Hydrothermal history of (162173) Ryugu’s parent body inferred from remote-sensing data
Eri Tatsumi1,2,3, Naoya Sakatani4, Lucie Riu5, Moe Matsuoka5, Rie Honda6, Tomokatsu Morota3, Shingo Kameda4, Tomoki
Nakamura7, Michael Zolensky8, Rosario Brunetto9, Takahiro Hiroi10, Sho Sasaki11, Sei’ichiro Watanabe12, Satoshi Tanaka5,13,
Jun Takita14, Cédric Pilorget9, Julia de León1,2, Marcel Popescu15, Juan Luis Rizos1,2, Javier Licandro1,2, Ernesto Palomba16,Deborah Domingue17, Faith Vilas17, Humberto Campins18, Yuichiro Cho3, Kazuo Yoshioka3, Hirotaka Sawada5, Yasuhiro
Yokota5, Masahiko Hayakawa5, Manabu Yamada19, Toru Kouyama20, Hidehiko Suzuki21, Chikatoshi Honda22, KazunoriOgawa5, Kohei Kitazato22, Naru Hirata22, Naoyuki Hirata23, Yuichi Tsuda5,13, Makoto Yoshikawa5,13, Takanao Saiki5, Fuyuto
Terui5, Satoru Nakazawa5, Yuto Takei5, Hiroshi Takeuchi5,13, Yukio Yamamoto5,13, Tatsuaki Okada5,3, Yuri Shimaki5, KeiShirai23, Fernando Tinaut1,2, Sunao Hasegawa5, Seiji Sugita3,191Instituto de Astrofisica de Canarias (etatsumi-ext@iac.es), 2Dept. Astrophysics, Univ. La Laguna, 3The University of Tokyo,4Rikkyo University, 5Institute of Space and Astronautical Science (ISAS), 6Kochi University, 7Tohoku University, 8NASA
Johnson Space Center, 9Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 10Brown University, 11OsakaUniversity, 12Nagoya University, 13SOKENDAI, 14Hokkaido Kitami Hokuto High School, 15Astronomical Institute of theRomanian Academy,16INAF, Instituto di Astrofisica e Planetologia Spaziali, 17Planetary Science Institute, 18University ofCentral Florida, 19Planetary Exploration Research Center,, Chiba Institute of Technology, 20National Institute of Advanced
Industrial Science and Technology, 21Meiji University, 22The University of Aizu, 23Kobe University.
1. Summary
Small rubble pile asteroids record the thermal evolution of their much larger parent bodies. However, recent space
weathering and/or solar heating create ambiguities between the uppermost layer observable by remote-sensing and the pristinematerial from the parent body. Hayabusa2 remote-sensing observations find that on the asteroid (162173) Ryugu both northand south pole regions preserve the least space-weathered material, which is spectrally blue carbonaceous chondritic material
with a 0 – 3% deep 0.7-µm band absorption, indicative of Fe-bearing phyllosilicates [1]. We report that spectrally blue
Ryugu’s parent body experienced intensive aqueous alteration and subsequent thermal metamorphism at 570 – 670 K (300 –400 ˚C), suggesting that Ryugu’s parent body was heated by radioactive decay of short-lived radionuclides possibly because ofits early formation 2-2.5 Ma. The samples being brought to Earth by Hayabusa2 will give us our first insights into this epoch insolar system history. Moreover, we found the NUV-VIS spectral similarity between Ryugu and Polana–Eulalia family
members, suggesting plausible origin from inner main belt predicted by the dynamical simulation [2].
2. Observation on polar regions
Ryugu could be an ideal body to study the thermal history and water/rock ratio of pre-disruption parent bodies, muchlarger (~100 km) than Ryugu. Our objective is to find the most pristine material on Ryugu and to study evidence for proposed
thermal metamorphism of its original parent body and consequent processes after its catastrophic disruption and reaccumulation. Stratigraphic analyses have suggested that the possible surviving, unprocessed materials are bluer than theaverage Ryugu reflectance spectrum [3,4]. Slightly bluer materials than the average were found on the equatorial ridge, whichmight be uncovered by regolith migration from the ridge to the middle latitudinal regions. Furthermore, the global observations
obtained by the Hayabusa2 spacecraft discovered that the bluest materials are distributed at the polar regions of Ryugu [3].
This motivated us to conduct detailed surveys of the polar regions to investigate the unprocessed materials. Effects after theformation of Ryugu, such as solar wind irradiation, micrometeoroid bombardment, and radiative heating caused by close
encounter to the Sun, need to be deconstructed.
Spectrally blue (negative visible spectral slope) material is concentrated on both poles, as clearly shown. Furthermore,
blue material is associated with a relatively deeper 0.7-µm band absorption. Phyllosilicates in CM/CI chondrites become
progressively enriched in Mg (and depleted in Fe) as aqueous alteration proceeds [5]. Thus, Fe-bearing phyllosilicates showing0.7-µm band absorption are a strong indication of the specific water/rock ratio condition during the parent body formation. To
examine the cause of the distribution of blue materials with 0.7-µm band absorption, the maximum temperature in anasteroidal year and the normalized solar photon dose were calculated based on the TIR measurements [6], the shape model andcurrent orbital elements. The comparison indicates that areas and facets with low solar wind irradiation fluxes exhibit blue
spectra and relatively deeper 0.7-µm absorption, while the influence of solar heating does not clearly correlate with the spectral
characteristics. On the north pole, the concentration of the material with blue spectral slope, and deeper 0.7-µm band
absorption were well correlated with the regions experiencing the lowest temperatures and least solar wind irradiation due tothe large incidence angle during the whole asteroidal year. The correspondence between the color variation and thoseprocesses shows that solar wind irradiation is a more likely cause for the color changing from blue to red and the decrease in
the 0.7-µm band absorption depth
3. Possible connection with (3200) Phaethon(3200) Phaethon, the target of the DESTINY+ Mission, exhibits blue spectra in the visible wavelength range and turn-up
in the UV. Recently, many ground-based observations were made over a wide wavelength range, which reported that thevariation in visible spectral slope depends on rotational phase. The range of spectral slope variation in one rotation is -0.5 to0.05 µm-1 and that of the relative R-band magnitude is ± 0.15 [7,8]. Moreover, a correlation between brighter and bluer spectra
was also observed [7]. The similarity for both Ryugu and Phaethon, that neither exhibits a strong UV nor 2.7-µm OH-band
absorption for the entire rotational phases [9], might result in similar spectral changes due to space weathering on bothasteroids. Thus, the majority of Phaethon’s surface could be explained by freshness due to rejuvenation caused by the recent
encounter with the Sun, i.e. fresh cometary activity.
4. Search for F types in the main belt
Ryugu is an F-type asteroid with a flat spectral shape in near-ultraviolet (0.4 – 0.55 µm) [10]. In order to put in context
the results from the Hayabusa2 spacecraft and of its returned sample analyses, it is critical to know the abundance of F-types
across the Solar System. However, after Eight Color Asteroid Survey (ECAS) [11], there is no major spectroscopic survey thatcovers the near-ultraviolet wavelength range. We started ground-based near-ultraviolet observations using telescopes at Roquede Los Muchacho Observatory, La Palma, Spain. Also we revisited our previous observations and reanalyzed the data in thiswavelength range. We collected the spectral data of Themis, Polana, and Eulalia family members from our observations andECAS. The largest members of these families are known to have negative visible spectral slope. Polana and Eulalia families
are the plausible origin of Ryugu based on the dynamical calculation [2]. When we compared only the visible slope, thosefamily members cannot be distinguished. However, if we use near-ultraviolet spectral slope, Themis family members are quite
different from Polana and Eulalia family members. While Themis members show deep absorption in near-ultraviolet, Polanaand Eulalia members exhibit flatter in this region. Ryugu is consistent with variation of the Polana and Eulalia members.
Moreover, Bennu, the target asteroid of NASA’s OSIRIS-REx [12], is also consistent with the Polana and Eulalia group. We
found the connection between Ryugu and Polana and Eulalia members spectroscopically. Thus, now the idea that Ryugu isoriginated from the inner main belt is supported also from spectroscopy.
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