The Mars Human Precursor Science Steering Group was chartered by MEPAG in June 2004 to analyze the priorities for precursor investigations, measurements, and technology/infrastructure demonstrations that would have a significant effect on the cost and risk of the first human mission to Mars. Based on this analysis, the MHP SSG proposes the following revised phrasing for MEPAG's Goal IV, Objective A, and within it the investigations that follow (in priority order). The measurements needed to carry out these investigations are described in the subsequent sections of this white paper. All of the measurements listed below, which are listed in priority...