From the revolutionary astronomical achievements to the social activism and inspiration for generations of scientists - how Carl Sagan shaped science and our understanding of the universe.
By Jean-Luc Margo, Professor of Earth, Planetary and Space Sciences, University of California, Los Angeles
On November 9, 2024, the world celebrated Carl Sagan's 90th birthday - but unfortunately without his presence, as he died in 1996 at the age of 62.
Most people remember him as the creator and presenter of the TV series "Cosmos" from 1980, which was watched by hundreds of millions around the world. Others read the science fiction book Contact or the Pulitzer Prize winning book Dragons of Eden. Millions more watched him spread the study of astronomy on "The Tonight Show".
What most people don't know about Sagan, and what has been somewhat dimmed by his publication, is his profound scientific influence, which still resonates today. Sagan was not only an unparalleled science communicator, sharp advocate and prolific author, but also an extraordinary scientist.
Sagan contributed to the advancement of science in at least three significant ways. He published impressive results and insights in over 600 scientific papers, enabled the flourishing of new scientific fields and inspired generations of scientists. As a planetary scientist, I believe that such a combination of talents and achievements is rare and may only occur once in a lifetime.
Scientific achievements
In the 60s, very little was known about Venus. Sagan investigated how the greenhouse effect in the atmosphere saturated with carbon dioxide might explain the unbearable temperatures of Venus - about 465 degrees Celsius. His research is a warning about the dangers of greenhouse gas emissions here on Earth.
Sagan offered a convincing explanation for the seasonal changes in the brightness of Mars, which had previously been wrongly explained as related to vegetation or volcanic activity. He showed that the changes are due to a spirit-driven struggle.
Sagan and his colleagues studied how changes in the albedo (reflectance of light) of the Earth's surface affect our climate. They looked at how detonating nuclear bombs could introduce a huge amount of soot into the atmosphere that would lead to a long period of significant cooling, a phenomenon known as "nuclear winter".
With an extraordinary breadth of knowledge in astronomy, physics, chemistry and biology, Sagan promoted the field of astrobiology - the study of life in the universe. Together with researcher Bishon Kahara at Cornell University, Sagan conducted innovative laboratory experiments and showed that certain components of prebiotic chemistry, such as tholins and amino acids, were formed naturally under conditions simulating planetary environments.
He also studied how prebiotic molecules (prebiotics containing the building blocks of life) might have reached the early Earth via asteroids and comets, and was involved in the biological experiments carried out on Viking rovers on Mars. Sagan even hypothesized the possible existence of balloon-like organisms in the atmospheres of Venus and Jupiter.
The search for extraterrestrial life has gone far beyond our solar system. Sagan was a major proponent of the Search for Extraterrestrial Intelligence (SETI) and took part in a systematic search for extraterrestrial radio signals. He also co-designed gold links attached to the Pioneer Voyager probes that feature sounds of Earth, including greetings in 55 languages.
advocacy
Sagan's research has repeatedly led him to become an eloquent advocate on issues of social and scientific importance. He testified before Congress about the dangers of climate change, opposed the development of nuclear defense systems and the strategic defense initiative known as "Star Wars," and encouraged cooperation between the United States and the Soviet Union in space exploration.
But perhaps the greatest gift he gave to society was the encouragement of critical thinking and the search for truth. He urged people to develop humility and inner discipline to examine their most cherished beliefs and rely on scientific evidence. His book "Haunted World: Science as a Fiddle in the Dark" is an important source for anyone trying to navigate the age of disinformation.
effect
A scientist's influence can be estimated by the number of citations his research accumulates. According to Sagan's Google Scholar page, his work still garners over 1,000 citations a year.
In fact, his citation rate today exceeds that of many members of the National Academy of Sciences, one of the highest honors in science. Sagan was proposed for election to the Academy in the 1991-1992 cycle, but his candidacy was rejected, when more than a third of the members voted to expel him from the Academy. "It is jealousy that prevented your acceptance," an observer wrote to him at that yeshiva, and this was confirmed by other participants. To me, the Academy's failure to accept Sagan is a lasting stain on that institution.
No amount of jealousy can diminish the depth of Sagan's legacy. He not only influenced scientifically but also inspired generations of scientists and made science accessible to the masses. Sagan showed what was possible in the field of science, communication and advocacy. On his 90th birthday, a renewed commitment to these values would be the appropriate way to honor his memory.
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