Manganese Hydroxides and Their Role in Phosphorus Nutrient Cycling on Mars

Phosphorous (P) is an essential element for life due to its use in many important biomolecules (DNA, ATP). In-situ and Martian meteorite data suggests that Mars is phosphate rich compared to Earth; however, phosphate is generally not biologically available due to its low solubility in seawater, posing an obstacle for life. Studies have shown that phosphate readily adsorbs to metal hydroxides, e.g. iron hydroxide, in seawater and at hydrothermal vent settings. Additionally, studies suggest that iron (II) chloride and iron (III) chloride mixtures can facilitate phosphite oxidation to phosphate due to Fenton chemistry that occurs in the presence of strong oxidants sometimes resulting in condensed polyphosphates. Similar to iron, Manganese (Mn) has multiple oxidation states, precipitates in hydroxide and oxide minerals in aqueous settings, and may participate in Fenton style reactions. The likely presence of Mn minerals including Mn-oxides on Mars, as well as the presence of various reactive oxidants (e.g. perchlorate, chlorate, nitrate) in the Martian surface, suggests that Mars may have the elements required for a Mn-facilitated P redox cycle that could affect geochemical availability of P for a putative biosphere. We explored these processes by synthesizing Mn-hydroxides in the laboratory and reacting them with phosphate/phosphite and oxidants. Products were analyzed using ³¹P liquid NMR and colorimetry; mineralogy was analyzed using Raman and near infrared spectroscopy. Results show that the presence and type of oxidants affects phosphate or phosphite adsorption into Mn-hydroxides, even when no P redox chemistry occurs, and Mn-hydroxides formed under different chemical conditions exhibit distinct crystalline appearances and spectral features. These results demonstrate the high adsorptive capacity of Mn-hydroxides and variability of oxidizer effects despite the absence of polyphosphate generation. These results suggest Mn hydroxides as well as surface oxidants may play a role in sequestration and concentration of P on Mars, affecting habitability of near-surface environments.