5:15 PM - 6:45 PM
[MZZ45-P03] Reconstruction of the aqueous environment of Ryugu's parent body using synchrotron radiation X-ray microscopy
Keywords:Ryugu, asteroid, XANES
A wide variety of organic matter has been found in the Hayabusa2 return sample and in carbonaceous chondrite meteorites that originated from a parent body similar to Ryugu. Carbonaceous chondrites are thought to fall to Earth frequently during the Late Heavy Bombardment Period, and many researchers believe that the organic matter contained in these meteorites may form the origin of life. Therefore, what kind of chemical reactions are taking place in the parent bodies of carbonaceous chondrite meteorites is an important problem for understanding how life on Earth formed and what kind of conditions exist in the habitable zone where life could exist.
Aqueous alteration that occurs in the parent bodies of carbonaceous chondrite meteorites, such as serpentinization reactions, is thought to proceed under reducing conditions. Since many of the materials formed in such environments are easily oxidized, oxidation by the Earth's atmosphere would be a fatal problem. Prior to the Hayabusa2 sample return mission, information on the parent bodies of carbonaceous chondrite meteorites was obtained from the analysis of meteorites that fell to Earth. However, because these meteorites were oxidized by oxygen in the Earth's atmosphere when they fell, it was impossible to determine whether or not they represented the actual chemical composition. In particular, saponite, the main component of the matrix of carbonaceous chondrite meteorites, is a reductive material that is very easily oxidized by oxygen, and in meteorite samples it has been altered by oxidation. On the other hand, the return sample from Ryugu was recovered with almost no exposure to the Earth's atmosphere. Therefore, by analyzing these samples, we can investigate how the alteration progressed under the reductive environment of Ryugu's parent body.
During aqueous alteration, various substances dissolve into the water from the surrounding minerals. The dissolved species cause various chemical reactions in the water, and as a result, phyllosilicates such as smectite (saponite) and serpentinite are the major minerals in Ryugu. (Nakamura et al. 2022; Yokoyama et al. 2022) The Eh-pH of water has a significant influence on the results of chemical reactions in water, but few studies have quantitatively examined these in Ryugu water. It is because Ryugu samples are now dry, and thus water at that time cannot be obtained directly from the samples. There is an attempt to estimate the aqueous environment by adding water to Ryugu samples and analyzing the eluted materials (Yoshimura et al. 2023), but there is a problem that the minerals and eluted materials in the samples are strongly affected by the added water, resulting in a different aqueous environment from that of the actual water at Ryugu. Therefore, in order to reconstruct the aqueous environment at the time of alteration, it is necessary to analyze what chemical species and how much of each element is present in the Ryugu sample, and to perform chemical calculations that take into account the conditions for the formation of these chemical species. The interlayer of saponite is charged negatively and has the property of adsorbing cations in water. The adsorption ratio of each cation depends on the concentration ratio of dissolved ions. Using this relationship, the cation ratios adsorbed between the layers of Ryugu's phyllosilicate can be used to determine the dissolved ions in Ryugu's water at the last stage of its water alteration.
We analyzed the chemical forms of each element in the Ryugu samples by measuring bulk XANES using X-ray spectroscopy with the high-intensity X-rays of the synchrotron radiation beamline. We also estimated the cation composition of the interlayer of Ryugu's phyllosilicate by SEM-EPMA analysis of the Ryugu sample embedded in Epofix resin and polished by film polishing. Based on the obtained results and previous studies, the concentrations of various dissolved ions in water coexisting with Ryugu's phyllosilicate were calculated using SpecE8, a chemical calculation application in The Geochemist's Workbench. The results showed that the water environment of Ryuguu, especially Eh and pH, were estimated.
The results indicate that Ryugu's water was strongly basic and highly reducing, with a pH of about 10-11 and an Eh of -0.7 to -0.9 V. These conditions were more reducing than the water present. These conditions are more reducing than the conditions for the presence of water, suggesting that hydrogen may have been generated in the Ryugu parent body during aqueous alteration.
Aqueous alteration that occurs in the parent bodies of carbonaceous chondrite meteorites, such as serpentinization reactions, is thought to proceed under reducing conditions. Since many of the materials formed in such environments are easily oxidized, oxidation by the Earth's atmosphere would be a fatal problem. Prior to the Hayabusa2 sample return mission, information on the parent bodies of carbonaceous chondrite meteorites was obtained from the analysis of meteorites that fell to Earth. However, because these meteorites were oxidized by oxygen in the Earth's atmosphere when they fell, it was impossible to determine whether or not they represented the actual chemical composition. In particular, saponite, the main component of the matrix of carbonaceous chondrite meteorites, is a reductive material that is very easily oxidized by oxygen, and in meteorite samples it has been altered by oxidation. On the other hand, the return sample from Ryugu was recovered with almost no exposure to the Earth's atmosphere. Therefore, by analyzing these samples, we can investigate how the alteration progressed under the reductive environment of Ryugu's parent body.
During aqueous alteration, various substances dissolve into the water from the surrounding minerals. The dissolved species cause various chemical reactions in the water, and as a result, phyllosilicates such as smectite (saponite) and serpentinite are the major minerals in Ryugu. (Nakamura et al. 2022; Yokoyama et al. 2022) The Eh-pH of water has a significant influence on the results of chemical reactions in water, but few studies have quantitatively examined these in Ryugu water. It is because Ryugu samples are now dry, and thus water at that time cannot be obtained directly from the samples. There is an attempt to estimate the aqueous environment by adding water to Ryugu samples and analyzing the eluted materials (Yoshimura et al. 2023), but there is a problem that the minerals and eluted materials in the samples are strongly affected by the added water, resulting in a different aqueous environment from that of the actual water at Ryugu. Therefore, in order to reconstruct the aqueous environment at the time of alteration, it is necessary to analyze what chemical species and how much of each element is present in the Ryugu sample, and to perform chemical calculations that take into account the conditions for the formation of these chemical species. The interlayer of saponite is charged negatively and has the property of adsorbing cations in water. The adsorption ratio of each cation depends on the concentration ratio of dissolved ions. Using this relationship, the cation ratios adsorbed between the layers of Ryugu's phyllosilicate can be used to determine the dissolved ions in Ryugu's water at the last stage of its water alteration.
We analyzed the chemical forms of each element in the Ryugu samples by measuring bulk XANES using X-ray spectroscopy with the high-intensity X-rays of the synchrotron radiation beamline. We also estimated the cation composition of the interlayer of Ryugu's phyllosilicate by SEM-EPMA analysis of the Ryugu sample embedded in Epofix resin and polished by film polishing. Based on the obtained results and previous studies, the concentrations of various dissolved ions in water coexisting with Ryugu's phyllosilicate were calculated using SpecE8, a chemical calculation application in The Geochemist's Workbench. The results showed that the water environment of Ryuguu, especially Eh and pH, were estimated.
The results indicate that Ryugu's water was strongly basic and highly reducing, with a pH of about 10-11 and an Eh of -0.7 to -0.9 V. These conditions were more reducing than the water present. These conditions are more reducing than the conditions for the presence of water, suggesting that hydrogen may have been generated in the Ryugu parent body during aqueous alteration.