In the matrices of carbonaceous chondrites, aqueous alteration affects the accreted assemblage of organic matter and minerals. Hence, co-evolution of organic and inorganic components needs to be considered. In other words, the occurrence of minerals influences the evolution of organic matter and organic matter influence the mineralogical evolution. This results in a complex mineral-organic association at the nm scale. A mixture of amorphous silicate and simple organic molecule was subjected to aqueous conditions typical of carbonaceous asteroids. The formation of clay minerals intimately associated with organic matter was observed (Viennet et al. 2022).
The PAH of Kolang, Mukundpura, Tarda and Aguas Zarcas chondrites were extracted and analyzed. Molecular distribution and concentrations were compared to reported data on Murchison and Paris chondrites. C and H isotopes on the most abundant compounds were also measured, to confirm an extraterrestrial origin. In parallel, a series of experiments were conducted to document the molecular evolution of PAHs along with their isotope signatures during aqueous alteration. While C isotope composition constitutes a robust proxy of the origin of these molecules, their D/H ratio is strongly affected by aqueous alteration. This was reported in Lecasble et al. 2022 and 2023 and presented in several international meetings.
The insoluble organic matter of the same chondrites was isolated and characterized by solid state NMR, FTIR, Raman, EPR, LDI-FTICR and XANES. This allowed us to investigate the impact of aqueous alteration on the CM parent body on the molecular properties of the IOM. Its N and H isotope composition was investigated by EA-irMS and NanoSIMS, revealing the influence of fluid circulation on these isotope compositions. Using ultra-high mass resolution mass spectrometry (FTICR), several thousands of molecule formula could be identified in each sample. Principal component analysis revealed patterns that are related to molecular evolution under aqueous conditions on top of other patterns related to the synthesis source of IOM precursors. Despite reactions that may have occurred on the parent body, the IOM can then preserve molecular components related to the accreted organic precursors. This was reported in Laurent et al. 2022a.
An experiment was conducted on the IOM of the Paris chondrite at 150°C in presence of pure water (Laurent et al. 2022b). The molecular structure and isotope properties of experimental product were compared to those of the starting material, to document the influence of hydrothermal conditions. A similar experiment was performed on intact powder of the Orgueil chondrite to assess the influence of clay minerals on the alteration of organic macromolecules during aqueous alteration.
Experiments were conducted to evaluate the evolution of amino acids and nucleobases under hydrothermal conditions, in presence of minerals (He et al. 2024a). Results showed that guanine and uracil exhibit high stability, while xanthine undergoes decarboxylation. Saponite traps about 25% of xanthine, 53% of uracil, and nearly all guanine, promoting nucleobase preservation. The C and N-isotope values of nucleobases remained constant through aqueous alteration, suggesting that their isotope signatures are related to synthesis in cold environments in the proto solar nebula or the parent molecular cloud. The relative stability of guanine during aqueous alteration explains its abundance in meteorites. The reported concentrations of uracil and xanthine in CCs are not correlated with the alteration degree; hence, differences in relative abundance are either related to different parent body reservoirs or synthesis/migration during aqueous alteration events.
Six different amino acids were subjected to aqueous conditions at 150°C in presence or absence of phyllosilicates (He et al. 2024b). Glycine and α-alanine exhibit a rather high stability, which is consistent with the measured abundances of α-alanine and glycine in chondrites having experienced various degrees of aqueous alteration. In the meantime, the evolution of β-alanine under hydrothermal conditions leads to the formation of a new compound, which likely results from the decarboxylation and deamination of β-alanine followed by recombination. More than 95 % of γ-ABA was transformed into 2-pyrrolidione though self-cyclization during the aqueous alteration. Moreover, clay minerals tends to scavenge amino acid, with a preference of branched amino acids over chained amino acids. The δ13C values of amino acids have not changed significantly during the experiments, even with the presence of silicates. Thus, the δ13C values of amino acids reported in CR and CM chondrites likely relate to synthetic conditions or the origin of their precursors (i.e. inherited from the pre-accretion processes).
Similar experiments were conducted in pure D2O. Analysis by GC-MS allowed us to determine that H atoms bonded to the carbon atom in -position in amino acids are more prone to isotope exchange that the other methylene and methyl H, as reported in (He et al., 2025). We could identify the specific molecular positions where D-H exchange progressively occurred during the experiment (that lasted from a few to 10 days). The tendency of the H bonded to Cα explains, partly, the range of D/H compositions of amino acids within single carbonaceous chondrites.
