the geologically recent giant impact basins at vesta s south pole since 17 july 2011 nasa s spacecraft dawn has been orbiting the asteroid vesta the second most massive and the third largest asteroid in the solar system see the cover russell et al p 684 1 use dawn s observations to confirm that vesta is a small differentiated planetary body with an inner core and represents a surviving proto planet from the earliest epoch of solar system formation vesta is also confirmed as the source of the howardite eucrite diogenite hed meteorites jaumann et al p 687 2 report on the asteroid s overall geometry and topography based on global surface mapping vesta s surface is dominated by numerous impact craters and large troughs around the equatorial region marchi et al p 690 3 report on vesta s complex cratering history and constrain the age of some of its major regions based on crater counts schenk et al p 694 4 describe two giant impact basins located at the asteroid s south pole both basins are young and excavated enough amounts of material to form the vestoids a group of asteroids with a composition similar to that of vesta and hed meteorites de sanctis et al p 697 5 present the mineralogical characterization of vesta based on data obtained by dawn s visual and infrared spectrometer revealing that this asteroid underwent a complex magmatic evolution that led to a differentiated crust and mantle the global color variations detailed by reddy et al p 700 6 are unlike those of any other asteroid observed so far and are also indicative of a preserved differentiated proto planet 1 lookup doi 10 1126 science 1219381 2 lookup doi 10 1126 science 1219122 3 lookup doi 10 1126 science 1218757 4 lookup doi 10 1126 science 1223272 5 lookup doi 10 1126 science 1219270 6 lookup doi 10 1126 science 1219088

the geological nature of dark material on vesta and implications for the subsurface structure abstract deposits of dark material appear on vesta s surface as features of relatively low albedo in the visible wavelength range of dawn s camera and spectrometer mixed with the regolith and partially excavated by younger impacts the material is exposed as individual layered outcrops in crater walls or ejecta patches having been uncovered and broken up by the impact dark fans on crater walls and dark deposits on crater floors are the result of gravity driven mass wasting triggered by steep slopes and impact seismicity the fact that dark material is mixed with impact ejecta indicates that it has been processed together with the ejected material some small craters display continuous dark ejecta similar to lunar dark halo impact craters indicating that the impact excavated the material from beneath a higher albedo surface the asymmetric distribution of dark material in impact craters and ejecta suggests non continuous distribution in the local subsurface some positive relief dark edifices appear to be impact sculpted hills with dark material distributed over the hill slopes dark features inside and outside of craters are in some places arranged as linear outcrops along scarps or as dark streaks perpendicular to the local topography the spectral characteristics of the dark material resemble that of vesta s regolith dark material is distributed unevenly across vesta s surface with clusters of all types of dark material exposures on a local scale some craters expose or are associated with dark material while others in the immediate vicinity do not show evidence for dark material while the variety of surface exposures of dark material and their different geological correlations with surface features as well as their uneven distribution indicate a globally inhomogeneous distribution in the subsurface the dark material seems to be correlated with the rim and ejecta of the older veneneia south polar basin structure the origin of the dark material is still being debated however the geological analysis suggests that it is exogenic from carbon rich low velocity impactors rather than endogenic from freshly exposed mafic material or melt exposed or created by impacts

exogenic olivine on vesta from dawn framing camera color data abstract in this paper we present the results of a global survey of olivine rich lithologies on 4 vesta we investigated dawn framing camera fc high altitude mapping orbit hamo color cubes 60 m pixel resolution by using a method described in thangjam et al thangjam g nathues a mengel k hoffmann m schafer m reddy v cloutis e a christensen u sierks h le corre l vincent j b russell c t 2014b meteorit planet sci arxiv 1408 4687 astro ph ep in total we identified 15 impact craters exhibiting olivine rich 40 wt ol outcrops on their inner walls some showing olivine rich material also in their ejecta and floors olivine rich sites are concentrated in the bellicia arruntia and pomponia region on vesta s northern hemisphere from our multi color and stratigraphic analysis we conclude that most if not all of the olivine rich material identified is of exogenic origin i e remnants of a or and s type projectiles the olivine rich lithologies in the north are possibly ejecta of the 90 km diameter albana crater we cannot draw a final conclusion on their relative stratigraphic succession but it seems that the dark material nathues a hoffmann m cloutis e a schafer m reddy v christensen u sierks h thangjam g s le corre l mengel k vincent j b russell c t prettyman t schmedemann n kneissl t raymond c gutierrez marques p hall i buttner i 2014b icarus 239 222 237 and the olivine rich lithologies are of a similar age the origin of some potential olivine rich sites in the rheasilvia basin and at crater portia are ambiguous i e these are either of endogenic or exogenic origin however the small number and size of these sites led us to conclude that olivine rich mantle material containing more than 40 wt of olivine is basically absent on the present surface of vesta in combination with recent impact models of veneneia and rheasilvia clenet h jutzi m barrat j a gillet ph 2014 lunar planet sci 45 1349 jutzi m asphaug e gillet p barrat j a benz w 2013 nature 494 207 210 which predict an excavation depth of up to 80 km we are confident that the crust mantle depth is significantly deeper than predicted by most evolution models 30 km mittlefehldt d w 2014 asteroid 4 vesta a fully differentiated dwarf planet nasa technical reports server 20140004857 pdf or alternatively the olivine content of the upper mantle is lower than our detection limit which would lead to the conclusion that vesta s parent material was already depleted in olivine compared to ci meteorites

vesta s shape and morphology vesta s surface is characterized by abundant impact craters some with preserved ejecta blankets large troughs extending around the equatorial region enigmatic dark material and widespread mass wasting but as yet an absence of volcanic features abundant steep slopes indicate that impact generated surface regolith is underlain by bedrock dawn observations confirm the large impact basin rheasilvia at vesta s south pole and reveal evidence for an earlier underlying large basin veneneia vesta s geology displays morphological features characteristic of the moon and terrestrial planets as well as those of other asteroids underscoring vesta s unique role as a transitional solar system body

three dimensional spectral analysis of compositional heterogeneity at arruntia crater on 4 vesta using dawn fc abstract we introduce an innovative three dimensional spectral approach three band parameter space with polyhedrons that can be used for both qualitative and quantitative analyzes improving the characterization of surface compositional heterogeneity of 4 vesta it is an advanced and more robust methodology compared to the standard two dimensional spectral approach two band parameter space the dawn framing camera fc color data obtained during high altitude mapping orbit resolution 60 m pixel is used the main focus is on the howardite eucrite diogenite hed lithologies containing carbonaceous chondritic material olivine and impact melt the archived spectra of heds and their mixtures from relab hoserlab and usgs databases as well as our laboratory measured spectra are used for this study three dimensional convex polyhedrons are defined using computed band parameter values of laboratory spectra polyhedrons based on the parameters of band tilt r0 92 m r0 96 m mid ratio r0 75 m r0 83 m r0 83 m r0 92 m and reflectance at 0 55 m r0 55 m are chosen for the present analysis an algorithm in idl programming language is employed to assign fc data points to the respective polyhedrons the arruntia region in the northern hemisphere of vesta is selected for a case study because of its geological and mineralogical importance we observe that this region is eucrite dominated howarditic in composition the extent of olivine rich exposures within an area of 2 5 crater radii is 12 larger than the previous finding thangjam g et al 2014 meteorit planet sci 49 1831 1850 lithologies of nearly pure cm2 chondrite olivine glass and diogenite are not found in this region although there are no unambiguous spectral features of impact melt the investigation of morphological features using fc clear filter data from low altitude mapping orbit resolution 18 m pixel suggests potential impact melt features inside and outside of the crater our spectral approach can be extended to the entire vestan surface to study the heterogeneous surface composition and its geology

the violent collisional history of asteroid 4 vesta vesta is a large differentiated rocky body in the main asteroid belt that accreted within the first few million years after the formation of the earliest solar system solids the dawn spacecraft extensively imaged vesta s surface revealing a collision dominated history results show that vesta s cratering record has a strong north south dichotomy vesta s northern heavily cratered terrains retain much of their earliest history the southern hemisphere was reset however by two major collisions in more recent times we estimate that the youngest of these impact structures about 500 kilometers across formed about 1 billion years ago in agreement with estimates of vesta asteroid family age based on dynamical and collisional constraints supporting the notion that the vesta asteroid family was formed during this event

surface lithologic heterogeneity of asteroid 4 vesta abstract high precision spectrophotometry of 4 vesta the third largest asteroid was used to establish the surface composition of this body and to investigate mineralogical variations across its surface the average surface of vesta is analogous to howardite and or polymict eucrite assemblages regolith derived members of the hed meteorite suite which consist of a eucrite matrix containing differing amounts of a diogenite component color and or spectral changes which exhibit a consistent relationship to rotational phasing are evident in 16 of 18 studies of vesta dating back to 1929 including the present work after elimination of possible sources of spurious spectral or color changes it is concluded that these variations arise from hemispheric variations in surface materials and hence provide a means of spatially resolving subhemispheric compositional units on the surface of vesta the background surface of vesta is a relatively dark howardite or polymict eucrite pyroxene plagioclase assemblage with several compositionally distinct bright regions clustered in one hemisphere viewed around the maximum in vesta s lightcurve these include what appears to be an olivine bearing unit suggested name leslie formation located near vesta s equator which probably represents an impact basin and or its associated ejecta that penetrated through the basaltic crust other high albedo compositional units including an apparently low calcium eucrite region and several diogenite pyroxenite regions at least one located near the southern pole may be smaller shallower impact basins by analogy to the eucrite meteorites which represent surface flows or shallow intrusions and which constitute the major component of the regolith derived howardites and polymict eucrites we conclude that the howardite polymict eucrite units represent a regolith gardened original surface of vesta it is probable that the low albedo of the background surface on vesta is due to an age related darkening effect similar to that inferred from the galileo images of gaspra and ida this mechanism is consistent with the correlation of absorption feature intensity with the lightcurve vesta appears to have an old eucritic surface darkened with age and represented among the meteorites by the regolith derived howardites and polymict eucrites on which several impacts on one hemisphere have exposed fresher brighter diogenite and olivine bearing material based on qualitative analysis of the mineralogical variations as a function of rotation a generalized lithologic map of vesta was produced there is reasonable control on the longitude of lithologic features but little control on their latitudes except where specifically noted in producing this map discrete circular features were used on the plausible assumption that impacts have been the most important geologic process on this surface for most of the age of the solar system the shape of the features has thus been assumed rather than derived and is subject to future revision as data improve

dawn completes its mission at 4 vesta the dawn mission was designed to test our hypothesis about the origin and evolution of the early solar system by visiting the largest differentiated basaltic asteroid 4 vesta believed to be a survivor from the earliest times of rocky body formation observations from orbit show that vesta is the parent body of the howardite eucrite diogenite meteorites vesta has an iron core and a eucritic diogenitic crust its surface is characterized by abundant impact craters but with no evident volcanic features it has two ancient impact basins in the southern hemisphere that are associated with circum planetary troughs the northern hemisphere is the more heavily cratered and contains the oldest terrains the surface of vesta is diverse with north south and east west dichotomies in the eucrite to diogenite ratio its surface contains both very bright and very dark material and its color varies strongly from region to region both the mineralogical and the elemental compositions agree with that expected for the hed parent body significant oh or h may be present in the upper crust and the presence of pits in fresh craters is consistent with the devolatilization of the surface after a collision either brought to or tapped a source of water on vesta the presence of dark material on the surface of vesta suggests efficient transport pathways for organic material and the mixing of the dark material with the more pristine pyroxene explains the varying albedo across the surface vesta has proven to be a reliable witness to the formation of the solar system

grosvenor mountains 95 howardite pairing group insights into the surface regolith of asteroid 4 vesta regolithic howardites are analogs for the surface materials of asteroid 4 vesta recently mapped by the dawn spacecraft rigorously evaluating pairing of howardites recovered in 1995 in the grosvenor mountains gro 95 antarctica enables an examination of a larger more representative regolith sample previous work on two of the howardites studied here concluded that gro 95602 and gro 95535 are solar wind rich surface regolith samples and that they are not paired with each other leading to uncertainty regarding pairing relationships between the other gro 95 howardites based on petrology cosmic ray exposure history and terrestrial age four gro 95 howardites are paired the paired howardites gro 95534 95535 95574 95581 were from a meteoroid with radius of 10 15 cm a preatmospheric size comparable to that of kapoeta the largest known regolithic howardite the paired gro 95 howardites contain clasts of at least 18 separate hed lithologies providing evidence they were assembled from diverse source materials the total eucrite diogenite mixing ratio ratio of all eucrite lithologies to all diogenite lithologies in the paired gro 95 howardites is 2 1 petrographically determined basaltic eucrite cumulate eucrite ratios in regolithic howardites studied here and previously vary more widely than total eucrite diogenite ratios relative to eucritic pyroxene plagioclase is depleted in these howardites which provides evidence that plagioclase is preferentially comminuted in the vestan regolith the extent of plagioclase depletion could be an indicator of regolith maturity

space weathering on airless bodies space weathering refers to alteration that occurs in the space environment with time lunar samples and to some extent meteorites have provided a benchmark for understanding the processes and products of space weathering lunar soils are derived principally from local materials but have accumulated a range of optically active opaque particles oaopq that include nanophase metallic iron on in rims formed on individual grains imparting a red slope to visible and near infrared reflectance and larger iron particles which darken across all wavelengths such as are often found within the interior of recycled grains space weathering of other anhydrous silicate bodies such as mercury and some asteroids produce different forms and relative abundance of oaopq particles depending on the particular environment if the development of oaopq particles is minimized such as at vesta contamination by exogenic material and regolith mixing become the dominant space weathering processes volatile rich bodies and those composed of abundant hydrous minerals dwarf planet ceres many dark asteroids outer solar system satellites are affected by space weathering processes differently than the silicate bodies of the inner solar system however the space weathering products of these bodies are currently poorly understood and the physics and chemistry of space weathering processes in different environments are areas of active research

composition of the northern regions of vesta analyzed by the dawn mission abstract the surface composition of the northern regions of vesta observed by the dawn spacecraft offers the possibility to test several hypotheses related to impact related processes we used mostly imaging spectrometry in the visible and near infrared to assess the distribution of mafic lithologies hydrated components and albedo properties and use the link with howardite eucrite and diogenite meteorites heds to investigate the origin of those materials we established that rheasilvia ejecta reached part of the northern regions and have a diogenitic rich composition characteristic of the lower crust investigations of the antipodes of the two major impact basins rheasilvia and veneneia did not reveal any correlation between geographic location geological features and the surface composition the northern wall of mamilia crater which is one of the freshest craters above 22 n contains relatively pure eucritic rich diogenitic rich and dark hydrated materials which are representative of the rest of the northern regions and most of vesta with the exception of an olivine like component found in bellicia crater by ammannito et al ammannito e et al 2013a nature 504 7478 122 125 we determined that similar types of materials are found in various proportions over a large region including bellicia arruntia and pomponia craters and their origin does not seem to be related to rheasilvia ejecta these materials are hydrated which could indicate an exogenous origin and not as dark as expected for carbonaceous chondrites which likely compose the majority of dark hydrated materials on vesta spectral mixture analysis reveals that mixtures of pyroxenes hypersthene pigeonite and diopside could offer an alternative interpretation to olivine in this area

spectroscopic characterization of mineralogy and its diversity across vesta the mineralogy of vesta based on data obtained by the dawn spacecraft s visible and infrared spectrometer is consistent with howardite eucrite diogenite meteorites there are considerable regional and local variations across the asteroid spectrally distinct regions include the south polar rheasilvia basin which displays a higher diogenitic component and equatorial regions which show a higher eucritic component the lithologic distribution indicates a deeper diogenitic crust exposed after excavation by the impact that formed rheasilvia and an upper eucritic crust evidence for mineralogical stratigraphic layering is observed on crater walls and in ejecta this is broadly consistent with magma ocean models but spectral variability highlights local variations which suggests that the crust can be a complex assemblage of eucritic basalts and pyroxene cumulates overall vesta mineralogy indicates a complex magmatic evolution that led to a differentiated crust and mantle

elemental mapping by dawn reveals exogenic h in vesta s regolith using dawn s gamma ray and neutron detector we tested models of vesta s evolution based on studies of howardite eucrite and diogenite hed meteorites global fe o and fe si ratios are consistent with hed compositions neutron measurements confirm that a thick diogenitic lower crust is exposed in the rheasilvia basin which is consistent with global magmatic differentiation vesta s regolith contains substantial amounts of hydrogen the highest hydrogen concentrations coincide with older low albedo regions near the equator where water ice is unstable the young rheasilvia basin contains the lowest concentrations these observations are consistent with gradual accumulation of hydrogen by infall of carbonaceous chondrites observed as clasts in some howardites and subsequent removal or burial of this material by large impacts

olivine on vesta as exogenous contaminants brought by impacts constraints from modeling vesta s collisional history and from impact simulations the survival of asteroid vesta during the violent early history of the solar system is a pivotal constraint on theories of planetary formation particularly important from this perspective is the amount of olivine excavated from the vestan mantle by impacts as this constrains both the interior structure of vesta and the number of major impacts the asteroid suffered during its life the nasa dawn mission revealed that olivine is present on vesta s surface in limited quantities concentrated in small patches at a handful of sites not associated with the two large impact basins rheasilvia and veneneia the first detections were interpreted as the result of the excavation of endogenous olivine even if the depth at which the detected olivine originated was a matter of debate later works raised instead the possibility that the olivine had an exogenous origin based on the geologic and spectral features of the deposits in this work we quantitatively explore the proposed scenario of a exogenous origin for the detected vestan olivine to investigate whether its presence on vesta can be explained as a natural outcome of the collisional history of the asteroid over the last one or more billion years to perform this study we took advantage of the impact contamination model previously developed to study the origin and amount of dark and hydrated materials observed by dawn on vesta a model we updated by performing dedicated hydrocode impact simulations we show that the exogenous delivery of olivine by the same impacts that shaped the vestan surface can offer a viable explanation for the currently identified olivine rich sites without violating the constraint posed by the lack of global olivine signatures on vesta our results indicate that no mantle excavation is in principle required to explain the observations of the dawn mission and support the idea that the vestan crust could be thicker than indicated by simple geochemical models based on the howardite eucrite diogenite family of meteorites

photometric mapping of asteroid 4 vesta s southern hemisphere with hubble space telescope abstract we present the surface mapping of the southern hemisphere of asteroid 4 vesta obtained from hubble space telescope hst from 105 images of vesta through four filters in the wavelengths best to characterize the 1 m pyroxene band we constructed albedo and color ratio maps of vesta these new maps cover latitudes 50 to 20 the southern hemisphere of vesta displays more diverse albedo and color features than the northern hemisphere with about 15 new albedo and color features identified the overall longitudinal albedo and color variations in the southern hemisphere are comparable with that of the northern hemisphere with a range of about 20 and 10 respectively the eastern hemisphere is brighter and displays more diogenitic minerals than the western hemisphere correlations between 1 m band depth and band width as well as between 1 m band depth and albedo are present on a global scale attributed to pyroxene composition variations the lack of correlations between albedo and the spectral slope indicates the absence of globalized space weathering the lack of a global correlation between 1 m band depth and topography suggests that the surface composition of vesta is not completely controlled by a single impact the distribution of compositional variation on vesta suggests a possible large impact basin evidence of space weathering is found in regions including the bright rim of the south pole crater where the steepest gravitational slope on vesta is and a dark area near a gravitationally flat area we propose to divide the surface of vesta into six geological units different from the background according to their 1 m absorption features and spectral slopes including two eucrite rich units a low ca eucrite unit a diogenite rich unit a space weathered unit and a freshly exposed unit no evidence of olivine rich area is present in these data

vsp in crystalline rocks from downhole velocity profiling to 3 d fracture mapping abstract vsp surveys have been carried out at several potential nuclear waste disposal sites in finland since the mid 1980s to date more than 200 three component profiles have been measured the main purpose of the surveys was to detect fracture zones in the crystalline bedrock and to determine their positions most seismic events could be linked to zones of increased fracturing observed in the borehole logs the more pronounced seismic reflectors could be correlated with hydro geologically significant zones which have been the main targets in the investigations processing and interpretation methods have been developed specifically for vsp surveys in crystalline rocks weak reflections from thin fracture zones are enhanced by multi channel filtering techniques based on the radon transform the position and orientation of the fracture zones are determined by polarisation analysis and by combining data from several shot points the compilation of the results from several boreholes gives a comprehensive image of the fracture zones at the scale of the whole site the discussion of the methodology is based on examples from the olkiluoto site in sw finland

the contamination of the surface of vesta by impacts and the delivery of the dark material abstract the dawn spacecraft recently observed the presence of dark material which in turn proved to be associated with the presence of oh and h rich material on the surface of vesta the source of this dark material has been almost unanimously identified with the low albedo asteroids likely analogous to the carbonaceous chondrites found on earth that impacted on vesta over its lifetime however it is still a matter of debate whether the delivery of the dark material is associated with a few large impact events to micrometeorites or to the continuous secular flux of impactors on vesta the continuous flux scenario in particular predicts that a significant fraction of the exogenous material accreted by vesta should be due to non dark impactors likely analogous to ordinary chondrites which instead represent only a minor contaminant in the howardite eucrite diogenite meteorites in this work we explored the continuous flux scenario and its implications for the composition of the vestan regolith taking advantage of the data from the dawn mission and the howardite eucrite diogenite meteorites to constrain the contamination history of vesta we developed a model for the delivery of the exogenous material to vesta and verified how the results it supplies are sensitive to the different parameters we consider we calibrated the flux of impactors predicted by our model with the number of dark craters observed inside the rheasilvia basin and we tested the assumptions on the impact conditions by studying the formation of cornelia crater and of its dark deposits with a hydrocode simulation we used our calibrated model to show that the stochastic events scenario and the micrometeoritic flux scenario are just natural consequences of the continuous flux scenario we then used the model to estimate the amounts of dark and hydroxylate materials that were delivered on vesta since the late heavy bombardment and we showed how our results match well with the values estimated by the dawn mission we finally used our model to assess the amount of fe and siderophile elements that the continuous flux of impactors would mix in the vestan regolith concerning the siderophile elements we focused our attention on the role of ni the results we obtained are in agreement with the data available on the fe and ni content of the howardite eucrite diogenite meteorites and can be used as a reference frame in future studies of the data from the dawn mission and of the howardite eucrite diogenite meteorites our model cannot yet provide an answer to the conundrum of the fate of the missing non carbonaceous contaminants but we discuss some possible reasons for this discrepancy with the otherwise coherent picture described by our results

the spectral parameter maps of vesta from vir data abstract here we present the spectral parameter maps used in this surface composition of vesta special issue the use of spectral parameters have been important since the first ground based observations of vesta as they can describe single mineralogic aspects as abundances or change in surface composition mapping these parameters over the surface shows the mineralogic diversity across the asteroid vesta in this work we discuss the development of the maps which are produced from the data of the visible and infrared mapping spectrometer onboard nasa s dawn mission we describe how we processed the vir spectra and how we produced the geometries of data acquisition spectra and geometries are used to design a geographic information system procedure to mosaic these data we conclude the article with a description of the trends of the pyroxene related spectral parameters across the asteroid and some statistics on the spectral parameters of each map within the quadrangle based scheme used in this special issue

vesta as the howardite eucrite and diogenite parent body implications for the size of a core and for large scale differentiation if vesta is the parent body of the howardite eucrite and diogenite hed meteorites then geo chemical and petrologic constraints for the meteorites may be used in conjunction with astronomical constraints for the size and mass of vesta to 1 determine the size of a possible metal core in vesta and 2 model the igneous differentiation and internal structure of vesta the density of vesta and petrologic models for hed meteorites together suggest that the amount of metal in the parent body is 25 mass with a best estimate of 5 assuming no porosity for a porosity of up to 5 in the silicate fraction of the asteroid the permissible metal content is 30 these results suggest that any metal core in the hed parent body and vesta is not unusually large a variety of geochemical and other data for hed meteorites are consistent with the idea that they originated in a magma ocean it appears that diogenites formed by crystal accumulation in a magma ocean cumulate pile and that most noncumulate eucrites excepting such eucrites as bouvante and statinem formed by subsequent crystallization of the residual melts modelling results suggest that the hed parent body is enriched in rare earth elements by a factor of 2 5 3 5 relative to ci chondrites and that it has approximately chondritic mg si and al sc ratios stokes settling calculations for a vesta wide nonturbulent magma ocean suggest that early crystallizing magnesian olivine orthopyroxene and pigeonite would have settled relatively quickly permitting fractional crystallization to occur but that later crystallizing phases would have settled or floated an order of magnitude more slowly allowing instead a closer approach to equilibrium crystallization for the more evolved eucritic melts this would have inhibited the formation of a plagioclase flotation crust on vesta plausible models for the interior of vesta which are consistent with the data for hed meteorites and vesta include a metal core 130 km radius an olivine rich mantle 65 220 km thick a lower crustal unit 12 43 km thick composed of pyroxenite from which diogenites were derived and an upper crustal unit 23 42 km thick from which eucrites originated the present shape of vesta with 60 km difference in the maximum and minimum radius suggests that all of the crustal materials and possibly some of the underlying olivine from the mantle could have been locally excavated or exposed by impact cratering

the composition of vesta from the dawn mission abstract vesta s surface composition has been of special interest since early disk integrated telescopic spectral observations indicated that it is basaltic differentiated and similar to the hed howardite eucrite diogenite class of meteorites the dawn mission orbiting vesta provided a large and varied set of unique observations on the detailed mineralogy molecular and elemental composition and their distributions in association with surface features and geology the set of articles contained in this special issue is the first treatment of the entire surface composition of vesta using the complete dawn vesta data set and the calibrations from the entire campaign most articles treat a region of vesta within the context of the entire body but there are several articles that treat global or technical topics as a whole these articles provide a current and comprehensive view of vesta s composition using all the relevant data that is available vesta s surface composition is consistent with the upper layer being created by igneous processes while a more mafic lithology generally associated with a mantle is surprisingly limited there is evidence of contamination by low velocity infall of several types of objects dark hydrated hydroxylated material and probably fe mg silicates differing from vesta s isolated blocks of differing compositions seen especially in crater walls could indicate incomplete melting and mixing during the differentiation process and retention of some evidence of the original building blocks of the accreted vesta this lead article introduces and provides the context for the following articles presents a summary of the various findings and integrates them into overall conclusions

mineralogical and spectral analysis of vesta s gegania and lucaria quadrangles and comparative analysis of their key features abstract this work is aimed at developing and interpreting infrared albedo pyroxene and oh band depths and pyroxene band center maps of vesta s gegania and lucaria quadrangles obtained from data provided by the visible and infrared vir mapper spectrometer on board nasa s dawn spacecraft the gegania and lucaria quadrangles span latitudes from 22 s to 22 n and longitudes from 0 e to 144 e the mineralogical and spectral maps identify two large scale units on this area of vesta which extend eastwards and westward of about 60 e respectively the two regions are not associated to large scale geological units which have a latitudinal distribution rather than longitudinal but are defined by different contents of carbonaceous chondrites cc the eastern region poor in ccs is brighter and oh depleted whereas the western one rich in ccs is darker and oh enriched a detailed analysis of the small scale units in these quadrangles is also performed almost all the units show the typical correspondence between high albedo deep pyroxene bands short band centers and absence of oh and vice versa only a few exceptions occur such as the ejecta from the aelia crater where dark and bright materials are intimately mixed the most characteristic features of these quadrangles are the equatorial troughs and the lucaria tholus the equatorial troughs consist of graben i e a depression limited by two conjugate faults the graben do not present their own spectral signatures but spectral parameters similar to their surroundings in agreement to their structural origin this is observed also in graben outside the gegania and lucaria quadrangles however it is possible to observe other structural features such as tectonic grooves characterized by a changing composition and hence an albedo variation this result is confirmed not only by mineralogical maps of vesta but also by analyzing the virtis rosetta observations of lutetia the albedo change is instead a typical behavior of geomorphic grooves finally ridges are characterized by a bluer color and in some cases shorter band centers than their surroundings suggesting that they are composed of fresher materials we also performed a comparative analysis between the three tholi of vesta i e lucaria which gives the name to its quadrangle aricia in the marcia quadrangle and brumalia numisia quadrangle whereas brumalia tholus is a young magmatic intrusion the absence of diogenites the low albedo and the orientation of aricia and lucaria tholi suggest that they are older features which are covered by dark materials and therefore experienced a different geological history than brumalia

the sextilia region on asteroid 4vesta stratigraphy and variegation abstract mapping vesta surface composition in the sextilia region offers the unique possibility to investigate stratigraphic relationship of the surface compounds throughout the transition from rheasilvia vesta s prominent impact basin to vesta s northern equatorial region the vir data point to an upper eucrite dominated layer overlaying extended deposits of diogenite in the subsurface which reaches the surface at matronalia rupes the uplifted rim of the rheasilvia impact structure the dominance of diogenite in matronalia rupes suggests the existence of an extended diogenite rich layer and supports the magma ocean model as the most plausible formation model for vesta s interior local enrichment of diogenite outside of the rheasilvia impact basin are interpreted as outcrops of diogenite dominated breccia excavated during the rheasilvia impact event and re excavated by more recent impacts on the contrary the asymmetry in the distribution of diogenite rich outcrops in the norther parts i e the depletion of these deposits in the veneneia region is at odds with the equally distributed plutonic intrusions of diogenite rich material as proposed in the serial magmatism model but could be explained by an uneven distribution of its ejecta on vesta s surface due to an oblique rheasilvia impact

mineralogy of carbonaceous chondrite clasts in hed achondrites and the moon the majority of the carbonaceous chondrite clasts found in howardites eucrites and diogenites are cm2 material a lesser proportion is cr2 material and other rare types are present a single clast that was found on the moon and called the bench crater meteorite is apparently shocked cm1 material the cm2 clasts are matrix supported mixtures of olivine pyroxene phyllosilicate sulfide bearing aggregates loose olivines and pyroxenes sulfides carbonates and sinuous spinel phyllosilicate diopside calcium aluminum rich inclusions cais magnetite and metal are rare some aggregates have fine grained rims of material resembling matrix the opaque fine grained matrix consists predominantly of serpentine of extremely variable composition and sulfides tochilinite is occasionally present the trace element data for one jodzie clast from this study and the average of similar clasts from kapoeta support a cm classification volatiles are depleted relative to ci and enriched relative to cr material the cr2 clasts are found in small numbers in only four howardites bholghati jodzie kapoeta and y793497 petrographically they are matrix supported mixtures of olivine aggregates sometimes containing sulfides loose olivines pyrrhotite pentlandite low ca pyroxene minor hedenbergite rare kamacite rare and only found within olivine ca carbonates and abundant magnetite framboids and plaquets phyllosilicates are fine grained and largely confined to matrix they are mixtures of serpentine and saponite the matrix of cr2 clasts also contains pyrrhotite pentlandite chromite and a significant fraction of poorly crystalline material with the same bulk composition as matrix phyllosilicate there is evidence of heating in a substantial number of clasts both cm2 and cr2 including 1 corrugated serpentine flakes 2 pseudomorphs of anhydrous ferromagnesian material after flaky phyllosilicates and 3 hedenbergite rims on calcite while the timing of the hedenbergite rims is debatable the destruction of phyllosilicates clearly occurred at a late stage plausibly during impact onto the hed asteroid s and moon and required peak heating temperatures on the order of 400 c we note that in general cm2 material was the most common carbonaceous chondrite lithology impacting the hed asteroids with howardites and eucrites taken together as it is for the earth today a total of 61 out of 75 carbonaceous chondrite clasts from hed meteorites belong to the cm clan petrologic grade 2 this is also supported by published siderophile and volatile element data on howardites eucrites and diogenites that are taken to indicate that cm like materials were the most common impactors on the hed asteroid s the ratio of cr cm clasts in hed asteroids is essentially the same as for modern falls at earth this may indicate that the ratio of disaggregated cm2 to cr2 asteroidal material has been approximately constant through the history of the solar system finally our results are also compatible with type 2 carbonaceous chondrites being equivalent to or from the same source as the material that originally accreted to form the hed asteroid

a magma ocean on vesta core formation and petrogenesis of eucrites and diogenites abstract available evidence strongly suggests that the hed howardite eucrite diogenite meteorites are samples of asteroid 4 vesta abundances of the moderately siderophile elements ni co mo w and p in the hed mantle indicate that the parent body may have been completely molten during its early history during cooling of a chondritic composition magma ocean equilibrium crystallization is fostered by the suspension of crystals in a convecting magma ocean until the crystal fraction reaches a critical value near 0 80 when the convective system freezes and melts segregate from crystals by gravitational forces the extruded liquids are similar in composition to main group and stannern trend eucrites and the last pyroxenes to precipitate out of this ocean before convective lockup span the compositional range of the diogenites subsequent fractional crystallization of a main group eucrite liquid which has been isolated as a body of magma produces the nuevo laredo trend and the cumulate eucrites the predicted cumulate mineral compositions are in close agreement with phase compositions analyzed in the cumulate eucrites thus eucrites and diogenites are shown to have formed as part of a simple and continuous crystallization sequence starting with a magma ocean environment on an asteroidal size parent body that is consistent with vesta

is vesta an intact and pristine protoplanet abstract it is difficult to find a vesta model of iron core pyroxene and olivine rich mantle and hed crust that can match the joint constraints of a vesta s density and core size as reported by the dawn spacecraft team b the chemical trends of the hed meteorites including the depletion of sodium the feo abundance and the trace element enrichments and c the absence of exposed mantle material on vesta s surface among vestoid asteroids or in our collection of basaltic meteorites these conclusions are based entirely on mass balance and density arguments independent of any particular formation scenario for the hed meteorites themselves we suggest that vesta either formed from source material with non chondritic composition or underwent after its formation a radical physical alteration possibly caused by collisional processes that affected its global composition and interior structure

geologic mapping of vesta from 1994 hubble space telescope images we utilize the 56 images of asteroid 4 vesta obtained through four mineralogically diagnostic filters by the hubble space telescope zellneret al 1997 icarus to construct a geologic map vesta s surface is found to be geologically diverse and to be dichotomous at a hemispheric scale the eastern hemisphere longitude definition from thomaset al 1997 icarus is dominated by units interpreted to be impact excavated plutonic material composed of magnesium rich and calcium poor pyroxene the mineralogy of this region is most analogous to diogenite meteorites several eastern hemisphere units which have the deepest and broadest 1 m absorption bands may contain a substantial olivine component the locations of these units are consistent with a previous map based on rotationally resolved groundbased spectroscopy gaffey 1997 icarus the western hemisphere is dominated by units interpreted to consist of a single component of iron rich and relatively calcium rich pyroxene analogous to surface basalts such as eucrite meteorites we investigate the spectral properties of 20 representative surface regions and find a correlation between the 1 m absorption band depth and albedo the units with lower albedos have shallower band depths such a relation could arise from differences in lithologies differences in surface particle sizes or from a weathering effect which decreases albedo and band depth over time because of vesta s geologic diversity and hemispheric dichotomy it is apparent that any major impact events such as those possibly related to the formation of the vesta family binzel and xu 1993 science260 186 191 did not globally resurface the planet the spectral evidence for remnants of a basaltic crust implies that the oldest units on the surface date back to the time of their emplacement most likely 4 5 byr ago if a weathering process exists which alters the albedo then the lowest albedo units such as the feature with the proposed name olbers most likely represent remnants of vesta s ancient basaltic crust

differentiation and magmatic activity in vesta evidenced by 26al 26mg dating in eucrites and diogenites abstract the 26 al 26 mg short lived chronometer has been widely used for dating ancient objects in studying the early solar system here we use this chronometer to investigate and refine the geological history of the asteroid 4 vesta ten meteorites widely believed to come from vesta 4 basaltic eucrites 3 cumulate eucrites and 3 diogenites and the unique achondrite asuka 881394 were selected for this study all samples were analyzed for their 26 mg and 27 al 24 mg ratios in order to construct both whole rock and model whole rock isochrons mineral separation was performed on 8 of the hed s in order to obtain internal isochrons while whole rock al mg analyses of hed s plot on a regression that could be interpreted as a vestan planetary isochron internal mineral isochrons indicate a more complex history crystallization ages obtained from internal 26 al 26 mg systematic in basaltic eucrites show that vesta s upper crust was formed during a short period of magmatic activity at 2 66 0 58 1 39 million years ma after calcium aluminum inclusions after cai we also suggest that impact metamorphism and subsequent age resetting could have taken place at the surface of vesta while 26 al was still extant cumulate eucrites crystallized progressively from 5 48 0 60 1 56 to 7 25 ma after cai model ages obtained for both basaltic and cumulate eucrites are similar and suggest that the timing of differentiation of a common eucrite source from a chondritic body can be modeled at 2 88 0 12 0 14 ma after cai i e contemporaneously from the onset of the basaltic eucritic crust based on their cumulate texture we suggest cumulate eucrites were likely formed deeper in the crust of vesta diogenites have a more complicated history and their 26 al 26 mg systematics show that they likely formed after the complete decay of 26 al and thus are younger than eucrites this refined chronology for eucrites and diogenites is consistent with a short magma ocean stage on 4 vesta from which the basaltic eucrites rapidly crystallized in order to explain the younger age and the complex history of diogenites we postulate that a second episode of magmatism was possibly triggered by a mantle overturn we bring a refined chronology of the geological history of vesta that shows that the asteroid has known a more complex differentiation than previously thought

distinctive space weathering on vesta from regolith mixing processes whereas space weathering of some airless bodies such as the moon occurs through the accumulation on regolith of nanophase metallic particles spectroscopic data show that space weathering of the asteroid vesta occurs through the small scale mixing of diverse surface components which gradually generates locally homogenized upper regolith between 16 july 2011 and 5 september 2012 nasa s space probe dawn was orbiting vesta a protoplanet thought to have survived virtually intact since an early phase of solar system formation in this issue of nature two groups report on the encounter carle pieters and co workers find that space weathering on vesta has followed a different course from that observed on the moon and on itokawa the asteroid sampled in an earth return mission on vesta weathering involved fine scale regolith soil mixing that has removed clear traces of recent impact deposits there are no signs of the nanophase metallic particle deposits seen on the moon and itokawa thomas mccord and co authors describe two main types of material on vesta s surface bright and dark the bright material may be uncontaminated indigenous vesta basaltic soil with the darker material derived from low albedo impactors dawn has now moved on and is due to rendezvous with the protoplanet ceres in february 2015 the surface of the asteroid vesta has prominent near infrared absorption bands characteristic of a range of pyroxenes confirming a direct link to the basaltic howardite eucrite diogenite class of meteorites1 2 3 processes active in the space environment produce space weathering products that substantially weaken or mask such diagnostic absorption on airless bodies observed elsewhere4 5 and it has long been a mystery why vesta s absorption bands are so strong analyses of soil samples from both the moon6 and the asteroid itokawa7 determined that nanophase metallic particles commonly nanophase iron accumulate on the rims of regolith grains with time accounting for an observed optical degradation these nanophase particles believed to be related to solar wind and micrometeoroid bombardment processes leave unique spectroscopic signatures that can be measured remotely8 9 10 but require sufficient spatial resolution to discern the geologic context and history of the surface which has not been achieved for vesta until now here we report that vesta shows its own form of space weathering which is quite different from that of other airless bodies visited no evidence is detected on vesta for accumulation of lunar like nanophase iron on regolith particles even though distinct material exposed at several fresh craters becomes gradually masked and fades into the background as the craters age instead spectroscopic data reveal that on vesta a locally homogenized upper regolith is generated with time through small scale mixing of diverse surface components

large scale troughs on vesta a signature of planetary tectonics abstract images of vesta taken by the dawn spacecraft reveal large scale linear structural features on the surface of the asteroid we evaluate the morphology of the vesta structures to determine what processes caused them to form and what implications this has for the history of vesta as a planetary body the dimensions and shape of these features suggest that they are graben similar to those observed on terrestrial planets not fractures or grooves such as are found on smaller asteroids as graben their vertical displacement versus length relationship could be evaluated to describe and interpret the evolution of the component faults linear structures are commonly observed on smaller asteroids and their formation has been tied to impact events while the orientation of the large scale vesta structures does imply that their formation is related to the impact events that formed the rheasilvia and veneneia basins their size and morphology is greatly different from impact formed fractures on the smaller bodies this is consistent with new analyses that suggest that vesta is fully differentiated with a mantle and core we suggest that impact into a differentiated asteroid such as vesta could result in graben while grooves and fractures would form on undifferentiated asteroids

color and albedo heterogeneity of vesta from dawn multispectral images 0 44 to 0 98 m of asteroid 4 vesta obtained by the dawn framing cameras reveal global color variations that uncover and help understand the north south hemispherical dichotomy the signature of deep lithologies excavated during the formation of the rheasilvia basin on the south pole has been preserved on the surface color variations band depth spectral slope and eucrite diogenite abundance clearly correlate with distinct compositional units vesta displays the greatest variation of geometric albedo 0 10 to 0 67 of any asteroid yet observed four distinct color units are recognized that chronicle processes including impact excavation mass wasting and space weathering that shaped the asteroid s surface vesta s color and photometric diversity are indicative of its status as a preserved differentiated protoplanet

olivine and pyroxene from the mantle of asteroid 4 vesta a b s t r a c t a number of meteorites contain evidence that rocky bodies formed and differentiated early in our solar system s history and similar bodies likely contributed material to form the planets these differentiated rocky bodies are expected to have mantles dominated by mg rich olivine but direct evidence for such mantles beyond our own planet has been elusive here we identify olivine fragments mg 80 92 in howardite meteorites these mg rich olivine fragments do not correspond to an established lithology in the howardite eucrite diogenite hed meteorites which are thought to be from the asteroid 4 vesta their occurrence in howardite breccias combined with diagnostic oxygen three isotope signatures and minor element chemistry indicates they are vestan the major element chemistry of these mg rich olivines suggests that they formed as mantle residues in crustal layered intrusions or in mg rich basalts the trace element chemistry of these mg rich olivines supports an origin as mantle samples but other formation scenarios could be possible interpreted as mantle samples the range of mg rich olivine compositions indicates that vesta s structure differs from that predicted by conventional models vesta has a chemically heterogeneous mantle that feeds serial magmatism the range of olivine major element chemistries is consistent with models of an incompletely melted mantle such as in the model proposed by wilson and keil 2013 rather than a whole mantle magma ocean for vesta trace element chemistries of mg rich pyroxenes mg 85 92 provide support that some of these pyroxenes may represent initial

first disk resolved spectroscopy of 4 vesta abstract vesta the second largest main belt asteroid will be the first to be explored in 2011 by nasa s dawn mission it is a dry likely differentiated body with spectrum suggesting that is has been resurfaced by basaltic lava flows not too different from the lunar maria here we present the first disk resolved spectroscopic observations of an asteroid from the ground we observed 4 vesta with the eso vlt adaptive optics equipped integral field near infrared spectrograph sinfoni as part of its science verification campaign the highest spatial resolution of 90 km on vesta s surface was obtained during excellent seeing conditions 0 5 in october 2004 we observe spectral variations across vesta surface that can be interpreted as variations of either the pyroxene composition or the effect of surface aging we compare vesta s 2 m absorption band to that of howardite eucrite diogenite hed meteorites that are thought to originate from vesta and establish particular links between specific regions and hed subclasses the overall composition is found to be mostly compatible with howardite meteorites although a small area around 180 e longitude could be attributed to a diogenite rich spot we finally focus our spectral analysis on the characteristics of vesta s bright and dark regions as seen from hubble space telescope s visible and keck ii s near infrared images

mars high resolution gravity fields from mro mars seasonal gravity and other dynamical parameters abstract with 2 years of tracking data collection from the mro spacecraft there is noticeable improvement in the high frequency portion of the spherical harmonic mars gravity field the new jpl mars gravity fields mro110b and mro110b2 show resolution near degree 90 additional years of mgs and mars odyssey tracking data result in improvement for the seasonal j 3 gravity changes which compares well to global circulation models and odyssey neutron data and mars rotation and precession 7594 10 mas year once atmospheric dust is accounted for in the spacecraft solar pressure model solutions for mars solar tide are consistent between data sets and show slightly larger values k2 0 164 0 009 after correction for atmospheric tide compared to previous results further constraining core models an additional 4 years of mars range data improves the mars ephemeris determines 21 asteroid masses and bounds solar mass loss dgmsun dt

palynological assessment of organic tissues and metallic minerals in the jerritt canyon gold deposit nevada u s a abstract organic tissues and metallic minerals were studied in acid residues and thin section of the jerritt canyon bell mine gold deposit the purpose was to compare gold bearing and gold free rocks characterize their differences and assess a possible relationship between the invisible gold and the organic tissues in these early paleozoic carbonaceous limestones dolomites and claystones the most visually abundant acid resistant component in the sooty unoxidized gold bearing rocks is a black rectangular carbon bearing mineraloid in x ray diffraction it is poorly crystalline in nuclear magnetic resonance it is a highly aromatized structure the increase in gold content that accompanies the increase in content of this protographite suggests that it may be the major gold bearing compound less abundant among the acid resistant residues are uncarbonized organic tissues that are typical of marine settings the tissues not in the vicinity of silicified veins are medium to dark brown the dominant tissues are the amorphous remains of algae and their zooplankton predators these uncarbonized indigenous tissues indicate that temperatures were lower than those determined by geochemical and fluid inclusion methods the presence of different kinds of carbon bearing compounds could account for the discrepancy between the palynological and geochemical data the uncarbonized organic tissues comprise such a small percentage of these rocks that they could be easily missed by bulk analytical methods the presence of uncarbonized tissues helps to constrain the duration of the high heat flow that is shown by the fluid inclusion data the presence of mixed signals suggests that the protographite may be the product of chemical rather than thermal reactions