The outburst triggered by the Deep Impact collision with Comet Tempel 1 ★

Time‐variations in velocities and relative amount of observed particles (mainly icy particles with diameter d < 3 μ m) ejected from Comet 9P/Tempel 1 are studied based on analysis of the images made by Deep Impact (DI) cameras during the first 13 min after the collision of the DI impactor with the comet. Analysis of maxima or minima of plots of the time‐variations in distances of contours of constant brightness from the place of ejection allowed us to estimate the characteristic velocities of particles at several moments in time, t e , of ejection after impact for t e ≤ 115  s. Other approaches for estimates of the velocities were also used. All these estimates are in accordance with the same exponential decrease in velocity. The estimates of time‐variations in the relative amount of ejected particles were also based on results of the analysis of time‐variations in the size of the bright region of ejected material. At t e ∼ 10  s, the morphology of the ejecta (e.g. the location and brightness of the brightest pixel) changed and the rate, r te , of ejection of observed material increased. Between 1 and 3 s after the impact and between 8 and 60 s after the impact, more small bright particles were ejected than expected from crater excavation alone. An outburst triggered by the impact could cause such a difference. The sharp (by a factor of 1.6) decrease in the rate of ejection at 55 < t e < 72  s could be caused by a decrease in the outburst that began at 10 s. Analysis of observations of the DI cloud and of the outbursts from some comets testifies in favour of the proposition that there can be large cavities, with material under gas pressure, below a considerable fraction of a comet’s surface. Internal gas pressure and material in the cavities can produce natural and triggered outbursts and can cause splitting of comets.