Evidence for Global Structure Within 433 Eros

Asteroid formation and evolution models suggest a high probability of significant fracture and possibly reassembly structures, an expectation strongly supported by meteoritic evidence. NEAR's year-long orbital mission about 433 Eros has provided image and gravity data that provide unprecedented information regarding an asteroid's interior structure. Spectral studies with the Multispectral Imager (MSI) and the Near Infrared Spectrometer (NIS) suggest that there is very little compositional variation on the surface. Deep structural forms such as faults and fractures are inferred on the basis of linear grooves within the regolith. The search for a global fabric within a body relies chiefly on finding alignments of features that are not chance arrangements. Testing for alignment is carried out by seeing how well features can be described as planar, and whether different planar features are parallel or coplanar. This requires the use of large structures. The most distinctive linear structure on Eros is Rahe Dorsum, an 18 km long ridge. Rahe Dorsum is interpreted as a compressional feature along most of its length, with possible extension near the ends. A second distinctive feature is a ridge and trough set (proposed name: Calisto Fossae) at 25S, 150-170W. Tests of the planarity of Rahe Dorsum show that it can be fit by a plane with residuals (distance perpendicular to plane) of up to 500 m and an average absolute residual of 170 m. Putting Rahe and the Calisto Fossae in the same solution gives a result little different from the original Rahe fit, with a pole at 52.9S, 180W. Thus, Rahe Dorsum and the Calisto Fossae appear to be coplanar. Linear structural features thus suggest one example of nearly global fabric. One large area of Eros west of Shoemaker crater has a faceted appearance. A plane fit to this facet reveals an orientation that is only about 7 degrees from the Rahe-Calisto Fossae plane. These observations indicate that there is a fabric running nearly the length of Eros that affects some mechanical responses such as fracture orientation, and which has no correlation with variations in surface composition. Nearly planar fabric might arise from layering or pressure-related phenomena in much larger objects. Those precursor bodies would have to be at least many tens of km in radius (and Eros derived from similar radii). Developing them in Eros in nearly its present shape would appear to be difficult due to the necessity of obtaining principal stress orientations that are not related to the overall shape.