Carl Sagan and the Search for Life

Part of the Cosmic Horizons Curriculum Collection.

He was a leading planetary astronomer, a pioneer in the search for extraterrestrial biology, a spellbinding teacher, and the most effective public advocate for the values of science the world has ever seen.
Carl Sagan (1934-1996), American planetary astronomer, exobiologist, popular educator, and advocate for science. Portrait of Carl Sagan courtesy of Andy Levin/PARADE.

To hundreds of millions of people, Sagan communicated his passion for the universe of science. “When you’re in love,” he said, “you want to tell the world.”

Carl Sagan (1934–1996) grew up in a working-class family in Brooklyn. At age seven, he went to the public library to find out what the stars are. The answer—that the stars are suns, only very far away, and the Sun is a star, but close up—opened boundless vistas in his young mind. He understood that if those countless stars are suns, they might have their own planets. The universe could be teeming with life. The idea was delectable.

Sagan also learned about a powerful method, called science, that could help him explore such ideas. He knew then what he wanted to do with his life, and he prepared himself well. He went to the University of Chicago, where he studied biology and physics, and earned his Ph.D. in astrophysics in 1960. His mentors were the geneticists Hermann Muller and Joshua Lederberg, the geochemist Harold Urey, and the planetary astronomer Gerard Kuiper. Three of them were Nobel Laureates.

"Ask courageous questions. Do not be satisfied with superficial answers. Be open to wonder and at the same time subject all claims to knowledge, without exception, to critical scrutiny. Be aware of human fallibility. Cherish your species and your planet."—Carl Sagan 

As part of his wide-ranging Ph.D. thesis, Sagan solved an outstanding puzzle in solar system astronomy: Why is Venus such a strong source of microwave radiation? At the time, it was widely assumed that Venus had a warm and wet climate, a plausible enough environment for life. But Sagan calculated that the dense carbon dioxide atmosphere of Venus sustains an extreme greenhouse effect, which keeps the surface hot enough to melt lead and to emit the observed microwaves. Many years later the Pioneer Venus spacecraft verified this explanation. Surely nothing could live near the surface of Venus.

After teaching genetics at Stanford University’s School of Medicine, Sagan joined the astronomy faculty at Harvard University, where he gave a series of popular lectures called “Planets as Places.” This was a radical idea at the time. Few scientists had thought seriously about the geology and climates of other worlds. Few if any had recognized that the study of other planets could provide vital clues for understanding the Earth. By 1963, Sagan was already concerned that increasing carbon dioxide in the Earth’s atmosphere would lead to serious global warming.

These were the years when the spacecraft exploration of the solar system was just beginning. Sagan became a familiar figure at NASA’s Jet Propulsion Laboratory in Pasadena, where he was a principal investigator in every American spacecraft mission to the planets, including the Mariner flybys of Venus and Mars, the Viking orbiters and landers sent to Mars, and the Pioneer and Voyager missions to explore the outer solar system.

In 1967, Sagan and James Pollack, his first graduate student, solved another major mystery of the solar system: What causes the seasonal “wave of darkening” observed on Mars? The most popular view ascribed the phenomenon to seasonal changes of vegetation on the planet. But Sagan and Pollack proposed instead that seasonal winds alternately deposit light-colored Martian dust on darker highland rock and then remove it again. This explanation was later verified by the Viking spacecraft in orbit around Mars.

In 1968, Sagan joined the astronomy department at Cornell University. There he established and ran a laboratory, taught popular courses (including one on “Critical Thinking“), edited Icarus (which he turned into the leading scientific journal of solar system studies), supervised graduate students, and maintained a prodigious output of publications. He authored or co-authored two dozen books and more than a hundred scientific papers, many which were seminal, including forty on planetary atmospheres, fifty on other solar system topics, thirty-three on astrophysical and laboratory syntheses of organic molecules, thirty on extraterrestrial biology and SETI (the search for extraterrestrial intelligence), and others on science policy.

At Cornell, Sagan directed an extensive series of laboratory experiments to simulate the atmospheric and surface chemistry of planets, moons, and comets. The results showed that, under a wide range of observed conditions in the solar system, prevailing sources of energy (such as ultraviolet light and electrical discharge) will stimulate the production of complex organic molecules, including the chemical building blocks of life, in high yields. These results were regarded with some skepticism at the time. Today, we know that such substances exist in giant interstellar clouds and on the surfaces of many worlds in the outer solar system. The stuff of life appears to be common in the universe. Sagan assumed that life itself was also widespread.

But what about intelligent life? And advanced civilizations? Despite confident assertions on all sides of the question, no one knows whether they are numerous, rare, or nonexistent. One point however seems clear: Other things being equal, we should expect that the number of advanced civilizations in the universe will be proportional to their average lifetime. If the average civilization lasts no more than a few centuries, then at any given time there will not be very many of them. But if some survive for many millions of years, they will be more common. In that case, the nearest civilizations might be close enough for us to detect with radio telescopes. The only way to find out is to make the necessary observations. With that in mind, Sagan took part in and worked to build public and institutional support for a number of SETI projects.

As the nuclear arms race began to escalate again in the late 1970s, Sagan became increasingly concerned about the life expectancy of our own civilization. In March 1983, he very nearly died during a ten-hour emergency operation to replace his esophagus. While still in intensive care, he learned about President Reagan’s call to build a space-based anti-missile “shield.” This he regarded as a technically hopeless scheme that would destabilize nuclear deterrence and perhaps lead to the very war it was supposed to prevent. From his hospital bed, Sagan promptly drafted a petition to Congress opposing the project. Many leading American scientists signed the petition, and Sagan remained a strong critic of “missile defense.”

In the same year Sagan also participated in an extensive scientific study of the atmospheric consequences of nuclear war. He and his colleagues calculated that smoke from firestorms in cities might reach the stratosphere and block enough sunlight to cool the Earth, causing a catastrophic “nuclear winter.” Their analysis used techniques previously developed to model the cooling of the Earth resulting from major volcanic eruptions and the more drastic cooling due to dust lofted by the asteroid impact that destroyed the dinosaurs. Nuclear winter was at once plausible and controversial. Later, more detailed studies suggested that the climatic consequences of nuclear war would be less severe than calculated, but still sufficient to cripple agriculture in the northern hemisphere.

The widespread discussion of nuclear winter contributed to a substantial rethinking of nuclear war doctrines, particularly in the Soviet Union. In 1986, Sagan briefed the Soviet Central Committee on the subject. Some of those present later said that his effect was profound. Gorbachev personally told Sagan that he had studied the nuclear winter research and it bolstered the case for deep cuts in the nuclear arsenals. Some Russian scientific colleagues credit Sagan with having a major influence on ending the cold war.

Sagan regarded the prevalence of scientific ignorance in a technological society as a prescription for disaster. To promote public understanding of and support for science, he created the popular Cosmos television series, co-founded The Planetary Society (a non-profit public interest group) and used countless articles and interviews to popularize the values of reason, curiosity, critical thinking, and an unbiased search for the truth. While sharply critical of pseudoscience, nationalism, chauvinism, fundamentalism, and other irrational beliefs, he consistently defended the widest freedom of thought and expression. Sagan was never afraid to entertain extraordinary ideas, but he always insisted that “extraordinary claims require extraordinary evidence.”

He taught that the unrivaled success of science is due to its combination of openness to new ideas with the obligation to subject those ideas to the most critical scrutiny. He believed that science and democracy share essential values: a free exchange of ideas and information, accountability, and the questioning of authority. Sagan resolutely accepted the verdicts of science even when they contradicted his own fondest hopes and expectations. While he would have liked nothing better than to find evidence for life on other worlds, he instead argued the case that the surface of Venus must be lifeless and that seasonal changes on Mars have nothing to do with life. In both instances, he was guided by a hard-headed analysis of the evidence. By his life and works, he taught that we must always follow the evidence rather than accept uncritically what we merely wish to believe. He maintained that this principle is as valid in the social and political worlds as in the sciences.

In December 1996, after a courageous two-year struggle, Sagan died of a rare bone marrow disease. The Federation of American Scientists issued a tribute, which noted that in the midst of a life dedicated to scholarship and the popularization of science, Carl Sagan “found the time, and had the courage, to be an intellectual gladiator on issues involving the planet’s survival and, in particular, on the prevention of nuclear war.” Like a magnificent comet, he illuminated the lives of millions, and we will not see his like again.

—Steven Soter

This is an excerpt from COSMIC HORIZONS: ASTRONOMY AT THE CUTTING EDGE, edited by Steven Soter and Neil deGrasse Tyson, a publication of the New Press. © 2000 American Museum of Natural History.