Organic matter in Itokawa particles
Q. H. S. Chan1,2, A. Stephant2, I. A. Franchi2, X. Zhao2, R. Brunetto3, Y. Kebukawa4, T. Noguchi5, D. Johnson2,6, M. C. Price7,K. H. Harriss7, M. E. Zolensky8, M. M. Grady2,9, E. T. Parker10, D. P. Glavin10, and J. P. Dworkin10
1
Royal Holloway University of London, Egham TW20 0EX, Surrey, UK. 2The Open University, Walton Hall, Milton Keynes
MK7 6AA, UK. 3IAS-Université Paris-Saclay, 91405 Orsay, France. 4Yokohama National University, Yokohama 240‑8501,Japan. 5Kyushu University 744, Motooka, Nishi‑ku, Fukuoka 819‑0395, Japan. 6University of Exeter, Penryn, Cornwall TR109FE, UK. University of Kent, Canterbury CT2 7NH, Kent, UK. 8NASA Johnson Space Center, Houston, TX 77058, USA. 9The
Natural History Museum, London SW7 5BD, UK. 10NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.
The first Hayabusa mission returned samples from the near-Earth S-type asteroid 25143 Itokawa to Earth in 2010 [1].
Although Itokawa has a lithology related to ordinary chondrites (OCs) that typically have low organic contents, severalItokawa particles were found to contain organic matter (OM) [2-5]. However, there was not an explicit conclusion to the originof the observed OM in these early studies. We have extended our search for OM into other Itokawa grains. Here, we report
extraterrestrial OM (macromolecular carbon and amino acids) observed in six Itokawa particles (including a category 1
particle: RA-QD02-0162 [#62; also nicknamed “Amazon”], and five category 3 carbon-rich particles: RA-QD02-0012 [#12],RA-QD02-0078 [#78], RB-CV-0029 [#29], RB-CV-0080 [#80] and RB-QD04-0052 [#52]).
All allocated Itokawa samples were initially analysed by spot and point-by-point mapping Raman spectroscopic
analysis at the Open University, UK. Amazon was then transferred and mounted into indium on an aluminium stub, which wasstudied with a NanoSIMS 50L ion microprobe for its H,C,N isotopic compositions. The rest of the samples were mounted insterile gold foils, and the amino acid contents of their acid hydrolysed hot water extracts were obtained with a liquid
chromatography with tandem fluorescence and accurate mass detection at NASA Goddard Space Flight Center, USA.
Based on the observation of the Raman parameters (e.g. the peak locations and widths of the defect (D) and graphite(G) bands) [6], a significant variety of carbonaceous materials has been observed in Amazon. The carbonaceous materials
include primitive and unaltered OM that shares similarity with the IOM in primitive (CI,CM,CR) carbonaceous chondrites, aswell as OM that has been heavily graphitised. The organic structure of the heated material is best represented bynanocrystalline graphite, comparable to that observed for metamorphosed meteorites (e.g., L3–6 Inman, Tieschitz and New
Concord, CV3 Allende, and EH4 Indarch), suggesting peak metamorphic temperatures (PMT) of >~600°C. The thermal
history recorded in the graphitic OM agrees with PMT estimates for returned Itokawa regolith grains (600–800°C) [7].
We have obtained the H,C,N isotopic compositions for the primitive OM in Amazon, which exhibits unambiguously
extraterrestrial isotopic signatures (δD = +4868 ± 2288‰; δ13C = −24 ± 5‰; δ15N = +344 ± 20‰), contrasting to the typicallynegative isotopic values obtained for terrestrial organic matter [8]. The δD and δ13C values of the organic material in Amazon
are comparable to OCs, however, the δ15N value is higher than that typically observed for OCs (δ15N = −47 to +36‰), and issimilar to that of CRs (δ15N = +153 to +309‰) [9]. Our data suggest a genetic link between the primitive organic materialobserved in Itokawa to CRs and IDPs for they share similar D, 13C and 15N enrichments [10].
The high carbon contents of the five category 3 Itokawa particles suggest potentially higher OM abundances, hence
we extracted and analysed amino acids in these samples. Although terrestrial contamination was observed primarily as Lprotein amino acids, several terrestrially uncommon non-protein amino acids were also observed at low abundances, such as βaminoisobutyric acid (β-AIB), β-amino-n-butyric acid (β-ABA), and β-alanine. Itokawa amino acid content observed here wasdissimilar to thermally altered OCs, but preliminarily analogous to more aqueously altered CR2s.
Continued evolution of Itokawa is evident by the infall of primitive organic material derived from CRs/IDPs,accounting for a complex interplay between the remnant Itokawa silicates with exogenous organics. The results reported here
are the first evidence of extraterrestrial OM in asteroid material from a sample-return mission, showcasing a working protocol
for analysing samples returned by the Hayabusa2 and OSIRIS-REx missions.
References[1] Yada, T., et al. Meteoritics & Planetary Science, 2014. 49(2): p. 135-153. [2] Ito, M., et al. Earth, Planets and Space, 2014.
66(1): p. 91. [3] Yabuta, H., et al. Earth, Planets and Space, 2014. 66(1): p. 1-8. [4] Uesugi, M., et al. Earth, Planets and Space,2014. 66(1): p. 1-11. [5] Kitajima, F., et al. Earth, Planets and Space, 2015. 67(1): p. 1-12. [6] Tuinstra, F. and J.L. Koenig.
Journal of Chemical Physics, 1970. 53(3): p. 1126-1130. [7] Nakamura, T., et al. Science, 2011. 333(6046): p. 1113-1116. [8]Pizzarello, S. Accounts of Chemical Research, 2006. 39: p. 231-237. [9] Alexander, C.M.O.D., et al. Geochimica etCosmochimica Acta, 2007. 71: p. 4380-4403. [10] Busemann, H., et al. Science, 2006. 312: p. 727-730.