Role of stationary and transient waves in CO₂ supersaturation during northern winter in the Martian atmosphere revealed by MGS radio occultation measurements

The Martian atmosphere, which mainly consists of carbon dioxide (CO₂), is characterized by extremely low temperatures that cause CO₂ gas to freeze and dry ice to form. To date, temperatures below the CO₂ saturation temperature, which can be attributed to the effects of atmospheric waves, have been observed in the polar winter and in the mesosphere. Using data from Mars Global Surveyor (MGS) radio occultation measurements, we investigated the role of large-scale atmospheric waves including stationary and transient waves at northern high latitudes in winter on CO₂ supersaturation. A distinct longitudinal dependence of CO₂ supersaturation was observed at altitudes higher than the pressure level of 200–400 Pa, where a stationary wave with a wave number of 2, whose temperature amplitude had minima at 30–100 Pa, lowered the background temperature to a level close to the CO₂ saturation temperature. However, the stationary wave alone was not sufficient to cause CO₂ supersaturation. Additional temperature disturbances caused by transient waves, namely, superposition of both waves, had a significant role in CO₂ supersaturation. The longitudinal dependence of the occurrence of CO₂ supersaturation revealed by our study might affect the longitudinal distribution of CO₂ snowfall and the formation of the seasonal polar ice cap.