Mixed hydrothermal fluids and the origin of the Martian soil

The Martian soil consists mainly of weathered basaltic rock and a "salt" component enriched in volatile and mobile elements, primarily sulfur, chlorine, sodium, and potassium. Since the discovery of this mobile element component by the Viking X-ray fluorescence spectrometer, volcanic aerosols have been considered a likely source for these enrichments. Alternative sources of the mobile element component are fluids supplied to the surface by hydrothermal processes related to impact craters and volcanism. We show that a mixed hydrothermal fluid model can successfully explain the composition of the mobile element component of the soil. This model is constructed using terrestrial analogs constrained by the Viking and Pathfinder measurements of the Martian soil chemistry. The potential hydrothermal fluid contributions to the soil are tightly constrained because the relative abundances of the mobile elements in hydrothermal fluids are nearly constant. The two end-member types of hydrothermal fluids considered in the model are the neutral-chloride and the acid-sulfate type. The neutral-chloride fluids have high Cl and Na contents and a significant amount of K, but their S abundance relative to Cl (S/Cl ratio) is too low to match the Martian soil. However, acid-sulfate hydrothermal systems characterized by substantial vapor transport and high S/Cl ratios are likely on Mars due to the low abundance of water. Indeed, as the availability of water declines in a hydrothermal system, the neutral-chloride system can evolve to a vapor-dominated acid-sulfate system. A combination of the two types of fluids can quantitatively explain the S and Cl abundance in the soil and provide reasonable abundances of other mobile elements, including Na, K, and Br, without predicting an excess of any element measured in the soil. However, the hydrothermal fluids are unlikely to be involved in the formation of the magnetic minerals in the soil because the Fe contents of the fluids are too low. Although the acid-sulfate fluids have Fe concentrations a factor of 100 larger than the neutral-chloride fluids, the acid-sulfate fluids would contribute less than 1% of the total Fe in the soil. The volcanic aerosol models are poorly constrained in contrast to the hydrothermal fluid models because in volcanic aerosols the relative abundances of the mobile elements are highly variable. Therefore, while volcanic aerosols can explain the mobile element component of the soil, significant contributions from other sources cannot be excluded. Determining the relative contributions of hydrothermal components compared to volcanic aerosols will require measurements in the Martian soil of trace elements that provide signatures of the hydrothermal fluids, such as high lithium contents that are associated with neutral-chloride fluids.