Multiple space exploration missions have recently revealed that asteroids have complex and variable surfaces. Geomorphological features are clues for mass transport processes occurring on small bodies. For example, the NEAR Shoemaker mission discovered a loose regolith layer on asteroid Eros. There are direct evidences of downslope motions, such as flat-floored sediments ponds in craters, a deficit of small craters, and large blocks surrounded by debris aprons (e.g., Thomas et al., 2002; Veverka et al., 2001). Miyamoto et al. (2007) report from images taken by Hayabusa that the surface regolith on Itokawa is segregated. However, transporting processes of asteroid regolith in both lateral and vertical directions remain ambiguous.