SPECTRAL VARIATION ON ASTEROID ITOKAWA
SUGGESTING REGIONAL RESURFACING.
S. C. Koga1, S. Sugita1, S. Kamata2, M. Ishiguro3, T. Hiroi4 and S.
Sasaki5, 1Dept. of Complexity Sci. and Engr., Univ. of Tokyo,
2
Dept. of Earth and Planetary Sci., UC Santa Cruz, 3Dept. of Phys.
and Astr., Seoul National Univ., 4Dept. of Geological Sci., Brown
Univ., 5Dept. of Earth and Space Sci., Osaka Univ., E-mail: koga@astrobio.k.u-tokyo.ac.jp.
Introduction: Itokawa samples brought back by Hayabusa
mission have been extensively analyzed, leading to a variety ofresults, such as cosmic exposure age [1] and evidence for space
weathering [2]. However, accurate interpretation of these datarequire geologic context within the Itokawa surface variation. Inorder to obtain such geologic context of the sampling site, we
examine the spectral variation on Itokawa surface in this study.
Principal component analysis method: Many spectroscopic
taxonomical schemes of asteroids have been constructed using
principal component (PC) analysis [e.g., 3,4]. We decomposed
multi-band visible spectra of Itokawa surfaces, obtained byAMICA (Asteroid Multi-band Imaging Camera, [5,6]), with thePC’s defined by 540 asteroids observed in ECAS (Eight-ColorAsteroid Survey) [7] consisting mainly of main-belt asteroids(MBA’s). We also decomposed and compared meteorite spectrain pulse-laser irradiation experiments that simulated space weathering by micrometeoroid bombardment [8,9] in the PC space.
Itokawa spectral variation: All the spectra of Itokawa surfaces, ECAS asteroids, and the meteorites were plotted in thePC1-2 space. Itokawa surface spectra turned out to span widelyalong a line from around Q-type area, spectrally young [10], tothe central area of the S-type cluster, spectrally more matured(more space-weathered). The spectroscopic distribution of Itokawa surfaces along a narrow linear trend in the PC1-2 space defined by the MBA spectra coincides with the spectral change ofmeteorites due to laser irradiation [11]. PC score spatial distribution reveals that some regions, such as a crater interior and thetop of large boulders, exhibit lower degree of weathering and that
other areas, such as smooth terrains, more weathered. The shiftdistance of meteorites spectra along Itokawa distribution in thePC space was approximately logarithmic function of cumulative
irradiation energy. The spectral age of the most mature areas on
Itokawa was estimated to be 2-4 times as old as the freshest.
Implication for regional resurfacing on asteroids: Global
resurfacing due to tidal forcing during Earth encounters has been
proposed for young spectral age for Q-type asteroids [12,13].
Such a process, however, would not explain heterogeneous degree of space weathering found on Itokawa. Regional or local
processes, such as impact cratering and land slides, on an asteroid
may also play an important role in resurfacing asteroid surfaces.
An accurate understanding of such regional variations would be
important for interpretations for the analyses of Itokawa samples.
References: [1] Nagao, K. et al. 2011. Science 333:11281131. [2] Noguchi, T. et al. 2011. Science 333:1121-1125.
[3] Tholen, D. J. 1984. Doctoral Thesis, Univ. of Arizona. [4]Bus, S. J. and Binzel, R. P. 2002. Icarus 158:146-177. [5] Saito, J.
et al. 2006. Science 312:1341-1344. [6] Ishiguro, M. et al. 2010.
Icarus 207:714-731. [7] Tedesco, E. F. et al. 1982. Astron. J.
87:1585-1592. [8] Hiroi, T. et al. 2007. LPSC, #1048. [9] Noble,S. K. et al. 2011. LPSC, #1382. [10] Binzel, R. P. et al. 2004. Icarus 170:259–294. [11] Koga, S. et al. 2014. LPSC, #1721. [12]Nesvorny, D. et al. 2005. Icarus 173:132-152. [13] Binzel, R.P.
et al. 2010. Nature 463:331-334.