LONG-TERM EVOLUTION OF COMET SL-9 IMPACT FEATURES - JULY 1994 SEPTEMBER 1996

We present a two year study of the evolution of SL9 impact aerosol deb ris we observed between 0.4 and 0.9 micrometers with continuous high t emporal 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 wa velengths in the region covered by the impact debris show the contribu tions of different mechanisms in producing the complicated morphologic al evolution of the sites during the first month, Long-term horizontal aerosol transport was mainly due to the zonal jets in the upper tropo sphere with extreme measured velocities of -10 and 20 m s(-1). A compa rison 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 filt ers, indicating a low vertical wind shear in the upper troposphere. Th e spread of the aerosols resulted from the meridional and vertical she ars of the zonal winds, Rapid initial outward expansions (speeds of si milar to 30 to 60 m s(-1)) and interactions with nearby vortices (spee ds of similar to 10 to 25 m s(-1)) also contributed to the dispersion of particulates. Using methane band images we have measured a steady p oleward and equatorward meridional transport of the particulates with velocities of similar to-6 and 40 cm s(-1), respectively, Particulates were detected up to similar to-20 degrees by August 1995. Limb bright ening in the 890-nm methane band was observed up to similar to-30 degr ees during the last observation (September 1996) reported here, indica ting that a small population of aerosols was still present two years a fter impact, Photometric observations in the 890-nm band, together wit h a radiative transfer model, allowed us to calculate the evolution of the aerosol optical depth in the main impact core areas and in the su bsequent SL9 band. We found a rapid decrease in optical depth in the l argest impacts during July and August 1994 (from approximate to 3.2 to 2.1), followed by a gradual decrease during the next two years to app roximate to 0.3 (June 1996). This behavior can be explained by simple models of debris horizontal dispersion by the wind shear and by sedime ntation. Calculations of the characteristic times related to the micro physical processes in the aerosols (sedimentation, coagulation, and co alescence) together with their observed residence times (greater than or equal to 2 years) indicates that this persistent population of part icles had sizes less than or equal to 0.1 micrometers during 1995 and 1996. (C) 1998 Academic Press.