sedimentary and structural evolution of the northwest arabian red sea margin abstract an investigation of the northwestern margin of the arabian peninsula midyan region provides new insights into the two stage development of the red sea graben system the early red sea suez stage and the later red sea aqaba stage the early stage is characterized by the passive subsidence of a continental graben since the early oligocene after a period of enhanced subsidence of the basin in the early miocene this stage ended at the langhian serra valian boundary 14 ma the later stage was initiated by a release of stresses by strike slip movement along the newly formed aqaba levant transform creation of the new plate boundary was accompanied by stagnation of extension in the gulf of suez branch of the early rift stage strike slip motion on the aqaba levant structure is younger than 14 ma and is recorded by the post langhian compressional structures in the area of the midyan peninsula which developed by transpression along the curved path of a left lateral strike slip fault

petrogenesis detrital zircon shrimp u pb geochronology and tectonic implications of the upper paleoproterozoic seosan iron formation western gyeonggi massif korea abstract this study involves investigations on the upper paleoproterozoic iron formation viz seosan iron formation from the seosan group gyeonggi massif of the southwestern korean peninsula it occurs as thin banded layers within meta arkosic sandstone formed by alternating processes of chemical hydrothermal and detrital depositions under a shallow marine environment it mainly consists of alternating layers of iron oxides mostly hematite and quartz minor amounts of magnetite surrounded by muscovite clinopyroxene and amphibole indicate hydrothermal alteration since its formation meta arkosic sandstone is composed of recrystallized or porphyroclastic quartz and microcline with small amounts of hematite and pyrite clusters the seosan iron formation has high contents of total fe2o3 and sio2 with positive eu anomalies similar to those of other precambrian banded iron formations and its formation is clearly related to hydrothermal alteration since its deposition detrital zircon shrimp u pb geochronology data from a meta arkosic sandstone sn 1 and an iron formation sn 2 show mainly two age groups of ca 2 5 ga and ca 1 9 1 75 ga this together with intrusion age of the granite gneiss ca 1 70 1 65 ga clearly indicate that the iron formations were deposited during the upper paleoproterozoic the dominant paleoproterozoic detrital zircon bimodal age peaks preserved in the seosan iron formation compare well with those from the south china craton sedimentary basins reflecting global tectonic events related to the columbia supercontinent in east asia

further evidence for a mass movement origin of the olympus mons aureole analysis of the ridge graben and fracture patterns on the aureole material surrounding olympus mons lends further support to an origin by gravity sliding of the outer flanks of the shield the patterns provide evidence of the nature of sliding especially in the last stages of emplacement no part of the slide need have traveled the whole extent of the aureole for the slides to occur on a shallow slope reduction of shear strength of the materials involved is required possibly by melting of ground ice triggering mechanisms could be volcanically related marsquakes and possibly stresses induced on the surrounding area by the growing olympus mons volcano

possible long term decline in impact rates 1 martian geological data abstract quantin et al quantin c allemand p mangold n delacourt c 2004a icarus 172 555 572 tabulated crater count data for 56 landslides along the walls of valles marineris under the assumption of a constant cratering rate after about 3 gyr ago as used in the 1999 2005 iterations of the crater chronology isochron system of hartmann and in the hartmann and neukum system these data indicate a regularly increasing rate of landslides which would be difficult to explain we suggest that these data may support a decline in inner solar system cratering rates by about a factor of 3 since 3 gyr ago not unlike predictions based on asteroid belt collision models such a decline is also supported by our review of data on lunar impact melts and glass spherules in a companion paper hartmann w k quantin c mangold n 2007 icarus 186 11 23 such models produce not only a more uniform rate of landslides over the last 3 gyr but also a more uniform rate of resurfacing processes which also had an apparent increase under the assumption of a constant cratering rate

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

theoretical analysis of secondary cratering on mars and an image based study on the cerberus plains the issue of crater retention age estimates on planetary surfaces is discussed with an attempt to quantify the effect of overlapping primary and secondary impact crater populations in restricted crater diameter ranges the approach to this problem is illustrated with a simple model production function where the secondary crater input is artificially enhanced extrapolation of such a secondary crater model distribution to a global record results in extraordinarily high crater frequencies that do not exist on mars and implies the need of detailed studies of the size frequency distribution for remote secondary craters to date poorly known a key case the martian crater zunil and its secondary crater field illustrate that reasonable predictions for the secondary crater size frequency distribution at small 100 m crater diameters affected the standard model crater retention age for the cerberus plains less than the statistical uncertainty these observations show that age determination based on appropriate crater counting statistics is valid in a wide primary crater diameter range

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

eruption history of the elysium volcanic province mars abstract volcanism has played a significant role in shaping the surface of mars volatiles released during eruptions profoundly altered the composition and density of the martian atmosphere in the past quantifying the amounts of volcanic outgassing has been attempted regionally and globally however volume estimates of erupted volcanic material in time are still poorly constrained here we show the first eruption frequency record of a volcanic region the elysium volcanic province which is the second largest volcanic province on mars studied lava flows show model ages ranging from 3 4 ga to 60 ma together with observed resurfacing ages the visible record of volcanic activity extends back to 3 9 ga based on our 190 model ages continuous volcanic activity is apparent with a major peak at 2 2 ga in the last 1 gyr the activity rapidly waned although the majority of volcanic material including volatiles released to the atmosphere erupted within a short time span of less than 200 ma activity continued until the very recent past extrapolating the eruption record it appears likely that this region of mars is still volcanically active today our derivation of the eruption record has profound implications for global thermal and atmospheric modeling indicating sustained heat production melt generation and magma ascent at a single location over most of the planet s geological history

temporally transitional mantle convection implications for mars 1 the thermal evolution of terrestrial planets such as earth mars and venus is strongly dominated by the convective processes in the planet s silicate mantle the actual style of convection controls the efficiency of heat transport and thus the cooling behavior of the whole planet in the present study we investigate the heat transport properties of variable viscosity convection focusing on the temporally transitional behavior discovered recently while the difference of the newly found convective regime to the already known stagnant lid and episodic behavior has been elaborated in our previous study the present work investigates the applicability of the observed intermittent behavior to the thermal evolution of terrestrial planets a 3 d numerical mantle convection code is applied and calculations are carried out in the parameter range for which the temporally transitional behavior has been found using the described approach it is possible to investigate the transition from a temporarily mobilized to a stagnant surface in a fluid dynamically consistent manner while such a scenario has been suggested for mars early history it has so far been investigated only by means of parameterized convection models we show that the sporadic surface mobilization events may indeed occur on time scales relevant for mars in order to assess their influence on the subsequent thermal evolution of planetary bodies an internal heating of the mantle and a secular cooling of the core are additionally taken into account the obtained results are compared to the findings of thermal evolution studies employing parameterized convection models we show that the thermal consequences of a temporal transition from a mobile to a stagnant surface are indeed correctly described by parameterized models as done in previous studies

tectonic evolution of korean peninsula and adjacent crustal fragments in asia introduction abstract the tectonic relationship between korea china and japan has been a topic of debate since several decades several recent studies have brought to light new aspects of korean geology including the finding of late permian to triassic eclogite facies metamorphism from the hongseong area there has also been substantial progress on the understanding of the tectonic history of east china new reports of paleozoic and triassic high p t metamorphic belts have also come from japan this thematic issue of gondwana research presents a timely integration of new information that aids in reconstructing the tectonic relationship between korea china and japan this issue begins with a set of four papers dealing with the tectonic relationship between korea china and japan the next five papers deal with various aspects of korean geology from late proterozoic to cenozoic subsequent five papers are related to the geology of japan the next three papers deal with the tectonic interpretation of the sino korean plate evidence for permo triassic collision between the indo china and south china blocks is reported in the next paper we thank reviewers for providing thorough and constructive reviews on the manuscripts for this issue we also thank the international association for gondwana research for providing us the opportunity to publish this thematic issue

tyrrhena patera geologic history derived from mars express high resolution stereo camera 1 we used mars express high resolution stereo camera images of the tyrrhena patera volcano to assign cratering model ages to material units defined in the vikingorbiter based geologic mapping cratering model ages are generally consistent with their stratigraphy we can identify three key intervals of major activity at tyrrhena patera 1 formation of the volcanic edifice in the noachian period 3 7 4 0 ga shortly following the hellas impact 4 ga and coincident with the formation of hadriaca patera 3 9 ga 2 modification of the edifice and formation of the caldera rille and channels in the hesperian period possibly extending into the amazonian period and 3 a final stage of modification in the late amazonian epoch 0 8 1 4 ga early to mid hesperian activity on tyrrhena patera is consistent with similar activity on hadriaca patera at 3 3 3 7 ga the most recent dateable event on tyrrhena patera is modification on the upper shield caldera rille and channel floors at 800 ma this coincidence of resurfacing in three units suggests a widespread process es which we speculate involved preferential aeolian erosion of small craters on these flatter surfaces relative to the other units on the volcano alternatively some combination of pyroclastic flow emplacement on the upper shield and fluvial activity in the caldera rille and channels followed by differential aeolian erosion and deposition could have produced the present surface regardless major geologic resurfacing ended at tyrrhena patera nearly a billion years ago

solving the martian meteorite age conundrum using micro baddeleyite and launch generated zircon the age of the representative martian meteorite nwa 5298 is determined using spatially correlated electron beam nanostructural and uranium lead isotopic measurements of microminerals resolving a paradox of different age interpretations for the evolution of martian crust a few of the many meteorites that fall to earth are of martian origin the true age of these rare samples of the martian surface has been the subject of a decades long debate with interpreted ages differing by up to 4 billion years desmond moser and colleagues resolve this problem using a new approach to dating meteorite launch events through nanoscale investigation of crystal growth zoning and structures their analysis of the resistant micromineral baddeleyite and host igneous minerals in the highly shocked martian meteorite northwest africa 5298 reveals it as a crystallization product of martian volcanism of the past 400 million years previous estimates of a 4 billion year age of formation have actually dated a remnant signature of the ancient mantle melting event from which the magma was derived these findings confirm the presence of an ancient non convecting mantle beneath a relatively young volcanic martian crust invaluable records of planetary dynamics and evolution can be recovered from the geochemical systematics of single meteorites1 however the interpreted ages of the ejected igneous crust of mars differ by up to four billion years1 2 3 4 5 6 a conundrum7 due in part to the difficulty of using geochemistry alone to distinguish between the ages of formation and the ages of the impact events that launched debris towards earth here we solve the conundrum by combining in situ electron beam nanostructural analyses and u pb uranium lead isotopic measurements of the resistant micromineral baddeleyite zro2 and host igneous minerals in the highly shock metamorphosed shergottite northwest africa 5298 ref 8 which is a basaltic martian meteorite we establish that the micro baddeleyite grains pre date the launch event because they are shocked cogenetic with host igneous minerals and preserve primary igneous growth zoning the grains least affected by shock disturbance and which are rich in radiogenic pb date the basalt crystallization near the martian surface to 187 33 million years before present primitive non radiogenic pb isotope compositions of the host minerals common to most shergottites1 2 3 4 do not help us to date the meteorite instead indicating a magma source region that was fractionated more than four billion years ago9 10 11 12 to form a persistent reservoir so far unique to mars1 9 local impact melting during ejection from mars less than 22 2 million years ago caused the growth of unshocked launch generated zircon and the partial disturbance of baddeleyite dates we can thus confirm the presence of ancient non convecting mantle beneath young volcanic mars place an upper bound on the interplanetary travel time of the ejected martian crust and validate a new approach to the geochronology of the inner solar system

age of feiran basement rocks sinai implications for late precambrian crustal evolution in the northern arabian nubian shield basement exposed on the perimeter of the red sea was created during the pan african event at the end of the precambrian pre pan african crust in the northern part of this region has not yet been identified this paper reports the results of rb sr whole rock and u pb zircon dating of gneisses and related basement units from the wadi feiran area in the sinai peninsula where the existence of such older basement has previously been suggested a post tectonic extensional dyke gives a rb sr age of 591 9 ma with an initial 87 sr 86 sr ratio of 0 7034 0 0002 rb sr whole rock and thin slab dating of paragneisses gives ages of c 610 ma with an initial 87 sr 86 sr ratio of 0 7035 a u pb zircon age of 632 3 ma is interpreted as either the time of formation of these gneisses or the age of the crust sampled by protolith sediments granodiorite to the east gives a u pb zircon age of 782 7 ma and is interpreted as representing the westernmost extent of a 780 50 ma terrane that extends across sinai into jordan uplift and erosion of the 780 50 ma terrane supplied detritus to flanking terranes in n and se sinai this region thus acted as a foreland to the younger accretionary and extensional units to the south and west that were active later in the pan african event there is still no evidence for pre pan african basement in the precambrian units around the northern red sea east of the nile

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

origin of the infracambrian salt basins of the middle east abstract during the infra and lower cambrain the arabian gulf and zagros mountains hormuz formation south oman ara formation kerman in central iran ravar formation and the salt range province in pakistan salt range formation were all sites of salt precipitation in subsiding rift basins along the middle eastern edge of gondwanaland also from 600 to 540 ma a rift system extended westwards from the sinai peninsula and north egypt across ne africa while a second rift system about 600 to perhaps 510 ma extended northwards from the sinai peninsula across the jordan valley and dead sea into se turkey the development of these rifts was contemporaneous with the left lateral najd strike slip fault system which dislocated the arabian peninsula by 300 km the najd system may be interpreted as a transform fault system which accommodated rifting between ne egypt the jordan valley north pakistan kerman and south oman the najd system is parallel to the zagros line and the latter is also the eastern depositional limit of the infracambrian hormuz formation the infracambrian zagros line may be interpreted as a right lateral strike slip fault system which caused infracambrian rifting in the arabian gulf and zagros mountains palaeontological and radiometric age constraints together with stratigraphic considerations suggest that the syn rift evaporites were first precipitated during a transgression which commenced in the latest precambrian 600 ma and again during a lower cambrian regression the regression terminated a period dominated by marine carbonates and evaporites and initiated the deposition of progradational alluvial to marginal marine sediments finally in the late lower cambrian and middle cambrian transgressive marine deposits swept across the middle east

venus timing and rates of geologic activity we employ the accumulation of the impact crater population as a geochronometer to examine the observable record of the history of venus 1 b y analyzing age relationships between craters and the different geologic units we show that the earlier suite of units from heavily deformed tesserae through slightly deformed regional plains occurred during a time period an order of magnitude shorter than the subsequent period from the end of emplacement of the wrinkle ridge network until the present these results imply high global rates of endogenic volcanic activity during the first era comparable to that of mid oceanic ridge volcanism of earth and much lower global rates of endogenic activity by two orders of magnitude for the second period

temporal variations in the convective style of planetary mantles investigations of mantle convection with temperature and strain rate dependent viscosity have shown the existence of fundamentally different convective styles by varying e g the rayleigh number the viscosity contrast or the strain rate dependency of viscosity the planform of convection in the asymptotic stationary state changes from the so called stagnant lid regime to an episodic behaviour and further to a state characterised by a permanently mobilised surface our studies suggest that this transition may not only be induced by a change of parameters but also occurs temporally for fixed parameters we have in fact observed convective systems in the stagnant lid regime that show isolated events of surface mobilisation occurring out of a thermally equilibrated state we use a 3d numerical mantle convection model to investigate mantle convection and surface dynamics as a coupled fluid dynamical system our studies focus on the existence of a transitional regime in which temporal variations between the stagnant lid and the episodic regime occur we were able to deduce a mobilisation criterion that describes the stability of the stagnant surface thus allowing for a quantitative analysis of the transition to a temporarily mobilised surface this criterion is also suitable to predict the occurrence of surface mobilisation events

3d structure of the gusev crater region abstract gusev crater lies within the aeolis quadrangle of mars at the boundary between the northern lowlands and southern highlands the ancient valley ma adim vallis dissects the highlands south of gusev crater and is thought to have fed the crater with sediments high resolution stereo camera data and digital elevation models were used to construct a geologic geomorphic map 173 5 178 5 e 10 18 s and cross sections complemented by data from mars orbiter camera mars orbiter laser altimeter and thermal emission imaging system three geologic domains are recognised the highlands in the south gusev crater and lowlands in the north twelve units are mapped with thicknesses ranging from hundred meters to several kilometres thicknesses of units and their bedding attitude are estimated combining the geologic map and topographic information relative ages are determined from crater counts ranging from early noachian for highland units to middle amazonian for units in gusev crater and in lowlands episodes of intense geologic activity deposition volcanism deformation occur at around 4 0 ga 3 7 ga and 3 5 ga comparing the geometry of the gusev crater with similar sized filled and un filled martian craters suggests that the columbia hills are relics of the original central peak of gusev crater

the relation of catastrophic flooding of mangala valles mars to faulting of memnonia fossae and tharsis volcanism detailed stratigraphic relations indicate two coeval periods of catastrophic flooding and tharsis centered faulting producing memnonia fossae in the mangala valles region of mars major sequences of lava flows of the tharsis montes formation and local lobate plains flows were erupted during and between these channeling and faulting episodes first late hesperian channel development overlapped in time the tharsis centered faulting that trends north 75 to 90 e next late hesperian early amazonian flooding was coeval with faulting that trends north 55 to 70 e in some reaches resistant lava flows filled the early channels resulting in inverted channel topography after the later flooding swept through both floods likely originated from the same graben which probably was activated during each episode of faulting faulting broke through groundwater barriers and tapped confined aquifers in higher regions west and east of the point of discharge the minimum volume of water required to erode mangala valles about 5 1012 m3 may have been released through two floods that drained a few percent pore volume from a relatively permeable aquifer the peak discharges of the floods may have lasted from days to weeks the perched water discharged from the aquifer may have been produced by hydrothermal groundwater circulation induced by tharsis magmatism tectonic uplift centered at tharsis montes and compaction of saturated crater ejecta due to loading by lava flows

martian cratering 9 toward resolution of the controversy about small craters abstract the malin et al malin m c edgett k posiolova l mccolley s noe dobrea e 2006 science 314 1573 1577 report of ongoing production of decameter scale primary craters matches within an order of magnitude the crater production curves of hartmann hartmann w k 2005 icarus 174 294 320 and appears to contradict models that say our curves grossly overpredict primary production nonetheless such models may still predict correct order of magnitude secondary production the new observations if correct require geologically recent martian surface modification in some areas on timescales comparable to the cycles of obliquity changes suggesting episodic martian climatic environmental change driven by the obliquity cycles

tectonic evolution of the northwestern red sea margin as related to basement structure abstract at the northwestern egyptian margin of the red sea a crystalline basement of pan african late proterozoic age is exposed beneath a phanerozoic sedimentary cover this tectonic situation provided an opportunity to study the younger red sea tectonic evolution in relation to features that are inherited from or reactivated in an underlying basement two structural maps compiled from published sources and our data covering the eastern egyptian basement from hurghada to the south show the most important structural elements the major central and southern parts of the basement in eastern egypt ced and sed are dominated by low angle thrusts other fractures n s e w nw se are distinctly less frequent and of local importance nw se oriented strike slip faults najd system overprinted the low angle thrusts of the northern ced these fault are related to ne sw trending normal faults which characterize the northern part of the basement ned before discussing general relationships between basement structure and red sea evolution a detailed example of the post pan african phanerozoic evolution of the low angle thrust dominated domains of the ced and the sed from a traverse south of 25 n is presented there detailed structural mapping showed phanerozoic normal faults close to the red sea trending nnw sse and at the margin of the phanerozoic cover east of the nile trending nnw sse and nw se leading to a horst like position of the basement within this horst deformation is restricted to a local sinistral strike slip fault trending n s the normal faults frequently used and reactivated fractures parallel or normal to the axial surface of earlier pan african folds igneous activity is confined to the western part of the basement where mafic dykes mafic to trachytic volcanics the alkaline gabal abu khrug ring complex and trachyte plugs in that time sequence from 104 to 80 ma are exposed mafic dykes are dependent on host rock properties distributed mainly in relatively brittle granitoid rocks trachyte intrusions are generally not related to particular host rocks but may follow pre existing structures crosscutting relationships with the igneous rocks imply a late cretaceous age for the faults in the west of the traverse however in the east at the red sea coast faults cut middle miocene sediments and are thus younger tectonic activity in the traverse area is therefore confined to two distinct episodes a late cretaceous event of alkaline magmatism and normal faulting at the western margin of the basement and a miocene or younger event of normal faulting at the present red sea coast in contrast to earlier views neither transform faults nor large scale riedel shears related to red sea tectonics have been met in the area this holds also true for the ced and sed as a whole where only local faults remain for potential reactivation during red sea tectonics except perhaps the duwi shear zone at quseir the prevailing n s e w or nw se directions of these faults preclude an interpretation as possible transform faults normal to the spreading axis of the red sea therefore it is no longer possible to assume a continuation of red sea transform faults into the continental crust

ages and stratigraphy of mare basalts in oceanus procellarum mare nubium mare cognitum and mare insularum accurate estimates of mare basalt ages are necessary to place constraints on the duration and the flux of lunar volcanism as well as on the petrogenesis of lunar mare basalts and their relationship to the thermal evolution of the moon we performed new crater size frequency distribution measurements in order to investigate the stratigraphy of mare basalts in oceanus procellarum and related regions such as mare nubium mare cognitum and mare insularum we used high resolution clementine color data to define 86 spectrally homogeneous units within these basins which were then dated with crater counts on lunar orbiter iv images our crater size frequency distribution measurements define mineralogical and spectral surface units and offer significant improvements in accuracy over previous analyses our data show that volcanism in the investigated region was active over a long period of time from 3 93 to 1 2 b y a total of 2 7 b y volumetrically most of the basalts erupted in the late imbrian period between 3 3 and 3 7 b y and we see evidence that numerous units have been resurfaced during the eratosthenian period significantly less basalt was erupted depending on the absolute model ages that one can assign to the lunar chronostratigraphic systems five units might be of copernican age younger basalts are generally exposed in the center of the investigated area that is closer to the volcanic centers of the aristarchus plateau and marius hills older basalts occur preferentially along the northwestern margin of oceanus procellarum and in the southeastern regions of the studied area i e in mare cognitum and mare nubium combining the new data with our previously measured ages for basalts in mare imbrium serenitatis tranquillitatis humorum australe and humboldtianum we find that the period of active volcanism on the moon lasted 2 8 b y from 4 b y to 1 2 b y on the basis of the basalts dated so far which do not yet include the potentially young basalts of mare smythii e g schultz and spudis 1983 we conclude that oceanus procellarum not only exhibits the widest range of ages of all investigated basins but probably also is the location of some of the youngest basalts on the lunar surface

elysium planitia lava flows crater count chronology and geological implications mars global surveyor mgs and viking images allow analysis of chronological and geological relations among lava flows in southern elysium planitia based on crater populations mgs and viking images clearly show morphological features of lava flows with some extremely sparsely cratered young flow units atop somewhat older surfaces there is no evidence of substantial dust mantling on the young flows and hence we infer that the crater populations date not sediment accumulation but the lava flows themselves the youngest flows have some of the lowest crater densities that we have seen on mars some 1 of lunar mare values at crater diameters below about 180 to 500 m we infer that thin scattered volcanic flows some tens of meters thick have been emplaced atop stratigraphic units within the last 100 myr and possibly within the last 10 myr volcanism is thus a continuing process in the recent geologic history of mars and this must constrain geophysical models of the planet

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

concordant rb sr sm nd and ar ar ages for northwest africa 1460 a 346 ma old basaltic shergottite related to lherzolitic shergottites abstract multiple lines of evidence show that the rb sr sm nd and ar ar isotopic systems individually give robust crystallization ages for basaltic or diabasic shergottite northwest africa nwa 1460 in contrast to other shergottites nwa 1460 exhibits minimal evidence of excess 40ar thus allowing an unambiguous determination of its ar ar age the concordant rb sr sm nd and ar ar results for nwa 1460 define its crystallization age to be 346 17 ma 2 in combination with petrographic and trace element data for this specimen and paired meteorite nwa 480 these results strongly refute the suggestion by others that the shergottites are 4 1 ga old current crystallization and cosmic ray exposure cre age data permit identification of a maximum of nine ejection events for martian meteorites numbering more than 50 unpaired specimens as of 2008 and plausibly as few as five such events although recent high resolution imaging of the martian surface has identified limited areas of sparsely cratered terrains the meteorite data suggest that either these areas are representative of larger areas from which the meteorites might come or that the cratering chronology needs recalibration time averaged 87rb 86sr 0 16 for the mantle source of the parent magma of nwa 1460 480 over the 4 56 ga age of the planet is consistent with previously estimated values for bulk silicate mars in the range 0 13 0 16 and similar to values of 0 18 for the lherzolitic shergottites initial end for nwa 1460 480 at 350 16 ma ago was 10 6 0 5 which implies a time averaged 147sm 144nd of 0 217 in the martian mantle prior to mafic melt extraction similar to values of 0 211 0 216 for the lherzolitic shergottites these time averaged values do not imply a simple two stage mantle melt evolution but must result from multiple episodes of melt extractions from the source regions much higher late stage end values for the depleted shergottites imply similar processes carried to a greater degree thus nwa 1460 480 the lherzolitic shergottites and perhaps eet 79001 give the best albeit imperfect estimate of the sr and nd isotopic characteristics of bulk silicate mars

extent age and resurfacing history of the northern smooth plains on mercury from messenger observations abstract messenger orbital images show that the north polar region of mercury contains smooth plains that occupy 7 of the planetary surface area within the northern smooth plains nsp we identify two crater populations those superposed on the nsp post plains and those partially or entirely embayed buried the existence of the second of these populations is clear evidence for volcanic resurfacing the post plains crater population reveals that the nsp do not exhibit statistically distinguishable subunits on the basis of crater size frequency distributions nor do measures of the areal density of impact craters reveal volcanically resurfaced regions within the nsp these results suggest that the most recent outpouring of volcanic material resurfaced the majority of the region and that this volcanic flooding emplaced the nsp over a relatively short interval of geologic time perhaps 100 my or less stratigraphic embayment relationships within the buried crater population including partial crater flooding and the presence of smaller embayed craters within the filled interiors of larger craters and basins indicate that a minimum of two episodes of volcanic resurfacing occurred from the inferred rim heights of embayed craters we estimate the nsp to be regionally 0 7 1 8 km thick with a minimum volume of volcanic material of 4 10 6 to 10 7 km 3 because of the uncertainty in the impact flux at mercury the absolute model age of the post plains volcanism could be either 3 7 or 2 5 ga depending on the chronology applied

possible juventae chasma subice volcanic eruptions and maja valles ice outburst floods on mars implications of mars global surveyor crater densities geomorphology and topography 1 this article discusses image topographic and spectral data from the mars global surveyor mgs mission that provide new information concerning the surface age geomorphology and topography of the juventae chasma maja valles system our study utilizes data from two instruments on board mgs images from the mars orbiter camera moc and topography from the mars orbiter laser altimeter mola within maja valles we can now observe depositional bars with megaripples that unequivocally show catastrophic floods occurred in the channel viking impact crater densities indicated the chasma and channel floor areas were all one age late hesperian to amazonian however moc data indicate a marked difference in densities of small craters between juventae chasma maja valles and the channel debouchment area in chryse planitia basin although other processes may contribute to crater variability young resurfacing events in the chasma and episodes of recent erosion at maja valles channel head may possibly account for the disparate crater densities along the chasma channel system relatively young volcanic eruptions may have contributed to resurfacing as in juventae chasma a small possible volcanic cone of young dark material is observed moc data also indicate previously unknown interior layered deposit mounds in the chasma that indicate at least two periods of mound formation finally mola topography shows that the entire floor of the chasma lies at the same elevation as the channel debouchment area in chryse basin resulting in a 3 km high barrier to water flow out of the chasma blocked ponded water would rapidly freeze in the current and likely past climate of mars for catastrophic flow to occur in maja valles some process is required to melt ice and induce floods out of the chasma we suggest subice volcanic eruption and calculate estimates of water discharges and volumes that these eruptions might have produced

recent and episodic volcanic and glacial activity on mars revealed by the high resolution stereo camera the large area coverage at a resolution of 10 20 metres per pixel in colour and three dimensions with the high resolution stereo camera experiment on the european space agency mars express mission has made it possible to study the time stratigraphic relationships of volcanic and glacial structures in unprecedented detail and give insight into the geological evolution of mars here we show that calderas on five major volcanoes on mars have undergone repeated activation and resurfacing during the last 20 per cent of martian history with phases of activity as young as two million years suggesting that the volcanoes are potentially still active today glacial deposits at the base of the olympus mons escarpment show evidence for repeated phases of activity as recently as about four million years ago morphological evidence is found that snow and ice deposition on the olympus construct at elevations of more than 7 000 metres led to episodes of glacial activity at this height even now water ice protected by an insulating layer of dust may be present at high altitudes on olympus mons

noachian hesperian geologic history of the echus chasma and kasei valles system on mars new data and interpretations abstract new high resolution datasets have prompted a mapping based study of the 2500 km long echus chasma and kasei valles system that utilizes geomorphic details stratigraphic relations and cratering statistics derived from the new datasets our results suggest that between the hesperian and amazonian epochs on mars 3 7 ga to recent the study area was affected by at least 4 episodes of widespread volcanic activity and 4 periods of episodic fluvio glacial activity this paper discusses the noachian to hesperian 2 6 ga history of the study area during which time in the hesperian three of the volcanic episodes took place along with the first two episodes of fluvio glacial activity highlights of our new findings and refinements from this time period include 1 two ancient western floods sourced from tharsis that occurred around 3 61 and 3 4 ga and carved the east trending path of north kasei valles 2 uranius dorsum with 58 aligned cones along its length that was emplaced in north kasei around 3 6 ga between these periods of early flooding 3 possible ash deposits surrounding echus chasma one of which contains narrow dendritic valley systems that may have been carved over a period of about 10 000 yr and 4 evidence that suggests glaciers and near surface ice may have contributed to erosion of local units

interior channels in martian valleys constraints on fluvial erosion by measurements of the mars express high resolution stereo camera 1 in high resolution stereo camera hrsc images of the mars express mission a 130 km long interior channel is identified within a 400 km long valley network system located in the lybia montes ages of the valley floor and the surroundings as derived from crater counts define a period of 350 myrs during which the valley might have been formed based on hrsc stereo measurements the discharge of the interior channel is estimated at 4800 m3 s corresponding to a runoff production rate of 1 cm day mass balances indicate erosion rates of a few cm year implying the erosion activity in the valley to a few thousand years for continuous flow or one or more orders of magnitude longer time spans for more intermittent flows therefore during the hesperian relatively brief but recurring episodes of erosion intervals are more likely than sustained flow

lunar mare basalt flow units thicknesses determined from crater size frequency distributions accurate lava flow unit thicknesses estimates are necessary to place constraints on volcanic flux estimates we refine the technique of using the shape of crater size frequency distribution csfd curves to estimate the thickness of individual lunar mare flow units we find that a characteristic knee often observed in csfd curves is reasonably interpreted to represent the presence of two lava flow units separated in time and that the diameter at which this knee occurs is related to the thickness of the overlying flow unit examination of 58 curves with this characteristic knee in several lunar nearside basins oceanus procellarum imbrium tranquillitatis humorum cognitum nubium and insularum allowed us to identify flow units that have not been detected in low sun images we found that the range of flow unit thickness is 20 220 m and the average is 30 60 m this technique expands considerably the ability to assess lava flow unit thicknesses and volumes on the moon and planets

tempe fossae mars a planetary analogon to a terrestrial continental rift tempe terra the northeastern part of the tharsis region on mars is characterized by several extensional structures differing in style and age the tempe fossae a particularly well developed system of graben striking n45 e have been studied for the first time in detail on the basis of viking orbiter imagery and mars observer laser altimeter mola topographic data the graben system appears to be unique in the whole region because of its extent the remarkable width and depth of the graben the varying graben pattern and the associated volcanism single graben can be up to 35 km wide and 120 km long while their depth can reach nearly 3000 m they typically show an asymmetric architecture often in halfgraben or step fault style the graben system widens and changes its style along strike from ne to sw from a single very deep and narrow graben to a complex set of several shallower sinuous graben and halfgraben crustal extension was measured in the northeastern part from the observable throw and amounts to 2 5 3 1 km volcanic structures can be found at several locations along the graben in the sw the graben system seems to be connected with the tempe terra volcanic province marked by flood basalts and plains volcanism craters were counted on volcanic units unaffected by extensional tectonics crater statistics indicate an absolute crater model age for the upper end of the extensional deformation of 3 5 ga the dimensions and the characteristics of the mapped graben system resemble those of terrestrial continental rifts and a comparison between them and the kenya rift reveals striking similarities therefore the tempe fossae are interpreted as a martian analog to a continental rift associated with an underlying mantle plume this hypothesis seems to be supported by recent geophysical models based on topography and gravity data from mars global surveyor which indicate regional uplift for tempe terra

late proterozoic crustal evolution of the midyan region northwestern saudi arabia three volcano sedimentary sequences and related intrusive suites ranging in age from pre 700 ma to about 570 ma are recorded in the midyan region of northwestern saudi arabia crustal history involved island arc accretion and compressional tectonics up to about 625 ma and extensional tectonics and rifting after that date possible correlatives of the midyan stratigraphy as well as ophiolitic rocks of a probable suture zone may exist in southeastern sinai although doubt exists because of the conflicting views of workers in that area

evidence for recent volcanism on mars from crater counts impact craters help characterize the age of a planetary surface because they accumulate with time they also provide useful constraints on the importance of surface erosion as such processes will preferentially remove the smaller craters earlier studies of martian crater populations revealed that erosion and dust deposition are important processes on mars they disagreed however on the age of the youngest volcanism these earlier studies were limited by image resolution to craters larger than a few hundred metres in diameter here we report an analysis using new images obtained by the mars global surveyor spacecraft of crater populations that extend the size distribution down to about 16 m our results indicate a wide range of surface ages with one region lava flows within the arsia mons caldera that we estimate to be no older than 40 100 million years we suggest that volcanism is a continuing process on mars

u pb isotopic systematics of shock loaded and annealed baddeleyite implications for crystallization ages of martian meteorite shergottites abstract shock recovery and annealing experiments on basalt baddeleyite mixtures were undertaken to evaluate shock effects on u pb isotopic systematics of baddeleyite shock pressures up to 57 gpa caused fracturing of constituent phases mosaicism of olivine maskelynitization of plagioclase and melting but the phase transition from monoclinic baddeleyite structure to high pressure temperature polymorphs of zro 2 was not confirmed the u pb isotopic systems of the shock loaded baddeleyite did not show a large scale isotopic disturbance the samples shock recovered from 47 gpa were then employed for annealing experiments at 1000 or 1300 c indicating that the basalt baddeleyite mixture was almost totally melted except olivine and baddeleyite fine grained euhedral zircon crystallized from the melt was observed around the relict baddeleyite in the sample annealed at 1300 c for 1 h the u pb isotopic systems of baddeleyite showed isotopic disturbances many data points for the samples annealed at 1000 c plotted above the concordia both radiogenic lead loss uranium gain and radiogenic lead gain uranium loss were observed in the baddeleyite annealed at 1300 c complete radiogenic lead loss due to shock metamorphism and subsequent annealing was not observed in the shock loaded annealed baddeleyites studied here these results confirm that the u pb isotopic systematics of baddeleyite are durable for shock metamorphism since shergottites still preserve fe mg and or ca zonings in major constituent phases i e pyroxene and olivine the shock effects observed in martian baddeleyites seem to be less intense compared to that under the present experimental conditions an implication is that the u pb systems of baddeleyite in shergottites will provide crystallization ages of martian magmatic rocks

hadriaca patera insights into its volcanic history from mars express high resolution stereo camera 1 high resolution stereo camera hrsc images of hadriaca patera mars in combination with mars orbiter camera moc mars orbiter laser altimeter mola and thermal infrared imaging system themis data sets reveal morphologic details about this volcano and enable determination of a chronology of the major geologic events through new cratering age assessments new topographic measurements of the hadriaca edifice were also made from a hrsc based high resolution 125 m digital terrain model dtm and compared to the mola dtm we find evidence for a complex formation and erosional history at hadriaca patera in which volcanic fluvial and aeolian processes were all involved crater counts and associated model ages suggest that hadriaca patera formed from early shield building volcanic likely explosive pyroclastic eruptions at 3 7 3 9 ga with caldera formation no later than 3 5 ga a variety of geologic activity occurred in the caldera and on the northern flank and plains at 3 3 3 5 ga likely including pyroclastic flows that partially filled a large crater nw of the caldera and plains to the ne and differential erosion deposition by aeolian and or fluvial activity there were some resurfacing event s in the caldera and on the eastern flank at 2 4 2 6 ga in which the eastern flank s morphology is indicative of fluvial erosion the most recent dateable geologic activity on hadriaca patera includes caldera resurfacing by some process most likely differential aeolian erosion deposition in the amazonian period as recent as 1 5 ga this is coincident with the resurfacing of the heavily channeled south flank by fluvial erosion unlike the tharsis shields major geologic activity ended at hadriaca patera over a billion years ago

chemical dieferentiation of a convecting planetary interior consequences for a one plate planet such as venus partial melting to generate the crust of a planet creates compostionally buoyant residual mantle in the absence of mantle flow associated with plate tectonics this buoyant refractory layer may collect at the top of the mantle with important implications for the evolution of the interior and surface in this study models of the thermal and chemical evolution of a planetary interior demonstrate the possible consequences of a chemically buoyant depleted mantle layer as the depleted layer thickens the melting temperature at the top of the underlying convecting mantle also increases and the degree of partial melting of mantle added to the depleted layer decreases as less depleted mantle with less positive compositional buoyancy is added negative thermal buoyancy of the layer eventually exceeds its positive compositional buoyancy the depleted layer then sinks into and mixes with the convecting interior the top of the convecting mantle then moves to a shallower depth larger degrees of melting resume and a new depleted layer accumulates this accumulation and instability of the depleted layer occurs repeatedly over a substantial portion of the planet s evolution with a period of 300 500 myr on venus the population of impacts craters is indistinguishable from a random distribution over the surface and give a surface age of about 500 myr we speculate that the mechanism described above may explain this episodic global resurfacing of venus

a review of meteorite evidence for the timing of magmatism and of surface or near surface liquid water on mars 1 there is widespread photogeological evidence for ubiquitous water flowing on the surface of mars however the age of surface and near surface water cannot be deduced with high precision from photogeology while there is clear evidence for old and young fluvial features in the photogeologic record the uncertainty in the absolute calibration of the martian crater flux results in uncertainties of 1 5 gyr in the middle period of martian geologic history aqueous alteration of primary igneous minerals produces secondary minerals in martian meteorites here we use the ages of secondary alteration minerals in martian meteorites to obtain absolute ages when liquid water was at or near the surface of mars aqueous alteration events in martian meteorites occurred at 3929 37 ma carbonates in alh84001 633 23 ma iddingsite in nakhlites and 0 170 ma salts in shergottites furthermore these events appear to be of short duration suggesting episodic rather than continuous aqueous alteration of the meteorites the martian meteorites appear to be contaminated by martian surface pb characterized by a 207pb 206pb ratio near 1 lead of this composition could be produced by water based alteration on the martian surface the high 129xe 132xe ratio in the martian atmosphere compared to martian meteorites indicates fractionation of i from xe within 100 myr after nucleosynthesis of 129i such fractionation is difficult to achieve through magmatic processes however water very efficiently fractionates i from xe raising the intriguing possibility that mars had a liquid water ocean within its first 100 myr 1

the circum hellas volcanic province mars assessment of wrinkle ridged plains abstract wrinkle ridged plains common on the terrestrial planets are inferred to form by tectonic deformation of volcanic or sedimentary surfaces if wrinkle ridged plains adjacent to volcanic features originated from volcanic activity associated with the features then the plains should have similar ages to the volcanic features as well as other morphological or compositional indicators of volcanism we test this hypothesis on several regions of wrinkle ridged plains bounding the circum hellas volcanic province chvp we constructed themis daytime ir mosaics spatial resolution 100 m pixel and performed crater counts to determined cratering model formation ages of these regions we found ages of 3 4 3 7 ga for se malea planum 3 6 3 7 ga for w promethei terra 3 5 3 7 ga for hesperia planum and 3 6 3 8 ga for the eastern hellas basin floor all of which are contemporaneous with the formation of the major volcanic edifices 3 6 3 8 ga in the chvp although examination of available moc ctx hirise themis and hrsc data shows few examples of features definitive of a volcanic origin of these wrinkle ridged plains hrsc true color images 50 100 m pixel show that large areas of these plains have a dark blue black color consistent with previous viking and mars express hrsc and omega color spectral studies indicative of basaltic compositions however in some areas this color may be due to aeolian mantles rather than underlying bedrock thus we cannot demonstrate unequivocally that these wrinkle ridged plains are volcanic beyond the synchronicity of their formation ages with those of the chvp volcanoes nevertheless if these wrinkle ridged plains were originally formed by volcanic activity or other processes induced by volcanic activity then the inclusion of these regions brings the size of the chvp to 4 86 106 km 2 comparable in size to the elysium volcanic province thus the chvp may have been a dominant source of volcanism early in martian history

volatiles in the atmosphere of mars the effects of volcanism and escape constrained by isotopic data abstract we study the long term evolution of the conditions on the surface of mars through the modeling of the effects of volcanic degassing and atmospheric non thermal escape during the last four billion yr we propose to use the recent advances due to observation and modeling to constrain possible evolutions of the atmosphere of mars with the help of isotopic data from carbon nitrogen and argon the history of argon is studied through direct calculation of its degassing and escape whereas for other species the analysis is an integration backwards in time from present day situation in our calculation we do not consider early impact erosion hydrodynamic escape or carbonate formation volcanic degassing is obtained from crust production models observation of the surface and realistic volatile contents of the lavas aspera analyzer of space plasma and energetic atoms measurements and modeling of the escape rates produced by ionic escape sputtering and dissociative recombination constitute the sink of volatiles we constrain the maximum escape flux of co 2 with the evolution of argon and the 40 ar 36 ar ratio in the atmosphere and measurements of the present day situation this imposes restricted escape flux consistent with the recent lowering of the expected escape efficiency on mars our model is able to reproduce present day 36 ar abundance and 40 ar 36 ar ratio we also show that the present day atmosphere of mars is likely to be constituted by a large part of volcanic gases with a low co 2 concentration in the magma 150 ppm present atmosphere is constructed of 50 of volcanic gases emitted since 3 7 billion yr ago we oppose this late volcanic atmosphere to the early atmosphere in place during noachian and composed of primitive volatile brought during accretion magma ocean phase and pre 3 7 ga volcanism likewise the mean age of the atmosphere is estimated to be no more than 1 9 to 2 3 billion yr atmospheric pressures and variations on mars are predicted to be low 50 mbar as the result of degassing and non thermal escape this seems in line with the assumption of a big loss of volatiles during the first 500 myr isotopic ratios lead us to propose that nitrogen is probably old in the martian atmosphere and has been subjected to the fractionation of atmospheric escape the 12 c 13 c on the other hand is more stable and indicates that carbon is younger water could have existed on mars through the last 4 billion yr evolution within a factor of 1 6 times greater to 3 times less than the present day inventory including polar caps depending on the volcanic degassing it is however unlikely to reside in the atmosphere or in liquid form unless large scale perturbations occur changes in obliquity and large input of greenhouse gases due to a short burst of volcanism

topography of valley networks on mars from mars express high resolution stereo camera digital elevation models 1 martian valley networks have been identified mainly in the noachian heavily cratered uplands the geometry of valley networks can be studied using mars orbiter laser altimeter mola altimetry which is sufficient to map large valleys without a detailed 3 d shape of valley networks imaging from the mars express high resolution stereo camera hrsc is used to generate digital elevation models dems with resolution 50 m and vertical accuracy 60 m we studied valleys near huygens crater and in the aeolis region both in the noachian bedrock and on the west echus plateau in hesperian bedrock hrsc dems in these areas show that 1 drainage density is 3 times higher than is observed in mola data 2 degree of ramification is 1 order more than with mola 3 transverse valley profiles show a v shape more accurately and a minimum depth of 20 m and 4 higher drainage density shows greater headward extension that is not correlated to greater valley depth the deepest valleys 400 m are found in the huygens region where the density in the dem is 0 1 km 1 compared to shallow valleys 100 m of the echus region where the density is higher 0 3 km 1 these regional differences are due to spatially variable preservation and bedrock lithology longitudinal profiles suggest variations in duration of activity profile concavity is only developed in some noachian terrains valleys visible in hrsc images correspond to topographic features in dems showing the same geometry as terrestrial valleys thought to be formed by overland flows and seepage

ages of valles marineris mars landslides and implications for canyon history the chronology of landslides of valles marineris the equatorial trough system of mars has been investigated by a crater population study valles marineris landslides have widespread debris aprons which offer a remarkable opportunity to study the crater population with high resolution images from mars orbiter camera moc and from mars odyssey thermal emission imaging system themis sixty six ages were determined within valles marineris including 56 landslide ages and 10 ages of the previous termcanyonnext term floor results reveal that landslides of valles marineris system of previous termcanyonsnext term occurred during a widespread period of time between 3 5 gy and 50 my in some locations the previous termcanyonnext term floor has an apparent age of 3 5 gy suggesting that at least locally within valles marineris no major refreshing processes have occurred for 3 5 gy the temporal repetitivity of landslides implies that the triggering mechanisms of the landslides are reproducible in time landslides have the same features whatever their age the dynamic of these landslides is probably the same either with intervention of water up to recently the last 100 my or without water since 3 5 gy

rift zone volcanism and associated cinder cone field in utopia planitia mars 1 we analyzed a small cone field in sw utopia planitia that shows striking similarities to volcanic rift zones on earth this is of particular interest as the study area lies off any of the volcanic centers of mars in the northern hemisphere lowlands and is surrounded by material of the amazonian aged vastitas borealis formation vbf we believe that the study area could be the first direct evidence for volcanic activity in utopia planitia prior to the resurfacing by vbf material the most striking characteristics that we observed are 1 a set of broad eruptive fissures showing signs of ongoing extension during cone formation 2 parallel dike swarms 3 magmatic intrusions and 4 a number of lava flows that have erupted from the fissures and pitted cones that surround a larger cone in the center of the study area and that are often associated with or aligned along the fissures and dikes cratering model ages indicate that the study area is of late noachian to early hesperian age and is thus distinctly older than the surrounding vbf falling into the time frame of the main phases of activity in the volcanic centers of syrtis major elysium planitia and the southern highlands

the case for old basaltic shergottites abstract the crystallization age of shergottites is currently not agreed upon although mineral 87 rb 87 sr 147 sm 143 nd 176 lu 176 hf and u pb isochrons all give very young ages typically in the range of 160 180 ma 207 pb 206 pb data support a much older crystallization age at 4 1 ga which is consistent with published whole rock 87 rb 87 sr data on basaltic shergottites different isotopic systems present different complexities but crater counting chronology which shows that a substantial fraction of the martian surface was resurfaced during the late heavy bombardment is in favor of an old martian lithosphere with ages in accordance with pb pb and rb sr isotopic data a 4 1 ga pb pb age of shergottites also agrees with the 142 nd and 182 w anomalies found in these rocks and concur with the presence of an actively convecting mantle during the first 500 myr of the planet s history we here present new sm nd lu hf and pb pb mineral isochrons for the basaltic shergottites shergotty and los angeles complementing our previous results on zagami bouvier a blichert toft j vervoort j d and albarede f 2005 the age of snc meteorites and the antiquity of the martian surface earth planet sci lett 240 221 233 the internal 147 sm 143 nd and 176 lu 176 hf isochrons give young ages of respectively 172 40 mswd 2 0 and 188 91 mswd 3 1 for shergotty and 181 13 mswd 0 14 and 159 42 mswd 0 01 for los angeles in contrast the pb isotope compositions of the leached whole rock fragments and maskelynite separates of shergotty and los angeles fall on the whole rock isochron previously established for zagami and other shergottite samples and collectively yield a pb pb age of 4050 70 ma for the crystallization of the basaltic shergottite suite the contrast between the 170 ma ages of internal isochrons and the 4 1 ga age supported by pb pb and 87 rb 87 sr on whole rocks simply reflects that the younger age dates the perturbation of a suite of rocks of noachian age the internal rb sr sm nd lu hf and u pb errorchrons are heavily biased by the presence of disseminated phosphate minerals and inclusions for which d h ratios d 4600 indicate strong interaction with martian subsurface waters in contrast baddeleyite occasionally present in sncs and also having extremely young u pb ages reflect resetting under the shock conditions that prevailed either during shergottite extraction from the planet or from impacts associated with a major break up event of planetesimals in the main asteroid belt we finally also re examine 39 ar 40 ar data on snc meteorites and suggest that they as well support old crystallization ages

geochronological constraints on multiple deformations of the honam shear zone south korea and its tectonic implication abstract the ne trending honam shear zone south korea is a tectonic scale shear zone consisting of several parallel mylonite zones jeonju sunchang and yecheon shear zones here we report the sensitive high resolution ion microprobe shrimp u pb zircon ages from the mylonitic precambrian orthogneiss and hornblende biotite granitoid near the myeongho area within the yecheon shear zone a representative segment of the honam shear zone the core and overgrowth parts of the zircons from the foliated hornblende biotite granitoid yielded ca 187 ma and ca 178 ma respectively the former is interpreted as a timing of emplacement whereas the latter as a timing of subsequent mylonitization which is similar to zircon overgrowth age of ca 179 ma obtained from mylonitized quartz feldspar orthogneiss muscovite and biotite k ar ages of mylonitized precambrian rocks and foliated granitoids range from 162 168 ma 148 155 ma and 136 ma our geochronological results combined with those of previous geochronological and structural studies suggest that the honam shear zone is a tectonic element created during middle jurassic daebo or early yanshanian orogeny and lasted possibly as late as at early cretaceous in relation to the subduction of the izanagi plate under the asian continent

evidence for a massive phreatomagmatic eruption in the initial stages of formation of the mangala valles outflow channel mars 1 new data reveal the presence of a system of parallel dune like ridges extending in a band for up to 25 30 km around the eastern margin of the source trough for the mangala valles outflow channel system we interpret these ridges to be formed during initial phreatomagmatic activity caused by dike emplacement cryospheric cracking magmagroundwater mixing and explosive eruption to the surface the erupted material consisted of fragmented magma steam and country rock which expanded from a choked state at the surface vent to form a near ballistic io like eruption plume outward flow of the plume at velocities of 430 m s from a 30 60 km long section of the graben is interpreted to have formed the system of dune like ridges subsequent outpouring of groundwater formed the mangala valles outflow channel system index terms 5400 planetology solid surface planets 5407 planetology solid surface planets atmospheres evolution 5416 planetology solid surface planets glaciation 6225 planetology solar system objects mars citation wilson l and j w head iii 2004 evidence for a massive phreatomagmatic eruption in the initial stages of formation of the mangala valles outflow channel mars geophys res lett 31 l15701 doi 10 1029 2004gl020322

analysis of impact crater populations and the geochronology of planetary surfaces in the inner solar system analyzing the density of impact craters on planetary surfaces is the only known technique for learning their ages remotely as a result crater statistics have been widely analyzed on the terrestrial planets since the timing and rates of activity are critical to understanding geologic process and history on the moon the samples obtained by the apollo and luna missions provide critical calibration points for cratering chronology on mercury venus and mars there are no similarly firm anchors for cratering rates but chronology models have been established by extrapolating from the lunar record or by estimating their impactor fluxes in other ways this review provides a current perspective on crater population measurements and their chronological interpretation emphasis is placed on how ages derived from crater statistics may be contingent on assumptions that need to be considered critically in addition ages estimated from crater populations are somewhat different than ages from more familiar geochronology tools e g radiometric dating resurfacing processes that remove craters from the observed population are particularly challenging to account for since they can introduce geologic uncertainty into results or destroy information about the formation age of a surface regardless of these challenges crater statistics measurements have resulted in successful predictions later verified by other techniques including the age of the lunar maria the existence of a period of heavy bombardment in the moon s first billion years and young volcanism on mars

the age of snc meteorites and the antiquity of the martian surface we report new sm nd lu hf and pb pb mineral and whole rock isotope data for the basaltic shergottite zagami as well as pb pb whole rock isotope data for the basaltic shergottite los angeles the lherzolitic shergottite dar al gani 476 dag 476 and the clinopyroxenite nakhla in agreement with previous findings our new sm nd and lu hf mineral ages on the martian meteorite zagami are young 155 and 185 ma respectively the 207pb 206pb 204pb 206pb compositions of the insoluble fractions of shergottites zagami los angeles and literature data for shergotty and eeta79001 form an excellent alignment indicative of a 4 0 ga crystallization age the range of pb isotope compositions observed in the leachates of these samples attests to negligible contamination of the shergottites by terrestrial pb and argues against mixing relationships the age of 4 048 0 017 ga mswd 1 5 provided by the pb isotope compositions of the zagami whole rock and residues is therefore taken to date the crystallization of this rock which so far was believed to be only not vert similar 180 ma old based on this result we argue that the lithosphere of mars is extremely old and that most mineral ages were reset recently by acidic aqueous solutions percolating through the martian surface this interpretation is consistent with photographic interpretations of erosional features on mars it also relieves the constraint imposed by the presence of anomalies of 142nd and 182w both products of extinct radioactive nuclides that the martian mantle should have preserved primordial isotopic heterogeneities thus allowing for the planet interior to be actively convecting

geomorphic evidence for former lobate debris aprons at low latitudes on mars indicators of the martian paleoclimate 1 circumferential depressions enclosing mesas and plateaus in the northern kasei valles and in the tartarus colles regions of mars are interpreted as indicators of the former extent of lobate debris aprons thought to be mixtures of ice and clastic particles these former lobate debris aprons existed about 1 ga ago and were embayed by lavas or other flow deposits after the lobate debris aprons had been removed by sublimation and deflation topographic depressions with a depth of 50 m and a width of several kilometers were left behind between the mesa or plateau scarp and the solidified flow materials these depressions or moats are located equatorward of 30 at significantly lower latitudes than generally observed for occurrences of modern intact lobate debris aprons this observation provides evidence that the paleoclimate at that time was different than today probably due to a higher averaged obliquity of the planet s rotational axis

tectonic and deposition model of late precambrian cambrian arabian and adjoining plates during the late precambrian the terranes of the arabian and adjoining plates were fused along the northeastern flank of the african plate in gondwanaland this phase which ended approximately 640 to 620 ma was followed by continental failure 620 to 580 ma and intracontinental extension 600 to approximately 550 ma during the infracambrian extensional phase a triple junction may have evolved near the sinai peninsula and may have consisted of the 1 jordan valley and dead sea rift branch 2 sinai and north egypt rift branch and 3 the najd wrench rift branch the najd hawasina and zagros fault systems may have been transverse faults that accompanied rifting in the arabian gulf and zagros mountains southern oman pakistan and kerman in central iran while the area was extending and subsiding the tethys ocean flooded the eastern side of the arabian plate and iran and deposited calcareous clastics carbonates and evaporites including the hormuz and ara halites this transgression extended into the western part of the arabian plate via the najd rift system the termination of the extensional phase during the late early cambrian was accompanied by a major regression and terrestrial conditions on the arabian peninsula however by the early ordovician as sea level peaked to a highstand the arabian plate was blanketed with marginal marine sediments

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

crater based dating of geological units on mars methods and application for the new global geological map abstract the new post viking generation of mars orbital imaging and topographical data provide significant higher resolution details of surface morphologies which induced a new effort to photo geologically map the surface of mars at 1 20 000 000 scale although from unit superposition relations a relative stratigraphical framework can be compiled it was the ambition of this mapping project to provide absolute unit age constraints through crater statistics in this study the crater counting method is described in detail starting with the selection of image data type locations both from the mapper s and crater counter s perspectives and the identification of impact craters we describe the criteria used to validate and analyse measured crater populations and to derive and interpret crater model ages we provide examples of how geological information about the unit s resurfacing history can be retrieved from crater size frequency distributions three cases illustrate short intermediate and long term resurfacing histories in addition we introduce an interpretation independent visualisation of the crater resurfacing history that uses the reduction of the crater population in a given size range relative to the expected population given the observed crater density at larger sizes from a set of potential type locations 48 areas from 22 globally mapped units were deemed suitable for crater counting because resurfacing ages were derived from crater statistics these secondary ages were used to define the unit age rather than the base age using the methods described herein we modelled ages that are consistent with the interpreted stratigraphy our derived model ages allow age assignments to be included in unit names we discuss the limitations of using the crater dating technique for global scale geological mapping finally we present recommendations for the documentation and presentation of crater statistics in publications

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

the rotation of arabia and the levant fault system abstract we establish the kinematics of the rotation of arabia with respect to africa and show that the rotation rate of africa increased by a factor of about four in late serravallian time about 12 13 ma ago we discuss evidence that oceanic accretion started simultaneously in the gulf of aden and in the red sea at this time it is also at this time that the levant and anatolian fault systems were initiated our initial reconstruction of the red sea rift in late oligocene early miocene time approximately superposes the present shore lines which coincide with the mid point of the zones of rapid crustal thinning the future continental slopes the amount of extension during the early phase of continental rifting is obtained by assuming that the total volume of continental crust is conserved during extension and by checking these estimates through closure around the sinai triple junction in the late miocene the southern evant fault zone was a zone of distributed shear deformation through counterclockwise block rotation extending soul h ward within the northernmost red sea since the early pliocene the shear motion has been localized within the gulf of aqaba changing the tectonic pattern in the northern red sea

jarosite argon diffusion and dating aqueous mineralization on earth and mars abstract jarosite 40 ar 39 ar ages can be used to date surface processes such as weathering and environmental transitions i e aridification on earth and mars to better interpret jarosite ages from a thermochronological perspective the diffusion kinetics of argon in jarosite were determined incremental fractional loss measurements indicate an activation energy e of 37 8 1 5 kcal mol and a log d o a 2 of 5 68 0 63 s 1 corresponding to a closure temperature of 143 28 c assuming a cooling rate 100 c ma downward extrapolation of these parameters to martian surface temperatures 22 c predicts 40 ar 39 ar age spectra using the least retentive e d o a 2 pairs predict that if held at 22 c or less for 4 0 ga supergene jarosite would preserve original growth ages manifest as plateau ages consisting of 95 of the gas release because of its susceptibility to mineralogical breakdown 40 ar 39 ar ages on preserved martian jarosite will reflect the time since water was present at a location that has since undergone aridification and remained hydrologically inactive and thermally quiescent

geologically recent tectonic volcanic and fluvial activity on the eastern flank of the olympus mons volcano mars 1 we show evidence of very recent 25 40 myr geologic activity on the eastern flank of olympus mons volcano that includes a suite of fluvial channel networks volcanic emplacement of lava flows and dikes and tectonic wrinkle ridges and troughs processes the combination and youth of these features confirms the importance of geological activity continuing to the present on mars

continental margin evolution of the northern arabian platform in syria copyright aapg 1993 reprinted by permission of the aapg whose permission is required for further use see also http atlas geo cornell edu syria best 1993 html

geochronology of the martian meteorite zagami revealed by u pb ion probe dating of accessory minerals abstract the precise chronology of geological events on mars is hampered by the lack of absolute ages for the martian timescale and the significant uncertainties that result from the extrapolation of the lunar timescale to mars hartmann and neukum 2001 martian meteorites represent the only samples of mars currently available attempts to identify source craters for the meteorites have thus far proven inconclusive hamilton et al 2003 lang et al 2009 mouginis mark and boyce 2012 precluding their use in constraining the absolute martian timescale the majority of the known martian meteorites are basalts shergottites all dated shergottites have mineral separate rb sr or sm nd ages of borg et al 2005 here we report a 238u 206pb age of 182 7 6 9 ma by ion microprobe analysis of baddeleyite zro2 grains in the zagami shergottite there is no correlation between discordancy and baddeleyite grain location relative to shock metamorphism mineral petrography demonstrates that baddeleyite is the result of late stage igneous crystallization and raman spectroscopy shows that baddeleyite has not been transformed by shock into preservable high pressure polymorphs supported by an age of 153 81 ma for phosphate grains obtained using the same method we conclude that zagami crystallized at 180 ma in agreement with previous results from mineral separate geochronology therefore the shergottites represent igneous rocks preferentially ejected from young terrains on mars in a small number of ejection events

density and lithospheric thickness of the tharsis province from mex mars and mro gravity data 1 radio science tracking of mars express mars experiment and mars reconnaissance orbiter produced high resolution martian gravity data applying localized spectral analysis on the new gravity data sets we study the surface density and lithospheric elastic thickness in the tharsis province the gravity signal is predicted with geophysical models either including bottom loading top bottom model or taking into account the loading history top top model volcanic shields are mainly composed of high density lava but their construction could have begun with lower density lava with the exception of ascraeus mons which has a top of lower density buoyant bottom loading may have been present in the form of a mantle plume under olympus mons the elastic thickness was much larger at olympus mons than at other volcanoes suggesting large spatial variations of heat flux during the hesperian alternatively small elastic thicknesses could be artifacts reflecting the presence of very localized high density crustal intrusions beneath the volcanoes thaumasia highlands were probably supported by a mantle plume at the time of formation in valles marineris top bottom models predict low densities and large elastic thicknesses in conflict with the basaltic rock composition and hesperian age of the valley dense mafic dikes underlie the western part of the valley the top top model serves to test another scenario in which the trough is formed with sedimentary infilling removed much later by erosion the elastic thickness increasing in between at the large volcanoes the relation between gravity and topography is anisotropic probably because of density variations

the geologic evolution of mars episodicity of resurfacing events and ages from cratering analysis of image data and correlation with radiometric ages of martian meteorites we have investigated 12 prominent regions on mars in detail with respect to their geologic evolution through time on the basis of detailed geologic mapping exercises and determining age relationships through crater counting techniques using imagery new data in combination with previously obtained data have been analyzed by way of a refined method of cratering age extraction that also gives fine details of periods of resurfacing we have found that there has been volcanic and fluvial glacial geologic activity on the martian surface at all times from 4 ga ago until today this activity shows episodic pulses in intensity of both volcanic and fluvial glacial processes at 3 8 3 3 ga 2 0 1 8 ga 1 6 to 1 2 ga 800 to 300 m y 200 m y and 100 m y and a possible weaker phase around 2 5 2 2 ga ago in between these episodes there was relative quiescence of volcanic and or fluvial glacial activity the episodes we find on the martian surface in the crater frequency analyses of hrsc moc and themis data coincide with some age groups of the martian meteorites 1 3 ga 600 300 m y 170 m y it appears that the surface activity expressions and their episodicity are related to the interior evolution of the planet when convection in the asymptotic stationary state changes from the so called stagnant lid regime to an episodic behavior similarities in episodic behavior are found for the other terrestrial planets venus the earth s moon and the earth itself suggesting a common general relationship in the evolutionary tracks

the arabian infracambrian extensional system abstract during the late precambrian and early cambrian about 600 540 ma extensive left lateral faulting along the complex nw se trending najd fault system cut across the arabian shield this tectonic episode was accompanied by nw se directed extension in northern egypt and the sinai peninsula based on these relationships stern 1985 interpreted the najd fault system as a set of transform faults emanating from a rift basin in ne egypt contemporaneous to this system in the western part of the arabian plate and northern egypt extensive evaporitic basins developed in northern pakistan southern oman and the arabian gulf and zagros mountains these basins are generally bounded by nw trending faults such as the najd hawasina and zagros faults and ne trending faults such as the ghudun khasfah and dibba faults these evaporitic basins are interpreted as rift basins such that stern s 1985 extensional model in northern egypt may be expanded regionally to include the eastern part of the arabian plate and the indian subcontinent this broader interpretation implies that after the e w directed collisional tectonics of the late precambrian in western arabia and africa which terminated around 640 600 ma the arabian plate experienced nw se directed crustal extension and continental break up

glacial deposits on the rim of a hesperian amazonian outflow channel source trough mangala valles mars 1 mangala valles located in tropical regions of mars is interpreted to have formed by dike induced cracking of the cryosphere to produce a linear graben and catastrophic release of groundwater sequestered under hydrostatic pressure outpouring of water and downcutting led to erosion and widening of the graben to produce a trough patterns of ridges and lobes along the outer rims of the trough are interpreted to have formed primarily by local accumulations of snow and ice on the graben rim derived from exposed ponded groundwater in the graben and glacial like outward flow of ice lobes resulting in development of moraines and tills along the north and south rims of the trough these observations and interpretations support the concept that the climate of mars was a hyper arid cold polar like desert similar to today in the late hesperian early amazonian period

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

martian cratering vi crater count isochrons and evidence for recent volcanism from mars global surveyor this paper develops a methodology to establish absolute martian ages by deriving isochrons on a plot of martian impact crater density vs crater diameter calibrated by lunar crater age data the isochrons illustrated here are based on a mars moon cratering ratio of 1 6 at constant size but there is a factor of 2 to 4 uncertainty in this ratio and the consequent model ages martian crater diameter distributions are determined in several areas down to diameter d 16 45 m the shapes of the curves in young areas are found to be close to that of the predicted isochrons and close to the standard production function found by neukum the youngest areas studied here display the lunar like production function down to d 30 m where saturation equilibrium sets in model crater retention ages of several volcanic units are found to be a few hundred million years or less with estimated uncertainties ranging from a factor of 2 lower to a factor of 4 higher the results are consistent with martian meteorite ages volcanism on mars has probably persisted into the last 10 to 15 of the planet s history and is likely ongoing because surfaces as young as a few hundred million years have reached crater saturation equilibrium at d 60 to 100 m mars is likely to have widespread impact produced regoliths at least a few meters deep and this may contribute to the widespread mobile dust and boulder fields of mars

amazonian geologic history of the echus chasma and kasei valles system on mars new data and interpretations abstract new high resolution datasets have prompted a mapping based study of the 2500 km long echus chasma and kasei valles system that utilizes geomorphic details stratigraphic relations and cratering statistics derived from the new datasets our results suggest that between the hesperian and amazonian epochs on mars 3 7 ga to recent the study area was affected by at least 4 episodes of widespread volcanic activity and 4 periods of episodic fluvio glacial activity this paper discusses the amazonian

an episodic hypothesis for venusian tectonics it is suggested that episodic plate tectonics occurs on venus episodes of rapid plate tectonics are separated by periods of surface quiescence for the last 500 200 m y it is postulated that the surface of venus has been a single rigid plate that has been thickening due to conductive cooling a near uniform surface age is consistent with observed crater densities and the relatively small number of craters modified by surface tectonics or embayed by lava flows a lithosphere that has conductively thickened for some 500 m y has a thickness of about 300 km nearly an order of magnitude greater than the thickness associated with steady state conductive heat loss such a thick lithosphere can support the high topography and associated gravity anomalies on venus as well as the unrelaxed craters studies of lithospheric flexure at coronae are also consistent with a thick elastic lithosphere incipient subduction associated with large coronae may represent the onset of a new episode of rapid plate tectonics on the earth 75 90 of mantle heat transport is attributed to the creation of new oceanic lithosphere at ocean ridges this process is not operative on venus this paper suggests that episodic plate tectonics on venus constitutes the primary mechanism for mantle heat transport on that planet

uranium lead age of baddeleyite in shergottite roberts massif 04261 implications for magmatic activity on mars 1 the issue of the duration of martian magmatic activity is one of the controversial debates recently whether the young radiometric ages of 180 ma for shergottites a class of martian meteorites reflect the timing of crystallization from a magma or rather record later events related to shock metamorphism in order to address the timescale over which mars was geologically active we have undertaken u pb isotopic studies on baddeleyite in the roberts massif rbt 04261 shergottite baddeleyite in rbt 04261 is usually associated with ilmenite and shows monoclinic structure suggesting that the baddeleyite could have formed by crystallization from a residual liquid and not by shock metamorphism in situ u th pb isotopic analyses of baddeleyite yield a young238u 206pb age of 200 ma since the u pb system of baddeleyite is considered to be more resistant to resetting during reheating events and olivine in rbt 04261 preserves an igneous calcium zoning the young age of baddeleyite could be interpreted as a crystallization age of rbt 04261 the present results imply that martian magma was still forming only 200 ma ago and that mars had been geologically active until the recent past

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