Interior of top-shaped asteroids with cohesionless surface

Multiple sources of evidence suggest that asteroids ranging from hundreds meters to few kilometers in size are rubble piles, i.e. gravitational aggregates of loosely consolidated material. However, no direct data on their internal structure is available to date. Cohesion between rubble-pile building blocks has been invoked in the past to explain the stability of top-shaped asteroids, which in most cases would not be capable of maintaining their large-scale shape features (low flattening, and a pronounced equatorial ridge) otherwise. However, the physical origin of cohesion is unclear and there is no direct evidence of it. Recent close-range imaging and local sampling of the surfaces of top-shaped Near Earth Asteroids (NEA) suggest the presence of very porous surface structure with minimal strength and nearly no cohesion. This raises new questions about the internal structure of such objects, with important implications on their origin and evolution. Here we show by numerically simulating the dynamics of irregular rocky fragments, that the presence of a rigid core within the asteroid’s rubble-pile structure can explain the top shape and surface features observed recently on Bennu and Ryugu, without the need of cohesion between building blocks. Also, we find that the rigid core model produces more easily equatorial mass shedding, which is thought to be responsible for satellite formation. The presence of a rigid core has never been revealed so far, but is consistent with the accretion history of those objects, and with recent estimates of their internal mass distribution. Our findings will be tested directly by ESA’s Hera mission, which will scan the interior of Dimorphos, the small moon of Didymos binary system, providing for the first time direct data on the interior of a NEA.