Spectrally blue hydrated parent body of asteroid (162173) Ryugu

Understanding the evolution from dust to planetesimals in the early solar system is crucial. Asteroids, remnants of planetesimals, offer insights into the thermal history and water/rock ratios of the early solar system. Missions like NASA's Dawn to Ceres and Vesta have revealed evidence of subsurface liquids on Ceres, suggesting aqueous alteration in carbonaceous protoplanets. Ryugu, a rubble-pile asteroid visited by Hayabusa2, formed from the catastrophic disruption of a larger parent body. Studying Ryugu can shed light on the parent body's thermal history and water/rock ratio. Previous research suggested that less-processed, pristine materials on Ryugu are spectrally bluer than the average reflectance spectrum, with such materials found at the equatorial ridge and, more prominently, in the polar regions where space weathering and solar heating are minimized. This study focuses on detailed surveys of Ryugu's polar regions to investigate these unprocessed materials and deconstruct the effects of solar wind, micrometeoroid bombardment, and solar heating.

Studies of carbonaceous chondrites, like CM and CI types, reveal that phyllosilicates' Mg/Fe ratios change with the degree of aqueous alteration. Fe-bearing phyllosilicates, exhibiting 0.7-µm band absorption, are indicative of specific water/rock ratio conditions during parent body formation. The 2.7 µm OH-band absorption also reflects hydration states, with peak positions shifting from 2.8 to 2.7 µm due to Fe-rich to Mg-rich phyllosilicate phase changes. Previous global observations of Ryugu suggested two compositional scenarios: (1) aqueously altered and thermally metamorphosed carbonaceous material, and (2) incipiently altered interplanetary dust particle or cometary material. This study aims to resolve this ambiguity by focusing on the least processed materials.

The study utilized remote-sensing data from Hayabusa2's Telescopic Optical Navigation Camera (ONC-T) and Near-Infrared Spectrometer (NIRS3). ONC-T provided multi-band images of Ryugu's poles, enabling the creation of spectral index maps showing visible spectral slope (b-x) and 0.7-µm band absorption depth. NIRS3 provided near-infrared spectra, focusing on the 2.72-µm OH-band absorption. The analysis included detailed investigation of Otohime Saxum, a large boulder at the south pole, examining its different facets for spectral variations. The depth of 0.7-µm absorption was assessed for statistical significance, considering the ONC-T's sensitivity calibration ambiguity. To assess the influence of solar heating and space weathering (solar wind irradiation and micrometeoroid bombardment), maximum temperature and normalized solar photon dose were calculated based on Ryugu's shape model and orbital elements. The results were compared with spectral properties of other asteroids, such as Polana, Eulalia, Phaethon, and Bennu, to understand Ryugu's parent body origin and evolution. The study also analyzed aqueously altered and thermally metamorphosed carbonaceous chondrites (ATCCs) to constrain the parent body's thermal history.

Spectrally blue materials, associated with deeper 0.7-µm band absorption, are concentrated in Ryugu's polar regions, indicating less space weathering. The bluest materials (Otohime Facet A and the north pole) show absorption depths of 1.24 ± 0.11% and 1.28 ± 0.06%, significantly deeper than the average Ryugu value (0.98%). NIRS3 observations of the north pole show a slightly deeper 2.72-µm band absorption compared to the average Ryugu spectrum. Analysis of solar heating and solar wind irradiation indicates a stronger correlation between blue spectra/deeper 0.7-µm absorption and low solar wind irradiation, suggesting that space weathering reduces absorption features. Ryugu's color variation, attributed to space weathering, points to an originally bluer B-type composition. Comparison with other asteroids suggests a possible link with the Polana or Eulalia families, but differences in hydration and space weathering with Bennu suggest different parent bodies. Ryugu's spectral characteristics, specifically the weak 2.7-µm band and the presence of the 0.7-µm feature, point to a scenario where its parent body underwent aqueous alteration and subsequent thermal metamorphism at 570–670 K (300–400 °C). The low thermal inertia of Ryugu suggests that the parent body might have been ~100km in diameter with heterogeneous thermal metamorphism. The heating is likely caused by radioactive decay of short-lived radionuclides like 26Al, indicating early accretion about 2-2.5 Ma after Calcium-Aluminum-rich Inclusions (CAIs).

The findings support a scenario where Ryugu's parent body formed under water/rock ratio conditions similar to CM chondrites, but with earlier accretion leading to thermal metamorphism. The observed spectral features, including the presence of Fe-bearing phyllosilicates and a relatively weak OH-band, indicate aqueous alteration followed by partial dehydration due to heating. The highly porous nature of Ryugu is consistent with a heating process driven by primordial radiogenic decay rather than impacts. The study's results provide constraints on the formation conditions of water-rich carbonaceous asteroids and suggest that Ryugu's parent body might have had a layered structure with varying degrees of thermal metamorphism.

This research reveals that the spectrally blue, least-processed materials on Ryugu's poles reflect a parent body's history of extensive aqueous alteration and subsequent thermal metamorphism at 570–670 K (300–400 °C), likely caused by the decay of short-lived radionuclides. The findings support a layered structure for the parent body and highlight the potential of returned samples to further elucidate the early solar system's thermal evolution. Future research could focus on detailed mineralogical and isotopic analyses of the returned samples to refine the thermal and aqueous alteration history and constrain the parent body's formation timeline.

The study relies primarily on remote-sensing observations. The absolute values of 0.7-µm band absorption depths have uncertainties due to ONC-T's calibration. While the relative differences are statistically significant, in-situ analyses of returned samples will provide more precise measurements. The modelling of temperature and photon dose is based on current orbital parameters, and past orbital configurations might have influenced surface properties. Finally, the absence of a perfect spectral match with known meteorite samples suggests potential variations in composition not yet represented in existing meteorite collections.