  The distance between the pre-impact surface of Comet 9P/Tempel 1 and the
upper border of the largest cavity excavated during ejection of material after
the collision of the impact module of the Deep Impact spacecraft with the comet
is estimated to be about 5-6 metres if the diameter of the DI transient crater
was about 150-200 m. The estimated distance was 4 m at the diameter was 100 m.
This result suggests that cavities containing dust and gas under pressure
located a few metres below surfaces of comets can be frequent.
