Wave-Induced Eddy Diffusion and the Turbopause Height in the Martian Atmosphere

Abstract In this paper, we examine the altitudes of the middle and upper atmosphere of Mars, specifically the mesospheric–thermospheric range of 80–140 km. These altitudes include a transition zone called the turbopause, where the diffusion regime of atmospheric parcels changes from turbulent to molecular. Variations of the turbopause altitude and temperature are influenced by seasonal variability and vertical energy transfer due to global circulation and atmospheric gravity waves. Our research is based on the analysis of temperature and CO 2 density vertical profiles measured during solar occultations by the Atmospheric Chemistry Suite (ACS) on board the Trace Gas Orbiter (TGO) of the ExoMars-2016 mission. The statistics of the analyzed profiles cover seasons spanning 3.5 Martian years (MY), from the middle of MY 34 (May 2018) to the end of MY 37 (November 2024), with atmospheric altitudes ranging from 10 to 170 km. Based on the processed vertical wave profiles, the coefficients of molecular and turbulent diffusion were estimated assuming saturated gravity waves. The derived turbopause height varies from 90 km at aphelion to 135 km at perihelion with its CO 2 density ranging from 10 10 to 10 12 cm –3 and diffusion coefficients ranging between 10 6 and 5 × 10 7 cm 2 /s. The estimated average wave-induced phase wind speed is 43 m/s while the mean-square thermal speed is 270 m/s at the turbopause layer. The obtained results could be utilized to refine circulation models in calculations of vertical parcel transport in the upper atmosphere.