ISOTROPIC ABUNDANCE OF METEORITIC LITHIUM
Determination of the isotopic abundance of meteoritic lithium by mass spectrometric techniques
STONY METEORITE↗
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Compilation of cosmogenic radionuclides in meteorites
All available data for the concentration of cosmogenic nuclides Mn-53(t(1/2) = 3.7 x 10(6) years), Al-26 (7.05 x 10(5) years), Be-10 (1.6 x 10(6) years), Cl-36 (3.0 x 10(5) years) and Ne-21, and Ne-22/Ne-21 ratios in stony, iron, and stony-iron meteorites were compiled. For iron meteorites, the He-4/Ne-21 ratio was adopted instead of Ne-22/Ne-21 ratio, because the He-4/Ne-21 ratio in iron meteorites indicates the shielding condition of the sample. The compilation contains over 2000 different analyses for four cosmogenic radionuclides.
Nishiizumi, K.↗
Origin of differentiated meteorites
The larger asteroids with low inclination in the inner portion of the asteroid belt are evaluated as sources of differentiated meteorites (irons, stony irons and achondrites). It is found that the calculated earth impact rate is in agreement with the observed flux of these bodies. The frequent occurrence of elapsed times before impact of 0-1000 m.y. is in agreement with the observed typical exposure ages of iron meteorites, and the observed occurrence of some very young (less than 100 m.y.) exposure ages. The absence of many greater than 1000 m.y. iron meteorite exposure ages and greater than 50 m.y. achondrite exposure ages must be attributed to collisional destruction as the calculations predict a significant number of these older ages. Chemical and isotopic data on these bodies suggest a relationship to the earth and moon, and it may be that they represent residual material from earth's accretional zone.
Wetherill, G. W.↗
Circumstellar material in meteorites - Noble gases, carbon and nitrogen
In addition to preserving a record of isotopically distinct reservoirs in the early solar system, some primitive meteorites contain discrete grains of presolar origin. Such grains are distinguished by the isotopically anomalous noble-gas components they contain. One such component consists of monoisotopic Ne-22, produced by decay of radioactive Na-22 with a 2.6 yr half-life. Two xenon components have also been identified: one synthesized apparently in a supernova, the other probably in a red giant star. Most of the grains that carry these noble-gas components are carbonaceous and contain isotopically anomalous C, N, or both. They include diamond and silicon carbide. Two unidentified carriers of isotopically anomalous nitrogen, unaccompanied by noble gases, occur in the brecciated stony iron meteorite, Bencubbin.
Anders, Edward↗
Possible correlation between classifications and potassium-argon ages of chondrites.
Potassium-argon and uranium-helium age measurements for l and h group chondrites
STONY METEORITE↗
Interaction of Meteoroids with the Atmosphere. I - Stony Micrometeorites
Process of energy dissipation through radiation and evaporation, deceleration by drag, and ablation through evaporation of spherical stony micrometeorites in earth atmosphere
ENERGY DISSIPATION↗
Achievements in planetology, 1967
Planetology, terrestrial geochemistry and geophysics, impact cratering and metamorphism, lunar probes, stony-iron meteorites, and tektites - literature review for 1967
Source record↗
Shock history of mesosiderites.
The (still largely incomplete) shock histories of 18 stony-iron meteorites (classified as mesosiderites) are studied. The crystallographic character of kamacite in each (polished and etched) sample was determined by a back-reflection X-ray diffraction technique. All specimens were studied metallographically, and some by reflected light microscopy. The shock history of each sample is determined in kbar.
Jain, A. V.↗
Minor planets and related objects. XX - Polarimetric evidence for the albedos and compositions of 94 asteroids
Polarimetric observations are presented and analyzed for a total of 94 asteroids. The data include numerical parameters describing the polarization-plane curve, the maximum depth of the negative branch, geometric albedo, and diameter. With few exceptions, the polarizations are found to be repeatable from hour to hour, night to night, and apparition to apparition at the same solar phase angle within an observational accuracy of plus or minus 0.05%. It is shown that over a range of more than a factor of 50 in diameter or a factor of 100,000 in mass, all the asteroids seem to have microscopically very rough or particulate surface textures. In terms of their polarimetric properties, the asteroids are classified as follows: 48 in the broad S class, with compositions corresponding to stony-iron meteorites or ordinary chondrites; 34 in the C class, probably corresponding to carbonaceous chondrites; three to five M asteroids with surfaces rich in free metal; two with low-opacity surfaces (class E) attributable to pure enstatite; and five of other types.
Zellner, B.↗
Visual and infrared photometry of asteroids
Results are reported for a survey of asteroids at 0.56, 1.6, and 2.2 microns. The observations of 30 asteroids are reduced to relative reflectances at 1.6 and 2.2 microns (such that the relative reflectance at 0.56 micron is scaled to unity). These relative reflectances have important implications for the classification of asteroids. Low-albedo asteroids show a significant range in their infrared relative reflectances, but carbonaceous chondritic-type material remains a good candidate for their surface composition. Many S-type asteroids are found to have significantly brighter infrared relative reflectances than M-type asteroids. Lunarlike dark glass on the surfaces of these objects is probably ruled out. A metallic phase is the most plausible candidate, which implies a multiple-component surface composition perhaps similar to certain stony-iron meteorites. The data suggest that such a metallic phase may be common in the inner asteroid belt.
Veeder, G. J.↗
The diameter and albedo of 1943 Anteros
The results of broadband visual and infrared photometry of the Apollo-Amor asteroid 1943 Anteros during its 1980 apparition are reported. By means of a radiometric model, a diameter of 2.3 + or - 0.2 km and a visual geometric albedo of 0.13 + or - 0.03 is calculated. The albedo and reflectance spectrum of Anteros imply that it is a type S asteroid. Thus, Anteros may have a silicate surface similar to other Apollo-Amor asteroids as well as some stony-iron meteorites.
Veeder, G. J.↗
First spacecraft encounter with an asteroid approaches
During the course of the Galileo spacecraft's journey to Jupiter it will make two excursions through the steroid belt situated between Mars and Jupiter. The first excursion involves an encounter with the asteroid 951 Gaspra, which will take place on October 29, 1991. Gaspra is a small (about 15 km diameter) asteroid near the outer edge of the main asteroid belt. It's spectral classification is S, suggesting a composition similar to those of stony-iron meteorites. A figure is given showing the brightness of this asteroid as a function of time.
Tholen, David J.↗
Mesosiderite clasts with the most extreme positive europium anomalies among solar system rocks
Pigeonite-plagioclase gabbros that occur as clasts in mesosiderites (brecciated stony-iron meteorites) show extreme fractionations of the rare-earth elements (REEs) with larger positive europium anomalies than any previously known for igneous rocks from the earth, moon, or meteorite parent bodies and greater depletions of light REEs relative to heavy REEs than known for comparable cumulate gabbros. The REE pattern for merrillite in one of these clasts is depleted in light REEs and has a large positive europium anomaly as a result of metamorphic equilibration with the silicates. The extreme REE ratios exhibited by the mesosiderite clasts demonstrate that multistage igneous processes must have occurred on some asteroids in the early solar system. Melting of the crust by large-scale impacts or electrical induction from an early T-Tauri-phase sun may be responsible for these processes.
Mittlefehldt, David W.↗
More shock recovery experiments on mesosiderite analogs
Mesosiderites, a small but unique group of stony-iron meteorites with affinities to howardites, eucrites, and pallasites, remain enigmatic in terms of their petrogenesis. They are composed of approximately equal weight proportions of Fe-Ni metal plus troilite and gabbroic, basaltic, and orthopyroxenitic materials. The metal and silicates, which display variable grain sizes and shapes, are delicately intermingled, forming irregular grain boundaries that have been attributed to a wide range of origins from subsolidus metamorphism to supersolidus igneous processes. Perhaps the most relevant question regarding the petrogenesis of mesosiderites is: what is the source and duration of heating that could produce the unequilibrated textures and chemistry of these meteorites? A leading candidate appears to be impacts of metallic core fragments with a differentiated asteroidal surface. This provides not only a suitable source of heat, but also the metal component uniquely required by mesosiderites. A series of shock recovery experiments on mesosiderite analogs has been continued. Textural and chemical similarities have been found that support an impact-derived origin for these unusual meteorites.
Rowan, L. R.↗
Formation of Mesosiderites: Fragmentation and Reaccretion of a Large Differentiated Asteroid
We propose that these stony-iron meteorites formed when a 50-150 km diameter projectile disrupted a 200-400 km diameter asteroid with a molten core. Several mineralogical features of mesosiderites need reinterpreting if our model is correct. Additional information is contained in the original extended abstract.
Scott, Edward R. D.↗
Phosphates in pallasite meteorites as probes of mantle processes in small planetary bodies
Trace element analyses of the phosphates minerals in stony-iron pallasite meteorites are used here to investigate the magmatic history of the silicate portions of pallasites. In Eagle Station and seven other pallasites, the phosphates have relatively low concentrations of REEs and are strongly enriched in heavy relative to light REE. These patterns are consistent with formation of phosphate by subsolidus reactions between metal and silicate, in which phosphate inherits the REE pattern of olivine. In Springwater and Santa Rosalia, calcium-rich phosphates have higher concentrations of REE, are enriched in light relative to heavy REE, and have negative europium anomalies. These patterns are consistent with crystallization of phosphate from a europium-depleted chondritic liquid. This is unlikely to have happened near the base of the differentiating parent-body mantle; it suggests that some pallasites may come from regions of their parent bodies much nearer the surface than the core-mantle boundary.
Davis, Andrew M.↗
Geologic History of Asteroid 4 Vesta
Some types of meteorites - most irons, stony irons, some achondrites - hail from asteroids that were heated to the point where magmatism occurred within a very few million years of the formation of the earliest solids in the solar system. The largest clan of achondrites, the howardite, eucrite and diogenite (HED) meteorites, represent the crust of their parent asteroid]. Diogenites are cumulate harzburgites and orthopyroxenites from the lower crust whilst eucrites are basalts, diabases and cumulate gabbros from the upper crust. Howardites are impact-engendered breccias mostly of diogenites and eucrites. There remains only one large asteroid with a basaltic crust, 4 Vesta, which is thought to be the source of the HED clan. Differentiation models for Vesta are based on HED compositions. Proto-Vesta consisted of chondritic materials containing Al-26, a potent, short-lived heat source. Inferences from compositional data are that Vesta was melted to high degree (≥50%) allowing homogenization of the silicate phase and separation of a metallic core. Convection of the silicate magma ocean allowed equilibrium crystallization, forming a harzburgitic mantle. After convective lockup occurred, melt collected between the mantle and the cool thermal boundary layer and underwent fractional crystallization forming an orthopyroxene-rich (diogenite) lower crust. The initial thermal boundary layer of chondritic material was replaced by a mafic upper crust through impact disruption and foundering. The mafic crust thickened over time as additional residual magma intrudes and penetrates the mafic crust forming plutons, dikes, sills and flows of cumulate and basaltic eucrite composition. This magmatic history may have taken only 2-3 Myr. This magma ocean scenario is at odds with a model of heat and magma transport that indicates that small degrees of melt would be rapidly expelled from source regions, precluding development of a magma ocean. Constraints from radiogenic Mg-26 distibutions suggest that the parent asteroid of HEDs was much smaller than Vesta. Thus, first-order questions regarding asteroid differentiation remain.
Mittlefehldt, David W.↗
A source mechanism for meteorites controlled by the Yarkovsky effect
Some previous difficulties associated with attributing an asteroid-belt origin to meteorites are briefly reviewed. To overcome these, a two-step mechanism is proposed by which small fragments produced by asteroid collisions are gradually sent into eccentric earth-crossing orbits, while the larger parent bodies themselves remain relatively unaffected. Central to this mechanism is the Yarkovsky effect, which arises from the asymmetric reradiation emitted by an illuminated rotating body. Not only can the Yarkovsky effect be three orders of magnitude greater than the Poynting-Robertson drag, but the Yarkovsky acceleration can also be either positive or negative, depending on the sense of the body's rotation. The second stage of the proposed mechanism becomes important only when either secular acceleration causes the orbital elements of the body to evolve into a secular resonance with Jupiter. The Yarkovsky acceleration is explicitly calculated for both cylindrical and spherical bodies. When the orbital consequences of this acceleration acting alone on small asteroid-belt fragments are determined, the results are found to be reasonably consistent with both the relative and absolute cosmic-ray exposure ages of stony and iron meteorites.
Peterson, C.↗