Abstract
Abstract 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 ≈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 km2—and the number density is nearly 103/km2. 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 ≈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).
Highlights
The existence of boulders on asteroids provides an opportunity to study the physical properties and the geological evolution of asteroid surfaces and, in particular, their collisional history
The ratio of the total volume of the boulders to the total excavated volume of the craters on Itokawa is ∼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 images taken by the Asteroid Multi-band Imaging Camera (AMICA) on board the Hayabusa spacecraft showed that Itokawa is covered with a surprisingly large number of boulders (Fujiwara et al, 2006; Saito et al, 2006)
Summary
The existence of boulders on asteroids provides an opportunity to study the physical properties and the geological evolution of asteroid surfaces and, in particular, their collisional history. The boulders are formed by impact cratering and/or catastrophic disruption of the parent asteroid. The boulders on small bodies, such as the Martian satellites and the two asteroids 243 Ida and 433 Eros explored by previous space missions, are thought to be attributed to impact cratering (e.g., Lee et al, 1996; Thomas et al, 2001). Itokawa is an extremely small body (mean diameter ∼320 m) compared with asteroids explored previously (mean diameter ∼12–50 km) and the escape velocity is very small (sub-meter per second); before the arrival of the Hayabusa spacecraft, most scientists thought that there would be only a few boulders on its surface. The images taken by the Asteroid Multi-band Imaging Camera (AMICA) on board the Hayabusa spacecraft showed that Itokawa is covered with a surprisingly large number of boulders (Fujiwara et al, 2006; Saito et al, 2006). Based on the appearance of Itokawa’s surface and the sizes of the boulders, Fujiwara et al (2006) and Saito et al (2006) suggested that such numerous boulders were produced when Itokawa was generated by a catastrophic disruption of its parent body
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