Seismology of rubble pile asteroids
It is expected that a large subclass of asteroids are actually a gravitational aggregate of smaller asteroid fragments. Such a system of macroscopic particles where the strongest interactions are mutual collisions can be described as a granular material. It has been found that granular materials exhibit strongly non-linear behavior while undergoing a jamming transition which is triggered by a change in particle density. At densities above the critical density particle cannot avoid one another and are forced against each other building up strain energy. Because of the (relatively weak) gravitational attraction the particles in a rubble pile asteroid form a slightly compressed, jammed state. For sufficiently small deformations this system should react in a linearly elastic manner while for larger deformations irreversible rearrangements occur.
A granular material model has been implemented in C++ with both hard and soft sphere interactions. Particles are attracted toward eachother by their mutual gravity. After equilibration we can probe the linear response to small deformations by solving the eigenvalue problem of the systems Hessian matrix. The question under study is: Can we deduce the internal composition of the asteroid (particle size, particle density, etc.) from the spectrum of eigenvalues? Initial results indicate sensitivity of the eigenvalue spectrum to composition changes. It is, however, not clear yet if the variations in spectra are strong enough to make asteroid seismology practical.