Near-Misses and Collisions With Comets

Details Near-Misses and Collisions With Comets Near-Misses and Collisions With Comets

Sep 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999dps....31.0608a&link_type=abstract

American Astronomical Society, DPS meeting #31, #06.08

Astronomy and Astrophysics

Astrophysics

Scientific paper

Levison et al. (1999) estimate the present impact rate R_I on Jupiter due to ecliptic comets (ECs) with diameters >2 km at 1/1300 years. Assuming this comet population has declined as 1/time since the origin, the averaged impact rate over the last 4 Gyr is higher by log {[4.56/(4.56 - 4)]} = 2.1. Thus, over 4 Gyr, \begin{equation} R_I = 1.6 \times 10^{-3}/year. \label{eq: ri} From Voyager observations of crater chains on Callisto, Schenk et al. (1996) estimated the rate of tidal disruption of comets by Jupiter, averaged over 4 Gyr, as \begin{equation} R_T = 3.7 \times 10^{-3}/year. \label{eq: rt} Asphaug and Benz (1996) predict threshold catastrophic tidal disruption of a comet near Jupiter for perijove distance q < 1.37 R_J/rho (1/3) , where R_J is the radius of Jupiter, and rho is comet density in g/cm(3) . ECs are gravitationally focused by Jupiter, implying uniform distribution of perijove distances; hence \begin{equation} R_T/R_I = 1.37/\rho^{1/3} - 1. \label{eq:ratio} Substituting from (1) and (2) gives rho = 0.07 g/cm(3) , significantly lower than the density for Shoemaker-Levy 9 derived by Asphaug and Benz (1996). The consideration of comet strength makes this density an upper limit. Alternatively, if we use the higher present-day impact rate (1/330 years) of Levison and Duncan (1997), we find rho = 0.64 g/cm(3) , in good agreement with Asphaug and Benz. These issues are tied to the cratering versus catenae record on the Galilean moons, which adds a third independent element for constraint, to be presented. Asphaug, E. and Benz, W. (1996). Icarus 121, 225--248. Levison, H.F. and Duncan, M.J. (1997). Icarus 127, 13--32. Levison, H.F., Duncan, M.J., Zahnle, K., and Dones, L. (1999). Submitted to Icarus. Schenk, P.M., Asphaug, E., McKinnon, W.B., Melosh, H.J., and Weissman, P.R. (1996). Icarus 121, 249--274.

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