Phoenix addresses the overarching NASA theme:Follow the Water.

   Odyssey scientists have reported that ice underlies the high latitude regions in great abundance. The TES team has documented the global water vapor cycle showing dramatic enhancements in the near polar regions during the summer season. From a "follow-the-water" perspective this is where the action is in the current epoch.

   The current Martian water cycle is driven by atmosphere-surface (including polar caps) interactions that control the sublimation and condensation of water ice on the Martian surface. The surface is both a source and a sink for water depending on local conditions (e.g. ice abundance, temperature, porosity). The atmosphere transports the water vapor from location to location removing it from some sites, depositing it at others. The details depend on a complex interplay of surface characteristics (porosity, composition, albedo), atmospheric properties (temperature, circulation patterns), and insolation (variable on daily, seasonal, and geological times scales). Many variables that control water transport into and out of the regolith (e.g. soil porosity, the ice-to-soil mixing ratio, stratigraphy, relative humidity) cannot be measured well from orbit. Recently, large abundances of ground ice at high latitudes in the uppermost ~1 m of the surface have been measured by the Odyssey spacecraft . This region has in the past been suspected of containing near surface ice based on landforms, including a recently proposed thin (meters) mantle that is currently ice-rich.

   At the season we are landing (northern summer), large amounts of water vapor have been observed in the atmosphere. Water vapor transport and the water cycle are central to the question of why the permanent north polar cap consists of water ice and the southern one is carbon dioxide. Multiple hypothesis and models for the transport process have been proposed, including obliquity driven processes, scavenging of water by ice-clouds tied to seasonal cloud formation, and topographically forced asymmetry.

   Adding further complexity to the history of water in the northern hemisphere are geologic processes that may have contributed stochastically to the cycle by introducing large amounts of water at specific locations through outflow channels, perhaps even creating oceans. The fate of these oceans (and indeed of all the water released by the outflow channels even if "oceans" did not form) is currently unknown. Did the water sublimate away? Is it buried under the surface? Understanding the transport processes today will be an important first step in answering the question about where the water is now.

   The “follow the water” theme goes beyond geology and climate studies. Water sustains life. Phoenix will land where its robotic arm can reach the ice layer. The instrument package on Phoenix has been chosen to characterize the soil and ice and will also assess the habitability of the ice-soil zone just a fraction of a meter below the surface. The regolith offers protection from harsh ionizing UV radiation and desiccation from drying winds. Have there been warmer periods in Mars’ history when this ice layer has melted? Does the ice represent a habitable zone where life forms could grow and reproduce during moist conditions? Continuing the Viking quest, but in an environment known to be water-rich ,  Phoenix will search for signatures of past life that may be found in disequilibrium isotopic ratios and fossil microbe communities.