Global dust opacity mapping for Mars has been carried forward using the approach described by Martin (1986) for Viking IR Thermal Mapper data. New maps are presented for the period from the beginning of Viking observations, until LS 210¿ in 1979 (1.36 Mars years). This range includes the second and more extensive planet-encircling dust storm observed by Viking, known as storm 1977b. Improvements in approach result in greater time resolution and smaller noise than the earlier work. A strong local storm event filled the Hellas basin at LS 170¿, prior to the 1977a storm. Dust is retained in equatorial regions following the 1977b storm far longer than in mid-latitudes. Minor dust events appear to raise the opacity in northern high latitudes during northern spring. Additional mapping with high time resolution has been done for the periods of time near the major storm origins in order to search for clues to the mechanism of storm initiation. The first evidence of the start of the 1977b storm is pushed back to LS 274.2¿, preceding signs of the storm in images by about 15 hours. ¿ American Geophysical Union 1993