Mineralogy, Abundance, and Hydration State of Sulfates and Chlorides at the Mars Pathfinder Landing Site

soil-free rock rocks Introduction: Detection of elevated concentrations of S and Cl at the landing sites of Viking 1 and 2 [1], and Mars Pathfinder (MP) [2-5] reveals the presence of sulfates and chlorides in soil and rock samples [1-10]. These data are consistent with the findings of Ca sulfates and NaCl in Martian meteorites [11,12], and with Earth-based spectroscopic observations [13,14] tentatively indicating the presence of sulfates on Mars. Although the correlation of S and Mg in Viking and MP samples could reveal the occurrence of Mg sulfate [1-10], the mineralogy of sulfates and chlorides remains unclear. The hydration state of surface sulfates and chlorides is also unknown, which makes a long list of salts possible, as shown in Table 1. Taking into account likely aqueous origins of sulfates and chlorides [e.g., 7,15,16], periodic (seasonal and daily) presence of water ice condensates at a significant part of the surface, and recycling of mineral dust through polar regions could account for hydration of salts, at least at high latitudes and at depth [17]. Hydration of surface salts, which could account for up to 15-25 vol. % in Viking soils [7], could partially account for bound water in the regolith. In fact, from 0.5 wt. % to 4 wt. % of bound water is present in the soil based on ground [e.g., 13,18,19], Hubble Space Telescope [20], Mariner 6 [21-23] and Phobos 2 [24] spacecraft spectroscopic data in the near-infrared (3 μm), Mars Odyssey neutron spectroscopy [25], a Viking biological experiment [26], APXS analyses of soils and rocks at the MP landing site (based on “excess” oxygen) [4], and the clear absorption of bound water (6.1 μm) at the landing site of Spirit rover (Mini-TES data) [27]. Both hydrated silicates (e.g., zeolites, clays) and salts (sulfates, carbonates, chlorides) could be host minerals for Martian bound water.