We are indebted to conversations with many members of the Mars Architecture Definition Team. This work was funded in part by the National Aeronautics and Space Administration Exobiology and Astrobiology programs.
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Article published online before print: Proc. Natl. Acad. Sci. USA, 10.1073/pnas.040539497.
Article and publication date are at www.pnas.org/cgi/doi/10.1073/pnas.040539497