Size-frequency statistics of boulders on global surface of asteroid 25143 Itokawa

The surface of asteroid 25143 Itokawa is covered with numerous boulders although gravity is very small compared with that of other asteroids previously observed from spacecraft. Here we report the size-frequency statistics of boulders on the entire surface of Itokawa based on high-resolution images (1 pixel similar to 0.4 m) obtained by the Hayabusa spacecraft. There are 373 boulders larger than 5 m in mean horizontal dimension on the entire surface-0.393 km(2)-and the number density is nearly 10(3)/km(2). The cumulative boulder size distribution on the entire surface has a power-index of -3.1 +/- 0.1. For the east and west sides and the head and body portions of Itokawa, the power-index of the size distributions and the number densities of boulders of these areas are thought to be similar from the statistical point of view. A global mapping of boulders shows that there is no apparent correlation in the locations of boulders and craters. The ratio of the total volume of the boulders to the total excavated volume of the craters on Itokawa is similar to 25% when only craters larger than 50 m in mean diameter are considered, and this ratio is extremely larger than that on Eros and the Moon, respectively. The origin of boulders on the surface of Itokawa was examined quantitatively by calculating the number of boulders and the size of the largest boulder using a model based on impact cratering experiments. The result indicated that the boulders on the surface of Itokawa cannot solely be the product of craters. Our results suggest that the boulders originated from the disruption of the larger parent body of Itokawa, as has been described in previous papers (Fujiwata et al., Science, 312, 1330-1334, 2006; Saito et al., Science, 312, 1341-1344, 2006).