Infrared observations of Mars by the Viking infrared thermal mapper (IRTM) are presented for both conditions of a relatively clear and a dust-laden atmosphere. The 7-, 9-, 11-, and 20-μm bands of IRTM respond differently to radiation emitted through and by a dusty atmosphere, permitting characterization of the global atmospheric state, monitoring of secular changes, and derivation of optical depth information. Surface temperature behavior is found to be greatly modified by the diminution of insolation and thermal blanketing resulting from global dust storms. Brightness temperature at 7 μm (T7) is employed to estimate surface temperatures in the presence of dust absorption. The difference T7-T9 is strongly indicative of airborne dust when thermal contrast exists between the surface and atmosphere. The diurnal behavior of T7-T9 reveals changes in that contrast; the sign of the differential reverses as the surface, warmer than the atmosphere in daytime, becomes cooler than the atmosphere at night. IRTM observations of local areas at varying emission angle yield optical depths indicative of global trends. Two global dust storms in 1977 produced large optical depth changes; at 9 μm the optical depth became as large as 2.0.