  Context. The space weathering timescale of near-Earth S-type asteroids has
been investigated by several approaches (i.e., experiments, sample analyses,
and theoretical approaches), yet there are orders of magnitude differences.
Aims. We aim to examine the space weathering timescale on a near-Earth S-type
asteroid, Itokawa using Hayabusa-AMICA images and further investigate the
evolutional process of the asteroid. Methods. We focused on bright mottles on
the boulder surfaces generated via impacts with interplanetary dust particles
(IDPs). We compared the bright mottle size distribution with an IDP flux model
to determine the space weathering timescale. Results. As a result, we found
that the space weathering timescale on Itokawa's boulder surfaces is 10$^3$
years (in the range of 10$^2$-10$^4$ years), which is consistent with the
timescale of space weathering by light ions from the solar wind. Conclusions.
From this result, we conclude that Itokawa's surface has been weathered shortly
in 10$^3$ years but portions of the surface are exposed via seismic shaking
triggered by a recent impact that created the Kamoi crater.
