Figure 5. A bright-field TEM image of the rim of an altered olivine grain from the pulse-laser irradiated pellet sample (30 mJ x 5). There are many nanophase particles whose size is several to 20 nm. The rim is probably deposited from recondensation.

We also irradiated pulse laser on the flat surface of olivine crystal. Olivine crystal samples hardly changed optically, although surface evaporation was confirmed during laser irradiation. Irradiated crystal samples were investigated by TEM but no structural change on the crystal surface was observed. Evaporated materials should escape quickly from a flat crystal surface, whereas on pellet samples the evaporated phase could recondense and deposit on nearby grain surfaces. The presence of a regoligh-like surface would be essential for effective space weathering on asteroids. If very small asteroids are free from regolith, their surface could not be weathered easily.

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