regolith and megaregolith formation of h chondrites thermal constraints on the parent body abstract spectral reflectivity data and its location near an orbital resonance suggest that asteroid 6 hebe may be the source body for h chondrites the second largest meteorite group recent spacecraft images of asteroids and theoretical modeling indicate that contrary to previous ideas asteroids can retain thick regoliths we model the thermal evolution of a hebe sized object coated with a thick insulating regolith and heated by26al and other long lived radionuclides the heat conduction equations for spherically symmetric objects were solved using finite difference approximations we assumed a three layer structure with regolith and megaregolith overlying a rocky core the three layers differed in bulk density porosity and thermal conductivity interior peak temperatures were set to match metamorphic temperatures of h6 chondrites the regolith has a major influence on thermal history and the results are very different from those for a simple rocky body published by various authors regolith insulation produces a uniform interior peak temperature of 1250 k and moves the petrographic type boundaries close to the surface of the parent body petrologic types 3 6 can be produced within 10 km of the asteroid s surface with only moderate 1 km regolith thicknesses the calculations indicate that h4 h6 formation would be consistent with the cooling rate estimates and pb pb formation ages if the material originated in the near surface regions we suggest that many if not all h chondrites could have been formed in a megaregolith and thick regolith their observed properties are consistent with this environment especially the abundance of regolith breccias and h chondrites of all petrologic types with implanted solar wind gases

thermal history of the h chondrite parent body implications for metamorphic grade and accretionary time scales abstract multiple temperature age constraints for eight h chondrite samples have been used to provide insight into the thermal history of their parent body through combination with numerical models of thermal evolution assuming internal heating by 26al and conductive cooling the effect of spreading accretion out over time is the principal focus of this work a wide range of body size and date of accretion is systematically tested for different values of accretion rate in order to quantify and illustrate the parameter space that is consistent with the available thermo chronological data we conclude that the h chondrite samples considered have a thermal history consistent with a parent body that at some stage had a concentric onion shell internal structure that body had a radius no larger than 130 km and accretion most probably took place over a time interval on the order of 0 0 0 2 myr approximately 2 myr after cai condensation in any case the time interval of accretion is unlikely to have been more than 0 5 myr supporting evidence in favour of rapid accretion possibly through reassembly of the fragments of an earlier generation of bodies furthermore the h chondrites studied here are inferred to have come from a wide depth range within the body where they experienced metamorphism indicating that preservation of the onion shell structure is unlikely the presence of an insulating regolith does not modify this conclusion as appropriate thermal histories for the three h6 samples considered cannot be reproduced at depths near the surface asteroid 6 hebe may be the parent body of the h chondrites but the high bulk density of the latter is difficult to reconcile with a rubble pile structure of pure h chondrite material finally optimized thermal histories are used to constrain the temperatures characterizing boundaries between petrological types 800 1000 and 1140 k for the h3 4 h4 5 and h5 6 boundaries respectively in detail the type 6 samples studied here are inferred to have experienced a similar peak temperature on the order of 1180 k but spent different times at that temperature while samples of lower metamorphic grade experienced different peak temperature but spent more or less the same time within 10 k of peak temperature on the order of 1 ma these results constitute a quantitative framework within which variations in petrographic and textural properties may be interpreted

unraveling the evolution of chondrite parent asteroids by precise u pb dating and thermal modeling u th pb isotopic data are reported for mineral fractions individual chondrules and fractions of chondrule fragments from the equilibrated ordinary chondrite richardton h5 chondrules and milligram sized fractions of pyroxene rich chondrule fragments contain highly radiogenic pb and concordant or nearly concordant u th pb isotopic systems and are suitable for precise pb pb age determinations olivine and sulfide have low u concentrations and contain less radiogenic pb the ages of individual chondrules pyroxene rich and phosphate fractions are determined using u pb and pb pb isochron and model date calculations the pb pb isochron date of 4562 7 1 7 ma of the richardton chondrules and chondrule fragments is resolved from the pb pb isochron date of 4550 7 2 6 ma obtained from multiple phosphate fractions possible biases of the isochron dates due to single stage approximation of multi stage evolution contamination with modern common pb and disturbance to the system by reheating are examined and are found to be insignificant the chondrule and phosphate dates are interpreted as the timing of cessation of pb diffusion during cooling following metamorphism in chondrite parent bodies the difference in estimated closure temperatures 950 1150 k for pyroxenes and 700 800 k for phosphates temperature estimates are based on published diffusion rates for pb in pyroxenes and apatite allows evaluation of the average cooling rate at 26 13 k million years for the richardton parent body over the period of 4563 4551 my thermal modeling of the h chondrite parent body which is assumed to be asteroid 6 hebe heated by decay of 26al suggests a scenario in which accretion initiated at 1 7 m y after formation of calcium aluminum rich inclusions and continued for 3 5 m y

the mineralogy of ordinary chondrites and implications for asteroid spectrophotometry abstract the normative mineralogies of 94 ordinary chondrites have been calculated from high quality bulk chemical analyses relative proportions of olivine orthopyroxene and metal are more variable within the h l and ll classes than has been recognized previously mineralogy within each chondrite class varies systematically with oxidation state these data have important implications for the design of reflectance spectroscopy studies that match asteroid and meteorite spectral features and for quantitative spectral interpretations of asteroids they also provide a tool with which to predict mineral abundances in unsampled asteroids that formed under different redox conditions

the wood lake ontario h4 ordinary chondrite a new canadian meteorite1 the wood lake meteorite was found in the muskoka region of ontario in july 2003 we examine the mineralogy physical properties shock and weathering state of this recently reported canadian meteo

radiative transfer mixing models of meteoritic assemblages 1 a spectral mixing model for the expected conditions on asteroid surfaces based on hapke s radiative transfer theory for intimate mixtures is presented this model calculates the visible near infrared reflectance spectrum of an intimate mixture incorporating the modal mineralogy the mineral chemistry the particle size and the degree of space weathering as factors and includes an improved treatment for the spectral effects of coarse grained fe ni metal this model can reproduce the spectra of geologically plausible mixtures of silicate components and meteorites with known mineral abundances chemistry and particle sizes we compare the model spectral effects of the coarse grained fe ni metal naturally present on asteroid surfaces to the spectral effects of submicroscopic iron produced by lunar style space weathering the model spectral effects of large asteroid surface concentrations of coarse grained fe ni metal are similar to the spectral effects of smfe in some cases we model the composition of asteroid 4 vesta and find it to be consistent with a minimally space weathered eucritic composition finally we predict the spectral properties of some example of ordinary chondrite parent bodies when modeled with particle sizes matching the optically dominant particle size of the lunar surface and minimal abundances of smfe these model ordinary chondrite parent bodies plot outside the s iv asteroid field on a band area ratio versus band i center plot we find that a combination of larger particle sizes and increased smfe abundances is the most effective way to move these models into the s iv field

s type asteroids ordinary chondrites and space weathering the evidence from galileo s fly bys of gaspra and ida new observations of the s type asteroids gaspra and ida especially by the galileo spacecraft demonstrate that a space weathering process operates which modifies the reflectance spectra of fresh material to be redder straighter and have shallower absorption bands the weathering process appears similar to but less potent than that observed on the moon it operates in the sense that it would tend to convert spectra of ordinary chondrites oc to having the spectral traits of s type asteroids these results appear to resolve the major obstacles of the long standing s type conundrum about the provenance of ordinary chondrite meteorites a wide body of recent research which is reviewed here builds on previous meteoritical evidence to support a new developing consensus that the larger s type asteroids are a diverse assortment of silicaceous assemblages which includes the ordinary chrondite parent bodies recent fairly realistic laboratory simulations of space weathering processes have changed oc spectra to resemble s type spectra j f bell s highly regarded earlier paradigm that oc parent bodies would be found only among sub 10 km main belt asteroids has been tested by an extensive observational survey by r binzel and his proteges the result is that no small main belt analog for the ocs has been found not even the somewhat oc like object boznemcova this article reviews the history of the s type conundrum which set the stage for galileo s historic flybys i review the findings about gaspra and ida including results about their geology their geophysical structure and probable origins and about ida s small moonlet dactyl density constraints on ida set by dactyl s orbit rule out for ida at least the classic view of s types as metallic stripped cores of differentiated precursor asteroids new analysis of the galileo spectral images of ida is presented that provides strong evidence that space weathering occurs on ida and that ida and the koronis family asteroids in general are plausibly oc like in composition after reviewing 1990s developments on the s type conundrum i advocate a new perspective that the ordinary chondrite parent bodies are among the s type asteroids a diverse grouping that also contains other silicaceous and silicate metal assemblages presumably including various stony irons and primitive achondrites represented in meteorite collections gaspra may be such a metal rich assemblage

importance of the accretion process in asteroid thermal evolution 6 hebe as an example widespread evidence exists for heating that caused melting thermal metamorphism and aqueous alteration in meteorite parent bodies previous simulations of asteroid heat transfer have assumed that accretion was instantaneous for the first time we present a thermal model that assumes a realistic incremental accretion scenario and takes into account the heat budget produced by decay of 26al during the accretion process by modeling 6 hebe assumed to be the h chondrite parent body we show that in contrast to results from instantaneous accretion models an asteroid may reach its peak temperature during accretion the time at which different depth zones within the asteroid attain peak metamorphic temperatures may increase from the center to the surface and the volume of high grade material in the interior may be significantly less than that of unmetamorphosed material surrounding the metamorphic core we show that different times of initiation and duration of accretion produce a spectrum of evolutionary possibilities and thereby highlight the importance of the accretion process in shaping an asteroid s thermal history incremental accretion models provide a means of linking theoretical models of accretion to measurable quantities peak temperatures cooling rates radioisotope closure times in meteorites that were determined by their thermal histories

metal silicate mixtures spectral properties and applications to asteroid taxonomy the reflectance spectra of combinations of olivine orthopyroxene and iron meteorite metal show systematic variations in spectral properties which can be used to constrain many of the physical and chemical properties of the assemblages the presence of metal most noticeably affects band area ratios peak peak and peak minimum reflectance ratios and band widths these parameters are also affected to varying degrees by the abundance and grain size of the metal and silicates and the composition of the silicates band width and band areas are useful for determining metal abundance in olivine metal and orthopyroxene metal assemblages respectively mafic silicate grain size variations are best determined using band depth criteria band centers are most useful for determining mafic silicate composition analysis of the reflectance spectrum of the s class asteroid 8 flora on the basis of these spectral parameters indicates a surface composed of approximately 50 wt metal 40 wt olivine fa 35 10 10 wt orthopyroxene fs 30 25 35 and perhaps a few weight percent of clinopyroxene a substantial amount of the surface seems to consist of a fine grained 45 m size component analysis of the reflectance spectrum of the a class asteroid 446 aeternitas suggests a surface assemblage of approximately 35 wt metal 55 wt olivine fa 20 10 7 wt clinopyroxene fs 17 and 3 wt orthopyroxene fs 45 a substantial fine grained 45 m size surface component is also indicated for this asteroid

extensive impact melting on the h chondrite parent asteroid during the cataclysmic bombardment of the early solar system evidence from the achondritic meteorite dar al gani 896 dag 896 is an olivine rich microporphyritic rock of komatiitic composition both the olivine composition fa 17 5 2 1 mn mg 0 0061 and the bulk oxygen isotopic composition delta 17 o 2 55 delta 18 o 3 50 indicate that dag 896 is a sample of the h chondrite parent body the bulk chemistry shows an h chondritic distribution of lithophile elements whereas chalcophile and siderophile elements are strongly depleted indicating formation through whole rock melting or nearly so of h chondrite material nearly complete loss of the metal plus sulfide component and crystallization without significant igneous fractionation superheated severely shocked chondritic relics similar to10 vol typically in the form of corroded lithic fragments 100 mum in size intimately distributed within the igneous lithology indicate that melting was triggered by a highly energetic impact which possibly induced shock pressures of similar to80 100 gpa the relatively young 3 704 0 035 ga ar 40 ar 39 crystallization age is consistent with the impact melting origin as magmatism in the asteroid belt was active only in the first hundred million years of solar system history 1ased on textural data and thermodynamic crystallization modelling we infer that dag 896 crystallized from a liquidus temperature of similar to1630degreesc under relatively slow cooling rates similar to10degreesc h 1 to 1300degreesc before quenching the two stage cooling history indicates that a reasonable formation environment might be a dike intruding cooler basement below a crater floor metal silicate fractionation may have been accomplished at least at the centimeter scale of the studied meteorite sample through differential acceleration of immiscible liquids of different density during the intense flow regimes associated with the excavation and modification stages of the cratering mechanism alternatively dag 896 may represent a surface sample of a differentiated melt body at the floor of an impact crater as gravitational settling appears to be an effective process at the surface of a chondritic parent asteroid for metal particles 1 to 10 mm in size typically observed in partially differentiated impact melt rocks stokes law indicates a settling velocity 1 in h 1 during the first few hours of crystallization on asteroidal bodies of 25 kin radius the similar to3 7 ga age of dag 896 nearly overlaps with the slightly older resetting ages of h chondrites all impact melts available from the literature indicating that the h chondrite parent asteroid underwent extensive impact melting at the enduring of the cataclysmic bombardment of the early solar system such an age overlap may also indicate early disruption of the initial h chondrite parent asteroid the close similarity between the bulk composition and degassing age of dag 896 and silicate inclusions in he iron meteorites is further evidence in support of a common origin by impact melting and metal silicate segregation on the h chondrite parent asteroid our new high precision oxygen isotopic measurements of h chondrites delta 17 o 0 77 0 04 should be extended to iies to verify this possible petrogenetic link

oxygen isotope relationships in iron meteorites iron meteorites with oxygen bearing phases can be classified in terms of their oxygen isotopic abundances these iron meteorite classes are isotopically similar to various stony meteorite classes which may indicate a common origin the group iab and iiicd irons may be related to the winonaites group iie irons may be related to h chondrites group iva irons may be related to l or ll chondrites

the porosities of ordinary chondrites models and interpretation densities and porosities for 285 ordinary chondrites have been assembled and analyzed measured chondrite porosities are bimodal finds have an average porosity of 30 we conclude that mild degrees of weathering fill pore spaces lowering grain densities and porosities without significantly changing the bulk size or mass of the sample by assuming an original pristine grain density as a function of the meteorite s mineralogy determined by its class we can derive model pristine porosities these model porosities cluster around an average value of 10 for all classes of ordinary chondrites ordinary chondrites do not show any correlation of porosity model or measured with petrographic grade or sample size over a range from 0 2 g to 2 kg however we do see a correlation between shock state and porosity shock blackened meteorites are less porous than other meteorites furthermore less severely shocked meteorites show a much broader range of porosities with the maximum porosity seen among meteorites of a given shock class falling linearly as a function of that shock class this is consistent with the idea that shock compresses and closes pore space analysis of meteorite porosity provides a lower bound to the fine scale porosity of asteroids our densities even with 10 primordial porosity are significantly higher than inferred densities of possible asteroid parent bodies these asteroids are probably loose piles of rubble

a survey of clasts and large chondrules in ordinary chondrites we report the results of a survey of clasts and large 5 mm chondrules macrochondrules within the 833 ordinary chondrites of the natural history museum collection thirty six macrochondrules and 24 clasts were identified and studied macrochondrules have textures and mineral assemblages like normal chondrules and so share a common origin clasts show evidence for fracturing from larger bodies and can be classified as either 1 chemically fractionated if they have major and trace element compositions differing substantially from most chondrules and clasts 2 impact melt clasts if they have microporphyritic textures and signs that indicate they are derived from shock melted chondritic material 3 microporhyritic clasts if they are similar to the last category but lack evidence for derivation through shock melting or 4 indeterminate clasts forming a diverse class that includes all those clasts that do not fit into the other categories

revised model calculations for the thermal histories of ordinary chondrite parent bodies recent measurements of ordinary chondrite physical and thermal properties along with new geothermometry studies have provided the necessary parameters for updating a previously proposed model miyamoto et al 1981 for the thermal evolution and internal structure of ordinary chondrite parent bodies model calculations assumed a heat source term derived from the decay of 26al justification is provided differences from the previous model include varying the thermal diffusivity parameter with increasing temperature and decreasing porosity using variable physical and thermal parameters to provide end member models and incorporating a shortened thermal history of 60 ma obtained from new pb pb chronology of phosphates rather than 100 ma times of isotopic closure in chondrite phosphates overlap the thermal model estimates and postmetamorphic cooling rates from the model approximately coincide in both trend and magnitude with metallographic and fission track cooling rate data model calculations attempt to match peak metamorphic conditions in the central portions of these bodies and yield accretion ages between 1 4 to 3 1 ma after calcium aluminum inclusion cai formation model calculations also predict that both the h and the l chondrite parent asteroids consisted of 80 equilibrated and 20 unequilibrated chondritic material and that their original radii ranged from 80 to 95 km

39ar40ar age and petrology of chico large scale impact melting on the l chondrite parent body our studies of the 105 kg chico l chondrite show that it contains 60 impact melt and the largest volume of impact melt recognized in stony meteorites we suggest that it is part of a much larger dike complex that formed when chondritic impact melt was intruded into host chondrite during a large if not catastrophic impact on the l chondrite parent body at about 0 5 ga petrologic and 39ar 40ar dating studies were made on several lithologies including the massive melt zone host chondrite and melt chondrite boundaries for the purpose of studying the melting and thermal histories associated with impacts on small bodies and their effects on the k ar chronometer the chondritic host is shocked to stage s6 and contains pockets and veins of melt there are no unmelted clasts in the interior of the melt coalesced metal troilite nodules reach up to 2 cm in size melt near the contact with the host chondrite contains numerous clasts and quenched more rapidly metal troilite textures suggest cooling rates of 0 1 c s in the interior of the melt dike during crystallization secondary kamacite rims indicate cooling at 0 01 1 c y over the range of 700 500 c consistent with an impact heated volume of up to a kilometer in thickness compositions of olivines and pyroxenes are generally similar in melt and chondritic host reflecting rapid crystallization not metamorphic equilibration the interior melt shows an overall depletion in k whereas the melt near the boundary is enriched in k the 39ar 40ar release spectra during stepwise heating of both melt and chondrite samples can be divided into two parts based on ar diffusion properties and k ca ratios the low temperature high k ca phase of both melt and host chondrite show ages of 0 54 0 78 ga ages of the high temperature low k ca phase of the melt are comparable or higher 0 61 1 35 ga whereas those of the host chondrite are even higher 0 87 1 86 ga due to lesser degrees of degassing isochron plots for several melt samples suggest an age of 0 53 ga and the presence of variable amounts of excess 40ar not completely degassed by the impact even this age however is significantly higher than the previously reported rb sr isochron age of 0 467 015 ga the apparent retention of radiogenic 40ar in the chico impact melt in spite of its relatively large size absence of clasts and moderately slow cooling rate below 700 c raises questions as to the reliability of using melts for 39ar 40ar dating of meteoritic impact events

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

spectral properties and geologic processes on eros from combined near nis and msi data sets from april 24 to may 14 2000 the near earth asteroid rendezvous near shoemaker mission s near infrared spectrometer nis obtained its highest resolution data of 433 eros high signal to noise ratio nis reflectance spectra cover a wavelength range of 8002400 nm with footprint sizes from 213 x 427 m to 394 x 788 m this paper describes improvement in instrument calibration by remediation of internally scattered light derivation of a pseudo channel for nis at 754 nm using multispectral imager msi eros approach maps at 951 and 754 nm synthesis of a 3127 spectrum high resolution data set with the improved calibration and expanded wavelength coverage and investigation of global and localized spectral variation with respect to mineralogy composition and space weathering of eros comparing the findings with previous analyses scattered light removal reduces the red slope of eros spectra though not to the level seen by telescopic observations the pseudo channel completes sampling of eros 1 micron band i absorption feature enabling direct comparison of nis data with other asteroid and meteorite spectra without additional scaling or correction following scattered light removal and wavelength range extension the spectral parameters of average eros plot well inside the s iv field of gaffey et al 1993 and are consistent with the l6 chondrite meteorite fields of gaffey and gilbert 1998 although eros shows no evidence of mineralogical heterogeneity modest spectral variations correlate with morphologically and geographically distinct areas of the asteroid eros bright to dark spectral ratios are largely consistent with laboratory space weathering experiment results and modeling of space weathering effects eros brightness variation unaccompanied by significant spectral variation departs from lunar type where band depths slopes and albedoes all correlate and ida type where significant spectral variation is unaccompanied by corresponding brightness variation the brightest areas on eros steep crater walls have lesser spectral slope and deeper band i consistent with exposure of fresher less space weathered materials bright crater slope materials have opx opx olv of 0 240 29 and may be more representative of the subsurface mineralogy than average eros which is probably affected by space weathering the floors of the large craters psyche and himeros have lower albedo and contain the most degraded or altered looking materials nis spectra retain a red spectral slope at greater than 2 microns the recalibrated and expanded nis spectra show better agreements with mixing models based on space weathering of chondritic mixtures

koronis asteroid dust within antarctic ice here i report the first discovery of large numbers of ordinary chondrite derived micro meteorites mms recovered from antarctic ice ordinary chondrite derived mms comprise 70 coarse grained igneous particles and are identified on the basis of their distribution of petrologic types accessory mineralogy and minor element compositions all of which are very similar to those of millimeter sized igneous objects known as chondrules from ordinary chondrites and largely distinct from those of other meteorite groups the majority of ordinary chondrite derived mms are unequilibrated materials however 15 are equilibrated indicating parent body metamorphism and have compositions consistent with both h high iron and l low iron chondrites the total abundance of ordinary chondrite derived mms of 18 is similar to predictions by numerical calculations for the abundance of dust generated by the recent breakup of the karin group asteroids of the koronis asteroid family suggesting that these asteroids are the main source of the particles these mms represent the first to be associated with a known asteroid and imply that the karin group progenitor asteroid was a rubble pile asteroid that sampled different depths within the original internally metamorphosed koronis parent body

analyses of the chondritic meteorite orvinio h6 insight into the origins and evolution of shocked h chondrite material we have studied the petrography reflectance spectra and ar ar systematics of the orivinio meteorite orvinio is an h chondrite not an l chondrite as sometimes reported the material in the meteorite was involved in several impact events one impact event produced large swaths of impact melt from h chondrite material surrounding relict clasts of chondrule bearing material not only were portions of a bulk h chondrite planestesimal melted during the impact event but shock redistribution of metal and sulfide phases in the meteorite dramatically altered its reflectance spectra both the melt and relict clasts are darker than unshocked h chondrite material bearing spectral similarities to some c class asteroids such shock metamorphism which lowers the albedo of an object without increasing its spectral slope may partially explain some of the variation among s class asteroids and some of the trends seen on asteroid 433 eros noble gases record the evidence of at least two and perhaps three impact events in the meteorite and its predecessor rocks the most significant evidence is for an event that occurred 600 ma ago or less perhaps 325 ma ago or less there is also a signature of 4 2 ga in the ar ar systematics which could either reflect complete degassing of the rock at that time or partial degassing of even the most retentive sites in the more recent event

petrologic evidence for collisional heating of chondritic asteroids abstract the identification of the mechanism s responsible for heating asteroids is among the major problems in planetary science because of difficulties with models of electromagnetic induction and the decay of short lived radionuclides it is worthwhile to evaluate the evidence for collisional heating localized impact heating is responsible for forming shock veins melt pockets metal sulfide mixtures vugs agglutinates and a wide variety of melt rock clasts new evidence for localized impact heating comes from the high proportion of relict type 6 material among impact melt bearing ordinary chondrites oc this relict material was probably metamorphosed by residual heat within large craters olivine aggregates composed of faceted crystals with 120 triple junctions occur within the melted regions of the chico and rose city oc melt rocks the olivine aggregates formed from shocked mosaicized olivine grains that underwent contact metamorphism large scale collisional heating is supported by the correlation in oc between petrologic type and shock stage no other heating mechanism can readily account for this correlation the occurrence of impact melt rock clasts in oc that have been metamorphosed along with their whole rocks indicates that some impact events preceded or accompanied thermal metamorphism such impact events occurring during or shortly after accretion are probably responsible for substantially melting 0 5 of oc these events most have heated a larger percentage of oc to subsolidus temperatures sufficient to have caused significant metamorphism if collisional heating is viable then oc parent asteroids must have been large large oc asteroids in the main belt may include those of the s iv spectral subtype collisional heating is inconsistent with layered onion shell structures in oc asteroids wherein the degree of metamorphism increases with depth but the evidence for such structures is weak it seems likely that collisional heating played an important role in metamorphosing chondritic asteroids

oxygen and asteroids hundreds of thousands of asteroids have been discovered in the asteroid belt and in near earth space oxygen is an abundant element in meteorites and presumably in most asteroids spectral reflectance measurements of asteroids in the visible and near infrared can identify oxygen bearing minerals such as those found in the olivine pyroxene and serpentine groups due to their distinctive absorption features interpretation of the mineralogy of asteroids is complicated by the effects of space weathering which tends to redden and darken the surfaces of asteroids asteroids are primarily classified into a number of taxonomic classes and subclasses according to their spectral properties in the visible wavelength region however asteroids with similar spectral properties in the visible may have different spectral properties in the near infrared and therefore different interpreted mineralogies definite trends in the abundances of different taxonomic classes versus heliocentric distance are apparent these trends appear to be a function of both composition differences and degree of heating however dynamical processes have significantly affected these trends close observation of asteroids by spacecraft such as near shoemaker to 433 eros and hayabusa to 25143 itokawa are currently the best way to discern the mineralogies of individual asteroids

chronology and petrology of silicates from iie iron meteorites evidence of a complex parent body evolution iie iron meteorites contain silicate inclusions the characteristics of which suggest a parent body similar to that of h chondrites however these silicates show a wide range of alteration ranging from netschaevo and techado the inclusions of which are little altered to highly differentiated silicates like those in kodaikanal weekeroo station and colomera which have lost metal and sulfur and are enriched in feldspar we find these inclusions to show varying degrees of shock alteration we made 39ar 40ar age determinations of watson techado miles colomera and sombrerete watson has an ar ar age of 3 677 0 007 gyr similar to previously reported ages for kodaikanal and netschaevo we suggest that the various determined radiometric ages of these three meteorites were probably reset by a common impact event the space exposure ages for these three meteorites are also similar to each other and are considerably younger than exposure ages of other iies 39ar 40ar ages inferred for the other four meteorites analyzed are considerably older than watson and are techado 4 49 0 01 gyr miles 4 405 0 012 gyr colomera 4 470 0 010 gyr and sombrerete 4 541 0 0012 gyr these ages are in fair agreement with previously reported rb sr isochron ages for colomera and weekeroo station although several mechanisms to form iie meteorites have been suggested it is not obvious that a single mechanism could produce a suite of meteorites with very different degrees of silicate differentiation and with isotopic ages that differ by 0 8 gyr we suggest that those iies with older isotopic ages are a product of partial melting and differentiation within the parent body followed by mixing of silicate and metal while both were relatively hot netschaevo and watson may have formed by this same process or by impact mixing 4 5 gyr ago but their isotopic ages may have been subsequently reset by shock heating kodaikanal apparently is required to have formed more recently in which case impact melting and differentiation seems the only viable process we see no compelling reasons to believe that iie silicate and metal derived from different parent bodies or that the parent body of iies was the same as that of h chondrites

exploring exogenic sources for the olivine on asteroid 4 vesta abstract the detection of olivine on vesta is interesting because it may provide critical insights into planetary differentiation early in our solar system s history ground based and hubble space telescope hst observations of asteroid 4 vesta have suggested the presence of olivine on the surface these observations were reinforced by the discovery of olivine rich hed meteorites from vesta in recent years however analysis of data from nasa s dawn spacecraft has shown that this olivine bearing unit is actually impact melt in the ejecta of oppia crater the lack of widespread mantle olivine exposed during the formation of the 19 km deep rheasilvia basin on vesta s south pole further complicated this picture ammannito et al ammannito e et al 2013a nature 504 122 125 reported the discovery of local scale olivine rich units in the form of excavated material from the mantle using the visible and infrared spectrometer vir on dawn these sites are concentrated in the walls and ejecta of craters arruntia 10 5 km in diameter and bellicia 41 7 km in diameter located in the northern hemisphere 350 430 km from rheasilvia basin s rim here we explore alternative sources for the olivine in the northern hemisphere of vesta by reanalyzing the data from the vir instrument using laboratory spectral measurements of meteorites our rationale for using the published dataset was to bypass calibration issues and ensure a consistent dataset between the two studies our analysis of the vir data shows that while the interpretation of their spectra as an olivine rich unit is correct the nature and origin of that olivine could be more complicated we suggest that these olivine exposures could also be explained by the delivery of olivine rich exogenic material this hypothesis is supported by meteoritical evidence in the form of exogenic xenoliths containing significant amount of olivine in some of the hed meteorites from vesta previous laboratory work on heds show that potential sources of olivine on vesta could be different types of olivine rich meteorites either primitive achondrites acapulcoites lodranites ureilites ordinary chondrites h l ll pallasites or carbonaceous chondrites e g cv based on our spectral band parameters analysis the lack of correlation between the location of these olivine rich terrains and possible mantle excavating events and supported by observations of hed meteorites we propose that a probable source for the olivine seen in the northern hemisphere corresponds to remnants of impactors made of olivine rich meteorites the best curve matching results with laboratory spectra suggest these units are hed material mixed with either ordinary chondrites or with some olivine dominated meteorites such as r chondrites

mineralogical characterization of near earth asteroid 1036 ganymed abstract we present a mineralogical assessment of near earth asteroid 1036 ganymed using data obtained may 18 2006 ut combined with 24 color asteroid survey data to cover the spectral interval of 0 3 2 45 m results of the analysis indicate 1036 ganymed is an s vi asteroid with a surface silicate assemblage consisting primarily of orthopyroxene fs 23 5 wo 3 3 consistent with calculated band centers and band area ratios bar 1036 ganymed appears to be once part of a large mesosiderite containing howardite eucrite and diogenite hed pyroxenes mixed with metal that was broken apart and dispersed the calculated composition of the average pyroxenes in the surface material of 1036 ganymed is consistent with mesosiderite pyroxenes in particular the diogenites a second possibility could be 1036 ganymed is not yet represented in the meteorite collection our investigation has confirmed ganymed is not a parent body of the ordinary chondrites and is not genetically related to 433 eros

petrogenesis of silicate inclusions in the weekeroo station iie iron meteorite differentiation remelting and dynamic mixing the weekeroo station iie iron meteorite contains a variety of felsic and mafic inclusions enclosed in an feni metal host petrographic emp and sims data suggest that the petrogenesis of the silicates was complex and included differentiation remelting feo reduction and dynamic mixing of phases differentiation produced a variety of olivine free inclusion assemblages ranging from pyroxene 1 plagioclase 1 tridymite with peritectic compositions to coarse orthopyroxene to plagioclase 1 tridymite and its glassy equivalent individual phases have similar trace element abundances and patterns despite large variations in inclusion textures modes and bulk compositions probably as a result of mechanical separation of pre existing phases in an impact event that dynamically mixed silicates with the metallic host trace element data imply that augite and plagioclase grains in different inclusions crystallized from the same precursor melt characterized by relatively unfractionated ree abundances of 20 30 3 ci chondrites except for a negative eu anomaly such a precursor melt could have been produced by 2 5 equilibrium partial melting of an h chondrite silicate protolith or by higher degrees of partial melting involving subsequent fractional crystallization glass appears to have formed by the remelting of pre existing plagio clase and orthopyroxene in a process that involved either disequilibrium or substantial melting of these phases during remelting silicate melt reacted with the feni metal host and feo was reduced to fe metal following remelting and metal silicate mixing inclusions apparently cooled at different rates in a near surface setting on the parent body glass or pigeonite bearing inclusions cooled more rapidly 2 5 c hr between 1000 850 c than pigeonite free largely crystalline inclusions the results of this study point to two likely models for forming iie iron meteorites both involving collision between an feni metal impactor and either a differentiated or undifferentiated silicate rich target of h chondrite affinity each model has difficulties and it is possible that both are required to explain the diverse iie group copyright 1999 elsevier science ltd

formation conditions of igneous regions in ordinary chondrites chico rose city and other heavily shocked h and l chondrites we report microprobe and radiochemical neutron activation analysis rnaa data for hosts and igneous inclusions in the heavily shocked ordinary chondrites rose city h5 yanzhuang h6 farmington l5 malakal l5 chantonnay l6 chico l6 and tuan tuc l6 based on these analytical results equilibrium crystallization calculations ghiorso and sack 1995 phase equilibrium analysis e g sack and ghiorso 1994a b sack et al 1994 and experimental cooling rate studies e g walker et al 1976 we have assessed the metamorphic and magmatic histories of these seven heavily shocked chondrites we infer that 1 unfractionated chondritic liquids were intruded to depths 0 1 km in the parent asteroid of rose city 2 early chondritic liquids experiencing 5 10 olivine fractionation were erupted onto the surface of the parent asteroid s of yanzhuang and chantonnay and 3 near surface crystallization is also indicated for the liquids in tuan tue and farmington with chico and malakal crystallizing at slightly greater depths in all but chantonnay liquids appear to have derived from melting of chondrite types corresponding to their hosts in the l chondrite chantonnay the intrusive liquids derive from melting an h chondrite source region in which chondritic melts were stored for a sufficient time to produce pigeonite in zoned pyroxene xenocrysts heating effects are also reflected in the trace element contents of the chondrites our rnaa data for rose city seem to reflect only the siderophile lithophile fractionation evident in the metal distribution the yanzhuang rnaa data are generally similar to those of other h chondrites severe shock not involving phase transport seems to leave h chondrites unaffected compositionally contents of the four most mobile elements cd bi t1 in and cs in chico indicate loss so that this assemblage experienced extended low temperature cooling after injection of dike material into the chico host rnaa data for the other four l chondrites examined indicate more rapid cooling

asteroidal source of l chondrite meteorites abstract establishing connections between meteorites and their parent asteroids is an important goal of planetary science several links have been proposed in the past including a spectroscopic match between basaltic meteorites and 4 vesta that are helping scientists understand the formation and evolution of the solar system bodies here we show that the shocked l chondrite meteorites which represent about two thirds of all l chondrite falls may be fragments of a disrupted asteroid with orbital semimajor axis a 2 8 au this breakup left behind thousands of identified 1 15 km asteroid fragments known as the gefion family fossil l chondrite meteorites and iridium enrichment found in an 467 ma old marine limestone quarry in southern sweden and perhaps also 5 large terrestrial craters with corresponding radiometric ages may be tracing the immediate aftermath of the family forming collision when numerous gefion fragments evolved into the earth crossing orbits by the 5 2 resonance with jupiter this work has major implications for our understanding of the source regions of ordinary chondrite meteorites because it implies that they can sample more distant asteroid material than was previously thought possible

a critical evaluation of oxidation versus reduction during metamorphism of l and ll group chondrites and implications for asteroid spectroscopy modal mineralogies of individual equilibrated petrologic type 4 6 l and ll chondrites have been measured using an electron microprobe mapping technique and the chemical compositions of coexisting silicate minerals have been analyzed progressive changes in the relative abundances and in the molar fe mn and fe mg ratios of olivine low ca pyroxene and diopside occur with increasing metamorphic grade variations in olivine low ca pyroxene ratios ol px and in metal abundances and compositions with petrologic type support the hypothesis that oxidation of metallic iron accompanied thermal metamorphism in ordinary chondrites modal ol px ratios are systematically lower than normative ol px ratios for the same meteorites suggesting that the commonly used c i p w norm calculation procedure may not adequately estimate silicate mineral abundances in reduced chondrites ol px ratios calculated from visnir reflectance spectra of the same meteorites are not in agreement with other ol px determinations possibly because of spectral complexities arising from other minerals in chondrites characteristic features in visnir spectra are sensitive to the proportions and compositions of olivine and pyroxenes the minerals most affected by oxidative metamorphism this work may allow spectral calibration for the determination of mineralogy and petrologic type and thus may be useful for spectroscopic studies of asteroids

core formation in asteroids chemical arguments based on the s concentrations in the metallic magmas that formed magmatic iron meteorites and on the lack of pyroxene in pallasites suggest that core formation in asteroids was accompanied by 40 melting of the associated silicates the existence of plagioclase free residues from partial melting such as lodranite meteorites indicates that metal did not segregate from silicates when the amount of partial melting was only 15 thus core formation in asteroids requires substantial melting of an asteroid high interfacial energies between metallic melts and silicates require that metallic spherules sink through partially molten silicates to form cores in asteroids and planetesimals the globule sinking mechanism is not very efficient because silicate crystal liquid mixtures have high yield strengths making it difficult for the globules to sink calculations suggest that the melt fraction must reach 50 before globules can sink alternatively porous flow of molten metal is possible when the amount of metal is high enough to form an interconnected network about 50 this can be achieved by high initial abundances of metal or by gradual removal of silicate melt thereby enriching the residue in the metal sulfide assemblage this also requires removal of 50 silicate partial melt though not necessarily all at once such high amounts of melting were not always attained in asteroids so many asteroids probably consist of partially differentiated silicates and metallic masses that did not segregate to a core s asteroids might represent such partially differentiated bodies

oxygen isotope studies of ordinary chondrites several stages in the evolution of ordinary chondritic meteorites are recorded in the oxygen isotopic composition of the meteorites and their separable components chondrules fragments clasts and matrix the whole rock isotopic compositions reflect the iron group of the meteorite h l or ll isotopic uniformity of h3 to h6 and l3 to l6 are consistent with closed system metamorphism within each parent body ll3 chondrites differ slightly from ll4 to ll6 implying a small degree of opensystem aqueous alteration and carbon reduction on the scale of individual chondrules the meteorites are isotopically heterogeneous allowing recognition of the solar nebular processes of chondrule formation chondrules for all classes of ordinary chondrites are derived from a common population which was separate from the population of chondrules in carbonaceous or enstatite chondrites chondrules define an isotopic mixing line dominated by exchange between 16o rich and 16o poor reservoirs the oxygen isotopic compositions of chondrites serve as fingerprints for identification of genetic association with other meteorite types achondrites and iron and for recognition of source materials in meteoritic breccias

igneous inclusions from ordinary chondrites high temperature cumulates and a shock melt we report microprobe instrumental neutron activation analysis and radiochemical neutron activation analysis data for three large igneous inclusions in the yamato y 75097 y 793241 and y 794046 ordinary chondrites the inclusions in the first two chondrites are troctolitic cumulates that have undergone appreciable reactions with their hosts either during emplacement and or cooling olivine spinel fe mg exchange pairs in these two inclusions record equilibration temperatures of about 710 c and these temperatures are similar to those exhibited by mineral pairs in the y 75097 and y 793241 hosts the inclusion in y 794046 is texturally unique consisting of fine grained randomly distributed olivines coarse 2 mm fascicular pyroxene laths and angular pockets of maskelynite plagioclase feldspar the phase compositions are readily interpreted as having resulted from extremely rapid essentially isochemical cooling to temperatures 1670 c we suggest that this igneous inclusion formed in situ by shock

a petrologic and isotopic study of lodranites evidence for early formation as partial melt residues from heterogeneous precursors abstract we have conducted petrologic chemical and isotopic studies of lodranites in an attempt to constrain their genesis lodran gibson y 791491 y 791493 y 74357 y 8002 y 75274 mac 88177 lew 88280 eet 84302 fro 90011 and que 93148 are classified as lodranites lodranites and acapulcoites are indistinguishable on the basis of oxygen isotopic compositions but are distinct in average grain sizes of their mafic silicates with lodranites being significantly coarser grained lodranites exhibit a diverse range of petrologic and mineralogic features they range widely in mafic silicate compositions fa3 13 plagioclase 0 11 4 vol fe ni metal 0 5 20 vol and troilite 0 2 5 3 vol contents and shock levels s1 s4 they appear to have experienced high peak temperatures and rapid cooling in the temperature range recorded by metallographic cooling rates i e 700 350 c the only dated lodranite gibson cooled to ar closure temperatures at 4 49 0 01 ga lodranites formed from chemically and isotopically heterogeneous precursors in which the mineral and oxygen isotopic compositions were correlated heating of their parent body to temperatures between 1050 1200 c resulted in formation of fe ni fes and basaltic partial melts depletions of troilite and or plagioclase in most lodranites testify to the removal of some of these partial melts although melt migration was complex lodranites appear to have experienced a complex cooling history of slow cooling at high temperatures followed by rapid cooling at intermediate temperatures possibly related to breakup of the parent body lodranites were liberated from their parent body during 1 3 impact events with most having cosmic ray exposure ages of 5 5 7 ma the acapulcoites are samples from the same parent body but were heated to lower temperatures and thus experienced lower degrees of partial melting

the porosity and permeability of chondritic meteorites and interplanetary dust particles abstract we have investigated the porosity of a large number of chondritic interplanetary dust particles idps and meteorites by three techniques standard liquid gas flow techniques a new noninvasive ultrasonic technique and image processing of backscattered images the latter technique is obviously best suited to sub kilogram sized samples we have also measured the gas and liquid permeabilities of some chondrites by two techniques standard liquid gas flow techniques and a new nondestructive pressure release technique we find that chondritic idps have a somewhat bimodal porosity distribution peaks are present at 0 and 4 porosity a tail then extends to 53 type 1 3 chondrite matrix porosities range up to 30 with a peak at 2 the bulk porosities for type 1 3 chondrites have the same approximate range as exhibited by the matrix which indicates that other components of the bulk meteorites including chondrules and aggregates have the same average porosity as the matrix these results reveal that the porosities of primitive materials at scales ranging from nanogram to kilogram are similar which implies that similar accretion dynamics operated through 12 orders of size magnitude permeabilities of the investigated chondrites vary by several orders of magnitude and there appears to be no simple dependence of permeability with degree of aqueous alteration chondrite type or porosity

the distribution of basaltic asteroids in the main belt abstract we present the observational results of a survey designed to target and detect asteroids whose photometric colors are similar to those of vesta family members and thus may be considered as candidates for having a basaltic composition fifty basaltic candidates were selected with orbital elements that lie outside of the vesta dynamical family optical and near infrared spectra were used to assign a taxonomic type to 11 of the 50 candidates ten of these were spectroscopically confirmed as v type asteroids suggesting that most of the candidates are basaltic and can be used to constrain the distribution of basaltic material in the main belt using our catalog of v type candidates and the success rate of the survey we calculate unbiased size frequency and semi major axis distributions of v type asteroids these distributions in addition to an estimate for the total mass of basaltic material suggest that vesta was the predominant contributor to the basaltic asteroid inventory of the main belt however scattered planetesimals from the inner solar system a 2 0 au and other partially fully differentiated bodies likely contributed to this inventory in particular we infer the presence of basaltic fragments in the vicinity of asteroid 15 eunomia which may be derived from a differentiated parent body in the middle main belt 2 5 a 2 8 we find no asteroidal evidence for a large number of previously undiscovered basaltic asteroids which agrees with previous theories suggesting that basaltic fragments from the 100 differentiated parent bodies represented in meteorite collections have been battered to bits burbine t h meibom a binzel r p 1996 meteorit planet sci 31 607 620

oxygen isotope and petrological study of silicate inclusions in iie iron meteorites and their relationship with h chondrites abstract the origin of silicate bearing irons especially those in groups iab iiicd and iie is poorly understood as silicate should have separated rapidly from molten metal here we report the results of high precision oxygen isotope analysis of silicate inclusions in eleven group iie meteorites and a petrological study of silicate inclusions in ten iie irons including those in garhi yasin and tarahumara which have not been described in detail before oxygen isotopes have also been analysed in 20 h chondrites to investigate their possible relationship with the iie irons based on petrographic observations and mineral analysis the silicate bearing iie meteorites have been divided into four types according to the nature of their silicate inclusions 1 primitive chondritic 2 evolved chondritic 3 differentiated with 10 vol orthopyroxene and 4 differentiated with 17 o variation a wide range of values is seen in the dataset as a whole group iie irons with differentiated silicates with the exception of colomera have a range of mean 17 o values that is essentially identical to those of the h4 6 chondrites 0 60 0 77 and 0 61 0 76 respectively colomera inclusions which are differentiated with 17 o value and plot 2 away from the next nearest iie iron however in view of the textural similarities to other iie inclusions a separate source for colomera is deemed unlikely three iie irons with primitive chondritic inclusions garhi yasin netschaevo and techado have relatively low mean 17 o values of 0 56 0 57 as well as relatively reduced silicates with fa 15 17 olivine which have been called hh chondrites given the significant overlap in their oxygen isotope compositions a genetic relationship between iie irons and h chondrites is supported by our new data however derivation of both groups from one parent body seems unlikely instead both groups probably sampled similar precursor materials and accreted at a similar nebular location our data suggest that the iie meteorites formed on an internally heated h hh chondrite like body that experienced the initial stages of differentiation in response to radiogenic heating however prior to full differentiation the iie parent body experienced a major hit and run style collision that resulted in silicate metal mixing the initial stages of this event involved a phase of rapid cooling that prevented unmixing of metal and silicates reassembly of the iie parent body produced a large regolith blanket that facilitated subsequent slow cooling the iie parent body has probably experienced numerous subsequent less catastrophic collisions the development of alkali glass textures in some differentiated inclusions is probably the result of one of these later events

near s flyby of 253 mathilde images of a c asteroid on 27 june 1997 the near earth asteroid rendezvous near spacecraft flew within 1212 kilometers of asteroid 253 mathilde mathilde is an irregular heavily cratered body measuring 66 kilometers by 48 kilometers by 46 kilometers the asteroid s surface is dark estimated albedo between 0 035 and 0 050 and similar in color to some cm carbonaceous chondrites no albedo or color variations were detected the volume derived from the images and the mass from doppler tracking of the spacecraft yield a mean density of 1 3 0 2 grams per cubic centimeter about half that of cm chondrites indicating a porous interior structure

evidence for an asteroid comet continuum from simulations of carbonaceous microxenolith dynamical evolution abstract micrometeoroids with 100 and 200 m size dominate the zodiacal cloud dust such samples can be studied as micrometeorites after their passage through the earth atmosphere or as microxenoliths i e submillimetric meteorite inclusions microxenoliths are samples of the zodiacal cloud dust present in the asteroid main belt hundreds of millions years ago carbonaceous microxenoliths represent the majority of observed microxenoliths they have been studied in detail in howardites and h chondrites we investigate the role of carbonaceous asteroids and jupiter family comets as carbonaceous microxenolith parent bodies the probability of low velocity collisions of asteroidal and cometary micrometeoroids with selected asteroids is computed starting from the micrometeoroid steady state orbital distributions obtained by dynamical simulations we selected possible parent bodies of howardites vesta and h chondrites hebe flora eunomia koronis maria as target asteroids estimates of the asteroidal and cometary micrometeoroid mass between 2 and 4 au from the sun are used to compute the micrometeoroid mass influx on each target the results show that all the target asteroids except koronis receive the same amount within the uncertainties of asteroidal and cometary micrometeoroids therefore both these populations should be observed among howardite and h chondrite carbonaceous microxenoliths however this is not the case carbonaceous microxenoliths show differences similar to those existing among different groups of carbonaceous chondrites e g ci cm cr but two sharply distinct populations are not observed our results and the observations can be reconciled assuming the existence of a continuum of mineralogical and chemical properties between carbonaceous asteroids and comets

partial melting and melt migration in the acapulcoite lodranite parent body we review petrologic evidence that the acapulcoites and lodranites formed by 1 vol to probably 20 vol whole rock partial melting of a chondritic precursor material at low degrees of partial melting only fe ni fes cotectic melting occurred migration distances for partial melts were short resulting in the formation of acapulcoites with essentially chondritic troilite and plagioclase contents but achondritic textures at high degrees of partial melting both fe ni fes and basaltic plagioclase pyroxene partial melts formed and the melts may have migrated out of the source rock the partial melt residues which are more or less depleted in fe ni fes and plagioclase are the lodranites melt migration was complex most acapulcoites which experienced relatively low degrees of partial melting lost little if any of the partial melt one acapulcoite lew 86220 represents a unique case in which fe ni fes and basaltic partial melts appear to have migrated from a lodranite source region into a cooler acapulcoite region where they were trapped in cases of the relatively high degrees of partial melting experienced by lodranites melts may have been partly selectively or totally removed from the rocks and fe ni fes and or basaltic partial melts may have been removed to different degrees and may in fact have been trapped on occasion in greater than chondritic proportions we model vein and dike formation and melt migration by calculating the excess pressures and vein and dike sizes for varying degrees of partial melting our calculations are broadly consistent with observations indicating that melt migration is inefficient at low degrees of partial melting and extremely efficient at high degrees of partial melting although the size of the acapulcoite lodranite parent body and the volatile contents of the chondritic precursor rocks are poorly constrained the lack of basaltic rocks in the world s meteorite collections complementary to the lodranites suggests that basaltic partial melts may have been accelerated off the body by explosive volcanism of the type envisioned by wilson and keil 1991 and ejected into space the diversity of rocks from the acapulcoite lodranite parent body may provide a basis for better understanding the diverse range of spectral subtypes recognized among the s type asteroids

mega chondrules and large igneous textured clasts in julesberg l3 and other ordinary chondrites vapor fractionation shock melting and chondrule formation abstract a petrographic microprobe study of large metal poor igneous textured objects in julesberg l3 and other ordinary chondrites suggests that they can be classified into two petrographic types mega chondrules and large lithic clasts and into two chemical types na poor and na rich mega chondrules show textural evidence of having solidified as freely floating melt droplets whereas lithic clasts formed by the fragmentation of larger objects possibly still larger mega chondrules barred olivine or barred olivine pyroxene textures are most common for mega chondrules whereas a variety of textures occur in large lithic clasts the two petrographic types cannot be distinguished on the basis of modal bulk or phase compositions sodium poor objects are characterized by 1 plagioclase or glass with mainly bytownite composition an70 90 typical 2 subchondritic na al ratios and typically subchondritic volatile element na k and mn abundances and 3 bulk chemical trends that resemble those expected for vapor fractionation processes some na poor objects may have formed by the melting of precursors that formed as condensates or vaporization residues others may have formed by the melting of precursors that formed by fractional condensation or fractional vaporization following vapor fractionation na poor objects or their precursors appear to have reequilibrated at lower temperatures which raised the bulk na content of the objects but not to the levels seen in na rich objects enrichments of fe2 na k and p on the margins of some na poor objects suggest that they partly reacted with volatile rich surroundings both before and after brecciation sodium rich objects are characterized by 1 plagioclase or glass with oligoclase or albite composition an2 25 2 roughly chondritic na al ratios and volatile element na k mn abundances and 3 bulk chemical trends similar to those shown by melt pocket glasses in ordinary chondrites sodium rich melt objects could have formed by the shock melting of chondritic precursors feldspathic compositions for some na rich melt objects can be explained by preferential shock melting of feldspar or feldspathic glass in chondritic target materials literature data imply that the same two chemical populations of objects na poor and na rich occur among smaller normal sized chondrules in type 3 ordinary chondrites suggesting that the same processes that affected large melt objects also affected chondrules

potentially hazardous asteroid 2007 le compositional link to the black chondrite rose city and asteroid 6 hebe abstract the research is an integrated effort beginning with telescopic observations and extending through detailed mineralogical characterizations to provide constraints on the albedo diameter composition and meteorite affinity of near earth object potentially hazardous asteroid neo pha 2007 le results of the analysis indicate a diameter of 0 56 kilometers km and an albedo of 0 08 2007 le exhibits a 1 m absorption feature without a discernible band ii feature compositional analysis of 2007 le reveal fs 17 and fa 19 values which are consistent with the fa and fs values for the h type ordinary chondrites fs 14 5 18 and fa 16 20 and of asteroid 6 hebe fs 17 and fa 15 spectroscopically 2007 le does not appear like the average h chondrite spectra exhibiting a reddened spectrum and subdued absorption feature further investigation of the meteorite classes yielded a black chondrite rose city which is both similar in mineralogy and spectrally to pha 2007 le dynamical analysis could not directly link the fall of the rose city meteorite to 2007 le as it stands 2007 le and rose city have a compositional link and both could come from the same parent body possible family one known source of the h chondrites is 6 hebe

characterization of carbonaceous matter in xenolithic clasts from the sharps h3 4 meteorite constraints on the origin and thermal processing abstract primitive xenolithic clasts often referred to as dark clasts are well known in many regolith breccias the sharps h3 4 ordinary chondrite contains unusually large dark clasts up to 1 cm across poorly graphitized carbon pgc with fe ni metal and described as carbon rich aggregates has been reported in these clasts brearley 1990 we report detailed analyses of carbonaceous matter in several identical sharps clasts using ftir raman c xanes and tem that provide insight on the extent of thermal processing and possible origin of such clasts we also prepared acid residues of the clasts using the hcl hf method and conducted mass spectrometric analysis of the entrained noble gases carbonaceous matter is often used to infer thermal history due to its sensitivity to thermal processes the ftir spectra of the acid residue from the sharps clast suggest that carbonaceous matter in the clast contains less hydrogen and oxygen compared to acid residues from typical type 3 4 ordinary chondrites the metamorphic temperatures obtained by raman spectroscopy ranges between 380 c and 490 c tem observations indicate that the clasts experienced a peak temperature of 300 c to 400 c based on the carbon d002 layer lattice spacing of c rich aggregates these estimates are consistent with an earlier estimate of 330 50 c that is also estimated by the d002 layer lattice spacing brearley 1990 it should be noted that the lattice spacing thermometer is based on terrestrial metamorphose rocks and thus temperature was probably underestimated meanwhile the c xanes spectra of the c rich aggregates show high exciton intensities indicative of graphene structures that developed at around 700 800 c following an extensive period of time millions of years however the surrounding matrix areas experienced lower temperatures of less than 300 500 c noble gas analysis of the acid residue from the sharps clasts shows that the residue is almost identical with some material reported in carbonaceous chondrites i e heavily enriched in the q gas component as well as hl gas from presolar diamonds and ne e h from presolar sic these results indicate that the c rich aggregates in the sharps clasts formed under relatively high temperature conditions up to 800 c and were subsequently mixed with lower temperature matrix probably in a different parent body before they were incorporated into the final sharps lithology by collision

chronology and sources of lunar impact bombardment abstract the moon has suffered intense impact bombardment ending at 3 9 gyr ago and this bombardment probably affected all of the inner solar system basin magnetization signatures and lunar crater size distributions indicate that the last episode of bombardment at about 3 85 gyr ago was less extensive than previously thought we explore the contribution of the primordial mars crosser population to early lunar bombardment we find that mars crosser population initially decays with a 80 myr half life with the long tail of survivors clustering on temporarily non mars crossing orbits between 1 8 and 2 au these survivors decay with half life of about 600 myr and are progenitors of the extant hungaria asteroid group in the same region we estimate the primordial mars crosser population contained about 0 01 0 02 earth masses such initial population is consistent with no lunar basins forming after 3 8 gya and the amount of mass in the hungaria group as they survive longer and in greater numbers than other primordial populations mars crossers are the best candidate for forming the majority of lunar craters and basins including most of the nectarian system however this remnant population cannot produce imbrium and orientale basins which formed too late and are too large to be part of a smooth bombardment we propose that the imbrian basins and craters formed in a discrete event consistent with the basin magnetization signatures and crater size distributions this late impactor shower would be triggered by a collisional disruption of a vesta sized body from this primordial mars crossing population wetherill g w 1975 proc lunar sci conf 6 1539 1561 that was still comparable to the present day asteroid belt a 3 9 gya this tidal disruption lead to a short lived spike in bombardment by non chondritic impactors with a non asteroidal size frequency distribution in agreement with available evidence this body wetherill s object also uniquely matches the constraints for the parent body of mesosiderite meteorites we propose that the present day sources of mesosiderites are multi km sized asteroids residing in the hungaria group that have been implanted there soon after the original disruption of their parent 3 9 gyr ago

surface composition and taxonomic classification of a group of near earth and mars crossing asteroids abstract in the past constraining the surface composition of near earth asteroids neas has been difficult due to the lack of high quality near ir spectral data 0 7 2 5 m that contain mineralogically diagnostic absorption bands here we present visible 0 43 0 95 m and near infrared 0 7 2 5 m spectra of nine neas and five mars crossing asteroids mcs the studied neas are 4055 magellan 19764 2000 nf5 89830 2002 ce 138404 2000 ha24 143381 2003 bc21 159609 2002 aq3 164121 2003 yt1 241662 2000 ko44 and 2007 ml13 the studied mcs are 1656 suomi 2577 litva 5407 1992 ax 22449 ottijeff and 47035 1998 ws the observations were conducted with the ntt at la silla chile the 2 2 m telescope at calar alto spain and the irtf on mauna kea hawai i the taxonomic classification bus system of asteroids showed that all observed mc asteroids belong to the s complex including the s sr and sl classes seven of the neas belong to the s complex including the s sa sk and sl classes and two neas were classified as v types the classification of the nea 164121 2003 yt1 as a v type was made on the basis of its near infrared spectrum since no visible spectrum is available for this asteroid a mineralogical analysis was performed on six of the asteroids those for which near ir spectra were obtained or previously available we found that three asteroids 241662 2000 ko44 19764 2000 nf5 138404 2000 ha24 have mafic silicate compositions consistent with ordinary chondrites while three others 4055 magellan 164121 2003 yt1 5407 1992 ax are pyroxene dominated basaltic achondrite assemblages in the case of 5407 1992 ax we found that its basaltic surface composition contrasts its taxonomic classification as a s type

importance of space weathering simulation products in compositional modeling of asteroids 349 dembowska and 446 aeternitas as examples based on recent progress in simulating space weathering on asteroids using pulse laser irradiation onto olivine and orthopyroxene samples detailed analyses of two of the a and r type asteroid reflectance spectra have been performed using reflectance spectra of laser treated samples the visible near infrared spectrum of olivine is more altered than that of pyroxene at the same pulse laser energy suggesting that olivine weathers more rapidly than orthopyroxene in space the same trend can be detected from reflectance spectra of the asteroids where the more olivine an asteroid has the redder its 1 m band continuum can become comparison of the 1 m band continuum slope and the 2 1 m band area ratio between the asteroids and olivine and pyroxene samples including the laser treated ones suggests that asteroids may be limited in the degree of space weathering they can exhibit possibly due to the short life of their surface regolith their pyroxenes may also have a limited chemical composition range fitting the visible continuum shape and other parts of the spectra especially the 2 m part has been impossible with any combination of common rock forming minerals such as silicates and metallic irons however this study shows for the first time excellent fits of reflectance spectra of an a asteroid aeternitas and an r asteroid dembowska including their visible spectral curves band depths and shapes and overall continuum shapes our results provide estimates that aeternitas consists of 2 fresh olivine 93 space weathered olivine 1 space weathered orthopyroxene and 4 chromite and that dembowska consists of 1 fresh olivine 55 space weathered olivine and 44 space weathered orthopyroxene these results suggest that space weathering effects maybe important to the interpretation of asteroid reflectance spectra even those with deep silicate absorption bands modified gaussian model deconvolutions of the laser irradiated olivine samples show that their identity as olivine remained the most recent submicroscopic mineralogical analyses have revealed that the laser irradiated olivine samples contain nanophase iron particles similar to those in space weathered lunar samples

nea 162385 2000 bm19 near infrared compositional analysis of a mining target abstract the research is an integrated effort beginning with telescopic observations and extending through detailed mineralogical characterizations to provide constraints on the albedo diameter composition and meteorite affinity for the near earth asteroid 162385 2000 bm19 results of the analysis indicate a diameter ranging from 0 46 to 0 57 km km using albedo values of 0 20 0 25 and 0 30 2000 bm19 exhibits 1 and 2 m absorption features from which a compositional analysis was conducted revealing fs23 and fa28 values which are consistent with the fa and fs values for the ll chondrites

sylvite and halite on particles recovered from 25143 itokawa a preliminary report we observed cross sectional ultra thin sections near the surface of 12 particles recovered from the s type asteroid itokawa by the hayabusa spacecraft in 2010 using spherical aberration corrected stem and conventional tem although their mineralogy is almost identical to the equilibrated ll chondrites and therefore basically anhydrous micrometer to submicrometer sized sylvite was identified on the surface of itokawa particle ra qd02 0034 separately micrometer sized halite was also identified on the surface of itokawa particle ra qd02 0129 detailed inspection of the sample processing procedures at the jaxa s planetary materials sample curation facility and textural observation of the sylvite and halite indicate that they were clearly present on two itokawa particles before they were removed from clean chamber 2 at jaxa however there is no direct evidence for their extraterrestrial origin at present if the sylvite and halite are extraterrestrial their presence suggests that they may be more abundant on the surface of s type asteroids than previously thought

mainbelt asteroids dual polarization radar observations observations of 20 asteroids in the main belt between mars and jupiter provide information about the nature of these objects surfaces at centimeter to kilometer scales at least one asteroid pallas is extremely smooth at centimeter to meter scales each asteroid appears much rougher than the moon at some scale between several meters and many kilometers the range of asteroid radar albedos is very broad and implies substantial variations in porosity or metal concentration or both the highest albedo estimate for the asteroid psyche is consistent with a surface having porosities typical of lunar soil and a composition nearly entirely metallic

bulk density of asteroid 243 ida from the orbit of its satellite dactyl during its reconnaissance of the asteroid 243 ida the galileo spacecraft returned images of a second object 1993 243 1 dactyl1 the first confirmed satellite of an asteroid sufficient data were obtained on the motion of dactyl to determine its orbit as a function of ida s mass here we apply statistical and dynamical arguments to constrain the range of possible orbits and hence the mass of ida combined with the volume of ida2 this yields a bulk density of 2 6 0 5 g cm 3 allowing for the uncertainty in the porosity of ida this density range is consistent with a bulk chon dritic composition and argues against some but not all classes of meteoritic igneous rock types that have been suggested as compositionally representative of s type asteroids like ida

anatomy of a partially differentiated asteroid a near sighted view of acapulcoites and lodranites with the insertion of the near spacecraft into orbit around the asteroid 433 eros a new stage of asteroid exploration has begun with the return of spatially resolved mineralogical geological and chemical information from specific asteroids in this paper we outline the features of a partially differentiated asteroid using the acapulcoite and lodranite meteorites as a guide such a body would likely contain a variety of lithologies including chondritic rocks largely unaltered by melting residues of partial melts and basaltic and fe ni fes rocks crystallized from the partial melts the presence or nature of the regolith remains largely unconstrained mineralogical and isotopic heterogeneities preserved in these rocks suggest spatial separation of these rock types by distances in excess of 200 m spacecraft based remote sensing instrumentation should detect substantial spectral heterogeneity most prominently manifested in the band area ratio and chemical heterogeneity observation of these various lithologies in situ would provide important new constraints on the processes involved during the earliest differentiation of asteroids

on the origin of shocked and unshocked cm clasts in h chondrite regolith breccias cm chondrite clasts that have experienced different degrees of aqueous alteration occur in h chondrite and hed meteorite breccias many clasts are fragments of essentially unshocked cm projectiles that accreted at low relative velocities to the regoliths of these parent bodies a few clasts were heated and dehydrated upon impact these objects most likely accreted at higher relative velocities we examined three clasts and explored alternative scenarios for their formation in the first scenario we assumed that the h and hed parent bodies had diameters of a few hundred kilometers so that their high escape velocities would effectively prevent soft landings of small cm projectiles this would imply that weakly shocked cm clasts formed on asteroidal fragments family members associated with the h and hed parent bodies in the second scenario we assumed that weakly shocked cm clasts were spall products ejected at low velocities from larger cm projectiles when they slammed into the h and hed parent bodies in both cases if most cm clasts turn out to have ancient ages e g 3 4 4 1 ga a plausible source for their progenitors would be outer main belt objects some which may have been dynamically implanted 3 9 ga ago by the events described in the so called nice model on the other hand if most cm clasts have recent ages 200 ma a plausible source location for their parent body would be the inner main belt between 2 1 2 2 au in that case the possible source of the cm clasts progenitors parent fragments would be the breakup 160 ma ago of the parent body 170 km in diameter of the baptistina asteroid family baf

records of the moon forming impact and the 470 ma disruption of the l chondrite parent body in the asteroid belt from u pb apatite ages of novato l6 novato a newly observed fall in the san francisco bay area is a shocked and brecciated l6 ordinary chondrite containing dark and light lithologies we have investigated the u pb isotope systematics of coarse cl apatite grains of metamorphic origin in novato with a large geometry ion microprobe the u pb systematics of novato apatite reveals an upper intercept age of 4472 31 ma and lower intercept age of 473 38 ma the upper intercept age is within error identical to the u pb apatite age of 4452 21 ma measured in the chelyabinsk ll5 chondrite this age is interpreted to reflect a massive collisional resetting event due to a large impact associated with the peak arrival time at the primordial asteroid belt of ejecta debris from the moon forming giant impact on earth the lower intercept age is consistent with the most precisely dated ar ar ages of 470 6 ma of shocked l chondrites and the fossil meteorites and extraterrestrial chromite relicts found in ordovician limestones with an age of 467 3 1 6 ma in sweden and china the lower intercept age reflects a major disturbance related to the catastrophic disruption of the l chondrite parent body most likely associated with the gefion asteroid family which produced an initially intense meteorite bombardment of the earth in ordovician period and reset and degassed at least approximately 35 of the l chondrite falls today we predict that the 470 ma impact event is likely to be found on the moon and mars if not mercury

a nonmagmatic origin of group iie iron meteorites abstract new neutron activation data on 10 elements in 12 iie and iie related irons lead to a reclassification of several irons seymchan and lonaconing are removed from iie and leshan added four iie members are designated iie an to call attention to some anomalous properties the eight normal iie members define element ni trends generally similar to those in the nonmagmatic group iab the small negative slopes on w ni and ir ni diagrams are strongly indicative of a nonmagmatic origin of the iie irons we propose that iie irons like iab irons originated as individual pools of impact produced melt in the near surface region of a chondritic parent body the positive as ni and au ni trends are the only evidence suggesting fractional crystallization but their slopes are lower than those in magmatic group iiiab and only slightly higher than those of cu and sb in iab we suggest that the s and c contents of the iie precursor materials were much lower than those of the iab precursors thus higher temperatures were required to generate enough metallic melt to segregate into pools these higher temperatures are also reflected in the nonchondritic compositions of the silicate inclusions

relationship of e type apollo asteroid 3103 1982 bb to the enstatite achondrite meteorites and the hungaria asteroids visible and near infrared observations confirm that apollo asteroid 3103 1982 bb has a reflectance spectrum consistent with its previous assignment to taxonomic type e it has been generally believed that the e type asteroids are analogous and perhaps even genetically related to the enstatite achondrite meteorites aubrites correlated variations of previously published visible and thermal infrared lightcurves and the lack of rotational color variations in the present data indicate that this is an elongated object with no substantial surface albedo variations apollo asteroid 3103 1982 bb is presently in an orbital resonance 3 5 with earth and appears to be a relatively long lived member of the earthapproaching population its orbit is consistent with both the fall dates and long cosmic ray exposure ages of the aubrites combined with the rarity of e type asteroids in the near earth asteroid population this indicates that 3103 1982 bb is probably either a major or the primary immediate source body of the aubrites the aphelion location and the mineralogy of 3103 1982 bb indicate that it was most probably derived from the hungaria region at the innermost edge 1 9 au of the asteroid belt although the exact dynamical mechanism for producing its present orbit is not clear these results represent the first plausible direct link of a set of meteorites aubrites or enstatite achondrites back to a particular source region in the asteroid belt this link allows the composition of the solar nebula to be constrained at that heliocentric distance during the time interval when the parent assemblage of the aubrites was isolated from contact with the nebular gas by accretion into its parent planetesimal 1992 academic press inc

properties of the guin ungrouped iron meteorite the origin of guin and of group iie irons abstract guin is an ungrouped iron meteorite containing 45 g g ga and 186 g g ge it is a coarse octahedrite with an unusually high ni content 92 5 mg g its metal composition is generally similar to irons in groups iab and iiicd but its closest relative is the cruz del aire ungrouped iron guin contains 6 3 vol silicate inclusions that are petrologically and compositionally similar to those in iie irons the largest inclusion 2 4 cm consists primarily of a shock melted plagioclase rich matrix surrounding large 1 15 mm partly melted augite grains the oxygen isotopic composition of the inclusion 18 o 5 29 17 o 3 87 is near the ll chondrite field the guin inclusion is similar in bulk composition to selected melt pocket glasses in ordinary chondrites produced in situ by preferential melting of plagioclase by shock compression we suggest that the guin assemblage formed by impact melting on a chondritic parent body silicate inclusions in iie irons share many petrologic and compositional but not oxygen isotopic characteristics with the guin inclusions suggesting that iie irons also formed by impact melting of chondritic materials individual iie irons were probably derived from separate cratering events

high calcium pyroxene as an indicator of igneous differentiation in asteroids and meteorites our analyses of high quality spectra of several s type asteroids 17 thetis 847 agnia 808 merxia and members of the agnia and merxia families reveal that they include both low and highcalcium pyroxene with minor amounts of olivine 0 4 high calcium pyroxene is a spectrally detectable and petrologically important indicator of igneous history and may prove critical in future studies aimed at understanding the history of asteroidal bodies the silicate mineralogy inferred for thetis and the merxia and agnia family members requires that these asteroids experienced igneous differentiation producing broadly basaltic surface lithologies together with 4 vesta and its smaller vestoid family members and the main belt asteroid 1489 magnya these new asteroids provide strong evidence for igneous differentiation of at least five asteroid parent bodies based on this analysis of a small subset of the near infrared asteroid spectra taken to date with spex at the nasa irtf we expect that the number of known differentiated asteroids will increase consistent with the large number of parent bodies inferred from studies of iron meteorites

acapulco and lodran like achondrites petrology geochemistry chronology and origin we have performed petrologic and geochemical studies of five primitive achondrites alha81187 and alha81261 acapulco like eet 84302 transitional but acapulco like and lew 88280 and mac 88177 lodran like we have also performed 39ar 40ar chronology on alha81187 alha81261 and eet 84302 lew 88280 and mac 88177 contain more ferroan olivines orthopyroxenes clinopyroxenes and chromites than do the acapulco like achondrites plagioclase is present in only alha81187 alha81261 and eet 84302 its composition does not track that of the mafic silicates eet 84302 with the most calcic plagioclase contains olivine and pyroxene with an mg intermediate between alha81187 and alha81261 similarly spinels in eet 84302 have the same mg as those in alha81187 while pyroxenes in the former are more ferroan than in the latter acapulco like achondrites have sm sc ratios between 0 8 1 5 times h chondrites and na sc ratios are between 0 81 0 times h chondrites indicating that silicate partial melts have not been lost from these rocks siderophile and chalcophile elements are fractionated and show a trend of increasing ir ni ratios with decreasing se co ratios this variation is consistent with fractionation by partial melting in the fe ni s system eet 84302 is very depleted in se and troilite is a accessory phase indicating essentially total loss of the low temperature fe ni s melt in contrast the lodran like achondrites are depleted in the highly incompatible lithophile elements relative to more compatible elements sm sc ratios are 0 2 0 7 times and na sc ratios are 0 04 0 18 times h chondrites these data are consistent with a model for formation of the lodran like achondrites as partial melting residues however the low ir ni and high se co ratios in two lodran like achondrites suggest the metal troilite in these rocks is dominated by the low melting fraction in the fe ni s system this metal troilite was added to the silicates after partial melting depleted them in a basaltic fraction the ar ar release spectra yield good plateau ages of 4 507 0 024 ga alha81187 4 511 0 007 ga alha81261 and 4 519 0 017 ga eet 84302 in good agreement with previously determined ar ar ages for other acapulco like achondrites together the ar ar ages demonstrate that metamorphism on the acapulco lodran parent body occurred 4 51 0 02 ga ago roughly 50 ma after its formation the geochemical and petrologic evidence on the acapulco and lodran like achondrites suggest that heating on the parent body was localized and heterogeneous in space and time the parent body was probably not subjected to parent body wide magmatism in contrast to the case for 4 vesta and the hed meteorites

relict chondrules in primitive achondrites remnants from their precursor parent bodies abstract we studied the petrography analyzed the chemical compositions constrained the closure temperatures via geothermometry and determined the oxidation states of relict chondrules in campo del cielo iab iron meteorite graves nunataks gra 98028 acapulcoite and netschaevo iie iron meteorite to constrain their formation conditions and investigate links to known meteorite groups despite having been thermally metamorphosed mineral phases within relict chondrules retain information about their precursor compositions the sizes and textures of relict chondrules and silicate and chromite compositions indicate that campo del cielo gra 98028 and netschaevo had distinct parent bodies that were similar to but different from known chondrite groups to determine the utility of relict chondrule sizes in thermally metamorphosed meteorites we determined the chondrule size distributions in the ll chondrites semarkona ll3 00 soko banja ll4 siena ll5 and saint severin ll6 and the h chondrites clovis no 1 h3 6 kesen h4 arbol solo h5 and estacado h6 as expected mean chondrule diameters increase with degree of thermal metamorphism we find that campo del cielo and gra 98028 were reduced during thermal metamorphism consistent with previous studies indicating that their precursors were initially more feo rich than their current compositions in contrast to previous studies we find no evidence for reduction of silicates in netschaevo normal zoning of olivine in netschaevo is consistent with crystallization and suggests its silicates are near their primary feo contents the presence of elongated chromite grains along olivine grain boundaries in netschaevo indicates formation during thermal metamorphism under oxidizing conditions due to the absence of reduction and the composition of chromite being distinct from that of metamorphosed h chondrites we conclude that netschaevo and by extension the iie iron meteorites are not from the h chondrite parent body

impact cratering on the h chondrite parent asteroid 1 this paper reports petrological data for lapaz icefield 02240 03922 031125 031173 031308 04462 and 04751 which are meteoritic samples of clast rich impact melt rocks from the h chondrite parent asteroid the size distribution and metallographic characteristics of fe ni metal in the melts indicate very rapid 1 to 40 c s cooling in the temperature range between 1500 and 950 c when the clast melt mixtures were thermally equilibrating cooling slowed to values between 10 3 and 10 2 c s in the temperature range between 700 and 400 c when the melt rocks were cooling to their surroundings these data suggest that the rocks cooled near the surface of the h chondrite asteroid within suevitic impact deposits integrating these data with the petrologic characteristics of other h chondrite melt rocks and their radioisotopic ages indicates that the h chondrite asteroid suffered at least one large impact event while still cooling from endogenous metamorphism at 4500 ma this impact must have degraded the asteroid s integrity but did not cause shattering impact events in the era between 4100 and 3600 ma produced melt volumes large enough to allow segregation of metal and troilite from silicate melts possibly within continuous impact melt sheets contained in craters the impact record after 3600 ma does not display such assemblages which suggests a decrease in the rate of large impact events or a catastrophic size reduction of the h chondrite parent asteroid at around this time

vertical propagation of information in a middle atmosphere data assimilation system by gravity wave drag feedbacks 1 the mesospheric response to the 2002 antarctic stratospheric sudden warming ssw is analysed using the canadian middle atmosphere model data assimilation system cmam das where it represents a vertical propagation of information from the observations into the data free mesosphere the cmam das simulates a cooling in the lowest part of the mesosphere which is accomplished by resolved motions but which is extended to the mid to upper mesosphere by the response of the model s non orographic gravity wave drag parameterization to the change in zonal winds the basic mechanism is that elucidated by holton consisting of a net eastward wave drag anomaly in the mesosphere during the ssw although in this case there is a net upwelling in the polar mesosphere since the zonal mean mesospheric response is shown to be predictable this demonstrates that variations in the mesospheric state can be slaved to the lower atmosphere through gravity wave drag

detection and rapid recovery of the sutter s mill meteorite fall as a model for future recoveries worldwide the sutter s mill c type meteorite fall occurred on 22 april 2012 in and around the town of coloma california the exact location of the meteorite fall was determined within hours of the event using a combination of eyewitness reports weather radar imagery and seismometry data recovery of the first meteorites occurred within 2 days and continued for months afterward the recovery effort included local citizens scientists and meteorite hunters and featured coordination efforts by local scientific institutions scientific analysis of the collected meteorites revealed characteristics that were available for study only because the rapid collection of samples had minimized terrestrial contamination alteration this combination of factors rapid and accurate location of the event participation in the meteorite search by the public and coordinated scientific investigation of recovered samples is a model that was widely beneficial and should be emulated in future meteorite falls the tools necessary to recreate the sutter s mill recovery are available but are currently underutilized in much of the world weather radar networks scientific institutions with interest in meteoritics and the interested public are available globally therefore it is possible to repeat the sutter s mill recovery model for future meteorite falls around the world each for relatively little cost with a dedicated researcher doing so will significantly increase the number of fresh meteorite falls available for study provide meteorite material that can serve as the nuclei of new meteorite collections and will improve the public visibility of meteoritics research

evolutionary history of the mesosiderite asteroid a chronologic and petrologic synthesis abstract recent petrologic chemical and isotopic analyses of mesosiderite clasts and whole rocks allow reconstruction of the evolutionary history of the mesosiderite parent body mpb chronologic measurements indicate that initial melting of the mpb occurred shortly after accretion 4 56 ga ago at about the same time that the eucrites and angrites formed significant crustal remelting 4 47 ga ago probably associated with metal silicate mixing is responsible for forming abundant gabbroic clasts with extreme positive eu anomalies localized impact melting 4 5 3 9 ga ago produced quench textured clasts and matrices and may be responsible for causing mesosiderite metamorphism as well as the extensive melting of type 4 mesosiderites collisional disruption and gravitational reassembly 3 9 ga ago is indicated by thorough ar degassing at this time metallographic cooling rates of 1 c ma were likely initiated by this event the history of higher temperature events measured by thermoluminescence was annealed out impact excavation and ejection of deeply buried breccias occurred 3 9 ga ago the 10 160 ma cosmic ray exposure ages of mesosiderites represent the epoch when individual mesosiderites were excavated from the mpb or from a 10 m size fragment it is unlikely that mesosiderites and hed howardite eucrite diogenite samples come from the same parent body there is no evidence for metal silicate mixing with attendant redox effects or major crustal remelting on the hed body bulk howardites have flat chondrite normalized rare earth element ree patterns whereas many bulk mesosiderites have ree patterns with positive eu anomalies and relative enrichments in light ree the nature of the heating mechanism which caused the initial differentiation of mesosiderites and howardites is problematic

constraints on the role of impact heating and melting in asteroids received 1996 august 30 accepted in revised form 1997 january 30 abstract imaging of asteroids gaspra and ida and laboratory studies of asteroidal meteorites show that im pacts undoubtedly played an important role in the histories of asteroids and resulted in shock metamorphism and the formation of breccias and melt rocks however in recent years impact has also been called upon by numerous authors as the heat source for some of the major geological processes that took place on asteroids such as global thermal metamorphism of chondrite parent bodies and a variety of melting and igneous events the latter were proposed to explain the origin of ureilites aubrites mesosiderites the eagle station pallasites acapulcoites lodranites and the iab iiicd and iie irons we considered fundamental observations from terrestrial impact craters combined with results from laboratory shock experiments and theoretical consider ations to evaluate the efficiency of impact heating and melting of asteroids studies of terrestrial impact craters and relevant shock experiments suggest that impact heating of aster oids will produce two types of impact melts 1 large scale whole rock melts total melts not partial melts at high shock pressure and 2 localized melts formed at the scale of the mineral constituents mineral specific or grain boundary melting at intermediate shock pressures the localized melts form minuscule amounts of melt that quench and solidify in sifu thus preventing them from pooling into larger melt bodies partial melting as defined in petrology has not been observed in natural and experimental shock metamor phism and is thermodynamically impossible in a shock wave induced transient compression of rocks the total impact melts produced represent a minuscule portion of the displaced rock volume of the parent crater internal differentiation by fractional crystallization is absent in impact melt sheets of craters of sizes that can be tolerated by asteroids and impact melt rocks are usually clast laden thermal metamorphism of country rocks by impact is extremely minor experimental and theoretical considerations suggest that 1 single dis ruptive impacts cannot raise the average global temperature of strength or gravity dominated asteroids by more than a few degrees 2 cumulative global heating of asteroids by multiple impacts is ineffective for asteroids less than a few hundred kilometers in diameter 3 small crater size low gravity and low impact velocity suggest that impact melt volume in single asteroidal impacts is a very small 0 014 1 fraction of the total displaced crater volume 4 total impact melt volume formed during the typical lifetime of an asteroid is a small fraction 0 001 of the volume of impact generated debris and 5 much of the impact melt generated on asteroidal targets is ejected from craters with velocities greater than escape velocity and thus not retained on the asteroid the inescapable conclusion from these observations and calculations is that impacts cannot have been the heat source for the origin of the meteorite types listed above and we must turn to processes other than impact such as decay of short lived radionuclides or electromagnetic induction during an early t tauri phase of the sun to explain heating and melting of the parent bodies of these meteorites

size scales over which ordinary chondrites and their parent asteroids are homogeneous in oxidation state and oxygen isotopic composition abstract literature data demonstrate that on a global asteroid wide scale plausibly on the order of 100 km ordinary chondrites oc have heterogeneous oxidation states and o isotopic compositions represented respectively by the mean olivine fa and bulk 17 o compositions of equilibrated samples samples analyzed here include a two h5 chondrite antarctic finds alha79046 and til 82415 that have the same cosmic ray exposure age 7 6 ma and were probably within 1 km of each other when they were excavated from the h chondrite parent body b different individual stones from the holbrook l ll6 fall that were probably within 1 m of each other when their parent meteoroid penetrated the earth s atmosphere and c drill cores from a large slab of the estacado h6 find located within a few tens of centimeters of each other our results indicate that oc are heterogeneous in their bulk oxidation state and o isotopic composition on 100 km size scales but homogeneous on meter decimeter and centimeter size scales on kilometer size scales oxidation state is heterogeneous but o isotopes appear to be homogeneous the asteroid wide heterogeneity in oxidation state and o isotopic composition was inherited from the solar nebula the homogeneity on small size scales was probably caused in part by fluid assisted metamorphism and mainly by impact gardening processes which are most effective at mixing target materials on scales of 1 m

asteroid 6 hebe the probable parent body of the h type ordinary chondrites and the iie iron meteorites the s iv type asteroid 6 hebe is identified as the probable parent body of the h type ordinary chondrites and of the iie iron meteorites the ordinary chondrites are the most common type of meteorites falling to earth but prior to the present study no large mainbelt source bodies have been confirmed hebe is located adjacent to both the v6 and 3 1 resonances and has been previously suggested as a major potential source of the terrestrial meteorite flux hebe exhibits subtle rotational spectral variations indicating the presence of some compositional variations across its surface the silicate portion of the surface assemblage of hebe is consistent both in overall average and in its range of variation with the silicate components in the suite of h type chondrites the high albedo of hebe rules out a lunar style space weathering process to produce the weakened absorption features and reddish spectral slope in the s type spectrum of hebe linear unmixing models show that a typical ni fe metal spectrum is consistent with the component that modifies an h chondrite spectrum to produce the s type spectrum of hebe on the basis of the association between the h chondrites and the he iron meteorites our model suggests that large impacts onto the relatively metal rich h chondrite target produced melt bodies sheets or pods that differentiated to form thin laterally extensive near surface layers of ni fe metal fragments of the upper silicate portions of these melt bodies are apparently represented by some of the igneous inclusions in h chondrite breccias alternately masses of metal could have been deposited on the surface of hebe by the impact of a core or core fragment from a differentiated parent body of h chondrite composition subsequent impacts preferentially eroded and depleted the overlying silicate and regolith components exposing and maintaining large masses of metal at the optical surface of hebe in this interpretation the nonmagmatic iie iron meteorites are samples of the ni fe metal masses on the surface of hebe whereas the h chondrites are samples from between and or beneath the metal masses

discovery and physical properties of dactyl a satellite of asteroid 243 ida observations of stellar occultations by asteroids have suggested that some may have satellites1 but given the absence of any confirmatory evidence the prevailing view has been that although such satellites probably do exist they are likely to be rare2 here we report the discovery3 by the galileo spacecraft of a satellite associated with the asteroid 243 ida although the satellite dactyl is only 1 6km across it has been imaged with sufficient resolution for geological analysis we describe the physical properties of dactyl with emphasis on its notably smooth shape its crater population which includes a crater chain and its photometric properties we find that spectroscopically dactyl resembles both ida and the other members of the koronis asteroid family implying a similar composition small spectral differences may reflect a space weathering process that slightly alters the colours with time we argue that dactyl originated during the breakup of the koronis parent body and that satellites could be common around other asteroids particularly members of asteroid families

first images of asteroid 243 ida the first images of the asteroid 243 ida from galileo show an irregular object measuring 56 kilometers by 24 kilometers by 21 kilometers its surface is rich in geologic features including systems of grooves blocks chutes albedo features crater chains and a full range of crater morphologies the largest blocks may be distributed nonuniformly across the surface lineaments and dark floored craters also have preferential locations ida is interpreted to have a substantial regolith the high crater density and size frequency distribution 3 differential power law index indicate a surface in equilibrium with saturated cratering a minimum model crater age for ida and therefore for the koronis family to which ida belongs is estimated at 1 billion years older than expected

rotational spectral variations of asteroid 8 flora implications for the nature of the s type asteroids and for the parent bodies of the ordinary chondrites abstract the new interpretive calibrations olivine pyroxene abundance spectral signature of chondritic metal grains have been applied to visible and near infrared spectral reflectance data for the typical s type asteroid 8 flora chondritic metal grains do not exhibit the steeply reddened spectral signature previously associated with a nickel iron nife metal phase the conditions required to produce a s type spectral curve from a chondritic material are much more restrictive and improbable than previously supposed five mineralogic and patrologic parameters have been derived for the surface material of flora from the analysis of the spectral reflectance and rotational spectral variations of this object these include bulk mineralogy nife metal olivine pyroxene mafic mineral abundance ol py 2 8 metal nature abundant 50 and coarse grained in the substrate pyroxene composition orthopyroxene f s 50 and or abundant clinopyroxene and spatial mineralogic variations olivine pyroxene ratio and pyroxene composition do not covary in a chondritic manner all of the diagnostic mineralogic properties of the flora surface material indicate that this is a differentiated body none of the derived surface properties unambiguously support a chondritic type material and several contradict any type of undifferentiated assemblage flora is most probably the residual core of an intensely heated thermally evolved and magmatically differentiated planetesimal which was subsequently disrupted the present surface samples layers formed at and near the core mantle boundary in the parent body several lines of evidence suggest that the s type asteroids as a class are predominantly the core fragments of disrupted differentiated planetesimals a main belt source of the ordinary chondrites remains elusive

surface movements of emplaced lava flows measured by synthetic aperture radar interferometry lava flows continue to move after they have been emplaced by flow mechanisms this movement is largely vertical and can be detected using differential synthetic aperture radar sar interferometry there are three main components to this motion 1 movement of surface scatterers resulting in radar phase decorrelation 2 measurable subsidence of the flow surface due to thermal contraction and clast repacking and 3 time dependent depression of the flow substrate these effects act in proportion to the thickness of the lava flow and decay with time although there is a time lag before the third component becomes significant we explore these effects using sar data from the ers satellites over the etna volcano sicily phase decorrelation on young thick a a lava flows persists for a few years and probably results from surface block rotations during flow contraction maximum measured subsidence rates of the 1991 1993 lava flow over a period of 70 days are about 0 7 mm day 1 but are potentially greater in areas of data decorrelation these rates fall to 2 7 10 2 mm day 1 after about 20 years in flows about 50 m thick sooner for thinner flows comparison with measured subsidence rates on kilauean lava lakes suggests that thermal contraction only accounts for about one third of the observed subsidence the remaining motion is thought to come from surface clast repacking during cooling and from creep mechanisms in the flow substrate measurements of postemplacement surface movement provide new constraints on the thermomechanical properties of lava flows and have cautionary implications for the interpretation of interferometric sar data of volcanoes 2001 american geophysical union

oxidation during metamorphism of the ordinary chondrites subtle but progressive changes in the compositions and relative abundances of olivine pyroxene and metal with increasing metamorphic grade in equilibrated types 4 6 ordinary chondrites indicate that these meteorites experienced oxidation of metallic fe during heating oxygen fugacities calculated from these mineral assemblages increase with petrologic type the hypothesis of oxidation during chondrite metamorphism differs from a commonly accepted idea that chondritic fe was reduced by graphite the mineralogies of unequilibrated type 3 chondrites do not conform to the progressions in equilibrated chondrites allowing the possibility that fe may first have been reduced through reaction with graphite at the onset of metamorphism although there are conflicting observations supporting both oxidation and reduction in these meteorites as temperatures increased to levels appropriate for types 4 6 chondrites the activity of graphite may have been lowered by its dissolution in taenite and in any case oxygen fugacities appear to have been well within the stability field of graphite precluding its reaction at these temperatures oxidation state was largely controlled by equilibrium between ferromagnesian silicates and metal oxygen fugacities calculated from chondrite mineral equilibria are 2 3 log units below previously reported intrinsic o2 measurements we propose that progressive oxidation during metamorphism was promoted by interaction with small amounts of an oxidizing vapor derived by heating ices originally accreted into the parent asteroids assuming that this vapor was pure h2o the watenrock weight ratio required to account for the observed oxidation of fe in h and l chondrites is very modest less than several per mil progressive oxidation in the metamorphic sequence resulted from different degrees of reaction with such a fluid possibly caused by temperature variations or evolution of the composition of the fluid as it permeated through the body the presence of even minor amounts of a vapor phase during metamorphism affects interpretations of the volatile trace element and oxygen isotopic fractionations observed in ordinary chondrites and inferred to have formed by nebular or parent body processes