diviner lunar radiometer observations of cold traps in the moon s south polar region diviner lunar radiometer experiment surface temperature maps reveal the existence of widespread surface and near surface cryogenic regions that extend beyond the boundaries of persistent shadow the lunar crater observation and sensing satellite lcross struck one of the coldest of these regions where subsurface temperatures are estimated to be 38 kelvin large areas of the lunar polar regions are currently cold enough to cold trap water ice as well as a range of both more volatile and less volatile species the diverse mixture of water and high volatility compounds detected in the lcross ejecta plume is strong evidence for the impact delivery and cold trapping of volatiles derived from primitive outer solar system bodies

indicators of aqueous alteration in cm carbonaceous chondrites microtextures of a layered mineral containing fe s o and ni a petrographic and transmission electron microscopy study of the mighei murchison and murray cm carbonaceous chondrites shows that much of the cm matrix material was probably produced by aqueous alteration of olivine pyroxene sulfide and metal the amount of cm matrix appears to be proportional to the degree of alteration as suggested by mcsween 1979 and microtextures of pcp poorly characterized phase provide evidence of the progressive alteration pcp is divided into two major types one occurs in chondrules and aggregates and consists largely of an fe ni s o phase type i and the other occurs in matrix and consists of the fe ni s o phase and cronstedtite in various proportions type ii microtextures of pcp suggest that it resulted from a three stage alteration process 1 1 type i was produced by alteration of kamacite in chondrules and aggregates presumably early in the parent body regolith 2 2 as the alteration advanced olivine and pyroxene were converted to serpentine type i pcp separated from chondrules and aggregates into the matrix during regolith gardening simultaneously the fe ni s o phase reacted with si released by alteration of olivine and pyroxene producing well formed platy cronstedtite and coherent intergrowths of the fe ni s o phase and cronstedtite the fe ni s o phase also recrystallized into platy and pod like crystals fe s ni cr and p were leached out of type i pcp and were deposited as small grains of fe ni sulfides magnetite chromite and a mineral unidentified containing fe ni cr and p as a result pcp came to consist primarily of the fe ni s o phase and cronstedtite i e type ii pcp 3 3 during continued alteration the well formed crystals of the fe ni s o phase cronstedtite and their intergrowths in type ii pcp were replaced by poorly formed fibers in comparison to other cm chondrites mighei murchison and murray are relatively unaltered their matrices retain abundant amounts of the fe ni s o phase and cronstedtite commonly as distinctive pcp grains which account for a large proportion of the fe in these meteorite matrices in more altered cm chondrites much of the fe ni s o phase was probably consumed to produce cronstedtite magnetite and sulfides with further alteration cronstedtite itself reacted with the serpentine to form ferroan serpentine thus the cm matrix was increasingly enriched in mg with alteration and pcp was increasingly degraded and intimately mixed with the magnesian phyllosilicates

the cr renazzo type carbonaceous chondrite group and its implications abstract a petrologic geochemical and oxygen isotopic study of eight meteorites identified as cr renazzo type chondrites reveals that they have a set of similar properties that distinguishes them from other chondrite groups and confirms cr as a carbonaceous chondrite group characteristics of the cr group include 1 1 abundant large multilayered fe ni metal rich type i chondrules 2 2 abundant matrix and dark inclusions di that contain microchondrules microfragments and magnetite framboids and platelets 3 3 unique assemblages of serpentine and chlorite rich phyllosilicates and ca carbonates 4 4 ca carbonate rims on chondrules 5 5 abundant fe ni metal with a positive ni vs co trend and a solar ni co ratio 6 6 low abundance of refractory rich inclusions 7 7 amoeboid olivine aggregates aoa with mn rich and mn poor forsterite and 8 8 oxygen isotopic compositions that form a unique mixing line the cr chondrites are breccias consisting of two major components the large layered chondrules and the matrix dark inclusions al rais is an anomalous member in having a higher abundance of the dark inclusion component 70 vol and this accounts for its having a higher volatile content than other members of the group the conditions that prevailed during aqueous alteration of the cr chondrites differed from those of other hydrated chondrite groups and may be the result of a higher temperature environment than that for the ci and cm chondrites the cr chondrite components chondrules matrix dark inclusions etc experienced aqueous alteration to differing degrees prior to assembly of the host chondrites and dark inclusions represent a lithology that was altered by water to a greater degree than the rest of the meteorite the overall degree of hydration in cr chondrites varies among meteorites with al rais being the most hydrated the oxygen isotopic variation of the whole rocks chondrules and matrices may be related to degree of hydration indicating that cr chondrites interacted with a water rich component having heavy oxygen and that was the same as ci and cm water

anticipated electrical environment within permanently shadowed lunar craters 1 shadowed locations near the lunar poles are almost certainly electrically complex regions at these locations near the terminator the local solar wind flows nearly tangential to the surface and interacts with large scale topographic features such as mountains and deep large craters in this work we study the solar wind orographic effects from topographic obstructions along a rough lunar surface on the leeward side of large obstructions plasma voids are formed in the solar wind because of the absorption of plasma on the upstream surface of these obstacles solar wind plasma expands into such voids producing an ambipolar potential that diverts ion flow into the void region a surface potential is established on these leeward surfaces in order to balance the currents from the expansion limited electron and ion populations we find that there are regions near the leeward wall of the craters and leeward mountain faces where solar wind ions cannot access the surface leaving an electron rich plasma previously identified as an electron cloud in this case some new current is required to complete the closure for current balance at the surface and we propose herein that lofted negatively charged dust is one possible nonunique compensating current source given models for both ambipolar and surface plasma processes we consider the electrical environment around the large topographic features of the south pole including shoemaker crater and the highly varied terrain near nobile crater as derived from goldstone radar data we also apply our model to moving and stationary objects of differing compositions located on the surface and consider the impact of the deflected ion flow on possible hydrogen resources within the craters

widespread oxidized and hydrated amorphous silicates in cr chondrites matrices implications for alteration conditions and h2 degassing of asteroids abstract the cr chondrites carry one of the most pristine records of the solar nebula materials that accreted to form planetesimals they have experienced very variable degrees of aqueous alteration ranging from incipient alteration in their matrices to the complete hydration of all of their components in order to constrain their chemical alteration pathways and the conditions of alteration we have investigated the mineralogy and fe oxidation state of silicates in the matrices of 8 cr chondrites from type 3 to type 1 fe l edge x ray absorption near edge structure xanes was performed on matrix fib sections using synchrotron based scanning transmission x ray microscopy stxm the fe 3 fe ratio of submicron silicate particles was obtained and coordinated with tem observations in all the least altered cr chondrites que 99177 eet 87770 eet 92042 lap 02342 gra 95229 and renazzo we find that the matrices consist of abundant submicron fe rich hydrated amorphous silicate grains mixed with nanometer sized phyllosilicates the fe 3 fe ratios of both amorphous and nanocrystalline regions are very high with values ranging from 68 to 78 in the most altered samples al rais and gro 95577 fine grained phyllosilicates also have a high fe 3 fe ratio around 70 whereas the coarse micrometer sized phyllosilicates are less oxidized down to 55 and have a lower iron content these observations suggest the following sequence submicron fe2 amorphous silicate particles were the building blocks of cr matrices after accretion they were quickly hydrated and oxidized leading to a metastable amorphous gel like phase nucleation and growth of crystalline phyllosilicates was kinetically limited in most type 3 and 2 crs but increased as alteration became more extensive in al rais and gro 95577 the decreasing fe 3 fe ratio is interpreted as a result of the transfer of fe3 from silicates to oxides during growth while aqueous alteration progressed higher temperature longer duration change of fluid composition in a fully closed system equilibrium thermodynamics suggest that the water to rock ratios typically assumed to be low fe 3 fe ratio such a high fe 3 fe value could be accounted for however if the system was partially open at least with respect to h2 and other gases as well rapid degassing of the fluid would have favored more oxidizing fluid conditions recently proposed scenarios involving some degree of water d h increase through rayleigh isotopic fractionation are supported by these results

aqueous alteration in carbonaceous chondrites mass balance constraints on matrix mineralogy abstract bulk chemical compositions of matrix material in antarctic cm chondrites and other non antarctic cm and ci chondrites have been determined using microprobe defocused beam techniques these are used along with the results of previously published mineralogical studies to provide mass balance constraints on the relative proportions of intergrown and intermixed phyllosilicate phases in carbonaceous chondrite matrices results of these calculations indicate differing amounts of pcp a mixture of approximately 25 tochilinite and 75 cronstedtite and serpentines mg rich and fe rich varieties in varying proportions or intermediate compositional varieties additional sulfide phases are also probably necessary to account for excess ni and s fe si ratios for matrices of individual meteorites range from 1 21 to 2 77 corresponding to pcp pcp serf ratios of 0 16 to 0 58 progressive aqueous alteration of matrix appears to have occurred by formation of tochilinite then cronstedtite and mg rich serpentine and finally fe rich serpentine and sulfides cm matrix clearly did not behave as an isolated system during alteration ci chondrite matrices appear to contain little if any pcp this may be a natural consequence of the absence of chondrule associated metal from which pcp forms in the unaltered precursor material these data provide a more quantitative picture of low temperature aqueous alteration processes in carbonaceous chondrite parent bodies than has heretofore been possible from tem studies alone

fine grained rims in the allan hills 81002 and lewis cliff 90500 cm2 meteorites their origin and modification antarctic cm meteorites allan hills alh 81002 and lewis cliff lew 90500 contain abundant fine grained rims fgrs that surround a variety of coarse grained objects fgrs from both meteorites have similar compositions and petrographic features independent of their enclosed objects the fgrs are chemically homogeneous at the 10 m scale for major and minor elements and at the 25 m scale for trace elements they display accretionary features and contain large amounts of volatiles presumably water they are depleted in ca mn and s but enriched in p all fgrs show a slightly fractionated rare earth element ree pattern with enrichments of gd and yb and depletion of er gd is twice as abundant as er our results indicate that those fgrs are not genetically related to their enclosed cores they were sampled from a reservoir of homogeneously mixed dust prior to accretion to their parent body the rim materials subsequently experienced aqueous alteration under identical conditions based on their mineral textural and especially chemical similarities we conclude that alh 81002 and lew 90500 likely have a similar or identical source

olivine in terminal particles of stardust aerogel tracks and analogous grains in chondrite matrix abstract the dearth of both major and minor element analyses of anhydrous silicate phases in chondrite matrix has thus far hindered their comparison to the wild 2 samples we present 68 analyses of olivine fa 0 97 in the coarse grained terminal particles of stardust aerogel tracks and a comprehensive dataset 10 3 analyses of analogous olivine grains 5 30 m isolated in ci cm cr ch co cv3 oxidized cv3 reduced c3 ungrouped acfer 094 and ningqiang l ll 3 0 4 eh3 and kakangari chondrite matrix these compositions reveal that wild 2 likely accreted a diverse assortment of material that was radially transported from various carbonaceous and ordinary chondrite forming regions the wild 2 olivine includes amoeboid olivine aggregates aoas refractory forsterite type i and type ii chondrule fragments and or microchondrules and rare relict grain compositions in addition we have identified one terminal particle that has no known compositional analog in the meteorite record and may be a signature of low temperature aqueous processing in the kuiper belt the generally low cr content of feo rich olivine in the stardust samples indicates that they underwent mild thermal metamorphism akin to a petrologic grade of 3 05 3 15

ci chondrite like clasts in the nilpena polymict ureilite implications for aqueous alteration processes in ci chondrites abstract we have carried out a detailed petrographic and mineralogical study of carbonaceous chondrite matrix clasts in the nilpena polymict ureilite the bulk compositions of a number of clasts determined by electron microprobe show that they have close affinities to ci chondrite matrix and differ substantially from cm matrix material transmission electron microscopy studies show that the mineralogy of the clasts is also consistent with ci chondrite matrix indicated by the presence of saponitic smectite clays serpentine magnetite pentlandite pyrrhotite and ferrihydrite phases such as tochilinite and cronstedtite diagnostic of cm matrix are absent based on the oxygen isotopic composition of the matrix the phase chemistry and the bulk clast chemistry we suggest that the matrix clasts in nilpena represent material which is less altered than orgueil the difference in oxygen isotopic composition of the nilpena matrix clasts from ci chondrites may be because the clasts underwent alteration at a lower water rock ratio increased alteration of nilpena type material should result in the oxidation of fe bearing saponite and sulfides to produce ferrihydrite and more mg rich saponite such as that in orgueil matrix although the presence of ci like matrix clasts in the polymict ureilites suggests that ci matrix may have been a precursor to the ureilites in detail this seems highly unlikely the trace element and carbon isotopic compositions of ureilites are incompatible with a significant ci chondrite precursor we suggest that the ci like matrix clasts in nilpena represent a component of a late accreting veneer on the ureilite parent body

sulfur isotopic composition of fe ni sulfide grains in ci and cm carbonaceous chondrites in situ secondary ion mass spectrometry analyses of 32 s 33 s and 34 s in iron nickel sulfide grains in two ci1 chondrites and six cm chondrites were performed the results show a wider range of both enrichment and depletion in 34 s relative to troilite from the canyon diablo meteorite cdt than has been observed in previous studies all data points lie within error of a single mass dependent fractionation line sulfides from ci1 chondrites show 34 scdt from 0 7 to 6 8 while sulfide grains in the cm1 chondrite are generally depleted in heavy sulfur relative to cdt 34 s from 2 9 to 1 8 cm2 chondrites contain sulfide grains that show enrichment and depletion in 34 s 34 scdt from 7 0 to 6 8 sulfates forming from sulfide grains during aqueous alteration on the chondrite parent body are suggested to concentrate light sulfur leaving the remaining sulfide grains enriched in the heavy isotopes of sulfur the average degree of enrichment in 34 s in cm chondrite sulfides is broadly consistent with previously suggested alteration sequences

hydrogen isotopic composition of water from fossil micrometeorites in howardites we have measured the hydrogen isotopic composition d h ratios of the water from 13 carbonaceous chondritic microclasts ccms size 1 mm trapped in two howardites kapoeta and yamato 793497 early in the evolution of solar system the division into tochilinite rich magnetite rich olivine poor magnetite rich olivine rich ccm types is corroborated by the hydrogen isotopic compositions both mineralogy and hydrogen isotopic compositions demonstrate that tochilinite rich ccms represent cm2 chondritic matter in contrast there is no good match between the isotopic and mineralogical properties of the magnetite rich ccms and the known groups of carbonaceous chondrites suggesting that magnetite rich ccms represent a new kind of chondritic matter not yet sampled in meteorite collections this demonstrates that the view of the asteroid belt revealed by the collection of meteorites is incomplete the study of micro clasts offers a unique opportunity to better decipher the nature and relative abundance of asteroids the average hydrogen isotopic composition of water belonging to ccms d h 152 0 4 8 x 10 6 1 sigma m is similar to that of antarctic micrometeorites amms d h 161 2 3 8 x 10 6 1 sigma m the similarity in terms of mineralogy and hydrogen isotopic composition between ccms and amms demonstrates that the composition of the micrometeorites has not been modified over the whole history of the solar system it indicates that the composition of the micrometeorite flux onto earth has been and is dominated by a mixture of cm2 like magnetite rich olivine poor magnetite rich olivine rich carbonaceous chondritic matter exemplified by ccms found in howardites because ccms have not suffered atmospheric entry they provide an abundant source of pristine micrometeorites the average d h ratio of the whole population of ccms is identical within errors to that of the earth 149 3 x 10 6 the match between the ccms d h ratio and that of the earth is especially remarkable because 1 three different populations of ccms are needed to make the d h ratio of the earth 2 there is no single carbonaceous chondrite group for which a similar match exists this observation suggests that ccms population might be representative of the late veneer agent s that delivered water to the earth

mineralogy and composition of matrix and chondrule rims in carbonaceous chondrites abstract the degree of compositional variation of fine grained minerals displayed by the members within any carbonaceous chondrite group i e ci cm cv cr is a direct reflection of the range of aqueous alteration assemblages present matrix and fine grained chondrule rims within any particular carbonaceous chondrite are mineralogically nearly identical to one another but not necessarily similar in bulk elemental composition even though they have subsequently experienced post accretional secondary processing aqueous alteration under identical conditions this may indicate preferential alteration of one component brecciation exchange of components between rims and matrix and or accretion of matrix and rims from mineralogically or compositionally distinct materials we propose that co chondrites experienced parent body conditions of low f o2 low water rock ratios and temperatures below 50 c cr chondrites experienced higher water rock ratios potentially higher temperatures 150 c and a wide range of f o2 the alteration mineralogy of cv chondrites indicates water rock ratios at the high end at least of the range for cr chondrites essebi and mac 87300 cm chondrites experienced temperatures below 50 c low f o2 and low water rock ratios except eet 83334 which probably experienced relatively higher f o2 and b 7904 and y 86720 which experienced postalteration temperatures in the range 500 700 c most ci chondrites experienced temperatures between 50 and 150 c relatively high water rock ratios and variable f o2 with a late stage increase in the latter due possibly to brecciation and consequent release of methane y 82162 witnessed post alteration heating possibly as high as 400 c

accretionary dust mantles in cm chondrites evidence for solar nebula processes abstract the texture and modal composition of fourteen carbonaceous chondrites of the cm group were studied in detail to unravel the origin of their various components and the evolution of their parent body ies the analysis and mapping of complete thin sections by scanning electron and optical microscopy revealed textural units in most samples in which all coarse grained primordial chondritic components chondrules chondrule fragments refractory inclusions particles rich in hydrous silicates pcp and various mineral fragments are coated by fine grained serpentine and tochilinite rich dust the dust mantled particles have spheroidal shapes and form a special lithology in which they are densely aggregated and compacted without any interstitial clastic material the dust mantles termed accretionary dust mantles and their host lithology called primary accretionary rock are interpreted as products of accretion processes in the solar nebula an origin by processes on the parent body is excluded for textural and mineralogical reasons most cm chondrites are affected by secondary impact induced brecciation of variable intensity except for yamato 791198 which is composed entirely of a primary accretionary rock obviously unaltered by secondary parent body processes such as aqueous alteration or brecciation the cm chondrites y74662 haripura cold bokkeveld kivesvaara mighei pollen nogoya y793321 alh 83100 murray and murchison consist of clasts of primary accretionary rock embedded in a fine grained clastic matrix derived from primary rock material the volume percentage of the clastic matrix ranges from 5 y74662 to 76 murchison bells and essebi are brecciated and affected by aqueous alteration to an extent that fragments of primary rock are not preserved the evaluation of literature data indicates that solar wind implanted noble gases are absent in the primary accretionary rock component of cm chondrites they are only present in brecciated samples it must be concluded from the relation between the content of trapped solar wind and degree of brecciation that 1 the solar noble gases are residing exclusively in the clastic matrix and 2 some cm chondrites are regolith breccias and others are fragmental or monomict breccias both types were formed on the parent body although the latter were never exposed to the solar wind based on the results of textural chemical and mineralogical investigations we conclude that a major portion of the water bearing phases in cm chondrites were produced by the hydration of refractory phases prior to the formation of dust mantles and prior to the formation of the recent cm parent body this process requires special physicochemical conditions which allow the crystallization of oh bearing silicates e g higher density of the nebula gas or special precursor planetesimals of the cm chondrite parent body

petrologic study of the belgica 7904 carbonaceous chondrite hydrous alteration oxygen isotopes and relationship to cm and ci chondrites abstract seven components were separated from the belgica 7904 b7904 carbonaceous chondrite in order to learn more about the genetic relationships between the petrology hydration reactions and the oxygen isotopic compositions these components include two partially altered chondrules two phyllosilicate clasts two olivine fragments and one matrix sample the partially altered chondrules have a core rim structure cores have igneous textures and are unaltered and rims consist mainly of talc component bearing phyllosilicates and unaltered olivine clear textural evidence indicates that the phyllosilicates in the rims were produced by replacing pyroxene and plagioclase while the olivine remained unaltered during the process on the other hand the phyllosilicate clasts consist mainly of fine grained serpentine produced from olivine as well as the pyroxene and plagioclase the olivine fragments derived by disaggregation of large porphyritic olivine chondrules are unaltered the matrix sample consists mainly of fine grained phyllosilicates if the hydration reactions took place with liquid water in the parent body olivine would have produced talc component free phyllosilicates before pyroxene and plagioclase reacted this is not the case for the phyllosilicates in the chondrule rims therefore we suggest that the chondrule rims formed by hydration in the nebula prior to accretion rather than with liquid water in the parent body thermodynamic calculations show that the reactions of pyroxene and plagioclase with a gas would produce the talc component bearing phyllosilicates at temperatures higher than that at which olivine reacts to produce serpentine assuming that total gas pressure was 10 3 bars the chondrule rims may have been produced by reactions with a nebular gas having an h 2 o h 2 ratio of 0 052 at temperatures ranging from 560 to 330 k the phyllosilicate clasts where olivine also reacted may have been produced by reactions with this nebular gas at temperatures near 300 k and the matrix phyllosilicates may have been in equilibrium with the gas at temperatures higher than 250 k there is no evidence for liquid water interacting with the chondrite the cm mixing line in the three oxygen isotope plot mayeda et al 1991 which includes the separated components and whole rock composition of b7904 is explained by hydration in nebular gas ci chondrites may have formed from materials similar to the b7904 matrix by reactions with liquid water in a parent body cm chondrites have experienced two stage alteration including both hydrous reactions with a gas in the nebula and aqueous alteration with liquid water in their parent body

sensitivity of the late saalian 140 kyrs bp and lgm 21 kyrs bp eurasian ice sheet surface mass balance to vegetation feedbacks this work uses an atmospheric general circulation model agcm asynchronously coupled to an equilibrium vegetation model to investigate whether vegetation feedbacks could be one of the reasons why

spillage of lunar polar crater volatiles onto adjacent terrains the case for dynamic processes we present an investigation of the release and transport of lunar polar crater volatiles onto topside regions surrounding the cold traps the volatiles are liberated via surface energization processes associated with the harsh space environment including solar wind plasma sputtering and impact vaporization we find that some fraction of these volatiles can migrate from crater floors onto topside regions those regions directly adjacent to and above the polar crater floors and that these surrounding terrains should contain a sampling of the material originating within the crater itself it is concluded that the nature of the volatile content on crater floors can be obtained by sampling the surface volatiles that have migrated or spilled out onto the adjacent terrain this spillage effect could make human or robotic prospecting for crater resources significantly easier since an assessment may not require direct entry into the very harsh polar crater environment we also suggest that there are dynamic processes actively operating on the crater floors and we estimate their source rates assuming dynamic equilibrium of the observed water frost and our modeled loss rates

aqueous alteration and brecciation in bells an unusual saponite bearing cm chondrite the petrological and mineralogical characteristics of the unusual cm2 chondrite bells have been investigated in detail by scanning electron microscopy sem electron microprobe analysis epma and transmission electron microscopy tem bells is a highly brecciated chondrite which contains few intact chondrules a very low abundance of refractory inclusions and is notable in having an unusually high abundance of magnetite which is disseminated throughout the fine grained matrix fragmental olivines and pyroxenes are common and based on compositional data appear to have been derived from chondrules as a result of extensive brecciation the fine grained mineralogy of matrix in bells differs considerably from other cm chondrites and has closer affinities to matrix in ci chondrites the dominant phases are fine grained saponite interlayered with serpentine and phases such as tochilinite and cronstedtite which are typical of cm chondrite matrices are entirely absent pentlandite pyrrhotite magnetite anhydrite calcite and rare ti oxides also occur as accessory phases based on its oxygen and noble gas isotopic compositions zadnik 1985 rowe et al 1994 bells can be considered to be a cm2 chondrite although its bulk composition shows some departures from the typical range exhibited by this group however these variations in bulk chemistry are entirely consistent with the observed mineralogy of bells the unusual fine grained mineralogy of bells matrix can be reasonably attributed to the combined effects of aqueous alteration and advanced brecciation in a parent body environment extensive brecciation has assisted aqueous alteration by reducing chondrules and mineral grains into progressively smaller grains with high surface areas which are more susceptible to dissolution reactions involving aqueous fluids this has resulted in the preferential dissolution of fe rich chondrule olivines which are now completely absent in bells although present in other cm chondrites the formation of saponite in bells probably resulted from the dissolution of relatively silica rich phases such as pyroxene and olivine that were derived from chondrules the result of such dissolution reactions would be to increase the activity of silica in the fluid phase at least on a localized scale stabilizing saponite in preference to serpentine an increase in asio2 would also have destabilized preexisting cronstedtite which may have reacted to form magnetite and mg fe serpentine under conditions of constant o2

recursive plasma wake formation on the moon and its effect on polar volatiles abstract solar wind plasma flows almost horizontally in near terminator regions of the moon one question is how does this plasma flow around topographic features like near terminator mountains and polar craters we present kinetic simulations of plasma inflow into a complicated polar crater having a second child crater within the parent crater i e a crater in a crater model we find that the effect on the inflowing ions of this second child feature is a function of its position downstream relative to the primary crater wall close to the wall where the solar wind is occulted there is little ion inflow and thus not a significant perturbation from the child obstruction however beyond about two crater wall heights downstream there is substantial ion flow and the child obstruction will create a second ambipolar wake region similar to that created by the first obstruction we also present a general analytical formalism that explains the most critical properties of the inflow perturbations finally we consider the effect of the ion inflow into polar craters with 1 water frost mixed within the top micron layers of the regolith we find that in order to maintain equilibrium a persistent frost most likely has to be maintained by a dynamic water source of 10 9 molecules m 2 s 1 in polar craters to offset solar wind ion inflow sputtering losses

matrix and fine grained rims in the unequilibrated co3 chondrite alha77307 origins and evidence for diverse primitive nebular dust components abstract the mineralogical and chemical characteristics of dark matrix and fine grained rims in the unequilibrated co3 chondrite alha77307 have been investigated in detail by scanning electron microscopy electron microprobe analysis and transmission electron microscopy matrix and fine grained rims on chondrules and other objects are compositionally homogeneous on the scale of 10 m and there are no compositional differences between matrix and rims mineralogically both the matrix and rims are extremely diverse and consist of a highly unequilibrated assemblage of si and fe rich amorphous material olivine pyroxene fe ni metal magnetite pentlandite pyrrhotite anhydrite and mixed layer phyllosilicate phases several distinct components can be recognized within the matrix and rims based on their textural and compositional characteristics which appear to represent basic fine grained units of nebular dust the microstructures of these different components show that they have experienced significantly different thermal histories unlike the ordinary chondrites an origin for any significant component of the matrix or rims in alha77307 from chondrules is improbable based on compositional and microstructural evidence the matrix and rim components in alha77307 formed by disequilibrium condensation processes as fine grained amorphous dust that is represented by the abundant amorphous component in the matrix such condensation could have occurred under a variety of conditions at different times and locations within the solar nebula or possibly earlier in a circumstellar environment subsequent thermal processing of this primitive condensate material in a variety of environments in the nebula resulted in partial or complete recrystallization of the fine grained dust the intimate association of fine grained components with disparate compositions and thermal histories shows that mixing of fine grained dust within the nebula must have been extremely thorough

matrix mineralogy of the orgueil ci carbonaceous chondrite a petrographic and transmission electron microscopic study of the orgueil ci carbonaceous chondrite shows that the matrix consists mainly of fe bearing mg rich serpentine and smectite saponite as well as a poorly crystallized fe rich material that contains minor variable s and ni the fe rich material is probably ferrihydrite a ferric hydroxide the ferrihydrite occurs in small particles 80 a in diameter and the s and ni are probably adsorbed on its surfaces the serpentine and saponite occur in poorly crystallized fine crystallites fine phyllosilicates that are intimately associated with the ferrihydrite coarsely crystallized phyllosilicates coarse phyllosilicates occur in clusters relatively free of ferrihydrite the ferrihydrite is a major matrix constitutent and probably accounts for most of the fe3 as well as the superparamagnetic fraction in the orgueil matrix the textures suggest that the fine phyllosilicates resulted from alteration of the coarse phyllosilicates the alteration probably involved a substantial amount of water and coincided with the period of sulfate vein formation during the aqueous alteration magnetite grains disaggregated from framboids and fe ni sulfides were replaced by ferrihydrite simultaneously aqueous solutions reacted with the coarse phyllosilicates and precipitated ferrihydrite producing intimate pervasive mixtures with the fine phyllosilicates the matrix mineralogy of the orgueil chondrite suggests that orgueil experienced considerably different alteration conditions from the cm chondrites the mineralogical and petrological differences between ci and cm chondrites can not simply be explained as the result of different degrees of aqueous alteration

oxygen isotopes in separated components of ci and cm meteorites oxygen isotopic compositions have been determined for whole rock and separated components from the ci chondrites alais ivuna and orgueil and from the atypical cm chondrites bells and essebi the composition of ci matrix is in agreement with previous measurements and is systematically different from that of cm matrix probably due to warmer and wetter conditions of aqueous alteration for ci matrix although bells matrix is like that of other cm chondrites the matrix of essebi falls in the ci field essebi also contains abundant magnetite like cis and unlike other cms the isotopic compositions of magnetite in alais ivuna orgueil and essebi define a trend with 17o of 1 3 to 1 8 distinctly out of isotopic equilibrium with the phyllosilicates in each meteorite the magnetite serves as an isotopic tracer for the fluid which brought about aqueous alteration and oxidation and thus provides additional evidence for a gaseous reservoir in the solar nebula which was more 16o depleted than the earth

ordered mixed layer structures in the mighei carbonaceous chondrite matrix abstract high resolution transmission electron microscopy of the mighei carbonaceous chondrite matrix has revealed the presence of a new mixed layer structure material this mixed layer material consists of an ordered arrangement of serpentine type s and brucite type b layers in the sequence sbbsbb electron diffraction and imaging techniques show that the basal periodicity is 17 a discrete crystals of sbb type material are typically curved of small size

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

nanometer scale measurements of iron oxidation states of cronstedtite from primitive meteorites we report the first nanometer scale measurements of the iron iii to total iron fe 3 fe ratios from primitive meteorites these ratios from the matrices and fine grained rims fgrs of the murchison murray and cold bokkeveld cm chondrites fall within a tight range from 0 45 to 0 54 0 02 the measurements were made using electron energy loss spectroscopy eels on cronstedtite which is a product of aqueous alteration early in the history of the solar system the results indicate that the alteration of these meteorites which display a broad range of alteration intensity occurred under similar redox conditions and further that alteration likely occurred in situ on asteroidal bodies rather than in the solar nebula

refractory inclusions in the murchison meteorite mineralogical and petrographic studies of a wide variety of refractory objects from the murchison c2 chondrite have revealed for the first time melilite rich and feldspathoid bearing inclusions in this meteorite but none of these is identical to any inclusion yet found in allende blue spinel hibonite spherules have textures indicating that they were once molten and thus their sio2 poor bulk composition requires that they were exposed to higher temperatures 1550 c than those deduced so far from any allende inclusion melilite rich inclusions are similar to allende compact type a s but are more al ti rich one inclusion much 1 consists of a delicate radial aggregate of hibonite crystals surrounded by alteration products and probably originated by direct condensation of hibonite from the solar nebular vapor the sinuous nodular and layered structures of another group of inclusions spinel pyroxene aggregates suggest that these also originated by direct condensation from the solar nebular gas each type of inclusion is characterized by a different suite of alteration products and or rim layers from all the other types indicating modification of the inclusions in a wide range of different physico chemical environments after their primary crystallization all of these inclusions contain some iron free rim phases these could not have formed by reaction of the inclusions with fluids in the murchison parent body because the latter would presumably have been very rich in oxidized iron other rim phases and alteration products could have formed at relatively low temperatures in the parent body but some inclusions were not in the locations in which they were discovered when this took place some of these inclusions are too fragile to have been transported from one region to another in the parent body indicating that low temperature alteration of these may have occurred in the solar nebula