Presentation Information
[PPS12-P06]Speciation of transition metals in the C-type asteroid Ryugu and implication for prebiotic chemistry
*Ayu Takemoto1, Tomohiro OHNO1, Takahiro Kawai1, Akiko Yamaguchi1, Mika Yoshioka1, Shunpei Abe2, Yoko Kebukawa2, Hiroyuki Kagi1, Yoshio Takahashi1 (1.Tokyo University, 2.Institute of Science Tokyo)
Keywords:
Ryugu,Transition metal,XAFS,aqueous alteration
Carbonaceous chondrites that experienced aqueous alteration are rich in amino acids and diverse organics have been found in such as Orgueil meteorite and in Ryugu samples (Burton et al., 2012). During aqueous alteration in such extraterrestrial, prebiotic chemistry relevant environments, metal ions may have acted as catalysts for organic reactions (Aithal et al., 2023). Among them, 3d transition metals are of particular interest because they exhibit diverse coordination chemistry and are relatively abundant compared with many other d-electron elements. However, systematic constraints on aqueous speciation and dissolved concentrations of 3d transition metals during Ryugu parent body alteration remain scarce. In this study, we investigated the speciation of divalent transition metals (TM : Mn, Ni, Cu, Zn) during aqueous alteration on Ryugu and evaluated implications for prebiotic chemistry, using a returned sample collected by the JAXA Hayabusa2 mission. We conducted: (1) estimation of partitioning behavior and dissolved concentrations (water solubility) of transition metals relative to iron sulfide (pyrrhotite), and (2) laboratory assessment of the catalytic effects of transition metals on plausible organic reactions in the Ryugu aqueous environment, namely a sugar forming reaction (formose reaction) and an amino acid polymerization reaction.Transition metals were assumed to partition into solid phases while maintaining equilibrium with the aqueous phase during alteration. Dissolved concentrations were estimated by combining: (i) bulk XAFS constraints on average speciation and host-phase partitioning, (ii) laboratory adsorption experiments to determine distribution coefficients (Kd) onto host phases, and (iii) micro-XRF quantification of elemental abundances in host phases. Bulk XAFS indicates that major host phases for these metals are pyrrhotite and phyllosilicates including saponite. We separated alteration into an early (neutral) and late (alkaline) stage and estimated dissolved concentrations for both pH regimes. Zn K-edge EXAFS in pyrrhotite is better explained by an adsorbed Zn component on Fe sulfide rather than ZnS precipitation, suggesting that adsorption onto host sulfides largely controlled dissolved transition-metal levels. From the derived solid-phase abundances and Kd values, estimated dissolved concentrations follow Ni > Mn > Zn > Cu, implying that Ni may have been comparatively more available for aqueous organic chemistry.
To evaluate impacts on prebiotic chemistry, we performed formose reaction and amino acid polymerization experiments with added metals. In the formose reaction, Ca and Ni added systems yielded higher amounts of C4 sugars, indicating that metal driven enhancement can occur even under near neutral conditions, consistent with potential sugar formation during early stage alteration. When comparing catalytic contribution across elements (catalytic efficiency and concentration), Ca was estimated to contribute ~8 more than Ni. Thus, if the formose reaction proceeded in early stage aqueous environments, Ca may have been the more effective catalyst overall, while Ni remains the most plausible catalytic transition metal. In the amino acid polymerization experiments, we compared systems with amino acid (L-alanine monomer) + transition metal, with and without added saponite, and quantified dimer production. Zn addition significantly increased dimer yields both as dissolved Zn2+ and as clay adsorbed Zn, whereas Ni addition did not increase dimer yields beyond analytical uncertainty. Together, these results suggest that although dissolved Zn concentrations were lower than Ni in Ryugu water, Zn could have exerted a strong catalytic influence especially because Zn is expected to be dominated by clay adsorbed species in the bulk speciation budget, making that Zn form the most important contributor to catalysis.
To evaluate impacts on prebiotic chemistry, we performed formose reaction and amino acid polymerization experiments with added metals. In the formose reaction, Ca and Ni added systems yielded higher amounts of C4 sugars, indicating that metal driven enhancement can occur even under near neutral conditions, consistent with potential sugar formation during early stage alteration. When comparing catalytic contribution across elements (catalytic efficiency and concentration), Ca was estimated to contribute ~8 more than Ni. Thus, if the formose reaction proceeded in early stage aqueous environments, Ca may have been the more effective catalyst overall, while Ni remains the most plausible catalytic transition metal. In the amino acid polymerization experiments, we compared systems with amino acid (L-alanine monomer) + transition metal, with and without added saponite, and quantified dimer production. Zn addition significantly increased dimer yields both as dissolved Zn2+ and as clay adsorbed Zn, whereas Ni addition did not increase dimer yields beyond analytical uncertainty. Together, these results suggest that although dissolved Zn concentrations were lower than Ni in Ryugu water, Zn could have exerted a strong catalytic influence especially because Zn is expected to be dominated by clay adsorbed species in the bulk speciation budget, making that Zn form the most important contributor to catalysis.