The Harsh Electro-chemical Environment in Martian Dust Storms

Introduction: Mars is a planet of many apparent dichotomies in topography, atmospheric evolution , hydrology, and geomagnetism. We describe yet another set of extremes in the area of atmospheric chemistry. While solar photons in fair-weather are a known and well-studied driver of Martian chemistry, there is increasing evidence that electro-chemistry unique to dust storms may be a second new driver of chemical reactions. This new chemical pathway features the formation of harsh, reactive trace species via recombination of products formed following electron dissociation. This new pathway may explain the presence of peroxides in the soil (Viking inferences) and possibly the low levels of atmospheric methane. The electro-chemistry has its source in the tribo-electric dust grain interactions occurring in dust devils and storms. Large E-fields are suspected to be present in dust storms by the spatial separation of small negative grains (at storm top) and large, positive grains (near surface). These E-fields in the low pressure CO 2 gas would then energize ambient electrons between < 1 eV and 50 eV, which in turn would create new electrons via CO 2 impact ionization (> 14 eV), thereby creating a collisional plasma consisting of CO 2 + ions and electrons. This plasma then drives the new chemistry. In this presentation, we review the fundamental elements of this new chemical pathway and discuss implications for the Mars environment. Electrostatic Fields in Dust Storms: Dust storms on Mars are expected to generate and maintain large-scale electric dipole fields, with interior E-field values approaching the atmospheric breakdown levels of ~25 kV/m [1-4]. The E-fields form via grain-grain contact electrification that tends to leave smaller dust particles with a net negative charge and larger sandy grains with a net positive charge [5]. Vertical winds in the storm then mass-stratify (and charge-stratify) the grain distribution as a function of height, creating a downward-directed electric dipole moment and large-scale dipole electric field structure [1,6,7]. Evidence for this charge creation/separation process is in the form of analytical models [1-4] and terrestrial analog studies [6, 8-11]. The former model-ing studies all suggest that E-fields at many tens of kV/m can be generated in a convective Martian storm with charging exponential growth time scales on the