First Detection and Thermal Characterization of Terminator CO2 Ice Clouds With ExoMars/NOMAD

We present observations of terminator CO2 ice clouds events in three groups: Equatorial dawn, Equatorial dusk (both between 20 degrees S and 20 degrees N) and Southern midlatitudes at dawn (45 degrees S and 55 degrees S east of Hellas Basin) with ESA ExoMars Trace Gas Orbiter's Nadir and Occultation for MArs Discovery instrument. CO2 ice abundance is retrieved simultaneously with water ice, dust, and particle sizes, and rotational temperature and CO2 column profiles in 16 of 26 cases. Small particles (<0.5 mu m) prevail at dusk, while water ice likely provides most source nuclei at dawn. Clouds east of Hellas are found to be dominantly nucleated on surface-lifted dust. CO2 ice is sometimes detected in unsaturated air together with dust nuclei at dawn, suggesting ongoing sublimation. Depending on latitude and local time, the interplay between particle precipitation and the lifetime of temperature minima (i.e., cold pockets) determines CO2 ice properties.

Automated summary

The study observed terminator CO2 ice cloud events at different locations and times, finding that water ice may provide most of the cloud nuclei at dawn, while small particles dominate at dusk. Additionally, it was found that some clouds are nucleated on surface-lifted dust.