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Posted by baalke on April 20, 2007, 12:21 pm
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Contact Information:
Hirdy Miyamoto, Planetary Science Institute/
The University Museum, University of Tokyo;
Tel/Fax +81-3-5841-2830
hm@um.u-tokyo.ac.jp
http://www.um.u-tokyo.ac.jp/hp/miyamoto/e.html
Dr. Robert Gaskell
Planetary Science Institute
Tel: 1-520-622-6300
rgaskell@psi.edu
Images and additional information: http://www.psi.edu/press
PSI SCIENTISTS FIND MIGRATING REGOLITH ON TINY ASTEROID ITOKAWA
For immediate release
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.
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
nd Sorting on Asteroid Itokawa, published online 19 April on Science
Express, 10.1126/science.1134390. http://www.sciencexpress.org
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