EOLIAN GEOLOGY: THE ORIGIN OF SAND AND DUST ON MARS (Invited Presentation)

Eolian features and bedforms are ubiquitous on Mars, indicating a large supply of particles that can be moved by the wind. Global thermophysical data and surface exploration at seven landing sites show that sand is the dominant particle size of soils that make up the surface layer on Mars, which is a global layer of order a meter thick. Unlike the Earth, sand on Mars is dominated by basaltic particles and primary igneous minerals, which are particularly susceptible to aqueous alteration, indicating that the production of sand on Mars is primarily due to physical processes. Study of the Spirit and InSight landing sites reveal a ~10 m thick regolith that developed on top of Hesperian basalt flows dominantly by impact and eolian processes. Microscopic images of sand on Mars show that the larger grains that can be resolved are rounded to sub-rounded and equant and have been rounded by saltation collisions. Measurements of rock and sand size-frequency distributions (10 m to 0.6 mm) at the landing sites can be described by negative binomial functions from fragmentation that is controlled by the number of fragmentation events (meteorite impacts) and the probability of failure during each event. The size-frequency distribution of smaller particles down to ~100 nm can be fit by a process in which multiple collisions have a very small probability of successful fragmentation from saltation of sand size particles. This agrees with arguments that collisional attrition during saltation rapidly decreases as the kinetic energy of sand particles decreases with size, which leads to poorly sorted eolian deposits dominated by fine sand. Collisions of sand grains during saltation can easily chip off the sharp edges of the particles thereby creating the dust particles (1-10 μm) and producing the rounded sand particles. A similar process occurs on Earth, where saltation of quartz sand in northern Africa, creates deposits of quartz rich desert loess (silt) downwind as well as dust aerosols (1-10 μm) that are suspended in the atmosphere and deposited elsewhere. Mars dust is also entrained and cycled through the atmosphere and preferentially deposited in dusty regions. These results indicate that meteorite impact can efficiently produce coarse sand size particles and that eolian saltation reduces the particles down to dominantly fine sand and produces dust.