Investigating the ammonium-bearing phase in Ryugu samples
M. Ferrari1, L. Folco2, S. De Angelis1, M. Masotta2, A. Raponi1, M.C. De Sanctis1, E. Mugnaioli2,
R. Borriello2, M. Ciarniello1, E. Ammannito3, E. La Francesca1, L. Rossi1, M. Pedone3.
1
INAF-IAPS, Via del Fosso del Cavaliere 100, 00133, Rome, Italy (marco.ferrari@inaf.it); 2Dep. Earth Science, University ofPisa, Via Santa Maria, 53, 56126, Pisa, Italy; 3ASI, Via del Politecnico, 00133, Rome, Italy.
Introduction
The JAXA/Hayabusa2 mission collected 5.4 g of samples from two different sites (i.e., A and C) on the surface of the C-type
asteroid (162173) Ryugu [1; 2]. This asteroid is characterized by a low albedo and by the presence of hydrous andcarbonaceous materials [3]. The Ryugu surface spectra collected by the Hayabusa2/NIRS 3 spectrometer reveal a narrowabsorption feature at 2.72 μm [4] resembling that observed on the surface of Ceres by the Dawn/VIR spectrometer [5; 6].
Laboratory analysis of returned grains confirmed that Ryugu samples contain hydrated silicates, sulfides, oxides, carbonates,and organics and have spectroscopic similarities to CI chondrites [7]. MicrOmega hyperspectral microscope observation [8]revealed the presence of a band at 3.06 μm, indicating the possible presence of NH-bearing phases in most of the returned
grains [9]. This ammonium-related absorption feature was confirmed by FTIR measurements [10]. The 3.06 μm band was also
observed in the spectra of Ceres by telescopic observations [11] and by the Dawn/VIR spectrometer that accurately
characterized its distribution on the dwarf planet's surface [12]. Current interpretations of Ceres spectra suggest that the mostlikely mineralogical phases hosting ammonium are phyllosilicates [13; 14], but the presence of ammonium salts has also beeninferred in specific areas of the Cerean surface [15; 16]. Hydrated silicates represent the main reservoir of water in Ryugu
samples, and their presence is indicative of extensive aqueous alteration in the parent body, likely the alteration path that
occurred on Ceres. Even if phyllosilicates are the most likely carriers, the specific ammonium-bearing phase determining the3.06 μm band on Ryugu samples and the surface of Ceres is not fully constrained, nor are the processes that led to ammonium
enrichment on these C-type bodies.
Sample selection
Our selection criterion of the samples among those available in the first allocation (2022A) was the presence of the absorption
band at 3.06 µm as observed in the spectroscopic data obtained from the preliminary screening by FTIR and MicrOmega. We
have considered: 1. Similarities of the 3.06 µm absorption between Ryugu samples and Ceres spectra, in terms of wavelength
range and band center position; 2. The presence of surface features or morphological structures suggests some mineralogicaldiversity.
Through the first allocation, we obtained two samples: A0198 and C0091. The preliminary FTIR data provided by JAXA
collected on particle C0091 show the presence of a remarkable absorption close to 3.06 μm. On the contrary, the spectroscopic
data collected on particle A0198 show a shallow absorption ascribed to the presence of ammonium and several surface featuresthat comply with our scientific requirements. Moreover, the two assigned samples were collected at different sites which willallow us to make further considerations.
Objectives and research plan
With this work, we aim to contribute to the definition of the physical and mineralogical characteristics of the Ryugu asteroid tounderstand the initial stages of formation and evolution of C-type bodies; particular attention will be given to the identificationof the ammonium-bearing phase(s) and its/their interplay with hydrated silicates. In addition, we aim to define Ryugu's path ofalteration and its similarities with Ceres.
The following laboratory analyses will be used to characterize the samples:•VIS-IR imaging spectroscopy (0.4-5 μm) will be carried out with the SPectral IMager (SPIM) facility [17] on whole samples.
Data will allow a first characterization of the mineralogic composition of their external surface, including an assessment of thepresence of water, organic matter, and ammonium in addition to the minerals. Furthermore, by using the SPIM facility (i.e., theDawn/VIR replica), we can perform a compelling comparison between the data collected on the samples with those provided
by the Dawn/VIR spectrometer on Ceres;•X-ray microscopy (XRM). This non-destructive technique has been used to obtain a high-resolution (submicron scale) 3D
tomography of the samples, revealing their internal structure (including pore structure and fracturing), as well as themorphology and composition of the individual grains contained in them; XRM data will also enable the location of regions ofinterest (ROIs) within the samples to be analyzed by transmission electron microscopy (TEM);

•Raman µ-spectroscopy, to obtain compositional maps of the surface of the samples at the micro-metric scale;•Focused ion beam-field emission gun-scanning electron microscopy (FIB-FEG-SEM) will be used to identify the mainmineralogical phases and to extract the electron-transparent lamellae for TEM analyses;•Transmission Electron Microscopy (TEM), electron diffraction, and energy-dispersive X-ray spectroscopy (EDS) will be usedto identify cryptocrystalline mineral phases and crystal defects through high-resolution techniques.
Results
The VIS-NIR spectra obtained with the SPIM facility on the surface of the two particles show the almost ubiquitous presenceof a deep V-band centered at lengths slightly above 2.7 μm, and absorptions near 3.4 μm and in the range 3.7-3.9 μm, however,limited at specific points on the surface returning an inhomogeneous surface composition at the scale of ~40 µm (SPIM pixelsize). These absorption features can be related to the presence of phyllosilicates and carbonates, respectively.
In addition, at some specific points, the band at 3.06 μm is found indicating the presence of ammonium-bearing compounds.
The Raman spectra acquired on the two particles are characterized by a high fluorescence background and by the presence of
the Raman D and G-bands related to the presence of poorly ordered carbonaceous matter. The spectral parameters derived
from the mathematical fitting of the acquired spectra as band positions, and band intensity ratio do not differ significantly
between the two particles. Raman analysis identifies dolomite as the most common carbonate on both particles.
The interpretation of spectral data acquired by both particles is currently in progress. However, at the moment there do notappear to be notable differences in spectral terms (VIS-NIR and Raman) between the two particles coming from the twodifferent sampling sites.
The two samples (A0198 and C0091) are characterized by a generally similar composition in terms of phase abundance andcomposition. Energy Dispersive X-ray spectroscopic (EDX) data obtained using the FE-SEM confirmed the presence of
hydrated silicates, sulfides (Ni-rich and Fe-rich), carbonates, fosfates, and oxides (mainly magnetite). Sulfides form
dominantly euhedral grains (up to 5 µm in size), whereas oxides form submicrometric framboid, platy, or columnar structures.
The internal distribution of silicates, sulfides, and oxide in the interior of the two samples, as revealed by XRM data, is rather
homogeneous. Notably, sample C0091 displays a slightly smaller grain size and a slightly higher porosity compared to particle
A0198.
Conclusions
The notable similarities of the spectra of some Ryugu samples with Ceres' average spectrum [18] sparked scientific curiosity.
To date, interpretation of remotely sensed data coupled with spectroscopic investigation of analogs has not enabled a fullunderstanding of the mechanisms of ammonium enrichment and the nature of the mineral assemblage hosting ammonium on
the surface of C-type objects. Furthermore, studies on meteorites have not provided morphological and compositional
information on the mineral phases that preceded the alteration of their progenitor body.
Therefore, we consider that spectroscopic measurements and nanoscale mineralogical investigations that we are conducting on
the samples returned from Ryugu represent a unique opportunity to definitively establish the link, if any, between ammonium
and the mineralogy of this type of object.
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