Numerical N-body models adopting self-gravitating spherical particles have proven to have interesting properties and are very useful for describing some aspects of the expected behaviour of rubble-piles. In fact, particle interlocking can simulate a certain degree of shear strength (corresponding to a non-zero critical slope in the Mohr-Coulomb approach). At the same time, if the critical slope angle is exceeded for some reason (thus temporarily breaking the sphere packing) the shape of the body can readjust to a new equilibrium. Usually, this re-organisation produces a shape closer to the theoretical fluid equilibrium predicted by theory, at the corresponding angular momentum. We have recently compared this reshaping process of ellipsoidal gravitational aggregates to the field of potential energy associated with incompressible fluid shapes. The gradient of this field suggests the evolutionary track that a reshaping body undergoing a slow (quasi-stationary) modification should follow. The same applies to the gradient of the maximum slope present on the surface. The results show the overall tendency to readjust along the potential slope. However, other effects are at work, making the interpretation more complex - among them we can identify: the granularity of the shape model; the relative flatness of the potential field; and a certain anisotropy of the most compact spherical packings. The results offer an interesting insight into the detailed behaviour of this model, allowing us to characterize its applicability in more realistic situations, such as restructuring due to tidal forces, impact reshaping, or shape build-up during gravitational reaccumulation.
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