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.

Support for this project was provided by Contract NASW 01001 between the National Academy of Sciences and the National Aeronautics and Space Administration. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the sponsor.

Cover: Cover design by Penny E. Margolskee. The lower half of the cover is an image of a microbial mat community found at a depth of about 15 meters in Lake Vanda, Wright Valley, Antarctica; photo courtesy of Dale Andersen, SETI Institution. The upper half of the cover is a composite image of the Orion Nebula made by combining data from the Hubble Space Telescope and the Spitzer Space Telescope; image courtesy of NASA/Jet Propulsion Laboratory-California Institute of Technology, T. Megeath (University of Toledo), and M. Robberto (Space Telescope Science Institute). The crescent on the center right is an ultraviolet image of the planet Venus as seen by the Hubble Space Telescope; image courtesy of NASA/JPL/Space Telescope Science Institute.

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