planetary surface dating from crater size frequency distribution measurements partial resurfacing events and statistical age uncertainty abstract we describe the procedure to fit a cumulative production function polynomial to a partial crater size frequency distribution the technique is of particular use in deriving ages for surfaces which have undergone partial resurfacing events namely erosional or depositional events which have affected a limited diameter range of the crater population we demonstrate its use in obtaining times for both the surface formation and the resurfacing event we give a practical outline of the method for making age measurements from crater counts and how to identify resurfacing effects in the results we discuss the conversion of production function polynomials between common presentations and the statistical uncertainty of the determined ages with respect to the non linear chronology function and a minor refinement of data binning

was the mangala valles system incised by volcanic flows 1 the mangala valles system of channels heads at one of the graben that comprise memnonia fossae extends northward nearly 1000 km across noachian and hesperian highlands and terminates at basins contiguous with amazonis planitia the mangala valles system has previously been interpreted to have formed through one or more catastrophic aqueous floods on the basis of similarities between the characteristics of its channel landforms and those of terrestrial systems including the channeled scabland of washington although aqueous mechanisms for formation of mangala valles are broadly congruous with known characteristics of the channel system an alternative volcanic hypothesis for formation of the system appears to be worthy of consideration on the basis of 1 its consistency with the volcanotectonic nature of the system and 2 commonalities between the basic nature of the system and that of large volcanic channels of the inner solar system estimates based on thermal considerations suggest that formation of mangala valles could conceivably have taken place through eruption of a lava volume of 2 105 km3 or roughly the total volume of the terrestrial columbia river basalt group the volcanic hypothesis for formation of mangala valles and a hybrid hypothesis involving formation of mangala valles through aqueous processes followed by or in concert with substantial modification of the system by volcanic erosion and deposition appears viable and worthy of future consideration

global vector and scalar magsat magnetic anomaly maps empirical and analytical techniques for modeling ionospheric fields in magsat data have been developed that facilitate ionospheric field removal from uncorrected anomalies to obtain better estimates of regional lithospheric anomalies this task has been accomplished for equatorial x z and b component and polar z and b component measurements the techniques for modeling ionospheric fields have been integrated into a processing sequence that incorporates some of the important data processing techniques developed during the last decade data processing techniques include retention of common signal in a correlation analysis of adjacent passes analysis of field differences between dawn and dusk data at points where their orbits cross and retention of common signal in a covariant spherical harmonic analysis procedure results suggest that implementation of the above processing scheme leads to the mapping of the most robust magnetic anomalies of the lithosphere vector components as well as scalar

scalar magnetic anomaly maps of earth derived from pogo and magsat data a new pogo scalar magnetic anomaly map at 400 km altitude is presented which consists of spherical harmonics of degree 15 60 on the basis of the common features of this map with two new magsat anomaly maps dawn and dusk two scalar magnetic anomaly maps of the earth are presented using two selection criteria with different levels of stringency these selection criteria suppress the noncrustal components of the original maps by different amounts the more stringent selection criteria seek to eliminate as much contamination as possible at the expense of suppressing some anomaly signal this map is represented by spherical harmonics of degree 15 60 the less stringent selection criteria seek to retain as much crustal signal as possible at the expense of also retaining some contaminating fields this map is represented by spherical harmonics of degree 15 65 the resulting two maps are highly correlated with degree correlation coefficients greater than 0 8

episodes of fluvial and volcanic activity in mangala valles mars a new mapping based study of the 900 km long mangala valles outflow system was motivated by the availability of new high resolution images and continued debates about the roles of water and lava in outflow channels on mars this study uses photogeologic analysis geomorphic surface mapping cratering statistics and relative stratigraphy results show that mangala valles underwent at least two episodes of fluvial activity and at least three episodes of volcanic activity during the late amazonian the occurrence of scoured bedrock at the base of the mapped stratigraphy in addition to evidence provided by crater retention ages suggests that fluvial activity preceded the deposition of two of the volcanic units crater counts performed at 30 locations throughout the area have allowed us to construct the following timeline 1 formation of noachian highlands and possible initial flooding event s before 1 ga 2 emplacement of tharsis lava flows in the valley from 700 to 1000 ma 3 a megaflooding event at 700 800 ma sourced from mangala fossa 4 valley fill by a sequence of lava flows sourced from mangala fossa 400 500 ma 5 another megaflooding event from 400 ma 6 a final phase of volcanism sourced from mangala fossa 300 350 ma and 7 emplacement of eolian sedimentary deposits in the northern portion of the valley 300 ma these results are consistent with alternating episodes of aqueous flooding and volcanism in the valles this pattern of geologic activity is similar to that of other outflow systems such as kasei valles suggesting that there is a recurring and perhaps coupled nature of these processes on mars

considerations of variations in ionospheric field effects in mapping equatorial lithospheric magsat magnetic anomalies summary longitudinal seasonal and altitude dependent variability of magnetic fields is investigated in equatorial latitudes to determine their effect on the isolation of lithospheric magsat magnetic anomalies an estimate of ionospheric effect was compiled by averaging the total intensity magsat anomalies as a function of dip latitudes called dip latitude averages from dawn and dusk data sets grouped according to longitudes time months and altitudes unanticipated seasonal variations were observed in dusk magsat data over the indian sector that may contribute to an improved understanding of the equatorial electrojet the amplitudes of the dawn dip latitude averages are small in comparison to the dusk averages and they are of opposite sign as anticipated from the westward and eastward flowing currents at the dip equator at dawn and dusk time respectively however longitudinal variation in the equatorial amplitudes of the dawn diplatitude averages is not entirely consistent with present knowledge of the electrojet field in the past dawn magsat anomalies have been considered to be largely free of ionospheric fields however in the preparation of the lithospheric component maps neglecting to remove the dawn dip latitude averages from dawn magsat data set leaves conspicuous e w trending anomaly artifacts in certain regions of the world removal of both the dawn and the dusk dip latitude averages from their respective data sets substantially improves the agreement between the separately prepared dawn and dusk lithospheric component magsat maps despite the agreement between the resultant lithospheric anomaly maps dip latitude averages do appear to contain some contribution from the lithospheric fields thus processing of the data with the aid of the dip latitude averages unavoidably removes a small degree of e w oriented lithospheric signal as well

equatorial electrojet from orsted scalar magnetic field observations 1 previous studies of the longitudinal variation of the local noon electrojet have yielded doubtful results either because of the poor data quality or because the local times of equatorial crossings occurred in the early morning or late afternoon the recent launch of the orsted satellite in a near circular orbit with slow drift in local time of equatorial crossing has provided the opportunity for researchers to study the electrojet more accurately most studies remove the main field using a spherical harmonic model and then search the daytime equatorial passes for the distinctive electrojet trough in total intensity the present study examines the electrojet for two consecutive 6 month periods and consequently two local time ranges pure signal processing is used to remove the main field directly the residuals are binned separately for night and day passes on a 1 by 1 grid to enhance the signal to noise ratio and are bin centered by a least squares fitted linear model to compensate for the variations in satellite altitude thereafter for each period the compensated night and day binned values are subtracted from each other to produce a difference set global plots of the subsequently spatially filtered difference sets reveal an almost constant electrojet 1 e half width of 3 as seen at satellite altitude apart from a region in the western pacific there are four maxima in the electrojet amplitude at 0 30 e 90 120 e 180 220 e and 260 290 e in each local time range

the equatorial electrojet an integrated treatment of observations and models of the undisturbed equatorial electrojet is presented previous reviews have concentrated on particular aspects of the electrojet phenomenon such as ground magnetic variations the counterelectrojet or e region electron density irregularities and associated plasma instabilities here accomplishments during the past 15 years involving all aspects of the phenomenon are covered and where possible interrelated observational and theoretical studies during recent years have emphasized the local time longitude vertical and latitudinal structure of electric fields and currents in the electrojet this review similarly places its emphasis along these lines and by necessity involves a detailed examination of electrodynamic coupling with the neutral atmosphere tidal oscillations in temperature composition and winds the review concludes with an assessment of current knowledge concerning the equatorial electrojet and suggestions for the future direction of observational and theoretical research on electrodynamical processes in the equatorial ionosphere

longitudinal variation of the e region electric fields caused by atmospheric tides 1 polarization electric fields created by the e and f region dynamos cause the uplift of f region plasma the subsequent redistribution of that plasma along the magnetic field lines creates the equatorial ionospheric anomaly eia observations of the post sunset eia made by the image and timed satellites are compared here with champ orsted and sac c observations of the noontime equatorial electrojet eej during magnetically quiet periods around equinox the eia and eej show a remarkably similar four peaked wave like longitudinal variation its structure is consistent with the longitudinal variation in the strength of diurnal tides that drive the e region dynamo this indicates a strong vertical coupling between the ionosphere and troposphere because the four peaked tidal structure is driven by tropospheric weather furthermore the dayside ionospheric conditions are found to perform the global scale longitudinal structure of the post sunset ionosphere at low latitudes

anisotropy in the deep earth abstract seismic anisotropy has been found in many regions of the earth s interior its presence in the earth s crust has been known since the 19th century and is due in part to the alignment of anisotropic crystals in rocks and in part to patterns in the distribution of fractures and pores in the upper mantle seismic anisotropy was discovered 50 years ago and can be attributed for the most part to the alignment of intrinsically anisotropic olivine crystals during large scale deformation associated with convection there is some indication for anisotropy in the transition zone particularly in the vicinity of subducted slabs here we focus on the deep earth the lower mantle and core where anisotropy is not yet mapped in detail nor is there consensus on its origin most of the lower mantle appears largely isotropic except in the last 200 300 km in the d region where evidence for seismic anisotropy has been accumulating since the late 1980s mostly from shear wave splitting measurements recently a picture has been emerging where strong anisotropy is associated with high shear velocities at the edges of the large low shear velocity provinces llsvps in the central pacific and under africa these observations are consistent with being due to the presence of highly anisotropic mgsio 3 post perovskite crystals aligned during the deformation of slabs impinging on the core mantle boundary and upwelling flow within the llsvps we also discuss mineral physics aspects such as ultrahigh pressure deformation experiments first principles calculations to obtain information about elastic properties and derivation of dislocation activity based on bonding characteristics polycrystal plasticity simulations can predict anisotropy but models are still highly idealized and neglect the complex microstructure of polyphase aggregates with strong and weak components a promising direction for future progress in understanding the origin of seismic anisotropy in the deep mantle and its relation to global mantle circulation is to link macroscopic information from seismology and microscopic information mineral physics through geodynamics modeling anisotropy in the inner core was proposed 30 years ago to explain faster p wave propagation along the direction of the earth s axis of rotation as well as anomalous splitting of core sensitive free oscillations there is still uncertainty about the origin of this anisotropy in particular it is difficult to explain its strength based on known elastic properties of iron as it would require almost perfect alignment of iron crystals indeed the strongly anomalous p travel times observed on paths from the south sandwich islands to alaska may or may not be due to inner core anisotropy and will need to be explained before consensus can be reached on the strength of anisotropy in the inner core and its origin

crustal magnetic anomalies this quadrennial report departs in format from its predecessors earlier reports provided brief descriptions of the entire range of topics on crustal magnetic anomalies following guidelines established by the agu we instead focus our discussion on only a few selected topics our abbreviated list of references reflects this restricted focus a more comprehensive bibliography can be obtained from the authors as noted subsequently the past quadrennium was distinguished by a significant change in the scale and scope of crustal magnetic anomaly research advances in instrumentation navigation and data processing techniques resulted in increasing use of two dimensional anomaly maps rather than profile data although maps are commonly used in aeromagnetic studies of continental areas their use in the ocean basins has been limited until recently the other important change has been a trend towards increased integration of magnetic anomaly data with gravity seismic geochemical remote sensing and geologic data to enhance the interpretation and reduce the ambiguity of magnetic field data

seismic anisotropy in the earth s innermost inner core testing structural models against mineral physics predictions using an updated data set of ballistic pkikp travel time data at antipodal distances we test different models of anisotropy in the earth s innermost inner core imic and obtain significantly better fits for a fast axis aligned with earth s rotation axis rather than a quasi equatorial direction as proposed recently reviewing recent results on the single crystal structure and elasticity of iron at core conditions we find that an hcp structure with the fast c axis parallel to earth s rotation is more likely but a body centered cubic structure with the 111 axis aligned in that direction results in very similar predictions for seismic anisotropy these models are therefore not distinguishable based on current seismological data in addition to match the seismological observations the inferred strength of anisotropy in the imic 6 7 implies almost perfect alignment of iron crystals an intriguing albeit unlikely situation especially in the presence of heterogeneity which calls for further studies

stability of iron crystal structures at 0 3 1 5 tpa abstract ab initio molecular dynamics simulations carried out for tetragonal and orthorhombic distortions of iron closely follow the results of static lattice electronic structure calculations in revealing that the body centered cubic bcc phase of fe is mechanically unstable at pressures of 0 3 1 5 tpa and temperatures up to 7000 k crystal structural instabilities originate in the static lattice for the bcc configuration and are consistent with recent results from both static and dynamic high pressure experiments both theory and experiment thus show that the close packed hexagonal hcp and face centered cubic fcc crystal structures of iron are those relevant to the cores of earth like planets

generation of martian chaos and channels by debris flows abstract a debris flow mechanism is proposed to account for the formation of chaos and the large channels debouching into chryse planitia from the adjacent southern uplands of mars the debris is thought to have originated through a mechanism of collapse in the chaotic terrains which exist at the head of these channels as well as locally along the channels this proposition is based on the detailed morphologic similarities between martian channel source areas and the heads of both subaerial and subaqueous terrestrial debris flows the downslope movement of the debris produced the channels through a modification of earlier collapse areas b active bed erosion and c loading induced collapse the large scale channel geometry and the assemblage of related morphologic features on mars correspond tto that observed in subaqueous debris flow chutes on the mississippi delta front through various mechanisms of strain dependent viscosity decrease the debris flow gained mobility downstream turned into a debris avalanche and moved onto chryse planitia at very high velocities this high velocity avalanche eroded a series of streamlined remnants near the channel mouths and deposited its load as a thin blanket over a large area of the basin creating virtually no depositional relief

ab initio calculations of the elasticity of iron and iron alloys at inner core conditions evidence for a partially molten inner core abstract ab initio finite temperature molecular dynamics simulations have been used to calculate the elasticity of hcp fe bcc fe fes and fesi at core conditions the calculated compressional wave velocities are in excellent agreement with the most recent experimental data however the calculated shear wave velocities of all phases studied are significantly higher than those inferred from seismology this discrepancy can only be explained if the inner core is partially molten containing more than 8 liquid

new global vector magnetic anomaly maps derived from magsat data new global maps of the vector magnetic anomaly field of earth are derived from magsat data improvement on earlier published vector and scalar maps is achieved with the following procedure 1 the noise level of vector measurements associated with the discontinuities of the in flight sensor attitude determination is reduced by selecting the data 2 a monthly model of the main field is computed to account for secular variation using only quiet day data 3 the time dependence of the ring current effect is taken into account using hourly values of the dst index based on ground obseivations 4 sectorial indices based on ground observations are introduced to select profiles corresponding to low magnetic activity these indices provide a better discrimination of quiet data than the global kp index usually used for that purpose 5 a time and longitude dependent dynamic model of the equatorial electrojet is derived and used to correct magnetic measurements at low latitudes anomaly maps of x and z components are obtained using selected measurements within the reduced altitude range 350 425 km these maps are compared with earlier magsat anomaly maps and their respective resolutions are discussed these maps evidence large anomalies of lithospheric origin within the oceanic domain in particular the z map displays a characteristic pattern of long wavelength anomalies in these areas it is first shown that some of these anomalies particularly in the north atlantic and indian oceans may be correlated with the age of the lithosphere then a significant similarity between magnetic and geoid anomalies above ocean basins is evidenced this suggests that long wavelength oceanic magnetic anomalies may also be attributed to variations of the thickness of the magnetized layer associated with ocean floor topography

new evidence of a meridional current system in the equatorial ionosphere an interesting variation in the geomagnetic d component has been found by analysis of the magsat data characteristics of this variation are that it appears everyday on the low latitude duskside and is antisymmetric about the dip equator negative in the northern hemisphere that the magnitude from 5 to 25 nt depends upon the geographic longitude and altitude and that no corresponding change can be seen on the ground magnetic data the change suggests a meridional current system in the equatorial ionosphere produced in association with the electrojet current along the dip equator this effect should be taken into account in the study of geomagnetic anomaly of internal origin with the magsat data

a refined chronology of catastrophic outflow events in ares vallis mars abstract we investigate the geomorphology and chronology of catastrophic flooding in a major martian outflow channel ares vallis we use recently acquired stereo and colour images and derived topographic data with grid spacing of 50 m from the high resolution stereo camera hrsc onboard mars express to constrain the detailed flood geomorphology of proximal ares vallis we identify morphologic evidence for 6 distinct erosion surfaces that were likely formed by multiple episodes of catastrophic flooding these surfaces occur at distinct topographic levels exhibit multiple sets of flood grooves and contain distinct post flood crater populations crater count statistics on the flood eroded surfaces from mro context camera ctx and thermal emission imaging system themis visible images indicate that flooding was initiated during the late noachian early hesperian at 3 6 ga the morphology data confirm that the oldest flooding events occurred on the topographically highest erosion surfaces while subsequent floods abandoned the upper surfaces and continued to modify craters in the topographically lowest main channel of ares vallis into the late hesperian early amazonian 2 5 ga our results suggest that release of water to ares vallis likely occurred at repeated intervals over a long period of geological time thus supporting recent numerical modelling studies that suggest a recharging water source

origin of martian outflow channels the eolian hypothesis the ancient outflow channels on mars exhibit fossil flow patterns similar to current flow patterns in terrestrial desert regions eroded by wind we have modeled outflow channel formation by a cascade process in which sand sized particles produced by rock weathering in the chaotic terrain source areas of the outflow channels were transported by the wind across the adjacent plateau surfaces and sand blasting and deflating the surface comparisons of model predictions with photogeologic data suggest that rates of rock breakdown by physical and chemical action along the channels during the channel forming era must have been much higher than they have been on similar terrains formed after the channel era ended a tenuous atmosphere comparable to the present one could have transported materials sufficiently rapidly to excavate outflow channels in about 106 years however there may be difficulties in explaining how abrading particles could survive high velocity impacts in a thin atmosphere over the many hundreds or even thousands of kilometers that they were transported in outflow channels it is difficult to avoid the conclusion that a climate change affecting rock breakdown rates atmospheric density or both occurred early in martian history to end the era of outflow channel formation particulate materials generated during this era may be important components of the north circumpolar dune field and the polar layered deposits

athabasca valles mars a lava draped channel system athabasca valles is a young outflow channel system on mars that may have been carved by catastrophic water floods however images acquired by the high resolution imaging science experiment camera onboard the mars reconnaissance orbiter spacecraft reveal that athabasca valles is now entirely draped by a thin layer of solidified lava the remnant of a once swollen river of molten rock the lava erupted from a fissure inundated the channels and drained downstream in geologically recent times purported ice features in athabasca valles and its distal basin cerberus palus are actually composed of this lava similar volcanic processes may have operated in other ostensibly fluvial channels which could explain in part why the landers sent to investigate sites of ancient flooding on mars have predominantly found lava at the surface instead

ice sculpture in the martian outflow channels many landforms in martian outflow channels have characteristics that suggest sculpture by glaciers ice streams or ice sheets viking orbiter and terrestrial satellite images were examined at similar resolution to compare features of the martian outflow channels to features produced by the movement of ice on earth many resemblances were found they include the anastomoses sinuosities and u shaped cross profiles of valleys hanging valleys and linear scour marks on valley walls grooves and ridges on valley floors and streamlining of bedrock highs the question of whether ice could have moved in the martian environment is investigated since gradients on martian channel floors are extremely low or even reversed a certain thickness of ice is required to initiate glacial motion if ice filled the martian channels to the level of interior plateaus flow could have occurred even under present climatic conditions although warmer climates in the past would have been more favorable additionally if the channel fluids were brines ice might have been wet based and would have moved readily by slipping over the ground it is envisioned that springs or small catastrophic outbursts discharged fluids from structural outlets or chaotic terrains the fluids built icings that may have grown into substantial masses and eventually flowed like glaciers down preexisting valleys alternatively the fluids may have formed rivers or floods that formed ice jams and consolidated into icy masses in places where obstacles blocked their flow these masses of ice or slush were probably episodic and temporary but where they occurred within the channels they may have produced landforms that are characteristic of glacial sculpture on earth

the near earth magnetic field at 1980 determined from magsat data data from the magsat spacecraft for november 1979 through april 1980 and from 91 magnetic observatories for 1978 through 1982 are used to derive a spherical harmonic model of the earth s main magnetic field and its secular variation at epoch 1980 0 the model is called gsfc 12 83 constant coefficients are determined through degree and order 13 and secular variation coefficients through degree and order 10 the first degree external terms and corresponding induced internal terms are given as a function of dst preliminary modeling using separate data sets at dawn and dusk local time showed that the dusk data contain a substantial field contribution from the equatorial electrojet current the final data set was therefore selected first from dawn data and then augmented by dusk data to achieve a good geographic data distribution for each of three time periods 1 november december 1979 2 january february 1980 and 3 march april 1980 a correction for the effects of the equatorial electrojet was applied to the dusk data utilized the solution included calculation of fixed biases or anomalies for the observatory data although similar in many respects gsfc 12 83 differs from international geomagnetic reference field 1980 by 3 6 nt in the g10 term and shows a slightly negative b in the northern polar region as well as other differences in secular variation pattern

magsat scalar anomaly map of south america a scalar magnetic anomaly map for south america and adjacent marine areas was prepared from magsat data preparation of the map posed problems notably in separating external field and crustal anomalies and in reducing data to a common altitude external fields are manifested in a long wavelength ring current effect a medium wavelength equatorial electrojet and short wavelength noise the ring current is corrected through use of a standard ring current equation augmented by further wavelength filtering because the electrojet is confined primarily to dusk profiles its effect is minimized by drawing the data set from dawn profiles only the noise is reduced by selecting profiles from quiet periods k 2 the optimum filter cutoff level is determined by a statistical regression technique to be approximately 50 degrees wavelength when wavelengths longer than this are rejected resultant profiles from redundant satellite orbits correlate well with differences due primarily to satellite altitude the

the intermediate wavelength magnetic anomaly maps of the north atlantic ocean derived from satellite and shipborne data summary two intermediate wavelength magnetic anomaly maps of the north atlantic ocean are derived based on satellite and marine magnetic observations the satellite map is produced from the covariant features of pogo and magsat dawn and magsat dusk magnetic anomalies the marine map is compiled using shipborne data collected since 1965 the two maps are in excellent agreement over well defined anomalies both in location and amplitude suggesting that these anomalies are real and reflect magnetization contrasts in the oceanic lithosphere both maps show the intermediate wavelength seafloor spreading anomalies of the cretaceous quiet zone of the central north atlantic ocean as well as the negative seafloor spreading anomaly of the labrador sea caused by dominantly reversed polarity periods the iceland hotspot alpha ridge azores high king s trough and the rockall microcontinent have positive anomalies there is no consistent magnetic signature associated with the ocean continent boundary although the deep sedimentary basin of nova scotia is delineated by a well defined negative anomaly

structure and origin by injection of lava under surface crust of tumuli lava rises lava rise pits and lava inflation clefts in hawaii tumuli are positive topographic features that are common on hawaiian pahoehoe lava flow fields particularly on shallow slopes and 75 measured examples are presented here to document the size range tumuli form by up tilting of crustal plates without any crustal shortening and are thus distinguished from pressure ridges which are up buckled by laterally directed pressure the axial or star like systems of deep clefts that characterize tumuli are defined here as lava inflation clefts their tips advanced into red hot lava and they widened as uplift proceeded and while the lava crust was thickening flat surfaced uplifts formed like tumuli by injection of lava under a surface crust were previously called pressure plateaus but lava rise is proposed instead the pits that abound among lava rises previously attributed to collapse or subsidence are generally formed because the lava around them rose and the name lava rise pit is proposed unique examples of tumuli and lava rises from which lava drained out under a surface crust 1 5 to 2 5 m thick are described from kilauea caldera these examples show that in tumuli and lava rises the crust floats on considerable bodies of fluid lava and is able to do so because of its higher vesicle content the fluid lava loses many of its gas bubbles during residence beneath the crust the bulk densities of samples from tumuli show a general downward increase the form of the density profile is consistent with the relationship that for any given crustal thickness the density of fluid lava closely matched the average density of that crust suggesting that the lava was stably density stratified it is inferred that stable stratification was regulated by out flows of the more vesicular lava fractions loss of bubbles through the lava inflation clefts and entry of injected lava at its level of neutral buoyancy below the uppermost meter the downward decrease in vesicularity closely conforms with that expected by compression of a uniform mass of gas per unit mass of lava

mean ionospheric field correction for magsat data to obtain a meaningful anomaly map showing magnetic fields of crustal origin from satellite observations it is necessary to account adequately for and subtract the fields generated by the currents flowing in the ionosphere conventionally corrections are made separately for each path by empirical methods of fitting and subtracting linear quadratic or harmonic functions from the original data this procedure however lacks a sound physical basis we examined the field components after the main field and the magnetospheric field were subtracted from magsat data and found certain structures in the residual field the residual fields at 0600 and 1800 local times were separately averaged over longitude to obtain the effective contribution of ionospheric current system at these local times which is called the mean ionospheric field mif to obtain the crustal anomalies the mif was subtracted from the residual fields to account for the day to day variation in the ionospheric current system and for the difference in satellite altitude anomaly maps are constructed on 5 5 grid averages the dawn and dusk anomaly maps after the correction agree well with each other showing that the effect of the ionospheric field is well taken care of by the mif correction the amplitude of anomalies is large even at comparatively long wavelength and the maps successfully represent these crustal anomalies which have been greatly reduced in the previous maps

mosaicking of global planetary image datasets 1 techniques and data processing for thermal emission imaging system themis multi spectral data 1 the mosaicking of global planetary data sets allows for the examination of local regional and global scale processes on all planetary bodies processing techniques that allow us and other users to crate mosaics of tens of thousands of images are documented along with the associated errors introduced by each image processing algorithm these techniques e g non uniformity correction running contrast stretches line and row correlated noise removal and random noise removal were originally developed for the 2001 mars odyssey thermal emission imaging system themis infrared multispectral imager data but can be adapted and applied to other data sets by the alteration of input parameters the techniques for mosaicking planetary image data sets e g image registration blending and normalization are also presented along with the generation of qualitative and quantitative products these techniques are then applied to generate themis daytime and nighttime infrared viking context imager ctx and mars orbiter camera moc visible mosaics using a variety of input and output types at a variety of scales by creating mosaics of the same area using different data sets such as those that illustrate compositional diversity thermophysical properties or small scale morphology it is possible to view the surface of the planet and geologic problems through many different perspectives in addition to the techniques used to create large scale seamless mosaics we also present the themis daytime and nighttime relative temperature global mosaics which are the highest resolution 100m pixel global scale data sets available for mars to date

primary centers and secondary concentrations of tectonic activity through time in the western hemisphere of mars five main stages of radial and concentric structures formed around tharsis from the noachian through the amazonian as determined by geologic mapping of 24 452 structures within the stratigraphic framework of mars and by testing their radial and concentric orientations tectonic activity peaked in the noachian stage 1 around the largest center claritas an elongate center extending more than 20 in latitude and defined by about half of the total grabens which are concentrated in the syria planum thaumasia and tempe terra regions during the late noachian and early hesperian stage 2 extensional structures formed along the length of present day valles marineris and in thaumasia with a secondary concentration near warrego vallis radial to a region just to the south of the central margin of valles marineris early hesperian stage 3 radial grabens in pavonis syria ulysses and tempe terra and somewhat concentric wrinkle ridges in lunae and solis plana and in thaumasia sirenum memnonia and amazonis are centered northwest of syria with secondary centers at thaumasia tempe terra ulysses fossae and western valles marineris late hesperian early amazonian stage 4 structures around alba patera the northeast trending alignment of tharsis montes and olympus mons appears centered on alba patera stage 5 structures middle late amazonian represent the last pulse of tharsis related activity and are found around the large shield volcanoes and are centered near pavonis mons tectonic activity around tharsis began in the noachian and generally decreased through geologic time to the amazonian statistically significant radial distributions of structures formed during each stage centered at different locations within the higher elevations of tharsis secondary centers of radial structures during many of the stages appear related to previously identified local magmatic centers that formed at different times and locations throughout tharsis

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

an assemblage of lava flow features on mercury 1 in contrast to other terrestrial planets mercury does not possess a great variety of volcanic features its history of volcanism instead largely manifest by expansive smooth plains however a set of landforms at high northern latitudes on mercury resembles surface flow features documented on earth the moon mars and venus the most striking of such landforms are broad channels that host streamlined islands and that cut through the surrounding intercrater plains together with narrower more sinuous channels coalesced depressions evidence for local flooding of intercrater plains by lavas and a first order analysis of lava flow rates the broad channels define an assemblage of flow features formed by the overland flow of and erosion by voluminous high temperature low viscosity lavas this interpretation is consistent with compositional data suggesting that substantial portions of mercury s crust are composed of magnesian iron poor lithologies moreover the proximity of this partially flooded assemblage to extensive volcanic plains raises the possibility that the formation of these flow features may preface total inundation of an area by lavas emplaced in a flood mode and that they escaped complete burial only due to a waning magmatic supply finally that these broad channels on mercury are volcanic in nature yet resemble outflow channels on mars which are commonly attributed to catastrophic water floods implies that aqueous activity is not a prerequisite for the formation of such distinctive landforms on any planetary body

planetary surface dating from crater size frequency distribution measurements multiple resurfacing episodes and differential isochron fitting abstract the analysis of crater size frequency distributions and absolute densities forms the basis of current approaches for estimating the absolute and relative ages of planetary surfaces users of the neukum system of crater dating have conventionally used a cumulative presentation of the data but because of the recent proliferation of interest in identifying resurfacing ages it is worth emphasising the utility of the differential presentation of crater data in identifying resurfacing events and particularly in distinguishing the signature of short lived events such as volcanic flows from long acting processes such as aeolian erosion the work describes some additional considerations for making isochron fits to differentially plotted crater populations with respect to the removal of a binning bias for incrementally differentially plotted data the hartmann approach has not typically employed the fitting of isochrons but differential fitting would be a natural choice for this system and is implemented in the craterstats software a revised calculation of the martian epoch boundary times in both chronology systems is provided