Like most astronomers, Carl Sagan relied on indirect measurements of faraway planets and galaxies for his research. Years later, even as more precise tools have emerged, his work on planetary atmospheres remains relevant. His passion also led to him creating the Planetary Society.
Take his description of Venus's atmosphere, for example. In the early 1960s, Sagan tackled why the second planet in our solar system trapped heat in its atmosphere. He hypothesized that a greenhouse effect kept Venus at a toasty 900 degrees Fahrenheit (500 degrees Celsius). As sunlight hits the surface of the planet, gases in its atmosphere trap the heat rather than let it escape. Sagan's conclusions opened the door to discovering a similar phenomenon on other planets -- even Earth.
By studying other planets' atmospheres and chemical cycles, Sagan discovered that many of the same processes were at play on our home planet. Despite Earth's early, icy history, geology shows that not all water was bound up in ice. But what propelled the planet to the warmer, wetter place it is today? That's what puzzled Sagan and his colleague George Mullen, who labeled the issue the faint young sun paradox in 1972. Since stars gain energy and luminosity as they age, the sun emitted more and more energy that could have helped Earth thaw. Yet Sagan and Mullen suggested that an increasingly powerful sun couldn't have been entirely responsible for melting young Earth; rather, other greenhouse gas cycles may have accelerated warming. Scientists are still trying to piece together the puzzle today, but Sagan helped draw attention to the paradox.
Mars enchanted Sagan, too, driving his strong desire to explore the red planet. He hammered out the logistics of space missions at the NASA Jet Propulsion Laboratory and described Mars' seasons, citing the planet's seasonal storms as causes behind its shifting features.
The famous astronomer also believed there was more to Saturn's moon Titan than meets the telescoped eye. He hypothesized that complex organic molecules lent a reddish hue to the distant moon, which ended up being true.