venus topography a harmonic analysis a model of venusian global topography has been obtained by fitting an eighteenth degree harmonic series to pioneer venus orbiter radar altimeter data the mean radius is 6051 45 0 04 km the corresponding mean density is 5244 8 0 5 kg m 3 the center of figure is displaced from the center of mass by 0 339 0 088 km towards 6 6 10 1 n 148 8 7 7 e the figure of venus is distinctly triaxial but the orientation and magnitudes of the principal topographic axes correlate rather poorly with the gravitational principal axes however the higher degree harmonics of topography and gravity are significantly correlated the topographic variance spectrum of venus is very similar in form to those of the moon mars and especially earth we suggest that this spectral similarity simply reflects a statistical balance between constructional and degradational geomorphic processes venus and earth are particularly similar and differ from the moon and mars in that the larger bodies both exhibit a significant low degree deficit relative to the extrapolated trend of the higher harmonics

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

constraints on the martian lithosphere from gravity and topography data localized spectral admittances of the large martian volcanoes are modeled by assuming that surface and subsurface loads are elastically supported by the lithosphere in order to model the case where the load density differs from that of the crust a new method for calculating gravity anomalies and lithospheric deflections is developed the modeled gravity anomalies depend upon the elastic thickness crustal thickness load density and crustal density and these parameters were exhaustively sampled in order to determine their effect on the misfit between the observed and modeled admittance function we find that the densities of the martian volcanoes are generally well constrained with values of 3200 100 kg m 3 which is considerably greater than those reported previously these higher densities are consistent with those of the martian basaltic meteorites which are believed to originate from the tharsis and elysium volcanic provinces the crustal density is constrained only beneath the elysium rise to be 3270 150 kg m 3 if this value is representative of the northern lowlands then pratt compensation is likely responsible for the approximately 6 km elevation difference between the northern and southern hemispheres the elastic thicknesses of the major martian volcanoes when subsurface loads are ignored are found to be the following elysium rise 56 20 km olympus mons 93 40 km alba patera 66 20 km and ascraeus mons 105 40 km we have also investigated the effects of subsurface loads allowing the bottom load to be located either in the crust as dense intrusive material or in the mantle as less dense material we found that all volcanoes except pavonis are better modeled with the presence of less dense material in the upper mantle which is indicative of either a mantle plume or a depleted mantle composition an active plume beneath the major volcanoes is consistent with recent analyses of cratering statistics on olympus mons and the elysium rise which indicate that some lava flows are as young as 10 30 myr as well as with the crystallization ages of the shergottites of which some are as young as 180 myr

mars global surveyor and mars odyssey accelerometer observations of the martian upper atmosphere during aerobraking 1 mars global surveyor and mars odyssey accelerometer measurements of martian upper atmospheric densities reveal the large scale and small scale structure of the thermosphere in unprecedented detail meridional density gradients which are seasonally dependent are greater on the dayside than the nightside indicating stronger meridional winds on the dayside density scale heights do not vary greatly with latitude densities and density scale heights are lower on the nightside than on the dayside but the day night contrast is not as great as at venus thus atmospheric circulation plays a greater role in determining the state of the thermosphere on mars than on venus oscillations in density along individual density profiles probably due to gravity waves are common and their amplitudes are tens of percent step like changes in density are also observed but their origin is less clear data coverage is extensive enough to permit studies of weather not just climate

postglacial rebound observed by lageos and the effective viscosity of the lower mantle sixty four observations of the orbital node made by the lageos satellite over a five year time interval reveal an acceleration of 8 1 or 1 8 x 10 to the 8 power arcseconds day 2 due to a source which is not presently modeled in the geodyn orbit determination computer program this acceleration cannot be explained by the ocean tide with 18 6 year period assuming it to be an equilibrium tide instead it seems to be due to postglacial rebound which changes the j 2 coefficient in the spherical harmonic expansion of the earth s gravitational field at the rate of 8 2 or 18 x 10 to the 19th power s this in turn accelerates the node this rate does not agree with the 32 x 10 to the 19th power s predicted by wu and peltier s 1982 l2 model which has upper and lower mantle effective viscosities of 10 to the 21st and 22nd powers pa s respectively it does agree well with their l1 model which gives about 10 x 10 to the 19th power s since the effective viscosity is 10 to the 21st power pa s throughout the entire mantle in the l1 model the results support the contentions that the efective viscosity is near 10 to the 21st power pa s everyhere in the mantle and this relatively low value for the effective viscosity may have permitted several degrees of polar wander due to glaciation during the quaternary ice age previously announced in star as n84 13705

an improved gravity model for mars goddard mars model 1 doppler tracking data of three orbiting spacecraft have been reanalyzed to develop a new gravitational field model for the planet mars goddard mars model 1 gmm 1 this model employs nearly all available data consisting of approximately 1100 days of s band tracking data collected by nasa s deep space network from the mariner 9 and viking 1 and viking 2 spacecraft in seven different orbits between 1971 and 1979 gmm 1 is complete to spherical harmonic degree and order 50 which corresponds to a half wavelength spatial resolution of 200 300 km where the data permit gmm 1 represents satellite orbits with considerably better accuracy than previous mars gravity models and shows greater resolution of identifiable geological structures the notable improvement in gmm 1 over previous models is a consequence of several factors improved computational capabilities the use of optimum weighting and least squares collocation solution techniques which stabilized the behavior of the solution at high degree and order and the use of longer satellite arcs than employed in previous solutions that were made possible by improved force and measurement models the inclusion of x band tracking data from the 379 km altitude near polar orbiting mars observer spacecraft should provide a significant improvement over gmm 1 particularly at high latitudes where current data poorly resolve the gravitational signature of the planet

the formation of valles marineris 1 tectonic architecture and the relative roles of extension and subsidence 1 the formation of the valles marineris troughs on mars is widely held to involve some combination of horizontal extension and vertical subsidence or collapse but the specific tectonic mechanism is poorly understood this study uses boundary element models of valles marineris formation to evaluate a range of fault dip angles for two end member models of trough formation simple extensional tectonism and extension together with vertical accommodation at the base of the lithosphere representing either viscous lower crustal flow or some other form of deep seated collapse the models are constrained by the lack of footwall uplift on the plateau surface outside the troughs and the lack of significant tectonic failure of the plateau in response to the stress changes induced by valles marineris formation the results demonstrate that trough formation by extensional tectonism alone generates large surface uplifts in the surrounding plateau and enormous stresses in the lithosphere in conflict with the observations the only mechanism that is compatible with both the observed topography and the limitations imposed by the finite strength of the lithosphere is trough formation through displacement along steeply dipping to subvertical border faults together with vertical accommodation at the base of the subsiding fault block fault dips are constrained to be greater than or equal to 85 leading to a ratio of vertical subsidence to horizontal extension exceeding 5 7 and a maximum extension of less than 4 6 km these results confirm earlier studies suggesting that trough formation involved a substantial component of vertical subsidence or collapse with only modest amounts of total extension

an improved solution of the gravity field of mars gmm 2b from mars global surveyor a spherical harmonic solution of the mars gravity field to degree and order 80 goddard mars model 2b gmm 2b has been developed using x band tracking data of mars global surveyor mgs from october 1997 to february 2000 and altimeter crossovers formed from the mars orbiter laser altimeter mola data between march and december 1999 during the mapping mission mgs was located in a near polar 92 9 inclination and near circular orbit at a mean altitude of 400 km the tracking data from this orbit provide a detailed global and high resolution view of the gravity field of mars mars gravity solutions are stable to 60 60 even without application of a kaula power law constraint the valles marineris is resolved distinctly with lows reaching 450 mgals olympus mons and its aureole are both separately resolved and the volcano has a peak anomaly of 2950 mgals the global correlation of the gmm 2b gravity coefficients with mola derived topography is 0 78 through degree 60 and the correlation remains above 0 6 through degree 62 the global gravity anomaly error predicted from the gmm 2b error covariance through 60 60 is 11 mgal the global geoid error from gmm 2b through 60 60 is 1 8 m mgs orbit quality using gmm 2b as measured by overlapping orbital arcs is 1 m in the radial direction and 10 m in total position

density of mars south polar layered deposits both poles of mars are hidden beneath caps of layered ice we calculated the density of the south polar layered deposits by combining the gravity field obtained from initial results of radio tracking of the mars reconnaissance orbiter with existing surface topography from the mars orbiter laser altimeter on the mars global surveyor spacecraft and basal topography from the mars advanced radar for subsurface and ionospheric sounding on the mars express spacecraft the results indicate a best fit density of 1220 kilograms per cubic meter which is consistent with water ice that has 15 admixed dust the results demonstrate that the deposits are probably composed of relatively clean water ice and also refine the martian surface water inventory

thickness of the martian crust improved constraints from geoid to topography ratios 1 the average crustal thickness of the southern highlands of mars was investigated by calculating geoid to topography ratios gtrs and interpreting these in terms of an airy compensation model appropriate for a spherical planet we show that 1 if gtrs were interpreted in terms of a cartesian model the recovered crustal thickness would be underestimated by a few tens of kilometers and 2 the global geoid and topography signals associated with the loading and flexure of the tharsis province must be removed before undertaking such a spatial analysis assuming a conservative range of crustal densities 2700 3100 kg m 3 we constrain the average thickness of the martian crust to lie between 33 and 81 km or 57 24 km when combined with complementary estimates based on crustal thickness modeling gravity topography admittance modeling viscous relaxation considerations and geochemical mass balance modeling we find that a crustal thickness between 38 and 62 km or 50 12 km is consistent with all studies isotopic investigations based on hf w and sm nd systematics suggest that mars underwent a major silicate differentiation event early in its evolution within the first 30 ma that gave rise to an enriched crust that has since remained isotopically isolated from the depleted mantle in comparing estimates of the thickness of this primordial crust with those obtained in this study we find that at least one third of the martian crust has an origin dating from the time of accretion and primary differentiation subsequent partial melting of the depleted mantle would have given rise to the remaining portion of the crust while we predict that a large portion of the crust should be composed of ancient enriched materials a representative sample of this primordial crust does not currently exist among the known martian meteorites index terms 6225 planetology solar system objects mars 5417 planetology solid surface planets gravitational fields 1227 5430 planetology solid surface planets interiors 8147 1227 geodesy and gravity planetary geodesy and gravity 5420 5714 6019 keywords mars crustal thickness crustal evolution geoid topography

internal structure and early thermal evolution of mars from mars global surveyor topography and gravity topography and gravity measured by the mars global surveyor have enabled determination of the global crust and upper mantle structure of mars the planet displays two distinct crustal zones that do not correlate globally with the geologic dichotomy a region of crust that thins progressively from south to north and encompasses much of the southern highlands and tharsis province and a region of approximately uniform crustal thickness that includes the northern lowlands and arabia terra the strength of the lithosphere beneath the ancient southern highlands suggests that the northern hemisphere was a locus of high heat flow early in martian history the thickness of the elastic lithosphere increases with time of loading in the northern plains and tharsis the northern lowlands contain structures interpreted as large buried channels that are consistent with northward transport of water and sediment to the lowlands before the end of northern hemisphere resurfacing

elastic thickness estimates for the northern lowlands of mars abstract the northern lowlands cover approximately one third of mars surface and are a fundamental part of the geologic evolution of mars we examine the admittance signature the ratio of gravity to topography data in the spectral domain for four regions in the northern lowlands that are both well resolved in the gravity data and have well constrained lithospheric parameters these regions also have topographic power spectra similar to many highlands regions a cartesian multitaper method is used to calculate admittance we compare the observed admittance signatures to those predicted from models of lithospheric flexure on the basis of these comparisons we estimate the thickness of the martian elastic lithosphere t e required to support the observed topographic load since the time of loading we use both top and bottom loading models to derive values of elastic thickness and crustal thickness or apparent depth of compensation although all four regions are best fit by bottom loading models we obtain best fit elastic thickness estimates between 10 and 25 km with an acceptable error range of 0 to 45 km these low elastic thickness estimates are consistent with the formation in the noachian when heat flow was high the consistency in t e estimates between the noachian highlands and lowlands basement suggests that both regions of the crust formed at similar times the paucity of crustal magnetization in the lowlands is thus more likely a result of demagnetization than formation following shut down of the dynamo

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

gravity field of mars a 75th degree and order model radiometric tracking data collected by nasa s deep space network dsn for mariner 9 viking 1 and 2 orbiters and mars global surveyor mgs have been analyzed to determine the gravity field of mars the gravity solution mgs75d is a complete spherical harmonics model to degree and order 75 also parts of the solution are the gravitational constants of mars phobos and deimos the significant improvement in the mgs75d mars gravity model is mainly a consequence of globally distributed data set collected from the mgs mapping orbits the use of optimal weighting and constrained least squares solution techniques and improved force and measurement models in the orbit determination process the gravity anomaly map reveals significantly more detail and larger amplitudes than any previous mars gravity map over the major impact and volcanic areas on mars olympus mons remains the largest known gravity anomaly in our solar system and isidis planitia the highest amplitude mascon there are mascons in the hellas argyre and utopia basins valles marineris has distinctive negative anomalies and all volcanic features are positive gravity anomalies over several craters have also been resolved the overall correlation of the mgs75d gravity anomaly map with topography derived from the mars orbiter laser altimeter mola is very high while the polar regions display relatively poor correlation

geologic evolution of the martian dichotomy in the ismenius area of mars and implications for plains magnetization 1 the origin of the martian dichotomy which divides highlands from lowlands is unknown we examine a section of the dichotomy 50 90e defined by steep scarps and normal faults stratigraphy and age relationships preclude the formation of the 2 5 km high boundary via erosion the abrupt disappearance of topographic knobs 300 500 km to the northeast is interpreted as a buried fault alignment of the buried fault with grabens stratigraphy and age determinations using crater counts indicate that the lowland bench is down faulted highlands crust the estimated local strain 3 5 and fault pattern are broadly consistent with gravitational relaxation of a plateau boundary magnetic and gravity anomalies occur on either side of the buried fault admittance analysis indicates isostatic compensation although nonunique a model with a 10 km thick intracrustal block under the lowland bench a 20 km thick block under the plains and an excess density of 200 kg m3 provides a good fit to the isostatic anomaly a good fit to a profile of the magnetic field perpendicular to the dichotomy is produced using uniformly polarized intracrustal blocks 10 20 km thick an intensity of 6 am m a field inclination of 30 and gaps aligned with the isostatic anomalies one interpretation is that high density intrusions demagnetized the crust after dynamo cessation and that low lying magnetized areas could be down faulted highlands crust another model inclination of 30 has magnetized crust beneath the isostatic anomalies separated by gaps the gaps could result from hydrothermal alteration of the crust along fault zones

an improved jpl mars gravity field and orientation from mars orbiter and lander tracking data abstract the mars gravity field resolution is mostly determined by the lower altitude mars reconnaissance orbiter mro tracking data with nearly four years of additional mro and mars odyssey tracking data since the last jpl released gravity field mro110c and lander tracking from the mer opportunity rover the gravity field and orientation of mars have been improved the new field mro120d extends the maximum spherical harmonic degree slightly to 120 improves the determination of the higher degree coefficients as demonstrated by improved correlation with topography and reduces the uncertainty in the corresponding mars orientation parameters by up to a factor of two versus previously combined gravity and orientation solutions the new precession solution is 7608 3 2 1 mas yr and is consistent with previous results but with a reduced uncertainty by 40 the love number solution k2 0 169 0 006 also shows a similar result to previous studies

mars time variable gravity and its determination simulated geodesy experiments 1 the seasonal carbon dioxide co2 cycle on mars results in a time variable global redistribution of mass these large scale variations are associated with changes in the gravity field mainly in the two zonal gravity coefficients and which have been recently evaluated from doppler tracking data of the mars global surveyor mgs spacecraft in the present study we calculated these variations from the mass redistribution obtained from outputs of two general circulation models gcm as well as from co2 thickness measurements by the high energy neutron detector hend instrument on board the mars odyssey spacecraft and compared them to the observations tracking observations provide one of the most direct measures of the global scale atmospheric mass cycle however the associated uncertainties are relatively large partly because the low degree zonals obtained from a single orbiter tracking analysis are contaminated by higher degree harmonics which are shown to have nonnegligible seasonal variations thus we investigated possibilities to improve the determination of the time variable gravity field by means of simulated geodesy experiments additional radio tracking of a second spacecraft with suitable orbital characteristics was shown to be able to separate the higher degree geodetic signatures radio links between landers on the martian surface and a near polar orbiter can further better estimate especially the even zonals

density structure of the upper thermosphere of mars from measurements of air drag on the mars global surveyor spacecraft we present measurements of the density of the martian atmosphere between 170 and 180 km altitude above the high northern latitudes over a 6 month period in 1998 when the solar cycle was beginning to rise out of its activity minimum these measurements were made from the observed orbital decay of the mars global surveyor mgs spacecraft during its science phasing orbits spo april to september 1998 using x band doppler tracking observations the densities that we retrieve are comparable to model values given by culp and stewart 1984 stewart 1987 mars gram 3 7 justus et al 1996 and recent mars thermospheric global circulation model mtgcm simulations bougher et al 2000 however the spo period can be divided into two time periods separated at ls 355 0 that are characterized by significantly different orbit to orbit variability that is not predicted by these earlier models the first time period corresponds to the time during which the mgs orbit perifocus moved toward the north pole while the local solar time was 1000 1100 during this period orbit to orbit variability is 50 70 and our average measured density at 175 km is 0 018 0 007 kg km 3 between 67 and 72 n and ls 315 to 320 the second time period corresponds to the time during which the orbit perifocus moved south from the north pole and the local time was 1700 1730 during this period orbit to orbit variability is 40 20 and our average measured density at 175 km is 0 024 0 004 kg km 3 between 62 and 69 n and ls 17 to 28 for both time periods the observed latitudinal gradient of density on a constant altitude surface exhibited a factor of 3 4 decrease between 60 and 90 n this gradient is comparable to that expected by the polar vortex high latitude wind effect modeled by the mtgcm for solar medium conditions at southern summer solstice bougher et al 2000 a southern hemisphere dust storm that the mgs thermal emission spectrometer tes observed at ls 309 is distinguishable in our data set as a 100 rise in density at 180 km above the 70 northern latitudes 7 days later ls 313

mars dynamics from earth based tracking of the mars pathfinder lander measurements of mars rotational variations can be conducted via earth based radio tracking observations of the mars pathfinder lander during an extended mission two way range measurements between an earth tracking station and the lander will enable precise monitoring of the planet s orientation allowing details of mars internal structure and global surface atmosphere interactions to be determined an analysis has been performed to investigate the accuracy with which key physical parameters of mars can be determined using the earth based radio tracking measurements acquisition of such measurements over one martian year should enable determination of mars polar moment of inertia to 1 or better providing a strong constraint on radial density profiles and hence on the iron content of the core and mantle and on long term variations of the obliquity which influences the climate variations in mars length of day and polar motion should also be detectable and will yield information on the seasonal cycling of carbon dioxide between the atmosphere and the surface

channel network extraction from high resolution topography using wavelets 1 the availability of high resolution topography from lidar offers new opportunities for objectively extracting the channels directly from a dem using local topographic information instead of inferring them indirectly based on global criteria such as area or area slope threshold relationships here we introduce the use of wavelet filtering to delineate threshold curvatures for defining valleys and threshold slope direction change for defining probable channeled portions of the valleys this approach exploits the topographic signatures uniquely found in high resolution topography and reveals the fuzzy topographic transition in which local weakly convergent areas lie at the transition between hillslopes and valleys

an alternative explanation for the buried channels on mars the gravity signal from a sharp boundary on partially compensated long wavelength topography 1 a trough observed in the free air gravity of mars along the edge of tempe terra has been interpreted as a large channel that has been completely buried by sediments here we show that this trough is likely real and not a product of ringing in a truncated harmonic expansion but that it is poorly resolved a detailed examination of the high resolution topographic data displays no evidence for a buried channel instead showing that the trough straddles the highlands lowlands boundary as an alternative we propose this gravity trough in a large way arises from the surface topography due to an effect that occurs when there exists a relatively sharp boundary on a long wavelength partially compensated feature

the gravity field of mars results from mars global surveyor observations of the gravity field of mars reveal a planet that has responded differently in its northern and southern hemispheres to major impacts and volcanic processes the rough elevated southern hemisphere has a relatively featureless gravitational signature indicating a state of near isostatic compensation whereas the smooth low northern plains display a wider range of gravitational anomalies that indicates a thinner but stronger surface layer than in the south the northern hemisphere shows evidence for buried impact basins although none large enough to explain the hemispheric elevation difference the gravitational potential signature of tharsis is approximately axisymmetric and contains the tharsis montes but not the olympus mons or alba patera volcanoes the gravity signature of valles marineris extends into chryse and provides an estimate of material removed by early fluvial activity

crossover analysis of mars orbiter laser altimeter data in its first 15 months of continuous operation the mars orbiter laser altimeter mola instrument aboard mars global surveyor ranged to mars over 330 million times generating more than 5000 orbital profiles with a ranging precision of 0 4 m over smooth terrain the accuracy of the profiles depends on knowledge of the spacecraft position orientation and observation time which are subject to errors we model these errors via the analysis of over 24 million altimetric crossovers a quasiperiodic once per revolution adjustment of the ground tracks as a function of time in three locally orthogonal directions minimizes the altimetric residuals via least squares using a sparse matrix technique computational effort scales linearly with the number of crossovers and only marginally with the number of parameters orbital errors mainly result from poor modeling of spacecraft thrusting events in the absence of tracking seasonal effects likely due to changing thermal environment as well as residual miscalibrations are evident in the pointing solutions incorporating multiple parameters per revolution significantly improves crossover residuals and resolves pointing aberrations during orbital transitions from night to day altimetry from the adjusted tracks generates a topographic model whose accuracy is typically better than 1 m vertically with respect to the center of mass of mars the centroid position of each mola shot is typically accurate to 100 m horizontally terrain models from accurately located lidar data can be gradient shaded to illuminate geological structures with 1 in 1000 slopes that are invisible to cameras temporal changes in elevation e g frost deposition ablation at decimeter levels may also be assessed using crossovers but results must be interpreted with caution due to uncertainties in range walk correction

the formation of valles marineris 2 stress focusing along the buried dichotomy boundary 1 although valles marineris is widely regarded as an extensional tectonic feature the source of stress responsible for its formation remains unknown this study argues that the tensile stresses that triggered valles marineris tectonism are a result of its location south of and subparallel to the buried crustal dichotomy boundary beneath tharsis the emplacement of the tharsis volcanic load straddling the pre existing topographic step of the crustal dichotomy boundary would have resulted in an abrupt change in the thickness of the load causing differential subsidence and extension across the boundary thin shell flexural models predict a narrow belt of focused tensile stresses south of the buried dichotomy boundary coinciding with the location of present day valles marineris the interaction of these boundary generated stresses with the competing stress fields associated with tharsis loading can explain the formation of noctis labyrinthus in the west and the deflection of the valles marineris troughs away from the buried boundary toward the east finite element models demonstrate that the magnitudes and vertical variations of stress at valles marineris are sensitive to the timing of loading and flexure in tharsis the incremental loading and flexure expected for a large volcanic rise results in the maximum tensile stress at valles marineris occurring at depth with tensile stresses through the majority of the lithospheric column dikes forming within this tensile stress belt would propagate through the full vertical extent of the lithosphere due to the stress release associated with the dilation of the dikes playing a crucial role in the formation of the valles marineris troughs

genesis of the auriferous c quartz tourmaline vein of the siscoe mine val d or district abitibi subprovince canada structural mineralogical and fluid inclusion constraints the c quartz tourmaline vein of the siscoe deposit is a classic example of a high grade archean gold lode and averages 45 g t au locally attaining 221 g t au this vein cuts all dikes the main regional foliation s2 and several auriferous quartz carbonate veins and is interpreted to be a shear vein which formed during the development of a late to post d2 reverse fault the vein is composed of alternating tourmaline and quartz rich layers with minor amounts of rutile scheelite apatite chlorite muscovite carbonate mainly calcite pyrite chalcopyrite tetradymite and native gold vein filling occurred during the development of a reverse fault through a process involving repeated episodes of opening mineral deposition and plastic and brittle deformation the vein geometry was controlled by local rheologic heterogeneities or by a west northwest east southeast compressive event the wall rock alteration is characterized by the occurrence of tourmaline pyrite and calcite the timing of gold deposition and the gold fineness in the c quartz tourmaline vein differ from those of the wall rock in the vein native gold 4 7 7 3 wt ag was the latest phase to deposit and commonly occurs in fractures that cut across the quartz and tourmaline layers whereas in the wall rock gold 7 9 13 2 wt ag started to precipitate just before or synchronously with pyrite consequently the textural and mineral chemical evidence suggests that the fluids in the vein and wall rock were not in equilibrium with each other during gold deposition and that the mechanisms of precious metal precipitation were different in the two environments in the wall rock gold started to precipitate synchronously with sulfides due to reaction of hydrothermal fluids with magmatic ilmenite this reaction caused the destabilization of gold bisulfide complexes and prompted gold deposition however sulfides are very rare in the c quartz tourmaline vein and gold deposited after the main episode of vein filling suggesting that its precipitation was not controlled by pyrite deposition the occurrence of aqueous carbonic ac fluid inclusions in healed fractures that radiate from gold grains suggests that these fluids were associated with the ore forming process two types of aqueous carbonic fluid inclusions were identified h2o nacl co2 bearing salinities between 4 and 7 wt nacl equiv and h2o nacl co2 ch4 bearing salinities varying between 7 and 9 wt nacl equiv both types homogenize at t 190 c based on structural textural and fluid inclusion evidence we propose that gold deposition in the c quartz tourmaline vein took place after the main episode of vein filling as a result of continued fracturing and mixing of these two fluids

geodetic constraints on the composition of mars the mean density and mean moment of inertia are remotely accessible geodetic parameters which provide two integral constraints on the radial density profile of mars the mean density is presently well known but until the axial precession rate is better determined the amount of inertia will remain uncertain values anywhere in the interval from 0 345 mr sup 2 to 0 365 mr sup 2 can be obtained from the observed gravitational field depending on how that field is partitioned into hydrostatic and nonhydrostatic components even if the moment were accurately known a considerable degree of nonuniqueness would still remain concerning the density and composition this nonuniqueness is illustrated by construction of a family of orthogonal polynomials in the normalized radius which make no contribution to either the mean density or the mean inertial moment if the mean moment is near to the lower end of its plausible range the nonuniqueness in density and composition is considerably diminished in order to avoid unrealistically high densities in the deep interior the martian mantle must have a density appreciably lower than the terrestrial mantle and must consequently be significantly depleted in iron on the other hand if the mean moment is near the upper end ofmore its plausible range a much broader range of density profiles can be accommodated including but not restricted to iron enriched mantles less

overview of the mars global surveyor mission the mars global surveyor spacecraft was placed into mars orbit on september 11 1997 and by march 9 1999 had slowly circularized through aerobraking to a sun synchronous near polar orbit with an average altitude of 378 km the science payload includes the mars orbiter camera mars orbiter laser altimeter thermal emission spectrometer ultrastable oscillator for radio science experiments and magnetometer electron reflectometer package in addition the spacecraft accelerometers and horizon sensors were used to study atmospheric dynamics during aerobraking observations are processed to standard products by the instrument teams and released as documented archive volumes on 6 month centers by the planetary data system significant results have been obtained from observations of the interior surface and atmosphere for example mars does not now have an active magnetic field although strong remanent magnetization features exist in the ancient crust these results imply that an internal dynamo ceased operation early in geologic time altimetry and gravity data indicate that the crust is thickest under the south pole thinning northward from the cratered terrain to the northern plains analysis of altimetry data demonstrates that mars is egg shaped with gravitational equipotential contours that show that channel systems in the southern highlands drained to the north largely to the chryse trough a closed contour in the northern plains is consistent with the existence of a great northern ocean emission spectra of low albedo regions show that basaltic rocks dominate spectral signatures on the southern highlands whereas basaltic andesites dominate the northern lowlands the bright regions show nondiagnostic spectra similar to that of dust in the atmosphere signatures of aqueous minerals e g clays carbonates and sulfates are noticeably absent from the emission spectra high spatial resolution images show that the surface has been extensively modified by wind and that layering is nearly ubiquitous implying that a complex history of events is recorded in surface and near surface materials altimetry data imply that both permanent caps are composed of water ice and dust with seasonal covers of carbon dioxide frost finally the altimetry data coupled with thousands of atmospheric profiles are providing new boundary conditions and dynamic controls for the generation and testing of more realistic dynamic models of the global circulation of the atmosphere

density variations within the south polar layered deposits of mars 1 the south polar layered deposits spld constitute the largest known reservoir of water on mars previous studies solved for the best fit uniform density of the deposits using a forward approach here we invert for the lateral density variations in the layered deposit using gravity data from radio tracking of mars reconnaissance orbiter topography from mola on board mars global surveyor and radar sounding data from marsis on board mars express we use the gravity anomalies outside the spld to construct a wiener filter which is applied to the gravitational signature of the spld to remove the short wavelength anomalies over the spld that are spectrally consistent with an origin in the crust or mantle we then use a constrained inversion for the vertically averaged density within the spld as a function of position the results suggest significant density variations within the spld an inverse relationship between the density and thickness of the spld suggests that thicker portions of the cap contain less dust alternatively the dorsa argentea formation may extend beneath the spld and result in the observed high gravity anomaly in the marginal area of the spld we find these conclusions to be robust against the choice of inversion constraint and perturbations to the applied filter a synthetic test is also performed to verify the recoverability of the density variation in our approach

potential anomalies on a sphere applications to the thickness of the lunar crust a new technique for calculating potential anomalies on a sphere due to finite amplitude relief has been developed we show that by raising the topography to the nth power and expanding this field into spherical harmonics potential anomalies due to topography on spherical density interfaces can be computed to arbitrary precision using a filter for downward continuing the bouguer anomaly we have computed a variety of crustal thickness maps for the moon assuming both a homogeneous as well as a dual layered crust the crustal thickness maps for the homogeneous model give plausible results but this model is not consistent with the seismic data petrologic evidence and geoid to topography ratios all of which suggest some form of crustal stratification several dual layered models were investigated and it was found that only models with both upper and lower crustal thickness variations could satisfy the gravity and topography data these models predict that the entire upper crust has been excavated beneath the major nearside multiring basins additionally significant amounts of lower crustal material was excavated from these basins especially beneath crisium this model also predicts that mantle material should not have been excavated during the south pole aitken basin forming event and that lower crustal material should be exposed at the surface in this basin

mars reconnaissance orbiter radio science gravity investigation 1 the objectives of the mars reconnaissance orbiter mro radio science gravity investigation are to improve knowledge of the static structure and characterize the temporal variability of the martian gravitational field relevant to the planet s internal dynamics the structure and dynamics of the atmosphere and the orbital evolution of spacecraft at mars the investigation will utilize range rate and range measurements from x band and when available ka band tracking systems of the mro spacecraft mro will enable a considerable improvement in the spatial resolution and quality of mars global gravity field the low orbital periapsis of mro 255 km will yield gravity maps suitable for study of regional 102 km structure of the crust and lithosphere the addition of tracking data from the mars global surveyor and mars odyssey spacecraft also currently orbiting mars will be useful in decorrelating errors in spherical harmonic coefficients of the gravity field that will improve the quality of the static field studies of the low degree gravity field combined with measurements of rotational dynamics will permit insight about the structure of mars deep interior changes in the low degree spherical harmonic coefficients of the martian gravity field and in polar mass anomalies will be used to track the seasonal cycle of co2 exchange with the surface measurements of spacecraft drag will be used to estimate density variations in the atmosphere relevant to weather patterns and aerobraking of future spacecraft tracking observations will also be used to improve the ephemeris of mars and the masses of the martian moons

results of the mars pathfinder atmospheric structure investigation we report on a thorough analysis of the mars pathfinder atmospheric structure investigation asi accelerometer data spanning the altitude range 161 8 9 km entry descent and landing occurred within 850 km of the viking 1 lander and somewhat later in northern summer the early morning entry 0300 hours provided the first opportunity to study mars nighttime atmospheric structure the close proximity to the viking 1 site has permitted a search for changes in atmospheric structure during the 21 years between the landings our results confirm and refine the major features of the atmospheric structure discovered in the initial analysis of the asi results reported by schofield et al 1997 the current analysis has yielded a much better definition of the temperature structure above 90 km altitude pathfinder measured a thermospheric peak temperature of 153 k at 134 km which is 30 k colder than the value found by viking 1 between 110 and 88 km pathfinder found a broad nearly isothermal plateau at 127 k which correlates very well with a similar structure evident in viking 1 data at the same pressures although the pathfinder layer is 25 k colder at levels above 88 km wavelike oscillations with a wavelength 5 km and amplitudes of 2 3 k are evident possibly reflecting gravity or planetary wave activity between 90 and 60 km the average temperature is 20 30 k cooler than viking 1 values large amplitude 10 20 k long wavelength 20 40 km oscillations which show a remarkable correspondence to equivalent structures found in the viking 1 profile at the same pressures occur in this region and are likely due to a diurnal tidal mode between 85 and 77 km a minimum of one oscillation results in temperatures up to 7 k below the saturation temperature of co2 well outside the uncertainties in the measurements and the vapor pressure curve and comparable to observed supersaturations of water in earth s mesosphere between 55 and 16 km temperatures are close to or warmer than viking 1 values superposed long wavelength oscillations correspond well with similar structures observed by viking 1 between 16 and 9 9 km the temperature profile shows an unexpected strong thermal inversion which may reflect radiative cooling due to a water cloud at the base of the inversion temperatures begin to increase abruptly until the last measurement at 8 9 km the viking like temperatures in the lower atmosphere are consistent with the viking like dust optical depths observed by the mars pathfinder imager the cool temperatures in the middle and upper atmosphere may be indicative of significant nighttime cooling at these levels although other interpretations are possible

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

the use of laser altimetry in the orbit and attitude determination of mars global surveyor altimetry from the mars observer laser altimeter mola an instrument on board the mars global surveyor mgs spacecraft has been analyzed for the period of the mgs science phasing orbit 1 spo 1 mission phase altimeter ranges have been used to improve significantly the orbit and attitude knowledge of the spacecraft by the use of crossover constraint equations derived from short passes of the mola data these constraint equations differ from traditional crossover constraints and exploit the small footprint associated with laser altimetry the rationale for using this technique with laser altimetry over sloping terrain is laid out and evidence of the resulting benefit is presented

utopia and hellas basins mars twins separated at birth 1 using topography and gravity data as constraints we formulate spherical harmonic thin elastic shell models to determine the subsurface structure of the hellas and utopia basins for hellas we show that our model is consistent with the elastic thickness results of mcgovern et al 2002 2004 the thin elastic lithosphere at the time of formation implies that hellas is close to isostatic since utopia formed earlier we argue that an isostatic assumption is justified for the utopia basin before it was filled from this supposition we derive a system of equations that allows us to solve for the amount of fill the prefill topography and the amount of flexure due to the fill within the utopia basin an analysis of the parameter space shows that the fill density and the amount of fill is strongly dependent on the elastic thickness at the time of infilling a thinner elastic lithosphere favors a denser fill while a thicker lithosphere will allow for less dense material likewise larger crustal thickness values lead to smaller fill density values the presence of quasi circular depressions interpreted as impact craters within the utopia basin indicates that the majority of the material within utopia was deposited prior to 4 04 4 11 ga the early timing for the deposition combined with the heat imparted by the basin forming event argues for a thinner lithosphere which could in turn suggest fill densities that are more consistent with a volcanic load than with pure sediment or ice rich material these results are supported using an alternative method of determining the amount of fill and flexure within utopia this model assumes that hellas and utopia were initially identical and that the only difference in their subsequent evolution was the addition of material in the utopia basin the volume of material needed to fill utopia is immense on the order of 50 million km3 or more the high density obtained for the fill requires that it contain a large igneous component the source of which is problematic relaxing the isostatic assumption to a reasonable degree perturbs the density bound only slightly

seasonal and static gravity field of mars from mgs mars odyssey and mro radio science abstract we present a spherical harmonic solution of the static gravity field of mars to degree and order 120 gmm 3 that has been calculated using the deep space network tracking data of the nasa mars missions mars global surveyor mgs mars odyssey ody and the mars reconnaissance orbiter mro we have also jointly determined spherical harmonic solutions for the static and time variable gravity field of mars and the mars k 2 love numbers exclusive of the gravity contribution of the atmosphere consequently the retrieved time varying gravity coefficients and the love number k 2 solely yield seasonal variations in the mass of the polar caps and the solid tides of mars respectively we obtain a mars love number k 2 of 0 1697 0 0027 3 the inclusion of mro tracking data results in improved seasonal gravity field coefficients c 30 and for the first time c 50 refinements of the atmospheric model in our orbit determination program have allowed us to monitor the odd zonal harmonic c 30 for 1 5 solar cycles 16 years this gravity model shows improved correlations with mola topography up to 15 larger at higher harmonics l 60 80 than previous solutions

the relationship between martian gravity and topography the relationship between gravity and topography of various regions of mars is used to estimate their effective elastic thicknesses te using direct measurements of line of sight velocity rather than spherical harmonic coefficients estimates of te vary from 70 km for tharsis 29 km for elysium to 14 5 km for the southern hemisphere and show that the thickness of the martian lithosphere increases with age as the radioactive isotopes of k th and u decay a simple parameterised model of the convective thermal history is used to estimate the temperature structure of the lithosphere and shows that the base of the elastic layer has a temperature of 300 50 c or similar to the value for terrestrial continents in both cases the rheology is probably affected by the presence of water the short wavelength behaviour of the gravity field allows the density of the rocks that form the topography to be estimated and gives values of about 3 0 mg m3 for tharsis and elysium this value is substantially greater than that of 2 7 mg m3 obtained for earth and is in agreement with estimates from snc shergottites nahklites chassigny meteorites of 3 3 mg m3 the density of the topography of valles marineris is only 2 35 mg m3 and suggests that ice may be present below the surface in the heavily bombarded southern hemisphere isostatic compensation occurs at wavelengths as short as 700 km which requires the effective compensation depth to be no more than 10 km the gravity field with wavelengths greater than 1500 km may be supported dynamically by a plume rising beneath the tharsis region the difference in temperature between the solidus and the present areotherm is less than 250 c so melt generation can occur in rising plumes

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

radio science investigations with mars observer mars observer radio science investigations focus on two major areas of study the gravity field and the atmosphere of mars measurement accuracies expressed as an equivalent spacecraft velocity are expected to be of the order of 100 m s for both types of investigations from use of an improved radio transponder for two way spacecraft tracking and a highly stable on board oscillator for atmospheric occultation measurements planned gravity investigations include a combination of classical and modern elements a spherical harmonic or equivalent field model of degree and order in the range 30 50 will be obtained while interpretation will be in terms of internal stress and density models for the planet using the topography to be obtained from the mars observer laser altimeter atmospheric investigations will emphasize precision measurement of the thermal structure and dynamics in the polar regions which are regularly accessible as a result of the highly inclined orbit studies based on the measurements will include polar processes cycling of the atmosphere between the poles traveling baroclinic disturbances small scale waves and turbulence the planetary boundary layer and possibly the variability and altitude of the ionosphere as the radio occultation is insensitive to dust in the atmosphere per se and measures only the resulting change in thermal structure it is expected that the radio technique can contribute to understanding of dust storm phenomena mutual observations of the atmosphere by means of radio occultation and by the pressure modulator infrared radiometer and the thermal emission spectrometer are expected to strengthen the reliability and accuracy of all three investigations