Long-Term Evolution of Comet SL-9 Impact Features: July 1994–September 1996

Abstract

We present a two year study of the evolution of SL9 impact aerosol debris we observed between 0.4 and 0.9 micrometers with continuous high temporal coverage from July 1994 through September 1996 and at 1.7 and 2.3 micrometers during three observing runs in July 1994 and March and August 1995. Temporal cylindrical map projections at red continuum wavelengths in the region covered by the impact debris show the contributions of different mechanisms in producing the complicated morphological evolution of the sites during the first month. Long-term horizontal aerosol transport was mainly due to the zonal jets in the upper troposphere with extreme measured velocities of −10 and 20 m s−1. A comparison of the zonal drift of the core sites in the red continuum and in the 890-nm methane band (sensitive to higher levels) during the first month do not show significant velocity differences between these filters, indicating a low vertical wind shear in the upper troposphere. The spread of the aerosols resulted from the meridional and vertical shears of the zonal winds. Rapid initial outward expansions (speeds of ∼30 to 60 m s−1) and interactions with nearby vortices (speeds of ∼10 to 25 m s−1) also contributed to the dispersion of particulates. Using methane band images we have measured a steady poleward and equatorward meridional transport of the particulates with velocities of ∼−6 and 40 cm s−1, respectively. Particulates were detected up to ∼−20° by August 1995. Limb brightening in the 890-nm methane band was observed up to ∼−30° during the last observation (September 1996) reported here, indicating that a small population of aerosols was still present two years after impact. Photometric observations in the 890-nm band, together with a radiative transfer model, allowed us to calculate the evolution of the aerosol optical depth in the main impact core areas and in the subsequent SL9 band. We found a rapid decrease in optical depth in the largest impacts during July and August 1994 (from ≈3.2 to 2.1), followed by a gradual decrease during the next two years to ≈0.3 (June 1996). This behavior can be explained by simple models of debris horizontal dispersion by the wind shear and by sedimentation. Calculations of the characteristic times related to the microphysical processes in the aerosols (sedimentation, coagulation, and coalescence) together with their observed residence times (≥2 years) indicates that this persistent population of particles had sizes ≤0.1 micrometers during 1995 and 1996.