The American astronomer and popularizer of science Carl E. Sagan (1934-1996) studied the surfaces and atmospheres of the major planets, conducted experiments on the origins of life on earth, made important contributions to the debate over the environmental consequences of nuclear war, and wrote a number of popular books explaining developments in astronomy, biology, and psychology.

Carl Edward Sagan was born November 9, 1934, in New York City. Pursuing a boyhood fascination with the stars, he studied astronomy at the University of Chicago, receiving his undergraduate degree in 1954 and his doctorate in 1960. After holding teaching and/or research posts at the University of California-Berkeley, Harvard University, the Smithsonian Astrophysical Observatory, and Stanford University, Sagan became director of Cornell University's Laboratory for Planetary Studies and David Duncan Professor of Astronomy and Space Science (1970). In addition to his academic appointments Sagan served as a consultant to the National Aeronautics and Space Administration (NASA) and was closely associated with the unmanned space missions to Venus, Mars, Jupiter, and Saturn. Sagan's work in the popularization of science, which brought him public recognition as author, lecturer, andtelevision personality, won for him the Pulitzer Prize in 1978.

Carl Sagan's main contributions to science were made in the fields of planetary studies and the origin of life. His first major research effort was an investigation of the surface and atmosphere of Venus. In the late 1950s the prevailing scientific view was that the surface of Venus was relatively cool, life of some sort might exist on the planet, and the observed Venusian radio emissions had their origins in the activity of charged particles located in an atmospheric layer. Sagan (1961) overturned this by showing that the emissions could be explained by simply assuming that the Venusian surface was very hot, over 300 degrees Centigrade, and therefore hostile to life. He accounted for the high temperatures by positing the existence of a "greenhouse effect" that resulted from the sun's heat being trapped between the Venusian surface and the planet's carbon dioxide cloud cover. This hypothesis was confirmed by an exploratory space vehicle sent to Venus by the Soviet Union in 1967.

Solar System Research

The physical characteristics of the surface of Mars have long interested astronomers and science fiction writers. Telescopic observation of the planet revealed distinctive bright and dark areas on its surface. This led some to speculate that large regions of Mars were covered with vegetation subject to seasonal changes. So matters stood until the mid-20th century, when radar and other new means of surveillance were used to gather information on the topography, temperature, wind velocities, and atmosphere of Mars. Reviewing this newly collected data, Sagan concluded that the bright regions were lowlands filled with sand and dust blown by the wind and that the dark areas were elevated ridges or highlands. Hence there was no need to assume the existence of extensive Martian plant growth.

Sagan's scientific interest in planetary surfaces and atmospheres led him to investigate the origins of life on earth and to champion the study of exobiology (the biology of extraterrestrial life). In the mid-1950s Harold Urey and Stanley Miller successfully produced key organic compounds in the laboratory by simulating the physical and chemical conditions that prevailed upon earth shortly before the first forms of life appeared. Building upon this research, Sagan irradiated a mixture of methane, ammonia, water, and hydrogen sulfide. In these experiments he was able to produce amino acids and adenosine triphosphate (ATP), complex chemical compounds which are crucial to living cells. Sagan's foray into biochemistry contributed to a better understanding of the nature and origins of terrestrial life and testified to his competence in fields of science beyond astronomy.

Popular Writing

It is not his scientific achievements but his popular books and his television appearances that have made Sagan a well-known public figure. In 1973 he published The Cosmic Connection, a lively introduction to space exploration and the search for extraterrestrial life. Four years later there appeared his Pulitzer Prize winning book on the evolution of human intelligence, The Dragons of Eden. Drawing upon recent work in neuro-physiology and exploring the brain-computer analogy and studies of sleep and dreaming, as well as interpretations of mythology, Sagan created a highly readable, original, and witty account of the development of the human intellect. Another of Sagan's books, Cosmos (1980), deserves notice because it was written in conjunction with his well-received television series of the same name. In this work Sagan offered a summary history of the physical universe, showed how the cosmos came to be understood with the help of modern science, and warned that the earth was in danger of being destroyed by a nuclear holocaust.

In December of 1983 Sagan, with colleagues R. P. Turco, O. B. Toon, T. P. Ackerman, and J. B. Pollack, published "Nuclear Winter: Global Consequences of Multiple Nuclear Explosions," an article which transformed the world-wide public debate over nuclear policy. The authors claimed that in a nuclear war tremendous quantities of soot and dust would be injected into the atmosphere to form a gigantic black cloud covering most of the Northern Hemisphere. This cloud would reduce the incoming sunlight by more than 95 percent for a period of several weeks and affect the climate on earth for a number of years thereafter. During the cold, dark nuclear winter the vegetation which animals and humans need for sustenance would be seriously depleted and great harm would be done to the ecosystem and to human society. In this instance, as so often during the course of his career, Sagan drew upon his extensive knowledge of the forces operating in the atmosphere. However, that knowledge was not used to explain the features of some remote planet but to send a message warning the entire human race of the terrible consequences of nuclear warfare.

Sagan continued to work and proselytize for the furthurance of science until his death in Seattle on December 20, 1996, of pneumonia brought on by a rare bone marrow disease. In July of the following year, upon successful touchdown and deployment on the surface of Mars, the Pathfinder Lander was renamed The Dr. Carl Sagan Memorial Station.

Associated Works

Further Reading

• Sagan's career in science and public life to the mid-1970s is covered in Henry S. F. Cooper, Jr., "Profiles (Carl Sagan--I, II)," The New Yorker, June 21 and 28, 1976. In 1999 two biographies were published: Carl Sagan: A Life and Carl Sagan: A Life in the Cosmos. Among Sagan's many publications see: Communication with Extraterrestrial Intelligence (1973), The Cosmic Connection (1973), The Dragons of Eden (1977), Broca's Brain (1979), Cosmos (1980), Comet (1985), and Contact (1985).
• Goodell, Rae, The Visible Scientist, Little, Brown, 1975.
• Sagan, Carl and Richard Turco, A Path Where No Man Thought: Nuclear Winter and the End of the Arms Race, Random House, 1990.
• Sagan, Carl, Pale Blue Dot: A Vision of the Human Future in Space, Random House, 1994.
• Sagan, Carl, The Demon-Haunted World: Science as a Candle in the Dark, Random House, 1996.
• Sagan, Carl, Billions and Billions : Thoughts on Life and Death at the Brink of the Millennium, Random House, 1997.