(469219) Kamo'oalewa, A Space-Weathering-Matured LL-chondrite-like Small NEA: Target of the Tianwen-2 Sample Return Mission

Introduction: Now, the China National Space Administration has proposed an asteroid mission, Tianwen-2, which plans to return a sample of a sub-hundred-meter Earth quasi-satellite (469219) 2016 HO3 Kamooalewa. Early studies suggested Kamooalewa originated from the Moon. However, here, we will report that Kamooalewa is a space-weathering (SW)-matured LL-chondrite-like object.Results: We first determined the composition of Kamooalewa by comparing Kamooalewas reflectance spectrum (which was previously reported by 1) with that of meteorites. As a result, Kamooalewa shows an absorption center at 0.984 m, only falling into the range of LL-chondrites. (Fig. 1), suggesting that Kamooalewa resembles LL chondrites in composition rather than other meteorite types.Then we used an orbital dynamical calculation method 2 to trace the source region of Kamooalewa. As a result, Kamooalewa shows a probability of 72 5% originating from the 6 secular resonance. Given that Flora family adjacent to the 6 secular resonance has been known as the major source region of LL-chondrite-like NEAs, such a high probability, therefore, emphasizes the possibility that Kamooalewa is an LL-chondrite-like asteroid.Particularly, Kamooalewa shows an extremely red spectral slope (0.726, calculated within 0.45-2.194 m) when compare with NEAs and main belt asteroids (MBAs), implying that Kamooalewa is a strongly space-weathered asteroid. Our nanosecond laser irradiation experiment on LL5/6 chondrite Kheneg Ljouds powder has successfully produced a slightly redder spectrum than Kamooalewa (Fig. 2), proving that Kamooalewas extremely red-sloped spectrum can indeed be contributed by SW processes. Furthermore, employing the radiative transfer mixing model 3-4, our calculation suggests that 0.29 0.05 wt.% SMFe0 (sub-microphase metallic iron, a major SW product that darkens and reddens silicate asteroids) in Kamooalewas regolith is required. This is higher than the average content of SMFe0 in the regolith of Itokawa (~ 0.2 wt.% 5), suggesting that Kamooalewa is indeed a SW-matured object. This is also consistent with our taxonomy of Kamooalewa as S-type rather than Sq- or Q-type.We also noted that Kamooalewas spectrum is redder than the mean spectrum of its source region Flora family (which has an exposure age of 0.5-1 109 year). Given that the SW rate at 1 AU area is about 10 times that of the main belt area, Kamooalewas SW timescale is hence estimated as at least 0.5-1 108 year. This exceeds the timescale of rapid reddening by solar wind irradiation (106 yr 6) and the average dynamical lifetimes of NEAs (106 year 7), indicating that Kamooalewa broke as a fragment in the inner main belt very early and still retains most of the previous (non-near-Earth-space) SW information without significant later surface refreshing.We also estimated Kamooalewas rotation period as ~27 min (meaning that it is a single rock), size as 69.45 m 58.49 m 51.78 m, and its regolith size on 75.38 % of surface area was lower than 2 cm, suggesting that fine-sized grains dominate Kamooalewas surface. Meanwhile, when we assumed Kamooalewa has been accelerated to current rotation period with a uniform angular acceleration within the Flora family, the estimation suggests that YORP spin-up lifetime is 4.23 104 to 4.23 105 yr. It means that the loss of large-sized grains (fresher) may have started very early and significant accumulation of small-sized grains/dust (maturer) has continued over a very long time (107 to 108 yr).Discussion: We explain that Kamooalewas extremely red spectrum can be comprehensively contributed by long-term SW and weak resurface process: (1) long-term loss of young large-sized grains and the accumulation of mature small-sized materials, (2) small size of Kamooalewa decreases the likelihood of surface refreshing caused by impact, (3) non-rubble pile structure may effectively avoid surface rejuvenation that would be driven by the inside-out movement of materials driven by spin-up and matter mixing driven by meteoroid impact, (4) Kamooalewa did not underwent resurfacing by Earth encounters, because its minimum Earth orbit intersection distance (0.0345 AU) and perihelion (0.898 AU) is much larger than the range of Earth encounters (5-16 times Earth radius 8), and quasi-satellites generally do not experience flybys with Earth as close as those observed for other co-orbital types.We further predict that sub-hundred-meter, rapidly spinning silicate-rich NEAs with small perihelion may generally exhibit redder spectral slopes and SW matured surfaces. This is different from the current observation that the Q-type/S-type ratio increases with decreasing perihelion distance 9-10.Fig. 1 Comparison of band I center and band area ratio (Band II/Band I) of Kamooalewa with meteorites, the band I center of Kamooalewa (0.984 m) best matches to LL chondrites.Fig. 2 Comparison of spectra of Kamooalewa with fresh (blue line) and laser irradiated (red line) LL5/6 chondrite Kheneg Ljoud. After irradiation, Kheneg Ljouds spectrum significantly steeps and slightly steeper than Kamooalewa, suggesting that Kamooalewa-like extremely red spectra can indeed be contributed by long-term SW processReference: 1 Sharkey et al. (2021) Commun Earth Environ, 2, 1-7. 2 Granvik and Brown (2018) Icarus, 311, 271-287. 3 Lawrence et al. (2007) JGR: Planets, 112. 4 Lucey et al. (2011) Icarus, 212, 451-462. 5 Binzel et al. (2001) Meteorit Planet Sci, 36, 1167-1172. 6 Vernazza et al. (2009) Nature, 458, 993-995. 7 Nesvorn et al. (2017) AJ, 155, 42. 8 Nesvorn et al. 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