polar wander on mars evidence in the geoid the geoid of mars is dominated by its equilibrium figure and by the effect of the tharsis rise to investigate the rotational stability of mars prior to the rise of tharsis we produced a residual non hydrostatic geoid without tharsis first the hydrostatic component of the present day flattening was removed this procedure was performed using a 6 non hydrostatic component of flattening a value set by the spin axis precession rate of mars then zonal spherical harmonics up to degree 6 centered on tharsis were removed finally the resultant residual geoid was evaluated for rotational stability by comparing polar and equatorial moments at 4050 trial pole positions if the spin axis of ancient mars was secularly stable our analysis indicates that substantial polar wander has occurred with the rise of tharsis stable spin axis positions on the non hydrostatic residual figure of mars are 15 to 90 from the present day poles this result is consistent with previously proposed paleopoles based on magnetic anomalies geomorphology and grazing impacts

syndeformational fluid trapping in quartz determining the pressure temperature conditions of deformation from fluid inclusions and the formation of pure co2fluid inclusions during grain boundary migration observations and microthermometric data on fluid inclusions from a terrane that underwent deformation following peak metamorphic conditions show that grain boundary migration recrystallization favours the entrapment of carbonic inclusions whereas microfracturing during brittle deformation favours the infiltration and eventual entrapment of aqueous fluids our results imply that pure co2 fluid inclusions in metamorphic rocks are likely to be the residue of deformation recrystallization process rather than representing a primary metamorphic fluid where the temperature of deformation can be deduced by other means the densities of fluid inclusions trapped during recrystallization which we call recrystallization primary fluid inclusions can be used to constrain the ambient pressure during deformation using these constraints the data imply that the post metamorphic hercynian exhumation in sardinia brought rocks at 300 c to within 3km of the surface this conclusion is similar to that described for the rapidly uplifted southern alps in new zealand

survival of impact induced thermal anomalies in the martian mantle 1 a number of geophysical arguments point out the possibility that global mantle convection could have been extremely sluggish or even absent during a large part of martian history this implies that early mantle thermal anomalies produced during planetary formation might not have been quickly erased by vigorous convection one likely mechanism of early thermal inhomogeneity is large impacts at the end of heavy bombardment we suggest that tharsis province might be related to such an impact induced thermal anomaly rather than a convective plume the shape of the present day geoid for our preferred model is in good agreement with the geoid measured by mars global surveyor mgs

is the martian crust also the martian elastic lithosphere 1 correlations between topography gravity and areoid on mars are used to constrain the crustal and lithospheric thicknesses on the planet assuming that the hellas basin is in isostatic equilibrium with airy compensation point correlations between areoid anomalies and topography are used to obtain the mean crustal density and the crustal thickness we find that the crustal thickness at the reference zero elevation is 90 10 km we also find that the mean crustal density is 2960 50 kg m 3 we have also used several approaches to constrain the thickness of the elastic lithosphere using the spherical harmonic coefficients of the gravity potential and topography as a function of degrees a relatively weak constraint on the globally averaged thickness of the elastic lithosphere is obtained an improved constraint is obtained using wavelet transform analyses of great circle tracks of gravity and topography the gravity topography admittance as a function of wavelet wavelength constrains the globally averaged thickness of the elastic lithosphere to be in the range 90 10 km the observation that the mean thicknesses of the crust and elastic lithosphere are likely to be equal suggests that a rheologically tougher crust is the elastic lithosphere

thermal convection in a volumetrically heated infinite prandtl number fluid with strongly temperature dependent viscosity implications for planetary thermal evolution parameterized models of the thermal evolution of planets are usually based on the assumption that the lithosphere convecting mantle boundary can be defined by an isotherm at a temperature below which viscosity is infinite on geologic timescales recent experimental results argue against this assumption we have investigated both the definition of the lithosphere convecting mantle boundary and the power law relation describing convecting heat transfer based on numerical experiments of thermal convection in a volumetrically heated fluid with temperature dependent viscosity other recent studies have treated only the heating from below but volumetric heating is likely to be the dominant mode of heating in planetary mantles either as a consequence of radioactive heating or as a proxy for secular cooling convection can occur either in the whole box or be located under a stagnant lid in the lid regime convection is driven by a temperature contrast depending on the rheology of the fluid and the interior temperature this result in agreement with experimental studies indicates that boundary between the stagnant lid and the convecting layer similar to the lithosphere convecting mantle boundary cannot be defined as a fixed isotherm during thermal evolution of planets the viscosity contrast in the convecting mantle remains constant not the temperature at the bottom of the lithosphere we present an example showing that the evolution of planets is strongly dependent on the criterion chosen to define the lithosphere convecting mantle boundary for reasonable values of the activation energy for thermally activated creep the temperature defining the lithosphere convecting mantle boundary the mantle temperature and the thickness of the lithosphere could be larger than expected from previous models which treat the base of the lithosphere as a fixed isotherm

obliquity variations due to climate friction on mars darwin versus layered models in response to periodic variations in the obliquity giant coe ice caps may form in the polar regions of mars the delayed response of the equatorial bulge of mars to these periodic loads may introduce a secular term in the time evolution of the obliquity up to now the variations in the flattening of mars induced by the polar ice caps have been estimated using the darwin model which simply consists of a uniform sphere with newtontan rheology for earth like viscosities of 0 3 x 1021 pa s the darwin model maximizes the friction and predicts a secular obliquity variation comparable to the currently observed value 0 24o our main purpose here is to address the role of two well recognized features of mars a thick lithosphere and a core on the response of its equatorial bulge and thus on the secular changes of obliquity driven by climate friction this is done using a layered model with maxwell viscoelastic rheology we show that for a wide range of admissible mantle viscosities the obliquity variation is sensibly smaller by a factor of 0 2 than the one predicted by the darwin model on the basis of this finding we can rule out the possibility that the present day obliquity of mars totally results from climate friction

multiphase carbonate salt immiscibility in carbonatite melts data on melt inclusions from the krestovskiy massif minerals polar siberia minerals of olivine melilite and olivine monticellite rocks from the krestovskiy massif contain primary silicate salt carbonate salt and salt melt inclusions silicate salt inclusions are present in perovskite i and melilite thermometric experiments conducted on these inclusions at 1 230 1 250 c showed silicate carbonate liquid immiscibility globules of composite carbonate salt melt rich in alkalies p s and cl separated in silicate melt carbonate salt globules in some inclusions from perovskite ii at 1 190 1 200 c separated into immiscible liquid phases of simpler composition carbonate salt and salt inclusions occur in monticellite melilite and garnet and homogenize at close temperatures 980 780 c they contain alkalies ca p so3 cl and co2 according to the ratio of these components and predominance of one of them melt inclusions are divided into 6 types i hyperalkaline cao na2o k2o 1 carbonate melts ii moderately alkaline cao na2o k2o 1 carbonate melts iii sulfate alkaline melts iv phosphate alkaline melts v alkali chloridic melts and vi calc carbonate melts joint occurrence of all the above types and their syngenetic character were established some inclusions demonstrated carbonate salt immiscibility phenomena at 840 800 c a conclusion in made that the origin of carbonate melts during the formation of intrusion rocks is related to silicate carbonate immiscibility in parental alkali ultrabasic magma the separated carbonate melt had a complex alkaline composition under unstable conditions the melt began to decompose into simpler immiscible fractions different types of carbonate salt and salt inclusions seem to reflect the composition of these spatially isolated immiscible fractions liquid carbonate salt immiscibility took place in a wide temperature range from 1 200 1 190 c to 800 c the occurrence of this kind of processes under macroconditions might most likely cause the appearance of different types of immiscible carbonate salt melts and lead to the formation of different types of carbonatites alkali phosphatic alkali sulfatic alkali chloridic and most widespread calcitic ones

chromite in the platreef bushveld complex south africa occurrence and evolution of its chemical composition the regional distribution and chemical composition of massive and disseminated chromitites through a platreef sequence and along a strike distance of over 20 km were investigated to correlate them both within the framework of the northern limb and to the eastern and western limbs of the bushveld complex the chromitite layers and seams of the platreef form two main chromite bearing zones the upper chromitite that occurs as two to three discontinuous seams in feldspathic pyroxenite at approximately 20 m below the platreef top contact and the lower chromitite that is composed of multiple seams in feldspathic harzburgite pyroxenite and norite close to the bottom contact of the platreef with footwall electron micro probe analyses reveal that the chemical composition of chromite depends on the host rock type norite and pyroxenite host chromite with the highest cr2o3 content while harzburgite hosted chromites are cr and mg poor the wide range in chromite compositions is explained by the influence of late magmatic processes including post cumulus growth and re equilibration interaction with fluid and sulphide saturated magmatic liquid and contact metamorphism each of these processes is characterised by its own distinct geochemical signature but generally they lead to a decrease in mg and al and an increase in both di and tri valent fe in the chromite the occurrence of chromitite locally on the different distance from the contact between the upper platreef sills and the overlying main zone magma suggests erosion of the upper platreef by the main zone as it was emplaced the localisation of chromitites supports an independent development of the northern limb during the critical zone emplacement although the chemical composition of chromite and co existing silicates from ultramafic rocks suggest a critical zone affinity with the eastern and western limbs of the bushveld complex

na rich carbonate inclusions in perovskite and calzirtite from the guli intrusive ca carbonatite polar siberia carbonate phases some rich in na2o and comparatively rich in sro and bao occur as inclusions in perovskite and calzirtite ca2zr5ti2o16 in the carbonatite of the guli complex siberia this is the first record of alkali carbonates akin to nyerereite na2ca co3 2 in plutonic igneous rocks the inclusion populations suggest that the parental magma of the complex was ca rich but developed na rich differentiates during the latest stages this points to the dominant calcic carbonatites of the complex not being derivatives of alkali rich parental carbonatites these alkali rich carbonate inclusions and rare inclusions of djerfisherite have been preserved due to the resistance of perovskite and calzirtite to processes of leaching hydrothermal alteration and weathering

polar wandering in mantle convection models we calculate polar motion in models of 3 d spherical mantle convection at rayleigh numbers up to 108 which include internal heating radial viscosity variations and an endothermic phase change isoviscous models yield rapid polar motion of order 3 myr but a factor of 30 increase in viscosity with depth reduces the rate of polar motion to about 0 5 myr due to stabilization of the large scale pattern of convection avalanching due to an endothermic phase change causes pulsating inertial interchange polar excursions of order 80 110 and of duration 20 70 myr a layered viscosity model with an endothermic phase change yields only one inertial interchange event in 600 million years these models show that the slow observed rate of post paleozoic true polar wander is not incompatible with higher rates inferred for earlier times

long term rotation and mantle dynamics of the earth mars and venus in response to internal and surface tectonic processes the terrestrial planets have the ability to displace the axis of rotation with respect to the mantle this behavior is quantified by means of a nondimensional number defined here as the rotational number ro that allows classification of the planets into two categories the first containing mars and the earth where true polar wander is a feasible mechanism and the second to which venus belongs where rotation equilibrium is attained by means of mega wobbles driven by internal mass anomalies the number ro is related to the timescale characterizing the readjustment of the equatorial bulge during long term polar motion and to the length of the sidereal day if these two timescales are well separated the planet experiences true polar wander nonlinear liouville equations for stratified viscoelastic bodies with linear maxwell rheology are solved in order to assess the relevance of surface and mantle processes in driving long term rotation instabilities in the terrestrial planets the amount of true polar wander estimated for the earth and mars is reproduced correctly by our modeling with mantle viscosities and lithospheric thickness consistent with other studies the major difference between the earth and mars is the driving mechanism subduction for the earth and lithospheric loading for mars when mantle viscosities similar to those of the earth are considered for venus the most updated estimate for the offset of about 0 5 between the rotation axis and the axis of maximum inertia is well reproduced during the mega wobbles induced by internal mass redistribution we show that the degree 2 topography of these three planets can be affected by their rotation which is responsible for the dominance of the sectorial component on the earth and mars and for the zonal component on venus

deformation of chromite s e m investigations abstract the scannning electron microscope operated in the back scattered electron mode has been applied in a microstructural analysis of chromites samples from the oman vourinos greece and tiebaghi new caledonia ophiolites displaying a variety of mantle flow and emplacement structures have been examined and compared with undeformed chromite from the bushveld intrusion and deformed chromite from the high grade metamorphic fiskenaesset complex channelling contrast and electron channelling patterns provide information on grain boundaries subgrains inclusions and cracks as well as defects in the chromite lattice these microstructures suggest that dislocation mechanisms are active in chromite deforming under mantle conditions recovery is quick whereas nucleation is slow and accordingly recrystallization of chromite occurs by migration of grain boundaries driven either by strain energy or by interfacial energy increasing the grain size during deformation dislocation mechanisms however only provide a secondary mode of deformation compared to brittle fracturing and passive rotation of chromite grains and fragments in the weaker silicate matrix diffusion accommodated flow may also be significant

deformation fabric and microstructures in ophiolitic chromitites and host ultramafics sultanate of oman harzburgite and dunite from the oman ophiolite display a strong olivine fabric and microstructures typical for high temperature mantle deformation the syntectonic magmatically formed chromitites occurring throughout the mantle sequence vary from almost undeformed resembling chromitite from stratiform complexes to highly deformed usually the interstitial silicates in the deformed ore possess a fabric similar to those of the host rocks whereas a weak or poikilitic fabric is observed on relatively undeformed deposits chromite grains are elongated and flattened defining strong foliations and lineations in disseminated ore and pull apart like structures perpendicular to the lineation form in the coarser massive ore no preferred crystallographic orientation of chromite has been detected with the x ray texture goniometer and it is proposed that chromite mainly deforms by passive rotation in the weaker silicate matrix cataclasis and diffusion creep

interior structure and seasonal mass redistribution of mars from radio tracking of mars pathfinder doppler and range measurements to the mars pathfinder lander made using its radio communications system have been combined with similar measurements from the viking landers to estimate improved values of the precession of mars pole of rotation and the variation in mars rotation rate the observed precession of 7576 35 milliarc seconds of angle per year implies a dense core and constrains possible models of interior composition the estimated annual variation in rotation is in good agreement with a model of seasonal mass exchange of carbon dioxide between the atmosphere and ice caps

polymineralic inclusions in oxide minerals of the afrikanda alkaline ultramafic complex implications for the evolution of perovskite mineralisation the exceptional accumulation of perovskite in the alkaline ultramafic afrikanda complex kola peninsula russia led to the study of polymineralic inclusions hosted in perovskite and magnetite to understand the development of the perovskite rich zones in the olivinites clinopyroxenites and silicocarbonatites the abundance of inclusions varies across the three perovskite textures with numerous inclusions hosted in the fine grained equigranular perovskite fewer inclusions in the coarse grained interlocked perovskite and rare inclusions in the massive perovskite a variety of silicate carbonate sulphide phosphate and oxide phases are assembled randomly and in variable proportions in the inclusions our observations reveal that the inclusions are not bona fide melt inclusions we propose that the inclusions represent material trapped during subsolidus sintering of magmatic perovskite the continuation of the sintering process resulted in the coarsening of inclusion rich subhedral perovskite into inclusion poor anhedral and massive perovskite these findings advocate the importance of inclusion studies for interpreting the origin of oxide minerals and their associated economic deposits and suggest that the formation of large scale accumulations of minerals in other oxide deposits may be a result of annealing of individual disseminated grains

deriving indicators of soil degradation from soil aggregation studies in southeastern spain and southern france soil degradation is perceived as a major threat in the mediterranean region due to changes in land use and possible future climate change soil aggregation parameters are used here to demonstrate their potential as a key indicator for land degradation studies the monitoring of these indicators offers a means of establishing the vulnerability and resilience of geo ecosystems soil aggregation stability and distribution were studied on soils with an open shrubby vegetation cover from several places in southeastern spain and southern france by applying drop tests and determining aggregate size distributions aspect and vegetation cover were incorporated in the soil sampling several indices were derived from these analyses to indicate the degree of soil aggregation this was done by referencing to a base level of aggregation bare soil aggregation it was found that soil aggregates were more stable and were often coarser under vegetation when compared to their immediate surrounding bare areas a similar slightly less clear effect was noted on n facing exposed slopes when compared to s facing exposed slopes long term changes were found by studying cultivated land abandoned fields and land covered by semi natural vegetation on comparable substrate and comparable land units it is clear that soil aggregation and aggregate stability increases with time years it is argued that soil aggregation indices can be used as a key indicator for degradation processes at a fine scale with implications for runoff and sediment generating processes at the hillslope scale

bebedourite from its type area salitre i complex a key petrogenetic series in the late cretaceous alto paranaiba kamafugite carbonatite phoscorite association central brazil abstract bebedourite is a cumulate rock composed of variable but roughly equant amounts of diopside apatite magnetite phlogopite and a ca ti phase mostly perovskite more rarely titanite and or ti garnet other minerals may be modally important such as olivine and k feldspar in the least and most evolved members of the bebedourite series respectively the magmatic evolution in bebedourites is accompanied by a progressive increase in sio2 activity which results in the transformation of perovskite into titanite and titanite into ti garnet although the sio2 increase may in some cases result from crustal contamination it seems to be a localized effect and cannot account for the evolution of the whole bebedourite series crystal fractionation is supported by the chemical variation of key mineral phases such as pyroxene and phlogopite the salitre complex is an ultrapotassic carbonatite and phoscorite bearing plutonic complex belonging to the late cretaceous alto paranaiba igneous province apip and consisting of three main bodies salitre i ii and iii the complex is composed mainly of bebedourite with lesser amounts of carbonatite and phoscorite in its central north portion a particular type of bebedourite where the main ca ti phase is ti garnet dominates the southern part of salitre i and also occurs as dikes crosscutting older bebedourites suggesting that ti garnet bebedourites form an independent intrusion sr and nd isotopic data indicate that the parental magmas to the bebedourites in the salitre complex originated in a metasomatized sub continental lithospheric mantle similar to that involved in the origin of the rest of the apip

on the origin of mascon basins on the moon and beyond 1 mascon basins on the moon are large craters that display significant positive free air and bouguer gravity anomalies an important question is why is not every large crater a mascon as less than half have been previously determined to be we detrend the free air topographic and bouguer gravity anomalies and find that most large basins 28 of 41 display mascon characteristics e g strong positive bouguer anomalies narrower than the surface rims negative free air gravity annuli surrounding the central highs generally are absent in the bouguer gravity implicating surface topography we propose that beneath a forming large basin the relatively narrow transient crater drives mantle uplift while upward and inward collapse forms the surface topography furthermore the nonmascon basins are all ancient and heavily degraded indicating a postimpact evolutionary process our results suggest that mascon formation is the standard for large impacts on the moon and by extension on other terrestrial planets

influence of soil properties on the aggregation of some mediterranean soils and the use of aggregate size and stability as land degradation indicators abstract soil aggregation in relation to other soil properties was studied along a climatological transect in the southeast of spain three sites were selected along this transect ranging from semiarid to subhumid climatological conditions the aggregate size distribution the macro and microaggregate stability of the superficial soil horizon and their relations with other soil properties were analysed large aggregates 10 10 5 5 2 mm were present in highest proportions in the most arid of the studied areas these large aggregates were associated with high values of water stable microaggregates however they did not improve soil structure and are related to high bulk densities and low water retention aggregates 1 0 105 mm were positively correlated to medium fine very fine sand and silt fractions and to organic matter aggregates water stability of microaggregates showed a positive correlation with clay content while the stability of the macroaggregates depended on the organic matter content when the organic matter content was greater than 5 or 6 below that threshold the carbonate content was strongly correlated with aggregate stability a general conceptual scheme of associations between aggregate size distribution water stable microaggregation and textural characteristics for the area was developed the land use history affecting soil overlaps the pattern of climatological situations and has to be taken into account aggregate size distribution and stability can be used as indicator of soil degradation but not as a unique parameter

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

geological and exploration models of beach placer deposits integrated from case studies of southern australia abstract the processes leading to the formation of beach placer deposits generally begin inland and terminate at the coast including source rocks being weathered eroded and then transported by streams and rivers to the coast where the sediments are deposited in a variety of coastal environments the coastal sediments are reworked by the action of waves tides longshore currents and wind which are effective processes for sorting the mineral grains based on differences in their size and density resulting in laminated or lens shaped packages of sediments up to tens of meters thick that are rich in heavy minerals detailed studies of sedimentary basins as well as peripheral paleo valleys that drained sediment source areas are important tools in the exploration for heavy mineral resources knowledge of the paleo basin associated valley architecture and the source of heavy minerals concentrated in the shorelines and valleys are useful guides to the potential for and location of economic deposits evidence from sedimentology can be combined with that from other geological and geophysical characteristics to arrive at a general reconstruction of basin and paleovalley architecture and depositional environments complex paleogeography of the shorelines can influence or determine the sites of heavy mineral concentration heavy mineral sands tend to concentrate in certain shoreline settings during storm activity repeated storm erosion and reworking over centuries e g the southeastern coast of australia or millennia e g the eucla and murray basins of australia can progressively enrich heavy mineral sand deposits preservation of these deposits over a geological timeframe of millions of years can ensue through subsidence of coastal sediments and during sea level change that results in either shorelines migrating inland marine transgression or seaward marine regression potentially burying or stranding earlier deposits or reworking them to form younger deposits refinements in remote sensing and geophysical techniques data processing sedimentology and computer aided interpretations provide effective economic and efficient methods for modeling coastal reconstructions and for exploring provinces and terrains prospective for heavy mineral sand deposits landscape topography analysis combined with geophysical methods that can resolve physical property contrasts between the shoreline sediments and underlying sequences are increasingly used in mineral exploration to locate and to reconstruct paleoshorelines and paleovalleys australia has modern and ancient beach placer deposits both of which show many similar geologic features the formation of these heavy mineral deposits provides one of the best examples of applying knowledge of modern systems e g the west and east coasts of australia as an analogue to interpret and understand the geology and form of ancient deposits e g the eucla and murray basins of southern australia this study provides descriptive and exploration models of australia s heavy mineral sand deposits formed in coastal environments which can be applied to similar settings worldwide

aggregate stability in range sandy loam soils relationships with runoff and erosion abstract the spatial variability of soil aggregate stability and its relationship to runoff and soil erosion were examined in a catena of soils and vegetation in a semiarid environment at the rambla honda field site tabernas almeria se spain to evaluate the validity of structural stability as a soil erosion indicator in sandy loam range soils the influence of soil properties and topography on the variability of aggregate stability was also examined methods include 1 aggregate stability assessment at 12 sites 3 repetitions per site on the hillslope by two methods a aggregate size distribution by dry sieving b water drop test 2 soil organic carbon content 3 particle size distribution determination 4 terrain attributes derived from a digital elevation model 1 m resolution 5 monitoring runoff and erosion for nearly 3 years in eight 10 2 m plots distributed over the hillslope results 41 of the average soil mass is formed by 2 mm aggregates however wet aggregate stability is poor with a mean of a total of 1440 aggregates of only 26 drop impacts necessary to break up a wet aggregate pf 1 significant relationships were found in the number of water drops required for aggregate breakdown and runoff and erosion rates however no significant relationships between the mean weight diameter of aggregates under dry conditions and runoff or erosion rates were observed the relationships of aggregates with other soil properties hillslope position and proximity to plants are also analysed the most significant correlation found was between the number of drop impacts and soil organic matter content the stability of topsoil aggregates seems to be a valuable indicator of field assessed runoff and inter rill erosion of sandy loam range soils under semiarid conditions

textural and compositional features of chromite in the lower and critical zones of the bushveld complex south of potgietersrus chromite is developed as an accessory mineral in a 1 700 m thick sequence of lower and lower critical zone cumulates in the area south of potgietersrus and as a major constituent in a number of chromitite layers within this sequence the lower zone consists of a 1 500 m thick sequence of ultramafic rocks characterized by varying proportions of olivine orthopyroxene and chromite which allows the pile of cumulates to be subdivided into 37 cyclic units the critical zone on the other hand is only 350 m thick of which only the lower 150 m contains chromite bearing lithologies electron microprobe analyses of chromite and coexisting silicates indicate a strong positive correlation between the mg mg fe super 2 and cr fe super 2 fe super 3 ratios of chromite and the mg mg fe super 2 ratio of the mafic silicates the composition of the chromite is also strongly influenced by the mineralogical composition of the host rock and probably reflects conditions within the parental magma during crystallization of these rocks the analytical data have also shown the tio 2 content of the disseminated chromite to be the most informative indicator of the degree of differentiation of the magma a detailed investigation of the chromite across the contact of a thin chromitite layer has shown that the chromitite layers crystallized in response to an influx of magma and that chromite crystallization could have been enhanced by an increase in the f sub o 2 and a rise in the cr 2 o 3 content of the liquid thereby causing an increase in the cr 2 o 3 content of the chromite within the chromitite layers postcumulus modification of the chromitite layers is extensive and primary textural features have been obliterated essentially by sintering sintering took place in the presence of a reactive mg rich liquid which resulted in an increase in the mg mg fe super 2 ratio of the chromite this process resulted in a densification of the chromitite layers from an estimated initial porosity of 30 to 45 percent to less than 10 percent

early mars serpentinization derived ch4 reservoirs h2 induced warming and paleopressure evolution ch4 has been observed on mars both by remote sensing and in situ during the past 15 yr it could have been produced by early mars serpentinization processes that could also explain the observed martian remanent magnetic field assuming a cold early mars a cryosphere could trap such ch4 as clathrates in stable form at depth the maximum storage capacity of such a clathrate cryosphere has been recently estimated to be 2 x 10 19 to 2 x 10 20 moles of methane we estimate how large amounts of serpentinization derived ch4 stored in the cryosphere have been released into the atmosphere during the noachian and the early hesperian due to rapid clathrate dissociation and photochemical conversion of ch4 to h 2 these episodes of massive ch4 release may have resulted in transient h 2 rich atmospheres at typical levels of 10 20 in a background 1 2 bar co2 atmosphere the collision induced heating effect of h 2 present in such an atmosphere has been shown to raise the surface temperature above the water freezing point we show how local and rapid destabilization of the cryosphere can be induced by large events such as the hellas basin or tharsis bulge formation and lead to such releases our results show that the early mars cryosphere had a sufficient ch4 storage capacity to have maintained h 2 rich transient atmospheres during a total time period up to several million years or tens of million years having potentially contributed to the formation of valley networks during the noachian early hesperian

mars without tharsis the significant power in the martian gravitational field due to the tharsis rise may mask or modify gravitational signatures that contain important information on martian geophysical processes in order to isolate gravity signals in regions where the field is significantly affected by tharsis as well to investigate characteristics of the global field we present a numerical technique to remove tharsis from the martian gravitational field our analysis will show that to first order tharsis can be represented as a sixth degree spherical harmonic zonal gravitational feature in a reference frame in which the axis of symmetry runs through the center of the province we produced the mars without tharsis mwt field by subtracting the gravitational signature of tharsis from the full field and rotating the spherical harmonics back to the original coordinate system in which the z axis is coincident with the planetary rotation axis our study yields limits on the moment of inertia factor of 0 361 c mr2 0 366 for the range of allowable non hydrostatic contributions to the flattening the analysis indicates that the formation of tharsis may have resulted in re orientation of the spin axis though if the planet retained rotational fossil bulge due to a thick elastic lithosphere then changes in the global figure could have been inhibited the zonal and tesseral components of the degree 2 field in the gmm i and rotated configurations indicates that tharsis is 5 times more axisymmetric about its pole than mars as a whole is about the rotation axis with the removal of tharsis long wavelength gravitational power regional structures become better defined in a spatial sense while maintaining their local dynamic range the olympus mons volcanic shield displays clear evidence of a gravitational flexural moat our method for removing tharsis may also be applied to analyze other global scale geophysical features that may be considered axisymmetric to first order such as mantle plumes major volcanic constructs or large impact basins

postcumulus modification of magnetite grains in the upper zone of the bushveld complex south africa abstract a detailed electron microprobe study has been made of magnetite grains from magnetitite layer 1 of the upper zone of the bushveld complex in order to establish if there is any evidence for postcumulus processes having affected magnetite compositions representative composition profiles obtained from detailed step traverses are presented for touching magnetite grains and where magnetite is in contact with either ilmenite or plagioclase magnetite grains in all textural settings are compositionally heterogeneous touching magnetite grains display systematic compositional zonations the rims of grains show a marked decrease in al and mg towards the grain boundary the cores are characterised by a distinctive peak and trough pattern for these elements these variations can be related to the positions of exsolved phases exsolutions in the cores are either pleonaste or an fe 3 bearing al mg spinel whereas those at the grain boundaries are generally more al rich and include corundum similar exsolution related compositional variations for mg and al were found in magnetites in contact with ilmenite and plagioclase a model has been developed which illustrates the possible sequence of postcumulus events during the interval between accumulation of the magnetite and the cessation of subsolidus reactions the essential feature of the model is the delicate interplay between depletion of the host magnetite by corundum pleonaste and the fe 3 bearing al mg spinel exsolutions and diffusion of al and mg into the depleted areas from the surrounding magnetite data for cr contents in magnetites show that although this particular trace element is apparently resistant to redistribution by exsolution processes it is affected by subsolidus re equilibration between magnetite and ilmenite or magnetite and intercumulus liquid

polar wander of mars evidence from giant impact basins abstract we investigate the polar wander of mars in the last 4 2 ga we identify two sets of basins from the 20 giant impact basins reported by frey frey h 2008 geophys res lett 35 l13203 which trace great circles on mars and propose that the great circles were the prevailing equators of mars at the impact times monte carlo tests are conducted to demonstrate that the two sets of basins are most likely not created by random impacts also fitting 63 771 planes to randomly selected sets of 5 6 or 7 basins indicated that the identified two sets are unique we propose three different positions for the rotation pole of mars besides the present one accordingly tharsis bulge was initially formed at 50 n and moved toward the equator while rotating counterclockwise due to the influence of the two newly forming volcanic constructs alba patera and elysium rise the formation of the giant impact basins subsequent mass concentrations mascons in argyre isidis and utopia basins and surface masses of volcanic mountains such as ascraeus pavonis arsia and olympus caused further polar wander which rotated tharsis bulge clockwise to arrive at its present location the extensive polar motion of mars during 4 2 3 9 ga implies a weak lithosphere on a global scale deduced from a total of 72 000 polar wander models driven by tharsis bulge alba patera and elysium rise as the major mass perturbations different compensation states 0 100 are examined for each of the surface loads and nine different thicknesses are considered for an elastic lithosphere the lithosphere must have been very weak with an elastic thickness of less than 5 km if the polar wander was driven by these mass perturbations

upwelling plumes superswells and true polar wander summary the geological evolution of the rotational axis of the earth is most likely controlled by internal mass redistribution within the mantle palaeomagnetic observations suggest that it is episodic in nature with periods of quasi standstill alternating with periods of faster wander here we investigate two models for the influence of mantle plumes that vary at different spatial wavelengths on the time variations of the rotational axis true polar wander tpw in the first model we represent an upwelling plume as a sphere whose radius varies as a function of the flux of material in the conduit and that traverses the mantle at the stokes velocity such a plume produces very little wander of the rotational axis we then study the effects of two superswells that mimic the ones observed with seismic tomography and conclude that a doming regime within the mantle involves significant polar wander some of the features of this tpw that are directly linked to the periodicity of doming are reminiscent of observed phases of slow and fast tpw with similar peak velocities

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

giant impact basins trace the ancient equator of mars 1 it is shown in this paper that five giant impact basins of mars argyre hellas isidis thaumasia and utopia are located on a single great circle this suggests that the projectiles orbital planes coincided with the equator of mars at the time of impact the projectiles were larger fragments of a heliocentric asteroid that broke apart as it entered the roche limit of mars the mass of the asteroid was at least 9 8 1020 kg 1 5 10 3 times the mass of mars

obliquity oblateness feedback on mars a simple model is presented for the coupled dynamics of the orbit rotation climate system of mars changes in the orientation of the spin pole relative to the orbit pole influence the spatiotemporal pattern of incident radiation and thus drive climatic mass transport into and out of the polar regions on a variety of timescales changes in the mass distribution occur from direct climatic forcing and compensating viscous flow in the interior the net change in mass distribution influences the rate of spin axis precession and thereby influences obliquity the rate of secular obliquity drift depends on several poorly known parameters including the magnitudes and response times of volatile inventories and viscosity structure within mars even relatively modest secular obliquity drift can lead to trapping in nearby resonances the dissipative nature of the coupled dynamical system makes reconstruction of past evolution much more difficult than for a purely inertial system the long term obliquity history of mars is dominated by climate

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

magmatic evolution of impact induced martian mantle plumes and the origin of tharsis 1 tharsis province is a major center of martian volcanic activity characterized by large gravity and topography anomalies the origin of tharsis is debated one hypothesis is that the province was produced by melting associated with a mantle plume from the coremantle boundary an alternative hypothesis is that tharsis formed by a plume associated with an impact recent studies have shown that this hypothesis is plausible from a geodynamical point of view and that long lived impact plumes might play a role in areoid evolution in this study the magmatic evolution of impact induced thermochemical mantle plumes is investigated with fully three dimensional spherical shell simulations of mantle convection melt volumes and emplacement rates predicted by the model can satisfy observational constraints on tharsis development index terms 6225 planetology solar system objects mars 5430 planetology solid surface planets interiors 8147 5455 planetology solid surface planets origin and evolution 8121 tectonophysics dynamics convection currents and mantle plumes keywords impact plumes mantle dynamics mars

impact induced convection as the main mechanism for formation of lunar mare basalts 1 using a suite of numerical models we show that impact induced convection in the moon can explain the formation of lunar mare basalts the depth range of their source region the observed delay between impact basin formation and starting time of mare flows and the long duration of the basaltic flows the effects of an impact on the thermal evolution of the moon and melt production in the mantle are investigated using convection calculations in an axisymmetric cylindrical coordinate system an ascending mantle plume is allowed to melt as it crosses the depth of the solidus temperature we consider two different models permeable and impermeable five different viscosity models and three different impact basin sizes are examined the total amount of melt produced by the permeable model with 1000 times viscosity contrast across the computation domain is comparable to the observed mare flows in imbrium and orientale basins moreover the starting time of major melting in the mantle and its duration are also compatible with the observations the model also allows a rigid lithosphere to develop beneath the basins that is capable of supporting the mascons largely created during the peak mare flow period the model for south pole aitken basin results in a substantial amount of melting in the mantle which does not seem to be compatible with the observations a potassium rare earth elements and phosphorus kreep layer with high concentration of radioactive elements incorporated directly beneath the crust has minor effects on the melt production in the mantle

na ti zr h2o rich mineral inclusions indicating postcumulus chrome spinel dissolution and recrystallization in the western laouni mafic intrusion algeria disseminated cumulus chrome spinel in the lower most olivine orthocumulates from the western laouni intrusion southern hoggar algeria contains inclusions of silicates enriched in mg ti na and h2o and fe ti zr rich oxides occurring either as numerous micro inclusions or as large solitary cavities regardless of the inclusion type titanian pargasite or kaersutite and the sodium analog of phlogopite predominate over orthopyroxene en86 to en92 and na rich plagioclase an50ab50 to an7ab93 oxide inclusions are magnesian ilmenite up to 37 mol mgtio3 rutile loveringite ca ree ti fe cr mg zr 21o38 and accessory magnesian pseudo brookite kennedyite apatite fe ni cu sulfides and alteration products saussurite bastite are also present apart from kennedyite all minerals included in chrome spinel form larger intercumulus crystals in the host rock as well the micro inclusions were trapped as the consequence of chrome spinel dissolving against the intercumulus liquid 150 to 300 c below its liquidus temperature the solitary cavities are attributed to fluid assisted solid state recrystallization of chains of spinel crystals the role of capillary fluids is demonstrated by both the hydrous nature of solid inclusions and a strong positive correlation between the amphibole content and the abundance of solid inclusions in the rock phlogopite locally showing similar optical orientation within and outside the inclusions indicates that spinel was still recrystallizing down to 950 c in the temperature range 1100 900 c determined for the formation of inclusions chrome spinel probably trapped various combinations of crystals liquids and fluids the respective quantities of which varied greatly over distances of few tens of micrometres in a single spinel the volume ratio of solids to liquids or to fluids must have been low enough to permit magmatic or hydrothermal reactions which are otherwise never possible for the larger intercumulus crystals hence the lack of olivine and clinopyroxene as inclusions in spinel while abundant in the host rock suggests that as trapped they completely reacted with the residual liquid to form phlogopite and pargasite respectively likewise plagioclase and phlogopite have been enriched in na against hydrothermal fluids to form albite and na phlogopite in the mineral inclusions

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

a numerical study of a mantle plume beneath the tharsis rise reconciling dynamic uplift and lithospheric support models 1 the tharsis rise is an area of extensive volcanism containing the most significant long wavelength topographic and areoid anomalies on mars the mechanism for supporting this large topographic expression and correlating areoid remains controversial the two main competing ideas are dynamic support by a deep mantle plume and a volcanically constructed lithosphere followed by lithospheric flexure while both of these models separately can account for specific features of tharsis neither explain its entirety most of the support of the tharsis topographic anomaly is best explained by a volcanic structure modifying the lithosphere however this raises the important question of the origin of the heat source for volcanic construction leading to possible small scale convection this demonstrates that the remaining topography and areoid not accounted for in lower end members of volcanically constructed surface loads is consistent with a deeper mantle plume source our results show we can produce about 15 and 18 of the total tharsis long wavelength areoid and topography anomaly with a plume forming in a strong temperature dependent rheology additionally our plume model has a 165 km thick rheological lithosphere within the range for present day estimates on mars and generates about 1 partial melting more than sufficient to provide a heat source for recent volcanism this implies both a model with dynamic support from a plume and a volcanically constructed lithosphere not only can coexist but offer an explanation for both the surface anomalies and heat needed for the minor amount of recent volcanic activity at tharsis rise

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

martian precession and rotation from viking lander range data the spatial orientation of the martian pole of rotation and axial rotation parameters have been determined at a midpoint in the viking epoch january 1 1980 the mean obliquity e and node angles are e 25 1894 0 0001 35 4796 0 0002 2 the mean precession rate is o 7 83 0 30 arc sec yr 1 the corresponding moment estimate is c mre2 0 355 0 015 and covers two extreme theoretical estimates seasonal annual and semiannual variations in mars rotation angle have also been observed and are 279 100 and 311 100 milliarc second of angle mas respectively after correcting for a general relativity effect a simple model has been constructed for the main contribution involving seasonal mass exchange between the ice caps and atmosphere where the air pressure at the viking 1 lander site is a proxy for the global ice cap air pressure histories the annual term is about 190 mas smaller and differs in phase by 30 compared to the model prediction the semiannual term is 100 mas larger and also differs in phase by 30 compared to the model a combination of tidally driven rotation changes zonal winds polar motion and ice cap model deficiencies may account for any discrepancy the upcoming pathfinder lander and mars global surveyor missions carry ranging transponders which can dramatically improve our knowledge of mars interior through precise determination of the precession rate and detection of core and tidal effects we advocate that plans be developed for a coordinated campaign of simultaneous ranging and doppler tracking to each transponder

the interior structure of mars implications from snc meteorites two end member models of mars present interior structure are presented the first model is optimized to satisfy the geochemical data derived from the snc meteorites in terms of the bulk chondritic ratio fe si 1 71 while the second model is optimized to satisfy the most probable maximum value c 0 366 x m p r p 2 of the polar moment of inertia factor hydrostatic equilibrium and stationary heat transfer are assumed and the basic differential equations for the mechanical and thermal structure are solved numerically together with an isothermal murnaghan birch type equation of state truncated in eulerian strain at forth order we obtain the radial distribution of mass hydrostatic pressure gravity temperature and heat flow density along with the corresponding density stratification viscosity profiles and the global seismic velocity structure of model mars the first model being consistent with the geochemical requirement produces c 0 357 x m p r p 2 whereas the second model commensurate with the geophysical constraint gives fe si 1 35 the calculated central pressure is about 40 gpa in both models and the central temperature is in the 2000 to 2200 k range the model calculations suggest a fe ni fes core a little less than one half of the planetary radius in size surrounded by a silicate mantle subdivided into lower spinel and upper olivine layers and overlain by a 100 to 250 km thick basaltic crust and a surface heatflow density of 25 to 30 mw m 2 in both models the pressure in the mantle is not sufficient for the spinel to perovskite transition to occur the present thermal lithosphere is estimated to be about 500 km thick and to be subdivided into a 300 km thick outermost rheological lithosphere and an underlying thermal boundary layer of mantle convection given the core sulfur content of 14 wt as derived from snc meteorites the martian core is found to be entirely molten implying the nonoperation of a self sustained dynamo due to the absence of sufficiently vigorous convection

correction to localized gravity topography admittance and correlation spectra on mars implications for regional and global evolution 1 in the paper localized gravity topography admittance and correlation spectra on mars implications for regional and global evolution by patrick j mcgovern sean c solomon david e smith maria t zuber mark simons mark a wieczorek roger j phillips gregory a neumann oded aharonson and james w head journal of geophysical research 107 e12 5136 doi 10 1029 2002je001854 2002 the thickness of the lithosphere and lithospheric heat flow for a number of regions of mars and as functions of time were inferred on the basis of gravity topography admittance spectra observed admittances derived from spherical harmonic expansions localized with the scheme of simons et al 1997 were compared with those predicted from models for the flexural response to lithospheric loading e g turcotte et al 1981 gravity was calculated according to the finite amplitude scheme of wieczorek and phillips 1998 estimates for the thickness of the elastic lithosphere te at the time of loading for each region were converted to equivalent thermal gradient dt dz and heat flux q by means of an elastic plastic stressenvelope formalism mcnutt 1984 here we describe a correction required in the calculation of the modeled gravity anomalies we report new estimates of te load density rl dt dz and q from corrected model admittances and we discuss the implications of the new results 2 the source of the required correction is a difference in reference radius values as defined by mcgovern et al 2002 the planetary shape was taken to equal the radius from the center of mass of mars to the martian surface expressed as a spherical harmonic expansion and referenced to the mean equatorial radius req 3396 km

reorientation of planets with elastic lithospheres abstract the orientation of a planet is controlled by the positions of the principal axes of the inertia tensor relative to the planetary surface using the theory for the deflection of thin elastic shells the principal axes are computed after emplacement of an arbitrary axisymmetrical load the partial compensation of the load and the partial relaxation of rotational flattening are included in the computation it is found that the amount of reorientation is independent of lithosphere thickness the parameters controlling the amount of reorientation are the location of the load and the size of the load compared to the rotational flattening the results indicate that the tharsis rise has probably reoriented mars by only 3 to 9 and certainly less that 18 the position of the caloris basin on mercury indicates that if the surrounding lava sheet controls the planetary orientation then the lava sheet is probably less than 2000 m thick

the chromite deposits of the troodos complex cyprus evidence for the role of a fluid phase accompanying chromite formation the troodos ophiolite of cyprus hosts chromite deposits both as podiform bodies within the basal harzburgite and as lesser layers and schlieren within the overlying dunites and clinopyroxene dunites of the metacumulate sequence all chromitite types have been studied from both field and mineralogical aspects with particular emphasis on the chemistry of the ubiquitous silicate inclusions within chromite grains two distinct inclusion assemblages have been documented chromites from podiform bodies contain abundant hornblende serpentine ortho and clinopyroxene and lesser olivine while those from the metacumulates host pargasite albite serpentine and rare pyroxenes this implies that no single silicate inclusion assemblage in chromite can be considered typically ophiolitic the presence of an alkalirich fluid phase during chromite crystallization as evidenced by the unusual mineralogy of these inclusions has already been widely discussed in the literature however the marked stratigraphic control of inclusion types in chromite as reported here enables more complete modelling of the evolution of this phase to be made

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