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Earth Planets Space, Vol. 60 (No. 1), pp. 13-20, 2008

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

Tatsuhiro Michikami1, Akiko M. Nakamura2, Naru Hirata3, Robert W. Gaskell4, Ryosuke Nakamura5, Takayuki Honda2, Chikatoshi Honda6, Kensuke Hiraoka2, Jun Saito7, Hirohide Demura3, Masateru Ishiguro8, and Hideaki Miyamoto9

1Fukushima National College of Technology, Iwaki, Fukushima 970-8034, Japan
2Graduate School of Science, Kobe University, Kobe, Japan
3University of Aizu, Aizu-Wakamatsu, Fukushima, Japan
4Planetary Science Institute, Tucson, Arizona, USA
5National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan
6Institute of Space and Astronautical Science, Japanese Aerospace Exploration Agency, Kanagawa, Japan
7PASCO Corporation, Meguro-ku, Tokyo, Japan
8Seoul National University, Seoul, Korea
9The University Museum, University of Tokyo, Tokyo, Japan

(Received February 18, 2007; Revised June 24, 2007; Accepted July 29, 2007; Online published February 12, 2008)

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).
Key words: Boulder, size distribution, Itokawa, asteroid.


Corresponding author E-mail: mitikami@fukushima-nct.ac.jp


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