ACIDIC VOLATILES AND THE MARS SOIL

Large portions of Mars' surface are covered with deposits of fine, hom ogeneous, weathered dusty-sail material. Nanophase iron oxides, silica te mineraloids, and salts prevail in the soil. The mode of formation o f this somewhat peculiar type of soil is still far from being clear. O ne scenario suggests that weathering took place during early epochs wh en Mars may have been ''warm and wet.'' The properties of the soil are not easily reconciled with this scenario. We propose another possible scenario that attributes, in dart, the peculiar nature of the Martian dust and soil to a relatively ''young'' weathering product formed dur ing the last few hundreds of millions of years in a process that invol ves acidic volatiles. We tested this hypothesis in an experimental stu dy of the first step of acidolytic weathering of a partly palagonitize d volcanic tephra of hawaiitic lava origin, using sulfuric, hydrochlor ic and nitric acids and their mixtures. The tephra effectively ''neutr alize'' the added acidity. The protonic acidity added to the tephra at tacks the primary minerals, releasing Fe, Al, and Mg, which control th e pH, acting as Lewis-acid species of varying acid strengths. The full amount of acidity added to the tephra is stored in it, but only a ver y small fraction is preserved as the original protonic acidity. The ma jority of the added sulfate and chloride were present as salts and eas ily solubilized minerals. Well-crystallized sulfate salt minerals of a luminum and calcium were detected by powder X ray diffractometry, wher eas secondary magnesium and iron minerals were not detected, due proba bly to lack of crystallinity. The presence-of gypsum (CaSO4(.)2H(2)O) and alunogen (Al-2(SO4)(3)(.)17H(2)O) is probably responsible for the observed increased hygroscopicity of the acidified tephra and their te ndency to form hardened crusts. We suggest that if this mechanism is o f importance on Mars, then the chemically weathered component of the M artian soil consists of a salt-rich mineral mixture containing the sal ts of the anionic-ligands SO4 and CI resulting from volatiles emitted from volcanoes during more recent eruptions (up to 10(9) years B.P.). The lack of liquid water on Mars surface during that time slowed or ha lted mineralogical evolution into highly crystallized minerals having large mineral grains. The chemically weathered components are mixed wi th the products of physical weathering. The recently formed soil may c over and coat more evolved, hydrothermally modified mineral deposits f ormed in earlier epochs of Mars.