PerspectivePlanetary Science

Sunshine on a Cloudy Forecast

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Science  30 Sep 2011:
Vol. 333, Issue 6051, pp. 1832-1833
DOI: 10.1126/science.1212490

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Atmospheric water vapor was the first water detected on Mars (1). There is too little of it to bolster the weak martian greenhouse effect or to drive moist convection (2), yet the ice clouds that condense from it affect the intensity of Mars' atmospheric circulation through radiative feedback (3). At the same time, atmospheric transport in both the solid and vapor phases mediates exchange between various ice deposits at and under the surface (4). Modelers of the martian water cycle have so far relied on observations of the total amount of water vapor and water ice in the atmospheric column to validate their simulations (4). However, no single model can simultaneously capture cloud radiative feedback while matching the seasonal cycles of water vapor and clouds, a limitation fundamentally rooted in cloud overprediction (46). On page 1868 of this issue, Maltagliati et al. (7) may have discovered the reason for these overly cloudy forecasts by demonstrating that portions of the martian atmosphere are supersaturated with respect to water vapor. Models assume that water vapor condenses as ice whenever air is saturated with respect to water vapor (4, 7). If this assumption is wrong, cloud prediction problems have an obvious explanation.