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.

Abstract

INTRODUCTION

Organic compounds in asteroids and comets contain information about the early history of the Solar System. They could also have delivered organic material to early Earth. The Hayabusa2 spacecraft visited the carbonaceous asteroid Ryugu and collected samples of its surface materials, which were brought to Earth in December 2020.

RATIONALE

We investigated the macromolecular organic matter in the Ryugu samples, measuring its elemental, isotopic, and functional group compositions along with its small-scale structures and morphologies. Analytical methods used included spectro-microscopies, electron microscopy, and isotopic microscopy. We examined intact Ryugu grains and insoluble carbonaceous residues isolated by acid treatment of the Ryugu samples.

RESULTS

Organic matter is abundant in the Ryugu grains, distributed as submicrometer-sized organic grains and as organic matter dispersed in matrix. The Ryugu organic matter consists of aromatic carbons, aliphatic carbons, ketones, and carboxyls. The functional group compositions are consistent with those of insoluble organic matter (IOM) from primitive carbonaceous CI (Ivuna-type) and CM (Mighei-type) chondritic meteorites. Those meteorites experienced aqueous alteration (reactions with liquid water) on their parent bodies, which implies that the Ryugu organic material was also modified by aqueous alteration on the asteroid parent body.

The functional group distributions of the Ryugu organic matter vary on submicrometer scales in ways that relate to the morphologies: nanoparticulate and/or nanoglobular regions are aromatic-rich, whereas organic matter associated with Mg-rich phyllosilicate matrix and carbonates is IOM-like or occurs as diffuse carbon. The observed macromolecular diversity provides further evidence that the organics were modified by aqueous alteration on Ryugu’s parent body. The diffuse carbon is similar to clay-bound organic matter that occurs in CI chondrites and the ungrouped C2-type meteorite Tagish Lake. No graphite-like material was found, which indicates that the Ryugu organic matter was not subjected to heating events on the parent body.

The bulk hydrogen and nitrogen isotopic ratios of the Ryugu grains are between the bulk values of CI chondrites and the IOM in CI chondrites. Some carbonaceous grains showed extreme deuterium (D) and/or nitrogen-15 ( ¹⁵ N) enrichments or depletions. These indicate an origin in the interstellar medium or presolar nebula. The bulk hydrogen isotopic ratios of insoluble carbonaceous residues from the Ryugu samples are lower than those in CI and CM chondrites. The range of D enrichments are consistent with the ranges of CI, CM, and Tagish Lake chondrites. The nitrogen isotopic ratios of the IOM from Ryugu samples were close to those in CI chondrites.

CONCLUSION

The organic matter in Ryugu probably consists of primordial materials that formed during (or before) the early stages of the Solar System’s formation, which were later modified by heterogeneous aqueous alteration on Ryugu’s parent body asteroid. Although the surface of Ryugu is exposed to solar wind, impacts, and heating by sunlight, the macromolecular organics in the surface grains of Ryugu are similar in their chemical, isotopic, and morphological compositions to those seen in primitive carbonaceous chondrites. The properties of Ryugu’s organic matter could explain the low albedo of the asteroid’s surface.