Water in Ryugu as a property of processes in its parent body

¹ Instituto de Astrofísica de Canarias (IAC), University of La Laguna, La Laguna, Tenerife, Spain
² Institut d’Astrophysique Spatiale, Université Paris-Saclay, CNRS, Orsay, France
³ Institut Universitaire de France, Paris, France
⁴ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan
⁵ Marine Works Japan Ltd, Yokosuka, Japan
⁶ National Institute of Polar Research, Tachikawa, Japan
⁷ University of Tokyo, Tokyo, Japan
⁸ European Space Agency, European Space Astronomy Centre, Madrid, Spain
⁹ Department of Astrophysics, University of La Laguna, La Laguna, Tenerife, Spain
¹⁰ The Graduate University for Advanced Studies (SOKENDAI), Hayama, Japan
¹¹ University of Tokyo, Kashiwa, Japan
¹² Nagoya University, Nagoya, Japan

^★ Corresponding author; tlepivert@iac.es

Received: 3 December 2024
Accepted: 17 February 2025

Abstract

Context. Samples from the asteroid Ryugu provide key information on the initial composition and evolutionary processes of primitive bodies.

Aims. This study aims to detect H₂O spectral features associated with inter-layer water in a statistically significant number of Ryugu grains protected from terrestrial contamination.

Methods. We analysed hyperspectral data of 186 millimetre-sized Ryugu grains (1.5–5.7 mm) obtained using the MicrOmega instrument at the Curation Facility. Water signatures were identified by computing the spectral distance from a reference Ryugu grain that exhibits a prominent 3 µm feature, which is associated with the presence of H₂O.

Results. We detect rare and small water-rich regions (typically <90 × 90 µm²). No significant difference in water content is observed between grains collected from chamber A (surface collection) and chamber C (artificial crater ejecta). The water-rich regions exhibit band widths similar to those of several primitive asteroids measured by the AKARI survey, whereas the average, millimetre-scale spectra of Ryugu grains show narrower bands.

Conclusions. We confirm the low abundance of inter-layer water in Ryugu samples across a statistically significant number of grains. The spectral differences between Ryugu and the primitive asteroids analysed in this study reflect differences in composition that may indicate distinct formation reservoirs or evolutionary processes.