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INTERNAL CONSTITUTION AND MECHANISMS OF
A V/VA BRECHER
It has been generally assumed in the past that the fragmentation of asteroidal bodies and the production of meteorites are solely the result of collision events. (See Dohnanyi, 1969; Hartmann and Hartmann, 1968; Wetherill, 1967.)
A possible mechanism of noncollisional fragmentation will be proposed below, its proper framework of applicability will be defined, and evidence suggesting and supporting its existence will be adduced. Briefly, it is shown that the presence of even trace amounts of hydrogen in meteoritic metal phases (Edwards, 1955) may have caused the parent bodies of iron meteorites to undergo, spontaneously, delayed brittle fracture under the action of prolonged slow stresses, the imprint of which has been recorded in the phase structure of meteorites (Baldanza and Pialli, 1969). This phenomenon, termed “hydrogen embrittlement,” has been amply documented in the literature on the metallurgy of ferrous metals (Bernstein, 1970; Tetelman, 1969).
INTERNAL CONSTITUTION OF ASTEROIDAL BODIES
Inferences on the internal constitution of asteroids are based on several lines of evidence.
First, an average density can be obtained if independent determinations of the mass and diameter of the body are made. Such data exist for Vesta and Ceres, yielding p - 5 + 1 g/cm3 in the latest estimate by Schubart." These densities are compatible with a high content of metallic nickel/iron, corresponding on the average to mesosideritic or pallasitic (p - 5 g/cm3) composition (~50 percent by volume of meteoritic Ni/Fe).
Second, recent spectral reflectivity data (Chapman, Johnson, and McCord;2 McCord, Adams, and Johnson, 1970) may yield information on the surface composition of the asteroids. The identification of the ferromagnesian silicate pyroxene on Vesta and the similarity of the overall spectrum to that of basaltic
achondrites do not rule out a high metal content for Vesta because basaltic achondrites (stony meteorites) are similar in composition to the silicate component of mesosiderites (stony iron meteorites). The possibility of high metal content is supported also by the fairly high density inferred for Vesta and is consistent with its very high albedo (Hapke?). Compositional differences between Ceres, Pallas, and Vesta are also apparent in spectral reflectivity data (McCord, Adams, and Johnson, 1970). This may be the rule in the asteroid belt, rather than the exception, as Levin (1965) suggested. An expectation of compositional diversity arose with the study of meteorites, which provides the largest body of evidence brought to bear, by implication, on the structure and composition of asteroidal bodies. The view that meteorites originated in asteroidal bodies (Anders, 1964) is entirely consistent with the assumption that many observed properties of meteorites are primordial and thus reflect the conditions prevailing during the condensation and accretion of small bodies in the solar system (Anders, 1964; Arrhenius and Alfvén, 1971). Such an assumption is particularly important with regard to metallic (Nisfe) phases in all meteorites, which have been used extensively and exclusively to determine cooling rates and parent body sizes (Buseck and Goldstein, 1968; Goldstein and Short, 1967; Powell, 1969; Wood, 1964; Wood, 1967). It was recently shown (Fricker, Goldstein, and Summers, 1970) that for very slowly cooled classes of meteorites such as the pallasites (0.5 to 2 K/10° yr) or mesosiderites (~0.1 K/106 yr) the parent body size cannot be specified uniquely and may be larger than asteroidal. In any case, the cooling rates of various classes of meteorites alone provide a strong argument against an origin of stony irons in the same (differentiated) parent body with iron or stony meteorites (Buseck and Goldstein, 1968; Fricker, Goldstein, and Summers, 1970; Powell, 1969). Sizes of iron meteorite parent bodies, however, based on cooling rates of 0.5 to 500 K/106 yr, encompass the range 10 to ~450 km in radius and are still compatible with observed sizes of asteroids, although the models are inadequate for discrimination between a “core” or a “raisin” origin (Fricker, Goldstein, and Summers, 1970; Levin, 1965). The evidence from meteorites clearly suggests an origin in a multiplicity of parent bodies (required by the existence of discrete chemical groups and different cooling rates for various classes of meteorites, as well as by the scatter in cooling rates within a class) of relatively large sizes (as required by slow cooling rates and by the large-scale continuity of Widmanstätten patterns in iron meteorites), which were fragmented in a few, discrete, large events (as indicated by the conspicuous clustering of cosmic-ray ages). (See Anders, 1964; Hartmann and Hartmann, 1968.) Mass balance arguments (Arnold, 1965), as well as the longer cosmic-ray exposure ages of iron meteorites, make an origin of these objects in fairly massive asteroids compatible with most evidence to date. Yet the study of size
and mass distributions of various groups of asteroids (Anders, 1965; Brecher and Alfvén, 1969-70; Hartmann and Hartmann, 1968) indicates that very few major collision events have altered the reconstituted primordial distribution and that the long collision lifetimes (T P 10° yr for m > 1018 kg) preclude frequent collisional disruption of massive bodies; moreover, secondary collisions are also less frequent than expected (Dohnanyi, 1969), as reflected in the relatively narrow spread in cosmic-ray exposure ages of meteorite classes (Anders, 1964; Anders, 1965; Arnold, 1965; Hartmann and Hartmann, 1968). An interesting alternative to their destruction in collision events can be conjectured for some massive parent bodies of iron meteorites (with a low probability of collisional destruction but with a considerable amount of strong Ni/Fe) and for parent bodies with a mesosideritic or pallasitic structure; i.e., with the metal (Nisfe) phase continuous in three dimensions conferring structural strength. It involves spontaneous brittle failure of the parent body due to hydrogen embrittlement of the Nisfe phases.
HYDROGEN EMBRITTLEMENT AND THE PRODUCTION OF
In a recent study of the mechanical properties of iron meteorites, Gordon (1970) found the internal structure of about 150 samples to have preserved surprising perfection over large dimensions for bodies presumed to have resulted from violent collisions. Moreover, no evidence was found of the large-scale plastic deformation or ductile fracture expected on the basis of shock loading experiments on the Odessa iron. He concluded that the iron meteorites appear to have formed in brittle fracture events; yet the metal of the Gibeon octahedrite was remarkably ductile and strong. Moreover, no tendency was found in the samples studied for preferential fracture along octahedral planes nor for embrittlement due to inclusions. The fact that the meteoritic metal was not intrinsically brittle (and it did not become brittle at any testing temperature down to 100 K) forced Gordon to conclude that “a mechanism of embrittlement must function for all meteorites having a Widmanstätten structure if these are to be considered fragments of a larger metal mass.” Unable to find such a mechanism, he assumed that small, meteorite-size masses of Nisfe must have been embedded in intrinsically brittle silicates.
A well-known embrittling agent of ferrous metals is gaseous hydrogen, and the phenomenon of hydrogen embrittlement has been extensively reviewed. (See Barth and Steigerwald, 1970; Bernstein, 1970; Groeneveld, Fletcher, and Elsea, 1966; Nelson, Williams, and Tetelman, 1971; Tetelman, 1969.) The loss in ductility caused by the introduction of hydrogen into Nisfe alloys to levels of a few parts per million is not detectable under impact loading conditions (i.e., at collisions), but only at very low strain rates of € 30.05 per minute (Tetelman and McEvily, 1967) and under static or sustained stresses. Thus, the susceptibility of meteoritic metal to structural hydrogen embrittlement could not have been detected at the relatively high strain rates (é < 0.3 per minute) in Gordon's (1970) tests. Nor is the hardness or the yield strength of iron meteorites affected by the presence of hydrogen, so that data available for the Gibeon (Gordon, 1970), Odessa, Sikhote-Alin, Canyon Diablo, and Henbury irons and for the Brenham pallasite (Baldanza and Pialli, 1969; Knox, 1970) may reflect the intrinsic, structure-dependent properties of meteoritic Fesni alloy phases (Baldanza and Pialli, 1969, table 1). The presence of hydrogen, however, reduces the fracture strength so that microcracks can start to propagate unstably after an incubation time during which the internal hydrogen reaches a critical configuration (Tetelman, 1969). Thus the body undergoes seemingly spontaneous brittle fracture, often after having withstood previous dynamic impacts or high loads. The incubation time before failure is relatively insensitive to stress level but is sensitive to stress rate. The embrittlement is promoted not only by low strain rates or prolonged quasi-static loading but also by concentration gradients in hydrogen. The physical picture of hydrogen embrittlement is briefly that of local stress fields in the metal lattice caused by screened protons at interstitial sites, by atomic H pinned at defects and grain interfaces, and even by H2 molecules recombined in internal voids. The amounts sufficient to cause brittle failure in steel can be less than 1 ppm by weight of average H content. Details about solubility in the o and y phases of Nisfe, and about the possible mechanisms of embrittlement will be given elsewhere (Brecher, 1971). Suffice it to say that a variety of environments can supply the internal and/or external hydrogen necessary for brittle fracture of a metal body, such as corrosive atmospheres of H2S and H2O (which may have existed at various stages of the formation of meteorites), fields of accelerated protons implanting hydrogen such as the solar wind, or partially ionized and dissociated low-pressure interplanetary gas media containing atomic hydrogen as an important constituent (Arrhenius and Alfvén, 1971; Nelson, Williams, and Tetelman, 1970). The seed of self. destruction may have been planted in parent bodies of iron meteorites at birth as hydrogen was occluded during the grain condensation and growth stages, in the presence of abundant hydrogen. Moreover, continuous surface implantation of solar-wind protons may have provided the local hydrogen pressure gradients and local strains known to initiate microcracks and thus may have promoted failure by brittle fracturing under unstable crack propagation. Is there evidence for the presence of hydrogen in iron meteorites? The old work on thermal release patterns of gases reported by Farrington (1915) showed that hydrogen was the most abundant gas phase released from iron meteorites at levels of 3 to 55 ppm; by chemical methods, Nash and Baxter (1947) detected minimal levels of a few tenths of a part per million of H2. More sophisticated determinations by Edwards (1953, 1955) revealed surprisingly high levels of hydrogen in iron meteorites (up to ~33 ppm average content, and up to ~55 ppm in the fine-grained fraction), leading Edwards (1955) to conclude that the hydrogen “must have been originally incorporated during the formation of the meteorites.” The H/D ratios typical of iron