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PSI Scientists Find Migrating
Regolith on Tiny Asteroid Itokawa
04/23/2007
From http://www.psi.edu/press/
(Additional comments below)
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 and Sorting on Asteroid Itokawa, published online 19 April
on Science Express, 10.1126/science.1134390.
http://www.sciencexpress.org
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