[PPS04-P07] Estimation of abrasion rates of regolith particles on Itokawa and the Moon based on abration experiments.
Keywords:Abrasion, Hayabusa mission, Impact
The samples used were quartz, olivine (Fo~90) and basalt. They were crushed and grains 1~2 mm in size were selected except for some to examine grain size effect (0.5~1 mm and 2~4 mm). These grains (~6.5g) were put into a vessel (10 mL) of agate, dunnite or basalt with ~50% fraction. Then the vessel was shaken with vertically-vibrational motion in a mill (Multi-beads-shocker: YASUIKIKAI Co.). Powder with the size less than 1/6 of the grain size (e.g., <250 mm for 1~2 mm grains) produced by abrasion was removed by sieving. The amount of abrasion, P, was defined as the difference between the grain masses before and after the abrasion, which was normalized by the initial grain mass.
For quartz grains, P increases with abrasion duration, t, (20 sec to 360 min) with the power of ~0.25 irrespective of vibration rate, Ω (100 to 2500 rpm). The amount of abrasion in the first 1 min, P1, increases with Ω (100 to 3000 rpm) for quartz, olivine and basalt. The powers of P1 - Ω relation, mi, at Ω < ~700 rpm (m = 0.1~0.4) are smaller than those at Ω > ~700 rpm (mi = 1.2~2). P1 increases with the grain size, d, at Ω = 2000 rpm for quartz and the power value is temporally ~1.6. If we assume that the power relation between P and t is applicable to different sample materials and that between P1 and d to different sample materials and Ω, we can obtain the following equation; P = Ai x Ωmi x d1.6 x t0.25 (Eq.1), where Ai is the proportionality and the suffix i denote the sample materials. SEM observation together with the P – t relation showed that the abrasion advances by chipping grain corners (chipping) for P < ~1% while grain corners and edges become rounded (wearing) for P > ~1%.
The maximum accelerations by impact on Itokawa and the Moon can be estimated to be ~1 and ~100 m/s2 based on the impact experiments , which correspond to Ω of ~100 and ~700 rpm, respectively. This gives Ai’s in Eq.1 with d = 1.5 mm and t = 1 min are ~0.2 % (olivine) on Itokawa and ~0.5 % (basalt) on the Moon. If typical grain convection time by impact of ~180 s on Itokawa  is adopted as the maximum value of t, P is estimated to be ~0.003 and ~0.1 % for d = 100 μm and 1 mm, respectively. As the Itokawa grains have rounded edges (wearing), >~4×109 and >~3000 times of impact are required, respectively, for P > 1 %. If the impact frequency on Itokawa  is considered, these numbers of impact required for wearing are too large, suggesting that grain abrasion on Itokawa is not expected and impact on the larger parent body (>~20 km)  should be necessary. In contrast, on the Moon, Ai ~ 0.5% suggests possible abrasion because of longer residence time in a regolith layer than Itokawa.
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