  Time variations in velocities and relative amount of observed particles
(mainly icy particles with diameter d<3 {\mu}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 minutes 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 te of ejection after impact for te $\leq$ 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 based also on
results of the analysis of time variations in the size of the bright region of
ejected material. At te~10 s, the morphology of the ejecta (e.g. the location
and brightness of the brightest pixel) changed and the rate rte of ejection of
observed material increased. Between 1 and 3 seconds after the impact and
between 8 and 60 seconds 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<te<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.
