3D Detection and Analysis of Lithologies in Ryugu: Insights into its Complex Geological Formation

INTRODUCTION & CONTEXT
Ryugu is a C-type asteroid, rich in carbon and organic compounds, and is considered to have evolved relatively little since its formation. It therefore represents a major scientific target for understanding the origins and early formation of Solar System bodies. As a relatively accessible near-Earth asteroid, it was selected by the Japanese Space Agency (JAXA) to be sampled as part of the Hayabusa2 mission. In December 2020, approximately 5 grams of material were successfully returned to Earth, enabling the development of various scenarios regarding Ryugu’s formation and evolution.

Ryugu is thought to originate from a larger parent body, estimated to have had a diameter ranging from several tens to several hundreds of kilometers [1, 2]. This body likely formed between 1.8 and 2.9 million years after the first solid components of the Solar System, beyond 3–4 AU, in a cold region where water (H2O) and carbon dioxide (CO2) could exist in the form of ice [2]. The parent body, mostly composed of ice, amorphous silicates (GEMS-like material), some minerals (metals, sulfides, and anhydrous silicates) and organics [3, 4, 5], would have experienced internal heating due to the radioactive decay of aluminum-26 leading to the melting of ices and the onset of mineralogical aqueous alteration depending on the location within the body [6]. These processes created local heterogeneities enriched in secondary minerals. Major impact events also contributed to the brecciation of the surface and the possible formation of second-generation asteroids, such as rubble piles, of which Ryugu is believed to be one [7].

The breccias that resulted from these catastrophic events are composed of fragments (or clasts) of various mineralogical assemblages cemented within a fine-grained matrix [8]. The lithological diversity observed in these breccias reflects the complex geological evolution of planetary surfaces and bodies in the Solar System, making their study essential for better understanding the processes that shaped these objects.

In this context, and to overcome the limitations of traditional 2D analyses, we developed a semi-automated method called Local Histogram (LH) segmentation ([9], publication in prep.), applied to a millimetric grain from Ryugu using Synchrotron-Radiation X-Ray Micro-Computed Tomography (SR-μXCT) dataset. This approach enables the 3D identification and visualization of mineralogical heterogeneities while minimizing manual intervention.

RESULTS & PERSPECTIVES
The analysis of a single grain revealed a composite of five distinct lithologies. Three of them (Lith I, II, and III) are matrix-dominated but differ in their contents of carbonates, magnetite, calcium phosphates, and sulfides. The widespread presence of hydrated phyllosilicates attests to an aqueous alteration process that led to the formation of these lithologies. Their current juxtaposition could be the result of a brecciation event that brought together materials from different depths within the parent body, consistent with rubble-pile formation simulations [10].

The 3D segmentation, combined with fracture analysis, revealed a clear fracture separating Lith I and Lith II, strengthening the hypothesis of a brecciation event. However, the contiguous (Lith I, II, then III) and sometimes concentric distribution in 3D also suggests a progressive, potentially radial alteration process, affecting each lithology differently (from the less altered Lith I to the carbonate-rich Lith III). Among the other detected lithologies are (i) a millimetric carbonate vein [11] (Lith IV), which appears to crosscut Lith II and III, suggesting that the aqueous event responsible for its formation occurred after the event(s) that juxtaposed or differentially altered these lithologies, and (ii) aggregates of large opaque minerals (mainly magnetite), associated with matrix and/or carbonates (Lith V), found within both Lith II and III, whose relative origin remains to be determined through further analysis. The 3D analysis enabled the formulation of several hypotheses concerning lithology formation; however, additional studies are required to converge on a consistent and coherent scenario. Thanks to 3D segmentation, specific zones of interest were precisely targeted for further investigation.

Two sections were prepared using Xe-pFIB and subsequently analyzed by scanning electron microscopy (SEM) and infrared spectroscopy. Preliminary results suggest a possible genetic link between the fracture network and the crystallization of the carbonate vein, providing new insights into fragmentation processes.In conclusion, this work highlights the complex thermal, aqueous, and mechanical histories that lead to the formation of meteoritic breccias. It underscores the crucial contribution of 3D analysis in reconstructing the geological evolution of small Solar System bodies.

ACKNOWLEDGEMENT
We thank JAXA for providing the Ryugu A0159 sample during the first Hayabusa2 AO. We acknowledge K. Hatakeda, M. Matsumoto, S. Pont, F. Borondics, C. Sandt, C. Le Guillou, F. Brisset, C. Boukary, D. Baklouti, Z. Djouadi, C. Lantz, and O. Mivumbi for their contributions. This work involved collaboration between IAS, CNRS, Université Paris-Saclay, and JAXA, with funding from CNES, ANR (LARCAS project, ANR-22-CE49-0009-01), Region Ile-de-France (DIM-ACAV), and SOLEIL.

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