The Soils of Mars: Physical, Elemental, and Mineralogic Properties.

Historical telescopic observations, fifty years of flyby and orbital spacecraft data, and now nearly twenty years of in situ measurements from landers and rovers have painted a detailed and complex picture of fine-grained regolith materials on Mars. Collectively, these materials are often referred to as "soils" in the planetary science community, taking advantage of a broader definition of the term that does not necessarily require the involvement of organic matter. Martian soils occur in a variety of now-common classes. These include: (1) Extremely fine-grained (clay-sized) dust particles that can be entrained in the thin atmosphere and transported both locally and globally in the current climate regime by dust devils and larger-scale storm systems. While chemical and mineralogic analyses of dust has proven problematic, these micron-sized (on average) grains appear to be predominantly basaltic in composition volumetrically, with perhaps only thin surface veneers of oxidized ferric iron giving them their characteristic reddish-brown color. (2) Well-sorted, dark basaltic sand grains, transported locally and regionally by aeolian saltation into an impressive variety of ripple and dune bedforms, many of which appear to be actively migrating today, albeit extremely slowly.  Compositionally, these sand grains have been studied in detail at several rover locations, including with XRD. Their crystalline component is basaltic in composition (plagioclase, olivine, pyroxene), but these grains also contain a significant X-ray amorphous component, potentially nanophase ferric oxide. (3) A variety of hydrated sulfate- and/or silica-rich soils occurring in very localized deposits in the shallow subsurface (and exposed by rover wheels) that might have been formed as either leachates or sinter in past ancient aqueous, and potentially hydrothermal, environments.  Other less common kinds of fine-grained regolith deposits have been encountered on Mars, including localized sand, silt, and gravels associated with mechanical erosion of rocks (including sandstones and mudstones), conglomerates consisting of sand and gravel bonded by a still-unknown cementing agent, and 1-3 mm-sized iron-rich spheroidal grains widely interpreted as concretions. Just as on Earth, the detailed study of Martian soils, and of their progenitor rocks, is providing new insights on past climate and habitability, as well as important details about the ways that these materials might be useful as natural resources for the future human habitability of the Red Planet.