09:45 〜 10:00
[PPS07-21] 火星は厚いCO2大気を持っていなかった？マンガン酸化実験から探る初期火星の大気組成
キーワード：火星、惑星進化、大気組成
Introduction
Both CO2 and O2 are important atmospheric components for climate and chemical evolution on early Mars. Several lines of geological and geomorphological evidence show that early Mars has been once warm sufficient to hold liquid water on the surface at least episodically in the late Noachian and early Hesperian [1]. Although early Mars would not be warmed sufficiently by CO2 alone, climate models presume the presence of a thick CO2 atmosphere to decrease outgoing longwave radiation and to cause effective collision-induced absorption. However, pCO2 on early Mars is poorly constrained by geochemical evidence thus far. On the other hand, the Curiosity rover has discovered Mn oxides in fracture-filling materials in sandstones of the Kimberley region of the Gale crater [2]. Given pO2 capable for deposition of Mn oxides (pO2 > ~0.01 bar) [3], the findings of Mn oxides indicate that early Mars had a substantial O2 in the atmosphere.
The present study aims to further constrain the composition of early Mars’ atmosphere, especially the CO2/O2 mixing ratio, at the time when the Mn oxides were formed. We performed laboratory experiments to generate Mn precipitates from Mn2+ in solutions by introducing CO2/O2 gas mixtures. We investigated the compositions of Mn precipitates under various compositions of CO2/O2.
Materials & Methods
The Mn2+ starting solution with 20 mM and pH 8–9 was prepared in an Ar-purged glovebox, where pO2 remained < 10-12 bar. The starting solution was deaerated by pure Ar gas for more than 6 hours prior to the use. Then, we introduced gas mixtures of pure CO2 and artificial air (N2/O2 = 4; pCO2 < 1ppm) into the starting solution at four different mixing ratios (CO2/O2 = 2, 0.2, 0.02, and artificial air) in the glovebox. Note that MnO2 is thermochemically stable under all of these conditions. Solution samples were collected in several times during the experiments. The samples were filtered through a membrane with pore size of 220 nm. After the reactions, Mn precipitates were collected by filtering the rest of the solutions using a membrane with 220 nm. Mn2+ concentrations of the filtered solution samples were measured using inductively-coupled plasma atomic emission spectroscopy (ICP-AES). The collected Mn precipitates were analyzed with X-ray absorption fine structure (XAFS) and X-ray diffraction (XRD).
Results
Our results of the ICP-AES analysis show that Mn2+ concentrations in the filtered solutions decrease over reaction time, which indicate that a part of dissolved Mn2+ was converted into solid precipitates. Despite both the wide range in CO2/O2 ratios and thermochemical stability of MnO2 under the experimental conditions, the results of XAFS analyses show that all of the Mn solid precipitates formed under these conditions are mainly composed of Mn carbonate, namely MnCO3. These results are consistent with our XRD results. Our results show that MnCO3 precipitated before the formation of MnO2 even very low CO2/O2 of 0.02. This suggests that kinetics of formation of MnCO3 and Mn oxides are the critical factor. On the other hand, the major peaks of the XANES spectra for the collected solid precipitates at CO2/O2 = 0 (namely, pure artificial air) would be a mixture of Mn oxides and Mn(OH)2.
Discussion
Our results show that, in order to form MnO2 in Mn2+ solutions by reactions with CO2/O2 gas mixtures, the CO2/O2 ratio should be lower than 0.02. Assuming pO2 of ~0.01–0.2 bar, which is capable to form and preserve MnO2 in sediments [3], the observations of both a lack of MnCO3 and presence of MnO2 in Gale infer that pCO2 on early Mars would have been less than 0.004 bar, or 4 mbar. This implies that early Mars may have possessed a low-CO2 and high-O2 atmosphere.
[1] Ehlmann, B.L. et al. (2011). Nature 479, doi:10.1038/nature10582.
[2] Lanza, N.L. et al. (2016). Geophys. Res. Lett., 43, 7398-7407.
[3] Shaw, T. et al. (1990). Geochim. Cosmochim, Acta 54, 1233-1246.
Both CO2 and O2 are important atmospheric components for climate and chemical evolution on early Mars. Several lines of geological and geomorphological evidence show that early Mars has been once warm sufficient to hold liquid water on the surface at least episodically in the late Noachian and early Hesperian [1]. Although early Mars would not be warmed sufficiently by CO2 alone, climate models presume the presence of a thick CO2 atmosphere to decrease outgoing longwave radiation and to cause effective collision-induced absorption. However, pCO2 on early Mars is poorly constrained by geochemical evidence thus far. On the other hand, the Curiosity rover has discovered Mn oxides in fracture-filling materials in sandstones of the Kimberley region of the Gale crater [2]. Given pO2 capable for deposition of Mn oxides (pO2 > ~0.01 bar) [3], the findings of Mn oxides indicate that early Mars had a substantial O2 in the atmosphere.
The present study aims to further constrain the composition of early Mars’ atmosphere, especially the CO2/O2 mixing ratio, at the time when the Mn oxides were formed. We performed laboratory experiments to generate Mn precipitates from Mn2+ in solutions by introducing CO2/O2 gas mixtures. We investigated the compositions of Mn precipitates under various compositions of CO2/O2.
Materials & Methods
The Mn2+ starting solution with 20 mM and pH 8–9 was prepared in an Ar-purged glovebox, where pO2 remained < 10-12 bar. The starting solution was deaerated by pure Ar gas for more than 6 hours prior to the use. Then, we introduced gas mixtures of pure CO2 and artificial air (N2/O2 = 4; pCO2 < 1ppm) into the starting solution at four different mixing ratios (CO2/O2 = 2, 0.2, 0.02, and artificial air) in the glovebox. Note that MnO2 is thermochemically stable under all of these conditions. Solution samples were collected in several times during the experiments. The samples were filtered through a membrane with pore size of 220 nm. After the reactions, Mn precipitates were collected by filtering the rest of the solutions using a membrane with 220 nm. Mn2+ concentrations of the filtered solution samples were measured using inductively-coupled plasma atomic emission spectroscopy (ICP-AES). The collected Mn precipitates were analyzed with X-ray absorption fine structure (XAFS) and X-ray diffraction (XRD).
Results
Our results of the ICP-AES analysis show that Mn2+ concentrations in the filtered solutions decrease over reaction time, which indicate that a part of dissolved Mn2+ was converted into solid precipitates. Despite both the wide range in CO2/O2 ratios and thermochemical stability of MnO2 under the experimental conditions, the results of XAFS analyses show that all of the Mn solid precipitates formed under these conditions are mainly composed of Mn carbonate, namely MnCO3. These results are consistent with our XRD results. Our results show that MnCO3 precipitated before the formation of MnO2 even very low CO2/O2 of 0.02. This suggests that kinetics of formation of MnCO3 and Mn oxides are the critical factor. On the other hand, the major peaks of the XANES spectra for the collected solid precipitates at CO2/O2 = 0 (namely, pure artificial air) would be a mixture of Mn oxides and Mn(OH)2.
Discussion
Our results show that, in order to form MnO2 in Mn2+ solutions by reactions with CO2/O2 gas mixtures, the CO2/O2 ratio should be lower than 0.02. Assuming pO2 of ~0.01–0.2 bar, which is capable to form and preserve MnO2 in sediments [3], the observations of both a lack of MnCO3 and presence of MnO2 in Gale infer that pCO2 on early Mars would have been less than 0.004 bar, or 4 mbar. This implies that early Mars may have possessed a low-CO2 and high-O2 atmosphere.
[1] Ehlmann, B.L. et al. (2011). Nature 479, doi:10.1038/nature10582.
[2] Lanza, N.L. et al. (2016). Geophys. Res. Lett., 43, 7398-7407.
[3] Shaw, T. et al. (1990). Geochim. Cosmochim, Acta 54, 1233-1246.