The atmosphere of Planet Mars is much thinner and contains a lot less water vapor than that of Earth’s. Even so, understanding water vapor’s behavior on Mars is important because of its role in the hydrological cycle. Maltagliati et al. (p. 1868; see the Perspective by Heavens) measured the vertical distribution of atmospheric water vapor during portions of the northern spring and summer of one Mars year and compared them with the predictions from global climate models. The amount of water vapor at 20- to 50-kilometer altitude was much higher than expected, in excess of saturation, implying that the water vapor’s profile height is not controlled by the saturation vapor pressure. This supersaturation is likely to be the result of inefficient condensation of water vapor into ice because of the presence of clouds.
SOURCE : SCIENCE MAGAZINE VOL333