NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.

This study was supported by Contracts No. NASW-96013 and No. NASW-99037 between the National Academy of Sciences and the National Aeronautics and Space Administration. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the agency that provided support for the project.

Cover: “First Light,” by Pat Rawlings and commissioned by NASA, depicts the first human travelers to Mars exploring the enormous Noctis Labyrinthus canyon system. Just after sunrise, early morning fog masks the canyon floor 4 miles below. These scientists-explorers conduct geological and meteorological research in order to help us better understand the characteristics of our sister planet and possibly our own Earth. Reproduced courtesy of the artist and NASA.

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