exotic crust formation on mercury consequences of a shallow feo poor mantle the range in density and compressibility of mercurian melt compositions was determined to better understand the products of a possible mercurian magma ocean and subsequent volcanism our experiments indicate that the only mineral to remain buoyant with respect to melts of the mercurian mantle is graphite consequently it is the only candidate mineral to have composed a primary floatation crust during a global magma ocean this exotic result is further supported by mercury s volatile rich nature and inexplicably darkened surface additionally our experiments illustrate that partial melts of the mercurian mantle that compose the secondary crust were buoyant over the entire mantle depth and could have come from as deep as the core mantle boundary furthermore mercury could have erupted higher percentages of its partial melts compared to other terrestrial planets because magmas would not have stalled during ascent due to gravitational forces these findings stem from the feo poor composition and shallow depth of mercury s mantle which has resulted in both low melt density and a very limited range in melt density responsible for mercury s primary and secondary crusts the enigmatically darkened yet low feo surface which is observed today can be explained by secondary volcanism and impact processes that have since mixed the primary and secondary crustal materials

investigating the origin of candidate lava channels on mercury with messenger data theory and observations received 12 april 2012 revised 31 august 2012 accepted 1 november 2012 published 31 march 2013 1 volcanic plains identified on mercury are morphologically similar to lunar mare plains but lack constructional and erosional features that are prevalent on other terrestrial planetary bodies we analyzed images acquired by the messenger spacecraft to identify features on mercury that may have formed by lava erosion we used analytical models to estimate eruption flux erosion rate and eruption duration to characterize the formation of candidate erosional features and we compared results with analyses of similar features observed on earth the moon and mars results suggest that lava erupting at high effusion rates similar to those required to form the teepee butte member of the columbia river flood basalts 0 1 1 2 10 6 m 3 s 1 would have been necessary to form wide valleys 15km wide observed in mercury s northern hemisphere first by mechanical erosion to remove an upper regolith layer then by thermal erosion once a lower rigid layer was encountered alternatively results suggest that lava erupting at lower effusion rates similar to those predicted to have formed rima prinz on the moon 4400 m 3 s 1 would have been required to form via thermal erosion narrower channels 7km wide observed on mercury although these results indicate how erosion might have occurred on mercury the observed features may have formed by other processes including lava flooding terrain sculpted during the formation of the caloris basin in the case of the wide valleys or impact melt carving channels into impact ejecta in the case of the narrower channels citation hurwitz d m j w head p k byrne z xiao s c solomon m t zuber d e smith and g a neumann 2013 investigating the origin of candidate lava channels on mercury with messenger data theory and observations j geophys res planets 118 471 486 doi 10 1029 2012je004103

thermal evolution of mercury effects of volcanic heat piping 1 a 1d thermal evolution model of mercury is presented it accounts for stagnant lid convection mantle differentiation and inner core growth early messenger results indicate that contrary to prior conclusions drawn from mariner 10 imagery volcanism has indeed played a significant role in mercury s past to study the effects of mantle heat bypassing the stagnant lid by means of volcanic heat piping contrasting end member models are considered results show how break down of mantle convection and onset of inner core growth are influenced by the mode of heat removal structural models of present day mercury are presented their dependence on the core sulphur contents predominates that on the choice of mantle heat removal however the latter clearly controls the timing of thermal history events

dark spots on mercury a distinctive low reflectance material and its relation to hollows 1 orbital images acquired by the mercury surface space environment geochemistry and ranging messenger spacecraft reveal a distinctive low reflectance material on the surface of mercury such material occurs in small isolated and thin surficial units we term these features dark spots dark spots have the lowest average reflectance yet documented on the planet in every case observed at sufficiently high resolution dark spots feature hollows at their centers not all hollows however are surrounded by a dark spot dark spots have been found on low reflectance smooth plains intercrater plains heavily cratered terrain and impact craters at almost all longitudes on mercury but they have not been documented on high reflectance smooth plains material dark spots are one of the youngest endogenic features on mercury and some postdate craters with distinctive rays sulfides may be the phase responsible for the low albedo of dark spot material we propose that dark spots form during the initial stages of hollow formation perhaps in a manner similar to intense outgassing events that feature exit velocities in excess of 100 m s such outgassing could contemporaneously produce a depression that constitutes an embryonic hollow under this scenario dark spot material is subsequently removed or modified by regolith gardening or other surface processes on time scales shorter than the lifetime of the central hollow

mercury s albedo from mariner 10 implications for the presence of ferrous iron mariner 10 clear filter 490 nm images of mercury were recalibrated and photometrically normalized to produce a mosaic of nearly an entire hemisphere of the planet albedo contrasts are slightly larger than seen in the lunar highlands excluding maria variegations indicative of compositional differences include diffuse low albedo units often overlain by smooth plains the high albedo smooth plains of borealis planitia and high albedo enigmatic crater floor deposits a higher level of contrast between immature crater ejecta and average mature material on mercury compared to the moon is consistent with a more intense space weathering environment on mercury that results in a more mature regolith immature lunar highlands materials are 1 5 times higher in reflectance than analogous immature mercurian materials immature materials of the same composition would have the same reflectance on both bodies thus this observation requires that mercury s crust contains a significant darkening agent either opaque minerals or ferrous iron bearing silicates in abundances significantly higher than those of the lunar highlands if the darkening agent is opaque minerals e g ilmenite or ulvospinel mercury s crust may contain significant ferrous iron and yet not exhibit a 1 m absorption band

late cenozoic uplift of mountain ranges and global climate change chicken or egg the high altitudes of most mountain ranges have commonly been ascribed to late cenozoic uplift without reference to when crustal thickening and other tectonic processes occurred deep incision and recent denudation of these mountain ranges abundant late cenozoic coarse sediment near them and palaeobotanical evidence for warmer climates where high mountain climates today are relatively cold have traditionally been interpreted as evidence for recent uplift an alternative cause of these phenomena is late cenozoic global climate change towards lower temperatures increased alpine glaciation a stormier climate and perturbations to humidity vegetative cover and precipitation

mariner 10 multispectral images of the eastern limb and farside of the moon we present the first analysis of mariner 10 multispectral image data for the eastern limb and farside of the moon a new rigorous calibration of these data is used that provides results that are consistent with independently derived data sets from these newly calibrated data a color ratio image 0 48 0 58 m is produced to show relative tio2 abundances in mare units on the eastern limb and farside of the moon our results indicate that the farside lunar mare deposits examined have tio2 contents that are intermediate to low 2 5 tio2 relative to mare deposits on the nearside we find no evidence for areally extensive deposits of either very low titanium vlt or high to very high titanium vht basalts in these portions of the moon our findings support previous interpretations that the source regions for lunar mare basalts are laterally heterogeneous additionally examination of highland units reveals a relatively blue region that corresponds with a previously proposed cryptomare suggesting enhanced tio2 for this unit finally based on these mariner 10 data telescopic spectral data galileo multispectral image data and lunar meteorite analyses we speculate that volcanism in farside mare deposits did not include high titanium basalts 8 wt

a new type of pyroclastic deposit on the moon containing fe spinel and chromite 1 we present details of the identification of sites that show an absorption band at visible wavelengths and a strong 2 m band using the selene spectral profiler all the sites exhibiting the visible feature are found on the regional dark mantle deposit dmd at sinus aestuum all the instances of the visible feature show a strong 2 m band suggestive of fe and cr rich spinels which are different from previously detected mg rich spinel since no visible feature is observed in other dmds the dmd at sinus aestuum is unique on the moon the occurrence trend of the spinels at sinus aestuum is also different from that of the mg rich spinels which are associated with impact structures this may suggest that the spinel at sinus aestuum is a different origin from that of the mg rich spinel

a mariner 10 color study of mercurian craters 1 we have used recalibrated mariner 10 color image data uv and orange to examine spectral trends associated with crater features on the incoming hemisphere of mercury spacecraft images of the moon galileo and clementine and lunar sample laboratory spectra provide a framework for interpreting the mercurian data two spectral parameter images were constructed one reveals opaque mineral abundance and the other provides a map of variations related to optical maturity and feo content the crater kuiper and its rays are bright mainly because they are fresh immature but also because kuiper has excavated material with a lower opaque content than the surroundings under the influence of space weathering kuiper will eventually lose its rays and evolve into a bright floored crater like lermontov lermontov and nearby smaller craters are examples of craters that are likely mature but remain bright because of the low opaque material exposed on their floors in this portion of the planet it appears that a surface layer with moderate opaque abundance overlies low opaque material found at depth on the basis of estimates of crater excavation depths we suggest that the moderate opaque surface layer is 3 4 km thick this two layer stratigraphy is reminiscent of the lunar highland crust and may have originated in similar magma ocean processes we also describe a feature that exposes dark material with a relatively high opaque content this feature could be a mercurian analog to a lunar dark halo impact crater or perhaps a pyroclastic deposit this implies that opaque rich possibly mafic magmas were generated in the interior and reached the surface through effusive or explosive activity

magmatic evolution of the moon although incomplete because of the imperfect and somewhat random sampling of rock types by the apollo and luna missions 1969 1976 the history of lunar magmatism has been reconstructed by numerous researchers over the past three decades these reconstructions have illustrated the continuous nature of lunar magmatism from 4 6 to approximately 2 0 ga and the large influence of early differentiation and catastrophic bombardment on lunar mantle dynamics magmatism and eruptive style in this review we group magmatism into multiple stages of activity based on sampled rock types and evaluate the models for each stage stage 1 is early lunar differentiation and associated magmatism partial melting of the moon soon after accretion was responsible for producing an anorthositic crust and a differentiated lunar interior the extent of lunar melting and mantle processing depends strongly on the mechanisms that induced melting estimates for the time over which melting and crystallization occurred range from tens to hundreds of millions of years stage 2 is the disruption of lunar magma ocean cumulates soon after the crystallization of most of the lunar magma ocean the cumulate pile experienced gravitational overturn this resulted in transport of late forming cumulates into the deep lunar mantle and mixing of magma ocean cumulates on a variety of scales stage 3 is the post magma ocean highland magmatism whereas the ferroan anorthositic crust was probably produced during the crystallization of a magma ocean the slightly younger mg suite and alkali suite plutonic rocks may have been generated by decompressional melting of early magma ocean cumulates during cumulate pile overturn a kreep and crustal signature was incorporated into these primitive basaltic magmas through assimilation near the base of the lunar crust or through melting of a hybridized mantle the alkali suite could represent either the differentiation products of mg suite parental magmas or a separate but contemporaneous episode of basaltic magmatism stage 4 is pre basin volcanism sample analysis and remote sensing data indicate that early lunar volcanism kreep basalts and high alumina basalts was contemporaneous with periods of highlands plutonism and catastrophic bombardment of the lunar surface the relationship between early stages of lunar volcanism and the contemporaneous plutonism is not clear the kreep basalts may be volcanic equivalents to both the mg suite and alkali suite stage 5 is the late remelting of magma ocean cumulates and eruption of mare basalts basin associated eruption of mare basalts occurred during and following the late stages of catastrophic bombardment this volcanic activity was possibly an extension of the thermal event that initiated pre basin volcanism mare basalts exhibit a wide range of composition resulting from near surface fractionation of chemically distinct primary basaltic magmas most likely mare basalts were produced by small to moderate degrees of partial melting of hybrid cumulate sources in the deep lunar mantle alternatively the mixed chemical signatures observed in many mare basalts may be interpreted as indicating assimilation of late stage evolved cumulates by melts produced deep in the cumulate pile the wide range of compositions exhibited by the mare basalts compared with earlier episodes of basaltic magmatism may reflect the thermal regime in the lunar mantle that limited the extent of partial melting and melt source homogenization

the formation of mercury s smooth plains there has been extensive debate about whether mercury s smooth plains are volcanic features or impact ejecta deposits we present new indirect evidence which supports a volcanic origin for two different smooth plains units in borealis planitia stratigraphic relations indicate at least two distinct stages of smooth plains formation at least one of these stages must have had a volcanic origin in the hilly and lineated terrain petrarch and several other anomalously shallow craters apparently have been volcanically filled areally extensive smooth plains volcanism evidently occurred at these two widely separated areas on mercury these results combined with work by other researchers on the circum caloris plains and the tolstoi basin show that smooth plains volcanism was a global process on mercury present data suggest to us that the smooth and intercrater plains may represent two distinct episodes of volcanic activity on mercury and that smooth plains volcanism may have been triggered by the caloris impact high resolution and multispectral imaging from a future mercury spacecraft could resolve many of the present uncertainties in our understanding of plains formation on mercury

constraints on mercury s surface composition from messenger neutron spectrometer data abstract the composition of mercury s surface is poorly known but the messenger mercury surface space environment geochemistry and ranging mission has provided a wealth of new data from three flybys in particular messenger neutron spectrometer ns observations reveal a surface enriched in neutron absorbing elements consistent with interpretations of color and albedo observations suggesting a surface composition enriched in fe mg ti oxides in this study we have computed the neutron absorption cross sections for all of the available proposed surface compositions of mercury and evaluated the plausibility of each surface composition based on the neutron absorption cross section observed by messenger for identified plausible compositions the implications for the thermal and magmatic evolution of mercury are discussed the measured macroscopic neutron absorption cross section of mercury is inconsistent with a crust formed from partial melting of plausible bulk mantle compositions flotation in a magma ocean or adiabatic melting of upwelling cumulates during magma ocean overturn however the observed neutron absorption is consistent with model compositions of late stage magma ocean cumulates and some proposed compositions from spectral modeling and equilibrium modeling this suggests that the enrichment of neutron absorbing elements may be indicative of the processes that acted to form mercury s crust the enrichment in neutron absorbing elements in combination with spectral observations that constrain feo in silicates

valley incision and the uplift of mountain peaks simple end member models describe the effect of isostatically compensated valley incision on the elevation of mountain peaks these models allow calculation of the maximum portion of mountain peak elevations attributable to the development of observed relief and predict general downstream patterns of ridgetop uplift in response to erosion by fluvial processes analyses of topographic profiles across major drainages of the central sierra nevada and the tibetan plateau indicate that isostatically compensated valley incision could account for at most 5 10 of the present elevation of mountain peaks in contrast analyses of topographic profiles across the himalaya show that as much as 20 30 of the present elevation of himalayan peaks could be explained by isostatically compensated valley incision the degree to which valley incision is compensated locally however depends on the flexural rigidity of the lithosphere the size of the range and its tectonic boundary conditions these results show that while a tectonic control is required for late cenozoic surface uplift of the sierra nevada a significant portion of the elevation of himalayan peaks may reflect late cenozoic valley incision on the margin of the tibetan plateau

the magma ocean concept and lunar evolution the model of lunar evolution in which the anorthositic plagioclase rich oldest crust of the moon is formed over a period of 300 myr or less by crystallization as it floats on a global ocean of magma tens or hundreds of km thick is examined in a review of petrological and theoretical studies consideration is given to the classification of lunar rocks the evidence for primordial deep global differentiation constraints on the depth of the molten zone the effects of pressure on mineral stability relationships mainly liquid vs mainly magmifer ocean models and the evidence for multiple ancient differentiation episodes a synthesis of the model of primordial differentiation and its aftereffects is presented and the generalization of the model to the earth and to mars mercury venus and the asteroids is discussed

lunar magma ocean crystallization revisited bulk composition early cumulate mineralogy and the source regions of the highlands mg suite abstract crystallization of the lunar magma ocean lmo has been numerically modeled and its products inferred from sample observations but it has never been fully tested experimentally this study is a reexamination of the lmo hypothesis by means of the first experimental simulation of lunar differentiation two end member bulk moon compositions are considered one enriched in refractory lithophile elements relative to earth and one with no such enrichment a two stage model of magma ocean crystallization based on geophysical constraints is simulated and features early crystal suspension and equilibrium crystallization followed by fractional crystallization of the residual magma ocean an initially entirely molten moon is assumed part 1 of this study presented here focuses on stage 1 of this model and considers the early cumulates formed by equilibrium crystallization differences in mantle mineralogy resulting from different bulk moon compositions and implications for the source regions of the highlands mg suite refractory element enriched bulk moon compositions produce a deep mantle that contains garnet and trace cr spinel in addition to low ca pyroxene and olivine in contrast compositions without refractory element enrichment produce a deep dunitic mantle with low ca pyroxene but without an aluminous phase the differences in bulk composition are magnified in the residual melt the residual lmo from the refractory element enriched composition will likely produce plagioclase and ilmenite earlier and in greater quantities both compositions produce mg rich early cumulate piles that extend from the core mantle boundary to 355 km depth if 50 equilibrium crystallization and whole moon melting are assumed these early lmo cumulates provide good fits for the source regions for a component of the high mg ni and co poor parental magmas of the mg suite cumulates if certain conditions are called upon the olivine in early lmo cumulates produced by either bulk moon composition is far too rich in cr to be reasonable for the source regions of the mg suite meaning either core formation in the presence of s and or c must be invoked to deplete the lmo and the crystallizing olivine in cr or that current estimates of the bulk lunar cr content are too high we infer that melts meeting the criteria of the mg suite parents could be produced from early lmo cumulates by solid state kreep and plagioclase hybridization near the base of the crust and subsequent partial melting additionally we propose a revised model for mg suite petrogenesis

the evolution of mercury s crust a global perspective from messenger mapping the distribution and extent of major terrain types on a planet s surface helps to constrain the origin and evolution of its crust together messenger and mariner 10 observations of mercury now provide a near global look at the planet revealing lateral and vertical heterogeneities in the color and thus composition of mercury s crust smooth plains cover approximately 40 of the surface and evidence for the volcanic origin of large expanses of plains suggests that a substantial portion of the crust originated volcanically a low reflectance relatively blue component affects at least 15 of the surface and is concentrated in crater and basin ejecta its spectral characteristics and likely origin at depth are consistent with its apparent excavation from a lower crust or upper mantle enriched in iron and titanium bearing oxides

mercury s hollows constraints on formation and composition from analysis of geological setting and spectral reflectance 1 landforms unique to mercury hollows are shallow flat floored irregular depressions notable for their relatively high reflectance and characteristic color here we document the range of geological settings in which hollows occur most are associated with impact structures simple bowl shaped craters to multiring basins and ranging from kuiperian to calorian in age hollows are found in the low reflectance material global color unit and in low reflectance blue plains but they appear to be absent from high reflectance red plains hollows may occur preferentially on equator or hot pole facing slopes implying that their formation is linked to solar heating evidence suggests that hollows form because of loss of volatile material we describe hypotheses for the origin of the volatiles and for how such loss proceeds intense space weathering and solar heating are likely contributors to the loss of volatiles contact heating by melts could promote the formation of hollows in some locations lunar ina type depressions differ from hollows on mercury in a number of characteristics so it is unclear if they represent a good analog we also use messenger multispectral images to characterize a variety of surfaces on mercury including hollows within a framework defined by laboratory spectra for analog minerals and lunar samples data from messenger s x ray spectrometer indicate that the planet s surface contains up to 4 sulfur we conclude that nanophase or microphase sulfide minerals could contribute to the low reflectance of the low reflectance material relative to average surface material hollows may owe their relatively high reflectance to destruction of the darkening agent sulfides the presence of alteration minerals and or physical differences in particle size texture or scattering behavior

hollows on mercury materials and mechanisms involved in their formation recent images of the surface of mercury have revealed an unusual and intriguing landform sub kilometre scale shallow flat floored steep sided rimless depressions typically surrounded by bright deposits and generally occurring in impact craters these hollows appear to form by the loss of a moderately volatile substance from the planet s surface and their fresh morphology and lack of superposed craters suggest that this process has continued until relatively recently and may be on going hypotheses to explain the volatile loss have included sublimation and space weathering and it has been suggested that hollow forming volatiles are endogenic and are exposed at the surface during impact cratering however detailed verification of these hypotheses has hitherto been lacking in this study we have conducted a comprehensive survey of all messenger images obtained up to the end of its fourth solar day in orbit in order to identify hollowed areas we have studied how their location relates to both exogenic processes insolation impact cratering and solar wind and endogenic processes explosive volcanism and flood lavas on local and regional scales we find that there is a weak correlation between hollow formation and insolation intensity suggesting formation may occur by an insolation related process such as sublimation the vast majority of hollow formation is in localised or regional low reflectance material within impact craters suggesting that this low reflectance material is a volatile bearing unit present below the surface that becomes exposed as a result of impacts in many cases hollow occurrence is consistent with formation in volatile bearing material exhumed and exposed during crater formation while in other cases volatiles may have accessed the surface later through re exposure and possibly in association with explosive volcanism hollows occur at the surface of thick flood lavas only where a lower reflectance substrate has been exhumed from beneath them indicating that this form of flood volcanism on mercury lacks significant concentrations of hollow forming volatiles

tectonostratigraphic development of the interior sudan rifts central africa abstract in the muglad melut and blue nile rift basins of interior sudan three major episodes of rifting concomitant subsidence and nonmarine nonvolcanic sedimentation are recognized these three rifting cycles which spanned 140 to 95 ma fl 95 to 65 ma f2 and 65 to 30 ma f3 resulted in the accumulation of up to 5400 4200 and 5400 m of sediments respectively in the muglad basin the best known and largest of the sudan rift basins each rifting cycle consists of 1 a basal sandstone unit at least near rift margins that is followed by 2 an upward coarsening section of lacustrine shale grading through marginal lacustrine mudstone and sandstone into fluvial mudstone and sandstone and 3 a capping blanket of fluvial and alluvial sandstone the shale dominated portions of these cycles were deposited in a closed drainage basin during active faulting the fluvial and alluvial blanket sands were deposited in an open drainage basin during the thermal sag phase following each tectonic cycle the early cretaceous f1 intracontinental rifts of interior sudan were linked to major rifts spreading centres in the proto south atlantic by the dextral wsw trending central african shear zone and to the indian ocean via the nw trending anza rift in kenya in the muglad basin f1 deformation involved high strain rates rapid syn rift crustal stretching and subsidence and the formation of deep fault bounded tensional and transtensional pull apart basins during the f2 and f3 deformations the rates of subsidence and stretching were much lower and were focused within smaller geographic areas structural elements include asymmetric half grabens and less common full grabens with central highs the three superimposed tectonic episodes resulted in the subsidence of nnw to nw trending rift sub basins this gave rise to a wide variety of normal fault geometries displacements and growth histories planar domino style and listric normal f1 fault arrays are modeled the rotated f1 basement blocks typically are asymmetric and low standing and favour ne directed growth and material transport the f2 and f3 normal faults which have both ne and sw directed polarities often are listric and decouple younger syn rift strata from older f1 rotated and locked planar basement faults palinspastic restorations and forward modeling of three regional cross sections suggest that f1 faults account for 65 80 of total crustal extension stretching factors across the interior sudan rifts vary between 1 25 and 1 40 this range is considered to be a minimum since restored sections do not cross f3 depocenters total crustal extension across the muglad plus the melut rift basins exceeds 75 km

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

matching martian crustal magnetization and magnetic properties of martian meteorites magnetic properties of 26 of 32 unpaired martian meteorites sncs are synthesized to further constrain the lithology carrying martian magnetic crustal sources magnetic properties of ultramafic cumulates i e chassigny allan hills alh 84001 and lherzolitic shergottites alh 77005 lewis cliff lew 88516 are one or two orders of magnitude too weak to account for the crustal magnetizations assuming magnetization in an earth like field nakhlites and some basaltic shergottites which are the most magnetic sncs show the right intensity titanomagnetite is the magnetic carrier in the nakhlites 7 meteorites whereas in most basaltic shergottites 11 meteorites it is pyrrhotite dhofar dho 378 los angeles and nwa 480 1460 and 2046 are anomalous basaltic shergottites as their magnetism is mainly due to titanomagnetite pyrrhotite should be among the candidate minerals for the magnetized noachian crust

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

the lunar rock and mineral characterization consortium deconstruction and integrated mineralogical petrologic and spectroscopic analyses of mare basalts the lunar rock and mineral characterization consortium lrmcc has conducted coordinated mineralogy petrography spectroscopy analyses of a suite of pristine lunar basalts four basalt slabs two low ti two high ti and paired thin sections were analyzed thin sections were analyzed for mineralogy petrography while the slabs were used to prepare particulate separates of major mineral phases and bulk samples mineral separates and particulate bulk samples were crushed to controlled grain sizes and their reflectance spectra measured in the nasa relab at brown university the resulting data set provides an essential foundation for spectral mixing models offers valuable endmember constraints for space weathering analyses and represents critical new ground truth results for lunar science and exploration efforts

emplacement and tectonic deformation of smooth plains in the caloris basin mercury abstract messenger s first flyby of mercury provided the first complete view of the caloris basin and its interior smooth plains multispectral imaging shows that the interior plains are spectrally distinct from surrounding terrain with a reflectance about 15 20 higher and a steeper spectral slope than the global average the caloris basin rim massifs and hummocky material in contrast have lower reflectance and a shallower spectral slope than the global average and extend around the entire basin isolated relatively red patches along the margins of caloris each surround irregularly shaped scalloped edged rimless depressions that are interpreted to be volcanic vents some with associated pyroclastic deposits a nearly continuous annulus of smooth plains exterior to the basin displays spectral characteristics that contrast sharply with those of the basin interior smooth plains the exterior smooth plains are lower in reflectance and spectrally similar to the caloris rim massifs and the hummocky materials the rims and floors of larger impact craters in the interior plains expose low reflectance material there is widespread evidence of contractional and extensional tectonic deformation of the caloris interior smooth plains a set of linear radial graben pantheon fossae radiates outward from a zone near the basin center and some of these graben extend to and intersect a set of generally basin concentric graben distributed along the outer margin of the caloris basin floor the average areal extensional strain expressed by the mapped system of graben is 0 08 wrinkle ridges also deform the interior smooth plains and predate both basin radial and basin concentric graben the average areal contractional strain expressed by the mapped wrinkle ridges is 0 07 the density of wrinkle ridges on the western interior smooth plains is less than on the eastern interior plains in contrast to the more even distribution of graben a similar difference is seen between the density of wrinkle ridges on the western and eastern exterior smooth plains with far fewer wrinkle ridges per unit area on the western annular plains superposition relations spectral contrast and the occurrence of volcanic vents and possible pyroclastic deposits all support a volcanic origin for the caloris interior plains crater counts indicate that the caloris exterior plains and hummocky material are younger than the caloris rim and interior plains and are therefore also likely volcanic in origin the contrast in spectral properties between the interior and exterior plains indicates a difference in the composition of the two expanses of volcanic material models for extensional tectonic deformation of the caloris basin interior inferred from the distribution of graben viewed by mariner 10 must be reevaluated to account for the discovery of pantheon fossae

compositions of mercury s earliest crust from magma ocean models article i nfo article history the size of the mercurian core and the low ferrous iron bearing silicate content of its crust offer constraints on formation models for the planet here we consider a bulk composition that allows endogenous formation of the planet s large core and by processing the mantle through a magma ocean would produce a low iron oxide crust consistent with observations more earth like bulk compositions require silicate removal perhaps by a giant impact to create the planet s large core fraction we find that the endogenous model can produce a large core with either a plagioclase flotation crust or a low iron oxide magmatic crust because a magma ocean creates a gradient in iron oxide content in the resulting planetary mantle the parts of the mantle removed by a putative giant impact could result in either a high iron oxide mantle in contradiction to current crustal measurements or a low iron oxide mantle consistent with the current understanding of mercury if a giant impact cannot preferentially remove shallow mantle material then the proto mercury must have had a bulk low iron oxide composition thus a specific bulk composition is required to make mercury endogenously and either a specific process or a specific composition is required to make it exogenously through giant impact measurements taken by the messenger mission when compared to predictions given here may help resolve mercury s formation process

rainfall patterns and the distribution of coals and evaporites in the mesozoic and cenozoic abstract paleoclimatologists have adequately demonstrated over the last twenty years that the distribution of climatically sensitive sediments can be related to present day climatic zones that are roughly parallel to latitude for example coals were deposited at paleolatitudes that correspond to the present day rainy zones at the equator and at about 55 n and 55 s indicating that those zones were present in the past from this it can be concluded that atmospheric circulation has not been radically different from its present configuration despite some apparently great differences in some climatic parameters such as the equator to pole temperature gradient nevertheless the zonal model for predicting the distribution of climatically sensitive sediments is not wholly adequate although general atmospheric circulation may not have been radically altered from its present pattern it would have differed in some respects because the paleogeography was different from the present geography therefore we have constructed what we believe are more realistic maps of the distribution of rainfall through time these maps are based on maps of atmospheric circulation that have been successfully used to predict the distribution of some petroleum source beds and phosphorites we have compared the predicted distribution of rainfall to the distribution of coals and evaporites for each of seven geologic stages the circulation rainfall maps were more successful than the zonal model for predicting the distribution of evaporites and coals for times when the paleogeography would have maximized the development of a monsoonal circulation which most effectively disrupts the general zonal pattern for times when a monsoonal circulation would not have been important the zonal model adequately predicted the distribution of these rocks

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

return to mercury a global perspective on messenger s first mercury flyby in january 2008 the mercury surface space environment geochemistry and ranging messenger spacecraft became the first probe to fly past the planet mercury in 33 years the encounter revealed that mercury is a dynamic system its liquid iron rich outer core is coupled through a dominantly dipolar magnetic field to the surface exosphere and magnetosphere all of which interact with the solar wind messenger images confirm that lobate scarps are the dominant tectonic landform and record global contraction associated with cooling of the planet the history of contraction can be related to the history of volcanism and cratering and the total contractional strain is at least one third greater than inferred from mariner 10 images on the basis of measurements of thermal neutrons made during the flyby the average abundance of iron in mercury s surface material is less than 6 by weight

laboratory chemical remanent magnetization in a natural claystone a record of two magnetic polarities summary a record of two magnetic polarities during the acquisition by heating of a laboratory chemical remanent magnetization crm in a natural pyrite rich claystone was investigated the samples were heated for 22 hr at 250 ci na controlled magnetic field 1 0 mt under an argon atmosphere we interpret the origin of crm as the surface oxidation of pyrite to magnetite which in turn is progressively oxidized into haematite we carried out experiments under a constant polarity magnetic field and under two opposite polarities the resulting crm was measured after cooling in zero field it is parallel to the applied field and has the direction of the last polarity thermal demagnetization under an argon atmosphere isolates an unexpected lowunblocking temperature component t ub 220 c this component probably results from thermal alteration of magnetic carriers during subsequent thermal demagnetization in the onepolarity experiments thermal demagnetization of crm above 220 c isolates a well defined component parallel to the imparted field direction in the experiments with two magnetic polarities thermal demagnetization of crm reveals two components of opposite polarity the component which is parallel to the direction of the last applied field is well defined while the other component which has the polarity of the first applied field is ill defined oppositely directed components are also detected by using alternating field demagnetization

experimental evaluation of thermal recording of successive polarities during uplift of metasediments summary the discovery of a post tectonic natural remanent magnetization nrm carried by pyrrhotite in the dauphinoise zone of the western alps allows us to test the ability of these rocks to record a sequence of magnetic polarity versus temperature acquired during their slow cooling several laboratory experiments composite partial thermal remanent magnetizations ptrms thellier thellier experiments trm thermal demagnetization as well as nrm demagnetization were performed this shows that the pyrrhotite grains are single domain sized and that they are able to record successive independent ptrms the nrm thermal demagnetization analysis demonstrates the occurrence of several antiparallel components a comparison between nrm and trm thermal demagnetizations is attempted in order to highlight small ptrms naturally recorded in a temperature interval of only a few degrees we attempt to retrieve the temperature at which each reversal occurs during the post metamorphic cooling this method called thermopalaeomagnetism is discussed and developed in order to delineate the sequence of polarity chrons recorded versus temperature

petroleum geology of cretaceous tertiary rift basins in niger chad and central african republic this overview of the petroleum geology of rift basins in niger chad and central african republic car is based on exploration work by exxon and partners in the years 1969 1991 the work included 50 000 km of modern reflection seismic 53 exploration wells 1 000 000 km2 of aeromagnetic coverage and about 10 500 km of gravity profiles the results outline ten cretaceous and tertiary rift basins which constitute a major part of the west and central african rift system wcars the rift basins derive from a multiphased geologic history dating from the pan african approximately 750 550 ma to the holocene main rifting was probably caused by the stretching and subsidence of african crustal blocks during the breakup of gondwana in the early cretaceous and by r activated plate movements in the early tertiary wcars in the study area is divided into the west african rift subsystem was and the central african rift subsystem cas was basins in niger and chad are chiefly extensional and are filled by up to 13 000 m of lower cretaceous to holocene continental and marine clastics the basins contain five oil 19 43 api and two oil and gas accumulations in upper cretaceous and eocene sandstone reservoirs the hydrocarbons are sourced and sealed by upper cretaceous and eocene marine and lacustrine shales the most common structural styles and hydrocarbon traps usually are associated with normal fault blocks cas rift basins in chad and car are extensional and transtensional and are filled by up to 7500 m of chiefly lower cretaceous continental clastics the basins contain eight oil 15 39 api and one oil and gas discovery in lower and upper cretaceous sandstone reservoirs the hydrocarbons are sourced by lower cretaceous shales and sealed by interbedded lacustrine and flood plain shales structural styles range from simple fault blocks through complex flower structures the main hydrocarbon traps are in contractional anticlines geological conditions favor the discovery of potentially commercial volumes of oil in wcars basins of niger chad and car

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

explosive volcanism on mercury analysis of vent and deposit morphology and modes of eruption abstract the messenger mission revealed for the first time conclusive evidence of explosive volcanism on mercury several previous works have cataloged the appearance and location of explosive volcanism on the planet using a variety of identifying characteristics including vent presence and deposit color as seen in multispectral image mosaics we present here a comprehensive catalog of vents of likely volcanic origin our classification scheme emphasizes vent morphology we have analyzed the morphologies of all vents in our catalog and recognize three main morphologies simple vent pit vent and vent with mound the majority of vents we identify are located within impact craters the spatial distribution of vents does not correlate with the locations of volcanic smooth plains deposits in contrast to the moon nor do vents correlate with the locations of large impact basins except for the caloris and tolstoj basins using the degradation state of the vent host crater as a proxy for maximum age we suggest that vent formation has been active through the mansurian and into the kuiperian periods although the majority of vents were likely formed much earlier in mercurian history the morphologies and locations of vents are used to investigate a set of plausible formation geometries we find that the most likely and most prevalent formation geometry is that of a dike stalled at depth which then explosively vents to the surface we compare the vent and deposit size of mercurian pyroclastic deposits with localized and regional lunar pyroclastic deposits and find a range of possible eruption energies and corresponding variations in eruption style localized lunar pyroclastic deposits and the majority of mercurian pyroclastic deposits show evidence for eruption that is consistent with the magmatic foam at the top of a dike reaching a critical gas volume fraction a subset of mercurian vents including the prominent copland rachmaninoff vent to the northeast of the rachmaninoff basin indicates eruption at enhanced gas volume fractions this subset of vents shows a similar eruptive behavior to the lunar orientale dark mantle ring deposit suggesting that the dikes that formed these vents and deposits on mercury underwent some form of additional volatile build up either through crustal volatile incorporation or magma convection within the dike there also exists a population of mercurian vents that no longer retain a visible associated pyroclastic deposit we hypothesize that the visible signature of the pyroclastic deposit has been lost through space weathering and regolith mixing processes together these results provide a comprehensive analysis of explosive volcanism on mercury and inform continued research on the thermal history of mercury and magma composition and evolution

recalibrated mariner 10 color mosaics implications for mercurian volcanism recalibration of mariner 10 color image data allows the identification of distinct color units on the mercurian surface we analyze these data in terms of opaque mineral abundance iron content and soil maturity and find color units consistent with the presence of volcanic deposits on mercury s surface additionally materials associated with some impact craters have been excavated from a layer interpreted to be deficient in opaque minerals within the crust possibly analogous to the lunar anorthosite crust these observations suggest that mercury has undergone complex differentiation like the other terrestrial planets and the earth s moon

the origin of boninites on mercury an experimental study of the northern volcanic plains lavas abstract phase equilibrium experiments were conducted on a synthetic rock composition matching that of the northern volcanic plains of mercury as measured by the mercury surface space environment geochemistry and ranging spacecraft messenger the northern volcanic plains are smooth plains of suspected volcanic origin that cover more than 6 of the surface area of mercury the northern volcanic plains are less cratered than their surroundings and reported to be the product of flood volcanism making them a prime candidate for experimental study the bulk composition of the northern volcanic plains is that of an alkali rich boninite and represents the first silica enriched crustal terrane identified on an extraterrestrial planet from orbital data phase equilibrium experiments were conducted over the pressure range of the mercurian mantle 0 5 5 gpa at very low oxygen fugacity iw0 to 7 using a piston cylinder apparatus p 0 5 1 7 gpa and a walker style multi anvil device p 2 5 gpa our results indicate the origin of the northern volcanic plains lavas boninites are best explained by high degrees of partial melting of an olivine dominant pyroxene and plagioclase bearing mantle source at low pressure 1 4 gpa and does not require hydrous melting to achieve the silica enriched melt composition the formation mechanism for boninites on mercury contrasts substantially with terrestrial boninites which typically occur in oxidized and hydrous arc environments associated with subduction zones instead mercurian boninites form at exceptionally low oxygen fugacity and do not require melting of hydrated source materials the nvp lavas represent a novel mechanism by which planetary bodies can form silica enriched secondary crusts without the aid of water

nonmarine cretaceous palynology of northern kordofan sudan with notes on fossil salviniales water ferns a palynological investigation of 164 samples from 18 water wells in northern kordofan sudan enabled the recognition of five informal zones based on pollen and spore assemblages ranging in age from albian to maastrichtian the youngest late campanian maastrichtian assemblages are restricted to the bagbag basin whereas albian cenomanian to turonian sediments are widespread to the east and west of the bagbag area impressions of salvinia floating leaves from outcrops of the upper hamrat el wuz formation western part of the study area are among the oldest occurrences of this water fern and indicate a campanian maastrichtian age for these sediments

sulfide mineralogy and redox conditions in some shergottites magmatic sulfide mineralogy has been studied in 2 olivine phyric shergottites dag 476 and sau 005 and 4 basaltic shergottites zagami shergotty los angeles and nwa 480 modal abundances of magmatic sulfides as estimated by image analysis on thin section are high 0 16 to 0 53 area percent and correlate positively with abundances of fe ti oxides sulfides are mesostasis minerals being mostly interstitial grains or locally enclosed in post cumulus melt inclusions e g sau 005 in olivine sulfides in shergottites are composed of major pyrrhotite containing pentlandite exsolutions associated with minor amounts of cu sulfides chalcopyrite and or cubanite hot desert finds e g dag 476 show abundant fracture filling iron oxy hydroxides of probable terrestrial origin unaltered sulfides show metal rich hexagonal pyrrhotite compositions with metal sulfur m s ratio ranging between 0 936 0 005 and 0 962 0 01 this compositional range corresponds to the two phase structural domain 2c nc of the fe s system however the high temperature disordered hexagonal 1c pyrrhotite structure would be in better agreement with magnetic properties of shergottites ni contents in pyrrhotite increase from los angeles 0 05 at to shergotty zagami and nwa 480 0 2 0 5 at and dag 476 and sau 005 up to 3 at the higher ni values of pyrrhotite in olivine phyric shergottites correlate with the abundance of pentlandite exsolutions both reflecting more primitive ni rich sulfide liquids where abundant olivine crystallized this result and the strong correlation between sulfide abundances and fe ti oxides argue for a primary magmatic origin of these sulfides although they reproduce the trend of magmatic oxygen fugacity conditions determined from fe ti oxide pairs observed pyrrhotite compositions are systematically more metaldeficient compared to those calculated from the fe s o system this suggests post magmatic oxidation during cooling on mars followed by terrestrial weathering for hot desert finds

optical effects of space weathering the role of the finest fraction the optical properties of lunar softs are different than those of rocks from which they are derived as a consequence of lunar space weathering soils are darker and exhibit a distinctive red sloped continuum and weaker mineral absorption bands the accumulation of dark glass welded aggregates agglutinates has been thought to account for these optical effects of space weathering on lunar soils spectroscopic analyses of agglutinate separates and size fractions for a suite of lunar softs presented here indicate that the agglutinate paradigm is insufficient to fully account for lunar optical alteration it is the finest fraction of lunar soils 25 gm which constitute 25 wt that dominates the optical properties of the bulk soil unlike size fractions of most silicates for which the finest fraction is the brightest the lunar soil size fractions all have comparable albedos in the short wavelength visible in the near infrared however it is the finest fraction that exhibits the steep red continuum and weak absorption bands the properties of the finest fraction cannot be duplicated by preparing a fine fraction by grinding larger agglutinate rich soil particles these results suggest space weathering on airless bodies is dominated by surface correlated processes perhaps associated with the development of fine grained fe o on or near the surface of grains and the accumulation of the larger agglutinates is not necessarily required to account for lunar optical alteration

an analysis of the mariner 10 color ratio map of mercury abstract the results of a geological analysis of the mariner 10 orange uv color ratio man of mercury b hapke c christman b rava and j mosher proc lunar planet sci conf 11th 1980 pp 817 822 are given certain errors that occured in reproducing the published version of the 1980 map are pointed out the relationships between color and terrain are distinctly nonlunar there is no correlation between color boundaries and the smooth plains on mercury in contrast to the strong correlation between color and maria highlands contacts on the moon there are no large exposures of low albedo blue material that could be considered to be mercurian analogs of high fe ti lunar maria basalts on any part of mercury imaged by mariner 10 three lines of evidence imply that the crust is low in fe2 and ti4 rays and ejecta blankets are bluer than most areas on mercury the fe2 band in mercury s reflectance spectrum is very weak or nonexistent and the albedo contrasts are smaller than those on the moon there is no evidence in the spectral or albedo data that a lunar type of second wave of melting ever occured on mercury rather the observations are most consistent with the hypothesis that the smooth plains are extrusive landforms derived from local material possibly mobilized by the caloris event in several places correlations between color and topography can be explained if older redder higher fe materials underlie younger bluer lower fe surfaces there is some evidence of late fe rich pyroclastic like activity

phase ratio images of the surface of mercury evidence for differences in sub resolution texture abstract analysis of images that were obtained at different phase angles can give clues to the texture of a planetary surface here we describe messenger phase ratio images for features on the surface of mercury including hollows on the floor of eminescu basin a pyroclastic deposit found within the caloris basin and a dark impact melt flow produced by the impact that formed waters crater the results indicate that hollows and the pyroclastic material are characterized by finer particle size or smoother sub resolution roughness than the ordinary impact generated regolith the impact melt flow appears to have a surface that is rougher or consists of coarser particle sizes than the nearby background

nature of opaque components on mercury insights into a mercurian magma ocean 1 analysis of mariner 10 and messenger data sets reveal the importance of opaque components on mercury s surface a global darkening agent suggested to be ilmenite or other fe ti bearing opaque mineral has been invoked to explain the lower albedo of mercury relative to the lunar highlands separately a low reflectance material lrm has been recognized as one of three dominant color terrains we present laboratory reflectance spectra of ilmenite size separates and other candidate fe ti bearing oxide minerals these oxides cannot sufficiently darken mercury without violating neutron spectrometer constraints on surface iron content the spectra of all samples exhibit negative spectral slopes shortward of 500 nm consistent with the lrm we review models of crystallization of an feo poor mercurian magma ocean and show that lack of a plagioclase flotation crust could lead to a thin quench crust with near surface layers of incompatible and ti rich late stage cumulates consistent with mercury s albedo and lrm citation riner m a p g lucey s j desch and f m mccubbin 2009 nature of opaque components on mercury insights into a mercurian magma ocean geophys res lett 36 l02201 doi 10 1029 2008gl036128

reflectance and color variations on mercury regolith processes and compositional heterogeneity multispectral images of mercury obtained by the messenger spacecraft reveal that its surface has an overall relatively low reflectance with three large scale units identified on the basis of reflectance and slope 0 4 to 1 0 micrometer a higher reflectance relatively red material occurs as a distinct class of smooth plains that were likely emplaced volcanically a lower reflectance material with a lesser spectral slope may represent a distinct crustal component enriched in opaque minerals possibly more common at depth a spectrally intermediate terrain probably forms most of the upper crust three other spectrally distinct but spatially restricted units include fresh crater ejecta less affected by space weathering than other surface materials high reflectance deposits seen in some crater floors and moderately high reflectance relatively reddish material associated with rimless depressions

evidence for a basalt free surface on mercury and implications for internal heat microwave and mid infrared observations reveal that mercury s surface contains less feo tio2 and at least as much feldspar as the lunar highlands the results are compatible with the high albedo brightness of mercury s surface at visible wavelengths in suggesting a rock and soil composition that is devoid of basalt the primary differentiate of terrestrial mantles the occurrence of a basalt free highly differentiated crust is in accord with recent models of the planet s thermal evolution and suggests that mercury has retained a hot interior as a result of a combination of inefficient mantle convection and minimal volcanic heat loss

multispectral images of mercury from the first messenger flyby analysis of global and regional color trends abstract we have analyzed messenger multispectral images of mercury in order to gain information on the nature of materials present on the planet s surface the quantitative analysis tools of lucey which are built on the hapke conceptual framework for the reflectance of a silicate regolith allow us to derive spectral parameter images that are sensitive to the abundance of ferrous iron in silicates the abundance of opaque phases and the state of maturity of the surface consistent with previous estimates we conclude that the silicate portion of mercury s surface is low in ferrous iron with a small range of variation the spectral changes that accompany regolith maturation on mercury are consistent with lunar style space weathering via the accumulation of nanophase iron the spectral character of mercury s major surface units high reflectance plains intermediate terrain and low reflectance material can be attributed to variations in the abundance of an opaque phase s a class of minor color unit the red spots occurs in at least three morphological varieties two of which are associated with deposits interpreted to be of pyroclastic origin the bright crater floor deposits a second type of minor unit have extremely shallow blue spectral slopes that are unrelated to the presence of immature material

impact induced thermal effects in the lunar and mercurian regoliths while the moon and mercury are similar objects in a geomorphologic sense they also possess important differences particularly in the context of small scale impact phenomena not only is the surface of mercury notably hotter than that of the moon but the impact flux is also more intense at mercury due to higher impact velocities and a greater spatial density of micrometeoroids by extrapolating the terrestrial micrometeoroid flux to the moon and mercury it is found that the impact rate at mercury is 5 5 times greater and the mean impact velocity more than 60 percent higher than at the moon a model of impact melting and vaporization applied to the lunar and mercurian environments indicates that almost 14 times more impact melt and over 20 times more vapor are generated per unit time in the mercurian regolith although the surface temperature plays a role in determining the melt and vapor volumes the difference in impact velocity appears to be markedly more important average craters formed by identical projectiles under the two velocity distributions on the two planets should be very similar in size because differences in the kinetic energy of the average impactors will be offset by the differences in gravitational acceleration as a result variations in regolith mixing should depend more on the actual impact rates than on disparities in excavated volumes each average impact on mercury however will produce more than twice as much impact melt as its lunar counterpart introducing more glass into the mercurian regolith although substantial quantities of impact vapor are produced on both planets mixing of the regolith should occur quickly enough in each case to spread the deposited vapor over large surface areas the resulting vapor coatings should be negligibly small for most purposes a much larger agglutinate population should exist on mercury and individual agglutinates should be similar to those of the lunar highlands reflectance spectra and visual albedos indicate that the mercurian crust possesses relatively small quantities of fe perhaps similar to those of apollo 16 light and dark matrix breccias laboratory studies have demonstrated that as the magnetic fractions which are correlated with agglutinate abundances of soils derived from these breccias increase the differences in 0 565 mm visible light reflectivity between the soils diminish should the regional variations in fe content of the mercurian crust be similar to those of these apollo 16 samples then the intense agglutination environment could account for the lack of optical contrast across the mercurian surface at the same time the very large fraction of impact glass in the regolith would imply that most pyroxenes have been fused and incorporated into these glasses seriously attenuating the fe2 crystal field absorption near 0 9 mm supporting this picture are the most recently published spectral observations of mercury which provide little or no evidence of such an absorption feature

crustal magnetization of mars controlled by lithology or cooling rate in a reversing dynamo 1 to explain the lack of magnetization in large portions of the noachian crust a lithological origin has been put forward corresponding to the lack of formation of magnetic minerals in the crustal rocks an alternative hypothesis is proposed here with constant amount of magnetic minerals but variable reduction of total magnetization over the magnetic thickness due to variable record of successive polarities of the magnetizing field large crustal cooling rates would correspond to strongly magnetized zone and slow cooling rates to poorly magnetized zone based on this model we derive an explanation for the proposed confinement of magnetized zones within the inter tropical band

magnetic properties of variably serpentinized abyssal peridotites we have compiled new and published data on the magnetic properties of 245 serpentinized abyssal peridotites from seven deep sea drilling project and ocean drilling program sites the magnetic susceptibility k of these samples does not increase linearly with the degree of serpentinization s instead k remains modest in partially serpentinized samples s 75 peridotites in the oceanic crust moderately serpentinized peridotites s 75 have nrm values 5 a m and k 0 05 si low temperature oxidation of magnetite is found to lower the nrm and k values of seafloor samples and of cataclastically deformed drilled samples

spectral effects of space weathering on mercury the role of composition and environment 1 the unique composition and space environment of mercury provides an opportunity to test models of space weathering and can potentially provide constraints on the relative importance of physical and chemical processes involved in space weathering in this work we apply a space weathering model to multispectral images of mercury and incorporate the geologic context of spectra into the interpretation of the model results we find that albedo variations on mercury s surface cannot be explained by the addition of an opaque mineral alone instead differential accumulation of space weathering products related to composition contributes significantly to albedo variations on mercury s surface even relatively immature crater ejecta on mercury have several times more space weathering derived iron than mature lunar soils complicating interpretations of mercury s surface composition

thermochronometry and cooling rates deduced from single sample records of successive magnetic polarities during uplift of metamorphic rocks in the alps france summary pyrrhotite bearing metamorphic calcschists from the western alps very probably carry a trm acquired during slow cooling of the unit during the late tertiary sequences of successive polarities have been recorded and can be unravelled by very detailed thermal demagnetization polarity versus temperature profiles on single samples are compared to the polarity time scale allowing on a statistical basis the estimation of a cooling rate of soocmyr at the time of trm acquisition 20 25 ma low apparent palaeointensities i0 pt are estimated by comparison with artificial trm

phase equilibria of ultramafic compositions on mercury and the origin of the compositional dichotomy measurements of major element ratios obtained by the messenger spacecraft using x ray fluorescence spectra are used to calculate absolute element abundances of lavas at the surface of mercury we discuss calculation methods and assumptions that take into account the distribution of major elements between silicate metal and sulfide components and the potential occurrence of sulfide minerals under reduced conditions these first compositional data which represent large areas of mixed highreflectance volcanic plains and low reflectance materials and do not include the northern volcanic plains share common silica and magnesium rich characteristics they are most similar to terrestrial volcanic rocks known as basaltic komatiites two compositional groups are distinguished by the presence or absence of a clinopyroxene component melting experiments at one atmosphere on the average compositions of each of the two groups constrain the potential mineralogy at mercury s surface which should be dominated by orthopyroxene protoenstatite and orthoenstatite plagioclase minor olivine if any clinopyroxene augite and tridymite the two compositional groups cannot be related to each other by any fractional crystallization process suggesting differentiated source compositions for the two components and implying multi stage differentiation and remelting processes for mercury comparison with high pressure phase equilibria supports partial melting at pressure o10 kbar in agreement with last equilibration of the melts close to the crust mantle boundary with two different mantle lithologies harzburgite and lherzolite magma ocean crystallization followed by adiabatic decompression of mantle layers during cumulate overturn and or convection would have produced adequate conditions to explain surface compositions the surface of mercury is not an unmodified quenched crust of primordial bulk planetary composition ultramafic lavas from mercury have high liquidus temperatures 1450 1350 1c and very low viscosities in accordance with the eruption style characterized by flooding of pre existing impact craters by lava and absence of central volcanoes