Abundance and properties of Ryugu diffuse organic matter
Rhonda Stroud1,2, Bradley De Gregorio2, Jens Barosch3, Larry Nittler1,3
The Hayabusa2-initial-analysis Organic Macromolecule Team and Core Team1
School of Earth and Space Exploration, Arizona State University, USA,
2Materials Science and Technology Division, US Naval Research Laboratory, USA,
3
Earth and Planets Laboratory, Carnegie Institution of Washington, USA

Initial Analysis studies have revealed that Ryugu regolith particles contain abundant organic carbon (3 wt%) [1,2],and that organic matter has the same range of morphologies as previously observed in primitive chondrites, including
solid and hollow nanoglobules, nanodiamonds, dense, irregularly shaped particles, and diffuse materials with poorlydefined boundaries [3]. The diffuse organics form coatings on and fill the space between mineral grains, and intercalate
into phyllosilicates. Because the diffuse carbon can be very thin (single nm) and intimately mixed with the nanoscale
minerals, especially phyllosilicates, it is below the spatial limit for many techniques. Many analyses focus instead on
globules or other organic particles > 200 nm. However, the diffuse carbon represents a substantial portion, possibly
50% or more, of the total organic carbon, and must be considered for a complete understanding of the Ryugu organics.
To better understand the abundance and properties of diffuse carbon, we used correlated scanning transmission x-ray
microscopy (STXM), scanning transmission electron microscopy (STEM) and nanoscale secondary ion mass
spectrometry (NanoSIMS) to examine samples from Chambers A and B.
Figure 1A shows a bright-field STEM image montage of a focused ion beam (FIB) lift-out section from particle
A108-?? pressed into a Au mount for NanoSIMS analysis. The BF STEM image shows a mixture sulfides andphyllosilicates with domain sizes of 10s to 100s of nm, but no discernable carbonaceous particles. However, STEMbased energy dispersive x-ray spectroscopy (EDS) mapping of the section Fig. 1B reveals carbon distributed
throughout. Quantification of the C:Si molar ratio with Cliff-Lorimer routines for areas > 1µm2 for no visible
nanoglobules indicate values of ~ 0.8 ±0.3. Similar STEM-EDS analysis of additional FIB sections shows comparable
C:Si ratios, confirming that a large portion of the organic carbon is present as diffuse matter rather than globular
carbon in general, not only in this specific particle. However, it is difficult to obtain a precise determination of fractionof the diffuse vs. particulate carbon because the ability to resolve individual organic carbon particles < 100 nm depends
greatly on the sample preparation and analysis conditions. In 100-nm thick FIB sections such as this, amorphous
carbon particles as large as 50 nm could be hidden in images by the greater intensity variation of the phyllosilicatesand sulfides, and only revealed by spectroscopy.
Measurement of the carbon K edge x-ray absorption near edge structure (XANES) with STXM can provide theaverage functional chemistry distribution of the diffuse carbon (Fig. 1D, Fig. 1E). Notably, there are four prominent
peaks in the diffuse carbon spectrum from regions across this FIB section, with the typical aromatic (C=C), ketone(C=O), carboxyl (COOH) groups commonly seen in chondritic IOM, plus the CO3 carbonate peak. Although thecarbonate peak is very intense, the additional x-ray absorption peaks associated with crystalline carbonates are absent.
In addition, the STEM-EDS shows Ca is absent, and no signs of Fe or Mg carbonate nanocrystals are observed in theSTEM images. XANES spectra extracted from smaller ROIs show variability in the relative intensity of the fourfunctional groups and in some cases distinct aliphatic-rich regions can be found (Fig.1D blue box, 1E blue spectrum).
To better constrain the nature of the diffuse carbon, it is also helpful to analyze samples of particle prepared byultramicrotome and as demineralized acid-insoluble organic matter (IOM) isolates. Those date are discussed but notshown here due to space constraints, but will be presented at the meeting. Some, but not all, of the diffuse organic
matter is preserved in the IOM samples, and comprises the fine-grained porous material referred to as “fluffy”, distinct
from the nanoglobules. Notably, the prominent carbonate peak, easily observed in the FIB sections, is not present inthe IOM XANES data. The CO3 peak is present though difficult to measure in the XANES data from microtomed

particles. Electron energy loss spectroscopy in the STEM, coupled with EDS, shows the carbonate peak is associatedwith organic carbon in direct contact with phyllosilicates. This suggests that different portions of the diffuse IOM
have distinct chemical origins, with the molecular carbonate signature possibly due to reaction of C-rich fluids withphyllosilicates.

Figure 1. A Bright field scanning transmission electron
microscope image montage of a FIB section from particle 108xx. B. Energy dispersive x-ray spectroscopy composite
elemental map of C, Si, Au and Ga. The Au is the substrate,and Ga from FIB carbon deposition. C. Extracted sum spectrum
from the area outlined in white in (C). D. Scanningtransmission x-ray absorption microcopy image of the FIB
section at 288.9 eV. E. Extracted normalized spectrum in
optical density units from representative regions, revealing
pervasive molecular carbonate chemistry and localizedenhancement in aliphatic content.

References[1] Yokoyama T et al. 2022. Science abn7850. [2] Yabuta H. et al. 2022. Abstract #2241. 53d LPSC. [3] Stroud R.
M., et al. 2022. Abstract #2052. 53d LPSC.
The Hayabusa2-initial-analysis Organic Macromolecule Team and Core Team: H. Yabuta, G. D. Cody, C.
Engrand, Y. Kebukawa, B. De Gregorio, L. Bonal, L. Remusat, R. Stroud, E. Quirico, L. R. Nittler, M. Hashiguchi,
M. Komatsu, E. Dartois, J. Mathurin, J. Duprat, T. Okumura, Y. Takahashi, Y. Takeichi, D. Kilcoyne, S. Yamashita,
A. Dazzi, A. Deniset-Besseau, S. Sandford, Z. Martins, Y. Tamenori, T. Ohigashi, H. Suga, D. Wakabayashi, M.
Verdier-Paoletti, S. Mostefaoui, G. Montagnac, J. Barosch, K. Kamide, M. Shigenaka, L. Bejach, T. Noguchi, H.
Yurimoto, T. Nakamura, R. Okazaki, H. Naraka, K. Sakamoto, S. Tachibana, S. Watanabe, and Y. Tsuda