High temperatures in returning ejecta from the R impact of comet SL9

Observations from ground-based observatories and with the Galileo spacecraf t suggest that the flares from the SL9 impacts resulted from ejecta falling back onto Jupiter in ballistic plumes. This explanation is supported by co mparing the plume height as a function of time in HST images with the flare light curve. We show that the rotational temperature of CO in the shock fr om the R impact rose from less than 2000 K near the beginning of the main f lare to about 5000 K at its end. This behavior agrees with a simple physica l model of ballistic plumes with a mean molecular weight indicating they ar e 50% or more jovian air, Alternate models involving formation of molecules at the original impact site, or formation of dust grains to initiate the h are, are inconsistent with these measurements. The energy is emitted primar ily as a hot continuum, supporting the possibility that finely divided dust grains are heated in the reentry shock and emit to create the flare. Scali ng such models to the energy of the KIT event supports proposals that balli stic plumes were responsible for the global disaster associated with it, (C ) 1999 Academic Press.