Evidence of a dust storm passing over the Viking 1 landing site captured by the lander's camera. The sols indicate the number of martian days spent at the surface by the lander. The storm was captured by the camera a little bit more than 2 and half year after Viking 1 landed on Chryse Planitia. Notice how much the visibility and light is reduced at the surface. Dust storms, and aeolian phenomena in general, will be serious environmental hazards to consider for future human missions to Mars.
Source: NASA/Viking Science Team