Atmospheric CO2 levels from 2.7 billion years ago inferred from micrometeorite oxidation.

Earth's atmospheric composition during the Archean eon of 4 to 2.5 billion years ago has few constraints. However, the geochemistry of recently discovered iron-rich micrometeorites from 2.7 billion-year-old limestones could serve as a proxy for ancient gas concentrations. When micrometeorites entered the atmosphere, they melted and preserved a record of atmospheric interaction. We model the motion, evaporation, and kinetic oxidation by CO2 of micrometeorites entering a CO2-rich atmosphere. We consider a CO2-rich rather than an O2-rich atmosphere, as considered previously, because this better represents likely atmospheric conditions in the anoxic Archean. Our model reproduces the observed oxidation state of micrometeorites at 2.7 Ga for an estimated atmospheric CO2 concentration of >70% by volume. Even if the early atmosphere was thinner than today, the elevated CO2 level indicated by our model result would help resolve how the Late Archean Earth remained warm when the young Sun was ~20% fainter.

Fine-grained Material Associated with a Large Sulfide returned from Comet 81P/Wild 2.

In a consortium analysis of a large particle captured from the coma of comet 81P/Wild 2 by the Stardust spacecraft, we report the discovery of a field of fine-grained material (FGM) in contact with a large sulfide particle. The FGM was partially located in an embayment in the sulfide. As a consequence, some of the FGM appears to have been protected from damage during hypervelocity capture in aerogel. Some of the FGM particles are indistinguishable in their characteristics from common components of chondritic-porous interplanetary dust particles (CP-IDPs), including glass with embedded metals and sulfides (GEMS) and equilibrated aggregates (EAs). The sulfide exhibits surprising Ni-rich lamellae, which may indicate that this particle experienced a long-duration heating event after its formation but before incorporation into Wild 2.

Possible in situ formation of meteoritic nanodiamonds in the early Solar System.

Grains of dust that pre-date the Sun provide insights into their formation around other stars and into the early evolution of the Solar System. Nanodiamonds recovered from meteorites, which originate in asteroids, have been thought to be the most abundant type of presolar grain. If that is true, then nanodiamonds should be at least as abundant in comets, because they are thought to have formed further out in the early Solar System than the asteroid parent bodies, and because they should be more pristine. Here we report that nanodiamonds are absent or very depleted in fragile, carbon-rich interplanetary dust particles, some of which enter the atmosphere at speeds within the range of cometary meteors. One interpretation of the results is that some (perhaps most) nanodiamonds formed within the inner Solar System and are not presolar at all, consistent with the recent detection of nanodiamonds within the accretion discs of other young stars. An alternative explanation is that all meteoritic nanodiamonds are indeed presolar, but that their abundance decreases with heliocentric distance, in which case our understanding of large-scale transport and circulation within the early Solar System is incomplete.

Identification of iron sulphide grains in protoplanetary disks.

Sulphur is depleted in cold dense molecular clouds with embedded young stellar objects, indicating that most of it probably resides in solid grains. Iron sulphide grains are the main sulphur species in cometary dust particles, but there has been no direct evidence for FeS in astronomical sources, which poses a considerable problem, because sulphur is a cosmically abundant element. Here we report laboratory infrared spectra of FeS grains from primitive meteorites, as well as from pyrrhotite ([Fe, Ni](1-x)S) grains in interplanetary dust, which show a broad FeS feature centred at approximately 23.5 micrometres. A similar broad feature is seen in the infrared spectra of young stellar objects, implying that FeS grains are an important but previously unrecognized component of circumstellar dust. The feature had previously been attributed to FeO. The observed astronomical line strengths are generally consistent with the depletion of sulphur from the gas phase, and with the average Galactic sulphur/silicon abundance ratio. We conclude that the missing sulphur has been found.

An infrared spectral match between GEMS and interstellar grains.

Infrared spectral properties of silicate grains in interplanetary dust particles (IDPs) were compared with those of astronomical silicates. The approximately 10-micrometer silicon-oxygen stretch bands of IDPs containing enstatite (MgSiO3), forsterite (Mg2SiO4), and glass with embedded metal and sulfides (GEMS) exhibit fine structure and bandwidths similar to those of solar system comets and some pre-main sequence Herbig Ae/Be stars. Some GEMS exhibit a broad, featureless silicon-oxygen stretch band similar to those observed in interstellar molecular clouds and young stellar objects. These GEMS provide a spectral match to astronomical "amorphous" silicates, one of the fundamental building blocks from which the solar system is presumed to have formed.

STARDUST: finessing expensive cometary sample returns.

The STARDUST Discovery mission will collect samples of cometary coma and interstellar dust and return them to Earth. Five years after launch in February 1999, coma dust in the 1- to 100-micrometers size range will be captured by impact into ultra-low-density silica aerogel during a 6 kms-1 flyby of Comet Wild 2. The returned samples will be investigated at laboratories where the most critical information on these primitive materials is retained. The Jet Propulsion Laboratory will provide project management with Lockheed Martin Astronauts as the spacecraft industrial partner. STARDUST management will aggressively and innovatively achieve cost control through the use of Total Quality Management principles, the chief of which will be organization in a Project Engineering and Integration Team that "flattens" the traditional hierarchical structure by including all project elements from the beginning, in a concurrent engineering framework focusing on evolving Integrated Mission Capability.

A direct measurement of the terrestrial mass accretion rate of cosmic dust.

The mass of extraterrestrial material accreted by the Earth as submillimeter particles has not previously been measured with a single direct and precise technique that samples the particle sizes representing most of that mass. The flux of meteoroids in the mass range 10(-9) to 10(-4) grams has now been determined from an examination of hypervelocity impact craters on the space-facing end of the Long Duration Exposure Facility satellite. The meteoroid mass distribution peaks near 1.5 x 10(-5) grams (200 micrometers in diameter), and the small particle mass accretion rate is (40 +/- 20) x 106 kilograms per year, higher than previous estimates but in good agreement with total terrestrial mass accretion rates found by geochemical methods. This mass input is comparable with or greater than the average contribution from extraterrestrial bodies in the 1-centimeter to 10-kilometer size range.

An Interplanetary Dust Particle Linked Directly to Type CM Meteorites and an Asteroidal Origin.

Tochilinite, an ordered mixed-layer mineral containing Mg, Al, Fe, Ni, S, and O, has been identified in an interplanetary dust particle (IDP). This mineral is found in only one other class of meteoritic materials, type CM carbonaceous chondrites. The presence of tochilinite in an IDP provides a direct petrogenetic link between a member of the layer-silicate subset of IDPs and a specific class of meteorites and thus establishes that some IDPs collected in the stratosphere have an asteroidal origin. The scarcity of this IDP type suggests that materials with CM mineralogy are not abundant among the dust-producing asteroids.

Placers of cosmic dust in the blue ice lakes of greenland.

A concentration process occurring in the melt zone of the Greenland ice cap has produced the richest known deposit of cosmic dust on the surface of the earth. Extraterrestrial particles collected from this region are well preserved and are collectable in large quantities. The collected particles are generally identical to cosmic spheres found on the ocean floor, but a pure glass type was discovered that has not been seen in deep-sea samples. Iron-rich spheres are conspicuously rare in the collected material.

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.

Discovery of nucler tracks in interplanetary dust.

Nuclear tracks have been identified in interplanetary dust particles (IDP's) collected from the stratosphere. The presence of tracks unambiguously confirms the extraterrestrial nature of IDP's, and the high track densities (10(10) to 10(11) per square centimeter) suggest an exposure age of approximately 10(4) years within the inner solar system. Tracks also provide an upper temperature limit for the heating of IDP's during atmospheric entry, thereby making it possible to distinguish between pristine and thermally modified micrometeorites.

Carbon compounds in interplanetary dust: evidence for formation by heterogeneous catalysis.

Associations of carbonaceous material with iron-nickel alloy, carbides, and oxides were identified by analytical electron microscopy in ten unmelted chondritic porous micrometeorites from the earth's stratosphere. These associations, which may be interpreted in terms of reactions between a carbon-containing gas and catalytically active dust grains, suggest that some of the carbon in the chondritic porous subset of interplanetary dust was emplaced through heterogeneous catalysis.

Interplanetary dust: trace element analysis of individual particles by neutron activation.

Although micrometeorites of cometary origin are thought to be the dominant component of interplanetary dust, it has never been possible to positively identify such micrometer-sized particles. Two such particles have been identified as definitely micrometeorites since their abundances of volatile and nonvolatile trace elements closely match those of primitive solar system material.

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.

Silicate spherules from deep-sea sediments: confirmation of extraterrestrial origin.

Silicate spherules produced by atmospheric melting of meteoric bodies are probably the most common form of extraterrestrial material on the earth. It has never been possible to positively identify such particles although it has been known for more than a century that silicate spherules of suspected extraterrestrial origin are present in deep-sea sediments. One such spherule has been identified as definitely extraterrestrial since its abundances of nonvolatile trace elements closely match those of primitive solar system material.

Detection of 4He in stratospheric particles gives evidence of extraterrestrial origin.

Whipple predicted the existence of micrometeorites, small interplanetary particles which enter the Earth's atmosphere without being melted by frictional heating. During the past 2 yr, large numbers of micrometre sized stratospheric particles have been collected which are believed to be extraterrestrial, because their elemental compositions closely match those of primitive meteorites. We report here the detection of large concentrations of 4He in some of the particles. This not only suggests an exposure to solar wind, but also indicates that these particles are true micrometeorites in the sense that they were not strongly heated by entry into the atmosphere. Strong heating would have caused much of the helium to escape.

Stratospheric aluminum oxide.

Balloons and U-2 aircraft were used to collect micrometer-sized strato-spheric aerosols. It was discovered that for the past 6 years at least, aluminum oxide spheres have been the major stratospheric particulate in the size range 3 to 8 micrometers. The most probable source of the spheres is the exhaust from solid-fuel rockets.

Micrometeorite craters discovered on chondrule-like objects from kapoeta meteorite.

Craters attributable to hypervelocity impacts of micrometeorites have been discovered on rare chondrule-like objects from the gas-rich meteorite Kapoeta. These chondrule-like objects, probably generated by impacts themselves, provide further evidence for the regolith origin of Kapoeta. The micrometeorite flux at the time of formation of the meteorites was probably an order of magnitude higher than the present flux, but the solar luminosity could not have been higher than 1.7 times its present value.