buried topography of utopia mars persistence of a giant impact depression knobs partially buried craters ring fractures and some mesas permit a qualitative determination of the topography buried beneath younger northern plains materials these features are widely distributed in the utopia area but are absent in a large roughly circular region centered at about 48 degree n 240 degree w this implies the existence of a circular depression about 3 300 km in diameter buried beneath utopia planitia that is here interpreted to represent the central part of a very large impact basin the presence of buried curved massifs around part of this depression and a roughly coincident mascon lend further support present topography areal geology and paleotopography of buried surfaces all point to the persistence of this major depression for almost the entire history of mars

a gravity survey of type 2 coronae on venus 1 coronae are roughly circular tectonic features believed to form as a result of small scale mantle upwellings we examine the admittance signature for all the 32 type 2 coronae defined as having less than 50 complete fracture annuli that are both well resolved in the gravity data and have fairly well constrained lithospheric parameters we use top and bottom loading models to derive values of elastic thickness and crustal thickness or apparent depth of compensation both cartesian and spatiospectral methods are used to calculate admittance overall we find reasonable agreement between the two methods the shape of the filter used in the spatiospectral method simons et al 1997 provides a means of isolating the signature of coronae that occur in close proximity to other geologic features the narrow filters used in the cartesian approach are useful for identifying transitions between different types of compensation as well as for estimating error we obtain a wide range of elastic thicknesses from 25 to 80 km larger values are derived from bottom loading models which have not been used in most prior studies of venus fifteen of the coronae have elastic thickness values between 0 and 20 km and are probably isostatically compensated estimates of crustal and elastic thicknesses and apparent depth of compensation obtained for type 2 coronae span the range obtained for venus globally neither the thickness of the elastic lithosphere nor the crust appear to control whether type 1 coronae type 2 coronae or volcanoes form over small scale mantle upwellings

lithospheric flexure and the origin of the dichotomy boundary on mars the crustal dichotomy between the ancient highlands of the southern hemisphere and the young appearing northern lowlands is a defining feature of mars in the eastern hemisphere the dichotomy boundary is marked by a prominent scarp and extensional and compressional tectonic features topographic data across the boundary returned from the mars global surveyor indicate lithospheric flexure of the southern highlands the topography of the boundary can be fit by a universal lithospheric deflection profile that corresponds to an elastic thickness of 31 36 km flexure of the southern highlands may be due to late noachian early hesperian vertical loading of the northern lowlands fracturing and normal faulting along the boundary may be in response to bending stresses while thrust faulting may result from a combination of stresses due to flexure erosion and global contraction

the shape and internal structure of the moon from the clementine mission global topographic and gravitational field models derived from data collected by the clementine spacecraft reveal a new picture of the shape and internal structure of the moon the moon exhibits a 16 kilometer range of elevation with the greatest topographic excursions occurring on the far side lunar highlands are in a state of near isostatic compensation whereas impact basins display a wide range of compensation states that do not correlate simply with basin size or age a global crustal thickness map reveals crustal thinning under all resolvable lunar basins the results indicate that the structure and thermal history of the moon are more complex than was previously believed

early plate tectonics versus single plate tectonics on mars evidence from magnetic field history and crust evolution 1 the consequences of an early epoch of plate tectonics on mars followed by single plate tectonics with stagnant lid mantle convection on both crust production and magnetic field generation have been studied with parameterized mantle convection models thermal history models with parameterized mantle convection not being dynamo models can provide necessary but not sufficient conditions for dynamo action it is difficult to find early plate tectonics models that can reasonably explain crust formation as is required by geological and geophysical observations and allow an early magnetic field that is widely accepted as the cause for the observed magnetic anomalies dating of crust provinces and topography and gravity data suggest a crust production rate monotonically declining through the noachian and hesperian and a present day crust thickness of more than 50 km plate tectonics cools the mantle and core efficiently and the core may easily generate an early magnetic field given a sufficiently weak mantle rheology plate tectonics can explain a field even if the core is not initially superheated with respect to the mantle because the crust production rate is proportional to temperature however an early efficient cooling will frustrate later crust production and therefore cannot explain for example the absence of prominent magnetic anomalies in the northern crustal province and the northern volcanic plains in the early hesperian voluminous crust formation following plate tectonics is possible if plate tectonics heat transfer is inefficient but then the crust growth rate has a late peak about 2 ga b p which is not observed these models also require a substantial initial superheating of the core to allow a dynamo if one accepts the initial superheating then as we will show a simple thermal evolution model with monotonic cooling of the planet due to stagnant lid mantle convection underneath a single plate throughout the evolution can better reconcile early crust formation and magnetic field generation

crustal history of north central arabia terra mars detailed geological studies across the highland lowland boundary in north central arabia terra mars provide constraints on the age and nature of processes responsible for a complex array of structures and landforms the area studied is bounded by 27 5 and 47 5 n 330 and 335 w highland basement was formed in early noachian and was extensively resurfaced in late noachian materials underlying highland plateaus were emplaced in middle noachian erosion responsible for fretted terrane and fretted channels most likely occurred in early hesperian but definitely prior to the middle of the hesperian and during or after middle noachian there is good evidence for the involvement of liquid water in fretted channel formation within the area mapped the presence of fretted terrane correlates with evidence for extensive development of large grabens all lowland materials were emplaced after the erosion that formed fretted terrane and channels the earliest of these materials were emplaced in mid hesperian and the youngest were emplaced in late amazonian morphological evidence supports debris flow or rock glacier processes for the formation of the youngest materials debris flow materials on the floors of fretted channels are much too young to be related to channel formation the entire region exhibits a northerly tilt of 0 1 0 15 that formed after deposition of the oldest lowland materials but before emplacement of upper hesperian and younger debris flow rock glacier materials the nature and ages of structures and landforms in this region are not consistent with the presence of a late noachian to early hesperian convergent plate boundary in northern arabia terra

relaxation of the martian dichotomy boundary faulting in the ismenius region and constraints on the early evolution of mars 1 the martian dichotomy is a global feature separating the cratered southern highlands and smooth northern lowlands parts of the boundary are defined by steep slopes with elevation differences of 2 6 km here we use constraints from the well preserved section of the dichotomy in the ismenius region we study the relaxation of the boundary using both finite element and semianalytical techniques to better constrain the thermomechanical conditions in early mars we argue that the steep slopes are the product of faulting resulting from relaxation of the edge of the boundary by lower crustal flow this relaxation requires that the lower crust must be at least 10 km thick and have a viscosity of 1021 pa s during the first 100 300 myr of relaxation we show that a wet crustal rheology and a thermal gradient of 15 20 k km at 4 ga allows for both the partial relaxation of the martian dichotomy and preservation of long wavelength topography as observed on mars this result is generally consistent with a stagnant lid model of cooling of martian interior the thermal gradient at 4 ga is also consistent with the estimated elastic thickness on early mars our modeling constrains the original slope of the boundary to approximately 2 we cannot constrain the crustal thickness more tightly than the current estimates of 50 100 km on the basis of our semianalytical modeling the relaxation of the boundary can contribute to the general north south slope observed on mars

lithospheric flexure and the evolution of the dichotomy boundary on mars 1 the boundary of the martian crustal dichotomy in the eastern hemisphere is one of the most striking topographic features on the planet the long wavelength topography of much of the boundary is expressed by a broad rise and an arched ramp that slopes downward from the southern highlands into the northern lowlands and often ends in a steep scarp lithospheric flexure of the southern highlands for both a continuous and broken plate boundary with the northern lowlands lithosphere is modeled we find that the long wavelength topography of the boundary is best fit by a lithospheric deflection profile of a broken lithosphere for reasonable values of the flexural parameter the lithosphere near the dichotomy boundary may have been weakened by tectonic stress associated with the formation of the crustal dichotomy by subcrustal transport caused by overturn of an early magma ocean

dust and ice deposition in the martian geologic record abstract the polar layered deposits of mars demonstrate that thick accumulations of dust and ice deposits can develop on the planet if environmental conditions are favorable these deposits appear to be hundreds of millions of years old and other deposits of similar size but of greater age in nonpolar regions may have formed by similar processes possible relict dust deposits include from oldest to youngest noachian intercrater materials including arabia mantle deposits noachian to early hesperian south polar pitted deposits early hesperian hellas and argyre basin deposits late hesperian electris deposits and the amazonian medusae fossae formation these deposits typically are hundreds of meters to a couple kilometers thick and cover upward of a million or more square kilometers the apparent persistence of dust sedimentation at the south pole back to the early hesperian or earlier and the early growth of tharsis during the late noachian and perhaps earlier indicates that extensive polar wandering is unlikely following the middle noachian a scenario for the overall history of dust and perhaps ice deposition on mars includes widespread voluminous accumulations perhaps planetwide during the noachian as impacts volcanism and surface processes generated large amounts of dust the arabia deposits may have formed as ice availability and dust accumulation waned during the early hesperian thick dust sedimentation became restricted to the south pole and the deep hellas and argyre basins the north polar sedimentary record prior to the amazonian is largely obscured deposits at electris and medusae fossae may have resulted from local sources of fine grained material perhaps volcanic eruptions

serpentinization of the martian crust during noachian abstract this paper proposes a model of serpentinization of the southern martian crust that may explain the topographic dichotomy the absence of an associated free air gravity anomaly and the presence of strong magnetic anomalies in the southern hemisphere the thermodynamical conditions for serpentinization were likely met in the lithosphere during the noachian period this process may have decreased the density in the southern crust and created the topographic dichotomy different reactions of serpentinization that can form magnetite have been considered assuming an intense magnetic field core dynamo we obtain chemical remanent magnetizations that are in the order of the estimates deduced from martian magnetic anomaly studies the pertinence and the implications of our model concerning the early thermal evolution of mars are discussed with emphasis on the intensity of the paleo magnetic field

thermoremanent magnetization of the martian lithosphere 1 this paper investigates the thermoremanent magnetization trm of the martian lithosphere assuming that the upper part of the lithosphere acquired trm in the early history of the planet and in the presence of the core field the primary magnetization whereas the lower part has been gradually magnetized by the magnetic field of the upper part as it has cooled below the curie temperature secondary magnetization the secondary magnetization is relatively weak if the magnetic properties of the lower lithosphere are taken to be similar to that of the extrusive basalt near the oceanic ridge axes on earth however if the martian lower lithosphere is more magnetic e g by an order of magnitude the secondary magnetization can be significant although still weaker than the primary magnetization of the upper part such a highly magnetic lower lithosphere does not seem plausible by terrestrial standards and it remains to be confirmed whether it is geologically plausible for mars i also examine a lithospheric model with a strongly magnetic layer beneath the crust 25 times more magnetic than the extrusive basalt representing a possible ilmenite rich layer with a considerable amount of hematite ilmenite lamellae i assume that the layer was not magnetized by the core field because it was hotter than the curie temperature of the lamellae when the core dynamo was active it acquired secondary magnetization later in the presence of the magnetic field of the crust despite adopting an extraordinarily high magnetic layer which seems implausible by terrestrial standards the secondary magnetization of the layer has minor contributions to the observed magnetic anomalies in all models considered the primary magnetization is 20 30 a m and has the dominant contribution to the observed magnetic anomalies it is worth noting that the radial components of the primary and secondary magnetization are parallel whereas the tangential components are in opposite directions because the strong magnetic anomalies in the south likely arise from the tangential magnetization considering the secondary magnetization requires even stronger primary magnetization

estimating hydraulic conductivity of a fine textured soil using tension infiltrometry abstract tension infiltrometry has become quite popular to determine near saturated hydraulic properties of soils and to examine the effects of macropores on infiltration the unsaturated hydraulic conductivity has traditionally been determined from steady state tension infiltrometer data using wooding s analytical solution recently hydraulic parameters are increasingly being estimated from transient data using a numerical solution of the flow problem and nonlinear parameter optimisation this inverse approach is convenient but its accuracy will vary with soil type in this study tension disc infiltration data obtained on a tilled clay soil located in the basilicata region in southern italy were analysed using both the traditional analytical and the numerical inversion approach three different parametric models were used for the unsaturated hydraulic conductivity for wooding s solution the two line exponential and the bimodal piecewise continuous models yielded similar results while the mualem van genuchten mvg model provided a somewhat poorer description of the cumulative infiltration the estimates for k s were one to two orders larger than those reported for core samples in soil databases the optimised hydraulic functions revealed the presence of a bimodal pore system with a break point in pressure head at 30 mm excellent agreement between measured and fitted infiltration curves was obtained for the numerical inversion procedure the hydraulic conductivities obtained with wooding s solution were systematically higher than those obtained from the inverse solution the difference is presumably due to a lack of steady state conditions and was greater than found in previous studies on coarser soils the greatest difference about 50 occurred at lower pressure heads the inverse solution may produce better results if another parametric model is used for the hydraulic properties instead of the mualem van genuchten function

transitional morphology in west deuteronilus mensae mars implications for modification of the lowland upland boundary abstract west deuteronilus mensae which lies along the lowland upland boundary of mars at 45 latitude 345 longitude exhibits both gradational l a rossbacher 1985 in models in geomorphology woldenberg ed pp 343 372 allen unwin boston and fretted terrain r p sharp 1973 j geophys res 78 4073 4083 boundary types to the west the boundary is gradational and to the east it is defined by fretted terrain the two types of boundaries do not merge with one another within the west deuteronilus mensae region however the gradational boundary materials appear to overlap the fretted terrain by about 500 km contacts between units associated with the gradational boundary and the fretted terrain boundary can both be recognized in the region of overlap the fretted terrain can be identified as much as 300 km north of the gradational boundary in this region lowland units associated with the gradational boundary embay canyons of the fretted terrain in a topographically conformal fashion changes in the fretted terrain across the gradational boundary from uplands to lowlands include a reduction of canyon wall slopes and depths such that the fretted terrain north of the gradational boundary appears mantled but not obscured there are at least two major classes of processes which might explain the lateral overlap 1 erosion of stratified upland terrain and 2 deposition of plains materials onto the sloping upland margin and fretted terrain erosion of stratified upland terrain does not adequately explain the plainward decrease in crater densities across the gradational boundary nor is it consistent with evidence that lowland plains material appears to onlap the sloping upland margin in several places of the possible plains emplacement mechanisms eolian deposition would not produce the sharp apparently topographically conformal gradational unit contacts volcanics plains emplacement would not preserve the complex geometry of the underlying fretted terrain sediment deposition in either a liquid or ice covered sea could produce the draped appearance of the fretted terrain outflow channels along the lowland upland boundary particularly those of the circum chryse and west elysium regions may have flooded the northern lowlands to depths of tens to hundreds of meters the gradational unit contacts may represent the shorelines of such a sea the scale of characteristic morphologies along these contacts may require an unfrozen condition of sufficient duration to allow lacustrine style wave erosion and redistribution of material along the contacts our limited understanding of the martian paleoclimate and h 2 o inventories allows the possibility of clement periods in the past and other geological evidence e g small valley networks and outflow channels strongly suggests an extensive role of liquid water

the martian hemispheric dichotomy may be due to a giant impact mars is divided into two fundamentally different geological provinces of approximately hemispherical extent1 3 the more southerly province is heavily cratered contains relatively old geological units and superficially resembles the lunar and mercurian highlands the northern province is relatively lightly cratered and contains younger geological units including extensive plains volcanic edifices and volcanic calderas except for the tharsis and elysium regions and other large volcanoes most of the younger northern province consists of lowlands which lie an average of 3 km below the highlands lowlands occupy about one third of mars they are separated from the highlands by a distinct scarp or by a sloping transitional zone as much as 700km wide in which highland materials have been disrupted and partly replaced by lowland deposits fig 1 the transition is characterized by a variety of landforms unknown on other planets the highlands and lowlands are in isostatic equilibrium across the transitional zone4 no generally accepted explanation for the cause of the highland lowland dichotomy has been proposed although thinning of the lithosph re in the northern hemisphere by internal processes has been suggested5 large impacts which were a feature of early solar system history have also been suggested as an important cause of such early planetary surface heterogeneities3 6 7 subcircular lowlands such as argyre chryse hellas and isidis planitia and the plains south of tharsis occupy previously recognized impact basins8 10 we propose here that the largest expanse of lowlands also results from formation of a large impact basin early in mars history that has profoundly shaped the planet s surface

the benefit of hh and vv polarizations in retrieving extreme wind speeds for an ascat type scatterometer the wind retrieval performance of a fixed fan beam scatterometer operating at c band in vv polarization advanced scatterometer ascat type is well established in the range of 0 25 m s this work evaluates prospective extensions with hh and vh polarized beams aimed at improving retrievals at extreme wind speeds from 25 to 65 m s the geophysical model functions for c band vv hh and vh polarized backscatter used in the wind retrieval simulations are defined along with an objective assessment of the quality scores ascribed to the optional beam configurations our study finds that the introduction of a vh capability in the midbeam improves the determination of wind speed over the entire scatterometer swath with wind speed root mean square rms errors of about 0 5 m s the introduction of hh beams in the fore aft antennas improves the determination of the ocean surface wind vector wind speed and direction but over a more limited portion of the outer swath with wind vector rms errors as low as 6 m s but conditioned to a priori information to resolve the directional ambiguity if the determination of wind speed primes over the determination of wind vectors at high wind speeds then the introduction of a single vh capability in the midbeam antenna comes forth as the most simple and cost effective manner to extend the wind retrieval capabilities of an ascat type scatterometer into the domain of extreme winds

evidence for igneous activity and implications for the origin of a fretted channel in southern ismenius lacus mars northern arabia terra mars is characterized by a complex landscape that includes fretted terrain fretted channels completely closed canyons shallow troughs and fracture bounded depressions the importance of ground ice in the initiation and development of these features and of large impact structures in controlling channel courses has long been recognized the development of the fretted channel systems including the integration of closed canyons craters and surface depressions into throughgoing channels has previously been attributed primarily to mass wasting processes the two channel systems in southern ismenius lacus quadrangle mamers valles and ismeniae fossae incorporate craters of probable impact origin but both also include irregular crater like features and scalloped channel reaches that have characteristics not consistent with an impact origin some of these are by analogy with morphologically similar features on earth and elsewhere on mars reasonably interpreted as of volcanic origin the detailed morphologies of a few diagnostic features associated with ismeniae fossae suggest a hydrovolcanic origin an inference supported by the probable presence of abundant ground ice or groundwater at the time of their formation the evidence described here for hydrovolcanism supports the inference that in addition to mass wasting processes explosive excavation intrusion driven sublimation subsurface erosion and erosion by running water may have played important roles in channel formation and evolution as well all of these processes are consistent with magma interacting with groundwater or ground ice

growth of the hemispheric dichotomy and the cessation of plate tectonics on mars 1 although mars is currently not tectonically active it may have experienced plate tectonics early in its history the southern hemisphere of mars possesses a thick crust which probably renders the lithosphere positively buoyant in this paper we present numerical and scaling arguments which show that if the area of positively buoyant lithosphere grows beyond a critical fraction 50 for mars plate tectonics will stop heat transfer through the buoyant lithosphere is inefficient which causes mean mantle temperatures to increase as the surface area of buoyant lithosphere increases the resulting reduction in mantle viscosity reduces shear stresses if these shear stresses drop below the yield strength of the lithosphere plate motions will cease and the planet will behave as a one plate system thus the end of plate tectonics on mars is a natural consequence of the growth of the southern highlands similar arguments for the earth predict that it should operate in the plate tectonic regime now but that it may have experienced stagnant lid convection in the past index terms 5475 planetology solid surface planets tectonics 8149 5430 planetology solid surface planets interiors 8147 5418 planetology solid surface planets heat flow 5455 planetology solid surface planets origin and evolution keywords hemispheric dichotomy mantle convection thermal evolution rheology

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

climate variability on the south eastern tibetan plateau since the lateglacial based on a multiproxy approach from lake naleng comparing pollen and non pollen signals a multi proxy lateglacial environmental record is described from lake naleng 31 10 degrees n 99 75 degrees e 4200 m above sea level situated on south eastern tibetan plateau to gain deeper insights into the hydrological and palaeoclimate development since 17 7 cal ka bp palynological reconstructions of variations in mean annual precipitation map and temperature mat sedimentological data and sediment chemistry including weathering indicators provide a multi faceted picture of local and regional environmental changes since the lateglacial principal component analyses of all parameters provide information on interrelationships between each parameters which help to evaluate their traceability to temperature and precipitation and to estimate their usability as proxy indicators for local and or regional variations during the lateglacial from 17 7 to 14 0 cal ka bp lake naleng experienced cold and dry climate conditions with low biological productivity and supply of unaltered fine grained material due to the high supply of glacier milk during the second half of the lateglacial climate conditions changed abruptly increases in mat from 4 to 2 2 degrees c and map from 500 mm to 820 mm between 14 0 and 13 0 cal ka bp indicate a climate amelioration this time interval can be correlated to the bolling allerod b a warming period in the north atlantic region and is followed by the younger dryas cold reversal indicated by abrupt decreases of mat from 2 2 to 5 degrees c and map from 820 to 650 mm the onset of the holocene at about 11 5 cal ka bp is indicated by rises in reconstructed mat from 5 to about 03 degrees c and map from 600 mm to 950 mm which led to an increased supply of weathered material and higher biological productivity between 5 0 and 3 0 cal ka bp mat increases to about 0 2 degrees c and map rises to maximum values of about 1000 mm followed by slightly decreasing mat and map between 3 0 and 0 cal ka bp the biogeochemical parameters total organic carbon toc c n delta c 13 org and weathering indicators e g the chemical index of alteration cia and sr ba are directly erosion of soils or indirectly changing provenance related to moisture availability on the south eastern tp and shows matching regional climate oscillations since the lateglacial in comparison to other lateglacial records from the tp map reconstructions from lake naleng indicate wetter climate conditions in the south eastern part of the tp and dryer conditions farther away from moisture sources c 2015 elsevier ltd all rights reserved

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

hemispheres apart the crustal dichotomy on mars the hemispheric dichotomy is a fundamental feature of mars expressed by a physiographic and geologic divide between the heavily cratered southern highlands and the relatively smooth plains of the northern lowlands the origin of the dichotomy which may have set the course for most of the subsequent geologic evolution of mars remains unclear internally driven models for the dichotomy form the lowlands by mantle convection plate tectonics or early mantle overturn externally driven models invoke one giant impact or multiple impacts areal densities of buried basins expressed by quasicircular depressions and subsurface echoes in radar sounding data suggest that the dichotomy formed early in the geologic evolution of mars tectonic features along the dichotomy boundary suggest late stage modification by flexure or relaxation of the highlands after volcanic resurfacing of the northern lowlands subsequent deposition and erosion by fluvial aeolian and glacial processes shaped the present day dichotomy boundary

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

early transient cooling of mars previous evolution models for one plate planets have commonly assumed steady state cooling beneath a rigid lid recent 3d numerical models indicate however that conductive lid cooling exhibits early transient stages where the quasi static assumption is not valid we apply scaling laws for these transient stages to mars a wide range of initial conditions for the martian mantle reflects the possible consequence of the early differentiation of the planet our results display large variations for the timing of early transient cooling from several 100 ma up to a few ga indicating that classical quasi static models cannot describe a significant part of the planet s evolution this suggests new scenarios for mars early history with initial heat fluxes in good agreement with values inferred from both gravity and topography data from the mgs mission

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

origin of the martian global dichotomy by crustal thinning in the late noachian or early hesperian the marked dichotomy in topography surface age and crustal thickness between the northern lowland and southern upland of mars has been explained as due to an initially inhomogeneous crust a single mega impact event several overlapping large basin impacts and first order convective overturn of the martian mantle all of the published hypotheses propose that the dichotomy was formed early in martian history before the end of the primordial heavy bombardment a primordial origin is inherent in the initial crustal inhomogeneity hypothesis and required for both impact hypotheses endogenic hypotheses are not so constrained geological data indicate episodes of fracturing and faulting in the late noachian and the early hesperian this fracturing and faulting occurred primarily within the northern lowland and along the boundary between lowland and highland igneous activity also peaked in the late noachian and early hesperian these data suggest a tectonic event near the noachian hesperian boundary characterized by enhanced heat loss and extensive fracturing including formation of the faults that define much of the highland lowland boundary we argue that the major result of this tectonic event was formation of the dichotomy by thinning of the crust above a large convection cell or plume

evidence for extensive denudation of the martian highlands high resolution topographic data from the mars orbiter laser altimeter reveal evidence for widespread denudation from margaritifer sinus to northern arabia terra an area of 1 107 km2 a major resurfacing event is indicated by 1 a heavily degraded landscape hosting numerous inliers 2 truncation or absence of valley networks and 3 a break in slope at the edge of early to middle noachian plateau materials geomorphic mapping was completed on the type locale of denudation 0 30 s 0 30 w superposition relations and crater counts for geomorphic units indicate that large scale resurfacing took place in the late noachian eroding the martian highlands and resulting in transportation and deposition of 4 5 106 km3 of sediment in the northern plains this is equivalent to a 120 m thick uniform layer of sediment on the surface of mars north of 30 n geomorphic mapping and crater counts limit the timing of denudation to the late noachian an interval of 350 500 m y using this limit we estimate a minimum rate of denudation of 2 0 m yr comparable to denudation of typical slopes in a temperate maritime climate on earth the morphology rate of denudation and extensive nature of upland degradation suggest that precipitation fed surface runoff is the most likely geomorphic agent capable of such a process indicative of a warm wet mars during the late noachian epoch

degree 1 mantle convection and the crustal dichotomy on mars the surface of mars consists of an old heavily cratered and elevated southern hemisphere and younger resurfaced and depressed northern hemisphere a feature often termed the crustal dichotomy the global crustal structure zuber et al 2000 revealed by topography and gravity data from the mars global surveyor spacecraft and the possible late formation of the boundary zone between the hemispheres mcgill and dimitriou 1990 have been proposed to indicate an endogenic origin for the dichotomy however degree 1 mantle convection that is required for any endogenic process to be viable cannot be produced with conventional mantle convection models schubert et al 1990 we have studied the role of radially stratified viscosity on mantle deformation by using rayleigh taylor instability analyses in a spherical shell geometry our analyses reveal that when mantle viscosity is stratified with a weak asthenosphere deformation at long wavelengths is more efficient than that at short wavelengths the weaker the asthenosphere the longer the wavelength at which the deformation is the most efficient a thicker asthenosphere also favors the deformation at long wavelengths both the rayleigh taylor instability analyses and numerical modeling of mantle convection show that degree 1 convection can be produced within the martian mantle provided that the mantle had a weak asthenosphere v500 km thick and v10 2 times weaker than the underlying mantle early in planetary history the degree 1 convection causes preferential heating of one hemisphere that may explain the primary features associated with the dichotomy in crustal structure fl 2001 elsevier science b v all rights reserved

the density and porosity of meteorites from the vatican collection abstract we have developed a nondestructive technique to measure the densities and porosities of large 50 g to several kilogram meteorite samples and have begun a systematic measurement program starting with meteorites from the vatican observatory collection our technique utilizes a modified archimedean method 40 m glass spheres substituting for the fluid to determine bulk density and an unusually large he pycnometer to determine grain density from these the porosity is calculated we report here our first results for 89 samples nearly doubling the published database on porosities of meteorite hand samples including porosities of 35 different meteorites these include several types not previously reported eight iron meteorites six stony iron meteorites an enstatite chondrite the ci carbonaceous chondrite orgueil and the shergottite nakhlatite chassignite snc meteorite nakhla

dual polarized c and ku band ocean backscatter response to hurricane force winds 1 airborne ocean backscatter measurements at c and ku band wavelengths and h and v polarizations at multiple incidence angles obtained in moderate to very high wind speed conditions 25 65 m s 1 during missions through several tropical cyclones are presented these measurements clearly show that the normalized radar cross sections nrcs response stops increasing at hurricane force winds for both frequency bands and polarizations except for high incidence angles at c band and h polarization the results also show the mean nrcs departing from a power law behavior for all the presented frequency bands polarizations and incidence angles suggesting a reduction in the drag coefficient the overall flattening of the azimuthal response of the nrcs is also very apparent in all cases a new set of geophysical model functions gmfs at c and ku band are developed from these direct ocean backscatter observations for ocean surface winds ranging from 25 to 65 m s 1 the developed gmfs provide a much more accurate characterization of the nrcs versus wind speed and direction and their implementation in operational retrieval algorithms from satellite based scatterometer observations would result in better wind fields the differences between these measurements and other currently available gmfs such as quikscat nscat2 cmod4 and cmod5 are reported the implementation of these gmfs in retrieval algorithms will result in better wind fields from satellite based scatterometers measurements

origin of the martian crustal dichotomy evaluating hypotheses abstract any hypothesis for the origin of the martian global dichotomy should survive three elementary tests 1 it must account for the observed plan shape and apparent depth of the martian northern lowland 2 it must be physically consistent and 3 it must be compatible with available geological and geophysical data at present there are three contending types of hypotheses for the origin of the dichotomy creation by some endogenic process or processes creation by a single mega impact or creation by several overlapping large impacts none of these hypotheses can survive all three tests without the incorporation of additional processes the endogenic and mega impact hypotheses require the presence of additional impact basins to explain many of the topographic details of the martian northern lowland and of the dichotomy boundary and the mega impact hypothesis probably also requires extensive primordial erosion the multiple impact hypothesis requires an additional process or processes to account for the large portion of the northern lowland that is external to the rims of the basins inferred to be the cause of the lowland while we believe that aspects of the multiple impact hypothesis are required to account for some topographic details of the boundary and of the lowland other processes appear better able to account for the dichotomy as a whole these could include an early mega impact endogenic processes or some combination of these

phase transitions in the martian mantle implications for partially layered convection abstract numerical simulations of mantle convection in mars using an axisymmetric spherical shell model show partial layering caused by the two exothermic olivine spinel phase transitions an extended boussinesq approximation has been used in which viscous dissipation adiabatic heating and cooling and latent heat are included the rayleigh number ra has been varied between 5 105 and 108 the partial layering with the vertical velocity at the exothermic phase transitions varying strongly in space and time is the result of two opposing effects the enhanced buoyancy of the phase boundaries by thermal anomalies and the impeding influences from the latent heat release or consumption the effect of the latent heat is stronger in mars than the earth because of the comparatively low pressure gradient in the martian mantle and the smaller excess temperature of upwellings and downwellings the time series of the mean vertical mass transport across the phase transitions show oscillations between blocking and acceleration of the flow the amplitude and the oscillations in the time series increase with increasingra because of the partial layering the planet will cool more slowly and less uniformly than suggested by thermal evolution models with parameterized convection in addition the number of strong mantle plumes is reduced to only a few upwellings such a pattern is suggested for mars by the existence of two pronounced volcanic centers tharsis and elysium this could also cause a strong time dependence in the martian volcanic activity the latent heat release causes the mantle temperature to increase across each transition by about 50 k and produces a hot lower mantle and a liquid core we have tested the case of a 85 350 km thick perovskite layer at the core mantle boundary a layer thicker than about 300 km would convect separately and induce leaking to the mantle above at a significantly smaller rate compared to the layers induced by the olivine spinel phase boundaries for a perovskite layer smaller than about 300 km the convective vigor near the core mantle boundary decreases with the layer thickness

a dynamic origin for the global asymmetry of lunar mare basalts we propose that the hemispheric asymmetry fig 1 of mare basalts may be explained as a result of hydrodynamic instabilities associated with a layer of mixed ilmenite rich cumulates ic and olivine orthopyroxene opx overlying a metallic core this mixed layer mic should form shortly after the solidification of the magma ocean p c hess and e m parmentier earth planet sci lett 134 1995 501 514 because of gravitational differentiation of chemically dense ic material that is expected to form below the anorthositic crust in the final stages of magma ocean crystallization a e ringwood and s e kesson proc lunar planet sci conf 7 1976 1697 1722 ic material is rich in heat producing elements and thermal expansion due to radiogenic heating causes the mic layer to become less dense than overlying mantle the time required for the mic layer to become thermally buoyant may explain a delay of mare volcanism until about 500 ma after solidification of the magma ocean our analyses of the resulting rayleigh taylor instability and numerical modeling of thermo chemical convection show that the instabilities produce spherical harmonic degree 1 thermal and compositional structure if a lunar metallic core is sufficiently small less than 250 km in radius fl 2000 elsevier science b v all rights reserved

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

strength of martian lithosphere beneath large volcanoes this paper provides an estimate of the thermal state of martian lithosphere established since the formation of the shield volcanoes comparison of the spherical harmonic models of martian topography derived using harmonics truncated at degrees 10 11 12 and 14 shows that the volcanoes have almost no contribution from harmonic coefficients of the topography less than degree 11 therefore intermediate scale surface topography and gravity anomalies of mars specified by harmonics of degree 11 50 are considered in this study these harmonics are dominated by shield volcanoes alba patera and isidis basin the central part of valles marineris is also well defined by these harmonics the strong correlation of the topography and gravity anomalies over these harmonics and the positive bouguer anomalies of the volcanoes suggest significant excess masses beneath these surface features in addition to their topographic masses this indicates that the martian lithosphere has been strong enough to support both the topography and the internal excess masses we determine the rigidity of the lithosphere beneath a given surface feature the lithosphere is modeled by a thin elastic spherical shell that overlies an inviscid interior of higher density and is subjected to both surface and internal loads the thickness of the shell and the magnitude of the internal load are calculated such that the deformed structure resulting from the flexure of the shell under both surface and internal loads gives rise to the observed topography and gravity anomaly it is shown that the lithosphere beneath elysium mons alba patera and the central part of valles marineris can be modeled by elastic shells subjected to topographic loads alone whereas the lithosphere beneath olympus and tharsis montes cannot internal excess masses of at least 40 and more likely 50 of the topographic masses are required and the lithosphere beneath must have an elastic core of 100 80 km to support these volcanoes the mean thermal gradient beneath this region of tharsis is estimated to be 7 10 k km on the basis of the strength envelopes calculated using diabase rheology for the crust of 50 km thickness and olivine rheology for the underlying mantle

buried mass anomalies along the hemispheric dichotomy in eastern mars implications for the origin and evolution of the dichotomy 1 gravity observations indicate the presence of buried high density material along the hemispheric dichotomy in eastern mars this material is unrelated to present day topography and is probably the result of localized thinning of the crust this thinning may be the result of an epoch of edge driven convection that occurred shortly after the dichotomy formed initiation of edge driven convection requires that lateral variations in lithospheric structure be created on a timescale that is shorter than the conductive cooling time for the lithosphere a few tens of million years at most this timescale cannot be achieved if the dichotomy boundary is created solely by large scale convective flow formation or modification of the boundary by large impact basins such as utopia can create the required lithospheric structure in a geologic instant this suggests that large impacts were important in shaping the dichotomy at least on a regional scale

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

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

martian plate tectonics the northern lowlands of mars may have been produced by plate tectonics preexisting old thick highland crust was subducted while seafloor spreading produced thin lowland crust during late noachian and early hesperian time in the preferred reconstruction a breakup margin extended north of cimmeria terra between daedalia planum and isidis planitia where the highland lowland transition is relatively simple south dipping subduction occurred beneath arabia terra and east dipping subduction beneath thatsis montes and tempe terra lineations associated with gordii dotsum are attributed to ridge parallel structures while phelegra montes and scandia colles are interpreted as transform parallel structures or ridge fault fault triple junction tracks other than for these few features there is little topographic roughness in the lowlands seafloor spreading if it occurred must have been relatively rapid quantitative estimates of spreading rate are obtained by considering the physics of seafloor spreading in the lower c 0 4 g gravity of mars the absence of vertical scarps from age differences across fracture zones and the smooth axial topography to the first order the height of vertical scarps across fracture zones does not involve gravity crustal thickness at a given potential temperature in the mantle source region scales inversely with gravity thus the velocity of the rough smooth transition for axial topography also scales inversely with gravity plate reorganizations where young crust becomes difficult to subduct are another constraint on spreading age possible plate reorganizations for example the end of spreading through alba patera occur when the ridge axis is far from the trench that is rapid plate motions are inferred to have placed young oceanic crust far from the ridge axis the preferred full spreading rate 900 from the plate pole is 80 mm yr 1 plate tectonics if it occurred dominated the thermal and stress history of the planet a geochemical implication is that the lower gravity of mars allows deeper hydrothermal circulation through cracks and hence more hydration of oceanic crust so that more water is easily subducted than on the earth age and structural relationships from photogeology as well as median wavelength gravity anomalies across the now dead breakup and subduction margins are the data most likely to test and modify hypotheses about mars plate tectonics

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

impact of alpine meadow degradation on soil hydraulic properties over the qinghai tibetan plateau summary alpine meadow soil is an important ecosystem component of the qinghai tibetan plateau however the alpine meadow soil is undergoing serious degradation mainly due to global climate change overgrazing human activities and rodents in this paper spatial sequencing was chosen over time succession sequencing to study the changes of soil hydraulic properties under different degrees of alpine meadow degradation soil saturated hydraulic conductivity ks and gardner both at the surface and at 40 50 cm depth were investigated in the field using tension infiltrometers soil physical and chemical properties together with the root index at 0 10 cm and 40 50 cm soil layer depths were also analyzed pearson correlations were adopted to study the relationships among the investigated factors and principal component analysis was performed to identify the dominant factor results show that with increasing degree of degradation soil sand content increased while soil ks and gardner as well as soil clay content soil porosity decreased in the 0 10 cm soil layers and organic matter and root gravimetric density decreased in both the 0 10 cm and 40 50 cm soil layers however soil moisture showed no significant changes with increasing degradation with decreasing pressure head soil unsaturated hydraulic conductivity reduced more slowly under degraded conditions than non degraded conditions soil ks and gardner were significantly correlated p 0 01 with bulk density soil porosity soil organic matter and root gravimetric density among these soil porosity is the dominant factor explaining about 90 of the variability in total infiltration flow under non degraded conditions the infiltration flow principally depended on the presence of macropores with increasing degree of degradation soil macropores quickly changed to mesopores or micropores the proportion of total infiltration flow through macropores and mesopores significantly decreased with the most substantial decrease observed for the macropores in the 0 10 cm soil layer the substantial decrease of macropores caused a cut in soil moisture and hydraulic conductivity this study improves the understanding and prediction of alpine meadow soil and ecosystem changes and provides guidelines for improving water flow modeling under the background of global climate change over the qinghai tibetan plateau and similar regions

new piecewise continuous hydraulic functions for modeling preferential flow in an intermittent flood irrigated field modeling water flow in macroporous field soils near saturation has been a major challenge in vadose zone hydrology using in situ and laboratory measurements we developed new piecewise continuous soil water retention and hydraulic conductivity functions to describe preferential flow in tile drains under a flood irrigated agricultural field in las nutrias new mexico after incorporation into a two dimensional numerical flow code chain 2d the performance of the new piecewise continuous hydraulic functions was compared with that of the unimodal van genuchten mualem model and with measured tile flow data at the field site during a number of irrigation events model parameters were collected estimated by site characterization e g soil texture surface subsurface saturated unsaturated soil hydraulic property measurements as well as by local and regional scale hydrologic monitoring including the use of groundwater monitoring wells piezometers and different surface irrigation and subsurface drainage measurement systems comparison of numerical simulation results with the observed tile flow indicated that the new piecewise continuous hydraulic functions generally predicted preferential flow in the tile drain reasonably well following all irrigation events at the field site also the new bimodal soil water retention and hydraulic conductivity functions performed better than the unimodal van genuchten mualem functions in terms of describing the observed flow regime at the field site

three dimensional models of martian mantle convection with phase transitions we have employed a three dimensional compressible convection model to study the dynamics of phase transitions in the martian mantle a large core model with two exothermic phase transitions the olivine to spinel and the to spinel transition and a small core model including also the endothermic spinel to perovskite transition have been considered the two exothermic transitions create thermal barriers for small upwellings due to the latent heat consumption from the phase change upwelling plumes lose part or all of their buoyancy which causes the formation of one stable area full of plumes this tendency for the merging of plumes increases with internal heating this type of convective planform is consistent with the relatively few large volcanic centers the presence of a 175 km thick perovskite layer above the core mantle boundary cmb yields a similar flow pattern albeit with an even smaller number of plumes however the excess temperatures of the plumes and the mantle flow velocities in the lower mantle are smaller than those found in models without perovskite layer the phase transitions cause an increase of temperature near the cmb which prevents the lower mantle and the core from extensive cooling a model with a perovskite layer decreasing in thickness with time can account for a peak in volcanic and magnetic activity early in the martian history

thrust faults along the dichotomy boundary in the eastern hemisphere of mars 1 mars orbiter laser altimetry mola data have been used to characterize lobate scarps along the dichotomy boundary in the eastern hemisphere of mars these structures are the surface expression of thrust faulting of the southern highlands displacement on the thrust faults is generally less than 1 km except in the case of the amenthes rupes thrust fault where the maximum displacement is on the order of 3 km the ratio of maximum displacement to fault length of the population of thrust faults is 6 2 10 3 n 26 consistent with previous estimates for martian thrust faults based on photoclinometrically derived topography and other planetary thrust fault populations thrust faulting is roughly coincident with late noachian to early hesperian fracturing and normal faulting along the dichotomy boundary this suggests that the formation of the present day dichotomy boundary involved compressional and extensional deformation the long wavelength topography suggests that the dichotomy boundary in the eastern hemisphere may have been formed by flexure of the southern highlands lithosphere lithospheric flexure alone cannot account for the thrust faulting of the highlands along the dichotomy boundary a combination of stresses due to bending erosion and global contraction may have formed the lobate scarp thrust faults

the mola topographic signature at the crustal dichotomy boundary zone on mars mola data collected during the aerobraking hiatus provide good longitudinal sampling of the crustal dichotomy boundary zone in eastern mars and reveal a significant topographic signature associated with the transition from cratered highlands to lowland plains total relief between the two major units is always 2 5 km and sometimes 6 km median elevation differences are about 4 km regional slopes are very low 0 02 in both cratered terrain south and in lowland plains north of the boundary within the transition zone where cratered terrain changes to detached plateaus mesas and knobby terrain slopes are 50 100 times steeper and over a few hundred kilometers average about 1 the topographic signature of the boundary zone is best described as a 2 4 km step function between two nearly flat surfaces similar in slope and amplitude to some terrestrial passive margins this supports previous conclusions of a significant difference in crustal properties thickness composition or both between the lowland plains and cratered terrain and suggests this difference occurs over a relatively short distance of a few to several hundred kilometers

the cause for the north south orientation of the crustal dichotomy and the equatorial location of tharsis on mars the crustal dichotomy and the tharsis rise are the most prominent topographic features on mars the dichotomy is largely an expression of different crustal thicknesses in the northern and southern hemispheres while tharsis is centered near the equator at the dichotomy boundary however the cause for the orientation of the dichotomy and the equatorial location of tharsis remains poorly understood here we show that the crustal thickness variations associated with the dichotomy may have driven true polar wander establishing the north south orientation of the dichotomy very early in martian history such a reorientation that placed the dichotomy boundary near the equator would also have constrained the tharsis region on the dichotomy boundary to have originated near the equator we present a scenario for the early generation and subsequent reorientation of the hemispheric dichotomy although the reorientation is independent of the formation mechanism our results also have implications for the sharply different remanent magnetizations between the two hemispheres

gravitational spreading of high terrain in ishtar terra venus magellan altimetry measurements indicate that the mountain belts and plateau margins of ishtar terra have topographic slopes in the range 2 30 magellan radar images reveal that numerous sets of narrow closely spaced troughs and lineations which we interpret as graben and normal faults are located in many of these regions of high slope the orientation of many of these graben sets perpendicular to the downslope direction suggests that they formed as a result of gravitational spreading in several of the mountain belts the extensional structures are parallel to the apparent direction of shortening consistent with the interpretation that gravitational spreading occurred while mountain building was still active to explore the implications of this spreading for the tectonic evolution of ishtar terra we model the process by means of a finite element algorithm for viscoelastic deformation in a vertical section of the crust at the margin of a plateau or broad mountain range flow in the crust is assumed to be governed by a nonlinear depth dependent rheology we predict brittle failure and relaxation of topographic relief as functions of time for ranges of crustal thickness 10 30 km and thermal gradient 5 25 k km and for observed ranges in topographic elevation 1 6 km and slope 1 30 this parameter range predicts a large spread in the time scales for initial failure and significant relaxation of relief for a crustal thickness greater than 10 km a thermal gradient of 15 k km or more and average values of relief 3 km and slope 3 the topography relaxes to 25 of its original height in less than 10 m y although initial failure in the form of normal faulting on the highlands and margin and occasional shallow thrusting in the lowlands is predicted to occur much earlier values of horizontal surface strain that are likely to be observable 1 do not accumulate until significant relaxation of relief begins given the predicted rate of topographic relaxation either the crust of ishtar terra must be anomalous in some way either much stronger than predicted by standard flow laws or very thin or cold or the topographic relief and high slopes have been actively built and maintained until times significantly younger than the average crater retention age for venus of 500 ma the large apparent depth of compensation the abundance of volcanism and the uncertainty in the local age favor the view that mountain building in ishtar terra has occurred until times at least as recent as 10 ma

admittance estimates of mean crustal thickness and density at the martian hemispheric dichotomy 1 admittance estimates from line of sight los acceleration profiles of the mars global surveyor spacecraft are used to constrain the mean crustal thickness and surface density centered on the hemispheric dichotomy from 110 e to 220 e 40 s to 20 n models with uniform crustal properties predict lower than expected bulk crustal densities two layer models with loading only at the surface and the moho produce satisfactory fits to the data the best fit surface density crustal thickness tc and elastic thickness te are 2 5 mg m 3 27 km and 61 km respectively higher elastic thicknesses require lower crustal thicknesses and vice versa the best fit ratio of moho to surface loading f is close to 1 models with no bottom loading f 0 provide a poor fit to the data underestimates in f result in underestimates of both tc and te the surface density is lower than that measured from martian meteorites and by admittance analyses of young volcanoes but is well constrained by the short wavelength admittance values for misfits up to 1 5 times the minimum value and a fixed surface density of 2 5 mg m 3 the ranges of tc te and f are 1 75 km 37 89 km and 0 4 2 6 respectively the apparently compensated nature of the large impact basins has been used to infer a lower bound on southern hemisphere crustal thickness of 45 km if this estimate is correct the likely mean crustal thickness in the area considered is 55 20 km

comparison of composite bragg theory and quad polarization radar backscatter from radarsat 2 with applications to wave breaking and high wind retrieval 1 depolarized de pol radar backscatter is now produced by many spaceborne satellites analysis of radarsat 2 r2 quad polarization quad pol data with collocated in situ ocean wind measurements reveals that the de pol radar backscatter does not saturate in high winds this is a significant development for radar wind sensing because wind retrieval with copolarized co pol backscatter suffers from problems of incidence and azimuth angle dependent signal saturation and dampening in high winds we present a study comparing satellite quad pol measurements with the composite surface bragg cb theory of radar backscattering from the ocean surface the co pol data are in good agreement with the cb theory de pol data are more sensitive to wind speed compared to theoretical prediction thus retrieval of high winds is more accurate using the de pol return the cubic wind speed dependence of de pol returns in high winds reflects the significant breaking wave contributions the relationship can be used to obtain wave breaking properties from space

large impact basins and the mega impact origin for the crustal dichotomy on mars the origin of the fundamental crustal dichotomy on mars remains the outstanding unsolved problem in martian geologic evolution the hypothesis that this dichotomy is due to a single giant mega impact early in martian history is tested by comparing the observed number of large impact basins on mars with the number expected from a d 2 distribution this test is based on the assumption that the mega impact borealis basin is the largest member of such a distribution the difference between observed and expected basins is the number of missing basins that might have been obscured by subsequent geologic events if the borealis basin were the largest member of a d 2 impact population there would be a very large number of missing basins on mars 1 larger than 5000 km 10 larger than 2000 km and 40 50 larger than about 1000 km diameter the area occupied by these missing basins allowing for overlap is too large to be hidden by the younger surface units a large population of presently unrecognized large impact basins would have to be located in the well preserved cratered highlands by contrast if chryse is the largest member of a d 2 impact population the more modest number of missing basins could in principle be confined to areas of mars that have been resurfaced or reworked by subsequent geologic processes a possible alternative to the mega impact hypothesis is that the crustal dichotomy is due to the cumulative effects of multiple overlapping large but not giant impacts

water on early mars possible subaqueous sedimentary deposits covering ancient cratered terrain in western arabia and sinus meridiani western arabia and northern sinus meridiani 30 n 10 s 40 w 40 e are almost entirely covered by wind eroded horizontally bedded sedimentary layers that lap against local topographic features this portion of mars ancient cratered terrain is relatively low 1 km elevation and flat sloping westward 0 7 m km over 3 000 km the region lacks valley network channels we propose that at some time in early martian history this region was under water the water hypothesis is strengthened by the presence of 1 a shore like contact between smooth surfaced deposits and ancient cratered terrain in central sinus meridiani 2 polygonal structures in northern sinus meridiani and 3 sand which cannot be from airfall possibly reworked by wind from the underlying sedimentary units the september 1997 arrival of mars global surveyor s thermal emission spectrometer offers an opportunity to begin to test this hypothesis by searching for carbonate and evaporite minerals among the sediment covering the region

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

stony meteorite porosities and densities a review of the data through 2001 in this review we summarize the data published up to december 2001 on the porosity and density of stony meteorites these data were taken from 925 samples of 454 different meteorites by a variety of techniques most meteorites have densities on the order of 3 to 4 g cm 3 with lower densities only for some volatile rich carbonaceous meteorites and higher densities for stony irons for the vast majority of stones porosity data alone cannot distinguish between different meteorite compositions average porosities for most meteorite classes are around 10 though individual samples can range as high as 30 porosity unbrecciated basaltic achondrites appear to be systematically less porous unless vesicles are present the measured density of ordinary chondrites is strongly controlled by the amount of terrestrial weathering the sample has undergone with porosities steadily dropping with exposure to the terrestrial environment a theoretical grain density based on composition can model pre weathered porosities the average model porosity for h and ll chondrites is 10 while l chondrite model porosities average only 6 a statistically significant difference

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

magnetization of the martian crust 1 this paper presents the lateral variations of the vertically averaged magnetization model of the martian crust determined from the degree 50 spherical harmonic model of the magnetic potential the model potential is derived using the mars global surveyor low altitude magnetic data acquired within elevations of 85 200 km the potential model is first verified by comparing the three orthogonal components of the corresponding vector field with the observed high altitude 380 km data using the paleopole positions calculated through modeling 10 isolated small magnetic anomalies the model potential is inverted to determine the vertically averaged magnetization of a nominal 50 km thick crust the crust is highly magnetic with a magnetization contrast peak to trough as high as 35 a m the lack of distinct magnetic signatures for the giant impact basins hellas argyre and isidis and the absence of a magnetic anomaly associated with the north south topographic dichotomy indicate that the upper parts of the crust are low magnetic and that the strong magnetic source bodies are in the lower parts

formation ages and evolution of shergotty and its parent planet from u th pb systematics the isotopic composition of lead was determined in different phases from shergotty zagami and eeta 79001 using phases with low 238 u 204 pb the initial leads of these meteorites could be well defined and are distinct from each other and have high 204 pb 206 pb from 0 0652 to 0 0739 these leads evolved in different reservoirs over most of solar system history in a low 238 u 204 pb 5 environment it follows that the parent planet of the shergottites has a high 204 pb concentration relative to u and must also be high in other volatiles if the parent planet of the shergottites is mars then this planet must be volatile rich compared to the earth by a factor of 1 8 this implies that mars lost its original atmosphere and that the present atmosphere was derived from a previously outgassed martian upper mantle the u th pb isotopic relationships involving phases with high 238 u 204 pb are quite regular and give ages for three shergottites which indicate an event of u th pb fractionation at 200 m y the initial leads at that time were highly evolved relative to the primordial lead the absence of substantial additions of radiogenic lead above the initial lead in high u phases precludes an older age for the u th pb fractionation however details of the data arrays are complex the young age is in general agreement with some of the ages obtained by other methods but precise concordance between the different methods is not established it is suggested that the shock event that produced maskelynite may possibly be associated with the late stage breakup at 2 5 m y and not necessarily related to the 180 m y record the major event of chronometer resetting is taken to be 180 m y this event does not appear to be an endogenic igneous event but an almost complete equilibration during a major impact on more ancient igneous rocks which were contained in the target problems of the polymetamorphic history of meteorites appear as complicated as those of polymetamorphic terrestrial terranes

quantification of waste morphology in martian fretted terrain extensive geomorphic mapping in the northern martian fretted terrain of deuteronilus and protonilus mensae shows details of the wastedominated landform association in this region wasting appears to have occurred along the cratered terrain boundary ctb at the expense of old cratered terrain north of the boundary remnants of old cratered terrain occur as island mesas surrounded by extensive debris aprons quantitative study using principal components analysis pca reveals distinct spatial variations in the morphology of waste forms in the egion and supports the qualitative interpretations made during geomorphic mapping these organized spatial patterns occur with respect to longitude and latitude and therefore with reference to distance from the ctb and provide clues to the degradational history of the region analyses of the individual landforms irrespective of their location to the ctb indicate strong structural control of their form mesas appear to evolve from equant forms to narrow ridges with large debris aprons after they become detached from the old cratered terrain the success of pca in explaining patterns in degradational landforms suggests that pca may be an important technique in quantifying the geomorphic evolution of the martian surface

the geomorphology sediment distribution soil formation nexus on the northeastern qinghai tibetan plateau implications for landscape evolution abstract the interactions between geomorphology sediment distribution and soil development in alpine regions are poorly understood in this study we analyzed the spatial variations of different sediment types and corresponding soils in 15 pedosedimentary profiles across three major geomorphic units in an alpine ridge valley basin of the qilian mountains northeastern qinghai tibetan plateau particle size distribution was used to trace the sediment types inorganic and organic carbon concentrations were included as metrics of soil processes aeolian silts from both distant and proximal sources cover floodplains hillslopes and some positions on alluvial fans while the distribution of fluvial sediments and periglacial slope deposits are much more limited due to the coverage of loess the soils vary in response to sediment types and climatic conditions under the influence of their geomorphic settings the vertical distribution patterns of carbonates in soil profiles show considerable differences among different geomorphic units reflecting their contrasting water balance and migration pathways soil organic carbon accumulates most extensively in aeolian silts with sufficient water supply a special pedogenic soil horizon the mattic epipedon is formed primarily in loessial sediments in kobresia ecosystems the complex entanglement of fine earths and massive grassroots make the mattic epipedon extremely resilient which helps to preserve the soils and to shape the landscapes by integrating geomorphology sedimentology ecology and pedology the outcomes of this study promote the understanding of the evolution of typical landscapes and ecosystems of the northeastern qinghai tibetan plateau and many other periglacial settings

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

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