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Water vapor is a trace gas in the Martian atmosphere and has huge spatial, diurnal and seasonal variability. Measurements made by Viking orbiter in the late 1970s suggested that the entire global total mass of water vapor is equivalent to about 1 to 2 km3 of ice. More recent measurements by "Mars Express" orbiter showed that the globally annually-averaged column abundance of water vapor is about 10-20 precipitable microns (pr. μm). Maximum abundance of water vapor (50-70 pr. μm) is found in the northern polar regions in early summer due to the sublimation of water ice in the polar cap.