Viking image mosaic of the beginnings of the 1977 Mars global dust storm. In this mosaic, a regional dust storm is sweeping south off the Thaumasia Plateau. This storm began in the polar Acidalia Planitia region and swept south across Valles Marineris towards the Argyre impact basin. Upon reaching the Martian southern highlands it exploded in size. This is a typical storm track for global dust storms. This image was acquired on February 17, 1977, during Viking Orbiter 2's 176th revolution around Mars. The mosaic makes use of three pairs of RED/VIO filter images.