Engineering topics

Wood, J. A.

Publications and source records attributed to Wood, J. A..

At least 19 records

NASA NTRS1994

Major unresolved issues in the formation of chondrules and CAIs

So much has been learned and published about chondrites and so many models have been floated (but neither proven nor refuted) that the subject has become extremely diffuse and amorphous. It is impossible to remain simultaneously aware of all the many constraints, from many different subdisciplines, that bear on nebular processes. This paper attempts to impose some structure on the situation and loosen the interpretational logjam by breaking the chondrite problem into a series of well-defined questions. Most are open questions; those that some might consider closed by now may deserve formal reexamination.

Wood, J. A.↗

NASA NTRS1992

Mineral equilibria and the high radar reflectivity of Venus mountaintops

The relationship between altitude and microwave emissivity in 10 highland regions of Venus is investigated on the basis of the Magellan data set. Highlands on Venus are found to display high radar reflectivity. The required change in surface electrical properties occurs abruptly at a 'critical altitude,' whose value varies from one highland area to another. Critical altitudes range from 4.75 km to 2.49 km. Differences in reflectivity are caused by differences in the surface mineral assemblage, which determines the dielectric constant of surface material. The mineral responsible for high radar reflectivity on mountaintops is pyrite, which occurs in weathered mineral assemblages at high altitudes. Conductive pyrite occurs dispersed in insulating materials, forming a loaded dielectric material.

Klose, K. B.↗

NASA NTRS1992

Venus: Geochemical conclusions from the Magellan data

Though the Magellan mission was not designed to collect geochemical or petrological information, it has done so nonetheless. Since the time of the Pioneer Venus mission it has been known that high-altitude (greater than 2.5-5 km) mountainous areas on Venus exhibit anomalously low radiothermal emissivity (e less than 0.6). Magellan has greatly refined and extended these observations. The low emissivity requires surface material in the uplands to have a mineralogical composition that gives it a high bulk dielectric constant, greater than 20. The dielectric constant of dry terrestrial volcanic rocks seldom exceeds 7. The high-dielectric character of high-altitude surface material cannot be a primary property of the local volcanic rock, because there is no reason why rock having the required special mineralogy would erupt only at high altitudes. Therefore it is a secondary property; the primary Venus rock has reacted with the atmosphere to form a mineralogically different surface layer, and the secondary minerals formed are controlled by the ambient temperature, which decreases with altitude on Venus. A further investigation of venusian mineralogy is presented.

Wood, J. A.↗

NASA NTRS1991

Magellan - Initial analysis of Venus surface modification

Images of the Venus surface provided by the Magellan mission make it possible to see the fine-scale features diagnostic of weathering, erosion, and deposition. These include ejecta deposits extending up to 1000 km to the west of several impact craters, windblown deposits, features containing both obstacles and a source of particulate material, and evidence for degradation by atmosphere-surface interactions and mass movements. Initial Magellan observations pertaining to the nature, rate, and history of surficial processes are analyzed. Emphasis is placed on radar imaging, but results from radiometry and altimetry observations are also discussed.

Arvidson, R. E.↗

NASA NTRS1986

Interstellar material in the solar system

All the substance of the Earth and other terrestrial planets once existed in the form of interstellar grains and gas. A major aspect of solar system formation (and undoubtedly of star formation generally) is the complex series of processes that converted infalling interstellar grains into planets. A cryptic record of these processes is preserved in certain samples of planetary materials, such as chondritic meteorites, that were preserved in a relatively unchanged form since the beginning. It is to be expected that some of these primitive materials might contain or even consist of preserved presolar interstellar grains. The identification and study of such grains, the ancestors of our planetary system, is a matter of intense interest. Types of primitive material accessible or potentially accessible, and component of or relationship to presolar interstellar grains are discussed.

Wood, J. A.↗

NASA NTRS1985

Mineral chemistry and origin of spinel-rich inclusions in the Allende CV3 chondrite

The present electron probe microanalysis of 20 spinel-rich inclusions in Allende indicates that the mineralogy of spinel-rich Allende inclusions is similar to that of fine grained Ca, Al-rich inclusions (CAIs) in ALH-77003 and rims on coarse grained CAIs in Allende. The mineralogy, mineral chemistry, and bulk chemistry of the inclusions indicate that they are not primitive aggregates of crystalline nebular condensates, but rather aggregates of minute concentric objects that are presently interpreted to be a fractionated distillation residue that lost Ca, Si-rich partial melt. It is suggested that interstellar dust aggregates were melted or distilled into Allende inclusions by aerodynamic drag heating, in regions of the nebula where the dust/gas ratio was so enhanced that the local oxygen fugacity was raised by several order of magnitude and liquids became thermodynamically stable.

Kornacki, A. S.↗

NASA NTRS1985

The identification of group II inclusions in carbonaceous chondrites by electron probe microanalysis of perovskite

The technique developed by Kornacki (1984) for identifying group II Ca/Al-rich inclusions in carbonaceous chondrites by electron-microprobe analysis of the ZrO2 or Y2O3 content of their perovskite component is demonstrated using material from 20 Allende inclusions. The results are presented in tables and graphs and compared with findings obtained by other procedures. Group II inclusions are found to have perovskites generally containing less than 0.10 wt pct ZrO2 and/or Y2O3 (average of several grains), while those of groups I, III, V, and VI have more than 0.25 wt pct ZrO2. Analysis of data on eight Allende Ca/Al-rich inclusions shows that 75 percent of the fine-grained inclusions belong to group II. The implications of these findings for fractionation processes in the primitive solar nebula are indicated.

Kornacki, A. S.↗

NASA NTRS1985

Meteoritic constraints on processes in the solar nebula

The origin of chondritic meteorites is discussed. The characteristics of the chondrite subtypes are shown, and the fine structure of chondrites is described in terms of the three principal categories of aggregated objects present in chondrites: chondrules, Ca,Al-rich inclusions (CAIs), and matrix. Four possibilities for the origin of these components are considered, leading to four important constraints on the chondrite-forming processes, based on mineralogy and astrophysics. It is concluded that the chondrules and CAIs were formed in the solar nebula at a radial distance of about 3 AU. Some type of transient high-energy event or process, operating in zones of dust enrichment, furnished the high temperatures needed to process the chondrules and CAIs. These objects cooled on a time scale of minutes, then on a similarly short time scale began to aggregate, together with dust, into chondritic masses having approximately the solar proportions of major condensable elements.

Wood, J. A.↗

NASA NTRS1984

On the formation of meteoritic chondrules by aerodynamic drag heating in the solar nebula

During the formation of the solar nebula, interstellar grains were falling into the nebula with velocities of the order of 10 km/s at the radial distance where the meteorites were to form. This kinetic energy is 20 times the amount of thermal energy needed to melt the grains. The grains were decelerated by aerodynamic drag in the nebula. Where grain-rich parcels of interstellar material fell into the nebula, heat generated by drag could not be radiated away because of the opacity imparted to the system by the grains, and high temperatures were reached. In this situation presolar aggregations of grains would melt to form chondrules. Many of the properties of chondrules (and also Ca/Al-rich inclusions) are consistent with their formation by this means. The infall-heating concept provides a new framework in which the formation and significance of chondritic meteorites can be understood.

Wood, J. A.↗

NASA NTRS1984

The mineral chemistry and origin of inclusion matrix and meteorite matrix in the Allende CV3 chondrite

The mineralogy and mineral chemistry of the inclusion and meteorite matrices in the Allende CV3 chondrite are described, and the physical and chemical parameters of the conventional equilibrium condensation model of the origin of chondrite meteorites are evaluated. An alternative model of the origin of the mafic constituent of Allende inclusions is presented, on the basis of a new model of chondrule petrogenesis and the physical evolution of the primitive solar nebula. The model shows that the mineral chemistry of the olivine matrix in Allende CV3 seems to preserve a good record of nebular and planetary processes, including: (1) vapor-to-solid condensation under relatively oxidizing nonequilibrium conditions; (2) Fe/Mg equilibration in the meteorite parent body; and (3) recrystallization and incipient melting in the solar nebula.

Kornacki, A. S.↗

NASA NTRS1984

Mineralogic and petrologic studies of meteorites and lunar samples

During a 13 year period beginning in 1971, the Extraterrestrial Petrology Group examined lunar soils from all 6 Apollo missions and those returned by the Soviet Luna 16, Luna 20, and Luna 24 missions. In addition, the properties and apparent origin of the carbonaceous chondrites were examined. Chondrules, calcium-aluminum-rich inclusions (CAI) and the fine grained matrix materials that accompany chondrules and CAI's in primitive meteorites were investigated. The effects of planetary hydrothermal alteration of matrix materials in the C1 chondrite was also investigated. Full length papers and extended abstracts published during the grant are listed chronologically.

Wood, J. A.↗

NASA NTRS1984

Petrography and classification of Ca, Al-rich and olivine-rich inclusions in the Allende CV3 chondrite

The results of a detailed, systematic petrographic survey of Ca, Al-rich and olivine-rich inclusions in the Allende CV3 chondrite are reported, and a new classification system based on clearly defined and readily applied petrographic criteria is presented. Most Allende inclusions are aggregates containing one or more of three distinct constituents: (1) rimmed concentric objects enriched in Al- and Ti-rich oxide minerals and various amounts of Ca-rich silicates; (2) porous, 'fine-grained' chaotic material enriched in Ca-rich silicates, especially clinopyroxenes and garnets; and (3) porous, 'fine-grained', mafic inclusion matrix, enriched in olivine, pyroxene, and feldspathoids. Two texturally distinct varieties of inclusions consist primarily of inclusion matrix: unrimmed olivine aggregates and rimmed olivine aggregates. Ca, Al-rich inclusions are classified on the basis of the size and abundance of their constituent concentric objects. Some fundamental relationships among Allende inclusions that previusly have not been emphasized are discussed.

Kormacki, A. S.↗

NASA NTRS1984

Formation of chondrules by drag heating in dust-enriched environments

The solar system which had a beginning, 4.6 billion years ago is discussed. The atoms that make up the solar system are much older than that, however: they were also part of the universe that existed before the solar system formed. The presence and location of the atoms of silicon, iron, calcium, that make up the Earth and other planets before there was a solar system is questioned. Astronomical it is shown that they existed in tiny dust grains which, along with molecules of gas, wandered about in the vast reaches of space between the stars of our galaxy. The solar system was formed when one particular cloud of interstellar dust and gas became dense enough to be gravitationally unstable: the gravitational pull that dust grains and gas molecules exerted on one another caused the cloud to collapse.

Wood, J. A.↗

NASA NTRS1984

Meteoritic constraints on processes in the solar nebula

High-temperature events or processes in the solar nebula were required to form the chondrules and CAI's that make up the bulk of the substance of most chondrites. There is no consensus in the meteoritical community as to the nature of these high-temperature events or processes. In spite of this major area of uncertainty several conclusions can be reached about properties of the nebula. The chondrules and CAI's were formed in regions where the dust/gas ratio was orders of magnitude above the cosmic value. After these high-temperature objects were formed, they began to accrete very promptly (a time scale of hours, probably). Formation and aggregation of the chondrite components happened at the same radial distance in the nebula where chondrites reside today (the asteroid belt). The ambient temperature of the nebula at that time and place was relatively low ( 700 K), and was not responsible for the thermal processing of the chondrules and CAI's.

Wood, J. A.↗

NASA NTRS1983

Petrography and classification of refractory inclusions in the Allende and Mokoia CV3 chondrites

Results are reported for a comprehensive petrographic survey of the refractory inclusions in the Allende and Mokoia CV3 chondrites. More than 600 refractory inclusions in 22 thin sections of the meteorites were studied by optical and scanning-electron microscopy. Olivine-rich inclusions and Ca, Al-rich inclusions (CAIs) are aggregates of various combinations of three fundamental petrographic constituents: rimmed concentric objects, Ca, Si-rich chaotic material, and mafic inclusion matrix. A new classification system for refractory inclusions is developed that is based on the size and abundance of these three fundamental constituents. The new classification system avoids several problems that are inherent in other classification systems, which use the term 'coarse-grained' too restrictively for many simple CAIs and inaccurately for most mililite-rich complex CAIs.

Kornacki, A. S.↗

NASA NTRS1983

Mineralogy and petrology of chondrules and inclusions in the Mokoia CV3 chondrite

The inclusions and chondrules of the Mokoia CV3 chondrite are studied systematically and compared with those in the Allende meteorite. Five polished thin sections of Mokoia were examined by optical microscopy, backscattered scanning electron microscopy, and electronprobe microanalysis, and objects greater than about 100 microns in apparent diameter were measured and classified petrographically. Three major types of olivine chondrules are distinguished: igneous chondrules, which evidently crystallized from droplets of silicate melt; recrystallized chondrules, apparently metamorphosed at relatively high temperatures; and accretional aggregates, which are probably fragments of igneous chondrules. Refractory inclusions in Mokoia are generally similar to those found in Allende, although Mokoia inclusions include phyllosilicates rather than feldspathoids and melilite-rich Ca, Al-rich inclusions (CAIs) are more abundant in Allende. The small, fine-grained CAIs, which are more abundant than coarse-grained CAIs in both meteorites, are observed to represent aggregates of three distinct constituents: concentric objects, chaotic material and inclusion matrix. It is concluded that most of the CAIs probably formed during metamorphism, partial melting, and incomplete distillation of primitive dust aggregates heated in the solar nebula.

Cohen, R. E.↗

NASA NTRS1981

The interstellar dust as a precursor of Ca,Al-rich inclusions in carbonaceous chondrites

Multiple dust components are considered as a source of the observed anomalies in meteoritic oxygen isotope compositions. It is pointed out that these anomalies are not due to an incomplete mixing of several dust or gas-plus-dust components in the solar nebula. If they were, other elements would display similar anomalies. The anomalies must therefore stem from differing degrees of incomplete exchange of oxygen isotopes between the primordial gas and dust components of the nebula. The dust is more likely to have been the component which was enriched with 0-16. Since the isotopic difference between dust and gas probably could not have been preserved if the dust was ever completely vaporized in the nebula, the Ca,Al-rich inclusions in carbonaceous chondrites are unlikely to be condensates, but instead are distillation residues.

Wood, J. A.↗

NASA NTRS1980

Mineralogic and petrologic studies of lunar samples and meteorites

Experimental and thermodynamic research on the pressure temperature limits of the stability of the mineral assemblages found in pristine, spinel bearing lunar highland lithologies demonstrated the likelihood that the minerals originated in the lower stratigraphic levels of the primordial crust. The phase equilibrium in silicate solid/liquid systems of planetary importance were thermochemically interpreted in order to model the early formation of the crusts and maneles of Earth and Moon sized planets. The petrography and chemical composition of coarse grained gabbro, the chemical analysis and age dating of clasts from Apollo 16 breccia, the analysis of glass particles from Apollo 16 soil samples, the study of Allende and Mokoia meteorites as a source of information about events in the solar nebula, and the hydrothermal alteration of amorphous materials were also investigated. The capabilities of a model for addressing the problem of the origin of the Earth's moon by the disruptive capture mechanism are examined as well as models of the thermal evolution of hypothetical meteorite bodies. Progress in determining the composition of stony meteorite specimens collected at the Allan Hills site during the Antarctic field exploration is reported.

Wood, J. A.↗