As previously mentioned, dust storms occurred in the southern hemisphere during the extended mission, creating large dust clouds which eventually covered much of the planet. In addition, as the northern winter approached and temperatures declined, ice condensed on atmospheric dust particles. These two effects, which could change over a few hours, conspired to make prediction of apparent Sun brightness very difficult. We encountered a problem well known to all photographers: trying to guess light levels. Our problem was unusually difficult, however, because we had to prepare the camera commands a week or more in advance of the actual picture. Sometimes we guessed wrong and pictures were either overexposed or underexposed.
Figure 172, taken during the extended mission, is generally darker and shadows are more diffuse than equivalent images taken during the early mission. This is the result of increased atmospheric opacity and scattering of light by suspended dust particles. Note, in particular, the "soft" circular shadow of the S band antenna.
Figures 173, 174, and 175 record variations in apparent brightness of the Sun, imaged directly with the Sun diode. Because only the Sun is bright enough to be recorded, the pictures are relatively unimpressive, although scientifically useful. Figure 173 shows the Sun in a relatively clear sky. Figure 174 shows a darkening by a factor of more than 2, the Sun actually being invisible. Figure 175 shows a partial return to clear conditions.