It has been twenty years since astronomer Carl Sagan’s death on December 20, 1996. Sagan was a planetary scientist of the first rank, with tremendous gifts for public speaking and popular writing. He used these gifts not only to advocate for the importance and excitement of science, but also for the centrality of reason, the settling of disagreements where possible by appeals to evidence, and the essential similarity of humans across ethnic divides. His greatest commitments today stand in stark antithesis to the worst of the current national and world situation.
Carl Sagan made key contributions to our understanding of the Solar System and Earth within it. Perhaps most important was his recognition, with George Mullen, of the “early faint Sun paradox”: if our Sun had evolved like a typical G-class star—the conservative assumption—then its luminosity would have been lower in the past. This in turn means that prior to about 2 billion years ago, Earth would have been too cold for our current greenhouse atmosphere to have kept surface temperatures high enough for liquid water—in contradiction to abundant evidence. The problem gets more severe the further back in time one goes, posing a challenge for our understanding of the origin of life. Earth’s early atmosphere therefore presumably provided a much more powerful greenhouse than is the case today, and understanding whether and why this was the case became a major topic within the geosciences.
Read more:
https://blogs.scientificamerican.com/guest-blog/carl-sagans-extraordinary-career/
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