Biology
Scientific paper
May 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000dda....31.0605d&link_type=abstract
American Astronomical Society, DDA Meeting #31, #06.05; Bulletin of the American Astronomical Society, Vol. 32, p.862
Biology
Scientific paper
The cumulative mass distribution of small bodies in the Solar System is often taken to be a power law, N(>m) m-γ , with γ < 1. A theoretical strength-independent collisional cascade has γ {5/ 6} (J. S. Dohnanyi 1969, J. Geophys. Res. 74, 2531; D. R. Williams and G. W. Wetherill 1994, Icarus 107, 117; H. Tanaka et al. 1996, Icarus 123, 450; D. D. Durda et al. 1998, Icarus 135, 431). Present-day impacts on the Earth and Moon are generally thought to be dominated by asteroids. For Near-Earth Asteroids (NEAs) with diameters d > 0.2 km, the Spacewatch and NEAT surveys found γ = 0.66 and 0.58, respectively (D. L. Rabinowitz 1993, Ap. J. 407, 412; D. L. Rabinowitz et al. 2000, Nature 403, 165). The distribution apparently steepens to γ 0.8--0.9 for NEAs with d > 5 km (W. F. Bottke et al. 2000, Science, in press). Finally, the application of impact scaling relations to crater counts on the Moon, Mars, and Venus yields values of γ ranging from 0.5 to 0.9 (K. J. Zahnle and N. H. Sleep 1997, in Comets and the Origin and Evolution of Life). Impactor distributions with γ < 1 are top-heavy; much of the total mass is likely to be delivered by one or a few bodies, and the characteristic total mass that has struck a planet after N impacts scales as N1/γ (S. Tremaine and L. Dones 1993, Icarus 106, 335; A. D. Anbar et al. 2000, JGR Planets, submitted). As part of a study of one model for the lunar Late Heavy Bombardment (H. F. Levison et al. 2000, Icarus, in preparation), we are investigating the statistical properties of the largest impactors on the Moon and Earth. Here we ask: Based upon the Moon's post-mare cratering record, how big was the largest body to strike the Earth in the last 3 billion years? Our preliminary results suggest that at least one Imbrium-scale impact ( 2 x 1033 ergs, some 100 times more energetic than the K-T impact 65 Myr ago) is likely if γ < 0.9. Assuming an asteroidal impactor, the mass of the putative impactor is 1021 g, which is at least 10 times the mass of the largest present-day NEA, 1036 Ganymed (P. Michel et al. 1999, Astron. Astrophys. 347, 711). We will discuss the sensitivity of these results to our assumptions about the transport of asteroids from the main belt to 1 AU, and will touch on the implications for the history of life on Earth. This work was supported by the NASA Origins and Exobiology programs.
Bottke William F. Jr.
Dones Luke
Levison Harold F.
Zahnle Kevin J.
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