Infrared absorption spectra from organic matter in the asteroid Ryugu samples: Some unique properties compared to unheated carbonaceous chondrites

Abstract

The infrared spectral characteristics of organic‐rich acid residues prepared from Ryugu samples returned by the JAXA Hayabusa2 mission generally match those from unheated carbonaceous chondrite meteorites, but the residues from Ryugu are richer in methyl and methylene functional groups and have higher CH2/CH3 ratios. Moreover, two distinct outlier carbonaceous phases are found; one with spectral characteristics of N‐H functional groups, likely amides, and a second phase containing less nitrogen. Such infrared characteristics of Ryugu organic matter might indicate the pristine nature of the freshly collected samples and reflect the near‐surface chemistry in the parent asteroid.

Nanoscale Spectroscopic Identification and Characterization of Minerals and Organic Matter in Ryugu Particles

“…Since the 1,250 cm −1 distribution does not overlap with other organics (1,600 and 1,325 cm −1 ), the 1,250 cm −1 compounds are potentially organic matter with C‐O and/or N‐H bonds and less aromatic or inorganic components. Interestingly, the 1,250 cm −1 peak observed in some of our Ryugu nano‐FTIR spectra is absent in the IR absorption spectra of unheated carbonaceous chondrites (CI, CM, and CR) (Dartois et al., 2023; Kebukawa et al., 2023; Yabuta et al., 2023). In any case, the nature of this peak is not well understood at this time.…”

“…(2023), and Kebukawa et al. (2023). These IR spectra represent average compositions of typically a few tens of microns.…”

“…In general, the acid residues of Ryugu samples are similar to the insoluble organic matter (IOM) of the unheated carbonaceous chondrites (CI, CM, and CR) (Kebukawa et al., 2023‐a); the characteristic C = O and aromatic C = C peaks at ∼1,700 and ∼1,600 cm −1 , respectively, smaller aliphatic C‐H peaks at ∼1,450 and ∼1,375 cm −1 , a broad C‐O and the aromatic skeletal peak at ∼1,230 cm −1 , and the aliphatic C‐H peaks at ∼2,900 cm −1 are present. However, there are some differences between Ryugu residues and chondrite IOM; Ryugu residues have a peak at 1,660 cm −1 that could be potentially attributed to C = O from the unsaturated ketones/aldehydes or amides (Kebukawa et al., 2023; Yabuta et al., 2023). In addition, two minor “outlier phases” were found in the Ryugu acid residues (Kebukawa et al., 2023‐a).…”

“…However, there are some differences between Ryugu residues and chondrite IOM; Ryugu residues have a peak at 1,660 cm −1 that could be potentially attributed to C = O from the unsaturated ketones/aldehydes or amides (Kebukawa et al., 2023; Yabuta et al., 2023). In addition, two minor “outlier phases” were found in the Ryugu acid residues (Kebukawa et al., 2023‐a). One has characteristic IR peaks at 1,660 and 1,635 cm −1 , which are potentially from primary amide‐like compounds, and the other has characteristic IR peaks at 1,510, 1,245, 1,180, 1,040, and 830 cm −1 , likely due to unknown aromatic‐carbonyl‐ether compounds (Kebukawa et al., 2023‐a).…”

“…Fourier transform infrared micro‐spectroscopy (micro‐FTIR) measurements were conducted on bulk Ryugu particles, as explained in the methods described in Kebukawa et al. (2023). IR absorption spectra were collected from each diamond window with a micro‐FTIR instrument (JASCO FT/IR‐6100+IRT‐5200) equipped with a ceramic IR light source, a germanium‐coated KBr beam splitter, a liquid‐nitrogen cooled mercury‐cadmium‐telluride (MCT) detector, and ×16 Cassegrain mirrors.…”