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    Like its namesake in ancient mythology, the 2007 Phoenix Mars Scout mission is reborn out of fire; created from the embers of previous missions. Phoenix returns to flight the Mars Surveyor Programís 2001 (MSPí01) lander with a rich and diverse scientific payload chosen from the 1998 Mars Polar Lander (MPL'98) and the MSPí01 missions. This complement of spacecraft and payload is ideally suited to perform a scientific analysis of the Martian arctic soils for clues to its geologic history and potential for biology. Phoenix is the first fully competed and investigator lead mission as part of NASA's new Mars Scout program. 

    The Mars programís crosscutting theme to "follow the water" proves to be difficult on a desert planet. Besides the traces of water vapor in the thin atmosphere and exposed water ice on the Northern Polar cap, only vestigial remnants mark the flowing rivers and crater lakes from ancient times. However, Odyssey scientists announced early in 2002 that large amounts of water ice are clearly seen by the Gamma Ray Spectrometer (GRS) in the circumpolar regions. Modeling the gamma ray and neutron fluxes, they predict that high concentrations of ice, up to 80% by volume, are to be found within 50 cm of the surface.

    The Phoenix will land in the northern plains between 65 and 75°N. Continuing GRS measurements verify that the Martian arctic, near 70°N, is the holy grail: water ice protected from solar UV by a thin layer of regolith. The signatures near longitude -120°W are nearly as strong as the exposed ice cap. In addition, high-resolution images from the Mars Orbiter Camera (MOC) on the Mars Global Surveyor (MGS) spacecraft show a "basketball-like" texture on the surface with low hummocks spaced 10ís of meters apart; polygonal terrain is also common. These geologic features are common in Earthís polar regions indicating permafrost and an active freeze-thaw cycle.

Depth To Ice Layer (cm)

     Phoenix will be the first scientific mission to study the history of water in all its phases. The polar regions are known from remote sensing to be critical to the seasonal transport of water and carbon dioxide. Quantifying the volatile inventory locked into the arctic soils and the water chemistry of wet soils, even at one location, is a giant step toward modeling the weather processes and climate history of Mars. As on Earth, the past history of water is written in the subsurface. Liquid water changes the soil chemistry in characteristic ways. Obliquity wander and precession are known to strongly influence the climate on time scales of 100,000 years or more. In particular, the water ice may melt and wet the overlying soil in cycles commensurate with orbital dynamics.

     Phoenix will search for evidence of a habitable zone and assess the biologic potential of the ice-soil boundary Microbial colonies can survive in a dormant state for eons. Recent work shows that water ice melts onto soil crystals at temperatures as cold as Ė20° C, thus activating dormant microbes. As temperatures increase, growth and reproduction may begin. The Phoenix lander brings instruments to the surface that acquire samples of this biological paydirt and test for signatures related to biology. Organic molecules will be easily detectable even in small concentrations. Other experiments will determine the chemical and paleo-hydrological properties of the soils under both dry and wet conditions.