Martian Subsurface Waters: Alkaline and Reduced throughout History

Introduction: Throughout the history of Mars, aqueous solutions should have existed at some depths below the surface. On early Mars, high geothermal gradients and intensive volcanic activity would have favored near-surface existence of aqueous solutions and sapping of ground water (e.g., [1]). During later epochs, solutions could have been stable deep below the surface, usually below ice-bearing rocks, and only episodically released on the surface, as at present. Valley networks and especially outflow channels demonstrate the effects of released waters on surface morphology. Release of subsurface waters should also have affected surface chemistry and mineralogy via processes of water-atmosphere interaction, dissolution of surface minerals, exchange of elements between solutions and minerals, and mineral deposition. Despite the importance for surface chemistry and mineralogy, the composition of deep martian subsurface waters is not known. Here we argue that in contrast to surface and nearsurface aqueous solutions, which could have been episodic, oxidized, and acidic; deep subsurface waters are likely to have been alkaline, reduced, and may have contained dissolved H2, CH4 and organic species of abiotic and/or biotic origin. Surface solutions: acidic, oxidizing, but episodic: Acid volcanic aerosols and near-surface aqueous oxidation of sulfides could have caused “acid weathering” [e.g., 2, 3] of near-surface martian rocks and may account for the formation of jarosite and other sulfates, and the deficiency of carbonates. Despite its potential influence on surface mineralogy, the impact of acid weathering could be very shallow. First, the amounts of SO2, H2S, and HCl in volcanic gases are limited by their solubilities in magmas. Only a minor fraction of volcanic rocks can be weathered by volcanic acids (aerosols, gases, or solutions) because of mass balance constraints. Second, rapid interactions of acidic solutions with minerals should result in neutralization. Third, acid lakes on Earth are only found in calderas in active volcanoes [4] or in locations sourced by acid springs [5] and they are not likely to be a widespread phenomenon on Mars. Fourth, the formation of acidic conditions near the surface is limited by low concentrations of O2 (needed to oxidize S and Fe), by the deficiency of massive sulfide deposits, as well as by a deficit of nearsurface aqueous solutions throughout history. Fifth, there are no obvious signs of acidic weathering in Martian meteorites. Finally, volcanic activity and weathering of massive sulfides occur locally on Earth and the same should be true on Mars. Therefore, acidic conditions were probably limited in space and time and “acid weathering” was probably a local nearsurface phenomenon. Subsurface waters: alkaline and reduced. Water-rock reactions, rather than interactions with atmospheric gases, determine the composition of deep subsurface solutions. The presence of olivine in the martian soil [6, 7] and the high abundance of mafic and ultramafic rocks on Mars inferred from remote sensing data and the compositions of martian meteorites imply that subsurface weathering on Mars probably resembles that on Earth. On Earth, low-temperature aqueous alteration of olivine-bearing rocks leads to dissolution of olivine and pyroxenes and formation of alkaline solutions that are often rich either in Mg (at early stages of alteration) or Ca, and also have elevated concentrations of Na and Cl [8-10], as illustrated in Table 1. Subsurface weathering of basalts also leads to moderately alkaline solutions [11, 12]. Aqueous alteration of terrestrial mafic, and especially ultramafic, rocks demonstrates strong correlations between solution chemistry and secondary mineralogy. Thermodynamic models are highly applicable to chemistry of aqueous solutions formed through alteration of ultramafic rocks, as shown by application of reaction path modeling [10, 13]. Our reaction path calculations, aimed at modeling weathering of olivine-bearing rocks on Mars demonstrate the formation of alkaline solutions and reveal a sharp difference in the oxidation state of solutions and in secondary mineralogy between subsurface alteration (closed system) and atmospherically influenced near-surface weathering (open system), as shown in Table 2. Note that magnetite is among the alteration products in the closed system, but that goethite is present in the open system results. This is consistent with the presence of H2 in the closed system fluid, and O2 in the open system fluid composition. Note also that bicarbonate, CO2, and various carbonate complexes are present in the open system results, but that graphite dominates the speciation of carbon in the closed system. The appearance of graphite in the calculations suggests that metastable organic compounds may also be reaction products. In fact, a variety of organic solutes appear in the results if the formation of graphite is suppressed (c.f. [14]), consistent with detection of CH4 and other hydrocarbons in serpentinites [9, 15, 16]. The results in Table 2 demonstrate that Fe in olivine can be oxidized even if O2 is not involved. In fact, water plays a major role in oxidation of ferrous silicates, as exemplified by oxidation of ferrous iron in olivine