William M. Farrell
Goddard Space Flight Center
Mars Dust Storms, Electric Fields, and Associated Chemistry
It is well-known that small particles of differing composition and size will exchange charge via contact electrification or “tribo-electrification”. On Earth, mixing grains in dust devils have been found to generate electricity via tribo-electric processes. When in collision with the ground or surface, smaller grains tend to obtain a negative charge polarity while heavy grains/surface contain an equal and opposite positive charge. Due to vertical winds, the small negative grains are lofted high into the storm thereby generating a charge separation into a dipole and thus creating macroscopic electric dipole moment within the storm. Electric fields in terrestrial dust devils have been measured to exceed 100 kV/m in a coherent, dipolar configuration that is consistent with a dipole moment oriented downward (negative charge at higher altitudes). We discuss some of our desert studies of such dust devils and show observational evidence for these large fields. It is believed that similar processes occur in Martian dust devils and storms, giving rise to large dust storm E-fields. Since electrical breakdown of a low pressure CO2 gas is 20-30 kV/m, we have suggested that the storm E-fields may become the source of a mildly-ionized gas forming a possible glow discharge. Such discharges have in fact been observed in the lab. We present a model of the electron avalanche process that is suspected to occur in such electrified dust devils/storms and discuss the associated plasma chemistry that may alter the lifetime of some critical Martian atmospheric species like methane.