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W.F. Bottke (SwRI), A. Morbidelli (Obs. Nice), D. Nesvorny (SwRI)
The largest impacts on Earth since the K/T event both occurred in the Late Eocene: the Popigai crater in Siberia (D = 100 km; 35.7 ±0.2 Ma) and the Chesapeake Bay crater off the eastern US seaboard (D = 40 km; 35.5± 0.6 Ma). At the same time, an unusually high flux of interplanetary dust particles was recorded by 3He measurements in well-dated marine sediments from the Indian/Atlantic Oceans. Farley et al. (1998) argued a comet shower produced these events. The impact melt at Popagai, however, was produced by an L-chondrite impactor (Tagle and Claeys 2004). Thus, the so-called Late Eocene event was likely produced by an ``asteroid shower". Note that asteroid showers were originally thought to be byproducts of main belt family-forming events, with the ejecta directly injected into nearby resonances (Zappala et al. 1998). A search by our team, however, indicates no known family can produce this event.
We postulate here a related but alternative mechanism to produce asteroid showers. Most large terrestrial impactors (D > 5 km) escape the main belt onto Mars-crossing orbits via tiny resonances in the inner main belt. Here they reside until close encounters push them into a powerful resonance (usually the v6 secular resonance) that quickly takes them to an Earth-crossing orbit. At the same time, these objects continue to pass through the main belt, where they are ``sitting ducks" for main belt projectiles. Using collision evolution model results, we find a D > 5 km asteroid in the v6 resonance should disrupt once every 70 My. The fragments, which often have low inclination orbits and thus high collision probabilities with Earth, have an impact probability distribution that is spiky; ~50% of those striking the Earth hit within 2 My. Finally, the dust produced by this event has a high likelihood of hitting Earth, consistent with the observed 3He spike.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.