constraints on the formation of the martian crustal dichotomy from remnant crustal magnetism the martian crustal dichotomy characterizing the topographic difference between the northern and southern hemispheres is one of the most important features on mars however the formation mechanism for the dichotomy remains controversial with two competing proposals exogenic e g a giant impact and endogenic e g degree 1 mantle convection mechanisms another important observation is the martian crustal remnant magnetism which shows a much stronger field in the southern hemisphere than in the northern hemisphere and also magnetic lineations in this study we examine how exogenic and endogenic mechanisms for the crustal dichotomy are constrained by the crustal remnant magnetism assuming that the dichotomy is caused by a giant impact in the northern hemisphere we estimate that the average thickness of ejecta in the southern hemisphere is 20 25 km while such a giant impact may cause crustal demagnetization in the northern hemisphere we suggest that the impact could also demagnetize the southern hemisphere via ejecta thermal blanketing impact demagnetization and heat transfer from the hot layer of ejecta thus posing a challenge for the giant impact model we explore how the pattern of magnetic lineations relates to endogenic theories of dichotomy formation specifically crustal production via degree 1 mantle convection we observe that the pattern of lineations roughly corresponds to concentric circles about a single pole and determine the pole for the concentric circles at 76 5 e and 84 5 s which nearly overlaps with the centroid of the thickened crust in the southern hemisphere we suggest that the crustal magnetization pattern magnetic lineations and crustal dichotomy i e thickened crust in the highlands can be explained by a simple endogenic process one plume convection causes melting and crustal production above the plume in the southern hemisphere and strong crustal magnetization and magnetic lineations are formed in the southern hemisphere as crustal production fronts spread radially out from the plume center and as the newly created crust cools in the presence of a dynamo with polarity reversals

records of an ancient martian magnetic field in alh84001 although mars does not presently appear to have a global dynamo magnetic field strong crustal fields have recently been detected by the mars global surveyor above surfaces formed 3 or more ga we present magnetic and textural studies of martian meteorite alh84001 demonstrating that 4 ga carbonates containing magnetite and pyrrhotite carry a stable natural remanent magnetization because 40 ar 39 ar thermochronology demonstrates that most alh84001 carbonates have probably been well below the curie point of magnetite since near the time of their formation weiss et al earth planet sci lett 2002 this issue their magnetization originated at 3 9 4 1 ga on mars this magnetization is at least 500 million years myr older than that known in any other planetary rock and its strong intensity suggests that mars had generated a geodynamo and global magnetic field within 450 650 myr of its formation the intensity of this field was roughly within an order of magnitude of that at the surface of the present day earth sufficient for magnetotaxis by the bacteria whose magnetofossils have been reported in alh84001 and possibly for the production of the strong crustal anomalies chromite in alh84001 may retain even older records of martian magnetic fields possibly extending back to near the time of planetary formation

the influence of degree 1 mantle heterogeneity on the past dynamo of mars abstract the hemispheric dichotomy in the crustal magnetic field of mars may indicate that the planet s past dynamo was influenced by a degree 1 heterogeneity on the outer boundary of its liquid metallic convecting core here we use numerical dynamos driven by purely volumetric internal heating with imposed degree 1 heat flux heterogeneities to study mantle control on the past dynamo of mars we quantify both south north and east west magnetic field dichotomies from time average properties that are calculated according to two different end member crust formation scenarios our results indicate that a moderate heat flux anomaly may have been sufficient for obtaining the observed dichotomy because of the excitation of a strong equatorial upwelling in the dynamo the efficiency of a mantle heterogeneity centered at the geographical pole in producing a south north dichotomy is much higher than that of an heterogeneity centered at the equator in producing an east west dichotomy these results argue against a significant true polar wander event with major planet re orientation after the cessation of the dynamo

imaging the transition between the region of mantle melt generation and the crustal magma chamber beneath the southern east pacific rise with short period love waves 1 beneath the southern east pacific rise resides a crustal magma chamber confined to the immediate vicinity of the rise axis yet the mantle melt production region is broad and asymmetric short period 4 17 s love waves propagating along the rise provide a means of probing the transition from the asymmetric melt production region to the rise centered crustal magma chamber low velocities beneath the rise act as a waveguide for love wave energy we model this complex wave propagation with a gaussian beam representation as part of our nonlinear inversion for velocity structure our results show that the asymmetry exists at the crust mantle interface or moho and increases downward the lowest velocities are located beneath the rise at the moho beneath the rise there is an asymmetric gap in the high velocity lid at the top of the mantle and there are lower asthenospheric velocities beneath the pacific plate than the nazca plate the transition from the broad upwelling region to the narrow crustal magmatic system occurs over a 15 km depth interval at the top of the mantle our observations indicate higher temperatures and greater melt production beneath the pacific plate east of the rise melting appears to abruptly shut off at shallow levels perhaps due to a component of downward mantle flow we suggest that melt percolates upward increasing in concentration until reaching a permeability barrier beneath the lithosphere it then moves up along the lithosphere to accumulate at the moho beneath the rise west of the rise higher temperatures and upward percolation of melt delay growth of the lithosphere

sedimentary geology of the palaeoarchaean buck ridge south africa and kittys gap western australia volcano sedimentary complexes abstract the two palaeoarchaean volcano sedimentary complexes of the buck ridge barberton greenstone belt south africa and kittys gap coppin gap greenstone belt east pilbara australia have a similar geological setting and age 3 45 ga the predominantly volcaniclastic sediments are concentrated at the top of these complexes and experienced thorough very early diagenetic silicification in many places the silicification process has led to excellent preservation of the primary sedimentary structures elsewhere it has resulted in their obliteration or replacement by diagenetic structures the buck ridge chert forms a regressive transgressive succession deposited around base level with lacustrine and littoral marine facies deposition of the kittys gap chert was also close to base level almost exclusively subaqueous with tidal influence and a regressive sequential trend in both volcano sedimentary complexes these low energy sediments are juxtaposed with high energy breccia pods and layers with often a high fe oxide content the breccias are interpreted as being the result of explosive hydrothermal activity sedimentation was strongly controlled by normal faulting

martian south polar layered deposit stratigraphy and implications for accumulation history 1 we examine the stratigraphy of the martian south polar layered deposits spld exposed on scarp walls throughout the deposit using themis visible 17 m pxl images from the mars odyssey spacecraft by correlating marker layers and packets of layers among multiple locations we are able to identify three sequences of layers within the spld each with a different areal extent while certain layers are correlatable over hundreds of km implying widespread deposition of polar material the absence of several identified layer sequences in some regions notably the promethei lingula region as well as the presence of several unconformities indicates that deposition and sublimation or erosion of these deposits has not been uniform around the spld this is further supported by the fact that individual layers display variations in thickness greater than a factor of 4 over hundreds of kilometers using these observations we propose an internal structure for the spld with individual layers roughly following a dome like shape toward the center of the deposit and becoming more planar toward the margins we infer that the spld underwent at least three major periods of layer accumulation each period is distinguished by a change in areal extent of net accumulation the changes in areal extent of the layer sequences may be due to changes in martian climate causing episodes of significant erosion of the spld between the deposition of each sequence or causing wide variations in accumulation zones of the spld over the history of these deposits

is the primordial crust of mars magnetized abstract a meteorite impact capable of creating a 200 km diameter crater can demagnetize the entire crust beneath and produce an appreciable magnetic anomaly at satellite altitudes of 400 km in case the pre existing crust is magnetized in this study we examine the magnetic field over all of the craters and impact related quasi circular depressions qcds with diameters larger than 200 km that are located on the highlands of mars excluding the tharsis bulge in order to estimate the mean magnetization of the highland crust using the surface topography and the gravity of mars we first identify those qcds that are likely produced by impacts the magnetic map of a given crater or impact related qcd is derived using the mars global surveyor high altitude nighttime radial magnetic data two extended ancient areas are identified on the highlands the south province and the tempe terra which have large number of craters and impact related qcds but none of them has an appreciable magnetic signature the primordial crust of these areas is not magnetized or is very weakly magnetized at most we examine some plausible scenarios to explain the weak magnetization of these areas and conclude that no strong dynamo existed in the first 100 myr of mars history when the newly formed primordial crust was cooling below the magnetic blocking temperatures of its minerals

early archean 3 2 ga fe oxide rich hydrothermal discharge vents in the barberton greenstone belt south africa ironstone pods fe 2 o 3 72 5 97 2 wt interpreted to have formed by early archean sea floor related hydrothermal activity are described from the ca 3 5 3 1 ga barberton greenstone belt south africa most of the pods are elliptical in shape and have their longest dimensions subparallel to the local stratigraphy they rest on silicified ultramafic rocks and are overlain by ferruginous shales silicified sand stones other coarse clastics and barite horizons the ironstone pods grade along strike into laminated iron oxide facies banded iron formation that is inferred to represent periodic discharge of iron oxide and silica rich flocculates from the pods the massive texture of the iron stone pods and their lack of internal sedimentary features suggest that they formed directly on the sea floor relict hydrothermal discharge chimney structures have been recognized honeycomb like cavities and possible fluid flow channel textures attest to primary porosity the iron stone pods are dominated by massive locally coarse grained specular hematite and goethite with lesser amounts of quartz and an x ray amorphous fe ai bearing silicate the pods are distinct from typical massive sulfide gossans in total trace element concentrations including precious and base metals indicating a different origin in contrast the ironstone pods have similarities in major oxide and trace metal concentrations to iron oxide deposits presently forming in part from low temperature hydrothermal fluids on the sea floor for example red seamount iron oxide deposit fluid inclusion studies on quartz show dominant type i primary fluid inclusions with salinities of 4 7 to 15 8 wt nacl equiv and homogenization temperatures t h of 90 to 150 c no evidence for boiling is seen type i fluid inclusions are most likely dominated by nacl cacl 2 h 2 o although their measured eutectic temperatures are consistent with the presence of fecl 2 these inclusions represent the end member hydrothermal fluid more saline 24 29 6 wt cacl 2 equiv lower temperature t h 33 109 c type ii and ila fluid inclusions represent pulses of a hydrothermal fluid of nacl cacl 2 fecl 2 or mgcl 2 h 2 o or possibly fecl 3 bearing composition mixing with ambient sea water in the case of type ii inclusions the various fluid types are interpreted to be indicative of intermediate compositions of an evaporitic brine the lowest salinity measurement for a type i inclusion of 3 1 wt nacl equiv provides a constraint on the salinity of unenriched by evaporation sea water the t h data give a minimum trapping pressure of 7 bars which equates to a minimum sea water depth of 60 m the lack of precise pressure estimates has not enabled calculation of maximum water depths above the pods quartz 18 o analyses average 17 3 0 6 1 n 5 calculated 18 o h 2 o for two samples are 1 4 and 1 2 respectively these estimates are within error of the postulated barberton early archean sea water value of 0 quartz hematite goethite mineral pairs yield temperatures inconsistent with fluid inclusion t h data suggesting isotopic disequilibrium

error distribution in regional inversion of potential field data summary an increasing number of regional inversions of gravity and magnetic field data have recently been computed some based on satellite and ground data some on satellite data only in each case it is important to quantify the errors resulting from the confined data region or the satellite parameters in this study we investigate the error distribution of regional inversions for scalar gravity and vector geomagnetic data from satellite data only we include the effects of satellite altitude size of the data region signal to noise ratio and spherical harmonic degree of an a priori global field model we evaluate our results on the basis of a regional inversion of the earth s magnetic field

thickness of the magnetic crust of mars 1 to estimate the thickness of the magnetic crust of mars six observational magnetic spectra are fitted with the theoretical spectrum expected from a novel bimodal distribution of magnetic sources observational spectra differ for each comes from a different map or model of variously selected and analyzed mars global surveyor magnetometer electron reflectometer measurements of the vector magnetic field around mars the new theoretical spectrum represents fields from both compact sources and extended laterally correlated sources on a spherical shell so the estimated shell depth can now be doubled to obtain layer thickness this typical magnetic crustal thickness is put at 47 8 8 4 km the extensive sources are enormous typically 650 km across and account for over half the magnetic energy at low degrees there is some indication that these sources are relatively shallow but the typical area remains about 330 000 km2 granted such extended sources represent magnetization of mars ancient crust in a core source field dominated by a reversing areocentric paleodipole each one arguably formed during a single polarity chron how did such vast regions of magnetic crust form a survey of many eligible mechanisms suggests magnetization of cooling igneous rock at minimal rates of about 1 to 0 1 km3 a during superchrons of order 15 to 150 ma long

meltback of hesperian aged ice rich deposits near the south pole of mars evidence for drainage channels and lakes ponding minimum volumes 10 12 m 3 overtopping downcutting and further drainage of material through a series of craters and into the prometheus basin near the edge of the current cap topography data show that water filled some craters to depths of at least 200 m and possibly as much as 600 m materials derived from melting of this lobe of the daf drained over a lateral distance of 600 km and a vertical height of 800 m partially emptying into the prometheus basin these combined observations imply that meltback of the older volatile rich deposit was a widespread event occurring on both sides of the deposit over a distance of at least 1800 km and in an area possibly as much as 2 9 10 6 km 2 candidate top down and bottom up melting scenarios are examined the presence of associated hesperian aged volcanic deposits and possible subglacial edifices suggests that local and regional basal heating was a major factor in generating the meltwater that formed the drainage features index terms 6225 planetology solar system objects mars 5416 planetology solid surface planets glaciation 5470 planetology solid surface planets surface materials and properties 3309 meteorology and atmospheric dynamics climatology 1620 5418 planetology solid surface planets heat flow keywords mars polar glaciers hesperian channels water

an origin for the linear magnetic anomalies on mars through accretion of terranes implications for dynamo timing the proposed existence of magnetic lineations in the terra cimmeria and terra sirenum regions of mars was initially explained by earth like sea floor spreading here we argue instead that these lineations could have been formed at a convergent plate margin through collision and accretion of terranes a similar process produced banded magnetic anomalies similar in geometry and even in size to those in earth s north american cordillera because only sparse and generally weak anomalies have been detected in the martian northern lowlands which could constitute an analog to the terrestrial oceanic crust it is possible that the magnetic field stopped its activity while crustal recycling was still active in mars

paleomagnetic poles of mars revisited 1 the huge amount of data acquired by mars global surveyor during its mapping period provides a unique opportunity to reassess the paleomagnetic pole positions of mars previously determined on the basis of the limited low altitude magnetic data we identify nine small and isolated magnetic anomalies on the basis of the global magnetic maps and model each anomaly using a vertical prism of elliptical cross section both high altitude 360 430 km and low altitude 100 200 km magnetic data are used simultaneously we calculate a paleomagnetic pole position assuming that the body is magnetized by a dipole core field although the new pole positions do not cluster as closely as the old ones the new cluster overlaps the older cluster the clustering suggests that mars rotation axis has likely wandered by 50 60 in the last 4 gyr the number of north and south poles in the cluster suggests at least one reversal of the core field during the time the source bodies acquired magnetization

magnetic lineations in the ancient crust of mars the mars global surveyor spacecraft in a highly elliptical polar orbit obtained vector magnetic field measurements above the surface of mars altitudes 100 kilometers crustal magnetization mainly confined to the most ancient heavily cratered martian highlands is frequently organized in east west trending linear features the longest extending over 2000 kilometers crustal remanent magnetization exceeds that of terrestrial crust by more than an order of magnitude groups of quasi parallel linear features of alternating magnetic polarity were found they are reminiscent of similar magnetic features associated with sea floor spreading and crustal genesis on earth but with a much larger spatial scale they may be a relic of an era of plate tectonics on mars

dike intrusion as a possible cause of linear martian magnetic anomalies a model in which the ancient 100 200 km wide magnetic lineations in the southern hemisphere of mars are formed by repeated dike intrusion is proposed on the basis of magnetizations observed at terrestrial mid ocean ridges a likely upper bound on the martian crustal magnetization is about 15 a m 1 this upper bound gives a minimum magnetized layer thickness of 35 60 km because the magnetic material is likely to be magnetite the long term stability of the anomalies probably requires ilmenite lamellae to have exsolved from the magnetite this exsolution process provides an upper bound on the cooling time available of 102 105 yr these bounds can be satisfied if the magnetic anomalies are created by successive intrusion of dikes from 20 to 2000 m wide production of a martian crust 35 60 km thick by rifting requires either mantle potential temperatures in excess of 1430 c or stretching factors greater than 5

upper mantle seismic wave velocity effects of realistic partial melt geometries we investigate seismic wave velocity reduction resulting from the presence of partial melt in the upper mantle the amount of shear and bulk modulus reduction produced by the presence of a connected network of realistically shaped and naturally organized melt inclusions is found using finite element calculations the geometries of the inclusions are taken directly from laboratory experiments of mantle melting with finite element meshes constructed to conform to these shapes the shear and bulk moduli of the composite material are found for both the unrelaxed isolated inclusions and relaxed pressure equalized inclusions cases by assigning appropriate material properties to the fluid modulus reduction from deformation simulations of a solid containing realistically shaped and ellipse shaped melt inclusions quantify the effect of melt pocket cuspateness and melt pocket organization on seismic velocity reduction the three dimensional response is estimated from two dimensional distributions of the melt phase by determining the mode ii and mode iii components of elastic modulus reduction separately and summing their effects in general cuspate and naturally organized melt inclusions cause greater velocity reduction it is shown that v and vs reduction per percent partial melt are at least 3 6 and 7 9 respectively even higher values for velocity reduction are possible above 1 melt fraction if melt exists only in tubules below 1 melt fraction the lower more conservative values of velocity reduction are 070 greater for vp and 84 greater for vs than the analytically determined values for ellipsoidal inclusions somewhat greater effects are possible if nonrandom organization of melt occurs on scales greater than our model

tectonic implications of mars crustal magnetism mars currently has no global magnetic field of internal origin but must have had one in the past when the crust acquired intense magnetization presumably by cooling in the presence of an earth like magnetic field thermoremanent magnetization a new map of the magnetic field of mars compiled by using measurements acquired at an 400 km mapping altitude by the mars global surveyor spacecraft is presented here the increased spatial resolution and sensitivity of this map provide new insight into the origin and evolution of the mars crust variations in the crustal magnetic field appear in association with major faults some previously identified in imagery and topography cerberus rupes and valles marineris two parallel great faults are identified in terra meridiani by offset magnetic field contours they appear similar to transform faults that occur in oceanic crust on earth and support the notion that the mars crust formed during an early era of plate tectonics

high resolution local magnetic field models for the martian south pole from mars global surveyor data we present two high resolution local models for the crustal magnetic field of the martian south polar region models sp130 and sp130m were derived from three component measurements made by mars global surveyor at nighttime and at low altitude 200 km the availability area for these data covers the annulus between latitudes 76 and 87 and contains a strongly magnetized region southern parts of terra sirenum adjacent to weakly magnetized terrains such as prometheus planum our localized field inversions take into account the region of data availability a finite spectral bandlimit spherical harmonic degree l 130 and the varying satellite altitude at each observation point we downward continue the local field solutions to a sphere of martian polar radius 3376 km while weakly magnetized areas in model sp130 contain inversion artifacts caused by strongly magnetized crust nearby these artifacts are largely avoided in model sp130m a mosaic of inversion results obtained by independently solving for the fields over individual subregions robust features of both models are magnetic stripes of alternating polarity in southern terra sirenum that end abruptly at the rim of prometheus planum an impact crater with a weak or undetectable magnetic field from a prominent and isolated dipole like magnetic feature close to australe montes we estimate a paleopole with a best fit location at longitude 207 and latitude 48 from the abruptly ending magnetic field stripes we estimate average magnetization values of up to 15 a m

crustal and uppermost mantle shear velocity structure adjacent to the juan de fuca ridge from ambient seismic noise 1 based on 6 months of obs data from the cascadia initiative experiment near the juan de fuca ridge we obtain rayleigh wave group and phase speed curves from 6 s to about 20 s period from ambient noise cross correlations among all station pairs we confirm the hypothesis that the dispersion data can be fit by a simple age dependent formula which we invert using a bayesian monte carlo formalism for an age dependent shear wave speed model of the crust and uppermost mantle between crustal ages of 0 5 and 3 5 ma igneous crustal structure is age invariant with a thickness of 7 km water depth varies in a prescribed way and sedimentary thickness and mantle shear wave speeds are found to increase systematically with crustal age the mantle model possesses a shallow low shear velocity zone lvz with a velocity minimum at about 20 km depth at 0 5 ma with lithosphere thickening monotonically with age minimum mantle shear velocities at young ages are lower than predicted from a half space conductively cooling model hscm and the lithosphere thickens with age faster than the hscm providing evidence for nonconductive cooling in the young lithosphere the shallow lvz is consistent with expectations for a largely dehydrated depleted harzburgite mantle with a small retained near ridge partial melt fraction probably less than 1 with melt extending to a lithospheric age of approximately 1 ma i e 30 km from the ridge

modeling of major martian magnetic anomalies further evidence for polar reorientations during the noachian abstract maps of the vector components of the mars crustal magnetic field are constructed at the mapping altitude 360 to 410 km using a selected set of data obtained with the mars global surveyor magnetometer during 2780 orbits of the planet in 1999 forward modeling calculations are then applied to six relatively strong and isolated dominantly dipolar magnetic anomalies for the primary purpose of estimating bulk directions of magnetization assuming that the magnetizing field was a dipolar core dynamo field centered in the planet paleomagnetic pole positions are calculated for the six primary source bodies together with that for a seventh anomaly analyzed earlier in agreement with several previous studies it is found that six of the seven pole positions are clustered in what is now the northern lowlands in a region centered northwest of olympus mons mean pole position 34 10 n 202 58 e assuming that the dynamo dipole moment vector was approximately parallel to the rotation axis the modeling results therefore suggest a major reorientation of mars relative to its rotation axis after magnetization was acquired such a reorientation may have been stimulated by internal mass redistributions associated with the formation of the northern lowlands and tharsis for example a comparison of the mean paleo magnetic equator to the global distribution of crustal fields shows that magnetic anomalies tend to occur at low paleolatitudes the same appears to be true for the noachian aged valley networks which exhibit a broad spatial correlation with the magnetic anomalies a possible interpretation is that the formation of magnetic anomalies and the valley networks was favored in the tropics where melting of water ice and snow was a stronger source of both surface valley erosion and groundwater recharge during the earliest history of the planet this would be consistent with models in which hydrothermal alteration of crustal rocks played a role in producing the unusually strong martian magnetic anomalies

mapping and modeling of magnetic anomalies in the northern polar region of mars vector crustal magnetic field maps of the northern polar zone 60 n to 90 n are constructed from selected mars global surveyor magnetometer data obtained during the period from may 28 to september 13 1998 two medium anomalies amplitudes 50 nt at 170 km altitude are mapped in locations consistent with earlier studies no visible surface features correlate with the anomalies suggesting that the sources lie beneath the visible veneer of polar deposits and volcanic lava flows if so then they formed prior to the immediate end of the heavy bombardment upper hesperian period modeling of anomaly vector field components combined with independent constraints on the depth to the curie isotherm yields lower limits on bulk magnetization intensities 0 4 0 9 a m that are significantly greater than those measured for martian snc meteorite samples rocks that contain substantially more titanomagnetite than snc meteorites or that contain magnetic phases in addition to titanomagnetite possibly resulting from hydrothermal alteration are therefore suggested alternatively remanence acquisition in a field of earthlike intensity 50 t rather than in the relatively weak inferred paleointensities for snc meteorites 1 10 t would also help to explain the relatively strong inferred remanent magnetizations the approximate south paleomagnetic pole positions corresponding to these two anomaly sources are located in a region between olympus mons and the present north rotational pole this region is adjacent to the approximate location predicted by melosh 1980 for the paleopole prior to the formation of the tharsis gravity anomaly

a 50 degree spherical harmonic model of the magnetic field of mars this paper presents a 50 degree and order spherical harmonic model of the magnetic field of mars derived at 120 km altitude using the three orthogonal components of the vector magnetic field data acquired by the mars global surveyor within the 80 to 200 km altitude range the downward continued vector magnetic field components to the surface of mars delineate details of the martian magnetic field the strong magnetic anomalies in the southern hemisphere do not show compelling evidence for magnetic lineations similar to those associated with the seafloor spreading on earth the anomalies are more or less equidimensional the magnetic signatures surrounding the impact basins hellas argyre and isidis show that the prominent magnetic anomalies are older than the impact events an average magnetization of 2 a m is estimated for the upper crust surrounding hellas basin four isolated magnetic anomalies are modeled by vertical prisms with circular cross sections of 170 to 190 km radius and 20 to 30 km thickness the magnetization of the model prisms is 5 a m more than that of the surroundings

implications of grain size evolution on the seismic structure of the oceanic upper mantle abstract we construct a 1 d steady state channel flow model for grain size evolution in the oceanic upper mantle using a composite diffusion dislocation creep rheology grain size evolution is calculated assuming that grain size is controlled by a competition between dynamic recrystallization and grain growth applying this grain size evolution model to the oceanic upper mantle we calculate grain size as a function of depth seafloor age and mantle water content the resulting grain size structure is used to predict shear wave velocity v s and seismic quality factor q for a plate age of 60 myr and an olivine water content of 1000 h 10 6 si we find that grain size reaches a minimum of 15 mm at 150 km depth and then increases to 20 30 mm at a depth of 400 km this grain size structure produces a good fit to the low seismic shear wave velocity zone lvz in oceanic upper mantle observed by surface wave studies assuming that the influence of hydrogen on anelastic behavior is similar to that observed for steady state creep further it predicts a viscosity of 10 19 pa s at 150 km depth and dislocation creep to be the dominant deformation mechanism throughout the oceanic upper mantle consistent with geophysical observations we predict larger grain sizes than proposed in recent studies in which the lvz was explained by a dry mantle and a minimum grain size of 1 mm however we show that for a 1 mm grain size diffusion creep is the dominant deformation mechanism above 100 200 km depth inconsistent with abundant observations of seismic anisotropy from surface wave studies we therefore conclude that a combination of grain size evolution and a hydrated upper mantle is the most likely explanation for both the isotropic and anisotropic seismic structure of the oceanic upper mantle our results also suggest that melt extraction from the mantle will be significantly more efficient than predicted in previous modeling studies that assumed grain sizes of 1 mm

an altitude normalized magnetic map of mars and its interpretation techniques developed for the reduction and analysisofterrestrialsatellitemagneticelddataareusedto better understand the magnetic eld observations made by mars global surveyor a global distribution of radial br magnetic eld observations and associated uncertainties is invertedfor an equivalentsource magnetization distribution and then used to generate an altitude normalized map of br at 200 km the observations are well represented by a potential function of crustal origin consistent with a rema nent origin for the martian magnetic features the correla tion between the 40546 br observations andbr calculated from the magnetization solution at observation locations is 0 978 for a magnetization distribution connedto a 50 km layer calculatedmagnetizationsrangefrom 22to 17a m we see correlations with tectonics that were only hinted at in earlier maps magnetic features appear to be truncated against valles marineris and ganges chasma suggestive of a major change in crustal properties associated with fault ing

estimates of martian crustal thickness from viscous relaxation of topography isostatically compensated crustal thickness variations and associated topographic contrasts at the surface of a planet result in lateral pressure gradients which may cause the lower crust to flow and reduce the relief areas of thicker crust are generally associated with more rapid relaxation of topography on mars topographic features such as impact basins and the hemispheric dichotomy have survived for 4 gyr we use a finite difference representation of depth dependent non newtonian lower crustal flow to investigate how topography decays with time for a dry diabase rheology total radiogenic concentrations 80 of terrestrial values and crustal radiogenic concentrations similar to terrestrial basalts we find that an upper bound on the mean planetary crustal thickness is 100 km in the probably unrealistic case where all the radiogenic elements are in the crust this maximum crustal thickness can be increased to 115 km the main uncertainty in these results is the total radiogenic abundances on mars comparing our results with the observed shape of the crustal dichotomy provides no evidence that this slope is primarily the result of lower crustal flow both hellas and the dichotomy are isostatically compensated if the mechanism is airy isostasy then the lower bound on mean crustal thickness is 30 km crustal thicknesses of 30 100 km on mars can be produced by mid ocean ridge spreading at potential temperatures of 1350 1600 c however for such crustal thicknesses the lithosphere is likely to be positively buoyant making subduction difficult

east west trending magnetic anomalies in the southern hemisphere of mars modeling analysis and interpretation abstract maps of the vector components of the martian crustal magnetic field over the strongly magnetized terra cimmeria sirenum region are constructed using mars global surveyor magnetometer data although pronounced east west trending anomalies are present on the radial and north field component maps at the mapping altitude 360 380 km these trends are much less prominent at the lower aerobraking altitude 90 150 km comparisons with similar maps produced using artificial data at the aerobraking altitude indicate that elongated sources in this region may have maximum lengths along the martian surface of 500 km and maximum aspect ratios of 2 iterative forward modeling of several relatively isolated anomalies in the mapped region yields paleomagnetic pole positions consistent with those estimated in previous studies of other anomalies using mapping phase and science phasing orbit data on this basis it is inferred that sources in the studied region are most probably magnetized primarily in northward or southward directions using this additional constraint iterative forward modeling is then applied to determine a magnetization distribution that is consistent with data at both the aerobraking altitude and the mapping altitude the model magnetization distribution which includes 41 discrete sources again indicates no highly elongated sources an examination of surface geology in the region as well as a consideration of the global distribution of anomalies suggests that magmatic intrusions e g subsurface dike swarms cooling in the presence of water are the most likely sources of the magnetic anomalies

ancient ocean on mars supported by global distribution of deltas and valleys the climate of early mars could have supported a complex hydrological system analysis of ancient deltaic deposits and valley networks reveals the presence of a planet wide equipotential surface in the northern lowlands indicative of the existence of a vast ocean on mars 3 5 billion years ago

confidence intervals for motion and deformation of the juan de fuca plate finite rotation parameters for 17 isochrons from anomalies 1 4a for the juan de fuca pacific plate pair are reported comparison of confidence intervals for plate rotations derived by different techniques indicates that a single plate misfit criterion significantly underestimates the confidence region defining the confidence interval by a threshold level of misfit determined by an f statistic gives similar results to the technique of bootstrap resampling for internally consistent data bootstrap resampling offers far greater robustness in the case of inconsistent data precision of about 1 km in the direction normal to isochrons is available for the younger anomalies this precision allows confident recognition of 2 5 km of eastward deformation of the pacific plate 50 100 km south of the active sovanco transform slight deformation of the northern juan de fuca plate adjacent to the nootka fault zone is also apparent east of the northern gorda ridge younger 3 ma anomalies do not show measurable deformation near the blanco fracture zone but older anomalies are up to 30 km east of their position predicted by rigid behavior of the juan de fuca plate reconstruction of the overlap geometry of past propagators shows no evidence that the extent of overlap has ever exceeded 70 km even with offset of 75 km in one case and 140 km in another new techniques for testing for intervals of constant plate motion require at least two changes in instantaneous motion at 99 confidence a clockwise change in motion direction at about 5 ma and an increase in the gradient of spreading rate since 2 ma two other changes in gradient since 5 ma are required at 95 confidence

questions about magnetic lineations in the ancient crust of mars connerney et al 1 1 claim to have discovered sea floor spreading type magnetic anomalies in the measurements of the martian magnetic field done by the mars global surveyor it is possible that sea floor spreading may have occurred on mars but the information that these authors use to

paleo pole positions from martian magnetic anomaly data magnetic component anomaly maps were made from five mapping cycles of the mars global surveyor s magnetometer data our goal was to find and isolate positive and negative anomaly pairs which would indicate magnetization of a single source body from these anomalies we could compute the direction of the magnetizing vector and subsequently the location of the magnetic pole existing at the time of magnetization we found nine suitable anomaly pairs and from these we computed paleo poles that were nearly equally divided between north south and mid latitudes these results suggest that during the existence of the martian main magnetic field it experienced several reversals and excursions

magnetic field and plasma observations at mars initial results of the mars global surveyor mission the magnetometer and electron reflectometer investigation mag er on the mars global surveyor spacecraft has obtained magnetic field and plasma observations throughout the near mars environment from beyond the influence of mars to just above the surface at an altitude of 100 kilometers the solar wind interaction with mars is in many ways similar to that at venus and at an active comet that is primarily an ionospheric atmospheric interaction no significant planetary magnetic field of global scale has been detected to date 2 1021 gauss cubic centimeter but here the discovery of multiple magnetic anomalies of small spatial scale in the crust of mars is reported

gravity and seismic study of crustal structure along the juan de fuca ridge axis and across pseudofaults on the ridge flanks variations in topography and seismic structure are observed along the juan de fuca jdf ridge axis and in the vicinity of pseudofaults on the ridge flanks left by former episodes of ridge propagation here we analyze gravity data coregistered with multichannel seismic data from the jdf ridge and flanks in order to better understand the origin of crustal structure variations in this area the data were collected along the ridge axis and along three ridge perpendicular transects at the endeavor northern symmetric and cleft segments negative mantle bouguer anomalies of 21 to 28 mgal are observed at the axis of the three segments thicker crust at the endeavor and cleft segments is inferred from seismic data and can account for the small differences in axial gravity anomalies 3 7 mgal additional low densities elevated temperatures within and or below the axial crust are required to explain the remaining axial mba low at all segments gravity models indicate that the region of low densities is wider beneath the cleft segment gravity models for pseudofaults crossed along the three transects support the presence of thinner and denser crust within the pseudofault zones that we attribute to iron enriched crust on the young crust side of the pseudofaults a 10 20 km wide zone of thicker crust is found reflection events interpreted as subcrustal sills underlie the zones of thicker crust and are the presumed source for the iron enrichment

continuous monitoring of nightside upper thermospheric mass densities in the martian southern hemisphere over 4 martian years using electron reflectometry details are presented of an improved technique to use atmospheric absorption of magnetically reflecting solar wind electrons to constrain neutral mass densities in the nightside martian upper thermosphere the helical motion of electrons on converging magnetic field lines through an extended neutral atmosphere is modeled to enable prediction of loss cone pitch angle distributions measured by the magnetometer electron reflectometer mag er experiment on mars global surveyor at 400 km altitude over the small fraction of mars southern hemisphere 2 5 where the permanent crustal magnetic fields are both open to the solar wind and sufficiently strong as to dominate the variable induced martian magnetotail field spherical harmonic expansions of the crustal fields are used to prescribe the magnetic field along the electron s path allowing least squares fitting of measured loss cones in order to solve for parameters describing the vertical neutral atmospheric mass density profile from 160 to 230 km results are presented of mass densities in the southern hemisphere at 2 a m lst at the mean altitude of greatest sensitivity 180 km continuously over four martian years seasonal variability in densities is largely explained by orbital and latitudinal changes in dayside insolation that impacts the nightside through the resulting thermospheric circulation however the physical processes behind repeatable rapid late autumnal cooling at mid latitudes and near aphelion warming at equatorial latitudes is not fully clear southern winter polar warming is generally weak or nonexistent over several mars years in basic agreement with mgs and mro accelerometer observations the puzzling response of mid latitude densities from 160 to 200 e to the 2001 global dust storm suggests unanticipated localized nightside upper thermospheric lateral and vertical circulation patterns may accompany such storms the downturn of the 11 year cycle of solar euv flux is likely responsible for lower aphelion densities in 2004 and 2006 mars years 27 and 28

error distribution in regional modelling of the geomagnetic field summary in this study we analyse the error distribution in regional models of the geomagnetic field our main focus is to investigate the distribution of errors when combining two regional patches to obtain a global field from regional ones to simulate errors in overlapping patches we choose two different data region shapes that resemble that scenario first we investigate the errors in elliptical regions and secondly we choose a region obtained from two overlapping circular spherical caps we conduct a monte carlo simulation using synthetic data to obtain the expected mean errors for the elliptical regions the results are similar to the ones obtained for circular spherical caps the maximum error at the boundary decreases towards the centre of the region a new result emerges as errors at the boundary vary with azimuth being largest in the major axis direction and minimal in the minor axis direction inside the region there is an error decay towards a minimum at the centre at a rate similar to the one in circular regions in the case of two combined circular regions there is also an error decay from the boundary towards the centre the minimum error occurs at the centre of the combined regions the maximum error at the boundary occurs on the line containing the two cap centres the minimum in the perpendicular direction where the two circular cap boundaries meet the large errors at the boundary are eliminated by combining regional patches we propose an algorithm for finding the boundary region that is applicable to irregularly shaped model regions

shear wave attenuation and dispersion in melt bearing olivine polycrystals 2 microstructural interpretation and seismological implications 1 the torsional forced oscillation tests of melt bearing olivine aggregates reported by jackson et al 2004 consistently show a peak in attenuation that is absent from melt free aggregates tested under similar conditions and grain sizes characterization by sem shows that the melt resides in triple junction tubules and larger pockets as previously described tem imaging and eds analysis reveals that olivine olivine grain boundaries are characterized by a region 1 nm wide which is structurally and chemically distinct from olivine grain interiors from the possible mechanisms that can produce an anelastic attenuation peak melt squirt can be eliminated for our samples and experimental conditions we attribute the observed attenuation peak to elastically accommodated grain boundary sliding requiring that the grain boundaries are weak relative to olivine grain interiors but have a significantly higher viscosity than bulk melt while the nanometer scale grain boundary structure in the melt bearing aggregates is essentially the same as for melt free aggregates studied previously elastically accommodated sliding in the latter is apparently inhibited by tight three grain edge intersections the exponentially increasing high temperature background attenuation in both types of aggregate is attributed to diffusionally accommodated grain boundary sliding extrapolation to mantle grain sizes shows that the broad peak may be responsible for nearly frequency independent attenuation in partially molten regions of the upper mantle

mapping crustal magnetic fields at mars using electron reflectometry 1 we present a kinetic model for the interaction of electrons precipitating along magnetic field lines into mars atmosphere and develop a technique for detecting crustal magnetic fields which act as magnetic mirrors reflecting some of the incident electrons back to spacecraft altitudes we present an initial map covering 20 of the planet which shows the topology of magnetic sources along the dichotomy boundary and reveals new magnetic sources in the northern lowlands implying the existence of preserved noachian crust beneath at least some regions of the apparently young plains

thermal history of the 3 5 3 2 ga onverwacht and fig tree groups barberton greenstone belt south africa inferred by raman microspectroscopy of carbonaceous material raman spectra of carbonaceous material were collected in situ from samples of cherts of the onverwacht and fig tree groups in the central barberton greenstone belt the spectra feature two dominant peaks characteristic of disordered carbon the d peak at 1310 cm 21 and the o peak at 1580 1600 cm 21 d peak positions and relative peak intensities and areas indicate that all samples have been altered to lower greenschist facies or above no correlation was observed between maximum temperature and stratigraphic position or degree of hydrothermal alteration implying that metamorphism in the central barberton greenstone belt was regional and unaccompanied by the flow of large quantities of hydrothermal fluids samples from the marble bar chert of the pilbara block western australia have been heated to the same extent as samples from barberton this study demonstrates the use of raman spectra of carbonaceous material as a sensitive geother mometer for low temperature metamorphic facies this application could also be used to establish the antiquity of putative microfossils from metamorphic terranes

magnetization paleomagnetic poles and polar wander on mars abstract using the geometry of an areocentric dipole field we have remodeled the anomalies observed in the terra sirenum and terra cimmeria regions by the mars global surveyor mgs for the origin of the anomalies we used both magnetic stripe and magnetic shell hypotheses for magnetic stripe models we assume that sea floor spreading formed the anomalies in the presence of a reversing axial dipole field we found that a simple magnetic stripe model with constant magnetization and crustal thickness results in poor agreement with the observed anomalies however by allowing the intensity of magnetization to vary freely between and along the stripes a reasonable fit to the observed data can be found while maintaining the observed polarity along the stripes applying the simple magnetic stripe model to the terra sirenum and terra cimmeria anomalies results in best fit spreading centers located well to the north of the surviving magnetic stripes this could possibly explain the apparent lack of symmetry that has been noted in the anomalies for the magnetic shell hypothesis we assume that a monopolarity dipole field magnetized the early crust of mars the anomalies that persist today would then be the result of partial demagnetization of the original magnetic shell by more recent processes the magnetic anomalies in terra sirenum and terra cimmeria are consistent with this model especially if the magnetization of the original magnetic shell was laterally inhomogeneous if the monopolarity magnetic shell hypothesis is correct best fit magnetic paleopole locations for the original magnetization lie in a sub equatorial belt centered near the prime meridian if the ancient magnetic pole of mars coincided with the spin pole this result implies that either apparent or true polar wander has occurred on mars apparent polar wander would be consistent with plate tectonics having occurred after the formation of the magnetic anomalies true polar wander has previously been proposed to explain certain morphologic features found on mars

extensive hesperian aged south polar ice sheet on mars evidence for massive melting and retreat and lateral flow and ponding of meltwater local and regional mars orbiter laser altimeter mola topographic data support the presence of an extensive hesperian aged volatile rich south polar deposit the dorsa argentea formation hd and related units underlying the present amazonian aged polar cap api residual ice and apl layered terrain and covering a surface area that could be as large as 2 94 106 km2 about 2 of the surface of mars over twice the area of the present amazonian aged south polar deposits the deposit characteristics indicate that it contained significant quantities of water ice in amounts comparable to present day polar deposits several lines of evidence for melting indicate that the ice sheet deposits underwent melt back and liquid water drainage into surrounding lows including a large valley near the crater schmidt and the argyre basin narrow sinuous ridges lie in a broad linear depression extending from a high near the present polar cap continuously downslope to near the distal portion of hd the new topographic data support the interpretation of these ridges as eskers representing meltwater distribution networks at the base of the receding deposit extensive development of large pits and depressions cavi have previously been interpreted as eolian etching or basal melting of ice rich deposits analysis of mola topography supports the interpretation that they represent basal melting of ice rich deposits and shows that they have links to the esker systems volumetric considerations and topographic lineations suggest that some of the basal melting occurred beneath regions presently occupied by apl and that some of the liquid water formed ponds and lakes in the distal parts of hd the presence of pedestal craters is further evidence of the removal of extensive volatile rich deposits and contributes to the quantitative measure of the former deposit thickness where did the melt products go inspection of the margins of the dorsa argentea formation reveals several large channels that begin there and drain downslope for distances between 900 and 1600 km onto the floor of the argyre basin some 3 5 4 0 km below their origin these channels do not exhibit tributaries their broad lateral distribution supports other evidence that deposit melting was areally very widespread and volumetrically significant and that a large part of the meltwater entered the surface distribution system and was deposited on the floor of the argyre basin over 1000 km away estimates of the present deposit thicknesses together with amounts of the deposit removed by meltback suggest that the original volume could have been as much as 5 9 106 km3 equivalent to a global layer of water 20 m deep if the deposit consisted of 50 volatiles a portion of these volatiles migrated across the surface to pond in adjacent valleys and basins and into the groundwater system a significant portion of the volatiles remain in the deposit representing a net removal from the atmosphere and from the active hydrologic system in early to middle mars history and forming an accessible record of aqueous conditions and possible biological environments dating from that time

pole wandering of mars evidence from paleomagnetic poles abstract we use the mapping phase high altitude magnetic measurements provided by mars global surveyor mgs between march 1999 and april 2003 to model nine relatively isolated magnetic anomalies of mars each anomaly is modeled with an elliptical prism each component of the observed magnetic field is modeled independently using an elliptical prism in order to assess the reliability of the results and suppress non crustal and nearby crustal source contaminations the paleomagnetic pole positions are obtained from the magnetization vectors of the model source bodies we clean the data by removing the bad tracks and then divide the entire data into two sets that are measured at different times applying covariance analysis in the fourier domain to two maps of the same magnetic component that are derived from the two sets provides a means to extract the most common features of the maps the quality of a model is evaluated and only good models are used in the final geophysical interpretation most poles that come from good models cluster in the tharsis region suggesting that mars experienced polar motion since the magnetic source bodies were magnetized

interpretation of mars southern highlands high amplitude magnetic field with total gradient and fractal source modeling new insights into the magnetic mystery of mars abstract using model studies the total gradient tg of the z component magnetic field is shown to be a useful quantity for delineating sources of satellite altitude magnetic anomalies this field is used to constrain the location and lateral boundaries of sources of high amplitude magnetic anomalies of southern highlands of mars the tg field suggests two parallel linear and oppositely magnetized sources of 1000 and 1800 km length separated by 1000 km of region of intervening non parallel sources the simplest interpretation of the long linear features is that they are zones of multitudinous crustal scale dikes formed in separate episodes of rifting and not features associated with the mechanism of seafloor spreading forward modeling with uniformly magnetized sources suggests that magnetizations of the order of 10 50 a m 40 km thickness over 100 km width in the case of the southern source and of 12 5 27 5 a m 40 km thickness and 200 km width for the northern source are necessary to explain the z component amplitudes and features of the tg field if the crustal magnetization on mars were to be distributed fractally as on earth magnetizations matching the largest amplitude features on mars may be spatially correlated from a 50 100 km distance range 3 to approaching nearly uniform magnetization 5 values to keep magnetization intensity as small as possible the higher end of values are preferred whereas small amplitude anomaly features could be generated from sources with 3 many of the mars anomaly features could be coalescence effects similar to the coalescence of anomaly features observed on http icarus cornell edu information keywords html earth

ironstone pods in the archean barberton greenstone belt south africa earth s oldest seafloor hydrothermal vents reinterpreted as quaternary subaerial springs bodies of iron oxide to 200 m across in the 3 5 3 2 ga barberton greenstone belt south africa have been interpreted as deposits of archean seafloor hydrothermal vents and have provided what are arguably key observations about surface environments on early earth these bodies termed ironstone pods have yielded what are putatively the oldest known complex organic compounds and have been used to estimate archean surface temperature ocean depth and volume and seawater composition and to deduce relationships between hydrothermal activity and seafloor sedimentation however the pods are composed largely of goethite a thermally unstable hydrated iron oxide mineral show abundant still open primary porosity are undeformed in a terrane of highly deformed rocks and show bedding and cavity filling dripstone indicating formation with their present orientation they are here interpreted to have formed as spring and shallow subsurface deposits of young quaternary groundwater and or low temperature hydrothermal systems iron was derived by the dissolution of siderite in surrounding archean sedimentary rocks along pod margins goethite locally replaces archean chert and silicified ultramafic rock however coarse vein and cavity fill quartz was less susceptible to replacement and remains as disrupted masses and rare remnant veins in the ironstone this quartz is of archean age but contains no more information about the archean surface environment than other late hydrothermal veins throughout the barberton greenstone belt rocks the presence of a well preserved modern iron oxide spring terrace confirms that these are deposits of young subaerial springs and contain no record of archean life or environments they do however provide a unique view of a remarkable young iron depositing hydrologic system

geological evidence for a migrating tharsis plume on early mars abstract the tharsis bulge is the largest magmatic volcanic center on mars and in the solar system having a volume of 3 10 8 km 3 with the majority of its mass emplaced more than 3 7 billion years ago the igneous history of tharsis has been implicated in the generation of an atmosphere and hydrosphere capable of clement conditions on early mars it has been proposed that an early plume migration from the southern highlands of mars relative to a one plate lithosphere led to the development of the tharsis bulge in its current location along the martian crustal dichotomy we used geologic mapping crustal magnetic data crater age dating and crater morphometry to detail a previously unidentified path of putative plume migration our results indicate that extensive volcanic resurfacing occurred from a location near the present south pole to the equator around 3 8 billion years ago obliterating older cratered terrains the resurfacing path is manifest as smooth volcanic plains embaying ancient massifs and infilling large impact craters as well as a lack of a magnetic signature in this portion of the crust our results have significant ramifications for mantle dynamics and the early geologic history of mars

a global lithospheric magnetic field model with reduced noise level in the polar regions 1 due to persistent ionospheric current systems at high latitudes satellite magnetic measurements have high noise levels in the polar regions consequently small scale lithospheric features are much more difficult to resolve there than at lower latitudes using linear combinations of spherical harmonics sh we define localized functions which are band limited to a given maximum degree thus a model s sh resolution can be varied over the globe we produce a lithospheric field model from champ satellite data which resolves features to sh degree 90 at low latitudes but only to sh degree 60 in the polar regions the new model can be downward continued from satellite altitude to the earth s surface without generating spurious high latitude anomalies citation lesur v and s maus 2006 a global lithospheric magnetic field model with reduced noise level in the polar regions geophys res lett 33 l13304

a spatially continuous magnetization model for mars 1 using a three component magnetic field data set at over 100 000 satellite points previously compiled for spherical harmonic analysis we have produced a continuously varying magnetization model for mars the magnetized layer was assumed to be 40 km thick an average value based on previous studies of the topography and gravity field the severe nonuniqueness in magnetization modeling is addressed by seeking the model with minimum root mean square rms magnetization for a given fit to the data with the trade off between rms magnetization and fit controlled by a damping parameter our preferred model has magnetization amplitudes up to 20 a m it is expressed as a linear combination of the green s functions relating each observation to magnetization at the point of interest within the crust leading to a linear system of equations of dimension the number of data points although this is impractically large for direct solution most of the matrix elements relating data to model parameters are negligibly small we therefore apply methods applicable to sparse systems allowing us to preserve the resolution of the original data set thus we produce more detailed models than any previously published although they share many similarities we find that tectonism in the valles marineris region has a magnetic signature and we show that volcanism south of the dichotomy boundary has both a magnetic and gravity signature the method can also be used to downward continue magnetic data and a comparison with other leveling techniques at mars surface is favorable

ironstone pods in the archean barberton greenstone belt south africa earth s oldest seafloor hydrothermal vents reinterpreted as quaternary subaerial springs comment and reply reply we respond to de ronde et al as per their numbered comments 1 de ronde et al suggest that there are two populations of ironstone pods in the barberton greenstone belt those on farm mendon that are archean in age and composed primarily of hematite and all other ironstone pods which are young

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

mars paleomagnetic field as the result of a single hemisphere dynamo mars crustal magnetic field was most likely generated by dynamo action in the planet s early history unexplained characteristics of the field include its strength concentration in the southern hemisphere and lack of correlation with any surface features except for the hemispheric crustal dichotomy we used numerical dynamo modeling to demonstrate that the mechanisms proposed to explain crustal dichotomy formation can result in a single hemisphere dynamo this dynamo produces strong magnetic fields in only the southern hemisphere this magnetic field morphology can explain why mars crustal magnetic field intensities are substantially stronger in the southern hemisphere without relying on any postdynamo mechanisms

a coherent model of the crustal magnetic field of mars 1 the immense amount of the mapping phase mars global surveyor mgs magnetic data allows us to derive a highly accurate magnetic map of mars the data acquired at nighttimes within the first 3 years of the mapping phase are divided into two almost equal parts and each part is expressed in spherical harmonics of degree up to 90 the two models are almost identical over harmonics of degree up to 62 but show appreciable differences over higher degree harmonics indicating that the higher degree harmonic coefficients have appreciable noncrustal contribution the most repeatable components of the two models are combined to produce an accurate 62 degree harmonic model of the magnetic field of the martian crust it is further demonstrated that the altitude of mgs is the major limiting factor of the resolution of the mapping phase data the small scale features of the data with wavelengths shorter than 400 km have significant contribution from noncrustal sources they are not useful for delineating the details of the magnetic source bodies in the crust

constraints on the depth of magnetized crust on mars from impact craters large diameter greater than 500 km martian impact basins are associated with observed magnetic fields which are statistically distinct from and smaller than fields associated with smaller craters we suggest that this effect arises because impacts cause shock heating and excavation reducing the magnetization of previously magnetized crust for a simple uniformly magnetized model the magnetic field at 100 km altitude is reduced by 50 when a crater shaped demagnetization zone reaches the base of the magnetized layer by analogy with terrestrial data we assume that in martian craters the zone of demagnetization extends to a depth of 0 04 0 15 crater diameters on the basis of this assumption the data suggest that the depth to the base of the magnetized layer on mars if uniform is 35 km with lower and upper bounds of 10 and 100 km respectively these bounds imply magnetizations of 5 40 a m 1 and are consistent with likely mars geotherms at 4 gyr b p

the global topography of mars and implications for surface evolution elevations measured by the mars orbiter laser altimeter have yielded a high accuracy global map of the topography of mars dominant features include the low northern hemisphere the tharsis province and the hellas impact basin the northern hemisphere depression is primarily a long wavelength effect that has been shaped by an internal mechanism the topography of tharsis consists of two broad rises material excavated from hellas contributes to the high elevation of the southern hemisphere and to the scarp along the hemispheric boundary the present topography has three major drainage centers with the northern lowlands being the largest the two polar cap volumes yield an upper limit of the present surface water inventory of 3 2 to 4 7 million cubic kilometers

thermal evolution of the martian core implications for an early dynamo mars is thought to have possessed a dynamo that ceased 0 5 b y after the formation of the planet a possible but ad hoc explanation is an early episode of plate tectonics which drove core convection by rapid cooling of the mantle we present an alternative explanation that the martian core was initially hotter than the mantle after core formation providing an initial high heat flux out of the core a core initially 150 k hotter than the mantle can explain the early dynamo without requiring plate tectonics recent experimental results suggest that potassium is likely to partition into the martian core potentially providing an extra source of energy to power a dynamo we find that the radioactive decay of 40 k cannot explain the inferred dynamo history without the presence of a hot core our results also suggest that core solidification is unlikely to have occurred because this process would have generated a long lived 1 b y dynamo if as we conclude the core is entirely liquid it must contain at least 5 wt sulfur an initially hot core is consistent with geochemical evidence for rapid core formation and incomplete thermal equilibration with the mantle thus the early history of planetary dynamos provides constraints on the processes of accretion and differentiation

low seismic velocities below mid ocean ridges attenuation versus melt retention the first comprehensive seismic experiment sampling subridge mantle revealed a pronounced low velocity zone between 40 and 100 km depth below the east pacific rise epr that has been attributed to substantial retained melt fractions of 0 3 2 such high melt fractions are at odds with low melt productivity and high melt mobility inferred from petrology and geochemistry here we evaluate whether seismic attenuation can reconcile subridge seismic structure with low melt fractions we start from a dynamic spreading model which includes melt generation and migration and is converted into seismic structure accounting for temperature pressure composition phase and melt dependent anharmonicity and temperature pressure frequency and hydration dependent anelasticity our models predict a double low velocity zone a shallow approximately triangular region due to dry melting and a low velocity channel between 60 and 150 km depth dominantly controlled by solid state high temperature seismic attenuation in a damp mantle with only a minor contribution of 0 1 melt we test how tomographic inversion influences the imaging of our modeled shear velocity features the epr experiment revealed a double low velocity zone but most tomographic studies would only resolve the deeper velocity minimum experimentally constrained anelasticity formulations produce vsas low as observed and can explain lateral variations in near ridge asthenospheric vs with 100 k temperature variations and or zero to high water content furthermore such qs formulations also reproduce low asthenospheric vs below older oceans and continents from basic lithospheric cooling models although these structures are compatible with global qs images they are more attenuating than permitted by epr data

cavi angusti mars characterization and assessment of possible formation mechanisms received 26 august 2002 revised 5 february 2003 accepted 20 february 2003 published 24 may 2003 1 cavi angusti represent a series of large irregular depressions localized in part of the south circumpolar area previously mapped as the hesperian aged dorsa argentea formation their origin has primarily been interpreted to be due to eolian deflation or subglacial melting we use mgs mola and moc data to analyze the largest of these features 100 50 km and up to about 1500 m deep these data reveal terraced interiors centrally located equidimensional and elongated edifices and lava flow like structures that strongly suggest that this basin formed as a result of magmatic intrusion and extrusion causing heating and melting of a volatile rich substrate and drainage and loss of the liquid water volume estimates and heat transfer calculations are consistent with a mechanism involving a combination of intrusion and extrusion very similar to that observed to be responsible for icelandic subglacial eruptions and meltwater generation mounds and ridges in the floors of other depressions suggest that this mechanism may have operated in at least several other features of the cavi eolian activity sublimation and solution are also likely to have played a role in further modification of these features meltwater from basin formation appears to have drained laterally and may also have reentered the regional subsurface groundwater system index terms 6225 planetology solar system objects mars 5480 planetology solid surface planets volcanism 8450 5462 planetology solid surface planets polar regions 5416 planetology solid surface planets glaciation keywords mars cavi angusti dorsa argentea formation

paleomagnetic pole positions and pole reversals of mars the paleomagnetic pole positions of mars obtained by modeling 10 small size isolated magnetic anomalies show considerable clustering 7 out of 10 are within a circle of 30 degrees radius centered at 230e 25n the directions of the magnetization of some nearby source bodies differ by 180 degrees and both north and south poles are found in the cluster suggesting the magnetic pole reversals of mars