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NASA NTRS1999

Lunar Meteorites and Implications for Compositional Remote Sensing of the Lunar Surface

Lunar meteorites (LMs) are rocks found on Earth that were ejected from the Moon by impact of an asteroidal meteoroid. Three factors make the LMs important to remote-sensing studies: (1) Most are breccias composed of regolith or fragmental material; (2) all are rocks that resided (or breccias composed of material that resided) in the upper few meters of the Moon prior to launch and (3) most apparently come from areas distant from the Apollo sites. How Many Lunar Locations? At this writing (June 1999), there are 18 known lunar meteorite specimens. When unambiguous cases of terrestrial pairing are considered, the number of actual LMs reduces to 13. (Terrestrial pairing is when a single piece of lunar rock entered Earth's atmosphere, but multiple fragments were produced because the meteoroid broke apart on entry, upon hitting the ground or ice, or while being transported through the ice.) We have no reason to believe that LMs preferentially derive from any specific region(s) of the Moon; i.e., we believe that they are samples from random locations. However, we do not know how many different locations are represented by the LMs; mathematically, it could be as few as 1 or as many as 13. The actual maximum is < 13 because in some cases a single impact appears to have yielded more than one LM. Yamato 793169 and Asuka 881757 are considered "source-crater paired" or "launch paired" because they are compositionally and petrographically similar to each other and distinct from the others, and both have similar cosmic-ray exposure (CRE) histories. The same can be said of QUE 94281 and Y 793274. Thus the 13 meteorites probably represent a maximum of 11 locations on the Moon. The minimum number of likely source craters is debated and in flux as new data for different isotopic systems are obtained. Conservatively, considering CRE data only, a minimum of about 5 impacts is required. Compositional and petrographic data offer only probabilistic constraints. An extreme, but not unreasonable viewpoint, is that such data offer no constraint. For example, if one were to cut up the Apollo 17 landing site (which was selected for its diversity) into softball-sized pieces, some of those pieces (e.g., sample 70135) would be crystalline mare basalts like Y 793169 whereas others (e.g., sample 73131 would be feldspathic regolith breccias like MAC 88104/ 88105. However, nature is not so devious. Warren argues that LMs come from craters of only a few kilometers in diameter. If so, even though CRE data allow, for example, that ALHA 81005 and Y 791197) were launched simultaneously from the same crater, the probability is nevertheless low because the two meteorites are compositionally and mineralogically distinct. Thus, within the allowed range (5-11) for the number of locations represented by the LMs, values at the high end of the range are probably more likely. Mare Meteorites: Three LMs consist almost entirely of mare basalt. Two, Y 793169 and Asuka 881757, are unbrecciated, low-Ti, crystalline rocks that are compositionally and mineralogically similar (but not identical) to each other; they probably derive from a single lunar-mare location. The third, EET 87521/96008, is a fragmental breccia consisting predominantly of VLT mare basalt. Thus, these LMs probably represent only two lunar mare locations. The basaltic LMs have mineral and bulk compositions distinct from Apollo mare basalts. The petrography of Calcalong Creek has not been described in detail, but compositionally it is unique in that it corresponds to a mixture (breccia) of about one-half feldspathic material (i.e., the mean composition of the feldspathic lunar meteorites, below), one-fourth KREEP norite, one-fourth VLT mare basalt (like EET 87521), and 1% CI chondrite. With 4 micro g/g Th and correspondingly high concentrations of other incompatible elements, it is the only lunar meteorite that is likely to have come from within the Procellarum KREEP Terrane (PKT). Yamato 793274 and QUE 94281 are together distinct in being fragmental breccias containing subequal parts of feldspathic highland material and VLT mare basalt. Jolliff et al. estimate a mare to highland ratio of 54:46 for QUE 94281 and 62:38 for Y 793274; this difference is well within the range observed for soils collected only centimeters apart (in cores) at interface site like Apollo 15 and 17 [11]. Although the two meteorites were found on opposite sides of Antarctica, they are probably launch-paired. The strongest evidence is that the pyroclastic glass spherules that occur in both are of two compositional groups and the two groups are essentially the same in both meteorites. Yamato 791197 is nominally a feldspathic lunar meteorite (below), but among FLMs, it probably contains the highest abundance of clasts and glasses of mare derivation. As a consequence, its composition is at the high-Fe, low-Mg end of the range for FLMs and is not included in the FLM average of Table 1. Its composition is consistent with about 10% mare-derived material. Similarly, the two small (Y 82) pieces of Y 82192/82193186032 are more mafic than the large (Y 86) piece, probably as a result of about 7% mare-derived material. All Apollo missions went to areas in or near the PKT, and, consequently, all Apollo regolith samples are contaminated with Th-rich material from the PKT. At the nominally "typical" highland site, Apollo 16, about 30% of the regolith (<1-mm fines) is Th-rich ejecta from the Imbrium impact and about 6% is mare material probably derived from mare basins. Thus Apollo 16 regolith is not typical of the highlands. Among Apollo rocks, the compositions of the FLMs correspond most closely to the feldspathic granulitic breccias of Apollo 16 and 17. (Additional information is contained in original)

Korotev, R. L.↗
NASA NTRS1999

Global Geochemical Variation on the Lunar Surface: A Three-Element Approach

We present a method for displaying the relative abundances of three important elements (Th, Fe, and Ti) on the same map projection of the lunar surface. Using Th-, Fe-, and Ti-elemental abundances from orbital geochemical data and assigning each element a primary color, a false-color map of the lunar surface was created. This approach is similar to the ternary diagram approach presented by Davis and Spudis with some important differences, discussed later. For the present maps, Th abundances were measured by the Lunar Prospector (LP) Gamma-Ray Spectrometer(GRS).The new LPGRS low-altitude dataset was used in this analysis. Iron and Ti weight percentages were based on Clementine spectral reflectance data smoothed to the LP low altitude footprint. This method of presentation was designed to aid in the location and recognition of three principal lunar compositions: ferroan anorthosite (FAN), mare basalts (MB), and the Mg suite/ KREEP-rich rocks on the lunar surface, with special emphasis on the highlands and specific impact basins. In addition to the recognition of these endmember rock compositions, this method is an attempt to examine the relationship between elemental compositions that do not conform readily to previously accepted or observed endmember rocks in various specific regions of interest, including eastern highlands regions centered on 150 deg longitude, and a northern highlands Th-rich region observed. The LP low-altitude data has full width at half-maximum spatial resolution of about 40 km. The Clementine spectral reflectance datasets were adapted using an equal-area, gaussian smoothing routine to this footprint. In addition, these datasets, reported in weight percent of FeO and of Ti02, were adjusted to Fe and Ti weight percentages. Each dataset was then assigned one of the three primary colors: blue for Th, red for Fe, and green for Ti. For each element, the data range was normalized to represent the ratio of each point to the maximum in the dataset. (To view the color table, go to http://cass.jsc.nasa.gov/meetings/moon99/pdf/8033.pdf.) The full range of lunar longitudes is represented, but due to the lack of coverage of the Clementine data for latitudes > 70 deg and <-70 deg, the data for these regions is excluded. The differences between this approach and the ternary diagram approach of Davis and Spudis eliminate some of the uncertainty and ambiguity of the ternary diagram approach. Rather than using a ratio of Th to Ti normalized to CI chondritic ratios, and a ternary diagram with ternary apexes located at specific endmember compositional values, elemental compositions were used independently, eliminating the errors resulting from dividing numbers that can have high uncertainties, especially at low concentration. The three elements used in this method of presentation were chosen for several reasons. One reason for the inclusion of Th in this study is that it is an accurate indicator of KREEP. Iron and Ti concentrations are both low in highland regolith, causing any small fluctuations in Th to stand out very well. In addition, Fe and Ti are good compositional indicators of different mare basalts. Mixed with red for Fe, the green for Ti produces a yellow signal in high-Ti basalts. While universally high in Fe relative to the surrounding highlands, mare basalts have a diverse range of Ti values, making Ti concentration a valuable asset to the classification and identification of different basalt types. Finally, an important constraint in element selection is the availability of the global data, both from LP and Clementine results. Data for Th, Fe, and Ti are among the highest quality of existing lunar remote-sensing data. In addition, LP data for Fe and Ti will become available, enabling these data to be incorporated into the analysis. Using upper-limit values for end member rock compositions calculated from Korotev et al., attempts were made to locate the different endmember compositions of terranes on this diagram. Most strikingly, ferroan anorthosite (Th < and = 0.37 micro g/g; Fe (wt%)< and =2.29; Ti (wt%) < and = 0.22), which should appear as an almost black, reddish color, does not appear on the diagram at any noticeable frequency. Based on this analysis, the suggestion of extensive FAN regions on the lunar surface is not strong, especially at the presently accepted values for Fe and Th. However, to make sure this effect is not due to systematic errors, a thorough investigation of the precision, accuracy, and uncertainties of the Fe, Ti, and Th abundances needs to be carried out, especially at low concentrations. A particular region of interest is an area of high Th concentrations relative to Fe and Ti content north and east of Humboldtianum Crater. First observed by Lawrence et al., this region does not coincide with any visible impact structure and comprises one of the closest approximations to pure blue (high Th, very low Ti and Fe) on the lunar surface. Such an elemental composition does not lend itself readily to classification, and presents something of an anomaly. More detailed analysis of this region is needed to understand its structure and origin. There seems to be a longitudinal asymmetry in the Th concentrations of the highlands regolith. High-Th, low-Ti, and Fe regions are located between 135 deg and 180 deg longitude and between -30 deg and +30 deg latitude. While the Th levels are not high enough to attract attention in a single elemental display, the variation in the abundance of Th relative to Fe and Ti abundances can be clearly seen. The composition that these data suggest is not well represented in the sample return suite. In addition, these regions were largely missed by the Apollo orbital ground tracks, which only covered the outer edge of the areas of interest. The LP orbital Th data represent the first information about the Th concentrations in these regions of the highlands. Additional information contained in original.

Thomsen, D. R.↗
NASA NTRS2000

Reliability and Maintainability (RAM) Training

The theme of this manual is failure physics-the study of how products, hardware, software, and systems fail and what can be done about it. The intent is to impart useful information, to extend the limits of production capability, and to assist in achieving low-cost reliable products. In a broader sense the manual should do more. It should underscore the urgent need CI for mature attitudes toward reliability. Five of the chapters were originally presented as a classroom course to over 1000 Martin Marietta engineers and technicians. Another four chapters and three appendixes have been added, We begin with a view of reliability from the years 1940 to 2000. Chapter 2 starts the training material with a review of mathematics and a description of what elements contribute to product failures. The remaining chapters elucidate basic reliability theory and the disciplines that allow us to control and eliminate failures.

Lalli, Vincent R.↗
NASA NTRS2001

Mineralogy and Petrology of Yamato 86029: A New Type of Carbonaceous Chondrite

Y-86029 resembles CI chondrites. Its matrix is very fine-grained. Olivine shows evidence of shock, which has rarely been observed in carbonaceous chondrites. Y-86029 experienced aqueous and thermal alteration during or after accretion in parent body. Additional information is contained in the original extended abstract.

Tonui, E.↗
NASA NTRS1994

On The Choice of a Zeroth-Order Hamiltonian for Second-Order Perturbation Theory with A CASSCF Reference Function

A new approach to perturbation theory based on a CASSCF reference function has been developed. The key to the approach is the definition of the zeroth order Hamiltonian, H(sub 0), which includes the full CI Hamiltonian for the active space. In the the inactive and secondary spaces, operators may be chosen which reduce to the usual Moller-Plesset or Epstein-Nesbet forms in the limit of a null active space. These operators are diagonal in the orbital indices and permit the block-diagonalization of H(sub 0). The reference is an eigenfunction of Ho without N-particle projection. H(sub 0) automatically incorporates denominator shifts in the style of those appearing in recent open-shell perturbation theories. Comparative results are presented for a few test cases.

Dyall, Kenneth G.↗
NASA NTRS1994

Intermediates in the Formation of Aromatics in Hydrocarbon Combustion

The formation of the first benzene ring is believed to be the rate limiting step in soot formation. Two different mechanisms have been proposed for formation of cyclic C6 species. The first involves the reaction of two acetylenes to give CH2CHCCH (vinyl acetylene), the loss of a H to give CHCHCCH (n-C41-13) or CH2CCCH (iso-C4H3), and addition of another acetylene to n-C4H3, followed by ring closure to give phenyl radical. Miller and Melius argue that only n-C4H3 leads to phenyl radical and since iso-C4H3 is more stable than n-C4H3 this mechanism is unlikely. An alternative mechanism proposed by them is formation of benzene from the dimerization of two CH2CCH (propargyl) radicals (formed by the reaction of singlet methylene with C2H2). We report reaction pathways and accurate energetics (from CASSCF/internally contracted CI calculations) for the reactions of CH(pi-2) and CH2-1 with acetylene, the reaction of vinylidene with acetylene, and the reaction of n-C4H3 and iso-C4H3 with acetylene. These calculations identify two new reactive intermediates CHCHCH ( a A"-2 ground state in Cs symmetry; spin coupling is a doublet from three singly occupied orbitals) and CHCCH (B-3 ground state in C2 symmetry) from the reaction of CH with acetylene. These species dimerize with no barrier to form benzene and para-benzyne, respectively. CHCCH is proposed as a reactive intermediate which can add to benzene to give higher polynuclear aromatic hydrocarbons or fullerenes. The addition of a C3H2 unit releases two C-C bond energies and thus the resulting addition product contains sufficient energy to break several CH bonds leading to a reduction in the H to C ratio as the cluster size increases. It is found that iso-C4H3 adds to acetylene to initially give a fulvene radical but that this species rearranges to phenyl radical. Thus, the reaction of acetylene with iso-C4H3 does lead to phenyl radical and the cyclization pathway may also contribute to formation of the initial benzene ring.

Walch, Stephen P.↗
NASA NTRS1994

Meteoritic Constraints on Models of the Solar Nebula: The Abundances of Moderately Volatile Elements

The "moderately volatile" elements are those which condense (or evaporate) in the temperature range 650 - 1350 K, as a mix of material with solar abundances is cooled (or heated) under equilibrium conditions. Their relative abundances in chondritic meteorites are solar (or "cosmic", as defined by tile composition of CI meteorites) to within a factor of several, but vary within that range in a way that correlates remarkably well with condensation temperature, independent of chemical affinity. It has been argued that this correlation reflects a systematically selective process which favored the accretion of refractory material over volatile material from a cooling nebula. Wasson and Chou suggested that condensation and settling of solids contemporaneously with the cooling and removal of nebular gas could produce tile observed abundance patterns, but a quantitative model has been lacking. We show that the abundance patterns of the moderately volatile elements in chondritic meteorites can be produced, in some degree of quantitative detail, by models of the solar nebula that are designed to conform to observations of T Tauri stars and the global conservation laws. For example, even if the local surface density of the nebula is not decreasing, condensation and accretion of solids from radially inflowing gas in a cooling nebula can result in depletions of volatiles, relative to refractories, like those observed. The details of the calculated abundance patterns depend on (but are not especially sensitive to) model parameters, and can exhibit the variations that distinguish the meteorite classes. Thus it appears that nebula characteristics Such as cooling rates, radial flow velocities, and particle accumulation rates can be quantitatively constrained by demanding that they conform to meteoritic data; and the models, in turn, can produce testable hypotheses regarding the time and location of the formation of the chondrite parent bodies and the planets.

Cassen, P.↗
NASA NTRS1994

Computed Potential Energy Surfaces and Minimum Energy Pathway for Chemical Reactions

Computed potential energy surfaces are often required for computation of such observables as rate constants as a function of temperature, product branching ratios, and other detailed properties. We have found that computation of the stationary points/reaction pathways using CASSCF/derivative methods, followed by use of the internally contracted CI method with the Dunning correlation consistent basis sets to obtain accurate energetics, gives useful results for a number of chemically important systems. Applications to complex reactions leading to NO and soot formation in hydrocarbon combustion are discussed.

Walch, Stephen P.↗
NASA NTRS1994

Computed Potential Energy Surfaces and Minimum Energy Pathways for Chemical Reactions

Computed potential energy surfaces are often required for computation of such parameters as rate constants as a function of temperature, product branching ratios, and other detailed properties. For some dynamics methods, global potential energy surfaces are required. In this case, it is necessary to obtain the energy at a complete sampling of all the possible arrangements of the nuclei, which are energetically accessible, and then a fitting function must be obtained to interpolate between the computed points. In other cases, characterization of the stationary points and the reaction pathway connecting them is sufficient. These properties may be readily obtained using analytical derivative methods. We have found that computation of the stationary points/reaction pathways using CASSCF/derivative methods, followed by use of the internally contracted CI method to obtain accurate energetics, gives usefull results for a number of chemically important systems. The talk will focus on a number of applications including global potential energy surfaces, H + O2, H + N2, O(3p) + H2, and reaction pathways for complex reactions, including reactions leading to NO and soot formation in hydrocarbon combustion.

Walch, Stephen P.↗
NASA NTRS2001

First Astronomical Use Of Multiplexed Transition Edge Bolometers

We present performance results based on the first astronomical use of multiplexed superconducting bolometers. The Fabry-Perot Interferometer Bolometer Research Experiment (FIBRE) is a broadband submillimeter spectrometer that achieved first light in June 2001 at the Caltech Submillimeter Observatory (CSO). FIBRE's detectors are superconducting transition edge sensor (TES) bolometers read out by a SQUID multiplexer. The Fabry-Perot uses a low resolution grating to order sort the incoming light. A linear bolometer array consisting of 16 elements detects this dispersed light, capturing five orders simultaneously from one position on the sky. With tuning of the Fabry-Perot over one free spectral range, a spectrum covering delta-lamda/lamda = 1/7 at a resolution of delta-lamda/lamda = 1/1200 can be acquired. This spectral resolution is sufficient to resolve doppler broadened line emission from external galaxies. FIBRE operates in the 350 micrometer and 450 micrometer bands. These bands cover line emission from the important PDR tracers neutral carbon [CI] and carbon monoxide (CO). We have verified that the multiplexed bolometers are photon noise limited even with the low power present in moderate resolution spectrometry.

Benford, D. J.↗
NASA NTRS1995

On the Abundances of Moderately Volatile Elements in Meteorites

The abundances of the moderately volatile elements (those which condense or evaporate in the temperature range 650-1350 K) in chondritic meteorites deviate from solar (or 'cosmic', as defined by CI meteorite composition) in a manner that strongly suggests that they were determined primarily by volatility, independent of chemical affinity. Wasson and collaborators have long argued that the observed correlation of relative abundance with condensation temperature reflects a systematically selective process which favored the accretion of refractory material over volatile material from a nebula cooling from a hot initial state. This view is supported by the detailed examination of chondrules and matrix in carbonaceous meteorites and the contrast between meteorite trace element abundance patterns with those produced in heating experiments. However, the idea that the moderately volatile abundances reflect global condensation has not previously been quantitatively tested or used as a constraint on nebula evolution. We have therefore constructed models of the solar nebula designed specifically for addressing meteoritic data. Calculations so far indicate that the abundance patterns of the moderately volatile elements in chondritic meteorites can be produced naturally in an evolving nebula that cools with the reduction in opacity associated with the accumulation of meteoritic bodies. Additional information is contained in the original extended abstract.

Cassen, Pat↗
NASA NTRS1995

The FEM-R-Matrix Approach: Use of Mixed Finite Element and Gaussian Basis Sets for Electron Molecule Collisions

For the calculation of electron molecule collision cross sections R-matrix methods automatically take advantage of the division of configuration space into an inner region (I) bounded by radius tau b, where the scattered electron is within the molecular charge cloud and the system is described by an correlated Configuration Interaction (CI) treatment in close analogy to bound state calculations, and an outer region (II) where the scattered electron moves in the long-range multipole potential of the target and efficient analytic methods can be used for solving the asymptotic Schroedinger equation plus boundary conditions.

Thuemmel, Helmar T.↗
NASA NTRS1995

Heating And Cooling of the Interstellar Medium

This talk will review the various heating and cooling processes in the interstellar medium. The most important heating processes include the photoelectric effect on dust grains and PAH molecules. Cooling of the gas in the interstellar medium is dominated by emission in the far infrared fine structure lines of OI, CII, SiII, and CI, and the rotational transitions of CO. Many of these lines have been observed from air borne platform, in particular the Kuiper Airborne Observatory. Which of these heating and cooling processes dominates in a given region depends on its physical and especially chemical conditions, which in turn depend themselves on the interstellar UV radiation field. As a result of the interplay of these processes, the interstellar medium is organized in various structures with distinctly different physical conditions (ie., temperature, density, and degree of ionization). Over the last decade, the dominant cooling lines of the neutral interstellar medium have been studied using the Kuiper Airborne Observatory. Because of sensitivity, these observations have concentrated on dense regions illuminated by strong UV fields from nearby stars, so-called Photo Dissociation Regions (PDRs). These observations allow us to study in detail the interaction of UV photons and the interstellar gas and dust. Detailed theoretical models have been developed for PDRs. These will be reviewed and compared to the observations. Finally, anticipated crucial results of future space-based missions (SIRTF, ISO) will be pointed out.

Tielens, Alexander G. G.↗
NASA NTRS1995

Minimum Energy Pathways for Chemical Reactions

Computed potential energy surfaces are often required for computation of such parameters as rate constants as a function of temperature, product branching ratios, and other detailed properties. We have found that computation of the stationary points/reaction pathways using CASSCF/derivative methods, followed by use of the internally contracted CI method to obtain accurate energetics, gives useful results for a number of chemically important systems. The talk will focus on a number of applications to reactions leading to NOx and soot formation in hydrocarbon combustion.

Walch, S. P.↗
NASA NTRS1996

Cooling of the Interstellar Medium

This talk will review the various heating and cooling processes in the interstellar medium. The most important heating processes include the photoelectric effect on dust grains and PAH molecules. Cooling of the gas in the interstellar medium is dominated by emission in the far infrared fine-structure lines of OI, CII, SiII, and CI, and the rotational transitions of CO. Which of these heating and cooling processes dominates in a given region depends on its physical and especially chemical conditions, which in turn depend themselves on the interstellar UV radiation field. As a result of the interplay of these processes, the interstellar medium is organized in various structures with distinctly different physical conditions (i.e., temperature, density, and degree of ionization). The dominant cooling lines of the neutral interstellar medium have been studied using the Kuiper Airborne Observatory, balloon-borne instruments, and recently space-based missions (IRTS, ISO). These observations have concentrated on dense regions illuminated by strong UV fields from nearby stars, so-called PhotoDissociation Regions (PDRs). These observations allow us to study in detail the interaction of UV photons observations allow us to study in detail the interaction of UV photons and the interstellar gas and dust. Detailed theoretical models have been developed for PDRs. These will be reviewed and compared to the observations. Recent space based missions have measured the dominant cooling line of the galaxy, the [CII] 158 micron line. These observations and their implications will be reviewed.

Tielens, A. G. G. M.↗
NASA NTRS2002

Broadband Direct Detection Submillimeter Spectrometer with Multiplexed Superconducting Transition Edge Thermometer Bolometers

We present performance results based on the first astronomical use of multiplexed superconducting bolometers as direct detectors (i.e., with cold electrons) for spectroscopy. The Fabry-Perot Interferometer Bolometer Research Experiment (FIBRE) is a broadband submillimeter spectrometer for the Caltech Submillimeter Observatory (CSO). FIBRE's detectors are superconducting transition edge sensor (TES) bolometers read out by a SQUID multiplexer. The Fabry-Perot uses a low resolution grating to order sort the incoming light. A linear bolometer array consisting of 16 elements detects this dispersed light, capturing 5 orders simultaneously from one position on the sky. With tuning of the Fabry-Perot over one free spectral range, a spectrum covering Delta lambda/lambda = 1/7 at a resolution of delta lambda/lambda = 1/1200 can be acquired. This spectral resolution is sufficient to resolve Doppler-broadened line emission from external galaxies. FIBRE has been operated in the 350 Am (850 GHz) band. These bands cover line emission from the important star formation tracers neutral carbon [CI] and carbon monoxide (CO).

Benford, D. J.↗
NASA NTRS2003

Blowing in the Wind: II. Creation and Redistribution of Refractory Inclusions in a Turbulent Protoplanetary Nebula

Ca-Al rich refractory mineral inclusions (CAIs) found at 1-6% mass fraction in primitive chondrites appear to be 1-3 million years older than the dominant (chondrule) components which were accreted into the same parent bodies. A prevalent concern is that it is difficult to retain CAIs for this long against gas-drag-induced radial drift into the sun. We reassess the situation in terms of a hot inner (turbulent) nebula context for CAI formation, using analytical models of nebula evolution and particle diffusion. We show that outward radial diffusion in a weakly turbulent nebula can overwhelm inward drift, and prevent significant numbers of CAI-size particles from being lost into the sun for times on the order of 10(exp 6) years. CAIs can form early, when the inner nebula was hot, and persist in sufficient abundance to be incorporated into primitive planetesimals at a much later time. Small (less than or approx. equal to 0.1 mm diameter) CAIs persist for longer times than large (greater than or approx. equal to 5mm diameter ones). To obtain a quantitative match t o the observed volume fractions of CAIs in chondrites, another process must be allowed for: a substantial enhancement of the inner hot nebula in silicate-forming material, which we suggest was caused by rapid inward drift of meter-sized objects. This early in nebula history, the drifting rubble would have a carbon content probably an order of magnitude larger than even the most primitive (CI) carbonaceous chondrites. Abundant carbon in the evaporating material would help keep the nebula oxygen fugacity low, plausably solar, as inferred for the formation environment of CAIs. The associated production of a larger than canonical amount of CO2 might also play a role in mass-independent fractionation of oxygen isotopes, leaving the gas rich in l60 as inferred from CAIs and other high temperature condensates.

Cuzzi, Jeffrey N.↗
NASA NTRS2003

Blowing in the Wind: II. Creation and Redistribution of Refractory Inclusions in a Turbulent Protoplanetary Nebula

Ca-A1 rich refractory mineral inclusions (CAIs) found at 1-6% mass fraction in primitive chondrites appear to be 1-3 million years older than the dominant (chondrule) components which were accreted into the same parent bodies. A prevalent concern is that it is difficult to retain CAIs for this long against gas-drag-induced radial drift into the sun. We reassess the situation in terms of a hot inner (turbulent) nebula context for CAI formation, using analytical models of nebula evolution and particle diffusion. We show that outward radial diffusion in a weakly turbulent nebula can overwhelm inward drift, and prevent significant numbers of CAI-size particles from being lost into the sun for times on the order of 10(exp 6) years. CAIs can form early, when the inner nebula was hot, and persist in sufficient abundance to be incorporated into primitive planetesimals at a much later time. Small (less than or approximately 0.1 mm diameter) CAIs persist for longer times than large (greater than or approximately 5mm diameter ones. To obtain a quantitative match to the observed volume fractions of CAIs in chondrites, another process must be allowed for: a substantial enhancement of the inner hot nebula in silicate-forming material, which we suggest was caused by rapid inward drift of meter-sized objects. This early in nebula history, the drifting rubble would have a carbon content probably an order of magnitude larger than even the most primitive (CI) carbonaceous chondrites. Abundant carbon in the evaporating material would help keep the nebula oxygen fugacity low, plausibly solar; as inferred for the formation environment of CAIs. The associated production of a larger than canonical amount of CO2 might also play a role in mass-independent fractionation of oxygen isotopes, leaving the gas rich in O-16 as inferred from CAIs and other high temperature condensates.

Cuzzi, Jeffrey N.↗