[PPS07-04] Morphological and spectral trends among boulders on asteroid 162173 Ryugu: implication for parent body processes
Keywords:Hayabusa2, Ryugu, C-type asteroid, boulder, space weathering
Approximately 100 largest boulders within the range of 40°N to 40°S and 260°E to 340°E taken at ~0.2 m/pix were used for our analysis. The rims of boulders were marked manually, and spectra were measured within the outlines. Though the resolution of the shape model used for the photometrical correction was ~10-20 times lower than the image resolution, the error in the normalized spectra was confirmed to be small: the error in b-x slope is <~30% the global variation.
Our result show that the red-to-blue trend among boulders follows the spectral transition by heating experiments of CM chondrites with temperatures between 800 and 900 °C . The same trend was seen with different observed phase angles, and thus is unlikely to be a manifestation of the phase reddening effect. The possible presence of multiple clusters in the spectral trend was investigated by conducting the k-means classification. The root-mean-square residual of the classification decreased with increasing number of classes, indicating no clear existence of large spectral clusters. This result is consistent with the existence of boulders which underwent different degrees of thermal metamorphism in the parent body. However, a small spectral class consistent with Type 3 boulders  exist, which particularly deviates from the thermal metamorphism trend. Also, the brighter half of the boulder population show a smaller size distribution. Thus, the darker boulders exhibit a volumetric preponderance over brighter boulders, which may also inherit the heterogeneity in the parent body.
We found that the power-law exponent of the 2D Fourier amplitude as a function of the spatial frequency could be an index for surface texture of boulders. The unimodal distribution of the exponent also shows no clear presence of morphological clusters. Thus, the rugged morphologies are consistent with a genomict-type breccia, which indicates a well-mixed nature of the material at the timing of brecciation (e.g. impact melting, cementation under aqueous environments).
High resolution imaging of the boulders revealed the spectral variation within individual boulders. These variations tend to be associated with different facets. Similar color variation was previously reported for the largest boulder Otohime . By calculating the direction of the bluer facet for each boulder, we found that bluer facets are likely to be directed toward the equator. This is consistent with the preferential vulnerability to space weathering and/or thermal degradation/exfoliation of facets facing a certain direction.
We conclude that majority of boulders on Ryugu exhibit a spectral and morphological trend within a single continuous cluster. The trend may have originated from boulders with different degree of thermal metamorphism together with the genomict-type brecciation processes on the parent body. In addition, observed intra-boulder spectral variation suggests superimposition of space weathering effects.
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