Distribution and Local Time Variability of Aphelion Equatorial Mesospheric Clouds on Mars

We show for the first time that aphelion equatorial mesospheric clouds on Mars form in regions characterized by both temperature minima and high temperature variance. We have identified high-altitude high-radiance aerosol layers in limb observations of the Mars Reconnaissance Orbiter's Mars Climate Sounder (MCS) in the aphelion season at multiple local times and compared cloud occurrence rates with retrieved temperature profiles from MCS. Clouds are observed at 3 primary longitudes which correspond to tidal temperature minima and increased temperature variance driven by gravity waves, revealing the role of waves in shaping the spatial distribution of this population. We show the probability of the temperature dropping below the CO2 frost point (calculated from the temperature mean and variance) represents the spatial distribution and seasonal evolution of the clouds reasonably well. A pause in clouds from Ls=45 to Ls=90 observed here and in previous work is associated with a decrease in temperature variance, suggesting decreased wave activity may inhibit mesospheric cloud formation near solstice.