oa Outbursts and cavities in comets

In 2005 the impact module of the Deep Impact (DI) spacecraft collided with Comet 9P/Tempel 1. Based on analysis of the images made by this spacecraft during the first 13 minutes after the impact, Ipatov and A'Hearn concluded that the triggered outburst of small particles and excavation of a large cavity with dust and gas under pressure began at te= 8s, where te is the time after the DI collision. Schultz et al. analyzed images of Comet Tempel 1 made by the Stardust spacecraft and supposed that the diameter of the transient DI crater (dtc) was about 150-200m. Some authors support smaller values of dtc (up to 50m). My recent studies were devoted to estimates of the distance between the upper border of the cavity (dcav) excavated at te= 8s and the pre-impact surface of the comet. In particular, I supposed that the depth of a growing crater is proportional to te^gamma (where gamma is about 0.25-0.4) during the intermediate stage of crater excavation. The most probable estimate of dcav was about 0.1dtc*(te/Te)^0.3+1 meters, where Te is the duration of the normal ejection (Te=500 s at dtc=150 m). Using this approach I obtained dcav to be 5 or 6 meters for dtc equal to 150 or 200 m (dtc is 3 or 4 m for dtc ~ 50-100 m). The obtained values of the depth are in accordance with the depth (4-20 m) of the initial sublimation front of the CO ice in the models of the explosion of Comet 17P/Holmes considered by Kossacki and Szutowicz. The porous structure of comets provides enough space for sublimation and testifies in favor of existence of cavities. Natural outbursts were observed for several comets. Our studies testify in favor of that cavities with dust and gas under pressure located a few meters below surfaces of comets can be common. Similarity of velocities of particles ejected at triggered and natural outbursts shows that these outbursts could be caused by similar internal processes in comets.