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PSI Scientists Find Migrating Regolith on Tiny Asteroid Itokawa

PSI Scientists Find Migrating Regolith on Tiny Asteroid Itokawa

Image of Itokawa
Eastern side of Itokawa showing both smooth and rough terrains. The longest axis of Itokawa is about 500m.

April 19, 2007: Unprecedentedly high-resolution images from the Hayabusa spacecraft, the first Japanese asteroid mission, show unexpected evidence of the migration of gravels covering the surface of asteroid Itokawa.

Hirdy Miyamoto (an Affiliate Scientist of PSI and an Associate Professor of University of Tokyo), Bob Gaskell (a Senior Scientist of PSI), and others studied the Hayabusa's high-resolution images with up to 6mm/pixel resolution and discovered that Itokawa was covered with unconsolidated millimeter-sized and larger gravels. The finest granules are in pebble size and are found only in the smooth-looking terrains that cover 20% of the surface. This is surprising because impact ejecta on a small asteroid is expected to spread globally over its surface resulting in continuous regolith. In a paper being published today in the journal Science, Miyamoto, Gaskell, and others propose that unconsolidated gravels have globally migrated and segregated due to fluidization caused by vibrations likely induced by impacts of small meteoroids.

Close up image of Itokawa
Close-up image between rough and smooth terrain. Note the alignments of boulders from left top to right bottom, indicating that gravels migrate into the direction of left bottom to right top. This is consistent with that the boulder at the center of this image is partly covered by fines. White circle on the left is a crater with its disrupted rim.

The key morphological evidence for the gravel migration is how gravels align in very close-up images. The directions of the longest axes of gravels might be randomly distributed if they are simply accumulated. However, statistic analyses based on mapping of the gravels indicate that Itokawa's gravels are generally aligned. Deposits of terrestrial riverbed or landslides often show similar alignments. The directions of these asteroidal gravel migrations exactly coincide with the directions of local gravitational slopes.

When gravel is vibrated, it can be fluidized and behave as granular fluid. The most popular phenomenon related to this is called the "Brazil nut effect"—the biggest particles end up on the surface when granular material is shaken. Thus, the stranding of boulders covering the rough terrain of Itokawa may have occurred as a result of this process. Granular processes may be a major resurfacing mechanism for all small asteroids possessing regolith.

The content of this story is covered by the following paper: Miyamoto, H., H. Yano, D.J. Scheeres, S. Abe, O. Barnouin-Jha, A.F. Cheng, H. Demura, R.W. Gaskell, N. Hirata, M. Ishiguro, T. Michikami, A.M. Nakamura, R. Nakamura, J. Saito, and S. Sasaki, Regolith Migration and Sorting on Asteroid Itokawa, published online 19 April on Science Express, 10.1126/science.1134390.

Directions of slopes
Directions of slopes estimated from gravity plus rotational acceleration based on the numerical shape model developed by Bob Gaskell.

Image credit: University of Tokyo and JAXA

Contact Information:
Hirdy Miyamoto, Planetary Science Institute/The University Museum, University of Tokyo;
Tel/Fax +81-3-5841-2830
hm [at]

Dr. Robert Gaskell
Planetary Science Institute
Tel: 520-622-6300
rgaskell [at]

The Planetary Science Institute is a nonprofit science research institute focusing on the exploration of the solar system. Our scientists are distributed in 12 states, Japan, Italy, the UK and Russia. We are headquartered in Tucson, Arizona, where PSI was founded in 1972. We are involved in numerous NASA missions, the study of Mars, asteroids, comets, interplanetary dust, the origin of the solar system, planet formation about other stars, dynamics, impact physics, and the rise of life. PSI scientists conduct field work in North America, Australia and Africa. They are also actively involved in science education and public outreach through school programs, children's books, popular science books and art.

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