The NASA-sponsored GeoMars conference held in Houston, Texas, U.S., in 2001 (http://cass.jsc.nasa.gov/meetings/geomars2001/) raised new issues and new questions that are challenges for Mars exploration today. Experts came from around the globe, contributing their ideas and experience to develop a roadmap for the next 5 or 10 years of Mars missions and to define a comprehensive geophysical program to map the planet-wide distribution of subsurface water. This exchange of information and ideas caused many of us to rethink a few things.

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Two basic problems have emerged with the current models for the subsurface of the Red Planet. The first,...

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