IMPACTS INTO POROUS ICE: THE DEEP IMPACT CRATERING EXPERIMENT ON COMET TEMPEL 1

On July 4, 2005 NASA’s Deep Impact mission collided a 366 kg spacecraft with comet Tempel 1 at a speed of 10.2 km/s. A flyby spacecraft recorded the event at a range of 500 km with a variety of instruments. The first indication of the impact was a bright flash at the impact site and an expanding plume of hot vapor and silicate droplets that expanded above the impact site over the first few seconds after impact. Subsequently a cone of ejecta was observed whose evolution indicated an average density of about 400 kg/m³ for the comet. The impact site was imaged 6 yr later by the NeXT mission in 2011 which indicated that the impact created a small crater, perhaps 50 m in diameter, although identification of the crater was difficult due to the limited resolution of the NeXT spacecraft’s imager. One unexpected result of the 2005 impact was a delay of 118 ms between the impact itself and the peak of the emitted light flash.

We modeled the impact event numerically using the iSALE hydrocode, which includes a porosity model and equations of state for ice and silicate (the comet) and copper (the impactor). We find that this projectile penetrates deeply into the porous, low-density ice/silicate comet, depositing its energy deep below the surface. The impact initially creates a nearly spherical cavity beneath the surface that has only a small opening from which light from the hot gases may be emitted. As the cavity, filled with water vapor and silicate melt, expands, its intersection with the surface grows rapidly and finally opens fully after about 100 ms, explaining the observed delay between the impact and the maximum emission of light. The hot gas and molten silicate droplets then vent from the crater at velocities comparable to the observed ca. 5 km/s incandescent plume. Impact cratering into porous icy targets thus displays qualitative differences from that into dense rocky surfaces, with implications for fast, distal ejecta on icy satellites.