Variations of organic functional chemistry in carbonaceous matter from the asteroid 162173 Ryugu

Abstract

Primordial carbon delivered to the early earth by asteroids and meteorites provided a diverse source of extraterrestrial organics from pre-existing simple organic compounds, complex solar-irradiated macromolecules, and macromolecules from extended hydrothermal processing. Surface regolith collected by the Hayabusa2 spacecraft from the carbon-rich asteroid 162173 Ryugu present a unique opportunity to untangle the sources and processing history of carbonaceous matter. Here we show carbonaceous grains in Ryugu can be classified into three main populations defined by spectral shape: Highly aromatic (HA), Alkyl-Aromatic (AA), and IOM-like (IL). These carbon populations may be related to primordial chemistry, since C and N isotopic compositions vary between the three groups. Diffuse carbon is occasionally dominated by molecular carbonate preferentially associated with coarse-grained phyllosilicate minerals. Compared to related carbonaceous meteorites, the greater diversity of organic functional chemistry in Ryugu indicate the pristine condition of these asteroid samples.

Fig. 1. Scanning transmission x-ray microscoy (STXM) and scanning transmission electron microscopy (STEM) imaging and x-ray absorption near-edge structure (XANES) spectroscopy of Ryugu insoluble organic matter (IOM).

Each set of images in A–C are a correlated STXM image at 290 eV, a false-color map of organic functional chemistry (magenta = aromatic C=C at 285 eV; green = ketone C=O at 286.7 eV; blue = carboxyl COOH at 288.5 eV), and a high-angle annular dark field (HAADF) STEM image. A IOM from Chamber A sample A0106. B, C IOM from Chamber C sample C0107. D Hierarchical clustering dendrogram of fitted XANES spectra from Ryugu IOM samples A0106 and C0107. The length of horizontal lines indicates the distance between spectral locations in a 21-dimensional space determined by Gaussian peak fitting. Samples labeled with IOM denote average spectra of surrounding IOM in the vicinity of the discrete grains (excluding other discrete grains). E XANES spectra from individual discrete grains and nanoglobules in A–C, compared to bulk, fluffy-textured IOM from A0106 and C0107, as well as IOM from the Orgueil (CI) and Murchison (CM) carbonaceous chondrites.

Fig. 2. Transmission electron microscopy (TEM) and Scanning transmission x-ray microscopy (STXM) imaging and x-ray absorption near-edge structure (XANES) spectroscopy of carbonaceous grains in Ryugu fine-grained matrix.

A TEM image of sample C0109-11 containing two carbonaceous grains (A1 and A2). B STXM false-color map of the same region showing the distinct organic functional chemistry of the two grains (magenta = aromatic C=C at 285 eV; green = ketone C=O at 286.7 eV; blue = carboxyl COOH at 288.5 eV). The surrounding phyllosilicate-rich matrix also contains diffuse C, visible as green and green-blue regions in this map. C High-angle annular dark field (HAADF) scanning TEM image of an alkyl-aromatic globular grain in a focused ion beam (FIB) section extracted from Ryugu particle A0108-3 (see Fig. 3). D XANES spectra of carbon grains and matrix shown in A–C. E Distribution of characteristic absorption peak area (plotted as a proportion of the total near-edge structure; see Methods) for carbonaceous grains and diffuse C in ultramicrotome and FIB sections. Solid color fields show the distributions for carbon grains identified in Ryugu insoluble organic matter (IOM) samples.

Fig. 3. Isotopic composition of Ryugu carbonaceous grains.

A High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) mosaic of a focused ion beam (FIB) lamella extracted from Chamber A particle A0108-3. B Nanoscale secondary ion mass spectrometry (NanoSIMS) elemental map of O, C, and S (in magenta, green, and blue, respectively). C Isotope ratio image of ¹³C/¹²C normalized to the solar value, also showing the location of a presolar SiC grain. D Isotope ratio image of δ¹⁵N/¹⁴N. Locations of carbonaceous grains in the section for which both x-ray absorption near edge structure (XANES) spectra and isotopic data were acquired are outlined in A–D. E C and N isotope compositions of carbonaceous grains. Grains with corresponding XANES spectra are color-coded by their shape classification. Dashed lines indicate “bulk” values for Ryugu. Carbon grains that are not correlated with scanning transmission x-ray microscopy (STXM) data (gray points) have error bars (2σ) similar in size to the colored points, but have been removed for clarity. The arrows in (A-E) indicate the location and isotopic composition of the carbon grain shown in Fig. 2C.

Fig. 4. Diffuse carbonaceous matter in Ryugu.

A X-ray absorption near-edge structure (XANES) spectra of Ryugu diffuse organic matter in ultramicrotome and focused ion beam (FIB) lamellae, compared with bulk Ryugu, Orgueil, and Murchison insoluble organic matter (IOM). B Scanning transmission x-ray microscopy (STXM) spectral map of a FIB lamella from Chamber A grain A0108-11 showing concentrations of diffuse organic matter with a molecular carbonate signature (magenta = carbon at 285 eV; green = carbonate CO₃ at 290.4 eV; blue = mineral carbonate at 302 eV). Mineral carbonates would be visible as a teal color in this map. C Bright-field scanning transmission electron microscopy (STEM) image of intermixed phyllosilicate sheets and brighter, interstitial organic matter from the region shown in B. Arrows denote locations of accumulations of CO₃-rich organic matter.