Role of stationary and transient waves in CO_2 supersaturation during northern winter in the Martian atmosphere revealed by MGS radio occultationmeasurements

摘要

The Martian atmosphere, which mainly consists of carbon dioxide (CO_2), is characterized by extremely low temperatures that cause CO_2 gas to freeze and dry ice to form. To date, temperatures below the CO_2 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 CO_2 supersaturation. A distinct longitudinal dependence of CO_2 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 CO_2 saturation temperature. However, the stationary wave alone was not sufficient to cause CO_2 supersaturation. Additional temperature disturbances caused by transient waves, namely, superposition of both waves, had a significant role in CO_2 supersaturation. The longitudinal dependence of the occurrence of CO_2 supersaturation revealed by our study might affect the longitudinal distribution of CO_2 snowfall and the formation of the seasonal polar ice cap.