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Planetary Science Seminar

CO2 Condensation and Supersaturation in the Martian Atmosphere Observed by Radio Occultation Measurements
Presented by Katsuyuki Noguchi
Nara Women's University

Monday, August 19, 2019
4:00 P.M. in 183-328 and on Webex

Abstract
The Martian atmosphere mainly consists of carbon dioxide (CO2). The total pressure of the Martian atmosphere changes seasonally by 20-30% due to extremely low temperatures in the polar night regions, where CO2 condenses to form dry ice clouds in the atmosphere and the polar caps on the surface. Supersaturation of CO2 is also frequently detected in those regions, and its influence on regional meteorology has been investigated by observations and numerical simulations.

Radio occultation (RO) is a powerful tool to investigate thermal structures of planetary atmospheres since RO can obtain vertical profiles of temperature with a precision of 1K or less and high vertical resolution (≤1 km). RO needs the information of atmospheric composition for the retrieval of temperature and typically utilizes a standard global composition for the Martian atmosphere. However, CO2 condensation in the winter polar regions lowers the local concentration of CO2 and increases the mixing ratio of the non-condensable species N2 and Ar. Hence RO measurements in the Martian polar night require the information of the local change in the atmospheric composition.

In the first half of this talk, I will show a method to estimate the composition of the Martian atmosphere during the polar nights. It utilizes the Ar measurements by the Gamma Ray Spectrometer aboard 2001 Mars Odyssey. The estimation of the composition changes was applied to rederive temperature profiles from Mars Global Surveyor RO measurements. The rederived profiles indicate that use of the standard global composition overestimated the temperature by ~5 K in midwinter, when the largest CO2 depletion occurred and the CO2 volume mixing ratio fell to 78%. We found a larger degree of CO2 supersaturation and a more frequent occurrence in the rederived temperature profiles than in the original MGS profiles.

In the second part of the presentation, I will talk about the results of the MGS RO data analysis that reveal the role of atmospheric waves on the CO2 supersaturation in the Martian polar regions. A distinct longitudinal dependence (wavenumber 2) of CO2 supersaturation was observed at a pressure level of 200–400 Pa or lower, where a stationary wave lowered the background temperature to a level close to the CO2 saturation temperature. However, the stationary wave alone was not sufficient to cause the observed CO2 supersaturation. Additional temperature disturbances caused by transient waves had a significant role in the creation of CO2 supersaturation.

JPL Contact: Armin Kleinboehl


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