TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 53 (No. 1), pp. 41-51, 2019

Bulk chemical characteristics of soluble polar organic molecules formed through condensation of formaldehyde: Comparison with soluble organic molecules in Murchison meteorite

Yuki Isono,1 Shogo Tachibana,1,2* Hiroshi Naraoka,3 François-Régis Orthous-Daunay,4 Laurette Piani5 and Yoko Kebukawa6

1Department of Natural History Sciences, Hokkaido University, N10W8, Sapporo, Hokkaido 060-0810, Japan
2UTokyo Organization for Planetary and Space Science, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan
3Department of Earth and Planetary Sciences, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
4Département Mesures Physiques, 17 Quai Claude Bernard, 38000 Grenoble, France
5Centre de Recherches Pétrographiques et Géochimiques, 15 rue Notre Dame des Pauvres, 54500 Vandoeuvre-lès-Nancy, France
6Faculty of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan

(Received May 24, 2018; Accepted November 20, 2018)

Abstract: Carbonaceous chondrites contain up to 2 wt% organic carbon, which is present as acid and solvent insoluble solid organic matter (IOM) and solvent soluble organic matter (SOM). The extraterrestrial organic matter should record chemical processes occurred in different environments in the early history of the Solar System, and the role of parent body aqueous alteration in the synthesis or subsequent modification of IOM and SOM still requires accurate constraints. We conducted hydrothermal experiments to simulate the synthesis of organic molecules during aqueous alteration on small bodies. Bulk chemical characteristics of soluble organic matter synthesized from formaldehyde in aqueous solutions were studied to compare them with that of chondritic SOM. We found that the redox state of synthesized organic molecules depends on temperature; the molecules become richer in hydrogen at higher temperatures. This can be explained by a cross-disproportionation reaction between organic molecules and formic acid, which occurs as a side reaction of the aldol condensation and works more effectively at higher temperatures. Comparison of the bulk chemical characteristics between the synthesized molecules and SOM extracted from the Murchison meteorite with methanol shows that the soluble organic molecules in Murchison are more reduced than the synthesized molecules. Considering the temperature condition for aqueous alteration on the CM parent body that is lower than or equivalent to the experimental temperatures, the reduced nature of Murchison organic molecules requires a reducing environment for them to be formed during hydrothermal alteration or imply that processes other than hydrothermal alteration were responsible for their synthesis. In case of hydrothermal synthesis, reducing conditions might be established by the interaction between water and iron-bearing silicates or metals on the parent body.
Key words: extraterrestrial material, organic matter, meteorite, small body, hydrothermal reaction

*Corresponding author E-mail: tachi@eps.s.u-tokyo.ac.jp

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