[128] In the course of the Viking mission the Landers, carefully cleaned and sterilized before leaving Earth, slowly accumulated a layer of dirt after arrival on Mars. Some of this was soil that was spilled on the upper deck when the surface sampler dumped sediment into the three funnels leading to the biology, GCMS, and inorganic chemistry instruments.

A grid had been painted on the upper deck of the Lander especially to create a distinctive background against which accumulation and movement of dust could be measured (figs. 168 and 169). In order to more clearly map the distribution of dust, parts of these two images have been rectified as though the viewer were looking straight down on the Lander grid (figs. 170 and 171). This also allows easy comparison between images acquired by the two cameras.

We recognized early the sediment spilled from sampling operations and some purposely dumped on the deck for analysis of soil properties. However, it was several months before we realized that the swirls of yellowish sediment were changing when no surface sampler activity had occurred.

 

Figures 168 [Left] & 169

The changing sediment patterns are caused by the martian wind. Why is sediment movement observed on the Lander but not in the natural scene? Several possible reasons come to mind. Surface materials may have been disaggregated during transportation to the spacecraft deck, creating finer grains easier to move. Constriction of air flow by spacecraft components may increase wind speed and turbulence. Vibration of the upper deck may cause particles to bounce, thereby becoming more susceptible to lateral transport by wind. Finally, a similar amount of dust transport may be occurring on the martian surface, undetectable in the absence of a background as contrasting in color as the Lander top.

During the extended mission giant dust storms occurred in the southern hemisphere and spread considerable material into the northern hemisphere.

Although wind velocities at both Viking sites remained below the critical velocity necessary for initiation of sediment transport, extensive dust clouds darkened the sky. There is an indication that fine material has been settling out from those clouds, progressively covering the spacecraft with a thin layer of fine dust. We also suspect that some dust has been redistributed throughout the Viking 1 site.

 

Figure 170

 

 

Figure 171