New Views of Soil and Dust on Mars

A combination of geologic processes including physical (meteoritic impact, wind abrasion) and chemical (aqueous alteration, oxidation/reduction) processing of local and regional basaltic materials produce soil on Mars. The finest fraction of the soil, i.e., dust, is suspended by wind and is transported at regional and global scales and remixed with surface soil. The arrival of the Curiosity rover has significantly enhanced our understanding of the chemical, mineralogical, volatile, and physical properties of soil and dust on Mars. X-ray amorphous materials dominate the "alteration" mineralogy of soils in Gale crater and, based upon datasets returned by previous missions (i.e., Mini-TES, Moessbauer Spectroscopy, APXS, Microscopic Imager, Atomic Force Microscope, and TEGA), we can refine the chemical, mineralogical, volatile, and physical properties of dust and soil on a global scale. Most basaltic soils have compositions similar to that of the average Martian crust, but these soils and dust have enrichments in the volatile elements S and Cl, with a consistent molar S/Cl = 3.4 ± 0.4. The S and Cl, along with nano-phase iron oxides (npOx), increase in soils with increasing dust content. Volatile components (oxychlorine compounds, nitrates, carbonates, and adsorbed/structural water) are confined to the dust or finer grain-sized materials in soils. Observations by landed imagers and the APXS indicate that the dust size fraction is <5 µm. Chemical and mineralogical datasets from across all missions were used in concert with microscopic images to constrain the dust or clay-size fraction (< 5µm) of a typical soil to be 15-25 wt. % with about 75-85 wt. % of the less than 2 mm material in the silt and sand fractions. Variations in particle size depend on the amount of dust mixed with local components, e.g., less dust in active eolian dune materials compared to "hollows" filled with fine dust and soils, e.g., Laguna class soils in Gusev crater. X-ray amorphous materials (e.g., npOx, short-range order materials, SO₄- and Cl-bearing phases, and nitrates) are components of the dust and have likely formed via limited water interactions with basaltic materials and volcanic gases.