Using observations from the SPICAM instrument on the European Space Agency's Mars probe Mars Express, French scientists discovered that the amount of water vapor in the upper Martian atmosphere sometimes far exceeds expectations. While some water vapor is formed from sublimation (evaporation) off ice on the Marian surface, the amount found in the upper atmosphere is greater than the temperature would predict. In terms of relative humidity, the humidity of the air was observed to approach almost as much as 1000%! Scientists call such air "supersaturated" in water, since the air is holding far more water than it would otherwise.
How could so much extra water end up in the atmosphere of Mars? The authors suggest that the low pressure and lack of dust particles in the upper atmosphere make condensation into ice very difficult, so that the water simply stays in vapor form. Simultaneous measurements of the amount of dust show that this could indeed be the case. These results fundamentally change scientists understanding of the water cycle on Mars as water vapor exists in much higher concentrations at higher altitudes than previous thought. Greater amounts of water vapor in the upper Martian atmosphere imply that a larger amount of water is able to escape Mars's gravity than previously thought. Models of the chemistry of the Martian atmosphere are also affected by water - despite its relatively low amounts (even with this result) water acts as a catalyst in many chemical cycles in the Martian atmosphere.
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