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vapor condensates. On the Moon, chondrulelike objects occur but they are relatively rare. To explain the striking difference in abundance, Whipple” has suggested a sorting mechanism acting in the meteorite parent environment.
Those lunar glass bodies that are formed with free surfaces range in geometry from perfect spheres to teardrops, dumbbells, and rods. Analysis of physical and chemical characteristics of these bodies (Isard, 1971) suggests that they were formed by breakup in flight of thin jets of impact-melted glass from the lunar surface. In contrast, meteorite chondrules practically always occur as spheroidal shapes of varying complexity. Hence it would seem that there are considerable differences in the formation of flight-cooled impact glass on the Moon on one hand and chondrules in the precursor environment of meteorites on the other. It is difficult to explain these differences on the basis of gravitational or compositional effects.
Generation and Crystallization of Melts
The lunar igneous rocks were found by numerous investigators to show textural and chemical similarities to a specific type of meteorite, the basaltic achondrites. (See, for example, Arrhenius et al., 1970; Duke et al., 1970; Reid et al., 1970). However, these two types of objects have a distinctly different oxygen isotope composition (Taylor and Epstein, 1970) suggesting their origin in different environments.
The frequently occurring grains in gas-rich meteorites that have been exposed to corpuscular irradiation in the range up to a few MeV, almost without exception show an all-sided exposure to this radiation (Lal and Rajan, 1969; Pellas et al., 1969; Wilkening et al., 1971). This has been interpreted by the discoverers of the phenomenon to be a result of exposure of the particles while they were freely suspended during the early stages of accretion. In contrast, such all-sided exposure is less common in the lunar regolith where a considerable fraction of particles, exposed to solar flare irradiation on the lunar surface, appear to have been irradiated mainly from one side before they were shielded by burial or a cohesive coating of fine dust.
One of the reasons for the occurrence of one-sided exposure of grains found on the lunar surface could be (Crozaz et al., 1970) that some of these grains received their irradiation while still part of exposed rock surfaces; the irradiated surfaces of these rocks subsequently would have disintegrated and the particles would have been transferred into the soil where shielding by material of the order of 10 to 30 pm thickness is sufficient to prevent further development of steep, high-density track gradients. The lack of the one-sided irradiation features in the achondrite crystals would then lead to the conclusion that in the parent environment of gas-rich achondrites, cohesive
rocks did not serve as a source of surface-exposed grains and hence probably were not present. To the extent that asteroids were formed in a way similar to these meteorite parent bodies, and provided that the mechanism proposed by Crozaz et al. is quantitatively important, similar conclusions would apply to the asteroidal precursor environment.
Another characteristic feature of the meteorite grains with direct surface exposure to corpuscular radiation is the gentleness of the process that has brought the grains together without destroying their highly irradiated surface skin (Wilkening et al., 1971), whereas other grains and aggregates in the same meteorite bear clear evidence of shock (Fredriksson and Keil, 1963; Wilkening et al., 1971).
At the time of the discovery of the skin implantation of low-energy cosmic-ray particles in grains now located in gas-rich achondrites (Eberhardt et al., 1965; Wänke, 1965), the isotropic distribution of impinging atoms, revealed by track techniques, was not known. Nor was the inhibited turnover behavior of aggregated particles in space yet known; this became evident only as a result of the lunar exploration (cf. the following section). Nonetheless, the perceptive suggestion was already at this stage made by Suess et al. (1964) that the irradiation took place while the individual particles were floating free in space, before their accretion into meteorite parent bodies. Lacking more direct evidence for this, and under the influence of the planetocentric reasoning of the time, the implantation process was relegated to surfaces of large bodies in most subsequent discussions.
The recent discovery of Lal and Rajan (1969) and of Pellas et al. (1969) returned the attention to the interesting alternative that the isotropic irradiation dates back to the largely unknown freeflight particle stage, preceding or concurrent with accretion. This interpretation avoids the difficulties associated with shielding at turnover of an accreted aggregate and is mechanically understandable in terms of theory and observation of particle streams in space (Alfvén, 1969, 1971; Alfvén and Arrhenius, 1970a,b; Danielsson;% Lindblad;7 Trulsenê). It must be remembered, however, that predictions from meteorites and lunar sediments constitute extrapolations, and the lesson drawn from the Moon suggests caution in the reliance on prediction in complex natural systems. Meteorites cannot be expected to furnish well-defined information on surface-related problems because the critical interface between the parent body and space, even if it were represented and preserved in the fragments that are captured by Earth, is destroyed at the passage through the atmosphere. Hence, actual samples collected in a controlled fashion on asteroids and comets and returned to Earth would be of unique value for the reconstruction of their surface evolution and of the preaccretive history of the materials.
SURFACE PROPERTIES AND SOURCE MATERIALS OF ASTEROIDS
The question of the physical behavior of fine grained particle aggregates in space is crucial for reconstructing the accumulation of primordial grains into planetesimal embryos and, until direct studies are possible, for postulating the conditions on the surface of the asteroids. In the time preceding lunar exploration, widely divergent estimates were made, ranging from vacuum welding of solid particles into a crunchy aggregate, to dispersion of particles by repulsive electrostatic forces into highly mobile, fluffy dust. Actual observations on the Moon have provided the first factual information and show that finely divided dielectric materials exposed to the space environment form a relatively dense, cohesive aggregate but without perceptible cold contact welding.
This marked cohesion is probably the reason why, as discussed above, lunar soil particles do not appear to turn around freely in the exposed surface monolayer of grains and that, as a result, surface grains with isotropically irradiated skins are in the minority on the Moon. Because this effect would appear to be independent of gravitation, a similar situation is likely to prevail on the surfaces of asteroids, regardless of their size.
IMPORTANCE OF FIELD RELATIONSHIPS
The materials that make up the asteroids and comets may be found, wholly or in part, to be similar to those that we already know from meteorites. It has been suggested (Anders”) that such an identification would be an embarrassment to the exploration effort. On the contrary, this would make it possible for us to apply the large body of experience in meteoritics to the problems of primordial solar system history in a more realistic fashion than is possible at the present time.
The critical information to be obtained from asteroid missions concerns not only the materials from which the objects are constructed. The explorations of Earth and the Moon have demonstrated that it is equally or more important to establish also the field relationships of these materials and the physical properties of the whole body. Only controlled probing and sampling of the asteroids will make it possible to seriously approach the problems of the original mechanism and timing of accretion, the relative role of breakup, the sequence of formation of material units, the possible effects of differentiation before and after accretion, the internal and surface structure of the bodies, and their record of the history of the asteroidal and Martian region, the Earth-Moon system, and the Sun.
The authors are grateful to the participants in the discussions at the Twelfth Colloquium of the International Astronomical Union in Tucson, Ariz., and for
further valuable criticism and comments from S. K. Asunmaa, A. Brecher, J. Z. Frazer, K. Fredriksson, D. Lal, D. Macdougall, and M. and J. Sinkankas. Generous support from NASA is gratefully acknowledged.
Alfvén, H. 1964, On the Formation of Celestial Bodies. Icarus 3, 57.
Reid, A. M., Frazer, J. Z., Fujita, H., and Everson, J. E. 1970, Apollo 11 Samples: Major Mineral Chemistry. Proc. Apollo 11 Lunar Sci. Conf. Geochim. Cosmochim. Acta 34, suppl. 1, vol. 1.
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