The Mineralogy of Asteroid Ryugu and its
Relationship to Highly Altered Extraterrestrial Materials
Ashley King1, Sara Russell1, Martin Suttle2, Catherine Harrison1,Paul Schofield1, Helena Bates1, and the Hayabusa2 “Stone” team
1
Natural History Museum, London, UK (a.king@nhm.ac.uk)
2
The Open University, Milton Keynes, UK
The Hayabusa2 mission returned to Earth with ~5.4 g of material collected from two sites on the surface of the Cb-type
asteroid (162173) Ryugu [e.g., 1]. In hand specimen, Ryugu particles are dark, often highly friable, and have bulk densities in
the range of ~1.2 – 1.8 gcm-3 [2–4]. Most Ryugu particles examined to date are breccias consisting of an abundant (~80 – 90vol.%) fine-grained (<1 μm) matrix of phyllosilicates (interlayered serpentine and saponite) in which coarser (~10’s – 100’sμm in size) grains, fragments, and clusters of oxides (~4 vol.%), sulphides (~3 vol.%), carbonates (~3 vol.%), and phosphates(<1 vol.%) are embedded [2–6]. Several CAI-like fragments (<30 μm) have been identified, as have small (<10 μm), rounded
objects with characteristics, such as barred textures, related to melting and crystallisation in chondrules [e.g., 4, 7]. The
mineralogy, petrography, and physical properties of Ryugu particles are consistent with them being having formed throughnear-complete, low temperature (<50°C) water-rock reactions during the first few million years of the Solar System [2–6].
If samples of Ryugu were to land on Earth as meteorites, we could classify them as petrologic type 1 [8]. Such materials werecompletely altered to secondary mineral assemblages by aqueous processing and contain various phyllosilicates, carbonates,sulphides, and magnetite. Type 1 materials are represented in four meteorite groups (CI, CM, CR, and CY), a handful ofungrouped chondrites (e.g., Flensburg [9]), as xenoliths within meteorites [e.g., 10], and as fine-grained micrometeorites [e.g.,11] and interplanetary dust particles (IDPs) [e.g., 12]. The mineralogy and chemistry of Ryugu particles are closely related tothe CI1 (“Ivuna-like”) carbonaceous chondrites, except for the presence of sulphates and ferrihydrite in the latter, which aremost likely terrestrial weathering products [13, 14]. However, type 1 samples exhibit a remarkably high diversity of starting
materials and alteration conditions, with variations in mineralogy, textures, and oxygen isotopic compositions pointing towards
multiple parent bodies. Despite type 1 materials being relatively rare in our collections, in part due to their fragile naturehindering survival during atmospheric entry, highly altered bodies appear to be common throughout the Solar System. Thesebodies could include primitive asteroids that accreted within the inner and/or outer Solar System, or transition objects such as
Main Belt comets or D-type asteroids. Characterising the sources of type 1 materials through laboratory analysis of meteorites
and Ryugu particles is therefore an important step towards understanding volatile reservoirs in the early Solar System.
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