Implications of the NEAR mission for internal structure of Mathilde and Eros

The Near Earth Asteroid Rendezvous (NEAR) performed a flyby of the C-type main belt asteroid 253 Mathilde and an orbital study of the S-type near-Earth asteroid 433 Eros. These asteroid datasets provide a basis for inferences regarding physical properties and internal structure. The NEAR flyby of Mathilde revealed a heavily cratered surface with at least five giant craters (close to geometric saturation). Mathilde's density was unexpectedly low at 1.3±0.3 g cm-3, indicating a high porosity. Such a high porosity may be consistent with a rubble pile structure and may favor a compressional style of cratering. There are structural features, such as a 20-km long scarp, and polygonal craters indicating that Mathilde is not completely strengthless. At least one of its structural components appears coherent over a few tens of km. NEARs study of Eros found an average density of 2.67±0.03 g cm-3, almost uniform within the asteroid. Several lines of evidence suggest a globally consolidated internal structure: topographic features indicating tectonic deformations, regional scale linear features with related orientations, and structural control of craters in an intermediate size range. Eros is interpreted to be extensively fractured, but it was not disrupted and reaccumulated gravitationally. Some constraints can be placed on its strength. The consolidated interior must support a shear stress at least on the order of a few bars. Crater morphologies can be interpreted as suggesting a 'strength' near the surface of a few tens of kPa. Macroscopic fractures within Eros should be filled with fines, so the low average density of Eros relative to ordinary chondrites is not simply explained by macroscopic void space.