[P-PS09] 火星と火星衛星

[PPS09-P15] Transformation of Ferrihydrite to Crystalline Fe Oxides in Salt Solutions: Implications for Effects of Water Chemistry on the Rare Detection of Goethite on Mars

*深谷 創¹、福士 圭介²、高橋 嘉夫³ (1.金沢大学 大学院自然科学研究科、2.金沢大学 環日本海域環境研究センター、3.東京大学 大学院理学系研究科 地球惑星科学専攻)

キーワード：火星、ヘマタイト、ゲーサイト、フェリハイドライト

Mars explorations by rovers and satellites have shown that liquid water once existed on Mars (e.g. Ehlmann et al., 2011). Iron oxides were formed by water/rock interaction during aqueous activities on early Mars. Ferrihydrite is a poorly crystalline iron oxide which is metastable with respect to hematite and goethite (Cornell and Schwertmann, 2003). Hematite and goethite are stable crystalline iron oxides produced by the transformation of ferrihydrite. Both crystalline iron oxides often form simultaneously and competitively from ferrihydrite because of their similar thermodynamic stabilities (Cornell and Schwertmann, 2003). It has been generally documented that the transformation of ferrihydrite to hematite and goethite is controlled by water chemistry. Hematite predominates at neutral or strongly acidic pH, while goethite predominates at acidic or alkaline pH conditions (Schwertmann and Murad, 1983). Fukushi et al. (2022) reported that ancient liquid water from Gale crater during temporary wet events at about 3-2 Ga was acidic at about pH 3-5. Although goethite formation should predominate under these pH condition, it has been poorly identified in the Gale sediment. In addition to pH, Fukushi et al. (2022) also characterized the liquid water at Gale crater as having a high salinity. It has been reported that salinity and cation/anion types of the solutions can affect the ratio of hematite and goethite produced by ferrihydrite (Torrent and Guzman, 1982; Baltpurvins et al., 1996; Zhang et al., 2018). Therefore, we hypothesized that the presence of goethite, which should form competitively with hematite, is scarcity in Martian sediments due to the effect of high salinity. However, the transformation of ferrihydrite where both pH and salinity (ionic strength) are controlled has not been studied and a systematic understanding of the transformation behavior is needed. In this study, we examined the transformation of ferrihydrite as a function of pH and ionic strength (I) with NaCl, MgCl₂, CaCl₂, and Na₂SO₄ media by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). This understanding may have implications for the scarcity of goethite and the ancient water chemistry on Mars.
Hematite was dominant at neutral pH conditions in all media independent of ionic strength. Under acidic pH conditions, goethite was predominant at lower ionic strength (I < 0.01 M) in NaCl, MgCl₂, and CaCl₂ media. However, the ratio of hematite increased systematically with increasing ionic strength, and only hematite was obtained at the highest ionic strength (I = 1 M). In addition to these media, hematite formation was more favored with ionic strength at acidic pH condition in Na₂SO₄ media. These results suggest that goethite formation can be prevented even under acidic condition by the effect of ionic strength. Previous studies have generally reported that hematite is formed by the combination of aggregation, dehydration, and atomic rearrangement processes of ferrihydrite particles (e.g. Cornell and Schwertmann, 2003). Under acidic condition, anions tend to adsorb onto the surface of ferrihydrite particles as the surface charge of ferrihydrite particles is positive, which can lead to rapid aggregation due to the increase in ionic strength (Liu et al., 2019). Therefore, our results obtained from acidic and high ionic strength conditions indicate that hematite formation was promoted by the strong aggregation of ferrihydrite particles. The poor detection of goethite on Mars can be explained by the presence of liquid water with relatively high salinity on ancient Mars.