  The long-lived spots caused by the impact of fragments of Comet S-L 9 on
Jupiter can be understood if clouds of dust are produced by the impact. These
clouds reside in the stratosphere, where they absorb visible light that would
ordinarily reflect from the cloud deck below, and reflect radiation at infrared
wavelengths that would ordinarily be absorbed by atmospheric methane. Here we
show that, provided that the nucleus of a fragment is composed substantially of
silicates and has a diameter greater than about 0.4 km, dust in the required
amounts will condense from the hot gas composed of cometary and Jovian material
ejected from the site where the fragment entered, and the dust will be
suspended in the stratosphere for long periods. Particles about 1$\;\mu$m in
radius can explain both the optical properties and longevities of the spots.
According to our model, a silicate band should be present in the $10 - \mu$m
spectra of the spots.
