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[PPS08-P07] Experiments simulating the formation of insoluble organic matter to establish organic indicators for the heliocentric distances of chondrite parent body accretion
Keywords:carbonaceous chondrites, small bodies, insoluble organic matter, ammonia, hexamethylenetetramine, aqueous alteration
Information on where small bodies formed is important for understanding the planet formation process. However, the current location of small bodies is not necessarily the same as the location of their
accretion[1]. The location of the accretion of small bodies is sometimes estimated using the isotopic ratios of materials in meteorites as an indicator. However, mixing of materials and reactions may complicate the processes, and no simple solutions was established.
In this study, we investigated the use of the chemical structure of insoluble organic matter (IOM) in extraterrestrial materials as an indicator of the accretion position of small bodies. IOM in meteorites may have been formed by aqueous alteration from simple molecules in the parent bodies[2]. Before alteration, the composition of the starting material is considered to depend on the accretion locations of the parent bodies. In this study, we synthesized materials that simulate IOM in extraterrestrial materials from starting materials of various compositions, particularly focusing on the differences in nitrogen sources, and examined whether the molecular structures of the products differ. NH3 and hexamethylenetetramine
(C6H12N4, HMT) were used as nitrogen sources. HMT is one of candidates of a precursor of organic
matter[3]. We investigated the possibility of using organic matter in extraterrestrial materials as an indicator of the locations of the accretion of small bodies. By comparing IOM in the chondrites with the experimental products, it may be possible to estimate what starting material IOM formed from. The relationship between the heliocentric distances and starting materials may then constrain the accretion locations of the chondrite parent bodies.
We considered that inside and outside the NH3 snow line, the nitrogen source in the starting material when IOM is formed is different. Assuming NH3 as the nitrogen source outside the NH3 snow line and HMT inside the NH3 snow line, the starting solutions with the compositions of H2O : HCHO : CH3OH : NH3 : HMT : glycolaldehyde = (1)100 : 4 : 2 : 1.5 : 0 : 0.5, (2)100 : 4 : 2 : 0 : 0 : 0.5, (3)100 : 4 : 2 : 0 : 0.375 : 0.5, and (4)100 : 0 : 2 : 0 : 0.375 : 0.5 were prepared. These compositions were determined with reference to the composition of molecules observed in comets. Ca(OH)2 was added to these samples as a catalyst and these samples were heated at 150 ℃ for 72 hours. After heating, the products were separated into solid and liquid phases. The solid products were washed with hydrochloric acid, pure water, CH3OH, and CH2Cl2 and dried. The C- and N-K-edge X-ray absorption near edge structure (C- and N-XANES) spectra of the solid products were measured using a scanning transmission X-ray microscope (STXM). The same experiments were performed under conditions without the addition of Ca(OH)2 and without the addition of glycolaldehyde to the starting material. The obtained C-XANES spectra were analyzed to calculate the proportion of C=C, C=O, aliphatic, and C(=O)O carbons. The N-XANES spectra were also analyzed to calculate the proportion of C=N, C≡N, and C-N nitrogen.
C-XANES spectral analysis confirmed that the proportion of C=C carbons in sample (2), which does not contain a nitrogen source in the starting material, tends to be larger than those in samples (1), (3), and (4), which contain a nitrogen source. The proportion of C=C, C=O, aliphatic, and C(=O)O carbons in sample (1), where the nitrogen source in the starting material is NH3, are relatively close to those in sample (3), where the nitrogen source is HMT.
For the N-XANES spectra obtained, there is a peak around 395-405 eV for samples (1), (3), and (4) with a nitrogen source in the starting material, but almost no peak around 395-405 eV for sample (2) without a nitrogen source. The proportion of C=N, C≡N, and C-N nitrogen in sample (1), in which the nitrogen source in the starting material is NH3, are relatively close to those in sample (3), in which the nitrogen source is HMT.
From these observations, IOM in small bodies accreted in regions where the starting material does not contain a nitrogen source may be characterized by a high proportion of C=C carbon (and no nitrogen). The IOM in small bodies accreted in regions where the starting material contains NH3 may have a chemical structure similar to that of IOM in small bodies accreted in regions where the starting material contains HMT. Our results may rather indicate that IOM-like compounds synthesized from C, H, O, N-bearing compounds resulted in similar molecular structures despite the differences in initial compounds.
[1] H. F. Levison et al., Nature 2009, 460, 7253.
[2] G. D. Cody et al., PNAS 2011, 108, 48.
[3] V. Vinogradoff et al., Icarus 2018, 305.
accretion[1]. The location of the accretion of small bodies is sometimes estimated using the isotopic ratios of materials in meteorites as an indicator. However, mixing of materials and reactions may complicate the processes, and no simple solutions was established.
In this study, we investigated the use of the chemical structure of insoluble organic matter (IOM) in extraterrestrial materials as an indicator of the accretion position of small bodies. IOM in meteorites may have been formed by aqueous alteration from simple molecules in the parent bodies[2]. Before alteration, the composition of the starting material is considered to depend on the accretion locations of the parent bodies. In this study, we synthesized materials that simulate IOM in extraterrestrial materials from starting materials of various compositions, particularly focusing on the differences in nitrogen sources, and examined whether the molecular structures of the products differ. NH3 and hexamethylenetetramine
(C6H12N4, HMT) were used as nitrogen sources. HMT is one of candidates of a precursor of organic
matter[3]. We investigated the possibility of using organic matter in extraterrestrial materials as an indicator of the locations of the accretion of small bodies. By comparing IOM in the chondrites with the experimental products, it may be possible to estimate what starting material IOM formed from. The relationship between the heliocentric distances and starting materials may then constrain the accretion locations of the chondrite parent bodies.
We considered that inside and outside the NH3 snow line, the nitrogen source in the starting material when IOM is formed is different. Assuming NH3 as the nitrogen source outside the NH3 snow line and HMT inside the NH3 snow line, the starting solutions with the compositions of H2O : HCHO : CH3OH : NH3 : HMT : glycolaldehyde = (1)100 : 4 : 2 : 1.5 : 0 : 0.5, (2)100 : 4 : 2 : 0 : 0 : 0.5, (3)100 : 4 : 2 : 0 : 0.375 : 0.5, and (4)100 : 0 : 2 : 0 : 0.375 : 0.5 were prepared. These compositions were determined with reference to the composition of molecules observed in comets. Ca(OH)2 was added to these samples as a catalyst and these samples were heated at 150 ℃ for 72 hours. After heating, the products were separated into solid and liquid phases. The solid products were washed with hydrochloric acid, pure water, CH3OH, and CH2Cl2 and dried. The C- and N-K-edge X-ray absorption near edge structure (C- and N-XANES) spectra of the solid products were measured using a scanning transmission X-ray microscope (STXM). The same experiments were performed under conditions without the addition of Ca(OH)2 and without the addition of glycolaldehyde to the starting material. The obtained C-XANES spectra were analyzed to calculate the proportion of C=C, C=O, aliphatic, and C(=O)O carbons. The N-XANES spectra were also analyzed to calculate the proportion of C=N, C≡N, and C-N nitrogen.
C-XANES spectral analysis confirmed that the proportion of C=C carbons in sample (2), which does not contain a nitrogen source in the starting material, tends to be larger than those in samples (1), (3), and (4), which contain a nitrogen source. The proportion of C=C, C=O, aliphatic, and C(=O)O carbons in sample (1), where the nitrogen source in the starting material is NH3, are relatively close to those in sample (3), where the nitrogen source is HMT.
For the N-XANES spectra obtained, there is a peak around 395-405 eV for samples (1), (3), and (4) with a nitrogen source in the starting material, but almost no peak around 395-405 eV for sample (2) without a nitrogen source. The proportion of C=N, C≡N, and C-N nitrogen in sample (1), in which the nitrogen source in the starting material is NH3, are relatively close to those in sample (3), in which the nitrogen source is HMT.
From these observations, IOM in small bodies accreted in regions where the starting material does not contain a nitrogen source may be characterized by a high proportion of C=C carbon (and no nitrogen). The IOM in small bodies accreted in regions where the starting material contains NH3 may have a chemical structure similar to that of IOM in small bodies accreted in regions where the starting material contains HMT. Our results may rather indicate that IOM-like compounds synthesized from C, H, O, N-bearing compounds resulted in similar molecular structures despite the differences in initial compounds.
[1] H. F. Levison et al., Nature 2009, 460, 7253.
[2] G. D. Cody et al., PNAS 2011, 108, 48.
[3] V. Vinogradoff et al., Icarus 2018, 305.