flood volcanism in the northern high latitudes of mercury revealed by messenger messenger observations from mercury orbit reveal that a large contiguous expanse of smooth plains covers much of mercury s high northern latitudes and occupies more than 6 of the planet s surface area these plains are smooth embay other landforms are distinct in color show several flow features and partially or completely bury impact craters the sizes of which indicate plains thicknesses of more than 1 kilometer and multiple phases of emplacement these characteristics as well as associated features interpreted to have formed by thermal erosion indicate emplacement in a flood basalt style consistent with x ray spectrometric data indicating surface compositions intermediate between those of basalts and komatiites the plains formed after the caloris impact basin confirming that volcanism was a globally extensive process in mercury s post heavy bombardment era

explosive volcanic eruptions on mercury eruption conditions magma volatile content and implications for interior volatile abundances images obtained by the messenger spacecraft have revealed evidence for pyroclastic volcanism on mercury because of the importance of this inference for understanding the interior volatile inventory of mercury we focus on one of the best examples determined to date a shield volcano like feature just inside the southwestern rim of the caloris impact basin characterized by a near central irregularly shaped depression surrounded by a bright deposit interpreted to have a pyroclastic origin this candidate pyroclastic deposit has a mean radius of 24 km greater in size than the third largest lunar pyroclastic deposit when scaled to lunar gravity conditions from the extent of the candidate pyroclastic deposit we characterize the eruption parameters of the event that emplaced it including vent speed and candidate volatile content the minimum vent speed is 300 m s and the volatile content required to emplace the pyroclasts to this distance is hundreds to several thousands of parts per million ppm of the volatiles typically associated with pyroclastic eruptions on other bodies e g co co2 h2o so2 h2s for comparison measurements of the exsolution of volatiles h2o co2 s from basaltic eruptive episodes at kilauea volcano hawaii indicate values of 1300 6500 ppm for the terrestrial mantle source evidence for the presence of significant amounts of volatiles in partial melts derived from the interior of mercury is an unexpected result and provides a new constraint on models for the planet s formation and early evolution

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

volcanism on mercury evidence from the first messenger flyby for extrusive and explosive activity and the volcanic origin of plains the first messenger flyby of mercury obtained images of 21 of the surface not seen by mariner 10 including the center and western half of the caloris basin and regions near the terminator that show details of the nature of smooth and intercrater plains these new data have helped to address and resolve a series of longstanding questions on the existence and nature of volcanism on mercury and the distribution of volcanic materials data from the mercury dual imaging system mdis on the messenger spacecraft have shown the following 1 numerous volcanic vents in the form of irregularly shaped rimless depressions are concentrated around the interior edge of the caloris basin 2 these vents appear to be sources for effusive volcanism that in one case built a shield in excess of 100 km in diameter and in some cases formed bright haloes around the vents that are interpreted to represent pyroclastic eruptions 3 lobate margins of plains units seen previously in mariner 10 data are documented in messenger images with more clarity and are often distinctive in morphology and color properties supporting the interpretation that these features are the edges of lava flow units 4 the interior of the caloris basin is filled with plains units spectrally distinctive from the rim deposits and comparison with the lunar imbrium basin and superposed impact crater stratigraphy provide evidence that these units are volcanic in origin detailed differences in the mineralogy of lava flow units so prominent in imbrium are not seen in the caloris interior 5 some of the smooth plains surrounding the exterior of the caloris basin show distinct differences in color and morphological properties supporting a volcanic origin 6 some smooth and intercrater plains units distant from the caloris basin show evidence of flooding and embayment relations unrelated to caloris ejecta emplacement local and regional geological and color relationships support a volcanic origin for these plains 7 large impact craters show a sequence of embayment of interior floor and exterior ejecta deposits that supports a volcanic origin for the embayment and filling processes 8 crater embayment and flooding relationships in selected areas suggest volcanic plains thicknesses of many hundreds of meters and local thicknesses inside impact craters of up to several kilometers 9 impact crater size frequency distributions for caloris exterior deposits including the facies of the caloris group and relatively high and low albedo smooth plains show that they are younger than plains interior to caloris and thus must be dominantly the product of post caloris volcanism these new data provide evidence that supports and confirms earlier hypotheses from mariner 10 data that volcanism was important in shaping the surface of mercury the emerging picture of the volcanic style of mercury is similar to that of the moon the other small one plate planetary body there are no major shield volcanoes e g comparable to tharsis montes on mars shallow magma reservoirs are rare and there is little evidence for surface deformation or long lived volcanic sources related to sites of upwelling mantle the close association of volcanic plains and surface deformation features suggests that future observations and analyses can help document the relation between the volcanic flux and the evolving state and magnitude of stress in the lithosphere of mercury

mineralogy of the mafic anomaly in the south pole aitken basin implications for excavation of the lunar mantle the mineralogy of south pole aitken basin spa the largest confirmed impact basin on the moon is evaluated using five color images from clementine although olivine rich material as well as basalts rich in clinopyroxene are readily identified elsewhere on the farside the dominant rock type observed across the interior of spa is of a very noritic composition this mineralogy suggests that lower crust rather than the mantle is the dominant source of the mafic component at spa the lack of variation in observed noritic composition is probably due to basin formation processes during which extensive melting and mixing of target materials are likely to occur

the formation of plagioclase chains during convective transfer in basaltic magma the basaltic rock in the lower part of the thick holyoke lava flow in connecticut and massachusetts has been shown to have a remarkable texture with crystals of feldspar linked together in a continuous three dimensional network of chains1 heating experiments have revealed that this network persists to temperatures where the rock is 75 liquid2 and therefore the network was interpreted to have formed at an early stage of crystallization and to have played an important role in the compaction of crystal mush in the lower part of the flow3 4 despite the texture s importance to our understanding of how such basalt flows form the origin of the texture has remained uncertain1 2 3 here we show that although the network is present in the lower third of the flow it was actually formed in the upper solidification front and was transported down in plumes of dense crystal mush convection of this type has been postulated for intrusive magma chambers but corroborative field evidence has been equivocal especially in lava lakes and flows preservation of the roof generated texture in the lower part of a thick flood basalt flow therefore constitutes important evidence for the role of convection in the solidification and differentiation of a simple magma sheet

olivine and pyroxene diversity in the crust of mars datafromthe observatoire pour la mineralogie l9eau les glaces et l9activite omega on the mars express spacecraft identify the distinct mafic rock forming minerals olivine low calcium pyroxene lcp and high calcium pyroxene hcp on the surface of mars olivine and hcp rich regions are found in deposits that span the age range of geologic units however lcp rich regions are found only in the ancient noachian aged units which suggests that melts for these deposits were derived from a mantle depleted in aluminum and calcium extended dark regions in the northern plains exhibit no evidence of strong mafic absorptions or absorptions due to hydrated materials

quantification of extraterrestrial lava flow effusion rates through laboratory simulations we have used carefully controlled laboratory simulations to develop a model which relates lava flow morphology to effusion rate and rheology through comparisons with measured and estimated eruption rates on earth this approach allows us to constrain eruptive styles and compositions of extraterrestrial lava flows by applying this model to lava flows on the moon mars and venus we have determined that all the common flow morphologies domes folds and levees on these planets could have been produced by basalt like or andesite like lavas through either continuous or episodic emplacement the presence of more evolved magma compositions on other planets is not required to explain any of the observed lava flow morphologies

the distribution and origin of smooth plains on mercury 1 orbital images from the messenger spacecraft show that 27 of mercury s surface is covered by smooth plains the majority 65 of which are interpreted to be volcanic in origin most smooth plains share the spectral characteristics of mercury s northern smooth plains suggesting they also share their magnesian alkali basalt like composition a smaller fraction of smooth plains interpreted to be volcanic in nature have a lower reflectance and shallower spectral slope suggesting more ultramafic compositions an inference that implies high temperatures and high degrees of partial melting in magma source regions persisted through most of the duration of smooth plains formation the knobby and hummocky plains surrounding the caloris basin known as odin type plains occupy an additional 2 of mercury s surface the morphology of these plains and their color and stratigraphic relationships suggest that they formed as caloris ejecta although such an origin is in conflict with a straightforward interpretation of crater size frequency distributions if some fraction is volcanic this added area would substantially increase the abundance of relatively young effusive deposits inferred to have more mafic compositions smooth plains are widespread on mercury but they are more heavily concentrated in the north and in the hemisphere surrounding caloris no simple relationship between plains distribution and crustal thickness or radioactive element distribution is observed a likely volcanic origin for some older terrain on mercury suggests that the uneven distribution of smooth plains may indicate differences in the emplacement age of large scale volcanic deposits rather than differences in crustal formational process

long lived explosive volcanism on mercury the duration and timing of volcanic activity on mercury are key indicators of the thermal evolution of the planet and provide a valuable comparative example for other terrestrial bodies the majority of effusive volcanism on mercury appears to have occurred early in the planet s geological history 4 1 3 55 ga but there is also evidence for explosive volcanism here we present evidence that explosive volcanism occurred from at least 3 9 ga until less than a billion years ago and so was substantially more long lived than large scale lava plains formation this indicates that thermal conditions within mercury have allowed partial melting of silicates through the majority of its geological history and that the overall duration of volcanism on mercury is similar to that of the moon despite the different physical structure geological history and composition of the two bodies

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

prediction of plagioclase melt equilibria in anhydrous silicate melts at 1 atm many models for plagioclase melt equilibria have been proposed over the past 30 years but the focus is increasingly on the effects of water content and pressure however many geological and petrological applications concern low pressure and low water systems such as the differentiation of large terrestrial basaltic magma chambers and lunar and asteroidal magmatism there is therefore a justified need to quantify the influence of anhydrous liquid composition on the composition of equilibrium plagioclase at 1 atm with this in mind a database of over 500 experimentally determined plagioclase liquid pairs has been created the selected low pressure anhydrous experiments include both natural and synthetic liquids whose compositions range from basalt to rhyolite four equations are proposed derived from this data the first is based on a thermodynamically inspired formalism explicitly integrating the effect of temperature this equation uses free energies and activities of crystalline anorthite available from the literature for the activity of anorthite in the liquid phase it is found that current models of the activity of individual oxides are insufficient to account for the experimental results we have therefore derived an empirical expression for the variation of anorthite activity in the liquid as a function of melt composition based upon inversion of the experimental data using this expression allows the calculation of plagioclase composition with a relative error less than 10 however in light of the fact that temperature is not necessarily known for many petrological applications an alternative set of t independent equations is also proposed for this entirely empirical approach the database has been divided into three compositional groups treated independently for regression purposes mafic ultramafic alkali rich mafic ultramafic and intermediate felsic this separation into distinct subgroups was found to be necessary to maintain errors below acceptable limits but results across group boundaries were found to be comparable overall 50 of plagioclase compositions are predicted to within 2 of the experimentally derived value and 90 to within 5 representing a significant improvement over existing models

evidence for geochemical terranes on mercury global mapping of major elements with messenger s x ray spectrometer abstract we have mapped the major element composition of mercury s surface from orbital messenger x ray spectrometer measurements these maps constitute the first global scale survey of the surface composition of a solar system body conducted with the technique of planetary x ray fluorescence full maps of mg and al together with partial maps of s ca and fe each relative to si reveal highly variable compositions e g mg si and al si range over 0 1 0 8 and 0 1 0 4 respectively the geochemical variations that we observe are consistent with those inferred from other messenger geochemical remote sensing datasets but they do not correlate well with units mapped previously from spectral reflectance or morphology location dependent rather than temporally evolving partial melt sources were likely the major influence on the compositions of the magmas that produced different geochemical terranes a large 5 10 6 km 2 region with the highest mg si ca si and s si ratios as well as relatively thin crust may be the site of an ancient and heavily degraded impact basin the distinctive geochemical signature of this region could be the consequence of high degree partial melting of a reservoir in a vertically heterogeneous mantle that was sampled primarily as a result of the impact event

silicate mineralogy at the surface of mercury nasa s messenger spacecraft has revealed geochemical diversity across mercury s volcanic crust near infrared to ultraviolet spectra and images have provided evidence for the fe2 poor nature of silicate minerals magnesium sulfide minerals in hollows and a darkening component attributed to graphite but existing spectral data is insufficient to build a mineralogical map for the planet here we investigate the mineralogical variability of silicates in mercury s crust using crystallization experiments on magmas with compositions and under reducing conditions expected for mercury we find a common crystallization sequence consisting of olivine plagioclase pyroxenes and tridymite for all magmas tested depending on the cooling rate we suggest that lavas on mercury are either fully crystallized or made of a glassy matrix with phenocrysts combining the experimental results with geochemical mapping we can identify several mineralogical provinces the northern volcanic plains and smooth plains dominated by plagioclase the high mg province strongly dominated by forsterite and the intermediate plains comprised of forsterite plagioclase and enstatite this implies a temporal evolution of the mineralogy from the oldest lavas dominated by mafic minerals to the youngest lavas dominated by plagioclase consistent with progressive shallowing and decreasing degree of mantle melting over time the messenger spacecraft has revealed geochemical diversity across mercury s surface magma crystallization experiments suggest a crustal mineralogy consistent with a transition towards shallower and cooler mantle melting conditions

remote sensing evidence for an ancient carbon bearing crust on mercury mercury appears darker globally than expected remote sensing evidence from the messenger spacecraft indicates that the planet s darkening agent is carbon and suggests that it originates from an ancient graphite rich crust

partial melting of the indarch eh4 meteorite a textural chemical and phase relations view of melting and melt migration to test whether aubrites can be formed by melting of enstatite chondrites and to understand igneous processes at very low o fugacities we have conducted partial melting experiments on the indarch eh4 chondrite at 1000 1500 c silicate melting begins at 1000 c and indarch is completely melted by 1500 c the metal sulfide component melts completely at 1000 c substantial melt migration occurs at 1300 1400 c and metal migrates out of the silicate charge at 1450 c and 50 silicate partial melting as a group our experiments contain three immiscible metallic melts si p and c rich two immiscible sulfide melts fe and femgmnca rich and silicate melt our partial melting experiments on the indarch eh4 enstatite chondrite suggest that igneous processes at low fo2 exhibit several unique features the complete melting of sulfides at 1000 c suggests that aubritic sulfides are not relics aubritic oldhamite may have crystallized from ca and s complexed in the silicate melt significant metal sulfide melt migration might occur at relatively low degrees of silicate partial melting substantial elemental exchange occurred between different melts e g s between sulfide and silicate si between silicate and metal a feature not observed during experiments at higher fo2 this exchange may help explain the formation of aubrites from known enstatite chondrites

silicate mineralogy of martian meteorites abstract basalts and basaltic cumulates from mars delivered to earth as meteorites carry a record of the history of that planet from accretion to initial differentiation and subsequent volcanism up to recent times we provide new microprobe data for plagioclase olivine and pyroxene from 19 of the martian meteorites that are representative of the six types of martian rocks we also provide a comprehensive wds map dataset for each sample studied collected at a common magnification for easy comparison of composition and texture the silicate data shows that plagioclase from each of the rock types shares similar trends in ca na k and that k2o na2o wt of plagioclase multiplied by the al content of the bulk rock can be used to determine whether a rock is enriched or depleted in nature olivine data show that meteorite y 980459 is a primitive melt from the martian mantle as its olivine crystals are in equilibrium with its bulk rock composition all other olivine bearing shergottites have been affected by fractional crystallization pyroxene quadrilateral compositions can be used to isolate the type of melt from which the grains crystallized and minor element concentrations in pyroxene can lend insight into parent melt compositions in a comparative planetary mineralogy context plagioclase from mars is richer in na than terrestrial and lunar plagioclase the two most important factors contributing to this are the low activity of al in martian melts and the resulting delayed nucleation of plagioclase in the crystallizing rock olivine from martian rocks shows distinct trends in ni co and cr systematics compared with olivine from earth and moon the trends are due to several factors including oxygen fugacity melt compositions and melt structures properties which show variability among the planets finally fe mn ratios in both olivine and pyroxene can be used as a fingerprint of planetary parentage where minerals show distinct planetary trends that may have been set at the time of planetary accretion although the silicate mineralogical data alone cannot support one specific model of martian magmatism over another the data does support the basic igneous reservoirs proposed for mars and may also be used to constrain some aspects of specific petrogenetic models examples include enriched and depleted reservoirs that can be identified by plagioclase k na and al composition multivalent element partitioning in olivine and pyroxene v cr elucidates oxygen fugacity conditions of the reservoirs and minor element concentrations i e cr in pyx show that proposed fractional crystallization models linking y 980459 to que 94201 will not work

a lamina by lamina analysis of grain size distribution in fine grained turbidites abstract a new technique using an ultrasonic vibrator for sieving fine grained sediments was adopted in the study of the grain size characteristics of fine grained turbidites from the okinawa trough important results of the grain size analysis were 1 a positive size grading in the non laminated part division e of a bed 2 very gentle inclination of the cm plot from division d through division e and 3 open s and half s shape of log probability plots showing positive skewness in both the coarse grained lamina and the fine grained lamina of division d some discussion and tentative interpretation are presented on these results for instance the last one denies the idea of generation of lamina by separation of sediments of different size from a population of log normal distribution it is apparent that detailed study of sediments including lamina by lamina analysis of the grain size distribution can throw a new light on an aspect of fine grained turbidite sedimentology

bright and dark polar deposits on mercury evidence for surface volatiles measurements of surface reflectance of permanently shadowed areas near mercury s north pole reveal regions of anomalously dark and bright deposits at 1064 nanometer wavelength these reflectance anomalies are concentrated on poleward facing slopes and are spatially collocated with areas of high radar backscatter postulated to be the result of near surface water ice correlation of observed reflectance with modeled temperatures indicates that the optically bright regions are consistent with surface water ice whereas dark regions are consistent with a surface layer of complex organic material that likely overlies buried ice and provides thermal insulation impacts of comets or volatile rich asteroids could have provided both dark and bright deposits

melting processes and mantle sources of lavas on mercury abstract the messenger spacecraft provided geochemical data for surface rocks on mercury in this study we use the major element composition of these lavas to constrain melting conditions and residual mantle sources on mercury we combine modelling and high temperature 1320 1580 c low to high pressure 0 1 to 3 gpa experiments on average compositions for the northern volcanic plains nvp and the high mg region of the intercrater plains and heavily cratered terrains high mg icp hct near liquidus phase relations show that the s free nvp and high mg icp hct compositions are multiply saturated with forsterite and enstatite at 1450 c 1 3 gpa and 1570 c 1 7 gpa respectively for s saturated melts 1 5 3 wt s the multiple saturation point msp is shifted to 1380 c 0 75 gpa for nvp and 1480 c 0 8 gpa for high mg icp hct to expand our experimental results to the range of surface compositions we used and calibrated the pmelts thermodynamic calculator and estimated phase equilibria of 5800 compositions from the mercurian surface and determined the p t conditions of liquid forsterite enstatite msp 1300 1600 c 0 25 1 25 gpa surface basalts were produced by 10 to 50 partial melting of variably enriched lherzolitic mantle sources the relatively low pressure of the olivine enstatite liquid msp seems most consistent with decompression batch melting and melts being segregated from their residues near the base of mercury s ancient lithosphere the average melting degree is lower for the young nvp 0 27 0 04 than for the older icp hct 0 46 0 02 indicating that melt productivity decreased with time the mantle potential temperature required to form mercurian lavas and the initial depth of melting also decreased from the older high mg icp hct terrane 1650 c and 360 km to the younger lavas covering the nvp regions 1410 c and 160 km this evolution supports strong secular cooling of mercury s mantle between 4 2 and 3 7 ga and explains why very little magmatic activity occurred after 3 7 ga

near surface temperatures on mercury and the moon and the stability of polar ice deposits abstract in order to assess the thermal stability of polar ice deposits we present model calculated temperatures of flat surfaces and surfaces within bowl shaped and flat floored polar impact craters on mercury and the moon our model includes appropriate insolation cycles realistic crater shapes multiple scattering of sunlight and infrared radiation and depth and temperature dependent regolith thermophysical properties unshaded water ice deposits on the surface of either body are rapidly lost to thermal sublimation a subsurface water ice deposit is stable within 2 latitude of the moon s poles meter thick water ice deposits require billions of years to sublime if located in the permanently shaded portions of flat floored craters within 10 latitude of the poles of mercury and 13 latitude of the poles of the moon results for craters associated with radar features on mercury are consistent with the presence of stable water ice deposits if a thin regolith layer thermally insulates deposits at lower latitudes and within smaller craters a regolith cover would also reduce losses from diffusion ion sputtering impact vaporization and h ly and is implied independently by the radar observations permanently shaded areas near the moon s poles are generally colder than those near mercury s poles but the moon s obliquity history its orbit through earth s magnetospheric tail and its radar opaque regolith may limit the volume and radar detectability of ice deposits there

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

the global population of large craters on mercury and comparison with the moon 1 we have compiled a near global catalog of impact craters on mercury for diameters 20 km from images obtained during the flybys of the mariner 10 and messenger spacecraft the observed variations in crater density suggest that the smooth plains within and around the caloris basin are the most prominent contiguous comparatively young regions on mercury no other comparably large area appears as young in more heavily cratered terrain even the most densely cratered regions on mercury are deficient in craters between 20 km and 100 km in diameter compared with the moon a result that extends an observation made from mariner 10 images of a smaller fraction of mercury s surface this deficit is interpreted to reflect crustal resurfacing of mercury early in its history most likely by volcanic processes for craters larger than 100 km in diameter the density of craters on mercury in a given size range is similar to that of the moon because such larger craters are less easily removed by volcanic resurfacing we interpret this similarity to be the result of impact saturation effects on both bodies

support of long wavelength topography on mercury inferred from messenger measurements of gravity and topography to explore the mechanisms of support of surface topography on mercury we have determined the admittances and correlations of topography and gravity in mercury s northern hemisphere from measurements obtained by nasa s mercury surface space environment geochemistry and ranging messenger spacecraft these admittances and correlations can be interpreted in the context of a number of theoretical scenarios including flexural loading and dynamic flow we find that long wavelength spherical harmonic degree l 15 surface topography on mercury is primarily supported through a combination of crustal thickness variations and deep mass anomalies the deep mass anomalies may be interpreted either as lateral variations in mantle density or as relief on compositional interfaces domical topographic swells are associated with high admittances and are compensated at 300 400 km depth in the lower reaches of mercury s mantle quasi linear topographic rises are primarily associated with shallow crustal compensation and are weakly correlated with positive mass anomalies in the mantle the center of the caloris basin features some of the thinnest crust on the planet and the basin is underlain by a large negative mass anomaly we also explore models of dynamic flow in the presence of compositional stratification above the liquid core if there is substantial compositional stratification in mercury s solid outer shell relaxation of perturbed compositional interfaces may be capable of creating and sustaining long wavelength topography

global resurfacing of mercury 4 0 4 1 billion years ago by heavy bombardment and volcanism analysis of craters on mercury s oldest most heavily cratered terrains shows that they were formed 4 0 4 1 billion years ago and that the planet s previous geological history was erased most probably by voluminous volcanism which may have been triggered by heavy asteroidal bombardment at that time the most heavily cratered terrains on mercury exhibit a lower density of craters smaller than about 100 km in diameter than on the moon a deficit that has been attributed to resurfacing by formation of ancient intercrater plains simone marchi et al used a crater areal density map based on data from the messenger spacecraft the colour coded foreground on cover with a global surface mosaic in the background to locate the oldest surfaces on mercury and interpret the crater populations in the framework of a recent lunar crater chronology they conclude that the oldest surfaces were emplaced just after the start of the late heavy bombardment 4 0 to 4 1 billion years ago the large impact basins not previously dated yield a similar surface age this agreement implies that resurfacing was global and due to volcanism perhaps aided by heavy bombardment as previously suggested the most heavily cratered terrains on mercury have been estimated to be about 4 billion years gyr old1 2 3 4 but this was based on images of only about 45 per cent of the surface even older regions could have existed in the unobserved portion these terrains have a lower density of craters less than 100 km in diameter than does the moon1 3 5 an observation attributed to preferential resurfacing on mercury here we report global crater statistics of mercury s most heavily cratered terrains on the entire surface applying a recent model for early lunar crater chronology6and an updated dynamical extrapolation to mercury7 we find that the oldest surfaces were emplaced just after the start of the late heavy bombardment lhb about 4 0 4 1 gyr ago mercury s global record of large impact basins8 which has hitherto not been dated yields a similar surface age this agreement implies that resurfacing was global and was due to volcanism as previously suggested1 5 this activity ended during the tail of the lhb within about 300 400 million years after the emplacement of the oldest terrains on mercury these findings suggest that persistent volcanism could have been aided by the surge of basin scale impacts during this bombardment

carbon cycle changes of the zechstein sea isotopic transition zone in the marl slate the carbon isotope composition of carbonate rocks is one parameter used to study the carbon sulphur and oxygen cycles through geological time1 2 the understanding of these interrelated cycles is important in quantitative flux modelling of sedimentary rocks the carbon isotope composition in the large reservoir of dissolved carbonate in ocean water is a result of mass balance between the amount of carbon in the reduced organic reservoir and the oxidized carbonate reservoir at any given stage the reduced carbon reservoir preferentially sequesters 12c owing to biological fractionation processes any change in the removal ratio of oxidized reduced carbon will be reflected by the change of the 13c value of the oceanic carbonate system here we present a detailed study of the change in 13c which took place at the base of the upper permian zechstein sequence

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

global inventory and characterization of pyroclastic deposits on mercury new insights into pyroclastic activity from messenger orbital data we present new observations of pyroclastic deposits on the surface of mercury from data acquired during the orbital phase of the mercury surface space environment geochemistry and ranging messenger mission the global analysis of pyroclastic deposits brings the total number of such identified features from 40 to 51 some 90 of pyroclastic deposits are found within impact craters the locations of most pyroclastic deposits appear to be unrelated to regional smooth plains deposits except some deposits cluster around the margins of smooth plains similar to the relation between many lunar pyroclastic deposits and lunar maria a survey of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on mercury over a prolonged interval measurements of surface reflectance by messenger indicate that the pyroclastic deposits are spectrally distinct from their surrounding terrain with higher reflectance values redder i e steeper spectral slopes and a downturn at wavelengths shorter than 400 nm i e in the near ultraviolet region of the spectrum three possible causes for these distinctive characteristics include differences in transition metal content physical properties e g grain size or degree of space weathering from average surface material on mercury the strength of the near ultraviolet downturn varies among spectra of pyroclastic deposits and is correlated with reflectance at visible wavelengths we suggest that this interdeposit variability in reflectance spectra is the result of either variable amounts of mixing of the pyroclastic deposits with underlying material or inherent differences in chemical and physical properties among pyroclastic deposits

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