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Wavelength (|xm)

Figure 3. Asteroid spectra and best match mixing of pulse-laser irradiated samples [11]. (a) 446 Aeternitas. Mixing ratio: 01 (Non-irradiated) 2.0%; 01 (15 mJ) 19.4%; 01 (30 mJ) 73.7%; En (30 mJ x 10) 0.6%; En (30 mJ x 20) 0.3%; Chromite 4.0%. (b) 349 Dembowska. Mixing ratio: En (30 mJ) 7.0%; En (30 mJ x 20) 37.3%; 01 (raw) 0.7%; 01 (15 mJ) 55.0%.

Figure 3. Asteroid spectra and best match mixing of pulse-laser irradiated samples [11]. (a) 446 Aeternitas. Mixing ratio: 01 (Non-irradiated) 2.0%; 01 (15 mJ) 19.4%; 01 (30 mJ) 73.7%; En (30 mJ x 10) 0.6%; En (30 mJ x 20) 0.3%; Chromite 4.0%. (b) 349 Dembowska. Mixing ratio: En (30 mJ) 7.0%; En (30 mJ x 20) 37.3%; 01 (raw) 0.7%; 01 (15 mJ) 55.0%.

Figure 4. As for S/V/A/R asteroids, the continuum slope of the l-|^m band is compared with the area ratio between the 1-nm and 2-\m\ bands [13]. Data from laser-irradiated olivine and pyroxene pellet samples (Figure 2) are also shown for comparison.

In space, the impact rate of dust particles of around 10"12 g (1 jam size) is about a few 10~4 m"2 s"1 at 1 AU [10]. Relatively high-velocity beta meteoroids would be effective in producing the space weathering. One dust particle loses energy 2xl0"7 J at the impact with velocity 20 km s"1. Then, the total energy deposition rate by dust impacts is about 103 J m~2 yr"1. Supposing a simple comparison between the irradiation energy by pulse laser and released energy at dust impacts, irradiation with a 30 mJ pulse laser in our experiments corresponds to a few 108 yr in the space. If energy efficiencies for changing the optical properties are the same between dust impact and laser irradiation, olivine-rich surfaces should be darkened and reddened in 108 yrs, whereas pyroxene-rich surface should need more time to be optically reddened.

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