Diurnal variations in optical depth at Mars

Abstract

Viking lander camera images of the Sun were used to compute atmospheric optical depth at the two sites over a period of 1 1 3 Martian years. The complete set of 1044 optical depth determinations is presented, with error estimates. Optical depths in the morning ( AM) are generally larger than in the afternoon ( PM). The AM-PM differences are ascribed to condensation of water into atmospheric ice aerosols at night and their evaporation in midday. A smoothed time series of these differences shows several seasonal peaks. These are simulated using a one-dimensional radiative-convective model that predicts Martian atmospheric temperature profiles. A calculation combining these profiles with water vapor measurement from the Mars atmospheric water detector (MAWD, on the Viking orbiters)is used to predict when diurnal variations of water condensation should occur. The model reproduces a majority of the observed peaks and shows the factors influencing the process. Diurnal variation of condensation is shown to peak when the latitude and season combine to warm the atmosphere to the optimum temperature, cool enough to condense vapor at night, and warm enough to cause evaporation at midday. The diurnal variation is enhanced by increased water vapor and is sometimes enhanced, sometimes diminished, by enhanced dust loading, depending on the other conditions. Often the model predicts condensation only at altitudes of 25 km or more, while at other times the condensation reaches ground level. Agreement between model and observations is also evident on a time scale of hours, when the data are available at more than two times in a single day.