Gravitational Aggregates: Evidence and Evolution

It has been suggested that asteroids between ~100 m and ~100 km in size may be gravitational aggregates of loosely consolidated material. Recently evidence has been mounting to support this idea. Comet breakups, crater chains, doublet craters, giant craters, grooves, asteroid spins, underdense asteroids, asteroid satellites, and unusual asteroid shapes can all be explained to some degree by the properties of aggregate structures. Moreover, laboratory and numerical experiments indicate that solid asteroids, if they existed, would likely be completely shattered by now, owing to their violent collisional histories. Once shattered, a body becomes increasingly difficult to disrupt since it can efficiently absorb energy deposited at the impact site. In this chapter we present the evidence for gravitational aggregates and discuss the implications of their existence for asteroid evolution. We also propose a new classification scheme for asteroid structure parameterized by fundamental quantities such as strength and porosity.