Macromolecular organic matter in samples of the asteroid (162173) Ryugu

Abstract

Samples of the carbonaceous asteroid (162173) Ryugu were collected and brought to Earth by the Hayabusa2 spacecraft. We investigated the macromolecular organic matter in Ryugu samples and found that it contains aromatic and aliphatic carbon, ketone, and carboxyl functional groups. The spectroscopic features of the organic matter are consistent with those in chemically primitive carbonaceous chondrite meteorites that experienced parent-body aqueous alteration (reactions with liquid water). The morphology of the organic carbon includes nanoglobules and diffuse carbon associated with phyllosilicate and carbonate minerals. Deuterium and/or nitrogen-15 enrichments indicate that the organic matter formed in a cold molecular cloud or the presolar nebula. The diversity of the organic matter indicates variable levels of aqueous alteration on Ryugu’s parent body.

Hydrogen Isotopic Composition of Hydrous Minerals in Asteroid Ryugu

“…These points, hereafter referred to as D-rich outliers, were also occasionally observed in previous measurements of unmetamorphosed carbonaceous chondrites (Piani et al , 2021. The relatively moderate C/H ratios of these D-rich outliers (Figures 2(A); A4) and the limited D-enrichments found for the IOM isolated from Ryugu's rocks compared to CI or CM chondrites (δD ∼ +400‰ obtained from bulk mass spectrometry in Yabuta et al 2023; δD ∼ +530‰ obtained by NanoSIMS in Ito et al 2022) preclude them from simply being interpreted as IOM-rich areas. The D-rich outliers could rather correspond to areas including one or several submicron-sized D-rich organic hot spots as observed by ion imaging both in Ryugu's bulk particles (either IOM or soluble organics) and in IOM (δD up to +8000‰; Yabuta et al 2023) and more generally in pristine carbonaceous chondrites (e.g., Remusat et al 2010).…”

“…The first reports for Ryugu indicated that its bulk hydrogen isotopic composition δD = 252‰ ± 13‰, expressed in δ notation (‰) relative to the standard mean ocean water ratio D/H SMOW = 155.76 × 10 −6 (Naraoka et al 2023), is higher than most of the values measured so far for bulk CI chondrites: Orgueil δD = +182‰ ± 44‰, Ivuna δD = +232 ± 60‰ and Alais δD = +89‰ ± 63‰ (±2 standard deviations; data compilation in Piani et al 2021). In contrast, the D/H of the insoluble organic matter (IOM) obtained after mineral removal by acid dissolution appears be lower in Ryugu (δD = +306‰ and +440‰ for sites A and C, respectively; Yabuta et al 2023) than in the IOM of CI chondrites (δD ≈ +1000‰; Alexander et al 2007).…”

“…Alternatively, the water could have been enriched in D by hydrogen isotopic exchange with D-rich IOMs. This process, if it was more efficient for Ryugu than for other CI chondrites, could explain the lower D/H measured for Ryugu's IOM compared to CI IOMs (Yabuta et al 2023) but would only marginally increase the water D/H (from about −100‰ to +59‰). It is difficult to draw conclusions with our data on the efficiency of such hydrogen isotopic exchanges, due to the large uncertainties of the D/H ratios (±120‰).…”

“…The relatively moderate C/H ratios of these D-rich outliers (Figures 2(A); A4) and the limited D-enrichments found for the IOM isolated from Ryugu's rocks compared to CI or CM chondrites (δD ∼ +400‰ obtained from bulk mass spectrometry in Yabuta et al 2023; δD ∼ +530‰ obtained by NanoSIMS in Ito et al 2022) preclude them from simply being interpreted as IOM-rich areas. The D-rich outliers could rather correspond to areas including one or several submicron-sized D-rich organic hot spots as observed by ion imaging both in Ryugu's bulk particles (either IOM or soluble organics) and in IOM (δD up to +8000‰; Yabuta et al 2023) and more generally in pristine carbonaceous chondrites (e.g., Remusat et al 2010). Interestingly, the highest D-enrichments were reported in the intact Ryugu grains instead of the isolated IOM (Yabuta et al 2023) and could thus correspond to soluble organic compounds.…”

“…The D-rich outliers could rather correspond to areas including one or several submicron-sized D-rich organic hot spots as observed by ion imaging both in Ryugu's bulk particles (either IOM or soluble organics) and in IOM (δD up to +8000‰; Yabuta et al 2023) and more generally in pristine carbonaceous chondrites (e.g., Remusat et al 2010). Interestingly, the highest D-enrichments were reported in the intact Ryugu grains instead of the isolated IOM (Yabuta et al 2023) and could thus correspond to soluble organic compounds. Ito et al (2022) also observed a generally higher D/H ratio (δD up to +840‰) for aliphatic-rich organics than in the entire C-rich region (δD up to +530‰), confirming the possible D-richest nature of some (possibly soluble) organic compounds.…”