Porosity Effects on Impact Processes in Solar System Materials

We will present calculations and experiments on the effect of porosity on the shock wave attenuation, dynamic strength and fragment distribution of porous, brittle materials. These results are part of our developing theory of porosity effects on impact processes in solar system materials. We will also examine the effects of porosity on crack propagation in ice and comet analog materials. The strength of comets, porous aggregates of ice and dust, is basically unknown with only one data point from the tidal breakup of Comet Shoemaker-Levy 9, which had an extremely low tensile strength of order 100 Pa. The porosity of comets has never been measured directly and models predict a wide range from 40-80%. Planned spacecraft missions, ESA’s Rosetta and NASA’s Deep Space 4, will measure the porosity and other physical properties of a couple of comets, and there is probably a large range of porosities found in comets. To isolate the effects of porosity on impact processes, we are conducting experiments on Plaster of Paris, a gypsum plaster, from which targets may be made with porosities ranging from 30-80%. The static tensile strength of the plaster is similar to solid ice, ranging from 2-4 MPa, depending on the porosity [1]. The outcome of impacts into icy bodies depends on the material strength and attenuation profile of the shock wave. Porosity will steepen the decay of the shock wave, reducing the volume effected by an impact compared to a solid of the same material. To disrupt a given volume of porous material compared to the same volume of solid of the same material strength, a stronger initial shock wave is necessary. In this way, a porous material may have stronger effective dynamic strength than a solid with similar static material strength.