Uracil in the carbonaceous asteroid (162173) Ryugu

Abstract

The pristine sample from the near-Earth carbonaceous asteroid (162173) Ryugu collected by the Hayabusa2 spacecraft enabled us to analyze the pristine extraterrestrial material without uncontrolled exposure to the Earth’s atmosphere and biosphere. The initial analysis team for the soluble organic matter reported the detection of wide variety of organic molecules including racemic amino acids in the Ryugu samples. Here we report the detection of uracil, one of the four nucleobases in ribonucleic acid, in aqueous extracts from Ryugu samples. In addition, nicotinic acid (niacin, a B3 vitamer), its derivatives, and imidazoles were detected in search for nitrogen heterocyclic molecules. The observed difference in the concentration of uracil between A0106 and C0107 may be related to the possible differences in the degree of alteration induced by energetic particles such as ultraviolet photons and cosmic rays. The present study strongly suggests that such molecules of prebiotic interest commonly formed in carbonaceous asteroids including Ryugu and were delivered to the early Earth.

Possible Ribose Synthesis in Carbonaceous Planetesimals

“…Furthermore, in the returned samples from the Hayabusa2 space mission that visited the near-Earth carbonaceous asteroid (162173) Ryugu, nicotinic acid was found in abundances of 49-99 ppb. [14] This reinforces the idea that these organics have been already present in the parent bodies of carbonaceous chondrites in the early history of the solar system. The nicotinic acid and nicotinamide could either have been formed in their interior by aqueous chemistry, or they could have been inherited by the solar nebula from the interstellar medium and incorporated into the source material of the planetesimals.…”

“…This might also explain why nicotinamide could only be detected by the more gentle method by Oba et al (2022), [13] using water extraction and ultra-sonication for 10 min at room temperature without involving high temperatures or acids. In their other study Oba et al ( 2023), [14] they could only find it in hot water extracts of the Orgueil CI meteorite at 105 • C over 20 h, which is at a lower temperature and over a shorter period in comparison to hot water extraction performed by Smith et al [12] In the same study Oba et al ( 2023), [14] acidhydrolyzed extraction (6 M HCl, same as acid-hydrolyzed extraction by Smith et al [12] ) of the same sample yielded no nicotinamide, but a slightly higher nicotinic acid abundance. This might indicate that in the acidic conditions of the extraction, nicotinamide was partially decomposed [116] and partially converted to nicotinic acid.…”

“…[52,64] Cb-type asteroids, e.g., (162173) Ryugu, are considered to originate from the same parent body as CI chondrites, such as the Orgueil meteorite. [14,[65][66][67][68]…”

“…The simulations were run at a pressure of 100 bar. From left to right, the plotted bars show the simulated molecular abundance of nicotinic acid (solid black line on the very left) for the whole temperature range of liquid water, as well as the abundances measured in samples of Cb asteroid (162173) Ryugu collected during the Hayabusa2 spacecraft mission, [14] in the CI chondrite Orgeil, [14] several Antarctic CM2 chondrites, [12] the CM2 chondrites Murchison and Murray, and the ungrouped C2 chondrite Tagish Lake [13] as lines and shaded ranges described in the legend. Each time, the measured nicotinic acid abundance and the sum of all isomers (nicotinic acid, isonicotinic acid, picolinic acid) are given, since thermochemical equilibrium simulations cannot distinguish between isomers.…”

“…Smith et al [12] also studied the hydrolysis of nicotinamide under the procedures employed to extract the organics from the meteorites. When using acid-hydrolyzed hot water (6 M HCl, 150 • C, 3 h) for extraction, a common method also used by Oba et al ( 2023), [14] purchased nicotinamide was 100 % converted to nicotinic acid. This might also explain why nicotinamide could only be detected by the more gentle method by Oba et al (2022), [13] using water extraction and ultra-sonication for 10 min at room temperature without involving high temperatures or acids.…”