Science Considerations Driving the Choice of the Phoenix Mission Landing Site

Introduction: The Phoenix mission will be the first of a series of planned Scout missions to Mars. Launching in August 2007, the spacecraft will land on the northern polar plains in May 2008. During the summer season, a robotic arm (RA) will dig a trench and provide samples to instruments on the deck of the spacecraft. A set of experiments is planned to help us understand the chemistry and mineralogy of the surface materials down to a layer where ice is stable. Because our landing site is selected with the expectation of finding water ice near the surface, the Phoenix mission may provide the first in situ examination of water on Mars. Science Objectives: Our science objectives are all based on NASA's crosscutting theme to " follow the water. " Previous missions have found this to be a difficult task; only tiny abundances of water vapor in the thin atmosphere and exposed water ice on the northern polar cap have been characterized. This situation changed early in 2002 when large amounts of water ice were clearly seen by the Odys-sey Gamma Ray Spectrometer (GRS) [1] in the cir-cumpolar regions. Modeling the gamma ray and neutron fluxes, the GRS team calculated that high concentrations of ice, up to 80% by volume, are to be found within 50 cm of the surface from the poles down to about 60° latitude. The Phoenix mission derives its goals from the scientific characterization of the subsurface ice: its interaction with the atmosphere, the effects on surface morphology, the history of the ice, and its biological potential. Phoenix science goals are achievable in the absence of ice-layers at our single landing site. The fact that we are in an ice-rich region with ice in close proximity is sufficient. Regional melting and atmospheric exchange will affect all the soils. Science goal #1: Study the history of water in all its phases. The circumpolar plains are active today and a primary reservoir in the cycle of water transport on Mars. Orbiter measurements show large seasonal variations in the atmospheric humidity and CO2 frost blanketing the surface. " Freeze-thaw " processes appear to be similar to those on Earth causing hum-mocky and polygonal terrain features. Quantifying the volatile inventory locked into the arctic soils and its seasonal exchange with the atmosphere through the soil overburden, even at one location, is a giant