On the Origin of U-shaped Structures in the Time Series of Meteorite Fall Frequencies

Details On the Origin of U-shaped Structures in the Time Series of Meteorite Fall Frequencies On the Origin of U-shaped Structures in the Time Series of Meteorite Fall Frequencies

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.582s&link_type=abstract

Meteoritics, vol. 30, no. 5, page 582

Astronomy and Astrophysics

Astronomy

Meteorites, Rings, Terrestrial, Roche Limit

Scientific paper

The time series of terrestrial accretionary events from 800 BC to 1750 AD seems to exhibit numerous periods of intensified accretion. Fall frequencies for several of the intensified accretionary periods, plotted against time, exhibit U-shapes. It has been suggested that each U-shaped fall frequency distribution reflects the presence of a temporary terrestrial ring, created from the break-up of a captured asteroidal body inside the Roche Limit [1,2]. We have investigated this hypothesis by applying a 3-D computer code to simulate the behavior of terrestrial rings. The model includes gravitational attraction from Earth, atmospheric drag from the terrestrial atmosphere and mutual collisions [3]. The time of meteorite falls, fire balls and shooting stars have been compiled from historical sources to constitute a time series of fall frequencies from 750 BC to AD 1750 [1,2]. In the time series 16 periods of intensified meteoritic flux with a U-shaped appearance are identified, corresponding to 36% of all intensified periods. The U-shape of the accretion profile somewhat resembles the structures found in the radial particle distribution in the rings of Saturn [4]. It would, thus, be reasonable to adopt a similar radial distribution for our ring model. It is also likely that tidal disruption of an asteroidal body will lead to a U-shaped particle disruption. As soon as the captured asteroid is inside the Roche limit, material will start to be spalled of the near-Earth and the far-Earth face of the asteroid. Material on the leading and trailing face of the asteroid will not be disrupted. Spallation continues until torque on the non-disrupted sides of the object will force the object to tumble, so that either the leading or trailing face will point towards Earth. The disruption process continues until the objected has become totally fragmented. As the major fraction of the asteroid volume is in the outer parts, spallation of the outer parts will create a large number of particles at the near-Earth and the far-Earth faces of the asteroid, while disruption of the central parts will only result in a small number of particles, thus creating a terrestrial ring with a U-shaped radial distribution. Model calculations show that under most conditions the accretion profile from a terrestrial ring will have only one maximum, despite a U-shaped radial start distribution [3]. In the first turbulent phase of the ring development with frequent collisions the particle orbits are often reorientated which leads to obliteration of the original radial distribution. For a very dilute ring, however, where even the first turbulent phase have relatively few collisions the radial distributions is not obliterated and the accretion profile reflects the original particle distribution, which means that the accretion profile will have two distinct maxima and a period between the maxima also with intensified accretion. With selected input parameters accretion profiles from our model are in accordance with the shape of the observed intensified accretionary events. The duration of the accretion period from a planetary ring varies from a few years to hundreds of years. Accretion from a dilute ring around the Earth is therefore a possible explanation of the U-shaped fall frequency distributions observed in the historical record. References: [1] Rasmussen K. L. (1990). Q. J. Astron. Soc., 31, 95-108. [2] Rasmussen K. L. (1991). Q. J. Astron. Soc., 32, 25-34. [3] Stage M. and Rasmussen K. L. (1995) Icarus, submitted. [4] Cuzzi J. N. et al. (1984) in Planetary Rings (R. Greenberg R. and A. Brahic, eds.), 73-199.

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