Electrostatic Precipitation in the Martian Environment

Because the Martian atmosphere is composed mainly of ${\mathrm{CO}}_2$, electrochemical conversion of ${\mathrm{CO}}_2$ to ${\mathrm{O}}_2$ is one of the most efficient methods of extracting oxygen, which is indispensable for astronauts to breathe and can be used as an oxidant for rocket engines. In order to realize a reliable oxygen production system, a large amount of ${\mathrm{CO}}_2$ gas must be accumulated and compressed in an electrolytic conversion system. However, dust present in the Martian atmosphere because of dust storms could damage the oxygen conversion system or cause it to malfunction. Thus, a dust removal system is necessary before ${\mathrm{CO}}_2$ gas can be introduced into the system. To this end, an electrostatic precipitator suitable for use in the low-pressure Martian atmosphere has been developed. First, a precipitator consisting of a wire and parallel-plate electrodes was constructed. In a preliminary study, the efficiency of dust removal was 75%–80% without corona discharge, and almost no dust was collected at the corona discharge region in the low-pressure (700 Pa) ${\mathrm{CO}}_2$ atmosphere that simulated the Martian atmosphere. In this case, dust was collected on the surface of the wire electrode, contrary to the case for a pressure of ${10}^5\mathrm{Pa}$ (1 atm). The mechanism of these phenomena was investigated by direct observation and numerical calculation of particle motion in the precipitator. It was clarified that the low charge density of particles in the low-pressure atmosphere caused a relatively large dielectrophoresis force in comparison with the Coulomb force, and the particles were attracted to the wire electrode, i.e., the dielectrophoresis forces dominate in the low-pressure regime, whereas Coulomb forces dominate in the high-pressure regime. Poor performance in the low-pressure atmosphere was caused by the low charge density and low electrostatic field owing to the low limited voltage. Although the performance deteriorated rapidly in the low-pressure atmosphere because of the deposition of dust on the fine wire electrode, it was easy to vibrate the wire electrode and remove dust on the wire.