A Comparative Carbon-XANES and -EELS study of Organic Material from Asteroid 162173
Ryugu and Ivuna.
H. G. Changela1,2, L. Petera1,3, J. C. Bridges4, Y. Kebukawa5, L. Hicks6, N. Topping4, C. S. Allen7, E. Chatzitheodoridis8, M
Ferus1
1 Department of Spectroscopy, J. Heyrovsky Institute of Physical Chemistry, Czech Academy of Sciences, Czechia. 2
Department of Earth & Planetary Science, University of New Mexico, New Mexico, USA. 3 Department of InorganicChemistry, Charles University in Prague, Czechia. 4 Space Research Centre, School of Physics and Astronomy, University ofLeicester, UK. 5 Department of Earth & Planetary Sciences, Tokyo Institute of Technology, Japan. 6 School of Geology,Geography, and the Environment, University of Leicester, UK. 7 electron Physical Science Imagine Centre, Diamond Light
Source, Oxford, UK. 8 School of Mining, National Technical University of Athens, Greece. Email: changela@unm.edu
Introduction The arrival of samples from carbonaceous asteroids 16273Ryugu [1] 101955Bennu [2] provide a window into
early Solar System organic evolution on a known asteroid. Organic material (OM) from carbonaceous asteroids is currently
most accurately characterised in situ by coordinating synchrotron based scanning transmission X-ray microscopy (STXM) withtransmission electron microscopy (TEM) on samples. Organic functional chemical variations can be measured by Carbon Kedge X-ray absorption near edge structure (XANES), whilst organic morphology and its mineral setting is characterised bysubsequent TEM. Carbon-energy electron loss spectroscopy (EELS) offers the same possibility as C-XANES but with theadvantage of being performed in a stand-alone TEM. Here we compare C-EELS with -XANES on OM from Asteroid Ryugu
and the Ivuna CI chondrite prepared by dual beam focused ion beam (FIB)- scanning electron microscopy (SEM) and ultra
microtome to assess the effects of electron beam dosage by all techniques.
Methods Two adjacent ~100 nm thick lamella were prepared using FIB-SEM (Fig. 1). Lamella 1 was prepared using Xeplasma FIB-SEM with the Helios 5 Hydra DualBeam (CEITEC, Masaryk University (MUNI), Czechia) and analysed by TEMC-EELS with the JEOL ARM200CF (ePSIC, Diamond Light Source, UK), followed by STXM-XANES at Beamline BL19Aof the KEK Photon Factory, Japan. Lamella 2 (Fig. 1) was prepared by Xe-plasma FIB-SEM with the TESCAN AMBER X(TESCAN ORSAY holding a.s., Brno, Czechia) followed by STXM-XANES (i.e. no TEM) at the photon factory. Microtome
samples of Ivuna were prepared using methods in [3] at CEITEC-MUNI with a Leica UC7. Organic material in a lamella fromRyugu grain C0105-03500000 provided by the Hayabusa-2 preliminary examination SAND team prepared using theUniversity of Leicester’s FEI Quanta 3D was also measured by C-EELS at ePSIC.
C-EELS FIRST SEM-TDL (5 KV)

Ivuna Grain SEM-ETD (5 KV)

C-XANES FIRST SEM-ETD (2 KV)

5 μm

1 μm

50 μm

Figure 1. SE image of a grain of Ivuna on carbonate sticky and coated with 5 nm Au. Large cross section at the front of the grain was thelocation where 2 adjacent FIB lamella were extracted: one with C-K edge EELS then XANES (left extraction) and the other direct XANES(right lamella). Blue and red rectangle mark the ROIs where the EELS and XANES maps were taken (Fig. 2).

Results Eelectron beam exposure to chondritic OM during SEM, TEM and EELS changes its functional chemistry, in both its
macromolecular and particularly in soluble/insoluble form within phyllosilicate (Fig. 2). A peak at ~290.4 eV attributed tocarbonate -O-C=O-O- bonds is observed under 200 KV electron doses by TEM, STEM imaging and by EELS spectral
mapping. This feature is absent from microtome samples measured by XANES. A bulge in the EELS spectra consistent withamorphous an carbon σ * peak (~288 – 315 eV) is found in organic particles. The carboxylic (288.5 eV) peak found in diffuse
OM within phyllosilicates (e.g. Graph 2) is replaced by the carbonate one at 290.4 eV (Graphs 1 & 4).

Figure 2. Carbon-EELS and -XANES on the Ivuna lamella and Ryugu sample.

Discussion The amount of radiation damage has been shown much lower in STXM-based XANES spectroscopy than in TEMbased EELS [8]. Our observations particularly of organo-carbonate bonding in diffuse OM shows its formation sensitive to ebeam exposure rather than X-ray exposure. This was also identified by [9] in terrestrial coals imaged with 5KV SEM during
FIB lamella preparation. Furthermore, Yabuta et al. 2023 [5] reported the lack of any carbonate peak in microtome samples
measured by XANES, but occurring in FIB sections with organic particles and diffuse OM in Ryugu samples (referred to as
clay bound OM in their study). Our microtome samples of CI chips also lack this peak in any XANES measurements,suggesting that e-beam exposure during FIB-SEM preparation also formed the organo-carbonate bonds observed in previous
STXM studies [e.g. 10] and recently in [5]. However, potential mixture of indigenous organo-carbonate cannot be ruled out.
Conclusions Chondritic OM is sensitive to electron radiolysis particularly by the formation of organo-carbonate bonding,
although the formation mechanism is unclear. Carbon-EELS of organic particles also convolute the 288.5 eV carboxyl peakby an amorphous bulge. This means that synchrotron XANES coordinated with subsequent TEM more accurately provides ameasure of functional chemical variation of chondritic OM than EELS-TEM.
FIB-SEM lamella preparation of chondritic matrices require electron energies lower than 5KV (e.g. 1.5KV) during imaging
to minimize e-beam damage by radiolysis. It is unclear though whether SEM lower than 5KV could prevent the formation ofthis carbonate -O-C=O-O- ~290.4 eV feature, and how other regions of C-XANES and -EELS spectra are effected.
References [1] Yada T. et al. 2014. Nature Astronomy 6:214-220. [2] Lauretta et al. 2015. Meteoritics & Planetary Science
50:834-849. [3] Noguchi et al. 2020. Life 10(8):135. [4] Yuan J. & Brown L. M. 2000. Micron 31: 515-525 [5] Yabuta H. et al.
2023 Science 679:6634. [6] Garvie and Buseck 2007. Meteoritics & Planetary Science 42(12):2111-2117. [7] Changela. H. G.
et al. 2023. Abstract #2848. 54th LPSC. [8] Wang J. et al. 2009. Journal of Electron Spectroscopy and Related Phenomena
170:25-36. [9] Bassim N. et al. 2012. Journal of Microscopy 245:288-301. [10] Changela. H. G. et al. (2018) Meteoritics &Planetary Science 53:1006-1029.
Acknowledgements Special thanks to the Hayabusa-2 preliminary examination SAND team for useful discussions and access to Ryugu
sample C0105-03500000 in this study. We thank Diamond Light Source and electron Physical Imaging Centre ePSIC (instrument EO1,proposal MG30752). We also thank TESCAN GROUP, a.s. for use of their instrumentation and Dr. Hana Tesarova for her help. We
acknowledge CF CryoEM of CIISB, Instruct-CZ Centre, supported by MEYS CR (LM2023042) and European Regional Development FundProject UP CIISB" (No. CZ.02.1.01/0.0/0.0/18_046/0015974).We thank KEK Photon factory for use of Beamline BL 19A for STXM.