Water and the Martian W cloud.

The diurnal brightening of the W cloud region of Mars during the flyby of Mariners 6 and 7 is likely due to the formation of water ice clouds. The water required in the cloud and as vapor in the atmosphere to produce the observed brightening is estimated to be roughly between 0.1 and 10 microns. Either a qualified local source with this daily production over the W cloud area or a more probable daily uplift and saturation of existing atmosphere are compatible with observations.