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Analysis of observations of natural and triggered outbursts from differentcomets testifies in favor of existence of large cavities with material undergas pressure below a considerable fraction of a comet's surface. Based onanalysis of images of the cloud of material ejected from Comet 9P/Tempel 1after the collision of the Deep Impact (DI) module with the comet, we studiedthe time variations in the rate and velocities of ejection of observedparticles (mainly icy particles with diameter d<3 micron). Comparison of thesedependencies with the theoretical dependencies allowed us to understand thetime variations in the rate of the outburst triggered by the impact. The lattervariations testify in favor of that there were cavities at the place of DIejection. The beginning of the increase of the main outburst at 8 s after theDI impact could be caused by excavation of a relatively large cavity thatcontained dust and gas under pressure. The upper boarder of the cavity could belocated at about 5-10 meters below the surface of the comet. This cavity couldbe deep because the excavation from the cavity could last for at least a fewtens of seconds. With the increase of the crater, more cavities could beexcavated. The outburst decreased at ~60 s after the impact. Besides the 'fast'outburst caused by ejection from cavities, there was a 'slow' outburstejection, which was similar to the ejection from a 'fresh' surface of a cometand 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 itcould significantly increase the fraction of particles ejected with velocities~100 m/s compared with theoretical models of ejection.

Cavities as a source of outbursts from comets