Search

Shock-darkening, the melting of metals and iron sulfides into a network of veins within silicate grains, altering reflectance spectra of meteorites, was previously studied using shock physics mesoscale modeling. Melting of iron sulfides embedded in olivine was observed at pressures of 40-50 GPa. This pressure range is at the transition between shock stage 5 (C-S5) and 6 (C-S6) of the shock metamorphism classification in ordinary and enstatite chondrites. To better characterize C-S5 and C-S6 with a mesoscale modeling approach and assess post-shock heating and melting, we used multi-phase (i.e. olivine/enstatite, troilite, iron, pores, and plagioclase) meshes with realistic configurations of grains. We carried out a systematic study of shock compression in ordinary and enstatite chondrites at pressures between 30 and 70 GPa. To setup mesoscale sample meshes with realistic silicate, metal, iron sulfide, and open pore shapes, we converted backscattered electron microscope images of three chondrites. The resolved macroporosity in meshes was 3-6%. Transition from shock C-S5 to C-S6 was observed through (1) the melting of troilite above 40 GPa with melt fractions of similar to 0.7-0.9 at 70 GPa, (2) the melting of olivine and iron above 50 GPa with melt fraction of similar to 0.001 and 0.012, respectively, at 70 GPa, and (3) the melting of plagioclase above 30 GPa (melt fraction of 1, at 55 GPa). Post-shock temperatures varied from similar to 540 K at 30 GPa to similar to 1300 K at 70 GPa. We also constructed models with increased porosity up to 15% porosity, producing higher post-shock temperatures (similar to 800 K increase) and melt fractions (similar to 0.12 increase) in olivine. Additionally we constructed a pre-heated model to observe post-shock heating and melting during thermal metamorphism. This model presented similar results (melting) at pressures 10-15 GPa lower compared to the room temperature models. Finally, we demonstrated dependence of post-shock heating and melting on the orientation of open cracks relative to the shock wave front. In conclusion, the modeled melting and post-shock heating of individual phases were mostly consistent with the current shock classification scheme (Stoffler et al., 1991, 2018).

The surfaces of airless planetary bodies are subject to a barrage of charged particles, photons, and meteoroids. This high-energy space environment alters the surfaces and creates a tenuous atmosphere of ejected particles surrounding the celestial bodies. Experiments with well characterized analogue materials under controlled laboratory conditions are needed to interpret the observations of these atmospheres and improve composition models of such bodies. This study presents methods to create and analyze mineral powder pellets for ion irradiation experiments relevant for rocky planetary bodies including the Moon and Mercury. These include the pyroxenes diopside and enstatite, the plagioclase labradorite and the non-analogue pyroxenoid wollastonite. First ion irradiation experiments with diopside, enstatite and wollastonite pellets were performed under UHV with 4 keV He+ at fluences of several 1021 ions m−2 (~100 and ~1000 years for Mercury and the Moon, respectively). The pellet's thermal IR reflectance properties were compared before and after irradiation showing monotonously shifting IR spectral features between 7 − 14 μm towards higher wavelengths. For all irradiated pellets, Reststrahlen bands shifted by ~0.03 μm. Surface abrasion was found to remove the sputter effect, which is restricted to the top few tens of nm of the surface. Additionally, ion irradiation experiments were performed in a quartz crystal microbalance catcher setup, where the mass sputtered from pellets was monitored. This proves, that the presented sample preparation method allows the study of irradiation induced sputtering and surface alteration on the surfaces of rocky planets under laboratory conditions.

Ground Penetrating Radar (GPR) is a terrestrial geophysical exploration method that has recently become one of the most promising technique for planetary, asteroidal and cometary subsurface exploration. The capability of GPR to sound Solar System's bodies relies on the electromagnetic properties of the constitutive materials. Enstatite and ordinary chondrites represent class of asteroids occurring in the inner asteroid belt whereas carbonaceous chondrites and their icy mixtures are reasonable analogues for cometary material as well as constituent of shallow part of some Jovian satellite crusts. Therefore, the knowledge of electromagnetic properties of meteorites is very important because it allows to estimate the radar response in terms of signal velocity and attenuation. In this work we measured the real and imaginary parts of the permittivity of a L5 chondrite meteorite as a function of frequency (20 Hz-1 MHz) by using a capacitive cell connected to a self-balancing bridge. We studied the spatial variability of dielectric properties of the sample that exhibits areas with different textures characterized by a darker appearance. In general, the meteorite sample shows a stronger dispersive behavior compared to terrestrial rocks with higher values for both real and imaginary part of permittivity. In particular, the occurrence of very small grains ( Ni metal, troilite and chromite scattered in some areas of the meteorite seems to be the cause of such behavior.

Chondrites are the likely building blocks of Earth, and identifying the group of chondrite that best represents Earth is a key to resolving the state of the early Earth. The origin of chondrites, however, remains controversial partly because of their puzzling major element compositions, some exhibiting depletion in Al, Ca, and Mg. Based on a new thermochemical evolution model of protoplanetary disks, we show that planetesimals with depletion patterns similar to ordinary and enstatite chondrites can originate at 1–2 AU outside where enstatite evaporates. Around the “evaporation front” of enstatite, the large inward flow of refractory minerals, including forsterite, takes place with a high pebble concentration, and the loss of those minerals results in depletion in Al, Ca, and Mg. The fractionation driven by the loss of forsterite would also create a complementary Mg-rich reservoir just inside the depleted region, creating two chemically distinct reservoirs adjacent to each other. The region around the evaporation front of enstatite has the highest dust concentration inside the snow line, and thus the streaming instability is most likely to be triggered therein. Planetesimals with two different major element compositions could naturally be created in the terrestrial region, which could evolve into parent bodies for Earth and chondrites. This can explain why Earth and enstatite chondrites share similar isotopic signatures but have different bulk compositions.

Salt-rich deposits may be more widespread on planetary surfaces than is generally appreciated. Remote observations, laboratory studies of meteorites, and cosmochemical constraints all point towards widespread occurrences of salts (including halides, sulfates, and (bi)carbonates) on asteroids, icy bodies, Mars, and elsewhere. We have investigated the mid-infrared (1.8–25 μm) reflectance spectral properties of mixtures of chondritic (ordinary, enstatite and carbonaceous) meteorites with potassium bromide; a mid-infrared transmissive salt like all halides. Our results demonstrate that halide-chondrite mixtures provide spectral signatures that either reveal the presence of transmissive materials or provide evidence for highly porous regolith. Previously, the nature of the surfaces of the asteroids 624 Hektor and 21 Lutetia was inferred using a limited range of spectra from halide-chondrite mixtures. Here, we provide an extensive dataset of halide-chondrite mixtures to encompass a wider set of possible surface compositions.

The Hungaria asteroids are interior to the main asteroid belt, with semimajor axes between 1.8 and 2 AU, low eccentricities and inclinations of 16-35 degrees. Small asteroids in the Hungaria region are dominated by a collisional family associated with (434) Hungaria. The dominant spectral type of the Hungaria group is the E or X-type (Warner et al, 2009), mostly due to the E-type composition of Hungaria and its genetic family. It is widely believed the E-type asteroids are related to the aubrite meteorites, also known as enstatite achondrites (Gaffey et al, 1992). Here we explore the hypothesis that aubrites originate in the Hungaria family. In order to test this connection, we compare model Cosmic Ray Exposure ages from orbital integrations of model meteoroids with those of aubrites. We show that long CRE ages of aubrites (longest among stony meteorite groups) reflect the delivery route of meteoroids from Hungarias to Earth being different than those from main-belt asteroids. We find that the meteoroids from Hungarias predominantly reach Earth by Yarkovsky-drifting across the orbit of Mars, with no assistance from orbital resonances. We conclude that the CRE ages of aubrites are fully consistent with a dominant source at the inner boundary of the Hungaria family at 1.7 AU. From here, meteoroids reach Earth through the Mars-crossing region, with relatively quick delivery times favored due to collisions (with Hungarias and the inner main-belt objects). We find that, after Vesta, (434) Hungaria is the best candidate for an asteroidal source of an achondrite group., Comment: Accepted for Icarus

We present a method to constrain the albedo and diameters of near-Earth asteroids (NEAs) based on thermal flux in their near-infrared spectra (0.7–2.5 μm) using the Standard Thermal Model. Near-infrared spectra obtained with the SpeX instrument on NASA Infrared Telescope Facility are used to estimate the albedo and diameters of 12 NEAs (1992 JE, 1992 UY4, 1999 JD6, 2004 XP14, 2005 YY93, 2007 DS84, 2005 AD13, 2005 WJ56, 1999 JM8, 2005 RC34, 2003 YE45, and 2008 QS11). Albedo estimates were compared with average albedo for various taxonomic classes outlined by Thomas et al. (Thomas, C.A. et al. [2011]. Astron. J. 142(3)) and are consistent with their results. Spectral band parameters, like band centers, are derived and compared to spectra of laboratory mineral mixtures and meteorites to constrain their composition and possible meteorite analogs. Based on our study we estimate the albedos and diameters of these NEAs and compare them with those obtained by other techniques such as ground-based mid-infrared, Spitzer thermal infrared and Arecibo radar. Our results are broadly consistent with the results from other direct methods like radar. Determining the compositions of low albedo asteroids is a challenge due to the lack of deep silicate absorption features. However, based on weak absorption features and albedo, we suggest possible meteorite analogs for these NEAs, which include black chondrites, CM2 carbonaceous chondrites and enstatite achondrites. We did not find any specific trends in albedo and composition among the asteroids we observed.

Using the S-band radar at Arecibo Observatory, we observed six new M-class main-belt asteroids (MBAs), and re-observed one, bringing the total number of Tholen M-class asteroids observed with radar to 19. The mean radar albedo for all our targets is σ ˆ OC = 0.28 ± 0.13 , significantly higher than the mean radar albedo of every other class (Magri, C., Nolan, M.C., Ostro, S.J., Giorgini, J.D. [2007]. Icarus 186, 126–151). Seven of these objects (Asteroids 16 Psyche, 129 Antigone, 216 Kleopatra, 347 Pariana, 758 Mancunia, 779 Nina, 785 Zwetana) have radar albedos indicative of a very high metal content ( mean σ ˆ OC = 0.41 ± 0.13 ) , and consistent with a remnant iron/nickel core interpretation (irons) or exotic high metal meteorite types such as CB. We propose designating these high radar albedo objects as Mm. Two asteroids, 110 Lydia and 678 Fredegundis, have more moderate radar albedos ( mean σ ˆ OC = 0.22 ) , but exhibit high values ( σ ˆ OC ∼ 0.35 ) at some rotation phases suggesting a significant metal content. The remaining 10 objects have moderate radar albedos ( σ ˆ OC = 0.20 ± 0.06 ) at all rotation phases. Most of our targets have visible/near-infrared spectra (Hardersen, P.S., Gaffey, M.J., Abell, P.A. [2005]. Icarus 175, 141–158; Fornasier, S., Clark, B.E., Dotto, E., Migliorini, A., Ockert-Bell, M., Barucci, M.A. [2009]. Icarus, submitted for publication) that indicate the presence of at least some silicate phases. All of the non-Mm asteroids show a positive correlation between visual and radar albedo but the reasons for this are not clear. All of the higher radar albedo targets (the 7 Mm asteroids, Lydia, and Fredegundis) show moderate to large variations in radar albedo with rotation phase. We suggest that their high radar reflectivity exaggerates irregularities in the asteroid shape to cause this behavior. One-third of our targets show evidence for asteroid-scale concavities or bifurcation. Based on all the evidence available, we suggest that most Tholen M-class asteroids are not remnant iron cores or enstatite chondrites, but rather collisional composites of silicates and irons with compositions more analogous to stony-iron meteorites and high-iron carbonaceous chondrites.

We have measured the thermal conductivity at low temperatures (5–300 K) of six meteorites representing a range of compositions, including the ordinary chondrites Cronstad (H5) and Lumpkin (L6), the enstatite chondrite Abee (E4), the carbonaceous chondrites NWA 5515 (CK4 find) and Cold Bokkeveld (CM2), and the iron meteorite Campo del Cielo (IAB find). All measurements were made using a Quantum Design Physical Properties Measurement System, Thermal Transport Option (TTO) on samples cut into regular parallelepipeds of ∼2–6 mm dimension. The iron meteorite conductivity increases roughly linearly from 15 W m−1 K−1 at 100 K to 27 W m−1 K−1 at 300 K, comparable to typical values for metallic iron. By contrast, the conductivities of all the stony samples except Abee appear to be controlled by the inhomogeneous nature of the meteorite fabric, resulting in values that are much lower than those of pure minerals and which vary only slightly with temperature above 100 K. The L and CK sample conductivities above 100 K are both about 1.5 W m−1 K−1, that of the H is 1.9 W m−1 K−1, and that of the CM sample is 0.5 W m−1 K−1; by contrast the literature value at 300 K for serpentine is 2.5 W m−1 K−1 and those of enstatite and olivine range from 4.5 to 5 W m−1 K−1 (which is comparable to the Abee value). These measurements are among the first direct measurements of thermal conductivity for meteorites. The results compare well with previous estimates for meteorites, where conductivity was derived from diffusivity measurements and modeled heat capacities; our new values are of a higher precision and cover a wider range of temperatures and meteorite types. If the rocky material that makes up asteroids and provides the dust to comets, Kuiper Belt objects, and icy satellites has the same low thermal conductivities as the ordinary and carbonaceous chondrites measured here, this would significantly change models of their thermal evolution. These values would also lower their thermal inertia, thus affecting the Yarkovsky and YORP evolution of orbits and spin for solid objects; however, in this case the effect would not be as great, as thermal inertia only varies as the square root of the conductivity and, for most asteroids, is controlled by the dusty nature of asteroidal surfaces rather than the conductivity of the material itself.

The high average density and low surface FeO content of the planet Mercury are shown to be consistent with very low oxygen fugacity during core segregation, in the range 3–6 log units below the iron–wustite buffer. These low oxygen fugacities, and associated high metal content, are characteristic of high-iron enstatite (EH) and Bencubbinite (CB) chondrites, raising the possibility that such materials may have been important building blocks for this planet. With this idea in mind we have explored the internal structure of a Mercury sized planet of EH or CB bulk composition. Phase equilibria in the silicate mantle have been modeled using the thermodynamic calculator p-MELTS, and these simulations suggest that orthopyroxene will be the dominant mantle phase for both EH and CB compositions, with crystalline SiO 2 being an important minor phase at all pressures. Simulations for both compositions predict a plagioclase-bearing “crust” at low pressure, significant clinopyroxene also being calculated for the CB bulk composition. Concerning the core, comparison with recent high pressure and high temperature experiments relevant to the formation of enstatite meteorites, suggest that the core of Mercury may contain several wt.% silicon, in addition to sulfur. In light of the pressure of the core–mantle boundary on Mercury (∼7 GPa) and the pressure at which the immiscibility gap in the system Fe–S–Si closes (∼15 GPa) we suggest that Mercury’s core may have a complex shell structure comprising: (i) an outer layer of Fe–S liquid, poor in Si; (ii) a middle layer of Fe–Si liquid, poor in S; and (iii) an inner core of solid metal. The distribution of heat-producing elements between mantle and core, and within a layered core have been quantified. Available data for Th and K suggest that these elements will not enter the core in significant amounts. On the other hand, for the case of U both recently published metal/silicate partitioning data, as well as observations of U distribution in enstatite chondrites, suggest that this element behaves as a chalcophile element at low oxygen fugacity. Using these new data we predict that U will be concentrated in the outer layer of the mercurian core. Heat from the decay of U could thus act to maintain this part of Mercury’s core molten, potentially contributing to the origin of Mercury’s magnetic field. This result contrasts with the Earth where the radioactive decay of U represents a negligible contribution to core heating.

Growth conditions and the resultant morphology of cosmic forsterite and enstatite particles condensed from the vapor were investigated experimentally as a function of growth temperature and supersaturation using flash-heating by CO2 laser irradiation. Forsterite particles grew dominantly, followed by a few pyroxenes. No silica minerals were observed. The forsterite morphology changed systematically depending on the temperature (T) and the vapor supersaturation (σ). As the temperature decreased and the vapor supersaturation increased, the forsterite morphology changed from a bulky type ( T = 1000 – 1450 °C , σ 97 ) to a platy type ( T = 700 – 1000 °C , σ = 97 – 161 ), then to a columnar needle shape ( T = 500 – 820 °C , σ = 131 – 230 ), and finally to a droplet type ( T 500 °C , σ > 230 ). Very thin polygonal growth steps, which suggest vapor–solid (VS) growth, were detected on the surface of the faceted bulky and platy forsterite particles. Transmission electron microscopic (TEM) observation showed that the tip of the columnar and needle-shaped forsterite was covered with an amorphous layer. This amorphous coverage illustrates that a liquid phase can be condensed from the vapor, even under the stable conditions where crystalline forsterite is stable. This amorphous layer plays a role in the vapor–liquid–solid (VLS) mechanism of the forsterite particles. The difference in the growth mechanism as a function of vapor supersaturation and temperature can be explained by metastable liquid condensation. The experimentally synthesized coexisting patterns of VS-grown and VLS-grown olivine particles resemble the pattern of matrix olivine or chondrule rims in primitive meteorites. Forsterite can crystallize more easily than either pyroxene or silica minerals, which is consistent with the dominance of forsterite in cometary dust particles, as suggested by its infrared spectrum.

We have conducted a radar-driven observational campaign of main-belt asteroids (MBAs) focused on X/M class asteroids using the Arecibo radar and NASA Infrared Telescope Facilities (IRTF). M-type asteroids have been identified as metallic, enstatite chondrites and/or heavily altered carbonaceous chondrites [Bell, J.F., Davis, D., Hartmann, W.K., Gaffey, M.J., 1989. In: Binzel, R.P., Gehrels, T., Matthews, M.S. (Eds.), Asteroids II. Univ. of Arizona Press, Tucson, pp. 921–948; Gaffey, M.J., McCord, T.B., 1979. In: Gehrels, T., Matthews, M.S. (Eds.), Asteroids. Univ. of Arizona Press, Tucson, pp. 688–723; Vilas, F., 1994. Icarus 111, 456–467]. Radar wavelength observations can determine whether an asteroid is metallic and provide information about the porosity and regolith depth. Near-infrared observations can help determine the grain size, porosity and composition of an object. Concurrent observations with these tools can give us a wealth of information about an object. Our objectives for this observation program were to (a) determine if there are any consistent relationships between spectra in the near-infrared wavelengths and radar signatures and (b) look for rotationally resolved relationships between asteroid radar properties and near-infrared spectral properties. This paper describes preliminary results of an ongoing survey of near-infrared observations of M-type asteroids and is a companion paper to radar observations reported by Shepard [Shepard, M.K., and 19 colleagues, 2008a. Icarus 195, 184–205]. In the analysis of 16 asteroid near-infrared spectra and nine radar measurements, we find a trend indicating a correlation between continuum slope from 1.7 to 2.45 μm and radar albedo—an asteroid with a steep continuum slope also has a bright radar albedo, which suggests a significant metal content. This may provide a means to use near-IR observations to predict the most likely metallic candidates for radar studies.

A total of 56 non-porphyritic pyroxene and pyroxene/olivine micro-objects from different unequilibrated ordinary chondrites were selected for detailed studies to test the existing formation models. Our studies imply that the non-porphyritic objects represent quickly quenched liquids with each object reflecting a very complex and unique evolutionary history. Bulk major element analyses, obtained with EMPA and ASEM, as well as bulk lithophile trace element analyses, determined by LA-ICP-MS, resulted in unfractionated (solar-like) ratios of CaO/Al2O3, Yb/Ce as well as Sc/Yb in many of the studied objects and mostly unfractionated refractory lithophile trace element (RLTE) abundance patterns. These features support an origin by direct condensation from a gas of solar nebula composition. Full equilibrium condensation calculations show that it is theoretically possible that pyroxene-dominated non-porphyritic chondrules with flat REE patterns could have been formed as droplet liquid condensates directly from a nebular gas strongly depleted in olivine. Thus, it is possible to have enstatite as the stable liquidus phase in a 800 × Cl dust-enriched nebular gas at a p tot of 10−3 atm, if about 72% of the original Mg is removed (as forsterite?) from the system. Condensation of liquids from vapor (primary liquid condensation) could be considered as a possible formation process of the pyroxene-dominated non-porphyritic objects. This process can produce a large spectrum of chemical compositions, which always have unfractionated RLTE abundances. Late stage and subsolidus metasomatic events appear to have furthered the compositional diversity of chondrules and related objects by addition of moderately volatile and volatile elements to these objects by exchange reactions with the chondritic reservoir (e.g., V, Cr, Mn, FeO as well as K and Na). The strong fractionation displayed by the volatile lithophile elements could be indicative of a variable efficiency of metasomatic processes occurring during and/or after chondrule formation. Histories of individual objects differ in detail from each other and clearly indicate individual formation and subsequent processing.

Near-Earth Asteroid (29075) 1950 DA may closely encounter Earth in 2880. The probability of Earth impact may be as high as 1/300, but the outcome of the encounter depends critically on the physical properties of the asteroid [Giorgini et al., 2002. Science 196, 132–136]. We have used Arecibo and Goldstone radar data and optical lightcurves to estimate the shape, spin state, and surface structure of 1950 DA. The data allow two distinct models. One rotates prograde and is roughly spheroidal with mean diameter 1.16 ± 0.12 km . The other rotates retrograde and is oblate and about 30% larger. Both models suggest a nickel–iron or enstatite chondritic composition. Ground-based observations should be able to determine which model is correct within the next several decades.

The first verifiable near-infrared absorption features in the ∼0.9-μm spectral region are reported for Asteroids 16 Psyche, 69 Hesperia, 110 Lydia, 125 Liberatrix, 201 Penelope, and 216 Kleopatra. These weak features ( ∼ 1 – 3 % ) are attributed to orthopyroxenes present on the surfaces of these asteroids. 16 Psyche and 125 Liberatrix have full rotational coverage while 69 Hesperia, 110 Lydia, 201 Penelope, and 216 Kleopatra have ∼ 75 % rotational coverage. Qualitative ∼2-μm absorption features are present, but are very weak ( 1 % ). Absorption band positions suggest relatively low abundances of calcium and iron in the pyroxenes. This indicates relatively reducing redox conditions for these asteroids, their parent bodies, and the nebular regions in which they formed. Four potential interpretations for these asteroids include: (1) they are exposed metallic cores or core fragments of differentiated parent bodies with residual orthopyroxene mantle material, (2) they are the result of a smelting-like reaction that converts olivine to pyroxene and metallic iron in the presence of carbon at high temperatures, (3) they are analogs to the primitive metal-rich Bencubbinite meteorites, or (4) they represent metallic surfaces which have accumulated silicate debris from external sources. Of the two original interpretations for the M-asteroids, the enstatite chondrite interpretation (Chapman and Salisbury, 1973, Icarus 19, 507–522; Gaffey and McCord, 1979, Mineralogical and petrological characterizations of asteroids. In: Gehrels T. (Ed.), Asteroids. Univ. of Arizona Press, Tucson, pp. 688–723) can be eliminated for these asteroids because the pyroxene in enstatite chondrites is iron-free and does not exhibit such absorption features. The iron meteorite interpretation remains valid, but with modification. For M-Asteroids 16 Psyche and 216 Kleopatra, these spectral results combined with previous determinations of high radar albedos indicate that these bodies are most probably exposed core fragments of differentiated bodies. M-Asteroids 69 Hesperia, 110 Lydia, 125 Liberatrix, and 201 Penelope exhibit similar spectral features consistent with exposed core fragments, but radar observations would be needed to confirm a high metal abundance. Observations of M-Asteroids 136 Austria and 325 Heidelberga suggest the absence of absorption features in the ∼0.4- to ∼2.5-μm region within the scatter of the data. Verification of the presence or absence of features across the surfaces of these two asteroids requires full rotational coverage. The interpretations for these “featureless” M-asteroids are not well-constrained, but remain consistent with the iron meteorite and enstatite chondrite interpretations.

The mid-infrared spectral evolution of amorphous metastable eutectic magnesiosilica smokes, obtained by kinetically controlled gas-to-solid condensation of a Mg–SiO–O2–H2 vapor, proceeded in three distinct phases as a function of increasing time and temperature. This paper reports the mineralogical and chemical properties of these same samples. We found a previously unanticipated size dependence of the petrologic development of the initially amorphous magnesiosilica smokes that may also be at least partially responsible for the previously reported spectroscopic changes. Condensed grains less than approximately 20 nm in diameter remained amorphous throughout the thermal annealing experiment. Mineralogical changes occurred only after fusion of condensed magnesiosilica grains and chemical homogenization of large amorphous agglomerates and ring structures. Kinetically favored nucleation and growth produced the thermodynamically unstable nanocrystalline assemblage forsterite + tridymite. Further mineralogical development was stalled until continued fusing of agglomerates, rings, and some fraction of condensed grains had produced smooth amorphous magnesiosilica sheets of 42 and 20 wt% MgO. In rare sheets with more than ∼55 wt% MgO large forsterite crystals had grown, while enstatite had nucleated in low-MgO sheets still in the presence of forsterite and tridymite. The mineralogical evolution of the samples is critically dependent on the mass of the structural entities in the condensed sample and seems to be restricted to fused agglomerates and ring structures larger than about 20 nm in diameter and the sheet materials. We discuss the implications of our study for the interpretation of similar astrophysical dust analog studies and for astrophysical applications.

A bright, slow moving, bolide observed during the 1989 AIDA campaign is shown to have been a very unusual near-Earth meteoroid. Data were obtained with five different instruments and these observations have now been extensively analyzed. Various indicators point to a stony object and spectroscopic observations in the near IR show two Fe I emissions with an excitation temperature of 3100 K. A third feature due to Na I gives an abundance ratio of Na to Fe of 1:1 by number. Ionized calcium (which is often strong in the near UV), manganese, and cobalt appear to have been weak or absent. Based on the spectrum analysis, we conclude that the object was an enstatite meteoroid and the lack of Mn compared with Fe and Na suggests an aubrite composition. The lightcurve, aerodynamic analysis, and video images show the object underwent extensive fragmentation at virtually all points of its trajectory.

Visible and near-IR observations are reported which confirm that Apollo asteroid 3103 (1982 BB) is a high-albedo E-type object with a reflectance spectrum consistent with the iron-free silicates, primarily enstatite, present in the aubrites. Visible and thermal IR light curves indicate that 3103 is an elongated object with no evidence of substantial surface albedo variations. It is presently in an orbital resonance with earth and is probably a relatively long-lived member of the earth-approaching population. This is consistent with the relatively long cosmic ray exposure ages of the aubrites compared to other stony meteorites. The fall geometries of the aubrites are consistent with their being derived from 3103. The ubiquitous brecciated nature of the aubrites and the indications of a single parent body for nearly all these meteorite suggest that 3103 is the sole near-earth parent body of this meteorite type.

Optical complex indices of refraction for the iron-sulfur semiconductors troilite and pyrrhotite have been measured between 0.3 and 1.1 μm, and at 1.5 and 3.0 cm in the microwave region. The behavior of the absorptive and refractive components of the index in the visual and ultraviolet regions suggests a reasonable match to published data on Saturn's rings. A combination of the iron sulfur with water ice and the orthopyroxene enstatite is consistent with an equilibrium condensation model for the formation of the solar system; the water ice and an orthopyroxene are also suggested by near-infrared observational data. A combination of these materials could explain all spectral features seen in the ring spectra to date. The microwave behavior of a small quantity of troilite embedded in water ice at low temperatures is consistent with the radar reflectivity and radio emissivity data.

Infrared transmission spectra from 53 meteorites in the spectral range from 2.5 to 25 μm were measured to permit comparisons with data of astronomical objects that are potential meteorite sources. Data were taken for 14 carbonaceous chondrites, 5 LL ordinary chondrites, 6 L ordinary chondrites, 10 H ordinary chondrites, 1 enstatite chondrite, 4 aubrites, 3 eucrites, 4 howardites, 1 diogenite, 1 mesosiderite, 2 nakhlites, 1 shergottite, and the anomalous achondrite Angra dos Reis. The CO and CV carbonaceous chondrites have spectra similar to each other, with 10-μm features characteristic of olivine. The CM carbonaceous chondrites have distinctive 10-μm features that are attributed to layer lattice silicates. Members of both the CI and CR classes have spectra distinct from those of other carbonaceous chondrites. The LL, L, and H ordinary chondrites have spectra that match those of olivine and pyroxene mixtures. The enstatite chondrites and enstatite achondrites (aubrites) all exhibit spectra diagnostic of the pyroxene enstatite. The angrite, howardites, aucrites, nakhlites, shergottite, and diogenite all have similar spectra also dominated by pyroxene. The single mesosiderite examined had a spectrum distinct from all the other meteorites.

It is noted that Cr, Mg, and Si are fractionated in chondritic material along with, but to a lesser extent than, a large group of more refractory elements. These patterns are reexamined with the reasoning that this might imply some unique distribution at the time of fractionation. It is shown that apparently two distinct fractionization patterns can be resolved: one involving ordinary and enstatite chondrites and the other involving carbonaceous chondrites, the earth, the moon, and the eucrite parent body. It is noted that the two trends intersect at C1 composition. Attention is given to the range in which the largest fraction of planetary matter condenses. It is concluded that the appearance of large amounts of condensed materials is conceivably in some way related to the fractionization process, although the exact relationship cannot be specified.

Shock wave and thermodynamic data for rock-forming and volatile-bearing minerals are used to determine minimum impact velocities (v_(cr)) and minimum impact pressures (p_(cr)) required to form a primary H_2O atmosphere during planetary accretion from chondritelike planetesimals. The escape of initially released water from an accreting planet is controlled by the dehydration efficiency. Since different planetary surface porosities will result from formation of a regolith, v_(cr) and p_(cr) can vary from 1.5 to 5.8 km/sec and from 90 to 600 kbar, respectively, for target porosities between 0 and ∼45%. On the basis of experimental data, hydration rates for forsterite and enstatite are derived. For a global regolith layer on the Earth's surface, the maximum hydration rate equals 6 × 10^(10) g H_2O sec^(−1) during accretion of the Earth. Attenuation of impact-induced shock pressure is modeled to the extent that the amount of released water as a function of projectile radius, impact velocity, weight fraction of water in the target, target porosity, and dehydration efficiency can be estimated. The two primary processes considered are the impact release of water bound in hydrous minerals (e.g., serpentine) and the subsequent reincorporation of free water by hydration of forsterite and enstatite. These processes are described in terms of model calculations for the accretion of the Earth. Parameters which lead to a primary atmosphere/hydrosphere are: an accretion time of ⪅ 1.6 × 10^8years, the use of an accretion model defined by Weidenschilling (1974, 1976), a mean planetesimal radius of 0.5 km, a hydration rate of 6 × 10^(10) g H_2O sec^(−1) inferred from a mean porosity of ∼ 10% for the upper 1 km of the accreting Earth, and values for the dehydration efficiency, DE, of 0.55 and 0.07 for the maximum and minimum pressure decay model, respectively. Conditions which prohibit the formation of a primary atmosphere include an accretion time much longer than 1.6 × 10^8 years, a hydration rate for forsterite and enstatite well in excess of 6 × 10^(10) g H_2O sec^(−1), and a dehydration efficiency DE < 0.07. We conclude that the concept of dehydration efficiency is of dominant importance in determining the degree to which an accreting planet acquires an atmosphere during its formation.

Pyroxene and plagioclase mixtures' spectral characteristics are investigated with a view to particle size and mineral proportion effects in the cases of variously proportioned labradorite/enstatite compositional mixtures. The amount of plagioclase that will allow its band to be observed in discrete absorption is particle size-dependent; plagioclase is detectable in the flattening of the pyroxene reflectance peak that lies between the 0.9 and 1.9 micron absorption bands, if sufficient plagioclase is present. Attention is given to the combined effects of mineral proportions and particle size for plagioclase/pyroxene mixtures in the case of analogs of mature and immature lunar highland soils.

A wide program of photometric and polarimetric observations for the largest M-type asteroids was carried out at the Astronomical Observatory of Kharkov University in 1978–1986. In order to interpret these and other observations we have made laboratory photometric and polarimetric measurements of the samples of meteorites, terrestrial silicates, and metals. All samples, including 13 meteorites (iron and all main types of chondrites and achondrites), have similar surface structure with grain sizes under 50 μm. Analysis of all available telescopic and laboratory data shows that surfaces of at least the largest M-type asteroids 16, 21, 22, 69, and 110 cannot consist of pure metal and contain considerable silicate-component-like stony-iron and enstatite chondrite meteorites.

It is pointed out that the primitive solar nebula may be modeled using the frictionally induced transport theory of Lynden-Bell and Pringle (1974) if the principal frictional mechanism within the nebula is turbulent viscosity. The present investigation is concerned with the construction of a model of a section of the primitive solar nebula as a basis for the study of nucleation and condensation processes within this section. The construction involves a relatively simple application of the Lynden-Bell and Pringle theory subject to steady mass flow conditions. The calculations which are conducted in connection with the investigation indicate that by the time the gas in the primitive solar nebula has become sufficiently supercooled to nucleate condensation centers, several different compounds, including the magnesium silicates forsterite and enstatite (MgSiO3), will probably be able to condense on the growing condensation center.

Na and Mn homogeneity in chondritic meteorites, discussing accretion processes, metamorphisms, weathering effects and specimens size

The 129 I− 129 Xe formation intervals of the Bjurbole, Bruderheim, Bruderheim chondrules, Renazzo, and the dark and light regions of Pantar are calculated by comparing the amount of radiogenic 129 Xe associated with iodine in each thermal fraction. Unlike the sharp 129 I− 129 Xe isochronism reported for the high-temperature minerals, the lower-temperature minerals show progressively longer formation intervals. A plot of formation times versus extraction temperatures yields an apparent 129 I− 129 Xe cooling rate for these chondrites. Below 800°C all of the chondrites except Renazzo and Pantar Dark display 129 I− 129 Xe cooling rates of 5–9°C per million years. Pantar Dark and Renazzo appear to have cooled rapidly to 600° and 400°C, respectively. An inversion of the I−Xe correlation of chondrites is noted in the slow temperature minerals of Shallowater. This enstatite achondrite displays a maximum 129 I− 129 Xe cooling rate of 2.4°C per million years.

Laboratory spectral reflectivities have been measured for 41 meteorites, including 22 ordinary chondrites of all chemical and petrologic types. They are compared with available spectral reflectivities for 36 asteroids. We find that absorption band depths, center positions, and breadths are diagnostic of the various types of ordinary chondrites, and that other meteorite types may also be distinguished by reflectivity characteristics. Asteroid spectral reflectivities approximately resemble those for meteorites (especially unequilibrated types), and the range of differences among asteroids is similar to that for meteorites. However, detailed comparisons show that few meteorites exactly match observed asteroids in spectral properties. Impact vitrification and shock can modify the spectral reflectance of materials, but vitrification, at least, is an unimportant process on asteroidal regoliths. Assuming the comparisons can be taken at face value, we conclude that ordinary chondrites either do not come from the main asteroid belt or they come from a few unusual asteroids. Some asteroidal/meteorite matches have been found for enstatite chondrites, a basaltic achondrite, an optically unusual L6 chondrite, and possibly a carbonaceous chondrite. Only 1685 Toro, an Earth-crossing asteroid, resembles typical ordinary chondrites.

A piece of the Odessa iron meteorite (a coarse octahedrite) was found to contain two types of nodules: globular and elongated ones. The globular nodules have a core of troilite surrounded by a shell of graphite and/or fine-grained graphite-troilite mixture containing some daubreelite, and a thin outer layer of schreibersite and cohenite, in some cases with further localization of schribersite and cohenite to one side. The relative proportions of the troilite and the intimate mixture of graphite and troilite vary greatly. The miniral content of the elongated nodules is more complex; they consist of troilite, graphite, schreibersite, cohenite, forsterite ,calcium-rich chromian clinopyroxene, enstatite, albite, chlorapatite, and daubreelite (listed in the approximate order of their abundances by volume). Magnetite and its small inclusions of nickel-rich iron are probably terrestrial alteration products. Again, schreibersite and cohenite are frequently concentrated on the same side of the elongated modules. The compositions of the silicates, magnetite, chlorapatite, nickel-rich iron, and daubreelite were measured by electron microprobe techniques. Plots of the small contents of iron in a number of olivine and pyroxene grains show ranges in composition that are narrow but that are outside the limits of uncertainty in the measurements. The consistent elongation in one direction of the nodules containing silicates may be interpreted as the result of solidification of the meteorite under the influence of a gravitational field and/or convection.

Enstatite luminescence response for bulk specimens, powders and individual grains, considering selenological implications