Chemical, Mineralogical, and Physical Properties of Martian Dust and Soil

Introduction: Global and regional dust storms on Mars have been observed from Earth-based telescopes, Mars orbiters, and surface rovers and landers. Dust storms can be global and regional. Dust is material that is suspended into the atmosphere by winds and has a particle size of 1-3 μm [1-4]. Planetary scientist refer to loose unconsolidated materials at the surface as “soil.” The term ‘‘soil’’ is used here to denote any loose, unconsolidated material that can be distinguished from rocks, bedrock, or strongly cohesive sediments. No implication for the presence or absence of organic materials or living matter is intended. Soil contains local and regional materials mixed with the globally distributed dust by aeolian processes [5,6]. Loose, unconsolidated surface materials (dust and soil) may pose challenges for human exploration on Mars. Dust will no doubt adhere to spacesuits, vehicles, habitats, and other surface systems. What will be the impacts on human activity? The objective of this paper is to review the chemical, mineralogical, and physical properties of the martian dust and soil. Chemical Properties: A host of lander and orbital missions have characterize the chemical composition of dust and soil. We will primarily focus on the results from the Alpha Particle X-ray Spectrometer (APXS) onboard the Mars Exploration Rovers Opportunity and Spirit. Opportunity has characterized the surface chemistry at Meridiani Planum for over 13 years, and Spirit obtained equivalent data over 6 years in Gusev crater. Basaltic soil and dust at all landing sites (Pathfinder, Spirit, Opportunity, and Curiosity) have similar compositions [6,7]. There are subtle differences in the alkaline and alkali earth cations, primary a reflection of different local basaltic mineralogies, e.g., feldspar vs. mafic mineralogy. Also, some soil shows enrichments of the local bedrock, e.g., the soil Doubloon in Gusev crater has elevated P from eroded high-P materials from the Wishstone/Watchtower rock classes [8]. Basaltic soil and dust on Mars have a composition similar to the average crustal composition [9]; however, soil and dust have enrichments in S and Cl (Table 1, [5,10]). Dust has a bit more Zn than soil (Table 1). The dust composition in Table 1 was derived from bright, undisturbed soils Desert_Gobi (Gusev crater) and MontBlanc_LesHauches (Meridiani Planum), from opposite sides of the planet. These surface materials have among the highest concentrations of nanophase iron oxides (npOx, see next section) and are thus our current best analyses of global aeolian dust [5]. Sulfur, Cl, and npOx have strong correlations in soil and dust (Fig. 1). These elements and phases are enriched in dust (Tables 1 & 2), suggesting that they are major components of the global dust. Recently, Berger et al. [10] have characterized the chemistry of materials collecting on the science observation tray onboard the Curiosity rover. These measurements by APXS confirmed that martian dust is enriched in S, Cl, and Fe compared to average Mars crustal composition and soil. Several unusual soils were discovered by Spirit while dragging a wheel through soil in Gusev crater. The Paso Robles class soil has high SO3 (~35 wt. %, Table 1) and the Kenosha Comets soil subclass contained very high SiO2 (~90 wt. %, Table 1) [8]. Although these types of soils are not common at other landing sites, human missions might encounter these unusual soils.