Introduction

A large number of asteroids are categorized into S-type based on their reflectance spectra. Those S-type asteroids are believed to be parent bodies of ordinary chondrites [1], However, the steep reddened slope of their reflectance spectra and the derived mineralogies of S-type asteroids are different from those of ordinary chondrites. The mismatch between spectra of ordinary chondrites and S-type asteroids is considered to be caused by so-called"space weathering", where high-velocity impacts of interplantary dust should change the optical properties of the uppermost surfaces of asteroids possibly covered with regolith. To explain optical property changes such as overall reflectance depletion, reddening, and weakened absorption bands, Hapke et al. [2] proposed formation of submicroscopic metallic (SMFe) iron particles (10 nm in size) within coatings produced by the recondensation of ferrous silicate vapor. Impacts of interplanetary dust particles are a plausible process to produce the vapor. Such nanophase iron particles were found on the vapor-deposited rims of lunar soils [3, 4], Recently it was shown theoretically that nanophase iron particles on the grain surface rim should decrease and redden reflectance spectra of ordinary chondrites and lunar soils [5]. However, realistic simulation of the space weathering is not easy: at present hyper-velocity (>10 km s"1) dust accelerators can hardly produce sufficient dust flux to simulate space weathering.

2. PULSE LASER IRRADIATION SIMULATING DUST IMPACT HEATING

Pulse laser beams have been used for small-scale short-duration heating of samples. Moroz et al. [6] observed changes of reflectance spectra of silicate samples after pulse laser heating. However, the pulse duration they used was 0.5-1 /zs, which is 1000 times the real timescale of micrometeorite (1-10 /zm size) impacts. To simulate space weathering by impact heating of dust particles as small as 1 /zm and impact velocity higher than 10 km s"1, nanosecond pulse duration is necessary. We irradiated powder and pellet samples of olivine and pyroxene by a pulse laser beam (1064 nm) with pulse duration 6-8 ns, which is comparable with a real dust impact [7, 8], To perform uniform scanning of pulse laser irradiation, we made a small chamber on an X-Y stage [8], The pulse energies, 1 and 30 mJ, can supply the total irradiation energy in unit area of 8 x 103 and 2.4 x 105 J m"2, respectively. Since the footprint diameter of the beam is focused to be as small as 500 /zm, the energy deposition rate under 30 mJ reached 1010 W cm"2, where evaporation and ion formation is strongly expected [9], After the laser irradiation, bidirectional reflectance spectra of samples were measured; a spectral range 250 -2600 nm was recorded at every 10 nm.

3. CHANGE OF REFLECTANCE SPECTRA

In the present experiments, olivine is San Carlos Olivine with 8.97 wt% FeO and pyroxene is an enstatite from Bamble, Norway with 9.88 wt% FeO. Figure 1 shows a pellet sample of olivine after the pulse laser irradiation. Reflectance of olivine is apparently decreased. Measured bidirectional reflectances of olivine and enstatite are shown in Figure 2. The reflectance of the mm-size region in the center of the sample (Figure 1) was measured. Laser-irradiated samples show significant reddening: reduction of spectra is much larger in the visible region than in near infrared region. Measured from the continuum slope, relative depth of the absorption at 1 /zm (and 2 /zm for pyroxenes) is not changed largely. Figure 3 shows comparison of two asteroid spectra with best-fit mixings of irradiated spectra [11], Treatment with pulse laser can reproduce those asteroid spectra very well. 446 Aeternitas is well explained by the irradiated olivine. 349 Dembowska, which is pyroxene-rich with distinct 1 and 2 /zm absorption bands, is also explained.

In Figure 2, the enstatite spectrum is changed only after repetitive irradiation. Reflectance of olivine should be more easily changed than that of pyroxenes: changes of hypersthene and diogenite are also smaller than that of olivine [8]. As seen in the raw reflectance samples in Figure 2, reflectance at 1064 nm is nearly the same (60%); the difference cannot be ascribed to the difference of absorbed energy. The degree of optical change on simulated space weathering by pulse laser heating depends on the mineral composition. We also compiled asteroid reflectance data using areas of 1 /zm and 2/zm absorption bands [11, 12]. Figure 4 shows that S-type asteroids with a weaker 2 /zm absorption band (relatively olivine-rich) show redder spectra than those with a distinct 2 /zm band (relatively pyroxene-rich). The difference of weathering degree according to mineral composition is also observed in the space.

Figure 1. An olivine pellet sample after pulse laser (30 mJ) irradiation. Each beam spot diameter is about 500 /¿m and the irradiated area is approximately 10 mm x 10 mm. Size of pellet with an aluminum ring is 20 mm. The samples are formed of particles smaller than 75 (im.

Figure 2. Reflectance spectra of laser-irradiated olivine and enstatite pellet samples, (a) Absolute olivine spectra, (b) Olivine spectra scaled at 560 nm. There are four spectra (raw, 15 mJ, 30 mJ, and 5 times 30 mJ). (c) Absolute enstatite spectra, (d) Enstatite spectra scaled at 560 nm. There are four spectra (raw, 30 mJ, 10 times 30 mJ pulse, and 20 times 30 mJ).

Figure 2. Reflectance spectra of laser-irradiated olivine and enstatite pellet samples, (a) Absolute olivine spectra, (b) Olivine spectra scaled at 560 nm. There are four spectra (raw, 15 mJ, 30 mJ, and 5 times 30 mJ). (c) Absolute enstatite spectra, (d) Enstatite spectra scaled at 560 nm. There are four spectra (raw, 30 mJ, 10 times 30 mJ pulse, and 20 times 30 mJ).

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