Laboratory investigation of Martian water ice cloud formation using dust aerosol simulants

The ice nucleation abilities of submicron aerosol particles of two Martian regolith analogs, the Mojave Mars simulant and Johnson Space Center Mars‐1, were investigated with the University of Toronto continuous flow diffusion chamber. The temperature range studied (> 200 K) is relevant to low‐altitude water ice cloud formation in the Martian atmosphere and the aerosol particles were suspended in air, in contrast to previous experiments. Both simulants were found to be active ice nuclei in the deposition nucleation mode between 223 K and 203 K. The Mojave Mars simulant particles were found to be better ice nuclei than the Johnson Space Center Mars‐1 particles requiring lower supersaturations to nucleate ice. It was observed that the critical supersaturation ( S crit ) to activate 1% of the particles increased with decreasing temperature in accord with previous low‐temperature studies, rising to a value of above 1.7 at 203 K. This corroborates literature results that there is a substantial barrier to ice nucleation at low temperatures, underlining the need for incorporating this effect in Martian cloud microphysical models. It was also found that S crit did not change when the size of the Mojave Mars simulant particles was increased from 240 to 400 nm. Comparison of the Martian simulants with other mineral dusts shows that the Johnson Space Center Mars‐1 analog behaves similarly to the well‐known terrestrial ice nuclei such as kaolinite and Arizona test dust particles, whereas the Mojave Mars simulant behaves closer to another clay, montmorillonite. The wettability parameter, m , was calculated to range from 0.955 to 0.959.