Multi-scale variations of the ~2.7µm feature in Ryugu samples, observed by MicrOmega
1Le Pivert-Jolivet, T.; 1,2Pilorget, C.; 1Brunetto, R.; 1Bibring, J.-P.; 3Nakato, A.; 1Hamm, V.; 3,4Hatakeda, K.; 1Lantz, C.;
Loizeau, D.; 1,5Riu, L.; 3Yogata, K; 1Baklouti, D.; 1Poulet, F.; 1Aléon-Toppani, A.; 1Carter, J.; 1Langevin, Y.; 3,6Okada, T.;
3
Yada, T.; 3,6Usui, T. and the MicrOmega ISAS Curation Team

11IAS, Université Paris-Saclay, CNRS, France, 2Institut Universitaire de France, 3Institute of Space and Astronautical Science,
Japan Aerospace Exploration Agency, 4Marine Works Japan, Ltd, 5ESAC, ESA, Madrid, Spain, 6University of Tokyo, Bunkyo,
Tokyo 113-0033, Japan

Hayabusa2 is the first space mission to study and collect samples from a C-type asteroid. In December 2020, the spacecraft
brought back to Earth ~5.4g of materials from the surface of asteroid (162173) Ryugu. The samples were collected from two
different sites TD1 and TD2 [1] at the surface of the asteroid. The second touchdown was performed near the artificial crater
created by the small carry-on impactor [2] to collect both surface and subsurface materials. The samples were delivered toJAXA (Japan Aerospace eXploration Agency) Extraterrestrial Curation Center for preliminary analyses. They were extracted
from chambers A and C, corresponding to TD1 and TD2 respectively, weighed and analyzed in a controlled N2 environment
by an optical microscope, a FTIR, and MicrOmega, a near-infrared (0.99-3.65 µm) hyperspectral microscope. MicrOmega
acquires images of 256x250 pixels with a spatial resolution of 22.5 µm. The total field of view covers ~5.7x5.7mm² [3]. The
first spectral characterization of the bulk samples within the Curation Facility [4,5] showed that the grains are extremely darkand exhibit absorption features at 2.72 µm, 3.1 µm and 3.4 µm due to phyllosilicates, NH-rich compounds, and organics and/orcarbonates respectively. The ~2.7 µm feature was also observed on the asteroid’s surface by the NIRS3 spectrometer [6]. In
addition to the bulk samples, observations of individual grains, extracted from the bulks, were performed with MicrOmega.
We investigate here the variations of the ~2.7 µm feature at different scales, from the millimeter scale to the hundred microns
scale, on 177 grains (typical size 1-7 mm) from chambers A and C.
In order to extract an average spectrum of each grain but also to delimit the region of interest to study the spectral
heterogeneities inside the grains, we developed a procedure using thermal emission maps measured by MicrOmega. First, eachgrain was isolated from the rest of the field of view (the sample holder) thanks to their difference in terms of thermal emission.
Then, we performed a study of the ~2.7µm band at two different scales. On one hand, all pixels within the mask whereaveraged to obtain the millimeter scale spectrum of each grain. On the other hand, to better understand the variations at
millimeter scale, we looked at smaller scales (<200 µm) to highlight possible heterogeneities within the grains. Two spectral
parameters were calculated to characterize the ~2.7 µm OH feature: the peak position was estimated using a gaussian fit, andthe band depth was calculated between the minimum of reflectance and a linear continuum.
At millimeter scale, the position of the ~2.7 µm OH feature is consistent with the position found in highly aqueously alteredcarbonaceous chondrites [7]. Contrary to the bulk spectra where the ~2.7 µm OH feature was very similar between the twochambers [5], the position of the band varies within an interval of 10 nm at individual grain scale and the peak position
distribution varies between the two chambers: there is an excess of grains from chamber A with a position at longer
wavelength. Another difference is that the band depth of the grains varies within a larger range in chamber A than in chamber
C. The analysis at smaller scales (<200 µm) is still ongoing and will be presented at the time of the conference.
We will discuss the spectral differences between the collected grains, in particular between chambers A and C, and what
information they carry about the composition of phyllosilicates and the space weathering processes affecting Ryugu’s surface
materials. Indeed, the OH feature position can change with the Mg/Fe ratio in phyllosilicates [8]. Moreover, space weatheringexperiments on carbonaceous chondrites [9] have shown that the band position was shifted towards longer wavelength afterirradiation. The slight variation of band position we observe at the grain scale is consistent with the shift observed on Ryugu’s
surface spectra, between the artificial crater and the surrounding surface [10]. We expect that the smaller scale analysis will
give hints on the involved processes.
References[1] Tachibana et al. (2020) LPS XXXXXI Abstract #2027. [2] Arakawa et al. (2017) Space Sci. Rev. 208, 187-212. [3] Bibringet al. (2017) Space Sci. Rev. 208, 401-412. [4] Yada et al. (2022) Nat Astron, 6, 214-220. [5] Pilorget et al. (2022) Nat Astron,

6, 221-225. [6] Kitazato et al. (2019) Science, 364, 272-275. [7] Takir et al. (2013) M & PS 48, 1618-1637. [8] Besson andDrits (1997), CCM, 45, 158. [9] Lantz et al. (2017) Icarus 285, 43-57. [10] Kitazato et al. (2021) Nat Astron, 5, 246-250.