Variability of the Martian Homopause: Insights from Global Climate Modelling and Empirical Estimates

The homopause marks the transition in a planetary atmosphere from turbulent mixing, which maintains a well-mixed composition, to molecular diffusion, which causes the diffusive separation of chemical species, impacting how these are distributed in the upper atmosphere and potentially escape to space. Here, we analyse simulations from the Mars Planetary Climate Model (Mars PCM) to investigate the variability of the Martian homopause in diurnal, seasonal and interannual timescales. The simulations reveal strong seasonal and latitudinal trends, with homopause altitudes peaking in the summer polar regions (∼120-130 km) and showing minimum values during the winter southern polar region (∼60-90 km). The simulations predict diurnal variations in the homopause altitude typically within 5-15 km and suggest that dust events can raise the value of the homopause altitude by 10-20 km, depending on the intensity of the event. When comparing the Mars PCM results with empirical estimates of the homopause altitude and density derived from Martian atmospheric data, we find that the model captures the overall magnitude and seasonal/latitudinal variability of the homopause, but appear to underestimate the strength of the diurnal cycle. Finally, we use the Mars PCM data to constrain the variability of the eddy diffusion coefficient, which can vary by one or two orders of magnitude across latitude and season. The derived parameterisation and variability of the eddy diffusion coefficient is suitable for use in one-dimensional models devoted to understanding seasonal difference in atmospheric photochemistry and escape.