Spectrally blue hydrated parent body of asteroid (162173) Ryugu

The study investigates how small rubble-pile asteroids like Ryugu record the thermal and aqueous histories of their larger parent bodies. Understanding asteroidal thermal history and water/rock ratios is crucial for reconstructing early solar system conditions. Prior spacecraft missions (e.g., Dawn at Ceres and Vesta) and models suggest carbonaceous parent bodies underwent aqueous alteration and potential differentiation. Ryugu likely formed from catastrophic disruption of a larger (~100 km) parent body, making it an ideal case to probe parent-body processes. The objective is to identify Ryugu’s least space-weathered, most pristine surface material and use its spectral properties to infer the parent body’s hydration state and subsequent thermal metamorphism, while decoupling effects of space weathering and solar heating. Observations indicate that bluer-than-average materials concentrate near Ryugu’s poles where solar irradiation and space weathering are minimal, motivating focused analyses there. Spectral indicators targeted include the 0.7-µm Fe-bearing phyllosilicate band and the 2.7-µm OH absorption, whose characteristics track hydration state and mineral chemistry.

- Spacecraft and planetary context: The Dawn mission’s observations of Ceres reveal subsurface liquids, supporting rock-ice differentiation and widespread aqueous alteration in large carbonaceous bodies. Ryugu is a rubble pile from catastrophic disruption of a larger parent body, making it a repository of earlier thermal history. - Prior Ryugu observations: Global color and stratigraphic analyses found slightly bluer materials at the equatorial ridge and suggested that the bluest materials reside at the poles where solar heating and space weathering are weakest. - Spectral indicators of hydration: The 0.7-µm band (Fe2+–Fe3+ charge transfer) is a proxy for Fe-bearing phyllosilicates (e.g., serpentine, saponite). As aqueous alteration proceeds in CM/CI chondrites, phyllosilicates become Mg-enriched; the OH band around 2.7–2.8 µm shifts accordingly from Fe-rich (~2.8 µm) to Mg-rich (~2.7 µm). Both 0.7-µm and 2.7-µm features are sensitive to thermal processing and space weathering. - Space weathering effects: Laboratory simulations and lunar observations show that solar wind and micrometeoroid bombardment can alter spectral slopes and attenuate some absorption bands. For hydrated carbonaceous materials, reproducing asteroid-like trends has been challenging and depends on material properties and irradiation conditions. - Asteroidal families and analogs: B-type asteroids (including F-types) display blue slopes and UV upturns; members of the Polana and Eulalia families exhibit spectral slopes similar to Ryugu’s range. Comparisons to Bennu and Phaethon highlight similarities and differences in UV-blue behavior and 3-µm hydration features.

- Imaging and spectrophotometry (ONC-T): The Hayabusa2 Telescopic Optical Navigation Camera (ONC-T) acquired seven-band images from 0.40 to 0.95 µm. After bias, updated flat-field, and radiometric calibration to I/F, images were photometrically corrected to standard geometry (i=30°, e=0°, a=30°) using a global shape model (SFM_3M_v20200815) and SPICE kernels. Pixels with i<70°, e≤70°, and I/F>0.005 were used. The visible spectral slope b–x (0.48–0.86 µm) and the 0.7-µm band depth indices were computed. High-latitude close-approach imaging enabled detailed regional analyses at the poles, including Otohime Saxum’s facets (A, B, C). To improve SNR, regional averages used 15×15 pixel ROIs; relative uncertainties in 0.7-µm depth across surface areas are ~0.1%, enabling detection of 0.3–0.7% spatial differences. - Near-infrared spectroscopy (NIRS3): NIRS3 spectra (1.8–3.2 µm) were extracted over ROIs (north polar plains and Otohime facets) and compared to same-day reference spectra outside the ROIs with similar viewing geometry. Spectra were normalized at 2.5 µm; the 2.72-µm OH band depth was measured after continuum removal using 2.6 µm and 2.9 µm anchors. Observations spanned multiple dates in 2019; datasets were processed with appropriate radiometric calibration coefficients. Photometric corrections were applied where trajectory reconstruction allowed; for one date, non-photometric data were used, justified by negligible impact on band depth for relative comparisons. - Solar photon dose and thermal modeling: Using SPICE and the high-resolution shape model (SHAPE_SFM_200k_v20200815), the normalized solar photon dose per facet over an asteroidal year was computed as cos(i)/(D/1 au)^2 and averaged, mapping space-weathering proxies (solar wind and micrometeoroid flux correlate with solar irradiation geometry). One-dimensional heat conduction was solved per facet to estimate maximum surface temperatures at perihelion (current orbit), adopting uniform thermophysical parameters: albedo 0.0146, emissivity 1.0, thermal inertia 200 J m−2 K−1 s−0.5, rotation period 7.63 h, and current pole orientation. Additional simulations explored closer heliocentric distances (e.g., 0.4 au) to assess potential historical heating, while noting current-orbit maximum temperatures do not reach phyllosilicate decomposition thresholds. - Correlative analyses: Observed spectral indices (b–x slope and 0.7-µm depth) were compared against modeled photon dose and peak temperatures, binned to evaluate trends and uncertainties. Morphological context (e.g., shielding by local topography) was considered in interpreting facet-level variations.

- Polar concentration of blue, hydrated materials: The bluest visible slopes and relatively deeper 0.7-µm absorptions occur at both poles. At the north pole and Otohime Facet A (south pole), b–x slopes are −0.17 µm−1 and −0.14 µm−1, respectively, compared with +0.10–0.12 µm−1 for the global average and Otohime Facet C. - 0.7-µm band depths: North pole 1.24 ± 0.11%; Otohime Facet A 1.28 ± 0.06%; typical Ryugu 0.98%; Otohime Facet C 0.63 ± 0.08%. Despite an absolute calibration ambiguity (~0–3%), spatial differences are statistically significant (~3–7σ) due to high relative precision (~0.1%). - 2.72-µm OH band: Slight, spatially coherent increases are observed but are modest. North polar plains: 8.1% vs 7.5% reference. Otohime Facet A: 8.85% vs 8.35% reference (high variance among facet spectra makes significance uncertain). Facet B: 7.7% vs 7.9% (slight decrease). Facet C: 8.5% vs 8.3%. No peak position shift was observed in polar spectra, contrasting with SCI crater observations. - Driver of color and band variations: Blue slopes and deeper 0.7-µm bands correlate with lower modeled solar photon dose (proxy for reduced solar wind/micrometeoroid space weathering), not with lower peak temperatures. Shielding by local topography (e.g., Otohime Facet A) reduces irradiation <20% of average, lengthening space-weathering timescales by >5×. This supports space weathering as the primary cause of reddening and 0.7-µm attenuation; the surface space-weathering timescale is on the order of exposure age (~10^5 years for the top ~1 m). - Parent-body processing: The presence of Fe-bearing phyllosilicates (0.7-µm) together with a relatively weak, short-wavelength 2.72-µm OH band suggests a CM-like aqueous alteration environment followed by mild thermal metamorphism that partially dehydrates and reduces band strengths. Laboratory constraints imply maximum heating of ~570–670 K (300–400 °C), below full phyllosilicate decomposition (870–970 K). - Source family and analogs: Ryugu’s spectral range matches blue B-type behavior and overlaps slopes seen in the Polana and Eulalia families, favoring these as possible source families. Bennu exhibits stronger 2.7-µm bands and different visible space-weathering trends (bluing), and no region on Ryugu shows Bennu-like strong OH bands, arguing against a shared immediate parent body. Phaethon’s blue slopes and UV upturn vary with rotation, potentially reflecting similar space-weathering processes. - Thermal history: The inferred metamorphism is consistent with radiogenic heating by 26Al due to early accretion (2–2.5 Ma after CAIs). High porosity and low compaction argue against impact heating as the dominant mechanism. The parent body likely had layered metamorphic degrees and remained largely undifferentiated or convected as a mud ball.

The study identifies and analyzes Ryugu’s least space-weathered materials at the poles, confirming that the original, fresher surface exhibits blue visible spectral slopes and detectable 0.7-µm Fe-phyllosilicate absorption. The correlation of bluer slopes and deeper 0.7-µm bands with low photon dose (reduced solar wind/micrometeoroid exposure) indicates space weathering, rather than solar heating, drives reddening and band attenuation across Ryugu. The combination of a weak but present 0.7-µm band and a modest, short-wavelength 2.72-µm OH feature constrains the parent body to have undergone significant aqueous alteration (CM-like water/rock conditions) followed by moderate thermal metamorphism (~570–670 K). This thermal overprint is consistent with internal radiogenic heating from short-lived radionuclides (e.g., 26Al) due to early accretion. Comparisons with B-type asteroids and inner-belt families (Polana/Eulalia) support Ryugu’s origin among spectrally blue inner main-belt populations. Differences with Bennu’s hydration strength and space-weathering trends suggest divergent histories or parentage. The spectral uniformity across Ryugu, despite low thermal inertia and extreme porosity, implies limited large-scale differentiation in the parent body or pervasive mixing (e.g., mud-ball convection). Overall, the findings address the research aim by linking the freshest observable surface materials to the parent body’s hydrothermal evolution and by disentangling ongoing space-weathering effects from inherited compositional signatures.

- The least space-weathered materials on Ryugu occur at the poles and exhibit blue visible slopes and measurable 0.7-µm absorptions diagnostic of Fe-bearing phyllosilicates. - Spatial correlations with low solar photon dose indicate that space weathering (solar wind and micrometeoroids) reddens the surface and diminishes the 0.7-µm band, whereas solar heating is not the dominant factor in the observed color distribution. - Spectral constraints imply Ryugu’s parent body experienced extensive aqueous alteration under CM-like water/rock conditions, followed by moderate thermal metamorphism to ~570–670 K, likely driven by radiogenic 26Al heating due to early accretion (2–2.5 Ma after CAIs). - Spectral slopes overlap those of the Polana and Eulalia families, suggesting a likely origin in the inner main belt among B-type populations; differences from Bennu’s hydration and space-weathering behavior imply distinct histories. - Anticipated analyses of Hayabusa2-returned samples will refine the heating mechanism, temperature-time paths, and the mineralogical basis for Ryugu’s spectral properties, and may resolve why laboratory space-weathering simulations have not yet replicated Ryugu’s spectral trends. Future work should quantify space-weathering rates, investigate parent-body heterogeneity, and compare returned samples with ATCCs and CM chondrites to calibrate spectral indicators of hydration and metamorphism.

- Absolute 0.7-µm band depth values are affected by calibration uncertainties (up to ~1.6% from standard star spectra), although spatial differences are precise (~0.1%). - NIRS3 2.72-µm band depth increases at the poles and Otohime Facet A are small, and facet-level variability limits statistical significance for some observations. - Thermal models use uniform thermophysical parameters and current orbital geometry; inferences about higher past temperatures rely on hypothesized closer perihelion passages, which are not directly observed here. - Laboratory space-weathering experiments on hydrated chondrites have not reproduced Ryugu’s exact spectral evolution, indicating possible differences in initial materials or space-weathering conditions. - Potential small-scale compositional or textural heterogeneities are suggested by TIR and ONC-T anomalies but are spatially limited; global extrapolation assumes overall uniformity. - No strong spatial variation in 2.7-µm OH band was detected at the poles, limiting constraints on hydration variability from NIR data.