Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27..238j&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 238
Other
Scientific paper
Inhomogeneity of cometary nuclei has been established through the observed disruptions of comets [1] and through the determination of dust particle composition during the encounter of the Vega and Giotto satellites with comet Halley [2,3,4]. The raisin bread model of cometary nuclei [5,6] assumes the presence of solid (rock) and dust particle material set in the volatile rich, ice- cemented material. Rock material may contribute to the formation of dust particles. Gombosi and Houpis [5] argued that only the composition of dust particles derived from the icy, volatile component of the comet were analyzed and implied thus that the third cometary component present (raisins/rocks) has not been examined. The compositions of the cometary (Halley) dust and the interplanetary dust particles (IDPs) are "chondritic" (Blanford et al., 1988). It is difficult, therefore to estimate the proportion of cometary to asteroid-derived dust in near Earth space, e.g., among the IDPs [7] unless other criteria are available. Bolide multistation photographic tracking allows the determination of the orbital preencounter parameters of solid bodies (0.01-100,000 kg in mass) with the Earth, and allows us to classify them according to their ablation coefficient (tau), penetration depth into the atmosphere (PE), theoretical densities (sigma), and terminal velocities (V(sub)E). Four groups are recognized (Table 1). Three of the type I bolides were recovered as ordinary chondrites (Pribram, Lost City, and Innisfree). Ceplecha [8] has shown that 38% of bolides (fireballs) come from cometary orbits (11% from highly eccentric orbits typical of new comets), but most of the bolides (62%) originate at asteroidal orbits. Seven of the 14 known meteoric showers could be attributed to known comets: N,S Taurids to 1970 P/Encke, Lyrids to 1861 I Thatcher-Beaker, Perseids to 1862 III Swift-Tuttle- Simons, Orionids to 1835 III P/Halley, Draconids to 1946 V P/Giacobini-Zinner, Leonids to 1966 I Tempel-Tuttle, and Leo Minorids to 1739 Zanotti. Geminids were related to asteroid 3200 Phaeton, considered to be an "extinct comet." Spurny [9], using ablation coefficient and penetration depth criteria, found that Geminids (frequently) and Taurids (rarely) contain bolides of types I and II. This may indicate that meteoric showers from "comets" on AAA orbits contain some portion of "rocky" material comparable to chondrites. These observations revive Opik's (1963) idea that comets may be captured in the asteroid belt on AAA orbits and may contain (and supply) chondritic meteorites to the Earth [10]. If the relationship among large solid particles "native to the asteroid belt" and those from the outer solar system can be established, they can be scaled and applied to IDPs. We have studied the records of 292 bolides (Prairie and European networks) with measured terminal velocities. We attempt to use the terminal velocity, calculated density, estimated terminal mass, and mechanical strength to correlate features with the meteorite features. We compare the meteorite fall frequency [11] with the bolide features. Two extreme hypotheses (Table 1) are examined: (A) bolides of types IIIa and IIIb do not have equivalents among the meteorites and (B) all four bolide types have meteoritic equivalents, and only IDPs do not produce bolides (fireballs). If the entry parameters of meteoroids are similar, bodies with lower density should reach terminal velocity at higher altitudes than those with higher density. If it is assumed that fragmentation is the same for dense (I and II) and less dense bodies (IIIa and IIIb), the calculated terminal altitudes show that among the bolides exist materials with lower densities than those of recovered meteorites and that model A of the correlation between meteorite falls and bolide observations is likely [12]. If, however, the less dense bodies were more easily fragmented than denser bodies, the correlation is better for hypothesis B. Table 1, which in the hard copy appears here, shows fireball observations. Using the value of terminal velocities and the average value of ablation coefficients the terminal (residual) masses (m(sub)E) can be calculated. Among the bodies studied, 99 were heavier than 0.1 kg and 153 heavier than 0.01 kg. The parameter (m(sub)E) indicates the end of ablation in the atmosphere, but it cannot distinguish between meteoroids that were totally disintegrated and those genuinely decelerated. Similarly the calculated terminal altitudes and mechanical strength values do not provide a unequivocal interpretation. Correlation of bolide properties with meteorite falls could well be accommodated by the hypothesis B in which each bolide type has a meteorite equivalent. This has, however, some "outrageous" implications: comets may carry chondrites, icy dust balls do not produce fireballs; the extremely primitive carbon-rich particles represented by the IDPs do not form larger discrete bodies (fireballs) of "asteroidal" size; and the asteroid belt is a mixture of "native fractionated old bodies" together with the captured comets. Hypothesis B therefore contradicts the "established" scheme of the asteroid belt in which the non-differentiated meteorites (CI, CM, and CV) form the outer part of the asteroid belt, whereas the fractionated metamorphosed and igneous meteorites characterize the inner asteroids. References: [1] Whipple F. L., 1987, Phil. Trans. R. Soc. Lond., A 323, 339. [2] Dikov Yu. P. et al., 1991, Geochemistry International, 29, 33-38. [3] Kissel J., et al., 1986, Nature, 321, 326. [4] Solc M., et al., 1987, Publ. Astr. Inst. Czechosl., 67, 47. [5] Gombosi T. I. and Houpis H. L. F., 1986, Nature, 324, 43. [6] Delsemme A. H., 1977, Comets, asteroids, meteorites, Univ. of Toledo Press. [7] Gibson E. K., 1992, J. Geophys. Res., 97, E3 3865. [8] Ceplecha Z., 1988, Bull. Astr. Inst. Czech., 39, 221. [9] Spurny P., 1991, doc. diss., Astr. Inst. Czechosl. [10] Opik E. J., 1963, Adv. Astr. Astrophys., 2, 219. [11] Hutchinson R., et al., 1977, Appendix to Catalogue of Meteorites, British Museum, London. [12] Ceplecha Z. and McCrosky R. E., 1976, J. Geophys. Res., 81, 6257.
Jakes Peter
Padevet V.
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