Two pictures taken a half-hour apart by the Viking 1 Orbiter show the development on Mars of early morning fog in low spots, such as crater and channel bottoms (see arrows on picture at right). Scene at left was photographed shortly after Martian dawn on July 24 from 12,400 kilometers (7700 miles) and, at right, 30 minutes later from 9800 kilometers (6100 miles). Slight warming of the sub-zero surface by the rising sun evidently has driven off a small amount of water vapor which has recondensed in the colder air just above the surface. Brightness measurements of the resulting fog patches indicate that a film of water about one micrometer (one-millionth of a meter) thick has condensed.. These fog patches are the first direct visible evidence as to where the exchange of water between the Martian surface and atmosphere is actually occurring
