Comet Shoemaker-Levy 9 Dust Size and Velocity Distributions

Abstract

Pre-impact observations of Comet Shoemaker-Levy 9 (S-L9) obtained with the Hubble Space Telescope are examined, and a model of an active, dust-producing comet is fitted to images of fragments G, H, K, and L. The model assumes steady isotropic dust emission from each fragment's sunlit hemisphere. Best-fit results indicate that the dominant light-scatterers in these fragments' comae were relatively large dust grains of radii 10 μm ≲ R ≲ 3 mm. The fragments' dust size distributions were rather flat in comparison to other comets, d N ( R) ∝ R^-2.3±0.1, and the dust ejection speeds were ∼0.5-1.5 m/s. The S-L9 fragments themselves were not detected directly, and upper limits on their radii are 1.0-1.5 km assuming an albedo a=0.04. However, these fragments' vigorous production of dust, which ranges from 6 to 22 kg/s, places a lower limit of ∼100 m on their radii at the moment of tidal breakup. Any fragments smaller than this limit, yet experiencing similar mass loss rates, would have dissipated prior to impact. Such bodies would fail to leave an impact scar at Jupiter's atmosphere, as was realized by fragments F, J, P ₁, P ₂, T, and U.