Mineralogy of chondritic porous aggregates : current status

Chondritic porous aggregates (CPA's) belong to an important subset of small particles (usually between 5 and 50 micrometers) collected from the stratosphere by high flying aircraft. These aggregates are approximately chondritic in elemental abundance and are composed of many thousands of small­er, submicrometer particles. CPA particles have been the subject of intensive study during the past few years [1-3] and there is strong evidence that they are a new class of extraterrestrial material not represented in the meteorite collection [3,4]. However, CPA's may be related to carbonaceous chondrites and in fact, both may be part of a continuum of primitive extraterrestrial materials [5]. The importance of CPA's stems from suggestions that they are very primitive solar system material possibly derived from early formed proto­ planets, chondritic parent bodies, or comets [3, 6]. To better understand the origin and evolution of these particles, we have attempted to summarize all of the mineralogical data on identified CPA's published since about 1976.

A multi-stage history for carbonaceous material in extraterrestrial chondritic porous aggregate W7029*A and a new cosmothermometer

A new set of primitive extraterrestrial materials collected in the Earth's stratosphere include Chondritic Porous Aggregates (CPA's) [1]. CPAs have a complex and variable mineralogy [1-3] that include 'organic compounds' [4,5] and poorly graphitised carbon (PGC)[6]. This study presents a continuation of our detailed Analytical Electron Microscope study on carbon-rich CPA W7029*A from the JSC Cosmic Dust Collection. This CPA is an uncontaminated sample that survived atmospheric entry without appreciable alteration [7] and which contains ~44% carbonaceous material. The carbonaceous composition of selected particles was confirmed by Electron Energy Loss Spectroscopy and Selected Area Electron Diffraction (SAED). Possible carbonaceous contaminants introduced by specimen preparation techniques are easily recognised from indigenous CPA carbon particles [8] and do not bias our interpretations.

Titanium-oxide magneli phases in four chondritic porous interplanetary dust particles

Detailed analytical electron microscope analyses of four fine-grained chondritic porous interplanetary dust particles (IDPs)reveal the presence of titanium oxide Magneli phases, TinO2n-1 (n=4,5,6), and rare Ti-metal. The titanium minerals are indigenous to these chondritic IDPs. The association of Magneli phases, Ti-metal, and carbonaceous material in chondritic IDPs, along with the grain size distributions support in situ solid carbon gasification in these extraterrestrial particles. The active catalyst in this process is titanium metal that we infer may be of interstellar origin. This favorable catalysis uniquely leads to the formation of Magneli phases. As chondritic IDPs may be solid debris of short-period comets, our data indicate that nuclei of short-period comets may show distinctive chemical reactions that lead to Ti-mineral assemblages that typically include Magneli phases. The proposed model provides a plausible mechnism to explain the higher solid carbon content of chondritic IDPs relative to bulk carbon abundances typical for carbonaceous chondrite matrices that represent another type of more evolved, that is, metamorphosed, undifferentiated solar system bodies.

Progress toward a cosmic dust collection facility on space station

Scientific and programmatic progress toward the development of a cosmic dust collection facility (CDCF) for the proposed space station is documented. Topics addressed include: trajectory sensor concepts; trajectory accuracy and orbital evolution; CDCF pointing direction; development of capture devices; analytical techniques; programmatic progress; flight opportunities; and facility development.

Targeted flight opportunities with large area collectors

A major factor in the stratospheric collection process is the relative density of particles at the collection altitude. With current aircraft-borne collector plate geometries, one potential extraterrestrial particle of about 10 micron diameter is collected approximately every hour. However, a new design for the collector plate, termed the Large Area Collector (LAC), allows a factor of 10 improvement in collection efficiency over current conventional geometry. The implementation of LAC design on future stratospheric collection flights will provide many opportunities for additional data on both terrestrial and extraterrestrial phenomena. With the improvement in collection efficiency, LAC's may provide a suitable number of potential extraterrestrial particles in one short flight of between 4 and 8 hours duration. Alternatively, total collection periods of approximately 40 hours enhance the probability that rare particles can be retrieved from the stratosphere. This latter approach is of great value for the cosmochemist who may wish to perform sophisticated analyses on interplanetary dust greater than a picogram. The former approach, involving short duration flights, may also provide invaluable data on the source of many extraterrestrial particles. The time dependence of particle entry to the collection altitude is an important parameter which may be correlated with specific global events (e.g., meteoroid streams) provided the collection time is known to an accuracy of 2 hours.

Possible comet samples : The NASA Cosmic Dust Program

Beginning in 1974, a limited effort to collect extraterrestrial dust samples from the stratosphere using impactors mounted on NASA U-2 aircraft was initiated at NASA Ames Research Center (1). Subsequent studies (e.g. 1-9) have clearly established an extraterrestrial origin for some of the material. Attrition of comets is considered to be one of the potential sources of extraterrestrial dust(1,5). Additionally, some of the particles appear to represent a type of primitive material not represented in meteorite collections. In order to provide a greater availability of these samples to the scientific community, NASA-Johnson Space Center (JSC) began in May of 1981 a program dedicated to the systematic collection and curation of cosmic dust for scientific investigation. Collections were made at 18 to 20 km altitude by means of collectors mounted under the wings of a WB57F. When the aircraft reaches operating altitude, the collector plates (impactors) are extended into the ambient airstream with the collection surface normal to the airflow. To prevent particles from bouncing off the surface, the impactors are coated with a film of high viscosity silicone oil. The impactors are sealed in canisters to minimize contamination when not collecting.

Refinements and developments on the stratospheric dust database and classification scheme

Greater than 750 individual particles have now been selected from collection flags housed in the JSC Cosmic Dust Curatorial Facility and most have been documented in the Cosmic Dust Catalogs [1]. As increasing numbers of particles are placed in Cosmic Dust Collections, and a greater diversity of particles are introduced to the stratosphere through natural and man-made processes (e.g. decaying orbits of space debris [2]), there is an even greater need for a classification scheme to encompass all stratospheric particles rather than only extraterrestrial particles. The fundamental requirements for a suitable classification scheme have been outlined in earlier communications [3,4]. A quantitative survey of particles on collection flag W7017 indicates that there is some bias in the number of samples selected within a given category for the Cosmic Dust Catalog [5]. However, the sample diversity within this selection is still appropriate for the development of a reliable classification scheme. In this paper, we extend the earlier works on stratospheric particle classification to include particles collected during the period May 1981 to November 1983.

High resolution transmission electron microscopy of the Murchison Carbonaceous Chondrite and the Kenna Ureilite

Fine-grained matrices in carbonaceous chondrites and small, micron-sized inclusions in achondrites can be characterized effectively using high resolution transmission electron micro­scopy (HRTEM).

Structural and textural variations in carbonaceous chondrite matrices

Recent studies of C2 carbonaceous chondrite matrices using high resolu­tion transmission electron microscopy (HRTEM)have shown that structural details of the matrix minerals can be imaged [1-4]. The Murchison and Mighei matrices contain minerals having ordered and disordered mixed-layer structures [1,3,4] in addition to chrysotile- and lizardite-type structures [2].

Scanning electron microscopy of condensation related minerals : the Bjurbole meteorite

Filamentary single crystals, blades, sheets, euhedral crystals and powders may form by vapor phase condensation depending on the supersauration conditions in the vapor with respect to the condensing species [1]. Filamentary crystal growth requires the operation of an axial screw dislocation [2]. A Vapor-Liquid-Solid (VLS) mechanism may also produce filamentary single crystals, ribbons and blades. The latter two morphologies are typically twinned. Crystals grown by this mechanism do not require the presence of an axial screw dislocation. Impurities may either promote or inhibit crystal growth [3]. The VLS mechanism allows crystals to grow at small supersaturation of the vapor. Thin enstatite blades, ribbons and sheets have been observed in chondritic porous Interplanetary Dust Partics (IDP's) [4, 5]. The requisite screw dislocation for vapor phase condensation [1] has been observed in these enstatite blades [4]. Bradley et al. [4] suggest that these crystals are primary vapor phase condensates which could have formed either in the solar nebula or in presolar environments. These observations [4,5] are significant in that they may provide a demonstrable link to theoretical predictions: viz. that in the primordial solar nebula filamentary condensates could cluster into 'lint balls' and form the predecessors to comets [6].

Electron microscopy of stratospheric particles

A recent NASA program to collect stratospheric dust particles using high-flying WB57 aircraft has made available many more potential candidates for the study of extraterrestrial materials. This preliminary report provides an interpretation of the types of particles returned from one flag (W7017) collected in August, 1981 using a subset of 81 allocated particles. This particular collection period is after the Mt. St. Helen's eruptions. Therefore, the flag may contain significant quantities of volcanic debris in addition to the expected terrestrial contaminants [1]. All particles were mounted on nucleopore filters and have been examined using a modified JEOL100CX analytical electron microscope. For most of the particles, X-ray energy dispersive spectra and images were obtained at 40kV on samples which have not received any conductive coating. However, in order to improve resolution (to ~30A) some images are recorded at 100kV. In addition, 16 samples have been coated with a thin layer (<50A) of Au/Pd.

A comparative study on the mineralogy, chemistry and chronology of the Apollo 16 crystalline impact melt breccias. Part II: Chronology.

The controversy on how to interpret the ages of lunar highland breccias has recently been discussed by James [1]. Are the measured ages testimony of true events in lunar history; do they represent the age of the ancient crustal rocks, mixed ages of unequilibrated matrix-phenocryst relationships, or merely thermal events subsequent to the formational event ? It is certain from analyses of terrestrial impact melt breccias that the melt matrix of whole impact melt sheets is isotopically equilibrated due to the extensive mixing process of the early cratering stage [2,3]. It has been shown that isotopic equilibration takes place between impact melt matrix and target rock clasts therein, with the intensity of isotopic exchange depending on the degree of shock metamorphism, thermal metamorphism and the size of the clasts [4]. Therefore, impact melt breccias - if they are relatively clast-poor and mineralogically well studied - can be considered to be the most reliable source for information on the impact history of the lunar highland.

Layered silicates in Chondritic Porous Aggregate W7029*A : a case of primary growth

An important subset of extraterrestrial particles that reach the Earth's stratosphere include the so-called Chondritic Porous Aggregates (CPA's) [1-3]. In general, CPA's have a fluffy morphology and consist of numerous (>104)subparticles that are often <100A in size [4]. Mineral species in CPA's include Mg-rich pyroxene and olivine, Fe- and (Fe,Ni)-sulphides, taenite, Fe,Ni-carbides, magnetite, Ti-metal, a Bi-phase (metal or oxide), and variable amounts of carbonaceous material [1, 5-7]. Hydrated silicates are rare in CPA's and are limited to aggregates that have not been severely altered (thermo-metamorphosed) during atmospheric entry [8]. The presence of hydrated silicates in one cosmic dust particle was established by X-ray diffraction [2] and has been inferred in others by infra-red spectroscopy [8]. If CPA's are cometary, their mineralogy and morphology suggest that at least two episodes of aggregation occurred and that variations in porosity may be related to local differences in ice-to-dust ratio [3].