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Deep Impact: An Outburst Triggered by an Impact?

Introduction: The Deep Impact (DI) mission impact into the comet Tempel 1 produced very unusual outcomes compared to current wisdom about "conventional" impact cratering; although both prior to and subsequent to the event, many researchers including ourselves [1] tried to fit the observations into the conventional cratering wisdom. Prior to DI little was known about cratering into the highly porous materials characteristic of Temple 1, which has an estimated density of only 0.4 g/cm. As a result of the DI observations, Shultz et al. [2] recently report extensive laboratory experiments in very porous materials which qualitatively reproduce many of the observed features. It is the quanitative features that remain puzzling if they are compared to "conventional" cratering. Specifically, the total mass thrown out is extremely large: many observations concluded that there was on the order of 10 kg or greater of material ejected at velocities on the order of 100 m/s. Those admittedly rough figures are compared to the known database for the ejecta [2] from impact craters shown in the figure here: