A GLOBAL VIEW OF THE NEAR-INFRARED REFLECTANCE PROPERTIES OF RYUGU AS SEEN BY THE NIRS3 SPECTROMETER ON HAYABUSA2. R.E. Milliken1, K. Kitazato2, L. Riu3, T. Iwata3, M. Abe3, M. Ohtake3, S. Matsuura4, T. Arai5, Y. Nakauchi3, T. Nakamura6, M. Mastuoka3, H. Senshu7, N. Hirata2, T. Hiroi1, C. Pilorget8, R. Brunetto8, F. Poulet8, J.-P. Bibring8, D. Takir9, D.L. Domingue10, F. Vilas10, M.A. Barucci11, D. Perna11,12, E. Palomba13, A. Galiano13, K. Tsumura7, T. Osawa14, M. Komatsu15, A. Nakato3, T. Arai7, N. Takato16, T. Matsunaga17, Y. Takagi18, K. Matsumoto16, T. Kouyama19, Y. Yokota3, E. Tatsumi20, N. Sakatani3, Y. Yamamoto3, T. Okada3, S. Sugita20, R. Honda21, T. Motora22, S. Kameda23, H. Sawada3, C. Honda2, M. Yamada7, H. Suzuki24, K. Yoshioka20, M. Hayakawa3, K. Ogawa25, Y. Cho20, Y. Takei3, T. Saiki3, S. Nakazawa3, S. Tanaka3, M. Yoshikawa3, S. Watanabe3,22, Y. Tsuda3. 1Brown University, Providence, RI, 02912, USA.2The University of Aizu, JP.3Institut of Space and Astronautical Science (ISAS/JAXA), JP. 4Kwansi Gakuin University, JP.5Ashikaga Universty, JP.6Tohoku University, JP.7Chiba Institute of Technology, JP.8Institut d’ Astrophysique Spatiale, FR.9Jacobs/NASA Johnson Space Center, USA.10Planetary Science Institute, USA.11LESIA, FR.12INAF, Osservatorio Astronomico di Roma, IT.13INAF, Istituto di Astrofisica e Planetologia Spaziali, IT.14Japan Atomic Energy Agency, JP.15SOKENDAI, JP.16National Astronomical Observatory of Japan, JP.17National Institute for Environmental Studies, JP.18Aichi Toho University, JP.19National Institute of Advanced Industrial Science and Technology, JP.20University of Tokyo, JP.21Kochi University, JP.22Nagoya University, JP.23Rikkyo University, JP.24Meiji University, JP.25Kobe University, JP. (Ralph_Milliken@brown.edu; kitazato@u-aizu.ac.jp)

Introduction: The Japanese Aerospace Exploration Agency (JAXA) Hayabusa2 spacecraft encountered the asteroid Ryugu in June 2018 and collected a wealth of data until its departure in late 2019. Prior to arrival, Ryugu was classified as a C-type asteroid and was anticipated to contain primitive materials similar to what is observed in carbonaceous chondrite meteorites, possibly including hydrous phases and/or organic compounds [1-3]. The Hayabusa2 spacecraft includes NIRS3, a point spectrometer with a 0.1° field of view that measures radiance over an effective wavelength range of ~1.8 – 3.2 µm [4-5]. NIRS3 successfully acquired many tens of thousands of spectra at a range of altitudes (and thus spot sizes) during the Ryugu encounter.

The highest spatial resolution data were acquired during various descent operations. Along with the global spectral properties derived from global mapping campaigns, these data can be used to assess the surface properties of Ryugu and its surface composition in particular. Hypotheses based on data such as NIRS3 will be tested in the coming year when the collected samples are returned to Earth in late 2020. The direct sampling of Ryugu will provide a significant step forward in understanding how to relate spectral properties of a C-type object to independently measured mineralogy and chemistry. ‘Global’ maps presented here are based on data acquired on July 11, 2018 (~40 m per spot) and July 19, 2018 (~20 m per spot) [5].

Data Analysis & Processing: When Ryugu is

illuminated by the Sun and observed by NIRS3, the measured radiance is a combination of reflected sunlight and thermally emitted radiation, where the latter strongly influences the longer wavelength range of the instrument (e.g., in the “3 µm” region that is sensitive to the presence of OH and H2O). Therefore, it is necessary to remove the thermal emission component in order to retrieve accurate surface reflectance values and to assess the presence or variations in OH/H2O features [5]. Raw data values (DN) for each wavelength are first converted to radiance through multiplication by the radiometric calibration

coefficient (RCC). The thermal contribution is then estimated by fitting a Planck function to the measured radiance value at a wavelength that is close to, but outside of, the longer wavelength OH/H2O region. This is similar to approaches that have been successfully applied to remote observations of the Moon [6]. The thermal contribution is then subtracted from the total radiance, and the residual (solar reflected) radiance is then converted to I/F or reflectance by accounting for the solar flux, viewing geometry, and Sun-Ryugu distance.

The initial RCC was based on pre-flight calibration measurements [4], but observations acquired after launch indicated an update to the RCC was necessary, and observations of the onboard calibration lamps were used for this purpose [5].

Results: Based on data from the ‘global’

mapping campaign, Ryugu appears to be quite spectrally homogenous at the ~20-40 m spatial scale. The entire surface of the asteroid is extremely dark, exhibiting an average albedo of ~0.017±0.002 [5]. This is darker than other primitive objects recently visited by spacecraft, including the nucleus of comet 67P/Churyumov-Gerasimenko measured by Rosetta. There are some albedo variations across the surface that may not be linked to photometric/viewing geometry effects (e.g., the equatorial region of Ryugu exhibits an increase in albedo relative to adjacent terrains, Figure 1), but absolute values of these variations are generally quite small [5].

The near-IR reflectance spectra of Ryugu are commonly linear with a slightly red (positive) spectral slope with increasing wavelength. The exact strength of this slope for wavelengths >2 µm is dependent on the accuracy of the thermal correction, and local geometry and physical properties can have strong links to surface temperature. Because of these complications and dependencies, weak variations in spectral slope that appear to be correlated with physical/morphologic properties of Ryugu are still under investigation.

There is currently no clear spectral evidence for the presence of pyroxene at the surface of Ryugu (i.e., no

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absorptions at ~2 µm due to Fe2+ in an M2 coordination site) [7]. Though not diagnostic, the lack of a 2 µm feature is common in certain types of C chondrites, particularly those in which primary silicates have been aqueously altered to form clay minerals (type C1/C2 meteorites). However, NIRS3 spectra of Ryugu also do not exhibit strong absorption features in the 3 µm region that indicate the object is rich in OH/H2O. Rater, all spectra of Ryugu exhibit a weak and narrow feature with a reflectance minimum at 2.72 µm that is interpreted to indicate the presence of OH, likely attached to Mg [5]. This absorption feature is consistent with the presence of Mg-serpentine.

The OH feature is observed in all spectra of Ryugu, but it is very weak when compared with lab spectra of typical aqueously altered C chondrites [8] and weaker than the hydration feature observed for Bennu by OSIRIS-REx [9]. The closest meteorite match to Ryugu, based on existing spectral libraries, is to several samples of thermally metamorphosed C chondrites and spectra of heated samples of Ivuna (Figure 2). As such, Ryugu may represent a primitive object that was aqueously altered and then thermally metamorphosed prior to (or as a result of) disruption and reformation as a rubble pile. Alternatively, space weathering processes may be operating at Ryugu’s optical surface to degrade hydrous materials and weaken the OH feature. In this scenario, material beneath the uppermost surface may be more hydrated. Indeed, preliminary analysis of NIRS3 spectra (<1 m per spot) acquired at the impact crater experiment location indicate material excavated from depth may

exhibit a deeper OH band that is shifted to slightly shorter wavelengths [10]. A third option is that Ryugu did not experience enough aqueous alteration to give rise to abundant hydrous phases, in which case the weak OH feature indicates low phyllosilicate abundance and/or OH formed by solar wind implantation.

Conclusions: At near-IR wavelengths, the surface of Ryugu is rather homogenous at the 20-40 m spatial scale. Major characteristics of Ryugu are that it is (1) extremely dark, (2) spectrally ‘flat’ with only a weak red slope, (3) exhibits a weak OH feature at all locations, (4) lacks clear evidence for pyroxene at a global scale, and (5) is most spectrally similar to lab data of thermally metamorphosed C chondrites [5].

Multiple samples were successfully acquired from the surface of Ryugu, including at the impact experiment site where weak variations in the OH band are observed. These samples are expected to be returned to Earth in late 2020, and detailed laboratory studies will allow testing of whether or not thermal metamorphism, space weathering, limited aqueous alteration or a combination of these processes best explains the spectral properties observed by NIRS3. It is clear that Ryugu is spectrally quite different from Bennu, and as such it provides a unique and distinct data point for improving our understanding of how to use reflectance data of C-type objects to infer their mineralogy, chemistry, and geological history.

References: [1] Binzel, R. et al. (2001), Icarus, 151, 139-149; [2] Vilas, F. (2008), Astron. J., 135, 1101-1105; [3] Tsuda, Y. et al. (2013), Acta Astronaut., 91, 356-362; [4] Iwata, T. et al. (2017), Space Sci. Rev., 208, 317-337; [5] Kitazato, K. et al. (2019), Science, 364, 272-275; [6] Li, S. and R.E. Milliken (2016), JGR, 121, doi:10.1002/2016JE005035; [7] Burns, R. (1993), Mineralogical Applications of Crystal Field Theory, 551pp.; [8] Hiroi, T. et al. (2017), LPSC 48, #108. [9] Hamilton et al. (2019), Nature Astronomy, 3, 332-340; [10] Kitazato et al. (2019), Asteroid Sci. in Age of Hayabusa2 and OSIRIS-REx, U. of Arizona, #2106.

Figure 2. Modified Fig. 3 from [5] showing comparison between typical Ryugu spectrum as acquired by NIRS3 and laboratory spectra of various C chondrites including thermally metamorphosed sample Meteorite Hills 01072. Within existing spectral libraries of meteorite samples, C chondrites that have experienced thermal metamorphism or alteration provide the closest spectral match in the near-IR.

Figure 1. Top: Albedo map of Ryugu, showing minor variations with average albedo of ~0.017 and increase in albedo near equatorial ridge. Bottom: Temperature map derived from NIRS3 data, warmer colors indicate higher temperatures approaching ~375K. Data from July 11 & 19, 2018.

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