Macroscopic Voids in Small Asteroids: Effects of Cohesion

The NEAR mission found surprisingly low densities for the C-asteroid Mathilde and the S-asteroid Eros, lower than bulk densities of likely meteorite analogs. A key question is whether there is significant macroscopic void space between major structural components of asteroids, which may cause the asteroid bulk densities to be significantly less than that of the components. If fines can drain into fractures, as appears to be the case at the surface of Eros, then the structural components must be underdense compared with meteorite analogs. Previous analyses have shown that friction does not prevent fines from filling fractures, but previous work has not considered cohesion. This paper shows that for fines to drain into the deep interior of an asteroid the size of Eros, the cohesion must be much smaller than that of lunar soils. The required cohesion to prevent filling of fractures decreases further for smaller asteroids. At the 400-meter mean diameter of 25143 Itokawa, which will be visited by MUSES-C in 2005, the cohesion can be up to approximately 10-3 of lunar values, or else fines cannot fall into cracks. A low density is therefore predicted for S-type Itokawa, less than or comparable to that of Eros, as Itokawa must have been shattered by impacts or perhaps turned into a rubble pile.