Physics
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..370h&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 370
Physics
Accretion, Hot, Chondrites, Ordinary, Chondrules, Solar System, Origin
Scientific paper
It is frequently assumed that chondrites represent material from a protosolar nebula that has not been reprocessed in planets and that the Sun and planetary bodies are coeval [1]. Woolfson [2] outlines five theories of the origin of the Solar System, each with its successes and problems. An important feature that any theory must explain is the tilt of the Sun's spin axis at 7 degrees from the normal to the ecliptic. This indicates that some (or all) of the matter that formed the planets was acquired from outside the Sun's mid-plane. A simple rotating nebular model is not in itself viable for the origin of the Sun and planets together. Interaction between the matter rotating in the Sun's midplane and that from outside probably led to collisions between early formed planets. Planetary impact, cited for the origin of the Moon [3], or asteroids [4], has been dismissed as a mechanism for chondrule formation [5], mainly on negative evidence. Variable refractory inter-element ratios (e.g., Ca/Al) indicate that some chondrules did not form by melting nebular dust [6]. Ordinary chondrites contain rock fragments with chemical signatures indicative of differentiation under gravity [7]; one formed when ^26Al was 'live' [8]. The products of different oxygen isotopic reservoirs co-exist in the ordinary chondrites, so the ingredients come from various sources and testify to impact and mixing of some planetary materials, plus a presolar component [9]. Textures show that some chondrules aggregated when hot and plastic. Magnetic [10] and metallographic [11] data indicate that ordinary chondrites accreted hot, rather than cold followed by prograde metamorphism. The counter arguments of [12] need not be valid because a fundamental premise, that chondrules are nebular, cannot be taken for granted. The prospects for future study are consequently brighter than before, because the ordinary chondrites provide a window on presolar conditions and contain samples of differentiated planetary bodies that may not now exist. References: [1] Kerridge J. F. and Matthews M. S. (1988) Meteorites and the early solar system, University of Arizona. [2] Woolfson M. M. (1993) J. R. Astr. Soc. 34, 1- 20. [3] Newsom H. E. and Taylor S. R. (1989) Nature, 338, 29-34. [4] Hughes D. W. (1991) J. R. Astr. Soc. 32, 133-145. [5] Taylor G. J. et al. (1983) In Chondrules and their origins (E. A. King, ed.), 262-278, LPI. [6] Hutchison R. et al. (1988) Phil. Trans. R. Soc. Lond., A325, 445-458. [7] Kennedy A. K. et al. (1992) EPSL, 113, 191-205. [8] Hutcheon I. D. and Hutchison R. (1989) Nature 337, 238-241. [9] Huss G. R. (1990) Nature, 347, 159-162. [10] Morden S. and Collinson D. W. (1992) EPSL, 109, 185-204. [11] Smith D. G. W. and Launspach S. (1991) EPSL, 102, 79-93. [12] Haack H. et al. (1992) Geophys. Res. Lett., 19, 2235-2238.
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