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Abstract: We determined the morphologies and dimensions of possible impact craters on the surface of Asteroid 25143 Itokawa from images taken by the Hayabusa spacecraft. Circular depressions, circular features with flat floors or convex floors, and circular features with smooth surfaces were identified as possible craters. The survey identified 38 candidates with widely varying morphologies including rough, smooth and saddle-shaped floors, a lack of raised rims and fresh material exposures. The average depth/diameter ratio was : these craters are very shallow relative to craters observed on other asteroids. These shallow craters are a result of (1) target curvature influencing the cratering process, (2) raised rim not being generated by this process, and (3) fines infilling the craters. As many of the crater candidates have an unusual appearance, we used a classification scheme that reflects the likelihood of an observed candidate''s formation by a hypervelocity impact. We considered a variety of alternative interpretations while developing this scheme, including inherited features from a proto-Itokawa, spall scars created by the disruption of the proto-Itokawa, spall scars following the formation of a large crater on Itokawa itself, and apparent depressions due to random arrangements of boulders. The size-frequency distribution of the crater candidates was close to the empirical saturation line at the largest diameter, and then decline with decreasing diameter. [Copyright &y& Elsevier]

Abstract: The surface topography of Asteroid 25143 Itokawa is explored using the LIght Detection And Ranging instrument (LIDAR). The data confirm the presence of a rough highland and a smooth lowland. The highland is dominated by boulders, but also possesses topography associated with surface lineaments and broad surface facets. The boulders ensure that the roughness of the highlands over short distances is typically greater relative to most surfaces on 433 Eros. Over larger distances, Itokawa is always smoother than Eros possibly because of its smaller size and weak rubble-pile structure. The lowlands of Itokawa are very smooth, and are typically devoid of boulders. Some transitional regions midway between the highlands and lowlands also exist. In these areas, craters that retain their regolith fill possess flat floors and resemble “ponds” seen on 433 Eros. Analyses of surface elevation, imagery and a quantitative measure of surface roughness are consistent with regolith flowing downhill from the highlands to fill in the low areas of Itokawa, probably covering up any pre-existing rough terrain. Using this interpretation, we find a minimum thick layer of regolith in the lowlands, which, if spread evenly across the entire asteroid, corresponds to a thick layer. It is very difficult to generate this amount of regolith with the population of craters seen on Itokawa. However, an Itokawa composed of several large masses may have retained this regolith during its formation. The presence of such large masses could account for the observed lineaments and what appear to be exposures of bedrock on the largest steep slope observed. [Copyright &y& Elsevier]