Variations in Vertical CO/CO2 Profiles in the Martian Mesosphere and Lower Thermosphere Measured by the ExoMars TGO/NOMAD: Implications of Variations in Eddy Diffusion Coefficient

Using the Nadir and Occultation for MArs Discovery instrument aboard Trace Gas Orbiter, we derived the CO/CO2 profiles between 75 and 105 km altitude with the equivalent width technique. The derived CO/CO2 profiles showed significant seasonal variations in the southern hemisphere with decreases near perihelion and increases near aphelion. The estimation of the CO/CO2 profiles with a one-dimensional photochemical model shows that an altitude-dependent eddy diffusion coefficient better reproduces the observed profiles than a vertically uniform one. Our estimation suggests that the eddy diffusion coefficient in L-s = 240-270 is uniformly larger by a factor of similar to 2 than that in L-s = 90-120 in the southern hemisphere, while they are comparable in the northern hemisphere. This fact demonstrates that the eddy diffusion coefficient is variable with season and latitude.

Automated summary

Using data from the Nadir and Occultation for MArs Discovery instrument aboard the Trace Gas Orbiter, we analyzed the CO/CO2 profiles in the Mars atmosphere between 75 and 105 km altitude. The derived profiles showed significant seasonal variations in the southern hemisphere, with decreases near perihelion and increases near aphelion. Our estimation using a one-dimensional photochemical model revealed that the eddy diffusion coefficient varies with altitude, showing larger values in the southern hemisphere compared to the northern hemisphere.