Minerologic and Petrologic Studies of Meteorites
This report provides a listing of the publications published by the Principal Investigator, J.A. Wood and co-worker M.I. Petaev during the reporting period
Boyce, J.↗
Engineering topics
Wood, John A.
Publications and source records attributed to Wood, John A..
A Study of the Electrical Properties and Mineralogy of the Surface of Venus
This final report contains a listing of published papers and abstracts which address the electrical properties and minerology of the surface of Venus. The topics of these papers include: Subduction on the margins of coronae on Venus: Evidence from radiothermal emissivity measurements; The weathering process on Venus takes 2-3 hundred million years: Evidence from radiothermal emissivity signatures at coronae; SO2 and CH4 levels in the Venusian atmosphere, measured by Pioneer Venus: Caused by plinian-style volcanic activity at Maat Mons?; Magellan reveals Venus; Recent volcanic activity on Venus: Evidence from radiothermal emissivity signatures; Occurrences of low-emissivity surface material at low altitudes on Venus: A window to the past; Must the Venus surface material contain hematite?; Rock weathering on the surface of Venus; and Comment on "The Rate of Pyrite Decomposition on the Surface of Venus."
Wood, John A.↗
Mineralogic and Petrologic Studies of Meteorites and Lunar Samples
In the period named, P.I. Wood and his Research Associate M.I. Petaev published or submitted for publication the following papers and abstracts, which address the subjects: Simulation of melting-crystallization relationships in chondritic and achondritic igneous systems. METEOMOD: A numerical model for the calculation of melting-crystalyization relationships in meteoritic igneous systems. Secondary Ca-Fe-rich minerals in the Bali-like and Allende-like oxidized CV3 chondrites and Allende dark inclusions. Silica minerals in the Gibeon IVA iron meteorite. Drusy vugs in the Albion iron meteorite: Mineralogy and textures. Search for exsolved ferromagnesian olivines: A meteoritic survey. Cr-bearing minerals in the Gibeon IVA iron: Indicators of sulfur and oxygen fugacities in the parent body. Lunar Planet. Computer modelling of ordinary chondrite melting. Drusy vugs in the Albion iron meteorite: Early speculation on the origin. Also Processing of chondritic and planetary material in spiral density waves in the nebula. Chondrite formation by turbulence and shock in the solar nebula.
Wood, John A.↗
A Study of the Electrical Properties and Mineralogy of the Surface of Venus
This annual progress report is a list of papers on a study of the electrical properties and mineralogy of the surface of Venus. These papers were written by P. I. Wood and C. A. Robinson. Titles include: Subduction on the margins of coronae on Venus: Evidence from radiothermal emissivity measurements; The weathering process on Venus takes 2-3 hundred million years: Evidence from radiothermal emissivity signatures at coronae; SO2 and CH4 levels in the Venusian atmosphere, measured by Pioneer Venus: Caused by plinian-style volcanic activity at Maat Mons; Magellan reveals Venus; Recent volcanic activity on Venus: Evidence from radiothermal emissivity signatures; Occurrences of low-emissivity surface material at low altitudes on Venus: A window to the past; Must the Venus surface material contain hematite; Rock weathering on the surface of Venus; Comment on "The Rate of Pyrite Decomposition on the Surface of Venus-by J. A. Wood, and R. Brett; and Complex dielectric constants and magnetic permeabilities of mineral mixtures-by De Roo, R.D, C.A. Robinson, F.T. Ulaby, and J.A. Wood.
Wood, John A.↗
Constraints placed on the chondrule-forming process by merrihueite in the Mezoe-Madaras chondrite
A few unusual chondrules in the Mezoe-Madaras L3 chondrite contain the peralkaline mineral merrihueite, (K, Na)2(Fe, Mg)5Si12O30, formed by reaction of free silica in the chondrules with K, Na, Fe-rich nebular gas. Vapor fractionation was required to create chondrules poor in Al and rich in Si, and the fractionated condensate produced had to aggregate into protochondrules very promptly (on the order of hours or less) to prevent it from mixing with dust of more generalized composition in the nebula. The traditional paradigm for chondrule formation, involving a series of unrelated events (chemical fractionation of precursor dust; clumping of the dust; melting of the dust-clumps), is inadequate for making these chondrules. A rapid process, all stages of which are related, is required. We propose that this occurred during the cooling and contraction of the volume of hot gas created by a small transient high-energy event in the nebula, of unspecified nature. Microdroplets of silicate melt created by condensation from the cooling, contracting gas had velocity vectors directed toward the center of the volume. Coalescence occurred in the crowded central zone where the microdroplets converged. The Mezoe-Madaras merriihueite-bearing chondrules may have been formed by an atypically low-energy nebular event, which achieved the needed chemical fractionations by only partially vaporizing precursor dust. More commonplace chondrule types may have been created by the same basic process, but involving higher-energy events in which precursor dust was completely vaporized and recondensed.
Wood, John A.↗
Mineral equilibrium in fractionated nebular systems
We investigated the equilibrium mineral assemblages in chemically fractionated nebular systems, using a computer routine that finds the set of minerals and gases which minimizes the Gibbs free energy of a system with stipulated elemental abundances. Diagrams are presented showing the equilibrium mineralogy, as a function of temperature (400-2300 K), for unfractionated solar material and five fractionated systems. The fractionated systems were defined by mixing, in various proportions, the following four volatility components that solar material can be divided into: refractory dust, carbonaceous matter, ices, and H2 gas. Dust enrichment is seen to increase temperatures of condensation/evaporation and the Fe(2+) content of mafic minerals and to permit existence of stable melt phases. Enrichment of dust and organic matter produces mineral assemblages that are similar in many ways to those of enstatite chondrites, but with mafic minerals that are far more reduced than those in primitive enstatite chondrites. Enrichment of dust, organics, and ices leads to highly ferrous mineralogies even at the highest temperatures but does not predict the stability of hydrous phases above about 450 K.
Wood, John A.↗
Recent volcanic activity on Venus - Evidence from radiothermal emissivity measurements
Radiothermal emissivity measurements are analyzed in order to study large volcanic constructs on Venus and to correlate details of the reflectivity/emissivity patterns with geological landforms and stratigraphy visible in corresponding SAR images. There appears to be a correlation between locations on Venus where high emissivity at high altitudes and low emissivity at low altitudes are observed. These phenomena are attributed here to relatively recent volcanic activity: the former to summit eruptions that have not had time to weather to the low-emissivity state, the latter to continuing emission of volcanic gases from neighboring small plains volcanoes. The pattern of reflectivity and emissivity on Maat Mons is examined in the light of these findings. It is concluded that Maat Mons has undergone the most recent episode of volcanic activity of all the volcanoes studied here.
Robinson, Cordula A.↗
Fluid outflows from Venus impact craters - Analysis from Magellan data
Many impact craters on Venus have unusual outflow features originating in or under the continuous ejecta blankets and continuing downhill into the surrounding terrain. These features clearly resulted from flow of low-viscosity fluids, but the identity of those fluids is not clear. In particular, it should not be assumed a priori that the fluid is an impact melt. A number of candidate processes by which impact events might generate the observed features are considered, and predictions are made concerning the rheological character of flows produce by each mechanism. A sample of outflows was analyzed using Magellan images and a model of unconstrained Bingham plastic flow on inclined planes, leading to estimates of viscosity and yield strength for the flow materials. It is argued that at least two different mechanisms have produced outflows on Venus: an erosive, channel-forming process and a depositional process. The erosive fluid is probably an impact melt, but the depositional fluid may consist of fluidized solid debris, vaporized material, and/or melt.
Asimow, Paul D.↗
What heated the parent meteorite planets?
The plausibility of the two most wide discussed mechanisms, decay of short-lived Al-26 and solar wind induction heating, for heating the small planetesimals in which the meteorites formed are examined and shown to have significant problems. The main problem for the Al-26 decay mechanism is the fact that eucritic lavas, melted by the mysterious heating mechanism in some early planetesimal, did not contain enough Al-26 to decay to radiogenic Mg-26 when they erupted to their planetesimal surface and cooled. It is necessary to postulate that the lavas lingered underground while their Al-26 decayed away. The solar wind induction heat concept has the problem that astrophysical evidence has made is seem increasingly unlikely that an intense solar wind flux blew past planetesimals in the early solar system. Instead, it was probably collimated in the direction of the sun's poles by the persistence of the solar nebula during the T Tauri epoch.
Wood, John A.↗
Refractory solids in chondrites and comets: How similar
The grains of ice, dust, and organic material that came together to form the solar system have been preserved to differing degrees in the most primitive solar system bodies, asteroids and comets. The study of samples of asteroids (in the form of chondritic meteorites) reveals that the dust component was extensively altered by high-temperature events and processes in the early solar system, before it was aggregated into chondritic planetesimals. The nature of these high-temperature events and processes is not known, but the evidence of their operation is pervasive and unequivocal. Are the refractory particles in comets likely to be similar to these chondrite components. Probably not (except for the presolar carbonaceous grains in chondrites), because the chondritic components are products of severe thermal processing, and all imaginable energy sources that could have provided the heat tend to diminish with distance from the sun. Every indication is that comets formed at much greater radial distances than asteroids, so the particles they incorporated would have experienced less heating. The possibilities cannot be completely ruled out that comets, too, formed inside the present orbit of Jupiter, or that thermally-processed grains were able to diffuse great radial distances before being incorporated in accreting objects, but it is far more likely that most of the refractory grains in comets have been spared the extreme thermal processing that shaped the character of chondritic components.
Wood, John A.↗
Physical processes and timescales in planet and small-body accretion
A series of processes in the solar nebula transforms infallen interstellar grains into planets. Somewhat surprisingly, samples of planetary material from the very first stages of this processing and aggregration were preserved, and are accessible for study in the terrestrial laboratory. These are the chondritic meteoroids, which passed the last 4.5 Gyr safely stored in asteroids. Thermal processing of interstellar grains, sticky agglomeration of chondrules, Goldreich-Ward instability, and gravitational accretion of planetesimals into planets are discussed.
Wood, John A.↗
Chondritic meteorites and the solar nebula
Present understanding of the formation of chondritic meteorites in the solar nebula is reviewed, focusing on the nebular history of chondritic material. The stages in the history of chondritic material are summarized. The processing of chondrules and refractory inclusions is considered, and the relatedness of chondrules and refractory inclusions is discussed.
Wood, John A.↗
A review of solar nebula models
The major changes that have occurred in thinking about protosolar nebula models are discussed. The concept favored by astrophysicists for the last decade, that of a viscous accretion-disk nebula, is examined. The properties of recent accretion-disk models that are most relevant to chondrite-forming processes are noted.
Wood, John A.↗
The origin of ferrous zoning in Allende chondrule olivines
Very similar major and minor element compositions are noted in the ferrous olivine occurring in chondrules at olivine grain boundaries, along cracks in olivine grains, interleaved with enstatite, and in the inner portions of exposed olivine grain surface rims; simultaneous formation by a single process is therefore suggested. The ferrous chondrule olivine probably formed by the reaction of chondrules with very hot nebular vapors over a period of several hours, followed by the condensation of residual metal vapors onto those olivine surfaces that were in direct contact with the gas as the system cooled. The ferrous chondrule olivine that occurs interleaved with enstatite in Allende does not have a composition idendical to, and is not the precursor of, matrix olivine.
Peck, Julia A.↗