31st Annual Meeting of the DPS, October 1999
Session 6. Celestial Mechanics of Planets and Comets
Contributed Oral Parallel Session, Monday, October 11, 1999, 10:30am-12:00noon, Sala Pietro d'Abano

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[6.08] Near-Misses and Collisions With Comets

E. Asphaug (UCSC), L. Dones (SwRI), P.M. Schenk (LPI)

Levison et al. (1999) estimate the present impact rate RI 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} RI = 1.6 \times 10-3/{\rm year}. \label{eq: ri} \end{equation}

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} RT = 3.7 \times 10-3/{\rm year}. \label{eq: rt} \end{equation}

Asphaug and Benz (1996) predict threshold catastrophic tidal disruption of a comet near Jupiter for perijove distance q < 1.37 RJ/\rho1/3, where RJ is the radius of Jupiter, and \rho is comet density in g/cm3. ECs are gravitationally focused by Jupiter, implying uniform distribution of perijove distances; hence

\begin{equation} RT/RI = 1.37/\rho1/3 - 1. \label{eq:ratio} \end{equation}

Substituting from (1) and (2) gives \rho = 0.07~g/cm3, 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/cm3, 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). {\em Icarus} 121, 225--248.

Levison, H.F. and Duncan, M.J. (1997). {\em Icarus} 127, 13--32.

Levison, H.F., Duncan, M.J., Zahnle, K., and Dones, L. (1999). Submitted to {\em Icarus}.

Schenk, P.M., Asphaug, E., McKinnon, W.B., Melosh, H.J., and Weissman, P.R. (1996). {\em Icarus} 121, 249--274.

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