Cavities as a source of outbursts from comets
  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. 
Sergei I. Ipatov
astro-ph.EP