Seasonal variation of aerosols in the Martian atmosphere

Reanalysis of Viking Lander (VL) visible and Viking Orbiter infrared optical depth measurements shows that the visible to infrared ratio of total extinction opacity varies with season. The ratio is near to its previously reported constant value, 2.5, during dust storm periods and higher during northern spring and summer. The increase in ratio is hypothesized to be due to seasonally varying water ice haze, which produces a higher optical depth in the visible than in the infrared. This differs significantly from previous analyses or VL visible opacities which have assumed that only dust contributes to the optical depth measured during the early afternoon. Consequently we suggest that the Martian atmosphere is clearer of dust, especially during northern spring and summer, than previously suggested based upon VL data. We find dust visible optical depths of 0.1-0.4 during the northern spring and summer seasons, compared to previous estimates of 0.4-0.6. We also find that water ice hazes can provide roughly 50% of the total visible opacity in these seasons. For southern spring and summer, dust optical depths are more variable, but generally greater than or equal to 0.4, with water ice opacity less than or equal to 0.1. The data suggest water ice optical depths are slightly higher and peak earlier (L-s=80 degrees - 90 degrees) at VL1 than at VL2 (L-s=115 degrees-130 degrees). We estimate average northern summer water (daytime minimum) ice masses to be roughly 0.1 - 0.5 precipitable microns, depending on the assumed particle size distribution and hence 1 - 5% of the total water column. The observation of significant and previously unrecognized amounts of water ice haze suggests a larger role for water in controlling atmospheric hearing rates and the vertical distribution of dust and water vapor than has been widely accepted to date.