CHEMICAL-COMPONENTS OF THE MARTIAN SOIL - MELT DEGASSING, HYDROTHERMAL ALTERATION, AND CHONDRITIC DEBRIS

On the basis of measurements by the Viking and Soviet Phobos missions the fine-grained surficial drift material on Mars is enriched in moder ately volatile and mobile elements such as S, Cl, K, and Br. The compo unds of these elements may amount to as much as 16 wt % of the soil, w hile the bulk of the soil appears to consist primarily of crustal mate rial whose composition is similar to that of the basaltic Mars meteori tes (Clark, 1993). Three possible geochemical sources (melt degassing, hydrothermal alteration, and accretion of chondritic material) may ha ve contributed to the enrichments. We have evaluated possible chemical signatures for these sources by examining the relative abundances of chemical elements in the different components to the measured K abunda nce in the Mars soil and have identified characteristic elements for e ach component. The relative abundances of mobile elements in the diffe rent components are much less variable than their absolute abundances, which strongly depend on conditions such as temperature and water com position. The volatility controlled elemental signature of the melt de gassing process includes enrichments of Zn, Mo, Cd, Ba, and W, relativ e to the elemental abundances in the other components and Martian mete orites. Hydrothermal alteration of minerals and glass, controlled by l ow-temperature solubility, makes up a second component with a characte ristic lithium enrichment signature. A chondritic component is enriche d in siderophile and chalcophile elements including Ni, compared to ma terial derived from the Martian mantle, which was depleted in sideroph ile elements due to core formation. Unfortunately, the existing data f or the Mars soil do not include any of these key signature elements, a nd the uncertainties for the Viking analyses could allow large amounts of each of these elements. The Mars Pathfinder inorganic analysis exp eriment and the 2001 orbital gamma ray experiment have the potential t o determine some of these characteristic elements and to reveal the po ssible contributions to the soil. The enriched components of the soil, especially Cd and Pb, could also be a potential hazard for future Mar tian explorers who will be exposed to the dust carried into their habi tats on their space suits.