Stratified Evolution of Ryugu’s Regolith Revealed by Spectroscopic Analyses of Two Hayabusa2 Collected Particles

We present Fourier transform infrared (FTIR) and Raman spectroscopic analyses of two particles (A0226 and C0242) returned from the C-type asteroid Ryugu by the Hayabusa2 mission. These particles, recovered from two different stratigraphic depths during the touchdown operations, provide a unique opportunity to investigate the compositional heterogeneity, alteration history, and space weathering processes of a primitive asteroid. All particles exhibit the 2.7 μ m OH absorption feature, consistent with phyllosilicate hydration. However, the subsurface particle C0242 shows a significantly deeper OH band and stronger spectral signatures of carbonates and organic matter, suggesting the preservation of primitive material compared to the surface of the A0226 particle, which is distinctly affected by space weathering. To characterize compositional diversity, we applied unsupervised machine learning methods (Uniform Manifold Approximation and Projection for dimensionality reduction and Hierarchical Density-Based Spatial Clustering of Applications with Noise for cluster detection), identifying five spectral clusters linked to variations in their surface evolution and surface texture. This provides an objective framework to resolve spectral heterogeneity beyond median analyses. Raman spectroscopy of a fragment of A0226 reveals broad D and G bands typical of disordered carbonaceous matter, with spectral variations attributable to space weathering effects, consistent with FTIR analyses. These results, integrated with Hayabusa2 remote sensing data, support a model of stratified regolith evolution on Ryugu shaped by impact gardening, mass wasting, and surface irradiation. By modeling the reduction of the 2.7 μ m band over time, we estimate surface exposure of 10 5 yr for C0242 and 10 6 for A0226.