples for testing are collected this is particularly valuable when working in fine grained deposits where stratigraphic position is not easily recognised by lithology alone and permits any data bank of geotechnical values and performance to be used without fear of confusing one stratigraphic layer with another construction in the london clay formation and its supporting research has necessitated an accurate and speedy method for identifying with a precision of centimetres the stratigraphic layer from which geotechnical data have been obtained the principles that enable biostratigraphy to be used for this purpose are explained and illustrated with reference to the london clay formation in london this approach can be used in most sedimentary strata an example of biostratigraphic data used in london is presented

the flood lavas of kasei valles mars abstract both the northern and southern arms of kasei valles are occupied by platy ridged flood lavas we have mapped these flows and examined their morphology to better understand their emplacement the lavas were emplaced as high flux turbulent flows exceeding 106 m3 s 1 lava in southern kasei valles can be traced back up onto the tharsis rise which is also the likely source of lavas in the northern arm these eruptions were similar to but somewhat smaller than the athabasca valles flood lava in elysium planitia with estimated volumes of 1200 km3 here and 5000 km3 in athabasca valles the flood lavas in both kasei and athabasca valles have evidence for distal inflation as well as widespread drainage or volume loss in medial areas this may be an important characteristic of many large recent martian eruptions despite their great size and flux the kasei valles flood lavas are only a late modification to the valley system capable of only modest local erosion the more vigorous athabasca valles lava may have been capable of somewhat more erosion in its smaller valley system

regional variations in spinel compositions an important key to the cretaceous tertiary event spinel found in spherules from cretaceous tertiary boundary sediments exhibits a wide range in composition and is distinguished from typical igneous spinel by high mg al and ni relatively low ti and cr and high fe2o3 feo solid solution compositions range from nearly pure magnetite to magnesioferrite to relatively pure mgal2o4 spinel the ni spinel trevorite is also a common component a regional variation in composition is also observed spinel from two north pacific sites has higher mg and al and lower ni and fe than spinel from two european sites and one south atlantic site the most probable source of the spinel is crystallization from molten silicate droplets produced from a major impact event the regional variations may provide a key to locating the impact site s

processes facies and architecture of fluvial distributary system deposits abstract there is evidence from the stratigraphic record of examples of fluvial deposits that were the products of deposition from river systems which had decreasing discharge down flow and transitions from proximal channelised to distal unconfined flow these deposits form fan shaped bodies several tens of kilometres in radius and their stratigraphic architecture is aggradational with no evidence of deep incision driven by base level fall the fluvial systems that generated these deposits formed under conditions for which there is no complete analogue today an endorheic basin with a relatively arid climate adjacent to an uplifted area with higher precipitation a conceptual model for fluvial systems of this type has therefore been built on the basis of outcrop examples and a consideration of the controls on sedimentation proximal areas are characterised by amalgamated coarse pebbly and sandy channel deposits with little preservation of overbank facies channel dimensions are generally smaller in the medial areas but sizes are variable deposits are of braided meandering and simple channels which show varying degrees of lateral migration the channel fills may be mud or sand with overbank flow processes playing an important role in filling channels abandoned on the floodplain after avulsion the proportion of overbank deposits increases distally with sheets of sand deposited as lateral and terminal splays by unconfined flow interconnection of sandstone bodies is poor in the distal areas because channel fill bodies are sparse small and are not deeply incised the radial pattern of the sediment body forms by the repeated avulsion of channels active channels build up lobes on the alluvial plain and rivers switch position to follow courses on lower lying areas the term fluvial distributary system is here used to describe a river system which has a downstream decrease in discharge and has a distal zone which is characterised either by terminal splays on to a dry alluvial plain or a lake delta during periods of lake highstand

a new global database of mars impact craters 1 km 1 database creation properties and parameters 1 impact craters have been used as a standard metric for a plethora of planetary applications for many decades including age dating geologic mapping and stratigraphic relationships as tracers for surface processes and as locations for sampling lower crust and upper mantle material utilizing craters for these and other investigations is significantly aided by a uniform catalog of craters across the surface of interest consequently catalogs of craters have been developed for decades for the moon and other planets we present a new global catalog of martian craters statistically complete to diameters d 1 km it contains 384 343 craters and for each crater it lists detailed positional interior morphologic ejecta morphologic and morphometric data and modification state information if it could be determined in this paper we detail how the database was created the different fields assigned and statistical uncertainties and checks in our companion paper robbins and hynek 2012 we discuss the first broad science applications and results of this work

a numerical simulation of underwater slope failures generated by salt diapirism combined with active sedimentation this paper describes advanced finite element simulations of a geologically active and periodically unstable 700 m high underwater escarpment located in the gulf of mexico that has been created by uplifting salt diapirs combined with sediment deposition the site s geological history soil permeability and clay shear strength characteristics govern current slope stability and determine the timing of any future failure the geological and geotechnical conditions are reported before presenting fully coupled large displacement numerical simulations employing critical state formulations of the slope s evolution over the last 600 000 years the latter form the basis for strain softening analyses of the pore pressure equalisation and progressive failure processes that control current slope stability the analyses also address the ground movement patterns that precede instability giving useful information for geohazard risk assessment and early warning monitoring system design

oceanic impacts a growing field of fundamental geoscience the importance of oceanic impacts of collisional events and resulting energy release are briefly described data collection methods from the eltanin a mesosiderite projectile deep water impact are presented

pliocene paleoenvironment and antarctic ice sheet behavior evidence from wright valley investigations in wright valley adjacent to the transantarctic mountains in east antarctica shed light on the question of whether high latitude pliocene climate was warm enough to cause widespread deglaciation of the east antarctic craton with a concurrent magellanic moorland like environment if pliocene age diatoms presently in glaciogenic deposits high in the transantarctic mountains had come from seaways on the east antarctic craton an expanding late pliocene ice sheet must have first eroded them from marine sediments and then deposited the diatoms at their present high altitude locations this hypothetical expanding glacier would have had to have come through wright valley glacial drift sediments from the central wright valley were mapped sampled analyzed and 40ar 39ar whole rock dated our evidence indicates that an east antarctic outlet glacier has not expanded through wright valley and hence cannot have overridden the dry valleys sector of the transantarctic mountains any time in the pa

origin and significance of decameter scale polygons in the lower peace vallis fan of gale crater mars abstract decameter scale polygons are extensively developed in the bedded fractured bf unit of the lower peace vallis fan the polygons occur in a likely extension of the gillespie lake member north of yellowknife bay the section first drilled by the mars science laboratory msl mission we examine hypotheses for the origin of these polygons to provide insight into the history of gale crater the polygons are 4 30 m across square to rectangular and defined by 0 5 4 m wide generally straight troughs with orthogonal intersections polygon networks are typically oriented orthogonal systems with occasional nearly circular patterns hundreds of meters across potential origins include cooling of lava and for sedimentary units syneresis unloading weathering desiccation impact processes and cold climate thermal contraction cold climate thermal contraction is the hypothesis most consistent with the sedimentary nature of the bf unit and the polygon morphology geometry networks and apparent restriction to the coarse grained gillespie lake member a periglacial setting further provides the best analogs for the circular networks and is consistent with geologic context and msl data most of the decametric polygons appear to be ancient they are confined to the hesperian bf unit and only a few of their bounding fractures extend into younger or recently exposed units in this regard they differ from the majority of proposed thermal contraction polygons on mars as those are generally thought to be young features and accordingly the history of formation preservation and reactivation of the decametric polygons is likely to be more complex than that of any proposed young polygons on mars the decametric polygons in the bf unit may represent landforms developed in a cold but still comparatively wet interlude between a clement early mars and the much drier and colder planet of today

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

determining chondritic impactor size from the marine osmium isotope record decreases in the seawater 187os 188os ratio caused by the impact of a chondritic meteorite are indicative of projectile size if the soluble fraction of osmium carried by the impacting body is known resulting diameter estimates of the late eocene and cretaceous paleogene projectiles are within 50 of independent estimates derived from iridium data assuming total vaporization and dissolution of osmium in seawater the variations of 187os 188os and os ir across the late eocene impact event horizon support the main assumptions required to estimate the projectile diameter chondritic impacts as small as 2 kilometers in diameter should produce observable excursions in the marine osmium isotope record suggesting that previously unrecognized impact events can be identified by this method

characterization of phyllosilicates observed in the central mawrth vallis region mars their potential formational processes and implications for past climate mawrth vallis contains one of the largest exposures of phyllosilicates on mars nontronite montmorillonite kaolinite and hydrated silica have been identified throughout the region using data from the compact reconnaissance imaging spectrometer for mars crism in addition saponite has been identified in one observation within a crater these individual minerals are identified and distinguished by features at 1 38 1 42 1 91 and 2 17 2 41 m there are two main phyllosilicate units in the mawrth vallis region the lowermost unit is nontronite bearing unconformably overlain by an al phyllosilicate unit containing montmorillonite plus hydrated silica with a thin layer of kaolinite plus hydrated silica at the top of the unit these two units are draped by a spectrally unremarkable capping unit smectites generally form in neutral to alkaline environments while kaolinite and hydrated silica typically form in slightly acidic conditions thus the observed phyllosilicates may reflect a change in aqueous chemistry spectra retrieved near the boundary between the nontronite and al phyllosilicate units exhibit a strong positive slope from 1 to 2 m likely from a ferrous component within the rock this ferrous component indicates either rapid deposition in an oxidizing environment or reducing conditions formation of each of the phyllosilicate minerals identified requires liquid water thus indicating a regional wet period in the noachian when these units formed the two main phyllosilicate units may be extensive layers of altered volcanic ash other potential formational processes include sediment deposition into a marine or lacustrine basin or pedogenesis

in situ benthos and paleo oxygenation in the middle cambrian burgess shale british columbia canada the middle cambrian burgess shale was deposited under variable levels of oxygenation during periods of oxygenation low diversity shell beds were formed and the muds were colonized by infauna under these circumstances pyrite was restricted to anoxic microenvironments and formed small 1 mm discrete aggregated masses soft bodied organisms were preserved under low oxygen conditions typified by evenly dispersed pyrite and an absence of trace fossils and shell beds

unmelted meteoritic debris collected from eltanin ejecta in polarstern cores from expedition ant xii 4 abstract a total of 1 7 g of unmelted meteorite particles have been recovered from fs polarstern piston cores collected on expedition ant xii 4 that contain ejecta from the eltanin impact event most of the mass 1 2 g is a large single specimen that is a polymict breccia similar in mineralogy and chemistry to howardites or the silicate fraction of mesosiderites most of the remaining mass is in several large individual pieces 20 75 mg each that are polymict breccias fragments dominated by pyroxene and an igneous rock fragment the latter has highly fractionated ree similar to those reported in mafic clasts from mesosiderites other types of specimens identified include fragments dominated by maskelynite or olivine these pieces of the projectile probably survived impact by being blown off the back surface of the eltanin asteroid during its impact into the bellingshausen sea

the influence of intense fissuring on the mechanical behaviour of clays the paper discusses the geological history intrinsic properties structural features and mechanical behaviour of three differently fissured clays outcropping within the apennine chain in southern italy based on a large experimental database the mechanical behaviour of the clays is investigated in the light of their different fissuring features which have been distinguished and characterised by means of a new chart the study assumed the soil to be a continuum despite the different fissuring features of the clay fabric therefore laboratory tests were carried out on both natural and reconstituted clay samples and the results were compared with those recognised in the literature to be typical of unfissured sensitive clays based on these comparisons a behavioural framework is proposed for clays possessing certain fissure structures the results of the analysis show that the mini to mesostructure of clays of fissuring intensity i5 i6 can be modelled as part of the structure variable controlling the c

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

predicting the stand up time of temporary london clay slopes at terminal 5 heathrow airport the paper describes the results of finite element analyses of the temporary slope geometries in london clay at london heathrow airport s terminal 5 the aims of the analyses were to examine the times before failures developed and to identify the failure mechanisms involved the brittle behaviour of the london clay was modelled and the effects of progressive failure were taken into account the possible presence of tectonic shears with their strength close to residual was considered by comparing analyses with and without a tectonic shear zone the predicted time to failure of the slopes and the form of the failure whether shallow or deep seated were determined by a combination of the assumed permeability profile and whether or not allowances were made for increases in permeability as the clay swelled the average surface suction the in situ k0 profile the depth of excavation and whether or not a low strength tectonic shear surface was present in the slope the analyses fall into the category of clas

paleohydrology and flood geomorphology of ares vallis ares vallis is a martian outflow channel which is about 1500 km long and locally exceeds 100 km in width and 1000 m in depth we estimate the channel s paleoflow capacity using paleohydraulic techniques at a constricted and unusually deep reach not affected by tributaries or secondary inflows the estimated possible maximum peak discharges are of the order of 108 109 m3 s these values are 1 to 2 orders of magnitude larger than any known terrestrial floods these high discharge flows were theoretically capable of transporting boulders larger than 10 m in diameter the downstream depositional plain where ares debauches into the chryse planitia displays geomorphological features indicative of high erosional capacity by the flooding in this area site of the proposed pathfinder landing we expect to find a complex sequence of sediments created by varying paleoflood flow hydraulics sediment transport characteristics and probable secondary modification of the primary flood landforms

variation in the position of the upwelling front on the oregon shelf 1 as part of an experiment to study wind driven coastal circulation 17 hydrographic surveys of the middle to inner shelf region off the coast of newport or 44 65 n from roughly the 90 m isobath to the 10 m isobath were performed during summer 1999 with a small towed undulating vehicle the cross shelf survey data were combined with data from several other surveys at the same latitude to study the relationship between upwelling intensity and wind stress field a measure of upwelling intensity based on the position of the permanent pycnocline is developed this measure is designed so as to be insensitive to density modifying surface processes such as heating cooling buoyancy plumes and wind mixing it is highly correlated with an upwelling index formed by taking an exponentially weighted running mean of the alongshore wind stress this analysis suggests that the front relaxes to a dynamic geostrophic equilibrium on a timescale of roughly 8 days consistent with a similar analysis of moored hydrographic observations this relationship allows the amount of time the pycnocline is outcropped to be estimated and could be used with historical wind records to better quantify interannual cycles in upwelling

geological record and reconstruction of the late pliocene impact of the eltanin asteroid in the southern ocean in 1995 an expedition on board the research vessel fs polarstern explored the impact site of the eltanin asteroid in the southern ocean the only known asteroidimpact into a deep ocean basin analyses of the geological record of the impact region place the event in the late pliocene 2 15 myr and constrain thesize of the asteroid to be 1 km the explosive force inferred for this event places it at the threshold of impacts believed to have global consequences and its studyshould therefore provide a baseline for the reconstruction and modelling of similar events which are common on geological timescales

chemical multispectral and textural constraints on the composition and origin of rocks at the mars pathfinder landing site rocks at the mars pathfinder site are probably locally derived textures on rock surfaces may indicate volcanic sedimentary or impact generated rocks but aeolian abration and dust coatings prevent unambiguous interpretation multispectral imaging has resolved four spectral classes of rocks gray and red which occur on different surfaces of the same rocks pink which is probably soil crusts and maroon which occurs as large boulders mostly in the far field rocks are assigned to two spectral trends based on the position of peak reflectance the primary spectral trend contains gray red and pink rocks maroon rocks constitute the secondary spectral trend the spatial pattern of spectral variations observed is oriented along the prevailing wind direction the primary spectral trend arises from thin ferric coatings of aeolian dust on darker rocks the secondary spectral trend is apparently due to coating by a different mineral probably maghemite or ferrihydrite a chronology based on rock spectra suggests that rounded maroon boulders constitute the oldest petrologic unit a flood deposit succeeded by smaller cobbles possibly deposited by impact and followed by aeolian erosion and deposition nearly linear chemical trends in alpha proton x ray spectrometer rock compositions are interpreted as mixing lines between rock and adhering dust a conclusion supported by a correlation between sulfur abundance and red blue spectral ratio extrapolations of regression lines to zero sulfur give the composition of a presumed igneous rock the chemistry and normative mineralogy of the sulfur free rock resemble common terrestrial volcanic rocks and its classification corresponds to andesite igneous rocks of this composition may occur with clastic sedimentary rocks or impact melts and breccias however the spectral mottling expected on conglomerates or breccias is not observed in any apxs analyzed rocks interpretation of the rocks as andesites is complicated by absence of a 1 m pyroxene absorption band plausible explanations include impact glass band masking by magnetite or presence of calcium and iron rich pyroxenes and olivine which push the absorption band minimum past the imager s spectral range the inferred andesitic composition is most similar to terrestrial anorogenic icelandites formed by fractionation of tholeiitic basaltic magmas early melting of a relatively primitive martian mantle could produce an appropriate parent magma supporting the ancient age of pathfinder rocks inferred from their incorporation in hesperian flood deposits although rocks of andesitic composition at the pathfinder site may represent samples of ancient martian crust inferences drawn about a necessary role for water or plate tectonics in their petrogenesis are probably unwarranted

numerical assessment of the effects of end restraints and a pre existing fissure on the interpretation of triaxial tests on stiff clays conventional laboratory triaxial tests apply axisymmetric boundary conditions to a cylindrical sample which has an axisymmetric geometry for a homogeneous sample this implies that the deformed sha

the mars observer laser altimeter investigation the primary objective of the mars observer laser altimeter mola investigation is to determine globally the topography of mars at a level suitable for addressing problems in geology and geophysics secondary objectives are to characterize the 1064 nm wavelength surface reflectivity of mars to contribute to analyses of global surface mineralogy and seasonal albedo changes to assist in addressing problems in atmospheric circulation and to provide geodetic control and topographic context for the assessment of possible future mars landing sites the principal components of mola are a diode pumped neodymium doped yttrium aluminum garnet laser transmitter that emits 1064 nm wavelength laser pulses a 0 5 m diameter telescope a silicon avalanche photodiode detector and a time interval unit with 10 ns resolution mola will provide measurements of the topography of mars within approximately 160 m footprints and a center to center along track foot print spacing of 300 m along the mars observer subspacecraft ground track the elevation measurements will be quantized with 1 5 m vertical resolution before correction for orbit and pointing induced errors mola profiles will be assembled into a global 0 2 deg x 0 2 deg grid that will be referenced to mars center of mass with an absolute accuracy of approximately 30 m other data products will include a global grid of topographic gradients corrected individual profiles and a global 0 2 deg x 0 2 deg grid of 1064 nm surface reflectivity

morphology of europan bands at high resolution a mid ocean ridge type rift mechanism 1 we utilize imaging data from the galileo spacecraft to investigate band formation on one of jupiter s moons europa bands are polygonal features first observed in voyager data close to europa s anti jovian point and represent areas where preexisting terrain has been pulled apart allowing new material to move up into the gap we examine the detailed morphology of several bands imaged at different resolutions and lighting geometries we identify several distinct morphological characteristics including central troughs hummocky textures and ridge and trough terrains some of which are common among the bands studied in many cases bands have initiated along segments of one or more preexisting double ridges ubiquitous within europa s ridged plains distinctive morphological features and high standing topography imply that the bands formed from compositionally or thermally buoyant ice rather than liquid water comparisons between europan band morphologies and features found on terrestrial mid ocean ridges reveal several similarities including axial troughs subcircular hummocks normal faults and indications of symmetrical spreading we conclude that terrestrial mid ocean ridge rifting is a good analogy for europan band formation if a terrestrial seafloor spreading model is applicable to europan bands we speculate that band morphologies might be related to the relative rate of spreading of each band bands may have contributed significantly to the resurfacing of europa europan bands we examine predate but do not postdate lenticulae and related features implying that the style of resurfacing on europa has changed over recent geological time in these regions

new evidence on the size and possible effects of a late pliocene oceanic asteroid impact debris from a late pliocene asteroid impact is spread across at least 600 kilometers of the ocean floor in the southeast pacific on the basis of iridium concentrations in sediments from six deep sea cores the asteroid diameter was at least 0 5 kilometer the impacting projectile may have been one of the largest in the last few million years the stratigraphic age of this impact is the same as that inferred for the onset of northern hemisphere glaciation

magnesioferrite spinel in cretaceous tertiary boundary sediments of the pacific basin remnants of hot early ejecta from the chicxulub impact abstract magnesioferrite spinel in sediments from six cretaceous tertiary k t boundary sites in the pacific basin are found to have relatively uniform compositions and textures they typically have much higher mgo al2o3 andfe2o3 feo than spinel from k t boundaries in western europe and the south atlantic or in meteor ablation debris these compositions are similar to those found in a few grains from the k t boundary at odp site 761 eastern indian ocean and in spherules from a late pliocene impact pacific spinel commonly exhibit unique textures with large porous anhedral grains which we believe reflect a myrmekitic intergrowth with another phase that has been destroyed by diagenetic alteration a prime candidate for this other phase in ni rich periclase mg ni fe o a mineral that has been observed as trace inclusions in several spinel grains from pacific ocean dsdp sites 577 and 596 the strewnfield for the pacific spinel extends over 5 of the earth s surface but it could be a much larger area these spinel must have crystallized from silicate liquids produced by a large impact event if their source is the impact that made the chicxulub structure on the yucata n peninsula mexico then there is a clear compositional asymmetry to the distribution of this compoment of the ejecta we speculate that a possible cause of this asymmetry could be a low angle impact if the projectile came in from the east the vapor cloud would deposit the earliest hottest materials over the pacific basin

numerical simulations of the mjolnir marine impact crater 1 the 40 km diameter mjolnir crater in the barents sea was created by a meteoroid impact into a 400 m shallow sea about 142 ma we have used a multimaterial hydrocode with depth depending target strength to model numerically the cratering and early modification stages of the mjolnir impact our models are constrained by the observed crater morphology and structure the best results were obtained for a composite target strength structure very low strength for the upper 3 km of sedimentary rocks and gradual increase from 3 to 6 5 km depth before using strength values typical for granitic rocks at greater depths the low strength of the target sediments at shallow depths led to exceptionally intensive slumping and gravitational collapse of the order of 2 0 2 5 considerably larger than the average expected values for typical terrestrial craters as a result we obtain a crater collapse in between simple and complex where slumping and gravitational collapse counteract the crater floor uplift and lead to suppression and burial of the central high the mjolnir impact in a shallow sea was responsible for a major disturbance of the water column following the impact induced outward water surge large amplitude tsunamis were formed and the rapid resurge of seawater into the excavated crater transported large amounts of ejecta and crater wall material back into the crater accounting for the redestribution of ejecta material and extensive infilling

first discovery of meteoritic events in deep antarctic epica dome c ice cores two distinct dust layers in the epica dome c ice core 75 06 s 123 21 e east antarctic plateau have been shown to relate to individual meteoritic events particles forming these layers investigated by electron microprobe show peculiar textural mineralogical and geochemical features and closely resemble extraterrestrial debris in deep sea sediments and polar caps preliminary estimates of cosmic debris input at the studied layers obtained from coulter counter measurements are 4 5 orders of magnitude greater than the yearly micrometeorite flux in east antarctic snow and ice the cosmic events are accurately dated through glaciological models at 434 6 and 481 6 ka respectively and are located in the core climatic stratigraphy near the mid brunhes event this is the first report of well dated cosmic horizons in deep antarctic ice cores it significantly improves the extraterrestrial record of antarctica and opens new correlation perspectives between long climatic records of the south polar region

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

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

unmelted meteoritic debris in the late pliocene iridium anomaly evidence for the ocean impact of a nonchondritic asteroid ir bearing particles have been recovered from 2 piston cores in the antarctic basin in the southeastern pacific in core e13 3 the particles closely correspond to the late pliocene ir anomaly and have a fluence of 100 mg cm 2 in core e13 4 120 km to the southwest the particle fluence is 4 mg cm 2 particles with diameters from 0 5 to 4 mm contain at least 35 of the ir in this horizon three types of particles have been identified 1 vesicular 2 basaltic and 3 metal the vesicular particles appear to be shock melted debris derived from the oceanic impact of a howarditic asteroid containing a minor metal component these particles have recrystallized from a melt and impact into the ocean has resulted in the incorporation of na k cl and radiogenic sr from the ocean water target the basaltic clasts appear to be unmelted fragments of the original asteroid which may have separated from the main body prior to impact combined vesicular and basaltic particles are believed to have formed by collisions in the debris cloud estimates of the diameter of the projectile range from 100 to 500 m by many orders of magnitude this is the most massive achondrite sampled by a single meteorite fall

the stratigraphy and history of mars northern lowlands through mineralogy of impact craters a comprehensive survey the basin filling materials of the northern lowlands which cover approximately one third of mars surface record the long term evolution of mars geology and climate the buried stratigraphy was inferred through analyses of impact crater mineralogy detected using data acquired by the compact reconnaissance imaging spectrometer for mars examining 1045 impact craters across the northern lowlands we find widespread olivine and pyroxene and diverse hydrated hydroxylated minerals including fe mg smectite chlorite prehnite and hydrated silica the distribution of mafic minerals is consistent with infilling volcanic materials across the entire lowlands 1 4 10 7 km 3 indicating a significant volume of volatile release by volcanic outgassing hydrated hydroxylated minerals are detected more frequently in large craters consistent with the scenario that the hydrated minerals are being excavated from deep basement rocks beneath 1 2 km thick mafic lava flows or volcaniclastic materials the prevalences of different types of hydrated minerals are similar to statistics from the southern highlands no evidence of concentrated salt deposits has been found which would indicate a long lived global ocean we also find significant geographical variations of local mineralogy and stratigraphy in different basins geological provinces independent of dust cover for example many hydrated and mafic minerals are newly discovered within the polar scandia region 60 n and chryse planitia has more mafic mineral detections than other basins possibly due to a previously unrecognized volcanic source

chryse planitia mars topographic configuration outflow channel continuity and sequence and tests for hypothesized ancient bodies of water using mars orbiter laser altimeter mola data many of the largest and most prominent outflow channels on mars debouch into chryse planitia pre mars global surveyor topographic data show chryse to be a closed depression almost 2000 km in diameter new mars orbiter laser altimeter mola data reveal the following 1 chryse is not a locally closed basin but instead opens into the north polar basin 2 the highly distinctive morphology of the six largest predominantly hesperian aged channels kasei maja simud tiu ares and mawrth disappears into the northern lowlands at average elevations that all occur within less than 170 m of a mean elevation of 3742 sd 153 m over a lateral distance in excess of 2500 km 3 the elevations where the distinctive morphology of each channel disappears all fall within 190 m of contact 2 a boundary mapped by parker et al 1993 and interpreted to represent an ancient shoreline and the mean elevation values of contact 2 and circum chryse channel termini fall within 18 m of each other in contrast the termini of seven later amazonian aged channels emerging from elysium into utopia planitia are spread over a vertical range of 1500 m 4 topographic evidence of the continuation of some of the outflow channels can be observed for distances of 250 450 km into the north polar basin but the morphology is subdued and distinctly different 5 the nature of this less distinctive topography and its crosscutting relationships show that simud and tiu are likely to represent the youngest activity specifically crosscutting ares valles 6 the distinctive change in channel morphology is consistent with rapid loss of energy encountered at base level subaerial submarine boundary and emplacement into a shallow submarine environment channel characteristics lack of distinctive deltas or lobes and continuation of subdued channel morphology suggest hyperpychnal flow and the possibility of density turbidity currents estimates of the volumes of individual channel events are wide ranging the minimum volume estimates of carr 1996 suggest that 46 such events would be required to fill the basin to the level of contact 2 and thus that the channels may have emptied into an existing standing body of water volume estimates of baker et al 1991 assume that single individual events may have filled the basin to the level of contact 2 thus requiring significant water loss between events and refilling during subsequent events to essentially the same level in both end member cases these observations are consistent with the presence of large standing bodies of water in the northern lowlands in hesperian early amazonian times

mars observer camera the mars observer camera moc is a three component system one narrow angle and two wide angle cameras designed to take high spatial resolution pictures of the surface of mars and to obtain lower spatial resolution synoptic coverage of the planet s surface and atmosphere the cameras are based on the push broom technique that is they do not take frames but rather build pictures one line at a time as the spacecraft moves around the planet in its orbit moc is primarily a telescope for taking extremely high resolution pictures of selected locations on mars using the narrow angle camera areas ranging from 2 8 km 2 8 km to 2 8 km 25 2 km depending on available internal digital buffer memory can be photographed at about 1 4 m pixel additionally lower resolution pictures to a lowest resolution of about 11 m pixel can be acquired by pixel averaging these images can be much longer ranging up to 2 8 500 km at 11 m pixel high resolution data will be used to study sediments and sedimentary processes polar processes and deposits volcanism and other geologic geomorphic processes the moc wide angle cameras are capable of viewing mars from horizon to horizon and are designed for low resolution global and intermediate resolution regional studies low resolution observations can be made every orbit so that in a single 24 hour period a complete global picture of the planet can be assembled at a resolution of at least 7 5 km pixel regional areas covering hundreds of kilometers on a side may be photographed at a resolution of better than 250 m pixel at the nadir such images will be particularly useful in studying time variable features such as lee clouds the polar cap edge and wind streaks as well as acquiring stereoscopic coverage of areas of geological interest the limb can be imaged at a vertical and along track resolution of better than 1 5 km different color filters within the two wide angle cameras permit color images of the surface and atmosphere to be made to distinguish between clouds and the ground and between clouds of different composition

compositional stratigraphy of clay bearing layered deposits at mawrth vallis mars phyllosilicates have previously been detected in layered outcrops in and around the martian outflow channel mawrth vallis crism spectra of these outcrops exhibit features diagnostic of kaolinite montmorillonite and fe mg rich smectites along with crystalline ferric oxide minerals such as hematite these minerals occur in distinct stratigraphic horizons implying changing environmental conditions and or a variable sediment source for these layered deposits similar stratigraphic sequences occur on both sides of the outflow channel and on its floor with al clay bearing layers typically overlying fe mg clay bearing layers this pattern combined with layer geometries measured using topographic data from hirise and hrsc suggests that the al clay bearing horizons at mawrth vallis postdate the outflow channel and may represent a later sedimentary or altered pyroclastic deposit that drapes the topography

modeling tsunami propagation and the emplacement of thumbprint terrain in an early mars ocean the identification of lobate debris deposits in arabia terra along the proposed paleoshoreline of a former northern ocean has renewed questions about the existence and stability of ocean sized body of water in the early geologic history of mars the potential occurrence of impact generated tsunamis in a northern ocean was investigated by comparing the geomorphologic characteristics of the martian deposits with the predictions of well validated terrestrial models scaled to mars of tsunami wave height propagation direction runup elevation and distance for three potential sea levels our modeling suggests several potential impact craters 30 50 km in diameter as the source of the tsunami events within the complex topography of flat floored valleys and plateaus along the dichotomy boundary the interference of the multiple reflected and refracted waves that are observed in the simulation may explain the origin of the arcuate pattern that characterizes the thumbprint terrain