The spatial distribution of soluble organic matter and its relationship to minerals in the asteroid (162173) Ryugu.

Abstract

We performed in-situ analysis on a ~1 mm-sized Ryugu grain A0080 returned by the Hayabusa2 spacecraft to investigate the relationship of soluble organic matter (SOM) to minerals. The DESI-HRMS (desorption electrospray ionization-high resolution mass spectrometry) imaging using methanol spray identified more than 200 soluble organic compounds, which were assigned as CHN, CHO, CHO-Na (sodium adducted), and CHNO in molecular composition. Heterogeneous spatial distribution was observed for different compound classes of SOM as well as among the same alkylated homologues on the sample surface. The A0080 sample showed similar mineralogy to that of CI chondrite and contained two different lithologies, which are rich in magnetite, pyrrhotite, and dolomite (lithology 1) and poor in those minerals (lithology 2). CHN compounds were relatively concentrated in lithology 1 than in lithology 2, on the other hand, CHO, CHO-Na, and CHNO compounds were distributed in both lithologies. Such different spatial distribution of SOM is the result of interaction of the SOM with minerals, during precipitation of the SOM via fluid activity, or could be due to difference in transportation efficiencies of SOMs in aqueous fluid. However, organic-related ions measured by ToF-SIMS did not coincide with the spatial distribution revealed by DESI-HRMS imaging, indicating that the ToF-SIMS data would be mainly derived from methanol-insoluble organic matter in A0080. Alkylated homologues of CHN compounds with large C number appeared more abundant in lithology 2 than lithology 1. In contrast, fragments of the Murchison meteorite showed different features to of A0080, implying different formation or growth mechanisms for the alkylated CHN compounds by interaction with fluid and minerals on Murchison parent body and asteroid Ryugu. This difference might be mainly attributed to the carbonate grains, which would have played as a catalyst for CH2 growth of CHN compounds. Future in-situ analysis of CI chondrite will provide more reliable constraints for the history of soluble organic compounds in asteroid Ryugu.