Comparison of comet 81P/Wild 2 dust with interplanetary dust from comets

The Stardust mission returned the first sample of a known outer solar system body, comet 81P/Wild 2, to Earth. The sample was expected to resemble chondritic porous interplanetary dust particles because many, and possibly all, such particles are derived from comets. Here, we report that the most abundant and most recognizable silicate materials in chondritic porous interplanetary dust particles appear to be absent from the returned sample, indicating that indigenous outer nebula material is probably rare in 81P/Wild 2. Instead, the sample resembles chondritic meteorites from the asteroid belt, composed mostly of inner solar nebula materials. This surprising finding emphasizes the petrogenetic continuum between comets and asteroids and elevates the astrophysical importance of stratospheric chondritic porous interplanetary dust particles as a precious source of the most cosmically primitive astromaterials.     

Noble gases in stratospheric dust particles: confirmation of extraterrestrial origin

Noble gas elemental and isotopic ratios were measured in a group of 13 "chondritic" stratospheric dust particles. Neon and argon are present in "solar" proportions; xenon appears to be dominated by contributions from "planetary" sources. The apparent xenon concentration is higher than that measured in any bulk meteorite, approaching the concentration found in the noble gas-rich, acid-insoluble residues from carbonaceous chondrites.     

Cometary particles: thin sectioning and electron beam analysis

Thin sections (500 to 1000 angstroms thick) of individual micrometeorites (5 to 15 micrometers) have been prepared with an ultramicrotome equipped with a diamond knife. Electron microscopic examination of these sections has revealed the internal structures of chondritic micrometeorites, and a subset of highly porous, fragile particles has been identified. Delicate meteoritic materials such as these are characteristic of debris from cometary meteors.     

Magnesium isotopic composition of interplanetary dust particles

The magnesium isotopic composition of some extraterrestrial dust particles has been measured. The particles are believed to be samples of interplanetary dust, a significant fraction of which originated from the disaggregation of comets and may contain preserved isotopic anomalies. Improvements in mass spectrometric and sample preparation techniques have made it possible to measure the magnesium isotopic composition of the dust particles, which are typically 10 micrometers in size and contain on the order of 10(-10) gram of magnesium. Of the 13 samples analyzed, nine have the terrestrial magnesium isotopic composition within 2 parts per thousand, and one shows isotopic mass fractionation of 1.1 percent per mass unit. A subset of the particles, described as chondritic aggregates, are very close to normal isotopic composition, but their normalized isotopic ratios appear to show nonlinear effects of 3 to 4 parts per thousand, which is near the present limit of detection for samples of this size. The isotopic composition of calcium was also determined in one particle and found to be normal within 2 percent. It is clear that the isotopic composition of interplanetary dust particles can be determined with good precision. Collection of dust particles during the earth's passage through a comet tail or an intense meteor stream may permit laboratory analysis of material from a known comet.     

Epsilon carbide: a low-temperature component of interplanetary dust particles

Transmission electron microscope study of a chondritic interplanetary dust particle has revealed the presence of epsilon iron-nickel carbide, a low- temperature carbide previously encountered only in metallurgical studies. In these studies in-carbide was synthesized by carburization of iron or nickel grains in a stream of carbon monoxide or carbon monoxide plus hydrogen. Similar carburization of an iron-nickel metal in situ may have produced in-carbide during particle heating on atmospheric entry or in solar orbit. Alternatively, the epsilon-carbide may be a by-product of Fischer-Tropsch reactions in the solar nebula. Such reactions have been proposed as the mechanism of hydrocarbon formation in the early solar system.     

The formation of chondrules: petrologic tests of the shock wave model

Chondrules are millimeter-sized rounded igneous rocks within chondritic meteorites. Their textures and fractionated mineral chemistries suggest that they formed by repeated, localized, brief (minutes to hours) melting of cold aggregates of mineral dust in the protoplanetary nebula. Astrophysical models of chondrule formation have been unable to explain the petrologically diverse nature of chondrites. However, a nebular shock wave model for chondrule formation agrees with many of the observed petrologic and geochemical properties of chondrules and shows how particles within the nebula are sorted by size and how rims around chondrules are formed. It also explains the volatile-rich nature of chondrule rims and the chondrite matrix.     

Late accretion on the earliest planetesimals revealed by the highly siderophile elements

Late accretion of primitive chondritic material to Earth, the Moon, and Mars, after core formation had ceased, can account for the absolute and relative abundances of highly siderophile elements (HSEs) in their silicate mantles. Here we show that smaller planetesimals also possess elevated HSE abundances in chondritic proportions. This demonstrates that late addition of chondritic material was a common feature of all differentiated planets and planetesimals, irrespective of when they accreted; occurring ≤5 to ≥150 million years after the formation of the solar system. Parent-body size played a role in producing variations in absolute HSE abundances among these bodies; however, the oxidation state of the body exerted the major control by influencing the extent to which late-accreted material was mixed into the silicate mantle rather than removed to the core.     

Ablation, flux, and atmospheric implications of meteors inferred from stratospheric aerosol

Single-particle analyses of stratospheric aerosol show that about half of the particles contain 0.5 to 1.0 weight percent meteoritic iron by mass, requiring a total extraterrestrial influx of 8 to 38 gigagrams per year. The sodium/iron ratio in these stratospheric particles is higher and the magnesium/iron and calcium/iron ratios are lower than in chondritic meteorites, implying that the fraction of material that is ablated must lie at the low end of previous estimates and that the extraterrestrial component that resides in the mesosphere and stratosphere is not of chondritic composition.     

Discovery of an unmelted h-chondrite inclusion in an iron meteorite

The link between H chondrites and silicate inclusions in group IIE iron meteorites has long been suspected, but direct evidence for a common parentage has remained elusive. The discovery of an unmelted chondritic inclusion in the Techado iron meteorite sheds light on the genetic relation between these two groups, providing clues on the origin of chondritic materials as inclusions in iron meteorites. It is proposed that the complex IIE iron meteorite breccias formed by collisions with several different bodies, followed by deep burial of metal and silicate fragments in the asteroidal megaregolith.     

Samarium-neodymium systematics in kimberlites and in the minerals of garnet lherzolite inclusions

The initial ratios of neodymium-143 to neodymium-144 in kimberlites ranging in age between 90 x 10(6) to 1300 x 10(6) years from South Africa, India, and the United States are different from the corresponding ratios in the minerals of peridotite inclusions in the kimberlites but are identical to the ratios in the basaltic achondrite Juvinas at the times of emplacement of the respective kimberlite pipes. This correlation between the kimberlites and Juvinas, which represents the bulk chondritic earth in rare-earth elements, strongly indicates that the kimberlite's source in the mantle is chondritic in rare-earth elements and relatively primeval in composition.     

Indium variations in a petrologic suite of L-group chondrites

Indium concentrations have been determined by neutron activation in four members of each of the L3, L4, L5, and L6 chondritic meteorite classes. The range of concentrations is found to be from 0.14 to 22 parts per billion, with the highest values in L3 chondrites and the lowest values in the L5 and L6 classes. Plots of indium concentration versus relative mean deviation of pyroxene iron content, total carbon concentration, and primordial argon-36 concentration show positive correlations to varying degrees. Indium concentration appears to be a valuable parameter relating to variable formation conditions of the chondritic meteorites during the early history of the solar system.     

Potassium, rubidium, strontium, barium, and rare-Earth concentrations in lunar rocks and separated phases

Concentrations of potassium, rubidium, strontium, barium, and rareearth elements have been determined by mass spectrometric isotope dilution for eight Apollo 11 lunar samples and for some separated phases. Potassiumn and ritbidium are at chondritic levels, strontium at 15 times, and barium and rare earths at 30 to 100 times chondritic levels. There are trace element similarities between the lunar samples and basaltic achondrites, terrestrial dredge basalts and the bulk earth. The trace element data appear to be consistent with these lunar samples being the result of limited partial fusion of some material similar to the brecciated eucrite meteorites.     

Chondritic meteorite fragments associated with the Permian-Triassic boundary in Antarctica

Multiple chondritic meteorite fragments have been found in two sedimentary rock samples from an end-Permian bed at Graphite Peak in Antarctica. The Ni/Fe, Co/Ni, and P/Fe ratios in metal grains; the Fe/Mg and Mn/Fe ratios in olivine and pyroxene; and the chemistry of Fe-, Ni-, P-, and S-bearing oxide in the meteorite fragments are typical of CM-type chondritic meteorites. In one sample, the meteoritic fragments are accompanied by more abundant discrete metal grains, which are also found in an end-Permian bed at Meishan, southern China. We discuss the implications of this finding for a suggested global impact event at the Permian-Triassic boundary.     

Neodymium isotope evidence for a chondritic composition of the Moon

Samarium-neodymium isotope data for six lunar basalts show that the bulk Moon has a 142Nd/144Nd ratio that is indistinguishable from that of chondritic meteorites but is 20 parts per million less than most samples from Earth. The Sm/Nd formation interval of the lunar mantle from these data is 215(-21)(+23) million years after the onset of solar system condensation. Because both Earth and the Moon likely formed in the same region of the solar nebula, Earth should also have a chondritic bulk composition. In order to mass balance the Nd budget, these constraints require that a complementary reservoir with a lower 142Nd/144Nd value resides in Earth's mantle.     

Direct detection of projectile relics from the end of the lunar basin-forming epoch

The lunar surface, a key proxy for the early Earth, contains relics of asteroids and comets that have pummeled terrestrial planetary surfaces. Surviving fragments of projectiles in the lunar regolith provide a direct measure of the types and thus the sources of exogenous material delivered to the Earth-Moon system. In ancient [>3.4 billion years ago (Ga)] regolith breccias from the Apollo 16 landing site, we located mineral and lithologic relics of magnesian chondrules from chondritic impactors. These ancient impactor fragments are not nearly as diverse as those found in younger (3.4 Ga to today) regolith breccias and soils from the Moon or that presently fall as meteorites to Earth. This suggests that primitive chondritic asteroids, originating from a similar source region, were common Earth-Moon-crossing impactors during the latter stages of the basin-forming epoch.     

Solar-type xenon: a new isotopic composition of xenon in the pesyanoe meteorite

Xenon in the Pesyanoe meteorite is a mixture of several componenits. Solar-type xenon is a new component deficient in the neutron-rich isotopes as compared to both trapped chondritic and terrestrial atmospheric xenon.     

Accretion rate of extraterrestrial matter: iridium deposited 33 to 67 million years ago

Iridium measured in 149 samples of a continuous 9-meter section of Pacific abyssal clay covering the time span 33 to 67 million years ago shows a well-defined peak only at the Cretaceous/Tertiary boundary. In the rest of the section iridium ranges from a minimum concentration near 0.35 nanograms per gram in the Paleocene to a maximum near 1.7 in the Eocene; between 63 and 33 million years ago the mean iridium accumulation rate is approximately 13 nanograms per square centimeter per million years. Correction for terrestrial iridium leads to an extraterrestrial flux of9 +/- 3 nanograms of iridium per square centimeter per million years, and an estimated annual global influx of 78 billion grams of chondritic matter, consistent with recent estimates of the influx of dust, meteorites, and crater-producing bodies with masses ranging from 10(-13) to 10(18 )grams. Combining the recent flux of objects ranging in mass from 10(6) to 10(7) grams with the flux of 10(14) - to 10(15) -gram objects indicates that the number of objects is equal to 0.54 divided by the radius (in kilometers) to the 2.1 power. Periodic comet showers should increase the cometary iridium flux by a factor of 200 to 600 on a time scale of 1 to 3 million years; the predicted iridium maxima (more than 30 times background) are not present in the intervals associated with the Cretaceous/Tertiary boundary or the tektiteproducing late Eocene events.     

Composition of the Earth

New estimates of solar composition, compared to earlier measurements, are enriched in Fe and Ca relative to Mg, Al, and Si. The Fe/Si and Ca/Al atomic ratios are 30 to 40 percent higher than chondritic values. These changes necessitate a revision in the cosmic abundances and in the composition of the nebula from which the planets accreted (which have been based on chondritic values). These new values imply that the mantle could contain about 15 weight percent FeO and more CaMgSi(2)O(6) than has been supposed. Geophysical data are consistent with a dense, FeO-rich lower mantle and a CaMgSi(2)O(6) (diopside)-rich transition region. FeO contents of 13 to 18 weight percent appear to be typical of the mantles of bodies in the inner solar system. The oldest komatiites (high-temperature MgO-rich magmas) have a similar chemistry to the derived mantle. These results favor a chemically zoned mantle.     

Organic matter on asteroid 130 elektra

Infrared absorption spectra of a low-albedo water-rich asteroid appear to show a weak 3.4-micrometer carbon-hydrogen stretching mode band, which suggests the presence of hydrocarbons on asteroid 130 Elektra. The organic extract from the primitive carbonaceous chondritic Murchison meteorite shows similar spectral bands.     

Chondrules: first occurrence in an iron meteorite

Complete chondrules and fragments of chondrules have been found within silicate inclusions from the octahedrite iron meteorite Netschaevo. The bulk chemical composition, mineralogy, and mineral chemistry indicate that this chondritic material has properties intermediate between those of the H-group chondrites and those of the enstatite chondrites.