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Returned Sample Identification of Organic Compounds from Asteroid Ryugu

The Japanese Space Agency (JAXA) launched a spacecraft named Hayabusa2 to study asteroid 162173 Ryugu on December 3, 2014. Hayabusa2 arrived at Ryugu on June 27, 2018 then successfully orbited, mapped and sampled the asteroid. On Dec. 6, 2020, the Hayabusa2 returned to earth and released a capsule containing a sample of the asteroid. I published an article entitled: Initial Sample Return Analysis from Asteroid Ryugu in the March 2022 issue of Meteorite-Times. Ryugu is classified as a C-type carbonaceous asteroid. (See figure 1) A total of 5.424 ±0.217 grams was collected from Ryugu and kept as physically and chemically pristine as possible, handled only in a vacuum of pure nitrogen. (See figure 2) Initial analysis of the samples indicated the presence of carbon-hydrogen (C-H) bonds typical of organic molecules. Hydroxyl (OH) was also identified which indicated the presence of water. Think of water (H2O as H-OH). (See References 1 & 4)

A recent paper by Hiroshi Naraoka et. al., published in the Feb 24, 2023 issue of Science has a more complete analysis of the organic compound inventory to date. (Reference 3) Organic refers to carbon that is covalently bonded to hydrogen, oxygen, nitrogen and sulfur (CHONS). Two samples were used in the study. The main analysis was performed on an aggregate sample designated (A0106) consisting of grains less than 1 mm in diameter with a total weight of 38.4 mg. The A0106 sample has typical mineralogy for Ryugu consisting mainly of hydrous silicate minerals, including serpentine and saponite, with other associated minerals such as dolomite, pyrrhotite, and magnetite which indicate extensive aqueous alteration. The second sample consisted of a single one millimeter sized grain designated (A0080) and was used to determine the spatial distribution of organic compounds on its surface. (See figure 3)

Organic compounds were analyzed using liquid chromatography and mass spectrometry. Liquid chromatography is a technique in analytical chemistry used to separate, identify, and quantify each component in a mixture. Mass spectrometers measure the mass-to-charge ratio (m/z) of one or more molecules present in a sample and determine their chemical structures.

A total of 15 amino acids were detected and quantitated. (See figure 4) The concentrations of each amino acid ranged from 0.01 to 5.6 nmol/g. There were α-, β-, γ-, and δ-amino acids detected, spanning a concentration range of 0.014 – 4.7 nmol/g. Protein amino acids included glycine (Gly), D,L-α-alanine (D,L-α-Ala), and D,L-valine (D,L-Val). Non-protein amino acids included β-alanine (β-Ala), D,L-α-amino-butyric acid (D,L-α-ABA), and D,L-βamino-isobutyric acid (D,L-β-AIB). Several non-protein C5 amino acids were also detected, including D,L norvaline (D,L-Nva), D,L-isovaline (D,L-Iva), δ-amino-n-valeric acid (δ-AVA), 3-amino-2,2- dimethyl butyric acid (3-A-2,2-DMBA), D,L-3-amino2-ethylpropanoic acid (D,L-3-A-2-EPA), and D, L-γamino-n-propanoic acid (D,L-γ-APA). The proteinogenic amino acids exhibit chirality- right (D) and left( L) handed forms. Biological organisms utilize only the L-amino acids. The A0106 Ryugu extract exhibited a racemic mixture of approximately equal D and L forms or enantiomers of amino acids which is indicative of a nonbiological origin. Many of the nonproteinogenic amino acids identified in the Ryugu extract are rare or nonexistent in terrestrial biology. The presence of a racemic mixture in all identified amino acids indicates that the returned Ryugu samples are pristine and free of contamination. (Reference 3, Figure 4)

Aliphatic (straight chain) amines and carboxylic acids were identified in the A0106 Ryugu samples. (See figure 4) Aliphatic amines detected were methylamine (CH3NH2) which was the most abundant, followed by ethylamine (C2H5NH2) , isopropyl amine [(CH3)2CHNH2], and n-propylamine (C3H7NH2). These amines are likely present as salts in the grains, because the free amines are highly volatile and reactive. Ammonium and amine salts are known to be the major reservoir of nitrogen on the dwarf planet Ceres and in comets. (Reference 3, Figure 4)

Monocarboxylic acids in the Ryugu samples included only formic acid HCOOH (5.7 mmol/g ) and acetic acid CH3COOH (9.5 mmol/g ) at the detection limits of the instrumentation. (See figure 4) The concentrations were high with low molecular weight diversity indicating low temperature hydrothermal processing on Ryugu’s parent body. This trend is also observed in highly aqueously altered carbonaceous chondrites such as Orgueil and Ivuna. (Reference 3, Figures 5 & 6)

Aromatic hydrocarbons were detected at below parts per million abundances, which included alkylbenzenes and polycyclic aromatic hydrocarbons (PAHs) (See figure 4) The highest abundance PAHs were fluoranthene and pyrene (which contain four benzene rings) followed by chrysene/triphenylene (also four rings) and methylated fluoranthene and pyrene. Smaller PAHs containing two rings (naphthalene) and three rings (phenanthrene and anthracene) were detected at lower abundances. Fluoranthene and pyrene are structural isomers (both have the formula C16H10) that are present in roughly equal amounts in CM chondrites. In the Ryugu sample fluoranthene is substantially less abundant than pyrene. In the CI meteorite Ivuna, both fluoranthene and pyrene are below the detection limits, although phenanthrene and anthracene are abundant. The difference in proportions of PAHs between Ryugu and carbonaceous chondrites could be due to different aqueous alteration effects on different parent bodies. (Reference 3, Figure 4)

The Fourier Transform InfraRed (FTIR) spectrum in the water extract of the A0106 grain has its strongest absorption band at 1000 cm–1 (10 mm) due to silicates (Si-O bonds). (See figure 3) Other bands are present at 750 to 1650 cm–1 (13.3 to 6.1 mm). Peaks at these wavelengths have often been observed in the interstellar medium and have been assigned to large polyaromatic hydrocarbons. The broad peak at 1400 cm–1 (7.14 mm) could also have a contribution from carbonates. The lack of the aromatic C–H stretching bands at 3030 cm–1 (3.30 mm) suggests that the PAHs present in the Ryugu water extract are highly depleted in hydrogen. The FTIR spectrum of the Ryugu sample is unlike those of carbonaceous chondrites. It is similar to astronomical observations of interstellar and presolar polyaromatic hydrocarbons that were incorporated into Ryugu’s parent body during its accretion, and then survived the subsequent aqueous alteration. (Reference 3, Figure 4)

Several classes of alkylated N-containing heterocyclic molecules were identified. (See figure 4) These alkylated N-heterocycles included pyridine, piperidine, pyrimidine, imidazole, or pyrrole rings with various amounts of alkylation. N-heterocyclic compounds can be synthesized through a reaction pathway using ammonia and formaldehyde which were abundant in the interstellar medium and the protosolar nebula. Therefore the Ryugu organic material might have inherited these characteristics from a molecular cloud environment. (Reference 3)

The diversity of Soluble Organic Material (SOM) in the Ryugu sample A0106 is as high as previously found for carbonaceous chondrites, and includes poly-sulfur-bearing species. By contrast, the diversity of low-molecular-weight compounds, including aliphatic amines and carboxylic acids, was lower in the Ryugu sample than previously measured in the Murchison meteorite. The total Soluble Organic Material concentration in the A0106 sample was less than that of Murchison and closer to those of the CI chondrites Ivuna and Orgueil. (See figure 5 & 6). The Ryugu organic matter seems to have been affected by aqueous alteration, which produced aromatic hydrocarbons. The SOM detected in the A0106 and A0080 samples indicates that Ryugu’s surface materials host organic molecules despite the harsh environment caused by solar heating, ultraviolet and cosmic-ray irradiation, as well as being in the vacuum of space. The uppermost surface grains on Ryugu protect organic molecules, unlike meteorites, for which atmospheric ablation during Earth entry removes or modifies analogous near-surface material. Organic compounds on asteroids can be ejected from the surface by impacts or other causes dispersing them through the Solar System as meteoroids or interplanetary dust particles. Therefore, organic material on C-type asteroids could be a source of organic compounds delivered to earth. (Reference 3)

NASA launched a similar sample return mission called OSIRIS-REx to asteroid Bennu on September 8, 2016. The spacecraft arrived at Bennu on December 3, 2018 and began mapping and analyzing the surface of Bennu. The spacecraft detected an infrared absorption at 3.4 µm (3.2 to 3.6 µm) on the surface of the asteroid. This absorption band is indicative of organic carbon, resulting from aliphatic (C-H), methylene (-CH2), and methyl (-CH3) symmetric and asymmetric stretching. (Reference 4) The samples were then collected on October 20, 2020 using the Touch and Go apparatus. OSIRIS-Rex departed Bennu on March 10, 2021 for return to earth. The sample return capsule safely touched down on September 24, 2023.

NASA’s curation team stated that they have removed and collected 70.28 grams (2.48 ounces) of Bennu material from the capsule so far and it hasn’t been fully opened yet. After multiple attempts at removal, the team discovered 2 of the 35 fasteners on the TAGSAM head could not be removed with the current tools approved for use in the OSIRIS-REx arsenal. At the time this article was published, NASA was currently working to develop and implement new approaches to extract the material inside the head, while continuing to keep the sample safe and pristine. The contents inside are estimated to weigh between 120 grams (4.2 oz) to 250 grams (8.8 oz). Preliminary analysis of the Bennu samples indicate the presence of water that is bonded to phyllosilicate clay, serpentine and sulfide minerals. Bennu contains a total of 4.7% carbon. The Bennu samples have a higher carbon content than previously analyzed carbonaceous chondrite meteorites. The identification of organic compound inventory in the Bennu samples is currently under investigation. (Reference 5)

In conclusion, both the Hayabusa 2 and OSIRIS-Rex sample return missions to the carbonaceous asteroids Ryugu and Bennu respectively were a complete success. The samples returned will give scientists insight into the formation of the solar system and the origins of life on earth.

 

Figure 1: Asteroid 162173 Ryugu photographed from a distance of approximately 12 miles (19 kilometers). Ryugu is a rather small asteroid measuring 0.87 km (0.54 miles) x 0.92 km (0.57miles) x 1.13 km (0.70 miles). The mass of Ryugu is estimated to be about 450 million tons with a volume of 0.377 ± 0.005 km and density of 1.19 ± 0.03 g/cm. Image credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu

 

Figure 2: The Ryugu samples totaling 5.424 ±0.217 g were separated into six separate containers. Optical microscopic images of bulk samples from Chambers A and C. Subunit samples (a) (b) (c) are those from the Chamber A while (d) (e) (f) are those from the Chamber C. The specimen weights for Chamber Aa are 0.79g, Ab 1.15g and Ac 1.16g. Weights for Chamber Cd are 0.56g, Ce 0.44g and Cf 0.51g. Container inner diameters are 21mm (0.827 inches). Both chamber samples are an aggregation of black millimeter-sized pebbles and sub-millimeter fine powder similar in size and composition. In total, more than five thousand particles of >100 µm are recognized in these images. Each chamber measures 25mm in diameter. From Yada et al 2021 (Reference 2)

 

Figure 3: Spatial distribution of CHN compounds on the surface of Ryugu grain A0080. (A and B) Optical images before sample preparation (A) and after embedding in an alloy (B). White arrow in (A) indicates the grain surface embedded in (B). Maps of organic molecule distribution for the CnH2n-6N+ series (n =14, 15) (C) and CnH2n-8N+ series (n =16, 17) (D) molecules. White outlines indicate the boundary between the A0080 grain and the surrounding metal. Scale bars, 500 mm. Credit: Hiroshi Naraoka 2023 (Reference 3)

 

Figure 4: Soluble organic molecules detected in surface samples of asteroid Ryugu. Chemical structural models are shown for example molecules from several classes identified in the Ryugu samples. Gray balls are carbon, white are hydrogen, red are oxygen, and blue are nitrogen. Clockwise from top: amines (represented by ethylamine), nitrogen containing heterocycles (pyridine), a photograph of the sample vials for analysis, polycyclic aromatic hydrocarbons (PAHs) (pyrene), carboxylic acids (acetic acid), and amino acids (b-alanine). The central hexagon shows a photograph of the Ryugu sample in the sample collector of the Hayabusa2 spacecraft. The background image shows Ryugu in a photograph taken by Hayabusa2 (Reference 3) CREDIT: JAXA, UNIVERSITY OF TOKYO, KOCHI UNIVERSITY, RIKKYO UNIVERSITY, NAGOYA UNIVERSITY, CHIBA INSTITUTE OF TECHNOLOGY, MEIJI UNIVERSITY, UNIV of AIST, NASA, Dan Gallagher

 

Figure 5: Fragment of ORGUEIL a CI carbonaceous chondrite from the authors personal collection. Orgueil fell on May 14, 1864 in Montauban, Tarn-et-Garonne 43°53’ N., 1°23’ E. A total of twenty stones totaling 14 kg were collected. The fragment weights 0.14 grams and measures 8mm x 3mm x 3mm.

 

Figure 6: Fragment of IVUNA a CI carbonaceous chondrite from the authors personal collection. Ivuna fell on December 16, 1938 in Tanzania, Africa 08°25’ S., 32°26’ E. A total of 705 grams were collected. The fragment weights 0.43 grams and measures 12mm x 8mm x 5mm.

 

The CI carbonaceous chondrites Orgueil (above) and Ivuna (below) are the closest meteoritic analogs to the Ryugu samples.

 

Figure 7: A view of the outside of the OSIRIS-REx sample collector. Sample material from asteroid Bennu can be seen on the middle right. Scientists have found evidence of both carbon and water in initial analysis of this material. The bulk of the sample is located inside. Note the similarity in appearance of the Bennu & Ryugu samples. (Image credit: NASA/Erika Blumenfeld & Joseph Aebersold)

 

 

References:

 

1) Shanos G.T. Initial Sample Return Analysis from Asteroid Ryugu March 1, 2022 (https://www.meteorite-times.com/2022/03/01/) pp. 51-55

2) Yada T., Masanao A., Tatsuaki O. et. al Preliminary analysis of the Hayabusa2 samples returned from C-type asteroid Ryugu. Nature Astronomy Letters 20 Dec 2021 open source https://doi.org/10.1038/s41550-021-01550-6

3) Hiroshi Naraoka, Yoshinori Takano , Jason P. Dworkin et. al. Soluble organic molecules in samples of the carbonaceous asteroid (162173) Ryugu Science 379, (pp. 10) February 24, 2023

4) H. H. Kaplan , A. A. Simon , V. E. Hamilton , M. S. Composition of organics on asteroid (101955) Bennu Astronomy & Astrophysics vol 653, Letter to the Editor (2021) pp 1-11

5) Lauretta, D, Glavin, D, McCovin, F. et. al. Revealing the OSIRIS-REx Asteroid Sample (Official NASA Broadcast in 4K) https://www.youtube.com/watch?v=oFvIuSpACQA

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