  Analysis of observations of natural and triggered outbursts from different
comets testifies in favor of existence of large cavities with material under
gas pressure below a considerable fraction of a comet's surface. Based on
analysis of images of the cloud of material ejected from Comet 9P/Tempel 1
after the collision of the Deep Impact (DI) module with the comet, we studied
the time variations in the rate and velocities of ejection of observed
particles (mainly icy particles with diameter d<3 micron). Comparison of these
dependencies with the theoretical dependencies allowed us to understand the
time variations in the rate of the outburst triggered by the impact. The latter
variations testify in favor of that there were cavities at the place of DI
ejection. The beginning of the increase of the main outburst at 8 s after the
DI impact could be caused by excavation of a relatively large cavity that
contained dust and gas under pressure. The upper boarder of the cavity could be
located at about 5-10 meters below the surface of the comet. This cavity could
be deep because the excavation from the cavity could last for at least a few
tens of seconds. With the increase of the crater, more cavities could be
excavated. The outburst decreased at ~60 s after the impact. Besides the 'fast'
outburst caused by ejection from cavities, there was a 'slow' outburst
ejection, which was similar to the ejection from a 'fresh' surface of a comet
and could be noticeable during 30-60 min. The 'fast' outburst with velocities
~100 m/s probably could continue for at least several tens of seconds, and it
could significantly increase the fraction of particles ejected with velocities
~100 m/s compared with theoretical models of ejection.
