Michael B. McElroy is Gilbert Butler Professor of Environmental Studies at Harvard University, where he also serves as Director of the Center for the Environment. He participated in the early Mariner missions to Mercury, Venus, Mars, Jupiter, and Saturn and the Viking mission to Mars. The author of more than 200 papers on atomic physics and planetary atmospheres, he is a Fellow of the American Academy of Arts and Sciences, the International Academy of Astronautics, the American Geophysical Union, and the American Association for the Advancement of Science.