Statistics – Computation
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27s.292s&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 292
Statistics
Computation
Scientific paper
Since the discovery of the Ir enrichment at the Cretaceous- Tertiary boundary a majority of the researchers have claimed a meteorite impact as origin of the event. But up till now the search for an impact crater has not been conclusive, and alternative explanations have been suggested, e.g., a volcanic origin (Hansen 1990). If, however, we maintain that the KT- boundary material is extraterrestrial, the missing crater constitutes a problem. The missing-crater-problem can be solved by postulating the existence of a temporary planetary ring around the Earth. We suggest the following scenario: an incoming asteroid is captured by the Earth inside the Roche limit, and the breakup of the asteroid creates a planetary ring. Atmospheric drag and partially inelastic collisions between particles cause the ring particles to lose energy and slowly accrete onto Earth. Once the asteroid is decomposed, the atmospheric drag on the ring particles will primarily drain the smaller particles from the ring. The figure shows residence times as a function of starting position. Each curve represents one particle size. Thus the needed amount of Ir is brought down to Earth as a gentle rain lasting perhaps thousands of years, without major crater production. Our 3D computer simulations of the ring dynamics show accretion profiles, which are comparable to the Ir profiles at the KT boundary. In our model partially inelastic collisions occur between ring particles (Brahic, 1976, 1977) and the particles experience a slight atmospheric drag (10^-14 atm at 0.75 Earth radii). The particles are injected into randomly oriented orbits near the Earth upper atmosphere, from 0.1 to 0.75 Earth radii. The number and the density profile of the inward spiralling particles are calculated, until the distance from the Earth is small enough to assure that they are lost to the Earth surface within a few hours. The profile reflects the composition of the ring, and thereby the asteroid. In conclusion we suggest that a planetary ring formed around the Earth prior to the KT boundary event, and that the Ir enrichment at the KT boundary layer is formed by a slow accretion from a planetary ring rather than from a giant impact. This explains the gradual rise in Ir content prior to the peak event at the boundary layer (Hansen et al. 1988) and the gradual decrease in Ir content found in the sediments after the peak event. References Brahic A. (1976) J. of Computational Physics 22, 171-188. Brahic, A. (1977) Astr. Astrophys. 54, 895-907. Hansen, H.J., Gwozdz, R., and Rasmussen, K.L. (1988). Revista Espanola Paleontologia. extra vol., 21-29. Hansen, H.J. (1990) Geological Soc. Am. Spec. Pap. 247, 417-423. Figure 1, which in the hard copy appears here, shows the atmospheric drag on ring particles.
Rasmussen Lars K.
Stage Michael
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